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Abstract: The site requirements for the methane total combustion, partial oxidation, and decomposition on LaFeO3 perovskite catalysts during the chemical looping process were addressed by a kinetic study through varying the O content and the crystal size. The exchange between the surface oxygen and lattice oxygen is rapid, and the active sites of Fe associated with the surface oxygen are highly dynamic in nature, which depends on the amount of O in the perovskite during reaction and the O binding energy. Three types of active sites are identified where Fe atoms highly coordinated with O lead to total combustion of methane, Fe moderately coordinated with O leads to partial oxidation, and Fe highly coordinated with vacancy sites leads to carbon formation. The crystal size of LaFeO3 perovskite plays a significant role in determining the O binding energy and their catalytic properties on the partial oxidation of methane by the chemical looping process. Different crystal sizes of perovskites have been obtained using various preparation methods. Larger crystal has a high selectivity to synthesis gas and high capacity of O removal, due to a lower O–Fe bond energy, as indicated by a smaller band gap measured by UV–Vis absorption spectroscopy. Nearly pure synthesis gas was produced by methane partial oxidation using chemical looping on LaFeO3 by tuning the surface O coverage through controlling the oxidation degree in the oxidation step. [Copyright &y& Elsevier]

Abstract: The effect of the crystal size of Ni on the growth of carbon nanofibers (CNFs) was studied in a tapered oscillating element microbalance reactor. Small Ni crystals yield a low growth rate and fast deactivation and thus a low final yield of CNF. Large Ni crystals reduce the growth rate because of low surface area. An optimum growth rate and yield of carbon nanofibers can be achieved on optimally sized Ni crystals (around 34 nm). A model has been proposed for interpreting the kinetic effects of the Ni crystal size based on a detailed mechanism of the carbon nanofiber growth. The reduced coking rate on a small-sized Ni crystal is a result of increased saturation concentration of CNF and thus a low driving force for carbon diffusion through the Ni crystals. As a consequence, the surface coverage of carbon increases, which enhances the formation of encapsulating carbon and thus the deactivation. Both the low initial coking rate and the fast deactivation result in a low yield of carbon nanofibers on small-sized crystals. The results also indicate that hydrogen has a significant effect on the formation of CNF, and an optimum partial pressure of hydrogen exists for the CNF growth. [Copyright &y& Elsevier]

[Display omitted] • The effect of ligand coordination on the mechanism and regioselectivity is investigated. • The free energy surfaces differ greatly for the bidentate and tridentate nitrogen ligands. • The electrostatics and orbital interactions are the dominant factors regulating the regioselectivity. The 3d transition metal-catalyzed cyclization of enynes provides a sustainable and useful strategy for the construction of cyclic compounds. Controlling the regioselectivity is a central challenge for the hydrofunctionalization of multiply unsaturated substrates such as 1,6-enynes. Herein, we report a computational mechanistic study of the cobalt-catalyzed hydroboration/cyclization of 1,6-enynes in the presence of three representative ligands including bidentate nitrogen ligand and phosphine ligand as well as tridentate nitrogen-based ligand. A detailed analysis is carried out on the ligand-substrate interactions to understand the effect of ligand coordination on the mechanism and regioselectivity. Our results indicate that although the initial catalyst-substrate complexes adopt the pseudo-trigonal bipyramidal geometries for the two nitrogen-based ligands, there is a remarkable difference in the free energy surfaces. The steric repulsions and hydrogen bonding interactions between the substrate and the ligand play important roles in regulating the competing migration insertion. Energy decomposition analyses are further performed for the subsequent σ-bond metathesis to illustrate the impacts of steric and electronic effects on the regioselectivity and the electrostatics and orbital interactions are disclosed to be the dominant factors. This work provides a deeper understanding of the specific structural and electronic properties of the cobalt complexes and their potential influences on the reactivity and selectivity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ir – TiO 2 interaction was dependent on Ir sizes and crystal plane of TiO 2. • Both activity and selectivity of the catalysts increased as the Ir size grew from single atom to nanoparticle. • End-on C O adsorption of crotonaldehyde was favored at the Ir-TiO x interface for Ir nanoparticles. • Crotonaldehyde adsorption geometry depends on H coverage on the catalyst surface. For the supported catalysts for the selective hydrogenation of α, β - unsaturated aldehyde with H 2 , tailoring of metal - support interfacial effect to improve reaction performance is a significant but challenging subject. In this work, we regulated both the size regimes of the Ir deposits (single atom, nanocluster and nanoparticle) and the crystal plane of anatase TiO 2 (TiO 2 (1 0 1), TiO 2 (1 0 0) and TiO 2 (0 0 1)) so as to alter the Ir-TiO x interfacial properties, in order to unravel reaction mechanism of selective hydrogenation of crotonaldehyde over the Ir/TiO 2 catalysts. Both the activity and selectivity to crotyl alcohol increased as the Ir size grew from single atom to nanoparticle, ascribed to high Ir0 content, high concentration of surface defects and prominent H-spillover effect on the Ir nanoparticles. Moreover, the in situ Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations demonstrated that an end-on C O adsorption mode for crotonaldehyde molecule was favored at the Ir-TiO x interface particularly at high H coverage, which thus facilitated the hydrogenation of C O bond to form crotyl alcohol. Moreover, the highest reactivity of the Ir nanoparticles was evidenced by the lowest reaction barrier. These results explicitly demonstrate that larger metal nanoparticles and adsorption geometry of the reactant at the metal - support interface are vital for improved catalytic performance, which would shed light on the ingenious catalyst design. [ABSTRACT FROM AUTHOR]

