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Gold stabilized aqueous sols immobilized on mesoporous CeO2–Al2O3 as catalysts for the preferential oxidation of carbon monoxide
Journal of Colloid and Interface Science 350 (2010) 435–442
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Gold stabilized aqueous sols immobilized on mesoporous CeO2–Al2O3 as catalysts for the preferential oxidation of carbon monoxide Loretta Storaro a,*, Maurizio Lenarda a, Elisa Moretti a, Aldo Talon a, Francesca Porta b, Bernardo Moltrasio b, Patrizia Canton c a b c
Department of Chemistry, University Ca’ Foscari of Venice, Via Torino 155/B, 30172 Venice, Italy Department of Inorganic, Organometallic and Analytical Chemistry, L. Malatesta, University of Milan, Via Venezian 21, 20133 Milan, Italy Department of Physical Chemistry, University Ca’ Foscari of Venice, Via Torino 155/B, 30172 Venice, Italy
a r t i c l e
i n f o
Article history: Received 26 April 2010 Accepted 27 June 2010 Available online 1 July 2010 Keywords: One-step synthesis Ordered mesopores Au sol THPS CO-PROX
a b s t r a c t Nanostructured Au/Al2O3–CeO2 catalysts with a low content of precious metal (0.9% wt.) were prepared immobilizing two different stabilized Au sols on a high surface area Al2O3–CeO2 mixed oxide with a uniform pore size distribution, synthesized by a one-pot methodology. The samples were characterized by elemental analysis, N2 physisorption, XRPD, TEM and 27Al-MAS NMR techniques. The catalytic activity of the two samples in the preferential oxidation of CO in excess of H2 (CO-PROX) was comparatively evaluated in the 35–110 °C temperature range. The Au-THPS/AlCe20 sample, prepared immobilizing a sol obtained reducing an aqueous solution of gold tetrachloroaurate salt with bis[tetrakis(hydroxymethyl)phosphonium sulfate], resulted very active and selective at low temperatures and its catalytic activity was correlated with the structural characteristics of the metal particles and of the ordered mesoporous support. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction One of the major problems for the introduction of polymer electrolyte membrane fuel cells (PEMFCs) as the energy source for hydrogen-based fuel cell processors, is the delivery of CO-free feed gas. Nevertheless, if H2 is generated by steam reforming or partial oxidation of fuels such as natural gas, gasoline, propane and alcohols (i.e. methanol), the resulting gas mixture (i.e. reformate gas), which usually comprises 75% H2 and 25% CO2, is contaminated with at least 1–2 vol.% CO. One of the most efficient processes that can be used to decrease the CO concentration below 100 ppm in the reformed gas, is the selective oxidation of CO to CO2 in the presence of H2, known as preferential CO oxidation (CO-PROX). Hence, the catalyst should be highly active in CO oxidation at low temperature (below 100 °C) and very selective towards CO2. Further the CO-PROX catalysts must tolerate the often high amounts of CO2 and water present in the reformate fuel. The more extensively studied catalysts for the CO-PROX are the Pt-group metals supported on oxides [1,2]. However, in the presence of H2O and CO2, these catalysts resulted not enough selective for fuel cell applications, and alternatives to platinum are desirable also to reduce the high cost associated to the use of a precious metal. Catalytic systems based on the couple CuO–CeO2 have been widely * Corresponding author. Fax: +39 0412346735. E-mail address: [email protected] (L. Storaro). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.06.062
studied as an alternative, because they are able to operate in the 100–200 °C temperature range and exhibit activity and selectivity comparable or superior to precious metals-based ones, being at the same time more convenient from an economical point of view. These catalytic systems show an intrinsic selectivity toward CO rather than H2 oxidation, usually attributed to the CuO–CeO2 close interaction [3–6]. Au has long been considered catalytically less active than other transition metals. Nevertheless, in recent years, nanostructured gold catalysts have attracted interest as potential candidates for CO removal processes. This was prompted by the seminal Haruta’s reports that demonstrated the very high activity for low temperature CO oxidation of gold nanoparticles supported on selected metal oxides [7,8]. Various nanoscale systems based on gold on simple and mixed oxides have been evaluated and found effective as CO-PROX catalysts [9–17]. Colloidal gold has a very long history, and the colorful sols have intrigued scientists through the ages, founding many applications outside catalysis, but very few within it. The truly scientific study of colloidal gold began with Turkevich [18,19] who, in one of the earliest applications of electron microscopy, investigated the size and shape of particles formed by various reducing agents and their properties. Unlike platinum, very small gold particles are not easily formed and, according to Turkevich, the best reducing agent resulted sodium citrate, which gave quite a narrow particle size distribution, peaking at 20 nm.
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Recent developments in preparative and analytical methods have led to the preparation of much smaller particles and to the determination of the intimate details of their structure. On the other hand, it is known that reduction of the chloroaurate ion by tetrakis(hydroxymethyl)phosphonium chloride gives a dark orange–brown sol of gold with particles, dimensionally ranging between 1 and 3 nm that have been successfully supported on titania, zirconia and alumina [20–22]. In previous papers [23–25] some of us described the preparation of alumina based multi-component systems with ordered mesoporosity. Following our studies on Ce/Al based catalysts, here we report the synthesis of nanostructured Au/CeO2–Al2O3 samples with high surface area, uniform pore size and low metal content (0.9 wt.%). Au nanoparticles were deposited on the support from stabilized Au sols [26]. The catalytic activity of the two samples in the preferential oxidation of CO to CO2 was comparatively studied in the 35–110 °C temperature range. 2. Experimental section 2.1. Samples preparation All the materials used in this paper are Aldrich products and no further purification was carried out. 2.2. AlCe20 Ce stearate (Ce(III) octadecanoate) was prepared analogously to what described previously [24]. The following molar ratio was used: 1 Al(sec-BuO)3:0.053 (C17H35COO)3Ce:24 n-C3H7OH:3H2O. In a typical synthesis, the amount of Ce(III) stearate required to obtain a final loading of 20 wt.% CeO2, was dissolved in n-propanol by sonication. After the addition of deionized water, the solution was stirred for 30 min; then Al tri-sec-butoxide was added. The resulting suspension was aged for 48 h in a Teflon-lined autoclave at 100 °C and autogenous pressure. After cooling to room temperature, the product was recovered by centrifugation, washed with ethanol and thermally treated, first at 410 °C in nitrogen flow for 6 h and then at 500 °C in a stream of air for 3 h. The cerium content determined by ICP-OES elemental analysis was 15.9 wt.%. Hereafter, the sample was identified by the acronym AlCe20, where the number 20 indicates the percentage of ceria. 2.3. Au-THPS/AlCe20 Gold nanoparticles were obtained by reduction of an aqueous solution of gold tetrachloroaurate salt by THPS, bis[tetrakis(hydroxymethyl)phosphonium sulfate], which acts as stabiliser and reducing agent, as already reported for THPC [tetrakis(hydroxymethyl)phosphonium chloride] [27]. THPS (0.0516 mmol, 0.062 M) was added to an aqueous solution of NaOH (0.033 M) under vigorous stirring. After 6 min a NaAuCl4 solution (0.01 mmol, 0.0126 M) was added dropwise. Immediately an orange–pink sol was obtained. After 20 min from sol generation, the colloid was immobilized on the oxide support, adding to the mixture a certain amount of AlCe20 under vigorous stirring. The amount of oxide was calculated in order to obtain a final metal loading of abt. 1 wt.%. After 2 h the slurry was filtered, the filtrate was washed thoroughly with distilled water (neutral mother liquors) and dried. ICP analysis was used to verify the Au loading on AlCe20, that resulted 0.9 wt.%. 2.4. Au-citrate/AlCe20 Gold nanoparticles were obtained by reduction of an aqueous solution of gold tetrachloroauric acid by NaBH4, in the presence
