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Structural and electronic promotion with alkali cations of silica-supported Fe(III) sites for alkane oxidation
Journal of Catalysis 296 (2012) 77–85
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Structural and electronic promotion with alkali cations of silica-supported Fe(III) sites for alkane oxidation Dario Prieto-Centurion, Andrew M. Boston, Justin M. Notestein ⇑ Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60202, USA
a r t i c l e
i n f o
Article history: Received 2 August 2012 Revised 10 September 2012 Accepted 11 September 2012 Available online 23 October 2012 Keywords: Alkali promoters Single-site catalyst Supported oxide EDTA ligand High dispersion
a b s t r a c t Promoters and precursors can control oxide phase, dispersion, and per-site reactivity of supported oxide catalysts. Previously, dispersed FeOx–SiO2 resulted from Fe3+ ethylenediaminetetraacetate (FeEDTA) precursors, with NaFeEDTA giving enhanced dispersion and oxidation rates vs. NH4FeEDTA. Here, catalysts were synthesized by sequential alkali deposition and Fe3+ impregnation. At up to 0.9 Fe nm2 from NH4FeEDTA and equimolar alkali, UV–visible and H2 TPR were consistent with isolated Fe3+ and small FeOx clusters. Omitting alkali, using Fe(NO3)3, or using Fe/alkali >1 gave evidence of larger agglomerates. For Fe/alkali 61 on non-porous SiO2, initial turnover frequencies in adamantane oxidation using H2O2 were independent of surface density. TOF increased as 6.3, 8.8, 15.4, and 20.9 (±0.3) ks1 for Li+, Na+, K+, and Cs+, respectively, increasingly linearly with decreasing electronegativity. These results give a synthesis–structure–function taxonomy with alkali as an electronic and structural promoter of dispersed FeOx species for alkane selective oxidation. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Supported metal oxide catalysts tend to agglomerate during the deposition step or the subsequent heat treatments employed to activate them for catalysis [1,2]. Extensive agglomeration of supported metal oxides lowers the number of active atoms exposed to reactants, decreasing the overall material efficiency of the catalyst, and also potentially alters the intrinsic reactivity of each exposed atom [1,2]. Much effort has been placed in developing synthesis methods and treatment techniques that create and preserve active phases with high dispersion – a significant challenge in the field [3] – however methodologies that rely on common precursors such as sulfates, nitrates, and acetoacetonates typically lead to a variety of surface species [4–8], while those that produce more uniform distributions of active structures can rely on cumbersome preparations or precursors [9,10]. The challenge is partially addressed by using ‘‘protected’’ metal cations as precursors [1,11,12]. Bulky organic ligands can prevent close approach of two cations and formation of M–O–M structures, and thereby maintain dispersion, although agglomeration can still occur if the ligands are removed because of the formation of mobile intermediates during oxidative heat treatment [1,11–14]. Heat treatment under inert gasses or under dilute NO has been shown to improve dispersion of the active phase, but small metal oxide agglomerates are still formed [13]. For optimal performance and ease of prepara⇑ Corresponding author. Fax: +1 847 491 3728. E-mail address: [email protected] (J.M. Notestein). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.09.004
tion, catalyst synthesis methods are desired that produce active phases with most metal cations exposed after heat treatment under stagnant air [3]. In addition to the practical and economical benefits of their greater material efficiency, heterogeneous catalysts with welldefined and highly dispersed surface species offer the opportunity to gain straightforward insight into structure–function relationships, including the effect of supports and structural vs. electronic promoters. In recent work by the authors, it was observed that impregnation of SiO2 with aqueous, anionic Fe3+ complexes with ethylenediaminetetraacetate (FeEDTA–) produced highly dispersed Fe3+ phases after calcination [14]. It was also noticed that the counter cation played a crucial role in FeOx speciation. Na+ led to catalysts with reactivity independent of Fe3+ surface density and evidence of atomic dispersion, while NHþ 4 led to the formation of amorphous FeOx agglomerates. In that system, the Fe/Na ratio was necessarily 1 by the precursor used. Select examples of promotion show effects on dispersion and activity at a variety of metal/promoter ratios. Pt on Na–modified SiO2 showed evidence of atomic dispersion and increased activity in water–gas shift reaction at Pt/Na ratios <0.33 [15]. Likewise, doping with La showed similar promotion effects for Cu2+ on Cu–Pd/a-Al2O3 catalysts for high-temperature removal of volatile organic compounds [16]. Promotion effects were observed for Cu/La ratios as high as 2.7, but the largest effects occurred at ratios of 1 or less. For VOx on SiO2, doping with phosphorous formed small VOPO4 surface species that were stable under reaction conditions and more selective toward butane partial oxidation to maleic
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acid, relative to unpromoted VOx on SiO2 [17]. In that study, the optimal V/P ratio was less than 1, the VOPO4 stoichiometry, because P also deposited elsewhere on the support. Doping SiO2 with small amounts of Cs+ before VOx deposition formed different surface species more selective toward the formation of alkenes during butane partial oxidation [18]. In that case, V/Cs molar ratios as high as 100 showed considerable promotion effects. The role of Cs+ was not ascribed to direct coordination, but rather to titration of acid sites that nucleated active VOx structures prone to complete oxidation. This work explores the effect of the amount and identity of alkali dopants on the dispersion and speciation of a supported Fe3+ oxide active phase derived from either a chelating Fe3+ precursor or Fe(NO3)3. First, sequential deposition of alkali then chelated Fe3+ as NH4FeEDTA or Fe(NO3)3 was compared to simultaneous deposition of alkali and Fe3+ via NaFeEDTA, as described in previous work. Second, the effects of Fe/Na ratios were determined at different absolute surface densities of alkali and Fe3+. Finally, multiple alkali cation species were compared to separate structural and electronic promotion.
2. Experimental 2.1. Materials and instruments All catalyst syntheses were performed under air at ambient conditions unless otherwise specified. Reagents were obtained from Sigma–Aldrich and used as received. Porous SiO2 supports were obtained from Selecto Scientific and dried at 120 °C under ambient pressure for >12 h before use. Electrospray Ionization Spectrometry (ESI-MS) measurements of Fe3+ chelates were taken using a Thermo Finnegan LCP instrument. Specific surface areas were calculated, using the BET equation, from N2 physisorption isotherms obtained with a Micromeritics ASAP 2010 instrument. All materials were degassed 8 h at <5 mTorr and 125 °C before measurements. Elemental analysis was done using a Varian MPX ICP-OES calibrated with standards of known concentration. Samples for elemental analysis were prepared by dissolving 50 mg solids in 1 mL HF, followed by dilution to 50 mL in H2O. Diffuse reflectance UV–visible (DRUV–Vis) spectra were recorded with a Shimadzu 3600 UV–visible–NIR spectrometer fitted with a Harrick Praying Mantis diffuse reflection attachment and using polytetrafluoroethylene (PTFE) as the baseline standard. All materials were ground in a mortar and pestle before acquiring spectra, and all diffuse reflectance spectra were converted to pseudo-absorption using the Kubelka–Munk transform [19]. Solution UV–Vis spectra were acquired using a standard 1 cm pathlength quartz cell. Thermogravimetric analyses (TGA) were carried out on a TA Instruments Q500 thermogravimetric analyzer with an evolved gas analysis furnace. Temperature-programmed reduction experiments were performed in a TA Instruments Q500 TGA fitted with a Pfeiffer Thermostar Q200 process mass spectrometer and using Ar as the internal standard. As-made materials were heated in 100 sccm of 90% O2 and 10% He from ambient temperature to 800 °C at 10 °C min1 and held for 30 min, cooled to near ambient temperature in 100 sccm He, then heated in 100 sccm of 4.5% H2, 4.5% Ar, balance He to 1000 °C at 10 °C min1. The m/z = 18 signal, H2O, was normalized by m/z = 40, Ar, and calibrated against CuO reduction. X-ray absorption near edge structure (XANES) spectroscopy studies were performed at Sector 5 of the Advanced Photon Source, Argonne National Laboratory, on the Dupont-Northwestern-Dow Collaborative Access Team (DND-CAT) bending magnet D beamline. Incident intensity and transmitted intensity were measured
using Canberra ionization chambers. Fe standards (Fe2O3 and FeO) were brushed on Capton tape and their spectra were measured in transmittance. Beam energies were calibrated against Fe foil standard also measured in transmittance. Supported Fe materials were pressed into 50 mg pellets 2.5 cm in diameter and mounted on a 9-pellet autosampler. Due to the low metal loadings of the pellets, spectra were measured as fluorescence intensity using two four-channel SII Vortex-ME4 detectors. Samples were mounted at incident angle h = 45 ± 5° with respect to beam and fluorescence detectors, which were perpendicular to one another. 2.2. NH4FeEDTA synthesis Synthesis of ammonium Fe(III) ethylenediaminetetraacetate, or NH4FeEDTA, was adapted from that by Meier and Heinemann [20]. Equimolar quantities of H4EDTA (10 mmol) and Fe(NO3)39H2O (10 mmol) were charged into a two-neck flask and dissolved in 25 mL of H2O at 60 °C under N2. Upon complete dissolution of Fe3+ precursor and ligand, four equivalents of NH4HCO3 (40 mmol) were slowly added. Following cooling to room temperature, the reaction solution was reduced to 5 mL by rotary evaporation and stored at 20 °C overnight. The resulting crystals were washed in acetone and dried under dynamic vacuum (20 mTorr) for 12 h. ESI-MS, in negative ion mode, showed m/z = 346 as the most prominent species, corresponding to the FeEDTA anion. TGA between 200 °C and 800 °C of the product showed a mass loss due to combustion corresponding to 99.3% purity according to the ratio of organic to inorganic components (Fig. S1). 2.3. Support preparation Non-porous SiO2 was prepared following a modification of the method described by Stöber et al. [21]. A polypropylene container was charged with 300 mL of 200 proof ethanol, 18 mL 30% aqueous NH4OH, and 6 mL H2O. The solution was heated to 60 °C and, once the temperature had remained stable for 1 h, 24 mL of tetraethoxysilane (TEOS) was added and the reaction solution was stirred overnight. The solvent was removed by rotary evaporation, and the resulting white solid was thoroughly ground and heated under ambient air to 600 °C for 6 h. Two batches of non-porous SiO2 with specific surfaces areas of 130 m2 g1 and 91 m2 g1 were prepared (marked d and e on Table 1). Non-porous and porous SiO2 were modified by incipient wetness impregnation (IWI) with aqueous solutions of alkali precursors, LiOH, NaHCO3, KHCO3, and CsHCO3 (0.1–0.6 M). The volumes required to wet the support were approximated by the total pore volume determined from N2 physisorption. Following impregnation, the modified supports were stored in partially covered containers at ambient conditions for 48 h, stirred approximately every 12 h, and then heated for 12 h at 120 °C to create alkali-modified SiO2. After drying the alkali-modified materials, pore volumes were re-measured and TGA redone, the latter showing no evidence of residual combustible or volatile groups on the surface. Likewise, FTIR showed no evidence of residual carbonates after the heat treatment. 2.4. Catalyst preparation Fe was deposited on alkali-modified SiO2 by IWI with (0.1– 0.6 M) aqueous solutions of NH4FeEDTA. Transmission UV–visible spectra of the impregnation solution showed no indication of dimeric FeEDTA complexes with characteristic peaks at 475 nm [22]. The impregnated supports, stored in partially covered containers at room temperature, were dried under ambient conditions for 48 h and under dynamic vacuum (20 mTorr) for 12 h. Subsequently, materials underwent heat treatment in ambient, static air
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2.5. Catalytic reactions
Table 1 Characteristics SiO2-supported Fe materials prepared for this study. Precursor
Alkali
Fe loading
lmol g FeEDTAa,b
Na
a,b,x a,b a,b c a,c c c a,d
Li
e a,d d,x
Na
d a,d a,d
K
e d e
Cs
e ad
, Fe(NO3)3d
d d
Fe2O3b
Li Na Cs None
142 278 403 502 154 297 391 509 113 98 186 100 144 191 115 89 189 66 85 180 88 90 85 240
1
Fe/alkali 2
wt.%
Fe nm
0.79 1.55 2.25 2.81 0.86 1.66 2.19 2.85 0.63 0.55 1.04 0.56 0.80 1.07 0.64 0.50 1.06 0.37 0.47 1.01 0.49 0.50 0.48 1.34
0.21 0.46 0.84 1.26 0.25 0.49 0.64 0.84 0.52 0.65 0.86 0.46 0.67 0.89 0.53 0.59 0.88 0.44 0.56 0.84 0.41 0.42 0.39 0.26
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1: 1: 1: 1: 1: 1: 3: 2: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1:
1 1 1 1 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0
a
Loadings confirmed by ICP-OES. Porous SiO2 support with varying Na loading, see Fig. S1 for specific surface areas. c Porous SiO2 support with constant Na loading, 367 m2 g1. d Non-porous SiO2, 130 m2 g1. e Non-porous SiO2, 91 m2 g1. x XANES spectra in Fig. 7.
