Catalyst Species Research Articles

Promoting Propane Dehydrogenation with CO2 over the PtFe Bimetallic Catalyst by Eliminating the Non-selective Fe(0) Phase

Fine tuning the structure of bimetallic nanoparticles is critical toward understanding structure–activity relationships and further improving the catalytic performance in propane dehydrogenation (PDH). Excessive Fe species in the PtFe bimetallic catalysts promote carbon deposition leading to low propylene selectivity, and it remains challenging to synthesize well-defined PtFe catalysts while selectively eliminating the excessive Fe. Herein, we show that the formation of coke can be significantly inhibited by introducing CO2 into the PDH over PtFe catalysts, where CO2 effectively eliminates the active Fe(0) coking sites without changing the catalytic surface structure of the PtFe alloy. With a CO2/C3H8 feeding ratio of 0.20, the Pt1Fe7/S-1 catalyst shows the highest propylene production rate and decreased amount of coke from 18.8 to 1.0 wt % compared with dehydrogenation without CO2. X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and 57Fe Mössbauer results indicate that it is the oxidation of excessive unalloyed Fe species during the CO2-PDH reaction, instead of the reverse Boudouard reaction (CO2 + C = 2CO), that significantly inhibits the carbon deposition. This work provides a promising strategy for tuning the structure of PtFe bimetallic catalysts under reaction conditions and improving the performance of the PDH reaction.

Evidence and Governing Factors of the Radical-Ion Photoredox Catalysis

Photoinduced electron transfer of a radical species provides a promising strategy for expanding the scope of photoredox catalysis. However, the mechanism underlying the radical-ion photoredox catalysis has yet been fully established. In the present research, we investigated the generation and annihilation of the key catalytic intermediate in the borylation reaction of aryl halides. To disentangle the intricate catalysis cycle, we conducted experiments using electrochemical, transient absorption and photoluminescence spectroscopy and quantum chemical calculation techniques. Our mechanistic studies provided convincing evidence that the active catalyst species is the excited-state radical anion. We also found that the catalyst activation is retarded by multiple annihilative processes, among which the intrinsic relaxation of the excited-state radical anion of the catalyst is the most destructive. We expect these mechanistic findings will guide the future development of photoredox catalysis of radical species.

Transition Metal Catalyzed Enantioselective Migratory Functionalization Reactions of Alkenes through Chain‐Walking

Comprehensive Summary“Chain‐walking” reactions could realize to modify a molecule at the position far away from the active site. It has been a hot topic in the research field of organic synthesis. As a number of achievements have been made by researchers, the products of transition metal‐catalyzed chain‐walking reactions could be obtained with good enantioselectivity. This review summarized the researches on the asymmetric chain‐walking reactions catalyzed by transition metal in the past decade. These works are classified according to the species of metal catalysts.

NOx Reduction over Smart Catalysts with Self-Created Targeted Antipoisoning Sites.

Selective catalytic reduction of NOx in the presence of alkali (earth) metals and heavy metals is still a challenge due to the easy deactivation of catalysts. Herein, NOx reduction over smart catalysts with self-created targeted antipoisoning sites is originally demonstrated. The smart catalyst consisted of TiO2 pillared montmorillonite with abundant cation exchange sites to anchor poisoning substances and active components to catalyze NOx into N2. It was not deactivated during the NOx reduction process in the presence of alkali (earth) metals and heavy metals. The enhanced surface acidity, reducible active species, and active chemisorbed oxygen species of the smart catalyst accounted for the remarkable NOx reduction efficiency. More importantly, the self-created targeted antipoisoning sites expressed specific anchoring effects on poisoning substances and protected the active components from poisoning. It was demonstrated that the tetrahedrally coordinated aluminum species of the smart catalyst mainly acted as self-created targeted antipoisoning sites to stabilize the poisoning substances into the interlayers of montmorillonite. This work paves a new way for efficient reduction of NOx from the complex flue gas in practical applications.

Kinetic Rationalization of Nonlinear Effects in Asymmetric Catalytic Cascade Reactions under Curtin-Hammett Conditions.

