Rh Particles Research Articles

Effect of pore structure on catalytic properties of mesoporous silica supported rhodium catalysts for the hydrogenation of cinnamaldehyde

Rhodium particles for catalyzing the hydrogenation were effectively confined in the mesopores of MCM-41 and MSU-H silica by using supercritical CO2 as impregnation solvent. It was found that the calcination was an important step to control the size of Rh particles, and its influence was strongly dependent on the pore size distribution of mesoporous silica support. The prepared catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption, and transmission electron microscopy (TEM). While irregularly big Rh particles were often found on MCM-41 with smaller pore size (2.7nm) during calcination at 873K, homogeneous and small Rh particles were maintained inside MSU-H with relatively larger pore size (8.4nm). The hydrogenation of cinnamaldehyde using the prepared catalysts was carried out in supercritical CO2. Rh/MSU-H which was calcined at 673K provided the highest catalytic activity among the studied conditions. The results indicated that the catalyst on the support with larger pore size and smaller size of Rh particles might be desirable to attain higher catalytic activity.

The Promoting Effect of Fe Doping on Rh/CeO2 for the Ethanol Synthesis

The effects of Fe promotion were investigated on the activity and selectivity of Rh/CeO2 catalyst for the direct synthesis of ethanol from syngas. The catalysts were comprehensively characterized by X-ray diffraction (XRD), N2 adsorption–desorption, CO uptake, H2-TPR, temperature programmed surface reaction (TPSR), and FT-IR. XRD, CO uptake and FT-IR measurements indicated that Rh particles were well dispersed. TPSR results revealed that the addition of Fe changed CO dissociation behavior over the catalysts. FT-IR results showed that Fe promoter stabilized both linear carbonyl and gem-dicarbonyl species, which could be responsible for the improved catalytic activity. The interaction between Rh and Fe species is proposed to play a crucial role in achieving higher ethanol selectivity.

Stable Rh particles in hydrotalcite-derived catalysts coated on FeCrAlloy foams by electrosynthesis

Rh-based structured catalysts for the Catalytic Partial Oxidation of CH4 to syngas were prepared by electrosynthesis of Rh/Mg/Al hydrotalcite-type compounds on FeCrAlloy foams and calcination. The effects of Rh content, total metal concentration, and partial replacement of Mg2+ by Ni2+ in the electrolytic solution on coating thickness, Rh speciation, metallic particle size, and catalytic activity were investigated by SEM/EDS, μ-XRF/XANES and tests under diluted and concentrated reaction conditions.The amount of Rh species, present as Mg (RhxAl1−x)2O4, depended on the thickness of the electrosynthesised layer as well as the Rh particle size and dispersion. Smaller and more dispersed particles were obtained by decreasing the Rh concentration in the electrolytic solution from Rh/Mg/Al=11/70/19 to 5/70/25 and 2/70/28 atomic ratio% (a.r.%) and in thinner rather than thicker layers. Despite the improvement in metallic particles features, the CH4 conversion was negatively affected by the low amount of active sites in the coating, the high metal support interaction and possibly the oxidation of metallic particles and carbon formation.A larger amount of solid containing well dispersed Rh particles was deposited by increasing the total metal concentration from 0.03M to 0.06M with the Rh/Mg/Al=5/70/25 a.r.%, and the catalytic performances were enhanced. The partial replacement of Mg2+ by Ni2+ gave rise to a very active bimetallic Rh/Ni catalyst, CH4 conversion and selectivity to syngas were above 90%, however, it slightly deactivated with time-on-stream.