Alkane oxidation was carried out in a 50 mL round-bottom flask fitted with a Liebig condenser, charged with 50 mg of catalyst, 70 mg (0.51 mmol) of adamantane, and 20 mL of acetonitrile (MeCN), and heated to 60 °C under vigorous stirring. Once the substrate fully dissolved, 1 mL of 30% aqueous H2O2 (9.5 mmol) was injected. At regular time intervals, samples of 300 lL were retrieved using syringes fitted with Whatman microfiber filters (0.7 lm pore size). All aliquots were then treated with Ag powder to decompose residual unreacted H2O2, which could further oxidize the reactants and products during or while awaiting analysis. Control experiments showed no loss of products or reactant due to adsorption or reaction following such sample treatment. Aliquots were analyzed by gas chromatography (GC) using a Shimadzu 2010 GC equipped with a Flame Ionization Detector (FID) and a Thermo-Nicolet TR-1 capillary column. Peak areas of the substrate and reaction products were quantified by comparison with standards of known concentration of adamantane and its expected oxidation products, 1-adamantanol (1-ADL), 2-adamantanol (2-ADL), and 2-adamantanone (2-ADN). Material balances, defined as sum of only these products over conversion of adamantane, were between 85% and 95% in all cases. The lower values were obtained with low Fe-loaded catalysts at low conversion and were thus attributed to limits of instrumental precision.
b
from room temperature to 800 °C at a ramp rate of 10 °C min1. Fe3+ surface densities, inferred from the loadings of the ligand as calculated by TGA, and confirmed by ICP, varied in the range 0.23–1.26 nm2 for porous SiO2 and 0.44–0.89 nm2 for nonporous SiO2 (Table 1, Fe loadings also in wt.% and lmol g1). The concentration of saturated aqueous solution NH4FeEDTA (0.6 M) and low interparticle volume of non-porous SiO2 (0.33 mL g1 and 0.24 mL g1 for samples of surface areas of 130 m2 g1 and 91 m2 g1, respectively) limited the maximum surface loading of Fe3+ to approximately 0.9 Fe nm2. Porous SiO2, with higher total volume (from 1.54 mL g1 to 0.71 mL g1 for surface areas between 554 m2 g1 and 240 m2 g1, Fig. S2) permitted the higher surface loadings shown on Table 1. The reader should note that maximum Fe3+ loadings of materials prepared by the sequential addition of alkali then NH4FeEDTA exceeded loadings reached in our previous work using NaFeEDTA [14]. This was the case expressing Fe3+ loading by mass (2.15 wt.% vs. 2.85 wt.%) and, particularly, surface density (0.39 Fe nm2 vs. 0.86 Fe nm2). The higher aqueous solubility of NH4FeEDTA relative to NaFeEDTA accounted for the higher loadings by permitting the deposition of larger amounts of Fe3+ with a single impregnation. The highest surface loadings observed in this work corresponded to approximately 60% of a FeEDTA- monolayer (1.5 FeEDTA nm2) based on the dimensions of the complex given elsewhere [20]. Transmission electron microscopy and electron diffraction of the heat-treated materials showed no regions of high contrast or crystallinity. Likewise, powder X-ray diffraction showed no diffraction peaks for 2h between 20° and 80°. The absence of any evidence for diffracting crystalline features was expected from the low Fe3+ weight loadings of these materials. Control catalysts were prepared by IWI of Li+, Na+, and Cs+ doped non-porous SiO2 with aqueous Fe(NO3)39H2O and through mechanical mixing of Fe2O3 with SiO2.
3. Results and discussion 3.1. Supports characterization As mentioned above, previous work showed that impregnation of SiO2 with aqueous NaFeEDTA produced highly dispersed Fe3+ phases, while the same procedure with NH4FeEDTA yielded lessreactive FeOx agglomerates [14]. However, NH4FeEDTA was significantly more soluble (0.6 M) relative to NaFeEDTA (0.35 M) and permitted higher Fe3+ loadings with a single impregnation. Here, we sought to use sequential deposition of alkali and NH4FeEDTA in a two-cycle impregnation with the objective of combining the high loadings attainable with the NHþ 4 counter cation and the high dispersions resulting from the Na+ counter cation. Accounting for the extra mass from the added alkali, specific surface areas of the alkali-functionalized porous SiO2 decreased with increasing loading, losing nearly 60% (554–240 m2 g1) at the highest Na+ loading (Fig. S2). This decrease in specific surface area was accompanied by a collapse of the pore structure, in which smaller diameter pores (20 Å) and then larger ones (45 Å) disappeared progressively as the Na+ loading increased (Fig. S3). It is well known that silicas readily dissolve in basic alkaline solutions and pore structures are especially vulnerable to such treatments. Non-porous SiO2 subjected to the same treatment showed less than 7% loss in specific surface area relative to their unmodified counterpart. These results indicated that the small volumes of alkaline solution used to modify SiO2 destroyed mesostructures but did not significantly agglomerate non-porous particulates. 3.2. Fe loadings Under oxidizing atmosphere, as-made Fe-containing materials underwent a one-step decomposition at 240 °C, comparable to that of pure H4EDTA (Fig. S4). Loadings of the NH4FeEDTA complex were determined by TGA assuming that all mass loss between 200 °C and 800 °C, relative to that of the support, corresponded to the loss of EDTA4– ligand (298 g mol1) and NHþ 4 counter cation (18 g mol1) except for 4 O atoms (64 g mol1) either from the ligand or from the O2 oxidant, giving a net fragment of 252 g mol1.
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Fe3+ loadings estimated by TGA using this fragment mass were within 1% agreement of the amount of Fe3+ added during IWI. The loading of selected materials was confirmed by ICP-OES (marked a in Table 1).
0.8 wt %
Fe/Na Ratio
3.3. UV–visible spectrometry
Nominal Fe loading 1.6 wt %
2.2 wt %
2.8 wt %
1/2
1/1
3/2
2/1
1/1
1/1
1/1
1/1
The solution UV–Vis spectrum of aq. NH4FeEDTA showed a characteristic absorption band at 280 nm (Fig. S5-a). The same band was observed in the DRUV–Vis spectrum of as-made SiO2supported NH4FeEDTA (Fig. S5-b), indicating that the complex maintained its structural integrity following impregnation and drying. The identity of the alkali promoter did not alter the spectrum of the supported Fe3+ complex. Dispersion of Fe3+ surface species on heat-treated materials was assessed by DRUV–Vis spectrometry. According to the literature [4,9,23,24], absorption bands below 300 nm correspond to isolated Fe3+ ions, between 300 nm and 500 nm to two-dimensional FeOx surface structures, and above 500 nm to three-dimensional Fe2O3 crystallites. As shown in Fig. 1, heat-treated materials derived from NH4FeEDTA on Na-doped porous SiO2 absorbed predominantly below 300 nm for Fe/Na of 1 or below. For Fe/Na = 1, no significant absorption above 300 nm was observed even at 1.26 Fe nm2 (2.81 wt.%), the highest possible loading with NH4FeEDTA as the precursor. For Fe/Na > 1, however, an absorption tail extended beyond 300 nm – into the two-dimensional regime – indicating the formation of agglomerates. These changes in structure were also readily apparent to the naked eye. Agglomerated FeOx or Fe2O3 surface species are generally accompanied by a red tint, while isolated Fe3+ ions are colorless [5,6,24]. As Fig. 2 shows, for Fe/Na at or below 1, Fe3+ supported on Na-doped porous SiO2 remained isolated (nearly colorless) up to the highest loading. However, once more Fe3+ than Na+ was present on the support, a deep red tint emerged. Again, this suggested that at least equimolar Na+ was needed to maintain high Fe3+ dispersion. This is similar to the example cited above where VOx/SiO2 doped with P required at least equimolar V/P to form a distinct VOPO4 surface species, more desirable for butane partial oxidation [17]. In contrast to the work here, Cs+ influenced
the surface speciation of VOx/SiO2 catalysts for partial oxidation by titrating nucleation sites of undesired species and, as a consequence, only small amounts of Cs+ were required to observe enhancements [18]. The same spectroscopic assessment was applied to heat-treated NH4FeEDTA supported on non-porous SiO2 doped with different alkali. As Fig. 3 shows, for Fe/alkali of 1 or below, absorption for all heat-treated materials was primarily below 300 nm. Using Li+ as the promoter, tails extended beyond 300 nm and into the twodimensional regime at higher Fe3+ loadings. Likewise, at the highest Fe3+ loadings, spectra of materials doped with Na+ and Cs+ also showed tails extending into the two-dimensional regime, albeit to a lesser extent than for Li+. For K+ doped materials, on the other hand, no spectral changes were observed as the Fe3+ loading was increased. These DRUV–Vis observations were again noticeable to the unaided eye. Following oxidizing heat treatment, all materials remained nearly colorless or lightly tinted regardless of the identity of the alkali. However, as shown in Fig. 4, it was apparent that the four alkali tested as promoters perform differently. The emergence of red tint was most delayed to higher Fe3+ loadings by K+ and least by Li+, with Na+ and Cs+ falling somewhere in
Fig. 1. DRUV–Vis spectra, after oxidative heat treatment, of materials prepared by IWI of Na-modified porous SiO2 with aqueous NH4FeEDTA. The top curves show the absorption of materials with Fe/Na molar ratios from 0.5 to 2 but constant Na loading of 0.5 atom nm2. For Fe/Na of 1 or lower absorption occurred almost entirely below 300 nm, but for Fe/Na higher than 1 absorption tails extended beyond 300 nm – suggesting agglomeration of Fe. The bottom curves show the absorption of materials with constant Fe/Na = 1 at for Fe loadings from 0.79 to 2.81 wt.%, for which absorption occurred almost entirely below 300 nm regardless of loading.
Fig. 3. DRUV–Vis spectra, after oxidative heat treatment, of materials prepared by IWI of aqueous NH4FeEDTA (black lines) and Fe(NO3)3 (gray lines) on alkalimodified non-porous SiO2 at constant Fe/alkali ratio of 1. The spectra shown represent materials with loadings from 0.5 to 0.9 Fe nm2 and equimolar amounts of alkali promoters, from top to bottom, Li, Na, K, and Cs. Regardless of Fe loading, materials prepared from NH4FeEDTA absorbed predominantly below 300 nm, characteristic of well-dispersed Fe3+ species. In contrast, even at low Fe loadings, materials prepared from Fe(NO3)3 showed considerable absorption above 300 nm, indicative of larger surface species.
Fig. 2. Materials prepared by IWI of aqueous NH4FeEDTA on Na-modified porous SiO2 after oxidative heat treatment. Highly dispersed Fe3+ surface species are expected to be colorless. Note the dark red tint characteristic of agglomerated species for Fe/Na above 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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between; a clear trend of larger alkali size corresponding to higher dispersion from Li+ to K+, but with Cs+ underperforming. We hypothesize that at the highest loadings, the large Cs+ cations occupied a sufficiently larger fraction of the support surface area that the Fe3+ oxide was forced into a higher effective Fe3+ surface density, which favored the formation of two-dimensional species. Note that, relative to NH4FeEDTA, the use of Fe(NO3)3 as precursor led to materials that showed spectral shoulders in the twodimensional regime (Fig. 3, gray lines) and greater red tint (Fig. 4) even at low Fe3+ loading. These observations strongly suggested the formation of dispersed Fe3+ surface species required both the presence of an alkali promoter and a bulky protecting ligand. 3.4. Temperature-programmed reduction DRUV–Vis indicated the Fe3+ surface species are predominantly single ions or small clusters, with FeOx aggregates forming whenever Fe/alkali exceeds 1 or Fe(NO3)3 was used as the precursor. Unfortunately, DRUV–Vis cannot provide quantitative information on the relative population of surface species whose molar absorptivities are unknown. Nevertheless, the degree of dispersion and agglomeration of surface structures is related to their redox properties. Single Fe3+ ions and small clusters interacting strongly with support are expected to be less reducible than higher nuclearity agglomerates with properties closer to the bulk oxide [24]. According to the literature, isolated Fe3+ single ions undergo reduction at temperatures between 600 °C and 700 °C [4,6]. In contrast, reduction of two-dimensional amorphous FeOx clusters occurs around 500 °C and of three-dimensional Fe2O3 aggregates around 400 °C, closer to that of the bulk oxide [6].
Fig. 5. TPR profiles of representative materials prepared by IWI NH4FeEDTA on Namodified porous SiO2 after oxidative heat treatment to 800 °C. For loadings below 0.5 Fe nm2 and Fe/Na of 1 or below (A), two reduction events with peaks around 420 °C and 620 °C were observed. Above loadings 0.5 Fe nm2 (B and C), materials showed a prominent third reduction event above 800 °C. The lower temperature reduction peaks of these highly loaded materials remained well separated for Fe/Na ratios of 1 (B), but merged for Fe/Na exceeding that value (C). Mechanically mixed Fe2O3 and porous SiO2 (D) showed the characteristic two-step reduction of Fe2O3 to Fe0 and the expected 2:3 M ratio of Fe to evolved water. The highest Fe-loaded (2.85 wt.%) material prepared from NH4FeEDTA on porous SiO2 (C) showed a ratio of Fe to evolved water of 1:0.63, indicating reduction from Fe3+ mainly to Fe2+ and, to some extent, Fe0. All other materials prepared with porous SiO2 showed of Fe to evolved water close 1:0.5, indicating a reduction from Fe3+ to Fe2+.
3.4.1. General behavior Figs. 5 and 6 show the TPR profiles, normalized by sample mass and Fe3+ loading, of representative materials prepared on porous and non-porous SiO2 after heat treatment under O2 to 800 °C. Mechanically mixed bulk Fe2O3 and SiO2 (profile D, Fig. 5) showed
Nominal Fe and alkali atom surface density 0.5 nm-2
0.7 nm-2
0.9 nm-2
Fe/Alkali = 1
Li
Na
Fig. 6. TPR profiles of representative materials prepared by IWI NH4FeEDTA on alkali-modified non-porous SiO2 after oxidative heat treatment to 800 °C. Materials prepared from NH4FeEDTA (A–D) showed two reduction events around 430 °C and 610 °C, regardless of the identity of the alkali promoter. Materials prepared by IWI of aqueous Fe(NO3)3 (E) showed multiple reduction events from 300 °C to 800 °C. All materials prepared with non-porous SiO2 produced a molar ratio of 0.5:1 of evolved H2O to Fe, indicative of a reduction from Fe3+ to Fe2+.
