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

The relations between particles morphology and surface properties for different kind of rutile polycrystalline samples have been deeply 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 nano-rutile crystals with well defined morphology, on nano-rutile from Aldrich and on the P25 rutile component. From these results the spectrum of CO on native P251 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 outwards 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 is providing the signature of surface relaxation. CO species interacting through the oxygen end were already studied for CO on zeolites exchanged with low polarizing alkaline cations, but not yet properly discussed for CO on metal oxides.