Palladium-catalyzed selective hydrogenation of nitroarenes: Influence of platinum and iron on activity, particle morphology and formation of β-palladium hydride

Journal of Catalysis 311 (2014) 153–160

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Palladium-catalyzed selective hydrogenation of nitroarenes: Influence of platinum and iron on activity, particle morphology and formation of b-palladium hydride K. Möbus a, E. Grünewald a, S.D. Wieland a, S.F. Parker b, P.W. Albers c,⇑ a b c

Evonik Industries, Business Line Catalysts, Rodenbacher Chaussee 4, D-63457 Hanau/Wolfgang, Germany ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom AQura GmbH, Rodenbacher Chaussee 4, D-63457 Hanau/Wolfgang, Germany

a r t i c l e

i n f o

Article history: Received 5 August 2013 Revised 7 November 2013 Accepted 19 November 2013 Available online 24 December 2013 Keywords: Catalyst–hydrogen interaction Selective hydrogenation Nitroarenes Palladium Alloying

a b s t r a c t The influence of alloying supported palladium particles of 2 nm size with platinum and iron on the catalytic activity for the selective hydrogenation of nitrobenzene (NB) was studied. We show that the use of a carbon black support of enhanced sp2 character has a marked ‘templating’ effect on the location of the palladium particles, which are preferentially deposited at the edge sites of the carbon support surface. Alloying with platinum and iron leads to disaggregation down to isolated primary particles, and this has a major effect on the catalytic activity (Pd 6.6, PdPt 36.9, PdPtFe 23.1 mmolNB min1), as well as on the relative amounts of b-palladium hydride formed (normalized peak integrals: Pd 127.0, PdPt 87.6, PdPtFe 27.2). The results enable a greater understanding of how better performance can be obtained in catalysts by appropriate choice of support, particle size, alloying and adjustment of hydrogen storage capability. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The catalytic hydrogenation of nitroarenes, for example, of nitrobenzene to aniline or of dinitrotoluenes (DNT) to toluene diamines (TDA), is a major industrial activity [1–3]. TDA is a key component in the production of toluene diisocyanate (TDI) for the manufacture of polyurethane (PU) [4]. These reactions illustrate the need and economic impact of steering and optimizing catalyst selectivity while maintaining adequate catalyst activity. It can be economically favorable to lower the reaction pressure and temperature by improving the catalyst’s properties. The reaction rate and selectivity in the hydrogenation can be enhanced by changing from nickel- to palladium-based catalysts. Usage of other noble metals such as platinum and additional promoters can further improve reaction efficiency and lower the amount of catalyst needed [5]. The addition of platinum to palladium increases activity, and the addition of iron increases selectivity: the enhancement of activity by the addition of platinum is moderated by iron in order to avoid aromatic ring hydrogenation [6–8]. The addition of Pt and Fe results in better selectivity and long-term stability on the large-scale production level as was reported by DuPont in the late 1960s that a Pd–Pt–Fe/C trimetallic catalyst was becoming a

⇑ Corresponding author. Fax: +49 6181 59 3554. E-mail address: [email protected] (P.W. Albers). 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.11.019

‘workhorse for the hydrogenation of aromatic nitro compounds’ [6]. Optimization of the carbonaceous support materials has led to the use carbon black-based catalysts which have a better precious metal–support interaction and improved catalytic activity as compared to the usual activated carbon-type supports [9]. The aim of the present work was to improve the understanding of the factors that affect the activity in palladium-based catalysts used for the hydrogenation of nitrobenzene. This requires knowledge of the subtle interdependence between the composition, the size and the size distribution function of the supported precious metal entities. In particular, the impact of alloying promoting additives on the catalyst particle morphology and the corresponding hydrogen interaction properties was the focus of this investigation. The transparency of most materials combined with the selectivity to hydrogen in neutron scattering experiments allows the determination of the hydrogen-related properties of macroscopic amounts of hydrogenated catalyst and support material. In the present investigation, the inelastic incoherent neutron scattering (IINS) measurements of 13–19 g of material provide statistically meaningful data on catalysts which have been produced and applied for decades. The multi-technique approach adopted here using IINS in combination with transmission electron microscopy (TEM), energy dispersive X-ray nano-analyses (EDX), electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and catalyst testing addresses all these aspects to build a comprehensive picture of the role of each of the components of the catalyst.