K
Cs From Fe(NO3)3
From NH4FeEDTA
Fig. 4. Materials prepared by IWI of NH4FeEDTA and Fe(NO3)3 on alkali-modified non-porous SiO2 after oxidative heat treatment and Fe/alkali = 1. Note that even for low loadings (0.5 Fe nm2), Fe(NO3)3 produces materials with the dark red tint characteristic of agglomerated Fe surface species. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the expected ratio of evolved H2O to Fe of 1.52, corresponding to the transition from Fe3+ to Fe0 characteristic of the complete reduction of the bulk oxide. Materials prepared by IWI of NH4FeEDTA and Fe(NO3)3 on porous and non-porous SiO2 showed a ratio of evolved H2O to Fe between 0.43 and 0.54 (see Table S1 for a complete list), with one notable exception discussed below. These H2O-to-Fe ratios corresponded to the one-electron transition from Fe3+ to Fe2+ and suggested strong Fe–support interactions stabilized Fe2+ and prevented complete reduction to Fe0 [6,25,26]. Interestingly, heat-treated NH4FeEDTA on porous SiO2 with Fe/Na = 2 (Fig. 5, profile C) displayed a H2O-to-Fe ratio of 0.63. This indicated that for
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excess Fe3+ relative to alkali on the support, approximately 10 mol% of the Fe3+ on the support was agglomerated Fe2O3 structures that reduced to Fe0. These results further illustrated that Fe/ alkali must be at or below 1 to maintain high dispersion of Fe3+ and the associated strong metal–support interactions. 3.4.2. Fe on Na-doped porous SiO2 For loadings below 0.5 Fe nm2 and Fe/Na less than or equal to 1, TPR profiles of catalysts derived from NH4FeEDTA on porous SiO2 were dominated by two reduction events centered around 415 °C and 615 °C (Fig. 4, profile A). The reduction events centered around 615 °C typically accounted for 60% of the H2O evolution and were attributed to isolated Fe3+ single ions, in accordance with the literature [4,6]. Reduction peaks centered on 415 °C were ascribed to two-dimensional oligomeric FeOx based on DRUV–Vis observations above; these materials showed no absorption corresponding to three-dimensional Fe2O3 crystallites. It is worth noting that, in our previous work, the more reducible (415 °C) species was not observed following the simultaneous deposition of Na+ and Fe3+ complex, that is, when using NaFeEDTA, even when comparing materials of the same nominal surface density [14]. We hypothesize that the large EDTA ligand resulted in good dispersion of both Na+ and Fe3+ when depositing NaFeEDTA. In the current work, alkali carbonates might have not resulted in as high a dispersion of alkali. If the alkali acts as an anchor for the ultimate Fe3+ site, lower dispersion of the former would be expected to result in lower, albeit still very good, dispersion of the latter after heat treatment. For the porous materials with loadings higher than 0.5 Fe nm2 and Fe/Na maintained at 1, the peaks at 415 °C and 615 °C were accompanied by the emergence of a third peak centered at 835 °C (Fig. 4, profile B), accounting for anywhere between 5% and 32% of the total evolved H2O. Such a high reduction temperature indicated the presence of highly stable or inaccessible Fe3+ species. In the literature, reduction temperatures as high as 900 °C have been attributed to highly stable Fe silicates [27,28]. Similarly, Fe3+ species sintered into the support or buried under a collapsed pore – both likely following harsh alkaline and heat treatments – would remain kinetically resistant to H2 reduction to high temperatures. The Fe atoms associated with the high-temperature reduction events were included in the calculated ratios of evolved H2O to Fe as well as turnover frequencies below. A detailed breakdown of extent and temperature of reduction by reduction event can be found in Table S1 of the Supporting information. At loadings higher than 0.5 Fe nm2 and Fe/Na higher than 1, the two lower temperature peaks merged into a poorly defined reduction event spanning 400 °C to 700 °C, while the hightemperature peak was centered at 895 °C (Fig. 4, profile C). At a Fe/Na ratio of 1.5, these peaks still integrated to H2O/Fe = 0.5 as for the other samples, but for Fe/Na = 2, the evolved H2O to Fe ratio was 0.63, which indicated 10 mol% of the Fe3+ on the support was present as Fe2O3 crystallites that completely reduced to Fe0. Additionally, the DRUV–Vis spectrum of this latter material (Fig. 5) showed absorption above 500 nm in the three-dimensional regime. These results indicated that larger FeOx agglomerates formed at high Fe3+ surface loadings when the Fe/Na ratio significantly exceeded 1. 3.4.3. Fe on alkali-doped non-porous SiO2 Materials prepared from NH4FeEDTA supported on non-porous SiO2 showed simpler reduction behavior relative to those on porous SiO2. On the more robust non-porous support, no reduction events were observed above 700 °C, suggesting that the highest temperature events seen for the porous materials were indeed due to inaccessible Fe3+ in collapsed pores. Regardless of Fe3+ loading or identity of the alkali promoter, TPR profiles for the non-porous supports displayed two reduction events centered at
465 °C and 605 °C (Fig. 6A–D). These reductions occurred at distinct enough temperatures that they could be assumed to correspond to different species [29], but no systematic evolution of one feature into another was observed. Based on literature, the higher reduction temperature was ascribed to ostensibly isolated Fe3+ ions tightly bound to the support [4,6]. Based on the absence of absorption bands above 500 nm – the three-dimensional regime – in the DRUV–Vis spectra of these materials, the lower reduction temperature was attributed to small, two-dimensional oligomeric FeOx. The relative population of these species followed no clear trend with Fe3+ surface loading or identity of alkali promoter (Table S1). Control materials derived from Fe(NO3)3 on alkali-modified non-porous SiO2 showed multiple reduction events from 300 °C to 800 °C (Fig. 6E) with the majority of the reduction occurring as a sharp event at lower temperatures. Such behavior was expected of more agglomerated surface species. Yet, the molar ratio of evolved H2O to Fe of materials prepared from Fe(NO3)3 was 0.46 ± 0.7, indicative of the one-electron reduction from Fe3+ to Fe2+ as also observed for materials prepared from NH4FeEDTA. This result suggested that supported FeOx produced by IWI of Fe(NO3)3 also closely interacted with the SiO2 support, thereby inhibiting complete reduction from Fe3+ to Fe0. As claimed previously [14], this suggests the formation of larger, amorphous sheets of twodimensional FeOx when using the nitrate precursor. 3.5. X-ray absorption spectrometry The pre-edge and XANES spectra were collected of select materials derived from NH4FeEDTA at Fe/Na = 1 on porous and nonporous SiO2 (marked x in Table 1) and are shown in Fig. 7a and b, respectively. In both cases, supported Fe3+ materials oxidized to 800 °C showed absorption edges of 7123 eV, identical to the bulk Fe2O3 used as a standard, indicating a Fe3+ oxidation state. Larger pre-edge features were present in the spectra of supported Fe3+, characteristic of higher fractions of undercoordinated species [30]. Following reduction to 1000 °C, Fe on porous SiO2 and nonporous SiO2 showed absorption edges at 7119 eV, like bulk FeO, indicating the presence of predominantly Fe2+ surface species. Like quantitative H2-TPR, XANES showed a one-electron partial reduction from Fe3+ to Fe2+ during reduction. Note that X-ray absorption measurements were taken after months of the reduced materials being stored in ambient air. Under such conditions, strong resistance to re-oxidation implies close contact and strong interactions between Fe2+ and support. No significant differences between the porous and non-porous supports were observed. 3.6. Oxidation of adamantane by H2O2 Heat-treated materials were tested as catalysts in the oxidation of adamantane by aqueous H2O2 in MeCN at 60 °C (Fig. 8). Supports showed no activity toward the reaction within the time interval of interest. Moreover, no reaction was observed in absence of H2O2, indicating the materials did not activate dissolved O2 under the utilized reaction conditions. Similar catalysts prepared in a previous study were determined to not leach soluble Fe species active in the reaction [14]. Fig. S6 shows total and individual product TON vs. time for a representative catalyst (0.44 Fe nm2 heattreated NH4FeEDTA on Cs-doped non-porous SiO2) for the first 15 min (1 ks) of reaction time. Conversions based on adamantane were less than 10% for all experiments. In subsequent discussion, only initial rates are reported because, as observed previously [14], these materials quickly deactivated under the reaction condition. Adamantane was initially selected as a substrate because the ratio of oxidation at tertiary to secondary carbons (3°/2°), on a
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Normalized absorption, a.u.
After TPO After TPR Fe(III) oxide Fe(II) oxide
A 7100
7110
7120
7130
7140
Photon Energy, eV
Normalized absorption, a.u.
After TPO After TPR Fe(III) oxide Fe(II) oxide
B 7100
7110
7120
7130
7140
Photon Energy, eV Fig. 7. Pre-edge and XANES spectra of SiO2-supported Fe (0.46 Fe nm2) promoted with Na (Fe/Na = 1) prepared from NH4FeEDTA on porous (A) and non-porous (B) SiO2 after oxidative heat treatment (black) and subsequent reduction (gray). The difference in edge energy between oxidized and reduced samples matched the difference between bulk Fe2O3 and FeO.
+ H2O2
Fe3+/SiO2
+
independent of Fe loadings, classical ‘‘single-site’’ behavior observed also with other protected metals [11,12], and displayed a constant value of 9.7 ks1. In that work, the upper bound on Fe3+ loading was determined by the aqueous solubility of NaFeEDTA. Higher loadings of Fe3+ on bare SiO2 were achieved with NH4FeEDTA, but agglomeration led to overall lower TOF0, which decreased as the Fe3+ surface density increased. Provided loadings remained below 0.5 Fe nm2, and Fe/Na at 1 or below, catalysts derived from NH4FeEDTA on Na-modified porous SiO2 showed constant TOF0 of 9.0 ± 0.2 ks1 in this study (Fig. 9). These results were very similar to those of the previous work and demonstrated that Na+ and Fe3+ need not be deposited simultaneously on the support to observe an enhanced reactivity independent of surface density. At surface densities on porous SiO2 above 0.5 Fe nm2, however, TOF0 decreased with loading and the loss in activity was particularly prominent when the Fe-to-Na ratio fell below 1. On Na-modified non-porous SiO2, NH4FeEDTA-derived catalysts showed no such drop in activity (Fig. 10) and the TOF0 remained constant at 8.8 ± 0.1 ks1 even at higher Fe3+ surface densities. This result suggested that the decrease in TOF0 observed at high Fe3+ loadings on porous SiO2 was due to inactive silicates or Fe3+ species inaccessible in collapsed pores correlated with high-temperature reduction events. Renormalizing turnover rates to exclude the species reducing above 800 °C did increase TOF0, but not up to the values observed at lower loadings, further suggesting that some of the FeOx in these porous materials are accessible to reduction by H2, but not the much larger H2O2 and adamantane reactants. As mentioned above, for non-porous with Fe/Na = 1, TOF0 was independent of Fe3+ surface density and constant at 8.8 ± 0.1 ks1. Moreover, TOF0 was independent of Fe3+ surface loading regardless of the identity of the alkali, albeit showing a different constant value for each promoter (Fig. 10). Indeed, TOF0 increased with size of the promoter; catalysts derived from NH4FeEDTA on non-porous SiO2 doped with Li+, Na+, K+, and Cs+ displayed TOF0 of 6.3 ± 0.3 ks1, 8.8 ± 0.1 ks1, 15.4 ± 0.3 ks1, and 20.9 ± 0.3 ks1, respectively, for Fe3+ surface densities in the range of 0.5–0.9 nm2. Note that these materials gave constant TOF0 with increasing Fe content only when counting total Fe; no particular trend was observed when normalizing TOF0 to account for either
+
60 °C, MeCN
Constant Fe/Na = 1 Constant Na Loading
Fig. 8. Adamantane oxidation by H2O2 over Fe3+/SiO2 in MeCN. Reaction products, from left to right, are 1-adamantanol (1-ADL), 2-adamantanol (2-ADL), and 2-adamantanone (2-ADN).
20
TOF0 , ks-1
C–H bond basis, could provide insight into the underlying mechanism of the reaction. In all cases, the observed ratio of 3°/ 2° products remained between 2.4 and 2.7 (Table S2) and these values did not change in any systematic way from sample to sample, nor did they change significantly as the reaction progressed (Fig. S7) due in part to the low conversions. This ratio is slightly lower (2.2–2.4) for the Fe(NO3)3-derived catalysts. The lower C–H dissociation energy of tertiary carbons, relative to secondary ones, leads to high 3°/2° selectivity in radical-mediated oxidations. Some molecular catalysts have shown 3°/2° selectivity as high as 10 [31,32], in contrast to the 3°/2° selectivity values of approximately 2 expected from indiscriminate OH radicals in the gas phase [33]. The materials prepared for this study behaved similarly to other oxide supported Fe3+ and Fe3+ salts, which have typically shown 3°/2° selectivity between 2.7 and 3.7 [9,34,35]. In previous work, the authors observed a first-order dependence of reaction rate on total Fe3+ in the system for catalysts derived from NaFeEDTA on porous SiO2 and Fe3+ surface densities below 0.5 nm2 [14]. Initial turnover frequencies (TOF0) were
25
15
10
5
0 0.0
0.5
1.0
1.5
-2
Loading, Fe-nm
Fig. 9. TOF0 in ks1 vs. Fe loading in atom nm2 for materials prepared from NH4FeEDTA on Na-modified porous SiO2. Materials showed constant TOF0 for loadings below 0.5 Fe nm2 and Fe/Na 6 1 but decreased rapidly at higher loadings regardless of the Fe/Na ratio. However, note that the loss in activity was lower when Fe/Na is maintained at 1.