[Display omitted] • In situ X-ray absorption spectroscopy (XAS) and operando UV–vis-NIR measurements. • Monitoring dynamic changing Cu active sites in the transient and steady state. • Correlating the Cu oxidation state with performance under the real reaction conditions. • The influence of La promoter studied by in situ XAS. The oxidation state of Cu species in ethylene oxychlorination is strongly correlated with catalytic performance. CuCl 2 and CuCl have been identified as the primary active sites in the reduction and oxidation step, respectively, according to a redox-type mechanism. The distribution of the active sites is governed by the kinetic equilibrium of the reduction, oxidation, and hydrochlorination rates at steady-state. Herein, in situ X-ray absorption spectroscopy (XAS) and operando ultraviolet–visible and near-infrared spectroscopy (UV–vis-NIR) were utilized to quantify the dynamic changes of the Cu active sites over the La-doped CuCl 2 / γ -Al 2 O 3 in transient and steady-state experiments. A higher CuCl 2 concentration plays a key role in dominating the higher activity, EDC selectivity, and stability. The in situ XAS result evidenced La was considered as a structural additive to increase the reducibility of the active CuCl 2 and modify the catalytic performance instead of directly involving the reduction-oxidation cycle. A deeper understanding of the complex role of the active species and promoter in the CuCl 2 / γ -Al 2 O 3 based-catalysts for ethylene oxychlorination was obtained. [ABSTRACT FROM AUTHOR]

[Display omitted] • Low H 2 dissociation barrier is obtained using N, S-doped Co single atom catalyst (Co-N 3 SN-def1); • Frustrated Lewis pair comprised of Co and pyridinic N sites facilitate the H 2 dissociation; • H transfer via the solvent water is the main diffusion pathway; • Moderate H adsorption strength at pyridinic N site contributes to the low breaking barrier of N-O bond in nitrobenzene. The limitation of isolated metal active sites in single atom catalysts (SAC) hinder their application in hydrogenation reactions, necessitating the creation of additional active site beyond the metal site. This study introduces pyridinic N and S atoms into the N/C sheet, optimally adjusting the local environment of the Co site. This adjustment synergistically enhances H 2 activation, H diffusion, and promotes the hydrogenation of nitrobenzene into aniline. The sterically hindered Co acidic site and N basic site, known as frustrated Lewis pairs, in defect-containing Co-N 3 SN-def1 model cooperatively facilitate the highly efficient cleavage of the H 2 molecule with a barrier of only 0.37 eV. The findings suggest that the pyridinic N site in the Co-N 3 SN-def1 model acts as a reservoir for *H, significantly contributing to the enhanced activity for the hydrogenation of nitrobenzene. The mechanism unveiled in this study offers valuable insights for designing efficient heterogeneous catalysts for target hydrogenation reactions. [ABSTRACT FROM AUTHOR]

