In situ identification of surface sites in Cu-Pt bimetallic catalysts: Gas-induced metal segregation

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 CuPtx/SBA-15 catalysts (x = 0-infinity) 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 H2 using small amounts of CO as a probe molecule. In situ x-ray absorption near-edge structure data obtained for CuPt0.2/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 similar to 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. Published under an exclusive license by AIP Publishing.

Automated summary

The effect of gases on the surface composition of Cu-Pt bimetallic catalysts has been investigated through in situ infrared and x-ray absorption spectroscopies. The results showed that Pt atoms can diffuse within the Cu-Pt nanoparticles in both vacuum and gaseous atmospheres. Reversible segregation and diffusion of Pt on the surface were observed when varying the temperature and gas environment. The surface composition of the catalysts is highly dependent on temperature.