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As a visible-light response photocatalyst, BiVO4 is widely used in photocatalytic oxygen evolution. In this study, a novel BiVO4−Bi6O6(OH)3(NO3)3 (BBN) heterostructure fabricates via a simple one-pot hydrothermal approach is certified to effectively restrain the recombination of carriers by efficient spatial charge separation. By employing BBN as a reductive-function photocatalyst, a solid-state Z-scheme is constructed to improve the photo-redox capacity of BiVO4 and hydrogen production is realized in the BiVO4−BBN heterostructure for the first time. The solid-state Z-scheme introduced in the BiVO4−BBN ensures the photoexcited carriers with the powerful redox capacity to participate in the photocatalytic reaction.

Plasma-assisted catalysis is an emerging technology for the atmospheric pressure and low bulk gas temperature synthesis of ammonia from molecular nitrogen and hydrogen. Direct evidence for plasma-i...

Ethanol is an important C2 platform molecule for producing value-added chemicals and distillate hydrocarbon fuels (e.g., jet and diesel). Among these, catalytic upgrading of ethanol to butenes can ...

Activation and surface reactions of CO and H2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions were (quasi) in situ studied using temperature programmed surface reaction spectra, diffuse reflectance Fourier transform infrared spectroscopy, inelastic neutron scattering spectroscopy and electron paramagnetic resonance. CO undergoes disproportion reaction to produce gaseous CO2 and surface carbon adatoms, and adsorbs to form surface formate species. H2 adsorption forms dominant irreversibly-adsorbed surface hydroxyl groups and interstitial H species and very minor surface Zn-H species. Surface formate species and hydroxyl groups react to produce CO2 and H2, while surface carbon adatoms are hydrogenated by surface Zn-H species sequentially to produce CH(a), CH2(a), CH3(a) and eventually gaseous CH4. The ZnO nanoplates, exposing a higher fraction of Zn-ZnO(0001) and O-ZnO(000–1) polar facets, are more active than the ZnO powders to catalyze CO hydrogenation to CH4. These results provide fundamental understanding of the reaction mechanisms and structural effects of CO hydrogenation reaction catalyzed by ZnO-based catalysts.

Summary Toward the preparation of industrial metal oxide catalysts, sacrificial organic templates, excessive solvents, complex impregnation, and drying steps are generally required. Here, we report a versatile rule for the simple synthesis of highly porous metal oxides with well-dispersed noble metal species. Porous metal oxides (Co3O4, FexOy, and Cr2O3) are obtained with some surface areas (e.g., Cr2O3: 224 m2·g−1) beyond the record value. Surprisingly, small noble metal nanoparticles (e.g., Pd: 3.1 and Pt: 3.2 nm) could be incorporated by this solid-state process simultaneously. Corresponding Rh-Co3O4, Pd-FexOy, and Pt-Cr2O3 exhibit excellent performance: CH4 combustion (T90 = ∼360°C and thermal stability: >100 h at 680°C), hydrogenation of nitrobenzene and derivatives (turnover number [TON] = 2.49 × 104, 300 mmol per run), and reversed water gas shift (RWGS) reaction (44% CO2 conversion with ∼98% CO selectivity and thermal stability: >100 h at 500°C), respectively. Therefore, current principle via a NaCl-based solid solution could provide a solid-state, fast, and efficient route for processing metal oxide catalysts.

It is highly desirable to convert bioethanol to value-added chemicals. As such, conversion of ethanol to propene (ETP) is attractive because propene is an important raw material for the production of plastics. In2O3 has shown promising catalytic performance for ETP conversion. However, the underlying mechanisms remain elusive. In this work, we use density functional theory (DFT) to investigate ETP reaction pathways on the In2O3 (110) surface. We find that the ETP reactions proceed through three major stages: ethanol to acetaldehyde, acetaldehyde to acetone, and acetone to propene. The ethanol-to-acetaldehyde step is kinetically facile. Comparing the two pathways from acetaldehyde to acetone, we show that the aldol reaction pathway via direct coupling of two acetaldehyde is more favorable than the acetate-ketonization pathway. The acetone-to-propene process is found to be the rate-limiting step of the overall reaction. This work provides a detailed mechanistic view of the ETP chemistry on In2O3(110) that paves the way for further exploration of effects such as surface termination, surface doping, and co-feeding of H2 and H2O on selectivity and catalyst stability.