Observations of nonlinear effects of catalyst enantiopurity on product enantiomeric excess in asymmetric catalysis are often used to infer that more than one catalyst species is involved in one or more reaction steps. We demonstrate here, however, that in the case of asymmetric catalytic cascade reactions, a nonlinear effect may be observed in the absence of any higher order catalyst species or any reaction step involving two catalyst species. We illustrate this concept with an example from a recent report of an organocatalytic enantioselective [10 + 2] stepwise cyclization reaction. The disruption of pre-equilibria (Curtin–Hammett equilibrium) in reversible steps occurring prior to the final irreversible product formation step can result in an alteration of the final product ee from what would be expected based on a linear relationship with the enantiopure catalyst. The treatment accounts for either positive or negative nonlinear effects in systems over a wide range of conditions including “major-minor” kinetics or the more conventional “lock-and-key” kinetics. The mechanistic scenario proposed here may apply generally to other cascade reaction systems exhibiting similar kinetic features and should be considered as a viable alternative model whenever a nonlinear effect is observed in a cascade sequence of reactions.

Highly stable Mo-based catalysts for selective hydrodeoxygenation to produce diesel-like hydrocarbons

Unsaturated hydrocarbons with C=C bonds prepared from bio-oils are excellent substitutes for petrochemical oils, which are characterized by high stability and high calorific value. Currently, the most effective way to prepare such compounds is hydrodeoxygenation of unsaturated oils and fats, and an ideal approach would be selective hydrodeoxygenation. Mo/C-based catalysts exhibit excellent performance in this type of reaction; however, their low oxidation resistance limits their stability, and hence their applicability. Herein, we report a novel Mo-based catalyst with high stability by exploiting the variable valences of transition metal dopants. A Cu-Mo/C catalyst proved to be most effective for the hydrogenation of methyl oleate to produce diesel-like hydrocarbons, showing excellent activity and stability. XRD, XPS, and EPR characterizations have indicated that the presence of oxygen vacancies is conducive to the preferential adsorption of carbon–oxygen bonds and avoids the hydrogenation of C=C bonds. Subsequently, the active species of the Mo-based catalyst promotes the cleavage of the carbon–oxygen bonds.

Spectroscopic Characterization of the Synergistic Mechanism of Ruthenium-Lithium Hydrides for Dinitrogen Cleavage.

Elucidating the role of alkali/alkaline earth metal hydrides in dinitrogen activation remains an important and challenging goal for spectroscopic studies of bulk systems, because their spectral signatures are often masked by the collective effects. Herein, mass-selected photoelectron velocity-map imaging spectroscopic and quantum chemical calculation techniques are utilized to explore the promotion mechanism of LiH in the Ru-based catalysts toward N2 activation. The RuHN2- anion is determined to be a N2-tagged complex. In contrast, the RuHN2(LiH)n- (n = 1 and 2) anions are characterized to have N≡N bond-cleaved ring structures. These observations indicate that the complexation of LiH to RuH- significantly facilitates N≡N bond cleavage. Theoretical analyses show that the synergy between Ru and LiH efficiently lowers the energy barrier of N≡N bond cleavage. These findings clarify the pivotal roles played by the LiH species in the transition metal catalysts for N2 activation and have important practical implications for the prospective design of high-performance catalysts via metal tuning strategies.

Improvement of interfacial interaction between poly(arylene sulfide sulfone) and carbon fiber via molecular chain grafting

An effective method for improving interfacial properties between carbon fiber and poly(arylene sulfide sulfone) (PASS) via grafting amine containing PASS (NH2-PASS) chains onto carbon fiber surface is reported here. X-ray photoelectron spectroscopy (XPS) results confirmed the successful grafting of molecular chains and obvious features including thin polymer layer and/or resin pitch were observed on the scanning electronic micrograph of carbon fiber surface. The contact angle obviously changed with the modification step of carbon fiber. And the obvious interface of composites can be observed on the atomic force micro-scopy (AFM). In addition, the effect of catalyst species and molecular weight of NH2-PASS chain on the interfacial properties were also studied. The interfacial shear test and macroscopic mechanical test show that grafting low-molecular-weight NH2-PASS onto carbon fiber with hexafluorophosphate azabenzotriazole tetramethyl uranium (HATU) (CF@AU-LPASS) as catalyst enhance the interfacial properties and composite performance to the largest extent. The interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of CF@AU-LPASS reinforced PASS (CF@AU-LPASS/PASS) increased significantly by 28.3% and 71.8% respectively.

