Number Of Catalytic Sites Research Articles

"Plug and Play" Photosensitizer-Catalyst Dyads for Water Oxidation.

We present a simple and easy-to-scale synthetic method to plug common organic photosensitizers into a cyanide-based network structure for the development of photosensitizer-water oxidation catalyst (PS-WOC) dyad assemblies for the photocatalytic water oxidation process. Three photosensitizers, one of which absorbs red light similar to P680 in photosystem II, were utilized to harvest different regions of the solar spectrum. Photosensitizers are covalently coordinated to CoFe Prussian blue structures to prepare PS-WOC dyads. All dyads exhibit steady water oxidation catalytic activities throughout a 6 h photocatalytic experiment. Our results demonstrate that the covalent coordination between the PS and WOC group not only enhances the photocatalytic activity but also improves the robustness of the organic PS group. The photocatalytic activity of “plug and play” dyads relies on several structural and electronic parameters, including the position of the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the PS with respect to the HOMO level of the catalytic site, the intensity and wavelength of the absorption band of the PS, and the number of catalytic sites.

Quantitatively evaluating activity and number of catalytic sites on metal oxide for ammonium perchlorate decomposition

AbstractAmmonium perchlorate (AP) catalytic thermal decomposition by metal oxides is complex and there is still no suitable quantitative evaluation method to characterize intrinsic catalytic activity. Here, a new method to quantitatively evaluate the catalytic capability of metal oxides for AP decomposition has been proposed. We introduce two indicators, intrinsic peak temperature () and catalytic density () in high‐temperature decomposition process, to uncouple activity and number of catalyst sites. Hematite (α‐Fe2O3) nanostructures with different shapes have been used as model catalysts to verify the feasibility of this method. Rod‐like α‐Fe2O3shows the best catalytic performance in AP decomposition with the lowest of 345.1°C and highest of 0.544, which indicate the largest number and highest activity of catalytic sites. Moreover, these new indicators are theoretically proven to be reasonable by density functional theory calculations.

Plant cell wall architecture guided design of CBM3-GH11 chimeras with enhanced xylanase activity using a tandem repeat left-handed β-3-prism scaffold

Effective use of plant biomass as an abundant and renewable feedstock for biofuel production and biorefinery requires efficient enzymatic mobilization of cell wall polymers. Knowledge of plant cell wall composition and architecture has been exploited to develop novel multifunctional enzymes with improved activity against lignocellulose, where a left-handed β–3-prism synthetic scaffold (BeSS) was designed for insertion of multiple protein domains at the prism vertices. This allowed construction of a series of chimeras fusing variable numbers of a GH11 β-endo-1,4-xylanase and the CipA-CBM3 with defined distances and constrained relative orientations between catalytic domains. The cellulose binding and endoxylanase activities of all chimeras were maintained. Activity against lignocellulose substrates revealed a rapid 1.6- to 3-fold increase in total reducing saccharide release and increased levels of all major oligosaccharides as measured by polysaccharide analysis using carbohydrate gel electrophoresis (PACE). A construct with CBM3 and GH11 domains inserted in the same prism vertex showed highest activity, demonstrating interdomain geometry rather than number of catalytic sites is important for optimized chimera design. These results confirm that the BeSS concept is robust and can be successfully applied to the construction of multifunctional chimeras, which expands the possibilities for knowledge-based protein design.

Metallic Ni3Mo3N Porous Microrods with Abundant Catalytic Sites as Efficient Electrocatalyst for Large Current Density and Superstability of Hydrogen Evolution Reaction and Water Splitting

