Catalytic Synergy Research Articles

Origin of Enhanced Activities for CO Oxidation and O2 Reaction over Composition-Optimized Pd50Cu50 Nanoalloy Catalysts

It has been shown experimentally that Pd50Cu50 nanoalloy achieves the maximum activity for CO oxidation (COox) and oxygen reduction reaction (ORR) on composition-tuned PdCu bimetallic catalysts, but the origin of this catalytic synergy remains unclear. In this work, results of our density functional theory (DFT) calculations show that the weakest adsorption strength of O2 in terms of the most pronounced charge transfer between Pd and Cu is responsible for the experimentally observed highest catalytic activity of Pd50Cu50 catalyst for both COox and ORR over a series of composition-tuned PdCu nanoalloys. For COox, the lowest barrier energy is attributed to the weakest adsorption strength of O2 on Pd50Cu50 catalyst. In ORR, the lowest barrier energy for O2 dissociation and also the weakest adsorption strength of O, OH, and OOH species are related to the weakest adsorption strength of O2 over the catalyst with a 50:50 ratio of Pd/Cu. Our work represents the first attempt to address an in-depth correlation bet...

Catalytic Synthesis of Propylene Carbonate from CO2 and Propylene Oxide on Fixed Bed

An efficient catalytic system is developed for propylene carbonate synthesis from carbon dioxide and propylene oxide on fixed bed reactor. The active component screening results make sure that the efficient catalyst is composed of methyl imidazole onium salt, zinc bromide and phenol formaldehyde resin. The prepared catalyst proves 72 h long running with 96 % yield and 98 % selectivity of propylene carbonate. The novel extruded catalyst for PC synthesis used in fixed bed is developed. PR, MimCl and ZnBr2 have the catalytic synergy for the reaction. The catalyst composition is optimized.

Unraveling the Catalytic Synergy between Ti3+ and Al3+ Sites on a Chlorinated Al2O3: A Tandem Approach to Branched Polyethylene

AbstractAn original step‐by‐step approach to synthesize and characterize a bifunctional heterogeneous catalyst consisting of isolated Ti3+ centers and strong Lewis acid Al3+ sites on the surface of a chlorinated alumina has been devised. A wide range of physicochemical and spectroscopic techniques were employed to demonstrate that the two sites, in close proximity, act in a concerted fashion to synergistically boost the conversion of ethylene into branched polyethylene, using ethylene as the only feed and without any activator. The coordinatively unsaturated Al3+ ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti3+ sites for the traditional ethylene coordination polymerization.

Unraveling the Catalytic Synergy between Ti(3+) and Al(3+) Sites on a Chlorinated Al2 O3 : A Tandem Approach to Branched Polyethylene.

An original step-by-step approach to synthesize and characterize a bifunctional heterogeneous catalyst consisting of isolated Ti(3+) centers and strong Lewis acid Al(3+) sites on the surface of a chlorinated alumina has been devised. A wide range of physicochemical and spectroscopic techniques were employed to demonstrate that the two sites, in close proximity, act in a concerted fashion to synergistically boost the conversion of ethylene into branched polyethylene, using ethylene as the only feed and without any activator. The coordinatively unsaturated Al(3+) ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti(3+) sites for the traditional ethylene coordination polymerization.

Synergistic catalytic properties of bifunctional nanoalloy catalysts in rechargeable lithium-oxygen battery

Abstract The understanding of factors influencing the performance of catalysts in the air cathode of a rechargeable lithium-oxygen battery, including overpotentials for oxygen reduction/evolution and discharge capacity, is essential for exploration of its ultimate application. This report describes new findings of an investigation of PdCu nanoalloys as cathode catalysts. Alloying Pd with oxophilic base metals such as Cu leads to reduction of the overpotentials and increase of the discharge capacity. The nanoalloy structures depend on the bimetallic composition, with an atomic ratio near 50:50 featuring mixed bcc and fcc structures. The discharge potential exhibits a maximum while the charge potential display a minimum in the range of 20–50% Cu, closer to 25% Cu, both of which correspond to a maximum reduction of the discharge-charge overpotentials. The discharge capacity displays a gradual increase with Cu%. This type of catalytic synergy is believed to be associated with a combination of ensemble and ligand effects. In particular, the activation of oxygen on Pd sites and oxygen oxophilicity at the alloyed Cu sites in the catalyst may have played an important role in effectively activating oxygen and maneuvering surface superoxide/peroxide species. These findings have implications for the design of multifunctional cathode catalysts in rechargeable lithium-oxygen batteries.

