OH spillover from a γ-Al2O3 support in the partial oxidation of methane over Ru/Al2O3

The microkinetics of the oxidation of methane on Ru/Al₂O₃ is developed using a TAP reactor. The shapes of the response curves of reactants and products are used to identify the elementary reactions and their rate parameters. The support, which strongly adsorbs H₂O, participates in the reaction sequence through its supply of hydroxyl to metal particles by spillover, and this step is of kinetic significance. The relative rates of the elementary reactions that give overall partial oxidation or total oxidation are discussed.     

Strong improvement on CH4 oxidation over Pt/γ-Al2O3 catalysts

A very strong promotion effect of the presence of 1000 ppmV C₃H₈ in the reaction feed on CH₄–O₂ reaction was found over unsulfated 1%Pt/γ-Al₂O₃ catalyst. This promotion was further increased on pre-sulfated 1%Pt/γ-Al₂O₃. The promoting effect of pre-sulfation on the activity of 1%Pt/γ-Al₂O₃ for propane combustion results in a further improvement on methane combustion due to propane combustion heat which is generated at lower temperatures, activating methane combustion over pre-sulfated 1%Pt/γ-Al₂O₃ at even lower temperatures relative to unsulfated 1%Pt/γ-Al₂O₃. These results suggest that small amounts of propane in the gas feed during CH₄–O₂ reaction over a pre-sulfated Pt/γ-Al₂O₃ catalyst may eliminate methane emissions at low temperatures from lean-burn NGV exhausts without being deactivated by sulfur poisoning as Pd supported catalysts.     

The effect of oxygen on the aromatization of methane over the Mo/HZSM‐5 catalyst

The aromatization of methane over a Mo/HZSM‐5 catalyst was carried out in the presence of oxygen. It is shown that the addition of a small amount of oxygen is beneficial to improve the durability of the catalyst. UV‐Raman spectra disclose that the carbonaceous deposits formed on the HZSM‐5 are mainly polyolefinic and aromatic, while that on the Mo/HZSM‐5 is mainly polyaromatic. The small amount of O₂ added may partly remove the coke deposits on the active sites and keep the catalyst as MoO_xC_y/HZSM‐5, thus resulting in an improvement of the catalytic performance of the Mo/HZSM‐5 catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Partial oxidation of methane to syngas over Co/MgO catalysts. Is it low temperature?

Co/MgO catalysts with high Co-loading (>28 wt%) are able to initiate the reaction of methane with oxygen at temperatures around 500 °C. High conversions of methane ( 70%) and very high selectivities for hydrogen and carbon monoxide ( 90%) are obtained at very high reactant gas space velocities (10⁵–10⁶ h^–1). The temperature of the catalyst at the conditions of partial oxidation of methane to form syngas was found to be extremely high (1200–1300 °C); it is about 600–850 °C higher than that previously reported by others. At these temperatures, high temperature homogeneous reactions may prevail. It is suggested that combustion of methane to carbon dioxide occurs on the catalyst with major heat release and that methane and water, respectively methane and carbon dioxide are reformed thermally in an endothermic reaction leading to syngas.     

Catalytic partial oxidation of methane to synthesis gas over γ-Al2O2-supported rhodium catalysts

The partial oxidation of methane was studied over -Al₂O₃-supported catalysts for Rh loadings between 0.01 and 5.0 wt%. It was found that the activity and selectivities for loadings between 0.5 and 5.0 wt% are almost the same. As an example, detailed information is presented for the 1.0 wt% Rh/-Al₂O₃, which provides at 750°C (furnace temperature) an activity of 82% and selectivities of 96% to CO and 98% to H₂, at a gas hourly space velocity (GHSV) of 720000 ml g^–1 h^–1. Its activity remained stable during our experiment which lasted 120 h. Possible explanations for this high stability are proposed based on TPR and XRD experiments. Pulse reactions with small pulses of CH₄ and CH₄/O₂ (2/1) were performed over the reduced and unreduced Rh catalysts to probe the mechanistic aspects of the reaction. The partial oxidation of methane to syngas was found to be initiated by metallic rhodium sites, since the CO selectivity increased with increasing number of such sites.     