Catalytic hydrogenation of carbon monoxide over Rh–Mn–Li/SiO2 catalyst for the synthesis of C2+ oxygenates: the remarkable effect of urea on the particle size of Rh

A series of Rh-Mn-Li/SiO2 catalysts were prepared by the impregnation of a silica support with solutions containing metal precursors with different amounts of urea. Catalyst performance was investigated by the hydrogenation of carbon monoxide. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, hydrogen chemisorption, diffuse reflectance ultraviolet-visible (DR UV-Vis) spectroscopy, and thermogravimetric analysis. The results revealed that the amount of urea had a significant effect on the particle size of Rh, which increased from approximately 2-5 nm with the increase in urea loading from 0 to 2.0 wt%. DR UV-Vis spectra indicated that the increase in the Rh particle size with increasing urea loading might be attributed to the coordination of Rh3+ with urea or its derivatives during calcination. At a urea loading of 1.0 wt%, the particle size of Rh was 3.7 nm, and the space time yield of C2+ oxygenates reached a maximum of 560.0 g kg(-1) h(-1).

Rh and Au deposited on ultrathin TiO~1.2 film formed on Rh(111) facets and the effects of CO exposure

Abstract This STM study deals with the structure and reactivity of Rh and Au deposited on a “pinwheel” TiO~ 1.2 ultrathin film (w-TiO-UTO) formed on Rh nanoparticles supported on TiO2(110). The deposition of a monolayer (ML) Au at 500 K disturbed the order of the w-TiO-UTO layer and led to the formation of 3D adparticles. The deposition of Rh (~ 1 ML) at 500 K and annealing up to 900 K led exclusively to the formation of 2D adparticles. It was found that 3D Au nanoparticles are gradually sintering in the range of 500–800 K. Upon a further raise of the temperature, a 3D→2D transformation takes place. The main difference is that on the top facet of the 2D Rh particles a w-TiO-UTO film forms, while the top facets of the 2D Au particles exhibit a clean gold metal structure. CO exposure of max. 10 min at 10 Pa causes no remarkable change of the encapsulated Rh particles, however, the 2D gold nanoparticles change significantly. The disintegration of Au particles and the spillover of Au atoms to the UTO surface accelerate at a CO pressure above 10− 1 Pa. This morphological process is reversible as an effect of annealing in UHV.

Structure and activity of flame made ceria supported Rh and Pt water gas shift catalysts

A series of ceria-supported Rh and Pt catalysts was synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions in a membrane reactor. The structural and catalytic properties were compared with a Pt/CeO2 catalyst prepared conventionally by incipient wetness impregnation. BET results show that compared to conventionally prepared catalysts the specific surface area of the flame made catalysts is larger and decreases with increasing metal content. This is in line with X-ray diffraction (XRD) results uncovering a high nanocrystallinity of the ceria, whose crystallite size slightly increases with higher noble metal loading. Both Pt and Rh particles are not detected by XRD due to the small crystallite size. In fact, transmission electron microscopy (TEM) images of the flame made catalysts with high loading showed Pt and Rh particles with diameters of ca. 3nm which remained stable after catalytic testing. X-ray absorption near edge spectra (XANES) at the Rh K- and Pt L3-edge showed that both Pt and Rh were in an oxidized state after preparation. In situ XANES during temperature programmed reduction in 5% H2/He indicated a lower reduction temperature than for the conventional Pt/CeO2 catalyst. Hence, Rh and Pt species are located on the surface of the CeO2 particles. Extended X-ray absorption fine structure (EXAFS) spectra provide further evidence for oxidized Rh, Pt before reduction and full reduction after activation in hydrogen, and small particle sizes in agreement with the TEM and XRD results. The catalytic activity measurements were performed at atmospheric pressure in two different gas mixtures by measuring CO conversion as a function of temperature. In all cases the Pt/CeO2 catalysts prepared by flame spray pyrolysis showed high catalytic activity and much higher selectivity than the Rh/CeO2 catalyst, where methane formation is observed above 300°C. Furthermore, the reaction rate in terms of turnover frequencies is much higher on the Pt-based catalysts, which makes them promising candidates for high temperature WGS catalysts in membrane and microreactors.

