Ethanol steam reforming: understanding changes in the activity and stability of Rh/MxOy catalysts as function of the support (original) (raw)
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Ethanol Steam Reforming on Rh Catalysts: Theoretical and Experimental Understanding
ACS Catalysis, 2014
Through combined theoretical and experimental efforts, the reaction mechanism of ethanol steam reforming on Rh catalysts was studied. The results suggest that acetaldehyde (CH 3 CHO) is an important reaction intermediate in the reaction on nanosized Rh catalyst. Our theoretical work suggests that the H-bond effect significantly modifies the ethanol decomposition pathway. The possible reaction pathway on Rh (211) surface is suggested as CH 3 CH 2 OH → CH 3 CH 2 O → CH 3 CHO → CH 3 CO → CH 3 + CO → CH 2 + CO → CH + CO → C + CO, followed by the water gas shift reaction to yield H 2 and CO 2. In addition, we found that the water-gas shift reaction, not the ethanol decomposition, is the bottleneck for the overall ethanol steam reforming process. The CO + OH association is considered the key step, with a sizable energy barrier of 1.31 eV. The present work first discusses the mechanisms and the water effect in ethanol steam reforming reactions on Rh catalyst from both theoretical and experimental standpoints, which may shed light on designing improved catalysts.
Effects of Support and Rh Additive on Co-Based Catalysts in the Ethanol Steam Reforming Reaction
ACS Catalysis, 2014
The effect of the nature of the support and the promotion achieved by a Rh additive on Co-based catalysts in the ethanol steam reforming reaction were studied. The catalysts with 2% Co loading were characterized by temperatureprogrammed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). In situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) identified the surface intermediates formed during the reaction, whereas gas phase products were detected by gas chromatography (GC). Upon heating in hydrogen to 773 K, cobalt could not be reduced to Co 0 on alumina, but on silica the reduction was almost complete. On ceria, half of the Co could be reduced to the metallic state. By the presence of a small amount (0.1%) of Rh promoter, the reduction of both cobalt and ceria was greatly enhanced. For Co on the acidic Al 2 O 3 support, the dehydration mechanism was dominant, although on the basic CeO 2 support, a significant amount of hydrogen was also formed. Addition of a small amount of Rh as promoter to the Co/CeO 2 catalyst resulted in a significant further increase in the hydrogen selectivity.
Ethanol steam reforming over MgxNi1−xAl2O3 spinel oxide-supported Rh catalysts
Journal of Catalysis, 2005
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 2 O 4 /Al 2 O 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 2 O 4 spinel was formed by solid-solid reaction between magnesia and alumina. MgAl 2 O 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 2 O/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.
Journal of Catalysis, 2006
This study focuses on the effects of the CeO 2 support properties on the catalyst properties and performance of bimetallic Ni-Rh/CeO 2 catalysts containing 5 wt% Ni and 1 wt% Rh for the oxidative steam reforming (OSR) of ethanol for hydrogen production and fuel cell applications. Three CeO 2 supports with different crystal sizes and surface areas were examined. The surface areas of these supports increases in the order of CeO 2 -I (74 m 2 /g) < CeO 2 -II (92 m 2 /g) < CeO 2 -III (154 m 2 /g), but their crystallite sizes were about 10.2, 29.3, and 6.5 nm, respectively. The properties of Ni-Rh/CeO 2 catalysts were investigated by XRD, TPR, H 2 chemisorption, and in situ XPS techniques. The Rh metal dispersion increased while the Ni metal dispersion decreased with decreasing crystallite sizes of CeO 2 . TPR studies revealed the existence of a Rh-CeO 2 metalsupport interaction as well as Ni-Rh interaction in the Ni-Rh bimetallic catalyst supported on CeO 2 -III with a crystallite size of about 6.5 nm. The in situ XPS studies corroborated the TPR results. The reduced Ni and Rh species were reversibly oxidized, suggesting the existence of Ni-Rh redox species rather than NiRh surface alloy in the present catalyst system. The Rh species became highly dispersed when the crystallite size of CeO 2 support was smaller. Comparing the catalytic performance in the OSR of ethanol with the properties of the catalysts demonstrated that both ethanol conversion and H 2 selectivity increased and the selectivity for undesirable byproducts decreased with increasing Rh metal dispersion. Best catalytic performance for OSR was achieved by supporting Ni-Rh bimetallic catalysts on the nanocrystalline CeO 2 -III. The Ni-Rh/CeO 2 -III catalyst exhibited stable activity and selectivity during on-stream operations at 450 • C and as well as at 600 • C.
Catalysis Today, 2015
The ethanol steam reforming (ESR) reaction has been tested over RhPd supported on polycrystalline ceria in comparison to structured supports composed of nanoshaped CeO2 cubes and CeO2 rods tailored toward the production of hydrogen. At 650-700 K the hydrogen yield follows the trend RhPd/CeO2cubes > RhPd/CeO2-rods > RhPd/CeO2-polycrystalline, whereas at temperatures higher than 800 K the catalytic performance of all samples is similar and close to the thermodynamic equilibrium. The improved performance of RhPd/CeO2-cubes and RhPd/CeO2-rods for ESR at low temperature is mainly ascribed to higher water-gas shift activity and a strong interaction between the bimetallic-oxide support interaction. STEM analysis shows the existence of RhPd alloyed nanoparticles in all samples, with no apparent relationship between ESR performance and RhPd particle size. X-ray diffraction under operating conditions shows metal reorganization on {1 0 0} and {1 1 0} ceria crystallographic planes during catalyst activation and ESR, but not on {1 1 1} ceria crystallographic planes. The RhPd reconstructing and tuned activation over ceria nanocubes and nanorods is considered the main reason for better catalytic activity with respect to conventional catalysts based on polycrystalline ceria.
