Effects of carboxylic acid adsorbates on CO adsorption and crotonaldehyde hydrogenation over Cu/Al₂O₃ catalyst

The hydrogenation of crotonaldehyde over Cu/Al2O3 which was modified by formic acid or acetic acid adsorption has been investigated, exposed Cu sites being characterised using infrared spectroscopy with CO as a probe molecule. Formic acid adsorption on reduced C sites generated adsorbed bidentate formate which blocked CO adsorption at many sites, induced cationic character at the remaining available sites, and partially poisoned hydrogenation reactions. Modification of the surface poisoned C=O hydrogenation more than C=C hydrogenation and hence promoted butanal as the dominant hydrogenation product. Desorption of formate by "flashing'' at high temperature resulted in a partly reconstructed surface which gave higher catalytic activities than an unmodified surface before formic acid treatment. However, crotyl alcohol remained a minor product. The effects of acetic acid adsorption were similar to those for formic acid except that attempts at desorption of the modifier resulted in carbonaceous residues which partially poisoned both CO adsorption and catalytic activity.