Rhodium Nanoparticle Shape Dependence in the Reduction of NO by CO

The shape dependence of the catalytic reduction of nitric oxide by carbon monoxide on rhodium nanopolyhedra and nanocubes was studied from 230 to 270 degrees C. The nanocubes are found to exhibit higher turnover frequency and lower activation energy than the nanopolyhedra. These trends are compared to previous studies on Rh single crystals.Chemistry, PhysicalSCI(E)EI21ARTICLE3-4317-32213

CO oxidation on ruthenium: The nature of the active catalytic surface

The oxidation of CO, i.e. CO + ½O2 → CO2, over metal surfaces is one of the most studied catalytic reactions. The details of the reaction mechanism under ultrahigh vacuum (UHV) conditions have been well understood for some time [1]. Under reducing or mildly oxidizing conditions for Pt, Pd, and Rh, metals used in automotive catalytic converters, the reaction proceeds via the Langmuir–Hinshelwood (L–H) mechanism between CO molecules and O atoms, both strongly chemisorbed to the metal surface. Under such conditions, the surface is predominantly covered by adsorbed CO and the reaction rate is controlled by the rate at which desorption of CO opens adsorption/dissociation sites for O2. The measured reaction kinetics at reducing or mildly oxidizing conditions for Pt, Pd, and Rh support this mechanistic picture showing a first order dependence on O2 pressure and a negative first-order dependence on CO pressure, with an apparent activation energy for CO2 formation on each metal approximately equal to the corresponding CO desorption energy [2]. On the other hand, CO oxidation on Ru, in particular the nature of the active catalytic surface, is controversial and the subject of this letter. Here we discuss whether the active surface at near atmospheric pressures and at typical reaction temperatures is a one monolayer oxygen-covered Ru surface or multilayer RuO2

Reply to comment on 'CO oxidation on ruthenium: The nature of the active catalytic surface' by H Over, M Muhler, AP Seitsonen

The premise of our Letter [1] was to state emphatically that under the conditions of the early studies of Peden and Goodman (PG) [2] RuO2 cannot be formed as the main active phase on Ru(0 0 0 1) as proved by recent SXRD studies [3], and that a monolayer oxygen-covered Ru(0 0 0 1) surface is the active phase for CO oxidation in the early PG studies. This monolayer oxygen-covered surface has recently been found on Rh, Pd and Pt to be significantly more active for CO oxidation than a predominately CO-covered metallic surface or surface oxide/bulk oxide with an oxygen coverage higher than one monolayer [4]. Contrary to the authors’ contention in their Comment [5], the reaction conditions addressed in the experiments of PG [2] spanned the temperature range 375–600 K, a pressure range of O2 and CO pressures from 0.5 to 500 Torr, and CO/O2 ratios from 60 to 0.03. The authors state in their introduction [5] that “the oxidation of Ru metal takes place to RuO2 only for temperatures higher than 500 K. Here we do not wish to address the oxidation of Ru in all morphologies, but rather the specific reaction conditions required for RuO2 to form on Ru(0 0 0 1), the catalyst used in the experiments of PG and the subject of our previous Letter [1]. Indeed, as pointed out in our Letter [1], Over and coworkers in a very definitive piece of work [3] showed that the critical conditions required for the formation of RuO2 on Ru(0 0 0 1) were far outside those used in the experiments of PG with respect to temperature and oxygen chemical potential. Based on our earlier studies [2] and the work of Over and coworkers [3] we concluded in our Letter that RuO2 could not have formed to any appreciable extent in the PG experiments

Characterization of NOx species in dehydrated and hydrated Na- and Ba-Y FAU zeolites formed in NO2 adsorption

Adsorbed ionic NO2 species formed upon the interaction of NO2 with dehydrated or hydrated Na- and Ba-Y, FAU zeolites were characterized using FT-IR/TPD, solid state NMR, and XANES techniques. NO2 disproportionates on both dehydrated catalyst materials forming NO+ and NO3- species. These ionic species are stabilized by their interactions with the negatively charged zeolite framework and the charge compensating cations (Ne+ and Ba2+), respectively. Although the nature of the adsorbed NOx species formed on the two catalysts is similar, their thermal stabilities are strongly dependent on the charge compensating cations. In the presence of water in the channels of these zeolite materials new paths open for reactions between NO+ and H2O, and NO2 and H2O, resulting in significant changes in the adsorbed ionic species observed. These combined spectroscopic investigations afforded the understanding of the interactions between water and NO2 on these zeolite catalysts. (c) 2005 Elsevier B.V. All rights reservedclose111