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FeEDTA on porous SiO2 FeEDTA on non-porous SiO2 Fe(NO3)3 on non-porous SiO2
25
Precursor
10
Na/Fe = 1 Li/Fe = 1
5
0 0.0
0.5
1.0
1.5
H 2O
25
Cs2O 20
TOF0 , ks
-1
K 2O 15
Na2O
10
Isolated ions
5
0 3.9
4.1
4.3
OH2
OH 2 OH2 Fe
High
O O O O O
Small clusters H2 O H2 O
H 2O OH 2 H 2O
Fe
Fe
O O
O O
OH 2
Fe
Low
O O O
Alkali/Fe(NO3) 3
n
2-dimensional sheets O Fe O Fe O
O Fe O
O
Fe
Fe
O Fe
O Fe O
Low O
3-dimensional crystallites Fig. 12. Graphical summary of Fe surface speciation on SiO2 based on this and previous work. Surface species active in alkane oxidation with H2O2 formed selectively when the EDTA protecting ligand and alkali promoter were present. Absence of either of the two led to the formation of larger Fe surface structures of lower activity. Sequential or simultaneous addition of the alkali promoter results in materials with similar catalytic activity but different surface speciation.
structural promoters that lead to highly dispersed Fe3+ surface species, but also electronic promoters that vary the overall rate at which those highly dispersed Fe3+ species activate H2O2 and adamantane. Relative to materials prepared from NH4FeEDTA on non-porous SiO2, those derived from Fe(NO3)3 showed lower TOF0 in all cases, regardless of the identity of the alkali promoter (Fig. 10 and Table S2). Additionally, no trend was observed between basic character of the alkali promoter and TOF0 (4.0 ks1, 5.8 ks1, and 4.4 ks1 for Li+, Na+, and Cs+, respectively). Thus, while Fe(NO3)3 with alkali promoters did not produce any crystalline Fe2O3, the bulkier NH4FeEDTA precursor was necessary to produce surface sites sufficiently dispersed to be active in adamantane oxidation.
4. Conclusions Li2O
3.7
High
O O O
Loading, Fe-nm
one of the two Fe surface species present on the support, as inferred from the two features in TPR (Fig. S8). Additionally, TOF0 normalized against all Fe monotonically decreased with increasing Sanderson electronegativity (increasing basic character) of the corresponding alkali oxide (Fig. 11) [36]. Although the mechanism of oxidation is not known, we propose, as previously [14], a coordinated hydroperoxide, Fe(III)OOH, as the initial reaction intermediate leading then to the formation of a high-valent Fe species capable of C-H bond activation. The literature on non-heme Fe complexes suggests the formation of Fe4+ or Fe5+ active centers [37], which could be stabilized by the additional electron density provided by the alkali. Such additional electron density could also promote the O–O homolytic cleavage required to form the highvalent Fe oxo species. This suggests that alkali species are not only
OH2
Fe
-2
Fig. 10. TOF0 in ks1 vs. Fe loading in atom nm2 for materials prepared from NH4FeEDTA and Fe(NO3)3 on alkali-modified SiO2. The loss in activity of materials prepared with porous supports at loadings exceeding 0.5 Fe nm2 was not observed when non-porous supports were used. All materials prepared from NH4FeEDTA and alkali-modified non-porous SiO2 showed constant TOF0 independent of Fe loading, which is characteristic of ‘‘single-site’’ behavior and expected of catalysts with uniform, highly dispersed, and equally accessible active sites. Materials prepared from Fe(NO3)3 on Cs, Na, and Li functionalized SiO2 showed generally lower activity than those prepared from NH4FeEDTA.
Oxidation Activity
Fe
H2 O
NH4FeEDTA
TOF0 , ks
-1
K/Fe = 1 15
Alkali/NH4FeEDTA > 1
Cs/Fe = 1 20
Surface Species
NaFeEDTA
84
4.5
4.7
Sanderson electronegativity, χ Fig. 11. Average TOF0 in ks1 vs. Sanderson electronegativity of alkali oxide for materials prepared from NH4FeEDTA on alkali-modified non-porous SiO2. TOF0 showed a clear increase with increasing basic character of the alkali promoter.
This work separated the roles of precursor ligand and alkali promoters in the synthesis of highly dispersed, SiO2-supported FeOx sites for alkane selective oxidation with H2O2. DRUV–Vis and H2-TPR experiments with chelating NH4FeEDTA and Fe(NO3)3 precursors deposited on doped SiO2 showed that both a bulky ligand and alkali promoter were necessary to produce highly dispersed species. The bulky EDTA ligand established an initial high dispersion which was maintained following the oxidative removal of the organic ligand due to the presence of the anchoring alkali. Maintaining high dispersion of Fe3+ ions on SiO2 after calcination
D. Prieto-Centurion et al. / Journal of Catalysis 296 (2012) 77–85
required a ratio of Fe/alkali of 1 or lower. The minimum requirement of equimolar amounts of Fe3+ and promoter is consistent with the idea that the alkali promoted dispersion by anchoring Fe3+ ions in place and did not simply titrate a small number of oxide nucleation sites [18] or otherwise alter the average surface chemistry. Likewise, surface densities were always below 0.9 nm2, which was well below the average SiOH surface density of these materials, so a simple Na+/H+ exchange would not be expected to be the origin of the stabilization. As such, no significant changes with these alkali loadings were seen in the FTIR OH stretching region of the final catalysts. Sequential addition of the alkali promoter, then impregnation of highly soluble Fe3+ precursors like NH4FeEDTA, followed by calcination, was an effective way to give higher Fe3+ surface densities than afforded by NaFeEDTA, while maintaining dispersion and high TOF0 in adamantane oxidation for all surface densities examined. It must be cautioned, however, that this protocol required the use of robust supports such as non-porous SiO2 particles. High-loaded porous supports lost activity in a way most consistent with collapse of the pore structure following alkaline and high-temperature treatments. DRUV–Vis indicated Fe3+ species remained well dispersed at high loadings, but the very poor reducibility of these species and the low TOF0 even when the sites were dispersed suggested that a fraction of the Fe3+ in these materials was inaccessible. The use of different alkali precursors may remedy this issue if porous materials are desired for the given application. The activity of alkali promoted supported Fe3+ in adamantane oxidation was altered by the identity of the promoter, with more basic alkali promoters leading to the higher activities. This effect was not seen when using Fe(NO3)3 as the precursor, again emphasizing that the bulky EDTA ligand was necessary to produce the surface species active in adamantane oxidation. As in our previous study, two Fe3+ surface active species of distinct reducibility were identified. Previously, we assigned these as isolated Fe3+ structures and a minority of very small FeOx oligomers present on catalysts derived from NaFeEDTA and NH4FeEDTA, respectively, and suggested by a renormalization of observed TOF0 that these displayed equal activity in adamantane oxidation by H2O2. Here, different alkali and surface densities led to variations in the relative populations of these species without any attendant changes in catalytic turnover frequencies or selectivities, providing a more-direct verification of their equal reactivity. Only the emergence of larger agglomerates (or inaccessible species in porous supports) resulted in decreases in TOF0. It is thus proposed that the catalytic behavior of these two highly dispersed species is the same because both make Fe3+ equally accessible to activating H2O2. These conclusions are summarized graphically in Fig. 12. Under such conditions, small changes in speciation showed no effect in catalyst activity and the main factor became electronic promotion following the trend in Fig. 11. It must be emphasized that Fe(NO3)3 at low loadings led to oligomeric FeOx with no evidence of the formation of Fe2O3, but these materials were much less active. Thus, it is not sufficient to use the absence of crystalline Fe2O3 as the sole criterion for quantifying dispersion and predicting reactivity. Finally, this methodology of two-cycle deposition of an alkali followed by a metal center ‘‘protected’’ by a bulky chelating species appears to be a viable route to generate other highly dispersed transition metal cations on different supports. Acknowledgments J.M.N. acknowledges support from U.S. Department of EnergyOffice of Basic Energy Sciences grant DE-SC0006718, Northwestern University, and a 3M Non-Tenured Faculty Grant. DPC acknowledges the support of Toyota Motor Engineering, Inc. and thanks
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Dr. Christian Canlas, Pria Young, and Todd Eaton for the helpful discussions. Portions of this work were performed with the valuable help of Dr. Qing Ma at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company and Northwestern University. Use of the APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2012.09.004. References [1] A.J. van Dillen, R.J.A.M. Terorde, D.J. Lensveld, J.W. Geus, K.P. De Jong, J. Catal. 216 (2003) 257. [2] T.M. Eggenhuisen, J.P.d. Breejen, D. Verdoes, P.E.d. Jongh, K.P.d. Jong, J. Am. Chem. Soc. 132 (2010) 18318. [3] A.T. Bell, B.C. Gates, D. Ray, Basic research needs: catalysis for energy, in: Basic Energy Sciences Workshop, US Department of, Energy, 2007. [4] F. Arena, F. Frusteri, L. Spadaro, A. Venuto, A. Parmaliana, Stud. Surf. Sci. Catal. 143 (2000) 1097. [5] F. Arena, G. Gatti, S. Coluccia, G. Martra, A. Parmaliana, Catal. Today 91–92 (2004) 305. [6] F. Arena, G. Gatti, G. Martra, S. Coluccia, L. Stievano, L. Spadaro, P. Famulari, A. Parmaliana, J. Catal. 231 (2005) 365. [7] A. Gervasini, C. Messi, A. Ponti, S. Canedese, N. Ravasio, J. Phys. Chem. C 112 (2008) 4635. [8] A. Gervasini, C. Messi, P. Carniti, A. Ponti, N. Ravasio, F. Zacheria, J. Catal. 262 (2009) 224. [9] C. Nozaki, C.G. Lugmair, A.T. Bell, T.D. Tilley, J. Am. Chem. Soc. 124 (2002) 13194. [10] A.W. Holland, G. Li, A.M. Shahin, G.J. Long, A.T. Bell, T.D. Tilley, J. Catal. 235 (2005) 150. [11] N. Morlanés, J.M. Notestein, J. Catal. 275 (2010) 191. [12] N. Morlanes, J.M. Notestein, Appl. Catal. A 387 (2010) 45. [13] P. Munnik, M. Wolters, A. Gabrielsson, S.D. Pollington, G. Headdock, J.H. Bitter, P.E. de Jongh, K.P. de Jong, J. Phys. Chem. C 115 (2011) 14698. [14] D. Prieto-Centurion, J.M. Notestein, J. Catal. 279 (2011) 103. [15] Y. Zhai, D. Pierre, R. Si, W. Deng, P. Ferrin, A.U. Nilekar, G. Peng, J.A. Herron, D.C. Bell, H. Saltsburg, M. Mavrikakis, M. Flytzani-Stephanopoulos, Science 329 (2010) 1633. [16] M. Ferrandon, E. Björnbom, J. Catal. 200 (2001) 148. [17] W.D. Harding, K.E. Birkeland, H.H. Kung, Catal. Lett. 28 (1994) 1. [18] L. Owens, H.H. Kung, J. Catal. 148 (1994) 587. [19] P. Kubelka, F. Munk, Z. Tech, Physics 12 (1931) 593. [20] R. Meier, F.W. Heinemann, Inorg. Chim. Acta 337 (2002) 317. [21] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [22] G. McLendon, R.J. Motekaitis, A.E. Martell, Inorg. Chem. 15 (1976) 2306. [23] S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal. 158 (1996) 586. [24] A. Parmaliana, F. Arena, F. Frusteri, A. Marinez-Arias, M.L. Granados, J.L. Fierro, Appl. Catal. A 226 (2002) 163. [25] Z. Gabelica, A. Charmot, R. Vataj, R. Soulimane, J. Barrault, S. Valange, J. Therm. Anal. Calorim. 95 (2009) 445. [26] S. Valange, R. Palacio, A. Charmot, J. Barrault, A. Louati, Z. Gabelica, J. Mol. Catal. A: Chem. 305 (2009) 24. [27] P.B. Amama, S. Lim, D. Ciuparu, Y. Yang, L. Pfefferle, G.L. Haller, J. Phys. Chem. B 109 (2005) 2645. [28] K. Yogo, S. Tanaka, T. Ono, T. Mikami, E. Kikuchi, Micropor. Mater. 3 (1994) 39. [29] A. Jones, B.D. McNicol (Eds.), Temperature-Programmed Reduction for Solid Materials Characterization, Marcel Dekker, Inc., New York, 1986. p. 174. [30] M. Wilke, F. Farges, P.-E. Petit, G.E. Brown, F. Martin, Am. Mineral. 86 (2001) 714. [31] A.N. Biswas, P. Das, A. Agarwala, D. Bandyopadhyay, P. Bandyopadhyay, J. Mol. Catal. A: Chem. 326 (2010) 94. [32] F. Minisci, F. Fontana, S. Araneo, F. Recupero, S. Banfi, S. Quici, J. Am. Chem. Soc. 117 (1995) 226. [33] J.K. Kochi, Free radicals, in: J.K. Kochi (Ed.), Reactive Intermediates in Organic Chemistry, Wiley, New York, 1973. [34] G. Roelfes, M. Lubben, R. Hage, J.L. Que, B.L. Feringa, Chem. Eur. J. 6 (2000) 2152. [35] B. Singh, J.R. Long, F.F.d. Biani, D. Gatteschi, P. Stavropoulos, J. Am. Chem. Soc. 119 (1997) 7030. [36] S.I. Lopatin, Glass Phys. Chem. 29 (2003) 390. [37] A. Bassan, M.R.A. Blomberg, P.E.M. Sieghahn, L. Que, J. Am. Chem. Soc. 124 (2002) 11056.