The low onset potential and large current density of electrocatalysts has always been important target for hydrogen evolution reaction (HER). In especial, the large current density (larger than 1000 mA cm−2) is an important criterion for the evaluation of electrocatalysts for industrial application. Usually, the number of catalytic sites in electrocatalyst limits the current density for HER. To overcome these problems, bimetallic nitride is controllably synthesized, and the corresponding catalytic sites are regulated by a bimetallic effect. Herein, an inexpensive electrocatalyst consisting of N-doped carbon-coated porous Ni3Mo3N microrods (NC/Ni3Mo3N/NF) is cultured on nickel foam using a hydrothermal reaction and subsequent nitriding process. The designed electrocatalyst consisting of porous NC/Ni3Mo3N microrods displays efficient catalytic activity for hydrogen evolution reaction (HER), with a small overpotential of 136 mV to achieve a cathodic current density of 100 mA cm−2. Density functional theory (DFT) calculations clarified that the Ni3Mo3N electrocatalyst possesses various HER catalytic active sites with suitable ΔGH* values (more than ten sites) due to the metallic semiconductor structure with special electronic structure. It is important that NC/Ni3Mo3N/NF possess perfect superstability for HER, with a large current density of 1100 mA cm−2 for 50 h. Lastly, NC/Ni3Mo3N/NF and NiMoO4·xH2O/NF are hired as the cathode and anode, respectively, to assemble the two-electrode electrolytic cell, thereby achieving excellent overall water splitting performance with a voltage of 1.58 V at 50 mA cm−2 in 1 M KOH aqueous solution.

Imprinting isolated single iron atoms onto mesoporous silica by templating with metallosurfactants

HypothesisOne of the main drawbacks of metal-supported materials, traditionally prepared by the impregnation of metal salts onto pre-synthesized porous supports, is the formation of large and unevenly dispersed particles. Generally, the larger are the particles, the lower is the number of catalytic sites. Maximum atom exposure can be reached within single-atom materials, which appear therefore as the next generation of porous catalysts. ExperimentsHerein, we designed single iron atom-supported silica materials through sol-gel hydrothermal treatment using mixtures of a non-ionic surfactant (Pluronic P123) and a metallosurfactant (cetyltrimethylammoniumtrichloromonobromoferrate, CTAF) as porogens. The ratio between the Pluronic P123 and the CTAF enables to control the silica structural and textural properties. More importantly, CTAF acts as an iron source, which amount could be simply tuned by varying the non-ionic/metallo surfactants molar ratio. FindingsThe fine distribution of iron atoms onto the silica mesopores results from the iron distribution within the mixed micelles, which serve as templates for the polymerization of the silica matrix. Several characterization methods were used to determine the structural and textural properties of the silica material (XRD, N2 sorption isotherms and TEM) and the homogeneous distribution and lack of clustering of iron atoms in the resulting materials (elemental analysis, magnetic measurements, pair distribution function (PDF), MAS-NMR and TEM mapping). The oxidation and spin state of single-iron atoms determined from their magnetic properties were confirmed by DFT calculations. This strategy might find straightforward applications in preparing versatile single atom catalysts, with improved efficiency compared to nanosized ones.

Recent advances in nanoporous materials for renewable energy resources conversion into fuels

The continuous growth in energy production from non-renewable resources in order to meet the ever-increasing energy demand has given rise to serious environmental issues and moving toward renewable energy resources is necessary. Heterogeneous catalysts play a key role in the conversion of renewable resources into fuels and chemicals. The performance of heterogeneous catalysts is directly linked to their surface area, since the number of catalytic sites as well as the activity of each catalytic site increase with increasing effective footprint area of a catalyst. Therefore, nanoporous heterogeneous catalysts are very attractive, owing to their high internal surface areas and high density of active sites generated by curved internal surfaces. The overall catalytic performance of nanoporous heterogeneous catalysts can reach orders of magnitude higher than that of planar catalysts counterparts. This paper reviews recent progress toward the applicability of three-dimensional bulk nanoporous metals and their composites in (electro-)catalytic conversion of renewable resources into fuels and value-added chemicals. The primary focus is given to metal-based materials fabricated through dealloying. Dealloyed nanoporous metals and their composites can be used either directly as high-performance (electro-)catalysts, or indirectly as three-dimensional bulk current collectors along with poorly conducting electro-catalyst materials. Limitations of these material systems such as cost, scalability, and long-term stability in-service are discussed.