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2. Experimental 2.1. Materials The catalysts were manufactured by Evonik Industries by strictly following the wet precipitation procedure reported and patented in [10]. All the catalyst samples in this study were prepared using the same lot of carbon black as support and using the same preparation procedure. Aniline (99%) and nitrobenzene (99%) were purchased from Sigma–Aldrich. Hydrogen (>99.999%) was supplied by Linde. 2.2. Hydrogen content The hydrogen content of the carbon black-type catalyst support was determined by hot extraction using a LECO TCH600 instrument. 2.3. Surface area The surface area was determined by the Brunauer, Emmett and Teller method (BET). 2.4. Transmission electron microscopy (TEM) A Jeol 2010F field emission transmission electron microscope was operated at 200 keV acceleration voltage. For spot analyses of the supported metal particles and characterization of binary or ternary alloying effects, energy dispersive X-ray (EDX) spot analyses at the nanoscale of the supported particles and the support particles were performed using a Noran SiLi detector with a 30-mm2 crystal and a Noran System Six device. For examination, a catalyst sample was dispersed in chloroform and transferred onto holey carbon foil supported by a 200-mesh copper grid. For the statistical evaluation of the primary particle sizes of the supported primary particles, the I-TEM software of Soft Imaging Systems (SIS, Münster, Germany) was utilized. The average primary particle sizes of the supported Pd-based catalyst particles were determined by statistical evaluation of 2000 particles in TEM images per catalyst sample. In the catalysts Pd1–Pd3, no additional fraction of coarse precious metal particles was observed in the TEM. The quality, stability and calibration of the TEM system were ensured by the use of the Magical No. 641 standard (Norrox Scientific Ltd., Beaver Pond, Ontario, Canada). 2.5. X-ray photoelectron spectroscopy (XPS) A Leybold MAX100 instrument with an EA200 electron energy analyzer was operated at 72 eV in the fixed analyzer transmission mode. A catalyst sample was introduced into a differentially pumped pre-chamber as a loose powder. Integral XPS spectra of an area of 2 mm  2 mm each were recorded.

stainless steel pipe and a welded bellows valve (Nupro) via an oxygen-free high conductivity (OFHC) copper gasket. A sealed can containing macroscopic amounts (ca. 15 g, precise values in Table 1) of catalyst was evacuated using a turbo-molecular pump which was backed by a dual-stage rotary pump with a zeolite trap to avoid back-diffusion of oil and other potential molecular contaminants. Additional information on IINS of related finely divided palladium entities is given in [11]. Each catalyst was subjected to slow, careful cycles of hydrogenation (99.999% hydrogen) and dehydrogenation at room temperature to avoid fast local heating due to a spontaneous excessive release of the heat of the dissociative absorption of hydrogen in the palladium in fast-step dosing. No heating during degassing of a hydrogenated catalyst by pumping down a sample can was performed, in order to avoid particle growth induced by heat in the presence of residual pressure of hydrogen. The hydrogen uptake of the supported palladium was monitored by capacitive pressure transducers (MKS/Baratron). Four cycles of hydrogenation/dehydrogenation were applied to each sample. Since in these cycles, the outgassing was restricted to room temperature, complete decomposition of the hydride phases was not achieved; thus, there is residual hydrogen in the samples. Each sample was measured at the MAPS spectrometer [12] at ISIS (STFC Rutherford Appleton Laboratory, UK) with incident energies of 5000 cm1 (C–H stretch region of the carbon support and also to check for hydroxyls), 2000 cm1 (surface Pd–H vibrations) and 1200 cm1 (b-PdHx). We have measured the catalysts in the hydrogenated state under 750 mbar hydrogen equilibrium pressure (Table 1) and after thermal decomposition of the hydride phases to check for the individual contribution of palladium hydride (ca. 480 cm1: b-phase, ca. 552 cm1: a-phase). To maximize sensitivity, we have used the S-chopper of MAPS rather than the A-chopper we used previously. This provides a factor of approximately three in sensitivity, albeit at a factor of two in resolution [11,12]. Background spectra were recorded after the removal of the hydrogen from samples Pd1 and Pd3 (Table 1) by a turbo-molecular pump at room temperature and subsequent heating to 120 °C with continuous evacuation. An evacuated empty can and the pure carbon black support which was used to manufacture the catalysts were also measured for background subtraction. For quantitative evaluation of the weight-corrected and normalized neutron scattering peak integrals, the following steps were used: (1) Subtraction of the low background of the stainless steel can (only relevant at 50–300 cm1). (2) Subtraction of the carbon support. (3) Normalization to 1 g Pd. (4) Integration over the range 350–600 cm1 using the endpoints as the baseline. This procedure enables the relative amounts of palladium hydride in the catalyst samples to be reliably compared. 2.8. Catalyst activity tests