Impact of Rh–CeOx interaction on CO oxidation mechanisms

We have investigated the impact of electronic metal–support interaction on catalytic properties of Rh/CeOx system, i.e. its capacity for the reverse and direct oxygen spillover, in the context of CO oxidation. The studies were carried out on two types of samples consisting of Rh particles supported on polycrystalline nearly stoichiometric and partially reduced cerium oxide films using temperature programmed reaction, temperature programmed desorption, and conventional X-ray and resonant photoelectron spectroscopies. We have found that the electronic Rh–CeOx interaction leads to a buildup of a net positive charge on Rh particles supported on stoichiometric cerium oxide, and a net negative charge on Rh particles on reduced cerium oxide. The effect of the Rh–CeOx interaction is manifested in suppression of the reverse and direct oxygen spillover on Rh particles supported on partially reduced ceria.

Kinetic model for modern double-layered Pd/Rh TWC as a function of metal loadings and mileage

The overall reaction kinetics for a modern double-layered Pd/Rh bimetallic TWC has been developed by combining the activity functions derived separately for the Pd-based (aPd) and Rh-based (aRh) TWCs with their respective reference reaction kinetics. A 3D activity function for the commercial Rh-based TWC (aRh) was developed on the basis of the metal surface area (MSA) of Rh, and subsequently used in combination with the detailed reaction kinetics of the Rh reference catalyst in predicting the catalytic performance variation with respect to the Rh loading from 2.5 to 15g/ft3 and the catalyst mileage from 4k to 100kmiles obtained from the laboratory aging program. Finally, the catalytic performance of a modern double-layered Pd80/Rh10 TWC monolith reactor was predicted with a reasonable accuracy over a wide range of the catalyst mileage from 4k to 100k by using the overall reaction kinetics for Rh developed in this work along with that for Pd developed previously. The excellent capability of the overall reaction kinetic model in predicting the catalytic performance of the Pd/Rh bimetallic TWC is attributable to the successful combination of the accurate initial activity profiles of Pd and Rh at 4kmiles, the accurate characterization of the deactivation kinetics of Pd and Rh, the detailed surface reaction kinetics employed, and the spatial separation of Pd and Rh particles in a double-layered catalyst configuration.

Comparison of precious metal doped and impregnated perovskite oxides for TWC application

Four high-surface-area (50–60m2/g) perovskite-based powder catalysts, Pd-doped LaFeO3, Pd-impregnated LaFeO3, Rh-doped CaTiO3, and Rh-impregnated CaTiO3, were characterized by scanning transmission electron microscopy and CO oxidation measurements in fresh and redox-aged (14h at 800°C) states. Both intrinsic catalytic activity and stability were significantly higher in Pd-doped LaFeO3 than in Rh-doped CaTiO3 under lean to stoichiometric reaction conditions. Activities of doped were initially lower than impregnated versions, though aging led to convergence in the catalytic performance for both systems. A mixture of metallic and cationic forms of Pd appear to contribute to activity in the Pd catalysts, whereas metallic Rh particles, which can easily segregate onto the surface of CaTiO3 in Rh-doped CaTiO3 upon aging, are the likely source of catalytic activity in the Rh catalysts.

Promoting effect of Al on tethered ligand-modified Rh/SiO2 catalysts for ethylene hydroformylation

A Rh/SiO2 catalyst with excellent activity and stability for ethylene hydroformylation was developed by modifying with tethered diphenylphosphinopropyl and doped with an Al promoter. The catalyst was characterized by means of N2 adsorption/desorption isotherms, transmission electron microscope, NH3 temperature programmed desorption, Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance. Experimental results showed that the existence of the Al promoter inhibited the growth of Rh particles, increased the number of exposed Rh atoms, changed the acidity of the catalyst surface, promoted in situ formation of active species that were similar to their corresponding homogeneous counterparts, and enhanced electron density of the P atom in the phosphine ligand.

Bimetallic IrRh/CeO2–ZrO2 as a Highly Active Catalyst for NO–CO–C3H6–H2–O2 Reactions under Stoichiometric Conditions

Abstract The catalytic activity of Rh/CeO2–ZrO2 for NO–CO–C3H6–H2–O2 reactions under stoichiometric conditions was significantly improved by the addition of Ir. The optimum Ir/Rh atomic ratio was 1/9. The improvement was ascribed to the formation of finely divided Ir species on the surface of Rh particles. IrRh/CeO2–ZrO2 with an IrRh loading of 0.3 wt % showed high catalytic performance, which is comparable with that of 0.5 wt % Rh/CeO2–ZrO2, leading to minimize the Rh usage.

