Restructuring of Cu-based Catalysts during CO Electroreduction: Evidence for the Dominant Role of Surface Defects on the C2+ Product Selectivity.

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.

Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction.

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

Transferable, Living Data Sets for Predicting Global Minimum Energy Nanocluster Geometries.

Modeling of nanocluster geometries is essential for studying the dependence of catalytic activity on the available active sites. In heterogeneous catalysis, the interfacial interaction of the support with the metal can result in modification of the structural and electronic properties of the clusters. To tackle the study of a diverse array of cluster shapes, data-driven methodologies are essential to circumvent prohibitive computational costs. At their core, these methods require large data sets in order to achieve the necessary accuracy to drive structural exploration. Given the similarity in binding character of the transition metals, cluster shapes encountered for various systems show a large amount of overlap. This overlap has been utilized to construct a living data set which may be carried over across multiple studies. Iterative refinement of this data set provides a low-cost pathway for initialization of cluster studies. It is shown that utilization of transferable structural information can reduce model construction costs by more than 90%. The benefits of this approach are particularly notable for alloy systems, which possess significantly larger configurational spaces compared to the pure-phase counterparts.

Unravelling CO Activation on Flat and Stepped Co Surfaces: A Molecular Orbital Analysis.

Structure sensitivity in heterogeneous catalysis dictates the overall activity and selectivity of a catalyst whose origins lie in the atomic configurations of the active sites. We explored the influence of the active site geometry on the dissociation activity of CO by investigating the electronic structure of CO adsorbed on 12 different Co sites and correlating its electronic structure features to the corresponding C-O dissociation barrier. By including the electronic structure analyses of CO adsorbed on step-edge sites, we expand upon the current models that primarily pertain to flat sites. The most important descriptors for activation of the C-O bond are the decrease in electron density in CO's 1π orbital , the occupation of 2π anti-bonding orbitals and the redistribution of electrons in the 3σ orbital. The enhanced weakening of the C-O bond that occurs when CO adsorbs on sites with a step-edge motif as compared to flat sites is caused by a distancing of the 1π orbital with respect to Co. This distancing reduces the electron-electron repulsion with the Co d-band. These results deepen our understanding of the electronic phenomena that enable the breaking of a molecular bond on a metal surface.

Descriptor for C2N-Supported Single-Cluster Catalysts in Bifunctional Oxygen Evolution and Reduction Reactions.

Developing highly active cluster catalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is significant for future renewable energy technology. Here, we employ first-principles calculations combined with a genetic algorithm to explore the activity trends of transition metal clusters supported on C2N. Our results indicate that the supported clusters, as bifunctional catalysts for the OER and the ORR, may outperform single-atom catalysts. In particular, the C2N-supported Ag6 cluster exhibits outstanding bifunctional activity with low overpotentials. Mechanistic analysis indicates that the activity of the cluster is related to the number of atoms in the active site as well as the interaction between the intermediate and the cluster. Accordingly, we identify a descriptor that links the intrinsic properties of the clusters with the activity of both the OER and the ORR. This work provides guidelines and strategies for the rational design of highly efficient bifunctional cluster catalysts.

Accelerated Synthesis of Nanolayered MWW Zeolite by Interzeolite Transformation.

Hierarchical zeolites can offer substantial benefits over bulk zeolites in catalysis. A drawback towards practical implementation is their lengthy synthesis, often requiring complex organic templates. This work describes an accelerated synthesis of nanolayered MWW zeolite based on the combination of interzeolite transformation (IZT) with a dual-templating strategy. FAU zeolite, hexamethyleneimine (HMI), and cetyltrimethylammonium bromide (CTAB) were respectively employed as Al source and primary zeolite, structure directing agent, and exfoliating agent. This approach allowed to reduce the synthesis of nanolayered MWW to 48 h, which is a considerable advance over the state of the art. Tracking structural, textural, morphological, and chemical properties during crystallization showed that 4-membered-ring (4MR) units derived from the FAU precursor are involved in the faster formation of MWW in comparison to a synthesis procedure from amorphous precursor. CTAB restricts the growth of the zeolite in the c-direction, resulting in nanolayered MWW. Moreover, we show that this approach can speed up the synthesis of nanolayered FER. The merits of nanolayered MWW zeolites are demonstrated in terms of improved catalytic performance in the Diels-Alder cycloaddition of 2,5-dimethylfuran and ethylene to p-xylene compared to bulk reference MWW sample.

