The catalysis of the parahydrogen conversion by tungsten

Source abstract

Abstract The parahydrogen conversion has been measured on a tungsten surface, as a function of the surface concentration of adsorbed oxygen, acting as a poison. This surface concentration was measured by a contact potential method. The oxygen would appear not only to cut down the fraction of surface available for reaction, but also to influence the reactivity of the neighbouring chemisorbed hydrogen. An important factor in this influence may be dipole-dipole interaction between the adsorbed atoms. A part at least of the oxygen adsorbed on tungsten can be removed at room temperature by reaction with hydrogen. Films of carbon monoxide, nitrogen and ethylene also poison the catalytic activity of tungsten and there is some evidence that the small conversion that does occur in their presence proceeds through the formation of a hydride surface on top of the film, rather than on gaps in it. Experiments with a number of different wires has led to evaluation of the constant A0 in k = A°e-E/RT. While Ao in two cases differs considerably from the average value, in all other six cases it approximates closely to an average. In the derived equation for the absolute rate the temperature independent factor is 2.6 x 1020 mol./sec./cm.2 of catalyst surface, at 1 mm. pressure of hydrogen. From this we estimate a value of this factor B0m = 1022 for the zero order reaction, which is more susceptible to simple theoretical treatment. This is considered in terms of the activated complex theory, and is shown to be more in accordance with a conversion mechanism based on an interchange between loosely adsorbed hydrogen molecules and chemisorbed hydrogen, rather than with the original idea of the recombination of chemisorbed hydrogen atoms. The H2 + D2 reaction has the same rate as the parahydrogen conversion, and so ‘tunnelling’ is an unimportant factor in this hydrogen reaction, even at — 150° C.