SULFUR POISONING OF NICKEL CATALYSTS: METHANE-DEUTERIUM EXCHANGE REACTIONS
CHAHAR, BHARAT SINGH
Doctor of Philosophy
The effects of sulfur poisoning on a nickel catalyst have been investigated using CH(,4)-D(,2) exchange reactions to monitor changes in the catalyst activity and selectivity. Mechanisms of the CH(,4)-D(,2) exchange reactions were also examined. The techniques employed include kinetic modeling, poisoning by pulse and flow methods, temperature programmed desorption (TPD), the thermogravimetric analysis (TGA). The catalyst used in the study contains 13% Ni (by weight) supported on a MgAl(,2)O(,4) carrier and has a BET surface area of 1.0 m('2)/g. Substantial increases in the catalyst activity were observed when the samples underwent a standard pretreatment in O(,2) at 475(DEGREES)C for 16 hours followed by a reduction in H(,2) at 525(DEGREES)C for 2 hours. The catalyst activation is believed to result from redispersion of nickel crystallites during oxidation. Two primary products, CD(,4) and CH(,3)D, were observed from the CH(,4)-D(,2) exchange reactions. The independent reactions responsible for the above products are multiple exchange and simple exchange. The multiple exchange is thought to occur via dissociative adsorption of CH(,4) on 7 nickel sites followed by rapid isotopic exchange of adsorbed H atoms with the gas phase deuterium. A kinetic expression based on the above model describes the experimental data quite well. It is proposed that CH(,3)D is the product of a reaction between the gas phase CH(,4), an adsorbed D atom, and an adjacent vacant Ni site. This model is supported by good agreement between the observed rates and the rates predicted by the developed kinetic expression. Sulfur poisoning by both pulse and flow methods (using H(,2)S as the poisoning compound) resulted in a linear decrease in the catalyst activity. However, no variations in either the selectivity (for CD(,4) and CH(,3)D) or activation energies of the exchange reactions were observed. Results from the poisoning experiments and H(,2)S absorption isotherms showed evidence of diffusion control for H(,2)S adsorption. A pore mouth poisoning model adequately describes the catalyst deactivation. The sulfur capacity of the nickel surface has been estimated as 8 x 10('14) S atoms/cm('2) Ni. The poisoned catalyst was partially regenerated by treatment in O(,2) at 475(DEGREES)C for an hour and reduction in H(,2) at 480(DEGREES)C for 2 hours. It was determined from the TPD experiments that, during this treatment, sulfur is removed in the form of SO(,2).