Dual-site Catalyst Research Articles

Metal–Support Interactions within a Dual-Site Pd/YMn2O5 Catalyst during CH4 Combustion

Catalytic combustion is a promising technology for removing unburnt CH4 from natural gas vehicle exhaust gas under lean-burn conditions. Supported Pd catalysts are widely applied and studied for use in CH4 oxidation at <500–550 °C, with their activities significantly affected by the interactions between Pd and the support, depending on the dispersion, Pd valence state, and properties of the support. However, whether metal–support interactions are favorable in catalytic CH4 oxidation is unclear. We therefore used YMn2O5 as a catalytically active support to prepare a dual-site catalyst, Pd/YMn2O5, which shows high CH4 catalytic activities in dry and wet atmospheres. Experimental and density functional theory studies reveal that the CH4 adsorption and lattice oxygen activity of the support are significantly enhanced due to the interactions between PdOx and YMn2O5. In situ X-ray photoelectron spectroscopy shows that Pdx+ (x = 2 and 4) and Mnx+ (x = 3 and 4) species undergo changes during CH4 oxidation. The PdOx sites are the main active sites, as the CH4 activation barrier is much lower than those at other sites. YMn2O5 and PdOx participate in CH4 activation and oxidation via active oxygen transfer.

Mitigating Coke Formations for Dry Reforming of Methane on Dual-Site Catalysts: A Microkinetic Modeling Study

Mitigating Coke Formations for Dry Reforming of Methane on Dual-Site Catalysts: A Microkinetic Modeling Study

Boosting CO desorption on dual active site electrocatalysts for CO2 reduction to produce tunable syngas

The electrochemical conversion of CO 2 to generate syngas (H 2 and CO) is regarded as a promising alternative technique to facilitate CO 2 reduction in the ambient atmosphere. However, it is still a great challenge to acquire high catalytic activity with an adjustable H 2 /CO ratio over a wide range. Here, we construct an electrocatalyst with Fe-containing dual active sites on N-doped porous carbon (Fe/FeN 4 C) to promote a CO 2 reduction reaction for tunable syngas production. The Fe/FeN 4 C catalysts have a positive onset potential (−0.18 V RHE ), approximately 100% of the sum of the Faradaic efficiency (FE) of CO and H 2 , a high total current density (>39.33 mA cm −2 ), and a wide H 2 /CO ratio (1.09∼7.08). Density functional-theory calculations suggest that the Fe single atoms dispersed into the N-doped carbon structure, along with the incorporation of Fe nanoparticles, may decrease the adsorption energy of ∗CO, thus synergistically enhancing the catalytic activity. Fe/FeN 4 C serves as a dual active site catalyst for CO 2 RR to syngas The Fe/FeN 4 C catalyst displays superior CO 2 RR performance toward syngas DFT study confirms the decrease in ∗CO adsorption energy benefits syngas production There is interest in developing active catalysts for the electrochemical conversion of carbon dioxide to syngas with a tunable H 2 /CO ratio. Here, Hua et al. report Fe-containing, N-doped, porous carbon to promote CO 2 reduction for tunable syngas production.

Atomic Co/Ni dual sites with N/P-coordination as bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries

Metal-nitrogen-carbon (M-N-C) single-atom catalysts exhibit desirable electrochemical catalytic properties. However, the replacement of N atoms by heteroatoms (B, P, S, etc.) has been regarded as a useful method for regulating the coordination environment. The structure engineered M-N-C sites via doping heteroatoms play an important role to the adsorption and activation of the oxygen intermediate. Herein, we develop an efficient strategy to construct dual atomic site catalysts via the formation of a Co1-PN and Ni1-PN planar configuration. The developed Co1-PNC/Ni1-PNC catalyst exhibits excellent bifunctional electrocatalytic performance in alkaline solution. Both experimental and theoretical results demonstrated that the N/P coordinated Co/Ni sites moderately reduced the binding interaction of oxygen intermediates. The Co1-PNC/Ni1-PNC endows a rechargeable Zn-air battery with excellent power density and cycling stability as an air-cathode, which is superior to that of the benchmark Pt/C+IrO2. This work paves an avenue for design of dual single-atomic sites and regulation of the atomic configuration on carbon-based materials to achieve high-performance electrocatalysts.

