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Universitat de Barcelona. Facultat de Química

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[eng] Developing advanced and efficient electrocatalytic energy conversion systems is of great and practical significance for propelling the efficient development of clean energy for the construction of new low-carbon power systems. Among them, electrocatalytic reduction reactions driven by renewable electricity to transform biomass-derived chemicals into biofuels and high value-added chemicals provide an effective way to improve the H/C ratio of biomass-derived chemicals and the stabilizations of bio-oil systems. However, the electrocatalytic reduction of organic compounds is more intricate compared to the electrocatalytic reduction of water molecules. It involves the adsorption of various organic functional groups, multi-step electron transfer, and the generation of organic intermediates. Meanwhile, organic electrocatalytic reduction calls for designing efficient, highly selective, and cost- effective electrocatalysts.
During a series conversion process of raw biomass, aldehydes are believed to be particularly troublesome for the aldol condensation and polymerization reactions. To avoid them, hydrogenation processes are necessary. As an alternative to traditional high-pressure and -temperature processing, we choose electrochemistry that can operate in ambient conditions for the conversion of benzaldehyde (BZH), which was chosen as a typical biomass-derived chemical. Another reason for choosing BZH is that the hydrogenation products benzyl alcohol (BA) and hydrobenzoin (HDB) are important industrial chemicals.
Based on the mentioned above, this work seeks to design highly efficient and high selective catalysts for the electrocatalytic conversion of the carbonyl group of BZH into BA, HDB or benzoic acid (BZA) in aqueous solution at pH＞5 (avoiding the deoxygenation product toluene). Additionally, this work screens the optimal reaction conditions for various products and speculates their most probable reaction pathways.
Chapter 4 focused on the electrocatalytic reduction of BZH into BA. Pd nanoparticles supported on a nickel metal-organic framework (MOF), Ni-MOF-74, are prepared and their activity towards the ECH of BZH in a 3M sodium acetate-acetic acid (pH 5.2) aqueous electrolyte is explored. An outstanding ECH rate up to 283 µmol cm-2 h-1 with a Faradic efficiency (FE) of 76% is reached. Besides, higher FEs of up to 96% are achieved using a step-function voltage. Materials studio and density functional theory calculations show these outstanding performances to be associated with the Ni- MOF support that promotes H-bond formation, facilitates water desorption, and induces a favorable tilted BZH adsorption on the surface of the Pd nanoparticles. In this configuration, BZH is bonded to the Pd surface by the carbonyl group rather than through the aromatic ring, thus reducing the energy barriers of the elemental reaction
 
steps and increasing the overall reaction efficiency.
Chapter 5 focused on the electrochemical reduction of self-coupling of BZH to HDB using semiconductor electrocatalysts with nanosheet morphologies. The effects of electrode potential and electrolyte pH on BZH self-coupling reaction were comprehensively studied on several semiconductor electrocatalysts. A correlation is observed between their band gap and the electrochemical potential necessary to maximize selectivity towards HDB in alkaline medium, which we associate with the charge accumulation at the semiconductor surface. N-type CuInS2 provides the highest conversion rate at 0.3 mmol cm−2 h−1 with a selectivity of 98.5% at -1.3 V vs. Hg/HgO in aqueous alkaline solution pH=14. Additional density functional theory calculations demonstrate a lower kinetic energy barrier at the CuInS2 surface compared with graphitic carbon, proving its catalytic role in the self-coupling reaction of BZH.
Based on the previous two works, we realize that even when selecting materials with poor HER performance, different voltages and pH values have a significant impact on the selectivity of HDB. This drives us towards the rational design of electrocatalysts for these two different reaction pathways. Chapter 6 employed material with exposed active sites Cu2S and the material Cu2S-OAm with ligands capped to catalyze the electrocatalytic reduction reaction of the biomass platform molecule BZH convert into BA and HDB. Cu2S particles are used as electrocatalysts for the BZH electrochemical conversion. We particularly analyze the effect of surface ligands, oleylamine (OAm), on the selective conversion of BZH to BA or HDB. The effect of the electrode potential, electrolyte pH, and temperature are studied. Results indicate that bare Cu2S exhibits higher selectivity towards BA, while OAm-capped Cu2S promotes HDB formation. This difference is explained by the competing adsorption of protons and BZH. During the BZH electrochemical conversion, electrons first transfer to the C in the C=O group to form a ketyl radical. Then the radical either couples with surrounding H+ to form BA or self-couple to produce HDB, depending on the available H+ that is in turn affected by the electrocatalyst surface properties. The presence of OAm inhibits the H adsorption on the electrode surface therefore reducing the formation of high-energy state Had and its combination with ketyl radicals to form BA instead promotes the outer sphere reaction for obtaining HDB.
Finally, we turn our attention to the anodic reaction in chapter 7. The electrooxidation of organic compounds offers a promising strategy for producing value-added chemicals through environmentally sustainable processes. A key challenge in this field is the development of electrocatalysts that are both effective and durable. In this study, we grow gold nanoparticles (Au NPs) on the surface of various phases of titanium dioxide (TiO2) as highly effective electrooxidation catalysts. Subsequently, the samples are tested for the oxidation of BZH to BZA coupled with a hydrogen
 
evolution reaction (HER). We observe the support containing a combination of rutile and anatase phases to provide the highest activity. The excellent electrooxidation performance of this Au-TiO2 sample is correlated with its mixed-phase composition, large surface area, high oxygen vacancy content, and the presence of Lewis acid active sites on its surface. This catalyst demonstrates an overpotential of 0.467 V at 10 mA cm-2 in a 1 M KOH solution containing 20 mM BZH, and 0.387 V in 100 mM BZH, well below the oxygen evolution reaction (OER) overpotential. The electrooxidation of BZH not only serves as OER alternative in applications such as electrochemical hydrogen evolution, enhancing energy efficiency, but simultaneously allows the generation of high-value byproducts such as BZA.

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[spa] El desarrollo de sistemas de conversión de energía electrocatalítica avanzados es crucial para la energía limpia y un sistema energético de bajo carbono. La reducción electrocatalítica de productos químicos de biomasa mejora la relación H/C y estabiliza los aceites biológicos, aunque es compleja debido a la transferencia de electrones y generación de intermediarios. Es esencial diseñar electrocatalizadores eficientes y selectivos. La hidrogenación de aldehídos en la biomasa cruda es necesaria para evitar reacciones no deseadas. Se utilizó la electroquímica para convertir benzaldehído (BZH) en productos industriales valiosos como alcohol bencílico (BA) e hidrobencoína (HDB). Este trabajo diseñó catalizadores eficientes para convertir BZH en BA, HDB o ácido benzoico (BZA) en solución acuosa con pH ＞ 5, optimizando las condiciones de reacción. En el Capítulo 4, se usaron nanopartículas de Pd en un marco metal-orgánico de níquel (Ni-MOF-74) logrando una alta eficiencia faradaica (FE) y mejor adsorción de BZH. El Capítulo 5 estudió el acoplamiento de BZH a HDB con electrocatalizadores semiconductores, destacando el CuInS₂ de tipo N por su alta selectividad y eficiencia. En el Capítulo 6, se usaron partículas de Cu₂S con y sin oleylamine (OAm), mostrando que OAm promueve la formación de HDB al inhibir la adsorción de protones. El Capítulo 7 se enfocó en la electrooxidación de BZH a BZA usando nanopartículas de oro (Au NPs) en dióxido de titanio (TiO₂), logrando alta actividad y eficiencia energética, generando además subproductos valiosos.