Temperature-Programmed Studies on Partial Oxidation of CH4 to Syngas over a Ni/Al2O3 Catalyst

Introduction Partial oxidation of CH4 to syngas has been a hotspot of research for the advantages comparing to steam reforming process, such as low H2/CO ratio (≈2), more energy-efficient, and high CH4 conversion and selectivities to CO and H2 with smaller reactor and short contact time (≤10s), etc . Many metal catalysts have been studied, such as supported Rh, Pt, Pd, Ru, Re, Ir, Ni, Fe and Co, etc. The supported noble Rh catalyst exhibits excellent activity and selectivity , but its price is too expensive. The supported Ni catalysts have similar performance with supported Rh, and the price of Ni is far lower than that of Rh. So the studies on supported Ni catalyst have attracted a great number of researchers . About the reaction mechanism, some authors 15,16-18 pointed out that partial oxidation of CH4 to syngas proceeds via indirect oxidation mechanism, namely: complete combustion of CH4 to CO2 and H2O and subsequently reforming reaction of the residual CH4 with CO2 and H2O to CO and H2. However, other authors 9,19,20,21 claimed that the reaction proceeds via direct oxidation mechanism: H2 is from the decomposition of CH4 and CO is the product of the reaction between surface carbon species and surface oxygen species. Our work 22 on the reaction over a Ni/Al2O3 catalyst supports direct oxidation mechanism. CH4 firstly decomposes on active metal Ni sites to H2 and NixC, and then NixC reacts with NiO formed by oxidation of metallic Ni by O2 to CO or CO2 depending on the concentration of NiO around NixC. Steady and non-steady state isotopic transient experiments showed that most of CO2 during the reaction is from the surface reaction between NixC and NiO, not from disproportionation of CO or further oxidation of CO . This also proves the mechanism we suggested. Furthermore, metal Ni is active sites of the Ni/Al2O3 catalyst. So the catalyst must be kept in reduced state, or the reaction mechanism will change and the conversion of CH4 and the selectivities to H2 and CO will be very low for CH4 can’t decompose on NiO . In this paper, the Ni/Al2O3 catalyst was evaluated and the effects of temperature and space velocity were investigated with on-line chromatography analysis. In addition, temperature-programmed technique was used to investigate the change of the catalyst surface state during the reaction at different temperature and its influence on the conversion and selectivities.