2.6. Electron energy loss spectroscopy (EELS) A Zeiss 912 LaB6 TEM instrument equipped with an in line Omega filter was used to probe the average sp2 character throughout the carbon primary particles and aggregates in transmission by means of the parallel-EELS technique to complement the TEM and XPS results. 2.7. Inelastic Incoherent Neutron Scattering (IINS) Each catalyst was loaded into a thin-walled (0.5 mm) stainless steel (1.4571) can which was sealed by a Conflat™ flange with a

Hydrogenation of nitrobenzene was carried out in a jacketed semi-batch stirred titanium autoclave (500 ml, Medimex) equipped with thermocouple, circulation thermostat, pressure sensor, gas inlet and outlet, sampling port, nitrobenzene supply port and gas entrainment stirrer (1700 rpm). At the beginning of each experiment, the reactor vessel was loaded with 50 mg of the catalyst and 150 ml of a mixture of aniline and water (stoichiometric mixture of the reaction products). After purging with nitrogen, the reactor was filled with hydrogen and heated to the operational temperature of 80 °C; the hydrogen pressure was adjusted to a value of 40 bar. Nitrobenzene was fed into the reactor by a Gison

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K. Möbus et al. / Journal of Catalysis 311 (2014) 153–160 Table 1 Sample numbers, composition, weight and final hydrogen equilibrium pressure in the IINS sample cans. Sample No.

Composition (wt.%)

IINS Sample weight (g)

PH2 (mbar)

Pd1 Pd2 Pd3

Pd(5%)/C Pd(4.5%)Pt(0.5%Pt)/C Pd(4.5%)Pt(0.5%)Fe(5%)/C

14.72 16.72 18.65

712 760 745

C

Unloaded carbon black support

13.05

–

Fig. 1. Top: TEM images of the carbon black used as support for the preparation of the hydrogenation catalysts Pd1–Pd3. Aggregates of intergrown primary particles with enhanced internal sp2-ordering. Bottom: EELS spectra of the carbon K-absorption edge recorded at different spots in different aggregates.

307 pump. Hydrogen pressure was kept constant using a pressflow gas controller (bpc 9901, Büchi Glas Uster AG). Hydrogen consumption was monitored online and was a measure of hydrogenation rate and, thus, catalytic activity. Catalytic activity [ml H2 s1] was calculated for each pulse by linear regression of hydrogen consumption plotted against time within the linear range of the plot.

3. Results and discussion The TEM images of the pure carbon black support in Fig. 1 shows that typical aggregate structures are present [13,14]. The inter-grown primary particles that are forming the aggregates

show graphite-type layers and edges at the surface and extended surface-near graphitic ordering. Plane-like oriented graphiticity, turbostratic disorder, corannulene-type curvature and sp2-sphericity is present which is characteristic of graphitized grades of carbon black. This is in line with the low hydrogen content of 0.018 ± 0.002 wt.% H, which is on the order of that of pure graphitic carbon. Details on the role and importance of non-hexagonal rings in the formation and spherical ordering of primary particles and aggregates of carbon blacks up to faceting are reviewed in [15]. The high degree of surface ordering and the absence of significant quantities of aliphatic matter in this carbon black support provide stable and uniform surface conditions for precious metal loading.

