(521cn) A Comprehensive Characterization of Catalytic Active Sites in Sodium-Modified Zirconia

Alkali metal incorporation has been adopted to manipulate the performance of solid acid catalysts, with previous reports that adding alkali metal cation could either decrease catalytic activity through active sites blockage, or introduce new catalytic capability via formation of basic sites. Despite numerous investigations, myths still exist on the nature of the alkali-modified active sites. Herein, we systematically investigate the catalytic active sites over unmodified zirconia (ZrO₂) and sodium-doped zirconia (Na-ZrO₂) through surface characterizations and kinetic measurement. A series of tetragonal ZrO₂ and Na-ZrO₂ catalysts with varying Na contents were synthesized, and pyridine FT-IR measurement showed only Lewis acid sites present on these catalysts. Using isopropanol as probe molecule for kinetic analysis, propene and acetone were the major products under reaction conditions, through dehydration and dehydrogenation of isopropanol. Decreased dehydration rate and increased dehydrogenation rate were observed with increasing Na content incorporated. Combination of activation energy measurement (Figure 1A and 1B) and quantitative TPSR indicates that the trend of dehydration/dehydrogenation rates with increasing Na content is not only due to changes in corresponding active site densities (more dehydrogenation sites and less dehydration sites), but also due to changes in reaction barriers resulting from the sodium present. Raman characterization revealed the formation of NaHCO₃/Na₂CO₃ surface layer over Na-ZrO₂ catalysts, likely due to the reaction between incorporated Na with CO₂ and water in air, which contributes to the formation of basic active sites for acetone synthesis. Additionally, co-feeding water with isopropanol decreased propene and acetone rates of formation over all catalysts reversibly, with dehydration more inhibited than dehydrogenation and such discrepancy increases at higher Na content (Figure 1C).