Green Chemistry Blog

Over the past 25 years, Green Chemistry has provided a unique forum for the publication of innovative research on the development of alternative sustainable technologies, efficient utilisation of resources and the concomitant minimisation of waste. We are delighted to bring together a very special issue containing articles by members of the green chemistry community as well as past and present Green Chemistry Board members, to mark and celebrate our first 25 years.

Among the contributions to this themed collection is a Paper on the promotive effect of Au and Sn incorporation into supported Pd nanoparticles for the direct synthesis of H₂O₂ from molecular H₂ and O₂  (DOI: 10.1039/D3GC03706A).

The direct synthesis of H₂O₂ from the elements represents an atom-efficient alternative to the current indirect industrial process, allowing for localised production of a major commodity chemical used in sectors as varied as chemical synthesis, bleaching and disinfection. However, despite over 100 years of research, few examples of highly selective catalysts (i.e. those that do not degrade H₂O₂ to H₂O), exist. Catalysts based on PdAu and PdSn active sites are an exception to this generality and this work compares and contrasts the efficacy of these two systems towards H₂O₂ production and demonstrates the excellent performance metrics which can be offered by these two distinct classes of materials.

Read our interview with Dr Richard J. Lewis, one of the authors, here.

How would you set this article in a wider context?

AuPd catalysts have been well studied in recent decades for a range of chemical transformations, including H₂O₂ synthesis. However, the replacement of Au with Sn, and the resulting improvement in catalytic efficiency that results, is a relatively new discovery, with earlier works in this area focussed on (i) the use of complex catalyst synthesis protocols to form Sn overlayers that encapsulate highly active yet unselective Pd species or (ii) the study of idealised, model PdSn catalysts in order to gain a fundamental understanding of the structure-reactivity relationships that exist in such systems.

Importantly the materials studied within this work are produced via a readily scalable synthesis protocol and compete with state-of-the-art materials reported within the academic literature, including those previously reported by our laboratory. Crucially they can achieve the high selectivity necessary for the direct route to H₂O₂ synthesis to compete with the current industrial approach to production.

What is the motivation behind this work?

Despite calls for uniformity in testing regimes, it is typical for stark differences in catalyst evaluation protocols and reaction conditions to exist between research groups. This is somewhat understandable given that researchers wish to evaluate catalytic performance under conditions idealised for their particular system, as well as allowing  for benchmarking against earlier works from their own laboratory. However, this often leads to confusion as to the current state-of-the-art, with comparison of catalysts, especially those from different laboratories, under standardised conditions rarely performed. In this work, we set out to address this concern, comparing and contrasting the performance of a series of catalysts based on the well-established AuPd formulation, against the emerging class materials centred around PdSn.

What aspects of this work are you most excited about at the moment and what do you find most challenging about it?

The performance of our 0.25%Pd–2.25%Sn/TiO₂ catalyst is particularly exciting, offering near-total selectivity towards H₂O₂ and product yields superior to the optimised AuPd formulation. Indeed, the performance of this catalyst can be considered among the state-of-the-art.  The mechanism by which this improved reactivity is achieved is also intriguing, with the addition of high quantities of Sn promoting Pd dispersion and the formation of highly active and selective, single atoms of Pd surrounded by Sn/SnO_x domains, rather than through the formation of PdSn alloys as may have been expected based on earlier works.

What is the next step? What work is planned?

While these systems are highly promising it is important to note that they represent only the first generation of materials developed in this project. As such our focus has now shifted towards the redesign and further optimisation of these catalyst formulations to ensure the key performance metrics (high reactivity and near-total selectivity towards H₂O₂) are maintained over industrially relevant lifetimes. We are also actively pursuing the translation of these research-grade catalysts into technical-grade materials to allow for further evaluation under realistic operating conditions, as well as use in alternative chemical transformations centred around the utilisation of in-situ synthesised H₂O₂.  We are also seeking a greater understanding of the dynamic nature of these systems during catalysis, through the use of operando and in-situ characterisation techniques. This is particularly exciting and allows us the opportunity to collaborate with colleagues from fields adjacent to catalysis, including spectroscopists and microscopists.

Please describe your journey to becoming part of the Green Chemistry community

Engineering a more sustainable world aligns perfectly with the aims of the Green Chemistry and Catalysis communities, as such it is easy to consider the members of these two fields as part of the same family, rather than belonging to two distinct communities whose goals are sometimes shared.  Therefore there has not been a journey towards a Green Chemistry community, rather there is a shared journey with friends and colleagues of one community.

Why did you choose to publish in Green Chemistry?

Green Chemistry is, without doubt, one of the flagship journals in the field of sustainability, built on the foundations laid out by Paul Anastas and Nicolas Eghbali and nurtured by past and present members of the journal Editorial Board. One of the major themes of  Green Chemistry is to find solutions to many of the grand challenges facing the chemical industries through the design, synthesis and evaluation of novel materials and alternative processes. We consider that our manuscript aligned very well with these ambitions and given the broad readership of the journal, we could think of no better home than Green Chemistry.

What do you think the Green Chemistry journal has done well in the past 25 years, and what do you think are the main challenges our community will face in the next 25 years?

For the past 25 years, Green Chemistry has been at the forefront of advances in sustainability and environmental protection, continually promoting the development of new technologies that improve efficiency and minimise the impact of the chemical sector, both to humans and the wider ecosystem. A major challenge facing the chemical sector has and will continue to be the transition towards manufacturing processes that better utilise raw materials and minimise the production of waste by-products. This challenge will only grow with the transition toward alternative, non-fossil-derived feedstocks supplied from a wide variety of sources.  The drive towards Net Zero poses both challenges and opportunities for the field, with the emergence of new feedstocks including sequestered CO₂, NH₃,  biomass and carbon-free H₂ requiring the development of new processes and modifications to existing technology. The associated shift towards electrification will also result in changes in supply/demand dynamics of critical elements that will undoubtedly result in increased utilisation of many earth-abundant metals (e.g. for use in energy storage), while the shift away from fossil fuel-based transit will have unpredictable effects on the supply of many of the precious metals that are utilised in petroleum refining and automotive exhaust treatment and have been the backbone of the chemical synthesis sector.