E.G.M. Kuijpers

The reversible decomposition of methane on a NiSiO2 catalyst

Abstract The interaction of CH 4 with a silica-supported nickel catalyst is studied at temperatures from 30 to 350 °C, both in continuous-flow and pulse-flow experiments. Even at 30 °C chemisorption is observed; the apparent activation energy for the chemisorption is estimated at 6 kcal mole −1 . At temperatures above 175 °C the methane which is adsorbed on the Ni catalyst dissociates completely into adsorbed carbon atoms and hydrogen. The hydrogen released shifts the equilibrium CH 4 (g) → C(ads) + 4H(ads) to the left side. The reactivity of the carbonaceous deposit with hydrogen is also investigated. At all temperatures in the range 30 to 450 °C the only product of the exothermic reaction is methane. The reactivity passes through a maximum at 200 °C and strongly decreases at temperatures above 300 °C.

Structure-sensitivity of the water-gas shift reaction over highly active Cu/SiO2 catalysts

Abstract Highly thermostable silica-supported copper catalysts prepared by deposition-precipitation from a homogeneous solution were found to be very active for the water-gas shift reaction. The reaction over this new catalyst type was established to be structure sensitive proceeding preferably on extremely small copper particles of dimensions below 20 to 10 nm. The experimental results point to a reduction-oxidation mechanism of the reaction.

Chemisorption of methane on silica-supported nickel catalysts: a magnetic and infrared study

The adsorption of methane on silica-supported nickel catalysts was studied at various constant temperatures (30 °C < T < 100 °C) and at increasing temperatures (30 °C < T < 300 °C) using a low-field magnetic method and infrared spectroscopy. In the entire temperature range the chemisorption of CH4 was found to be dissociative according to the reaction CH4 + 7Ni → Ni3C (“surface nickel-carbide”) + 4Ni-H. It was observed that, per unit surface area, small nickel crystallites were more reactive toward methane than were large crystallites.

Chemisorption of methane on NiSiO2 catalysts and reactivity of the chemisorption products toward hydrogen

The Chemisorption of hydrogen both on bare and carburized NiSiO2 catalysts was studied using a low-field magnetic method, infrared spectroscopy, and mass spectrometry. With a freshly reduced and evacuated sample of one of the catalysts, H2 chemisorption was investigated as a function of the temperature (30 < T < 100 °C). It was found that the slope of the magnetization-volume isotherm decreased with increasing temperature, which does not agree with the theory of superparamagnetism. The smaller slope at more elevated temperatures was ascribed to a more extensive coverage of the smaller nickel particles after admission of the initial H2 doses. Carburization of the catalysts was established by the decomposition of CH4 at temperatures from 30 to 300 °C. At low surface coverages the carbon was deposited as Ni3C strongly affecting the magnetization. At higher surface coverages CHx-complexes without any effect on the magnetization were chemisorbed. After the decomposition of CH4 the catalysts were evacuated at 250 °C, which was found to result in the conversion of a part of the carbonaceous deposit into methane. Also with the subsequent chemisorption of hydrogen on the carburized catalysts (T = 30 °C) the reaction between chemisorbed H-atoms and deposited carbon was apparent from the production of CH4. From a comparison of the magnetization-volume isotherms for H2 Chemisorption before and after the deposition of small amounts of carbon it was derived that the decomposition of methane preferentially proceeds on small nickel crystallites. Finally it was found that hydrogen was adsorbed not only on bare nickel (with magnetic effect) but also on nickel carbide (without magnetic effect).