Activity Enhancement of Bimetallic Rh-Ag/Al2O3 Catalysts for Selective Catalytic Reduction of NO by C3H6

The catalytic performance of bimetallic rhodium–silver clusters for the selective catalytic reduction of NO by propylene has been examined over 1%(Rh-Ag)/A₂O₃ catalysts of variable Ag content. It was found that substitution of small amounts of Rh by Ag results in a significant increase of the catalytic activity, which goes through a maximum for catalysts with metal compositions of ca. 95%Rh-5%Ag. This behaviour is tentatively attributed to the formation of rhodium-rich phase alloys, which may stabilize Rh in its reduced state at reaction conditions.     

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.     

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)₂ as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H₂ chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.     

LaMnO3 Perovskite Supported Noble Metal Catalysts for the Total Oxidation of Methane

Two series of LaMnO₃ supported noble metal (Pt, Pd, Rh) catalysts prepared by the citrate method and calcined in air at 600 and 800 °C, respectively, were investigated. The catalysts resulting from method A were prepared by simultaneous incorporation of the noble metals during perovskite preparation and those following method B were generated by impregnation of the calcined perovskites with the noble metal compounds. The noble metals form solid solutions with the perovskite lattice. Reduction of the catalysts with hydrogen prior to the catalytic reaction led to a significant enhancement of the catalytic activity. During the catalytic reaction, the noble metal clusters are partially transformed to highly dispersed noble metal oxides or nonstoichiometric noble metal oxide phases, which are the catalytically active phases for the total oxidation of methane. The best results were obtained with the Pd containing catalysts prepared by method B.     

Effects of Na, Cu, Ni and Co Modifications on the Activity and Characteristics of Rh/Al2O3 Catalysts for NO Reduction

This study used different metals to modify Rh/Al₂O₃ catalysts for NO reduction in a simulated waste incineration flue gas containing 6% O₂. The characteristics of the modified catalysts were analyzed using BET, TEM and XRD. The results of the experiment reveal that Na addition can significantly affect the properties of Rh/Al₂O₃ catalysts on the BET surface area and Rh metal dispersion. Furthermore, Na addition was found to significantly enhance the NO conversion of Rh/Al₂O₃ at 250–350 °C. On the contrary, Cu, Ni, and Co addition was found to have slight suppression effects.     

Partial oxidation of propylene MoO3/iron tellurite catalysts

The catalytic behaviour of the series Fe₂(TeO₃)₃ + × % MoO₃ (X = 0–30 mol. %) in the oxidation of propylene in the gaseous phase has been studied. It has been found that the addition of Mo0₃ to the iron tellurite activates the reaction towards the partial oxidation to acrolein. From the characterisation of solids it is possible to conclude that the addition of MoO₃ promotes the crystallinity of the iron tellurite. On the other hand, surface studies of the catalysts show that depending on the MoO₃ loading, this component is present in various states which contribute in different ways to the catalytic action. Some explanations for the observed behaviour are proposed.     

The effect of sodium on the catalytic activity of ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5 for the transesterification of vegetable oil with methanol

In order to elucidate the effect of sodium on the activity of ZSM-5 supported metal oxides catalysts (ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5) for the transesterification of soybean oil with methanol, ZSM-5 supported metal oxides were prepared with and without sodium hydroxide by impregnation. The metal compositions of the ZSM-5 supported metal oxide catalysts and the metal concentrations dissolved from the catalysts to the methylester phase were measured by SEM-EDS and inductive coupled plasma spectroscopy, respectively. The catalytic activity of ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5 containing sodium did not originate from surface metal oxides sites, but from surface sodium sites or dissolved sodium leached from the catalyst surface.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

Catalytic decomposition of N2O over supported rhodium catalysts: high activities of Rh/USY and Rh/Al2O3 and the effect of Rh precursors

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over Al₂O₃-supported and zeolite-supported Rh catalysts. The activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts prepared from Rh(NO₃)₃ were higher than those of Rh/ZSM-5 and Rh/ZnO reported in the literature, while the activity of a Rh/Al₂O₃ catalyst prepared from RhCl₃ was suppressed severely in spite of the high H/Rh and CO/Rh values. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. This revised version was published online in July 2006 with corrections to the Cover Date.     

