Fundamental studies

Kinetics, mechanisms and light-matter interactions

Fundamental studies are of the topmost importance in the field of materials chemistry. They allow us to understand the underlying processes of activation and deactivation, unravel mechanistic details of the catalytic reactions, and provide insights into the limitations of our nanostructures in terms of mechanical and chemical stability. Only by knowing these fundamental aspects can we design better nanostructures able to deliver excellent performance and long-term stability.

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:

 

Plot showing hydrogen evolution profile as a function of time of a platinum titania composite

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

 

Plot showing hydrogen evolution profile as a function of time of a nickel oxide titania composite

 

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.

 

Schematic illustration showing formation scheme of copper-based catalyst on the surface of titania

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.

Another important aspect of our researc his to unravel the mechanistic pathways of the photocatalytic processes. Only by understanding the factors that allow or limit the progress of the catalytic conversion will be be able to design better (photo)catalysts.

In the past years, several exemplary systems related to heterogeneous and homogeneuos catalysis have been studied in detail:

 

Schematic illustration showing mechanistic steps of the photocatalytic process on MIL-125 titanium metal-organic framework

In our recent contribution, opens an external URL in a new window on MOF-related photocatalysis, we looked into a series of isostructural MIL-125-Ti MOFs with various terephthalic-to-amino-terephthalic acid linker ratios following the evaluation of their photostability and performance towards photocatalytic H2 evolution in aqueous solutions using different hole scavengers. Detailed analysis with photoluminescence (PL) spectroscopy in solid-state and under simulated photocatalytic conditions, combined with life-time measurements via TRES, revealed different mechanisms of charge excitation and separation in MIL-125-Ti and NH2-MIL-125-Ti, which directly affect the respective HER performance.

 

Schematic illustration showing the catalytic cycle of a polyoxometalate-based photoredox system

In collaboration with the group of Prof. Rompel (Uni Wien), we recently conducted a study , opens an external URL in a new windowof three novel CoII-containing germanotungstate POMs and evaluated their photocatalytic performance towards light-driven water oxidation (WOC). Photoluminescence emission spectroscopy was employed to investigate the photocatalytic WOC mechanism and to understand the electron transfer kinetics between the reaction solution components, which overall suggested oxidative quenching to dominate under turnover conditions.

 

Plot of photoluminescence intensity and a schematic illustration showing the charge transfer and reaction mechanism of a surface-attached polyoxometalate cluster

In another study, opens an external URL in a new window, we aimed to explain the role of surface-immobilized [CoIIICoII(H2O)W11O39]7– polyoxometalate in water oxidation catalysis, for which we employed photoluminescence emission spectroscopy and used terephthalic acid (TA) as the fluorescence probe that can effectively trap ·OH. Our data (a) revealed that the more active WOC performance of the {CoIIICoIIW11}-APTES-TiO2 over TiO2 is related to the effective hole extraction by the POM cluster, which promotes charge separation and allows for a more efficient H2O oxidation by direct hole attack at the POM site (b).

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.

 

Schematic illustration showing the mechanisms of charge and energy transfer on a sandwich nanocarbon-inorganic hybrid prepared using atomic layer deposition

In our recent work, opens an external URL in a new window, we used ALD and designed a model hybrid in which ultrathin dielectric layers (Al2O3, ZrO2) were sandwitched between the hybrid’s components (ZnO, TiO2) and carbon nanotubes thus allowing for evaluating and tuning of interfacial charge transfer over an unusually long distance of at least 50 nm. Surprisingly, the transfer efficiency correlated linearly with the barrier layer thickness, indicating that electron conduction through the barrier layer constitutes the rate-limiting step. We also demonstrated that the charge transfer efficiency can be tuned by the type of interlayer and its degree of crystallinity, thus control-ling the hybrid’s performance in the photocatalytic production of hydrogen.