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Developing an earth-abundant catalyst that is sulfur-tolerant, active, and highly selective is of great interest for valorizing natural gas streams containing sour gas. A tin-modified alumina catal...

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...

Oxidative dehydrogenation of propane to propylene can be achieved using conventional, oxygen-assisted dehydrogenation of propane (O2–ODHP) or via the use of soft oxidants, such as CO2, N2O, S-conta...

A bulk γ-Al2O3 catalyst shows high selectivity for propane dehydrogenation upon pretreatment and co-feeding with H2S. The reaction kinetics, deactivation rates, and active sites for propane dehydrogenation on this catalyst were characterized using fixed bed conversion studies, NH3-TPD, O2-TPO, XPS, and density functional theory (DFT). Specifically, we observe that the selectivity to propylene was 94% at ca. 16% propane conversion at 560 °C for a C3H8:H2:H2S ratio of 1.1:1:0.1 on γ-Al2O3. Our results indicate that H2S can irreversibly modify the active sites of γ-Al2O3, postulated to be defect sites on the 110 facet comprised of a tri-coordinated Al atom, such that the modified site was more active and selective towards propylene and less inhibited by co-fed H2S. Along with XPS and O2-TPO, the dehydrogenation-regeneration experiments suggest the formation of sulfurous coke and strong adsorption of reaction products result in a less active catalyst. This study shows the potential of alumina/metal-sulfide as an earth-abundant and relatively benign class of catalysts for alkane activation, especially in the context of sour natural gas upgrading.

Strong metal–support interactions (SMSIs) and catalyst deactivation have been heavily researched for decades by the catalysis community. The promotion of SMSIs in supported metal oxides is commonly...

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.

The partial or complete blockage of active sites of metal nanoparticles (NPs) on supported-metal catalysts has been of interest for tuning the stability, selectivity, and rate of reactions. Here, w...

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.

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.

Ceria has been used as a hydrogenation catalyst especially in selective alkyne hydrogenation, but the reaction mechanism regarding the role of different surface hydrogen species remains unclear. In...

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.

Strontium titanate (SrTiO3) is an extensively investigated perovskite for various applications due to its optical, electrical and chemical properties. To gain an in-depth understanding of the active sites involved in heterogeneous catalysis over the broadly used SrTiO3 (STO), we studied a model reaction, isopropanol conversion, on three differently shape-controlled nanocrystals: cube, truncated cube and dodecahedra. SEM, XRD and XPS confirmed the morphology, phase and composition of STO shapes. Low energy ion scattering (LEIS) revealed the occurrence of surface reconstruction over STO shapes during O2 pretreatment at different temperatures. Based on the catalytic activities, scanning transmission electron microscopy images and density functional theory calculations, the step sites on STO derived from surface reconstruction were proposed to be the active sites for isopropanol conversion. This was further confirmed by steady state isotopic kinetic analysis (SSITKA) which demonstrated similar intrinsic turnover frequencies (TOFs) for the differently reconstructed STO shapes. It is concluded that the crystal facets impose an indirect effect on the catalysis of STO via controlling the degrees of surface reconstruction: the less stable STO (1 1 0) facet (dodecahedra) leads to more step sites after reconstruction and hence higher overall reaction rate than the more stable (1 0 0) facet (cube). This work highlights the important interplay between the crystal facet and surface reconstruction in controlling the nature and density of active sties and thus catalysis over complex oxides.

A series of metal silicates, NaMSi10Ox (M = Cu, Mn and Ni), were prepared by in situ doping of metals into mesoporous SBA-15 under a hydrothermal process, displaying a continuous framework of SiO4 structure with a narrow pore size distribution. These metal silicate materials were tested for CO2 adsorption behavior in the absence and presence of water. The results exhibited that the effect of H2O on the CO2 capture capability of metal silicates depends on the types of metal inserted into SBA-15. Compared to the dry condition, H2O addition enhances CO2 uptake dramatically for NaCuSi10Ox by 25%, and slightly for NaNiSi10Ox (∼10%), whereas little effect is shown on NaMnSi10Ox. The metal silicate materials are stable after adsorption of CO2 under wet conditions, which is benefited from their synthesis method, hydrothermal conditions. The improvement of CO2 uptake on metal silicates by H2O is attributed to the competitive and synergistic adsorption mechanism on the basis of IR investigations, where initially adsorbed H2O acts as a promoter for further CO2 capture through a hydration reaction, i.e., formation of bicarbonate and carbonates on the surface of the samples. These observations provide new possibilities for the design and synthesis of porous metal silicate materials for CO2 capture under practical conditions where moisture is present.

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.

Titanium dioxide (TiO2) represents a promising candidate for hydrogen production via photocatalysis. However, its large bandgap and fast charge recombination limits its efficiency. To overcome this...

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.

