DEVELOPMENT OF COKING-RESISTANT NI-BASED CATALYST FOR PARTIAL OXIDATION AND CO2-REFORMING OF METHANE TO SYNGAS

Addition of small amount of trivalent-metal oxides, Cr2O3 and La2O3, t o a Ni-Mg-O (Ni/Mg=1/1, mol/mol) catalyst for partial oxidation of met hane (POM) and CO2-reforming of methane (MCR) reactions has been found to improve the performance of the catalyst for coking-resistance. The POM operation at 1053 K for 50 h, or the MCR operation at 1100 K for 6 h, did not leave any detectable amount of carbon deposit on the surf ace of the catalyst. Studies of XRD, XPS, and HP-TPR spectroscopies sh owed that the doping of small amounts of Cr3+ and La3+ to the Ni-Mg-O system led to the formation of a host-dopant-type Ni-Mg-Cr-La-O solid solution, with a considerable number of Schottky defects in the form o f cationic vacancies. An increase in the degree of disorder in the sol id solution due to Cr2O3 and La2O3 dissolved in NixMg1-xO lattice woul d be expected to enhance the mobility of the lattice oxygen anions. Th is would be in favor of speeding up the reaction between the carbon-co ntaining species and reactive oxygen species via migration of the latt ice O2- so as to inhibit the deposition of carbon on the surface of th e catalyst. On the other hand, part of the Schottky defects in the for m of cationic vacancies may diffuse to the surface, where Ni+-species can be well accommodated and stabilized, thus, forming a rich-in-Ni (w ith mixed valence states) surface layer. As a result, the proportion o f the reducible Ni-species was pronouncedly increased, but the tempera ture for their reduction was considerably raised, so that the surface Ni-species were maintained with higher possibility in positive valence states under POM and MCR reaction conditions. This would, to some ext ent, lead to the reduction of the rate of deep dehydrogenation of meth ane to carbon, therefore tending to reduce, if not avoid, coking cause d by an excess of carbon on the surface. (C) 1998 Elsevier Science B.V .