Water splitting is thermodynamically and kinetically challenging mostly due to the water oxidation (OER) reaction. One alternative way to generate solar fuels is to replace water oxidation with another oxidative process which is thermodynamically more feasible, while at the same time makes economic sense. One such process is called photoreforming, it aims to convert waste organic compounds into solar fuels and potentially valuable chemicals. Our team develops this process from two perspectives: (a) design of active and selective photocatalysts and (b) mechanistic understanding of the process, which will lead to a more purposeful design of the photocatalyst and a more rational process engineering. 

​Collaborations involved:

Prof. Antje Potthast, BOKU, Vienna

Prof. Christian Pichler, TU Wien, Vienna

Publications:

Thus far, we have provided a detailed mechanistic study on the process of photoreforming polyethylene terephthalate (PET) microplastics into high-value gaseous and liquid compounds. Spearheaded by Madeline Weisweiller, the work has been published in Green Chemistry, opens an external URL in a new window. While we continue looking into the issues of microplastics and hard-to-process polymers, we are currently focusing on developing lignin photoreforming processes. 

The reactions of water splitting and carbon dioxide photoreduction involve complex multi-electron redox processes that require a rational design of the surface catalytic sites. When working with ill-defined inorganic surfaces, these sites are inevitably hard to study and understand on a truly fundamental level, which limits the rational synthesis of active and selective photocatalysts.

To tackle this challenge, we have been long inspired by the idea to merge the heterogeneous and homogeneous branches of photocatalysis and combine the excellent stability and optoelectronic properties of the solid-state supports with the structural and functional tunability of the molecular catalytic species, thus creating a new class of well-defined (i.e. controllable) multi-functional hybrid photocatalysts. Our unique way to approach this challenge is (a) the combination of solid-state and molecular photosystems and (b) systematic benchmarking of photocatalytic performance under homogeneous and heterogenized conditions.

Funding/Awards:

A part of the work was supported by the Austrian Science Fund (FWF) Stand-Alone Project P32801 (2020-2024, 378KEuro, PI: Alexey Cherevan) and is currently supported by the FWF Cluster of Excellence MECS (2023-2028, 1300KEuro, PI: Alexey Cherevan)

​Collaborations involved:

Prof. Carsten Streb, Johannes Gutenberg University Mainz, Germany

Prof. Annette Rompel, University of Vienna, Austria

Publications:

Two main works so far have featured the possibility to benchmark a cluster performance under homogeneous and heterogenized conditions: see our earlier publication by Sreejith Nandan in ACS Materials Au here, opens an external URL in a new window and our more recent work by Samar Batool on [Mo3S13]^2- cluster published in Sustainable Energy&Fuels here, opens an external URL in a new window

Over the past decades, polyoxometalates (POMs) have been studied extensively and have triggered a lot of attention in homogeneous catalysis due to their structural tunability and stability towards oxidative decomposition. Similar to POMs, polythiometalates (PTMs) have been known for decades, however, until very recently the main interest had focused on a purely synthetic point of view with respect to structural and supramolecular chemistry. This was mainly fuelled by the fact that PTMs are seen as zero-dimensional analogs to common MoS₂ and WS₂ nanostructures.

Recently, POMs and PTMs have been identified as promising candidates for water oxidation and water reduction catalysis. But the biggest challenge they face is that they suffer from quick deactivation due to degradation and self-aggregation under operational conditions.

In this project, we are wiring all-inorganic molecular oxide/sulfide clusters (POMs and PTMs) onto the surface of functional UV/visible-light active substrates with the ultimate aim to establish them as a platform for heterogeneous photocatalysis. These hybrids will further serve as model systems for fundamental studies to evaluate substrate effects on the stability, structure, and electronic properties of both components as well as on interfacial charge/heat transfer dynamics and catalytic steps.

Funding/Awards:

Austrian Science Fund (FWF) Stand-Alone Project P32801 (2020-2024, 378KEuro, PI: Alexey Cherevan)

Christiane Hörbiger Preis

​Collaborations involved:

Prof. Carsten Streb, Johannes Gutenberg University Mainz, Germany

Prof. Annette Rompel, University of Vienna, Austria

Prof. Annie K. Powell, Karlsruhe Institute of Technology (KIT), Germany

Publications:

We have started this journey with a dedicated analysis of the existing literature on POM heterogenization, which led us to write the first review , opens an external URL in a new windowthat summarized recent ground-breaking developments in the materials chemistry of supported polyoxometalates and established links between a molecular-level understanding of POM-support interactions and macroscopic effects including new or optimized reactivity, improved stability, and added functions. This review was published open-access in Advanced Science. More recently, we looked into the expanding field of thiomolybdate clusters, their prospects in catalysis and heterogenization studies. This review, opens an external URL in a new window was published in Advanced Materials.

​The first experimental work , opens an external URL in a new windowon the topic appeared in ACS Catalysis. Here, we for the first time demonstrate the immobilization of an all-inorganic thiomolybdate [Mo₃S₁₃]^2- cluster on various metal oxide surfaces and investigate its function as a co-catalyst for photocatalytic hydrogen evolution reaction. We show that the attachment of the cluster on TiO₂ is strong and irreversible and that it follows monolayer adsorption, whereas the surface coverage is directly proportional to the cluster loadings.

