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.     

Comparative study on hydrogen-assisted “one-step” methane conversion over Pd–Co/SiO2 and Pt–Co/NaY catalysts

In our laboratory, methane conversion to higher hydrocarbons in “one-step” process under non-oxidative condition at low temperature was first introduced and investigated over Pd–Co/SiO₂ prepared by sol/gel method [Guczi et al., Catal. Lett. 54 (1998) 33] and over Pt–Co/NaY [Guczi et al., Stud. Surf. Sci. Catal. 119 (1998) 295] bimetallic catalysts. It was found that methane conversion in one-step process is at least 2.5 times higher than that measured in “two-step” process on the same catalysts. In the present work, the two-step and one-step processes are compared. It has been established that in one-step process when methane dissociation occurs in the presence of hydrogen containing helium, not only the production of higher hydrocarbons increases but also the selectivity is shifted towards larger molecules. Palladium–cobalt system proved to be more efficient than the corresponding platinum–cobalt catalysts.     

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.     

Characteristics of methane combustion over La–Cr–O catalysts

The catalytic activities of methane combustion of La–Cr–O catalysts prepared with and without polyacrylic acid as a template have been compared. The polymer-templated catalyst had a higher BET surface area, 12.3 m² g⁻¹, than that obtained from the conventional precipitation method, 2.9 m² g⁻¹. The results of XRD and SEM experiments suggested that the structural characteristics were almost similar. Surprisingly, the areal rate of methane combustion over the catalyst with a small surface area was ten times larger than that of the catalyst with a large surface area. However, the site time yields (STY) based on the oxygen adsorption capacity were similar, independent of the surface area and preparation conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative study of CO and CO2 hydrogenation over supported Rh–Fe catalysts

The hydrogenation of CO, CO + CO₂, and CO₂ over titania-supported Rh, Rh–Fe, and Fe catalysts was carried out in a fixed-bed micro-reactor system nominally operating at 543 K, 20 atm, 20 cm³ min^− 1 gas flow (corresponding to a weight hourly space velocity (WHSV) of 8000 cm³ g_cat^− 1 h^− 1), with a H₂:(CO + CO₂) ratio of 1:1. A comparative study of CO and CO₂ hydrogenation shows that while Rh and Rh–Fe/TiO₂ catalysts exhibited appreciable selectivity to ethanol during CO hydrogenation, they functioned primarily as methanation catalysts during CO₂ hydrogenation. The Fe/TiO₂ sample was primarily a reverse water gas shift catalyst. Higher reaction temperatures favored methane formation over alcohol synthesis and reverse water gas shift. The effect of pressure was not significant over the range of 10 to 20 atm.     

Steam reforming of methanol over copper–manganese spinel oxide catalysts

The steam reforming of methanol was studied over a series of copper–manganese spinel oxide catalysts prepared with the urea–nitrate combustion method. All catalysts showed high activity towards H₂ production with high selectivity. Synthesis parameters affected catalyst properties and, among the catalysts tested, the one prepared with 75% excess of urea and an atomic ratio Cu/(Cu + Mn) = 0.30 showed the highest activity. The results show that formation of the spinel Cu_xMn_3 − xO₄ phase in the oxidized catalysts is responsible for the high activity. Cu–Mn catalysts were found to be superior to CuO–CeO₂ catalysts prepared with the same technique.     

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.     

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.     

The catalytic conversion of CO2 to hydrocarbons over Fe–K supported on Al2O3–MgO mixed oxides

Al₂O₃–MgO mixed oxides prepared by a co-precipitation method have been used as supports for potassium-promoted iron catalysts for CO₂ hydrogenation to hydrocarbons. The catalysts have been characterized by XRD, BET surface area, CO₂ chemisorption, TPR and TPDC techniques. The CO₂ conversion, the total hydrocarbon selectivity, the selectivities of C₂–C₄ olefins and C₅₊ hydrocarbons are found to increase with increase in MgO content upto 20 wt% in Fe–K/Al₂O₃–MgO catalysts and to decrease above this MgO content. The TPR profiles of the catalysts containing pure Al₂O₃ and higher (above 20 wt%) MgO content are observed to contain only two peaks, corresponding to the reduction of Fe₂O₃ to Fe⁰ through Fe₃O₄. However, the TPR profile of 20 wt% MgO catalyst exhibits three peaks, which indicate the formation of iron phase through FeO phase. The TPDC profiles show the formation of three types of carbide species on the catalysts during the reaction. These profiles are shifted towards high temperatures with increasing MgO content in the catalyst. The activities of the catalysts are correlated with physico-chemical characteristics of the catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simultaneous dehydrogenation and isomerization of n-butane to isobutene over Cr/WO3–ZrO2 catalysts

A series of WO₃-promoted Cr₂O₃-based catalysts were prepared and tested for the simultaneous dehydrogenation and isomerization of n-butane to isobutene. It is found that a Cr₂O₃/WO₃–ZrO₂ system is an effective catalyst for this reaction; however, the catalytic behavior is dependent on Cr₂O₃ and WO₃ contents, space velocity and temperature. 10 wt% Cr₂O₃/20 wt% WO₃–ZrO₂ can give high initial conversion and isobutene selectivity, but it deactivates rapidly due to the variation of surface properties and pore structure caused by carbon deposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Novel sol–gel‐derived Ag/SiO2–Al2O3 catalysts for highly selective oxidation of methanol to formaldehyde

