Isolating a Reaction Intermediate in the Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt(111)

The isolation and identification of surface intermediates is of the utmost importance for the elucidation of mechanisms and selectivity patterns in heterogeneous catalysis. However, the metastable nature of reaction intermediates makes their detection and differentiation from other species challenging. This work reports a combined variable temperature scanning tunneling microscopy (VT-STM) and van der Waals-corrected density functional theory (opt88-vdW DFT) study showing that a hydroxy intermediate (hy-TFAP) formed in the hydrogenation of 2,2,2-trifluoroacetophenone (TFAP) is trapped by parent TFAP to form a H-bonded bimolecular TFAP/hy-TFAP structure. The facile formation of the hydroxy intermediate, by residual hydrogen present in the ultrahigh vacuum chamber, was predicted based on a previous DFT study of the hydrogenation pathway for TFAP on Pt(111). The prediction is confirmed by comparison of calculated TFAP/TFAP and TFAP/hy-TFAP structures with STM images of bimolecular structures formed through TFAP adsorption and treatment at different temperatures.