CO-FTIR Diagnosis of Atomic Isolation in Dilute Limit Alloy Catalysts

The recent focus on single atom catalysts provides challenges not only in the development of synthesis methods but also in confirming the existence and characterizing the nature of single atom sites. One method of great potential is carbon monoxide probe Fourier transform infrared spectroscopy (CO-FTIR). Characteristic spectra of the adsorbed CO on metal surfaces has long been studied and depends primarily on the metal-carbon-oxygen bond strength. Since the synthesis method and conditions dictates the resulting catalyst, CO-FTIR spectra at various measurement conditions can serve to provide a unique spectra for analysis and synthesis evaluation. Hence, there exists a large number of CO-FTIR studies pertaining to metals commonly used in catalysis. Direct analysis of CO-FTIR spectra for monometallic and bimetallic with only one metal adsorbing CO can be made by fitting the wavenumbers associated with the metal-CO species present. However, difficulty in band wavenumber determination through fitting becomes evident in bimetallic system when both metal shows CO adsorption peaks. In this study, CO-FTIR was tested to investigate the presence of single atom sites on silica supported dilute limit alloy (DLA) bimetallic catalysts (Pd, Pt, Ru on Cu, Co, Ni) synthesized via simultaneously strong electrostatic adsorption (co-SEA) where both metal present CO vibration peaks. Gaussian parameters fitted on each of the monometallic catalyst were used to reasonably fit the spectra derived from each DLA bimetallic at various dilute limit concentration. Also, spectra corresponding to adsorption/desorption stages were utilized to determine contributing vibration peaks. Identified peaks were used in spectral fitting and eventually used to determine the unique wavenumber characteristic arising from single atom sites. Preliminary results shows a chemical shift in linearly adsorbed CO on palladium. A summary of the trends for all dilute limit alloys will be presented. These trends will be correlated to reactivity, which is currently being investigated.