ИСТИНА

Nowadays, bimetallic catalysts attract considerable attention from many researchers because their properties may significantly differ from monometallic ones, with the mixtures often exhibiting enhanced catalytic activity, stability and selectivity. Despite significant efforts origins of synergistic effects occurring upon the introduction of a second metal into monometallic catalysts in different systems are still under debate. The addition of a second metal is known to lead to active sites with a specific geometry and modified electronic properties. Therefore, the formation of specific surface configurations of bimetallic particles is primarily responsible for the synergistic effect. Due to this reason, many researchers believe that detailed study of their surface is a key for the comprehension of such effects. The chemical composition and structure of active sites are determined at the preparation step (relative amounts of two metals being deposited, the order of deposition, temperature, etc.). However, it can spontaneously change under reaction conditions or get deliberately tuned by a specific treatment of the surface sample in the reactive gas atmosphere. Enrichments of the nanoparticle surface by one of the components upon adsorption under catalytic reaction conditions, which are referred to as adsorption-induced segregation effects, are being paid ever augmenting attention by researchers dealing with heterogeneous catalysis. Though attempts of deliberate using these effects for the modification of surface composition are still rather rare, we believe the adsorption-induced segregation effects are greatly promising for fine surface composition tuning and consequent rational optimization of catalytic properties in low-temperature reactions [1, 2]. The use of model systems, where metal particles are deposited on a 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. In this work, a series of model bimetallic Pd–In catalysts with different ratios of metals and size distributions of supported particles has been prepared by successive metal deposition onto a modified surface of highly oriented pyrolytic graphite (HOPG). X-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM) were used to characterize the electronic and structural properties, as well as the morphology of nanoparticles at all stages of catalyst preparation. It was revealed that indium deposition onto Pd/HOPG sample led to spontaneous formation of intermetallic Pd-In nanoparticles or at least partial indium diffusion into palladium particles with emergence of In-Pd surface alloy. It was found that an additional cycle of oxidation-reduction treatment is necessary in order to form the uniform Pd-In alloy species. It was found that the prepared model Pd-In/HOPG catalysts were stable against sintering up to 500 °C in UHV and at least up to 100 °C under «realistic» conditions (200 mbar O2). Using the synchrotron radiation-based XPS, it was demonstrated that long-term contact of Pd- In/HOPG samples with air led to partial decomposition of intermetallic particles with the formation of Pd0 homogeneously distributed over their bulk, and the formation of surface indium oxide and bulk indium oxide. Mild oxidative treatment (0.25 mbar O2, 150 ̊C) leads to indium surface segregation and formation of indium oxide predominantly localized in the interior and metallic Pd0 uniformly distributed over the depth. The indium oxide is distributed homogeneously at further temperature increasing to 200 ̊C. It was also demonstrated that surface composition could be tuned using the O2-induced segregation phenomenon by varying both O2 partial pressure and exposure temperature. Thus, reversible oxidative-reductive transformation PdInIMC ⇄ Pd0 + InOx can be efficiently used to deliberately tune the nanoparticles surface composition/structure and respective catalytic characteristics.