of citrate, modifying a published methodology [28]. A HAuCl4 solution (0.0254 mmol, 0.0608 M) was added under stirring to an aqueous solution of sodium citrate (0.039 mmol, 0.0388 M), obtaining a yellow solution. After 2 min a NaBH4 solution (0.142 mmol, 0.1 M) was added. Immediately an orange–red sol was formed. After 2 min of sol generation, the colloid was immobilized adding AlCe20 to the mixture under vigorous stirring. The amount of support was calculated as having a total final metal loading of 1 wt.%. After 1 h the slurry was filtered, the catalyst washed thoroughly with distilled water (neutral mother liquors) and dried. The Au metal loading on AlCe20 was estimated by ICP analysis and resulted 0.9 wt.%. 2.5. Catalytic measurements Catalytic activity tests were carried out in a laboratory flow apparatus with a fixed bed reactor operating at atmospheric pressure. The catalyst, with a defined particle size (0.050–0.110 mm), was introduced into a tubular Pyrex glass reactor (5 mm i.d.), placed in an aluminum heating block. Before the catalytic experiments, the sample was heated in situ at 110 °C under flowing air for 1 h. The W/F value (where W is the catalyst weight and F the total flow rate) was W/F = 0.18 g s cm3, using k = 2, except when differently described. The feed consisted of 1.25% CO, 1.25% O2 and 50% H2 (vol.%) balanced with He (purchased from SIAD). The effect of CO2 and H2O was examined in separate runs after addition of 15 vol.% CO2 and 10 vol.% H2O in the feed gas. To evaluate the effect of water vapor on the reaction, 10 vol.% H2O was added to the dry gas stream gas through a HPLC pump (supplied by Jasco). Calibration of the GC was done with a gas mixture containing 1% CO, 1% CO2, 1% O2 in He. When the effect of CO2 was examined, the GC calibration was done with 15 vol.% CO2 in He. The gas lines were heated at 110 °C, to avoid water condensation before the reactor inlet. An ice-cooled water condenser was used to trap the excess of water downstream of the reactor. A HP6890 GC gas chromatograph equipped with a TC detector was used to analyze the outlet composition. A CP Carboplot P7 column was used, with helium as carrier. The detection limit for CO was 10 ppm. The temperature was varied in the 35–110 °C range, and measurements were carried out continuously till the steady state was achieved. Both methanation and reverse water gas shift reactions were found to be negligible in our experimental conditions. Reverse water gas shift (rWGS) was performed using as feed a CO2–H2 (25–25%, He balance) gas stream. The reaction was monitored measuring the CO formation. The carbon monoxide and oxygen conversions were calculated based on the CO (Eq. (1)) and O2 (Eq. (2)) consumption respectively:
CO conversion ð%Þ ¼
O2 conversion ð%Þ ¼
out nin CO nCO 100 nin CO out nin O2 nO2
nin O2
100
ð1Þ
ð2Þ
The selectivity towards CO2 was estimated from the oxygen mass balance as follows:
Selectivity ð%Þ ¼
out nin CO nCO 100 in 2 ðnO2 nout O2 Þ
ð3Þ
The excess oxygen factor (k) is defined as:
k ¼ 2
nin O2 nin CO
ð4Þ
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2.6. Elemental analysis
3. Results and discussion
Elemental analysis was determined by ICP analyses performed on a Jobin Yvon JV24.
3.1. Catalytic testing
2.7. N2 physisorption at 196 °C N2 adsorption–desorption measurements (BET method) were performed at liquid nitrogen temperature (196 °C) with an ASAP 2010 apparatus from Micromeritics. The analysis procedure is fully automated and operates with the static volumetric technique. Before each measurement, the samples (0.100 g) were outgassed first at 110 °C for 12 h at 5 103 Torr and then at room temperature for 2 h at 0.75 106 Torr. The N2 isotherms were used to determine the specific surface area using the BET equation (S.A.BET), and the specific pore volume (Vp), calculated at p/p0 = 0.98. The pore size distribution was calculated following the BJH method. 2.8. Transmission electron microscopy (TEM) TEM images were taken on a JEOL-JEM 3010 high-resolution microscope (point resolution at Scherzer defocus 0.17 nm), equipped with a lanthanum hexaboride (LaB6) gun, using an accelerating voltage of 300 kV. The images were taken with a CCD camera (Gatan, mod. 694). The powder was suspended in isopropyl alcohol and dropped on a holey carbon film grid. 2.9. X-ray powder diffraction (XRPD) X-ray patterns were obtained using a Philips X’Pert system with a Cu Ka radiation (k = 1.54184 Å). The samples were disc shaped pressed powders and spectra were collected after calcination. The average dimension of the crystallites was determined by the Scherrer’s equation. 2.10. 27Al-magic angle spinning nuclear magnetic resonance (27Al-MAS NMR) 27 Al-MAS NMR spectra of solid samples were recorded on a Brucker AVANCE 500 MHz, CPMASS spectrometer, spinning at 4 kHz and operating at room temperature.
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Fig. 1 shows CO conversion and selectivity towards CO2 of AuTHPS/AlCe20 and Au-citrate/AlCe20 catalysts for the preferential oxidation of CO (CO-PROX). The composition of the inlet feed stream was initially fixed to 1.25% CO, 1.25% O2 and 50% H2 (He balance), using W/F = 0.18 g s cm3 and k = 2. The two materials, that have the same metal loading, showed quite different reaction profiles in the whole studied temperature range. The Au-THPS/ AlCe20 sample catalyst shows already at 65 °C a conversion of CO to CO2 of about 100% (the CO detection limit was 10 ppm) with a 61% of selectivity. On the other hand, the Au-citrate/AlCe20 sample shows a very poor activity at low temperatures, reaching a 68% conversion only above 80 °C, never exceeding the 75% at higher temperatures. The better performing sample Au-THPS/AlCe20, was selected to study the effect of contact time on CO conversion and selectivity. The results are summarized in Fig. 2 that shows the effect of three different contact times: W/F = 0.06–0.12–0.18 g s cm3 on the conversion and selectivity of Au-THPS/AlCe20. A contact time decrease from 0.18 g s cm3 to 0.12 g s cm3apparently did not modify the CO conversion and the selectivity to CO2, while a significant conversion decrease was observed when the contact time was further reduced to 0.06 g s cm3. The selectivity to CO2 was found to remain substantially unmodified in all the studied W/F range. It is well known that CO-PROX catalysts must tolerate quite high amounts of CO2 and H2O, because these reagents are always present in a normal unpurified reformate fuel. The effect of the presence of 15% of CO2 in the reaction mixture on the catalytic performance of the Au-THPS/AlCe20 sample is shown in Fig. 3. As shown the CO conversion slightly decreases while selectivity remains unchanged. The effect is more important at temperatures below 65 °C. When also a 10% of water was added to the gas feed, in the 80–110 °C temperature range, the conversion was observed to slightly increase, remaining just below the CO conversion values obtained in the absence of water and carbon dioxide. This finding confirms the positive role played by water in the oxidative catalytic processes catalyzed by supported gold nanoparticles [9]. The selectivity resulted to increase in all the studied temperature range, with values higher than those observed with the standard
Fig. 1. (a) CO conversion and (b) selectivity towards CO2 of Au-THPS/AlCe20 and Au-citrate/AlCe20 catalysts for the preferential CO oxidation in excess of hydrogen. The composition of the inlet feed stream was: 1.25% CO, 1.25% O2 and 50% H2 (He balance), W/F = 0.18 g s cm3, using k = 2.
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Fig. 2. Dependencies of the: (a) CO conversion and (b) selectivity towards CO2 as a function of the reaction temperature, over Au-THPS/AlCe20 using three different contact times. Operating conditions: k = 2; 50% H2, 1.25% CO, 1.25% O2, He balance (% vol.); W/F = 0.06–0.12–0.18 g s cm3.
