Structural Effect of Nitrogen/Carbon on the Stability of Anchored Ru Catalysts for CO2 Hydrogenation to Formate

Abstract

CO2 hydrogenation to formic acid/formate is regarded as a promising strategy for achieving a sustainable circular-carbon economy. Herein, we report the catalytic behavior of nitrogen-doped carbon (NC)-immobilized ruthenium (RuCl3) catalysts for CO2 hydrogenation to formate. The NC supports were prepared at various temperatures by a facile one-step ionothermal carbonization process. The total N content and pyrrolic-N structures decreased with increasing carbonization temperature, whereas the graphitic-N structures increased. The optimized 2Ru/NC750 catalyst showed excellent turnover numbers of 4468 in a 2 h and 18,212 in a 12 h reaction at 120 °C and 8.0 MPa, and good stability, maintaining its original characteristics when recycled. The HRSTEM and XPS analyses revealed that ruthenium existed as highly dispersed atomic-scale Ru3+ species on the NC supports. Deactivation of the catalysts was attributed to (1) loss of Ru3+, (2) migration of Ru3+, and (3) reduction of Ru3+ to Ru0. It is induced that the origin of deactivation can be closely related to the binding of Ru on NC supports. DFT simulations showed that the pyrrolic-N structure was the most stable RuCl3 binding site, corresponding to experimental results. Thus, the superior performance of the 2Ru/NC750 catalyst was attributed to the strong binding of Ru to pyrrolic-N.