Moderate WO x surface density of Ni/WO x -ZrO 2 with higher Brønsted/Lewis acid sites ratio enhances LDPE hydrocracking and obtains the maximum liquid yield. [Display omitted] • The non-noble Ni catalysts achieves the LDPE hydrocracking process under a mild condition. • The Ni/WO x -Zr catalysts achieves 100 % LDPE conversion with 76.9 % liquid yield. • The WO x surface density shows a volcanic-shape relationship with the B/L ratio of catalyst. • The LDPE hydrocracking performance has a linear relationship with the B/L ratio of catalyst. Chemical upcycling through catalytic hydrocracking to fuels has potential to mitigate the serious environmental issues caused by plastic waste. Herein, non-noble tungstate-zirconia supported nickel catalyst (Ni/WO x -ZrO 2) with tunable Brønsted/Lewis (B/L) ratio was firstly proposed for the hydrocracking of low-density polyethylene (LDPE), which achieves hydrogenation/dehydrogenation on Ni sites and cracking/isomerization on WO x -ZrO 2 , respectively. Excellent 100 % LDPE conversion with 76.9 % liquid yield was achieved, which is comparable to noble Ru-based and Pt-based catalysts. Quantitative linear relationship between B/L ratio and hydrocracking performance was established, and the key to boost performance was keenly controlling the volcanic-shape relationship between surface WO x density and B/L ratio of catalyst. Moderate WO x surface density with higher B/L ratio enhances LDPE hydrocracking, and inhibits the hydrogenation of the primary products. However, excessive WO x polymerized and formed WO 3 crystals, compensating acid sites and inhibiting LDPE hydrocracking. This work is of prime scientific and industrial significance to the rational design of catalysts for industrial plastic hydrocracking. [ABSTRACT FROM AUTHOR]

[Display omitted] • Non-precious intermetallic compound catalysts for propane dehydrogenation. • Scaling relations are established using formation energies over terraces and steps. • Descriptor-based microkinetic analysis is carried out to obtain volcano curves. • Fe 3 Ga 1 is a good catalyst candidate with improved catalytic performance. By using a descriptor-based microkinetic analysis combined with results from density functional theory calculations over eight transition-metal (including Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) terrace and stepped surfaces, intermetallic compounds have been screened to search for inexpensive and environmentally friendly propane dehydrogenation (PDH) catalysts with enhanced catalytic performance. Based on the scaling relations, the formation energies of ethyl on terrace and methylidyne on stepped surfaces are identified as two descriptors to represent the energetics of other reaction intermediates and transition sates. The derived PDH activity and propylene selectivity volcano curves not only provide a rational interpretation of the previously experimentally reported catalyst candidates, but also predict several unexpected binary combinations by screening 1482 A 3 B 1 and 741 A 1 B 1 intermetallic compounds. The non-precious Fe 3 Ga 1 is then synthesized and evaluated. Its improved propylene selectivity and catalytic stability with respect to those of the Pt catalyst validates the theoretical predictions experimentally. [ABSTRACT FROM AUTHOR]

[Display omitted] • Atomically dispersed Pt-rTiO 2 catalyst shows good catalytic performance for PDH. • Single and dual Pt atoms can be tightly held by vacancies created on reduced TiO 2. • Dual-Pt-atom structure is the active ensemble for the C H bond activation. Tailoring atomically dispersed catalysts by various methods has been the subject of intense current research in heterogeneous catalysis. In this work, the structural stability and propane dehydrogenation performance of Pt-TiO 2 catalysts with single, dual, and triple Pt atoms doped on the reduced TiO 2 surface have been examined. Experimental results indicate the atomically dispersed Pt is thermally very stable and shows exceptionally high activity (TOF propane = 4.93 s−1) and selectivity (∼98.5 %) for the dehydrogenation reaction. The recently developed machine-learning-based SSW-NN method is then employed to explore the global potential energy surface of the atomically dispersed Pt atom-doped TiO 2 , which suggests that the single and dual Pt atoms can be tightly held by the vacancies created on the reduced transition-metal oxide, as confirmed by DFT + U calculated energetics. After that, advanced characterization techniques are used in conjunction with microkinetic analysis to establish the quantitative structure–activity relationships. It is found that the dual-Pt-atom structure with two Pt atoms embedded in adjacent Ti and O vacancies is the only active ensemble that dramatically promotes the C H bond activation and hydrogen recombination, as compared to single Pt atoms accommodated by either Ti or O vacancies. These results provide new and direct evidence in support of the idea that specific function can be imparted to specific active sites by tuning their coordination environments, which makes it possible to tailor the catalytic properties of metal oxides to a particular reaction such as propane dehydrogenation. [ABSTRACT FROM AUTHOR]

