The prospects of producing fuels and chemicals via CO2 methanation may simultaneously contribute to CO2 emission reduction and clean-energy storage solutions.1 However, CO2 activation remains a considerable scientific challenge.2 Supported Ni and Ru have been the industrial catalysts of choice for CO and CO2 methanation due to their high activity and methane selectivity.3 In recent years, the promotion of Ni-based catalysts with a second metal has been studied. Such bimetallic systems usually include expensive noble metals (e.g. Pt, Pd, Ru),4 therefore cheaper promoters are highly desired. A transition metal promoter that has received limited attention for Ni-based systems is Mn, this despite its importance in Co-based Fischer-Tropsch catalysts.5 In our study we report a promotional effect of Mn on Ni/TiO2 catalysts for CO2 and CO methanation.
2. Experimental
Mn-promoted Ni/TiO2 catalysts were prepared using wet-impregnation methods. Catalysts were characterized before reaction with TPR, chemisorption, ICP, and XRD. Particle sizes and metal distribution was studied with STEM-EDX. The reduction of unpromoted and Mn-promoted Ni/TiO2 was investigated using operando quick-EXAFS at both the Ni and Mn K-edges to determine the oxidation state and coordination of each metal. Catalytic activity and stability for gas phase CO2 and CO methanation were studied in high-throughput at atmospheric pressure between 200-400 °C. Temperature-programmed hydrogenation was used to study catalyst coking. In-situ FTIR was employed to investigate CO2 adsorption and observe intermediate species under methanation conditions, enabling us to develop a mechanistic understanding for TiO2 supported Ni and Ni-Mn catalysts. The mechanistic picture was completed by a combination of isotopic transient kinetic analysis, density functional theory (DFT) calculations and first-principles microkinetics simulations.
3. Results & Discussion
Prior to reduction, XRD and XANES/EXAFS measurements confirmed that Ni and Mn were present as highly dispersed oxides. Detailed analysis of the Ni-Mn/TiO2 samples’ reducibility was provided by operando XANES measurements. The addition of Mn to Ni/TiO2 catalysts led to a delay in the onset of Ni reduction from 300 °C to 400 °C as determined with operando XANES on the Ni K-edge (Figure 1a-b). This confirmed that Ni and Mn were in close proximity. STEM-EDX showed that all active catalysts consisted of bulk Ni nanoparticles, with Mn-promoted catalysts containing highly dispersed Mn, likely in the form of MnO. The decoration of the Ni particles with MnO led to an enhanced CO2 methanation activity (Figure 1c). Moreover, exposing Ni/TiO2 and Ni-Mn/TiO2 catalysts to CO methanation conditions led to the deactivation of all catalysts. However, the catalyst lifetime was clearly enhanced by the addition of Mn. An investigation into the deactivation pathways for these catalysts identified coking as the likely source. An in situ FTIR study verified that promotion with Mn leads to an increased CO2 adsorption capacity. Furthermore, Mn addition enhanced CO2 activation, ultimately leading to improved methanation activity.Mechanistically, we find that partial removal of O atoms from MnO in contact with Ni results in active sites for rapid C-O bond cleavage. Microkinetics simulations based on DFT calculations support this and point out C-O bond dissociation in adsorbed CO and CO2 at the interface between Ni and MnO contributes to the high catalytic performance. These data are consistent with operando IR measurements and 12CO2 " 13CO2 isotopic kinetic analysis.
4. Conclusions
The addition of Mn to Ni/TiO2 catalysts leads to enhanced CO2 methanation activity. The superior activity originates from the decoration of Ni nanoparticles by highly dispersed MnO, with higher Mn loadings yielding more active CO2 methanation catalysts, and improving the stability of the catalysts under CO methanation conditions. These findings demonstrate that Ni-based catalysts may be promoted with cheaper and more abundant transition metal oxides such as MnO, enabling better performance while ensuring a sustainable approach.
References
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(3) Wei, W.; Jinlong, G. Front. Chem. Sci. Eng. 2011, 5, 2–10.
(4) De, S.; Zhang, J.; Luque, R.; Yan, N. Energy Environ. Sci. 2016, 9, 3314–3347.
(5) Morales, F.; de Groot, F. M. F.; Gijzeman, O. L. J.; Mens, A.; Stephan, O.; Weckhuysen, B. M. J. Catal. 2005, 230, 301–308.