Photocatalysis relies on the energy of light photons – instead of thermal energy – to drive and accelerate chemical reactions. Photocatalysis is a broad research field with a number of applications, some of which have already made their way to commercial products as for example in water and air purification technologies as well as in self-cleaning and anti-bacterial surfaces.
Despite these advances, implementation of reductive photocatalysis – and especially processes related to solar fuel generation such as in photocatalytic water splitting and carbon dioxide photoreduction – is still inhibited by low efficiency, stability or poor selectivity of contemporary catalysts.
The field of heterogeneous photocatalysis relies on solid-state (nano)materials to access these red/ox processes. Here, since the discoveries of Fonda and Fujishima, targeted research attempts have been addressing each of the three main photocatalytic steps: light absorption, charge transport and redox catalysis (see scheme) - yielding fairly robust photocatalytic systems. In fact, most commonly used oxide-based photocatalysts are exceptionally stable under reaction conditions, exhibit low toxicity and can be easily nanostructured using inexpensive fabrication and processing techniques.
However, reliance on standard heterogeneous solid-state photocatalysts (TiO2, WO3, Ta2O5, CdS, C3N4 etc.) limits the us to poorly defined, rigid and thus unspecific inorganic surfaces that can barely accomplish complex multi-electron redox processes (step 3). This is especially relevant to water oxidation and CO2-to-fuel conversion reactions that pose additional requirements to reactant and product selectivity.
To address the catalytic challenge of heterogeneous photocatalysis and get access to high value products (H2 and hydrocarbons), we take the inspiration from nature and the field of artificial photosynthesis to combine stability and robustness of solid-state oxide photocatalysts with high selectivity and flexibility of molecular-sized artificial homogeneous catalysts.
Our way to bridge the fields of heterogeneous and homogeneous photocatalysis is to endow standard crystalline oxide photo-absorbers with well-defiled catalytic functions by:
Development of all-inorganic oxide cluster co-catalysts
Controlled surface-modification with redox sites
Immobilization of molecular catalysts on various photoactive matrices
Immobilization of Molecular Chalcogen-Based Metalates
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 analogues 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/sulphide 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.
Austrian Science Fund (FWF) Stand-Alone Project P32801
Christiane Hörbiger Preis
Prof. Carsten Streb, Institute of Inorganic Chemistry-1, Ulm University, Ulm, Germany
Prof. Annette Rompel, Institute of Biophysical Chemistry, University of Vienna, Vienna, Austria
Towards Single-Metal-Site Photocatalysis
The concept of single-site or single-atom catalysis is often referred to systems where individual hetero-atoms are immobilized on a given substrate surface and rightly finds itself on 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 unravelling their performance towards photocatalytic water splitting and CO2-to-fuel reactions.
We have recently investigated a set of oxide-based co-catalysts featuring Mn, Cu, Co, Fe and Ni prepared using a common wet impregnation – thermal decomposition route directly on TiO2 nanoparticles, used here as a model photoactive substrate. This study provided first insights into the dynamic nature and active state of these co-catalysts and now serves as a base to dive into the single-site concept. See JMCA 2019 for details.