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PtxY and PtxGd exhibit exceptionally high activity for oxygen reduction, both in the polycrystalline form and the nanoparticulate form. In order to understand the origin of the enhanced activity of these alloys, we have investigated thin films of these alloys on bulk Pt(111) crystals, i.e. Y/Pt(111) and Gd/Pt(111). These surfaces exhibit a 4-fold improvement over Pt(111). We observe the formation of a thick Pt overlayer after the electrochemical measurements, both on Y/Pt(111) and Gd/Pt(111). Using surface sensitive X-ray diffraction we revealed that crystalline closely packed Pt overlayers were formed. The diffraction experiments showed that the strain and crystallinity of the overlayers are strongly dependent on the electrochemical treatment, and in general show lateral compression.

Formation of meta-stable surface-alloys can be used as a way to tune the binding strength of reaction intermediates and could therefore be used as improved catalyst materials for heterogeneous catalysis. Understanding the role of adsorbates on such alloy surfaces can provide new insights for engineering of more active or selective catalyst materials. Dynamical surface changes on alloy surfaces due to the adsorption of reactants in high gas pressures are challenging to investigate using standard characterization tools. Here we apply synchrotron illuminated near ambient pressure X-ray photoelectron spectroscopy to probe the surface structure of a CuPt model system. We report an investigation of the formation of a CuPt surface alloy induced and stabilized by exposure to mbar pressures of CO. The location of Pt within the alloy is assessed by deconvolution of the Pt 4f core level spectra into surface Pt and bulk Pt contributions. The study provides direct evidence on how it is possible to monitor the surface structure under near operation conditions. (C) 2014 Elsevier B.V. All rights reserved. (Less)

Using temperature-programmed desorption experiments, we have studied the coordination dependent adsorption of CO on a platinum (Pt) single crystal, and mass-selected Pt nanoparticles in the size range of 3 to 11 nm, for CO dosing pressures in 10–7 mbar and mbar ranges. From low pressure CO adsorption experiments on the Pt(111) crystal, we establish a clear link between the degree of presputtering of the surface prior to CO adsorption, and the amount of CO bound at high temperature. It was found that for rougher surfaces, i.e., with more undercoordinated surface atoms, a feature appears in the CO desorption spectra at high temperature. The result is consistent with literature results from stepped single crystals that have found high temperature CO desorption features due to the presence of undercoordinated step and kink sites on the crystal facets. For the nanoparticles, a study of the dependence of the CO desorption profile with particle size found more prominent high temperature CO desorption features as...

The Pt(111)/electrolyte interface has been characterized during the oxygen reduction reaction (ORR) in 0.1 M HClO(4) using electrochemical impedance spectroscopy. The surface was studied within the potential region where adsorption of OH* and O* species occur without significant place exchange between the adsorbate and Pt surface atoms (0.45-1.15 V vs RHE). An equivalent electric circuit is proposed to model the Pt(111)/electrolyte interface under ORR conditions within the selected potential window. This equivalent circuit reflects three processes with different time constants, which occur simultaneously during the ORR at Pt(111). Density functional theory (DFT) calculations were used to correlate and interpret the results of the measurements. The calculations indicate that the coadsorption of ClO(4)* and Cl* with OH* is unlikely. Our analysis suggests that the two-dimensional (2D) structures formed in O(2)-free solution are also formed under ORR conditions.

We have prepared an yttrium modified Pt(111) single crystal under ultra-high vacuum conditions, simulating a bulk alloy. A Pt overlayer is formed upon annealing the crystal above 800 K. The annealed structure binds CO weaker than Pt(111), with a pronounced peak at 295 K in the temperature programmed desorption of CO. When depositing a large amount of yttrium at 1173 K, a (1.88 × 1.88)R30° structure relative to Pt(111) was observed by low energy electron diffraction. Such an electron diffraction pattern could correspond to a (2 × 2)R30° structure under 6% compressive strain. This structure is in agreement with the structure of the vacancies in a Pt Kagome layer in Pt5Y rotated 30° with respect to the bulk of the Pt(111). The Pt overlayer is relatively stable in air; however, after performing oxygen reduction activity measurements in an electrochemical cell, a thick Pt overlayer was measured by the angle resolved X-ray photoelectron spectroscopy depth profile. The activity of the annealed Y/Pt(111) for the oxygen reduction reaction was similar to that of polycrystalline Pt3Y.

To enable the development of low temperature fuel cells, significant improvements are required to the efficiency of the Pt electrocatalysts at the cathode, where oxygen reduction takes place. Herein, we study the effect of subsurface solute metals on the reactivity of Pt, using a Cu/Pt(111) near-surface alloy. Our investigations incorporate electrochemical measurements, ultrahigh vacuum experiments, and density functional theory. Changes to the OH binding energy, ΔE(OH), were monitored in situ and adjusted continuously through the subsurface Cu coverage. The incorporation of submonolayer quantities of Cu into Pt(111) resulted in an 8-fold improvement in oxygen reduction activity. The most optimal catalyst for oxygen reduction has an ΔE(OH) ≈ 0.1 eV weaker than that of pure Pt, validating earlier theoretical predictions.