Construction of fluffy MnFe nanoparticles and their synergistic catalysis for selective catalytic reduction reaction at low temperature

Mn-based catalysts had favorable low temperature SCR performance but it was restricted its practical application due to easy deactivation. Different proportions of Fe species were doped into Mn-MOFs as a precursor by a simple one-pot solvothermal method. A series of MnOx, Mn4FeOx, MnFeOx and MnFe4Ox catalysts with different structures were obtained by calcining under air. MnFeOx showed better catalytic activity and water resistance at low temperature. The NO conversion was 96% at 200 °C and kept above 85% in the range of 120–280 °C. Through a series of characterizations, it was found that manganese and iron species were highly dispersed in the MnFeOx catalyst, which was a vital factor for SCR reaction. Moreover, the MnFeOx catalyst not only possessed a large specific surface area and pore volume but also had a better reduction ability and abundant oxygen vacancies to adsorb and activate the reaction gases. Therefore, the catalyst possessed a satisfactory catalytic performance. More importantly, charging transfer between active manganese species and iron species in MnFeOx catalyst could effectively inhibit H2O poisoning and improve the stability of the catalyst. The results of in situ DRIFTs experiments showed that the low temperature NH3-SCR reaction of MnFeOx catalyst followed L-H mechanism. The effects of Fe doping content on the crystal structure of active species and SCR performance of manganese-based catalysts were revealed.

In situ formation of a nickel-iron-sulfur bifunctional catalyst within a porous polythiophene coating for water electrolysis

Developing readily scalable synthesis techniques for electrocatalysts is highly desirable for large-scale high-efficiency energy storage by water electrolysis. In this work, a coupled procedure of direct electrodeposition and in situ chemical transformation is presented to synthesize a nickel-iron-sulfur (Ni–Fe–S) composite catalyst. A polythiophene (PTh) coating with abundant micro/nano holes is directly deposited on graphite electrode at a constant potential. Two precursor solutions were injected onto and completely absorbed by the porous PTh coating, within which they spontaneously combine to form active species for catalysis. The PTh coating functions as a monolithic conductive matrix that well captures and disperses the catalyst species and thus decreases the contact resistance across the phase interfaces. The prepared catalyst shows a high catalytic performance for both hydrogen and oxygen evolution reactions. It requires a full cell voltage of about 2.0 V to afford a current density of 100 mA cm−2 in 1.0 M KOH, with no activity degradation at least for 24 h. The active species for the cathodic and anodic catalysis are different and discussed separately. This work indicates that in situ chemical synthesis within a porous conductive polymer coating is a promising approach for preparing high efficiency electrocatalysts.

Non-oxidative dehydrogenation of propane to propene over Pt-Sn/Al2O3 catalysts: Identification of the nature of active site

Supported platinum-tin over various oxide supports have been widely recognized as the efficient propane dehydrogenation catalysts due to their excellent activity, C3H6 selectivity and stability after oxidative regeneration. The nature of the active species of the Pt-Sn catalysts is still under debate. Here, by controlling the formation of Pt-Sn chloride complexes during the impregnation stage, we managed to tune spatial distribution of Pt and Sn species over Al2O3 and further influence the degree of alloying between reduced Pt and Sn. The structure characterization revealed the co-impregnation method tend to generate PtSn alloy active site, while sequential impregnation of Pt on pre-synthesized Sn/Al2O3 tend to form Sn partially modification of Pt. Performance evaluation demonstrated that the Pt clusters partially modified by reduced Sn exhibited much higher activity and longer life-time than PtSn alloy counterparts under similar C3H6 selectivity. With further optimization, the Pt-Sn/Al2O3 prepared by sequential impregnation method achieved over 64.1 μmol/gcat/h propylene formation rate at the 30% propane conversion with the C3H6 selectivity exceeding 99%.

