Emiel J. M. Hensen’s research while affiliated with Eindhoven University of Technology and other places

Controlling the size and composition of metal nanoparticles is of considerable interest, as these are essential to their catalytic properties. We report the controlled synthesis Rh nanoparticles by preorganisation of metal complexes inside Pt12L24 nanospheres based on complementary hydrogen bonds before the reduction step that leads to nanoparticle formation. The encapsulated RhI complexes (Rh‐s @ G‐sphere) led to reasonable size control (2.8 ± 0.9 nm). We also report the formation of Rh‐Ir alloyed nanoparticles with varying Rh/Ir compositions. These heterometallic particles were evaluated in the hydrogenation of cinnamaldehyde (7) as a probe reaction. Besides a high activity in this probe reaction, the Rh particles also catalyzed the conversion of the solvent (CH3CN). The formed basic amine leads to follow‐up reactions of the product and compatibility issues with the hosting nanosphere. The solvent hydrogenation was effectively suppressed by using the Rh:Ir alloyed nanoparticles, provided that they contain >66% Ir. The Rh:Ir alloyed nanoparticles displayed high catalytic activity, reaching optimal selectivity and activity at an 8:16 ‐ Rh:Ir ratio. The combined catalytic results illustrate that pre‐organisation of the metal complexes in the nanosphere before the reduction with hydrogen effectively facilitates the formation of Rh:Ir alloyed nanoparticles.

Controlled preparation of ultrafine metal nanoclusters (<2 nm) is challenging, yet important as the properties of these clusters are inherently linked to their size and local microenvironment. In the present work, we report the utilization of supramolecular pre-organization of organometallic complexes within well-defined M12L24 coordination spheres for the controlled synthesis of ultrafine Ir nanoclusters by reduction with molecular hydrogen. For this purpose, 24 sulfonate functionalized N-heterocyclic carbene (NHC) Ir complexes (Ir-s) were bound within a well-defined M12L24 nanosphere that is equipped with 24 guanidinium binding sites (G-sphere). Reduction of these pre-organized metal complexes by hydrogenation led to the templated formation of nanoclusters with a narrow size distribution (1.8 ± 0.4 nm in diameter). It was demonstrated through ¹H-DOSY-NMR and HAADF-STEM-EDX experiments that the resulting nanoclusters reside within the nanospheres. The reduction of similar non-encapsulated metal complexes in the presence of nanosphere systems (Ir-s + M-sphere or Ir-p + G-sphere) resulted in larger particles with a broader size distribution (2.3 ± 2.1 nm and 6.6 ± 3.2 nm for Ir-s + M-sphere and Ir-p + G-sphere respectively). The encapsulated nanoclusters were used as a homogeneous catalyst in the selective hydrogenation of 4-nitrostyrene to 4-ethylnitrobenzene and display absolute selectivity, which is even maintained at full conversions, whereas the larger non-encapsulated clusters were less selective as these also showed reduction of the nitro functionality.

Well-defined amorphous silica–alumina (ASA) with a relatively low Al loading were synthesized by homogeneous deposition-precipitation of Al³⁺ on SiO2 nanoparticles to understand the nature and formation of Brønsted acid sites (BAS). The amount of Al grafted relative to the silanol density was varied by variation of the size of SiO2 nanoparticles, reflected by their surface areas between 90 and 380 m²·g–1. Two sets of ASA were synthesized, one aiming at a SiOH/Al ratio of 3, corresponding to the maximum amount of BAS represented by Al³⁺ perturbation of SiOH groups, and the second one aimed at studying the impact of Al dispersion by using a constant Al loading (Si/Al ≈ 103). ²⁷Al MAS NMR spectroscopy confirmed that the first sample set only contained tetrahedral Al species. Calcination did not affect the Al coordination. CO IR spectroscopy revealed that the BAS concentration substantially varied in the 15–133 μmol·g–1 range by varying the Al loading and the SiO2 nanoparticle size. At equal Al loading, the BAS concentration increased from 15 to 46 μmol·g–1 with increasing SiO2 surface area. Less than 30% of all grafted Al sites gave rise to BAS, independent of the surface area and calcination temperature. The ASA samples were screened for their catalytic performance in pyrolytic cracking of ultrahigh molecular weight polyethylene in a thermogravimetric analysis apparatus. The performance in pyrolysis, as gauged by the temperature at which the weight loss rate was highest, increased with the Brønsted acidity. The cracking temperature decreased from 490 °C without a catalyst to 463 °C using the most acidic ASA. At equal Al loading, the pyrolysis temperature decreased with increasing surface area, indicating that, besides acidity, cracking also benefits from a higher surface area where the long polymer chains can adsorb. Compared to zeolite, ASA produced more liquid hydrocarbons and less coke.