feed and after 15% CO2 addition. Also if it is not easy to compare these results with literature data obtained with similar gold-based catalysts, owing to the often different operative conditions used by researchers (different Au loadings, gas feeds and contact times), a similar behavior was reported by Avgouropoulos et al. [15], on Au/CeO2 and Zn doped Au/CeO2 catalyst with a 3 wt.% Au loading and with gas feeds and contact times very similar to those used in the present work. The success of PROX in carbon monoxide removal, is also conditioned by the possible occurrence, in addition to the H2 oxidation, of other undesired reactions. The carbon balance of all experiments showed that the CO2 production was in good agreement with the CO consumption, ruling out coke formation. Additional reactions that may play a role in the CO-PROX reaction system are the methanation and the reverse water gas shift. Both the CO and CO2 methanation were found to be negligible in our experimental conditions, and the Au-THPS/AlCe20 catalyst was found also inactive for the rWGS at temperatures below 95 °C. 3.2. Catalysts and precursors characterization 3.2.1. N2 physisorption at 196 °C The nitrogen adsorption–desorption isotherms of the samples Au-THPS/AlCe20 and Au-citrate/AlCe20 and of the bare AlCe20 support are shown in Fig. 4. All the isotherms resulted to be type IV of the IUPAC classification. On the other hand the S.A.BET, the Vp and the distribution of the pore diameters, calculated according to the BJH model applied on the adsorption branch (Fig. 4 and Table 1), resulted very different for the AlCe20 based samples, prepared from the two different Au sols (Au-citrate, Au-THPS). The Au-citrate/AlCe20 sample was prepared by impregnation of the Al/Ce support with an aqueous sol composed by metal particles with a quite uniform distribution of diameters with a maximum observed value of 5 nm (determined by ultracentrifugation and HRTEM), while Au-THPS sample resulted from impregnation of the support with a sol with particles mainly of diameters <2 nm. As shown in Fig. 4 and Table 1, impregnation of the support AlCe20 with the Au-THPS colloidal gold sol (see Section 2.1) causes a slight decrease of the specific surface area (from 394 to 376 m2 g1), with the Vp value decreasing from 0.51 to 0.36 cm3 g1 (Table 1 and Fig. 4b). On the other hand, when the Au-citrate sol was used to prepare the Au-citrate/AlCe20 sample, the specific surface area
was found to decrease much more dramatically (from 394 to 286 m2 g1), with a 50% loss of pore volume and a severe alteration of the pore distribution (Table 1 and Fig. 4b). It clearly appears from the surface area and Vp data, that impregnation with gold sols of the mesoporous support oxide causes an overall decrease of these values, probably because of partial filling of the mesopores. The phenomenon is more dramatic for the Au-citrate/AlCe20 sample because the sol particles (<5 nm) are dimensionally slightly larger than the mean diameter of the oxide mesopores. The surface area of both samples did not change after the catalytic tests. 4.1. Transmission electron micrographs (TEM) It is well known that detection and evaluation of sizes distribution of gold nanoparticles on nanocrystalline ceria is very difficult because of disturbance and phase contrast by the oxide support [29,30]. TEM images of the Au-THPS/AlCe20 catalyst (Fig. 5a and b) showed the presence of dispersed and rounded Au particles ranging in size from 2 to 4 nm and that appear to only slightly aggregate after the CO-PROX catalytic test. TEM images of the Aucitrate/AlCe20 sample are unfortunately poorly resolved and difficult to analyze, because of the low quality of the sample, but it clearly appears that the gold particles have a wide distribution of diameters and a certain extent of aggregation (Fig. 5c and d). The more resolved TEM micrographs of this catalyst taken after reaction, confirm this finding and point out a more pronounced particles aggregation. 4.2. X-ray powder diffraction (XRPD) The X-ray diffraction profiles of the AlCe20 support, the Au-THPS/AlCe20 before and after the CO-PROX reaction and the Au-citrate/AlCe20 before and after the CO-PROX reaction are showed in Fig. 6. The patterns of all the samples contain broad diffraction peaks due to poorly microcrystalline c-alumina and signals attributable to the presence of fcc CeO2 with the fluorite structure, as expected for this type of oxide. The mean size of the ceria particles, for the bare AlCe20 support, was calculated from the half-width of the main peak at 2h = 28.6°, according to the Scherrer’s equation and resulted to be 3.2 nm. The presence of a nanocrystalline ceria phase and the low quality of XRPD signals attributable to the gold particles of Au-THPS/AlCe20 sample does
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Fig. 3. Effect of the addition of CO2 and H2O on the (a) activity and (b) selectivity towards CO2 as a function of the temperature over Au-THPS/AlCe20. Operating conditions: 1.25% CO, 1.25% O2, 50.0% H2, 0–15.0% CO2, 0–10.0% H2O and He balance; W/F = 0.18 g s cm3.
Fig. 4. N2 physisorption data of AlCe20, Au-THPS/AlCe20, Au-citrate/AlCe20: (a) isotherms (filled symbols for the adsorption branch and empty symbols for the desorption one); (b) BJH pore diameter distributions, estimated on the adsorption branch.
Table 1 BET specific surface area and specific pore volume of the prepared materials.
a b
a
2
Sample
S.A.BET (m g
AlCe20 Au-THPS/AlCe20 Au-citrate/AlCe20
394 ± 1 376 ± 1 285 ± 1
1
)
b
3
Vp (cm g
1
)
0.51 0.36 0.27
B.E.T. specific surface area. Specific pore volume determined at p/p0 = 0.98.
Table 2 27 Al-MAS NMR data calculated by integration of the peaks of AlCe20, Au-THPS/AlCe20 and Au-citrate/AlCe20 samples. Relative amounts (%)
AlCe20 Au-THPS/AlCe20 Au-citrate/AlCe20
AlIV
AlV
AlVI
21.6 22.1 19.5
12.4 20.9 20.5
66.0 57 60
not allow a careful quantitative determination of the crystallites dimensional distribution [15], nevertheless the mean size of the majority of the gold particles in the fresh Au-THPS/AlCe20 sample can be estimated 2.0 nm, and appears not to substantially change after the catalytic test (Fig. 6b and c), in agreement with what shown by the TEM images. On the other hand, the mean size of metallic gold in the Au-citrate/AlCe20 sample can be roughly estimated 4–5 nm before the CO-PROX, while the size distribution of the metallic particles in the used sample was found to become larger, with an increasing percentage of crystallites with diameter up to abt. 20 nm, as can be also roughly deduced from the shape of the (1 1 1) signal (Fig. 6d and e). Therefore we can conclude that, adsorbing the Au-THPS sol on the ordered mesoporous AlCe20 support, it is possible to prepare a sample with gold nanoparticles with a narrow dimensional distribution, centered at 2 nm, with a high thermal stability in the reaction conditions. The use of the Au-citrate sol, on the other hand, provides a catalyst with larger nanoparticles and wider
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Fig. 7. 27Al-MAS NMR spectra of the bare support AlCe20 and of the Au-THPS/ AlCe20 and Au-citrate/AlCe20 materials.
Fig. 5. TEM images of: (a) Au-THPS/AlCe20 before and (b) after CO-PROX reaction; (c) Au-citrate/AlCe20 before and (d) after catalytic test (1.25% CO, 1.25% O2, 50% H2, He balance, W/F = 0.18 g s cm3, k = 2).
Fig. 6. X-ray diffraction patterns of the samples: (a) AlCe20 support; (b) Au-THPS/ AlCe20 before and (c) after the CO-PROX reaction; (d) Au-citrate/AlCe20 before and (e) after the CO-PROX reaction. Operating conditions for the catalytic test: 1.25% CO, 1.25% O2, 50% H2, He balance, W/F = 0.18 g s cm3, k = 2.
dimensional distribution, with a largely reduced surface area and lower thermal stability in the reaction conditions. 4.3.
