The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 10(1) to 10(2) molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 +/- 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 10(2) to 10(0) nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 +/- 0.2 kcal/mol in the absence of CO and 11.4 +/- 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.