An uncoordinated tertiary nitrogen based tricarboxylate calcium network with Lewis acid-base dual catalytic sites for cyanosilylation of aldehydes.

The design and utilization of dual sites for synergistic catalysts has been recognised as an efficient method towards high-efficiency catalysis in the cyanosilylation of aldehydes, which gives key intermediates for the synthesis of a number of valuable natural and pharmaceutical compounds. However, most of the reported dual-site catalysts for this reaction were homogeneous, accompanied by potential deactivation through internal complexation of the dual sites. Herein, by the rational selection of an uncoordinated tertiary nitrogen based tricarboxylic ligand (tris[(4-carboxyl)-phenylduryl]amine, H3TCBPA), a new three-dimensional calcium-based metal-organic framework (MOF), Ca3(TCBPA)2(DMA)2(H2O)2 (1, where TCBPA = ionized tris[(4-carboxyl)-phenylduryl]amine and DMA = N,N-dimethylacetamide), possessing accessible dual catalytic sites, Lewis-basic N and Lewis-acidic Ca, has been designed and constructed by a one-pot solvothermal reaction. As expected, 1 is capable of dually and heterogeneously catalysing the cyanosilylation of aldehydes at room temperature, and can be reused for at least 6 runs with a maximum turnover number (TON) of 1301, which is superior to most reported cases. Additionally, 1 shows CO2 adsorption ability and conversion with epoxides, which is beneficial for the establishment of a sustainable society.

A New Dual Site Catalyst Design Concept for Promoting Reaction Kinetics of the Oxygen Reduction Reaction

Introduction Oxygen reduction reaction (ORR) accounts for a substantial portion of the energy loss of the proton exchange membrane fuel cell system used for vehicle application. Current state of art ORR catalysts are typically Pt-alloy nanoparticles supported on carbon, of which surface Pt is solely the active site and their activity improvement is fundamentally limited by a “scaling relationship” [1]. In this study, we report the preparation of a SnOx/Pt−Cu−Ni heterojunctioned nanostructure as one potential strategy to break the scaling relationship in ORR. Experimental results confirm a significant activity enhancement compared with pristine Pt−Cu−Ni. Theoretical study suggests a dual-site cascade mechanism wherein the first two steps occur on SnOx sites, followed by transfer of the intermediate to adjacent Pt sites for the subsequent steps. This new catalyst design offers a plausible new approach to achieve high ORR activity on Pt alloys by introducing a second active site. Materials and Methods Pt−Cu−Ni alloy nanoparticles were synthesized by a solid-state chemistry method involving electrochemical treatment to generate a clean Pt surface prior to use in this study [2]. The as prepared Pt-Cu-Ni catalysts were first de-alloyed and then followed by immersion in SnCl2 solution for various time to coat with SnOx [3]. The resulting heterojunctioned catalysts were characterized by HR-TEM, XPS, XANES, as well as in electrochemical measurement to determine the activity and durability. The density functional theory (DFT) simulations were performed with the Quantum ESPRESSO package to calculate the energy states of reaction intermediates on different surface sites. Results and Discussion The conceptual design of the dual site catalyst, as shown in Figure 1(a). involves a possible migration of reaction intermediates during the steps of ORR. Such a migration should be driven by the difference in free energy of the sites. For proof-of-concept, we synthesized Pt−Cu−Ni nanoparticles and then deposited SnOx on the surface to form a heterojunctioned nanostructure (Figure 1(b)). SEM -EDX and XPS analyses confirmed that the SnOx content increased with deposition time, while the Pt:Cu:Ni ratio remained nearly unchanged before and after the deposition. XANES and high resolution XPS characterization suggest the oxidation state of the SnOx is primarily 4+. A decrease of electrochemical active surface area (ECSA) was observed as the deposition time (td) increases (Figure 1 (c)). The mass activity and specific activity (SA) both exhibit a volcano trend with respect to td with the highest values at td=5 min. The apparent SA increased by 40% as compared to the pristine Pt-Cu-Ni, and if only considering the interfacial sites, the estimated specific activity enhancement may be up to 10 folds (Figure 1(d)). While these results provide unambiguous experimental evidence of SnOx’s promoting effect, DFT simulation of the reaction pathways was performed to obtain insight into the possible mechanism (Figure 1(e)). The results suggest the SnOx and the neighboring Pt sites may have cooperated to find the most kinetically favored pathway (Figure 1(f)), wherein the O2 first protonated on SnOx sites to form *OOH and *O intermediates, and then the *O transfers to Pt sites to complete the remaining steps. Conclusion This study demonstrates a strategy to bypass the energy barrier bottleneck of ORR, providing one more dimension of design flexibility towards developing highly active electrode catalysts for fuel cell applications. References Kulkarni, A. et al., Chem. Rev. 2018, 118 (5), 2302−2312.Zhang, C. et al., J. Am. Chem. Soc. 2014, 136 (22), 7805−7808.Shen, X. et al., J. Am. Chem. Soc. 2019, 141(24), 9463-9467. Figure 1

