Effect of Crystalline Phase and Composition on the ... - ACS Publications

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Effect of Crystalline Phase and Composition on the Catalytic Properties of PdSn Bimetallic Nanoparticles in the PROX Reaction Roberto Lanza,†,‡ Marco Bersani,*,‡,§ Luca Conte,∥ Alessandro Martucci,§ Paolo Canu,§ Massimo Guglielmi,§ Giovanni Mattei,⊥ Valentina Bello,⊥ Massimo Centazzo,∥ and Renzo Rosei∥ §

Department of Industrial Engineering, University of Padova, via F. Marzolo 9, 35131 Padova, Italy Department of Chemical Engineering and Technology, KTH − Royal Institute of Technology, Teknikringen 42, 100 44 Stockholm, Sweden ⊥ Department of Physics and Astronomy, University of Padova, via F. Marzolo 8, 35131 Padova, Italy ∥ QID Nanotechnologies Srl, via SS Felice e Fortunato 62, 36100 Vicenza, Italy †

ABSTRACT: We present a synthetic strategy for the preparation of palladium−tin alloy and intermetallic nanoparticles. Complexes of palladium(II) and tin(IV) precursors in oleylamine were thermally decomposed in an organic solution in the presence of reducing moieties, leading to the formation of monodispersed nanoparticles with varying crystallographic structures. We found that the nanoparticles crystalline structure closely follows the bulk material phase diagram. The nanoparticles were supported on Al2O3 and their reactivity tested as catalysts for the preferential oxidation of CO in excess of hydrogen (PROX). Different Pd/Sn and O2/CO ratios have been investigated, and the structure−reactivity correlation highlighted. With increasing tin content, the CO ignition temperature remarkably lowers and the CO conversion rate increases, up to the formation of intermetallic phases that concurrently determine a reduction in the catalyst activity; the selectivity of the pure palladium references is preserved.

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INTRODUCTION The synthesis of palladium-containing bimetallic nanoparticles (NPs) has always attracted great interest due to the interesting properties emerging from the synergistic behavior of the coupled metals.1−5 Studies focused on the correlation between the specific configuration of the elements in the NP and the correlation with reactivity. Consequently, bimetallic materials find a wide application in the field of catalysis with an extensive literature. In catalysis, coupling noble and non-noble metals has attracted a great deal of interest as it might be a convenient approach toward the reduction of the expensive precious metal loading,5,6 with the concurrent possibility of achieving improved catalytic performances, that is, higher selectivity and yield. Clearly, maintaining the original catalytic performances, mechanical and chemical stability, and simple preparation process is imperative while evaluating feasible alternatives to the currently used commercial catalysts. Examples of the preparation of bimetallic compounds and their properties and applications are reviewed in Bönnemann and co-workers.7 Palladium is widely employed for a variety of reactions both at the industrial and the experimental level. Its efficiency has been tested in combination with several other metals. Nowitzki and co-workers1 prepared monometallic Pd and Co catalysts as well as CoPd core−shell particles and investigated their activity in the decomposition of methanol. The reaction follows different pathways depending on the single metal used, while the bimetallic core−shell structure determines a mix of the two mechanisms followed by the monometallic catalysts. The bimetallic catalysts show a similar activity for all the © 2014 American Chemical Society

compositions, suggesting strong electronic effects when Pd and Co are used together in a core−shell structure. Son and coworkers3 also suggest using a core−shell structure with a Ni core and a Pd outer layer and tested their activity on Sonogashira coupling reactions. According to their findings, bimetallic samples exhibit much higher activity than monometallic Pd, suggesting that core−shell structures could be a viable solution to decrease the catalyst use of noble metal, preserving the reactivity. Wang and coauthors8 used Co in combination with Pd for the oxygen electrocatalytic reduction reaction. They sinthetized single-phase disordered fcc particles that exhibithed the best catalytic activity for a Pd:Co ratio of 2 (Pd2Co/C). They assigned the enhanced activity to the fact that a pair of Pd−Pd atoms in the bimetallic catalyst has the appropriate interatomic distance to host and reduce a molecule of O2. Sun and coworkers9 considered a wide range of Pd/Co ratios (from 21:79 to 95:5, molar basis) and tested their samples for the generation of H2 from ammonia borate hydrolysis. A Pd/Co ratio of 65:35 is indicated as optimal, and the improved catalytic activity is explained with the Sabatier principle, according to which the optimal catalytic activity is reached on a surface with median binding energies of reactive intermediates.10 Nabae and coworkers11 used nickel combined with Pd, reporting the formation of alloy, which is indicated as the reason for the Received: April 10, 2014 Revised: October 6, 2014 Published: October 6, 2014 25392

