Photocatalytic and Photoelectrochemical Carbon Dioxide Reduction

The increasing number of devastating environmental disasters in recent years is ascribed to a changing climate due to the steady increase in the concentration of carbon dioxide in the atmosphere. With this in mind, the search for ways to recycle and transform CO2 into more useful products has received significant attention and has become an issue of global concern. The activation of CO2 is, however, a very energy intensive endeavor and the use of a renewable energy source, such as sunlight, is a basic requirement. Strategies that aim at harnessing sunlight to produce fuels or chemicals are often assigned to the large research field of "artificial photosynthesis". Within this field it was realized that direct utilization of sunlight to produce electrical energy is often impractical because of its intrinsic intermittency and fluctuating intensity. Thus, converting light into chemical bond energy is highly desirable for a sustainable future fuel economy. The transformation of CO2 into different C1 building blocks, such as carbon monoxide, formic acid, methanol or methane, or compounds with a higher number of carbon atoms, all have their significance for industrial applications. Sunlight can either be applied directly or indirectly via a photovoltaic device to activate CO2. While indirect sunlight application is actually reducing the topic to a problem of electrocatalysis, direct application of sunlight can be roughly divided into homogeneous or heterogenous catalytic methods with some very interesting recent developments in the field of hybrid systems. Molecular compounds, with their well-defined structure, enable in-depth mechanistic studies and the deduction of structure–activity relationships, of which several are presented in this Special Collection. On the other hand, solid-state materials stand out with higher robustness and stability, although they usually show poor product selectivity. Nevertheless, the products obtained from systems incorporating solid-state materials are usually higher reduced forms of CO2 and Cn compounds (with n>1), which is attractive for further downstream industrial processes. As a novel synthetic development, hybrid systems, in which a molecular catalyst is fixed on a (most often) semiconducting surface, aim to merge the advantages of both worlds, and some interesting examples are also provided within this collection. In summary, in this Special Collection different developments to transform CO2 into value-added chemicals by using light as a clean energy source are presented. The research articles and reviews provide insight into existing state-of-the-art molecular, solid-state and hybrid systems. We hope that interested readers will take the presented work as stimulation and inspiration for the design of future photocatalytic or photoelectrochemical systems as well as other systems for solar energy conversion and storage. There is still the need to improve catalyst stability and selectivity, which will ensure continuation of research efforts to achieve a sustainable circular process. Any achievements will undoubtedly play an important role for the global society, economy and science in the future. Matthias Schwalbe studied chemistry in Jena (Germany) and Glasgow (UK). After a postdoctoral stay at MIT, Cambridge (USA), he started his independent career at the Humboldt-Universität zu Berlin (Germany) in 2010. His major research interest is in the design of metal complexes for the activation of small molecules, in particular CO2 and O2, for testing in electro- and photocatalytic applications. Hongwei Huang is a Professor at School of Materials Science and Technology, China University of Geosciences (Beijing, China). His current research mainly focuses on preparation of polar semiconductor photocatalysts for energy and environmental applications. Gonghu Li is currently a Professor of Chemistry at the University of New Hampshire (USA). His research involves the applications of surface chemistry and catalysis in solar energy conversion. Recent projects in his group include hybrid photocatalysts and single atom catalysts for solar CO2 reduction.