Insights into Acetic Acid Binding and Ketene Formation on Anatase TiO2(101)

Understanding the adsorption and reactivity of carboxylic acids on oxide surfaces is of great interest in catalysis for biomass upgrading via ketonization, a carbon-carbon coupling reaction. Herein, we investigate the adsorption and reaction of acetic acid on anatase TiO2(101) using scanning tunneling microscopy, infrared spectroscopy, temperature programmed reaction, and density functional theory calculations. We demonstrate the adsorption of acetic acid can form two intermediates: (1) dissociated, bidentate acetate with an associated bridging hydroxyl, and (2) molecular, monodentate acetic acid. The coexistence of ordered phases with increasing monolayer (ML) saturation coverages consisting of (1) pure acetate (0.5 ML), (2) mixed acetate/acetic acid (0.67 ML), (3) mixed acetate/acetic acid (1.0 ML) and (4) pure acetic acid demonstrates similar energetics for both acetate and acetic acid species. Under ultra-high vacuum conditions, the presence of both monodentate acetic acid and bidentate acetate was observed below room temperature, while solely bidentate acetate was observed up to 575 K. The deprotonation of acetic acid produces water at 280 K, while the thermal decomposition of bidentate acetate produces ketene and acetic acid at 645 K. This model study provides detailed insight into the stability and reactivity of carboxylic acid surface-bound intermediates, which could participate during ketonization reactions for biomass upgrading.

Automated summary

In this study, the adsorption and reaction of acetic acid on anatase TiO2(101) were investigated using various techniques. It was found that acetic acid can form two intermediates: dissociated, bidentate acetate and molecular, monodentate acetic acid. The presence of both intermediates was observed at low temperatures, while only the bidentate acetate was observed at high temperatures. This research provides important insights into the stability and reactivity of carboxylic acid surface-bound intermediates, which play a role in biomass upgrading via ketonization reactions.