dx.doi.org/10.1021/jp503531b | J. Phys. Chem. C 2014, 118, 25392−25402

The Journal of Physical Chemistry C

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explain the results as a geometric effect of dilution, which inhibits the double-bond isomerization ability of Pd. In the field of magnetic materials, Hu and co-workers20 investigated the effect of the Co/Pd ratio on the structure formed, by fabricating nanowires. They showed that, by adjusting the Co/Pd ratio, alloys are formed, with the presence of pure Co and Pd clusters. Furthermore, the presence of Pd favors the formation of the fcc phase, which is prevailing in a wide range of Co/Pd ratios. More recently, Panday and coworkers21 used Ni in combination with cobalt and also showed that, within certain composition limits, an alloy with a dominating fcc phase is formed. A critical review of the experimental works in the literature suggests that, notwithstanding the wide range of promoters and reaction investigated, there is a link between the Pd/Me molar ratio, the alloy/intermetallic phase formed, and its geometric and electronic structure and the catalytic properties of the materials. Theoretical studies also support the above observations. Indeed, the initial experimental observations on the absorption of CO on bimetallic surfaces by Rodriguez and Goodman,22 and the subsequent modeling proposed by Nørskov and co-workers,23,24 have demonstrated for alloys that the surface electronic density of states (DOS) strongly depends on the structural geometry of the bimetallic system and how it accounts for the strength of the bonding with chemisorbed molecules. In particular, they have found that the key parameter controlling the surface properties and, consequently, the catalytic performance, is the position of the metal d-band center. Therefore, the careful choice of a couple of metals and of their relative coordination can in principle be exploited to finely tune the reactivity of the system with respect to a specific reaction. In this work we studied the effect of the Pd/Sn ratio on the alloy and intermetallic phases formed and also how the latter affect the catalytic activity of the samples. DFT calculations suggested that the Pd−Sn system is expected to significantly improve palladium activity,25−28 in particular, in the Pd-rich part of the Pd−Sn phase diagram.29 Here, the catalytic activity of PdSn systems was tested on the preferential oxidation of CO, at small concentration in a large amount of H2 (PROX). PROX is an interesting and simple model reaction for the evaluation of noble metal-based heterogeneous catalysts.30 It is industrially relevant to allow using H2 rich syngas in PEM fuel cells. The main scope of this study is to evaluate the effect of the composition-phase correlation for the Pd−Sn system on its catalytic properties, and more specifically to assess the catalyst behavior as a function of the disordered alloy/ordered intermetallic atomic arrangement. This work does not have the ambition of presenting a catalyst with outstanding performance, but specifically focuses on the distinct catalytic behaviors related to different metallic structures and compositions. Palladium−tin bimetallic NPs were prepared by conventional colloidal synthesis technique and supported on alumina in a separate step. It is expected to obtain PdSn alloys within certain Pd/Sn ratios, while intermetallic PdxSny phases are expected outside the aforementioned Pd/Sn ratios. In most of the cited studies, the specific crystalline phase effectively involved in the catalytic process is not clearly or univocally identified, often because of the synthetic strategy adopted. The conventional impregnation route often used, in fact, does not allow a rigorous control over the particle size distribution and

superior catalytic activity of the bimetallic sample over the monometallic Pd one in the methane decomposition reaction. Harikumar and co-workers12 prepared and characterized Pdbased catalyst modified with Ni and also with Cu. In their work, that also includes Ni−Cu and Au−Ag samples, it is shown that, for these combinations of metals, alloying occurs when the metals are mixed with ratios that match those of the phase diagram. Kim and co-workers13 also used copper to modify a Pd/TiO2 catalyst used for the reduction of nitrate in water. Although the focus of their work is on the interaction between the metals and the support, the activity of the bimetallic catalyst is higher than monometallic Pd. Tin is another metal often used to modify the activity and selectivity of Pd for a number of different reactions. Awasthi and co-workers14 investigated the activity of tinmodified Pd catalysts, deposited on graphene nanosheets for the methanol electrooxidation. Among the different metal ratios considered, they found that samples containing 18−19 wt % Pd and 1−2 wt % Sn (on graphene) exhibit much higher activity and CO poisoning tolerance than monometallic Pd samples. Their findings are explained in terms of the different electronegativities of the two metals (lower for Sn): the overall effect is a decrease of the Pd−CO binding energy, hence, the rate of desorption and the rate of CO oxidation. An alternative explanation is also provided, according to which Sn increases the Pd−Pd bond distance, making it difficult for carbonaceous species (such as CO) to bind in bridge sites. Consequently, the accumulation of carbonaceous species is lowered and the catalyst is more tolerant to CO poisoning. Mobidebi and coworkers15 studied the PdSn/C and PdSnRu/C systems for the electrooxidation of ethanol. Both of the modified samples show higher activity and selectivity than the monometallic Pd catalyst, with the PdSn being the best among the three samples considered. The experimental results presented suggest that the current density of the binary system is higher than that of the one of the ternary and the monometallic systems, with this favoring the catalyst performance. Vicente and co-workers16 investigated the structural properties of the Pd−Sn/SiO2 system in relation to the catalytic performance for selective citral hydrogenation. They state that Pd is one of the least selective metals for such a reaction, but the addition of Sn considerably increases the selectivity toward the desired products. The improved selectivity (that comes at the cost of a lower conversion) is explained as the effect of the formation of the Pd3Sn intermetallic phase, which modifies both the geometric and the electronic properties of the Pd sites, favoring the formation of new sites for the citral reaction. Hammoudeh and Mahmoud17 also report enhanced selectivity of palladium, once Sn is added, when the catalyst is used in the selective hydrogenation of cinnamaldehyde. They investigate a range of the Pd/Sn molar ratio from 95:5 to 50:50 and reported similar findings to the aforementioned work of Vicente. Indeed, intermetallic PdxSn phases are detected that give the bimetallic system different geometric and electronic properties. Although the selectivity of the PdSn system is more than doubled, the catalyst activity linearly decreases (by two-thirds) as the tin content increases. Sales and co-workers18,19 studied the effect of Sn addition to Pd in the selective hydrogenation of hexadienes in two different works. In their investigation, the Pd/Sn molar ratio ranged from 87:13 to 49:51 and Pd−Sn solid solution was detected, as well as different PdxSny intermetallic phases. Again, the addition of tin increases the catalyst selectivity at the expense of its activity, except for high Pd/Sn ratio. The authors 25393