Engineering Platinum-Oxygen Dual Catalytic Sites via Charge Transfer towards Highly Efficient Hydrogen Evolution.

A dual-site catalyst allows for a synergetic reaction in the close proximity to enhance catalysis. It is highly desirable to create dual-site interfaces in single-atom system to maximize the effect. Herein, we report a cation-deficient electrostatic anchorage route to fabricate an atomically dispersed platinum-titania catalyst (Pt1 O1 /Ti1-x O2 ), which shows greatly enhanced hydrogen evolution activity, surpassing that of the commercial Pt/C catalyst in mass by a factor of 53.2. Operando techniques and density functional calculations reveal that Pt1 O1 /Ti1-x O2 experiences a Pt-O dual-site catalytic pathway, where the inherent charge transfer within the dual sites encourages the jointly coupling protons and plays the key role during the Volmer-Tafel process. There is almost no decay in the activity of Pt1 O1 /Ti1-x O2 over 300 000 cycles, meaning 30 times of enhancement in stability compared to the commercial Pt/C catalysts (10 000 cycles).

Novel SiO2‐Supported Chromium Oxide/Chromocene Dual Site Catalysts for Synthesis of Bimodal UHMWPE/HDPE in Reactor Alloys

AbstractPolyethylene (PE) with bimodal molecular weight distribution (MWD) has drawn attention from both academia and industry. Bimodal PE combines great processability with outstanding mechanical properties. The single‐reactor technology process through a dual site catalyst system is an optimal strategy considering the economic effect and the manufacturing technique. To satisfy the demand of the dual site catalyst system for producing bimodal PE, novel SiO2‐supported chromium oxide/chromocene dual site catalysts for ethylene polymerization are successfully synthesized and systematically investigated. The novel catalysts are prepared by using the residual hydroxyl groups on the surface of SiO2‐supported chromium oxide (CrOx) Phillips catalyst to introduce chromocene. It is found that the novel dual site catalysts combine the advantages of chromocene catalyst (S‐9 catalyst) and Phillips CrOx/SiO2 catalyst. The activities of the dual site catalysts are doubled at least compared to the S‐9 catalyst due to the high activity of the CrOx active center. The products of the dual site catalysts show significantly high molecular weight (MW) with bimodal MWD through ultrahigh molecular weight polyethylene/high‐density polyethylene in reactor alloys. The dual site catalysts show great hydrogen response according to the hydrogen experiment results, which lead to the easy control of MW.