Physical chemistry chemical physics : PCCP, 2016
Rh-Fe catalysts supported on Ca-Al2O3, MgO and ZrO2 were evaluated in ethanol steam reforming at 623 K and compared to Rh catalysts on the same supports without iron promotion. The metal-support interaction among the three entities, i.e. Rh ↔ Fe2O3 ← support (ZrO2, MgO and Ca-Al2O3) was investigated using H2-chemisorption, TEM, XPS and in situ techniques such as DRIFTS, temperature-resolved XRD and XAS. As compared to the unpromoted Rh catalysts on the same supports, the CO selectivity is depressed in the presence of iron on Rh/MgO and Rh/Ca-Al2O3, the latter being significantly superior. The role of metal-support interaction for CO-free hydrogen generation was unravelled using a combination of techniques. It was found that the reducibility of iron oxide determines the extent of the strong metal support interaction between Rh and Fe2O3 and the reducibility of iron oxide was affected by the support. On Rh-Fe/Ca-Al2O3, a good balance of the interaction between Rh, Fe2O3 and Ca-Al2O3 p...
The ethanol steam reforming (ESR) reaction has been tested over RhPd supported on polycrystalline ceria in comparison to structured supports composed of nanoshaped CeO2 cubes and CeO2 rods tailored toward the production of hydrogen. At 650-700 K the hydrogen yield follows the trend RhPd/CeO2cubes > RhPd/CeO2-rods > RhPd/CeO2-polycrystalline, whereas at temperatures higher than 800 K the catalytic performance of all samples is similar and close to the thermodynamic equilibrium. The improved performance of RhPd/CeO2-cubes and RhPd/CeO2-rods for ESR at low temperature is mainly ascribed to higher water-gas shift activity and a strong interaction between the bimetallic-oxide support interaction. STEM analysis shows the existence of RhPd alloyed nanoparticles in all samples, with no apparent relationship between ESR performance and RhPd particle size. X-ray diffraction under operating conditions shows metal reorganization on {1 0 0} and {1 1 0} ceria crystallographic planes during catalyst activation and ESR, but not on {1 1 1} ceria crystallographic planes. The RhPd reconstructing and tuned activation over ceria nanocubes and nanorods is considered the main reason for better catalytic activity with respect to conventional catalysts based on polycrystalline ceria.
Applied Catalysis A: General, 2015
In situ X-ray photoelectron spectroscopy (XPS) was carried out over model Rh 0.5 Pd 0.5 nanoparticles and Rh 0.5 Pd 0.5 nanoparticles supported on CeO 2 following exposure to oxygen at 573-823 K, to hydrogen at 573 K (activation of the catalyst), to a mixture of ethanol and water at 823 K simulating ethanol steam reforming (ESR) conditions, and to hydrogen at 823 K. The presence of the CeO 2 support had a strong influence on the atomic rearrangement and on the oxidation state of Rh 0.5 Pd 0.5 nanoparticles. Ceria exerted a quenching effect on the metal nanoparticles and inhibited atomic rearrangement under the different atmospheres tested except for ESR, where a partial segregation of Rh towards the surface of the nanoparticles was observed. When supported on CeO 2 , Rh 0.5 Pd 0.5 nanoparticles were significantly more oxidized due to metal-support interaction. By comparing these in situ XPS results with a previous operando near-ambient pressure XPS study conducted in a synchrotron facility at 0.05 mbar over the same samples, it is concluded that the information obtained for the unsupported Rh 0.5 Pd 0.5 nanoparticles is similar in both cases, whereas Rh 0.5 Pd 0.5 nanoparticles supported on CeO 2 are clearly more oxidized and enriched in Pd in the volume sampled under operando ESR conditions compared to that obtained in the in situ experiments. The study of catalytic systems under operando conditions appears essential to identify the active species at work during ESR, as the restructuring driven by the reforming environment induces strong changes in their architecture.
Design of Rh@Ce0.2Zr0.8O2–Al2O3 nanocomposite for ethanol steam reforming
Journal of Alloys and Compounds, 2008
Rh(1 wt.%)@Ce 0.2 Zr 0.8 O 2 (10 wt.%)-Al 2 O 3 nanocomposite has been investigated as active and thermally stable catalyst for ethanol steam reforming. Rh nanoparticles were synthesised by surfactant assisted route, using N-hexadecyl-N-(2-hydroxyethyl)-N,N-dimethyl ammonium bromide (HEAC16Br). Metal particles with average diameter of 2.1 nm were obtained at 0.53 Rh/HEAC16Br molar ratio, while increasing the amount of surfactant lead to formation of larger particles. The preformed Rh nanoparticles have been effectively embedded into a porous layer of nanocomposite oxides. Low temperature H 2 chemisorption experiments and activity data confirm that most of the Rh atoms are accessible to the reaction mixture. The Ce 0.2 Zr 0.8 O 2 mixed oxide inhibits the dehydration of ethanol to ethylene and favours the water gas shift reaction. The alumina ensures good thermal stability and high surface area of the catalyst. No significant deactivation is observed after repeated run-up and run-down experiments.
Novel zeolite-supported rhodium catalysts for ethanol steam reforming
Journal of Power Sources, 2008
Renewable bioethanol is an interesting hydrogen source for fuel cells through steam reforming, but its C–C bond promotes parallel reactions, mainly coke and by-products formation. In this way, good ethanol reforming catalysts are still needed, which explains current research and development efforts around the world. Most catalysts proposed for ethanol reforming are based on oxide-supported noble metals with surface area