Oxidation state of platinum clusters during the reduction of NOx with propene and propane

The oxidation state of platinum supported on mesoporous SiO₂ and Al₂O₃ with MCM-41 type structure during the reduction of NO_x with propene or propane was investigated using in situ X-ray absorption spectroscopy. Platinum supported on MCM-41 (SiO₂) was reduced at low and oxidized at high reaction temperatures when propene was used as reducing agent, while it was found to be always oxidized in Pt/MCM-41 (Al₂O₃). When propane was used as reducing agent significant NO conversion was not observed over Pt/MCM-41 (SiO₂) and on both supports platinum was in an oxidized state. At the successive adsorption of the reactants, the prereduced catalysts were oxidized after NO adsorption and reduced after addition of the hydrocarbons. Addition of oxygen re-oxidized the catalysts, while the presence of water vapor did not influence the oxidation state.     

Catalytic reduction of NOx over transition-metal-containing MCM-41

The catalytic properties of Pt, Rh and Co supported on mesoporous molecular sieves with MCM-41-type structure consisting of SiO₂ and Al₂O₃ were studied for the reduction of NO with propene. Pt supported on siliceous MCM-41 was the most active catalyst, however, significant quantities of undesirable N₂O were formed during the reaction. Pt supported on mesoporous Al₂O₃ and Rh supported on both mesoporous oxides showed a lower activity, but an improved selectivity towards N₂ formation. Co supported on MCM-41-type materials had only a low level of activity for the reduction of NO with propene. For Pt supported on MCM-41-type materials only a minor decrease in the activity was observed when water vapor was added into the reactant gas mixture, while on Rh- and Co-containing catalysts the activity strongly decreased. This revised version was published online in July 2006 with corrections to the Cover Date.     

Platinum catalysts supported on hydrothermally stable mesoporous aluminosilicate for the catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs)

Catalytic oxidation of polycyclic aromatic hydrocarbons (PAHs) was studied over platinum catalysts supported on the hydrothermally stable mesoporous aluminosilicate (SM-41). Naphthalene was chosen as a model reactant of PAHs, due to the simplest and the least toxic PAHs. Zeolite seeds crystallization method was used for synthesis of SM-41. Pt/SM-41 catalyst showed higher activity than Pt/MCM-41 for catalytic oxidation of naphthalene in the presence of 10 vol.% water vapor. Hydrothermal stability and hydrophobicity of Pt/SM-41 must be beneficial for the catalytic oxidation of naphthalene in the presence of water vapor.     

Influence of Catalyst Composition on NOX Trap Performances

In the present work the influence of the type of noble metals present in NO_X storage catalysts was investigated regarding the NO oxidation, NO_X storage and NO reduction performances. Monolith samples with combinations of platinum, palladium and rhodium were tested in a flow-reactor. From this study, it was concluded that platinum is necessary for NO oxidation while palladium shows a better ability to oxidize NO to NO₂ at high temperature. The presence of Rh on Pt/Rh and Pt/Pd/Rh was found to have a negative effect on NO oxidation reaction. Results suggest also that the storage capacity is partially linked to the oxidation function: Surface area developed by the storage material and the Pt–Ba interaction have an important influence on NO_X storage function. The rich step was not efficient to purge completely the trap and leads to accumulation of NO_X and so to performances degradation which was assigned to accumulation of NO_X after each cycle and storage on of Ba-bulk sites. In presence of Rh, a high oxygen storage capacity leads to high exothermic process between reducing agent and stored oxygen which improves the ability of catalyst to release NO_X.     