In2O3 is a promising electrocatalyst for CO2 electroreduction (CO2ER) to formate. In2O3 nanoparticles doped with Pd, Ni, Co, Zr, and Ce promoters using flame-spray pyrolysis were characterized and evaluated in a gas diffusion electrode for the CO2ER. Doping results in slight shifts of the In binding energy as probed by XPS, which correlates with a change of the Faradaic efficiency to formate (FEformate) in the order Ce-doped In2O3 > Zr-doped In2O3 > In2O3 > Pd-doped In2O3 > Ni-doped In2O3 > Co-doped In2O3. However, the differences in CO2ER performance are caused mainly by the different extent of In2O3 reduction. Co-doped In2O3 is prone to complete reduction to a stable Co–In alloy with a low FEformate due to a high hydrogen evolution activity. The stabilizing effect of Ce on In2O3 is further demonstrated by an X-ray absorption spectroscopy study of a set of Ce-doped In2O3 samples (10, 50, 90 at%), highlighting that reduction of In2O3 is suppressed with increasing Ce content. Optimum performance in terms of FEformate is obtained at a Ce content of 10 at%, which is attributed to the stabilization of In2O3 under negative bias up to −2 V. At higher Ce content, less active CeO2 is formed. The highest FEformate of 86% observed for In2O3 doped with 10 at% Ce, at a current density of 150 mA/cm², compares favorably with a FEformate of 78% for In2O3.

The ongoing energy transition will require a number of emerging technological concepts (e.g. Power-to-X and Hydrogen Economy, etc.) which will ultimately combine renewable energy, novel chemical production/conversion processes and innovative, integrated devices/systems to produce sustainable platform molecules, fuels and materials. In this book, readers are introduced to selected concepts, challenges, steps forward and necessities relating to the technologies required to deepen the integration between the energy and chemical sectors. Selected key technologies to support this integration will be discussed, with particular emphasis on the catalytic systems and devices required to enable the transition including electrochemical cells, CO2 hydrogenation and plasma-assisted processes. Several chapters will discuss evolving and emerging technologies and tools (e.g. LCA) that will be required to enable a green and successful energy transition. The book will be of interest to graduate students and researchers in renewable energy, catalysis, chemical engineering and chemistry, wishing to have an introduction to the topic and associated technologies.

Oil has long been the dominant feedstock for producing fuels and chemicals, but coal, natural gas and biomass are increasingly explored alternatives1–3. Their conversion first generates syngas, a mixture of CO and H2, which is then processed further using Fischer–Tropsch (FT) chemistry. However, although commercial FT technology for fuel production is established, using it to access valuable chemicals remains challenging. A case in point is linear α-olefins (LAOs), which are important chemical intermediates obtained by ethylene oligomerization at present4–8. The commercial high-temperature FT process and the FT-to-olefin process under development at present both convert syngas directly to LAOs, but also generate much CO2 waste that leads to a low carbon utilization efficiency9–14. The efficiency is further compromised by substantially fewer of the converted carbon atoms ending up as valuable C5–C10 LAOs than are found in the C2–C4 olefins that dominate the product mixtures9–14. Here we show that the use of the original phase-pure χ-iron carbide can minimize these syngas conversion problems: tailored and optimized for the process of FT to LAOs, this catalyst exhibits an activity at 290 °C that is 1–2 orders higher than dedicated FT-to-olefin catalysts can achieve above 320 °C (refs. 12–15), is stable for 200 h, and produces desired C2–C10 LAOs and unwanted CO2 with carbon-based selectivities of 51% and 9% under industrially relevant conditions. This higher catalytic performance, persisting over a wide temperature range (250–320 °C), demonstrates the potential of the system for developing a practically relevant technology.

