Adjustment of the Single Atom/Nanoparticle Ratio in Pd/CNT Catalysts for Phenylacetylene Selective Hydrogenation

Abstract

Pd/C catalysts are widely used for hydrogenation reactions in the chemical industry. One of the reasons for their high activity is the ability of Pd nanoparticles (PdNP) to dissociate H2 and promote H‐spillover. Nevertheless, for selective hydrogenation unpromoted Pd/C catalysts show disappointing results. The use of supported Pd single atom (PdSA) catalysts permits to achieve high selectivity. However, PdSA show low activity because they have difficulty in activating H2. A cooperative catalysis between PdNP and PdSA operates for the hydrogenation of alkenes thanks to the H‐spillover, which makes it possible to obtain active isolated PdSA−H species. Here, we present experimental and computational results obtained for phenylacetylene hydrogenation on Pd/CNT catalysts showing different PdSA/PdNP ratios. Tuning this ratio allows doubling the activity while reaching high selectivity to styrene at high conversion. DFT calculations suggest that the first coordination sphere of PdSA has a pronounced effect on their reactivity.

Mechanism of Hydrogen Spillover on Metal-Doped Carbon Materials: Surface Carboxylic Groups Are Key

“…Finally, COOH-assisted H diffusion was unfavorable in the case of C 9 H (average reaction energies and energy barriers above 14 and around 26 kcal·mol –1 , respectively, see Figure S31), which could indicate that if an H atom arrived at a carbon with this particular configuration, it would almost be trapped so as not to continue undergoing H-spillover. This specific configuration (H atom in para position with respect to the Pd SA anchored in a single vacancy) has recently been proposed by some of us as a potential active species contributing to the selective hydrogenation of phenylacetylene directly . Thus, in contrast to the unassisted hydrogen diffusion pathway, the carboxyl-group-assisted H-diffusion pathway appears to be kinetically favorable regardless of the migration position of C 3 H or C 4 H (except the C 9 H position).…”

“…As the C 9 H product is more thermodynamically stable, it is possible that more drastic reaction conditions could give rise to a hydrogen diffusion pathway through the carbon lattice. It is interesting to note that C 9 H is found in the vicinity of a single Pd atom in a single vacancy, a species of great interest in selective hydrogenation reactions of organic substrates . The H atom could reach Pd SA from this position ( para with respect to the metal) in a single step or by cascade diffusion through the meta and ortho relative positions.…”

“…This specific configuration (H atom in para position with respect to the Pd SA anchored in a single vacancy) has recently been proposed by some of us as a potential active species contributing to the selective hydrogenation of phenylacetylene directly. 105 Thus, in contrast to the unassisted hydrogen diffusion pathway, the carboxylgroup-assisted H-diffusion pathway appears to be kinetically favorable regardless of the migration position of C3H or C4H (except the C9H position). Consequently, the role of −COOH functional groups continues to be essential throughout the different steps of the H-spillover mechanism.…”

“…It is interesting to note that C9H is found in the vicinity of a single Pd atom in a single vacancy, a species of great interest in selective hydrogenation reactions of organic substrates. 105 The H atom could reach Pd SA from this position (para with respect to the metal) in a single step or by cascade diffusion through the meta and ortho relative positions. In any case, neither of the two paths seems to lead preferentially to the hydrogenated metal species Pd SA −H, mainly due to the high thermodynamic stability (at −14.1 kcal•mol −1 ) of the C9H position (both diffusion pathways are illustrated in Figure S28).…”

Tuning the electronic structure of Pd by the surface configuration of Al2O3 for hydrogenation reactions

“…5(f)). 45,50 In Fig. 5(e), a comparison of the styrene selectivity of the two catalysts based on the same phenylacetylene conversion level clearly shows that Pd/γ-Al 2 O 3 affords higher selectivity to styrene than Pd/η-Al 2 O 3 , which is derived from the weakened adsorption of styrene on the electron-rich Pd/γ-Al 2 O 3 hindering the complete hydrogenation of phenylacetylene to ethylbenzene.…”