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Angewandte Chemie. International Edition, 57 (47), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

The growing demand for plastics coupled with shrinking oil reserves has revived interest in the production of vinyl chloride (VCM), monomer to polyvinyl chloride, from coal-derived acetylene. However, the present acetylene hydrochlorination process relies on toxic mercuric chloride-based catalysts, the use of which will be banned from 2022, highlighting the urgency to implement sustainable and economically viable alternatives. Since decades, the search for a suitable replacement has been guided by the linear correlation between activity and the standard electrode potential of metal chlorides, directing research efforts primary towards carbon-supported gold catalysts. Still, several practical challenges and fundamental questions remain unsolved. While Au single atoms exhibit high initial activity, they rapidly agglomerate into inactive particles due to their insufficient stability on carbon. Furthermore, the established performance descriptor cannot provide reliable guidelines for the design of superior catalytic architectures, as speciation effects and stability of the metal nanostructures are not considered. Finally, the key role of carbon in generating active metal-based catalysts compared to any other support is still not understood. In fact, a complex interplay exists between the metal site and the carbon support, exhibiting as such notable activity in acetylene hydrochlorination, particularly upon functionalization with heteroatoms. This thesis disentangles the interplay between the metal nanostructure and the carbon support in acetylene hydrochlorination and unravels fundamental understanding on the active sites through speciation-performance analyses, providing guidelines for the optimal design of metal-free and nanostructured metal-based catalysts. To reach this goal, a holistic approach, combining precise material synthesis, in-depth characterization, quantitative catalytic evaluation, kinetic and transient mechanistic analyses, and density functional theory studies is adopted. Firstly, the potential of nitrogen-doped carbons (NC) as metal-free hydrochlorination catalysts is explored. By decoupling structural, compositional, and porous properties, an interplay of two activity descriptors is identified: (i) a high content of pyrrolic-N functionalities, being responsible for the adsorption of the reactants, and (ii) good electrical conductivity,x likely influencing the surface diffusion of adsorbed species. With this understanding, the first metal-free catalyst is developed that rivals the initial activity of gold-based systems at elevated reaction temperatures (473 K and 573 K, for metal-based and metal-free catalysts, respectively). However, the active sites promote extensive coking, leading to micropore blockage and rapid catalyst deactivation (deactivation constant kD = -20 h−1). The introduction of structurally more stable meso- and macropores results into a ca. 50-fold reduced deactivation rate of hierarchical NC at half the initial activity level compared to their purely microporous counterparts. Building on the acquired knowledge to functionalize carbons, host design strategies are developed to control the nuclearity and coordination environment of gold, platinum, palladium, ruthenium, rhodium, and iridium-based catalysts. Following this approach, metal speciation and host effects can be disentangled, enabling the derivation of generalized quantitative performance descriptors for acetylene hydrochlorination. Distinct active-site nanostructures were identified: (i) MClx single atoms of Au and Pt, (ii) metal oxide nanoparticles of Ru, Rh, and Ir and, (iii) metallic nanoparticles of Pd. The energy of acetylene adsorption is identified as speciation sensitive activity descriptor. Further, also the selectivity with respect to the formation of coke is mainly determined by the acetylene-affinity of the metals (i.e., maximized over Pd nanoparticles) and the functionalization of the carbon support (i.e., maximized over NC). Besides coking, chlorination and metal nuclearity changes are relevant deactivation mechanisms, originating from an interplay of two stability descriptors: (i) the single atom-carbon host interaction and (ii) the affinity towards chlorine. Specifically, all nanostructures of Au and Pd suffer from agglomeration on N-free carbon, while being sufficiently stabilized on NC. Oxidic nanoparticles of Ru, Rh, and Ir undergo chlorination and redispersion into fully chlorinated inactive single atoms, regardless of the host functionalization. In the case of Ru/NC, this process can be inhibited through encapsulation into single-layer graphene shells. In combination with optimized oxygen co-feeding to reduce coking while preserving the protective layer, the nanostructured Ru catalyst can achieve comparable activity and stability to Au single-atoms on NC (kD = -1.3 h−1). On non-functionalized carbon, Pt