Cerium oxide (ceria) has found a wide variety of applications in catalysis including as a catalyst, a modifier, or a support, largely thanks to its robust redox properties and versatile acid-base function. While it is often utilized for oxidation reactions, ceria has recently attracted intense research interest for its unusual ability to selectively hydrogenate alkynes to alkenes. The intriguing hydrogenation ability of ceria has sparked renewed research efforts to understand how pure ceria works as a hydrogenation catalyst. In this review, recent advances in both experimental and computational studies of ceria are summarized, focusing on the interaction of ceria with H2 and in hydrogenation reactions. Significant insights from various studies including in situ spectroscopy/microscopy and theoretic modeling of ceria in hydrogen-involved reactions are discussed, which shed light on the origin of the hydrogenation ability of ceria and the reaction mechanisms involved in ceria-catalyzed alkyne hydrogenation. Ways to further improve both the mechanistic understanding and catalytic performance of ceria-based materials for hydrogenation reactions are proposed at the end in the summary and outlook section.

Two-dimensional (2D)-structured photocatalysts with atomically thin layers not only have the potential to enhance hydrogen generation efficiency but also allow more direct investigations of the eff...

ABO3 perovskites are increasingly being explored as catalysts, but it is unclear how they behave as supports for single atoms and how the subsequent single-atom catalysts can be employed for important reactions such as methane activation. Here we examine the stability of Pt single atoms (Pt1) on the commonly exposed (100) surfaces of SrBO3 perovskites (B = 3d transition metals) and their methane-adsorption properties by first principles density functional theory. We find that the stability and charge state of Pt1 on the SrBO3(100) surfaces are termination-sensitive. Due to polar compensation, Pt1 is negatively charged on the A termination but positively charged on the B termination. This charge state greatly impacts methane adsorption: negatively charged Pt1 on the A-termination chemisorbs methane (in some cases, dissociatively), but positively charged Pt1 on the B-termination adsorbs methane physically. Analysis of the density of states of the negatively charged Pt1 reveals that its sp states are key to methane chemisorption and C–H activation. Our work shows that polar compensation on the perovskite surfaces can be used to tune the charge state of a single atom for methane chemisorption and C–H activation.

The nonpolar a-plane Mg-delta-doped Al0.6Ga0.4N/GaN superlattices (SLs) with enhanced hole concentration were successfully grown by using indium (In)-surfactant with metal organic chemical vapor deposition (MOCVD) technology. The effect of In-surfactant on the characteristics of the nonpolar a-plane Mg-doped AlGaN/GaN SLs were studied with scanning electron microscopy, atomic force microscopy, X-ray diffraction, and Hall effect measurements. It was found that the surface morphology was improved evidently and the hole concentration was enhanced effectively with the use of In-surfactant. In fact, a root-mean-square value as small as 0.8 nm and a hole concentration as high as 5.1 × 1017 cm-3 were achieved by carefully optimizing the TMIn mole flow rate in the MOCVD growth process. Moreover, it was inferred that the enhancement in hole concentration was due to the significant increase in the Mg incorporation efficiency and the decrease in the self-compensation effect induced by the proper usage of In-surfactant.

As petroleum-related products dominate the global fuel market, acid gas resistant materials are in demand. ZnO is one of the widely used materials as SO2 sorbent and catalyst/catalyst support. To u...

Ethane, the second major component of shale gas, is a prospective raw feedstock to valuable chemicals and fuels. Innovative pillared ZSM-5 with various lamellar thicknesses and Si/Al ratios were su...

The effects of the modulation in the NH3 flow duty-ratio in a three-way pulsed-flow epitaxial growth process on the structural and optical properties of non-polar a-plane Al0.34Ga0.66N epi-layers were studied intensively. It was revealed that the typical pyramidal defects originated from the anisotropy in growth rate could be evidently reduced with an optimized NH3 flow duty-ratio. In fact, the root-mean-square value determined with atomic force microscope was decreased from 12.8 to 3.1 nm when the NH3 flow duty-ratio was increased from 0 to 0.57. Moreover, the LO-phonon-assisted exciton emission observed in the photoluminescence (PL) spectrum was remarkably suppressed, and the linewidth of the near band edge PL emission peak was decreased by 47%, implying the significantly enhanced optical properties of the non-polar a-plane Al0.34Ga0.66N epi-layer.

Styrene is an important commodity chemical that is highly energy and CO2 intensive to produce. We report a redox oxidative dehydrogenation (redox-ODH) strategy to efficiently produce styrene. Facilitated by a multifunctional (Ca/Mn)1−xO@KFeO2 core-shell redox catalyst which acts as (i) a heterogeneous catalyst, (ii) an oxygen separation agent, and (iii) a selective hydrogen combustion material, redox-ODH auto-thermally converts ethylbenzene to styrene with up to 97% single-pass conversion and >94% selectivity. This represents a 72% yield increase compared to commercial dehydrogenation on a relative basis, leading to 82% energy savings and 79% CO2 emission reduction. The redox catalyst is composed of a catalytically active KFeO2 shell and a (Ca/Mn)1−xO core for reversible lattice oxygen storage and donation. The lattice oxygen donation from (Ca/Mn)1−xO sacrificially stabilizes Fe3+ in the shell to maintain high catalytic activity and coke resistance. From a practical standpoint, the redox catalyst exhibits excellent long-term performance under industrially compatible conditions., Styrene is an important commodity chemical that is highly energy and CO2 intensive to produce. Here, authors report a redox-oxidative dehydrogenation scheme and a tailored core-shell redox catalyst to convert ethylbenzene to styrene with up to 91.4% single-pass yield and 82% energy savings.

