Catalytic hydrogenation of CO2 to methane was investigated over Ru/rutile TiO2, focusing on the influence of the pretreatment temperature on the activity. Rutile TiO2 stabilized Ru nanoparticles even after pretreatment at 800 degrees C; therefore it was possible to establish the structure-activity relationship without considering the particle size effect. The CO2 turnover frequency initially increased with increasing pretreatment temperature from 300 to 800 degrees C, and reached a maximum value of 1.59 s(-1) on Ru/rutile TiO2 pretreated at 600 degrees C; it decreased to 1.16 s(-1) for the sample pretreated at 800 degrees C. The characterization results indicated that the activity depended on both the extent of encapsulation of Ru particles by TiOx layers and the number of hydroxyl groups on the TiOx surface. The mechanism of CO2 methanation was discussed based on in situ diffuse-reflectance infrared Fourier-transform spectroscopy results. (C) 2015 Elsevier Inc. All rights reserved.