Model Bimetallic Pd-Ag/HOPG and Pd-Cu/HOPG Catalysts: CO and Methanol Oxidations
Currently, bimetallic catalysts attract considerable attention of many researchers because their properties may significantly differ from monometallic ones, with the mixtures often exhibiting enhanced catalytic activity, stability and selectivity. However, the nature of synergistic effects in different catalytic reactions is still under debate. The addition of a second metal is known to lead to the formation of active sites with a specific geometry (ensemble effect) and modification of electronic properties of the active metal (ligand effect). Therefore, the formation of the specific
surface composition of bimetallic particles plays a crucial role in the synergistic effect. The use of the model systems, where metal particles are deposited on the planar support, together with in situ techniques can increase the information content and reliability of the results regarding the surface structure and chemical composition of active metal particles and their evolution in response to different treatment modes and reaction conditions.
A series of model bimetallic Pd–Ag and Pd–Cu catalysts with different ratios of metals and size distributions of supported particles was prepared by successive metal deposition onto a modified surface of highly oriented pyrolytic graphite (HOPG). X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the structural and electronic properties, as well as the morphology of nanoparticles at all stages of catalyst preparation. The use of the method made it possible to obtain bimetallic particles with various structures: alloyed, core–shell, and those consisting of individual monometallic particles. Using the synchrotron radiation-based XPS was shown that Pd–Ag and Pd–Cu alloys form when the particles deposited on HOPG were heated to ~450°C under high vacuum conditions irrespective of their initial structure (order of metal deposition). A mass-spectrometric study showed that the Pd–Ag/HOPG and Pd–Cu/HOPG bimetallic model catalysts exhibited catalytic activity in CO and methanol oxidations. The studies of the samples with different Pd/Me ratios on the surface of bimetallic particles demonstrated that the activity of the Pd–Ag/HOPG and Pd–Cu/HOPG catalysts depends on the initial ratio of metals on the surface of the planar support. The Pd–Ag/HOPG and Pd–Cu/HOPG samples are thermally stable under the
experimental conditions of the present work and can be used for subsequent in situ studies of the chemical state of metals during the catalytic reaction. We are grateful to the German-Russian Interdisciplinary Scientific Center (G-RISC) funded by the Federal Ministry of Foreign Affairs of Germany via the German Academic Exchange Service (DAAD) (Project T-2018a-1).