Operando Studies of the Catalytic Hydrogenation of Ethylene on Pt(111) Single Crystal Surfaces

Abstract

The hydrogenation of ethylene on Pt(111) single-crystal surfaces was studied by combining measurements of the kinetics of reaction using mass spectrometry detection with the simultaneous characterization of the species present on the surface using reflection− absorption infrared spectroscopy. The kinetics measured by us matches past reports on the same system, with zero-and first-order dependence on the partial pressures of ethylene and hydrogen, respectively, and extensive H−D exchange if D 2 is used instead of H 2 . The reaction takes place in the presence of an alkylidyne surface layer, which forms immediately upon exposure of the clean surface to the reaction mixture and can be removed by hydrogen or another olefin but at rates 1−2 orders of magnitude slower than the ethylene-toethane conversion. The nature of the alkylidyne surface species changes slightly upon being exposed to high pressures of hydrogen, with the carbon in the terminal methyl moiety acquiring some sp 2 character. Moreover, the alkylidyne hydrogenation rate shows an inverse relationship with H 2 pressure and is reduced by the presence of olefins in the gas phase. Turnover frequencies for the olefin hydrogenation reaction under pressures in the Torr range are high, as reported repeatedly in the past, but the corresponding reaction probabilities are quite low, below the 10 −4 range. In contrast, almost unit reaction probability was observed here in effusive collimated molecular beam experiments emulating intermediate pressure conditions.

Mechanistic Insights into Ethylene Transformations on Ir(111) by Density Functional Calculations and Microkinetic Modeling

“…The complexity of the reaction network makes it difficult to obtain the complete reaction mechanism even under well‐defined experimental conditions. Theoretical approaches based on first‐principles calculations are useful to clarify the detailed mechanism of ethylene transformations on the various transition metal surfaces, for example on Pd(111), and Pt(111) …”

“…Theoretical approaches based on first-principles calculations are usefult oc larify the detailed mechanism of ethylene transformationso nt he various transition metal surfaces, for example on Pd(111), [1,[7][8][9][10][11][12] and Pt(111). [13][14][15][16][17][18][19][20][21][22] Ethylene transformations on iridium-based catalysts hasr ecently attractedg reat interests. [23][24][25][26][27][28][29] Many research groups have been studying the growth of epitaxial graphene on the Ir(111)s urface from ethylene dehydrogenation and decomposition.…”

Identification of surface intermediates during ethylidyne formation on Pt(111) by calculation of infrared intensities and deuterium isotope shifts

“…A well-studied model reaction in this regard is the conversion of ethylene on transition metal surfaces [1][2][3][4]. On Pt(111), in particular, an important conclusion from these studies is that two types of adsorption configurations are possible for ethylene: a π-adsorbed species at very low temperatures and in certain coadsorbed systems [5], and a di-σ bonded species at temperatures higher than 52 K [6].…”

Shape‐Controlled Nanostructures in Heterogeneous Catalysis

“…[5,120,286] That factor was not considered explicitly in the original studies. In general, however, these are complex processes, and not all factors can be included in fundamental research using model systems; the final behavior of the catalysts may not always be what is predicted.…”

“…In this case, it is important to indicate that hydrocarbon conversions under catalytic conditions do not take place on clean surfaces but rather on surfaces covered with strongly bonded carbonaceous deposits. [5,120,286] That factor was not considered explicitly in the original studies. In addition, olefin isomerization steps compete with hydrogenation to the alkane, and the final balance among all the competing pathways may depend strongly on the reaction conditions and may not be always easy to predict; see, for instance, the case of the conversion of 2butenes on supported Pt nanocubes, in which the isomerization selectivity did not follow that predicted by the TPD results ( Figure 10).…”