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The primary particle size of the black is in the usual range of 30–40 nm, the average size of the aggregates is 300–650 nm, and the BET surface area is 65 m2/g1 [14]. Parallel-EELS in the TEM provides an average picture of the electronic state of the primary particles and the aggregates of the carbon black and thus spectroscopically complements the high-resolution TEM images and the surface-related results of XPS. Typical results are depicted in Fig. 1: the absorption edge as measured at different spots in different aggregates shows a strong p⁄ excitation band below 290 eV and the characteristic overall profile of sp2-type carbon. In agreement with the TEM results, there is no evidence for the presence of significant amounts of incorporated sp3-type carbon. The physical background of these characteristic fine structures is outlined in Ref. [16]. In analyzing the XPS C1s signals of the carbon black and the catalysts, a pronounced plasmon loss feature [17–19] around 290.7 eV contributed to the whole C1s region with ca. 7–8% of the integrated intensity as a signature for the high degree of graphiticity at the support’s surface and for the absence of significant amounts of adsorbed sp3 matter. Increasing sp2-ordering at the surface and in the surface-near ‘selvedge’ region of carbon black leads to a much more uniform and narrow distribution function of the surface energy, as shown by inverse gas chromatography at finite concentration conditions and static gas adsorption techniques [20–22]. Different fractions of different adsorption sites are discussed in literature: (I) graphitic planes (sp2), (II) amorphous carbon (sp3), (III) crystallite edges, (IV) slit-shaped cavities in the carbon black structure [Fig. 3 in Ref. 22]. Fig. 1 suggests that for the given carbon black type (I) and type (III) surface sites dominate. It follows that a narrow surface energy distribution function should be present. This leads to a more homogeneous and narrow adsorption site distribution and hence to a uniform deposition of particles onto this sp2-type carbon support, as compared to more disordered, paracrystalline and porous carbonaceous materials such as activated carbon [9]. Fig. 2 compares TEM images of a typical 5% Pd/activated carbon catalyst (left) and the 5% Pd/carbon black catalyst Pd1 (right). In the case of the activated carbon support, the Pd particles are comparatively more randomly distributed and more isolated. In the case of the carbon black support, the palladium particles are preferably located or targeted at the surface (Fig. 2, right, and Fig. 3) sites in edge decoration of type (III) sites [22]. This indicates an impact of a narrow adsorption site distribution of a carbon black support of enhanced sp2 character on the deposition of palladium entities onto certain surface sites. The high sp2-surface ordering suggests a templating effect of the support in the precious metal adsorption and deposition, nucleation, particle formation, growth

resulting in aggregation to chain-like entities during catalyst preparation. The high graphiticity of the carbon black (Fig. 1) provides a higher degree of local ordering, larger basic structural units and lower amount of micro- or ultramicroporosity (type IV sites) compared to typical microporous carbon blacks [23]. In the Pd(4.5%)Pt(0.5%)/C catalyst Pd2, much more isolated particles are encountered shown by TEM at different magnifications (Fig. 4, Table 2). EDX nanospot analyses reveal that all the supported particles are formed by a binary Pd/Pt alloy (Table 2). It appears that due to the intensive alloying even with small amounts of platinum, the surface free energy of the particles is changed compared to the palladium-only catalyst. A strong separation of the primary particles and a lower degree of aggregation are achieved on average. No isolated platinum-only or palladium-only entities were detected by TEM/EDX. Also chain-like alloy aggregates occur with lengths of up to 20 nm. It can be estimated that in the case of the palladium-only catalyst, about 90% of the particles are arranged as chain-like aggregates, whereas for the binary alloy catalyst, about 90% of the particles are isolated (Table 2). In the catalyst Pd3, Pd(4.5%)Pt(0.5%) Fe(5%)/C, the metal is mostly present as small isolated primary particles and as few chain-like aggregates (up to ca. 50 nm length) (Fig. 5). The relative amount of chain-like aggregates is also much lower than in the case of the palladium-only catalyst. Furthermore, many of the alloy particles are present in spherical clustering, agglomeration or nucleation zones. This suggests different interactions between the pure palladium and the carbon black support compared to the binary or the ternary palladium alloy particles. Broad area XPS (2 mm  2 mm) reveals an overall Pd/Pt/Fe composition of 1.8/0.1/1.0 (at.%) in the topmost surface region which suggests a partly lower dispersion of Fe compared to Pd. EDX in the TEM indicates that for the case of the binary alloy catalyst Pd2, a rather constant local Pd/Pt composition was achieved for the isolated particles as well as for the linear aggregates (Table 2). For the ternary system Pd3 nanospot, EDX analyses show that a larger variation in local Pd/Pt/Fe stoichiometry is observed; however, phases such as Fe3Pt are not present [24,25]. The occurrence of spherical clustering is correlated with enhanced local concentration of Fe. According to the DN and SDN values (Table 2), it can be concluded that the addition of iron to form a ternary alloy led – at the statistical average (2000 particles) – to smaller primary particle sizes and a narrowing of the primary particle size distribution and, hence, to the largest calculated TEM surface area for the metal component.

