Neutron Scattering Studies of Heterogeneous Catalysis.

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

Synthesis of Perdeuterated Alkyl Amines/Amides with Pt/C as Catalyst under Mild Conditions.

A convenient method for the synthesis of perdeuterated alkyl amides/amines is disclosed. Perdeuterated acetyl amides can be achieved by a hydrogen-deuterium (H/D) exchange protocol with Pt/C as a catalyst and D2O as a deuterium source under mild conditions. After removal or reduction of the acetyl group, this protocol can provide perdeuterated primary, secondary, and tertiary amines, which are difficult to achieve via other methods.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8 H0.2 Catalyst for CO2 Hydrogenation to Methanol.

Tuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal-support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3-fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8 H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8 H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High-pressure steady-state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8 H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3 . An operando high-pressure diffuse reflectance infrared spectroscopy (DRIFTS)-SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate-limiting. However, the catalytic reactivity of Cu/BaTiO2.8 H0.2 towards the formate species is much higher than Cu/BaTiO3 , likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Hydrogen-mediated polarity compensation on the (110) surface terminations of ABO3 perovskites.

Polar surfaces undergo polarity compensation, which can lead to significantly different surface chemistry from their nonpolar counterparts. This process in turn can substantially alter the binding of adsorbates on the surface. Here, we find that hydrogen binds much more strongly to the polar (110) surface than the nonpolar (100) surface for a wide range of ABO3 perovskites, forming a hydroxyl layer on the O24- termination and a hydride layer on the ABO4+ termination of the (110) surface. The stronger adsorption on the polar surfaces can be explained by polarity compensation: hydrogen atoms can act as electron donors or acceptors to compensate for the polarity of perovskite surfaces. The relative stability of the surface terminations is further compared under different gas environments and several perovskites have been found to form stable surface hydride layers under oxygen-poor conditions. These results demonstrate the feasibility of creating stable surface hydrides on perovskites by polarity compensation which might lead to new hydrogenation catalysts based on ABO3 perovskites.

Boosting the Activity of Pd Single Atoms by Tuning Their Local Environment on Ceria for Methane Combustion.

Supported Pd single atom catalysts (SACs) have triggered great research interest in methane combustion yet with contradicting views on their activity and stability. Here, we show that the Pd SAs can take different electronic structure and atomic geometry on ceria support, resulting in different catalytic properties. By a simple thermal pretreatment to ceria prior to Pd deposition, a unique anchoring site is created. The Pd SA, taking this site, can be activated to Pdδ+ (0<δ<2) that has greatly enhanced activity for methane oxidation: T50 lowered by up to 130 °C and almost 10 times higher turnover frequency compared to the untreated catalyst. The enhanced activity of Pdδ+ site is related to its oxygen-deficient local structure and elongated interacting distance with ceria, leading to enhanced capability in delivering reactive oxygen species and decomposing reaction intermediates. This work provides insights into designing highly efficient Pd SACs for oxidation reactions.

Engineering a Self-Grown TiO2 /Ti-MOF Heterojunction with Selectively Anchored High-Density Pt Single-Atomic Cocatalysts for Efficient Visible-Light-Driven Hydrogen Evolution.

A photocatalyst TiO2 /Ti-BPDC-Pt is developed with a self-grown TiO2 /Ti-metal-organic framework (MOF) heterojunction, i.e., TiO2 /Ti-BPDC, and selectively anchored high-density Pt single-atomic cocatalysts on Ti-BPDC for photocatalytic hydrogen evolution. This intimate heterojunction, growing from the surface pyrolytic reconstruction of Ti-BPDC, works in a direct Z-scheme, efficiently separating electrons and holes. Pt is selectively anchored on Ti-BPDC by ligands and is found in the form of single atoms with loading up to 1.8 wt %. The selective location of Pt is the electron-enriched domain of the heterojunction, which further enhances the utilization of the separated electrons. This tailored TiO2 /Ti-BPDC-Pt shows a significantly enhanced activity of 12.4 mmol g-1 h-1 compared to other TiO2 - or MOF-based catalysts. The structure-activity relationship further proves the balance of two simultaneously exposed domains of heterojunctions is critical to fulfilling this kind of catalyst.

