Supported Rh Particles Research Articles

Imaging Interface and Particle Size Effects by In Situ Correlative Microscopy of a Catalytic Reaction.

The catalytic behavior of Rh particles supported by three different materials (Rh, Au, and ZrO2) in H2 oxidation has been studied in situ by correlative photoemission electron microscopy (PEEM) and scanning photoemission electron microscopy (SPEM). Kinetic transitions between the inactive and active steady states were monitored, and self-sustaining oscillations on supported Rh particles were observed. Catalytic performance differed depending on the support and Rh particle size. Oscillations varied from particle size-independent (Rh/Rh) via size-dependent (Rh/ZrO2) to fully inhibited (Rh/Au). For Rh/Au, the formation of a surface alloy induced such effects, whereas for Rh/ZrO2, the formation of substoichiometric Zr oxides on the Rh surface, enhanced oxygen bonding, Rh-oxidation, and hydrogen spillover onto the ZrO2 support were held responsible. The experimental observations were complemented by micro-kinetic simulations, based on variations of hydrogen adsorption and oxygen binding. The results demonstrate how correlative in situ surface microscopy enables linking of the local structure, composition, and catalytic performance.

Reaction and Diffusion Paths of Water and Hydrogen on Rh Covered Black Titania

The reactions of H2O, H2 D2 and CO with clean and rhodium covered black titania have been investigated by TDS, AES and sensitive temperature programmed work function (TP-WF) measurements to elucidate the complex interactions with this narrow bandgap material promising for visible light energy harvesting. Water formed molecular and dissociative adsorption states with positive outward dipole moments on the reduced, r–TiO2 (110). Surface hydroxyl groups decomposed to H2 and recombined to H2O in a broad temperature range, characterized by TDS peaks at 300, 355–377 and 470 K, which has been associated with surface inhomogeneity. On a strongly reduced, sr–TiO2 (110), a part of H atoms arising from OHa species dissolved in the titania at 200–500 K, and desorbed as H2O with Tp = 570, 670 and 750 K. Sub-monolayer TiOx films produced by stepwise heating on r–TiO2 (110) supported Rh particles suppressed the adsorption of hydrogen, but allowed its spillover to the support. Co-adsorption experiments with D2, H2, H2O and CO on the Rh covered r–TiO2 (110) were also performed. Saturating the Rh by CO at 330 K blocked the uptake of hydrogen on the metal, eliminating its spillover to the support. At 270 K saturation CO exposure removed the pre-adsorbed hydrogen, while at 200 K replaced a part of it decreasing the adsorption bond energy of the rest remained adsorbed. Co-adsorption data proved that the hydrogen desorption states with Tp = 470 and 570 K belong to the decomposition of hydroxyl groups on the r–TiO2 (110) support and at the Rh–TiOx interface, respectively. It is remarkable that the latter state can evolve in the presence of adsorbed CO, which exhibits reactivity towards the same interface.

Time-resolved, in situ DRIFTS/EDE/MS studies on alumina-supported rhodium catalysts: effects of ceriation and zirconiation on rhodium-CO interactions.

The effects of ceria and zirconia on the structure–function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ-Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ-Al2O3. The structure–function correlations of ceria and/or zirconia-doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive extended X-ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ-Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.

A high pressure X-ray photoelectron spectroscopy study of oxidation and reduction of Rh(100) and Rh nanoparticles

We have studied the oxidation and reduction of Rh(100) and SiO2 supported Rh particles using high pressure X-ray photoelectron spectroscopy. We show that the formation and reduction of Rh bulk oxide can be followed in situ in O2 and CO pressures in the range of 0.1Torr. In general, the oxidation/reduction processes are similar on Rh(100) and the nanoparticles, but there are significant differences in temperature dependence. Already at a sample temperature of 140°C, the particles show clear signs of a thin bulk oxide, while an ultra-thin so-called surface oxide covers the single crystal at the same temperature. Both of these oxide films, however, hinder further oxidation, and a thick oxide is only found at a temperature of at least 300°C, for both samples. The reduction, in contrast, starts at a higher temperature on the particles as compared to the single crystal, but once started the particles are completely reduced at lower temperatures.

