Contributions and limitations of IR spectroscopy of CO adsorption to the characterization of bimetallic and nanoalloy catalysts

Abstract

This contribution reviews some of the structural features of supported bimetallic catalysts that could be unravelled using IR spectroscopy of CO adsorption. The few examples presented are focussed on active metals such as Au, Pd, Pt and Co and modifications by inert elements such as Sn or Zn. The difficulty in interpreting the IR spectra during CO adsorption over Au-Pt and Au-Pd is underlined, because of Au restructuring that leads to bands typically in the range associated with Pt and Pd carbonyls. Crucial aspects of metal dispersion and surface segregation can yet be obtained, such as active metal site isolation by the disappearance of bridged or multi-bonded CO. The use of IR spectroscopy of CO adsorption to investigate the surface of bimetallics or alloys remains yet challenging and not conclusive in many instances. Firstly, this results from the complexity of the spectra obtained (even over single metals) that depends on many parameters. Secondly, CO adsorption often results in modifications of both surface structure and composition of nanoalloys via reconstruction, as in the case of monometallic surfaces, or surface segregation when one of the metals exhibits a greater affinity for CO. CO dissociation near room temperature has also been documented on many metals, including cobalt and more surprisingly Pt, Ag and Au-based materials.