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Structural and electronic promotion with alkali cations of silica-supported Fe(III) sites for alkane oxidation Dario Prieto-Centurion, Andrew M. Boston, Justin M. Notestein ⇑ Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60202, USA
a r t i c l e
i n f o
Article history: Received 2 August 2012 Revised 10 September 2012 Accepted 11 September 2012 Available online 23 October 2012 Keywords: Alkali promoters Single-site catalyst Supported oxide EDTA ligand High dispersion
a b s t r a c t Promoters and precursors can control oxide phase, dispersion, and per-site reactivity of supported oxide catalysts. Previously, dispersed FeOx–SiO2 resulted from Fe3+ ethylenediaminetetraacetate (FeEDTA) precursors, with NaFeEDTA giving enhanced dispersion and oxidation rates vs. NH4FeEDTA. Here, catalysts were synthesized by sequential alkali deposition and Fe3+ impregnation. At up to 0.9 Fe nm2 from NH4FeEDTA and equimolar alkali, UV–visible and H2 TPR were consistent with isolated Fe3+ and small FeOx clusters. Omitting alkali, using Fe(NO3)3, or using Fe/alkali >1 gave evidence of larger agglomerates. For Fe/alkali 61 on non-porous SiO2, initial turnover frequencies in adamantane oxidation using H2O2 were independent of surface density. TOF increased as 6.3, 8.8, 15.4, and 20.9 (±0.3) ks1 for Li+, Na+, K+, and Cs+, respectively, increasingly linearly with decreasing electronegativity. These results give a synthesis–structure–function taxonomy with alkali as an electronic and structural promoter of dispersed FeOx species for alkane selective oxidation. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Supported metal oxide catalysts tend to agglomerate during the deposition step or the subsequent heat treatments employed to activate them for catalysis [1,2]. Extensive agglomeration of supported metal oxides lowers the number of active atoms exposed to reactants, decreasing the overall material efficiency of the catalyst, and also potentially alters the intrinsic reactivity of each exposed atom [1,2]. Much effort has been placed in developing synthesis methods and treatment techniques that create and preserve active phases with high dispersion – a significant challenge in the field [3] – however methodologies that rely on common precursors such as sulfates, nitrates, and acetoacetonates typically lead to a variety of surface species [4–8], while those that produce more uniform distributions of active structures can rely on cumbersome preparations or precursors [9,10]. The challenge is partially addressed by using ‘‘protected’’ metal cations as precursors [1,11,12]. Bulky organic ligands can prevent close approach of two cations and formation of M–O–M structures, and thereby maintain dispersion, although agglomeration can still occur if the ligands are removed because of the formation of mobile intermediates during oxidative heat treatment [1,11–14]. Heat treatment under inert gasses or under dilute NO has been shown to improve dispersion of the active phase, but small metal oxide agglomerates are still formed [13]. For optimal performance and ease of prepara⇑ Corresponding author. Fax: +1 847 491 3728. E-mail address: [email protected] (J.M. Notestein). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.09.004
tion, catalyst synthesis methods are desired that produce active phases with most metal cations exposed after heat treatment under stagnant air [3]. In addition to the practical and economical benefits of their greater material efficiency, heterogeneous catalysts with welldefined and highly dispersed surface species offer the opportunity to gain straightforward insight into structure–function relationships, including the effect of supports and structural vs. electronic promoters. In recent work by the authors, it was observed that impregnation of SiO2 with aqueous, anionic Fe3+ complexes with ethylenediaminetetraacetate (FeEDTA–) produced highly dispersed Fe3+ phases after calcination [14]. It was also noticed that the counter cation played a crucial role in FeOx speciation. Na+ led to catalysts with reactivity independent of Fe3+ surface density and evidence of atomic dispersion, while NHþ 4 led to the formation of amorphous FeOx agglomerates. In that system, the Fe/Na ratio was necessarily 1 by the precursor used. Select examples of promotion show effects on dispersion and activity at a variety of metal/promoter ratios. Pt on Na–modified SiO2 showed evidence of atomic dispersion and increased activity in water–gas shift reaction at Pt/Na ratios <0.33 [15]. Likewise, doping with La showed similar promotion effects for Cu2+ on Cu–Pd/a-Al2O3 catalysts for high-temperature removal of volatile organic compounds [16]. Promotion effects were observed for Cu/La ratios as high as 2.7, but the largest effects occurred at ratios of 1 or less. For VOx on SiO2, doping with phosphorous formed small VOPO4 surface species that were stable under reaction conditions and more selective toward butane partial oxidation to maleic
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acid, relative to unpromoted VOx on SiO2 [17]. In that study, the optimal V/P ratio was less than 1, the VOPO4 stoichiometry, because P also deposited elsewhere on the support. Doping SiO2 with small amounts of Cs+ before VOx deposition formed different surface species more selective toward the formation of alkenes during butane partial oxidation [18]. In that case, V/Cs molar ratios as high as 100 showed considerable promotion effects. The role of Cs+ was not ascribed to direct coordination, but rather to titration of acid sites that nucleated active VOx structures prone to complete oxidation. This work explores the effect of the amount and identity of alkali dopants on the dispersion and speciation of a supported Fe3+ oxide active phase derived from either a chelating Fe3+ precursor or Fe(NO3)3. First, sequential deposition of alkali then chelated Fe3+ as NH4FeEDTA or Fe(NO3)3 was compared to simultaneous deposition of alkali and Fe3+ via NaFeEDTA, as described in previous work. Second, the effects of Fe/Na ratios were determined at different absolute surface densities of alkali and Fe3+. Finally, multiple alkali cation species were compared to separate structural and electronic promotion.
2. Experimental 2.1. Materials and instruments All catalyst syntheses were performed under air at ambient conditions unless otherwise specified. Reagents were obtained from Sigma–Aldrich and used as received. Porous SiO2 supports were obtained from Selecto Scientific and dried at 120 °C under ambient pressure for >12 h before use. Electrospray Ionization Spectrometry (ESI-MS) measurements of Fe3+ chelates were taken using a Thermo Finnegan LCP instrument. Specific surface areas were calculated, using the BET equation, from N2 physisorption isotherms obtained with a Micromeritics ASAP 2010 instrument. All materials were degassed 8 h at <5 mTorr and 125 °C before measurements. Elemental analysis was done using a Varian MPX ICP-OES calibrated with standards of known concentration. Samples for elemental analysis were prepared by dissolving 50 mg solids in 1 mL HF, followed by dilution to 50 mL in H2O. Diffuse reflectance UV–visible (DRUV–Vis) spectra were recorded with a Shimadzu 3600 UV–visible–NIR spectrometer fitted with a Harrick Praying Mantis diffuse reflection attachment and using polytetrafluoroethylene (PTFE) as the baseline standard. All materials were ground in a mortar and pestle before acquiring spectra, and all diffuse reflectance spectra were converted to pseudo-absorption using the Kubelka–Munk transform [19]. Solution UV–Vis spectra were acquired using a standard 1 cm pathlength quartz cell. Thermogravimetric analyses (TGA) were carried out on a TA Instruments Q500 thermogravimetric analyzer with an evolved gas analysis furnace. Temperature-programmed reduction experiments were performed in a TA Instruments Q500 TGA fitted with a Pfeiffer Thermostar Q200 process mass spectrometer and using Ar as the internal standard. As-made materials were heated in 100 sccm of 90% O2 and 10% He from ambient temperature to 800 °C at 10 °C min1 and held for 30 min, cooled to near ambient temperature in 100 sccm He, then heated in 100 sccm of 4.5% H2, 4.5% Ar, balance He to 1000 °C at 10 °C min1. The m/z = 18 signal, H2O, was normalized by m/z = 40, Ar, and calibrated against CuO reduction. X-ray absorption near edge structure (XANES) spectroscopy studies were performed at Sector 5 of the Advanced Photon Source, Argonne National Laboratory, on the Dupont-Northwestern-Dow Collaborative Access Team (DND-CAT) bending magnet D beamline. Incident intensity and transmitted intensity were measured
using Canberra ionization chambers. Fe standards (Fe2O3 and FeO) were brushed on Capton tape and their spectra were measured in transmittance. Beam energies were calibrated against Fe foil standard also measured in transmittance. Supported Fe materials were pressed into 50 mg pellets 2.5 cm in diameter and mounted on a 9-pellet autosampler. Due to the low metal loadings of the pellets, spectra were measured as fluorescence intensity using two four-channel SII Vortex-ME4 detectors. Samples were mounted at incident angle h = 45 ± 5° with respect to beam and fluorescence detectors, which were perpendicular to one another. 2.2. NH4FeEDTA synthesis Synthesis of ammonium Fe(III) ethylenediaminetetraacetate, or NH4FeEDTA, was adapted from that by Meier and Heinemann [20]. Equimolar quantities of H4EDTA (10 mmol) and Fe(NO3)39H2O (10 mmol) were charged into a two-neck flask and dissolved in 25 mL of H2O at 60 °C under N2. Upon complete dissolution of Fe3+ precursor and ligand, four equivalents of NH4HCO3 (40 mmol) were slowly added. Following cooling to room temperature, the reaction solution was reduced to 5 mL by rotary evaporation and stored at 20 °C overnight. The resulting crystals were washed in acetone and dried under dynamic vacuum (20 mTorr) for 12 h. ESI-MS, in negative ion mode, showed m/z = 346 as the most prominent species, corresponding to the FeEDTA anion. TGA between 200 °C and 800 °C of the product showed a mass loss due to combustion corresponding to 99.3% purity according to the ratio of organic to inorganic components (Fig. S1). 2.3. Support preparation Non-porous SiO2 was prepared following a modification of the method described by Stöber et al. [21]. A polypropylene container was charged with 300 mL of 200 proof ethanol, 18 mL 30% aqueous NH4OH, and 6 mL H2O. The solution was heated to 60 °C and, once the temperature had remained stable for 1 h, 24 mL of tetraethoxysilane (TEOS) was added and the reaction solution was stirred overnight. The solvent was removed by rotary evaporation, and the resulting white solid was thoroughly ground and heated under ambient air to 600 °C for 6 h. Two batches of non-porous SiO2 with specific surfaces areas of 130 m2 g1 and 91 m2 g1 were prepared (marked d and e on Table 1). Non-porous and porous SiO2 were modified by incipient wetness impregnation (IWI) with aqueous solutions of alkali precursors, LiOH, NaHCO3, KHCO3, and CsHCO3 (0.1–0.6 M). The volumes required to wet the support were approximated by the total pore volume determined from N2 physisorption. Following impregnation, the modified supports were stored in partially covered containers at ambient conditions for 48 h, stirred approximately every 12 h, and then heated for 12 h at 120 °C to create alkali-modified SiO2. After drying the alkali-modified materials, pore volumes were re-measured and TGA redone, the latter showing no evidence of residual combustible or volatile groups on the surface. Likewise, FTIR showed no evidence of residual carbonates after the heat treatment. 2.4. Catalyst preparation Fe was deposited on alkali-modified SiO2 by IWI with (0.1– 0.6 M) aqueous solutions of NH4FeEDTA. Transmission UV–visible spectra of the impregnation solution showed no indication of dimeric FeEDTA complexes with characteristic peaks at 475 nm [22]. The impregnated supports, stored in partially covered containers at room temperature, were dried under ambient conditions for 48 h and under dynamic vacuum (20 mTorr) for 12 h. Subsequently, materials underwent heat treatment in ambient, static air
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2.5. Catalytic reactions
Table 1 Characteristics SiO2-supported Fe materials prepared for this study. Precursor
Alkali
Fe loading
lmol g FeEDTAa,b
Na
a,b,x a,b a,b c a,c c c a,d
Li
e a,d d,x
Na
d a,d a,d
K
e d e
Cs
e ad
, Fe(NO3)3d
d d
Fe2O3b
Li Na Cs None
142 278 403 502 154 297 391 509 113 98 186 100 144 191 115 89 189 66 85 180 88 90 85 240
1
Fe/alkali 2
wt.%
Fe nm
0.79 1.55 2.25 2.81 0.86 1.66 2.19 2.85 0.63 0.55 1.04 0.56 0.80 1.07 0.64 0.50 1.06 0.37 0.47 1.01 0.49 0.50 0.48 1.34
0.21 0.46 0.84 1.26 0.25 0.49 0.64 0.84 0.52 0.65 0.86 0.46 0.67 0.89 0.53 0.59 0.88 0.44 0.56 0.84 0.41 0.42 0.39 0.26
79
1: 1: 1: 1: 1: 1: 3: 2: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1: 1:
1 1 1 1 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0
a
Loadings confirmed by ICP-OES. Porous SiO2 support with varying Na loading, see Fig. S1 for specific surface areas. c Porous SiO2 support with constant Na loading, 367 m2 g1. d Non-porous SiO2, 130 m2 g1. e Non-porous SiO2, 91 m2 g1. x XANES spectra in Fig. 7.
