Alkane activation: Adsorption and reaction of n-butane on clean and carbided W(100)

Abstract

The adsorption and reaction of n-butane have been investigated on clean and carbided W(100) surfaces using temperature programmed reaction spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and isotopic exchange methods. For the clean surface, the initial adsorption probability is 1.0 at a crystal temperature of 130 K. At low coverages, butane adsorbed at 130 K converts to a dissociated state with unit probability below 200 K during temperature programmed reaction. Higher exposures result in a decrease in the adsorption probability and the onset of molecular butane desorption in the range of 150–200 K. A weakly adsorbed molecularly bound butane state is observed for all coverages on the [13 −10] and (5×1) carbide surfaces. Studies using n-butane-d10 show a dramatic isotope effect. The low coverage dissociation probability of n-C4D10 is significantly less than unity and the saturation amount of decomposition in a temperature programmed reaction experiment is less than for n-C4H10. This is evidence for a kinetic isotope effect in the decomposition process, with the activation barrier for C–D bond scission greater than the barrier for C–H bonds. For butane exposures made above 200 K, the initial sticking probability is lowered, and the saturation value for decomposition is also decreased. Preadsorption of deuterium suppresses greater than 90% of the butane decomposition, while postadsorption of D2 under the same conditions blocks less than 5%. These observations suggest that for low butane exposures, there is a substantial amount of dehydrogenation upon adsorption which eventually leads to complete decomposition. As the crystal temperature is increased, buildup of surface hydrogen and possibly carbon from further decomposition rapidly inhibits dissociation of additional butane and results in molecular desorption. A limited amount of butane decomposition occurs on the intermediate coverage [13 −10] carbide surface formed following temperature programmed reaction of a saturation exposure of butane. The amount of butane decomposition on the [13 −10] carbide is an order of magnitude less than on clean W(100). Likewise, no decomposition of butane is observed on the W(100)–(5×1)-C surface.