Understanding the Structural Deactivation of Ruthenium Catalysts on an Atomic Scale under both Oxidizing and Reducing Conditions

Abstract

The science and technology of catalysis are of central practical importance. About 80 % of all industrial chemicals are manufactured by utilizing (heterogeneous) catalysis. Besides activity and selectivity, catalyst deactivation during use is a key issue in practical catalysis. "The importance of understanding and being able to predict loss of activity during catalyst usage must not be under-estimated" [1] since replacement of a catalyst means high operational costs. Industrially used catalysts are, however, far too complex to allow for a microscopic understanding of why a catalyst deactivates. This knowledge calls rather for the use of model catalysts (such as single-crystalline surfaces) and their investigations under well-controlled ultrahigh vacuum conditions. The trade off for this so-called surface-science approach [2] is the introduction of a pressure and a materials gap by which catalytic properties determined under well defined conditions may not be extrapolated to those at realistic reaction conditions.[3]For a ruthenium-based catalyst, activity loss was reported for the CO oxidation reaction. In particular, under oxidizing reaction conditions the activity of supported ruthenium catalysts declines substantially. [4,5] This finding has been quite puzzling as recent investigations clearly indicate that RuO 2 is much more active than ruthenium in the oxidation of CO.[6] Since the pressure and materials gap for the CO oxidation over ruthenium are considered to be bridged [7] we can utilize the surface-science approach to clarify the microscopic processes determining the structural deactivation of ruthenium-based catalysts and how this atomic-scale knowledge is used to optimize the performance of practical ruthenium catalysts.We concentrate herein mainly on polycrystalline RuO 2 powder which is calcined at 573 K, resulting in a specific surface area of 0.9 m 2 g À1 . Complementary data of supported ruthenium catalysts are provided in the Supporting Information. The mean diameter of the particles in the RuO 2 powder is about 1 mm. Therefore RuO 2 powder represents a natural link between single-crystal ruthenium model catalysts and ruthenium catalysts supported on SiO 2 or MgO with an active surface area of 10 m 2 g À1 .[8] Applied partial pressures of CO and O 2 are in the range of 5-35 mbar.Figure 1 a displays the conversion of CO over oxidized RuO 2 polycrystalline powder as a function of time on stream, using a stepwise temperature variation and a CO/O 2 feed ratio of 1:2. During the first temperature cycle each temperature jump is accompanied by a rapid increase of the CO conversion followed by a transient decrease of the CO conversion whose steady state is not reached within 1 h. This deactivation process occurs faster at higher temperatures, whereas the extent of deactivation increases with higher concentrations of O 2 (see Supporting Information). For all investigated CO/O 2 feed stocks it is found that the conversion

Size Effect of Ruthenium Nanoparticles in Catalytic Carbon Monoxide Oxidation

“…In conjunction with this claim, Aßmann et al investigated structural deactivation of polycrystalline RuO 2 powder, and suggested that a core-shell particle composed of a RuO 2 shell layer formed on the metallic Ru core-shell is catalytically most reactive. 15 Based on this argument, they predicted that in supported Ru NP catalysts, catalytically active oxide species are more stable if the Ru particles become larger, which is in agreement with our results.…”

“…[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.…”

On the CO-Oxidation over Oxygenated Ruthenium

“…In the literature the deactivation particularly of oxidized samples is explained with an inactive state of the {110} and {100} facets, namely the reconstructed RuO 2 (100)-c(2 × 2) surface [8,10,11]. The occurrence of this reconstructed RuO 2 (100)-c(2 × 2) surface is nicely supported by our experiments: First, the irregular octagonal cross section habit of the calcined RuO 2 crystals in contrast to the obelisk-like habit shown in the literature [24] points to an equilibration of the (110) and (100) surface energies.…”

“…Whereas these aforementioned studies dealt with ruthenium single crystals, in more recent work on polycrystalline ruthenium dioxide [8] the authors proposed a core-shell model consisting of an about 1nm thick RuO 2 layer covering a metallic core as the most active state of RuO 2 in the CO oxidation. Again, the cus-oxygen was suggested as the active species, thus the authors concluded that the "pressure gap" was bridged [9,10].…”

Oxidation and reduction of Pd(100) and aerosol-deposited Pd nanoparticles

“…In the case of Pd(110), no surface oxide has been found so far [6,7]. In parallel, it has been reported that oxidation of CO to CO 2 over transition metal surfaces, is often more efficient on surfaces with a thin oxide (such as a surface oxide) than on the corresponding metallic surface [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Furthermore, in a recent theoretical paper [23], Rogal et al suggest that the surface oxide is indeed the most active phase for CO oxidation under conditions representative of technological catalysis.…”