Adsorption and methanation of carbon dioxide on a nickel/silica catalyst

The adsorption and methanation of carbon dioxide on a nickel/silica catalyst were studied using temperature-programmed desorption and temperature-programmed reaction. Carbon dioxide adsorption on nickel was found to be activated; almost no adsorption occurred at room temperature, but large coverages were obtained between 383 and 473 K. The data indicate CO2 dissociates upon adsorption at elevated temperatures to yield carbon monoxide and oxygen atoms. These oxygen atoms react with hydrogen at room temperature, so the methane and water peaks observed during programmed heating in flowing hydrogen are identical for adsorbed CO and adsorbed CO2. Single CH4 and H2O peaks, each with a peak temperature of 473 K, were observed. This peak temperature did not change with initial coverage, indicating methanation is first order in CO surface coverage. The activated adsorption of CO2 allowed these coverage variation experiments to be carried out. Thus, following adsorption, CO and CO2 methanation proceed by the same mechanism. However, the activated adsorption of CO2 may create a higher H2: CO surface ratio during steady-state hydrogenation, causing CO2 hydrogenation to favor methane over higher hydrocarbons.