Partial oxidation of methane to synthesis gas over α-Al2O3-supported bimetallic Pt–Co catalysts

The partial oxidation of methane to synthesis gas was studied at atmospheric pressure and in the temperature range of 550–800°C over -Al₂O₃-supported bimetallic Pt–Co, and monometallic Pt and Co catalysts, respectively. Both methane conversion and CO selectivity over a bimetallic Pt_0.5Co₁ catalyst were higher than those over monometallic Pt_0.5 and Co₁ catalysts. Furthermore, the addition of platinum in Pt–Co bimetallic catalysts effectively improved their resistance to carbon deposition with no coking occurring on Pt_0.5Co₁ during 80 h reaction. The FTIR study of CO adsorption observed only linearly bonded CO on bimetallic Pt–Co catalysts. TPR and XPS showed enhanced formation of a cobalt surface phase (CSP) in bimetallic Pt–Co catalysts. The origins of the good coking resistivity of bimetallic Pt–Co catalysts were discussed.     

Transient study of the dry reforming of methane over Pt supported on different γ-Al2O3

CO₂ reforming of methane has been studied over Pt/Al₂O₃ model catalysts in a temperature range of 600–800 °C using steady-state and transient methods (Transient Response Method (TRM) and DRIFT-MS). Pt-supported catalysts were prepared using two different alumina (γ-Al₂O₃(S) Sasol-Puralox and a synthesized γ-Al₂O₃(N) with nanofibrous structure). Catalysts and supports were characterized by conventional methods (XRD, TEM, A_BET, XPS) before and after reaction. Pt⁰ species are present in the catalysts, with a higher relative contribution for the catalyst that has a nanostructured support. Pt/γ-Al₂O₃(N) catalyst presented the best performance in reactivity and showed a low rate of carbon formation and a minimal water production. From TRM and DRIFT-MS results it can be concluded that, when CO₂ and CH₄ are fed separately into the reaction system, they are activated over the catalytic surface. Besides, when both reactants are fed contemporaneously the presence of CH_X species promotes the CO₂ activation that is responsible for the reforming reaction.     

The effect of the addition of Y2O3 to Ni/α-Al2O3 catalysts on the autothermal reforming of methane

The addition of Y₂O₃ to Ni/α-Al₂O₃ catalysts was investigated by BET surface area measurements, hydrogen chemisorption, X-ray diffraction, UV–vis diffuse reflectance spectroscopy, X-ray fluorescence, temperature programmed reduction, temperature programmed oxidation and cyclohexane dehydrogenation. Autothermal reforming experiments were performed in order to evaluate the methane conversion and proceeded through an indirect mechanism consisting of total combustion of methane followed by CO₂ and steam reforming generating the synthesis gas. The Y₂O₃·Al₂O₃ supported catalysts presented better activity and stability in autothermal reforming reaction. Temperature programmed oxidation analysis demonstrated that the addition of Y₂O₃ resulted in a change of the type or the location of coke formed during reaction. None of the prepared catalyst presented deactivation by sintering under the tested conditions. The improved stability of supported catalysts Y₂O₃·Al₂O₃ was the result of minimizing the formation of coke on the surface of nickel particles.     

Simultaneous removal of N2O and CH4 as the strong greenhouse‐effect gases over Fe‐BEA zeolite in the presence of excess O2

Simultaneous catalytic removal of N₂O and CH₄ as the strong greenhouse‐effect gases was found to be possible over an Fe‐ion‐exchanged BEA zeolite (Fe‐BEA) by the selective catalytic reduction (SCR) of N₂O with CH₄. The direct decomposition of N₂O (2N₂O → 2N₂ + O₂) and the oxidation of CH₄ (CH₄ + 2O₂ → CO₂ + 2H₂O) over Fe‐BEA zeolite required high temperature above 400 and 450 °C, respectively. Nevertheless, the catalytic reduction of N₂O by adding CH₄ over Fe‐BEA zeolite readily occurred at much lower temperatures (ca. 250–350 °C) whether in the presence of O₂ or not. No oxidation of CH₄ by O₂ took place at these temperatures. On the basis of these results and the kinetic studies, it was concluded that CH₄ reacted selectively with N₂O to produce N₂, CO₂ and H₂O over Fe‐BEA zeolite even in the presence of excess O₂. Overall stoichiometry of the SCR of N₂O with CH₄ was determined as follows: 4N₂O + CH₄ → 4N₂ + CO₂ + 2H₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ruthenium supported on new TiO2–ZrO2 systems as catalysts for the partial oxidation of methane

The partial oxidation of methane is studied at 673–873 K over new Ru-based catalysts supported on TiO₂–ZrO₂ with different TiO₂ content. Supports were prepared by a sol–gel method, and RuCl₃ and RuNO(NO₃)₃ were used as ruthenium precursors to prepare the catalysts (1–2 wt% Ru). The effect of the reaction temperature on the catalytic behavior is analyzed, along with the support composition and the Ru precursor used.     

