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  1. The adsorption of crotonaldehyde on Cu-Pt alloy surfaces was characterized by density functional theory (DFT). Two surfaces were considered: Cu2Pt/Cu(111) and Cu3Pt/Cu(111). It was determined that the presence of Pt on the surface, even when isolated as single atoms fully surrounded by Cu, provides additional stability for the adsorbates, increasing the magnitude of the adsorption energy by as much as 40 kJ/mol. The preferred bonding on both surfaces is via multiple coordination, with the most stable configuration being a cis arrangement with di-σ bonding of the C=O bond across a Cu–Cu bridge and an additional π bonding to a Pt atom. The fact that Pt significantly affects the adsorption of unsaturated aldehydes such as crotonaldehyde explains why the kinetics of their hydrogenation using single-atom alloy (SAA) catalysts vary with alloy composition, as we previously reported, and brings into question the simple model in which the role of Pt is only to promote the dissociation of H2. 
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  2. The effect of gases on the surface composition of Cu–Pt bimetallic catalysts has been tested by in situ infrared (IR) and x-ray absorption spectroscopies. Diffusion of Pt atoms within the Cu–Pt nanoparticles was observed both in vacuum and under gaseous atmospheres. Vacuum IR spectra of CO adsorbed on CuPt x /SBA-15 catalysts (x = 0–∞) at 125 K showed no bonding on Pt regardless of Pt content, but reversible Pt segregation to the surface was seen with the high-Pt-content (x ≥ 0.2) samples upon heating to 225 K. In situ IR spectra in CO atmospheres also highlighted the reversible segregation of Pt to the surface and its diffusion back into the bulk when cycling the temperature from 295 to 495 K and back, most evidently for diluted single-atom alloy catalysts (x ≤ 0.01). Similar behavior was possibly observed under H 2 using small amounts of CO as a probe molecule. In situ x-ray absorption near-edge structure data obtained for CuPt 0.2 /SBA-15 under both CO and He pointed to the metallic nature of the Pt atoms irrespective of gas or temperature, but analysis of the extended x-ray absorption fine structure identified a change in coordination environment around the Pt atoms, from a (Pt–Cu):(Pt–Pt) coordination number ratio of ∼6:6 at or below 445 K to 8:4 at 495 K. The main conclusion is that Cu–Pt bimetallic catalysts are dynamic, with the composition of their surfaces being dependent on temperature in gaseous environments. 
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