Composition-Tunable PtCu Alloy Nanowires and Electrocatalytic Synergy for Methanol Oxidation Reaction

The ability to impart Pt-based catalysts with high catalytic activity and low cost is essential for advancing fuel cell technologies. This report describes the synthesis of composition-tunable PtCu alloy nanowires (NWs) of ultrathin diameters (ca. 1 nm) to create synergistic catalytic sites along the nanowire surfaces. The bimetallic NWs exhibit composition-tunable fcc-type alloy phase. The electrocatalytic properties of the PtCu alloy NWs for methanol oxidation reaction were shown to display an intriguing composition-dependent catalytic synergy. The maximum mass activity for Pt32Cu68 NWs was about 2 times higher than that of Pt NWs. It also exhibited the highest stability and tolerance to CO poisoning. The enhanced activity and stability were attributed to a bifunctional synergy whereby the alloyed Cu atoms in the Pt lattice provides CO-maneuvering sites for reducing the poisoning effect of CO intermediate species on the active surface sites of the NWs.

New acetaminophen amperometric sensor based on ferrocenyl dendrimers deposited onto Pt nanoparticles

In this work, the electrocatalytical properties and kinetic characteristics of new electrodes modified with Pt nanoparticles (PtNPs) and three generations of ferrocene functionalized dendrimers have been investigated as new acetaminophen amperometric sensors. The catalytic synergy of PtNPs with the ferrocene groups is discussed in relation to the ferrocenyl dendrimer generation and their properties. The modified electrodes show excellent catalytic responses toward the oxidation of acetaminophen, with good reproducibility. The overpotential for oxidation of acetaminophen at pH 7 is decreased, and the current response significantly enhanced. The best dendrimer/PtNPs/Pt electrode shows several wide linear concentration ranges for the acetaminophen oxidation from 0 to beyond 17 mM. At 0.5 V vs. SCE, the first linear range extends from 0 to 100 μM (y = 0.0131x − 0.0028; R2 = 0.9996) and the last from 10 mM to at least 17 mM (y = 0.0024x + 26.6; R2 = 0.9977). This fact turns the developed acetaminophen sensor in the device with the widest application range. In addition, the sensor allows measuring acetaminophen in the presence of interfering substances as glucose, dopamine, uric acid, and ascorbic acid, and it has been successfully applied to the determination of acetaminophen in three pharmaceutical formulations.

Nanostructured Au/Ti bimetallic electrodes in selective anodic oxidation of carbohydrates

The anodic oxidation of species possessing potentially oxidisable functionalities has been studied on a non conventional electrode system based on a Ti substrate partially coated by electrodeposited Au nanoparticles. The results show that the electrocatalytic behaviour of the Au nanoparticles, in the frame of such a bimetallic system, is very different from that of pure Au. In particular, catalytic synergy between Au and Ti leads to the electro-oxidation of aldoses and aldehydes, while ketoses and monohydric alcohols result electrochemically inert. A different behaviour has been observed in the case of bulk Au surface: all the species under investigation undergo oxidation, so that no selectivity is activated. A correlation is found between the actual electroactivity and the nature of the functional group, when considered together with the chemical structure of the molecule.

PdCu Nanoalloy Electrocatalysts in Oxygen Reduction Reaction: Role of Composition and Phase State in Catalytic Synergy.

The catalytic synergy of nanoalloy catalysts depends on the nanoscale size, composition, phase state, and surface properties. This report describes findings of an investigation of their roles in the enhancement of electrocatalytic activity of PdCu alloy nanoparticle catalysts for oxygen reduction reaction (ORR). Pd(n)Cu(100-n) nanoalloys with controlled composition and subtle differences in size and phase state were synthesized by two different wet chemical methods. Detailed electrochemical characterization was performed to determine the surface properties and the catalytic activities. The atomic-scale structures of these catalysts were also characterized by high-energy synchrotron X-ray diffraction coupled with atomic pair distribution function analysis. The electrocatalytic activity and stability were shown to depend on the size, composition, and phase structure. With Pd(n)Cu(100-n) catalysts from both methods, a maximum ORR activity was revealed at Pd/Cu ratio close to 50:50. Structurally, Pd50Cu50 nanoalloys feature a mixed phase consisting of chemically ordered (body-centered cubic type) and disordered (face-centered cubic type) domains. The phase-segregated structure is shown to change to a single phase upon electrochemical potential cycling in ORR condition. While the surface Cu dissolution occurred in PdCu catalysts from the two different synthesis methods, the PdCu with a single-phase character is found to exhibit a tendency of a much greater dissolution than that with the phase segregation. Analysis of the results, along theoretical modeling based on density functional theory calculation, has provided new insights for the correlation between the electrocatalytic activity and the catalyst structures.

Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts.

A combined electronic structure computational and X-ray absorption spectroscopy study was used to investigate the nature of the active sites responsible for catalytic synergy in Co-Ti bimetallic nanoporous frameworks. Probing the nature of the molecular species at the atomic level has led to the identification of a unique Co-O-Ti bond, which serves as the loci for the superior performance of the bimetallic catalyst, when compared with its analogous monometallic counterpart. The structural and spectroscopic features associated with this active site have been characterized and contrasted, with a view to affording structure-property relationships, in the wider context of designing sustainable catalytic oxidations with porous solids.

Ultrafine Nanoparticle-Supported Ru Nanoclusters with Ultrahigh Catalytic Activity.

The design of an ideal heterogeneous catalyst for hydrogenation reaction is to impart the catalyst with synergetic surface sites active cooperatively toward different reaction species. Herein a new strategy is presented for the creation of such a catalyst with dual active sites by decorating metal and metal oxide nanoparticles with ultrafine nanoclusters at atomic level. This strategy is exemplified by the design and synthesis of Ru nanoclusters supported on Ni/NiO nanoparticles. This Ru-nanocluster/Ni/NiO-nanoparticle catalyst is shown to exhibit ultrahigh catalytic activity for benzene hydrogenation reaction, which is 55 times higher than Ru-Ni alloy or Ru on Ni catalysts. The nanoclusters-on-nanoparticles are characterized by high-resolution transmission electron microscope, Cs-corrected high angle annular dark field-scanning transmission electron microscopy, elemental mapping, high-sensitivity low-energy ion scattering, and X-ray absorption spectra. The atomic-scale nanocluster-nanoparticle structural characteristics constitute the basis for creating the catalytic synergy of the surface sites, where Ru provides hydrogen adsorption and dissociation site, Ni acts as a "bridge" for transferring H species to benzene adsorbed and activated at NiO site, which has significant implications to multifunctional nanocatalysts design for wide ranges of catalytic reactions.

Silver Triflate Catalyzed Cyclopropyl Carbinol Rearrangement for Benzo[b]oxepine and 2H‐Chromene Synthesis

AbstractAn efficient AgOTf‐catalyzed (TfO = trifluoromethanesulfonate) cyclopropyl carbinol rearrangement for the synthesis of benzo[b]oxepines and 2H‐chromenes is reported by starting from 2‐[cyclopropyl(hydroxy)methyl]phenols. The reactions proved to be general in scope and furnished a range of the corresponding functionalized oxygen heterocycles. The formation of the benzo‐fused six‐membered adduct is proposed to result from the ring‐contraction of the benzo[b]oxepine derivative. Control of the product selectivity is achievable by fine‐tuning the reaction time. The catalytic synergy between the silver(I) cation and the triflate anion was essential for optimal product yields.

Catalytic activity of bimetallic catalysts highly sensitive to the atomic composition and phase structure at the nanoscale.

The ability to determine the atomic arrangement in nanoalloy catalysts and reveal the detailed structural features responsible for the catalytically active sites is essential for understanding the correlation between the atomic structure and catalytic properties, enabling the preparation of efficient nanoalloy catalysts by design. Herein we describe a study of CO oxidation over PdCu nanoalloy catalysts focusing on gaining insights into the correlation between the atomic structures and catalytic activity of nanoalloys. PdCu nanoalloys of different bimetallic compositions are synthesized as a model system and are activated by a controlled thermochemical treatment for assessing their catalytic activity. The results show that the catalytic synergy of Pd and Cu species evolves with both the bimetallic nanoalloy composition and temperature of the thermochemical treatment reaching a maximum at a Pd : Cu ratio close to 50 : 50. The nanoalloys are characterized structurally by ex situ and in situ synchrotron X-ray diffraction, including atomic pair distribution function analysis. The structural data show that, depending on the bimetallic composition and treatment temperature, PdCu nanoalloys adopt two different structure types. One features a chemically ordered, body centered cubic (B2) type alloy consisting of two interpenetrating simple cubic lattices, each occupied with Pd or Cu species alone, and the other structure type features a chemically disordered, face-centered cubic (fcc) type of alloy wherein Pd and Cu species are intermixed at random. The catalytic activity for CO oxidation is strongly influenced by the structural features. In particular, it is revealed that the prevalence of chemical disorder in nanoalloys with a Pd : Cu ratio close to 50 : 50 makes them superior catalysts for CO oxidation in comparison with the same nanoalloys of other bimetallic compositions. However, the catalytic synergy can be diminished if the Pd50Cu50 nanoalloys undergo phase segregation into distinct chemically-ordered (B2-type) and disordered (fcc-type) domains. This finding is significant since it provides a rational basis for streamlining the design and preparation of Pd-based nanoalloy catalysts in terms of atomic structure and phase state.

