Hydrogenolysis sites. poisoned

Hydrogen sulfide also inhibits HDS activity by modifying the catalyst surface. For example, high concentrations of H2S are known to increase the density of Bronsted acid sites on a commercial catalyst. Several researchers have found H2S to poison mainly hydrogenolysis sites on a sulfided C0-M0/7-AI2O3 catalyst. ... 

Early workers in catalytic reforming discovered that a small amount of sulfur poisons hydrogenolysis sites and reduces coking. Studies with... 

The main minor product is ethane. (The distribution of the minor products on 0.48% Pt/Si02 catalyst CH4 = 2.6%, CO = 10.6%, C = 32.4%, C = 54.4%.) The minor products are produced by the decarbonylation of methyloxirane, but only the hydrocarbons desorb, the CO remaining adsorbed on the surface. During the decarbonylation process, C-D and C-C bond ruptures occur. It is well known that kink sites are the active sites of C-C hydrogenolysis, so it is understandable that decarbonylation will poison the kink sites. 

The discovery that kink sites in steps are effective in breaking C C bonds in addition to C-H and H H bonds, thereby initiating hydrogenolysis reactions, may also explain the effect of trace impurities or second component metals that introduce selectivity. Since these kink sites have fewer nearest neighbors than step or terrace sites, they are likely to bind impurities or other metal atoms with stronger chemical bonds. Thus, these sites are readily blocked by impurities. As a result selective poisoning of hydrogenolysis may be obtained by minute concentrations o veil-chosen impurities or another metal component. 

Temperature Increase Dynamics after the First Cycle. As with the start up of the bed, subsequent temperature cycles resulted in the formation of a mild hot spot. The occurrence of this temperature fluctuation is undesirable since the past history of the catalyst may be altered. The adsorption of thiophene upon the active hydrogenation sites was assumed to be irreversible and therefore unaffected by temperature. However, as will become apparent later, the effect of temperature may have altered the poison coverage/or profile. Lyubarski, et. al [73 determined that, as a result of the hydrogenation of thiophene and subsequent hydrogenolysis to butane, the adsorption capacity of a suported Ni... 

Thiophene metal poisoning as well as hydrogenation of ethylbenzene on metal catalysts require, as a first step, the chemisorption of both organic molecules on the metal active sites. Afterwards, catalyst deactivation can simply take place by the blocking of these sites or by further hydrogenolysis of thiophene and subsequent formation of an inactive surface metal sulfide. We believe that, in our conditions, this last mechanism is probably operating. This hypothesis is supported by the fact that butane was detected in our experiments and, furthermore, XPS analysis showed the formation of metal sulfides (S ) on the deactivated catalysts. 

The location of the potential sites of poison adsorption or, for ethane hydrogenolysis, hydrogen adsorption, must be specified. For single crystals, such information may be available from LEED studies. The poisoning entities may occupy a sublattice relative to the metal atoms. For instance, H may be adsorbed on the hollow sites centered among 4 atoms in a square lattice. 

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