Mechanism study of reduction of CO_2 into formic acid by in-situ hydrogen produced from water splitting with Zn: Zn/ZnO interface autocatalytic role

We have previously developed a new process of highly efficient conversion of CO_2 and water into formic acid with metallic Zn without the addition of catalyst, however, its mechanism is not clear, particularly in the catalytic role of Zn/ZnO interface. Herein, the autocatalytic role of Zn/ZnO interface formed in situ during the reduction of CO_2 into formic acid with Zn in water was studied by combining high resolution transmission electron microscopy(HRTEM), X-ray diffraction(XRD) and X-ray photoelectron spectroscopy(XPS) techniques and experimental data. The electron microscope results show that possible defects or dislocations formed on Zn/ZnO interface, in which plays a key role for Zn H-formation. Further XPS analyses indicate that oxygen vacancies on Zn/ZnO interface increased at short reaction times(less than 10 min). These analyses and experimental results suggest that a highly efficient and rapid conversion of CO_2 and water into formic acid should involve an autocatalytic role of the Zn/ZnO interface formed in situ, particularly at the beginning of the reaction.