Abstract
The selective hydrogenation of propyne over a Pd-black model catalyst was investigated under operando conditions at 1 bar making use of advanced X-ray diffraction (bulk sensitive) and photo-electron spectroscopy (surface sensitive) techniques. It was found that the population of subsurface species controls the selective catalytic semi-hydrogenation of propyne to propylene due to the formation of surface and near-surface PdCx that inhibits the participation of more reactive bulk hydrogen in the hydrogenation reaction. However, increasing the partial pressure of hydrogen reduces the population of PdCx with the concomitant formation of a β-PdHx phase up to the surface, which is accompanied by a lattice expansion, allowing the participation of more active bulk hydrogen which is responsible for the unselective total alkyne hydrogenation. Therefore, controlling the surface and subsurface catalyst chemistry is crucial to control the selective alkyne semi-hydrogenation.
Highlights
Catalytic hydrogenation of hydrocarbons is an important family of processes in chemical industry [1]
It was reported that the bulk H atoms are largely reactive species in hydrogenation reactions, while H atoms bound to the surface are less reactive pointing out the importance of bulk/subsurface species in heterogeneous catalysis [16], where the presence of carbonaceous species is determinant in the catalytic performance [17]
transmission electron microscopy (TEM) measurements indicate that the primary particle size is in the range of 5–15 nm and that the sponge-like structure comes from the aggregation of primary crystallites
Summary
Catalytic hydrogenation of hydrocarbons is an important family of processes in chemical industry [1]. It was reported that the bulk H atoms are largely reactive species in hydrogenation reactions, while H atoms bound to the surface are less reactive pointing out the importance of bulk/subsurface species in heterogeneous catalysis [16], where the presence of carbonaceous species is determinant in the catalytic performance [17] In this way, it was found that the carbon deposition strongly influences the hydrogen depth profile distribution promoting the diffusion of hydrogen into the palladium bulk [18].Thereby, the combination of in situ spectroscopy and DFT calculations has shown that the existence of sizable barriers for hydrogen emerging from the bulk through PdCx to the surface [19] is the origin for the alkynes’ selective hydrogenation. Ertl indicating that under such circumstances the effective pressure is much smaller than the hydrostatic pressure [20]
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