Size Effect in Ethylene Oxidation over Silver Nanoparticles Supported on HOPG

A preparation procedure for a model catalyst – stable silver nanoparticles with a narrow size distribution supported on highly oriented pyrolytic graphite (Ag/HOPG) – is presented. This procedure is carried out under ultra high vacuum conditions and consists of three stages: 1 – surface defect formation by soft Ar+-treatment; 2 – silver deposition; 3 – defect annealing by heating at T = 300 °C. The analysis of oxygen forms on a silver surface was carried out by in-situ X-ray photoelectron spectroscopy (XPS). The simultaneous presence of two oxygen species (electrophilic and nucleophilic) on the silver surface was found to be necessary for high activity of the model catalyst in ethylene epoxidation. An alternative explanation of the size effect in ethylene oxidation has been suggested: size dependence of the reaction rate may result from the existence of a ring-shaped zone at the edge of silver particles in which the surface concentration of nucleophilic oxygen is not uniform. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3514

State of the Active Component and Catalytic Properties of Pd/C Cata- lysts on the Selective Hydrogenation of Butadiene-1,3 into Butylenes

Palladium (0.25-5.8 wt.%) was supported on three different types of filamentous carbon in which the basal graphite planes are arranged along, crosswise and at a 45° angle to the axis of a nanofiber. HREM, XPS and X-ray techniques were used to study the state of high dispersed Pd particles supported on the filamentous carbon. It was established that the type of the carbon support affects the properties of the active component. The high disperse Pd particles most strongly interact with the prismatic planes of graphite to provide electron transfer from palladium to the support, which stabilizes palladium in a more dispersed state. Because of this strong interaction, Pd2+ is stabilized in the ionic state. A change in the properties of the palladium particles initiates changes in the catalytic properties of Pd/C catalysts in the reaction of selective hydrogenation of 1,3-butadiene into butylenes. An increase in the fraction of Pd2+ in the catalyst composition results in the decrease of both total activity and selectivity of Pd/C catalysts in the considered reaction

Near Ambient Pressure XPS and MS Study of CO Oxidation over Model Pd Au HOPG Catalysts The Effect of the Metal Ratio

In this study, the dependence of the catalytic activity of highly oriented pyrolytic graphite HOPG supported bimetallic Pd Au catalysts towards the CO oxidation based on the Pd Au atomic ratio was investigated. The activities of two model catalysts differing from each other in the initial Pd Au atomic ratios appeared as distinctly different in terms of their ignition temperatures. More specifically, the PdAu 2 sample with a lower Pd Au surface ratio 0.75 was already active at temperatures less than 150 C, while the PdAu 1 sample with a higher Pd Au surface ratio 1.0 became active only at temperatures above 200 C. NAP XPS revealed that the exposure of the catalysts to a reaction mixture at RT induces the palladium surface segregation accompanied by an enrichment of the near surface regions of the two component Pd Au alloy nanoparticles with Pd due to adsorption of CO on palladium atoms. The segregation extent depends on the initial Pd Au surface ratio. The difference in activity between these two catalysts is determined by the presence or higher concentration of specific active Pd sites on the surface of bimetallic particles, i.e., by the ensemble effect. Upon cooling the sample down to room temperature, the reverse redistribution of the atomic composition within near surface regions occurs, which switches the catalyst back into inactive state. This observation strongly suggests that the optimum active sites emerge under reaction conditions exclusively, involving both high temperature and a reactive atmospher

SRPES and STM data for the model bimetallic Pd In HOPG catalysts Effects of mild post synthesis oxidative treatments

Post synthesis treatment of bimetallic catalysts in different gas phases resulting in the adsorption induced segregation is among promising approaches to enhance their activity not compromising selectivity towards a number of low temperature reactions. Our recently published paper M.A. Panafidin, A.V. Bukhtiyarov, I.P. Prosvirin, I.A. Chetyrin, A.Yu. Klyushin, A. Knop Gericke, N.S. Smirnova, P.V. Markov, I.S. Mashkovsky, Y.V. Zubavichus, A.Yu. Stakheev, V.I. Bukhtiyarov, A mild post synthesis oxidative treatment of Pd In HOPG bimetallic catalysts as a tool of their surface structure fine tuning. Appl. Surf. Sci. reports on Pd In intermetallic formation regularities and their evolution after storage in air as well as during treatment in oxygen at submillibar pressures. The current paper gives an extended representation of experimental ex situ in situ synchrotron based photoelectron spectroscopy SRPES and scanning tunnelling microscopy STM data used to derive scientific conclusions in the paper quoted abov

Pd Cu HOPG and Pd Ag HOPG Model Catalysts in CO and Methanol Oxidations at Submillibar Pressures

The regularities of formation of alloyed Pd–Cu bimetallic particles deposited on highly oriented pyrolytic graphite (HOPG) were studied by STM and synchrotron-radiation-based XPS. Their chemical composition, structure, and the ranges of thermal stability under ultrahigh vacuum were determined. The Pd–Cu/HOPG model catalysts, as well as the Pd–Ag/HOPG samples whose preparation procedure was tested earlier, exhibited catalytic activity in CO and methanol oxidations at pressures typical of in situ XPS experiments and were stable at the temperatures of the catalytic reaction