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dropwise and absorbed by capillarity in three different steps; the solvent was removed by evaporation in a ventilated oven at 200 °C for 2 h in-between each step. The total metal loading was 1.5% w/w for all the PdSn samples prepared. A Pd sample was also prepared as a reference, using the same procedure previously described, with a loading of 1.5% w/w. The obtained powders of metal NPs/Al2O3 were suspended in ethanol and ball milled for 24 h. Finally, the suspension was washcoated on cordierite monoliths according to standard procedures and calcined at 400 °C for 4 h in N2. Each monolith has 400 cells per square inch (CPI), 10 mm length, square channels with 1.1 mm of internal size and wall thickness of 0.2 mm (after washcoating). In the following the samples will be labeled SnX, with X indicating the relative atomic percentage of Sn. The reference sample, pure Pd, is simply labeled as Pd catalyst. Nanoparticles’ Characterization. Field-emission transmission electron micrographs (FEI Tecnai F20) were acquired at 200 keV. A droplet of the NPs dispersion diluted to 0.01 M was deposited on a carbon-coated copper grid placed on filter paper so as to remove the excess solution. The grid was allowed to dry in air at room temperature before the analysis. X-ray diffractograms were acquired in air at room temperature (Philips PW1710) with a Cu anode operating at 30 KV/ 40 mA and a graphite monochromator at grazing incidence (1− 3°). A droplet of the NPs suspension was deposited on a glass slide and the solvent allowed to evaporate at room temperature. Dynamic light scattering measurements were performed on diluted NPs suspension in a quartz cuvette using a Malvern Zetasizer Nano S in backscattering configuration (178°). Activity tests of the wash-coated cordierite monoliths were carried out in a continuous flow reactor. A NDIR/O 2 multichannel instrument (NGA 2000 Rosemount Analytical MLT-1/2-Analyzer) was used to analyze CO, CO2, O2, CH4, and NH3. H2 was measured with a TCD integrated in the gas analyzer. An FID (JUM 300A) was used to detect the total hydrocarbon content. A more detailed description of the experimental setup is available.31 The Pd and Pd−Sn NPs supported Al2O3 and washcoated on monoliths have been tested for CO preferential oxidation in the presence of H2, with O2/CO ratio of 1, 2, and 5. The inlet concentration of H2 and CO were kept constant for each samples, while the O2/CO ratio was varied by changing the amount of O2 fed. The resulting gas mixture had the following composition: 10% H2, 0.5% CO, 0.5−2.5% O2 and N2 as balance gas. H2 is present in a large excess (20× the amount of CO) in order to challenge and better assess the selectivity of the catalysts. The thermal cycle consists of specular heating and cooling ramps with constant, equal heating and cooling rates, with an isothermal step of 20 min at the top temperature. The contact time (GHSV, defined as volumetric flow rate at RT/ exterior monolith volume) and heating rate (HR) were kept constant at 25000 h−1 and 6 °C min−1, respectively. Unless otherwise specified, results report conversions and yields as a function of the temperature of the gases at the inlet of the catalytic bed (Tin). This temperature was measured with a type K thermocouple placed 1 mm upstream of the monolith. Another thermocouple was placed 1 mm after the monolith, monitoring the temperature of the gases leaving the catalyst (Tout). A more detailed description of the experimental set up is available elsewhere.31 CO and H2 conversion (X) and CO selectivity (S) were calculated as follows:

crystalline phase, yielding an overall catalytic performance that is the average result of multiple complex and often opposing effects. The choice of a two-step preparation procedure consisting of the initial synthesis of the catalyst NPs and the successive impregnation of the preformed NPs on the ceramic support allows better control of the NPs formation. This work focuses, in particular, on the material composition and crystalline structure and their correlation with its catalytic performance.

