Tuning the crystallite size of monoclinic ZrO₂ to reveal critical roles of surface defects on m–ZrO₂ catalyst for direct synthesis of isobutene from syngas

The effects of crystallite size on the physicochemical properties and surface defects of pure monoclinic ZrO₂ catalysts for isobutene synthesis were studied. We prepared a series of monoclinic ZrO₂ catalysts with different crystallite size by changing calcination temperature and evaluated their catalytic performance for isobutene synthesis from syngas. ZrO₂ with small crystalline size showed higher CO conversion and isobutene selectivity, while samples with large crystalline size preferred to form dimethyl ether (DME) instead of hydrocarbons, much less to isobutene. Oxygen defects (O_Defects) analyzed by X-ray photoelectron spectroscopy (XPS) provided evidence that more O_Defects occupied on the surface of ZrO₂ catalysts with smaller crystalline size. Electron paramagnetic resonance (EPR) and ultraviolet–visible diffuse reflectance (UV–vis DRS) confirmed the presence of high concentration of surface defects and Zr³⁺ on m-ZrO₂-5.9 sample, respectively. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) analysis indicated that the adsorption strength of formed formate species on catalyst reduced as the crystalline size decreased. These results suggested that surface defects were responsible for CO activation and further influenced the adsorption strength of surface species, and thus the products distribution changed. This study provides an in-depth insight for active sites regulation of ZrO₂ catalyst in CO hydrogenation reaction.