The phase pure, unsupported intermetallic compound PtZn is intrinsically active and selective for chemoselective hydrogenation of crotonaldehyde to crotyl alcohol in the absence of any support effects or auxiliary phases. During un-promoted reaction conditions. the PtZn Surface becomes platinum enriched within the XPS sampling depth with respect to the 1:1 bulk stoichiometry. Catalyst modification by co-feeding ppm levels of bromoethane results in (i) surface enrichment by zinc, (ii) pronounced formation of a chemically distinct Zn delta+ species and (iii) substantial improvements in selectivity toward crotyl alcohol (up to 88% at 10% reactant conversion). Bromoethane promotion acts both by suppressing formation of the undesired products and by enhancing crotyl alcohol formation, likely due to activation of the C=O bond by coordination to the Zn delta+ sites. Haloalkanes containing F, Cl or I do not induce any significant effects in either surface composition or electronic structure, serving only to poison the system. (C) 2008 Elsevier Inc. All rights reserved.