A Bi-component Cu Catalyst for the Direct Synthesis of Methylchlorosilane from Silicon and Methyl Chloride

A bi-component catalyst comprising CuCl and metallic copper was used in the direct synthesis of methylchlorosilane to study the catalytic synergy between the different copper sources. The catalyst exhibited high activity and high selectivity of dimethyldichlorosilane (M2) in the stirred bed reactor. The effect of the proportion of CuCl used was studied and 10%-30% CuCl gave the best yield of M2. The use of CuCl decreased the induction period of reaction, improved the selectivity in the induction stage, and gave a longer stable stage. These results suggest that bi-component catalyst has advantages in the direct synthesis reaction.

Amperometric biosensors for NADH based on hyperbranched dendritic ferrocene polymers and Pt nanoparticles

The electrocatalytic performance of electrodes modified with Pt nanoparticles (PtNPs) and two dendritic hyperbranched carbosilane polymers, polydiallylmethylsilane (PDAMS) and polymethyldiundecenylsilane (PMDUS) with interacting ferrocenes, has been investigated for the NADH oxidation. The catalytic synergy of PtNPs with the interacting ferrocenes is discussed in relation with the polymer structure. This effect have allowed us to develop efficient biosensors capable of measuring NADH from +0.3V (vs. SCE) providing a total protection vs. the poisoning of the electrodes. The polymer/PtNPs/Pt electrodes tolerate wide linear concentration ranges for NADH to 2.5mM (R=0.9979) and 2.1mM (R=0.99849), with detection limits of 4.78μM and 6.18μM and sensitivities of 68.24 and 40.21μAmM−1cm−2 for the PDAMS/PtNPs/Pt and PMDUS/PtNPs/Pt respectively. In the light of the good results obtained, novel amperometric alcohol biosensors were also successfully prepared with alcohol dehydrogenase (ADH). These devices showed more affinity for methanol than for ethanol, with a wide linear range to 30mM and sensitivities of 0.957 and 0.756μAmM−1cm−2 for the ADH/PDAMS/PtNPs/Pt and ADH/PMDUS/PtNPs/Pt respectively. The oxidation potential of the NADH enzymatically produced was negatively shifted to +0.25V.

An in situ methylation of toluene using syngas over bifunctional mixture of Cr2O3/ZnO and HZSM-5

The in situ methylation of toluene using syngas (in situ methylation hereafter) was studied to produce xylenes over bifunctional catalysts comprising Cr2O3/ZnO (CrZ) and HZSM-5 (SiO2/Al2O3=30) in a fixed-bed down-flow reactor at 460psig. In this study, three basic aspects in the in situ methylation were investigated: (1) the catalytic activity of CrZ as the methanol synthesis catalyst with respect to the equilibrium conversion, (2) the evaluation of catalytic synergies in the in situ methylation over bifunctional catalyst, and (3) the effect of the ratio of catalytic functions regarding the ratio of methanol synthesis catalyst to HZSM-5, and their intimacy. The CrZ catalyst exhibited very low CO conversion to methanol in the methanol synthesis reaction, which is controlled by the equilibrium. However, the bifunctional mixture of CrZ and HZSM-5 catalysts exhibited significant catalytic synergies in the in situ methylation in terms of catalytic activity and product selectivities, with about a 10-times-higher CO conversion rate, and 2- to 3-times-higher toluene conversion and xylene production rates compared to the monofunctional catalysis of methanol synthesis and toluene disproportionation, respectively. The in situ methylation also gave rise to higher catalytic performance than in typical toluene methylation conducted with a molar feed ratio of methanol/toluene of 0.5. The catalytic synergy was manifested when both metallic and acidic functions were brought into close intimacy with a weight ratio of CrZ/HZSM-5 close to one in the in situ methylation.