Study on CO2 reforming of methane to syngas over Al2O3–ZrO2 supported Ni catalysts prepared via a direct sol–gel process

Ni-based catalysts supported on Al₂O₃–ZrO₂ (Ni/Al₂O₃–ZrO₂) were prepared by a direct sol–gel process with citric acid as the gelling agent. The evaluation of the catalyst prepared for methane reforming with CO₂ was carried out with thermal gravimetric analysis (TGA), infrared spectroscopy (IR), X-ray diffraction (XRD), microscopy analyses (SEM and TEM), temperature-programmed reduction (TPR) and in a micro-reactor system. The catalytic performance for CO₂ reforming of methane to synthesis gas in a continuous-flow micro-reactor under atmospheric pressure was investigated. TGA, IR, XRD and microscopy analyses show that the Ni particles have a nanostructure of around $5\mathrm{nm}$ and are uniformly dispersed on the Al₂O₃–ZrO₂ support, which exists as an amorphous phase. Catalytic tests using CO₂ reforming of methane to synthesis gas show that the catalytic activity increases with increasing metal loading, and the 20Ni/Al₂O₃–ZrO₂ (0.2 Ni/Al molar ratio) catalyst has excellent activity and stability, compared with that of the Al₂O₃ supported Ni catalyst, with 91.9% conversion of CO₂ and 82.9% conversion of CH₄ over $50\mathrm{h}$ at $1073\mathrm{K}$, atmospheric pressure, hourly space velocity of $11,200\mathrm{ml}{\mathrm{gcat}}^{-1}{\mathrm{h}}^{-1}$ and CH₄:CO₂:N₂ of 2:2:1. The excellent catalytic activity and stability is attributed to the very highly and uniformly dispersed small metallic Ni particles, the reducibility of the Ni oxides and an interaction between metallic Ni particles and the support Al₂O₃–ZrO₂.     

Influence of H2, CO and CO2 co-feeding on the catalytic activity of Rh/Ti–SiO2 during the partial oxidation of methane

The influence of the addition of 1, 2 or 5 vol.% of CO, H₂ or CO₂ to the feed during the partial oxidation of methane (POM) was studied over a Rh/Ti–SiO₂ catalyst. The addition of H₂ or CO decreases the conversion and syngas selectivity. This decrease of performance seems to be related to a higher reduction of the catalyst due to the co-feeding of H₂ or CO. The addition of CO₂ also appears unfavourable to the production of hydrogen but increases the CO yield. A combination of the dry reforming and the reverse water–gas shift reactions is suggested to explain the observed modifications in the product yields.     

Molecular mechanisms in partial oxidation of methane on Ir/α-Al2O3: reactivity dependence on catalyst properties and transport phenomena limitations

An in situ DRIFT and mass spectrometric study of catalytic partial oxidation of methane with Ir/-Al₂O₃ has enlightened relationships between the formation of surface metal carbonyl clusters and residence time and temperature conditions. Some cluster species produced during catalytic partial oxidation were also originated during CO₂ hydrogenation and CO₂ reforming experiments described in previous literature. An EXAFS analysis of the catalyst precursor, prepared through a solid-liquid reaction between Ir₄(CO)₁₂ clusters and the reactive surface sites of-Al₂O₃, is also included to discuss clusters structure produced at the surfaces.     

Mode of action of oxidation‐active centres in Mo–V mixed oxides on the partial oxidation of an unsaturated aldehyde

The mode of action of the oxidation‐active centres in Mo–V mixed oxides on the selective oxidation of an unsaturated aldehyde was investigated by non‐steady‐state methods. Various oxidation‐active centres, differing in activity and selectivity, were identified by non‐steady‐state methods. In addition, the influence of the crystallinity of the Mo–V mixed oxides on the activity and selectivity properties were investigated. It was shown that, in contrast to crystalline samples, only X‐ray‐amorphous Mo–V mixed oxides contained selective oxidation centres. The measurements were used to derive a model based on the interaction of active centres with various redox properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of CuO–ZnO–Al2O3 catalyst structure on the ethanol synthesis from syngas

CuO–ZnO–Al₂O₃ catalysts were prepared by complete liquid phase technology with different addition sequences. The results indicated that the catalyst prepared by the addition of (C₃H₇O)₃Al to Cu(NO₃)₂ and Zn(NO₃)₂ solutions shows excellent ethanol selectivity at the initial stages of reaction, reaching approximately 40%. X-ray photoelectron spectroscopy results showed that ethanol synthesis requires a higher Cu⁺ content and higher Cu/Zn ratio on the catalyst surface. The temperature-programmed reduction test revealed strong interactions between Cu species and zinc or aluminum oxide. The increase in the difficulty of catalyst reduction indicated higher ethanol selectivity.     

