Kinetic Studies on Pd/CexZr1-xO2 Catalyst for Methane Combustion

In recent years, catalytic combustion of CH4 has been extensively studied as an alternative option to conventional thermal combustion for the production of heat and energy in view of its capability to achieve effective combustion at much lower temperatures than in conventional oxidation processes, with high efficiency and reduced pollutants such as CO and NOx. In this paper a kinetic study-on 2% Pd over CexZr1-xO2 catalyst is presented. The catalyst was prepared by solution combustion synthesis, fully characterized and tested for CH4 catalytic combustion. A kinetic study was carried out in a continuous recycle reactor, operated in differential conditions. Twelve different possible rate equations belonging to Eley Rideal, Langmuir Hinshelwood, and Mars van Krevelen models were fitted to the experimental data. The best fitting belonged to a Mars van Krevelen mechanism, considering the concentration of the molecular O-2 adsorbed on a single active site, which successively dissociated creating an active site with atomic O-2. The obtained kinetic parameters were inside the range reported in literature for similar catalysts (E-act,E-1 = 23.8 kJ mol(-1); E-act.2 = 31.9 kJ mol(-1); = 5.4 kJ mol-1). For comparison, to better assess the catalytic role of Pd in the CH4 combustion, also the kinetic parameters of the sole carrier, CeZr,O-2, were calculated. The best rate equation for the carrier was based on a different Mars van Krevelen mechanism, a simpler one, involving the surface reaction between adsorbed molecular oxygen and methane from the gas phase (E-act.1 = 79.6 kJ mol(-1), E-act.2 = 114.6 kJ mol(-1)).