Single-atom catalysts usually comprise positively charged atomically dispersed metal cations on oxide supports. Neutral atoms on oxides are synthetically challenging, and their performance in catalytic reactions remains ambiguous. Here, we shed light on this question with the design of Pd single-atom catalysts on polyoxometalates. Depending on the composition of the support, Pd can either exhibit oxidation states of 0 or 2+. We show that this difference is decisive for the C-O bond hydrogenolysis while displaying negligible effects on the C=O bond hydrogenation. The selective conversion of 5-hydroxymethylfurfural (5-HMF) to 2,5-dimethylfuran (2,5-DMF), a key renewable fuel compound, was shown to occur at 253 K with reaction rates up to 71.0 per hour.

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.

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.

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 (

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...

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

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...

Propylene epoxidation with molecular oxygen has been proposed as a green and alternative process to produce propylene oxide (PO). In order to develop catalysts with high selectivity, high conversion, and long stability for the direct propylene epoxidation with molecular oxygen, understanding of catalyst structure and reactivity relationships is needed. Here, we combined atomic layer deposition and deposition precipitation to synthesize series of well-defined Au-based catalysts to study the catalyst structure and reactivity relationships for propylene epoxidation at 373 K. We showed that by decorating TiO2 on gold surface the inverse TiO2/Au/SiO2 catalysts maintained ∼90% selectivity to PO regardless of the weight loading of the TiO2. The inverse TiO2/Au/SiO2 catalysts exhibited improved regeneration compared to Au/TiO2/SiO2. The inverse TiO2/Au/SiO2 catalysts can be regenerated in 10% oxygen at 373 K, while the Au/TiO2/SiO2 catalysts failed to regenerate at as high as 473 K. Combined characterizations of ...

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.

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.

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 ...

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 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...

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...

Taming interfacial electronic effects on Pt nanoparticles modulated by their concomitants has emerged as an intriguing approach to optimize Pt catalytic performance. Here, we report Pt nanoparticles assembled on vacancy-abundant hexagonal boron nitride nanosheets and their use as a model catalyst to embrace an interfacial electronic effect on Pt induced by the nanosheets with N-vacancies and B-vacancies for superior CO oxidation catalysis. Experimental results indicate that strong interaction exists between Pt and the vacancies. Bader charge analysis shows that with Pt on B-vacancies, the nanosheets serve as a Lewis acid to accept electrons from Pt, and on the contrary, when Pt sits on N-vacancies, the nanosheets act as a Lewis base for donating electrons to Pt. The overall-electronic effect demonstrates an electron-rich feature of Pt after assembling on hexagonal boron nitride nanosheets. Such an interfacial electronic effect makes Pt favour the adsorption of O2, alleviating CO poisoning and promoting the catalysis., Tuning electronic properties of metallic catalysts is a useful way to improve their activity, however control over metal-support interactions is still challenging. Here the authors report a vacancy-induced interfacial electronic effect for Pt assembled on vacancy-abundant h-BN nanosheets leading to superior CO oxidation catalysis.

The impetus to explore biomass derived chemicals arises from a desire to enable renewable and sustainable commodity chemicals. To this end, we report catalytic production of propene, a building-block molecule, from 1-propanol. We found that zeolite catalysts are quite versatile and can produce propene at or below 230 °C with high selectivity. Increasing the reaction temperature above 230 °C shifted product selectivity toward C4+ hydrocarbons. Cu-ZSM-5 was found to exhibit a broader temperature window for high propene selectivity and could function at higher 1-propanol space velocities than H-ZSM-5. A series of experiments with 1-propan(ol-D) showed deuterium incorporation in the hydrocarbon product stream including propene suggesting that a hydrocarbon pool type pathway might be operational concurrent with dehydration to produce C4+ hydrocarbons. Diffuse reflectance infrared spectroscopy of 1-propanol and 1-propan(ol-D) over Cu-ZSM-5 in combination with deuterium labeling experiments suggest that deuteriu...

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.

Single-atom catalysts have recently been applied in many applications such as CO oxidation. Experimental in situ investigations into this reaction, however, are limited. Hereby, we present a suite of operando/in situ spectroscopic experiments for structurally well-defined atomically dispersed Rh on phosphotungstic acid during CO oxidation. The identification of several key intermediates and the steady-state catalyst structure indicate that the reactions follow an unconventional Mars-van Krevelen mechanism and that the activation of O2 is rate-limiting. In situ XPS confirms the contribution of the heteropoly acid support while in situ DRIFT spectroscopy consolidates the oxidation state and CO adsorption of Rh. As such, direct observation of three key components, i.e., metal center, support and substrate, is achieved, providing a clearer picture on CO oxidation on atomically dispersed Rh sites. The obtained information are used to engineer structurally similar catalysts that exhibit T20 values up to 130 °C below the previously reported Rh1/NPTA., Single-atom catalysts have been studied for CO oxidation, but experimental in situ investigations are limited. Here, the authors use a suite of in situ/operando spectroscopy to identify key intermediates and define design principles to enhance the CO oxidation activity of atomically dispersed Rh on heteropoly acids.