Ultrasonication-Induced Strong Metal-Support Interaction Construction in Water Towards Enhanced Catalysis.

The development of facile methodologies to afford robust supported metal nanocatalysts under mild conditions is highly desirable yet challenging, particularly via strong metal-support interactions (SMSI) construction. State-of-the-art approaches capable of generating SMSI encapsulation mainly focus on high temperature annealing in reductive/oxidative atmosphere. Herein, ultra-stable metal nanocatalysts based on SMSI construction were produced by leveraging the instantaneous high-energy input from ultrasonication under ambient conditions in H2 O, which could rapidly afford abundant active intermediates, Ti3+ ions, and oxygen vacancies within the scaffolds to induce the SMSI overlayer formation. The encapsulation degree could be tuned and controlled via the reducibility of the solvents and the ultrasonication parameters. This facile and efficient approach could be further extended to diverse metal oxide supports and noble metal NPs leading to enhanced performance in hydrogenation reactions and CO2 conversion.

Construction of Boron- and Nitrogen-Enriched Nanoporous π-Conjugated Networks Towards Enhanced Hydrogen Activation.

Boron-enriched scaffolds have demonstrated unique features and promising performance in the field of catalysis towards the activation of small gas molecules. However, there is still a lack of facile approaches capable of achieving high B doping and abundant porous channels in the targeted catalysts. Herein, construction of boron- and nitrogen-enriched nanoporous π-conjugated networks (BN-NCNs) was achieved via a facile ionothermal polymerization procedure with hexaazatriphenylenehexacarbonitrile [HAT(CN)6 ] sodium borohydride as the starting materials. The as-produced BN-NCN scaffolds were featured by high heteroatoms doping (B up to 23 wt. % and N: up to 17 wt. %) and permanent porosity (surface area up to 759 m2 g-1 mainly contributed by micropores). With the unsaturated bonded B species acting as the active Lewis acid sites and defected N species acting as the active Lewis base sites, those BN-NCNs delivered attractive catalytic performance towards H2 activation/dissociation in both gaseous and liquid phase, acting as efficient metal-free heterogeneous frustrated Lewis pairs (FLPs) catalysts in hydrogenation procedures.

Defect-Regulated Frustrated-Lewis-Pair Behavior of Boron Nitride in Ambient Pressure Hydrogen Activation.

The construction of heterogeneous frustrated Lewis pairs (FLPs) with performance comparable to or surpassing the homogeneous counterparts in H2 activation is a long-standing challenge. Herein, sterically hindered Lewis acid ("B" center) and Lewis base ("N" center) sites were anchored within the rigid lattice of highly crystalline hexagonal boron nitride (h-BN) scaffolds. The active sites were created via precision defect regulation during the molten-salt-involved (NaNH2 and NaBH4) h-BN construction procedure. The as-afforded h-BN scaffolds achieved highly efficient H2/D2 activation and dissociation under ambient pressure via FLP-like behavior, and attractive catalytic efficiency in hydrogenation reactions surpassing the current heterogeneous analogues.

Ammonia synthesis on BaTiO2.5H0.5: computational insights into the role of hydrides.

