Physicochemical Properties of Deuterated Compounds. 7^th Communication. The H/D exchange between hydrogen and methane over Ni Catalyst

Abstract

The isotopic exchange between hydrogen and methane over Ni catalyst has been studied as a function of temperature, isotopic substitution, and ratio of hydrogen to methane. The experimental results can be described by means of a model in which methane is adsorbed either as methyl or as methylene. The adsorbed species undergo one or several exchanges. The four required parameters are adjusted to the experimental results. The exchange of partially deuterated methane with H, or D2 allowed us to study the isotopic effect on the adsorption and the exchange processes. Introduction. -The exchange between D, and CH,, either on Ni films [2-41 or other types of catalysts [5][6], has been described in several publications. CH, is an especially interesting molecule for exchange studies because of its high symmetry and low reactivity [7]. On our catalyst, no exchange can be observed below 500 K, where the isotopic equilibrium for hydrogen is instantaneously attained. The speed of exchange is largely determined by the rate of adsorption of the hydrocarbon [8], i.e. the ease of formation of one or several C-Ni bonds.The major differences between the models proposed for the exchange between CH, and D, [23 [6] [9-141 are due to the nature of the adsorbed species and their interconversion on the metal surface. The aim was to achieve a good representation of the experimental results using a minimum of parameters, because very little is known about the elementary reactions on the surface. Usually, the following scheme is assumed (M-C and M-X describe simple or multiple bonding between the metal and C or H):

The H/D Exchange and Hydrogenolysis of Butane on an Alumina‐Supported Nickel Catalyst

“…The model developed in this laboratory that simulates the initial distribution of alkanes was described in [lo] [l 11 [21]; it gave satisfactory results for methane [21], and for heptane, methylcyclohexane, and cycloheptane [ 1 11 at moderate temperatures, i.e. below 70".…”

“…2 ) 'M + B' ( = 'mono-and biadsorbed species'): an alkyl ( a ) with a probability p or an alkene (a$') with a probability (I -p ) are initially formed [lo] [l I] [21]. The adsorbed species can either immediately desorb with a probability (1 -q ) and (1 -q'), respectively, or it will undergo a further exchange.…”

“…To test, whether the C-C splitting or C; desorption (where * designs a surface adsorbed species) is the rate-determining step in the butane/H, hydrogenolysis, we have compared the ratio R , defined as the methane production rate for hydrogenolysis to the exchange rates of methane measured in this laboratory at similar conditions [21]. With a value of R > 10 the product desorption is assumed to be the rate-limiting step [33].…”