Controlled oxygen chemisorption on an alumina supported rhodium catalyst. The formation of a new metal-metal oxide interface determined with EXAFS

Martens, Jha; Prins, R.; Koningsberger, DC Diek

doi:10.1021/j100345a059

Cited by 29 publications

(14 citation statements)

References 2 publications

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“…• C as a pretreatment was considered as simple rhodium oxide because almost rhodium was easily reduced to metallic state by the reduction at the relatively low temperature of 250 • C. [4,13] The dispersed rhodium particles have size distribution about 2 nm or less for fresh catalysts. [10] The reason for existence of more than one time constants of the reduction of the supported rhodium is that the reduction rate probably depends on the size of the rhodium particle.…”

Section: Resultsmentioning

confidence: 99%

Real‐time XAFS analysis of Rh/alumina catalyst

Dohmae

Nagai

Tanabe

et al. 2008

Surface & Interface Analysis

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Reduction rates of rhodium supported on alumina were investigated with in situ time-resolved X-ray absorption fine structure (XAFS). The catalytic sample was prepared with 0.5-wt% rhodium on the alumina powder by a usual impregnation method. Two types of XAFS techniques were applied for the experiments: energy-dispersive XAFS and usual step-scanning XAFS. The reduction rates of the rhodium supported on alumina after oxidizing treatment at 500• C were measured with dispersive XAFS. The reducing condition was with 3% H 2 in He at various temperatures from 100 to 250 o C. The spectra of Rh K-edge X-ray absorption near edge structure (XANES) were acquired every 0.2 s. The oxidation state of the rhodium was evaluated by the energy shift of Rh K-edge. The reduction rate of the rhodium was on a line in an Arrhenius plot. Reduction of the rhodium on the alumina in 3% H 2 in He at 900• C after high temperature oxidization at 900 and 1000• C were measured with step-scan XAFS. Though a portion of the rhodium was reduced in a few minutes, the residual rhodium was hardly reduced. The hardly reducible rhodium was considered to be reacted with the alumina support and lost catalytic activities. The ratio of the deactivated rhodium has been estimated as a function of the oxidization temperature and period.

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Section: Resultsmentioning

confidence: 99%

Real‐time XAFS analysis of Rh/alumina catalyst

Dohmae

Nagai

Tanabe

et al. 2008

Surface & Interface Analysis

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“…Such mechanisms are well established for noblemetal particles of a wide variety of sizes and situations. [18][19][20] In such a case, the variation in CN can simply reflect a progressively smaller metallic Pd core surrounded by a progressively larger oxide layer. To be sure of what we are actually observing we turn to a quantitative principle component analysis (PCA) [21] using the XANES (X-ray absorption near-edge structure) region of the dispersive EXAFS to elucidate the levels of oxidation to Pd 2+ that are found in each case during the oxidizing cycle.…”

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confidence: 99%

“Oxidationless” Promotion of Rapid Palladium Redispersion by Oxygen during Redox CO/(NO+O₂) Cycling

et al. 2007

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The study of the behavior of palladium, and noble metals in general, in respect of fundamental gas-metal interactions and reactive chemistry, has attracted a great deal of attention over a great many years. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] The intrinsic importance of a range of commercial applications of palladium-not least of which being automotive exhaust catalysis-coupled to a fundamental desire to understand the basic physics and chemistry of such interactions, are the prime drivers for this interest. In the exhaust catalysis application, the catalyst has to operate in conditions wherein the redox potential of the feedstock can change in a highly dynamic manner. Further, the interplay between oxidized and reduced phases of palladium is of central importance to palladium activity in methane oxidation catalysis. [8][9][10] As such, a comprehension of the dynamic redox behavior of Pd and other noble metals is of considerable importance.Recently, we demonstrated the presence of a highly dynamic size/shape variation occurring during CO/NO cycling over 1 wt % Pd/10 % (Ce,Zr)O x /Al 2 O 3 catalysts. [16] Part of this phenomenon comprises a non-oxidative, palladium-redispersion mechanism. Herein we contrast the behavior of chloride-free 2 wt % Pd/Al 2 O 3 catalysts at 673 K during redox cycling using CO as the reductant, and in the absence (NO only) and presence (5 O 2 :1 NO) of O 2 during the oxidizing part of the cycle. We show that, even in the presence of a relatively large amount of oxygen, oxidation cannot compete kinetically with the redispersion events. On the contrary, significant levels of O 2 in the oxidizing feed greatly enhance the degree of palladium redispersion possible before any formal oxidation of Pd 0 to Pd 2+ can be observed. Figure 1 shows the variation in observed Pd-Pd coordination number (CN) derived from analysis of energy-dispersive EXAFS (EDE) data (EXAFS = extended X-ray absorption fine structure spectroscopy; see also Supporting Information) in the two cases studied: the first (Figure 1, open circles) is obtained during cycling of a 2 wt % Pd/Al 2 O 3 sample between flows of 5 % CO/He and 5 % NO/He at 673 K. The second (Figure 1, filled circles) shows results obtained when the total flow in the oxidizing cycle now comprises a mix of 5 O 2 :1 NO. Figure 2 shows the result of translating this raw CN data into an average number of Pd atoms per particle. This translation is done within the framework proposed by Jentys [17] and, as such, a constant particle morphology and 0 % Pd oxidation is assumed.The overall behavior during CO/NO cycling over a 2 wt % Pd/Al 2 O 3 sample is as observed previously.[16] The presence of the CeZr phase is therefore not a prerequisite and these transformations appear intrinsic to Pd nanoparticles. It further appears that, even though there are considerable error