single atoms are identified as the only metal nanostructure with intrinsic stability on O/C defects. This endows them with unparalleled durability in acetylene hydrochlorination (kD = -0.1 h−1), ultimately surpassing the space-time-xi yield of state-of-the-art Au- and Ru-based catalysts and qualifying them as a new promising candidate for sustainable vinyl chloride production. The discovery of stable carbon-supported Pt single atoms further allowed to systematically vary the porous properties and surface functionalization of carbon while preserving the metal speciation, shedding light on the intrinsic role of the support. A high acetylene adsorption capacity in HCl-rich atmosphere is identified as the central activity descriptor, which is finely controllable through the porosity of the carbon host. The rate of coking as the main deactivation mechanism is decreased by reducing the density of acidic surface groups. With the aim to combine the high initial activity of Au single atoms with the unprecedented stability of their Pt-based analogs, synergies in bimetallic catalysts are explored. Thereby, we reveal the potential of Pt chloride in aqueous solution to disperse large gold agglomerates (>70 nm) on carbon carriers into single atoms, a phenomenon of practical relevance beyond the field of acetylene hydrochlorination. Furthermore, the formed bimetallic single-atom catalyst exhibits improved resistance against agglomeration, indicating cooperativity effects between gold and platinum atoms and giving exciting future prospects for multimetallic single-atom catalysis. This thesis demonstrates how catalysis can be enhanced via precise nanoscale engineering, giving momentum to future developments in acetylene hydrochlorination and single-atom catalysis.

Halogen-mediated alkane functionalization into haloalkanes and olefins and their subsequent conversion into liquid fuels and chemicals constitute an attractive technology with the potential to develop processes for the on-site valorization of natural gas. However, the industrial implementation of this route is contingent on the full recycling of the hydrogen halide (HX, X=Cl, Br), which results from the alkane activation and haloalkanes upgrading steps. Catalytic alkane oxyhalogenation, comprising the reaction of the alkane with HX and O2, allows to integrate alkane functionalization and halogen recovery in a single step. Still, a key challenge to be overcome for the potential application of this route is the availability of efficient catalytic materials as well as the development of advanced characterization techniques as tools enabling the design of active, selective, and stable catalysts. This thesis discovers new families of catalysts and unravels fundamental understanding in the oxychlorination and oxybromination of C1-C4 alkanes, providing guidelines for designing optimal catalytic systems, by combining precise material syntheses, in-depth characterization analyses, detailed kinetic assessments, and advanced spectroscopic techniques. Firstly, methane oxyhalogenation has been investigated over supported metal nanoparticles, revealing activity and selectivity differences as a function of the hydrogen halide. In particular, silica-supported palladium catalysts are identified as the most selective (&98.5%) towards desired halomethanes (CH3X CH2X2) at moderate methane conversion (20%), rivaling the performance of the best oxyhalogenation catalysts reported to date. Kinetic analyses indicate that CH4 oxyhalogenation might proceed via the surface-catalyzed generation of molecular halogen and its reaction with CH4 in the gas phase with free radicals. However, to unravel this mechanism, the detection of these short-lived intermediates under operating conditions is required. This thesis demonstrates operando photoelectron photoion coincidence (PEPICO) spectroscopy as a pivotal method to achieve this goal. In particular, this technique is conducted in methane oxyhalogenation, providing, not only the evidence for the formation of methyl, bromine, and chlorine radicals, but also a strong correlation between methyl halide production and methyl radicals formation. Finally, these findings enable to demonstrate that radical- and surface-based routes are equivalent in oxychlorination, while the observed rate in oxybromination is dominated by gas-phase pathways. In view of the functionalization of higher alkanes, the use of HCl as halogenating agent allows to design catalysts for selective alkane activation as compared to HBr, due to the possibility to confine the reaction to the catalyst surface. In particular, the oxychlorination of ethane and propane is found to selectively (below or equal to 98%) generate ethylene and propylene up to full and high (55%) ethane and propane conversion, respectively, over several catalytic systems, including titanium oxide (TiO2), activated titanium carbide silicon carbide composite (TiO2-TiC-SiC), vanadium phosphate ((VO)2P2O7), and particularly iron phosphate (FePO4), chromium phosphate (CrPO4), and