Kinetic studies of oxidative coupling of methane on samarium oxide

Kinetic behaviour of three samples of samarium oxide (cubic (Sm-1 ), monoclinic (Sm-3) and mixed cubic-monoclinic (Sm 2) ) were studied in the oxidative coupling of methane using a gradientless flow circulation system. The specific rate of C2- product formation differed by a factor of 6-8 for Sm-1 and Sm-3. The specific activity for CO formation did not depend upon the crystal structure of samarium oxide while the rate of formation of CO2 was different for the samples studied. It is proposed that formation of CO and CO2 occurs via different reaction routes. The rate of CO2 formation at high CHJO2 ratio is limited by oxidant activation or surface CO2-complex decomposition

Nitrogen containing species as intermediates in the oxidation of ammonia over silica supported molybdena catalysts

The behaviour of ammonia and oxygen over silica supported molybdena catalysts has been studied by means of thermal analysis techniques, such as thermo-gravimetry and heat-flow calorimetry. The composition of the reactants and products was determined by means of mass spectrometric analysis. Nitrogen- and hydrogen-containing species were observed to be present on the catalyst surface after reduction of the catalysts by ammonia and subsequent flushing in helium. Additional evidence for the presence of these species was provided by experiments in which labelled molecules were used as well as by means of X-ray photo-electron spectroscopy. The nitrogen- and hydrogen-containing species could be removed from the surface by O2. or NO. The reaction products were found to be N2 and H2O

Selective oxidation of n-butane to maleic anhydride under oxygen-deficient conditions over V-P-O mixed oxides

The selective oxidation of n-butane to maleic anhydride over V-P-O mixed oxides was studied under oxygen deficient conditions. The mixed oxides were prepared with P/V atomic ratios ranging from 0.7 to 1.0. Catalysts with P/V <1.0 did not show any selectivity to maleic anhydride formation, regardless of whether or not (VO)2P2O7 was present. For catalysts with P/V = 1.0, containing (VO)2P2O7 and/or the so-called. β-phase, the selectivity was strongly influenced by the actual surface V5+/V4+ ratio. This ratio is determined by the temperature, the crystal phases present in the catalyst and the composition of the gas mixture. Optimal selectivity was obtained at 425°C with 15% butane in air and a butane/oxygen ratio of 0.9

Zirconia as a support for catalysts: influence of additives on the thermal stability of the porous texture of monoclinic zirconia

A single-phase monoclinic zirconia (the thermodynamically stable modification up to a temperature of 1170°C), having a specific surface area of 67 m2g¿1 and a well-developed mesoporous texture, has been prepared by gel-precipitation followed by calcination at 450°C. A commercially available high-surface area monoclinic zirconia powder (SBET=71 m2g¿1) has also been studied. It was found that the specific surface area and pore volume of monoclinic zirconia both decreased markedly on increasing the calcination temperature; despite the fact that the crystal structure was that of the stable modification, this did not seem to impart any substantial resistance to thermal sintering. The thermal stability of monoclinic zirconia could however be improved significantly by addition (by an impregnation technique) of various oxides: CaO, Y2O3, La2O3 all led to an improvement in the thermal stability up to 900°C while MgO exhibited stabilizing properties only up to 700°C; the best results were obtained with La2O3. All the additives investigated other than MgO were found to bring about a partial transition of the monoclinic to a fluorite-like phase of zirconia upon heat treatment; this phase has been shown in the case of the CaO-doped sample to be cubic zirconia and in the cases of the Y2O3- and La2O3-doped samples to be tetragonal zirconia. As little as 20¿50% of a theoretical monolayer quantity of La2O3 was sufficient to give satisfactory thermal stability. The results can be explained by a model involving mass transport by a surface diffusion mechanism

The interaction between silver and N2O in relation to the oxidative dehydrogenation of methanol