Engineering strong metal–support interactions (SMSI) is an effective strategy for tuning structures and performances of supported metal catalysts but induces poor exposure of active sites. Here, we demonstrate a strong metal–support interaction via a reverse route (SMSIR) by starting from the final morphology of SMSI (fully-encapsulated core–shell structure) to obtain the intermediate state with desirable exposure of metal sites. Using core–shell nanoparticles (NPs) as a building block, the Pd–FeOx NPs are transformed into a porous yolk–shell structure along with the formation of SMSIR upon treatment under a reductive atmosphere. The final structure, denoted as Pd–Fe3O4–H, exhibits excellent catalytic performance in semi-hydrogenation of acetylene with 100% conversion and 85.1% selectivity to ethylene at 80 °C. Detailed electron microscopic and spectroscopic experiments coupled with computational modeling demonstrate that the compelling performance stems from the SMSIR, favoring the formation of surface hydrogen on Pd instead of hydride., Strong metal–support interactions (SMSI) are effective in tuning the structures and catalytic performances of catalysts but limited by the poor exposure of active sites. Here, the authors develop a strategy to engineer SMSI via a reverse route, which is in favor of metal site exposure while embracing the SMSI.

The nonpolar a-plane Mg-doped p-type AlGaN/GaN distributed Bragg reflectors (DBRs) were grown for the first time with metal organic chemical vapor deposition technology. The structural, optical, and electrical properties of the nonpolar p-DBRs were characterized with various kinds of technical tools. The measurement results demonstrate that a stopband centered at 390 nm with a peak reflectivity of 55% was achieved. Meanwhile, a relatively high reflectivity (>35%) could also be obtained within the wavelength range of 270–320 nm for the nonpolar p-DBR which was intended to be a reflector with a working wavelength at 280 nm. Additionally, a hole concentration higher than 2 × 1017 cm−3 was obtained with the nonpolar p-DBRs.

CO oxidation is an important reaction both experimentally and industrially, and its performance is usually dominated by the charge states of catalysts. For example, CO oxidation on the platinum (Pt) surface requires a properly charged state for the balance of adsorption and activation of CO and O2. Here, we present “Mott–Schottky modulated catalysis” on Pt nanoparticles (NPs) via an electron-donating carbon nitride (CN) support with a tunable Fermi level. We demonstrate that properly-charged Pt presents an excellent catalytic CO oxidation activity with an initial conversion temperature as low as 25 °C and total CO conversion below 85 °C. The tunable electronic structure of Pt NPs, which is regulated by the Fermi level of CN, is a key factor in dominating the catalytic performance. This “Mott–Schottky modulated catalysis” concept may be extended to maneuver the charge state on other metal catalysts for targeted catalytic reactions.

The semi-polar (11 2 ¯ 2) plane Al0.3Ga0.7N epi-layers with different background electron concentrations were deposited on (10 1 ¯ 0) m-plane sapphire substrates with metal-organic chemical vapor deposition technology, and characterized with Hall effect measurement, photoluminescence spectroscopy, high-resolution X-ray diffraction, and atomic force microscopy. It was found that there is a strong positive correlation between the background electron concentrations and the anisotropy in two-dimensional (2D)-growth rate of the triangular structures on the surface of semi-polar Al0.3Ga0.7N epi-layers. That is to say, the anisotropy in the 2D-growth rate is indeed the crucial influential factor on the background electron concentration. In fact, by carefully suppressing the anisotropy in 2D-growth rate, the background electron concentration in semi-polar Al0.3Ga0.7N epi-layers was reduced successfully from 1.38 × 1019 cm−3 to 4.20 × 1015 cm−3. This work should open a new way to realize the p-type semi-polar AlGaN epi-layers, which is of great significance for the manufacturing of semi-polar AlGaN-based DUV-LEDs.

Photocatalytic hydrogen evolution from water has received enormous attention due to its ability to address a number of global environmental and energy-related issues. Here, we synthesize 2D/2D Ti3C2/g-C3N4 composites by electrostatic self-assembly technique and demonstrate their use as photocatalysts for hydrogen evolution under visible light irradiation. The optimized Ti3C2/g-C3N4 composite exhibited a 10 times higher photocatalytic hydrogen evolution performance (72.3 μmol h-1 gcat-1) than that of pristine g-C3N4 (7.1 μmol h-1 gcat-1). Such enhanced photocatalytic performance was due to the formation of 2D/2D heterojunctions in the Ti3C2/g-C3N4 composites. The intimate contact between the monolayer Ti3C2 and g-C3N4 nanosheets promotes the separation of photogenerated charge carriers at the Ti3C2/g-C3N4 interface. Furthermore, the ultrahigh conductivity of Ti3C2 and the Schottky junction formed between g-C3N4/MXene interfaces facilitate the photoinduced electron transfer and suppress the recombination with photogenerated holes. This work demonstrates that the 2D/2D Ti3C2/g-C3N4 composites are promising photocatalysts thanks to the ultrathin MXenes as efficient co-catalysts for photocatalytic hydrogen production.