Morphological evolution of Ba(NO3)(2) supported on alpha-Al2O3(0001): An in situ TEM study

A key question for the BaO-based NOx storage/reduction catalyst system is the morphological evolution of the catalyst particles during the uptake and release of NOx. Notably, because the formed product during NOx uptake, Ba(NO3)(2), requires a lattice expansion from BaO, one can anticipate that significant structural rearrangements are possible during the storage/reduction processes. Associated with the small crystallite size of high-surface area gamma-Al2O3, it is difficult to extract structural and morphological features of Ba(NO3)(2) supported on gamma-Al2O3 by any direct imaging method, including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)(2) particles supported on single-crystal alpha-Al2O3, we have investigated the structural and morphological features of Ba( NO3)(2) as well as the formation of BaO from Ba(NO3)(2) during the thermal release of NOx, using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)(2) supported on alpha-Al2O3 possesses a platelet morphology, with the interface and facets being invariably the eight {111} planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)(2) and alpha-Al2O3, indicating that the interfacial energy is lower than the Ba( NO3) 2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is similar to 1/4 of the {111} surface free energy of Ba(NO3)(2). The orientation relationship between Ba(NO3)(2) and the alpha-Al2O3 is alpha-Al2O3[0001]// Ba(NO3)(2)[111] and alpha-Al2O3(1- 210)// Ba( NO3) 2( 110). Thus, the results clearly demonstrate dramatic morphology changes in these materials during NOx release processes. Such changes are expected to have significant consequences for the operation of the practical NOx storage/reduction catalyst technologyclose4

Synthesis, characterization, and catalytic function of novel highly dispersed tungsten oxide catalysts on inesoporous silica

The physical and chemical properties of tungsten oxide supported on SBA-15 rnesoporous silica prepared by a controlled grafting process through atomic layer deposition (ALD) were studied using complementary characterization methods. X-ray diffraction, optical absorption, and transmission electron microscopy showed that tungsten oxide species are highly dispersed on SBA-15 surfaces, even at 30 wt% WO, content (surface density, 1.33 WOx/nm(2)). ALD methods led to samples with much better thermal stability than those prepared via impregnation. Dehydration reactions of 2-butanol and methanol dehydration were used as probe reactions. Differences in reaction rates between the samples prepared by ALD and conventional impregnation may reflect the sintering resistance of catalysts prepared by ALD. Notably, temperature-programmed oxidation of spent catalysts showed that carbon formation was not responsible for the different dehydration rates in samples prepared by ALD and impregnation. (c) 2006 Elsevier Inc. All rights reservedclose778

NO Adsorption and Reaction on Aged Pd–Rh Natural Gas Vehicle Catalysts: A Combined TAP and Steady-State Kinetic Approach

SSCI-VIDE+ING+YSC:CMIInternational audienceA combined temporal analysis of products (TAP) and steady-state kinetic study was achieved to characterize the surface reactivity of fresh and aged bimetallic Pd-Rh/Al2O3 Natural-Gas Vehicle catalysts. Single NO pulse TAP experiments were performed on a stabilized surface after exposure to successive NO pulses until to get a steady-state NO conversion. Outlet flow curves recorded during such experiments show fast reaction steps taking place on noble metal particles and a slow process during NO desorption ascribed to the involvement of spill-over effect of chemisorbed NO molecules from the metal to the support. This slow process attenuates on the aged sample likely due to an alteration of the metal/support interface induced by particle sintering at high temperature. Thermal aging also alters the surface composition of bimetallic Pd-Rh particles which leads to changes in the products distribution from NO dissociation. A similar selectivity behavior is observed from steady-state kinetic measurements during the NO/H-2 reaction. Interestingly, a weak partial pressure dependency of the selectivity reflects a surface Rh enrichment of Pd-Rh particles during aging