The Role of SnO2 on Electrocatalytic Activity of PtSn Catalysts (original) (raw)
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Insight into the Effect of Sn on CO and Formic Acid Oxidation at PtSn Catalysts
The Journal of Physical Chemistry C, 2014
The role of Sn on the catalytic activity for CO and formic acid oxidation is studied by comparing the activities of differently treated PtSn/C and Pt/C catalysts. The catalysts are prepared by a microwave-assisted polyol synthesis method. As revealed by scanning tunneling and transmission electron microscopic (STM and TEM) characterization, the outcomes of the synthesis procedure for both Pt and PtSn are small particles, ∼1.5 nm in diameter. Upon deposition on the carbon support, the particle size increases to ∼2.5 nm due to sintering. X-ray diffraction (XRD) analysis shows that PtSn/C has a low alloying degree and is mainly composed of Pt and Pt 3 Sn phases. The remaining Sn is present in the form of very small tin oxide particles. Different surfaces are obtained by double-layer, oxide, and CO annealing of the Pt/C and PtSn/C catalysts and by modifying the CO-annealed surfaces with irreversibly adsorbed tin, Sn irr . The presence of Sn in any form (oxide, alloyed, or Sn irr ) on the surface shifts the onset potential for the CO oxidation negatively by more than 0.4 V in comparison to equivalently treated Pt/C catalysts. For the CO-annealed PtSn/C catalyst, a so-called skeleton structure, Sn is present only in the subsurface layers. The subsurface Sn has a mild effect on the CO activity, and hence the onset potential is only marginally shifted to cathodic potentials by ∼50 mV compared to that on Pt/C. The formic acid oxidation is enhanced at any of the PtSn/C surfaces with Sn in the surface layer. The activity enhancement is explained by a reduced CO poisoning of the surface Pt sites. As a consequence, the current is not entering plateau as on the Pt/C catalysts. Furthermore, the skeleton PtSn/C is ∼2 times more active than similarly treated Pt/C. The results have been substantiated and explained by comprehensive density functional theory (DFT) simulations. The DFT results indicate that the increased oxidation rates are not only due to surface Sn but also due to a weakened CO binding in the vicinity of the surface SnOH x moieties and SnO 2 particles. The work was designed by V.M.J. Electrochemical experiments were done by S.S. and D.T., who also synthesized the catalysts and performed STM measurements. D.M. and A.G. did XRD measurements. DFT calculations were rationalized and performed by V.T. V.M.J., V.T., and A.T. participated in writing the manuscript.
Insight of Sn influence on formic acid oxidation at Pt based catalysts
The role of Sn on the catalytic activity for CO and formic acid oxidation is studied by comparing the activities of differently treated PtSn/C and Pt/C catalysts. The catalysts are prepared by a microwave-assisted polyol synthesis method. As revealed by scanning tunneling and transmission electron microscopic (STM and TEM) characterization, the outcomes of the synthesis procedure for both Pt and PtSn are small particles, ∼1.5 nm in diameter. Upon deposition on the carbon support, the particle size increases to ∼2.5 nm due to sintering. X-ray diffraction (XRD) analysis shows that PtSn/C has a low alloying degree and is mainly composed of Pt and Pt 3 Sn phases. The remaining Sn is present in the form of very small tin oxide particles. Different surfaces are obtained by double-layer, oxide, and CO annealing of the Pt/C and PtSn/C catalysts and by modifying the CO-annealed surfaces with irreversibly adsorbed tin, Sn irr . The presence of Sn in any form (oxide, alloyed, or Sn irr ) on the surface shifts the onset potential for the CO oxidation negatively by more than 0.4 V in comparison to equivalently treated Pt/C catalysts. For the CO-annealed PtSn/C catalyst, a so-called skeleton structure, Sn is present only in the subsurface layers. The subsurface Sn has a mild effect on the CO activity, and hence the onset potential is only marginally shifted to cathodic potentials by ∼50 mV compared to that on Pt/C. The formic acid oxidation is enhanced at any of the PtSn/C surfaces with Sn in the surface layer. The activity enhancement is explained by a reduced CO poisoning of the surface Pt sites. As a consequence, the current is not entering plateau as on the Pt/C catalysts. Furthermore, the skeleton PtSn/C is ∼2 times more active than similarly treated Pt/C. The results have been substantiated and explained by comprehensive density functional theory (DFT) simulations. The DFT results indicate that the increased oxidation rates are not only due to surface Sn but also due to a weakened CO binding in the vicinity of the surface SnOH x moieties and SnO 2 particles. The work was designed by V.M.J. Electrochemical experiments were done by S.S. and D.T., who also synthesized the catalysts and performed STM measurements. D.M. and A.G. did XRD measurements. DFT calculations were rationalized and performed by V.T. V.M.J., V.T., and A.T. participated in writing the manuscript.