Tailoring the atomic structural configuration at metal and oxide interface offers an effective route for the development of catalysts with optimized properties. Here, we report the design of a unique structural configuration of yolk-shell-like FePt-FeOx nanoparticles (NPs), that exhibits notably enhanced activity and stability towards CO oxidation at relatively low temperatures (

The nonpolar a-plane p-AlxGa1-xN/GaN superlattices (SLs) were successfully grown with metal organic chemical vapor deposition technology. The surface morphology, carrier concentration, layer thickness, and Al composition for the nonpolar a-plane p-AlxGa1-xN/GaN SLs were characterized by various kinds of technical tools. It was demonstrated that the thickness of the AlxGa1-xN and GaN layers in SLs was calculated to be 12 and 14.5 nm, respectively and the Al composition of the AlxGa1-xN layers in SLs was around 30%. Meanwhile, the results reveal that the surface morphology and carrier concentration of the nonpolar p-SLs could be improved remarkably with the Al composition-graded AlGaN buffer layer and the Mg-δ-doping process. Consequently, a root mean square value of 0.6 nm and a hole concentration of 4.2 × 1017 cm−3 were achieved.

Methane conversion has received renewed interest due to the rapid growth in production of shale gas. Methane combustion for power generation and transportation is one of the alternatives for methan...

The mixed-ligand metal–organic framework (MOF) DMOF-1 has been successfully synthesized via ligand: 2,3,5,6-tetramethylterephthalic acid (TM), 9,10-anthracenedicarboxylic acid (ADC), 1,4-naphthalen...

The effect of dual nitridation processes for both r -plane sapphire and low temperature-grown AlN (LT-AlN) nucleation layer on non-polar a -plane AlGaN epi-layer was studied intensively. A root-mean-square value as small as 1.54 nm for a -plane Al 0.53 Ga 0.47 N epi-layer was achieved. It was revealed that the generation of AlN grains as well as the coalescence and recrystallization of LT-AlN islands were the key factors for growing a -plane AlGaN epi-layers with smooth surface morphology. Meanwhile, the evolution of surface morphology with varied nitridation processes and the mechanisms for improving surface morphology of a -plane AlGaN epi-layers were also investigated.

Molecular structures of the unpromoted and K2O-promoted supported WO3/Al2O3 catalysts were studied with in situ Raman and UV–vis spectroscopy. In situ Raman spectra revealed that supported 20% WO3/Al2O3 corresponds to near monolayer coverage of isolated and oligomeric surface WOx species on Al2O3. Above monolayer surface WOx coverage (21% WO3/Al2O3), crystalline WO3 nanoparticles are also present. The addition of K2O to the supported WO3/Al2O3 catalyst increased the concentration of isolated surface WOx species and did not form K2WO4 nanoparticles. The reducibility of the tungsten oxide structures depends on their structures (2D or 3D) and the K2O promoter. Their interaction with acidic CO2 and SO2 gases was also investigated. Adsorption of CO2 creates several surface carbonate species of varying acidity that were detected using a combination of in situ IR and mass spectroscopy. Adsorbed bicarbonate form on weakly basic surface sites on tungsten oxide monolayer WO3/Al2O3 catalyst as well as in the presence of low 2.5% K2O loading. At high 5% K2O loading, the presence of the strong surface basic sites yields adsorbed carbonates. After SO2 pretreatment, however, new strongly adsorbed sulfate appears on the surface that inhibits CO2 adsorption.

Hydrogen interaction with the TiO2 surfaces is important in many catalytic and photocatalytic reactions. However, the mechanisms of H2 activation on TiO2 surfaces remain unclear. Here, we study H2 ...

