Effect of reduction pretreatment on the structure and catalytic performance of Ir-In2O3 catalysts for CO2 hydrogenation to methanol

Abstract

In₂O₃ has been found a promising application in CO₂ hydrogenation to methanol, which is beneficial to the utilization of CO₂. The oxygen vacancy (O_v) site is identified as the catalytic active center of this reaction. However, there remains a great challenge to understand the relations between the state of oxygen species in In₂O₃ and the catalytic performance for CO₂ hydrogenation to methanol. In the present work, we compare the properties of multiple In₂O₃ and Ir-promoted In₂O₃ (Ir-In₂O₃) catalysts with different Ir loadings and after being pretreated under different reduction temperatures. The CO₂ conversion rate of Ir-In₂O₃ is more promoted than that of pure In₂O₃. With only a small amount of Ir loading, the highly dispersed Ir species on In₂O₃ increase the concentration of O_v sites and enhance the activity. By finely tuning the catalyst structure, Ir-In₂O₃ with an Ir loading of 0.16 wt.% and pre-reduction treatment under 300°C exhibits the highest methanol yield of 146 mg_CH3OH/(g_cat·hr). Characterizations of Raman, electron paramagnetic resonance, X-ray photoelectron spectroscopy, CO₂-temperature programmed desorption and CO₂-pulse adsorption for the catalysts confirm that more O_v sites can be generated under higher reduction temperature, which will induce a facile CO₂ adsorption and desorption cycle. Higher performance for methanol production requires an adequate dynamic balance among the surface oxygen atoms and vacancies, which guides us to find more suitable conditions for catalyst pretreatment and reaction.

Keywords: Carbon dioxide; Fixed-bed; Hydrogenation; In(2)O(3); Methanol; Reduction temperature.