Fig. 2. Left: TEM image of a 5% palladium on activated carbon catalyst. DN: 2.10 nm; SDN: 0.20 nm; DA: 2.17 nm; EMS: 230 m2/g (see Table 2); catalyst characterized in [11]. Right: catalyst Pd1: 5% palladium on carbon black.

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Fig. 3. TEM images of the catalyst Pd1: Pd(5%)/C. Predominantly chain-like aggregates of about 2-nm-sized primary particles on the graphitic planes and edges of the support.

Fig. 4. TEM images of the binary alloy catalyst Pd2: Pd(4.5%)Pt(0.5%)/C. Predominantly isolated alloy particles of about 2.2 nm size. For the typical local element concentrations of Pd and Pt see Table 2.

Table 2 Catalyst composition, hydrogenation activity (nitrobenzene (NB) to aniline), primary particle size, morphology and local composition (EDX, TEM), and relative amounts of supported b-phase palladium hydride (IINS). Catalyst

Pd1a

Pd2b

Pd3c

Catalytic activity (mmolNB min1) TEM average primary particle size DN (nm)d Standard deviation SDN (nm)d TEM average primary particle size DA (nm)e EMS (m2/g)f

6.6 2.0 0.24 2.1 243

36.9 2.2 0.31 2.3 216

23.1 1.9 0.19 2.0 254

10 90

90 10

Pd 100 127.0

Pd/Pt 85/15 to 93/7 87.6

60 10 30 Pd/Pt/Fe 71/5/24 to 86/6/8 26.3g/27.2h

TEM particle morphology (1) isolated particles (%), ca. (2) chain-like aggregates (%), ca. (3) spherical clustering (%), ca. EDX/TEM nano-composition, ranges (wt.%) Normalized IINS peak integrals of b-phase Pd-hydride (arb. units) a b c d e f g h

Pd(5%)/C. Pd(4.5%)Pt(0.5%)/C. Pd(4.5%)Pt(0.5%)Fe(5%)/C. DN and SDN: primary particle size (arithmetical average) and its standard deviation, DN = (Rnidi)/N. 3 2 DA: primary particle size averaged over the surface, DA ¼ ðRni di Þ=ðRni di Þ. EMS: calculated electron microscopic surface, EMS= 6000/(DA  q), qPd: 11.99 g cm3. After subtraction of the normalized spectrum of the pure carbon black support; After subtraction of the normalized spectrum of the dehydrogenated sample (12 h, 473 K, turbo-molecular pump).

In summary: (1) edge (type III) and surface (type I) [22] site decoration and two-dimensional, linear aggregation in the palladium-only catalyst. (2) predominantly isolated particles in the binary alloy case.

(3) predominantly isolated particles and three-dimensional, spherical clustering/agglomeration over the whole sp2-type support in the ternary alloy catalyst. The overall loading capacity of the support’s surface (65 m2/g) compared to ca. 1000–1500 m2/g of nitrogen area for typical

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Fig. 5. TEM images of the ternary alloy catalyst Pd3: Pd(4.5%)Pt(0.5%)Fe(5%)/C. Predominantly isolated alloy particles, spherical agglomerates and small amounts of chain-like aggregates.