Perovskite oxyhydrides such as BaTiO2.5H0.5 have been found to be able to catalyze NH3 synthesis, but the mechanism and the role of the catalyst's lattice hydrides in the catalytic reaction remain unknown. Here we employ first principles density functional theory to investigate the mechanism of ammonia synthesis and the role of lattice hydrides on a prototypical perovskite oxyhydride, BaTiO2.5H0.5 (BTOH). Two mechanistic hypotheses, the distal and alternating pathways, have been tested on the Ti2O2 termination of the BTOH (210) surface, previously determined to be the most stable surface termination under the reaction conditions considered. In the distal pathway, H atoms hydrogenate N2 to form the *N-NHx key intermediates, followed by N-N bond breaking. In the alternating pathway, H atoms hydrogenate N2 in an alternating fashion to form the *NHx-NHy intermediates before N-N bond breaking and formation of co-adsorbed *NHx/*NHy on the surface. We find that the subsurface hydride vacancy formed after reaction of *N2 with the lattice hydride is key to the distal pathway, leading to surface nitride formation after breaking the *N-NH3 bond, while the neighboring surface Ti sites are key to bridging and stabilizing the *NNH intermediate in the alternating pathway. In both pathways, desorption of NH3 is the most uphill in energy. Our results provide important insights into the role of hydrides and surface vacancies in hydrogenation reactions over BTOH, which will be useful to guide future spectroscopic experiments such as operando IR and inelastic neutron scattering to verify the key intermediates.

Correction: The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization.

Correction for 'The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization' by Ding Ma et al., Phys. Chem. Chem. Phys., 2005, 7, 3102-3109, https://doi.org/10.1039/B502794B.

Photoinduced Strong Metal-Support Interaction for Enhanced Catalysis.

Strong metal-support interaction (SMSI) construction is a pivotal strategy to afford thermally robust nanocatalysts in industrial catalysis, but thermally induced reactions (>300 °C) in specific gaseous atmospheres are generally required in traditional procedures. In this work, a photochemistry-driven methodology was demonstrated for SMSI construction under ambient conditions. Encapsulation of Pd nanoparticles with a TiOx overlayer, the presence of Ti3+ species, and suppression of CO adsorption were achieved upon UV irradiation. The key lies in the generation of separated photoinduced reductive electrons (e-) and oxidative holes (h+), which subsequently trigger the formation of Ti3+ species/oxygen vacancies (Ov) and then interfacial Pd-Ov-Ti3+ sites, affording a Pd/TiO2 SMSI with enhanced catalytic hydrogenation efficiency. The as-constructed SMSI layer was reversible, and the photodriven procedure could be extended to Pd/ZnO and Pt/TiO2.

Deep Learning Accelerated Determination of Hydride Locations in Metal Nanoclusters.

Although the coordinates of the metal atoms can be accurately determined by X-ray crystallography, locations of hydrides in metal nanoclusters are challenging to determine. In principle, neutron crystallography can be employed to pinpoint the hydride positions, but it requires a large crystal and a neutron source, which prevents its routine use. Here, we present a deep-learning approach that can accelerate determination of hydride locations in single-crystal X-ray structure of metal nanoclusters of different sizes. We demonstrate the efficiency of our method in predicting the most probable hydride sites and their combinations to determine the total structure for two recently reported copper nanoclusters, [Cu25 H10 (SPhCl2 )18 ]3- and [Cu61 (St Bu)26 S6 Cl6 H14 ]+ whose hydride locations have not been determined by neutron diffraction. Our method can be generalized and applied to other metal systems, thereby eliminating a bottleneck in atomically precise metal hydride nanochemistry.

Nature of Reactive Hydrogen for Ammonia Synthesis over a Ru/C12A7 Electride Catalyst.

Recently, there have been renewed interests in exploring new catalysts for ammonia synthesis under mild conditions. Electride-based catalysts are among the emerging ones. Ruthenium particles supported on an electride composed of a mixture of calcium and aluminum oxides (C12A7) have attracted great attention for ammonia synthesis due to their facile ability in activating N2 under ambient pressure. However, the exact nature of the reactive hydrogen species and the role of electride support still remain elusive for this catalytic system. In this work, we report for the first time that the surface-adsorbed hydrogen, rather than the hydride encaged in the C12A7 electride, plays a major role in ammonia synthesis over the Ru/C12A7 electride catalyst with the aid of in situ neutron scattering techniques. Combining in situ neutron diffraction, inelastic neutron spectroscopy, density functional theory (DFT) calculation, and temperature-programmed reactions, the results provide direct evidence for not only the presence of encaged hydrides during ammonia synthesis but also the strong thermal and chemical stability of the hydride species in the Ru/C12A7 electride. Steady state isotopic transient kinetic analysis (SSITKA) of ammonia synthesis showed that the coverage of reactive intermediates increased significantly when the Ru particles were promoted by the electride form (coverage up to 84%) of the C12A7 support rather than the oxide form (coverage up to 15%). Such a drastic change in the intermediate coverage on the Ru surface is attributed to the positive role of electride support where the H2 poisoning effect is absent during ammonia synthesis over Ru. The finding of this work has significant implications for understanding catalysis by electride-based materials for ammonia synthesis and hydrogenation reactions in general.

