Reaction mechanism using Ni-Ce-CaO for ICCU-RWGS

“To date, two main reaction mechanisms have been proposed: a redox process (Scheme 1) [[68][69][70]] and an associative “formate” mechanism (Scheme 2) [[71][72][73]].

Scheme 1:


Scheme 2:


“In the redox mechanism, the interaction of CO2 over oxygen vacancy results in the oxidation of catalyst and release of CO molecule. The oxidized catalyst is subsequently reduced by H2. In the associative mechanism, σ refers to sites on the oxide support. The main reaction intermediate is a bidentate formate, which decomposes to form CO and terminal hydroxyl groups. As shown in Fig. 8 and S5, the in-situ DRIFTS spectra show peaks around 1780 and 2120 cm−1 corresponding to the carbonyl, which is caused by the stretching mode of CO molecules bound at on-top metal sites. The characteristic peaks around 2450, 2850 and 2950 cm−1 are assigned to the formate species, indicating the co-existence of Scheme 1 and 2. The spectra of the Ca1Ni0.1 and Ca1Ni0.1Ce0.033 in Fig. 8 are quite similar to the spectra of CaO and Ca1Ni0.1Ce0.017 in Fig. S5. However, the performance of integrated CO2 capture and conversion exhibits a great difference between various DFMs. Thus, we propose that the surface formate and carbonyl observed by DRIFTS were not the main reaction intermediates to cause the difference between various DFMs, which has also been reported by Alexandre et al. [74]. Thus, a model for the reaction mechanism of the integrated CO2 capture and conversion over DFMs is proposed in Fig. 9. As for the DFMs without cerium doping, after switching the gas to H2, CO2 will spillover from the surface of the DFMs and react with Ni active sites resulting in the formation of CO and NiO (Eq. (8)). The reduction of NiO by the adsorbed H2 species is proposed by Eq. (9). In this case, Ni2+ becomes Ni and the oxygen reacts with hydrogen to form water. Ce3+ and Ce4+ represent reduced ceria (oxygen vacancy) and oxidized ceria, respectively. After the incorporation of cerium, the first step involves the interaction of CO2 over oxygen vacancy resulting in the oxidation of catalyst and release of CO molecule as represented by Eq. (10). The adsorbed H2 species extracts lattice oxygen from ceria, giving out H2O molecule along with the reduction of catalyst (Eq. (11)).”


Fig. 8. In-situ DRIFTS spectra of the species formed over (a) Ca1Ni0.1; (b) Ca1Ni0.1Ce0.033 after switching gas to H2 for 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20 min.”


Fig. 9. Reaction mechanism of the RWGS reaction over DFMs.”

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