Fabrication of oxygen vacancies through assembling an amorphous titanate overlayer on titanium oxide for a catalytic water–gas shift reaction

The water–gas shift (WGS) reaction is a key chemical process for CO removal and hydrogen generation in which CO and H2O are converted to CO2 and H2. The activation of H2O at the oxygen vacancy sites of a reducible oxide-supported catalyst is widely viewed as the critical step for this reaction. However, the strategy of rational fabrication of oxygen vacancies in reducible oxides to boost catalytic activity is rarely reported. Here, we report a facile method to fabricate oxygen vacancies through assembling an amorphous titanate overlayer on the surface of TiO2 support via alkali doping, aiming to improve its activity in WGS reaction. TEM, EPR, H2-TPR and XPS results verified that the titanate overlayer is in situ formed via alkali doping, where oxygen vacancies can be rationally regulated during reduction through tuning the amount of alkali. The ease of oxygen vacancy formation induced by alkali doping could be attributed to the lower Ti–O bond energy in titanate. It is shown that sodium-doped Ni/TiO2 with titanate overlayer on the support exhibited superior activity in WGS reaction compared with undoped catalyst, i.e., around threefold greater activity compared with undoped catalyst was achieved at 300 °C. Furthermore, the formation of CH4, a common side reaction in WGS on Ni-based catalysts, is suppressed on the sodium-doped Ni/TiO2. More importantly, the titanate overlayer strategy is successfully extended to other reducible oxide-supported catalysts, which provides a universal method to fabricate oxygen vacancies in reducible oxide-supported catalysts.