Radical Chemistry and Reaction Mechanisms of Propane Oxidative Dehydrogenation over Hexagonal Boron Nitride Catalysts.

Although hexagonal boron nitride (h-BN) has recently been identified as a highly efficient catalyst for the oxidative dehydrogenation of propane (ODHP) reaction, the reaction mechanisms, especially regarding radical chemistry of this system, remain elusive. Now, the first direct experimental evidence of gas-phase methyl radicals (CH3 . ) in the ODHP reaction over boron-based catalysts is achieved by using online synchrotron vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS), which uncovers the existence of gas-phase radical pathways. Combined with density functional theory (DFT) calculations, the results demonstrate that propene is mainly generated on the catalyst surface from the C-H activation of propane, while C2 and C1 products can be formed via both surface-mediated and gas-phase pathways. These observations provide new insights towards understanding the ODHP reaction mechanisms over boron-based catalysts.

Elucidation of the Reaction Mechanism for High-Temperature Water Gas Shift over an Industrial-Type Copper-Chromium-Iron Oxide Catalyst.

The water gas shift (WGS) reaction is of paramount importance for the chemical industry, as it constitutes, coupled with methane reforming, the main industrial route to produce hydrogen. Copper-chromium-iron oxide-based catalysts have been widely used for the high-temperature WGS reaction industrially. The WGS reaction mechanism by the CuCrFeO x catalyst has been debated for years, mainly between a "redox" mechanism involving the participation of atomic oxygen from the catalyst and an "associative" mechanism proceeding via a surface formate-like intermediate. In the present work, advanced in situ characterization techniques (infrared spectroscopy, temperature-programmed surface reaction (TPSR), near-ambient pressure XPS (NAP-XPS), and inelastic neutron scattering (INS)) were applied to determine the nature of the catalyst surface and identify surface intermediate species under WGS reaction conditions. The surface of the CuCrFeO x catalyst is found to be dynamic and becomes partially reduced under WGS reaction conditions, forming metallic Cu nanoparticles on Fe3O4. Neither in situ IR not INS spectroscopy detect the presence of surface formate species during WGS. TPSR experiments demonstrate that the evolution of CO2 and H2 from the CO/H2O reactants follows different kinetics than the evolution of CO2 and H2 from HCOOH decomposition (molecule mimicking the associative mechanism). Steady-state isotopic transient kinetic analysis (SSITKA) (CO + H216O → CO + H218O) exhibited significant 16O/18O scrambling, characteristic of a redox mechanism. Computed activation energies for elementary steps for the redox and associative mechanism by density functional theory (DFT) simulations indicate that the redox mechanism is favored over the associative mechanism. The combined spectroscopic, computational, and kinetic evidence in the present study finally resolves the WGS reaction mechanism on the industrial-type high-temperature CuCrFeO x catalyst that is shown to proceed via the redox mechanism.

An overview of photocatalysis facilitated by 2D heterojunctions.

Two-dimensional (2D) photocatalysts have attracted considerable research interest in the past decades due to their unique optical, physical and chemical properties. Constructing 2D/2D heterojunctions with large interface area has been considered as an effective approach to enhance the transfer rate and the separation efficiency of the charge carriers, leading to dramatic increase in the photocatalytic performance of the photocatalysts. Here, the state-of-the-art progress on heterojunctions based on 2D materials is reviewed, including the photocatalysis principles using 2D heterojunctions, the categories of 2D heterojunctions and their application in different photocatalytic reactions, and the theoretical studies of the 2D heterojunctions. Moreover, the advantages and disadvantages of the 2D heterojunctions are also discussed. Finally, the ongoing challenges and opportunities for the future development of 2D photocatalysts with built-in heterojunctions are proposed.

