Dispersion and metal-support interactions due to Ag promotion leads to accelerate CO₂ conversion to methanol over zeolite based Ag-Zn catalysts

The efficient use of carbon resources and the corresponding drop in atmospheric carbon dioxide (CO₂) concentration have made the thermo-chemical conversion of CO₂ into valuable chemical compounds an important area of study. Herein, we describe the efficient thermochemical conversion of CO₂ to methanol by the use of zeolite supported Ag-Zn catalysts. In the current work, a simple co-precipitation method was used in the synthesis of zeolite supported Ag-Zn bimetallic catalysts with different Ag loadings. To investigate the physicochemical characteristics of the synthesized catalysts, a number of analytical techniques were used. The mordenite nature of zeolite support and successful doping of Ag Zn metals were confirmed by Fourier Transform Infrared (FTIR) spectra by displaying functional groups absorption bands at relative wavelengths. Studies using X-ray diffraction (XRD) showed a crystalline structure of the zeolite support with highly dispersed phases of both Ag and Zn on the surface of the support. The N₂ adsorption-desorption studies revealed the existence of both mesopores and micropores in zeolite supported Ag-Zn catalysts. In addition, N₂ adsorption-desorption studied of zeolite supported Ag-Zn catalysts also pointed out the creation of surface defects by incorporating Ag concentration. Importantly surface chemistry of bimetallic catalysts was affected by Ag addition as demonstrated by X-ray Photoelectron Spectroscopy (XPS) results. Metal support interaction and reduction behavior of Ag-Zn/Z catalysts were assessed by hydrogen temperature program reduction (H₂-TPR). Catalytic activity of zeolite supported Ag Zn catalysts with different Ag concentration for methanol synthesis by CO₂ hydrogenation was evaluated in slurry Parr reactor. Methanol synthesis rate was accelerated from 18 to 27 g.meth.kg._cat. h by doubling the Ag concentrations from 1 to 2 wt%. The trend of increasing methanol synthesis rate was continued with further addition of Ag concentration and reached at maximum 39 g.meth.kg._cat.h for highest Ag loading. This linear trend in methanol synthesis rate as consequent of incrementing Ag concentration documented promoting role of Ag in zeolite based Ag-Zn catalysts for methanol synthesis by CO₂ hydrogenation. Structure-activity relationship clued the Ag Zn dispersion and metal-support interactions and creation of surface defects as the key parameters tailoring the promoting role of Ag in the current work.