Small Angle X-ray Scattering Analysis of Clostridium thermocellum Cellulosome N-terminal Complexes Reveals a Highly Dynamic Structure

Clostridium thermocellum produces the prototypical cellulosome, a large multienzyme complex that efficiently hydrolyzes plant cell wall polysaccharides into fermentable sugars. This ability has garnered great interest in its potential application in biofuel production. The core non-catalytic scaffoldin subunit, CipA, bears nine type I cohesin modules that interact with the type I dockerin modules of secreted hydrolytic enzymes and promotes catalytic synergy. Because the large size and flexibility of the cellulosome preclude structural determination by traditional means, the structural basis of this synergy remains unclear. Small angle x-ray scattering has been successfully applied to the study of flexible proteins. Here, we used small angle x-ray scattering to determine the solution structure and to analyze the conformational flexibility of two overlapping N-terminal cellulosomal scaffoldin fragments comprising two type I cohesin modules and the cellulose-specific carbohydrate-binding module from CipA in complex with Cel8A cellulases. The pair distribution functions, ab initio envelopes, and rigid body models generated for these two complexes reveal extended structures. These two N-terminal cellulosomal fragments are highly dynamic and display no preference for extended or compact conformations. Overall, our work reveals structural and dynamic features of the N terminus of the CipA scaffoldin that may aid in cellulosome substrate recognition and binding.

Toward Understanding the Catalytic Synergy in the Design of Bimetallic Molecular Sieves for Selective Aerobic Oxidations

Structure-property correlations and mechanistic implications are important in the design of single-site catalysts for the activation of molecular oxygen. In this study we rationalize trends in catalytic synergy to elucidate the nature of the active site through structural and spectroscopic correlations. In particular, the redox behavior and coordination geometry in isomorphously substituted, bimetallic VTiAlPO-5 catalysts are investigated with a view to specifically engineering and enhancing their reactivity and selectivity in aerobic oxidations. By using a combination of HYSCORE EPR and in situ FTIR studies, we show that the well-defined and isolated oxophilic tetrahedral titanium centers coupled with redox-active VO(2+) ions at proximal framework positions provide the loci for the activation of oxidant that leads to a concomitant increase in catalytic activity compared to analogous monometallic systems.

Cellulosomes: Highly Efficient Nanomachines Designed to Deconstruct Plant Cell Wall Complex Carbohydrates

Cellulosomes can be described as one of nature's most elaborate and highly efficient nanomachines. These cell bound multienzyme complexes orchestrate the deconstruction of cellulose and hemicellulose, two of the most abundant polymers on Earth, and thus play a major role in carbon turnover. Integration of cellulosomal components occurs via highly ordered protein:protein interactions between cohesins and dockerins, whose specificity allows the incorporation of cellulases and hemicellulases onto a molecular scaffold. Cellulosome assembly promotes the exploitation of enzyme synergism because of spatial proximity and enzyme-substrate targeting. Recent structural and functional studies have revealed how cohesin-dockerin interactions mediate both cellulosome assembly and cell-surface attachment, while retaining the spatial flexibility required to optimize the catalytic synergy within the enzyme complex. These emerging advances in our knowledge of cellulosome function are reviewed here.

ChemInform Abstract: In Situ Raman Spectroscopy - A Valuable Tool to Understand Operating Catalysts

Abstract Laser Raman spectroscopy (LRS) is one of the most powerful tools for the in situ study of catalytic materials and surfaces under working conditions. Raman characterizations can be carried out at temperatures as high as 1000 K and in controlled atmospheres. Modern high light-throughput spectrometers permit the recording of the whole spectral range from 100 to 4000 cm −1 at once and time resolutions in the subsecond regime for materials with high Raman cross-sections. Transient temperature or pressure response studies, e.g. pulse experiments with isotope labels, are thus possible, and kinetic and spectroscopic characteristics can be related. Modern quartz fiber optics render possible easy spectroscopic access to catalytic reactors of defined and well characterized operation conditions. Quantitative relation of real catalytic steady state operation, e.g. catalytic activity and selectivity, to changes in the catalyst structure is thus made possible. Several in situ LRS studies are discussed including the characterization of supported and unsupported Mo-based catalysts, confocal Raman microspectroscopy of mixed MoVW oxide catalysts, oxygen exchange in Sb 2 O 3 /MoO 3 oxide physical mixtures elucidating the catalytic synergy effects, and active surface intermediates during oxidative coupling of methane, and NO and N 2 O decomposition over Ba/MgO catalysts related to the catalytic reaction via transient pressure step experiments.