Alkali metals can promote the performance of MoS2 in methanethiol (CH3SH) synthesis from CO/H2/H2S mixtures. Recently, it has been found that alkali sulfides and most prominently Cs2S can also catalyze the reaction between CO and H2S to COS and H2, COS acting as an intermediate in CH3SH formation (M. Yu et al. J. Catal. 2022, 405, 116–128). Here, we study the nature of the active sites and the mechanism of the CO + H2S → COS + H2 reaction for the 6 low-index Miller planes of Cs2S. While CO adsorbs weakly, strong dissociative adsorption of H2S results in HS* and H* intermediates, which further stabilize the (0 0 1) facet as the dominant surface termination. The main reaction pathway towards COS involves the association of CO* and SH* to COSH* followed by its dehydrogenation in a Langmuir-Hinshelwood mechanism. Reactions of CO* with lattice S atoms have prohibitively high barriers due to the strong Cs-S bonds in Cs2S. Overall, the reaction rate is dominated by the (0 0 1) facet with small contributions of the (0 1 0), (0 1 1) and (1 0 1) surfaces. COSH* formation, its dehydrogenation to COS, and COS desorption compete as rate-controlling steps on these surfaces.

CO is the key reaction intermediate in the Cu-catalyzed electroreduction of CO2 to products containing C–C bonds. Herein, we investigate the impact of the particle size of CuO precursors on the direct electroreduction of CO (CORR) to C2+ products. Flame spray pyrolysis was used to prepare CuO particles with sizes between 4 and 30 nm. In situ synchrotron wide-angle X-ray scattering (WAXS), quasi-in situ X-ray photoelectron spectroscopy, and transmission electron microscopy demonstrated that, during CORR, the CuO precursors transformed into ∼30 nm metallic Cu particles with a crystalline domain size of ∼17 nm, independently of the initial size of the CuO precursors. Despite their similar morphology, the samples presented different Faradaic efficiencies (FEs) to C2+ products. The Cu particles derived from medium-sized (10–20 nm) CuO precursors were the most selective to C2+ products (FE 60%), while those derived from CuO precursors smaller than 10 nm displayed a high FE to H2. As the oxidation state, the particle and the crystallite sizes of these samples were similar after CORR, the differences in product distribution are attributed to the type and density of surface defects on the metallic Cu particles, as supported by studying electrochemical oxidation of the reduced Cu particles during CV cycling in combination with synchrotron WAXS. Cu particles derived from <10 nm CuO contained a higher density of more under-coordinated defects, resulting in a higher FE to H2 than Cu particles derived from 10 to 30 nm CuO. Bulk oxidation was most prominent and stable for Cu particles derived from medium-sized CuO, which indicated the more disordered nature of their surface compared to Cu particles derived from 30 nm CuO precursors and their lower reactivity compared to Cu particles derived from small CuO. Cu particles derived from <10 nm CuO initially displayed intense redox behavior, quickly fading during subsequent CVs. Our results evidence the significant restructuring during the electrochemical reduction of CuO precursors into Cu particles of similar size. The differences in CORR performance of these Cu particles of similar size can be correlated to different surface structures, qualitatively resolved by studying surface and bulk oxidation, which affect the competition between CO dimerization to yield C2+ products and undesired H2 evolution.

