Oxide-supported Rh Research Articles

Architecture of urchin-like TiO2 integrated ultrasmall Rh nanoparticles with oxygen vacancy-reinforced electronic metal-support interaction for boosting hydrogen production from ammonia borane hydrolysis

Metal oxide-supported Rh nanoparticles (NPs) can provide abundant catalytically active sites for hydrogen evolution from ammonia borane hydrolysis (ABH). Considering the scarcity and high cost, developing effective strategies to reduce the utilization of Rh and simultaneously improving its catalytic activity is absolutely needed. Here, we develop ultrafine Rh NPs anchored on oxygen vacancies (VO)-bearing urchin-like TiO2 (U-TiO2). VO can facilitate the formation of electron-rich metallic Rh surface and boost the H2O dissociation, leading to highly efficient hydrogen generation from ABH without alkali promoters. Noticeably, the temperature-controlled annealing of the Rh/U-TiO2 precursor at 450 °C in reducing atmosphere generates the maximum VO concentration. This arouses strong electronic metal-support interaction between the Vo and Rh NPs to yield electron-rich metallic Rh surface. The obtained Rh/UT-450Vo exhibits a high initial turnover frequency of 1729 min−1 for ABH without alkali promoters, outperforming that of the contrast samples and most of previously developed Rh catalysts. This finding offers an effective strategy to fabricate high-efficiency metal oxide-supported metal NPs toward catalytic hydrogen production.

Experimental and Theoretical Characterization of Rh Single Atoms Supported on γ-Al2O3 with Varying Hydroxyl Contents during NO Reduction by CO

Oxide-supported Rh catalysts are important components of commercial three-way catalysts for pollution abatement. Despite their universal application, many mysteries remain about the active structure of Rh on oxide supports as these materials often contain a mixture of nanoparticles and single-atom Rh species on the same support, even after aging. Probe molecule Fourier transform infrared (FTIR) spectroscopy in this work shows that atomically dispersed Rh on γ-Al2O3 prefer to strongly bind CO when exposed to NO and CO mixtures and that light-off of NO reduction occurs at temperatures similar to CO desorption, suggesting that the first and rate-determining step in NO–CO reactions may be the desorption of CO from single-atom Rh dicarbonyl complexes, Rh(CO)2. Two sets of symmetric and asymmetric stretching frequencies associated with distinct Rh(CO)2 species are observed in FTIR spectra at 2084/2010 and 2094/2020 cm–1. During temperature ramps, the latter pair of bands at 2094/2020 cm–1 converts to the 2084/2010 cm–1 bands at 463 K before all symmetric and asymmetric bands disappear at 573 K. Bands then appear in the range of 1975–1985 cm–1 associated with Rh monocarbonyl, Rh(CO), species upon the disappearance of the 2084/2010 cm–1 bands, suggesting that CO desorbs sequentially from Rh(CO)2 by forming Rh(CO) intermediates. Combined DFT and FTIR experiments suggest that local OH coverage on the γ-Al2O3 surface distinguishes the two Rh(CO)2 species: the higher frequency species resides on a less hydroxylated region and migrates to a more hydroxylated region at higher temperatures, causing the CO vibrational frequency to decrease by ∼10 cm–1. CO desorption occurs from this Rh(CO)2 structure with high local OH coverage, consistent with the DFT predicted trend of CO binding energies. Because of the coincidence of CO desorption with the light-off of NO reduction, local support hydroxylation of atomically dispersed Rh1/γ-Al2O3 catalysts likely affects both the Rh structure after CO desorption and the kinetics of NO reduction, studies of which are enabled by the Rh(CO)2 model developed here.

Adsorbate-mediated strong metal–support interactions in oxide-supported Rh catalysts

The optimization of supported metal catalysts predominantly focuses on engineering the metal site, for which physical insights based on extensive theoretical and experimental contributions have enabled the rational design of active sites. Although it is well known that supports can influence the catalytic properties of metals, insights into how metal-support interactions can be exploited to optimize metal active-site properties are lacking. Here we utilize in situ spectroscopy and microscopy to identify and characterize a support effect in oxide-supported heterogeneous Rh catalysts. This effect is characterized by strongly bound adsorbates (HCOx) on reducible oxide supports (TiO2 and Nb2O5) that induce oxygen-vacancy formation in the support and cause HCOx-functionalized encapsulation of Rh nanoparticles by the support. The encapsulation layer is permeable to reactants, stable under the reaction conditions and strongly influences the catalytic properties of Rh, which enables rational and dynamic tuning of CO2-reduction selectivity.