Novel Ag/SiO₂–Al₂O₃ catalysts with low silver content prepared by the sol–gel method exhibit excellent catalytic properties in the catalytic oxidation of methanol to formaldehyde. The silver content was as low as 2% and the yield of formaldehyde was achieved as 90.3%, which is 16% higher than that of pumice‐supported silver and even 5–6% higher than that of a commercial electrolytic silver catalyst. XRD, XPS and SEM results reveal that all silver was present as Ag⁺ before catalytic reaction and was partially reduced to the metallic state after the reaction. It was also found that silver was aggregated on the surface after its reduction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparative Study on Partial Oxidation of Methane over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2 Catalysts

The partial oxidation of methane has been studied by sequential pulse experiments with CH₄ O₂ CH₄ and simultaneous pulse reaction of CH₄/O₂ (2/1) over Ni/CeO₂, Ni/ZrO₂ and Ni/Ce–ZrO₂ catalysts. Over Ni/CeO₂, CH₄ dissociates on Ni and the resultant carbon species quickly migrate to the interface of Ni–CeO₂, and then react with lattice oxygen of CeO₂ to form CO. A synergistic effect between Ni and CeO₂ support contributes to CH₄ conversion. Over Ni/ZrO₂, CH₄ and O₂ are activated on the surface of metallic Ni, and then adsorbed carbon reacts with adsorbed oxygen to produce CO, which is composed of the main path for the partial oxidation of methane. The addition of ceria to zirconia enhances CH₄ dissociation and improves the carbon storage capacity. Moreover, it increases the storage capacity and mobility of oxygen in the catalyst, thus promoting carbon elimination.     

Characterization and catalytic properties of Ni and NiCu catalysts supported on ZrO2–SiO2 for guaiacol hydrodeoxygenation

ZrO₂–SiO₂ complex oxides with Si/Zr mole ratio of 3 (SZ-3) were synthesized. ZrO₂–SiO₂ supported Ni and NiCu catalysts were prepared by impregnation method. Their catalytic performances were evaluated in the hydrodeoxygenation (HDO) upgrading of model reactant guaiacol to hydrocarbons. The physicochemical properties of the support materials and catalysts were characterized by FTIR, XRD, TPD, TPR, and BET techniques. The addition of Cu significantly affected the acidity, and thus influenced their catalytic performance for product distributions. Over the Ni5Cu/SZ-3 catalyst, the cyclohexane selectivity of 80.8% and the methylcyclohexane selectivity of 12.4% were obtained with complete conversion of guaiacol under the 300 °C, 5.0 MPa H₂ pressure.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Low‐temperature oxidation of methane to form formaldehyde: role of Fe and Mo on Fe–Mo/SiO2 catalysts, and their synergistic effects

The partial oxidation of methane with molecular oxygen was performed on Fe–Mo/SiO₂ catalysts. Iron was loaded on the Mo/SiO₂ catalyst by chemical vapor deposition of Fe₃(CO)₁₂. The catalyst showed good low‐temperature activities at 723–823 K. Formaldehyde was a major condensable liquid product on the prepared catalyst. There were synergistic effects between iron and molybdenum in Fe–Mo/SiO₂ catalysts for the production of formaldehyde from the methane partial oxidation. The activation energy of Mo/SiO₂ decreased with the addition of iron and approached that of the Fe/SiO₂. The concentration of isolated molybdenum species (the peak at 1148 K in TPR experiments) decreased as the ion concentration increased and had a linear relationship with the selectivity of methane to formaldehyde. The role of Fe and Mo in the Fe–Mo/SiO₂ catalyst was proposed: Fe is the center for the C–H activation to generate reaction intermediates, and Mo is the one for the transformation of intermediates into formaldehyde. Those phenomena were predominant below 775 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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.     

Catalytic combustion of methane over M (Ni,Co, Cu) supported on ceria–magnesia

The catalytic activity of a series of M(= Ni, Co, Cu)/(CeO₂)_x–(MgO)_1 − x catalysts for methane combustion was investigated. (CeO₂)_x–(MgO)_1 − x supports were prepared by a sol-gel method. The influence of CeO₂ content and active components such as Ni, Co and Cu are discussed. The results indicate that the activity of the catalysts depends strongly on CeO₂ content. The Ni/(CeO₂)_0.1 − (MgO)_0.9 catalyst showed the highest catalytic activity and good thermal stability for methane combustion. The highly dispersed NiO is the main active site for methane combustion. Fresh M (Ni, Co and Cu)/(CeO₂)_0.1–(MgO)_0.9 catalysts showed that the activity of CuO for methane combustion was slightly higher than that of NiO and CoO, while the thermal stability increased in the order Cu < Co < Ni. Cu/(CeO₂)_0.1–(MgO)_0.9 catalyst was sintered after a second evaluation. Consequently, (CeO₂)_0.1–(MgO)_0.9 is deemed to be a good support for Ni.