Microemulsion solventing-out co-precipitation strategy for fabricating highly active Cu–ZnO/Al2O3 dual site catalysts for reverse water gas shift

A new strategy for the preparation of dual site catalysts is introduced, which combines microemulsion technology and anti-solvent extraction technology.

Dual Site Lewis‐Acid Metal‐Organic Framework Catalysts for CO2 Fixation: Counteracting Effects of Node Connectivity, Defects and Linker Metalation

AbstractThree Zr‐oxo‐cluster node and porphyrin‐linker based MOFs, MOF‐525, PCN‐222 and PCN‐224 exhibiting different linker connectivities of 12, 8 and 6 and their porphyrin‐linker metalated analogues, were synthesized and tested as catalysts for CO2 fixation by using the cycloaddition of CO2 and propylene oxide to propylene carbonate as the test reaction. In general, the catalytic activity correlates with the connectivity of the Zr‐oxo nodes. The lowest connected PCN‐224 (6‐fold) exhibits a superior catalytic activity in this series, while higher connected PCN‐222 (8‐fold) and MOF‐525 (12‐fold) are less active. Interestingly, the catalytic activity of the higher connected MOFs significantly depends on defects. The (ideally) 12‐connected MOF‐525, however exhibiting 16% of missing linker defects, features a higher catalytic activity compared to the 8‐connected PCN‐222 with less defects. The overall catalytic activity is increased in dual site catalysts when the porphyrin linkers are metalated with Mn(III) and Zn(II) centers, which are acting as additional Lewis acid sites. Here, the metalated MOFs with higher connectivity exhibit the highest activity.

The influence of modification methods on the catalytic cracking of LPG over lanthanum and phosphorus modified HZSM-5 catalysts

In the present study, the effect of various modification methods on the activity of ZSM-5 modified catalysts has been studied in catalytic cracking (CC) of LPG. Lanthanum (La) has been loaded in HZSM-5 catalyst using ion exchange, and wet impregnation, and phosphorus (P) has been introduced by wet impregnation and dry impregnation methods. Physico-chemical properties of modified samples were evaluated by using XRF, ICP-OES, XRD, FT-IR, physical N2 adsorption and NH3-TPD characterization methods. Studying the catalytic performance of the samples revealed that wet impregnation could result in better efficiency than dry impregnation and ion-exchange methods. Afterwards, the dual site catalysts containing both La and P have been prepared using wet impregnation and physical mixture processes. It was found that HZSM-5 catalyst modified by successive wet impregnation of La and P could led to the higher conversion and yield of light olefins (ethylene + propylene) compared to the other samples. At the reaction temperature of 650 °C and GHSV of 433 ml min−1/gcat over the mentioned catalyst (in the presence of N2) yield of total light olefins and selectivity was acquired to be 51% and 62%, respectively.

Direct synthesis of dimethyl carbonate from methanol and carbon dioxide: A thermodynamic and experimental study

The direct synthesis of dimethyl carbonate (DMC) from carbon dioxide and methanol is an atom economic, green and promising process. The present work focuses on this process using calcined hydrotalcite (CHT) supported on hexagonal mesoporous silica (HMS) as a catalyst and phosphonium based ionic liquid (IL) as a promoter. Phosphonium based ionic liquids are good solvents for carbon dioxide and are better alkali promoters. Different modified Keggin type cesium modified heteropoly acids (HPA) supported on HMS were also used as catalysts, which give good comparison between dual site catalysts with altered acidity or/and basicity. The conversion and selectivity were measured as a function of concentration of reactants, catalyst and temperature. The experimental results were compared with the thermodynamic calculations and simulated results. Kinetic model was developed by proposing a reaction mechanism. Sustainability of the process was justified by using a heterogeneous catalyst and by avoiding the use of harmful organic solvents in the presence of an ionic liquid with supercritical CO2.

