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Abstract: Mitigating climate change is one of the biggest challenges of today's society. The most direct way to achieve this goal is to capture and use CO2 as a source of energy and chemicals. This work, inspired by previous publications focused on homogeneous catalysis, proposes the transformation of the easy-to-prepare CO2 derivatives dialkylureas into C1 chemicals using Ru-MOFs as heterogeneous catalysts. This choice is due to (i) the well-known ability of Ru to catalyze hydrogenation reactions and (ii) that Ru-complexes were the pioneer homogenous catalyst in converting CO2 into an added-value C1 chemical, methanol. Apart from the already reported MOF Ru-HKUST-1, we have prepared a new Ru-MOF material, denoted Ru-BTC, analogous to the semiamorphous Fe-BTC. It has been found by XAS that Ru-BTC and Ru-HKUST-1 have different metal environment and oxidation states: only 3+ in Ru-BTC, a 50:50 mixture of 2+ and 3+ in Ru-HKUST-1. Both Ru-MOFs catalyzed the hydrogenation of N,N’-dimethylurea under relatively mild conditions, giving methane as the main product. Ru-BTC was particularly efficient: 67 % conversion and 96 % selectivity to CH4 at 150 ºC and 30 bars of H2 using a Ru/dimethylurea weight ratio of 1 %. Ru-MOFs were also able to transform CO2 into CH4, again being Ru-BTC the most effective catalyst, but giving much poorer selectivity to CH4. Ru-MOFs, particularly Ru-BTC, were damaged under reaction conditions, but no significant Ru leaching was observed.

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Abstract: Catalytic hydrogenation reactions represent one of the most important transformations in the chemical industry. In the light of the required defossilization of the chemical value chain, the development of new, highly selective hydrogenation catalysts is of utmost importance to deal with renewable resources, such as biomass, recycled feedstock, or CO2. In that regard, this thesis deals with the preparation, characterization and application of metal nanoparticles (NPs) on molecularly modified surfaces for thermo- and electrocatalytic hydrogenation reactions. In the thermocatalytic systems, Mn-based NPs are investigated (monometallic and bimetallic MnRu NPs), as this earth-abundant metal possesses a low environmental impact and generally low toxicity. In addition, the electrocatalytic hydrogenation of alkenes, aldehydes and ketones using a Pickering emulsion-based system is described. Here, Pd NPs on molecularly modified carbon nanotubes are evaluated as catalyst. Firstly, small (1-10 nm) Mn NPs are immobilized on different carbon-based supports, as well as on a supported ionic liquid phase (SILP). Different synthetic methods are developed in order to optimize the properties of the NPs on each respective support, which are investigated using various characterization methods. The materials are evaluated for different catalytic transformations, with the catalytic transfer hydrogenation of aldehydes and ketones using Mn@SILP being found to be best performing. This material is demonstrated to be more active than previously reported Mn NP-based systems. X-ray absorption studies showed that the catalytic activity is highly dependent on the oxidation state of Mn. Followingly, bimetallic MnRu@SILP materials are prepared and characterized. Tuning the metal ratio of Mn:Ru enables the selective targeting of different products for substrates containing multiple reducible moieties. Furthermore, the alloying state of these NPs is investigated in order to assess its importance for the performance of the catalysts. The last part of this thesis deals with electrocatalytic hydrogenation (ECH) reactions, which offer an attractive way for the use of renewable electricity for chemical transformations. As water is used as solvent and hydrogen donor, the application of this reaction type for the conversion of apolar, organic systems is rather limited. In order to overcome this challenge, a Pickering emulsion-based system is developed. Herein, the apolar substrates are located within oil droplets, which are surrounded by an aqueous phase, providing the protons and high conductivity. This emulsion is stabilized by Pd NPs on molecularly modified CNTs. These CNTs additionally conduct electricity to the interface where the reaction is catalyzed by the Pd NPs. The mechanism of the ECH of alkenes is investigated and the reaction rate and Faradaic efficiency surpassed state-of-the-art Pd membrane reactors. The ECH system is further extended to the ECH of aldehydes and ketones to the respective alcohols. Here, the subsequent deoxygenation products can be also be yielded, which was not demonstrated yet in the electrocatalytic application of Pd NPs. Overall, this work contributes towards the development of novel catalytic systems for selective hydrogenation reactions. It can be demonstrated that metal NPs on molecularly modified surfaces represent a class of highly tunable catalysts. Their NP composition, choice of support and molecular modifier are varied to design highly efficient and selective catalysts for thermo- and electrocatalytic hydrogenation reactions.

