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Understanding the underlying mechanism of improved selectivity in pd1 single-atom catalyzed hydrogenation reaction

Yan, Huan, Lv, Haifeng, Yi, Hong, Liu, Wei, Xia, Yujia, Huang, Xiaohui, Huang, Weixin, Wei, Shiqiang, Wu, Xiaojun, Lu, Junling
Journal of catalysis 2018 v.366 pp. 70-79
X-ray absorption spectroscopy, adsorption, ambient temperature, butanes, catalysts, catalytic activity, density functional theory, dissociation, graphene, heterolytic cleavage, hydrogen, hydrogenation
Supported metal single-atom catalysts (SACs) with maximized metal atom efficiency and unique catalytic properties have drawn tremendous attention in the catalysis field. In this work, we reported that in selective hydrogenation of 1,3-butadiene, graphene supported Pd₁ single atoms, synthesized by atomic layer deposition, exhibited expressively 100% butenes selectivity at 100% conversion at near ambient temperature, regardless of hydrogen partial pressures. The hydrogen reaction order was found to be about 1.2, indicating that hydrogenation dissociation is the rate-determining step. Combining with other structural characterization techniques, in situ X-ray absorption fine structure spectroscopy suggests that the Pd₁ single atom likely bonds to the graphene support through three PdC and one PdOC coordinations. Density functional theory calculations show that 1,3-butadiene adsorbs on Pd₁ via a mono-π adsorption mode (−0.98 eV), much stronger than that of H₂ (−0.30 eV), which makes Pd₁ to be predominantly covered with a 1,3-butadiene molecule during reaction, consistent very well with the results of H-D exchange reaction. Facilitated by the bridging O atom in the PdOC coordination, H₂ dissociates at the 1,3-butadiene covered Pd₁ atom in the heterolytic way, then hydrogenates the adsorbed 1,3-butadiene molecule by following the pseudo Horiuti-Polanyi mechanism. Distinct from its adsorption on extended Pd surfaces, such mono-π adsorption of 1,3-butadiene on Pd₁ favors 1-butene formation, but impedes the secondary hydrogenation to butane owing to intensified steric hindrance, thus yielding the expressively high butenes selectivity over Pd₁ SAC. These findings pave a new way to rational design SACs catalyst in selective hydrogenation reactions.