The interaction of N2O with pure silver at temperatures up to 900 °C has been studied using temperature-programmed reduction and desorption; the interaction is compared with that of oxygen with silver. The effect of addition of N2O, as well as of the complete replacement of oxygen by N2O, on the oxidative dehydrogenation of methanol on a silver catalyst has also been studied. It was found that the interaction of silver with N2O was much slower than that of O2; no atomic surface oxygen species were observed, probably because the formation of subsurface species was not complete; selective adsorption appears to take place on the surface defects and grain boundaries involved in the formation of the subsurface species. Addition of small amounts of N2O to the reaction mixture (CH3OH + O2) for the oxidative dehydrogenation of methanol had almost no influence on the conversion or on the product distribution measured. However, the conversions were considerably lower when oxygen was totally replaced by N2O; only above 600 °C was the N2O exhausted. At the same level of conversion of the methanol, the amount of CO2 produced was lowered compared to the case of O2. This is in agreement with the suggestion that CO2 is formed via weakly bound surface oxygen

The oxidative dehydrogenation of methanol to formaldehyde over silver catalysts in relation to the oxygen-silver interaction

The properties of silver in the oxidative dehydrogenation of methanol were studied in a flow reactor under near industrial conditions. The influences of temperature, concentration of both reactants, gas velocity, space velocity, the form of the silver catalyst and surface composition of the catalyst were studied. A model for the reaction is proposed which is based on the experimental observations and on the nature of the interaction of silver with oxygen. It issuggested that different oxygen species on the silver surface play different roles in the reactions to CO, CO2 and H2CO. Gas phase reactions only contribute to the conversion to CO

The silver-oxygen interaction in relation to oxidative dehydrogenation of methanol

The interaction of unsupported silver with oxygen at atmospheric pressure and at temperatures between 100 and 600°C has been studied using temperature programmed reduction and desorption experiments with temperatures ranging up to 900°C. In addition, the interaction of an oxygen-loaded silver surface with methanol has been studied using both these techniques and temperature programmed reaction. It appears that the silver-oxygen chemistry is influenced strongly by hydrogen dissolved in the silver during the pretreatment of the catalyst, the hydrogen giving rise to a new type of sub-surface species, possibly sub-surface OH groups, and also to an increase of the amount of sub-surface oxygen formed. Sub-surface oxygen can be converted into a strongly bound species that is not present to a measurable extent after normal oxidation. Defects, partly generated as a consequence of the interaction between oxygen and hydrogen in the sub-surface region of the silver, probably generate this strongly bound oxygen species. The presence of the sub-surface oxygen species appears to activate the silver for methanol dehydrogenation

The influence of hydrogen treatment and catalyst morphology on the interaction of oxygen with a silver catalyst

The interaction of an unsupported silver catalyst which had been pretreated by hydrogen at various temperatures with oxygen at 210°C has been studied using Temperature Programmed Reduction (TPR) over a temperature range up to 900°C. Hydrogen treatment at 500°C or above before the oxidation step causes the formation of extra species, thought to be OH groups in the sub-surface of the sample. A peak in the spectra attributable to oxygen strongly bound in the vicinity of surface defects is found to be dependent on the surface roughness and grain size of the silver sample used; hydrogen pretreatment causes the strongly bound oxygen in the vicinity of surface defects to be converted to sub-surface OH. It is also shown that the TPR measurements themselves influence the morphology of the sample and that these changes are comparable with structural changes which occur during the use of the catalysts for oxidative dehydrogenation of methanol. It is suggested that these structural changes are caused by the interaction of the sub-surface of the silver with both oxygen and hydrogen

Influence of preparation method on the performance of vanadia-niobia catalysts for the oxidative dehydrogenation of propane

The influence of various preparation methods on the performance of V-Nb-0 catalysts has been investigated. It was found that the activity and selectivity of a vanadium site depend on the nature of the neighbouring atoms. Vanadium neighbours provide activity, while niobium neighbours provide selectivity. Careful preparation of these catalysts ensures a homogeneous distribution and good mixing of the vanadium and niobium. It was also found that the vanadium becomes mobile upon reduction and this results in better distribution of vanadium in used catalysts