In-situ XAFS Analysis of Y Zeolite-Supported Rh Catalysts during High-Pressure Hydrogenation of CO2

In-situ XAFS (X-ray absorption fine structure) analysis under high-pressure, high-temperature reaction conditions was done to clarify the effect of Li doping on the structures of Rh species in Rh ion-exchanged NaY zeolite catalysts (RhY). The analysis showed that Rh species in RhY were reduced at a lower temperature than those in Li-doped RhY (Li/RhY). The reduction of Rh in RhY at a lower temperature was related to the formation of amorphous Rh oxide species during the initial stage of reduction, which was not observed in Li/RhY catalysts. The average particle size of metallic Rh species formed under the reaction conditions was 1.3 nm for RhY catalysts and 0.8 nm for Li/RhY catalysts. During exposure to either CO or air, the metallic Rh species in RhY were more easily subject to structural changes than the metallic Rh species in Li/RhY. These differences in the properties of Rh species between the RhY and Li/RhY catalysts resulted from the replacement of hydroxyl groups on the zeolite surface by O–Li compounds, and caused different catalytic activities during CO₂ hydrogenation.     

Rh/γ-Al2O3–sepiolite monolithic catalysts for decomposition of N2O traces

The behaviour of Rh/γ-AlO₃–sepiolite monolithic systems in the N₂O decomposition reaction using catalysts with rhodium contents between 0.2 and 0.8 wt.% were analysed. The observed increase of the catalytic activity with the metal loading was related to an enhancement in the number of active sites. The SEM–EDX results showed that rhodium was mainly located on the alumina particles. The variation of the reaction rate values with the inlet N₂O and O₂ concentrations, suggested an apparent first order equation with respect to N₂O and zero order for O₂. The presence of NH₃ traces or water vapour in the feed decreased the catalytic activity, probably due to the competitive adsorption of NH_x or OH species. The ammonia oxidation reaction took place, producing N₂O as a secondary product. The monolithic catalysts developed operate with a low pressure drop achieving a N₂O conversion higher than 90% at 773 K in the presence of oxygen, ammonia or water vapour in the stream.     

Surface reconstruction of supported Pd on LaCoO3: Consequences on the catalytic properties in the decomposition of N2O

This study deals with the catalytic decomposition of N₂O over noble metal based catalysts. The deposition of palladium on reducible supports such as LaCoO₃ leads to higher activity from temperature-programmed experiments in comparison with conventional supports such as alumina. Such a different catalytic behaviour cannot be completely explained by changes in the metal dispersion but also by the extent of the metal/support interaction. Interestingly, successive reductive (250 °C in H₂) and oxidative thermal treatments in the reactant mixture at high temperature enhance the conversion of N₂O particularly on perovskite support. Additional surface characterisations show a re-dispersion and the stabilisation of palladium species in unusual oxidation states which would originate a rate enhancement in the decomposition of N₂O.     

Conditioning of Rh/α-Al2O3 catalysts for H2 production via CH4 partial oxidation at high space velocity

Experimental evidence and literature indications suggest that the process of methane partial oxidation over Rh catalysts is structure sensitive. Crystal phases and Rh cluster size are thus expected to affect the final catalytic performance. In this work, it is observed that outstanding performances are obtained when the as-prepared catalysts are conditioned through repeated runs at increasing temperature and O₂/CH₄ = 0.56. Catalysts slowly activate, that is CH₄ conversion and synthesis gas selectivity progressively grow with time on stream. On the basis of TPO and CH₄ decomposition measurements, this phenomenon is herein explained as the result of a surface reconstruction driven by the repeated exposition to the reaction at high temperature; it is thought that such reconstruction tends to eliminate defect sites and disfavors C-deposition reactions (extremely fast over steps and kinks). Conditioning with O₂-enriched feed streams makes conditioning faster, since the accumulation of surface C-species is suppressed; however, the catalyst is eventually less active than a catalyst conditioned with standard feed mixtures. As an alternative, accumulation of carbon can be suppressed and surface reconstruction proceeds faster if the catalyst is directly exposed to the reaction at high temperature for several hours.     

