Site requirements in the kinetics of alkane transformations catalysed by metals

Abstract

Hydrogen pressure is often found to cause an inhibiting effect on the kinetics of alkane reactions on metal surfaces. The most common interpretation of such results is focussed on the degree of dehydrogenation of the active surface intermediate following a dissociative mechanism. A series of arguments will be given in favour of an associative mechanism in which the hydrocarbon reacts with an adsorbed hydrogen atom. In this mechanism, the adsorption site or ''landing site'' is composed of a chemisorbed hydrogen atom associated with an ensemble of ''Z'' potential sites. These sites are associated with the hydrogen chemisorption site. The validity of H 2 adsorption to determine the metallic dispersion will be also discussed.

Chemical Transient Kinetics Applied to CO Hydrogenation over a Pure Nickel Catalyst

“…Only for the second step, a (common) decay time can be evaluated. Actually, such a two-step process is in agreement with the role that hydrogen plays in exchange reactions between hydrocarbons and D 2 , isomerization and hydrogenolysis. , Accordingly, surface hydrogenation takes place first and depends on the availability of atomic hydrogen, whereas reactive desorption is second and encounters a hydrogen molecule interacting with respective surface precursors. Following this scenario, metallic fractions liberated in the first step gradually increase the availability of atomic hydrogen so as to reduce the masi intermediate, while in a second hydrogenation step the resulting precursor is transformed into a hydrocarbon.…”

Reactivity Theory of Transition-Metal Surfaces: A Brønsted−Evans−Polanyi Linear Activation Energy−Free-Energy Analysis

“…For an extensive treatment of the classical heterogeneous catalytic view of this reaction, we refer to an important paper by Garin. 74 Two essential mechanistic steps have to be distinguished: cleavage of the C-C bond between atoms R and β (Scheme 2, b3) and cleavage of the C-C bond through metal-carbon contact of atoms R and γ (Scheme 2, II). On 73 showed that the reactions occur most favorably on Pt at step edges as opposed to terrace sites.…”

“…While the barrier for NH 2 activation by coadsorbed O is unfavorable on Pt (111), the barrier for this same reaction on the Pt(100) surface is lower (Figure 47). 74 Activation of NH 2 with oxygen can now occur through a transition state that does not require metal atom sharing, as illustrated in Figure 40c.…”

Understanding of Novel Surface Events on a Supported Co2+-Ensemble Catalyst Promoted by Gas-Phase Molecules: Increases in the Amount and Reactivity of Adsorbed NO by CO Undetectable at the Catalyst Surface

“…Geminal dinitrosyl species are general species in metal complexes 46 and also in supported metal-ion catalysts. [47][48][49] The nitrogen atom of a NO adsorbate in the geminal I interacts with an adjacent Co 2+ ion, while the other NO adsorbate is free. In the geminal II the Co-O(III) bond at the surface is broken, where the Co-O(III) separation is elongated to 0.373 nm contrasted to the Co-O(III) bonding at 0.217 nm in the geminal I.…”