Alkane oxidation was carried out in a 50 mL round-bottom flask fitted with a Liebig condenser, charged with 50 mg of catalyst, 70 mg (0.51 mmol) of adamantane, and 20 mL of acetonitrile (MeCN), and heated to 60 °C under vigorous stirring. Once the substrate fully dissolved, 1 mL of 30% aqueous H2O2 (9.5 mmol) was injected. At regular time intervals, samples of 300 lL were retrieved using syringes fitted with Whatman microfiber filters (0.7 lm pore size). All aliquots were then treated with Ag powder to decompose residual unreacted H2O2, which could further oxidize the reactants and products during or while awaiting analysis. Control experiments showed no loss of products or reactant due to adsorption or reaction following such sample treatment. Aliquots were analyzed by gas chromatography (GC) using a Shimadzu 2010 GC equipped with a Flame Ionization Detector (FID) and a Thermo-Nicolet TR-1 capillary column. Peak areas of the substrate and reaction products were quantified by comparison with standards of known concentration of adamantane and its expected oxidation products, 1-adamantanol (1-ADL), 2-adamantanol (2-ADL), and 2-adamantanone (2-ADN). Material balances, defined as sum of only these products over conversion of adamantane, were between 85% and 95% in all cases. The lower values were obtained with low Fe-loaded catalysts at low conversion and were thus attributed to limits of instrumental precision.
b
from room temperature to 800 °C at a ramp rate of 10 °C min1. Fe3+ surface densities, inferred from the loadings of the ligand as calculated by TGA, and confirmed by ICP, varied in the range 0.23–1.26 nm2 for porous SiO2 and 0.44–0.89 nm2 for nonporous SiO2 (Table 1, Fe loadings also in wt.% and lmol g1). The concentration of saturated aqueous solution NH4FeEDTA (0.6 M) and low interparticle volume of non-porous SiO2 (0.33 mL g1 and 0.24 mL g1 for samples of surface areas of 130 m2 g1 and 91 m2 g1, respectively) limited the maximum surface loading of Fe3+ to approximately 0.9 Fe nm2. Porous SiO2, with higher total volume (from 1.54 mL g1 to 0.71 mL g1 for surface areas between 554 m2 g1 and 240 m2 g1, Fig. S2) permitted the higher surface loadings shown on Table 1. The reader should note that maximum Fe3+ loadings of materials prepared by the sequential addition of alkali then NH4FeEDTA exceeded loadings reached in our previous work using NaFeEDTA [14]. This was the case expressing Fe3+ loading by mass (2.15 wt.% vs. 2.85 wt.%) and, particularly, surface density (0.39 Fe nm2 vs. 0.86 Fe nm2). The higher aqueous solubility of NH4FeEDTA relative to NaFeEDTA accounted for the higher loadings by permitting the deposition of larger amounts of Fe3+ with a single impregnation. The highest surface loadings observed in this work corresponded to approximately 60% of a FeEDTA- monolayer (1.5 FeEDTA nm2) based on the dimensions of the complex given elsewhere [20]. Transmission electron microscopy and electron diffraction of the heat-treated materials showed no regions of high contrast or crystallinity. Likewise, powder X-ray diffraction showed no diffraction peaks for 2h between 20° and 80°. The absence of any evidence for diffracting crystalline features was expected from the low Fe3+ weight loadings of these materials. Control catalysts were prepared by IWI of Li+, Na+, and Cs+ doped non-porous SiO2 with aqueous Fe(NO3)39H2O and through mechanical mixing of Fe2O3 with SiO2.
3. Results and discussion 3.1. Supports characterization As mentioned above, previous work showed that impregnation of SiO2 with aqueous NaFeEDTA produced highly dispersed Fe3+ phases, while the same procedure with NH4FeEDTA yielded lessreactive FeOx agglomerates [14]. However, NH4FeEDTA was significantly more soluble (0.6 M) relative to NaFeEDTA (0.35 M) and permitted higher Fe3+ loadings with a single impregnation. Here, we sought to use sequential deposition of alkali and NH4FeEDTA in a two-cycle impregnation with the objective of combining the high loadings attainable with the NHþ 4 counter cation and the high dispersions resulting from the Na+ counter cation. Accounting for the extra mass from the added alkali, specific surface areas of the alkali-functionalized porous SiO2 decreased with increasing loading, losing nearly 60% (554–240 m2 g1) at the highest Na+ loading (Fig. S2). This decrease in specific surface area was accompanied by a collapse of the pore structure, in which smaller diameter pores (20 Å) and then larger ones (45 Å) disappeared progressively as the Na+ loading increased (Fig. S3). It is well known that silicas readily dissolve in basic alkaline solutions and pore structures are especially vulnerable to such treatments. Non-porous SiO2 subjected to the same treatment showed less than 7% loss in specific surface area relative to their unmodified counterpart. These results indicated that the small volumes of alkaline solution used to modify SiO2 destroyed mesostructures but did not significantly agglomerate non-porous particulates. 3.2. Fe loadings Under oxidizing atmosphere, as-made Fe-containing materials underwent a one-step decomposition at 240 °C, comparable to that of pure H4EDTA (Fig. S4). Loadings of the NH4FeEDTA complex were determined by TGA assuming that all mass loss between 200 °C and 800 °C, relative to that of the support, corresponded to the loss of EDTA4– ligand (298 g mol1) and NHþ 4 counter cation (18 g mol1) except for 4 O atoms (64 g mol1) either from the ligand or from the O2 oxidant, giving a net fragment of 252 g mol1.
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Fe3+ loadings estimated by TGA using this fragment mass were within 1% agreement of the amount of Fe3+ added during IWI. The loading of selected materials was confirmed by ICP-OES (marked a in Table 1).
0.8 wt %
Fe/Na Ratio
3.3. UV–visible spectrometry
Nominal Fe loading 1.6 wt %
2.2 wt %
2.8 wt %
1/2
1/1
3/2
2/1
1/1
1/1
1/1
1/1
The solution UV–Vis spectrum of aq. NH4FeEDTA showed a characteristic absorption band at 280 nm (Fig. S5-a). The same band was observed in the DRUV–Vis spectrum of as-made SiO2supported NH4FeEDTA (Fig. S5-b), indicating that the complex maintained its structural integrity following impregnation and drying. The identity of the alkali promoter did not alter the spectrum of the supported Fe3+ complex. Dispersion of Fe3+ surface species on heat-treated materials was assessed by DRUV–Vis spectrometry. According to the literature [4,9,23,24], absorption bands below 300 nm correspond to isolated Fe3+ ions, between 300 nm and 500 nm to two-dimensional FeOx surface structures, and above 500 nm to three-dimensional Fe2O3 crystallites. As shown in Fig. 1, heat-treated materials derived from NH4FeEDTA on Na-doped porous SiO2 absorbed predominantly below 300 nm for Fe/Na of 1 or below. For Fe/Na = 1, no significant absorption above 300 nm was observed even at 1.26 Fe nm2 (2.81 wt.%), the highest possible loading with NH4FeEDTA as the precursor. For Fe/Na > 1, however, an absorption tail extended beyond 300 nm – into the two-dimensional regime – indicating the formation of agglomerates. These changes in structure were also readily apparent to the naked eye. Agglomerated FeOx or Fe2O3 surface species are generally accompanied by a red tint, while isolated Fe3+ ions are colorless [5,6,24]. As Fig. 2 shows, for Fe/Na at or below 1, Fe3+ supported on Na-doped porous SiO2 remained isolated (nearly colorless) up to the highest loading. However, once more Fe3+ than Na+ was present on the support, a deep red tint emerged. Again, this suggested that at least equimolar Na+ was needed to maintain high Fe3+ dispersion. This is similar to the example cited above where VOx/SiO2 doped with P required at least equimolar V/P to form a distinct VOPO4 surface species, more desirable for butane partial oxidation [17]. In contrast to the work here, Cs+ influenced
the surface speciation of VOx/SiO2 catalysts for partial oxidation by titrating nucleation sites of undesired species and, as a consequence, only small amounts of Cs+ were required to observe enhancements [18]. The same spectroscopic assessment was applied to heat-treated NH4FeEDTA supported on non-porous SiO2 doped with different alkali. As Fig. 3 shows, for Fe/alkali of 1 or below, absorption for all heat-treated materials was primarily below 300 nm. Using Li+ as the promoter, tails extended beyond 300 nm and into the twodimensional regime at higher Fe3+ loadings. Likewise, at the highest Fe3+ loadings, spectra of materials doped with Na+ and Cs+ also showed tails extending into the two-dimensional regime, albeit to a lesser extent than for Li+. For K+ doped materials, on the other hand, no spectral changes were observed as the Fe3+ loading was increased. These DRUV–Vis observations were again noticeable to the unaided eye. Following oxidizing heat treatment, all materials remained nearly colorless or lightly tinted regardless of the identity of the alkali. However, as shown in Fig. 4, it was apparent that the four alkali tested as promoters perform differently. The emergence of red tint was most delayed to higher Fe3+ loadings by K+ and least by Li+, with Na+ and Cs+ falling somewhere in
Fig. 1. DRUV–Vis spectra, after oxidative heat treatment, of materials prepared by IWI of Na-modified porous SiO2 with aqueous NH4FeEDTA. The top curves show the absorption of materials with Fe/Na molar ratios from 0.5 to 2 but constant Na loading of 0.5 atom nm2. For Fe/Na of 1 or lower absorption occurred almost entirely below 300 nm, but for Fe/Na higher than 1 absorption tails extended beyond 300 nm – suggesting agglomeration of Fe. The bottom curves show the absorption of materials with constant Fe/Na = 1 at for Fe loadings from 0.79 to 2.81 wt.%, for which absorption occurred almost entirely below 300 nm regardless of loading.
Fig. 3. DRUV–Vis spectra, after oxidative heat treatment, of materials prepared by IWI of aqueous NH4FeEDTA (black lines) and Fe(NO3)3 (gray lines) on alkalimodified non-porous SiO2 at constant Fe/alkali ratio of 1. The spectra shown represent materials with loadings from 0.5 to 0.9 Fe nm2 and equimolar amounts of alkali promoters, from top to bottom, Li, Na, K, and Cs. Regardless of Fe loading, materials prepared from NH4FeEDTA absorbed predominantly below 300 nm, characteristic of well-dispersed Fe3+ species. In contrast, even at low Fe loadings, materials prepared from Fe(NO3)3 showed considerable absorption above 300 nm, indicative of larger surface species.
Fig. 2. Materials prepared by IWI of aqueous NH4FeEDTA on Na-modified porous SiO2 after oxidative heat treatment. Highly dispersed Fe3+ surface species are expected to be colorless. Note the dark red tint characteristic of agglomerated species for Fe/Na above 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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between; a clear trend of larger alkali size corresponding to higher dispersion from Li+ to K+, but with Cs+ underperforming. We hypothesize that at the highest loadings, the large Cs+ cations occupied a sufficiently larger fraction of the support surface area that the Fe3+ oxide was forced into a higher effective Fe3+ surface density, which favored the formation of two-dimensional species. Note that, relative to NH4FeEDTA, the use of Fe(NO3)3 as precursor led to materials that showed spectral shoulders in the twodimensional regime (Fig. 3, gray lines) and greater red tint (Fig. 4) even at low Fe3+ loading. These observations strongly suggested the formation of dispersed Fe3+ surface species required both the presence of an alkali promoter and a bulky protecting ligand. 3.4. Temperature-programmed reduction DRUV–Vis indicated the Fe3+ surface species are predominantly single ions or small clusters, with FeOx aggregates forming whenever Fe/alkali exceeds 1 or Fe(NO3)3 was used as the precursor. Unfortunately, DRUV–Vis cannot provide quantitative information on the relative population of surface species whose molar absorptivities are unknown. Nevertheless, the degree of dispersion and agglomeration of surface structures is related to their redox properties. Single Fe3+ ions and small clusters interacting strongly with support are expected to be less reducible than higher nuclearity agglomerates with properties closer to the bulk oxide [24]. According to the literature, isolated Fe3+ single ions undergo reduction at temperatures between 600 °C and 700 °C [4,6]. In contrast, reduction of two-dimensional amorphous FeOx clusters occurs around 500 °C and of three-dimensional Fe2O3 aggregates around 400 °C, closer to that of the bulk oxide [6].
Fig. 5. TPR profiles of representative materials prepared by IWI NH4FeEDTA on Namodified porous SiO2 after oxidative heat treatment to 800 °C. For loadings below 0.5 Fe nm2 and Fe/Na of 1 or below (A), two reduction events with peaks around 420 °C and 620 °C were observed. Above loadings 0.5 Fe nm2 (B and C), materials showed a prominent third reduction event above 800 °C. The lower temperature reduction peaks of these highly loaded materials remained well separated for Fe/Na ratios of 1 (B), but merged for Fe/Na exceeding that value (C). Mechanically mixed Fe2O3 and porous SiO2 (D) showed the characteristic two-step reduction of Fe2O3 to Fe0 and the expected 2:3 M ratio of Fe to evolved water. The highest Fe-loaded (2.85 wt.%) material prepared from NH4FeEDTA on porous SiO2 (C) showed a ratio of Fe to evolved water of 1:0.63, indicating reduction from Fe3+ mainly to Fe2+ and, to some extent, Fe0. All other materials prepared with porous SiO2 showed of Fe to evolved water close 1:0.5, indicating a reduction from Fe3+ to Fe2+.
3.4.1. General behavior Figs. 5 and 6 show the TPR profiles, normalized by sample mass and Fe3+ loading, of representative materials prepared on porous and non-porous SiO2 after heat treatment under O2 to 800 °C. Mechanically mixed bulk Fe2O3 and SiO2 (profile D, Fig. 5) showed
Nominal Fe and alkali atom surface density 0.5 nm-2
0.7 nm-2
0.9 nm-2
Fe/Alkali = 1
Li
Na
Fig. 6. TPR profiles of representative materials prepared by IWI NH4FeEDTA on alkali-modified non-porous SiO2 after oxidative heat treatment to 800 °C. Materials prepared from NH4FeEDTA (A–D) showed two reduction events around 430 °C and 610 °C, regardless of the identity of the alkali promoter. Materials prepared by IWI of aqueous Fe(NO3)3 (E) showed multiple reduction events from 300 °C to 800 °C. All materials prepared with non-porous SiO2 produced a molar ratio of 0.5:1 of evolved H2O to Fe, indicative of a reduction from Fe3+ to Fe2+.
