Symmetry-Breaking Sites for Activating Linear Carbon Dioxide Molecules

ConspectusCO2 is not only a greenhouse gas but also a pivotal carbon source as a promising supplement to fossil fuels. Both the environment and energy crises have compelled the researchers to explore how to efficiently transform CO2 into liquid fuels and value-added chemicals. As the industrialized approach nowadays, heterogeneous CO2 hydrogenation driven by thermal energy represents a potential strategy to help mitigate the greenhouse effect and reduce the reliance on fossil fuels. However, as the prerequisite for CO2 hydrogenation, CO2 activation is difficult due to the thermodynamic stability and chemical inertness of CO2 molecules. It is not proper to activate CO2 by directly increasing the reaction temperature, because CO2 hydrogenation into liquid products is an exothermic process where elevating the temperature decreases both the balanced conversion of CO2 and the balanced selectivity for target products. Therefore, the key scientific issue for CO2 hydrogenation lies in how to design catalysts which enable efficient activation of CO2. Up to date, a vast variety of active sites have been constructed for effective activation of CO2. These active sites including step sites, alloys, interface, substitution, vacancies, etc. are generally symmetry-breaking rather than perfect flat surfaces.Herein, we propose a catalyst design principle of constructing symmetry-breaking sites to activate nonpolar CO2 molecules. From the perspective of electronic properties, there is a prominent charge density gradient in a symmetry-breaking center, resulting in perturbing electronic structures of nonpolar CO2 and polarizing the adsorbed species. From the perspective of adsorption configuration, a symmetry-breaking site gives a local torque which enables more effective overlapping of atomic orbitals and thus more facilely bending of linear CO2 molecules, compared with symmetric sites. In this Account, we categorize the modes of CO2 activation and put forward the design principle of constructing symmetry-breaking sites. Moreover, we illustrate how to construct symmetry-breaking sites from the perspectives of local and global structures. Strategies to break the symmetry of local structures include surface substitution, surface adatom, and surface vacancy. Strategies to break the symmetry of global structures comprise surface modification with ligands, high-index surface, and phase reconstruction. In the future, further improvements, such as quantified descriptors, function for C-C coupling, and applicability to other nonpolar molecules, are necessary.