27
Al-MAS NMR
27 Al-MAS NMR spectra of calcined AlCe20 is shown in Fig. 7. The sample shows three resonance signals that can be assigned to 4, 5, 6-coordinated Al atoms respectively. The intense resonance at 0 ppm can be assigned to Al in octahedral coordination (AlVI), the signal at 60 ppm is assigned to tetrahedral framework aluminium (AlIV), and the signal at 29 ppm to pentahedral (AlV) one. The (AlVI)/(AlIV) ratio is about 3.0 (Table 2). As reported in the literature, a AlVI/AlIV ratio ranging from 2.4 to 3.8 is consistent with a mesoporous c-alumina structure [31–33]. The observation of an AlV signal is usually attributed to deviations from ideality of the cphase caused by the presence of Al sites with unsaturated coordi-
nation [31]. The relative amount of AlV was found to increase with the increasing of Ce content, in ceria-doped aluminas [34]. The presence of Ce cations in the AlCe20 structural network can then be responsible for the AlV peak, that represents the 12.4% of the 29 Al signal. No shifts of the resonance signals assigned to AlVI, AlIV and AlV were observed in the 29Al-MAS NMR spectra of Au-THPS/ AlCe20 and Au-citrate/AlCe20 but only a decrease of the AlVI/AlV ratio from 3.1 of the bare AlCe20 support to 2.9 and 2.7 of the Au-citrate/AlCe20 and Au-THPS/AlCe20 respectively (Table 2). The decrease of AlVI/AlV ratio, that appears almost the same in the two samples, is caused by a relative increase of the amount of AlV species and can be most probably attributed to a further distortion of the aluminum network induced by the adsorbed gold nanoparticles. Gold nanoparticles of less than 5 nm have been found to be active for low temperature complete CO oxidation (COX), water gas shift reaction and selective CO oxidation (CO-PROX) [9]. However, the data in the literature are quite variable, and the nature of the active sites remains quite obscure. It is, in any case, generally accepted that the catalytic activity of Au depends to a large extent on the size of the particles, even if other effects, such as the nature of the support material, the Au–support interface, the particle shape and metal–support charge transfer, must be considered of fundamental importance. The mechanisms for gold-catalyzed reactions have been critically reviewed by two of the founding fathers of the catalytic gold chemistry, Bond and Thompson in a recent paper [35], where reactions schemes for the WGS, COX and PROX catalyzed by gold on both ceramic and reducible oxide supports, are critically discussed. The mechanism proposed, almost a decade ago by Bond and Thompson [36] for CO oxidation by supported gold, involved Au particles assumed to contain both Au0 and Aux+ active sites, the latter located at the metal–support interface. Later studies, investigating the role of water on the catalyst activity, provided further corroborations to this hypothesis and underlined the role of –OH groups of the support coordinated to gold cationic sites in the catalytic cycle [17,37–39]. Nevertheless, the structure of the catalytically active gold species and the oxidation state/s of the metal is still a debated subject, mainly because of the heterogeneity of the methods used to prepare the studied samples [40–42]. The role of the support oxide is also still under discussion, in particular after the demonstration of the good reactivity shown by unsupported gold nanoparticles in the aerobic glucose oxidation [43]. The influence of the reducibility of the oxide on the catalytic performance of the metal–support system was the object of several studies, mainly related to the identification of the oxygen activation sites [9,44,45]. Results obtained from model catalysts,
L. Storaro et al. / Journal of Colloid and Interface Science 350 (2010) 435–442
prepared by sol deposition of almost identical metal particle size supported on various oxides, suggested that the metal–support interaction concept must be revised. It was proposed by Comotti et al. [22] that the reducibility of the support and its ability to activate oxygen is not a key factor. These observations confirmed what proposed by Lopez et al. [46], namely the catalytic activity in COPROX is considerably more affected by gold particle size than by the support. The differences in the CO oxidation activity between reducible or not reducible supports for gold nanoparticles of the same size were attributed to an interaction of gold clusters with the support that can result in a change in the shape of the particles [22]. It has been also reported [47] that cerium oxide is not a suitable support for gold-based catalysts in CO low temperature oxidation but, more recently, it was demonstrated that, if nanocrystalline CeO2 was used as support, the activity of gold nanoparticles increased by two orders of magnitude, achieving 100% CO conversion at 10 °C in excess of oxygen [48]. The studies on catalytic systems based on gold nanoparticles supported on ceria and alumina–ceria were also extended to low temperature WGS [49,50], complete hydrocarbon oxidation and CO-PROX [12,13,15,16]. Gold–ceria catalysts were found to be very active and stable systems. The good catalytic activity was attributed to the ceria structure modification caused by the insertion of the gold particles and the consequent formation of more Ce3+ and oxygen vacancies in close contact with small Au clusters. There is a quite general agreement in attributing the high activity of these systems to the enhanced electron transfer between defective ceria and partially charged gold particles via oxygen vacancies [48– 50]. The catalyzed reaction was suggested to proceed at the boundary between small gold clusters and ceria. The high stability of these systems was also explained by the gold ions stabilization in the ceria matrix through strong gold–ceria interparticles surface interaction, that therefore appeared to be of crucial importance [49,50]. In the ceria–alumina mixed oxides, the presence of alumina was found to both increase the ceria dispersion and the formation of oxygen vacancies. Alumina was found responsible for the higher stability of gold and ceria particles against agglomeration, during the reaction at higher temperatures [50]. Up to date the remarkable catalytic activity of very small gold nanoparticles is maybe the best explanation of nanoscale effects in catalysis and it was proposed that nanosized metallic Au is intrinsically catalytically active [51,52]. Nevertheless, the interaction between Au nanoparticles and the support oxide, in particular of reducible type, such as TiO2 or CeO2, that is still quite far from being totally understood, represents a key issue in the study of gold catalysis [53]. A very active gold–ceria–alumina based catalyst for COX or COPROX can be imagined as an ensemble of highly dispersed gold nanoparticles (1–2 nm) closely interacting with nanostructured ceria particles stabilized by alumina against aggregation. The highly uniform ordered mesoporous alumina, containing 20% of ceria used in this study (AlCe20), was prepared by a single step synthesis, using long-chain carboxylic acid salts that are structure-directing agents for the preparation of alumina samples with high surface area and organized mesopores [54,55]. It is in fact well known that carboxylic acids greatly influence the process of hydrolization of Al alkoxides [56,57], controlling the rates of hydrolysis and condensation. When carboxylates are used as templates, Ce stearate in our case, the process is supposed to proceed through a charge matching pathway SI+ [58]. In fact, after the dissociation of the salts, cerium ions act, together with the partially hydrolyzed and condensed Al species, as counterions of the anionic surfactant forming organic–inorganic ion pairs. All the species cooperate to the formation of the final framework. Consequently we can reasonably presume that, in the sample AlCe20, cerium ions are uniformly distributed on the pore walls of the resulting Al–Ce mesoporous mixed oxide system.