Selective catalytic reduction of NOx in lean-burn engine exhaust over a Pt/V/MCM-41 catalyst

The activities of Pt supported on various metal-substituted MCM-41 (V-, Ti-, Fe-, Al-, Ga-, La-, Co-, Mo-, Ce-, and Zr-MCM-41) and V-impregnated MCM-41 were investigated for the reduction of NO by C₃H₆. Among these catalysts, Pt supported on V-impregnated MCM-41 showed the best activity. The maximum conversion of NO into N₂+N₂O over this Pt/V/MCM-41 catalyst (Pt=1 wt.%, V=3.8 wt.%) was 73%, and this maximum conversion was sustained over a temperature range of 70 °C from 270 to 340 °C. The high activity of Pt/V/MCM-41 over a broad temperature range resulted from two additional reactions besides the reaction occurring on usual supported Pt, the reaction of NO with surface carbonaceous materials, and the reaction of NO occurring on support V-impregnated MCM-41. The former additional reaction showed an oscillation characteristic, a phenomenon in which the concentrations of parts of reactant and product gases oscillate continuously. At low temperature, some water vapor injected into the reactant gas mixture promoted the reaction occurring on usual supported Pt, whereas at high temperature, it suppressed the additional reaction related to carbonaceous materials. Five-hundred parts per million of SO₂ added to the reactant gas mixture only slightly decreased the NO conversion of Pt/V/MCM-41.     

Pt/MCM-41 catalyst for selective catalytic reduction of nitric oxide with hydrocarbons in the presence of excess oxygen

First results are reported on the use of MCM-41 mesoporous molecular sieve as the support for Pt for the selective catalytic reduction of NO by hydrocarbons in the presence of O₂. MCM-41 provided the highest specific NO reduction rates for Pt as compared with all other supports reported in the literature, i.e., Al₂O₃, SiO₂ and ZSM-5. This revised version was published online in July 2006 with corrections to the Cover Date.     

Basic Cs-Pt/MCM-41 Catalysts: Synthesis, Characterization and Activity in n-Hexane Conversion

Cs-Pt/MCM-41 catalysts with basic character have been prepared by selective adsorption of Pt(NH₃)₄(OH)₂ at pH = 8 on pure silica MCM-41 followed by incipient-wetness impregnation of CsNO₃, then oxidation and reduction. The influence of the amount of Cs (0, 2 and 4 wt%) on the size of the reduced Pt particles and on their catalytic behavior in the conversion of n-hexane has been investigated. Whatever the Pt content (0.7 or 2.0 wt% Pt), the addition of Cs results in a higher Pt dispersion, a higher initial activity and a higher selectivity for aromatization. On the more basic Pt-richest MCM-41 sample (4 wt% Cs, 2 wt% Pt) the selectivity to benzene reaches values close to those obtained on Cs-enriched Pt/Cs-BEA zeolite. However, the catalytic stability is lower. The catalytic trends are discussed in view of the physicochemical properties of the solids determined by chemical analysis, N₂ physisorption, XRD and TEM before and after catalytic test.     

Adsorption and interaction of NO, C3H6 and O2 on Pt, Zr, Nb-MCM-41—FTIR study

Adsorption of NO, O₂ and C₃H₆ on the MCM-41 matrices with Nb and Zr loaded with Pt has been studied by the FTIR spectroscopy to characterize these materials as catalysts in the selective reduction of NO with propene. Two types of the catalysts have been studied differing by the methods of Zr and Nb introduction: either by one-pot (group 1) or by post-synthesis impregnation (group 2) and hence by the location of Nb and Zr in the framework (group 1) or extra framework (group 2). It has been found that the positions of these metals in the MCM-41 matrix determine the platinum dispersion, acidic–basic properties and influence the interaction of NO + O₂ + C₃H₆ with the catalyst surfaces. The fact that the Pt dispersion is much higher in group 2 materials has been revealed by results of XRD patterns and TEM images. According to the explanation proposed, the presence of Lewis acid–base pairs in the group 2 of catalysts has strongly activated chemisorption of propene, whereas Lewis basicity, characterized by 2-PrOH dehydrogenation on the samples containing transition metals introduced during the synthesis (group 1), has enhanced chemisorption of nitrite species on platinum. It has been proved that nitrite species have not been stored on Pt/Zr/MCM-41 samples, whereas they have been stabilized on Pt/Zr/Nb/MCM-41 containing BrØnsted acidic centres.     

