Harvesting solar energy by converting abundant chemicals photocatalytically into valuable products is still far from being applied on a large scale. Aspirant applications are the production of hydrogen by water splitting, biomass reforming or the fixation and valorization of greenhouse gases such as carbon dioxide and methane. Investigations of model catalysts under ideal conditions, i.e. single crystals in ultra-high vacuum, give essential insights into fundamental photocatalytic processes. Yet, due to a phenomenon known as “pressure gap” conclusions drawn from UHV studies cannot always be readily transferred to environ-mental conditions. Reactions on a macroscopic scale are usually carried out at ambient conditions and require large amounts of photocatalyst (mg). However, the availability of active material might be limited, either due to intensive synthesis procedures, or due to a planar catalyst shape, which impedes a simple increase of catalyst amount in the photo reactor.
Our motivation is the design of a gas phase photoreaction setup operating at ambient conditions to investigate catalysts with planar shape or low sample amount with high sensitivity. This will enable the investigation of novel catalysts with very well defined properties to deepen the understanding of photocatalytic processes, which so far could not be assessed under these conditions due to their planarity. Potential substrates are (metal-decorated) single crystals as well as supported semiconductor nanorods synthesized in the scope of e-conversion.
Our strategy is:
- to downscale reactant gas flows to the µL/min range to achieve sufficient product to total gas ratio for quantitative mass spectrometry and
- to decrease reactor size to ensure adequate contact times as well as fluid dynamics.
Inspired by µ-reactors based on silicon, we plan to etch the reactor chamber as well as in- and outlet into the UV/VIS-transparent reactor lid. The planar substrate supported by a metal plate will serve as the reactor bottom. In contrast to tube reactors commonly used in thermal catalysis, the circular shape ensures maximum and homogeneous light intensity on the photocatalyst surface provided by LEDs or a laser.
As a proof of concept, alcohol reforming under exclusion of O2 and H2O will serve as an example for H2 production. In the first step, the powder-based Pt@TiO2 will be tested in a liquid phase test setup, which is established in literature. The same powder catalyst will be investigated in the gas-phase photoreactor setup in our laboratories. On the one hand this enables establishing a setup for gas phase photoreaction for low catalyst amounts. One the other hand, the influence of the reactant phase is elucidated this way. In another step a platinum-decorated titania single crystal will exemplify the testing of planar photocatalysts.
Finally, two setups, one for liquid and one for gas phase measurements, shall be available to eanble the study of newly designed materials in full water splitting and CO2 conversion. With this approach the evaluation and comparison of different photocatalysts under different conditions shall not only provide the identifcation of key properties of the materials (as e.g. chemical compositions, shapes or morphologies), but also enable the fundamental understanding of general mechanisms in photocatalysts, fully within the scope of e-conversion and beyond.
In the frame of Germany’s Excellence Strategy