Reaction Kinetics, Mechanisms and Catalysis, 2017
Alloy-type Sn-Pt/C electrocatalysts with desired Pt/Sn= 3.0 ratio have been prepared by Controlled Surface Reactions using home-made 20 wt.% Pt/C (20Pt/C) catalysts with different Pt dispersion. Reaction conditions were found for the preparation of highly dispersed 20Pt/C catalysts by modified NaBH 4-assisted ethylene-glycol reduction method using ethanol as a solvent. It has been demonstrated that the increase of the heating time in ethanol up to 2 h results in decreasing dispersion of Pt. Upon using highly dispersed 20Pt/C catalyst the exclusive incorporation of Sn onto the Pt sites was achieved resulting in exclusive formation of the Pt-Sn alloy phase. According to in situ XPS studies pre-treatment of the air exposed catalyst in H 2 even at 170°C resulted in complete reduction of the ionic tin to Sn 0 , suggesting alloy formation. In contrast, the catalyst with lower Pt dispersion cannot be completely reduced even at 350°C, as 10 % of tin still remains in the form of Sn 4+ surface species. The electrocatalytic performance of both Sn-20Pt/C catalysts in the CO electrooxidation and the oxygen reduction reaction is superior to that of the parent 20Pt/C catalysts. Our data obtained for the oxygen reduction reaction indicate that the small size of the bimetallic nanoparticles in the highly dispersed Sn-20Pt/C catalyst, along with their optimal surface composition, result in increased activity compared to the catalyst with lower dispersion.
Structure and chemical composition of supported Pt–Sn electrocatalysts for ethanol oxidation
Electrochimica Acta, 2005
Carbon supported PtSn alloy and PtSnO x particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0-3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnO x nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnO x clearly shows that the change of the spacing of Pt (1 1 1) plane is neglectable, meanwhile, SnO 2 nanoparticles, characterized with the nominal 0.264 nm spacing of SnO 2 (1 0 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (1 1 1) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnO x catalyst. PtSnO x catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnO x catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites.
Journal of Catalysis, 2005
The catalytic activity of three different carbon-supported Pt-Sn catalysts for the anodic oxidation of hydrogen, carbon monoxide, and H 2 /CO mixtures is correlated with the bimetallic microstructure established in Part I [V. Radmilovic, T.J. Richardson, S.J. Chen, P.N. Ross, Jr., J. Catal. 232 (2005) 199-209] of this study. These catalysts differ primarily by having differing amounts of Pt, Pt 3 Sn, PtSn, and SnO 2 phase nanoparticles distributed on the carbon support. Further surface chemical characterization of the Pt 3 Sn nanoparticles is provided by comparison of in situ vibrational spectra of CO adsorbed on the nanoparticles with CO adsorbed on Pt 3 Sn(hkl) surfaces [V. Stamenkovic, M. Arenz, B.B. Blizanac, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, Surf. Sci. 576 (2005) 145-157; V.R. Stamenkovic, M. Arenz, C.A, Lucas, M.E. Gallagher, P.N. Ross, N.M. Markovic, J. Am. Chem. Soc. 125 (2003) 2736-2745].