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Therefore, this compositionally homogeneous and structurally ordered mixed oxide, with a narrow pore distribution, that can be prepared with a high level of reproducibility, appears to be very suitable as support to prepare catalytic systems for CO-PROX oxidation. The two samples used in this comparative study were prepared by deposition on AlCe20, of the gold sols obtained by reduction of tetrachloroaurate ions by THPS or by NaBH4, in the presence of citrate, because, as pointed out by Comotti [22], this procedure allows the elimination of the influence of the support on the formation of the gold nanoparticles that can be created almost identical, independently of the support used. To achieve satisfactory activity for CO oxidation, the gold particles must be in the range of 1–7 nm and both the used methods allow the obtainment of gold sols with nanoparticles in this dimensional range. Nevertheless the catalytic data in the CO-PROX, graphically presented in Fig. 1 highlight the enormous difference both in activity and in selectivity between the samples Au-THPS/ AlCe20 and Au-Citrate/AlCe20. As shown in Fig. 4 and Table 1, the AlCe20 oxide constitutes a porous system with a quite narrow pore diameter distribution peaking at 3.3 nm. Therefore, the outstanding low temperature catalytic activity and selectivity of Au-THPS/ AlCe20 can be interpreted, imagining that the small nanoparticles of the Au-THPS sol, mainly with diameters <2 nm, can slip into the support mesopores and enter in close contact with the exposed surface of the ceria nanoparticles, realizing what are thought to be the optimum conditions for the creation of CO-PROX active sites. On the other hand, the immobilization on AlCe20 of the Au-citrate gold sol, that is mostly formed by nanoparticles dimensionally greater than the support pores diameter, probably causes the almost complete occlusion of a large part of the mesopores, as pointed out by the observed dramatic decrease of the specific pore volume Vp. The activity of Au-citrate/AlCe20 does not break down completely but it is strongly reduced (less than 50% at 65 °C), due to the aggregation of the gold nanoparticles situated at the interface of the support as shown by the comparison of XRD profiles of the Au-citrate/ AlCe20 sample before and after the CO-PROX reaction. 5. Conclusions A very active gold–ceria–alumina catalyst for CO-PROX can be prepared with highly dispersed gold nanoparticles (1–2 nm) closely interacting with nanostructured ceria particles stabilized by alumina against aggregation. The highly uniform ordered mesoporous alumina, containing 20 wt.% of ceria was prepared by onepot methodology, using Al alkoxide as alumina precursor and Ce stearate both as metal source and structure directing agent. The immobilization of a gold sol on this high surface area and uniform pore size ceria–alumina support allows the preparation of a nanostructured CO-PROX catalyst very active at low temperature, thermally stable and selective. The Au sol obtained reducing a solution of gold tetrachloroaurate salt with THPS, bis[tetrakis(hydroxymethyl)phosphonium sulfate], which acts as stabiliser and reducing agent, produces metal nanoparticles able to penetrate the support pores, allowing a very close interaction between the surfaces of ceria and gold nanoparticles, without pore clogging up. References [1] [2] [3] [4]
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Gold stabilized aqueous sols immobilized on mesoporous CeO2–Al2O3 as catalysts for the preferential oxidation of carbon monoxide Loretta Storaro a,*, Maurizio Lenarda a, Elisa Moretti a, Aldo Talon a, Francesca Porta b, Bernardo Moltrasio b, Patrizia Canton c a b c
Department of Chemistry, University Ca’ Foscari of Venice, Via Torino 155/B, 30172 Venice, Italy Department of Inorganic, Organometallic and Analytical Chemistry, L. Malatesta, University of Milan, Via Venezian 21, 20133 Milan, Italy Department of Physical Chemistry, University Ca’ Foscari of Venice, Via Torino 155/B, 30172 Venice, Italy
a r t i c l e
i n f o
Article history: Received 26 April 2010 Accepted 27 June 2010 Available online 1 July 2010 Keywords: One-step synthesis Ordered mesopores Au sol THPS CO-PROX
a b s t r a c t Nanostructured Au/Al2O3–CeO2 catalysts with a low content of precious metal (0.9% wt.) were prepared immobilizing two different stabilized Au sols on a high surface area Al2O3–CeO2 mixed oxide with a uniform pore size distribution, synthesized by a one-pot methodology. The samples were characterized by elemental analysis, N2 physisorption, XRPD, TEM and 27Al-MAS NMR techniques. The catalytic activity of the two samples in the preferential oxidation of CO in excess of H2 (CO-PROX) was comparatively evaluated in the 35–110 °C temperature range. The Au-THPS/AlCe20 sample, prepared immobilizing a sol obtained reducing an aqueous solution of gold tetrachloroaurate salt with bis[tetrakis(hydroxymethyl)phosphonium sulfate], resulted very active and selective at low temperatures and its catalytic activity was correlated with the structural characteristics of the metal particles and of the ordered mesoporous support. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction One of the major problems for the introduction of polymer electrolyte membrane fuel cells (PEMFCs) as the energy source for hydrogen-based fuel cell processors, is the delivery of CO-free feed gas. Nevertheless, if H2 is generated by steam reforming or partial oxidation of fuels such as natural gas, gasoline, propane and alcohols (i.e. methanol), the resulting gas mixture (i.e. reformate gas), which usually comprises 75% H2 and 25% CO2, is contaminated with at least 1–2 vol.% CO. One of the most efficient processes that can be used to decrease the CO concentration below 100 ppm in the reformed gas, is the selective oxidation of CO to CO2 in the presence of H2, known as preferential CO oxidation (CO-PROX). Hence, the catalyst should be highly active in CO oxidation at low temperature (below 100 °C) and very selective towards CO2. Further the CO-PROX catalysts must tolerate the often high amounts of CO2 and water present in the reformate fuel. The more extensively studied catalysts for the CO-PROX are the Pt-group metals supported on oxides [1,2]. However, in the presence of H2O and CO2, these catalysts resulted not enough selective for fuel cell applications, and alternatives to platinum are desirable also to reduce the high cost associated to the use of a precious metal. Catalytic systems based on the couple CuO–CeO2 have been widely * Corresponding author. Fax: +39 0412346735. E-mail address: [email protected] (L. Storaro). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.06.062
studied as an alternative, because they are able to operate in the 100–200 °C temperature range and exhibit activity and selectivity comparable or superior to precious metals-based ones, being at the same time more convenient from an economical point of view. These catalytic systems show an intrinsic selectivity toward CO rather than H2 oxidation, usually attributed to the CuO–CeO2 close interaction [3–6]. Au has long been considered catalytically less active than other transition metals. Nevertheless, in recent years, nanostructured gold catalysts have attracted interest as potential candidates for CO removal processes. This was prompted by the seminal Haruta’s reports that demonstrated the very high activity for low temperature CO oxidation of gold nanoparticles supported on selected metal oxides [7,8]. Various nanoscale systems based on gold on simple and mixed oxides have been evaluated and found effective as CO-PROX catalysts [9–17]. Colloidal gold has a very long history, and the colorful sols have intrigued scientists through the ages, founding many applications outside catalysis, but very few within it. The truly scientific study of colloidal gold began with Turkevich [18,19] who, in one of the earliest applications of electron microscopy, investigated the size and shape of particles formed by various reducing agents and their properties. Unlike platinum, very small gold particles are not easily formed and, according to Turkevich, the best reducing agent resulted sodium citrate, which gave quite a narrow particle size distribution, peaking at 20 nm.
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Recent developments in preparative and analytical methods have led to the preparation of much smaller particles and to the determination of the intimate details of their structure. On the other hand, it is known that reduction of the chloroaurate ion by tetrakis(hydroxymethyl)phosphonium chloride gives a dark orange–brown sol of gold with particles, dimensionally ranging between 1 and 3 nm that have been successfully supported on titania, zirconia and alumina [20–22]. In previous papers [23–25] some of us described the preparation of alumina based multi-component systems with ordered mesoporosity. Following our studies on Ce/Al based catalysts, here we report the synthesis of nanostructured Au/CeO2–Al2O3 samples with high surface area, uniform pore size and low metal content (0.9 wt.%). Au nanoparticles were deposited on the support from stabilized Au sols [26]. The catalytic activity of the two samples in the preferential oxidation of CO to CO2 was comparatively studied in the 35–110 °C temperature range. 2. Experimental section 2.1. Samples preparation All the materials used in this paper are Aldrich products and no further purification was carried out. 2.2. AlCe20 Ce stearate (Ce(III) octadecanoate) was prepared analogously to what described previously [24]. The following molar ratio was used: 1 Al(sec-BuO)3:0.053 (C17H35COO)3Ce:24 n-C3H7OH:3H2O. In a typical synthesis, the amount of Ce(III) stearate required to obtain a final loading of 20 wt.% CeO2, was dissolved in n-propanol by sonication. After the addition of deionized water, the solution was stirred for 30 min; then Al tri-sec-butoxide was added. The resulting suspension was aged for 48 h in a Teflon-lined autoclave at 100 °C and autogenous pressure. After cooling to room temperature, the product was recovered by centrifugation, washed with ethanol and thermally treated, first at 410 °C in nitrogen flow for 6 h and then at 500 °C in a stream of air for 3 h. The cerium content determined by ICP-OES elemental analysis was 15.9 wt.%. Hereafter, the sample was identified by the acronym AlCe20, where the number 20 indicates the percentage of ceria. 2.3. Au-THPS/AlCe20 Gold nanoparticles were obtained by reduction of an aqueous solution of gold tetrachloroaurate salt by THPS, bis[tetrakis(hydroxymethyl)phosphonium sulfate], which acts as stabiliser and reducing agent, as already reported for THPC [tetrakis(hydroxymethyl)phosphonium chloride] [27]. THPS (0.0516 mmol, 0.062 M) was added to an aqueous solution of NaOH (0.033 M) under vigorous stirring. After 6 min a NaAuCl4 solution (0.01 mmol, 0.0126 M) was added dropwise. Immediately an orange–pink sol was obtained. After 20 min from sol generation, the colloid was immobilized on the oxide support, adding to the mixture a certain amount of AlCe20 under vigorous stirring. The amount of oxide was calculated in order to obtain a final metal loading of abt. 1 wt.%. After 2 h the slurry was filtered, the filtrate was washed thoroughly with distilled water (neutral mother liquors) and dried. ICP analysis was used to verify the Au loading on AlCe20, that resulted 0.9 wt.%. 2.4. Au-citrate/AlCe20 Gold nanoparticles were obtained by reduction of an aqueous solution of gold tetrachloroauric acid by NaBH4, in the presence