Boron nitrides (BNs) are a class of materials with unique properties that exhibit promise for applications in CO2 capture. However, the surface electron-deficiency of BNs makes their interaction with Lewis acidic CO2 very weak. By utilizing the strong interaction between electron-deficient boron atoms and electron-donating amine groups, BN nanosheets were functionalized with polyethyleneimine (PEI) which is rich in amine density, through simple impregnation to improve their performance for CO2 capture. The important roles of the boron–amine interaction in the incorporation, distribution and stabilization of PEI, as well as the facilitation of CO2 adsorption and desorption were both experimentally and theoretically investigated. It is demonstrated that after functionalization with PEI, the capacity of pure CO2 on BN nanosheets was significantly improved (3.12 mmol g−1 for BN functionalized with 54.9 wt% of PEI vs. 0.29 mmol g−1 for pristine BN at 75 °C). Furthermore, the adsorbed CO2 can be facilely released through N2 purge at 75 °C, and the PEI-functionalized BN nanosheets exhibit high stability throughout consecutive cycles.

Precise control over the chemical structure of hard-matter materials is a grand challenge of basic science and a prerequisite for the development of advanced catalyst systems. In this work we report the application of a sacrificial metal–organic framework (MOF) template for the synthesis of a porous supported metal oxide catalyst, demonstrating proof-of-concept for a highly generalizable approach to the preparation of new catalyst materials. Application of 2,2′-bipyridine-5,5′-dicarboxylic acid as the organic strut in the Ce MOF precursor results in chelation of Cu2+ and affords isolation of the metal oxide precursor. Following pyrolysis of the template, homogeneously dispersed CuO nanoparticles are formed in the resulting porous CeO2 support. By partially substituting non-chelating 1,1′-biphenyl-4,4′-dicarboxylic acid, the Cu2+ loading and dispersion can be finely tuned, allowing precise control over the CuO/CeO2 interface in the final catalyst system. Characterization by X-ray diffraction, X-ray absorption fine structure spectroscopy, and in situ IR spectroscopy/mass spectrometry confirm control over interface formation to be a function of template composition, constituting the first report of a MOF template being used to control interfacial properties in a supported metal oxide. Using CO oxidation as a model reaction, the system with the greatest number of interfaces possessed the lowest activation energy and better activity under differential conditions, but required higher temperature for catalytic onset and displayed inferior efficiency at 100 °C than systems with higher Cu-loading. This finding is attributable to greater CO adsorption in the more heavily-loaded systems, and indicates catalyst performance for these supported oxide systems to be a function of at least two parameters: size of adsorption site and extent of interface. Optimization of catalyst materials thus requires precise control over synthesis parameters, such as is demonstrated by this MOF-templating method.

Selective conversion of bio-derived ethanol to benzene, toluene and xylenes (BTX) is desirable for producing renewable BTX. In this work, we show that addition of Ga to H-ZSM-5 leads to a two-fold increase in the BTX yield as compared with H-ZSM-5 when ethanol is converted over these zeolites at 450 °C and ambient pressure. Besides promoting BTX formation, Ga also plays an important role in enhancing molecular hydrogen production and suppressing hydrogen transfer reactions for light alkane formation. The ion exchange synthesis of Ga-ZSM-5 results in the majority of Ga at the outer surface of zeolite crystals as extra-zeolitic Ga2O3 particles and only a small fraction of Ga exchanging with the Bronsted acid sites which appears to be responsible for higher ethanol conversion to BTX. The interface between H-ZSM-5 and Ga2O3 particles is not active since H-ZSM-5 and the physical mixture of β-Ga2O3/H-ZSM-5 furnish an almost identical product distribution. Hydrogen reduction of the physical mixtures facilitates movement of Ga to ion exchange locations and dramatically increases the BTX yield becoming comparable to those obtained over ion-exchanged Ga-ZSM-5, suggesting that exchanged Ga(III) cations are responsible for the increased BTX production. A linear correlation between the BTX site time yield and exchanged Ga sites further confirms that Ga occupying cationic sites are active sites for enhancing BTX formation. Reduction of physical mixtures (β-Ga2O3/H-ZSM-5) also provides an economical and environmentally friendly non-aqueous method for large scale catalyst synthesis without sacrificing catalyst performance for ethanol conversion application.

