Development of coking-resistant Ni-based catalyst for partial oxidation and CO 2-reforming of methane to syngas

Abstract

Addition of small amount of trivalent-metal oxides, Cr 2O 3 and La 2O 3, to a Ni Mg O (Ni/Mg=1/1, mol/mol) catalyst for partial oxidation of methane (POM) and CO 2-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 surface of the catalyst. Studies of XRD, XPS, and H 2-TPR spectroscopies showed that the doping of small amounts of Cr 3+ and La 3+ 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 of cationic vacancies. An increase in the degree of disorder in the solid solution due to Cr 2O 3 and La 2O 3 dissolved in Ni x Mg 1− x O lattice would be expected to enhance the mobility of the lattice oxygen anions. This would be in favor of speeding up the reaction between the carbon-containing species and reactive oxygen species via migration of the lattice O 2− so as to inhibit the deposition of carbon on the surface of the catalyst. On the other hand, part of the Schottky defects in the form of cationic vacancies may diffuse to the surface, where Ni +-species can be well accommodated and stabilized, thus, forming a rich-in-Ni (with mixed valence states) surface layer. As a result, the proportion of the reducible Ni-species was pronouncedly increased, but the temperature 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 extent, lead to the reduction of the rate of deep dehydrogenation of methane to carbon, therefore tending to reduce, if not avoid, coking caused by an excess of carbon on the surface.