of citrate, modifying a published methodology [28]. A HAuCl4 solution (0.0254 mmol, 0.0608 M) was added under stirring to an aqueous solution of sodium citrate (0.039 mmol, 0.0388 M), obtaining a yellow solution. After 2 min a NaBH4 solution (0.142 mmol, 0.1 M) was added. Immediately an orange–red sol was formed. After 2 min of sol generation, the colloid was immobilized adding AlCe20 to the mixture under vigorous stirring. The amount of support was calculated as having a total final metal loading of 1 wt.%. After 1 h the slurry was filtered, the catalyst washed thoroughly with distilled water (neutral mother liquors) and dried. The Au metal loading on AlCe20 was estimated by ICP analysis and resulted 0.9 wt.%. 2.5. Catalytic measurements Catalytic activity tests were carried out in a laboratory flow apparatus with a fixed bed reactor operating at atmospheric pressure. The catalyst, with a defined particle size (0.050–0.110 mm), was introduced into a tubular Pyrex glass reactor (5 mm i.d.), placed in an aluminum heating block. Before the catalytic experiments, the sample was heated in situ at 110 °C under flowing air for 1 h. The W/F value (where W is the catalyst weight and F the total flow rate) was W/F = 0.18 g s cm3, using k = 2, except when differently described. The feed consisted of 1.25% CO, 1.25% O2 and 50% H2 (vol.%) balanced with He (purchased from SIAD). The effect of CO2 and H2O was examined in separate runs after addition of 15 vol.% CO2 and 10 vol.% H2O in the feed gas. To evaluate the effect of water vapor on the reaction, 10 vol.% H2O was added to the dry gas stream gas through a HPLC pump (supplied by Jasco). Calibration of the GC was done with a gas mixture containing 1% CO, 1% CO2, 1% O2 in He. When the effect of CO2 was examined, the GC calibration was done with 15 vol.% CO2 in He. The gas lines were heated at 110 °C, to avoid water condensation before the reactor inlet. An ice-cooled water condenser was used to trap the excess of water downstream of the reactor. A HP6890 GC gas chromatograph equipped with a TC detector was used to analyze the outlet composition. A CP Carboplot P7 column was used, with helium as carrier. The detection limit for CO was 10 ppm. The temperature was varied in the 35–110 °C range, and measurements were carried out continuously till the steady state was achieved. Both methanation and reverse water gas shift reactions were found to be negligible in our experimental conditions. Reverse water gas shift (rWGS) was performed using as feed a CO2–H2 (25–25%, He balance) gas stream. The reaction was monitored measuring the CO formation. The carbon monoxide and oxygen conversions were calculated based on the CO (Eq. (1)) and O2 (Eq. (2)) consumption respectively:
CO conversion ð%Þ ¼
O2 conversion ð%Þ ¼
out nin CO nCO 100 nin CO out nin O2 nO2
nin O2
100
ð1Þ
ð2Þ
The selectivity towards CO2 was estimated from the oxygen mass balance as follows:
Selectivity ð%Þ ¼
out nin CO nCO 100 in 2 ðnO2 nout O2 Þ
ð3Þ
The excess oxygen factor (k) is defined as:
k ¼ 2
nin O2 nin CO
ð4Þ
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2.6. Elemental analysis
3. Results and discussion
Elemental analysis was determined by ICP analyses performed on a Jobin Yvon JV24.
3.1. Catalytic testing
2.7. N2 physisorption at 196 °C N2 adsorption–desorption measurements (BET method) were performed at liquid nitrogen temperature (196 °C) with an ASAP 2010 apparatus from Micromeritics. The analysis procedure is fully automated and operates with the static volumetric technique. Before each measurement, the samples (0.100 g) were outgassed first at 110 °C for 12 h at 5 103 Torr and then at room temperature for 2 h at 0.75 106 Torr. The N2 isotherms were used to determine the specific surface area using the BET equation (S.A.BET), and the specific pore volume (Vp), calculated at p/p0 = 0.98. The pore size distribution was calculated following the BJH method. 2.8. Transmission electron microscopy (TEM) TEM images were taken on a JEOL-JEM 3010 high-resolution microscope (point resolution at Scherzer defocus 0.17 nm), equipped with a lanthanum hexaboride (LaB6) gun, using an accelerating voltage of 300 kV. The images were taken with a CCD camera (Gatan, mod. 694). The powder was suspended in isopropyl alcohol and dropped on a holey carbon film grid. 2.9. X-ray powder diffraction (XRPD) X-ray patterns were obtained using a Philips X’Pert system with a Cu Ka radiation (k = 1.54184 Å). The samples were disc shaped pressed powders and spectra were collected after calcination. The average dimension of the crystallites was determined by the Scherrer’s equation. 2.10. 27Al-magic angle spinning nuclear magnetic resonance (27Al-MAS NMR) 27 Al-MAS NMR spectra of solid samples were recorded on a Brucker AVANCE 500 MHz, CPMASS spectrometer, spinning at 4 kHz and operating at room temperature.
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Fig. 1 shows CO conversion and selectivity towards CO2 of AuTHPS/AlCe20 and Au-citrate/AlCe20 catalysts for the preferential oxidation of CO (CO-PROX). The composition of the inlet feed stream was initially fixed to 1.25% CO, 1.25% O2 and 50% H2 (He balance), using W/F = 0.18 g s cm3 and k = 2. The two materials, that have the same metal loading, showed quite different reaction profiles in the whole studied temperature range. The Au-THPS/ AlCe20 sample catalyst shows already at 65 °C a conversion of CO to CO2 of about 100% (the CO detection limit was 10 ppm) with a 61% of selectivity. On the other hand, the Au-citrate/AlCe20 sample shows a very poor activity at low temperatures, reaching a 68% conversion only above 80 °C, never exceeding the 75% at higher temperatures. The better performing sample Au-THPS/AlCe20, was selected to study the effect of contact time on CO conversion and selectivity. The results are summarized in Fig. 2 that shows the effect of three different contact times: W/F = 0.06–0.12–0.18 g s cm3 on the conversion and selectivity of Au-THPS/AlCe20. A contact time decrease from 0.18 g s cm3 to 0.12 g s cm3apparently did not modify the CO conversion and the selectivity to CO2, while a significant conversion decrease was observed when the contact time was further reduced to 0.06 g s cm3. The selectivity to CO2 was found to remain substantially unmodified in all the studied W/F range. It is well known that CO-PROX catalysts must tolerate quite high amounts of CO2 and H2O, because these reagents are always present in a normal unpurified reformate fuel. The effect of the presence of 15% of CO2 in the reaction mixture on the catalytic performance of the Au-THPS/AlCe20 sample is shown in Fig. 3. As shown the CO conversion slightly decreases while selectivity remains unchanged. The effect is more important at temperatures below 65 °C. When also a 10% of water was added to the gas feed, in the 80–110 °C temperature range, the conversion was observed to slightly increase, remaining just below the CO conversion values obtained in the absence of water and carbon dioxide. This finding confirms the positive role played by water in the oxidative catalytic processes catalyzed by supported gold nanoparticles [9]. The selectivity resulted to increase in all the studied temperature range, with values higher than those observed with the standard
Fig. 1. (a) CO conversion and (b) selectivity towards CO2 of Au-THPS/AlCe20 and Au-citrate/AlCe20 catalysts for the preferential CO oxidation in excess of hydrogen. The composition of the inlet feed stream was: 1.25% CO, 1.25% O2 and 50% H2 (He balance), W/F = 0.18 g s cm3, using k = 2.
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Fig. 2. Dependencies of the: (a) CO conversion and (b) selectivity towards CO2 as a function of the reaction temperature, over Au-THPS/AlCe20 using three different contact times. Operating conditions: k = 2; 50% H2, 1.25% CO, 1.25% O2, He balance (% vol.); W/F = 0.06–0.12–0.18 g s cm3.