Despite its promising performance for C 2+ product formation in electrochemical reduction of CO 2 (CO 2 RR), Cu-based catalysts exhibit a relatively poor selectivity towards important chemical intermediates such as ethylene and ethanol. Up to 16 different products can typically be formed during CO 2 RR. ¹ Cu is the only element that can form C-C bonds at an appreciable rate. ² Therefore, it is critical to better understand the active sites of Cu-based electrocatalysts, as it can lead to better catalyst design with improved selectivity towards desired products. A common approach is to investigate the reactivity of well-defined surface terminations of Cu, which can be achieved by employing shape-controlled (nano)particles. ³ Here, we expanded on such studies by evaluating the impact of Au on well-defined Cu 2 O surfaces for the electrochemical reduction of CO 2 . This promoter metal was chosen, because it can reduce CO 2 to CO, which is widely accepted as the key intermediate for C-C coupling. 2,4 In this study, 40 nm Cu 2 O nanocubes that predominantly expose the (100) facet were synthesized with a ligand-free method. The (100) facet is more active in the production of ethylene at the expense of unwanted methane formation, which takes place predominantly at the (111) facet. ³ The surface of the nanocubes was decorated with Au nanoparticles by a galvanic replacement reaction. HAADF-STEM images of the catalysts with various Au loadings are shown in Figure 1a-d . After addition of Au nanoparticles, the shape of the Cu 2 O nanocubes was preserved. However, the Au nanoparticles were highly dispersed at low loadings (1 mol % Au) whereas Au clusters were observed at higher loadings (10 mol % Au). This result was in line with synchrotron X-ray diffraction (XRD) results, whereby the Au(111) reflection appears for the 10Au/Cu 2 O sample. Besides, the formation of a Au-Cu alloy and CuO phase was confirmed as well. The highly dispersed nature of the Au nanoparticles at low loadings was further established by Au 4f X-ray photoelectron spectroscopy (XPS), whereby a shift towards higher binding energies was observed as compared to the samples with higher Au loadings. The electrocatalytic performance of the catalysts was assessed over a range of potentials in neutral electrolyte (0.1 M KHCO 3 ) in H-cell configuration. With highly dispersed Au as promotor, we found that the formation of ethylene and ethanol was significantly enhanced ( Figure 1e-f ). More specifically, the Faradaic effiency towards ethanol increased by roughly 1.5 times by introducing 1 mol % Au to the Cu 2 O nanocubes. Surprisingly, the 10Au/Cu 2 O sample displayed the lowest Faradaic efficiency for C 2 products. We believe that the dispersion of Au on the Cu 2 O surface plays an important role in the product formation, whereby CO can be formed at Au sites and diffuse to Cu sites in proximity for further reduction to C 2 products (CO spillover). To further study structure-activity relationships, we performed in-situ X-ray absorption spectroscopy (XAS) measurements. Figure 1g shows the normalized Cu K-edge X-ray absorption near-edge structure (XANES) of the Cu 2 O and Au/Cu 2 O samples in the final state. The evolution of the Cu(0), Cu(I) and Cu(II) fractions was followed by Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analysis. This analysis revealed that the final oxidation state of the catalysts is different. The Cu(I)/Cu(0) ratio shows the following trend 1Au/Cu 2 O > 5Au/Cu 2 O > Cu 2 O > 10Au/Cu 2 O. Based on these observations, we hypothesize that the oxidation state of Cu affects the product distribution and in particular, the formation of oxygenates. The presence of Cu(I) species and the formation of Cu(0) species during CO 2 electroreduction was further confirmed with in-situ XRD measurements. Finally, we performed quasi in-situ XPS measurements to study the reduction of the surface of the catalysts. Acknowledgements This publication is part of the research programme 'Reversible Large-scale Energy Storage' (RELEASE) with project number 17621 which is financed by the Dutch Research Council (NWO). References [1] Kuhl, K.P.; Cave, E.R.; Abram, D.N.; Jaramillo, T.F . Energy Environ. Sci. 2012 , 5 , 7050-7059. [2] Bagger, A.; Ju, W.; Varela, A.S.; Strasser, P.; Rossmeisl, J. ChemPhysChem 2017 , 18 , 1-9. [3] Grosse, P.; Gao, D.; Scholten, F.; Sinev, I.; Mistry, H.; Roldan Cuenya, B . Angew. Chem. Int. Ed . 2018 , 57 , 6192-6197. [4] Montoya, J.H.; Shi, C.; Chan, K.; Norskov, J.K. J. Phys. Chem. Lett . 2015 , 6 , 2032-2037. Figure 1