Beneficial effect of adding γ-AlOOH to the γ-Al2O3 washcoat of a PdO catalyst for methane oxidation

The beneficial effect of adding γ-AlOOH to the γ-Al₂O₃ washcoat of a ceramic cordierite (2MgO · 2Al₂O₃ · 5SiO₂) monolith, used to support a PdO catalyst, is reported for methane oxidation in the presence of water at low temperature (<500°C). The mini-monolith (400 cells per square inch (CPI), 1 cm diameter × 2.54 cm length; ~52 cells) was washcoated using a suspension of γ-Al₂O₃ plus boehmite (γ-AlOOH), followed by calcination and then deposition of Pd by wet impregnation. An optimum solid content of 25 wt% in the washcoat suspension was used to obtain a ~25 wt% washcoat on the monolith. The presence of γ-AlOOH enhanced the thermal and mechanical stability of the washcoat, provided that the γ-AlOOH content was <8 wt%. Temperature-programmed methane oxidation (TPO) showed that the addition of γ-AlOOH to the γ-Al₂O₃ washcoat decreased the catalyst activity. However, when H₂O (2 vol% and 5 vol%) was present in the feed gas, the γ-AlOOH improved the catalyst activity and stability. A γ-AlOOH content of ~5 wt% in the washcoat was determined to provide the highest catalyst activity and stability for CH₄ oxidation in the presence of water.     

NO reduction by CH4over La2O3: temperature‐programmed reaction and in situ DRIFTS studies

The chemistry between NO _x species adsorbed on La₂O₃ and CH₄ was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH₄ in the presence of O₂, NO ₃ ^- does not appear to activate CH₄, thus either an adsorbed O species or an NO ₂ ^- species is more likely to activate CH₄. In the absence of O₂, a different reaction pathway occurs and NO^- or (N₂O₂)^2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH₄ unidentate NO ₃ ^- , which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO₂ or H₂O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N₂ and N₂O from NO was observed at much lower temperature during the TPR experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Controlled partial oxidation of methane to methanol/formaldehyde over Mo–V–Cr–Bi–Si oxide catalysts

The non-catalytic and catalytic oxidations of CH₄ over Mo–V–Cr–Bi–Si oxide catalysts were investigated in a tubular reactor and the catalysts were characterized by XRD, XPS and TPR. Contents of Bi in the catalysts influenced the combination of Mo–V–Bi–O species and, consequently, influenced the TPR reduction temperature of the catalysts. The catalysts exhibited more selective production of methanol when the TPR reduction peaks shifted to lower temperature.     

Mechanistic studies of CO2/CH4 reforming over Ni–La2O2/5A

The mechanism of CO₂/CH₄ reforming over Ni–La₂O₃/5A has been studied. The results of the CO₂‐pulsing experiments indicated that the amount of CO₂ converted was roughly proportional to the amount of H present on the catalyst, implying that CO₂ activation could be H‐assisted. Pulsing CH₄ onto a H₂‐reduced sample and a similar sample pretreated with CO₂, we found that CH₄ conversion was higher in the latter case. Hence, the idea of oxygen‐assisted CH₄ dissociation is plausible. The fact that the amount of CO produced in 10 pulses of CO₂/CH₄ was larger than that produced in 5 pulses of CO₂ followed by 5 pulses of CH₄, indicated that CO₂ and CH₄ could activate each other synergistically. In the chemical trapping experiments, following the introduction of CD₃I onto a Ni–La₂O₃/5A sample pretreated with CH₄/CO₂, we observed CD₃COOH, CD₃CHO, and CD₃OCD₃. In the in situ DRIFT experiments, IR bands attributable to formate and formyl were observed under working conditions. These results indicate that formate and formyl are intermediates for syngas generation in CO₂/CH₄ reforming, and active O is generated in the breaking of a C–O bond. Based on these results, we suggest that during CO₂/CH₄ reforming, CO₂ activation is H‐promoted and surface O species generated in CO₂ dissociation reacts with CH_x to give CO. A reaction scheme has been proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of the crystalline structure of ZrO2 on the metallic properties of Pt in Pt/WO3–ZrO2 catalysts

The influence of the crystalline structure of ZrO₂ on the metallic properties of Pt, when supported on WO₃–ZrO₂, was studied. Pt supported on tetragonal zirconia loses its metallic properties while when supported on monoclinic zirconia it presents good metallic activities. WO₂,^2- deposited on amorphous Zr(OH)₄ before calcination generates an active material for n‐butane isomerization. The larger the fraction of the tetragonal phase of zirconia in this material, the higher the isomerization activity and the lower the metallic activity of Pt. This revised version was published online in July 2006 with corrections to the Cover Date.