feed and after 15% CO2 addition. Also if it is not easy to compare these results with literature data obtained with similar gold-based catalysts, owing to the often different operative conditions used by researchers (different Au loadings, gas feeds and contact times), a similar behavior was reported by Avgouropoulos et al. [15], on Au/CeO2 and Zn doped Au/CeO2 catalyst with a 3 wt.% Au loading and with gas feeds and contact times very similar to those used in the present work. The success of PROX in carbon monoxide removal, is also conditioned by the possible occurrence, in addition to the H2 oxidation, of other undesired reactions. The carbon balance of all experiments showed that the CO2 production was in good agreement with the CO consumption, ruling out coke formation. Additional reactions that may play a role in the CO-PROX reaction system are the methanation and the reverse water gas shift. Both the CO and CO2 methanation were found to be negligible in our experimental conditions, and the Au-THPS/AlCe20 catalyst was found also inactive for the rWGS at temperatures below 95 °C. 3.2. Catalysts and precursors characterization 3.2.1. N2 physisorption at 196 °C The nitrogen adsorption–desorption isotherms of the samples Au-THPS/AlCe20 and Au-citrate/AlCe20 and of the bare AlCe20 support are shown in Fig. 4. All the isotherms resulted to be type IV of the IUPAC classification. On the other hand the S.A.BET, the Vp and the distribution of the pore diameters, calculated according to the BJH model applied on the adsorption branch (Fig. 4 and Table 1), resulted very different for the AlCe20 based samples, prepared from the two different Au sols (Au-citrate, Au-THPS). The Au-citrate/AlCe20 sample was prepared by impregnation of the Al/Ce support with an aqueous sol composed by metal particles with a quite uniform distribution of diameters with a maximum observed value of 5 nm (determined by ultracentrifugation and HRTEM), while Au-THPS sample resulted from impregnation of the support with a sol with particles mainly of diameters <2 nm. As shown in Fig. 4 and Table 1, impregnation of the support AlCe20 with the Au-THPS colloidal gold sol (see Section 2.1) causes a slight decrease of the specific surface area (from 394 to 376 m2 g1), with the Vp value decreasing from 0.51 to 0.36 cm3 g1 (Table 1 and Fig. 4b). On the other hand, when the Au-citrate sol was used to prepare the Au-citrate/AlCe20 sample, the specific surface area
was found to decrease much more dramatically (from 394 to 286 m2 g1), with a 50% loss of pore volume and a severe alteration of the pore distribution (Table 1 and Fig. 4b). It clearly appears from the surface area and Vp data, that impregnation with gold sols of the mesoporous support oxide causes an overall decrease of these values, probably because of partial filling of the mesopores. The phenomenon is more dramatic for the Au-citrate/AlCe20 sample because the sol particles (<5 nm) are dimensionally slightly larger than the mean diameter of the oxide mesopores. The surface area of both samples did not change after the catalytic tests. 4.1. Transmission electron micrographs (TEM) It is well known that detection and evaluation of sizes distribution of gold nanoparticles on nanocrystalline ceria is very difficult because of disturbance and phase contrast by the oxide support [29,30]. TEM images of the Au-THPS/AlCe20 catalyst (Fig. 5a and b) showed the presence of dispersed and rounded Au particles ranging in size from 2 to 4 nm and that appear to only slightly aggregate after the CO-PROX catalytic test. TEM images of the Aucitrate/AlCe20 sample are unfortunately poorly resolved and difficult to analyze, because of the low quality of the sample, but it clearly appears that the gold particles have a wide distribution of diameters and a certain extent of aggregation (Fig. 5c and d). The more resolved TEM micrographs of this catalyst taken after reaction, confirm this finding and point out a more pronounced particles aggregation. 4.2. X-ray powder diffraction (XRPD) The X-ray diffraction profiles of the AlCe20 support, the Au-THPS/AlCe20 before and after the CO-PROX reaction and the Au-citrate/AlCe20 before and after the CO-PROX reaction are showed in Fig. 6. The patterns of all the samples contain broad diffraction peaks due to poorly microcrystalline c-alumina and signals attributable to the presence of fcc CeO2 with the fluorite structure, as expected for this type of oxide. The mean size of the ceria particles, for the bare AlCe20 support, was calculated from the half-width of the main peak at 2h = 28.6°, according to the Scherrer’s equation and resulted to be 3.2 nm. The presence of a nanocrystalline ceria phase and the low quality of XRPD signals attributable to the gold particles of Au-THPS/AlCe20 sample does
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439
Fig. 3. Effect of the addition of CO2 and H2O on the (a) activity and (b) selectivity towards CO2 as a function of the temperature over Au-THPS/AlCe20. Operating conditions: 1.25% CO, 1.25% O2, 50.0% H2, 0–15.0% CO2, 0–10.0% H2O and He balance; W/F = 0.18 g s cm3.
Fig. 4. N2 physisorption data of AlCe20, Au-THPS/AlCe20, Au-citrate/AlCe20: (a) isotherms (filled symbols for the adsorption branch and empty symbols for the desorption one); (b) BJH pore diameter distributions, estimated on the adsorption branch.
Table 1 BET specific surface area and specific pore volume of the prepared materials.
a b
a
2
Sample
S.A.BET (m g
AlCe20 Au-THPS/AlCe20 Au-citrate/AlCe20
394 ± 1 376 ± 1 285 ± 1
1
)
b
3
Vp (cm g
1
)
0.51 0.36 0.27
B.E.T. specific surface area. Specific pore volume determined at p/p0 = 0.98.
Table 2 27 Al-MAS NMR data calculated by integration of the peaks of AlCe20, Au-THPS/AlCe20 and Au-citrate/AlCe20 samples. Relative amounts (%)
AlCe20 Au-THPS/AlCe20 Au-citrate/AlCe20
AlIV
AlV
AlVI
21.6 22.1 19.5
12.4 20.9 20.5
66.0 57 60
not allow a careful quantitative determination of the crystallites dimensional distribution [15], nevertheless the mean size of the majority of the gold particles in the fresh Au-THPS/AlCe20 sample can be estimated 2.0 nm, and appears not to substantially change after the catalytic test (Fig. 6b and c), in agreement with what shown by the TEM images. On the other hand, the mean size of metallic gold in the Au-citrate/AlCe20 sample can be roughly estimated 4–5 nm before the CO-PROX, while the size distribution of the metallic particles in the used sample was found to become larger, with an increasing percentage of crystallites with diameter up to abt. 20 nm, as can be also roughly deduced from the shape of the (1 1 1) signal (Fig. 6d and e). Therefore we can conclude that, adsorbing the Au-THPS sol on the ordered mesoporous AlCe20 support, it is possible to prepare a sample with gold nanoparticles with a narrow dimensional distribution, centered at 2 nm, with a high thermal stability in the reaction conditions. The use of the Au-citrate sol, on the other hand, provides a catalyst with larger nanoparticles and wider
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Fig. 7. 27Al-MAS NMR spectra of the bare support AlCe20 and of the Au-THPS/ AlCe20 and Au-citrate/AlCe20 materials.
Fig. 5. TEM images of: (a) Au-THPS/AlCe20 before and (b) after CO-PROX reaction; (c) Au-citrate/AlCe20 before and (d) after catalytic test (1.25% CO, 1.25% O2, 50% H2, He balance, W/F = 0.18 g s cm3, k = 2).
Fig. 6. X-ray diffraction patterns of the samples: (a) AlCe20 support; (b) Au-THPS/ AlCe20 before and (c) after the CO-PROX reaction; (d) Au-citrate/AlCe20 before and (e) after the CO-PROX reaction. Operating conditions for the catalytic test: 1.25% CO, 1.25% O2, 50% H2, He balance, W/F = 0.18 g s cm3, k = 2.
dimensional distribution, with a largely reduced surface area and lower thermal stability in the reaction conditions. 4.3.
