(581a) Formation of Dilute Ti-Cu(111) Surface Alloys and Their High Selectivity Toward Ethanol Deoxygenation to Ethylene

Alloys comprised of an early transition metal dispersed in a coinage metal can provide opportunities for effecting selective chemical transformations of organic oxygenates. In this talk, I will discuss our recent work to synthesize dilute Ti-Cu(111) surface alloys in ultrahigh vacuum and characterize their structural and chemical properties using experiments and DFT. Scanning tunneling microscopy shows that Cu-capped, Ti-containing islands are preferentially generated on step edges of Cu(111) during Ti deposition below ~500 K, but that Ti atoms incorporate into the step edges during deposition above 500 K, generating a dilute Ti-Cu(111) surface alloy. In agreement with DFT, distinct C-O stretch bands were observed using surface vibrational spectroscopy for CO adsorbed on Cu-capped, Ti ensembles vs. isolated Ti atoms alloyed into the Cu(111) surface.

We also find that dilute Ti-Cu(111) surfaces are highly selective for the deoxygenation of ethanol. Temperature programmed reaction spectroscopy shows that ethanol deoxygenates on Ti-Cu(111) surfaces to produce only gaseous ethylene (C₂H₄) and H₂ near 400 K. The O released during reaction remains bound to Ti atoms, resulting in the formation of surface TiO_x moieties and a shift in reaction selectivity toward acetaldehyde formation. DFT calculations corroborate the high selectivity of metallic Ti-Cu(111) surfaces toward ethanol deoxygenation and predict that ethoxy deoxygenation and C₂H₄ desorption become significantly favored as the Ti ensemble size is increased from monomers to trimers, and that the O released during C-O bond cleavage causes destabilization of the adsorbed C₂H₄ product and hinderance of further dehydrogenation, both of which facilitate desorption of the desired C₂H₄ product. Our findings provide insights about the structural and chemical properties of dilute Ti-Cu(111) surfaces, revealing their propensity for promoting alcohol deoxygenation to alkenes and showing how the nuclearity and formation of reaction intermediates can influence the chemistry promoted by atomic-scale Ti ensembles in Cu.