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EXPERIMENTAL SECTION Nanoparticles’ Synthesis. The thermal reduction of metal-oleylamine complexes is a well-established route for the synthesis of metal NPs. Here we extend this synthetic strategy to the formation of Pd−Sn bimetallic NPs relying on the initial formation of oleylamine complexes of palladium(II) and tin(IV) from the β-diketonate and acetate precursors, respectively. A total of 1 mmol of metals was dissolved in 3 mL of oleylamine. The solvents used in this synthesis were degassed in a Schlenk apparatus through multiple freeze/thaw cycles and successively stored under inert gas. The whole synthesis was performed using standard Schlenk techniques. The precursors of the two metals are initially dissolved at room temperature in oleylamine, which acts both as coordinating solvent and reducing agent. Tin(IV) instead of tin(II) precursors are preferred to avoid possible redox pathways leading to the premature reduction of palladium(II) to its metallic state. At the same time, oleylamine forms coordinating complexes with the two metals that are stable at room temperature. After the precursors’ complete dissolution and stirring for 15 min, the solution is rapidly injected in 20 mL of oleylamine, which was preheated at 250 °C in a three-neck flask equipped with a reflux condenser, under vigorous stirring. At 250 °C, this is well above the decomposition temperature of the two metal precursors. Lower reaction temperatures yield tin-rich NPs, suggesting that tin precursors decompose more rapidly than palladium ones and that higher reaction temperatures are required to equalize the decomposition kinetics. Oleylamine is a good capping agent for the Pd−Sn system as it effectively binds both palladium and tin surface atoms while preventing their oxidation, allowing for the dispersion of the NPs in organic solvents that results in colloidal suspensions that are stable for months. After the injection, the hot solution turned dark brown within seconds. It was stirred for additional 30 min then allowed to cool to room temperature, without discontinuing the inert atmosphere. The NPs have been eventually recovered from the colloidal dispersion. The latter was diluted with an equivalent volume of hexane, and ethanol was used as a nonsolvent to precipitate the NPs, which were recovered by centrifugation. The supernatant solution was discarded, and the washing procedure repeated to remove unreacted species and excess oleylamine. After the final washing cycle, the NPs were resuspended with a 0.1 M concentration in toluene or chloroform, with the addition of 50 μL of oleylamine to improve stability of the colloidal suspension. Alloys with different Pd/Sn molar ratios were synthesized: 99:1, 90:10, 84:16, and 70:30. The catalysts were prepared by supporting the NPs on porous γ-Al2O3 (Sasol Puralox HP 14/ 150). The alumina was calcined in air at 500 °C for 5 h, with a heating rate of 5 °C/min. The resulting dry alumina powder was transferred to a beaker and 18 mL of the colloidal suspension in toluene containing 60 mg of metal were added 25394



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monophasic fcc alloy with a lattice parameter that decreases with increasing tin content. At the same time, it must be noticed that the pure palladium reference sample presents a significant deviation from the literature value for its lattice constant,32 with an experimental value of 3.99 ± 0.01 Å versus a reported value of 3.89 Å. While for NPs smaller than 5 nm a lattice constant contraction has been predicted and experimentally confirmed by several authors for Pd samples prepared by physical methods,33−35 a lattice expansion has been routinely reported for palladium NPs prepared by wet chemistry and colloidal methods.36−39 In the case of the lattice contraction, the surface tension plays a role, along with effects related to the NP shape. For the reported lattice expansion several explanations have been proposed. The presence of a tensile strain has been attributed to the presence of low-coordination number atoms on specific lattice planes,40 to the disorder of surface atoms,41 to the inclusion of impurities like C, O, CO during the synthesis,37 and more recently to the general inability at the NP level to terminate the surface with welldefined facets, thus, forcing a tensile strain across the whole nanocrystalline structure in order to accommodate the disordered surface atoms.36 In our case, the presence of tin atoms determines a progressive and linear reduction of the tensile strain, as shown in Figure 1, right. Interestingly, when the alloyed samples are treated at 300 °C in inert atmosphere to release the strain and rearrange their structure to the bulklike alloy one, the expected Vegard’s behavior is restored, with the lattice parameter linearly increasing with tin content (Figure 1, right). Remarkably, the tin-induced relaxation of the tensile strained lattice matches the tin-induced expansion of the unstrained bulk lattice at a composition that is very close to the fcc phase boundary at 16 at. % of tin. Such composition therefore appears to be structurally invariant with respect to possible strain arising from surface effects or doping, and therefore ideal for a stable catalytic behavior. When the composition is shifted to 30 at. % tin, X-ray diffractograms show the simultaneous presence of both Pd3Sn and Pd2Sn crystalline domains, again in agreement with the bulk phase diagram.29 No diffraction peaks that can be referred to oxidized phases were detected, and X-ray diffractograms were unchanged even when they were acquired on dried samples after 6 months of storage in air at room temperature.

[CO]out [CO]in [H 2]out X H2 = 1 − [H 2]in 1 [CO]in − [CO]out SCO = 2 [O2 ]in − [O2 ]out

XCO = 1 −

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RESULTS AND DISCUSSION Characterization. The phase diagram for the palladium− tin system in the bulk shows that in the palladium-rich region of the diagram the two metals form an alloy with the palladium fcc structure up to a 17 at. % of tin.29 At higher tin contents, up to the 30% tin investigated, intermetallic phases Pd3Sn, Pd2Sn, Pd20Sn13, Pd3Sn2 and PdSn or their mixtures are formed. Since such intermetallic phases are stable in a very narrow range close to the stoichiometric ratio, biphasic systems are expected to form at nonstoichiometric Pd/Sn ratios. X-ray diffraction data (Figure 1) show that NPs with compositions ranging from 1 to 16 at. % tin result in a

Figure 1. XRD patterns for samples of Pd and PdSn alloys (SnX, where X indicates the Sn at. %, left) and lattice parameter variation vs Sn content (right) for as-prepared samples (■) and for samples annealed in an inert atmosphere (□).