Sulfonated magnetic sugarcane bagasse as an efficient natural polymer-based catalyst for the synthesis of nitrogen-containing heterocyclic rings in water.

A new species of catalysts that are prepared from biocompatible materials is demonstrated. Sulfonated magnetic sugarcane bagasse has been synthesized as a novel biodegradable and robust heterogeneous catalyst for organic transformations. The catalyst was characterized by different techniques. Next, the efficiency of this acid catalyst was examined with multi-component reactions for the synthesis of some biologically active scaffolds of heterocyclic organic compounds such 2,3-dihydroquinazolin-4(1H)-ones and pyrido[2,3-d]pyrimidin-4-one derivatives. A wide range of these heterocycles was synthesized with excellent yields in short reaction times under green conditions. In all cases, sulfonated magnetic sugarcane bagasse could be simply collected using an external magnet and reused for several runs without any significant loss of catalytic activity.

Synthesis of mesoporous TiO2-Al2O3 composites supported NiW hydrotreating catalysts and their superior catalytic performance for heavy oil hydrodenitrogenation

Mesoporous TiO2-Al2O3 composites with different Ti/Al molar ratios were successfully synthesized via pre-hydrolysis co-precipitation method and the corresponding NiW supported hydrotreating (HDT) catalysts were prepared via incipient wetness co-impregnation method. The effects of Ti modification on the physicochemical properties of the prepared supports and catalysts were determined by the characterization techniques such as XRD, FTIR, N2 physical adsorption–desorption, SEM-EDS, ICP-OES, NH3-TPD, H2-TPR, HRTEM and XPS. Finally, quinoline, indole and coker gas oil (CGO) were used respectively as probes to investigate the Ti modification on the hydrodenitrogenation (HDN) performances of the corresponding catalysts. The results show that the TiO2-Al2O3 samples synthesized by the pre-hydrolysis co-precipitation method achieved the level of molecular composition, and the crystalline structures, pore properties and acidity properties of the TiO2-Al2O3 supports varied with the Ti/Al molar ratios. Ti modification can modulate the interaction between the active metals and the supports (MSI), and promote the formation of more so-called “type II” NiWS active phases. Furthermore, the existence of Ti3+ species in catalysts NiW/TA1 and NiW/TA2 enhanced the sulfidation degrees of active metals and weakened the bond energy between the active metals and sulfur atoms and further promoted the formation of more coordinatively unsaturated sites (CUS). The catalytic assessment results show that the HDN activities of the catalysts were effectively improved after Ti modification and the improvement was largely due to the promotion of the cleavage of the C(sp3)-N bond of basic nitrides and the C(sp2)-N bond of non-basic nitrides. The NiW/TA1 catalyst exhibited the highest performance in the HDT process of heavy oil, with a 15.8% and 12.4% improvement in HDN rate and hydrodesulfurization (HDS) rate compared with the unmodified catalyst, respectively, which was mainly attributed to its suitable pore properties, moderate MSI, large number of CUS, moderate stacking and highest active metal sulfidation degree of the active phase.

Mechanism, Stereoselectivity, and Role of O2 in Aza-Diels-Alder Reactions Catalyzed by Dinuclear Molybdenum Complexes: A Theoretical Study.

