Homogenous-Heterogenous Photo(Electro)Catalysis

Homogenous-Heterogenous Photo(electro)catalysis

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 – 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 us to poorly defined, rigid and thus unspecific inorganic surfaces that can barely accomplish complex multi-electron redox processes. This is especially relevant to water oxidation and CO2-to-fuel conversion reactions that pose additional requirements to reactant and product selectivity.

Our approach

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

 

Our current projects

A. 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 and catalytic steps.

Funding:

Austrian Science Fund (FWF) Stand-Alone Project P32801

Christiane Hörbiger Preis

Collaborations involved:

Prof. Carsten Streb, Institute of Inorganic Chemistry-1, Ulm University, Ulm, Germany

Prof. Annette Rompel, Institute of Biophysical Chemistry, University of Vienna, Vienna, Austria

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 that 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 (IF 15.8) and can be accessed following this link.

B. 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 unraveling their performance towards photocatalytic water splitting

and CO2-to-fuel reactions.

Publications:

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. In contrast to the early-stage deactivation discovered by our group a few years ago (2017), this study provided detailed insights into the in situ      Ni self-activation (2019, see below) and unraveled the active state and dynamic nature of Cu co-catalysts (2021) that undergo surface-to-bulk diffusion when subjected to thermal treatments.

This work further served as a platform for developing the concept of

single-site photocatalysis: to this end, we employed the so-called

“site-isolation strategy” (see right) and developed an adsorption-limited

impregnation protocol 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 results demonstrated a strong impact of substrate surface

modification (e.g. with inorganic acids) on the co-catalyst deposition

and structure and revealed a strong increase of HER TOF values –

corresponding to more single, isolated sites – when lower co-catalyst

loadings were used. See this publication for insights.

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