Tetrahydrofolate Modulates Floral Transition through Epigenetic Silencing.

Folates, termed from tetrahydrofolate (THF) and its derivatives, function as coenzymes in one-carbon transfer reactions and play a central role in synthesis of nucleotides and amino acids. Dysfunction of cellular folate metabolism leads to serious defects in plant development; however, the molecular mechanisms of folate-mediated cellular modifications and physiological responses in plants are still largely unclear. Here, we reported that THF controls flowering time by adjusting DNA methylation-regulated gene expression in Arabidopsis (Arabidopsis thaliana). Wild-type seedlings supplied with THF as well as the high endogenous THF content mutant dihydrofolate synthetase folypoly-Glu synthetase homolog B exhibited significant up-regulation of the flowering repressor of Flowering Wageningen and thereby delaying floral transition in a dose-dependent manner. Genome-wide transcripts and DNA methylation profiling revealed that THF reduces DNA methylation so as to manipulate gene expression activity. Moreover, in accompaniment with elevated cellular ratios between monoglutamylated and polyglutamylated folates under increased THF levels, the content of S-adenosylhomo-Cys, a competitive inhibitor of methyltransferases, was obviously higher, indicating that enhanced THF accumulation may disturb cellular homeostasis of the concerted reactions between folate polyglutamylation and folate-dependent DNA methylation. In addition, we found that the loss-of-function mutant of CG DNA methyltransferase MET1 displayed much less responsiveness to THF-associated flowering time alteration. Taken together, our studies revealed a novel regulatory role of THF on epigenetic silencing, which will shed lights on the understanding of interrelations in folate homeostasis, epigenetic variation, and flowering control in plants.

The Antibacterial Mechanism of Terpinen-4-ol Against Streptococcus agalactiae.

Streptococcus agalactiae, a highly contagious mastitis pathogen, caused huge economic losses; meanwhile, repeated use of antibiotics results in the emergence of serious antibiotic residues and drug resistance. Therefore, it is in great need to develop ecologically sustainable antimicrobial agents. In the study, the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), and action mechanism of terpinen-4-ol against S. agalactiae was investigated to evaluate antibacterial activity of terpinen-4-ol. Results showed the MIC and MBC of terpinen-4-ol were 98 and 196 µg/mL, respectively. Time-kill curves displayed that the antibacterial activity of terpinen-4-ol was in a concentration-dependent manner. Transmission electron micrographs showed that the cell membrane and wall of S. agalactiae were damaged, and plasmolysis and chromatins were inconspicuous. Release of Ca2+ and Mg2+ proved that terpinen-4-ol could increase cell membrane permeability. And the release of lactate dehydrogenase (LDH) suggested that cell wall was destroyed. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and 4',6-diamidino-2-phenylindole (DAPI) staining results showed that terpinen-4-ol could affect the synthesis of protein and DNA. These results suggested that terpinen-4-ol might be used as candidate for treating S. agalactiae infection.

Nature of Active Sites and Surface Intermediates during SCR of NO with NH3 by Supported V2O5--WO3/TiO2 Catalysts.

Time-resolved in situ IR was performed during selective catalytic reduction of NO with NH3 on supported V2O5-WO3/TiO2 catalysts to examine the distribution and reactivity of surface ammonia species on Lewis and Brønsted acid sites. While both species were found to participate in the SCR reaction, their relative population depends on the coverage of the surface vanadia and tungsta sites, temperature, and moisture. Although the more abundant surface NH4+,ads intermediates dominate the overall SCR reaction, especially for hydrothermally aged catalysts, the minority surface NH3,ads intermediates exhibit a higher specific SCR activity (TOF). The current study serves to resolve the long-standing controversy about the active sites for SCR of NO with NH3 by supported V2O5-WO3/TiO2 catalysts.