Atomistic Theory of Ostwald Ripening and Disintegration of Supported Metal Particles under Reaction Conditions

Understanding Ostwald ripening and disintegration of supported metal particles under operating conditions has been of central importance in the study of sintering and dispersion of heterogeneous catalysts for long-term industrial implementation. To achieve a quantitative description of these complicated processes, an atomistic and generic theory taking into account the reaction environment, particle size and morphology, and metal-support interaction is developed. It includes (1) energetics of supported metal particles, (2) formation of monomers (both the metal adatoms and metal-reactant complexes) on supports, and (3) corresponding sintering rate equations and total activation energies, in the presence of reactants at arbitrary temperature and pressure. The thermodynamic criteria for the reactant assisted Ostwald ripening and induced disintegration are formulated, and the influence of reactants on sintering kinetics and redispersion are mapped out. Most energetics and kinetics barriers in the theory can be obtained conveniently by first-principles theory calculations. This allows for the rapid exploration of sintering and disintegration of supported metal particles in huge phase space of structures and compositions under various reaction environments. General strategies of suppressing the sintering of the supported metal particles and facilitating the redispersions of the low surface area catalysts are proposed. The theory is applied to TiO(2)(110) supported Rh particles in the presence of carbon monoxide, and reproduces well the broad temperature, pressure, and particle size range over which the sintering and redispersion occurred in such experiments. The result also highlights the importance of the metal-carbonyl complexes as monomers for Ostwald ripening and disintegration of supported metal catalysts in the presence of CO.

New insights into catalytic CO oxidation on Pt-group metals at elevated pressures

Abstract Producing a definitive picture of the CO oxidation reaction (CO + O 2 → CO 2 ) on Pt-group metals (Rh, Pd, Pt, and Ru) across the ‘pressure gap’ has proved to be a challenging task. Surface-sensitive techniques amenable to high pressure environments (e.g. PM-IRAS) have sparked a renewed interest in this reaction under realistic pressures. Here, we review recent work in our laboratory examining CO oxidation kinetics on Pt-group single crystals using PM-IRAS, XPS, and mass spectrometry from low (10 −8 –10 −3 Torr) to high (1–10 2 Torr) pressures. These studies have shown that at both low and high pressures (a) Langmuir–Hinshelwood kinetics adequately describe CO oxidation kinetics on Pt-group metals (Pt, Pd, Rh) (i.e. there is no pressure gap) and (b) the most active surface is one with minimal CO coverage. Additionally, recent investigations of high pressure CO oxidation kinetics on SiO 2 film supported Rh particles prepared in situ are discussed.

Promotion Effects in the Oxidation of CO over Zeolite-Supported Rh Nanoparticles

Rh particles with an average diameter smaller than 1.5 nm have been supported on a series of zeolite Y samples. These zeolite materials contained different monovalent (H+, Na+, K+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations and were used as model systems to investigate the effect of promoter elements in the oxidation of CO over supported Rh particles in excess of oxygen. Infrared (IR) spectroscopy was carried out to monitor the electronic changes in the local environment of Rh-adsorbed CO. It was found that the bands corresponding to two Rh gem-dicarbonyl species, Rh+(CO)2−(Oz)2 and Rh+(CO)2−(Oz)(H2O), shift to lower wavenumbers with increasing ionic radius/charge ratio of the cation. In addition, the relative intensity of the bridge bonded CO as compared to the total absorbance of Rh-bonded CO species decreases with increasing Lewis acidity, as expressed by the Kamlet−Taft parameter α of the cation. This trend could be directly correlated to the Rh CO oxidation activity, since low temperatures at 50% CO conversion corresponded with catalyst materials with a high contribution of bridge-bonded CO species and hence with small α values. A lower Lewis acidity causes an increased electron density on the framework oxygen atoms and thus an increased electron density on the zeolite-supported Rh particles. Comparable trends have been observed previously on a similar series of cation containing zeolite supported Pt catalyst materials.

MgO-Supported Rhodium Particles and Films: Size, Morphology, and Reactivity

The chemical transformations of supported Rh particles, ranging in size from a few micrometers to a few nanometers, and nanocrystalline Rh films have been studied under identical oxidizing and reduction conditions by means of scanning photoelectron microscopy (SPEM), which allows determination of the chemical state of single Rh microparticles. Comparing the oxidation states attained by the Rh particles revealed substantial reactivity differences with particle size, as well as variations in the reactivity of particles with similar dimensions and within the same particle. The results are interpreted in terms of the variable morphology of the particles as verified by secondary electron microscopy and atomic force microscopy.