Mesoporous silica supported Rh catalysts for high concentration N2O decomposition

A set of Rh-containing catalysts (Rh-MCM-41, Rh-SBA-15, Rh-KIT-6 and Rh-MCF, nominal Rh content=1wt.%) has been prepared by wet impregnation of mesoporous silicas and tested for high concentration N2O abatement. The physico-chemical properties of the materials have been investigated by means of complementary techniques.The best performances, in terms of N2O decomposition, have been achieved for the Rh-MCF catalyst, due to the better textural properties of the MCF silica. In fact, the MCF-type support exhibits three-dimensional mesoporosity with ultra-large cells (up to 40nm), which allow a uniform distribution of small RhOx particles (≈1nm) over the high (internal) surface area of the MCF. Moreover, the Rh active sites are also readily accessible to N2O molecules.The most promising catalyst has shown the highest amount of Rh1+ species, the easiest rhodium reducibility and the greatest abundance of Rh surface sites. These important features reflect the different Rh particle sizes and play a role in catalytic activity.A remarkable relationship between the catalytic activity and the dimension of the RhOx particles has been observed in the 1–2.5nm size domain, thus confirming the dispersion-sensitivity of N2O decomposition over RhOx nanoparticles.

Hydrogen production from ethanol steam reforming over Rh/CeO2 catalyst

Hydrogen production from ethanol steam reforming over an Rh/CeO2 catalyst was investigated with a stoichiometric feed composition. Ethanol was entirely converted to hydrogen and C1 products (CO, CO2, CH4) at 400°C due to the remarkable C–C bond cleavage capacity of Rh species. The Rh/CeO2 catalyst exhibited stable activity and selectivity without the obvious deactivation during 70h on stream test. Structural analysis of the aged catalysts indicated that the strong interaction between Rh and ceria support efficiently inhibited Rh particles sintering (stable at around 2nm) and coke formation to guarantee catalyst stability.

Steam- and autothermal-reforming of n-butanol over Rh/ZrO2 catalyst

Steam reforming (SR) and autothermal reforming (ATR) of n-butanol was studied over 0.5wt% Rh/ZrO2 catalyst at 500, 600 and 700°C, atmospheric pressure, steam to carbon (S/C) molar ratio of 4 and oxygen to carbon (O2/C) molar ratio of 0.1. The influence of the O2/C ratio on performance was also carried out at 500°C. Catalyst performance, i.e. conversion, product distribution and short-term stability, was compared with plain ZrO2 support to derive a reaction network, including contributions of reactions in the gas phase, on the support and on Rh. At 500 and 600°C the catalyst deactivated very rapidly for WGS and hydro-isomerization reactions, slightly less rapidly for reforming and significantly slower for dehydration and dehydrogenation reactions. Increasing temperature and oxygen concentration improved catalyst stability. Rh/ZrO2 showed promising performance at 700°C with complete conversion and hydrogen yield, 70% of theoretical maximum, decreasing over 21h with not more than 8 and 1% for SR and ATR, respectively. Deactivation of the catalyst for reforming is caused by carbon deposition on and near the Rh particles. At 700°C, coke forms via gas phase reactions, while catalytic reactions on the surface mainly contribute at 500 and 600°C.

