Metal@carbon nitrogenated porous architectures as novel sustainable catalysts for electrocatalytic hydrogenation of biomass-derived organics

Abstract

The valorisation of biomass and waste feedstocks is a crucial strategy for reducing carbon emissions. Biomass stands out among renewables for its potential beyond energy production, serving as a sustainable source for high-value products such as fuels and chemicals. Biomass feedstock conversion often involves thermal catalytic hydrogenation (TCH), a common method for upgrading oxygenated compounds. However, TCH depends on costly precious metal catalysts, operates at high temperatures and pressures, and relies on fossil fuel-derived hydrogen. Conversely, electrocatalytic hydrogenation (ECH) offers a more sustainable alternative, generating hydrogen in situ and avoiding the need of extreme conditions and fossil fuels, while also ensuring compatibility with renewable energy sources. However, its viability depends on developing efficient and selective electrocatalysts. As a result, research efforts are ongoing to explore new classes of abundant and cost-effective electrocatalyst materials. Metal@carbon:nitrogen (M@C:N) architectures have gained attention in electrocatalysis due to their versatility and ability to incorporate non-critical, low-cost metals. These materials are already widely studied for cathodic reactions like the hydrogen evolution reaction (HER), which also represents a key competitive process for ECH. Therefore, given the strong link between HER and ECH, M@C:N structures could be a viable, low-cost alternative for hydrogenating biomass-derived organics. This thesis aims to position itself within this context exploring, for the first time, the use of low-cost and non-critical metals as active sites for the design, synthesis and characterisation of M@C:N architectures to be used for ECH applications. Three metals have been identified as potential candidates based on their activity towards HER: iron (Fe), molybdenum (Mo), and tungsten (W). Benzaldehyde (BZH) was instead selected as the model organic compound representative of biomass fraction for conducting ECH studies. In the opening section of this thesis (Chapter 1), the state-of-the-art of the use of biomass as renewable resource is discussed, including the most widely employed methods for its valorisation. The electrochemical principles essential for understanding electrocatalyst design are explored, with a particular focus on the mechanisms of HER and ECH. A deeper insight into the design of porous carbon-based materials is provided, with emphasis on M@C:N structures, along with an overview of the current findings on the ECH of benzaldehyde. Chapter 2 discusses the characterisation methods and their theoretical background used to study the prepared materials throughout this thesis. The first results chapter (Chapter 3) assesses Fe@C:N as the initial material for BZH hydrogenation, based on its promising performance in other cathodic reactions. Special attention is given to optimising catalyst preparation by evaluating different annealing processes and carbon nanoscaffolds, providing at the same time a comprehensive material characterisation and preliminary electrochemical studies. Chapter 4 explores the usability of these materials on larger conductive supports for quantitative studies, combining chronoamperometry with gas chromatography analysis. The development of a new stable electrode preparation is discussed, along with the evaluation of key ECH performance parameters, demonstrating the potential of Fe@C:N as a viable alternative to precious-metal-based electrocatalysts. Chapter 5 expands the study to include Mo- and W-based materials, investigating different metal concentrations to assess their impact on structural and electrochemical properties. The results emphasise that the HER/ECH competition play a critical role in regulating the activity during BZH hydrogenation. High metal concentration in M@C:N leads to excessive HER activity, which appears to limit their potential application for ECH. To clarify the role of the metal and its impact on ECH performance indicators, a more indepth study is conducted in Chapter 6, where Mo@C:N and W@C:N materials are evaluated alongside a metal-free benchmark electrocatalyst. Both Mo- and W-based electrocatalysts exhibit superior ECH performance compared to the metal-free catalyst, highlighting the significant enhancement provided by metal encapsulation. Notably, W@C:N outperformed Fe- and Mo-based materials, positioning itself as a cost-effective alternative to precious-metal catalysts for ECH applications.