• A 20-fold increase of catalytic activity via engineering the Pt-carbon interactions. • The electron transfer and interfacial bonding vary with the carbon support. • Pt/p-CNF shows the highest Pt binding energy with the least Pt-O-C linkages. • n EWG /n EDG is identified as a good descriptor of Pt/carbon catalyst. Understanding the metal-support interactions in heterogeneous catalysis is critical yet complicated to tailor-design the catalysts with desirable properties. Exemplified with Pt-catalyzed ammonia borane (AB) hydrolysis, a dramatic increase of 20 folds in the catalytic activity is achieved by engineering the Pt-carbon interactions via adopting four different carbon materials (AC, CNT, f-CNF and p-CNF) as the catalyst supports. Multiple characterization techniques reveal that the Pt-carbon electronic interactions, including electron transfer and interfacial bonding, are deemed to be mainly responsible for the remarkable enhancement in the hydrogen generation rate. The molar ratio of electron-withdrawing group to electron-donating group (n EWG /n EDG) is further identified as a descriptor of catalyst in terms of Pt binding energy, which exhibits an almost linear relationship with the catalytic activity. Moreover, a comparison of Pt catalyst pre-treatments, i.e., H 2 and AB reduction as well as Ar calcination, suggests that the Pt-O-C linkages within the Pt-carbon interactions are highly stable yet inferior to this reaction. As a result, combining the merits of the highest Pt binding energy as well as the minimum Pt-O-C linkages, the Pt/p-CNF delivers the highest catalytic activity. The insights presented here could shed new lights on the nature of Pt-carbon interactions, which could be extended to the design and manipulation of other metal-carbon catalysts. [ABSTRACT FROM AUTHOR]

• The PDDA and PVP co-functionalized CNT has been developed to support Pt catalyst. • A combined kinetics analysis, DFT calculations with multiple characterizations is conducted. • The co-functionalization improves the quantity (N i) and quality (TOF i) of Pt active sites. • The catalyst exhibits significantly enhanced H 2 generation activity and durability. Decorating the catalyst support with organic moieties to integrate multiple functionalities into metal particles offers a unique platform to promote the catalytic performance. Here, we report a mechanism-driven strategy to functionalize CNT with dual polymers, endowing the supported Pt catalyst with simultaneously enhanced hydrogen generation activity and durability. Kinetics analysis, multiple characterization and DFT calculations reveal that the PDDA acts as electron-acceptor to capture electrons from the Pt particles toward strengthened adsorption of reactants, while the PVP acts as structure-directing agent to induce the catalyst morphology evolution with a preferential exposure of Pt(1 1 1) active sites. Such electronic and geometric synergy by co-functionalizing PDDA and PVP gives rise to a 3-fold increase in the hydrogen generation activity, together with remarkably improved catalytic durability due to the suppressed adsorption of B(OH) 4 − over the Pt(1 1 1). The strategy reported here might shed new lights on establishing the functionalization protocols to design carbon supported metal catalysts with the targeted properties. [ABSTRACT FROM AUTHOR]

Increasing the utilization efficiency of noble metals by the synergy with less expensive metals is an important yet challenging issue in heterogeneous catalysis. Herein, exemplified with Pt-catalyzed hydrolytic dehydrogenation of ammonia borane, partial substitution of the Pt catalysts by over 11 times less expensive Ru is studied to obtain highly efficient and cost-effective Pt-based catalysts. It is observed that the Pt-Ru bimetallic catalysts, especially Pt 0.5 Ru 0.5 /CNT, deliver higher hydrogen generation activity and durability than the Pt/CNT and Ru/CNT catalysts, indicating a remarkable Pt-Ru synergy. The underlying nature of this synergy is elucidated by combining kinetic and isotopic analyses with multiple techniques such as HAADF-STEM, EDX, XRD, H 2 -TPR, HRTEM, XPS and ICP-AES. The appropriate activation energy and entropy of activation for the reaction are mainly responsible for the highly active Pt 0.5 Ru 0.5 /CNT catalyst, and a volcano-shaped relationship for the reactivity of these five catalysts as a function of entropy of activation is proposed. The insights revealed here might guide the rational design of catalysts for this reaction, and the methodology could be extended to unravel the nature of other bi- or multi-metallic catalysts. [ABSTRACT FROM AUTHOR]