europium oxychloride (EuOCl) in a stable manner for up to 150 h on stream. The comprehensive evaluation of selected benchmark systems in the oxychlorination of ethane, propane, and butane, in combination with in-depth material characterization and kinetic analyses reveals activity and selectivity descriptors as a function of the alkane substrate. In particular, the reactivity of the catalysts is found to correlate with the ability of the material to evolve chlorine, which weakens with increasing carbon number. On the other hand, the olefin selectivity is a function of the rate of dehydrochlorination of the alkyl chloride, which is believed to be the intermediate to the olefin, and of the propensity towards alkane cracking and combustion over the catalysts, both increasing from ethane to butane. Specifically, the ethylene selectivity in ethane oxychlorination is mainly driven by catalytic ethyl chloride dehydrochlorination, while olefin production in propane and especially butane oxychlorination is primarily determined by the tendency of the catalysts to combust and/or crack the alkane, which is found on average moderate for propane and particularly enhanced in the case of butane. Interestingly, the olefin selectivity in the oxybromination of ethane and propane is limited (below or equal to 60% and 30%, respectively) over all investigated systems, due to combustion, cracking, and coking, indicating that the differences in the observed halide-dependent selectivity patterns are catalyst independent and inherent to the oxyhalogenation reaction network. In order to unravel the mechanistic origin of these distinct product distributions, this thesis demonstrates operando PEPICO and prompt-gamma activation analysis (PGAA) as pivotal techniques to unravel mechanisms within complex reaction networks by enabling the detection of short-lived radical species and by quantifying halogen and metal contents over the catalyst surface during operating conditions, respectively. In particular, evidences from these techniques combined with kinetic analyses and detailed material characterization, which were further complemented by density functional theory calculations in the case of ethane oxyhalogenation, demonstrate that while the alkane activation leads to alkyl halide regardless of the halogen source, the major difference lies in where the activation occurs. A chlorinated surface results in the observed selectivity control in oxychlorination whereas the generation of alkyl bromide occurs in the gas phase with the evolved Br2 and Br radicals, promoting undesired side reactions. In addition, operando PEPICO enables the selective detection of propyl radical isomers and provides unprecedented insights into the coking and cracking mechanisms in propane activation via oxybromination, entailing the formation of allyl and propargyl radicals. These findings allowed us to develop a novel catalytic process for the production of ethylene and propylene, which surpasses any existing olefin generation technology, paving the way for the potential implementation of a chlorine-based route for on-site natural gas valorization and giving momentum to future development in the halogen-mediated functionalization of alkanes.

Avoiding the most serious effects of climate change is one of the greatest technical and socio-political challenges of our time. At the present rate of anthropogenic CO2 emissions, in less than two decades mankind will lose the opportunity to limit the global temperature increase to 2 °C (as stated in the Paris Agreement), a fact that makes carbon dioxide an urgent target for recycling efforts. In this context, the combination of the electrochemical reduction of CO2 (eCO2RR) with carbon-neutral energy sources opens the door for the valorization of carbon dioxide as a medium for energy storage and as a source for the production of building blocks in a fossil fuel-free chemical industry. In particular, the efficient reduction of CO2 to CO would provide a versatile compound for the production of liquid fuels and plastics by established industrial processes. However, a key challenge for the eCO2RR on its way toward technological viability is the development of highly active and robust electrocatalysts capable of targeting a single CO2 reduction product and of inhibiting the competing hydrogen evolution reaction (HER) in aqueous media. Theoretical insights indicate that the key to unlocking breakthrough advances in catalytic performance for the eCO2RR lies in breaking the sub-optimal scaling relations between reaction intermediates that exist on transition metal surfaces. In this context, this thesis work is aimed at exploring the emergence of synergistic interactions in multicomponent materials as a design strategy for the development of improved eCO2RR catalysts, with an emphasis on understanding how the introduction of p-block elements modulates the activity and selectivity of transition metal-based catalysts for this reaction. First steps are aimed at evaluating whether the intrinsic