activated carbons is lower due to the low porosity and surface area of the carbon black. This can explain the effects of spherical clustering at enhanced metal loading in Pd3 (Fig. 5). The proton dynamics of Pd1–Pd3 were compared by means of IINS. The high sp2-character and very low hydrogen content of the dry, non-porous carbon black support used for these catalysts enables direct, nondestructive investigation of the hydrogen storage capabilities in the ca. 2 nm mono-, bi- or tri-metallic particles present by IINS without the spectral interference from the hydrogenous entities found in an activated carbon support [11]. The high penetrating power of the neutron allows each catalyst to be investigated under in situ conditions: sealed in a stainless steel cuvette and hydrogenated up to ca. 750 mbar sorption equilibrium pressure or more. The normalized spectra reveal distinct differences in hydrogen absorption capacity between the catalyst samples (Fig. 6 and Table 2). The question arises as to how relevant these differences are to the catalyst under the operation conditions of 40 bar hydrogen pressure and 80 °C. In the case of palladium, the a-/b-phase transition region and the corresponding hydrogen storage window at constant hydrogen equilibrium pressures, well below 750 mbar, are very broad [26–30]. The b-hydride phase is formed and saturated at pressures also well below the 750 mbar hydrogen pressure used in our measurements. Further, the isotherms show a very steep increase with pressure once the b-phase is obtained up to 100 bar and more. Very similar isotherms are obtained at both 20° and 80 °C [Fig. 3.4 in Ref. 29 and Fig. 7 in Ref. 30]. This holds for coarse as well as for finely divided palladium. The result is that our measurements of the relative amounts of b-phase hydride present in the catalysts at 20 °C and 750 mbar are essentially those that would be obtained at 40 bar hydrogen pressure and 80 °C and hence are directly relevant to the industrial conditions. At the surface of the palladium particles, the hydrogen molecules are dissociatively chemisorbed [31,32,29] and the hydrogen atoms then diffuse into the bulk. The band at ca. 480 cm1 (Fig. 6) represents the vibrations of the electronically screened interstitial protons on the octahedral sites of the highly loaded b-phase [33,34]. The lattice expansion due to hydrogen uptake as a geometric factor is of influence on surface reactivity [35]. The width, fine-structure and filling of the d-band are also influenced by changes in the lattice constants and elastic interactions [36] due to hydrogenation as well as by alloying [37]. For the hydrogenation of ethyne, it is reported that the transition from b- to ahydride phase increases ethane selectivity [30] illustrating the electronic effect. Due to the high diffusion coefficient of hydrogen in palladium [29,38 and literature cited therein] and the small primary particle

Fig. 6. IINS spectra of the hydrogenated catalysts Pd1-Pd3 (top), recorded under ca. 750 mbar equilibrium pressure, and of the carbon black support (bottom). The signal around 480 cm1 is due to b-palladium hydride: vibrations of the interstitial protons in the octahedral sites of the supported palladium particles. The signal around 120 cm1 is due to the rotational transition of physisorbed molecular hydrogen at the catalyst surface. It is IINS allowed because neutron scattering results in a nuclear spin flip.

size in the catalysts Pd1–Pd3 and, therefore, the rate of hydrogen uptake and release, it can be expected that the different hydrogen storage capacities observed are of relevance for the different catalytic properties. The numerical results from evaluating the vibrational mode region of the b-palladium hydride phase are compared in Table 2. The signal assignment follows conclusions from the literature for the IINS signal of compact and of finely divided palladium hydrides [33,34,39–45]. It is known that: (A) The solubility of hydrogen in bulk and in supported palladium decreases linearly with the percentage of dispersion [46,47]. Furthermore, it is known that the hydrogen solubility in single-sized palladium clusters (stabilized by surfactant or polymer-embedding) decreases significantly with decreasing size of the isolated primary nanoparticles (5 ? 3 ? 2 nm): the shape of

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Fig. 7. Hydrogenation activity of Pd1–Pd3 in selective hydrogenation of nitrobenzene to aniline: addition of Pt to Pd strongly increases hydrogenation activity; addition of Fe leads to slightly lower activity compared to the binary system. The hydrogen consumption rates [ml H2 s1] were measured. Activity values are noted in Table 2.

the hydrogen absorption isotherms and the relative proportions of a- and b-phase palladium hydride are strongly affected [48–52]. The width of the phase transition region is much smaller than in the case of larger palladium particles [26–29,53]. It is also known that the hydrogen absorption capability of nanosized primary particles of palladium is lower than that of palladium aggregates which are formed from intergrown primary particles of comparable size (ca. 3–4 nm) [11]. (B) For the case of larger, isolated, stabilized Pd/Pt particles (about 6 nm size and more), the hydrogen solubility of the alloy was found to be higher than for pure Pd particles of equal size; however, differences were observed in comparing solid solution versus core/shell particles [54,55]. However, at higher Pt concentration (50% and more), the hydrogen absorption capability is lowered [56]. (C) For the Pd/Pt system, it is reported that melting temperature, melting enthalpy and catalytic activation energy decrease with size. The physicochemical properties including surface energy of bulk material, of individual constituents and of bimetallic nanoparticles differ [57,58].

