Towards single-site photocatalysis: design of non-noble-metal-based systems

Abstract

Due to the world’s current energy crisis and the inevitable depletion of fossil fuels, finding alternative energy sources has become ever more important. Photocatalysis, also known as artificial photosynthesis, allows the transformation of solar to chemical energy by facilitating non-spontaneous chemical reactions with the aid of light irradiation and a photocatalyst. Through photocatalysis, the sun’s energy – an inexhaustible energy source – can be stored in the chemical bonds of so-called “solar fuels” (e.g. hydrogen or hydrocarbons) by means of water splitting or CO2 reduction reactions, respectively. However, after 60 years of targeted research, real-world applications of heterogeneous photocatalysis are still limited by the insufficient efficiency and predictability of contemporary catalytic systems.Heterogenous single-metal-site catalysts (HSMSCs) feature many advantages over their bulkier counterparts as a result of their ability to bridge the qualities of homo- and heterogeneous catalysis. When relying on the concept of HSMSCs, maximized atom utilization efficiency and accessibility of the active sites renders excellent catalytic performances (homogeneous trait), while stability and recyclability stay preserved (heterogeneous trait).Applying HSMSCs in photocatalysis is a rather novel approach, especially considering earth-abundant co-catalysts, which are still underexplored. Besides, the use of HSMSCs allows to construct more unique and selective co-catalysts to address the challenges of complex multielectron catalytic reactions. Moreover, applying the single-site concept to noble metals – which show superior catalytic activities but are scarce in the earth’s crust – could make their implementation more feasible for large-scale applications.In this Thesis we synthesize heterogeneous single-metal-site photocatalysts following the “isolation strategy” through an adsorption-limited wet impregnation process by using TiO2 as model support and earth-abundant (Cu and Ni) and noble (Pt and Au) metals as co-catalysts. The catalysts are characterized by IR and UV-vis spectroscopy for qualitative analysis, TXRF for determination of the real loadings of the active species, and TEM for resolving the atomic structure and morphology of the samples. We investigate the different adsorption/deposition behaviors of the precursor species on the support and the influence of light during synthesis. Noble metals undergo photodeposition to form nanoparticles under ambient-light, whereas earth-abundant metals rather stay as complexed species on the substrate’s surface. To further promote the formation and stabilization of single-site species, we modify the support’s surface with PO4 groups. The resulting PO4/TiO2 shows enhanced adsorption of metal precursors and stronger structural changes in the co-catalyst species thus confirming a crucial role of the surface charge and chemistry on the single-site stabilization.Furthermore, we evaluate the performance of our photocatalysts through photocatalytic hydrogen evolution reaction (HER) experiments and discuss the obtained activity trends (in terms of H2 evolution rates and turnover frequency values) in light of different mathematical models. All investigated photocatalysts consistently follow our non-linear model upon decreasing the loadings, which speaks for an alteration in the co-catalyst’s size/morphology towards smaller, perhaps even single-site-like species. When using the modified PO4/TiO2 substrate, the activity trend of Pt > Au > Cu > Ni gets less pronounced, and the best-performing earth-abundant systems based on Cu become comparable to the Au-based samples. Finally, we demonstrate that the activity trend – when normalized per cost of precursor – gets inverted, hence, suggesting that earth-abundant co-catalysts could open up new frontiers for practical applications.