Improving the performance of ASA in the DAC of 2,5-DMF and ethylene.

A variety of methods are employed to synthesize amorphous silica-alumina (ASA) to resolve the role of Al speciation and surface area in the catalytic performance in the Diels-Alder cycloaddition reaction of 2,5-dimethylfuran and ethylene to p-xylene. ASA was prepared by homogeneous deposition-precipitation (HDP) of Al3+ on ordered mesoporous silica, i.e., SBA-15 and OMS prepared under hydrothermal synthesis conditions using an imidazole-based template, and one-step flame spray pyrolysis (FSP). IR spectroscopy and 27Al MAS NMR showed that the resulting ASA represented a set of materials with distinct textural and acidic properties. ASA prepared by grafting Al to ordered mesoporous silica led to a much higher concentration of Brønsted acid sites (BAS). These samples performed much better in the DAC reaction, with p-xylene yields higher than those obtained with a HBeta zeolite benchmark. Materials with Al partially in the bulk of silica (OMS, FSP) and containing significant alumina domains are less acidic and exhibit much lower p-xylene yields. These findings point to the importance of Brønsted acidity for p-xylene formation. This study shows that careful design of the Al speciation can lead to amorphous silica-alumina with similar DAC performance to microporous zeolites.

Al Promotion of In2O3 for CO2 Hydrogenation to Methanol.

In2O3 is a promising catalyst for the hydrogenation of CO2 to methanol, relevant to renewable energy storage in chemicals. Herein, we investigated the promoting role of Al on In2O3 using flame spray pyrolysis to prepare a series of In2O3-Al2O3 samples in a single step (0-20 mol % Al). Al promoted the methanol yield, with an optimum being observed at an Al content of 5 mol %. Extensive characterization showed that Al can dope into the In2O3 lattice (maximum ∼ 1.2 mol %), leading to the formation of more oxygen vacancies involved in CO2 adsorption and methanol formation. The rest of Al is present as small Al2O3 domains at the In2O3 surface, blocking the active sites for CO2 hydrogenation and contributing to higher CO selectivity. At higher Al content (≥10 mol % Al), the particle size of In2O3 decreases due to the stabilizing effect of Al2O3. Nevertheless, these smaller particles are prone to sintering during CO2 hydrogenation since they appear to be more easily reduced. These findings show subtle effects of a structural promoter such as Al on the reducibility and texture of In2O3 as a CO2 hydrogenation catalyst.

Mild Hydrogenation of 2-Furoic Acid by Pt Nanoparticles Dispersed in a Hierarchical ZSM-5 Zeolite.

Hydrogenation of biobased compounds can add value to platform molecules obtained from biomass refining. Herein, we explore the hydrogenation of 2-furoic acid (2-furancarboxylic acid, FCA), a derivative of furfural, with H2 generated in situ by NaBH4 hydrolysis at ambient conditions. Nearly complete conversion of FCA was obtained with tetrahydrofuroic acid (THFA) and 5-hydroxyvaleric acid (5-HVA) as the only two reaction products over Pt nanoparticles supported on hierarchical ZSM-5. Small Pt nanoparticles (2 to 3 nm) were stabilized by ZSM-5 nanosheets. At an optimized Pt loading, the Pt nanoparticles can catalyze the hydrolysis of NaBH4 and the subsequent hydrogenation of FCA with the assistance of Brønsted acid sites. Nanostructuring ZSM-5 into nanosheets and its acidity contributes to the stability of the dispersed Pt nanoparticles. Deactivation due to NaBO2 deposition on the Pt particles can be countered by a simple washing treatment. Overall, this approach shows the promise of mild hydrogenation of biobased feedstock coupled with NaBH4 hydrolysis.

Structure Sensitivity of CO2 Hydrogenation on Ni Revisited.

Despite the large number of studies on the catalytic hydrogenation of CO2 to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO2 hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2-12 nm), we found that the active sites responsible for the conversion of CO2 to CO are different from those for the subsequent hydrogenation of CO to CH4. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO2 hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO2 hydrogenation and demonstrated that direct dissociation of CO2 to CO is followed by the conversion of strongly bonded carbonyls to CH4. These findings provide essential insights into the much debated structure sensitivity of CO2 hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts.

Pt/CeO2 as Catalyst for Nonoxidative Coupling of Methane: Oxidative Regeneration.

