CO hydrogenation over a RhVO4/SiO2 catalyst after H2 reduction

The hydrogenation of CO over a RhVO₄/SiO₂ catalyst has been investigated after H₂ reduction at 773 K. A strong metal–oxide interaction (SMOI) induced by the decomposition of RhVO₄ in H₂ enhanced not only the selectivity to C₂ oxygenates but also the CO conversion drastically, compared with an unpromoted Rh/SiO₂ catalyst. The selectivity of the RhVO₄/SiO₂ catalyst was similar to those of conventional V₂O₅‐promoted Rh/SiO₂ catalysts (V₂O₅–Rh/SiO₂), but the CO dissociation activity (and TOF) was much higher than for V₂O₅–Rh/SiO₂, and hence the yield of C₂ oxygenates was increased. This revised version was published online in July 2006 with corrections to the Cover Date.     

The production of higher alcohols from syngas using potassium promoted Co/Mo/A12O3 and Rh/Co/Mo/A12O3

The optimized compositions K(4.9)/Co(2.7)/Mo(6.4)/A1₂0₃(gamma) and K(1.2)/ Rh(1.1)/Co(0.6)/Mo(5.7)/A1₂0₃(gamma) are both productive catalysts for higher alcohols. The incorporation of rhodium into the K/Co/Mo/A1₂0₃-system allows the space-time-yield for alcohols to be increased from 0.37 g alcohols (g catalyst)^–1 h^–1 to 1.1 g alcohols (g catalyst)–1 h–1. In addition, the higher productivity can be achieved with the rhodium-type catalyst at a lower reaction temperature (45°C lower), and so a higher selectivity to alcohols is observed. Potassium is a key promoter in both catalyst systems, and shifts the distribution of oxygenates towards higher alcohols. In IR measurements it is found that potassium stimulates bands for adsorbed CO around 1885 cm–. These bands are small for the K/Co/Mo/A1₂0₃ system, but are greatly stimulated by the incorporation of rhodium. It is suggested that these bands are indicative of the sites used to catalyze higher alcohols, thereby explaining the promotional effect of rhodium in the K/Co/Mo/A1₂0₃ catalyst system. Previous workers have suggested that the less-electron-rich sites (ionic) are important for methanol synthesis, while the more electron-rich sites (metallic) are important for higher alcohols and hydrocarbons synthesis. The IR measurements and reactivity data support these suggestions.     

Effect of Ba Addition on Catalytic Activity of Pt and Rh Catalysts Loaded on γ-Alumina

The catalytic activity of Pt catalyst loaded on -alumina was improved by Ba addition in simulated automotive exhaust gases. On the other hand, the result of Rh catalyst was the opposite. From the results of the partial reaction orders in C₃H₆–O₂ reaction and TPR, it was concluded that the Ba addition to Pt catalyst suppressed the hydrocarbon chemisorption on the Pt catalyst and therefore allowed the catalytic reaction to proceed smoothly. On the other hand, Ba addition to Rh catalyst caused such a strong oxygen adsorption on Rh that rejected the hydrocarbon adsorption and suppressed the reaction.     

EXAFS investigations of lanthana-promoted Rh/SiO2 catalysts

EXAFS investigations at the Rh K edge of lanthana-promoted Rh/SiO₂ catalysts showed that the local environment of the Rh ions in the oxidic catalyst precursor state did not depend on the La₂O₃ content and resembled that of Rh₂O₃. No LaRhO₃ formation could be detected. In the reduced state, EXAFS as well as H₂ and CO chemisorption demonstrated that La₂O₃ increased the Rh dispersion. Covering of the Rh metal particles by La₂O₃ was minor, because during catalyst preparation, La was impregnated prior to Rh.     