Parallel to this, our first publication , opens an external URL in a new windowrelated to POM immobilization came out in ACS Materials Au. Here, we show anchoring of a molecular all-inorganic [Co^IIICo^II(H₂O)W₁₁O₃₉]^7– Keggin-type polyoxometalate onto a model inorganic surface, employing a 3-aminopropyltriethoxysilane (APTES) linker to form a novel heterogeneous photosystem for light-driven water oxidation.

We are currently working on expanding this strategy to novel supports and novel clusters. 

The concept of single-site or single-atom catalysis is often referred to as systems where individual hetero-atoms are immobilized on a given substrate surface and rightly finds itself at the intersection of proper homogeneous catalysis and proper heterogeneous catalysis seeking to combine the advantages of both fields while addressing their drawbacks.

In this project, we apply the concept of heterogeneous single-metal-site catalysis to the contemporary challenges of heterogeneous photocatalysis by controllable modification of photocatalyst surface with atomically sized co-catalyst species, understanding the interaction with the support and unraveling their performance towards photocatalytic water splitting and CO2-to-fuel reactions.

Publications:

Our work on the design of noble-metal-free co-catalysts (see section above) further serves as a platform for developing the concept of single-site photocatalysis: to this end, we employ the so-called site-isolation strategy and develop adsorption-limited impregnation protocols aiming to downscale the co-catalyst species exemplifying the most promising earth-abundant Cu and Ni as well as noble Pt and Au systems. Our first publication , opens an external URL in a new windowdemonstrates a strong impact of the substrate surface modification (e.g. with inorganic acids) on the co-catalyst deposition and structure and reveals a strong increase of HER TOF values – corresponding to more single, isolated sites – when lower co-catalyst loadings are used.

While we are still developing single-atom-stabilization strategies for inorganic supports, we have also started active work using metal-organic frameworks as a matrix. The first results are yet to come.

While we are originally coming from the materials perspective, over the years we have shifted a lot and worked a lot with molecular (homogeneous) systems. A part of this work is covered by the topic of (a) all-inorganic molecular clusters: polyoxometalates and thiometalates desrcribed above. At the same time, while working with a lot of synthetic groups around the world, we have built expertise in evaluating photocatalytic properties of (b) new molecular photosensitizers and new molecular (photo)catalysts. 

Funding/Awards:

A part of the work is currently supported by the FWF Cluster of Excellence MECS (2023-2028, 1300KEuro, PI: Alexey Cherevan)

We also acknowledge the AKTION 102p15 project from the OeAD

​Collaborations involved:

Prof. Annette Rompel, University of Vienna, Austria

Prof. Jozef Krajčovič, Brno University of Technology, Czech Republic 

Dr. Mariusz Jozef Wolff, University of Vienna, Austria

Publications:

In our early works with novel POM clusters, we unraveled their photocatalytic activity towards water oxidation and HER leading to two collaborative publications in ChemSusChem here, opens an external URL in a new window and in Journal of Materials Chemistry C here, opens an external URL in a new window. Currently, we are working the design and understanding on novel molecular compounds.

The performance of many contemporary photocatalysts is often limited by the fast electron-hole recombination rates and poor/unselective catalytic sites on their surface. Several strategies have been explored to address these issues, such as the use of cocatalysts – surface-attached species – that provide better-suited catalytic sites and simultaneously promote the separation of photoexcited electrons and holes. The most widely used co-catalysts are Pt, Pd, and IrO₂, RuO₂ for the photocatalytic reduction and oxidation of water, respectively. Given their rare nature, to achieve the large-scale industrial application of photocatalytic water splitting, the development of novel co-catalysts based on cheap and widely available elements remains an important issue.

Driven by this objective, our group has been investigating oxide-based co-catalysts based on d-block transition metals such as Mn, Co, Fe, Ni, and Cu, which are known for their excellent catalytic properties and applications in industry, research, and nature. These elements – especially in their oxide form – can undergo quick and reversible redox shuttling, accept, accumulate and release electrons – conditions necessary to generate a self-recovering system. Besides this, their surface structure and chemistry can be varied through synthetic conditions (e.g. different oxides can be generated) allowing us to further tune adsorption/desorption properties and thus their catalytic function.

Funding/Awards:

Otto Vogl Prize for the Best Master Thesis in Chemistry from the Austrian Academy of Sciences (Jasmin Schubert)

​Collaborations involved:

Prof. Johanna Rosen and Dr. Shun Kashiwaya, Linköping University, Sweden

Publications:

To this end, we applied a wet impregnation route to prepare Mn, Co, Fe, Ni, and Cu-based co-catalysts immobilized on TiO₂ nanoparticles and systematically investigated their prospects in photocatalytic water splitting reactions. In contrast to the early-stage deactivation, opens an external URL in a new window discovered by our group a few years ago, we have recently provided detailed insights into the in situ Ni self-activation, opens an external URL in a new window and unraveled the active state of these co-catalysts. More recently, we took a close look into the Cu-based photosystem and unraveled thermally induced Cu diffusion, opens an external URL in a new window, which has a detrimental effect on photocatalytic performance. 

The range of studies conducted within the group or for external collaborators demands a dedicated facility where photocatalytic experiments can be conducted in a reliable, reproducible, benchmarkable and tunable way. Our PhotoLab is a state-of-the-art facility of the group enabling a range of light-driven catalytic and fundamental studies. This facility is recognized on the Austrian and international scale with a high number of cooperative research projects in which we produce reliable and in-depth investigations of photocatalytic properties.