Figure 2. Left: XRD pattern for Sn30 sample with the formation of intermetallic compounds Pd3Sn and Pd2Sn. Some traces of Pd20Sn13 can also be noticed. Inset: HRTEM of a single polycrystalline Sn30 nanoparticle. Right (top): DLS number size distribution for Sn30 samples. Right (bottom): EDS spectrum acquired from the single Sn30 NP. 25395



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The preparation of metal NPs by colloidal techniques was preferred to the conventional impregnation of metal salt precursors to better control the average particle size and crystalline structure of the active metal NPs. The homogeneity achieved in the colloidal solution is maintained throughout the whole subsequent processing, including the calcination and/or reduction steps. On the contrary, for porous alumina supports the approach relying on the metal salt impregnation typically results in a broad size distribution and compositional dishomogeneity42 due to the intrinsic irregular pore distribution and subsequent uneven dispersion of the metal salts after the solvent removal. To achieve a narrow particle size distribution, tailored supports,43 or specific techniques need to be employed, such as microemulsion44 and colloidal methods.45 TEM analyses (Figure 3) show well-dispersed, spheroidal mono-

Figure 4. Experimental (solid lines) and equilibrium (dotted lines) reactants and products conversions and yields as a function of the gases inlet temperature (Tin) detected during a PROX test with monometallic Pd; O2/CO = 2, GHSV = 25 000 h−1, HR = 6 °C/min.

not thermodynamically controlled. Indeed the thermodynamic equilibrium predicts a much higher H2 consumption and water production, particularly at lower temperature. Consistent with the reducing, N2-rich environment, NH3 is expected, according to the equilibrium predictions, but it was not experimentally detected. CO should be completely converted, but producing equivalent amounts of CH4, instead of CO2. The amount of H2 peaks when also CO conversion is the highest; still H2 conversion remains rather low (∼8%), but it increases at higher temperature, reaching a maximum of 12.5%. This indicates a change in the selectivity of the catalyst as the temperature rises above 250 °C, switching from CO to H2 oxidation. Overall the catalyst shows a fairly good selectivity, as confirmed by the comparison with the calculated thermodynamic equilibrium. In the following discussion, only CO and H2 concentration after the catalyst will be reported, as the other products and reactants follow very similar trends to those presented in Figure 4. Tests at O2/CO = 1:1. All the catalysts at different Pd/Sn ratios show a similar trend in the CO conversion, with an optimal temperature and a lower reactivity at higher and (quite obviously) lower temperatures, Figure 5a. An increasing amount of Sn significantly lowers the onset of reactivity: Ton, the temperature at which CO conversion reaches 5%, lowers from 215 °C for Pd, down to 186 °C for Sn1, 136 °C for Sn10, 125 °C for Sn16, and 112 °C for Sn30. All the Pd/Sn materials are more active than the reference, pure Pd, but the CO conversion increases for Sn at.% from 1 to 16, and it decreases considerably for the sample with the highest Sn content, Sn30. The maximum CO conversion measured, Figure 5c, rises from 43% for pure Pd, to 45 and 51% for Sn10 and Sn16. The sample with the highest Sn content shows a reduced max CO conversion of 36%. Clearly, an increasing Sn content improves the CO reactivity activity, showing a synergetic effect of the Pd−Sn alloy. The exception is represented by the sample with the highest Sn content; this is likely connected with the formation of Pd2Sn and Pd3Sn intermetallic phases.

Figure 3. TEM images for sample Sn10 (10 at. % Sn in Pd,left) with HRTEM of a single Sn10 NP (right).

crystalline NPs in the fcc alloy composition domain, while at 30 at. % Sn spherical polycrystalline NPs are observed (Figure 2, left inset). EDS data confirm that the NPs composition is consistent with what is expected by the given reactant ratios (Figure 2, bottom right). DLS data for the same polycrystalline sample Sn30 show that the NPs colloidal dispersion has a number-average hydrodynamic diameter of 13.1 nm (Figure 2, top right), which is consistent with TEM observations: the crystallite size as determined by Scherrer’s method applied to the XRD data shows crystallites with sizes of 4.9−7.2 nm, according to different crystallographic planes, again in agreement with TEM observations. Reactivity. Figure 4 shows a typical evolution of the gas composition after contacting a sample of monometallic Pd in a PROX test. The experimental data are compared with the predicted thermodynamic equilibrium, as calculated by the NASA CEA Code.46 The program determines the minimum total free energy configuration by changing the relative amounts of given species in a broad set of candidates. Obviously, equilibrium composition is the best achievable performance of a given catalyst. CO starts reacting above 200 °C and its concentration slowly decrease up to 230 °C, where a sudden increase of the reaction rate quickly leads to a significant conversion of about 80% at 250 °C (residual concentration of 0.1%). At higher temperatures, the CO concentration gradually raises, indicating the presence of competing reactions. The CO2 concentration exactly mirrors the CO behavior, being the only C-containing species at measurable levels. No methane or other hydrocarbons where detected, notwithstanding the lack of O2, the reducing environment and the consistent predictions of thermodynamics. As CO starts being converted, also H2 is activated and reacts to water. The competition between H2 and CO oxidation is clear and the whole process is kinetically and 25396



