Fundamental studies (kinetics, mechanisms, light-matter interactions)

One aspect of our research is related to the understanding of the short-term and long-term stability of our (photo)catalysts. For this, a variety of analytical and testing methods are in place with a special focus dedicated to in situ techniques, which allow following the state of the catalyst during the reaction, often coupled with on-stream activity evaluation.

In the past years, several exemplary systems have been studied in detail:

In our work , opens an external URL in a new windowpublished in ACS Catalysis, we unraveled a sudden deactivation of Pt/TiO2 during the initial stages of photocatalytic H2 evolution from aqueous solutions that, until now, has gone unnoticed. Utilizing a set of analytical techniques, we were able to attribute this deactivation to a shift in mechanism, accompanied by an increase in CO concentration. Key to this phenomenon is the ratio of Pt atoms to oxygen vacancies, which were created through ultrasonic pretreatment and in situ UV irradiation in the bulk and surface, respectively

More recently, we conducted a systematic study, opens an external URL in a new window of a series of non-noble-metal co-catalysts based on Co, Mn, Ni and Fe oxides that were prepared by wet impregnation of model TiO2 substrate and discovered light-driven activation of the catalytic activity. Complemented by XPS analyses, our detailed HER studies revealed the dynamic nature of the NiO/TiO2 photocatalyst whose Ni0 active HER sites were generated in situ upon light exposure.

Finally, as an example of the structural dynamics of such a system under photocatalytic conditions, we recently reported , opens an external URL in a new windowthermally induced bottom-up generation and transformation of a series of promising Cu-based co-catalysts. Supported by DFT modeling, our data higher temperatures (>200 °C) do not affect the Cu oxidation state but induce a gradual, temperature-dependent surface-to-bulk diffusion of Cu, which results in interstitial, tetra-coordinated Cu+ species. The disappearance of Cu from the surface and the introduction of new defect states is associated with a drop in HER performance.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is a complementary IR technique usually applied to solid-state samples and reactions proceeding in the gas phase. DRIFTS allows monitoring changes in vibrational modes of both the catalyst and the adsorbed reactants thus providing in-depth information about the adsorption behavior, reaction intermediates, catalytic mechanisms, and (de)activation phenomena e.g. potential surface poisoning.

Aiming to apply DRIFTS to photocatalytic reactions, we have established an in situ flow setup that allows conducting gas-phase reactions over the (photo)catalyst surface, while simultaneously measuring the IR signal of the gas-solid interface and quantifying the reaction products with gas chromatography. The reaction chamber is further coupled with a tunable light source that can trigger a light-activated photocatalytic process.

Currently, we focus on photocatalytic CO2 reduction studies but have the possibility to apply the setup for other light-triggered reactions including MeOH oxidation, water-gas shift reaction etc.

The charge and energy transfer processes in nanocarbon-inorganic hybrids are vital to the improved properties but are still not clearly understood. Specifically, what is the maximum distance from which the charge transfer can occur? What is the ideal nanocarbon type/surface chemistry? How can the electron-hole lifetime be maximized?

To investigate the electron transfer from the inorganic material to the CNTs, we chose ZnO as the inorganic material due to its strong photoluminescent and photocatalytic properties. To investigate the charge transfer, we conduct fluorescence quenching and lifetime experiments. Introducing an Al₂O₃ blocking layer with variable thickness allows for determining the distance dependence of the occurring interfacial processes. The required thin and conformal coatings are obtained by atomic layer deposition, opens an external URL in a new window (ALD), which has proven to be the method of choice.