Surface oxides on supported Rh particles: thermal decomposition of Rh oxide under high vacuum conditions

Reduction properties of alumina-supported Rh catalysts together with a polycrystalline Rh foil were studied by a stepwise thermal decomposition in high vacuum. To investigate the progress of the oxide layer decomposition in detail, the samples were annealed in vacuum at various temperatures. After each annealing step, the changes in the chemical state of the noble metal were detected by XPS measurements. Our results show marked differences in the rate of decomposition between catalysts of different Rh loadings. In thermal decomposition, the formation of molecular oxygen and the subsequent desorption of O 2 decreases the oxygen content within the information depth of the XPS measurement. On the other hand, diffusion of oxygen from the deeper layers brings more oxygen to the selvedge. The observed decomposition properties are found to correlate with the particle size. In small particles the average diffusion length of oxygen atoms is shorter than in larger particles and the surface area is larger. These factors favor the decomposition of smaller particles in comparison with larger ones.

Magnetism of small supported rhodium particles studied by means of 103Rhnuclear magnetic resonance

We present experimental evidence from 103Rh nuclear spin–lattice relaxationrates that small (average diameter 2.6 nm) supported Rh particles showantiferromagnetic susceptibility enhancement at temperatures below 80 K.Slightly larger particles (3.6 nm) have a density of states at the Fermi level onsurface sites that is higher than that in the bulk.

Temperature programmed desorption study of the reaction of C 2H 4 and CO on Rh supported on α-Al 2O 3(0001), YSZ(100) and CeO 2 thin films

The reaction of CO and ethylene on oxide-supported Rh particles was studied using temperature programmed desorption. The model systems investigated consisted of Rh particles on the following oxide supports: α-Al 2O 3(0001), yttria-stabilized zirconia (100) [YSZ(100)], CeO 2 thin film on α-Al 2O 3(0001) and CeO 2 thin film on YSZ(100). The results of this study demonstrate that both the ceria and YSZ supports are able to donate oxygen for reaction with carbon species adsorbed on the supported Rh particles. The reducibility of the ceria thin films was found to be influenced by the underlying oxide supports. Ceria films supported on YSZ(100) underwent reduction upon heating to 900 K, while those on α-Al 2O 3(0001) did not.

Oscillating Behavior in N2O Decomposition over Rh Supported on Zirconia-Based Catalysts: The Role of the Reaction Conditions

Rhodium particles supported on zirconia doped with neodymium oxide show an oscillating behavior in N2O decomposition in the presence of oxygen and water, but the oscillating behavior occurs only when the zirconia surface is hydrated. The phenomenon is ascribed to a dynamic process of surface accumulation of atomic oxygen (produced by N2O dissociation) on the Rh surface and reconstruction of these supported Rh particles above a level of oxygen coverage, with a rapid drop in the activity and release of gaseous dioxygen. This in situ reconstruction of Rh particles, however, may occur only on a hydrated surface.

The Effect of Li on Structure of Supported Rh Particles in Zeolite

We prepared Rh ion exchanged Y-type zeolite (RhY) and Li-doped RhY (Li/RhY) catalysts and studied the structure transformation of Rh intercalated species during CO adsorption and CO2 hydrogenation by EXAFS and TEM. We found that Rh particles in RhY initially supported inside the zeolite cage as 1.3 nm spherical or hemispherical particles. CO exposure of the fresh RhY caused formation of atomically dispersed Rh species. After CO2 hydrogenation and following reduction, the Rh particles aggregated to form 3.3 nm particles on the outer surface. Meanwhile, Rh particles in Li/RhY were embedded in the zeolite cages, which were narrowed by Li deposition on the wall. The size of the Rh particles was 0.8 nm in diameter. The Rh particles in Li/RhY were stable under reduction, CO adsorption, and CO2 hydrogenation conditions. We concluded Li additive changed the stability of Rh particles in the supercage by modifying surface hydroxyl group concentration.

An examination of praseodymia as an oxygen‐storage component in three‐way catalysts

The redox properties of praseodymia have been compared to those of ceria using temperature‐programmed desorption (TPD) of O2 from films prepared by vapor deposition onto an α‐Al2O3(0001) support, in order to assess the potential for praseodymia as an oxygen‐storage component in three‐way catalysts. Desorption of O2 was observed between 700 and 1000 K, and diffraction measurements confirmed that the films change from Pr6O11 to Pr2O3 in this temperature range. However, once reduced, the films could not be reoxidized by exposures to O2 in the vacuum system. Evidence was found for oxygen transfer from Pr6O11 to supported Rh particles. Following CO adsorption on Rh particles deposited onto Pr6O11 film, significant amounts of CO2 were observed in TPD until the film was completely reduced to Pr2O3. Following deposition of Rh particles onto Pr6O11, the oxygen desorption curves became identical to those normally observed for Rh. These results are compared with similar measurements performed on ceria films and the implications for automotive, emission‐control catalysis are discussed.

