Reversibility of strong metal-support interactions on Rh/TiO2

Hydrogenolysis of n-butane and CO hydrogenation were used to study the reversibility of the SMSI state in Rh supported on model nonporous TiO2 spheres. High-temperature reduction (HTR) caused a three-order-of-magnitude drop in hydrogenolysis activity and led to increased selectivity toward ethane. Oxidation at 573 K was sufficient to restore the hydrogenolysis activity in these catalysts. Variations in hydrogenolysis selectivity on RhTiO2 were similar to those on RhSiO2 subjected to comparable treatments. This suggests that site blocking by TiOx may be primarily responsible for the suppressed hydrogenolysis activity on RhTiO2. CO hydrogenation activity, on the other hand, was only marginally affected by the onset of SMSI and either increased or decreased after HTR depending on the prior treatment of the catalyst. High-resolution transmission electron microscopy showed the presence of 2- to 4-Å amorphous overlayers on both the Rh and the support after high-temperature reduction. The thickness of these overlayers was nonuniform and oxidation followed by low-temperature reduction led to partial removal of the overlayers and a roughening of the Rh metal surfaces.