27
Al-MAS NMR
27 Al-MAS NMR spectra of calcined AlCe20 is shown in Fig. 7. The sample shows three resonance signals that can be assigned to 4, 5, 6-coordinated Al atoms respectively. The intense resonance at 0 ppm can be assigned to Al in octahedral coordination (AlVI), the signal at 60 ppm is assigned to tetrahedral framework aluminium (AlIV), and the signal at 29 ppm to pentahedral (AlV) one. The (AlVI)/(AlIV) ratio is about 3.0 (Table 2). As reported in the literature, a AlVI/AlIV ratio ranging from 2.4 to 3.8 is consistent with a mesoporous c-alumina structure [31–33]. The observation of an AlV signal is usually attributed to deviations from ideality of the cphase caused by the presence of Al sites with unsaturated coordi-
nation [31]. The relative amount of AlV was found to increase with the increasing of Ce content, in ceria-doped aluminas [34]. The presence of Ce cations in the AlCe20 structural network can then be responsible for the AlV peak, that represents the 12.4% of the 29 Al signal. No shifts of the resonance signals assigned to AlVI, AlIV and AlV were observed in the 29Al-MAS NMR spectra of Au-THPS/ AlCe20 and Au-citrate/AlCe20 but only a decrease of the AlVI/AlV ratio from 3.1 of the bare AlCe20 support to 2.9 and 2.7 of the Au-citrate/AlCe20 and Au-THPS/AlCe20 respectively (Table 2). The decrease of AlVI/AlV ratio, that appears almost the same in the two samples, is caused by a relative increase of the amount of AlV species and can be most probably attributed to a further distortion of the aluminum network induced by the adsorbed gold nanoparticles. Gold nanoparticles of less than 5 nm have been found to be active for low temperature complete CO oxidation (COX), water gas shift reaction and selective CO oxidation (CO-PROX) [9]. However, the data in the literature are quite variable, and the nature of the active sites remains quite obscure. It is, in any case, generally accepted that the catalytic activity of Au depends to a large extent on the size of the particles, even if other effects, such as the nature of the support material, the Au–support interface, the particle shape and metal–support charge transfer, must be considered of fundamental importance. The mechanisms for gold-catalyzed reactions have been critically reviewed by two of the founding fathers of the catalytic gold chemistry, Bond and Thompson in a recent paper [35], where reactions schemes for the WGS, COX and PROX catalyzed by gold on both ceramic and reducible oxide supports, are critically discussed. The mechanism proposed, almost a decade ago by Bond and Thompson [36] for CO oxidation by supported gold, involved Au particles assumed to contain both Au0 and Aux+ active sites, the latter located at the metal–support interface. Later studies, investigating the role of water on the catalyst activity, provided further corroborations to this hypothesis and underlined the role of –OH groups of the support coordinated to gold cationic sites in the catalytic cycle [17,37–39]. Nevertheless, the structure of the catalytically active gold species and the oxidation state/s of the metal is still a debated subject, mainly because of the heterogeneity of the methods used to prepare the studied samples [40–42]. The role of the support oxide is also still under discussion, in particular after the demonstration of the good reactivity shown by unsupported gold nanoparticles in the aerobic glucose oxidation [43]. The influence of the reducibility of the oxide on the catalytic performance of the metal–support system was the object of several studies, mainly related to the identification of the oxygen activation sites [9,44,45]. Results obtained from model catalysts,
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prepared by sol deposition of almost identical metal particle size supported on various oxides, suggested that the metal–support interaction concept must be revised. It was proposed by Comotti et al. [22] that the reducibility of the support and its ability to activate oxygen is not a key factor. These observations confirmed what proposed by Lopez et al. [46], namely the catalytic activity in COPROX is considerably more affected by gold particle size than by the support. The differences in the CO oxidation activity between reducible or not reducible supports for gold nanoparticles of the same size were attributed to an interaction of gold clusters with the support that can result in a change in the shape of the particles [22]. It has been also reported [47] that cerium oxide is not a suitable support for gold-based catalysts in CO low temperature oxidation but, more recently, it was demonstrated that, if nanocrystalline CeO2 was used as support, the activity of gold nanoparticles increased by two orders of magnitude, achieving 100% CO conversion at 10 °C in excess of oxygen [48]. The studies on catalytic systems based on gold nanoparticles supported on ceria and alumina–ceria were also extended to low temperature WGS [49,50], complete hydrocarbon oxidation and CO-PROX [12,13,15,16]. Gold–ceria catalysts were found to be very active and stable systems. The good catalytic activity was attributed to the ceria structure modification caused by the insertion of the gold particles and the consequent formation of more Ce3+ and oxygen vacancies in close contact with small Au clusters. There is a quite general agreement in attributing the high activity of these systems to the enhanced electron transfer between defective ceria and partially charged gold particles via oxygen vacancies [48– 50]. The catalyzed reaction was suggested to proceed at the boundary between small gold clusters and ceria. The high stability of these systems was also explained by the gold ions stabilization in the ceria matrix through strong gold–ceria interparticles surface interaction, that therefore appeared to be of crucial importance [49,50]. In the ceria–alumina mixed oxides, the presence of alumina was found to both increase the ceria dispersion and the formation of oxygen vacancies. Alumina was found responsible for the higher stability of gold and ceria particles against agglomeration, during the reaction at higher temperatures [50]. Up to date the remarkable catalytic activity of very small gold nanoparticles is maybe the best explanation of nanoscale effects in catalysis and it was proposed that nanosized metallic Au is intrinsically catalytically active [51,52]. Nevertheless, the interaction between Au nanoparticles and the support oxide, in particular of reducible type, such as TiO2 or CeO2, that is still quite far from being totally understood, represents a key issue in the study of gold catalysis [53]. A very active gold–ceria–alumina based catalyst for COX or COPROX can be imagined as an ensemble of highly dispersed gold nanoparticles (1–2 nm) closely interacting with nanostructured ceria particles stabilized by alumina against aggregation. The highly uniform ordered mesoporous alumina, containing 20% of ceria used in this study (AlCe20), was prepared by a single step synthesis, using long-chain carboxylic acid salts that are structure-directing agents for the preparation of alumina samples with high surface area and organized mesopores [54,55]. It is in fact well known that carboxylic acids greatly influence the process of hydrolization of Al alkoxides [56,57], controlling the rates of hydrolysis and condensation. When carboxylates are used as templates, Ce stearate in our case, the process is supposed to proceed through a charge matching pathway SI+ [58]. In fact, after the dissociation of the salts, cerium ions act, together with the partially hydrolyzed and condensed Al species, as counterions of the anionic surfactant forming organic–inorganic ion pairs. All the species cooperate to the formation of the final framework. Consequently we can reasonably presume that, in the sample AlCe20, cerium ions are uniformly distributed on the pore walls of the resulting Al–Ce mesoporous mixed oxide system.
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Therefore, this compositionally homogeneous and structurally ordered mixed oxide, with a narrow pore distribution, that can be prepared with a high level of reproducibility, appears to be very suitable as support to prepare catalytic systems for CO-PROX oxidation. The two samples used in this comparative study were prepared by deposition on AlCe20, of the gold sols obtained by reduction of tetrachloroaurate ions by THPS or by NaBH4, in the presence of citrate, because, as pointed out by Comotti [22], this procedure allows the elimination of the influence of the support on the formation of the gold nanoparticles that can be created almost identical, independently of the support used. To achieve satisfactory activity for CO oxidation, the gold particles must be in the range of 1–7 nm and both the used methods allow the obtainment of gold sols with nanoparticles in this dimensional range. Nevertheless the catalytic data in the CO-PROX, graphically presented in Fig. 1 highlight the enormous difference both in activity and in selectivity between the samples Au-THPS/ AlCe20 and Au-Citrate/AlCe20. As shown in Fig. 4 and Table 1, the AlCe20 oxide constitutes a porous system with a quite narrow pore diameter distribution peaking at 3.3 nm. Therefore, the outstanding low temperature catalytic activity and selectivity of Au-THPS/ AlCe20 can be interpreted, imagining that the small nanoparticles of the Au-THPS sol, mainly with diameters <2 nm, can slip into the support mesopores and enter in close contact with the exposed surface of the ceria nanoparticles, realizing what are thought to be the optimum conditions for the creation of CO-PROX active sites. On the other hand, the immobilization on AlCe20 of the Au-citrate gold sol, that is mostly formed by nanoparticles dimensionally greater than the support pores diameter, probably causes the almost complete occlusion of a large part of the mesopores, as pointed out by the observed dramatic decrease of the specific pore volume Vp. The activity of Au-citrate/AlCe20 does not break down completely but it is strongly reduced (less than 50% at 65 °C), due to the aggregation of the gold nanoparticles situated at the interface of the support as shown by the comparison of XRD profiles of the Au-citrate/ AlCe20 sample before and after the CO-PROX reaction. 5. Conclusions A very active gold–ceria–alumina catalyst for CO-PROX can be prepared with highly dispersed gold nanoparticles (1–2 nm) closely interacting with nanostructured ceria particles stabilized by alumina against aggregation. The highly uniform ordered mesoporous alumina, containing 20 wt.% of ceria was prepared by onepot methodology, using Al alkoxide as alumina precursor and Ce stearate both as metal source and structure directing agent. The immobilization of a gold sol on this high surface area and uniform pore size ceria–alumina support allows the preparation of a nanostructured CO-PROX catalyst very active at low temperature, thermally stable and selective. The Au sol obtained reducing a solution of gold tetrachloroaurate salt with THPS, bis[tetrakis(hydroxymethyl)phosphonium sulfate], which acts as stabiliser and reducing agent, produces metal nanoparticles able to penetrate the support pores, allowing a very close interaction between the surfaces of ceria and gold nanoparticles, without pore clogging up. References [1] [2] [3] [4]
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