Kinetics and deactivation of a dual-site heterogeneous oxide catalyst during the transesterification of crude jatropha oil with methanol

In this work, a dual-site heterogeneous catalyst was used to study the kinetics of the transesterification of crude jatropha oil with methanol in batch experiments. Experimental data were obtained between 150 and 182°C at a constant molar ratio of alcohol to oil (11:1) and at a set catalyst concentration (3.32wt% based on weight of oil). Kinetic modelling was performed with an assumed pseudo-first order to describe the rate and catalyst deactivation, which suitably fit the experimental data. The model parameters were determined for both the mass transfer controlled and reaction regimes. The triglyceride (TG) conversion, methyl ester (ME) formation, and system thermodynamics were also evaluated. Based on the calculated values, within an acceptable range, the equilibrium constant, Ke=1.42; activation energy, Ea=161kJ/mol; and free energy, ΔG=−1286J/mol indicate that the developed model adequately described the methanolysis of the oil, which may be useful in reactor design and process simulation. The values that were calculated from the kinetic equations agree well with the experimental values, and the results help to understand and predict the behaviour of dual-site catalysts in practical applications for the production of biodiesel.

Dual-site hybrid catalysts for production of linear low-density polyethylene

Possibility of hybrid catalytic systems for ethylene dimerization and copolymerization including Ti (On-Bu)4, (C5H5)4Zr, and methylaluminoxane formation has been proved. Properties of copolymers formed in the presence of these systems were studied. It was shown that the main advantage of new catalytic systems is their flexibility that allows them to be used for production of a wide assortment of polyethylene. Developed catalytic systems make possible the production of polyethylene all branded assortment from ethylene as the only feedstock according to the one-reactor technological scheme.

Graphene-Supported Dual-Site Catalysts for Preparing Self-Reinforcing Polyethylene Reactor Blends Containing UHMWPE Nanoplatelets and in Situ UHMWPE Shish-Kebab Nanofibers

The catalytic ethylene polymerization on dual-site catalysts, supported on functionalized graphene, enables nanostructure formation in polyethylene reactor blends by in situ formation of uniformly dispersed ultrahigh molecular weight polyethylene (UHMWPE) nanoplatelets and in situ formed aligned UHMWPE shish-kebab nanofibers. For tailoring bimodal molar mass distributions, the quinolylsilylcyclopentadienylchromium(III) complex (Cr-1), producing UHMWPE with Mw > 3 × 106 g mol–1, is blended together with bisiminopyridine complexes of either chromium(III) (CrBIP), producing polyethylene (PE) wax (2 × 103 g mol–1), or iron(II) (FeBIP), producing PE with Mw = 2.0 × 105 g mol–1. Hence, the FeBIP/Cr-1 and CrBIP/Cr-1 molar ratios govern the PE/UHMWPE weight ratio without affecting the average molar mass of the individual PE fractions. In sharp contrast to conventional UHMWPE/PE reactor blends, the UHMWPE content is substantially increased up to 17 wt % without impairing melt processing. In the case of graphene-supported FeBIP/Cr-1, SEM and TEM analysis reveal that UHMWPE nanoplatelets are formed during polymerization. This is attributed to graphene-mediated mesoscopic shape replication. During injection molding, the UHMWPE nanoplatelets are transformed into aligned UHMWPE shish-kebab nanofibers, thus enabling efficient polyethylene matrix reinforcement.

Synthesis, Characterization, and Catalytic Activity of Alcohol-Functionalized NHC Gold(I/III) Complexes

A series of alcohol-functionalized NHC gold(I/III) complexes of the types [(L)Au(Cl or NTf2)] and [(L)AuCl3] (L = IPrEtOH, (1, 4, 5), IMesEtOH (2, 6), IMeEtOH (3, 7)) was readily synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The catalytic activity of the new complexes was assessed and compared with that of other gold catalysts, in the tandem 3,3-rearrangement–Nazarov reactions of enynyl acetate and the rearrangement reaction of alkynyloxirane. The new complexes appeared to be dual-site catalysts in both transformations, thanks to the presence of the primary alcohol function tethered to the NHC ligands.

