(621z) High-Temperature Water-Gas Shift Reaction over Ni/Xk/CeO2 Catalysts: Suppression of Methanation Via Formation of Bridging Carbonyls

The effect of potassium (K) loading on ceria-supported nickel (Ni/xK/CeO₂) catalysts over water-gas shift reaction has been investigated. An optimum loading of 5 wt% K was found to enhance the catalytic performance in terms of activity and selectivity. As evidenced by DRIFTS, the methane suppressing effect of K is attributed to the inhibition of formation of nickel subcarbonyl species through interaction of Ni and K, coupled with the strong adsorption of carbon monoxide (CO) on Ni via the formation of bridging carbonyls. Additionally, K was found to enhance reduction of CeO₂ via XANES and promote water dissociation on reduced CeO₂ to form hydroxyl (OH) groups which dissociates further into adsorbed oxygen that react with adsorbed CO on Ni to form adsorbed carbon dioxide (CO₂). A dual-site redox mechanism was proposed and a good fit of the kinetic data with R² = 0.91 was obtained with the proposed kinetic model.

From the present study, the following concluding remarks can be made for Ni/xK/CeO₂ catalysts on the water-gas shift reaction:

(a)    Effect of varying K loadings on Ni/xK/CeO₂ catalysts (x = 0-10 wt%) has been studied in detail to under the role of K in methane suppression and enhancement of WGS activity. An optimal K loading of 5 wt% was found to effectively suppress methanation during WGS reaction by enhancing CO adsorption on Ni via formation of bridging carbonyls as evidenced by in-situ DRIFTS studies. Strong adsorption of CO in the bridging modes inferred that CO disproportionation and subsequent methanation of carbon precursors were prevented. Besides the formation of bridging carbonyls, subcarbonyl species which are precursors for methane formation were not observed in the presence of K.

(b)     The role of K in enhancing WGS activity can be attributed to the provision of OH groups in the vicinity of Ni-Ce interface. As K is hygroscopic, it increases the affinity of water to the CeO₂ support, enhancing water dissociation and the subsequent formation of OH groups which are vital for forming adsorbed oxygen on the support that subsequently reacts with adsorbed CO on Ni to form the reaction products. Moreover, doping of K on CeO₂ causes charge imbalance and lattice distortion of CeO₂, thereby leading to greater concentration of surface lattice oxygen as shown by XPS. As evidenced by XANES and CO-TPR-MS, reduced CeO₂ exists predominantly in the Ce³⁺ state in the presence of K, producing oxygen species that are important for the WGS reaction.

(c)    Kinetic study on Ni/5K/CeO₂ catalyst revealed that the catalyst has a large positive dependency on H₂O and a great inhibitory effect imposed by H₂. Additionally, a dual-site redox mechanism was proposed and achieved a good fit to the kinetic data obtained.