Oxide-supported Rh catalysts for H2 generation from low-temperature ethanol steam reforming: effects of support, Rh precursor and Rh loading on catalytic performance

Rh4(CO)12-derived Rh/CeO2 is superior to the other oxide-supported Rh catalysts. Coking is the only cause of catalyst deactivation which affects the catalytic stability of Rh/CeO2. Both CeO2-supported Rh0 and Rh+ may participate in catalysis for ESR.

Ethanol steam reforming over Mg xNi 1− xAl 2O 3 spinel oxide-supported Rh catalysts

Two series of Rh/spinel catalysts supported on alumina were prepared. In the first series, magnesium, nickel, and aluminum nitrates were co-impregnated over γ-alumina beads (200 m 2 g −1), dried, and further calcined at 1000 °C to obtain Mg x Ni 1− x Al 2O 4/Al 2O 3 supports (≈100 m 2 g −1), where x ranged from 0 to 1. These supports were impregnated with aqueous solutions of Rh nitrate to obtain 0.1–0.2 wt% Rh catalysts. We prepared the second series by coating alumina beads with Mg acetate. The support was dried and calcined at 1000 °C. The MgAl 2O 4 spinel was formed by solid–solid reaction between magnesia and alumina. MgAl 2O 4 was impregnated with different Rh precursor salts (nitrate, chloride, acetate) to obtain 0.2–0.8 wt% Rh loading. Supports and catalysts were characterized by BET area, pore volume, XRD, TEM and SEM, CO 2 chemisorption (basicity), and dimethyl-3,3-but-1-ene isomerization (acidity). Rh dispersion was measured by H 2 chemisorption. The second series of catalysts was also characterized by FTIR of adsorbed lutidine and DRIFT. The activity of the catalysts was evaluated in the ethanol steam reforming at 700 °C under 1 or 11 atm (H 2O/ethanol molar ratio of 4, space velocity 24,000 h −1). Acidic and basic properties of the catalysts are crucial parameters inasmuch as they control the primary selectivity for ethylene or acetaldehyde. To avoid ethylene formation, which leads to a significant carbon deposit, all acidic sites should be neutralized. The effects of the precursors used in the support and catalyst preparation were investigated. The second preparation method gave less acidic materials with very high performances (activity and stability) in the ethanol steam reforming reaction. The morphology of the support, with a Mg-deficient spinel layer (thickness of about 8–9 nm) intimately covering all of the alumina grains (around 40 nm in size), can explain the neutralization of most acidic sites. Nitrate precursors should be avoided, since as we obtained the most acidic materials and the poorest stability. Rh acetate led to neutral catalysts with interesting performances in the ethanol steam reforming, and Rh chloride allowed the preparation of well-dispersed Rh catalysts. Although the catalysts prepared with Rh chloride were moderately acidic, they were very active and stable.

Influence of the nature of the noble metal (Rh,Pt) on the low‐temperature reducibility of a Ce/Tb mixed oxide with application as TWC component

AbstractCeria‐based mixed oxides have proved to be good alternatives to pure ceria as redox materials for advanced three‐way catalysts. In this work, the behaviour of a Ce0.8Tb0.2O2−x mixed‐oxide sample and the corresponding oxide‐supported Rh and Pt samples has been investigated. Special attention has been paid to the low‐temperature redox response. Temperature‐programmed desorption (TPD), temperature‐programmed reduction and oxygen buffering capacity were used as experimental techniques. The evolution of reducibility was also followed by tunable high‐energy x‐ray photoemission spectroscopy, focussing on the surface information provided by Tb 3d and Ce 3d core levels. The oxidation state of both Ce and Tb was followed during ultrahigh vacuum annealing of the three samples up to 750 K. Comparison of the XPS spectra for the different samples before and after annealing showed that the Ce4+ reduction was negligible but Tb4+ was gradually reduced to Tb3+. Furthermore, the TPD and XPS experiments are in good agreement, with the highest reduction rate corresponding to the Rh/CeTbOx sample. Copyright © 2002 John Wiley & Sons, Ltd.