The widespread use of low-temperature polymer electrolyte membrane fuel cells for mobile applications will require significant reductions in the amount of expensive Pt contained within their cathodes, which drive the oxygen reduction reaction (ORR). Although progress has been made in this respect, further reductions through the development of more active and stable electrocatalysts are still necessary. Here we describe a new set of ORR electrocatalysts consisting of Pd or Pt alloyed with early transition metals such as Sc or Y. They were identified using density functional theory calculations as being the most stable Pt- and Pd-based binary alloys with ORR activity likely to be better than Pt. Electrochemical measurements show that the activity of polycrystalline Pt(3)Sc and Pt(3)Y electrodes is enhanced relative to pure Pt by a factor of 1.5-1.8 and 6-10, respectively, in the range 0.9-0.87 V.

Polymer Electrolyte Membrane Fuel Cells (PEMFC) hold promise as a zero-emission source of power, particularly suitable for automotive vehicles. However, the high loading of Pt required to catalyse the Oxygen Reduction Reaction (ORR) at the PEMFC cathode prevents the commercialisation of this technology. Improving the activity of Pt by alloying it with other metals could decrease the loading of Pt at the cathode to a level comparable to Pt-group metal loading in internal combustion engines. PtxY and PtxGd exhibit exceptionally high activity for oxygen reduction, both in the polycrystalline form and the nanoparticulate form. [1,2,3,4]. Moreover, their negative alloying energy may make them inherently less prone to degradation via dealloying than the more commonly investigated alloys of Pt and late transition metals such as Ni, Co, Fe and Cu. In order to understand the origin of the enhanced activity of these alloys, we have investigated Y/Pt(111) [5] and Gd/Pt(111) single crystals, formed by depositing large amounts of Y and Gd on Pt(111) single crystals under Ultra-High Vacuum (UHV) conditions and annealing to high temperatures. We subsequently characterised the surface using low energy electron diffraction, ion scattering spectroscopy and temperature programmed desorption of CO. After the characterization in UHV, the ORR activity was measured. Angle resolved X-ray photoelectron spectroscopy measurements were carried out after the electrochemical measurements. These experiments revealed, that thick platinum overlayers had been formed, and that the structure formed under reaction conditions was significantly different from our initial expectations. The structures of the overlayers were investigated using surface sensitive X-ray diffraction using synchrotron radiation, and correlated to the oxygen reduction activity. [1] M. Escudero-Escribano, A. Verdaguer-Casadevall, P. Malacrida, U. Grønbjerg, B. P. Knudsen, A. K. Jepsen, J. Rossmeisl, I. E. L. Stephens, and I. Chorkendorff, Journal of the American Chemical Society, 134(40):16476–16479, Oct 10 2012. [2] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Nature Chemistry, 1 (2009) 552-556. [3] A. Velázquez-Palenzuela, F. Masini, A. F. Pedersen, M. Escudero-Escribano, D. Deian, P. Malacrida, T. W. Hansen, D. Friebel, A. Nilsson, I. E. L. Stephens, I. Chorkendorff, J. Catal. 2015, in press. [4] Hernandez-Fernandez, P., Masini F., McCarthy D. N., Strebel C. E., Friebel D., Deiana D., Malacrida P., Nierhoff A., Bodin A., Wise A. M., Nielsen J. H., Hansen T. W., Nilsson A., Stephens I. E. L., and Chorkendorff I. Nat Chem, 6(8): 732-738, Aug 2014 [5] T. P. Johansson, E. T. Ulrikkeholm, P. Hernandez-Fernandez, M. Escudero-Escribano,P. Malacrida, I. E. L. Stephens, and I. Chorkendorff. Physical Chemistry Chemical Physics, 16(27):13718–13725, 2014.

Polymer electrolyte membrane fuel cells (PEMFC) hold promise as a zero-emission source of power, particularly suitable for automotive vehicles. However, the high loading of Pt required to catalyse the oxygen reduction reaction (ORR) at the PEMFC cathode, prevents the commercialisation of this technology. Improving the activity of Pt by alloying it with other metals could decrease the loading of Pt. An earlier theoretical study conducted at our laboratory identified PtxY as an active and stable catalyst for oxygen reduction. Experiments conducted on sputter-cleaned polycrystalline Pt3Y confirmed that it showed exceptional activity for the ORR [1]. Later studies also revealed that Pt5Y, prepared in the same manner also showed similar high activity, as shown in Figure (a) below. However, our X-ray photoelectron spectroscopy measurements revealed that the structure formed under reaction conditions was significantly different from our initial expectations. In order to understand this phenomenon, we investigated a Y/Pt(111) single crystal, formed by depositing large amounts of Y om Pt(111) under ultra-high vacuum (UHV) conditions and annealing to high temperatures. We subsequently characterised the surface using electrochemical measurements, low energy electron diffraction, ion scattering spectroscopy, angle resolved X-ray photoelectron spectroscopy, temperature programmed desorption of CO, and synchrotron based X-ray absorption spectroscopy and surface sensitive X-ray diffraction. These measurements were supported by DFT calculations. From the experimental investigations of the crystal structure, it could be concluded that we have obtained a Pt5Y structure with a 5.5 % compressive strain compared to Pt. Electrochemical measurements show that the activity approximates that of sputter-cleaned Pt3Y, as shown in Figure A. Since there is no Y in the first few atomic layers, we attribute the high activity to the compressive strain exerted onto the surface by the alloy bulk. [1] J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T.P. Johansson, H.A. Hansen, T.F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J.K. Nørskov, Nature Chemistry, 1 (2009) 552-556.