Rutile Surface Properties Beyond the Single Crystal Approach: New Insights from the Experimental Investigation of Different Polycrystalline Samples and Periodic DFT Calculations

Abstract

The relationships between particle morphology and surface properties for different kinds of rutile polycrystalline samples have been thoroughly investigated, combining FTIR spectroscopy of CO adsorbed at 60 K through the carbon end on surface Ti4+ centers, electron microscopy, and periodic DFT calculations. We provide a complete assignment of the FTIR spectra of CO adsorbed on micro- and nanorutile crystals with well-defined morphology, on commercially sourced nanorutile, and on the P25 rutile component. On the basis of these results, the spectrum of CO on native P25(1) is revisited. Of special interest is the fact that the (110) rutile main surface undergoes a thermally induced relaxation process, leading to the shielding of exposed Ti4+ sites and consequently to the reduction of the polarizing power. This process can be reversed by inducing an outward relaxation of the shielded Ti atoms by treating the sample in water at 573 K. A red-shifted band ascribed to CO adsorbed through the oxygen end on the low polarizing sites of relaxed surfaces provides the signature of surface relaxation. CO species interacting through the oxygen end have already been studied for CO on zeolites exchanged with low polarizing alkaline cations but not yet properly discussed for CO on metal oxides.