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Figure 5. CO (a) and H2 (b) conversion. Maximum conversion and corresponding selectivity as a function of the Sn loading (c); O2/CO = 1, GHSV = 25000 h−1, HR = 6 °C/min.

clear from Figure 7a,b. However, the alloy samples are still active at a much lower temperature than the Pd reference. All the SnX samples show a higher activity compared to the reference with a much lower onset temperature and a considerably higher CO maximum conversion (up to 83% vs 72% of the reference), Figure 7a. The alloys outperform the reference (pure Pd) in maximum CO conversion, Figure 7c. Even the Sn30 sample shows a maximum comparable to pure Pd catalyst. We observed, both for CO and H2 oxidation, lower ignition temperatures with increasing of tin content at any O2/CO ratios. PdSn alloys with low Sn content show higher reactivity than pure Pd sample. There seem to be an exception in the case of O2/CO = 2. Indeed, from Figure 6c the Pd sample exhibits a slightly better performance compared to the alloys (78% conversion and 25% selectivity vs 71−78% conversion and 21− 24% selectivity for Sn1−Sn16). The reason for this quantitatively different trend lies in a small experimental imprecision. In all the tests presented, the actual O2/CO ratios are very close to the nominal value of 1, 2, and 5. However, when the ratio is 2, the Pd sample was tested with a slightly higher amount of oxygen compared to the alloys. For example, the difference in O2 concentration between the Pd and the Sn1 and Sn10 samples is 0.014 and 0.023%, respectively. This potentially corresponds to 5 and 8% higher CO conversion

For all the samples, CO conversion is limited by the competing reaction of O2 with H2 to yield H2O, whose progress is shown in Figure 5b. The H2 conversion plot shows that the drawback of the alloys increasing the reactivity of both CO and H2 is a slightly lower CO selectivity, as confirmed by Figure 5c, where selectivity at the maximum conversion is reported. The %Sn reported on the x axis of Figure 5c (and Figures 6c and 7c) refers to the metallic phase only. The Sn30 sample exhibits the worst selectivity, confirming that the Pd−Sn alloy is the preferable phase, while PdSn intermetallic compounds are detrimental in the catalysis of the PROX reaction. To further investigate the reactivity, tests with a larger O2/CO ratio have been performed, which challenges the selectivity. Tests at O2/CO = 2:1. When the O2/CO ratio is doubled, the increased reactivity proportional to the Sn at.% is confirmed, as measured by the progressively lower activation temperature, Figure 6a. Also, CO conversion increases. It is highest for the Pd reference and Sn16 (78%), while both Sn1 and Sn10 shows a slightly lower value of 73%, as shown in Figure 6c. Again, Sn30 is the worst sample reaching only 60% conversion. The increased H2 conversion, Figure 6b, slightly lowers the selectivity of the oxidation, Figure 6c, but its sensitivity to Sn amount is weaker. Tests at O2/CO = 5:1. Increasing the amount of oxygen available in the feed boosts the CO and H2 conversion, as is 25397



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The presence of tin atoms on the NP surface can influence the adsorption of CO in multiple ways: a common, widely studied effect consists of the shift of the d-band of Pd toward higher binding energies determined by the formation of the Pd−Sn bond. The concurrent weakening of the Pd 4d−CO 2π* interaction has been shown to be in direct correlation with the lowering of the desorption temperature of CO and with a higher CO surface diffusion coefficient.43 However, the overall catalytic performance is strongly dependent on the local surface geometry of the catalyst: we clearly observed a drop in the activity with the formation of ordered intermetallic compounds Pd2Sn and Pd3Sn. The disorder−order transition at the surface level has been shown to affect strongly the nature of the CO interaction with the catalyst, thereby determining its activity.43 In our case, the transition from the disordered alloy to the Pd3Sn and Pd2Sn phases brings about a progressive reduction of pure Pd 3-fold sites of the type responsible for strong CO chemisorption. Surface science studies combining TPD and LEED47 have proven that for Pd−Sn intermetallics on Pd (111) surfaces, the p(2 × 2)-Pd3Sn structure presents a CO desorption yield reduced to ≈40% of that on a clean Pd (111) surface, that is further decreased to ≈10% for the √3-Pd2Sn structure. Similar results were obtained for Pd (110) surfaces.48 These experimental observations are corroborated by the relative XPS measurements47,48 that have confirmed that these

reachable by the Pd sample. The higher amount of O2 available in the case of pure Pd and O2/CO ratio = 2:1 is the reason for the different quantitative results. This is confirmed by the oxygen conversion, which is complete or almost for the PdSn samples and it is only slightly higher than 90% with pure Pd, meaning that O2 acts as a limiting reactant for the alloys, therefore, limiting the maximum CO conversion. However, it is important to note that the qualitative trends are very similar in all the cases presented. Indeed, pure Pd exhibits similar conversion and selectivity to Sn1 and the performance of the bimetallic catalyst improve as the tin content increases, as long as the alloy phase is maintained. Furthermore, the onset temperature of the reaction is always lowered by the addition of Sn and follows the same trend in all the cases presented. When the tin content is such that an intermetallic phase is formed, instead, both conversion and selectivity drop to considerably lower levels compared both to the pure Pd and the alloy samples. We speculate that the formation of intermetallic compounds Pd2Sn and Pd3Sn is the cause of the lower reactivity. Such a behavior can be explained by considering in detail what happens at the atomic level when the system shifts from a composition where a disordered alloy is the stable phase to a composition where an atomic ordering is present due to the formation of intermetallic crystalline phases. 25398