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“…These nominally 20 ._, diameter particles possess a significant number of surface atoms (30%) occupying a variety of low-coordination sites (i.e., highly exposed edges and comers) [18]. With lower coordination, the binding energies are smaller so that these sites can be oxidized at lower potentials [19]. It is known that different crystalline faces (i.e., [111] or [100]) of bulk Pt electrodes have different potentials for oxidation [20,21].…”

Section: Resultsmentioning

confidence: 99%

Real Time Structural Electrochemistry of Platinum Clusters using Dispersive XAFS

et al. 1993

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Chemical reference tables state that the standard potential for the reaction of Pt with water, Pt + 2H2O → Pt(OH)2 + 2H+ + 2e−, is 0.98 V, and electrochemical studies propose that this reaction may occur at potentials as low as 0.8 V [1–5]. Using dispersive x-ray absorption finestructure (XAFS) spectroscopy [6], we have directly probed the structural evolution of a Pt catalyst operating in-situ in a polymer electrolyte [7] fuel cell during cyclic voltammetry. The changes in the number of Pt and O nearest-neighbors and the Pt charge demonstrate a close correspondence with features in the voltammogram. Because dispersive XAFS is very sensitive to detecting structural changes, we have been able to detect the presence of chemisorbed oxygen at potentials of 0.6–0.9 V in the anodic sweep. Since double-layer charging is regarded as the only process in this region for bulk Pt [3–5], these results may reflect a limitation of previous (indirect) studies on Pt electrochemistry, or they may indicate that these clusters are different from their bulk metal counterparts. Exploiting the timeresolving capability of dispersive XAFS, we also monitored changes in the Pt charge and the number of O and Pt nearest-neighbors during the electrochemical oxidation and reduction of the Pt clusters in real-time. The results are inconsistent with those expected from the placeexchange mechanism [8–11] for the formation of the surface oxide on bulk Pt electrodes in aqueous solution;Our current model for understanding these behaviors is that, relative to bulk Pt, unusual types of surface sites play a major role in determining the reactivity of these clusters.

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Controlled oxygen chemisorption on an alumina supported rhodium catalyst. The formation of a new metal-metal oxide interface determined with EXAFS

Cited by 29 publications

References 2 publications

Real‐time XAFS analysis of Rh/alumina catalyst

Real‐time XAFS analysis of Rh/alumina catalyst

“Oxidationless” Promotion of Rapid Palladium Redispersion by Oxygen during Redox CO/(NO+O₂) Cycling

Real Time Structural Electrochemistry of Platinum Clusters using Dispersive XAFS

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Controlled oxygen chemisorption on an alumina supported rhodium catalyst. The formation of a new metal-metal oxide interface determined with EXAFS

Cited by 29 publications

References 2 publications

Real‐time XAFS analysis of Rh/alumina catalyst

Real‐time XAFS analysis of Rh/alumina catalyst

“Oxidationless” Promotion of Rapid Palladium Redispersion by Oxygen during Redox CO/(NO+O2) Cycling

Real Time Structural Electrochemistry of Platinum Clusters using Dispersive XAFS

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“Oxidationless” Promotion of Rapid Palladium Redispersion by Oxygen during Redox CO/(NO+O₂) Cycling