Gold (Au) nanoclusters have recently emerged as ideal models for understanding Au catalysis, because the nanosized Au particles have precise atomic numbers and uniform size. In this work, we studied for the first time the support shape effect on the catalysis of Au nanoclusters by using CO oxidation as a model reaction. Au22(L8)6 (L = 1,8-bis(diphenylphosphino) octane) nanoclusters were supported on CeO2 rods or cubes, then pretreated at different temperatures (up to 673 K), allowing the gradual removal of the organic phosphine ligands. CO oxidation test over these differently pretreated samples shows that CeO2 rods are much better supports than cubes for Au22 nanoclusters in enhancing the reaction rate. In situ IR spectroscopy coupled with CO adsorption indicates that the shape of CeO2 support can impact the nature and quantity of exposed Au sites, as well as the efficiency of organic ligand removal. Although CeO2 rods are helpful in exposing a greater percentage of total Au sites upon ligands removal, the percentage of active Au sites (denoted by Auδ+, 0

An internal quantum efficiency (IQE) as high as 39% was achieved with the nonpolar a-plane AlGaN-based multiple quantum wells (MQWs) grown on the r-plane sapphire substrate with metal organic chemical vapor deposition technology. Evident fourth order X-ray diffraction satellite peak and intense MQW-related exciton emission peak at a wavelength of 279.2 nm were observed, implying the successful growth of high quality nonpolar a-plane AlGaN-based MQWs. It was found that the employment of the trimethyl-aluminum (TMAl) flow duty-ratio modulation method played a crucial role in the epitaxial growth of the nonpolar a-plane AlGaN MQWs with sharp heterointerfaces and high IQE. Moreover, the unambiguous absence of the blue-shift in the MQWs-related exciton emission peak was verified in spite of the increase in the excitation power during the measurement of the excitation power-modulated photoluminescence spectra, indicating a complete elimination of the quantum confined Stark effect in the nonpolar AlGaN-based MQWs.

Perovskites are interesting materials for catalysis due to their great tunability. However, the correlation of many reaction processes to the termination of a perovskite surface is still unclear. I...

Two-dimensional atomically thin hexagonal boron nitride (h-BN) monolayers have attracted considerable research interest. Given the tremendous progress in the synthesis of h-BN monolayers on transition metals and their potential as electrocatalysts, we investigate the electrocatalytic activities of h-BN/Ni, h-BN/Co, and h-BN/Cu interfaces for CO2 reduction by the first-principles density functional theory. We find that with the h-BN monolayer on the metal, electrons transfer from the metal to the interface and accumulate under the B atoms. By calculating the binding energies of three key intermediates (H, HCOO, and COOH) for hydrogen evolution and CO2 reduction, we find that H binding on the metal can be significantly weakened by the h-BN monolayer, preventing the hydrogen evolution reaction (HER). However, the binding strength of HCOO is strong on both the metal and h-BN/metal, especially for Ni and Co, promoting the CO2 reduction channel. On the basis of the free-energy diagrams, we predict that h-BN/Ni ...

Hydrogen generation from the direct splitting of water by photocatalysis is regarded as a promising and renewable solution for the energy crisis. The key to realize this reaction is to find an efficient and robust photocatalyst that ideally makes use of the energy from sunlight. Recently, due to the attractive properties such as appropriate band structure, ultrahigh specific surface area, and more exposed active sites, two-dimensional (2D) photocatalysts have attracted significant attention for photocatalytic water splitting. This Review attempts to summarize recent progress in the fabrication and applications of 2D photocatalysts including graphene-based photocatalysts, 2D oxides, 2D chalcogenides, 2D carbon nitride, and some other emerging 2D materials for water splitting. The construction strategies and characterization techniques for 2D/2D photocatalysts are summarized. Particular attention has been paid to the role of 2D/2D interfaces in these 2D photocatalysts as the interfaces and heterojunctions a...

The diversity of perovskites offers many opportunities for catalysis, but an overall trend has been elusive. Using density functional theory, we studied a large set of perovskites in the ABO3 formula via descriptors of oxygen reactivity such as vacancy formation energy, hydrogen adsorption energy, and the first C–H activation energy of methane. It was found that changing the identity of B within a period increases the oxygen reactivity from the early to late transition metals, while changing A within a group has a much smaller effect on oxygen reactivity. Within the same group, B in the 3d period has the most reactive lattice oxygen compared to the 4d or 5d period. Some perovskites display large differences in reactivity for different terminations. Further examination of the second C–H bond breaking on these perovskites revealed that larger A cations and non-transition metal B cations have higher activation energies, which is conducive to the formation of coupling products instead of oxidation to CO or CO2. Balance between the first C–H bond breaking and methyl desorption suggests a just right oxygen reactivity as described by the hydrogen adsorption energy. These insights may help in designing better perovskite catalysts for methane activation.