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O2 is consumed during the reaction. The most likely conditions to have hysteresis are when the O2/CO is the highest. Indeed at low O2/CO ratios, no hysteresis at all was detected. As shown in Figure 8, the conversion lines during heating and cooling overlaps at all the temperatures, for both pure Pd and Sn10. Only two samples are reported for clarity, but all the samples tested showed the same qualitative behavior. The only exception is a slight mismatch above 200 °C for the Sn10 sample. This confirms that no reduction or oxidation reaction is occurring during the catalytic tests. Tests at the lowest O2/CO ratio are not reported here, but similarly to Figure 8, no hysteresis could be noticed. Apparently, at low O2/CO ratios, the materials are stable, notwithstanding the temperature and atmosphere they are exposed to. The situation changes dramatically at the highest O2/CO ratio, as reported in Figure 9. In this case, a very wide hysteresis is detected. Furthermore, the maximum CO conversion recorded is much higher during the cooling step than during heating. A possible explanation for such a large hysteresis is the aforementioned redox reactions. However, the higher activity causes a larger rate of heat production, being both CO and H2 oxidation very exothermic. The temperature on the catalytic surface, inside the monolith channels, is likely to be significantly higher than that of the gases entering the monolith, but also of

charge transfer/orbital hybridization processes directly affect the metal → CO 2π* electron back-donation, in accordance to our earlier hypothesis; it should be noticed that, for both Pd (111) and Pd (110) surfaces, the Pd−Sn structures considered in the cited studies correspond to the unreconstructed terminations of the bulk intermetallics and are therefore structurally analogous to the systems we are presenting. Hysteresis. All the results reported in Figure 5, 6, and 7 refers to tests carried out with a constant heating rate. Outlet concentrations reported refer to the first heating step. The same tests have been continued with a specular cooling program, at the same rate, but opposite, used for heating (i.e., −6 °C/min) and the outlet gases concentrations recorded also during the cooling step. Such an experiment allows to monitor the stability of the Pd:Sn alloys when its environment (temperature and composition) is continuosly changing. Hysteresis cycles imply a different reactivity during heating and cooling. It is well-known and documented for Pd where it has been explained by redox reactions of the metal. The phenomena is reported both for CO49,50 oxidation and also for other reactions, such as methane combustion and partial oxidation.51 In our case, hysteresis could indicate changes in the alloy structure. Provided that there is always H2 and CO exceeding the O2 required for total oxidation, reducing conditions prevail at all the temperatures, and especially when 25399



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Figure 10. CO conversion as a function of the outlet temperature; O2/ CO = 5.

Figure 8. CO conversion recorded with different catalyst samples ramping the temperature up and down at equal heating and cooling rate (6 °C/min); O2/CO = 2.

be the same at lower O2/CO ratios, if the data were plotted using the outlet gas temperature, instead than the inlet one. This is not the case for the tests presented in this work, since the gas inlet and outlet temperatures are always matching or at least very close, except for the specific case illustrated in Figures 9 and 10. In Figure 11 the CO conversion detected during the same tests reported in Figures 5 and 6 is plotted as a function of the gas outlet temperature. It is very clear that both the qualitative and the quantitative information are the same as those obtained by plotting the conversion as a function of the gas inlet temperature. This fact further confirms that the hysteresis detected at the highest O2/CO ratio is a thermal phenomenon.

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CONCLUSIONS We prepared monodispersed bimetallic NPs of Pd and Sn by the thermal coreduction of a solution of palladium(II) and tin(IV) complexes in oleylamine. NPs have been characterized by TEM and XRD identifying either polycrystalline NPs, with Pd3Sn and Pd2Sn intermetallic phases, or fcc monocrystalline alloys, according to the bulk material phase diagram. The NPs have been tested for possible catalytic activity for the preferential oxidation of CO with respect to H2, in excess of hydrogen (PROX), after being supported on porous Al2O3 and subsequent washcoating on ceramic monoliths. For a given O2/CO ratio, increasing the Sn content in the catalysts determines a surprising improvement in the activity for the preferential oxidation of CO with respect to pure Pd, at least as long as the fcc disordered alloy structure is conserved. Larger Sn/Pd ratios (above 17 Sn at.%) lead to the formation of intermetallic compounds, as confirmed by TEM and expected by the phase diagram, and despite an even lower ignition temperature, they do not allow to achieve the maximum CO conversion, as obtained with the alloys and pure Pd. Remarkably, the selectivity is comparable to the one given by pure Pd, thus, providing an interesting structure− reactivity correlation to be exploited in similar reactions. For instance, other M/Sn systems, where M = Pt, Ni, Mo, and Cu, present phase diagrams where alloy/intermetallics transition are clearly defined for specific compositions and where similar behavior might be observed.