Metal-support interaction: titania-supported nickel-iron catalysts

The influence of the titania support and metal particle size on the performance of nickel-iron catalysts in the Fischer-Tropsch synthesis has been studied by varying the nickel-iron loading and, consequently, varying the nickel-iron particle size. Low-loaded titania-supported nickel-iron catalysts (2 wt%) turned out to be more selective towards higher hydrocarbons than high-loaded nickel-iron catalysts (20 wt%), which produce much more methane. From temperature-programmed hydrogenation experiments, magnetic measurements and Mössbauer spectroscopy it followed that different types of carbon are present on the catalysts depending on the metal loading. These types of carbon could be related to the selectivity in Fischer-Tropsch synthesis and to the deactivation of the catalysts with time on stream. These differences in catalytic behaviour are most likely due to the presence of TiO _x species on the surface of the active phase, which species decrease the CO and H₂ adsorption properties of the catalyst, but increase the dissociation of CO.     

Combination of flame synthesis and high-throughput experimentation: The preparation of alumina-supported noble metal particles and their application in the partial oxidation of methane

Mono and multi-noble metal particles on Al₂O₃ were prepared in one step by flame spray pyrolysis (FSP) of the corresponding noble metal precursors dissolved in methanol and acetic acid (v/v 1:1) or xylene. The noble metal loading of the catalysts was close to the theoretical composition as determined by WD-XRF and LA-ICP-MS. The preparation method was combined with high-throughput testing using an experimental setup consisting of eight parallel fixed-bed reactors. Samples containing 0.1–5 wt% noble metals (Ru, Rh, Pt, Pd) on Al₂O₃ were tested in the catalytic partial oxidation of methane. The ignition of the reaction towards carbon monoxide and hydrogen depended on the loading and the noble metal constituents. The selectivity of these noble metal catalysts towards CO and H₂ was similar under the conditions used (methane: oxygen ratio 2:1, temperature from 300 to 500 °C) and exceeded significantly those of gold and silver containing catalysts.Selected catalysts were further analysed using XPS, BET, STEM-EDXS and XANES/EXAFS. The catalysts exhibited generally a specific surface area of more than 100 m²/g, and were made up of ca. 10 nm alumina particles on which the smaller noble metal particles (1–2 nm, partially oxidized state) were discernible. XPS investigation revealed an enrichment of noble metals on the alumina surface of all samples. The question of alloy formation was addressed by STEM-EDXS and EXAFS analysis. In some cases, particularly for Pt–Pd and Pt–Rh, alloying close to the bulk alloys was found, in contrast to Pt–Ru being only partially alloyed. In situ X-ray absorption spectroscopy on selected samples was used to gain insight into the oxidation state during ignition and extinction of the catalytic partial oxidation of methane to hydrogen and carbon monoxide.     

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.     

Hydrogen production by ethanol reforming over Rh/CeO2–ZrO2 catalysts

Reforming of ethanol in excess of water (1–8 molar ratio) has been investigated on Rh/CeO₂, Rh/ZrO₂ and Rh/CeO₂–ZrO₂ (Ce/Zr=4, 2 and 1). Catalysts characterization was conducted by X-ray diffraction, BET surface area measurements, CO₂ adsorption, and temperature programmed reduction (TPR). At 400–500 °C all catalysts showed high activity and selectivity towards hydrogen production (between 5 and 5.7 mol of H₂ per mol of ethanol inlet) despite the considerable textural differences of the oxides (fluorite, monoclinic and tetragonal). The large variations of Rh dispersion (as monitored by TPR) between all catalysts had a small effect on H₂ production. Although it appears that the reaction is not sensitive to either the oxide or the metal structure Rh/CeO₂ (the most basic catalyst investigated) was the least reactive.     