Organometallic Rhodium Complexes Encapsulated in Mesoporous Molecular Sieves

Rh(III) complexes both dimmer [Cp*RhCl]₂(μ-Cl)₂ and monomer ([RhCp*(S)₃]²⁺) were encapsulated into MCM-41 channels. All silica MCM-41 molecular sieve and aminosililated MCM-41 matrix were used for rhodium complexes accommodation. Reactivity of Cp* rhodium complexes encapsulated in meso structure was estimated on the grounds of their susceptibility to interaction with CO molecules resulting in the formation of carbonyl complexes. Formation of Cp*Rh carbonyls was recorded by means of FTIR spectra. It was found that accommodation of Rh(III) complexes in MCM-41 molecular sieves activated the complex and led to the formation of Rh(III)Cp* carbonyls as a result of contact with CO. Contact of rhodium (III) complexes encapsulated in MCM-41 matrix with CO did not result in rhodium (III) reduction, whereas in the presence of amine groups in aminosililated MCM-41 the reduction of Rh(III) to Rh(I) occurred relatively easily and formation of Cp*Rh(CO)₂ complex containing Rh(I) was noted. Encapsulated rhodium complexes showed some activity in methanol carbonylation reaction carried out under heterogeneous conditions. For the most active catalyst the amount of methyl acetate reached about 8 mol.%, however, deactivation of catalyst occurred and after 2 h on stream methyl acetate was not found in the product.     

Catalytic activity of Pt and tungstophosphoric acid supported on MCM-41 for the reduction of NOx in the presence of water vapor

The catalytic activity of tungstophosphoric acid and Pt loaded MCM-41 for the catalytic reduction of NO_x with propene in the presence of water vapor was studied. Pt/MCM-41 was found to be most active, the loading with H₃PW₁₂O₄₀ led to an improved selectivity to N₂ formation and to an enhanced activity in the presence of water vapor. Hydrated tungstophosphoric acid generated additional sorption sites for C₃H₆ and led to a higher local concentration of the reducing agent on the catalyst surface, which was found to increase the activity of the Pt and H₃PW₁₂O₄₀ loaded catalysts in the presence of water vapor.     

Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H₂O⁺/H₃O⁺) cause image formation of the O₂–H₂ reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO–H₂ reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the <100> zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO + H₂ reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.     

Kinetic Study of Metal-Catalyzed Reaction of Solid Calcium Oxide with Gas Mixtures of Methane and Water Vapor to Form Hydrogen

With noble metal catalysts (Pd, Pt, Rh, Ir) present, hydrogen is formed by the interaction of solid calcium oxide with gas mixtures of methane and water vapor, according to CaO + CH₄ + 2H₂O CaCO₃ + 4H₂. Among the metals, Ir and Rh are so active that the reaction takes place at temperatures as low as 600 K. Rate data obtained with these metals show a nearly first order with respect to CH₄ pressure, while a negative order with respect to H₂O vapor pressure. The apparent activation energies are 171 and 217 kJ/mol for the Ir- and Rh-catalyzed reactions, respectively. On the other hand, Ni does not catalyze the reaction below 733 K, probably due to its strong interaction with H₂O vapor.     

Electrochemical removal of both NO and CH4 under lean-burn conditions

An electrochemical cell, Pd|YSZ|Pd, was constructed in order to remove both NO and CH₄ in the presence of excess oxygen. When direct current was supplied to the cell with a flow of a mixture of NO, CH₄, O₂, H₂O and CO₂ at 700° C, NO was reduced to nitrogen at the cathode, and CH₄ was oxidized to CO _x at both the anode and cathode. At the cathode, the reduction of NO and the oxidation of CH₄ proceeded with the removal of chemisorbed oxygen species from the Pd surface, and at the anode, the oxidation of CH₄ was enhanced by forming an active oxygen atom.     