The aza-Diels-Alder-type reaction between imines and functionalized alkenes is one of the most versatile approaches to obtain piperidine derivatives. When using the Lewis acid [Mo2(OAc)4] (CAT) as a catalyst, it was found that the activation of CAT by O2 was essential for an efficient reaction. In this paper, the mechanism and stereoselectivity of the aza-Diels-Alder reaction between aromatic acyl hydrozones 1 and Danishefsky diene 2 under uncatalyzed and catalyzed (CAT not activated by O2 and CAT activated by O2) conditions have been studied by density functional theory (DFT) calculation. The results show that the uncatalyzed reaction is difficult to proceed at room temperature due to the high energy barrier. The CAT not activated by molecular oxygen has catalytic activity but not too much. When CAT is activated by O2, CATO2 may be the correct catalytic species, which results in a dramatic increase of reaction activity. The reaction mechanisms with/without the catalyst are different. The uncatalyzed reaction is concerted for both the endo and exo pathways. For the CAT-catalyzed reaction, the endo pathway is concerted, but the exo pathway is nonconcerted and involves two steps. The endo product is the main product for the reaction catalyzed by CAT, while for reactions catalyzed by CATO1 and CATO2, the endo and exo products can be obtained. The reaction activity is directly correlated to the atomic charges of two coupling C atoms. Our work explains the experimental results, determines the structure of the O2-activated catalyst species, and provides predictions for the reaction activity and stereoselectivity controlling.

Oxalate regulate the redox cycle of iron in heterogeneous UV-Fenton system with Fe3O4 nanoparticles as catalyst: Critical role of homogeneous reaction

The redox cycle of iron is a well-known rate-determining step for hydroxyl radical generation in photo-Fenton system. In this study, oxalate was employed as regulator to enhance the degradation of Orange II in Fe3O4 magnetic nanoparticles (NPs)-catalyzed heterogeneous UV-Fenton system. Results showed that the oxalate could interact with the surface ≡FeIII species of catalyst, which weakened the bond of ≡FeIII–O and promoted the leaching of iron ions. Then the redox cycle of iron and generation of HO· would be accelerated via the homogeneous UV-Fenton reaction. The degradation rate constant of Orange II reached 0.220 min−1 when additional oxalate concentration was 0.4 mM, which was 2.5 times as high as that without oxalate in heterogeneous UV-Fenton system. In this case, the removal efficiencies of color and TOC were 99.3% and 92.0% after 30 and 120 min treatment, respectively. In addition, based on the results of XRD and XPS characterization, it could be deduced that the crystal structure and elemental configuration of Fe3O4 magnetic nanoparticles could be maintained after reaction. Besides, the results of FTIR and magnetization characterization indicated that the C2O42− on surface of catalyst could be degraded and the catalyst could be easily separated from aqueous by applying an external magnetic field. The Fe3O4 magnetic nanoparticles showed high catalytic stability and reusability under the regulation of oxalate due to the fact that the leached iron ions could be re-adsorbed on the catalyst after treatment.

Sulfur-resistant methane combustion invoked by surface property regulation on palladium-based catalysts

Improving sulfur resistance of palladium-based catalysts is crucial for the sustained and efficient methane combustion. Herein, the surface property of catalysts was rationally tailored to tune the dynamic behavior of sulfur species on active sites and supports, which was qualitatively and quantitatively studied through a combination of in-situ and on-line characterizations. Results reveal that the content of accumulated sulfur species in catalysts increased linearly (0.20–0.59 mmol SO2·g−1) with the rise of the surface basic sites, while it was restrained in catalysts with high surface acidity. The zirconium doped alumina supported palladium catalyst exhibited a moderate surface acidity/basicity, meanwhile it showed an excellent catalytic activity in the presence of SO2 and after regeneration. Its enhanced sulfur resistance derived from the proper interaction between supports and sulfur species, the facile decomposition of zirconium sulfates, the suppressed formation of surface/bulk aluminum sulfates, as well as the effective regeneration of PdO phase. The established quantitative correlation between accumulated sulfur species, surface acidity/basicity and catalytic performance paves the way to develop sulfur-resistant palladium-based catalysts for oxidation reactions.

Tuning Active Species in N-Doped Carbon with Fe/Fe3C Nanoparticles for Efficient Oxygen Reduction Reaction.