K
Cs From Fe(NO3)3
From NH4FeEDTA
Fig. 4. Materials prepared by IWI of NH4FeEDTA and Fe(NO3)3 on alkali-modified non-porous SiO2 after oxidative heat treatment and Fe/alkali = 1. Note that even for low loadings (0.5 Fe nm2), Fe(NO3)3 produces materials with the dark red tint characteristic of agglomerated Fe surface species. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the expected ratio of evolved H2O to Fe of 1.52, corresponding to the transition from Fe3+ to Fe0 characteristic of the complete reduction of the bulk oxide. Materials prepared by IWI of NH4FeEDTA and Fe(NO3)3 on porous and non-porous SiO2 showed a ratio of evolved H2O to Fe between 0.43 and 0.54 (see Table S1 for a complete list), with one notable exception discussed below. These H2O-to-Fe ratios corresponded to the one-electron transition from Fe3+ to Fe2+ and suggested strong Fe–support interactions stabilized Fe2+ and prevented complete reduction to Fe0 [6,25,26]. Interestingly, heat-treated NH4FeEDTA on porous SiO2 with Fe/Na = 2 (Fig. 5, profile C) displayed a H2O-to-Fe ratio of 0.63. This indicated that for
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excess Fe3+ relative to alkali on the support, approximately 10 mol% of the Fe3+ on the support was agglomerated Fe2O3 structures that reduced to Fe0. These results further illustrated that Fe/ alkali must be at or below 1 to maintain high dispersion of Fe3+ and the associated strong metal–support interactions. 3.4.2. Fe on Na-doped porous SiO2 For loadings below 0.5 Fe nm2 and Fe/Na less than or equal to 1, TPR profiles of catalysts derived from NH4FeEDTA on porous SiO2 were dominated by two reduction events centered around 415 °C and 615 °C (Fig. 4, profile A). The reduction events centered around 615 °C typically accounted for 60% of the H2O evolution and were attributed to isolated Fe3+ single ions, in accordance with the literature [4,6]. Reduction peaks centered on 415 °C were ascribed to two-dimensional oligomeric FeOx based on DRUV–Vis observations above; these materials showed no absorption corresponding to three-dimensional Fe2O3 crystallites. It is worth noting that, in our previous work, the more reducible (415 °C) species was not observed following the simultaneous deposition of Na+ and Fe3+ complex, that is, when using NaFeEDTA, even when comparing materials of the same nominal surface density [14]. We hypothesize that the large EDTA ligand resulted in good dispersion of both Na+ and Fe3+ when depositing NaFeEDTA. In the current work, alkali carbonates might have not resulted in as high a dispersion of alkali. If the alkali acts as an anchor for the ultimate Fe3+ site, lower dispersion of the former would be expected to result in lower, albeit still very good, dispersion of the latter after heat treatment. For the porous materials with loadings higher than 0.5 Fe nm2 and Fe/Na maintained at 1, the peaks at 415 °C and 615 °C were accompanied by the emergence of a third peak centered at 835 °C (Fig. 4, profile B), accounting for anywhere between 5% and 32% of the total evolved H2O. Such a high reduction temperature indicated the presence of highly stable or inaccessible Fe3+ species. In the literature, reduction temperatures as high as 900 °C have been attributed to highly stable Fe silicates [27,28]. Similarly, Fe3+ species sintered into the support or buried under a collapsed pore – both likely following harsh alkaline and heat treatments – would remain kinetically resistant to H2 reduction to high temperatures. The Fe atoms associated with the high-temperature reduction events were included in the calculated ratios of evolved H2O to Fe as well as turnover frequencies below. A detailed breakdown of extent and temperature of reduction by reduction event can be found in Table S1 of the Supporting information. At loadings higher than 0.5 Fe nm2 and Fe/Na higher than 1, the two lower temperature peaks merged into a poorly defined reduction event spanning 400 °C to 700 °C, while the hightemperature peak was centered at 895 °C (Fig. 4, profile C). At a Fe/Na ratio of 1.5, these peaks still integrated to H2O/Fe = 0.5 as for the other samples, but for Fe/Na = 2, the evolved H2O to Fe ratio was 0.63, which indicated 10 mol% of the Fe3+ on the support was present as Fe2O3 crystallites that completely reduced to Fe0. Additionally, the DRUV–Vis spectrum of this latter material (Fig. 5) showed absorption above 500 nm in the three-dimensional regime. These results indicated that larger FeOx agglomerates formed at high Fe3+ surface loadings when the Fe/Na ratio significantly exceeded 1. 3.4.3. Fe on alkali-doped non-porous SiO2 Materials prepared from NH4FeEDTA supported on non-porous SiO2 showed simpler reduction behavior relative to those on porous SiO2. On the more robust non-porous support, no reduction events were observed above 700 °C, suggesting that the highest temperature events seen for the porous materials were indeed due to inaccessible Fe3+ in collapsed pores. Regardless of Fe3+ loading or identity of the alkali promoter, TPR profiles for the non-porous supports displayed two reduction events centered at
465 °C and 605 °C (Fig. 6A–D). These reductions occurred at distinct enough temperatures that they could be assumed to correspond to different species [29], but no systematic evolution of one feature into another was observed. Based on literature, the higher reduction temperature was ascribed to ostensibly isolated Fe3+ ions tightly bound to the support [4,6]. Based on the absence of absorption bands above 500 nm – the three-dimensional regime – in the DRUV–Vis spectra of these materials, the lower reduction temperature was attributed to small, two-dimensional oligomeric FeOx. The relative population of these species followed no clear trend with Fe3+ surface loading or identity of alkali promoter (Table S1). Control materials derived from Fe(NO3)3 on alkali-modified non-porous SiO2 showed multiple reduction events from 300 °C to 800 °C (Fig. 6E) with the majority of the reduction occurring as a sharp event at lower temperatures. Such behavior was expected of more agglomerated surface species. Yet, the molar ratio of evolved H2O to Fe of materials prepared from Fe(NO3)3 was 0.46 ± 0.7, indicative of the one-electron reduction from Fe3+ to Fe2+ as also observed for materials prepared from NH4FeEDTA. This result suggested that supported FeOx produced by IWI of Fe(NO3)3 also closely interacted with the SiO2 support, thereby inhibiting complete reduction from Fe3+ to Fe0. As claimed previously [14], this suggests the formation of larger, amorphous sheets of twodimensional FeOx when using the nitrate precursor. 3.5. X-ray absorption spectrometry The pre-edge and XANES spectra were collected of select materials derived from NH4FeEDTA at Fe/Na = 1 on porous and nonporous SiO2 (marked x in Table 1) and are shown in Fig. 7a and b, respectively. In both cases, supported Fe3+ materials oxidized to 800 °C showed absorption edges of 7123 eV, identical to the bulk Fe2O3 used as a standard, indicating a Fe3+ oxidation state. Larger pre-edge features were present in the spectra of supported Fe3+, characteristic of higher fractions of undercoordinated species [30]. Following reduction to 1000 °C, Fe on porous SiO2 and nonporous SiO2 showed absorption edges at 7119 eV, like bulk FeO, indicating the presence of predominantly Fe2+ surface species. Like quantitative H2-TPR, XANES showed a one-electron partial reduction from Fe3+ to Fe2+ during reduction. Note that X-ray absorption measurements were taken after months of the reduced materials being stored in ambient air. Under such conditions, strong resistance to re-oxidation implies close contact and strong interactions between Fe2+ and support. No significant differences between the porous and non-porous supports were observed. 3.6. Oxidation of adamantane by H2O2 Heat-treated materials were tested as catalysts in the oxidation of adamantane by aqueous H2O2 in MeCN at 60 °C (Fig. 8). Supports showed no activity toward the reaction within the time interval of interest. Moreover, no reaction was observed in absence of H2O2, indicating the materials did not activate dissolved O2 under the utilized reaction conditions. Similar catalysts prepared in a previous study were determined to not leach soluble Fe species active in the reaction [14]. Fig. S6 shows total and individual product TON vs. time for a representative catalyst (0.44 Fe nm2 heattreated NH4FeEDTA on Cs-doped non-porous SiO2) for the first 15 min (1 ks) of reaction time. Conversions based on adamantane were less than 10% for all experiments. In subsequent discussion, only initial rates are reported because, as observed previously [14], these materials quickly deactivated under the reaction condition. Adamantane was initially selected as a substrate because the ratio of oxidation at tertiary to secondary carbons (3°/2°), on a
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Normalized absorption, a.u.
After TPO After TPR Fe(III) oxide Fe(II) oxide
A 7100
7110
7120
7130
7140
Photon Energy, eV
Normalized absorption, a.u.
After TPO After TPR Fe(III) oxide Fe(II) oxide
B 7100
7110
7120
7130
7140
Photon Energy, eV Fig. 7. Pre-edge and XANES spectra of SiO2-supported Fe (0.46 Fe nm2) promoted with Na (Fe/Na = 1) prepared from NH4FeEDTA on porous (A) and non-porous (B) SiO2 after oxidative heat treatment (black) and subsequent reduction (gray). The difference in edge energy between oxidized and reduced samples matched the difference between bulk Fe2O3 and FeO.
+ H2O2
Fe3+/SiO2
+
independent of Fe loadings, classical ‘‘single-site’’ behavior observed also with other protected metals [11,12], and displayed a constant value of 9.7 ks1. In that work, the upper bound on Fe3+ loading was determined by the aqueous solubility of NaFeEDTA. Higher loadings of Fe3+ on bare SiO2 were achieved with NH4FeEDTA, but agglomeration led to overall lower TOF0, which decreased as the Fe3+ surface density increased. Provided loadings remained below 0.5 Fe nm2, and Fe/Na at 1 or below, catalysts derived from NH4FeEDTA on Na-modified porous SiO2 showed constant TOF0 of 9.0 ± 0.2 ks1 in this study (Fig. 9). These results were very similar to those of the previous work and demonstrated that Na+ and Fe3+ need not be deposited simultaneously on the support to observe an enhanced reactivity independent of surface density. At surface densities on porous SiO2 above 0.5 Fe nm2, however, TOF0 decreased with loading and the loss in activity was particularly prominent when the Fe-to-Na ratio fell below 1. On Na-modified non-porous SiO2, NH4FeEDTA-derived catalysts showed no such drop in activity (Fig. 10) and the TOF0 remained constant at 8.8 ± 0.1 ks1 even at higher Fe3+ surface densities. This result suggested that the decrease in TOF0 observed at high Fe3+ loadings on porous SiO2 was due to inactive silicates or Fe3+ species inaccessible in collapsed pores correlated with high-temperature reduction events. Renormalizing turnover rates to exclude the species reducing above 800 °C did increase TOF0, but not up to the values observed at lower loadings, further suggesting that some of the FeOx in these porous materials are accessible to reduction by H2, but not the much larger H2O2 and adamantane reactants. As mentioned above, for non-porous with Fe/Na = 1, TOF0 was independent of Fe3+ surface density and constant at 8.8 ± 0.1 ks1. Moreover, TOF0 was independent of Fe3+ surface loading regardless of the identity of the alkali, albeit showing a different constant value for each promoter (Fig. 10). Indeed, TOF0 increased with size of the promoter; catalysts derived from NH4FeEDTA on non-porous SiO2 doped with Li+, Na+, K+, and Cs+ displayed TOF0 of 6.3 ± 0.3 ks1, 8.8 ± 0.1 ks1, 15.4 ± 0.3 ks1, and 20.9 ± 0.3 ks1, respectively, for Fe3+ surface densities in the range of 0.5–0.9 nm2. Note that these materials gave constant TOF0 with increasing Fe content only when counting total Fe; no particular trend was observed when normalizing TOF0 to account for either
+
60 °C, MeCN
Constant Fe/Na = 1 Constant Na Loading
Fig. 8. Adamantane oxidation by H2O2 over Fe3+/SiO2 in MeCN. Reaction products, from left to right, are 1-adamantanol (1-ADL), 2-adamantanol (2-ADL), and 2-adamantanone (2-ADN).
20
TOF0 , ks-1
C–H bond basis, could provide insight into the underlying mechanism of the reaction. In all cases, the observed ratio of 3°/ 2° products remained between 2.4 and 2.7 (Table S2) and these values did not change in any systematic way from sample to sample, nor did they change significantly as the reaction progressed (Fig. S7) due in part to the low conversions. This ratio is slightly lower (2.2–2.4) for the Fe(NO3)3-derived catalysts. The lower C–H dissociation energy of tertiary carbons, relative to secondary ones, leads to high 3°/2° selectivity in radical-mediated oxidations. Some molecular catalysts have shown 3°/2° selectivity as high as 10 [31,32], in contrast to the 3°/2° selectivity values of approximately 2 expected from indiscriminate OH radicals in the gas phase [33]. The materials prepared for this study behaved similarly to other oxide supported Fe3+ and Fe3+ salts, which have typically shown 3°/2° selectivity between 2.7 and 3.7 [9,34,35]. In previous work, the authors observed a first-order dependence of reaction rate on total Fe3+ in the system for catalysts derived from NaFeEDTA on porous SiO2 and Fe3+ surface densities below 0.5 nm2 [14]. Initial turnover frequencies (TOF0) were
25
15
10
5
0 0.0
0.5
1.0
1.5
-2
Loading, Fe-nm
Fig. 9. TOF0 in ks1 vs. Fe loading in atom nm2 for materials prepared from NH4FeEDTA on Na-modified porous SiO2. Materials showed constant TOF0 for loadings below 0.5 Fe nm2 and Fe/Na 6 1 but decreased rapidly at higher loadings regardless of the Fe/Na ratio. However, note that the loss in activity was lower when Fe/Na is maintained at 1.