selectivity for CO of silver electrodes can be enhanced by an interaction with indium, which is a poor HER catalyst. To this end, a comprehensive set of Ag-In electrocatalysts with different architectures is synthesized and tested, ranging from bulk intermetallic compounds to Ag nanoparticles supported on In2O3 and In(OH)3. Bulk Ag9In4 and AgIn2 alloys prepared by a succession of electrodeposition and annealing steps show a catalytic performance very similar to that of pure In, which is attributed to the surface enrichment of In in these materials. In contrast, the supported catalysts exhibit an enhanced current efficiency for CO at moderate overpotential, evidencing a synergistic effect between the metal nanoparticles and the oxidic supports. This effect is particularly marked with In(OH)3 as support, which unlike In2O3 is characterized by a kinetic impairment toward reduction to metallic In under eCO2RR conditions. In a following step, this approach is extended to the study of Cu-In catalysts. In this regard, the structure of Cu-In nanoalloys prepared by the in situ reduction of the CuInO2 delafossite and of In(OH)3-supported Cu nanoparticles evolves substantially over several electrocatalytic cycles, in parallel with an increase in the activity and selectivity for CO evolution. The detailed characterization of this process reveals that this evolving behavior is caused by the progressive segregation of Cu and In in these materials, resulting in the formation of a heterogeneous nanostructure of Cu-rich cores embedded within an In(OH)3 shell-like matrix. In addition, the presence of In(OH)3 in the equilibrated catalysts is shown to play a key role in ensuring a high selectivity toward CO. In the second part of this thesis, the modulating effect of p-block elements on the properties of Cu-based catalysts is further explored, along with an investigation of alternative synthetic routes. A Cu2O catalyst prepared by a solvothermal route shows enhanced performance compared to a commercially available material and to oxide-derived Cu bulk electrodes, highlighting the capacity of a simple and potentially scalable synthesis to prepare a Cu-based powder catalyst that can be readily incorporated into gas diffusion electrodes in view of practical applications. Moreover, the addition of a suitable precursor to the synthesis medium enables the preparation and comparison of Cu2O catalysts modified with other elements. Indium- and tin-promoted catalysts show even higher activity and selectivity toward CO compared to pristine Cu2O. On the other hand, the incorporation of aluminum into Cu2O negatively affects the production of CO but enhances the selectivity toward more reduced products, such as ethylene and alcohols, which based on detailed characterization is ascribed to a stabilizing effect of aluminum on copper(I) species under reaction conditions. Modification with sulfur results in a striking shift of the selectivity exclusively toward formate, implying the inhibition of the CO pathway that is characteristic of Cu-based catalysts. Sulfur-modified Cu electrocatalysts show profound surface restructuring under reaction conditions, attaining a surface configuration which is independent of the initial sulfur content of the fresh material. Apart from the fundamental interest that such a radical change in selectivity represents, sulfur-modified Cu outperforms all practical earth-abundant and non-toxic electrocatalysts reported to date for the production of formate via the eCO2RR. Despite recent advances, further progress in the design of more efficient eCO2RR catalysts would be greatly boosted by an expansion of the fundamental understanding. In this context, the last part of this thesis focuses on the development of a versatile photolithography-based microfabrication process for structured electrodes with controlled geometry and composition. This approach is aimed at gaining insights into the role of interfaces in multicomponent electrocatalysts and at deriving clear structure-performance relationships that can push forward the development of this class of materials. The microfabrication process is applied to the Cu-In system, and structured electrodes are produced in which well-defined arrays of microsized In2O3 and In islands are deposited on Cu and Cu2O substrates by the patterning of a sacrificial photoresist layer. This approach confirms the crucial role of the Cu2O substrate in attaining the synergistic effect, and the control over the geometry uncovers the fundamental dependence of the CO evolution activity on the interfaces between the components. Overall, the work in this thesis demonstrates that the introduction of p-block elements is a powerful strategy to modify the catalytic properties of transition metal-based eCO2RR catalysts typically used in this reaction, and the microfabrication of structured electrodes is shown as a powerful experimental platform for the rationalization of synergistic interactions and dynamic phenomena in multicomponent catalysts for CO2 reduction.