Transition metal-nitrogen-carbon (M-N-C) catalysts (M = Fe, Co, etc.) are the most promising substituents of Pt-based catalysts for oxygen reduction reaction (ORR). However, the insufficient active species in catalysts inevitably hamper their widespread applications. Herein, we report the regulation of the active species in the catalysts of multicomponent N-doped carbon with Fe/Fe3C nanoparticles by polydopamine (PDA) coating. It is found that the PDA is conducive to increasing the pyridinic, graphitic, and total N content in the carbon matrix. Benefiting from the chelating effects, the PDA further profits the formation of Fe-Nx structures and the implantation of Fe/Fe3C nanoparticles in the matrix during the pyrolysis. As expected, the resultant catalysts exhibit over 15 times mass activity toward ORR than nitrogen-doped carbon. Moreover, our developed catalysts show long-term stability as well as high methanol tolerance, which is superior to that of the commercial Pt/C electrode. This work provides a new avenue to explore a wider range of high-performance ORR electrocatalysts by regulating the active species.

Defect-induced formation of nickel silicates on two-dimensional MWW-type catalysts promoting catalytic activity for dry reforming of methane

The surface properties of a catalyst support not only influence the chemical states of the impregnated metals, but also the catalytic performance. Here, we systematically investigated the role of support morphology and defect concentration on the catalytic activity for dry reforming of methane (DRM) by comparing three-dimensional (3D) and two-dimensional (2D) aluminosilicate, borosilicate, and deboronated MCM tWenty-tWo (MWW)-type catalysts. Changes in the surface properties by delamination and/or deboronation as well as their influence on the formation of nickel silicates were experimentally demonstrated. Density functional theory (DFT) calculations were also performed to estimate the stabilization of metallic Ni clusters by the introduction of silanol defect sites on the MWW-type support. The delaminated–deboronated 2D MWW-type catalyst with the highest concentration of nickel silicates, and accordingly, the highest amount of silanol defect sites exhibited remarkable CH4 and CO2 conversions as well as stability for DRM. Unlike conventional Ni/γ-Al2O3, the Ni species in nickel silicate 2D MWW-type catalysts were readily reduced to metallic Ni even without pre-reduction by H2, which is advantageous for practical applications.

Analysis of the Continuous Feeding of Catalyst Particles during the Growth of Vertically Aligned Carbon Nanotubes by Aerosol-Assisted CCVD.

Aerosol-assisted catalytic chemical vapor deposition (AACCVD) is a powerful one-step process to produce vertically aligned carbon nanotubes (VACNTs), characterized by the continuous supply of the catalyst precursor (metallocene). The behavior of catalyst species all along the synthesis is essential for the continuous growth of VACNTs. It is there investigated through detailed observations and elemental analyses at scales of VACNT carpets and of individual CNTs. Our approach is based on two complementary experiments: quenching of the sample cooling, and sequential injection of two distinct metallocenes. Metal-based nanoparticles nucleated in the gas-phase during the whole synthesis duration are shown to diffuse in between the growing VACNTs from the top of the CNT carpet towards the substrate. They are much smaller than the catalyst particles formed on the substrate in the initial steps of the process and evidences are given that they continuously feed these catalyst particles at the VACNT roots. Particularly, the electron energy-loss spectroscopy (EELS) analyses of metal-based segments found into a single CNT show that the second injected metal is very gradually incorporated in the particle initially formed from the metal firstly injected. The feeding of the catalyst particles by the nanoparticles continuously nucleated in the gas-phase is therefore an essential feature of the base-growth of CNTs by AACCVD.

Hydrogenation of Esters by Manganese Catalysts

AbstractThe hydrogenation of esters catalyzed by a manganese complex of phosphine‐aminopyridine ligand was developed. Using this protocol, a variety of (hetero)aromatic and aliphatic carboxylates including biomass‐derived esters and lactones were hydrogenated to primary alcohols with 63–98% yields. The manganese catalyst was found to be active for the hydrogenation of methyl benzoate, providing benzyl alcohol with turnover numbers (TON) as high as 45,000. Investigation of catalyst intermediates indicated that the amido manganese complex was the active catalyst species for the reaction.magnified image