Supported-induced effect on the catalytic properties of Rh and Pt-Rh particles deposited on La2O3 and mixed α-Al2O3-La2O3 in the dry reforming of methane

Rhodium and platinum-rhodium based catalysts supported on La2O3 and Al2O3 doped with La2O3 were investigated in the dry reforming of methane. The textural and structural properties were studied on calcined, reduced and used catalysts from nitrogen physisorption, X-ray diffraction, XPS, SEM, H2- and O2-temperature-programmed experiments. Rh is intrinsically more active than Pt independently of the support composition. Different types of interactions occur when Rh is incorporated in Pt on La2O3 and La-modified α-Al2O3 supports with the observations of synergistic effects on the activity of Pt-Rh/Al2O3-La2O3 for methane conversion. Correlations between catalytic properties with structural and surface properties have been tentatively established and are discussed. A detrimental effect of alloyed Pt-Rh particles on La2O3 and a beneficial effect on the catalytic activity of Pt-Rh/Al2O3-La2O3 due to the formation of possible composite microstructures combining noble metals and the elements of the support is reported.

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.

Enhanced electrocatalytic oxidation of isoniazid at electrochemically modified rhodium electrode for biological and pharmaceutical analysis

A simple and sensitive electrochemical method has been proposed for the determination of isoniazid (INZ). For the first time, rhodium (Rh) modified glassy carbon electrode (GCE) has been employed for the determination of INZ by linear sweep voltammetry technique (LSV). Compared with the unmodified electrode, the proposed Rh modified electrode provides strong electrocatalytic activity toward INZ with significant enhancement in the anodic peak current. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) results reveal the morphology of Rh particles. With the advantages of wide linearity (70–1300μM), good sensitivity (0.139μAμM−1cm−2) and low detection limit (13μM), this proposed sensor holds great potential for the determination of INZ in real samples. The practicality of the proposed electrode for the detection of INZ in human urine and blood plasma samples has been successfully demonstrated using LSV technique. Through the determination of INZ in commercially available pharmaceutical tablets, the practical applicability of the proposed method has been validated. The recovery results are found to be in good agreement with the labeled amounts of INZ in tablets, thus showing its great potential for use in clinical and pharmaceutical analysis.

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.

Influence of Rh nanoparticle size and composition on the photocatalytic water splitting performance of Rh/graphitic carbon nitride

The effect of Rh co-catalyst nanoparticle size for photocatalytic water splitting using graphitic carbon nitride (g-C3N4) as light absorber was investigated. Rh nanoparticles with sizes in the 4–9 nm range were synthesized and deposited on g-C3N4. The light-absorption properties of the g-C3N4 and the particle size of Rh supported on g-C3N4 were also not influenced by the catalyst synthesis procedures. Rh/C3N4 is active in the photocatalytic splitting of water using visible light. The activity for H2 generation does not depend on Rh particle size. The results obtained point to two important design criteria for a successful photocatalyst: firstly, the surface of the semiconductor should support a sufficient number of Rh nanoparticles to remove the photogenerated electrons before their recombination with holes; secondly, the nanoparticles should be metallic in nature to catalyze the proton-electron transfer reaction to generate adsorbed H atoms. Surface oxidation of the Rh nanoparticles substantially lowers their photocatalytic activity.

Hydrogen production by ethanol steam reforming over Rh nanoparticles supported on lanthana/silica systems

The aim of the present work was to investigate Rh nanoparticle catalysts supported on La2O3–SiO2, with different contents of La2O3, in the ethanol steam reforming reaction. The size of Rh particles was determined by both XPS, applying the Davis model, and CO chemisorption measurements. Particle sizes between 1.2 and 3nm were estimated with these techniques. The catalyst with 15wt.% of La2O3 was the most stable one under the different reaction conditions under study, i.e. 773K and water to ethanol molar ratios of 3 and 5. In order to explain the deactivation observed in the other catalysts, the samples used in the reaction were characterized by XPS, TEM and in situ Raman spectroscopy. The TEM results allowed a direct visualization of the carbon deposits that presented different degrees of catalyst coverage. The surface of the most stable catalyst was not fully covered by carbon while the Rh/La2O3(40)–SiO2 sample, which deactivated partially, exhibited a carbon layer uniformly distributed over the surface. The oxidation of the carbon deposits was analyzed using in situ Raman spectroscopy. Results were in agreement with those obtained by TEM microscopy. DRIFTS experiments under reaction conditions suggested that the direct decomposition of ethoxy species would produce hydrogen, CO, and CHx species, which could result in carbon formation.