Direct nonoxidative coupling is a promising route for methane upgrading, yet its commercialization is hindered by the lack of efficient catalysts. Pt/CeO2 catalysts with isolated Pt species have attracted an increasing amount of interest in recent years. Herein, we studied the catalytic role and evolution of isolated Pt centers on CeO2 prepared by flame spray pyrolysis under the harsh reaction conditions of nonoxidative methane coupling. During the reaction at 800 °C, the isolated Pt sites sinter, leading to a loss of the ethylene and ethane yield. The agglomerated Pt can be redispersed by using an in situ regeneration strategy in oxygen. We found that isolated Pt centers are able to activate methane only at the initial reaction stage, and the CePt5 alloy acts as the active phase in the prolonged reaction.

Revealing Active Sites and Reaction Pathways in Methane Non-Oxidative Coupling over Iron-Containing Zeolites.

Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe3+ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe2+ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products.

Role of Strontium Cations in ZSM-5 Zeolite in the Methanol-to-Hydrocarbons Reaction.

The selectivity of the methanol-to-hydrocarbons (MTH) reaction can be tuned by modifying zeolite catalysts with alkaline earth metals, which typically increase propylene selectivity and catalyst stability. Here we employed Sr2+ as its higher atomic number in comparison to the zeolite T atoms facilitates characterization by scanning transmission electron microscopy and operando X-ray absorption spectroscopy. Sr2+ dispersed in the ZSM-5 micropores coordinates water, methanol, and dimethyl ether during the MTH reaction. Complementary characterization with nuclear magnetic resonance spectroscopy, thermogravimetric analysis combined with mass spectrometry, operando infrared spectroscopy, and X-ray diffraction points to the retention of substantially more adsorbates during the MTH reaction in comparison to Sr-free zeolites. Our findings support the notion that alkaline earth metals modify the porous reaction environment such that the olefin cycle is favored over the aromatic cycle in the MTH, explaining the increased propylene yield and lower deactivation rate.

Size of cerium dioxide support nanocrystals dictates reactivity of highly dispersed palladium catalysts.

The catalytic performance of heterogeneous catalysts can be tuned by modulation of the size and structure of supported transition metals, which are typically regarded as the active sites. In single-atom metal catalysts, the support itself can strongly affect the catalytic properties. Here, we demonstrate that the size of cerium dioxide (CeO2) support governs the reactivity of atomically dispersed palladium (Pd) in carbon monoxide (CO) oxidation. Catalysts with small CeO2 nanocrystals (~4 nanometers) exhibit unusually high activity in a CO-rich reaction feed, whereas catalysts with medium-size CeO2 (~8 nanometers) are preferred for lean conditions. Detailed spectroscopic investigations reveal support size-dependent redox properties of the Pd-CeO2 interface.

Optimizing Catalytic Depolymerization of Lignin in Ethanol with a Day-Clustered Box-Behnken Design.

Lignin is a potential resource for biobased aromatics with applications in the field of fuel additives, resins, and bioplastics. Via a catalytic depolymerization process using supercritical ethanol and a mixed metal oxide catalyst (CuMgAlOx), lignin can be converted into a lignin oil, containing phenolic monomers that are intermediates to the mentioned applications. Herein, we evaluated the viability of this lignin conversion technology through a stage-gate scale-up methodology. Optimization was done with a day-clustered Box-Behnken design to accommodate the large number of experimental runs in which five input factors (temperature, lignin-to-ethanol ratio, catalyst particle size, catalyst concentration, and reaction time) and three output product streams (monomer yield, yield of THF-soluble fragments, and yield of THF-insoluble fragments and char) were considered. Qualitative relationships between the studied process parameters and the product streams were determined based on mass balances and product analyses. Linear mixed models with random intercept were employed to study quantitative relationships between the input factors and the outcomes through maximum likelihood estimation. The response surface methodology study reveals that the selected input factors, together with higher order interactions, are highly significant for the determination of the three response surfaces. The good agreement between the predicted and experimental yield of the three output streams is a validation of the response surface methodology analysis discussed in this contribution.

Dual Atom Catalysts for Energy and Environmental Applications.

The pursuit of high metal utilization in heterogeneous catalysis has triggered the burgeoning interest of various atomically dispersed catalysts. Our aim in this review is to assess key recent findings in the synthesis, characterization, structure-property relationship and computational studies of dual-atom catalysts (DACs), which cover the full spectrum of applications in thermocatalysis, electrocatalysis and photocatalysis. In particular, combination of qualitative and quantitative characterization with cooperation with DFT insights, synergies and superiorities of DACs compare to counterparts, high-throughput catalyst exploration and screening with machine-learning algorithms are highlighted. Undoubtably, it would be wise to expect more fascinating developments in the field of DACs as tunable catalysts.