Perovskite catalysts have been extensively studied for reduction–oxidation (redox) reactions; however, their acid–base catalytic properties are still under-explored. This review collects work aiming to study the acid–base catalytic properties of perovskites. Reports regarding combined acid–base/redox catalysis over perovskites lie beyond the scope of the present review. For the characterization of acid–base properties, researchers have studied the interaction of probe molecules with perovskite surfaces by means of multiple techniques that provide information about the density, strength and type of adsorption sites. The top-surface composition of perovskites, which relates to the abundance of the acid–base sites, has been studied by means of low energy ion scattering (LEIS), and, the less surface sensitive, conventional X-ray photoelectron spectroscopy (XPS). Probe reactions, with the conversion of 2-propanol as the common choice, have also been employed for characterizing the acid–base catalytic properties of perovskites. The complex nature of perovskite surfaces, which explains the still absent fundamental relations between the structure of the catalyst and reaction rates/selectivity, encounters a great challenge due to the surface reconstruction of these materials. In this review, we devote a special section to highlight recent publications that report the impact of surface reconstruction and particle shape on acid–base catalysis over perovskites. In addition, we review promising catalytic performances of perovskite catalysts for other reactions of interest. Challenges in acid–base catalysis over perovskites focus on the development of time-resolved monolayer-sensitive characterization of surfaces under operando conditions and the discernment of combined acid–base/redox reaction mechanisms. Opportunities lay on tuning the acid–base characteristics of perovskites with computation-based catalytic descriptors to achieve desired selectivities and enhanced rates.

Atomically precise metal nanoclusters have recently emerged as a novel class of catalysts for the hydrogen evolution reaction. From first-principles density functional theory, we show that the eight coordinatively unsaturated (cus) Au atoms in the Au22(L8)6 cluster [L8 = 1,8-bis(diphenylphosphino) octane] can adsorb H stronger than Pt, thereby being a potentially promising catalyst for the hydrogen evolution reaction (HER). We find that up to six H atoms can adsorb onto the Au22(L8)6 cluster and they have close-to-zero Gibbs free adsorption energies (ΔGH). From the HOMO–LUMO gaps, frontier orbitals, and Bader charge analysis, we conclude that H behaves as a hydride or electron-withdrawing ligand in the Au22(L8)6 clusters, in contrast to the metallic H in thiolate-protected Au nanoclusters. Our study demonstrates that ligand-protected Au clusters with cus Au sites will be the most promising candidates for realizing Au–H nanoclusters and can act as excellent electrocatalysts for the HER.

To gain an in-depth understanding of the surface properties relevant for catalysis using ternary oxides, we report the acid–base pair reactivity of shape-controlled SrTiO3 (STO) nanocrystals for the dehydrogenation of ethanol. Cubes, truncated cubes, dodecahedra, and etched cubes of STO with varying ratios of (001) and (110) crystal facets were synthesized using a hydrothermal method. Low-energy ion scattering (LEIS) analysis revealed that the (001) surface on cubes of STO is enriched with SrO due to surface reconstruction, resulting in a high ratio of strong base sites. Chemical treatment with dilute nitric acid to form etched cubes of STO resulted in a surface enriched with Ti cations and strong acidity. Furthermore, the strength and distribution of surface acidic sites increase with the ratio of (110) facet from cubes to truncated cubes to dodecahedra for STO. Kinetic, isotopic, and spectroscopy methods show that the dehydrogenation of ethanol proceeds through the facile dissociation of the alcohol gro...

A metal catalyst supported on an inert substrate could consist of both metal nanoparticles and singly dispersed metal atoms. Whether these singly dispersed metal atoms are active and how different their catalytic mechanism could be in contrast to a supported metal catalyst are fundamentally important for understanding catalysis on a supported metal or oxide. By taking reduction of NO with CO on singly dispersed Rh atoms anchored on an inert support SiO2 as a probe system (Rh1/SiO2), here we demonstrated how singly dispersed metal atoms on an inert support could perform a complex multi-step catalytic cycle through a mechanism distinctly different from that for a supported metal nanoparticle with continuously packed metal sites. These singly dispersed Rh1 atoms anchored on SiO2 are active in reducing nitric oxide with carbon monoxide through two reaction pathways that are different from those of Rh nanoparticles. In situ IR studies show that a CO molecule and a NO molecule coadsorb on a singly dispersed Rh ...

cited By 0; International audience; Ceria is one of the world's most prominent material for applications in heterogeneous catalysis, as catalyst support or catalyst itself. Despite an exhaustive literature on the structure of reactive facets of CeO2 in line with its catalytic mechanisms, the temporal evolution of the atomic surface structure exposed to realistic redox conditions remains elusive. Here, we provide a direct visualization of the atomic mobility of cerium atoms on 100 surfaces of CeO2 nanocubes at room temperature in high vacuum, O2, and CO2 atmospheres in an environmental transmission electron microscope. Through quantification of the cationic mobility, we demonstrate the control of the surface dynamics under exposure to O2 and CO2 atmospheres, providing opportunities for a better understanding of the intimate catalytic mechanisms. © 2017 American Chemical Society.