Figure 9. Hysteresis cycles in CO conversion at O2/CO = 5.

the gas mixture flowing in the channels. If this was the case, the hysteresis would only be an artifact. It is not possible to measure the temperature on the catalyst surface inside the channels, but the temperature at the outlet of the catalyst bed was continuously monitored. A plot of the CO conversion as a function of the outlet temperature, Figure 10, shows considerably narrower hysteresis cycles. This is because the outlet temperature was significantly higher than the inlet one during the tests at the highest O2/CO ratio. The hysteresis does not disappear in Figure 10, but it decreases significantly, supporting the hypothesis that it is only an artifact and not a result of structural changes in the alloys. The temperature on the catalyst surface is certainly even higher than the outlet temperature of the gases and, if available, it would very likely make the hysteresis cycle disappear. Furthermore, XRD of the tested samples showed that the alloy structure was maintained, confirming that the catalyst is stable even after exposure to high oxygen concentrations. Provided the explanation given for the detected hysteresis, a legitimate doubt could rise on whether or not the results would 25400



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Figure 11. CO conversion as a function of the gases outlet temperature for O2/CO = 1 (a) and O2/CO = 2 (b). (7) Bönnemann, H.; Richards, Ryan M. Nanoscopic Metal Particles − Synthetic Methods and Potential Applications. Eur. J. Inorg. Chem. 2001, 2001, 2455−2480. (8) Wang, W.; Zheng, D.; Du, C.; Zou, Z.; Zhang, X.; Xia, B.; Yang, H.; Akins, D. L. Carbon-Supported Pd-Co Bimetallic Nanoparticles as Electrocatalysts for the Oxygen Reduction Reaction. J. Power Sources 2007, 167, 243−249. (9) Sun, D.; Mazumder, V.; Metin, O. N.; Sun, S. Catalytic Hydrolysis of Ammonia Borane via Cobalt Palladium Nanoparticles. ACS Nano 2011, 5, 6458−6464. (10) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Nørskov, J. K. Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Production. Nat. Mater. 2006, 5, 909−913. (11) Nabae, Y.; Yamanaka, I.; Hatano, M.; Otsuka, K. Catalytic Behavior of Pd−Ni/Composite Anode for Direct Oxidation of Methane in SOFCs. J. Electrochem. Soc. 2006, 153, A140−A145. (12) Harikumar, K. R.; Ghosh, S.; Rao, C. N. R. X-ray Photoelectron Spectroscopic Investigations of Cu−Ni, Au−Ag, Ni−Pd, and Cu−Pd Bimetallic Clusters. J. Phys. Chem. A 1997, 101, 536−540. (13) Kim, M.-S.; Chung, S.-H.; Yoo, C.-J.; Lee, M. S.; Cho, I.-H.; Lee, D.-W.; Lee, K.-Y. Catalytic Reduction of Nitrate in Water Over Pd− Cu/TiO2 Catalyst: Effect of the Strong Metal-Support Interaction (SMSI) on the Catalytic Activity. Appl. Catal., B 2013, 142−143, 354− 361. (14) Awasthi, R.; Singh, R. N. Optimization of the Pd-Sn-GNS Nanocomposite for Enhanced Electrooxidation of Methanol. Int. J. Hydrogen Energy 2012, 37, 2103−2110. (15) Modibedi, R. M.; Masombuka, T.; Mathe, M. K. Carbon Supported Pd−Sn and Pd−Ru−Sn Nanocatalysts for Ethanol ElectroOxidation in Alkaline Medium. Int. J. Hydrogen Energy 2011, 36, 4664−4672. (16) Vicente, A.; Lafaye, G.; Especel, C.; Marécot, P.; Williams, C. T. The Relationship between the Structural Properties of Bimetallic Pd− Sn/SiO2 Catalysts and their Performance for Selective Citral Hydrogenation. J. Catal. 2011, 283, 133−142. (17) Hammoudeh, A.; Mahmoud, S. Selective Hydrogenation of Cinnamaldehyde over Pd/SiO2 Catalysts: Selectivity Promotion by Alloyed Sn. J. Mol. Catal. A: Chem. 2003, 203, 231−239. (18) Sales, E. A.; de Jesus Mendes, M.; Bozon-Verduraz, F. LiquidPhase Selective Hydrogenation of Hexa-1,5-diene and Hexa-1,3-diene on Palladium Catalysts. Effect of Tin and Silver Addition. J. Catal. 2000, 195, 96−105. (19) Sales, E. A.; Jove, J.; de Jesus Mendes, M.; Bozon-Verduraz, F. Palladium, Palladium−Tin, and Palladium−Silver Catalysts in the Selective Hydrogenation of Hexadienes: TPR, Mö ssbauer, and Infrared Studies of Adsorbed CO. J. Catal. 2000, 195, 88−95.

The NP-based materials were stable under a wide range of conditions (temperature and gas composition), without showing hysteresis due to redox reactions or structure modifications. Only when the oxygen amount is large and exothermic reactions more intensely active, some hysteresis was measured. However, we prove it to be a mere effect of a mismatch between the measured temperature and the actual temperature on the surface of the catalyst. Trimming the Sn/Pd ratio allows to obtain different structures whose reactivity can be tuned with the Sn amout. Furthermore, a catalytic activity comparable (or better) than pure Pd with a substantial saving in raw materials’ cost is an intersting result for industrial applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 049 827 5634. Author Contributions ‡

These authors contributed equally to this work (R.L. and M.B.). Notes

The authors declare no competing financial interest.

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