Journal of Solid State Electrochemistry, 2004
The electrocatalytic activity of a spontaneously tin-modified Pt catalyst, fabricated through a simple ''dip-coating'' method under open-circuit conditions and characterized using surface analysis methods, was studied in electrooxidation reactions of a preadsorbed CO monolayer and continuous oxidation of methanol, formic acid, and formaldehyde in the potentiodynamic and potentiostatic modes. The catalytic activity of the tinmodified Pt surface is compared with that of a polycrystalline Pt electrode. Spontaneously Sn-modified Pt catalyst shows a superior activity toward adsorbed CO oxidation and thus can be promising for PEFC applications. The methanol oxidation rate is not enhanced on the Sn-modified Pt surface, compared to the Pt electrode. Formic acid oxidation is enhanced in the low potential region on the Sn-modified surface, compared to the Pt electrode. The formaldehyde oxidation rate is dramatically increased by modifying tin species at the most negative potentials, where anodic formaldehyde oxidation is completely suppressed on the pure Pt electrode. The results are discussed in terms of poisoning CO intermediate formation resulting from dehydrogenation of organic molecules on Pt sites, and oxidation of poisoning adsorbed CO species via the surface reaction with OH adsorbed on neighboring Sn sites.
Electrochimica Acta, 2012
Carbon supported Pt-Sn catalysts were prepared by reduction of Pt and Sn precursors with formic acid and characterized in terms of structure, morphology and surface properties. The electrocatalytic activity for ethanol oxidation was studied in a direct ethanol fuel cell (DEFC) at 70 • C and 90 • C. Electrochemical and physico-chemical data indicated that a proper balance of Pt and Sn species in the near surface region was necessary to maximize the reaction rate. The best atomic surface composition, in terms of electrochemical performance, was Pt:Sn 65:35 corresponding to a bulk composition 75:25 namely Pt 3 Sn 1 /C. The reaction products of ethanol electro-oxidation in single cell and their distribution as a function of the nature of catalyst were determined. Essentially, acetaldehyde and acetic acid were detected as the main reaction products; whereas, a lower content of CO 2 was formed. The selectivity toward acetic acid vs. acetaldehyde increased with the increase of the Sn content and decreased by decreasing the concentration of the reducing agent used in the catalyst preparation. According to the recent literature, these results have been interpreted on the basis of ethanol adsorption characteristics and ligand effects occurring for Sn-rich electrocatalysts. (A.S. Aricò).
Applied Catalysis B-environmental, 2010
Pt-SnO x /C catalysts were prepared in a polyol process under Ar, sequential Ar and air, and air atmospheres in combination with a high temperature reduction treatment. The composition, structure, morphology and oxidation state of the prepared catalysts were characterized by Inductively Coupled Plasma-Atom Emission Spectroscopy, X-ray diffraction, scanning transmission electron microscopy and X-ray photoelectron spectroscopy. The electrochemical activities were evaluated by CO stripping voltammetry and single cell test in combination with in situ IR reflection absorption spectroscopy (IRRAS). The polyolsynthesized Pt-SnO x /C catalysts prepared under different atmospheres had a similar bulk composition, particle size and lattice parameter, however, the Pt-SnO x /C catalyst prepared under Ar atmosphere possessed a greater proportion of Sn(II) species than the other Pt-SnO x /C catalysts. Electrochemical and in situ IRRAS measurements indicated that the Pt-SnO x /C catalyst under Ar atmosphere had the greatest CO tolerance in proton electrolyte fuel cell among the Pt-SnO x /C catalysts.
Fuel Cells, 2002
A comparative study between carbon supported Platinum± Tin (Pt±Sn syn ), synthesized via the carbonyl route, and the commercial (Pt 3 Sn) from E±TEK is reported. The electro-oxidation of methanol and adsorbed CO, in sulphuric acid medium, were used as probes to evaluate the performance of these electrocatalysts. In-line differential mass spectrometry (DEMS) was used for this purpose. Both nanoparticulate materials had a mean particle size of = 1.68 0.72 nm, and = 3.58 1.94 nm, respectively. It is demonstrated that, under the same experimental conditions, our home-made Pt±Sn syn is less sensitive to poisoning by CO. This observation was again verified during the oxidation of methanol. These results are discussed in terms of the local disorder of the particles surface atoms, favourably induced by size effect and the preparation route employed.