The Role of Carbonaceous Deposits in Hydrogenation Catalysis Revisited

Abstract

The effect of carbonaceous deposits on the performance of Pt(111) surfaces as catalysts for the hydrogenation of ethylene was tested by decoupling their preparation, which was done beforehand in an ultrahigh vacuum (UHV) environment, from the catalytic runs, which were carried out in an enclosed "high pressure" cell. The time evolution of the gas composition during reaction was followed continuously by mass spectrometry, and the nature of the surface species was determined by in situ reflection−absorption infrared absorption spectroscopy (RAIRS). Reaction rates were seen to vary by up to approximately 40% depending on the type of molecules preadsorbed at room temperature, with the maximum activity seen with a propylidyne layer, the result of propylene adsorption. The rate with ethylidyne-covered Pt( 111) is reduced by ∼20%, with butylidyne by ∼35%, and with benzyl moieties by ∼40%. These changes are reversible: the surface regains the activity expected when starting with the clean substrate after one or two catalytic runs to full conversion. RAIRS data show that this is because the initial species are slowly replaced by a new layer of the adsorbate that forms with the olefin in the reaction mixture (ethylidyne for ethylene hydrogenation). More significant changes are seen if the adsorbed layers are dehydrogenated at higher temperatures: the turnover frequencies for ethylene hydrogenation are reduced by more than an order of magnitude upon the conversion of propylidyne to C n H(ads) species, by annealing at temperatures between 500 and 650 K. Some activity is regained upon annealing to even higher temperatures, presumably because of the formation of graphitic layers, which have a smaller footprint on the surface. It was concluded that although the main role of carbonaceous deposits in catalytic hydrogenations is to block surface sites, their influence is also affected by the reversibility of their bonding to the surface in a hydrogenation atmosphere and by the structure of their carbon skeleton.

Kinetic Study of the Hydrogenation of Unsaturated Aldehydes Promoted by CuPtx/SBA-15 Single-Atom Alloy (SAA) Catalysts

“…The kinetic reaction orders implied by this analysis are unreasonably high and indicate that additional factors, besides the reaction rate dependence on [CMA], play a role in determining reaction rates. Two possibilities are considered in the following analysis: (1) the surface of the catalysts, primarily the Cu atoms, are poisoned by organic deposits that form and bind strongly on the surface during the course of the reaction, a common occurrence in hydrogenation catalysis; − and/or (2) the products of the reaction, CMO, HCMA, and HCMO, may compete for adsorption sites on the surface, blocking the CMA uptake and consequently inhibiting further catalytic activity.…”

The Surface Chemistry of Metal-Based Hydrogenation Catalysis

“…Recent experiments from our group support the general view of the role of carbonaceous deposits mentioned above but also point to other subtle effects. In particular, by independently controlling the nature of the carbonaceous layer, it was determined that their structure and reactivity do affect the performance of the catalyst . For one, ethylene hydrogenation on Pt(111) was found to be the fastest on a propylidyne-precovered surface and to exhibit rates that are lower by ∼20% with ethylidyne, by ∼35% with butylidyne, and by ∼40% with benzyl moieties (Figure ).…”

Selectivity in Hydrogenation Catalysis with Unsaturated Aldehydes: Parallel versus Sequential Steps

“…Selectivity is particularly difficult to control in hydrogenation processes with unsaturated organic molecules, because the surface of the catalyst in those cases, typically transition metal nanoparticles dispersed on a high-surface-area support, tends to be covered with strongly bonded carbonaceous deposits. − Those carbon-containing layers modify the behavior of the surface and can affect the adsorption mode of the reactants as well as the uptake and activation of hydrogen. , This is particularly critical if the molecule to be hydrogenated contains two or more unsaturations, in which case the initial adsorption mode may define the selectivity of their conversion …”