CO 2 reforming of CH 4 over Rh-containing catalysts

Two series Rh catalysts were prepared depending on the kind of the support: (i) Rh supported on zeolite and (ii) on oxide carriers. Rh/NaY zeolite catalysts were prepared by ion-exchange from an aqueous solution of [Rh(NH 3) 5]Cl 3. Rh catalysts supported on γ-Al 2O 3, Nb 2O 5 and TiO 2 were prepared by incipient wetness impregnation method of the carrier with an aqueous solution of RhCl 3·2H 2O. The catalysts were characterized by X-ray diffraction (XRD) spectroscopy, nuclear magnetic resonance (NMR), temperature-programmed desorption of hydrogen (TPD-H 2). Test reaction for Rh catalysts was CO 2 reforming of methane at different reaction temperatures. TPD data showed that the dispersion of metal particles depends on the pretreatment activation procedure; highest dispersion was observed for samples previously calcined and then activated in H 2. Catalysts activated directly in H 2, steam or submitted directly to the reaction conditions showed large metal particles. The effect of the protons on the destabilization of the zeolite framework during reaction was revealed by the higher Si/Al ratios, due to a dealumination of the zeolite. Neutralization of protons with solution of NaOH leaded to a stabilization of the dispersion and zeolite structure. Correlation between the dispersion and specific activity in CO 2 reforming was found for zeolite-supported Rh catalysts. When the activities are compared by turnover frequencies, oxide-supported Rh catalysts are significantly more active compared to zeolite-supported ones due to a higher degree of participation of the reverse water–gas shift reaction (WGSR). The difference in activity and thermal stability was related to the nature of the support.

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.

Oxide-supported Rh particle structure probed with carbon monoxide

Rh deposits in a wide range of metal exposures have been grown on a thin, well-ordered alumina film under ultra-high vacuum conditions. Their morphologies range from individual Rh particles with sizes below ten atoms to closed metal films, as determined by scanning tunneling microscopy (STM). CO chemisorption on these deposits has been studied by infrared reflection absorption spectroscopy (IRAS) and X-ray photoelectron spectroscopy (XPS). At low metal coverages, the formation of isolated Rh carbonyl species associated with surface defects is observed. Changes in the infrared spectra of CO adsorbed on annealed Rh aggregates are attributed to a thermally induced ordering of the Rh particle surface on a local scale, leading to a reduction in the number of low coordinated metal atoms. This is accompanied by a marked decrease of the CO chemisorption capacity. Adsorbed CO reduces the extent of this ordering process, while the adlayer itself reorganizes.

Temperature dependent XPS study of CO dissociation on small Rh particles

We have used temperature dependent X-ray Photoelectron Spectroscopy (XPS) to study the heating-induced CO dissociation on oxide-supported Rh particles through observation of changes undergone by the adsorbate CO and the formation of atomic carbon from the dissociation process. Different behaviour upon heating was observed for the two CO species on the islands. Our study indicates that one of the species is desorptive and that the other is dissociative.

CO2-Reforming of Methane on Supported Rh and Ir Catalysts

The effect of catalyst structure on CO2reforming of methane has been studied in the temperature range 550–850°C with oxide-supported Rh and Ir catalysts under conditions of reaction rate control. It is found that the rate of reaction expressed per unit metal surface area is constant over a wide dispersion range. There is no detectable effect of the pore structure or nature of the support on the reaction rate. By variation of feed composition through addition of steam or oxygen the H2/CO ratio in the product could be varied.

Molecular orbital calculations for surface-bound metal species: rhodium(I) dicarbonyls on alumina or silica

The oxide-supported Rh I(CO) 2 species, frequently generated on heterogeneous rhodium catalysts, has been modeled for extended Hückel molecular orbital calculations using, as oxide supports, simple oxyanion structures composed of Si or Al atoms in tetrahedral or octahedral oxygen environments. Calculations for different orientations of Rh I(CO) 2 with respect to the oxyanion structures showed the lowest energy configuration to be that in which the metal car bonyl fragment binds to two oxygen atoms, forming a square planar geometry about the Rh I. Rearrangement to a five-coordinate species with Rh I(CO) 2 bound to three oxygens requires between 0.51 and 1.94 eV of energy, depending upon the oxyanion structure and whether the central atom is Si or Al. The lowest energy for conversion from four- to five-coordination about Rh I occurs upon octahedral anions. The square planar→ five-coordinate conversion is discussed as a surface migration pathway, with relative activation barriers to the migration depending upon the energy of this conversion.

CO hydrogenation over rhodium supported on SiO 2, La 2O 3, Nd 2O 3 and Sm 2O 3

The kinetics of CO hydrogenation over Rh supported on SiO 2, La 2O 3, Nd 2O 3 and Sm 2O 3 were investigated. Large differences were observed between silica-supported Rh and the rare earth oxide-supported Rh catalysts, in the apparent activation energies for the synthesis of methane, methanol, and C 2-oxygenates. Large differences were also observed, between Rh/SiO 2 and the rare earth oxide-supported Rh catalysts, in the H 2 and CO partial pressure dependences for the synthesis of C 2-oxygenates. These differences are attributable to metal-support interactions. No clear correlation was found between the activity or selectivity for CO hydrogenation of the rare earth oxide-supported Rh catalysts and basicity of the support. For each of the catalysts studied, acetaldehyde was found to be the primary C 2-oxygenated product. Ethanol appears to be formed via the hydrogenation of acetaldehyde, a process which occurs much more readily over the rare earth oxide-supported catalysts than over the silica-supported catalyst.