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Abstract: The formation of C–C bonds through coupling reactions is an important industrial process. The ability of Au to catalyze such reactions has been reported, with both homogeneous and heterogeneous catalyst examples. Previous work has shown that carbon-supported cationic and nanoparticulate Au are active for the homocoupling of phenylboronic acid to biphenyl. However, the stability of supported cationic Au is short-lived, and the formed nanoparticles were suggested to be the active species. Through the synthesis of two types of supported cationic Au catalysts, utilizing either aqua regia or acetone solvents, we show that both catalysts develop nanoparticulate Au species early in the reaction; however, only the aqua regia prepared catalyst is active. We ascribe the activity of the aqua regia prepared Au catalyst to excess Cl and the presence of C–Cl surface species in combination with Au. Carbon treated with aqua regia was inactive; however, when used as a support for Au deposited with acetone or via a sol immobilization method, activity was comparable to the aqua regia prepared catalyst. The role of C–Cl and Au nanoparticles is discussed with respect to their correlation to the biphenyl yield, which is shown to be significant only when the C–Cl species are present on the catalyst.

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Abstract: The effects of global warming are devastating ecosystems and driving the transition from fossil fuels to renewable energy. Achieving a carbon-neutral energy system requires efficient devices for storing and converting renewable energy. Hydrogen, produced via water electrolysis, is a promising zero-emission energy carrier. However, its production is hindered by slow reaction kinetics, necessitating costly noble metal catalysts. Transition metals, abundant and cost-effective, are a viable alternative, especially when optimized as single-atom catalysts (SACs). SACs maximize catalytic activity, significantly enhancing efficiency compared to bulk materials and nanoparticles, offering a scalable solution for the energy transition. This PhD thesis centers on developing efficient and durable catalysts for green hydrogen production in alkaline electrolyzers, emphasizing the relationship between catalytic activity and precise control of active sites. By correlating material structure with performance through extensive characterizations, the work aims to establish a systematic framework for catalyst design. The ultimate objective is to achieve tailored catalytic properties for practical applications, linking nanoscale to macroscale characteristics. A progressive approach grounded in structure-activity principles guides the design and investigation of electrocatalysts throughout the thesis. Initially, a detailed investigation was conducted to understand the interactions between single metal atoms and their coordination environments. Single Ni atoms were stabilized in a carbon nitride matrix synthesized via a low-temperature method and evaluated as oxygen evolution reaction (OER) catalysts under alkaline conditions. Experimental and theoretical approaches provided an accurate model of the material's structure and properties, including its restructuring during catalysis. Results revealed a linear increase in activity with Ni loading, up to a maximum of 1 wt.% Ni, which the carbon nitride framework could effectively stabilize. These SAC-based catalysts demonstrated good stability with partial Ni site restructuring. Following the demonstration of carbon nitride's effectiveness in stabilizing Ni single atoms, efforts have focused on enhancing Ni centers' catalytic activity by engineering their coordination environments, akin to molecular catalysts. A novel triazine-thiadiazole-based organic polymer was synthesized to study sulfur incorporation into the Ni coordination sphere. This modification effectively enhances catalytic performance and stability by altering the electronic density of the Ni centers. To further improve the performance and industrial viability of SACs, the materials have been incorporated into composite structures with carbon nanotubes (CNTs). This integration allows to combine the high activity and stability of Ni stabilized in carbon nitride with the superior electrical conductivity and surface area of CNTs. The resulting composite materials have demonstrated markedly improved catalytic performance, as evidenced by a substantial increase in specific current density due to enhanced electrical conductivity, greater surface area, and improved accessibility of Ni active sites to the electrolyte. The final phase of this work focused on bridging fundamental research with industrial applications. In collaboration with Acca Industries S.r.l., chemically treated stainless steel was investigated as a scalable platform for applying structure-activity principles to enhance catalytic performance in alkaline electrolyzers. The improvements were linked to newly formed surface metal phases that promote beneficial electronic interactions among Fe, Cr, Ni, and Mo, enhancing charge transfer and catalytic pathways. This integrative and systematic approach highlights the potential of structure-activity relationships to advance scalable hydrogen production technologies, addressing the pressing global demand for sustainable energy solutions.

Open Access

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Abstract: A major challenge in hydrogen production from water electrolysis is the slow kinetics of oxygen evolution (OER). Applying an alternating magnetic field (AMF) to ferromagnetic metal nanoparticles on electrodes has gained attention due to the generation of a thermally activated electrocatalyst, which can boost OER performance. This work studies the influence of external parameters and intrinsic characteristics of carbon-encapsulated cobalt MOF-derived nanoparticles deposited onto graphite paper electrodes on the electrocatalytic AMF-OER coupled process. Specifically, the impact of AMF strength, the electrolyte composition (concentration and cation nature) and cobalt content on the electrocatalytic AMF-OER performance are thoroughly investigated. Results reveal that AMF significantly boosts OER activity of Co@C-based electrodes, their enhancement being strongly dependent on the electrolyte composition. Furthermore, both the heating capacity of the herein synthesized catalyst for magnetic hyperthermia and their structural features remain intact after an intense and prolonged electrocatalytic AMF-OER experiment. No signs of sintering, leaching, or particle size increase, which are typical issues observed when metal nanoparticles are subjected to an intense external magnetic field, have been found. This underscores the high operational stability of this catalyst. These findings provide new insights into thermal AMF-assisted alkaline water oxidation for developing high-performance catalysts for enhanced electrocatalysis.