Fig. 6 indicates that for the ca. 2 nm primary particles in catalyst Pd2 and Pd3, the influence of A) (low primary particle size of the Pd-based entities) together with the disaggregation effect caused by alloying with Pt and Fe does overcome the potential influence of B) (enhancement of hydrogen storage capability [54,55]) but still at a much lower level than for bulk palladium entities [26–29,53] which may be more significant at larger particle sizes. The catalysts Pd1–Pd3 were tested for the selective hydrogenation of nitroben-

zene to aniline in order to examine the influence of the additional components (Pt and Fe) on catalytic activity (Fig. 7). The activity of the Pd-only catalyst was the lowest. The Pd–Pt catalyst exhibited the highest average activity among the three catalysts. The activity of the Pd–Pt–Fe catalyst was significantly higher than the activity of the Pd-only catalyst but lower than that of the Pd–Pt catalyst. Clearly, the catalytic activity of supported Pd for the hydrogenation of nitroarenes can be optimized by alloying with Pt and moderating with Fe [6–8]. While this has been known (and used commercially) for nearly half-a-century, the underlying reasons for this behavior were unknown. In this work, we have shown that different templating influence of a carbon black support of enhanced sp2 character has a tremendous influence on the catalyst morphology in the case of the deposition of Pd-only entities on the one hand and of binary or ternary alloy particles on the other. For the given ternary catalyst, it is found that alloying Pd with small amounts of Pt and, furthermore, with Fe leads to a marked disaggregation effect, to a much higher amount of isolated supported primary particles in the 2-nm size regime and this has a major effect on the catalytic behavior: as the relative amounts of isolated metal or alloy particles are changed from 10 ? 90 ? 60% and the primary particle size distribution function is narrowed, in parallel the catalytic activity changes from 6.6 ? 36.9 ? 23.1 mmolNB min1 (Pd ? Pd–Pt ? Pd–Pt–Fe). For this catalyst system, there is a subtle balance between the disaggregation effect and the huge increase in activity by adding Pt and the moderation and stabilization of activity and selectivity by Fe. Undoubtedly, part of the moderating effect of Fe is the marked reduction in hydrogen storage capacity that it causes due to the incorporation of Fe into the crystal structure of the PdPt alloy: the long range phase coherence in b-phase hydride formation is strongly affected. There is a clear parallelism between the proportion of isolated particles, alloying and the catalytic activity (Table 2), suggesting that these are the most active components. This implies that it is conceivable that the same activity could be obtained with a reduced metal loading, provided that the metal particles are sufficiently dispersed and alloyed. 4. Conclusions The catalytic activity of supported Pd for the hydrogenation of nitroarenes can be optimized by alloying with Pt and moderating with Fe [6–8]. While this has been known (and used commercially) for nearly half-a-century, the underlying reasons for this behavior were unknown. In this work, we have shown that the use of a carbon black support of enhanced sp2 character has a marked ‘templating’ effect on the location of the catalyst particles, which are preferentially located at the edges of the carbon particles. The degree and type of aggregation depends on the alloying metal(s) and, in particular, changes the proportion of isolated metal particles in the catalyst, which are strongly correlated with the activity. Alloying reduces the availability of hydrogen in all cases, as shown by

Table 3 Ratios of normalized IINS peak integrals: quantification of the effect of alloying, disaggregation, moderation and controlled poisoning in supported palladium catalysts on the formation of b-palladium hydride.

a b c

Catalyst

Cause of change in hydrogen storage capability

Ratio of decrease in the formation of b-PdHx

Pd1/Pd2 Pd2/Pd3 Pd1/Pd3 Lindlar catalyst Pd/CaCO3/Pd/C

Disaggregation by alloying with Pta Moderating Pd/Pt with Fea Disaggregation, alloying and moderation with Fea Poisoning Pd with Pbb Aggregates vs. isolated primary particlesc

1.5 3.2 4.7 2.2 3.5

Calculated from IINS data in Table 2, support: carbon black. Pd(5%)/CaCO3/Pd(5%)Pb(3.5%)/CaCO3, both 10- and 15-nm-sized aggregates of ca. 3.7 nm primary particles, from Ref. [11]. 10–15-nm-sized aggregates of ca. 3.7 nm primaries versus isolated ca. 3.2 nm primaries, from Ref. [11]; C-support: activated carbon.

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