Acetic acid’s thermal decomposition over Pd (111) is a good model to study the stability of fatty acids and other oxygenates involved in biofuel production. The molecular modeling of this surface reaction could give important insights into how to tune the selectivity of the reactions. Previous density functional theory (DFT) studies on the thermal decomposition of acetic acid on Pd (111) suggest that carbon monoxide (CO) and carbon dioxide (CO<sub>2</sub>), are produced by two different reaction pathways: decarbonylation (DCN), and decarboxylation (DCX), respectively. To further understand these reaction mechanisms, we have studied this model system using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and mass spectrometry (MS). The experimental data analysis is supported by DFT calculations of the XPS spectra as well as calculations of the reactions mechanism and microkinetic modeling. Pd(111) exposed to acetic acid (at 1 mbar and room temperature) shows the presence of surface carbon, chemisorbed acetic acid, acetate, and CO, as well as gaseous acid. We also explored the effect of co-adsorbed water on the selectivity of the decomposition by dosing acetic acid in the presence of water. The addition of waters shows an increase in the coverage of acetic acid and a decrease in CO coverage. Moreover, gaseous components from the reaction show an increase in CO<sub>2</sub>/CO ratio from 45% to 80%, suggesting an enhancement of DCX over DCN when co-adsorbed water is present. This is further supported by our DFT free energy diagram analysis and microkinetic model which show that the presence of co-adsorbed water favors the DCX pathway over DCN pathway by lowering barriers for O-H bond breakings but increasing barriers for C-O bond breakage.

Synergistic study of thermal decomposition of acetic acid on Pd(111) using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT)

ACS Fall 2021

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