Study of induction period over K2CO3/MoS2 catalyst for higher alcohols synthesis

The K₂CO₃/MoS₂ catalyst for higher alcohols synthesis with synthesis gas as feedstock was prepared. The catalyst was characterized by TPR, in-situ XPS, XRD and SEM. Effects of pretreatment with H₂, CO or synthesis gas on activity and selectivity of the catalyst were investigated. Results showed that there was a remarkable induction period about 180 h at the initial reaction stage for the un-treated catalyst. The catalytic performances for alcohols synthesis changed notably during the induction period. The induction period was confirmed to be resulted primarily from the sulfur losing and K element dispersion on the surface of ADM catalysts. Pretreatment of the catalyst could remarkably shorten the time of induction period as well as promote the catalytic activity. Furthermore, the higher alcohols (C₂ + OH) content in the liquid products were enhanced after the catalyst pretreated by CO or synthesis gas which could be ascribed to the increasing of Mo⁴⁺ content on the surface of the catalyst.     

Selective synthesis of mixed alcohols from syngas over catalyst Fe2O3/Al2O3 in slurry reactor

The Fe₂O₃/Al₂O₃ catalyst was studied to selectively synthesize mixed alcohols from syngas in a continuously stirred slurry reactor with the oxygenated solvent Polyethylene Glycol-400 (PEG-400). The selectivity of mixed alcohols in the products reached as high as 95 wt.% and the C₂₊ alcohols (mainly ethanol) was more than 40 wt.% in the total alcohol products at the reaction conditions of 250 °C, 3.0 MPa, H₂/CO = 2 and space velocity = 360 ml/g_cat h. The hydrogen temperature programmed reduction (H₂-TPR) and X-ray photoelectron spectroscopy (XPS) measurements of the catalyst confirmed that the FeO phase was responsible for the high selectivity to mixed alcohols in the process. And the oxygenated solvent PEG-400 was also necessary for the selective synthesis of mixed alcohols in the reaction system.     

Activity and characterization of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for the SCR of NO with CO in the presence of SO2 and HCl

SO₂ and HCl are major pollutants emitted from waste incineration processes. Both pollutants are difficult to remove completely and can enter the catalytic reactor. In this work, the effects of SO₂ and HCl on the performance of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for NO removal were investigated in simulated waste incineration conditions. The characterizations of the catalysts were analyzed by BET, SEM/EDS, XRD, and ESCA. Experimental results indicated the 1%Rh/Al₂O₃ catalyst was significantly deactivated for NO and CO conversions when SO₂ and HCl coexisted in the flue gas. The addition of between 2 and 10 wt.% Na promoted the activity of the 1%Rh/Al₂O₃ catalyst for NO removal, but decreased the CO oxidation and BET surface area. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of RhCl₃. Adding Na to the Rh/Al₂O₃ catalyst decreased the inhibition of SO₂ because of the formation of Na₂SO₄, which was observed in the XRD and ESCA analyses. SEM mapping/EDS showed that more S was residual on the surface of the Rh-Na/Al₂O₃ catalyst than Cl.     

Catalyst deactivation and regeneration in low temperature ethanol steam reforming with Rh/CeO2–ZrO2 catalysts

Rh/CeO₂–ZrO₂ catalysts with various CeO₂/ZrO₂ ratios have been applied to H₂ production from ethanol steam reforming at low temperatures. The catalysts all deactivated with time on stream (TOS) at 350 °C. The addition of 0.5% K has a beneficial effect on catalyst stability, while 5% K has a negative effect on catalytic activity. The catalyst could be regenerated considerably even at ambient temperature and could recover its initial activity after regeneration above 200 °C with 1% O₂. The results are most consistent with catalyst deactivation due to carbonaceous deposition on the catalyst.     

CeO2–ZrO2 Solid Solution Catalyst for Selective Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide

CeO₂–ZrO₂ solid solution catalysts are very effective for the selective synthesis of dimethyl carbonate from methanol and CO₂. The activity was much dependent on the calcination temperature. The higher the calcination temperature, the higher the activity of the catalyst for DMC formation, though the BET surface area is lower on the catalyst calcined at higher temperature.     

Role of support in reforming of CH4 with CO2 over Rh catalysts

The reforming of CH₄ with CO₂ over supported Rh catalysts has been studied over a range of temperatures (550–1000 K). A significant effect of the support on the catalytic activity was observed, where the order was Rh/Al₂O₃>Rh/TiO₂>Rh/SiO₂. The catalytic activity of Rh/SiO₂ was promoted markedly by physical mixing of Rh/SiO₂ with metal oxides such as Al₂O₃, TiO₂, and MgO, indicating a synergetic effect. The role of the metal oxides used as the support and the physical mixture may be ascribed to the promotion in dissociation of CO₂ on the surface of Rh, since the CH₄ + CO₂ reaction is first order in the pressure of CO₂, suggesting that CO₂ dissociation is the rate-determining step. The possible model of the synergetic effect was proposed.     