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C₂H₄ over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C₂H₄ were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al₂O₃ for CO or C₂H₄ oxidation. For NO decomposition, the H₂-reduced Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C₂H₄, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al₂O₃ has been reduced to zero valence during reduction by H₂ at 450°C, but in Co/γ-Al₂O₃ only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl₂O₄-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl₂O₄ which cannot be reduced by H₂ at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al₂O₃ has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al₂O₃. This difference mainly results from the distorting tetrahedrally- coordinated Co²⁺ ions which have lower coordination symmetry than Co⁰ in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al₂O₃ makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co⁰ is the main active phase. For NO decomposition and selective reduction, Co⁰ is also catalytically active, but it can be oxidized into Co₃O₄ by oxygen at high reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Steam Reforming of Ethanol Over Cobalt Catalyst Modified with Small Amount of Iron

Steam reforming of ethanol was examined over Co/SrTiO₃ with addition of another metal—Pt, Pd, Rh, Cr, Cu, or Fe—for promotion of the catalytic activity. Ethanol conversion and H₂ yield were improved greatly by adding Fe or Rh at 823 K. Although Rh addition promoted CH₄ formation, Fe addition enhanced steam reforming of ethanol selectively. A suitable amount of Fe loading was in the window of 0.33–1.3 mol%. A comparative study of the reaction over a catalyst supported on SiO₂ was conducted, but no additional effect of Fe was observed on the Co/SiO₂ catalyst. High activity of Fe/Co/SrTiO₃ catalyst came from interaction among Fe, Co, and SrTiO₃.     

Mechanisms of electrochemical reduction and oxidation of nitric oxide

A summary is given of recent work on the reactivity of nitric oxide on various metal electrodes. The significant differences between the reactivity of adsorbed NO and NO in solution are pointed out, both for the reduction and the oxidation reaction(s). Whereas adsorbed NO can be reduced only to hydroxylamine and/or ammonia, it takes NO in solution to produce N₂O and N₂. From the reduction of NO on a series on stepped single-crystal Pt electrodes, it is concluded that NO_ads reduction is not a structure sensitive process. The protonation of the adsorbed NO is rate-determining; neither the NO adsorption strength nor the NO bond breaking play a significant role in its reduction rate. Whereas adsorbed NO on polycrystalline Pt can only be oxidized to nitrate, in the presence of NO in solution nitrous acid HNO₂ may also be formed, in a potential region where adsorbed NO is otherwise stable. Interestingly, on Pt(1 1 1) and Pt(5 5 4) NO_ads may be oxidized to HNO₂ in a surface-bonded redox couple. Whereas surface oxides appear to catalyze the oxidation of solution NO to HNO₂, the further oxidation to nitrate seems to be inhibited by the presence of surface oxides. Both the reduction and oxidation of solution NO appear to be not very metal-dependent reactions, as they take place with approximately equal rate on all electrode metals studied, including gold. This suggests the involvement of weakly adsorbed intermediates, and the relatively unimportant role of surface-bonded NO in the bulk NO reduction and oxidation activity.     

NO reduction by C3H6 in excess oxygen over fresh and sulfated Pt‐ and Rh‐based catalysts

The selective catalytic reduction (SCR) of NO with C₃H₆ was studied over three noble‐metal‐based catalysts: 2% Pt/γ‐Al₂O₃, 2% Rh/γ‐Al₂O₃ and 1.5% Rh/TiO₂(4% WO₃). The SO₂ effect on the catalyst activity was examined using sulfated samples of the above catalysts and SO₂‐containing feeds. Temperature‐programmed desorption and oxidation studies were carried out to examine the adsorption characteristics of NO and C₃H₆, respectively, in the absence or the presence of SO₂. The adsorption data were linked to variations in the NO reduction rates over fresh and sulfated samples. Modification of the support surface as a result of the SO₂ presence affects the NO and propene sorption characteristics, the NO oxidation and the propene consumption rates. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of Sulfur or Nitrogen Poisoning on the Activity and Selectivity of Y-Zeolite-Supported Pt–Pd Catalysts in the Hydrogenation of Tetralin

