Michael C.J. Bradford

Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity

The reforming of methane with carbon dioxide was studied over nickel supported on MgO, TiO2, SiO2, and activated carbon. The influence of the support on catalyst activity and carbon deposition resistivity was markedly different ineach case. Although considerable formation of filamentous carbon was observed over Ni/SiO2 (confirmed by TEM and TPO), there was negligible initial loss of catalytic activity. The catalytic activity of Ni/C was very similar to that of Ni/SiO2, but no filamentous carbon appeared to be formed. In contrast to Ni/SiO2, substantially less coking was observed over either the Ni/TiO2 or the Ni/MgO catalysts. Evidence of strong metal-support interaction (SMSI) in the Ni/TiO2 catalyst indicated that large ensembles of nickel atoms, active for carbon deposition, are deactivated or removed by the presence of mobile TiOx species. The identification of two TiOx phases and a Ni5TiO7 phase was made possible by direct measurement of crystalline d spacings with TEM. The Ni/MgO catalyst was both active and very stable for up to 44 h time on stream. Chemisorption, XRD and TEM results indicate the formation of a partially reducible NiO—MgO solid solution, which appears to stabilize the reduced nickel surfaces and provide resistance to carbon deposition.

Catalytic reforming of methane with carbon dioxide over nickel catalysts II. Reaction kinetics

The reforming of methane with carbon dioxide was studied over nickel supported on SiO2, TiO2, MgO and activated carbon. Specific activities on a turnover frequency basis were in the order: Ni/TiO2 > Ni/C > Ni/SiO2 > Ni/MgO. Interestingly, a 2-fold increase in activation energy for this reaction was observed over Ni/TiO2 after several hours time on stream. The reverse water-gas shift reaction was found to be close to thermodynamic equilibrium over all catalysts. Partial pressure dependencies were obtained with the Ni/C and Ni/SiO2 catalysts at 723 K for comparative purposes only, but a more thorough kinetic analysis was made with the Ni/MgO and Ni/TiO2 catalysts, which were shown previously to strongly inhibit carbon deposition. Partial pressure dependencies were obtained at 673, 698, and 723 K for Ni/TiO2 and at 773, 798, and 823 K for Ni/MgO. In situ DRIFTS studies clearly showed the presence of both linear and bridged carbon monoxide adsorption on Ni/SiO2 under reaction conditions; however, adsorbed carbon monoxide could not be identified on Ni/TiO2. A reaction model for CH4—CO2 reforming, based on CH4 activation to form CHx and CHxO decomposition as the slow kinetic steps, successfully correlated the rate data.

CO2 reforming of methane over vanadia-promoted Rh/SiO2 catalysts

The reforming of methane with carbon dioxide over rhodium dispersed on silica, Rh/SiO2, and vanadia-promoted silica, Rh/VOx/SiO2, was studied by kinetic test reactions under differential conditions in a temperature range from 723 to 773 K. Transmission infrared spectroscopy was applied to observe the interaction of CO2 with the catalysts and the formation of surface intermediates during the CO2–CH4 reforming reaction. To analyze carbon deposition XP spectroscopy and TPO was carried out. It has been shown that the promotion of Rh/SiO2 catalysts with vanadium oxide enhances the catalytic activity for CO2 reforming of methane and decreases the deactivation by carbon deposition. This is attributed to the formation of a partial VOx overlayer on the Rh surface, which reduces the size of accessible ensembles of Rh atoms required for coke formation and creates new sites at the Rh–VOx interfacial region that are considered to be active sites for the activation/dissociation of carbon dioxide.

Novel high‐temperature Calvet‐type calorimeter for investigating metal‐water reactions

The initial operation of a Calvet‐type calorimeter is reported. The instrument has been designed specifically to study lithium and lithium compound reactions with water at an elevated temperature. Additionally, the calorimeter test section incorporates an environment that resembles actual application conditions. Calibration procedures are explained for three important parts of the instrument, including (1) introduction of the water sample, (2) analysis of the effluent gas, and (3) measurement of the heat of reaction. Ten measurements of the heat of the lithium oxide‐water reaction yielded values from 110 to 140 kJ/mol H2O and averaged 124 kJ/mol H2O. The average is in perfect agreement with the theoretical heat value computed utilizing JANAF thermochemical data.