Achievement of complete CO removal

Achievement of complete CO removal at low temperature can avoid exothermic CO2 methanation at high temperatures. As a simple solution, an increase in the amount of exposed active species can enhance the CO methanation activity. Indeed, addition of a noble metal (such as Ru67,68 and Pt69) to Ni-containing mixed oxide catalysts boosted CO methanationat low temperature because H2 spillover from the noble metal enhanced the reducibility of the Ni species, as will be seen in section 5.1. The other answer is the control of CO adsorption and dissociation, since adsorption and dissociation on an active metal surface are key steps for CO methanation. It is considered that the electron density (ED) of the surface directly influences the activity of CO methanation. If the ED is enhanced, CO adsorbed on a metal surface with high ED is more easily dissociated by enhanced dπ–pπ* back bonding and consequently CO methanation is improved.33,70,71 Otherwise, the amount of dissociatively adsorbed H2 on the metal increases due to weak adsorption of CO on the surface, which favors the transformation of CO to CH4. 42 These investigations show the importance of finding the suitable ED for selective CO methanation. Basically the ED can be changed by the following processes: (i) change of support material,72 and (ii) addition of some promoters, such as alkali and alkali earth metals, to catalysts.73–77 In this section, we summarized them.