The effects of Mg-doping on the characteristics of semi-polar ( 1 1 2 ¯ 2 ) plane Al0.15Ga0.85N films grown on ( 1 0 1 ¯ 0 ) m-plane sapphire substrates with metal-organic chemical vapor deposition (MOCVD) technique were investigated intensively and systematically. The characterization results showed that the crystalline quality of the ( 1 1 2 ¯ 2 ) plane AlGaN films could be improved by optimizing the Mg-delta-doping process. Whereas, the surface morphology for the samples was monotonously degraded with increasing the Mg-doping level. Meanwhile, a hole concentration as high as 5.4 × 1017 cm−3 were achieved by optimizing the Mg-doping level. Furthermore, the luminescence mechanism of the samples was investigated by photoluminescence spectra at room temperature.

The reduction in the crystalline quality anisotropy and the in-plane strain for the non-polar a -plane GaN epi-layer with nano-scale island-like SiN x interlayer was studied extensively with scanning electron microscopy, high-resolution X-ray diffraction measurement, and Raman spectroscopy. It was demonstrated that the SiN x interlayer was powerful to suppress the crystalline quality anisotropy in the a -plane GaN epi-layer since the full width at half maximum values of the X-ray rocking curves for both c -direction and m -direction were reduced from 1108 to 780 arcsec and from 2077 to 806 arcsec, respectively. The Raman measurement results also reveal that the SiN x interlayer plays a crucial role in compensating the in-plane strains. In fact, the in-plane compressive strain along m -direction and the tensile strain along c -direction were found to be decreased by approximately 62% and 78%, respectively due to the insertion of the SiN x interlayer.

Selective catalytic reduction (SCR) of NOx with NH3 by supported vanadium oxide catalysts is an important technology for reducing acidic NOx emissions from stationary sources and mobile diesel vehicles. However, rational design of improved catalysts is still hampered by a lack of consensus about reaction pathways and kinetics of this critical technology. The SCR fundamentals were resolved by applying multiple time-resolved in situ spectroscopies (ultraviolet–visible light (UV-vis), Raman and temperature-programmed surface reaction (TPSR)) and isotopically labeled molecules (18O2, H218O, 15N18O, ND3). This series of experiments directly revealed that the SCR reaction occurs at surface V5+O4 sites that are maintained in the oxidized state by O2 and the rate-determining step involves the reduction of V5+O4 sites by NO and NH3, specifically the breaking of N–H bonds during the course of formation or decomposition of the NO–NH3 intermediate.

A new g-C 3 N 4 /SrTa 2 O 6 heterojunction photocatalyst was designed and prepared by chimie douce (soft chemistry) method where carbon nitride (g-C 3 N 4 ) was deposited over the metastable perovskite phase of SrTa 2 O 6 . The morphological study of the heterojunction using SEM and STEM revealed that g-C 3 N 4 nanofibers are dispersed uniformly on the surface of SrTa 2 O 6 plates leading to the intimate contact between them. The heterojunction could achieve a high and stable visible light photocatalytic H 2 generation of 137 mmol/h/mole of g-C 3 N 4 , which is much larger than the amount of hydrogen generated by one mole of pristine g-C 3 N 4 . A plausible mechanism for the observed enhanced photocatalytic activity for the heterojunction is proposed on the basis of effective charge separation of photogenerated electron-hole pairs, supported by band position calculations and photo-physical properties of g-C 3 N 4 and SrTa 2 O 6 .

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 Bronsted 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.

The Mg-delta-doping technique was employed in the epitaxial growth of the nonpolar a-plane p-AlGaN films on semi-polar r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). The structural, morphological, and electrical properties of the nonpolar a-plane p-AlGaN epi-layers were characterized with secondary ion mass spectroscopy (SIMS), high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM), and Hall effect measurement, respectively. The characterization results demonstrate that the crystalline quality, the surface morphology, and the efficiency of Mg incorporation in the nonpolar a-plane p-AlGaN films could be improved significantly by optimizing the pulse time of Cp2Mg mass flow in the Mg-delta-doping process. In fact, a hole concentration as high as 1.4 × 1018 cm−3 under an average Mg incorporation density of ∼1 × 1019 cm−3 was achieved.

The effect of surface structure of TiO2 nanocrystals on the structure, amount, and strength of adsorbed CO2 and resistance to SO2 was investigated using in situ IR spectroscopy and mass spectrometric techniques along with first-principles density functional theory (DFT) calculations. TiO2 nanoshapes, including rods {(010) + (101) + (001)}, disks {(001) + (101)}, and truncated octahedra {(101) + (001)}, were used to represent different TiO2 structures. Upon CO2 adsorption, carboxylates and carbonates (bridged, monodentate) are formed on TiO2 rods and disks, whereas only bidentate and monodentate carbonates are formed on TiO2 truncated octahedra. In general, the order of thermal stability of the adsorbed CO2 species is carboxylates ≈ monodentate carbonates > bridged carbonates > bidentate carbonates ≈ bicarbonates. TiO2 rods and disks adsorb CO2 more strongly than TiO2 truncated octahedra, which is explained by the larger number of low coordinated surface oxygen and oxygen vacancies on the rods and disks th...