CO2 Hydrogenation on Rh/TiO2 Previously Reduced at Different Temperatures

Hydrogenation of CO₂ was studied on 1% Rh/TiO₂ reduced at different temperatures. The interaction of CO₂ with the catalyst and that of the CO₂+H₂ mixture was also studied. FTIR and TPD measurements revealed that CO₂ dissociation depends on the reduction temperature of the catalyst. In the surface reaction, besides Rh carbonyl hydride, formate groups and different carbonates and surface formyl species were also formed. The surface concentration of the formyl group depended on the reduction temperature. The initial rate of CO₂ hydrogenation significantly increased with increasing reduction temperature but after some time it drastically decreased. The promotion effect of the reduction temperature was explained by the formation of oxygen vacancies on the perimeter of the Rh/TiO₂ interface, which can be re-oxidized by the adsorption of CO₂ and H₂O.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.     

Autothermal reforming of ethanol for hydrogen production over an Rh/CeO2 catalyst

Hydrogen production from ethanol by autothermal reforming over an Rh/CeO₂ catalyst was investigated with a stoichiometric feed composition. Ethanol as well as the reaction intermediates like acetaldehyde and acetone was entirely converted to hydrogen and C1 products at 673 K, and methane steam reforming and reverse water gas shift were the major reactions above 823 K. The Rh/CeO₂ catalyst exhibited stable activity and selectivity during 70 h on-stream operation at 823–923 K without obvious deactivation evidenced by the constant effluent gas composition. Structural analysis of the used catalyst revealed that CeO₂ prevented effectively the highly dispersed Rh particles with sizes of 1–3 nm from sintering and thus maintained sufficient Rh–CeO₂ interfacial areas, which facilitated coke gasification through the high oxygen storage-release capacity.     

The kinetics of CO2 hydrogenation on a Rh foil promoted by titania overlayers

Submonolayer deposits of titania on a Rh foil have been found to increase the rate of CO₂ hydrogenation. The primary product, methane, exhibits a maximum rate at a TiO _x coverage of 0.5 ML which is a factor of 15 higher than that over the clean Rh surface. The rate of ethane formation displays a maximum which is 70 times that over the unpromoted Rh foil; however, the selectivity for methane remains in excess of 99%. The apparent activation energy for methane formation and the dependence of the rate on H₂ and CO₂ partial pressure have been determined both for the bare Rh surface and the titania-promoted surface. These rate parameters show very small variations as titania is added to the Rh catalyst. The methanation of CO₂ is proposed to start with the dissociation of CO₂ into CO_(a) and O_(a), and then proceed through steps which are identical to those for the hydrogenation of CO. The increase in the rate of CO₂ hydrogenation in the presence of titania is attributed to an interaction between the adsorbed CO, released by CO₂ dissociation, and Ti³⁺ ions located at the edge of TiO _x islands covering the surface. Differences in the effects of titania promotion on the methanation of CO₂ and CO are discussed in terms of the mechanisms that have been proposed for these two reactions.     

Effect of H2S on the hydrogenation of carbon dioxide over supported Rh catalysts

The hydrogenation of CO₂ was studied on supported noble metal catalysts in the presence of H₂S. In the reaction gas mixture containing 22 ppm H₂S the reaction rate increased on TiO₂ and on CeO₂ supported metals (Ru, Rh, Pd), but on all other supported catalysts or when the H₂S content was higher (116 ppm) the reaction was poisoned. FTIR measurements revealed that in the surface interaction of H₂ + CO₂ on Rh/TiO₂ Rh carbonyl hydride, surface formate, carbonates and surface formyl were formed. On the H₂S pretreated catalyst surface formyl species were missing. TPD measurements showed that adsorbed H₂S desorbed as SO₂, both from TiO₂-supported metals and from the support. IR, XP spectroscopy and TPD measurements demonstrated that the metal became apparently more positive when the catalysts were treated with H₂S and when the sulfur was built into the support. The promotion effect of H₂S was explained by the formation of new centers at the metal/support interface.     