Molecular beam study of CO and O 2 sticking coefficients on Rh model catalysts

The adsorption of CO and O 2 on Rh foil and Rh small particles supported by an Al 2O 3 polycrystalline substrate were investigated by means of a molecular beam technique. The supported Rh particles of different sizes were prepared by evaporation using the principle of electron bombardment. The Rh foil used was plane annealed and Ar + ion bombarded. The effect of Rh surface morphology on the results is discussed. All the sticking coefficients obtained on small particles show that the part of adsorbed molecules reach the Rh particles by diffusion across the Al 2O 3 substrate. The diameters of the diffusion zones are calculated.

Structural Characterization of Rhodium-Containing Hydrodesulfurization (HDS) Catalysts Derived from a Laser Vaporization Cluster Source

A series of highly dispersed Rh-based materials have been prepared by deposition of laser-generated gas-phase clusters onto an industrial-grade alumina. While the diameter of the gas-phase metal clusters could be kept at or below 1 nm, the average diameter of the resulting supported Rh particles was always at least 1.5 nm. The reaction of the clusters with either O2or C2H4failed to prevent Rh aggregation on the alumina surface. The addition of O2upstream of the Rh target was sufficient to form bulk Rh2O3, whereas downstream addition of O2or C2H4to the Rh clusters was insufficient to form Rh oxide particles. Most of the Rh-containing particles further aggregated during either sulfiding or HDS testing. Nevertheless, the resulting rhodium sulfides were comparable in activity to sulfided, commercial Co–Mo or Ni–Mo catalysts on a per gram of catalyst basis and superior on a per gram of metal basis. At low loadings (0.1 wt%), the cluster-derived catalysts were more hydrogen-efficient than the commercial catalysts.

Support effects in the dissociation of CO on Rh/CeO2(111)

The adsorption and reaction of CO on Rh particles supported on stoichiometric and partially reduced CeO2(111) surfaces was studied using a combination of HREELS and TPD. A fraction of the CO adsorbed on the supported Rh particles was found to undergo dissociation to produce adsorbed C and O atoms. TPD results for isotopically labeled CO demonstrated that O atoms produced by CO dissociation rapidly exchange with the oxygen in the ceria lattice. The fraction of adsorbed CO which dissociated was found to increase significantly with the extent of reduction of the CeO2(111) surface, suggesting that oxygen vacancies on the surface of the support play a direct role in the CO dissociation reaction.

Characterization of ceria films on α-Al 2 O 3 (0001) and polycrystalline zirconia using O 2 TPD with labeled 18 O 2

The nature of weakly bound oxygen on ceria films was studied using temperature-programmed desorption with labeled 18O2. For α- Al2O3(0001)-supported ceria, a desorption feature between 800 and 1300 K is shown to result from partial reduction of ceria. However, this oxygen accounts for only a small fraction of the total oxygen in the ceria film and isotopic labeling studies suggest that this oxygen does not exchange freely with the remaining oxygen in the film. In contrast, results for zirconia-supported ceria demonstrate that much more oxygen desorbs in the low temperature regime below 1300 K, and that there is significant isotope exchange throughout the ceria film and with the zirconia substrate. Finally, exposure of reduced, zirconia-supported ceria to water at 670 K resulted in reoxidation of the ceria film. Oxygen from ceria was then shown to react with CO adsorbed on supported Rh particles, completing the catalytic cycle for the water-gas-shift reaction.

Size effect study of carbon monoxide oxidation by Rh surfaces

The catalytic oxidation of CO on Rh foil and small Rh particles supported by Al 2O 3 polycrystalline surfaces was studied by the transient experiment of molecular beam surface scattering technique. The supported Rh particles were prepared by evaporation, using the principle of electron bombardment. The results show the dependence of the CO 2 production rate on surface morphology and particle size. The defect rich Rh foil surface and smaller particles were found to be more active. The high oxygen adsorption capacity of 2.5 nm particles as well as their time dependent form of the CO 2 production rate is explained by the oxygen diffusion into the subsurface region.

Study of CO desorption and dissociation on Rh surfaces

The adsorption of CO on Rh foil and small Rh particles supported by Al2O3 polycrystalline surfaces was studied using temperature programmed desorption (TPD). The supported Rh particles were prepared by evaporation, using the principles of electron bombardment. The activation energy of desorption was calculated. The results show the dependence on the CO exposure of the sample as well as the size effect. Desorption and recombination peaks were measured. During CO desorption the CO2 production was followed. The dependence of CO oxidation rate on the kind of sample is discussed.