Ruthenium nanoparticles supported on magnesium oxide: A versatile and recyclable dual-site catalyst for hydrogenation of mono- and poly-cyclic arenes, N-heteroaromatics, and S-heteroaromatics

The development of catalysts capable of promoting hydrogenation of aromatics while being resistant to poisoning by nitrogen- and sulfur-containing species is of much interest in connection with hydrotreating of fossil fuels. We report a catalyst composed of ruthenium nanoparticles supported on magnesia, designed to promote heterolytic hydrogen splitting and surface ionic hydrogenation pathways. The catalyst, prepared through a one-pot procedure, promotes the hydrogenation of mono- and poly-cyclic arenes, as well as N- and S-heteroaromatics representative of fossil fuels components. Of particular significance are the superior activity and wider substrate scope of the catalyst, in relation to other known supported noble metals, and the excellent recyclability and long catalyst lifetime. Based on our experimental data, a dual-site catalyst structure and an associated dual-pathway mechanism are proposed, which may have interesting implications for the development of new poison-tolerant noble metal catalytic systems.

A dual site catalyst for mild, selective nitrile reduction

A novel ruthenium bis(pyrazolyl)borate catalyst exhibits useful reactivity in ammonia borane dehydrogenation and water oxidation. It also enables unprecedented cooperative reduction reactivity in which boron and ruthenium centers work in concert to effect surprisingly selective nitrile reduction.

Dinuclear Transition Metal Complexes Consisting of an α-Diimine Complex Unit and a Half Sandwich Complex Unit as Dual Site Catalysts for the Polymerization of Ethylene

Dinuclear complexes consisting of an α-diimine complex unit with different ortho substituents at the aryl rings and a half sandwich complex unit were prepared, activated with methylaluminoxane (MAO) and tested for the polymerization of ethylene. The dinuclear catalysts bearing zirconium centers showed higher ethylene polymerization activities than the analogous titanium catalysts. Among the dinuclear catalysts with the same metal centers, the bulkier ortho substituents at the aryl moiety gave lower polymerization activities. The GPC spectra of the polyethylenes produced with the dinuclear catalysts displayed bimodal molecular weight distributions due to different active sites.

Silica Nanofoam (NF) Supported Single- and Dual-Site Catalysts for Ethylene Polymerization with Morphology Control and Tailored Bimodal Molar Mass Distributions

Highly active single- and dual-site catalysts supported on silica nanofoams (NF) enable the control of both polyethylene (PE) morphology and tailoring of bimodal PE molar mass distribution in catalytic ethylene polymerization. In a templating process, aqueous polystyrene (PS) nano suspensions are mineralized and calcinated at 600 °C, thus producing NF with specific surface area of 1200 m2/g and average pore diameter varying between 20 and 80 nm. Mineralization of the aqueous PS nano suspensions in a water-in-oil emulsion affords spherical NF with average diameter of 40 μm and control of pore size. The methylalumoxane- (MAO-) tethered NF immobilizes single-site catalysts such as metallocene (nBuZr), bisiminopyridine iron(II) (Fe) and constrained geometry chinolyl cyclopentadienyl chromium(III) complexes (Cr-1). Immobilization of binary blends of Fe/Cr-1 and nBuZr/Cr-1 affords NF-supported dual-site catalysts. The catalyst activity, PE particle size and molecular weight distribution varies as a function of the NF pore size. Typically, macroporous NF (pore size of 75 nm) are effective supports for Cr-1, producing ultrahigh molecular weight polyethylene (UHMWPE, Mw > 106 g/mol). Dual -site catalysts such as nBuZr/Cr-1 on mesoporous NF (pore size of 20 nm) enable tailoring of bimodal PE molar mass distribution with UHMWPE content increasing with increasing Cr-1/nBuZr molar ratio.