The partial oxidation of methane (POM) to synthesis gas over SiO₂-supported Rh and Ru catalysts was studied by in situ microprobe Raman and in situ time-resolved FTIR (TR-FTIR) spectroscopies. The results of in situ microprobe Raman spectroscopic characterization indicated that no Raman band of Rh₂O₃ was detected at 500°C over the Rh/SiO₂ catalyst under a flow of a CH₄/O₂/Ar (2/1/45, molar ratio) mixture, while the Raman bands of RuO₂ can even be detected at 600°C over the Ru/SiO₂ catalyst under the same atmosphere. The experiments of in situ TR-FTIR spectroscopic characterizations on the reactions of CH₄ over O₂ pre-treated Rh/SiO₂ and Ru/SiO₂ catalysts indicated that the products of CH₄ oxidation over Rh/SiO₂ and Ru/SiO₂ greatly depend on the concentration of O²– species over the catalysts. On the catalysts with high concentration of O²–, CH₄ will be completely oxidized to CO₂. However, if the concentration of O²– species over the catalysts is low enough, CH₄ can be selectively converted to CO without the formation of CO₂. The parallel experiments using in situ TR-FTIR spectroscopy to monitor the reaction of the CH₄/O₂/Ar (2/1/45, molar ratio) mixture over Rh/SiO₂ and Ru/SiO₂ catalysts show that the mechanisms of synthesis gas formation over the two catalysts are quite different. On the Rh/SiO₂ catalyst, synthesis gas is mainly formed by the direct oxidation of CH₄, while on the Ru/SiO₂ catalyst, the dominant pathway of synthesis gas formation is via the sequence of total oxidation of CH₄ followed by reforming of unconverted CH₄ with CO₂ and H₂O. The significant difference in the mechanisms of partial oxidation of CH₄ to synthesis gas over Rh/SiO₂ and Ru/SiO₂ catalysts can be well related to the difference in the concentration of O²– species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the two metals.     

FTIR study of NO, C3H6 and O2 adsorption and interaction on gold modified MCM-41 materials

This paper is devoted to the detailed FTIR study of the adsorption, co-adsorption, and interaction of all the reagents used in NO HC-SCR process addressed to lean-burn engines with the surface of new gold catalysts based on ordered mesoporous materials. Gold was introduced into silicate and niobiosilicate matrices by the impregnation (Au/MCM-41 and Au/NbMCM-41, respectively) and via co-precipitation with siliceous and niobium sources (AuNbMCM-41). The in situ FTIR study allowed the estimation of the possible chemisorption of the reagents and their interaction towards intermediates, depending on the chemical composition of the catalyst and the way of gold introduction. It has been found that propene is chemisorbed, but not, NO, on gold species at room temperature. Chemisorbed C₃H₆ interacts with NO only in the presence of oxygen excess. Oxygen oxidizes NO to NO₂, the latter interacts with chemisorbed propene towards carboxylates (1570 cm⁻¹) and NO₂ is reduced to N₂O. At higher temperatures carboxylates interact with gaseous NO to carbonate, N₂O, CO and CO₂. The presence of niobium in the NbMCM-41 matrix enhances the oxidative properties of the catalysts and as a consequence the interaction between intermediates in NO reduction with propene in the oxygen excess. The co-precipitated AuNbMCM-41 exhibits higher NOx storage properties than the impregnated one.     

Complete Oxidation of Methane at Low Temperature over Pt Catalysts Supported on High Surface Area SnO2

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH₄ traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H₂S (100 vol.ppm). Their catalytic properties were compared to those of Pd/Al₂O₃ and Pt/Al₂O₃. In the absence of H₂S in the feed, Pt/SnO₂appears as a very promising catalyst for CH₄ oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al₂O₃. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO₂ is slightly less resistant than Pd/Al₂O₃. In the presence of H₂S, Pt/SnO₂catalysts are rapidly and almost completely poisoned, comparably to Pd/Al₂O₃and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO₂sites active in CH₄oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO₂.