Selective CO Methanation on Highly Active Ru/TiO2 Catalysts: Identifying the Physical Origin of the Observed Activation/Deactivation and Loss in Selectivity

Ru/TiO2 catalysts are highly active and selective in the selective methanation of CO in the presence of large amounts of CO2 but suffer from a considerable deactivation and loss of selectivity during time on stream. Aiming at a fundamental understanding of these processes, we have systematically investigated the physical reasons responsible for these effects, using catalysts with different surface areas and combining time-resolved kinetic and in situ/operando spectroscopy measurements as well as ex situ catalyst characterization. This allowed us to identify and disentangle contributions from different effects such as structural effects, adlayer effects, such as site blocking effects, and changes in the chemical (surface) composition of the catalysts. Operando X-ray absorption near edge spectroscopy (XANES)/extended X-ray absorption fine structure analysis (EXAFS) measurements revealed that an initial activation phase is largely due to the reduction of oxidized Ru species, together with a distinct change in the Ru particle shape, until reaching a state dominated by metallic Ru species (fraction RuO2 < 5%) with the highest Ru mass normalized activity. The losses of activity and selectivity during the subsequent deactivation phase are mainly due to slow Ru particle growth (EXAFS, transmission electron microscopy (TEM)). Surface blocking by adsorbed species such as surface formate/carbonate or surface carbon species, which are formed during the reaction, contributes little, as concluded from in situ infrared (IR), temperature -programmed oxidation (TPO), and X-ray photoelectron spectroscopy (XPS) data. Consequences on the selectivity for CO methanation, which decreases with time on stream for catalysts with larger surface area and for the distinct loss of adsorbed CO and surface formate species, as well as the role of the catalyst surface area in the reaction are discussed.