On the Nature of the Active State of Supported Ruthenium Catalysts Used for the Oxidation of Carbon Monoxide:  Steady-State and Transient Kinetics Combined with in Situ Infrared Spectroscopy

Abstract

The oxidation of CO over Ru/MgO and Ru/SiO2 catalysts was used as a simple model reaction to derive turnover frequencies at atmospheric pressure, which were observed to agree with kinetic data obtained under high-vacuum conditions with supported ruthenium catalysts and the RuO2(110) single-crystal surface. Thus, it was possible to bridge both the pressure and the materials gap. However, a partial deactivation was observed initially, which was identified as an activated process, both under net reducing and net oxidizing conditions. Temperature-programmed reduction (TPR) experiments were performed subsequently in the same reactor, to monitor the degree of oxidation, as a function of the reaction temperature and the CO/O2 reactant feed ratio. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, the structural changes of the ruthenium catalysts during the oxidation of CO were confirmed, under relevant reaction conditions. Under net reducing conditions, only domains of RuO2 seem to exist on the metallic ruthenium particles, whereas, under net oxidizing conditions, the ruthenium particles were fully oxidized to bulk RuO2 particles, which may expose less-active facets, such as the RuO2(100)−c(2 × 2) surface.

On the CO-Oxidation over Oxygenated Ruthenium

“…Since it is obviously very ambiguous, if not impossible, to grow solely one single surface oxygen-phase on Ru(0001) except for RuO 2 at very high temperatures and oxygen dosages, it is interesting to analyze catalytic experiments with fully oxidized RuO 2 . Several papers dealt with re-oxidized Ru particles as "core-shell" catalysts with a thin RuO 2 layer on a Ru metal core [10,24,25]. These catalysts should exhibit the highest formation rates of CO 2 [10].…”

Size Effect of Ruthenium Nanoparticles in Catalytic Carbon Monoxide Oxidation

“…[5][6][7][8][9][10][11] By contrast, studies of supported Ru NP catalysts for this reaction have been sporadically reported. [12][13][14][15] Particle-size dependence of CO oxidation, the main focus of this study, has not yet been investigated. The influence of metal particle size on catalytic reactivity has been a subject of continuous interest, due to its significance from fundamental and practical viewpoints.…”

Preparation of Nanoporous Ruthenium Catalyst and Its CO Oxidation Characteristics

“…The E a determined from the Arrhenius plots was 82 kJ mol ¹1 , and is similar to those obtained in previous CO oxidation experiments with various Ru catalysts, summarized by Assmann et al 41) including single crystal Ru, 2) polycrystalline Ru 42) and supported Ru nanoparticles. 9,43) Various factors affect the activity of catalysts, including the catalyst surface oxidation state which is crucially important for CO oxidation over Ru catalysts.…”

“…However, further analyses such as in-situ infrared spectroscopy are required to elucidate the CO oxidation mechanism over np-Ru, because the metallic Ru surface oxidation state is very sensitive to the atmosphere. 9,10,41) On the other hand, a measurable amount of residual Mn was also detected by XPS [ Fig. 8(b)], although strict elemental composition cannot be calculated due to overlap of Ru 3d and C 1s peaks, unfortunately.…”