Porous carbon materials have been modified by various metal oxides (CeO2, CuO, NiO and Mn3O4) with different weight loadings (10–30%). The obtained composite materials were characterized by scanning electron microscope (SEM), N2 adsorption isotherms, X-ray diffraction (XRD) and thermogravimetric analysis (TGA), and investigated as sorbents for selective capture and removal of carbon dioxide. The overall CO2 adsorption capacity of AMC decreases upon metal oxides modification due to the reduction in the total surface area of the composite. Interestingly, CO2 adsorption capacity of AMC at relative low pressure range (below 0.4 bar) is increased with high metal oxides loading (30%), especially for NiO modified AMC. This is explained by the increased interaction strength between CO2 and the metal oxides than with the carbon surface. The CO2/N2 selectivity values of the AMC modified with metal oxides are generally higher than the parent AMC, with the 30% NiO-modified AMC showing 72% increase in the selectivity. This enhancement in selectivity is attributed to an increased interaction between CO2 and the introduced metal oxides, which is supported by the increased isosteric heats of adsorption of CO2 after metal oxides modification. The results presented here demonstrate that the adsorption properties including capacity and selectivity of carbon materials are influenced by both their textural characteristics (such as surface area and pore volume) and the surface chemistry, which can be affected by the type and concentration of introduced metal oxides.

Although perovskites have been widely used in catalysis, tuning of their surface termination to control reaction selectivity has not been well established. In this study, we employed multiple surface-sensitive techniques to characterize the surface termination (one aspect of surface reconstruction) of SrTiO3 (STO) after thermal pretreatment (Sr enrichment) and chemical etching (Ti enrichment). We show, by using the conversion of 2-propanol as a probe reaction, that the surface termination of STO can be controlled to greatly tune catalytic acid/base properties and consequently the reaction selectivity over a wide range, which is not possible with single-metal oxides, either SrO or TiO2 . Density functional theory (DFT) calculations explain well the selectivity tuning and reaction mechanism on STO with different surface termination. Similar catalytic tunability was also observed on BaZrO3 , thus highlighting the generality of the findings of this study.

The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO2 nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO2. The two TiO2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO2 surfaces. Density functional theory calculations sho...

The ab initio density functional theoretical studies show that energetics favor CO oxidation on single Pd atoms supported on θ-alumina. The diffuse reflectance infra-red spectroscopy (DRIFTS) results show that carbonates are formed as intermediates when single supported Pd atoms are exposed to a gaseous mixture of CO + O2. The rapid agglomeration of Pd atoms under CO oxidation conditions even at 6 °C leads to the presence of Pd particles along with single atoms during CO oxidation experiments. Thus, the observed CO oxidation has contributions from both single Pd atoms and Pd particles.

The semi-polar $$\left(11\overline{2}2\right)$$ plane Al0.42Ga0.58N films were successfully grown on $$\left(10\overline{1}0\right)$$ -oriented m-plane sapphire substrates for the first time with an indium (In)-surfactant-assisted metal–organic chemical vapor deposition (MOCVD) technology. The crystal orientation, surface morphology, and electrical properties of the grown semi-polar $$\left(11\overline{2}2\right)$$ plane AlGaN epi-layers were characterized with high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM), and Hall effect measurements, respectively. The XRD scanning results showed that crystalline quality for the semi-polar $$\left(11\overline{2}2\right)$$ plane AlGaN epi-layer was improved with In-surfactant. The AFM results demonstrated that the root mean square value of the semi-polar AlGaN epi-layer samples decreased with increasing the TMIn mole flow rate. Furthermore, the native electron concentration of the unintentionally doped semi-polar $$\left(11\overline{2}2\right)$$ plane AlGaN epi-layers was decreased from 2.66 × 1017 to 2.97 × 1016 cm−3 due to the significant decrease in nitrogen vacancies (V N) with indium-surfactant-assisted growth process.

Although perovskite catalysts are well-known for their excellent redox property, their acid–base reactivity remains largely unknown. To explore the potential of perovskites in acid–base catalysis, we made a comprehensive investigation in this work on the acid–base properties and reactivity of a series of selected perovskites, SrTiO3, BaTiO3, SrZrO3, and BaZrO3, via a combination of various approaches including adsorption microcalorimetry, in situ FTIR spectroscopy, steady state kinetic measurements, and density functional theory (DFT) modeling. The perovskite surfaces are shown to be dominated with intermediate and strong basic sites with the presence of some weak Lewis acid sites, due to the preferred exposure of SrO/BaO on the perovskite surfaces as evidenced by low energy ion scattering (LEIS) measurements. Using the conversion of 2-propanol as a probe reaction, we found that the reaction is more selective to dehydrogenation over dehydration due to the dominant surface basicity of the perovskites. Furt...