Ca Cations Impact the Local Environment inside HZSM-5 Pores during the Methanol-to-Hydrocarbons Reaction.

The methanol-to-hydrocarbons (MTH) process is an industrially relevant method to produce valuable light olefins such as propylene. One of the ways to enhance propylene selectivity is to modify zeolite catalysts with alkaline earth cations. The underlying mechanistic aspects of this type of promotion are not well understood. Here, we study the interaction of Ca2+ with reaction intermediates and products formed during the MTH reaction. Using transient kinetic and spectroscopic tools, we find strong indications that the selectivity differences between Ca/ZSM-5 and HZSM-5 are related to the different local environment inside the pores due to the presence of Ca2+. In particular, Ca/ZSM-5 strongly retains water, hydrocarbons, and oxygenates, which occupy as much as 10% of the micropores during the ongoing MTH reaction. This change in the effective pore geometry affects the formation of hydrocarbon pool components and in this way directs the MTH reaction toward the olefin cycle.

Anomalous Glide Plane in Platinum Nano- and Microcrystals.

At the nanoscale, the properties of materials depend critically on the presence of crystal defects. However, imaging and characterizing the structure of defects in three dimensions inside a crystal remain a challenge. Here, by using Bragg coherent diffraction imaging, we observe an unexpected anomalous {110} glide plane in two Pt submicrometer crystals grown by very different processes and having very different morphologies. The structure of the defects (type, associated glide plane, and lattice displacement) is imaged in these faceted Pt crystals. Using this noninvasive technique, both plasticity and unusual defect behavior can be probed at the nanoscale.

The Promoting Role of Ni on In2O3 for CO2 Hydrogenation to Methanol.

Ni-promoted indium oxide (In2O3) is a promising catalyst for the selective hydrogenation of CO2 to CH3OH, but the nature of the active Ni sites remains unknown. By employing density functional theory and microkinetic modeling, we elucidate the promoting role of Ni in In2O3-catalyzed CO2 hydrogenation. Three representative models have been investigated: (i) a single Ni atom doped in the In2O3(111) surface, (ii) a Ni atom adsorbed on In2O3(111), and (iii) a small cluster of eight Ni atoms adsorbed on In2O3(111). Genetic algorithms (GAs) are used to identify the optimum structure of the Ni8 clusters on the In2O3 surface. Compared to the pristine In2O3(111) surface, the Ni8-cluster model offers a lower overall barrier to oxygen vacancy formation, whereas, on both single-atom models, higher overall barriers are found. Microkinetic simulations reveal that only the supported Ni8 cluster can lead to high methanol selectivity, whereas single Ni atoms either doped in or adsorbed on the In2O3 surface mainly catalyze CO formation. Hydride species obtained by facile H2 dissociation on the Ni8 cluster are involved in the hydrogenation of adsorbed CO2 to formate intermediates and methanol. At higher temperatures, the decreasing hydride coverage shifts the selectivity to CO. On the Ni8-cluster model, the formation of methane is inhibited by high barriers associated with either direct or H-assisted CO activation. A comparison with a smaller Ni6 cluster also obtained with GAs exhibits similar barriers for key rate-limiting steps for the formation of CO, CH4, and CH3OH. Further microkinetic simulations show that this model also has appreciable selectivity to methanol at low temperatures. The formation of CO over single Ni atoms either doped in or adsorbed on the In2O3 surface takes place via a redox pathway involving the formation of oxygen vacancies and direct CO2 dissociation.

Asymmetrical electrohydrogenation of CO2 to ethanol with copper-gold heterojunctions.

Copper is distinctive in electrocatalyzing reduction of CO2 into various energy-dense forms, but it often suffers from limited product selectivity including ethanol in competition with ethylene. Here, we describe systematically designed, bimetallic electrocatalysts based on copper/gold heterojunctions with a faradaic efficiency toward ethanol of 60% at currents in excess of 500 mA cm-2. In the modified catalyst, the ratio of ethanol to ethylene is enhanced by a factor of 200 compared to copper catalysts. Analysis by ATR-IR measurements under operating conditions, and by computational simulations, suggests that reduction of CO2 at the copper/gold heterojunction is dominated by generation of the intermediate OCCOH*. The latter is a key contributor in the overall, asymmetrical electrohydrogenation of CO2 giving ethanol rather than ethylene.