Highly Selective Activation of C-H Bond and Inhibition of C-C Bond Cleavage by Tuning Strong Oxidative Pd Sites

Improving the product selectivity meanwhile restrainingdeep oxidationstill remains a great challenge over the supported Pd-based catalysts.Herein, we demonstrate a universal strategy where the surface strongoxidative Pd sites are partially covered by the transition metal (e.g., Cu, Co, Ni, and Mn) oxide through thermal treatment of alloys.It could effectively inhibit the deep oxidation of isopropanol andachieve the ultrahigh selectivity (>98%) to the target productacetonein a wide temperature range of 50-200 degrees C, even at 150-200 degrees C with almost 100% isopropanol conversion over PdCu1.2/Al2O3, while an obvious decline in acetoneselectivity is observed from 150 degrees C over Pd/Al2O3. Furthermore, it greatly improves the low-temperature catalyticactivity (acetone formation rate at 110 degrees C over PdCu1.2/Al2O3, 34.1 times higher than that over Pd/Al2O3). The decrease of surface Pd site exposure weakensthe cleavage for the C-C bond, while the introduction of properCuO shifts the d-band center (epsilon(d)) of Pd upward andstrengthens the adsorption and activation of reactants, providingmore reactive oxygen species, especially the key super oxygen species(O-2 (-)) for selective oxidation, and significantlyreducing the barrier of O-H and beta-C-H bond scission.The molecular-level understanding of the C-H and C-Cbond scission mechanism will guide the regulation of strong oxidativenoble metal sites with relatively inert metal oxide for the otherselective catalytic oxidation reactions.

Automated summary

In this study, a universal strategy was demonstrated to inhibit the deep oxidation of isopropanol and achieve ultrahigh selectivity to acetone by partially covering the surface oxidative Pd sites with transition metal oxides through thermal treatment of alloys. The introduction of proper CuO shifted the d-band center of Pd upward and strengthened the adsorption and activation of reactants, resulting in more reactive oxygen species for selective oxidation and significantly reducing the barrier of bond scission.