The surface properties of CeO2, Pr-CeO2, and 5% and 15% Cu-doped Pr-CeO2 were investigated using methanol as a probe molecule through adsorption and desorption studies carried out using in situ DRIFTS. It was revealed that the surfaces of the 5% and 15% Cu materials were dominated by reduced cations/vacancies and that the 15% Cu material contained the highest concentration of these active species. The high oxygen storage capacity (OSC) of the 15% Cu material, as determined using TGA, reflects the available vacant sites for oxygen adsorption. Formates were formed on all materials, with those formed on the Cu-doped materials present at temperatures as low as 25 °C, hence showing their superior reactivity toward methoxy oxidation. During formate dehydrogenation, H2, CO, CO2, and H2O evolved as the surface cations were simultaneously reduced. It was also observed that, for the Cu-containing materials, H2 was not formed and the high surface mobility determined through isotopic exchange simultaneously generated...

The behavior of metal–organic frameworks (MOFs) in the presence of acid gases may be decisive in their suitability for industrial applications. In this study, MIL-125 and MIL-125-NH2 were investigated with SO2 exposure in dry, humid, and aqueous environments. MIL-125 was found to be unstable in both humid and aqueous acidic environments, while MIL-125-NH2 was stable under these exposure conditions, showing no change in textural properties or visual degradation, as observed through SEM. Both materials were stable in the presence of water and dry SO2, suggesting that the reaction of these molecules to form an acidic species is likely a key factor in the degradation of MIL-125. In situ IR experiments confirmed the presence of sulfite species, supporting the hypothesis that the presence of an acidic sulfur species likely leads to the degradation of the MIL-125 structure. Computational investigation of several potential reaction mechanisms in MIL-125 indicated reactions involving the bisulfite ion are favored ...

Investigation of atomically precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects on Au catalysis. Herein, we explored the catalytic behavior of a newly synthesized Au22(L8)6 nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO2, CeO2, and Al2O3. Stability of the supported Au22 nanoclusters was probed structurally by in situ extended X-ray absorption fine structure (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and their ability to adsorb and oxidize CO was investigated by IR absorption spectroscopy and a temperature-programmed flow reaction. Low-temperature CO oxidation activity was observed for the supported pristine Au22(L8)6 nanoclusters without ligand removal. Density functional theory (DFT) calculations confirmed that the eight uncoordinated Au sites in the intact Au22(L8)6 nanoclusters can chemisorb both CO and O2. Use of isotopically labeled O2...

Though carbon dioxide is a waste product of combustion, it can also be a potential feedstock for the production of fine and commodity organic chemicals provided that an efficient means to convert it to useful organic synthons can be developed. Herein we report a common element, nanostructured catalyst for the direct electrochemical conversion of CO2 to ethanol with high Faradaic efficiency (63 % at −1.2 V vs RHE) and high selectivity (84 %) that operates in water and at ambient temperature and pressure. Lacking noble metals or other rare or expensive materials, the catalyst is comprised of Cu nanoparticles on a highly textured, N-doped carbon nanospike film. Electrochemical analysis and density functional theory (DFT) calculations suggest a preliminary mechanism in which active sites on the Cu nanoparticles and the carbon nanospikes work in tandem to control the electrochemical reduction of carbon monoxide dimer to alcohol.

We present a new class of catalysts, InMo-ZSM-5, which can be prepared by indium impregnation of Mo-ZSM-5. The incorporation of indium dramatically decreases coke formation during methane dehydroaromatization. The benzene and C2 hydrocarbons selectivity among total hydrocarbons over InMo-ZSM-5 remains comparable to that of Mo-ZSM-5 despite reduced methane conversion due to decreased coke formation. We found 1 wt% indium to be optimal loading for reducing coke selectivity to half that of Mo-ZSM-5. Characterization methods were not helpful in discerning the interaction of In with Mo but experiments with bimetallic 1In2Mo-ZSM-5 and mechanical mixture 1In+2Mo-ZSM-5 suggest that In and Mo need to be in close proximity to suppress coke formation. This is supported by temperature programmed reduction experiments which show that In incorporation leads to lower Mo reduction temperature in In2Mo-ZMS-5.

Here, NiFe2O4 with an inverse spinel structure exhibits high activity for a complete oxidation of methane at 400 °C–425 °C and a higher temperature. The surface of the catalyst and its adsorbates were well characterized with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ infrared spectroscopy (IR). In situ studies of the surface of NiFe2O4 using AP-XPS suggest the formation of methoxy-like and formate-like intermediates at a temperature lower than 200 °C, supported by the observed vibrational signatures in in situ IR studies. Evolutions of C1s photoemission features and the nominal atomic ratios of C/(Ni + Fe) of the catalyst surface suggest that the formate-like intermediate is transformed to product molecules CO2 and H2O in the temperature range of 250–300 °C. In situ studies suggest the formation of a spectator, – Olattice – CH2 – Olattice –. It strongly bonds to surface through C–O bonds and cannot be activated even at 400 °C.