Photocatalysts for reforming of organic feedstocks

Hydrogen is named since many years as potential energy carrier of the future as it cannot be only burned but also used in fuel cells for the production of electricity. Currently the vast amount of hydrogen (more than 90%) is produced from natural gas, which is a fossil fuel. An alternative to this pathway would be the photocatalytic generation of hydrogen. In this approach water is split by light and a catalyst material in its individual parts: oxygen and hydrogen. The reaction itself is very challenging as the evolved oxygen is very reactive which is detrimental for the stability of the catalyst. Furthermore oxygen cannot be used in subsequent chemical process and thereby it becomes and unavoidable side product. In this project the generation of oxygen (oxidation) should be replaced by another reaction, namely the oxidation of renewable resources or chemicals. In this case bio-based chemicals or resources are used and oxidized instead of the oxygen. If this reaction can be performed in a controlled manner, chemical products can be generated with added economical value. This would increase the overall economic feasibility of the whole photocatalytic water splitting, because beside the desired hydrogen another value added chemical is generated. To reach this target a special catalyst material is designed. The catalyst is created by combining carbon nano dots (tiny carbon particles sized only a few nanometers) and titanium dioxide (TiO2). Both components alone were found to have special properties for photochemistry. Their combination shall result in a material with special activity and enable the photocatalytic production of hydrogen. Thereby the single tiny carbon nano dots are coated with a thin layer of titanium dioxide. In this assembly the carbon nano dots will absorb the light and the chemical reactions will take place on the surface of the titanium dioxide. Additionally metals can be deposited on the surface of the titanium dioxide, which would increase the overall activity of the catalyst. This material would not only be an active catalyst, it is also consisting solely of widely available and non-toxic elements, which is a difference to many other photocatalysts which are currently being investigated and used.

To reduce the global CO2 emissions, fossil resources must be replaced to produce fuels and chemicals. This can be achieved by using alternative resources. Waste streams (biomass, food or plastic waste) are especially appealing because by utilizing those streams, the use of fossil resources can be reduced and furthermore, if waste is converted in chemical processes, it will not contribute to landfilling or environmental pollution. Beside alternative resources also novel, sustainable conversion process should be used, that would reduce CO2 emissions. Photocatalysis can use the energy of sunlight and electrocatalysis uses electricity (in the best case from renewable sources) to perform chemical reactions. Both ways are sustainable alternatives for novel chemical processes. In my project I combined alternative resources with novel conversion processes. Waste streams such as biomass or food waste were converted via photocatalysis to form hydrogen (a useful energy vector) and in addition valuable organic molecules. The developed processes are more sustainable than comparable approaches because they work at lower temperatures and use less corrosive chemicals. Furthermore, the combined production of hydrogen and defined organic products from real world waste streams makes the processes more feasible. In addition to that another two-step process was developed that allowed the conversion of plastic waste into chemicals, that would allow the chemical recycling of those plastics. This works especially well for polyethylene, which is the plastic used in cheap packaging products, and especially difficult to recycle. The developed process combines a thermal step with a photo- or electrocatalytic one and compared to other processes the applied temperatures are much lower and it can potentially be applied at smaller decentralized scale.