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FeEDTA on porous SiO2 FeEDTA on non-porous SiO2 Fe(NO3)3 on non-porous SiO2
25
Precursor
10
Na/Fe = 1 Li/Fe = 1
5
0 0.0
0.5
1.0
1.5
H 2O
25
Cs2O 20
TOF0 , ks
-1
K 2O 15
Na2O
10
Isolated ions
5
0 3.9
4.1
4.3
OH2
OH 2 OH2 Fe
High
O O O O O
Small clusters H2 O H2 O
H 2O OH 2 H 2O
Fe
Fe
O O
O O
OH 2
Fe
Low
O O O
Alkali/Fe(NO3) 3
n
2-dimensional sheets O Fe O Fe O
O Fe O
O
Fe
Fe
O Fe
O Fe O
Low O
3-dimensional crystallites Fig. 12. Graphical summary of Fe surface speciation on SiO2 based on this and previous work. Surface species active in alkane oxidation with H2O2 formed selectively when the EDTA protecting ligand and alkali promoter were present. Absence of either of the two led to the formation of larger Fe surface structures of lower activity. Sequential or simultaneous addition of the alkali promoter results in materials with similar catalytic activity but different surface speciation.
structural promoters that lead to highly dispersed Fe3+ surface species, but also electronic promoters that vary the overall rate at which those highly dispersed Fe3+ species activate H2O2 and adamantane. Relative to materials prepared from NH4FeEDTA on non-porous SiO2, those derived from Fe(NO3)3 showed lower TOF0 in all cases, regardless of the identity of the alkali promoter (Fig. 10 and Table S2). Additionally, no trend was observed between basic character of the alkali promoter and TOF0 (4.0 ks1, 5.8 ks1, and 4.4 ks1 for Li+, Na+, and Cs+, respectively). Thus, while Fe(NO3)3 with alkali promoters did not produce any crystalline Fe2O3, the bulkier NH4FeEDTA precursor was necessary to produce surface sites sufficiently dispersed to be active in adamantane oxidation.
4. Conclusions Li2O
3.7
High
O O O
Loading, Fe-nm
one of the two Fe surface species present on the support, as inferred from the two features in TPR (Fig. S8). Additionally, TOF0 normalized against all Fe monotonically decreased with increasing Sanderson electronegativity (increasing basic character) of the corresponding alkali oxide (Fig. 11) [36]. Although the mechanism of oxidation is not known, we propose, as previously [14], a coordinated hydroperoxide, Fe(III)OOH, as the initial reaction intermediate leading then to the formation of a high-valent Fe species capable of C-H bond activation. The literature on non-heme Fe complexes suggests the formation of Fe4+ or Fe5+ active centers [37], which could be stabilized by the additional electron density provided by the alkali. Such additional electron density could also promote the O–O homolytic cleavage required to form the highvalent Fe oxo species. This suggests that alkali species are not only
OH2
Fe
-2
Fig. 10. TOF0 in ks1 vs. Fe loading in atom nm2 for materials prepared from NH4FeEDTA and Fe(NO3)3 on alkali-modified SiO2. The loss in activity of materials prepared with porous supports at loadings exceeding 0.5 Fe nm2 was not observed when non-porous supports were used. All materials prepared from NH4FeEDTA and alkali-modified non-porous SiO2 showed constant TOF0 independent of Fe loading, which is characteristic of ‘‘single-site’’ behavior and expected of catalysts with uniform, highly dispersed, and equally accessible active sites. Materials prepared from Fe(NO3)3 on Cs, Na, and Li functionalized SiO2 showed generally lower activity than those prepared from NH4FeEDTA.
Oxidation Activity
Fe
H2 O
NH4FeEDTA
TOF0 , ks
-1
K/Fe = 1 15
Alkali/NH4FeEDTA > 1
Cs/Fe = 1 20
Surface Species
NaFeEDTA
84
4.5
4.7
Sanderson electronegativity, χ Fig. 11. Average TOF0 in ks1 vs. Sanderson electronegativity of alkali oxide for materials prepared from NH4FeEDTA on alkali-modified non-porous SiO2. TOF0 showed a clear increase with increasing basic character of the alkali promoter.
This work separated the roles of precursor ligand and alkali promoters in the synthesis of highly dispersed, SiO2-supported FeOx sites for alkane selective oxidation with H2O2. DRUV–Vis and H2-TPR experiments with chelating NH4FeEDTA and Fe(NO3)3 precursors deposited on doped SiO2 showed that both a bulky ligand and alkali promoter were necessary to produce highly dispersed species. The bulky EDTA ligand established an initial high dispersion which was maintained following the oxidative removal of the organic ligand due to the presence of the anchoring alkali. Maintaining high dispersion of Fe3+ ions on SiO2 after calcination
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required a ratio of Fe/alkali of 1 or lower. The minimum requirement of equimolar amounts of Fe3+ and promoter is consistent with the idea that the alkali promoted dispersion by anchoring Fe3+ ions in place and did not simply titrate a small number of oxide nucleation sites [18] or otherwise alter the average surface chemistry. Likewise, surface densities were always below 0.9 nm2, which was well below the average SiOH surface density of these materials, so a simple Na+/H+ exchange would not be expected to be the origin of the stabilization. As such, no significant changes with these alkali loadings were seen in the FTIR OH stretching region of the final catalysts. Sequential addition of the alkali promoter, then impregnation of highly soluble Fe3+ precursors like NH4FeEDTA, followed by calcination, was an effective way to give higher Fe3+ surface densities than afforded by NaFeEDTA, while maintaining dispersion and high TOF0 in adamantane oxidation for all surface densities examined. It must be cautioned, however, that this protocol required the use of robust supports such as non-porous SiO2 particles. High-loaded porous supports lost activity in a way most consistent with collapse of the pore structure following alkaline and high-temperature treatments. DRUV–Vis indicated Fe3+ species remained well dispersed at high loadings, but the very poor reducibility of these species and the low TOF0 even when the sites were dispersed suggested that a fraction of the Fe3+ in these materials was inaccessible. The use of different alkali precursors may remedy this issue if porous materials are desired for the given application. The activity of alkali promoted supported Fe3+ in adamantane oxidation was altered by the identity of the promoter, with more basic alkali promoters leading to the higher activities. This effect was not seen when using Fe(NO3)3 as the precursor, again emphasizing that the bulky EDTA ligand was necessary to produce the surface species active in adamantane oxidation. As in our previous study, two Fe3+ surface active species of distinct reducibility were identified. Previously, we assigned these as isolated Fe3+ structures and a minority of very small FeOx oligomers present on catalysts derived from NaFeEDTA and NH4FeEDTA, respectively, and suggested by a renormalization of observed TOF0 that these displayed equal activity in adamantane oxidation by H2O2. Here, different alkali and surface densities led to variations in the relative populations of these species without any attendant changes in catalytic turnover frequencies or selectivities, providing a more-direct verification of their equal reactivity. Only the emergence of larger agglomerates (or inaccessible species in porous supports) resulted in decreases in TOF0. It is thus proposed that the catalytic behavior of these two highly dispersed species is the same because both make Fe3+ equally accessible to activating H2O2. These conclusions are summarized graphically in Fig. 12. Under such conditions, small changes in speciation showed no effect in catalyst activity and the main factor became electronic promotion following the trend in Fig. 11. It must be emphasized that Fe(NO3)3 at low loadings led to oligomeric FeOx with no evidence of the formation of Fe2O3, but these materials were much less active. Thus, it is not sufficient to use the absence of crystalline Fe2O3 as the sole criterion for quantifying dispersion and predicting reactivity. Finally, this methodology of two-cycle deposition of an alkali followed by a metal center ‘‘protected’’ by a bulky chelating species appears to be a viable route to generate other highly dispersed transition metal cations on different supports. Acknowledgments J.M.N. acknowledges support from U.S. Department of EnergyOffice of Basic Energy Sciences grant DE-SC0006718, Northwestern University, and a 3M Non-Tenured Faculty Grant. DPC acknowledges the support of Toyota Motor Engineering, Inc. and thanks
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Dr. Christian Canlas, Pria Young, and Todd Eaton for the helpful discussions. Portions of this work were performed with the valuable help of Dr. Qing Ma at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company and Northwestern University. Use of the APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2012.09.004. References [1] A.J. van Dillen, R.J.A.M. Terorde, D.J. Lensveld, J.W. Geus, K.P. De Jong, J. Catal. 216 (2003) 257. [2] T.M. Eggenhuisen, J.P.d. Breejen, D. Verdoes, P.E.d. Jongh, K.P.d. Jong, J. Am. Chem. Soc. 132 (2010) 18318. [3] A.T. Bell, B.C. Gates, D. Ray, Basic research needs: catalysis for energy, in: Basic Energy Sciences Workshop, US Department of, Energy, 2007. [4] F. Arena, F. Frusteri, L. Spadaro, A. Venuto, A. Parmaliana, Stud. Surf. Sci. Catal. 143 (2000) 1097. [5] F. Arena, G. Gatti, S. Coluccia, G. Martra, A. Parmaliana, Catal. Today 91–92 (2004) 305. [6] F. Arena, G. Gatti, G. Martra, S. Coluccia, L. Stievano, L. Spadaro, P. Famulari, A. Parmaliana, J. Catal. 231 (2005) 365. [7] A. Gervasini, C. Messi, A. Ponti, S. Canedese, N. Ravasio, J. Phys. Chem. C 112 (2008) 4635. [8] A. Gervasini, C. Messi, P. Carniti, A. Ponti, N. Ravasio, F. Zacheria, J. Catal. 262 (2009) 224. [9] C. Nozaki, C.G. Lugmair, A.T. Bell, T.D. Tilley, J. Am. Chem. Soc. 124 (2002) 13194. [10] A.W. Holland, G. Li, A.M. Shahin, G.J. Long, A.T. Bell, T.D. Tilley, J. Catal. 235 (2005) 150. [11] N. Morlanés, J.M. Notestein, J. Catal. 275 (2010) 191. [12] N. Morlanes, J.M. Notestein, Appl. Catal. A 387 (2010) 45. [13] P. Munnik, M. Wolters, A. Gabrielsson, S.D. Pollington, G. Headdock, J.H. Bitter, P.E. de Jongh, K.P. de Jong, J. Phys. Chem. C 115 (2011) 14698. [14] D. Prieto-Centurion, J.M. Notestein, J. Catal. 279 (2011) 103. [15] Y. Zhai, D. Pierre, R. Si, W. Deng, P. Ferrin, A.U. Nilekar, G. Peng, J.A. Herron, D.C. Bell, H. Saltsburg, M. Mavrikakis, M. Flytzani-Stephanopoulos, Science 329 (2010) 1633. [16] M. Ferrandon, E. Björnbom, J. Catal. 200 (2001) 148. [17] W.D. Harding, K.E. Birkeland, H.H. Kung, Catal. Lett. 28 (1994) 1. [18] L. Owens, H.H. Kung, J. Catal. 148 (1994) 587. [19] P. Kubelka, F. Munk, Z. Tech, Physics 12 (1931) 593. [20] R. Meier, F.W. Heinemann, Inorg. Chim. Acta 337 (2002) 317. [21] W. Stöber, A. Fink, E. Bohn, J. Colloid Interface Sci. 26 (1968) 62. [22] G. McLendon, R.J. Motekaitis, A.E. Martell, Inorg. Chem. 15 (1976) 2306. [23] S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola, G. Vlaic, J. Catal. 158 (1996) 586. [24] A. Parmaliana, F. Arena, F. Frusteri, A. Marinez-Arias, M.L. Granados, J.L. Fierro, Appl. Catal. A 226 (2002) 163. [25] Z. Gabelica, A. Charmot, R. Vataj, R. Soulimane, J. Barrault, S. Valange, J. Therm. Anal. Calorim. 95 (2009) 445. [26] S. Valange, R. Palacio, A. Charmot, J. Barrault, A. Louati, Z. Gabelica, J. Mol. Catal. A: Chem. 305 (2009) 24. [27] P.B. Amama, S. Lim, D. Ciuparu, Y. Yang, L. Pfefferle, G.L. Haller, J. Phys. Chem. B 109 (2005) 2645. [28] K. Yogo, S. Tanaka, T. Ono, T. Mikami, E. Kikuchi, Micropor. Mater. 3 (1994) 39. [29] A. Jones, B.D. McNicol (Eds.), Temperature-Programmed Reduction for Solid Materials Characterization, Marcel Dekker, Inc., New York, 1986. p. 174. [30] M. Wilke, F. Farges, P.-E. Petit, G.E. Brown, F. Martin, Am. Mineral. 86 (2001) 714. [31] A.N. Biswas, P. Das, A. Agarwala, D. Bandyopadhyay, P. Bandyopadhyay, J. Mol. Catal. A: Chem. 326 (2010) 94. [32] F. Minisci, F. Fontana, S. Araneo, F. Recupero, S. Banfi, S. Quici, J. Am. Chem. Soc. 117 (1995) 226. [33] J.K. Kochi, Free radicals, in: J.K. Kochi (Ed.), Reactive Intermediates in Organic Chemistry, Wiley, New York, 1973. [34] G. Roelfes, M. Lubben, R. Hage, J.L. Que, B.L. Feringa, Chem. Eur. J. 6 (2000) 2152. [35] B. Singh, J.R. Long, F.F.d. Biani, D. Gatteschi, P. Stavropoulos, J. Am. Chem. Soc. 119 (1997) 7030. [36] S.I. Lopatin, Glass Phys. Chem. 29 (2003) 390. [37] A. Bassan, M.R.A. Blomberg, P.E.M. Sieghahn, L. Que, J. Am. Chem. Soc. 124 (2002) 11056.