Molecular Understanding of Structure-Activity Relationships in Co-Catalyzed CO2 and CO Hydrogenation
Roos Kr枚sschell successfully defended her PhD thesis at the Department of Chemical Engineering and Chemistry on April 8th.
In her thesis, Roos explores the relationship between the structure of various cobalt (Co) catalysts and their reactivity in CO and CO2 methanation at the molecular level. To achieve this, she modelled a wide range of active sites, including various environments of the active sites, and correlated the site geometry with the reactivity of the site. Using density functional theory (DFT), we simulated the reaction pathways for CO+H2 and CO2+H2 conversions, analyzing stable states through density of states (DOS), crystal orbital Hamilton population (COHP), and charge analyses. Microkinetic modeling was employed to simulate the activity and selectivity of these active sites. By correlating the electronic structure of the site-adsorbate complex and its influence on the stabilization of intermediate and transition states, we gained deeper insights into catalytic behavior at the molecular level.
Roos and her colleagues found that the observed structure sensitivity in CO and CO2 methanation over various supported cobalt catalysts directly correlates with the structural factors governing Fischer-Tropsch synthesis. The analysis of CO dissociation pathways highlights the critical role of specific adsorption sites, particularly those facilitating the shift in the 1蟺 orbital during five- or sixfold CO adsorption, which is essential for efficient bond scission. Such sites are prevalent on larger cobalt particles but absent on smaller nanoparticles, resulting in higher activation barriers for direct CO scission.
In Fischer-Tropsch synthesis, where chain growth and hydrocarbon formation depend on the efficiency of CO dissociation and subsequent hydrogenation steps, these structural disparities between different nanoparticle sizes become crucial. The interfacial perimeter sites on smaller nanoparticles, which support CHO intermediate anchoring and activation through the mobility of Co atoms, may provide alternative pathways for C鈥揙 bond scission and catalytic activity. However, the absence of B5 sites and reduced availability of optimal adsorption sites on small nanoparticles inherently limit their catalytic efficiency for producing longer hydrocarbon chains. The low methane formation at the metal-support interface of larger NPs is due to these effects as well. Smaller nanoparticles and the NP interface predominantly promote the formation of unwanted methane, as the imbalance between the slow rate of CO dissociation and the rapid rate of hydrogenation favors methane over longer hydrocarbon chains. If the NPs are too small, they easily deactivate due to undercoordination of Co atoms. These particles show no appreciable catalytic activity at all. This interplay between particle size, adsorption site availability, and CO activation underscores the structural dependencies critical to catalytic performance in Fischer-Tropsch synthesis, with smaller nanoparticles and the metal-support interface exhibiting a bias toward methane over higher hydrocarbons.
Within the conventional structure sensitivity trends as discussed above, the assumption is that fully metallic sites dominate catalytic activity. However, a broader perspective that includes partially reduced species, rather than only purely metallic, can offer a more nuanced understanding of catalytic behavior and break from traditional structure sensitivity paradigms. Experiments show that partially reduced cobalt supported on ceria-zirconia, titania, or cobalt oxide is highly active in CO2 hydrogenation, yielding mostly methane and to a lesser extent CO. They observed a low CO2 conversion activity on single atom and few-atom clusters of Co/CoO, with 100% selectivity towards CO. This suggests that the experimentally observed CO originates from single atom and few-atom clusters, and methane is formed on larger NPs. The computed reaction energetics show that the mechanism of CO formation on single atom and few-atom clusters on a CoO(100) surface does not include hydrogen spillover nor oxygen vacancy formation.