The factor dominating olefin to paraffin ratio in cobalt-catalyzed Fischer-Tropsch synthesis has been identified via experimental kinetic studies and theoretical calculations including the van der Waals interaction between the adsorbates and Co surfaces. The olefin to paraffin ratio for each carbon number is expressed as a function of adsorption energies of olefin and activation energies of hydrogenation steps. Density functional theory calculations on Co (0 0 0 1) surface were performed to provide a comprehensive fundamental insight for the chemistry governing the olefin to paraffin ratio. The calculated olefin to paraffin ratio decreases with chain length except for ethylene, which agrees well with the experimental results. The differences introduced to the adsorption energies of olefin by van der Waals functional are correlated with d-band center changes of the Co surface. The carbon number dependent adsorption energies of olefin, rather than the activation energies of their hydrogenation reactions, are the driven force for the carbon number dependent olefin to paraffin ratio. The insights may delineate a new blueprint for the rational design of catalysts with enhanced olefin to paraffin ratio. [ABSTRACT FROM AUTHOR]

Au/TS-1 catalyst prepared by deposition–precipitation method is efficient for propene epoxidation with H 2 /O 2 , and its performance is significantly affected by the preparation parameters and the properties of support. In this work, effects of the slurry aging time and Si/Ti ratio of support on the location of Au nanoparticles inside or outside the micropores of TS-1 supports are examined by multi-techniques such as in situ characterization and molecular dynamics simulation, and the as-obtained catalysts are tested for propene epoxidation. It is found that longer slurry aging time facilitates the evolution of Au complex into smaller sized species which is easier to locate inside the micropores. Accordingly, the as-reduced catalyst has more tiny Au nanoparticles that are extremely active inside micropores and shows higher initial PO formation rate (g PO h −1 g Au −1 ). In addition, high Si/Ti ratio of TS-1 also favors the deposition of Au nanoparticles inside micropores, resulting in higher initial activity but lower stable activity. [ABSTRACT FROM AUTHOR]

Highlights: [•] CO hydrogenation studied at methanation conditions, 483K, 1.85bar, H2/CO=3–10. [•] Integrated transient and steady-state modeling as a powerful tool for useful kinetic parameters. [•] Combining experiments and DFT investigations of kinetic isotopic effect. [•] Dominating CO activation pathway through hydrogen-assisted CO dissociation. [•] Two carbon pools and two corresponding reaction pathways for CH4 formation. [ABSTRACT FROM AUTHOR]

Abstract: Dehydrogenation of propane (DHP) was studied over a series of xCr–SBA-15 and xCr–Al2O3 catalysts, prepared by incipient wetness impregnation, to gain a better understanding of the nature and distribution of Cr species and their catalytic function. To this end, the catalysts were characterized by N2-physisorption, X-ray diffraction (XRD), UV-Raman spectroscopy, scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM–EDXS) and X-ray absorption spectroscopy (XANES and EXAFS). All these characterization techniques indicate that, at ⩽1 wt% Cr, SBA-15 contains exclusively uniform distribution of chromium as isolated Cr(VI) species in tetrahedral () coordination, whilst on γ-Al2O3 a fraction of oligomers (including dimers) is present along with isolated species. At ⩾5 wt% Cr, SBA-15 is dominated by crystalline α-Cr2O3 particles, besides a fraction of isolated Cr(VI) species in coordination. Remarkably, γ-Al2O3 contains mainly oligomers with different degrees of nuclearity and a fraction of isolated species but no Cr2O3 particles. Among xCr–SBA-15 catalysts, those containing exclusively isolated Cr species (i.e., ⩽1 wt% Cr) exhibit higher activity and selectivity with respect to per mole of Cr than the catalyst dominated by crystalline α-Cr2O3 particles (⩾5 wt% Cr). The intrinsic activity of these isolated Cr species is higher than those observed on γ-Al2O3 (⩽1 wt% Cr). Interestingly, among xCr–Al2O3, the catalysts dominated by oligomeric species show the highest activity, indicating the important role of the species in the reaction. In situ XAS studies evidence that active Cr sites are apparently generated on-site during the reaction and that Cr is typically in (III) oxidation state. Based on ex situ and in situ characterization results and catalytic data, it appears that the activity of the Cr species is different in xCr–SBA-15 and xCr–Al2O3. For xCr–SBA-15, isolated Cr(III) sites with coordination number greater than four are more active, selective and stable than Cr sites on the surface of crystalline Cr2O3. In contrast, for xCr–Al2O3, oligomeric Cr species are more active and selective than the isolated Cr sites. [Copyright &y& Elsevier]