TEMPERATURE-PROGRAMMED OXIDATION OF SULFIDED NICKEL-MOLYBDATE ALUMINA CATALYSTS - CHANGE OF COMPOSITION AND STRUCTURE OF ACTIVE METALS

Temperature-programmed oxidation of nickel-molybdate, molybdate and nickel catalysts was carried out in thermogravimetric and fixed bed reactors. During the oxidation of sulfided Ni-Mo catalyst in air, the SO2 formation exhibited three maxima at about 433, 530 and 693 K, and about 11% of the total amount of sulfur remained after oxidation at 773 K. The SO2 formation for sulfided molybdenum and nickel catalysts indicated that the first and second maxima might be mainly from oxidation of nickel sulfide and MoS2-like sulfides, respectively. The promotion of nickel in molybdate catalysts resulted in an increase in the amount of residual sulfur, mainly as sulfate, after oxidation. The formation of nickel sulfate was inferred from thermodynamic data. XPS data revealed that the formation of the third SO2 maximum, accompanied by an exothermic reaction, might mainly result from oxidation of the unoxidized MoS2-like sulfides (Mo4+) and nickel sulfides remained in the bulk of the supported metals. The amount of residual sulfur could be reduced by decreasing the amount of oxygen in the oxidizing agent. A thermodynamic investigation supported this result.