Nanosized Pd/Pt and Pd/Rh Catalysts for Naphthalene Hydrogenation and Hydrogenolysis/Ring-opening

Pd/Rh and Pd/Pt catalysts supported on two different mesoporous materials – a Zr-doped MCM-41-type silica [Si/Zr = 5 w/w (SiZr)] and a commercial silica-alumina [Si/Al = 40:60 w/w (SiAl)] – were prepared by incipient wetness impregnation using nanosized suspensions of alloy particles prepared by polyol-mediated synthesis in diethylene glycol (DEG). The catalytic behaviour of these catalysts was investigated in the hydrogenation and hydrogenolysis/ring-opening of naphthalene at 6.0 MPa, by checking the role of both the main reaction conditions (temperature, contact time and H₂/naphthalene molar ratio) and increasing amounts of dibenzothiophene (DBT). The catalysts supported on SiAl showed higher activity than catalysts supported on SiZr, thus suggesting that activity is favoured by higher acidity of the support and/or higher interaction of the nanosized metal particles with the support. While using the SiZr support, weaker metal-support interactions took place by forming catalysts with bigger metal and/or metal oxide particles. Besides, the catalyst with lowest noble-metal content (0.3 wt.%) (SiAl-0.3Pd/Pt-5) had the greatest acidity and metal surface and, consequently, the highest activity. Furthermore, it exhibited a good thiotolerance in presence of increasing amounts of DBT in the feed, thus maintaining a high catalytic activity in the hydrogenation of naphthalene, although with decreased yield in trans- and cis-decalin (decahydronaphthalene or DeHN) and high-molecular-weight compounds (H.M.W.), with a corresponding increased yield in the partially hydrogenated tetralin (tetrahydronaphthalene or TeHN).     

CO2 hydrogenation reactivity and structure of Rh/SiO2 catalysts prepared from acetate,chloride and nitrate precursors

Silica-supported Rh catalysts (Rh/SiO₂) were prepared from acetate, chloride and nitrate precursors by an impregnation method and were applied to CO₂ hydrogenation reaction. CO₂ conversion over the catalyst prepared from chloride precursor was lower than that over acetate or nitrate one, because of fewer active sites on catalysts, as estimated by H₂ chemisorption. The main product was CO over the catalysts prepared from acetate and nitrate, but it was CH₄ over the catalyst prepared from chloride precursor. Characterization of catalysts by TEM, FT-IR and XPS was carried out in order to elucidate the effect of metal precursor on the CO₂ hydrogenation reactivity. The results of XPS showed that the O atomic ratio to Rh on surface hydroxyl groups increased in the order: chloride<nitrate<acetate precursor. The ratio of hydroxyl groups to Rh particles on SiO₂ surface was expected to have a significant influence on the reactivity.     

Modification of the catalytic properties of supported noble metal catalysts by carrier doping

The influence of altervalent cation doping of TiO₂ carriers on the chemisorptive and catalytic properties of supported Pt and Rh crystallites has been investigated. It was observed that doping of the carrier with higher valence cations leads to suppression of the H₂ and CO chemisorption capacity of Pt catalysts, while their activity in hydrogenation and oxidation reactions is significantly reduced. The opposite effects were observed in the case of Rh catalysts supported on higher valence doped TiO₂. These catalysts were found to possess higher activity in CO and CO₂ hydrogenation, in aromatics hydrogenation and in CO and C₂H₄ oxidation. Their stability characteristics were also found to be superior to those of the undoped Rh/TiO₂ catalyst. These effects are believed to originate from an electronic type interaction at the metal-support interface, induced by doping, which results in electron transfer from the support to the metal crystallites.