CO adsorption on pure, defective and mixed composition AlF3 and MgF2 surfaces

AlF3 and MgF2 are important catalysts for fluorination processes. In this study we characterise structures consisting of either AlF3, or MgF2 supported over AlF3, using Fourier transform infrared spectroscopy of surface absorbed CO. We interpret the results using atomic-scale density functional calculations, along with high-resolution electron microscopy and spectroscopy. By coupling theory to vibrational spectroscopy we are able to establish different stable facets in the host species, identifying both surface physisorbed CO binding to pristine facets, and chemisorbed CO binding to fluorine anion vacancy sites. In the case of MgF2 supported over AlF3 surfaces we suggest that a 50 : 50 mixed Mg-AlFx monolayer surface phase can form. Excess MgF2 deposition subsequently results in the formation of MgF2 nanoparticles on the surface. Mixed composition surface phases may play a critical role in Al/MgFx fluorine catalysis.