Relationship between oxidation states of copper supported on alumina and activities for catalytic combustion of NH3

The relationship between the oxidation state of Cu supported on an alumina catalyst (Cu/Al₂O₃) and the activity for combustion of NH₃ was investigated. Combustion of NH₃ on the catalyst treated in hydrogen at 800°C occurred at lower temperature than on the catalyst treated in air. It was also much better than Pt and Rh catalysts for the conversion of NH₃ to N₂. Characteristics of the catalyst were investigated by XRD, XPS, and the N₂O pulse injection method to understand the reason of its high catalytic activity. The reason of the high activity of the catalyst treated in hydrogen at the high temperature was attributed to the lower oxidation state of Cu in the catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative NOx Reduction Behavior of Pt, Pd, and Rh Supported Catalysts in Simulated Exhaust Gases as a Function of Oxygen Concentration

NO _x reduction activity on Pt and Pd catalysts had a maximum for S value as stoichiometry number at a fixed temperature, and the S value at the maximum NO _x conversion increased with decreasing temperature. NO _x conversion on Rh catalyst increased with decreasing S value, but independent of temperature. As for the effect of HC on NO _x reduction behavior, it was concluded that, for Pt and Pd catalysts, HC adsorbs strongly on the catalysts surface to cause the self-inhibition. Increasing O₂ concentration lead to oxidation of HC, but decreased the value of NO/O₂ ratio. The balance point of the two factors generated a maximum NO _x conversion. For Rh catalyst, the strongly adsorbed oxygen is more reactive with decreasing S value, and thus NO _x conversion is increased.     

High activity and stability of the Rh‐free Co‐based ex‐hydrotalcite containing Pd in the catalytic decomposition of N2O

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite‐like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co‐precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N₂ adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co‐based HTlc's improves considerably the catalytic activity. Co–Rh,Al‐HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex‐Co,Pd–La,Al‐HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh‐containing catalyst, both in a N₂O‐containing stream and in one containing also SO₂ and O₂, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO₂ and O₂ are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

From Electron Density Flow Towards Activation: Benzene Interacting with Cu(I) and Ag(I) Sites in ZSM-5. DFT Modeling

The effects of support pretreatment with nC₁–C₅ alcohols on the performance of Rh–Mn–Li/SiO₂ catalyst in the synthesis of C₂-oxygenates from syngas have been investigated by CO hydrogenation reaction, transmission electron microscopy (TEM), pulse adsorption of CO and H₂, and Fourier Transform infrared (FT-IR) spectroscopy. The catalysts prepared from the pretreated silica supports exhibited higher space time yields of C₂-oxygenates (STY_C2-oxy) and selectivities towards C₂-oxygenates (S_C2-oxy) than that prepared from the untreated silica support. The enhancement in the hydrophobicity of the pretreated silica supports would be favorable for increasing Rh dispersion and ratio of Rh⁺/Rh⁰ sites, therefore increasing the number of active sites, especially the active sites for CO insertion. Such variations are responsible for the improvements in the catalytic performance of the Rh–Mn–Li/SiO₂ catalyst.     

Highly active and stable Rh/MgOAl2O3 catalysts for methane steam reforming

Highly active and coke-resistant Rh catalysts were developed for methane steam reforming in microchannel chemical reactors. Rh loading was optimized on a stable MgOAl₂O₃ support to improve the volumetric productivity for methane conversion. Catalyst activities were stable over a wide range of steam/carbon ratios. In particular, experimental results demonstrated that Rh/MgOAl₂O₃ catalysts are extremely active for methane steam reforming and are resistant to coke formation at stoichiometric steam/carbon ratio of 1 for over 14 h time-on-stream with no sign of deactivation. Methane steam reforming activities on this catalyst is compared in both a microchannel reactor and a conventional micro-tubular reactor. Significant performance enhancement was observed in microchannel reactors owing to improved heat and mass transfer.