Facile Integration of Hierarchical Pores and N,P-Codoping in Carbon Networks Enables Efficient Oxygen Reduction Reaction

Effective integration of hierarchically porous architecture and high-density distribution of dopant sites into carbon is essential to improve the activity of heteroatom-doped carbon for oxygen reduction reaction (ORR). Herein, we present a general sealed space-assisted approach to fabricate N,P-codoped porous carbon networks (NP-PCN) by calcination of a self-assembled mixture of C6H14N2O2, NH4H2PO4, and NaCl. The NaCl assemblies serve as a space-enclosed nanoreactor for the effective formation of hierarchical pores with high surface area and dense N,P-induced dopants by preventing the loss of N,P-containing gasses from the decomposition of precursors during annealing. The NP-PCN is applied as an ORR catalyst and exhibits excellent performance with a positive half-wave potential of 0.82V vs. RHE, a low Tafel slope of 59mVdec−1, high tolerance to methanol, and high durability in alkaline media, making NP-PCN a promising alternative to costly platinum for the ORR. The enhanced performance is attributed to the combination of hierarchical pores and optimized N,P codoping, which significantly enhances the available number of catalytic sites by exposing the N,P-induced dopants and facilitates mass transfer in the ORR process. This finding opens a new avenue for the design of advanced porous carbons to improve their electrocatalytic activity.

Hybrid Semiconductor-Metal Nanorods as Photocatalysts.

Semiconductor-metal hybrid nanoparticles manifest combined and often synergistic properties exceeding the functionality of the individual components, thereby opening up interesting opportunities for controlling their properties through the direct manipulation of their unique semiconductor-metal interface. Upon light absorption, these structures exhibit spatial charge separation across the semiconductor-metal junction. A significant and challenging application involves the use of these nanoparticles as photocatalysts. Through this process, the charge carriers transferred to the metal co-catalyst are available as reduction or oxidation reagents to drive the surface chemical reactions. In this review, we discuss synthesis approaches that offer a high degree of control over the hybrid nanoparticle structure and composition, the number of catalytic sites and the interfacial characteristics, including examples of a variety of photocatalyst architectures. We describe the structural and surface effects with regard to the functionality of hybrid nanosystems in photocatalysis, along with the effects of solution and chemical conditions on photocatalytic activity and efficiency. We conclude with a perspective on the rational design of advanced semiconductor-metal hybrid nanoparticles towards their functionality as highly efficient photocatalysts.

Effects of Y3+ on properties of electroless NiYP coatings

NiYP alloy coatings were prepared on the surface of sintered NdFeB magnets by electroless composite plating. The effects of Y3+ concentration on the deposition rate, surface morphologies, element contents and corrosion resistance were investigated. The results showed that the deposition rate of NiYP alloy coatings decreased with the increasing Y3+ concentration. A thin NiYP film with adsorbed rare earth yttrium could be formed at higher concentration, which reduced the number of catalytic sites on the surface of intered NdFeB magnets. With the increase in Y3+ concentration up to 6 mmol L− 1, the self-corrosion potential Ecorr of the electroless NiYP coatings in 3.5%NaCl solution increased from − 672 to − 513 mV, and the self-corrosion current density Icorr decreased from 28.81 to 1.15 μA cm− 2. The mechanisms of corrosion resistance of the electroless NiYP coatings modified by Y3+ addition were discussed.

Steady-State NTPase Activity of Dengue Virus NS3: Number of Catalytic Sites, Nucleotide Specificity and Activation by ssRNA

Dengue virus nonstructural protein 3 (NS3) unwinds double stranded RNA driven by the free energy derived from the hydrolysis of nucleoside triphosphates. This paper presents the first systematic and quantitative characterization of the steady-state NTPase activity of DENV NS3 and their interaction with ssRNA. Substrate curves for ATP, GTP, CTP and UTP were obtained, and the specificity order for these nucleotides - evaluated as the ratio (kcat/KM)- was GTPATPCTP UTP, which showed that NS3 have poor ability to discriminate between different NTPs. Competition experiments between the four substrates indicated that all of them are hydrolyzed in one and the same catalytic site of the enzyme. The effect of ssRNA on the ATPase activity of NS3 was studied using poly(A) and poly(C). Both RNA molecules produced a 10 fold increase in the turnover rate constant (kcat) and a 100 fold decrease in the apparent affinity (KM) for ATP. When the ratio [RNA bases]/[NS3] was between 0 and 20 the ATPase activity was inhibited by increasing both poly(A) and poly(C). Using the theory of binding of large ligands (NS3) to a one-dimensional homogeneous lattice of infinite length (RNA) we tested the hypothesis that inhibition is the result of crowding of NS3 molecules along the RNA lattices. Finally, we discuss why this hypothesis is consistent with the idea that the ATPase catalytic cycle is tightly coupled to the movement of NS3 helicase along the RNA.

The effect of metal and support particle size on NiO/CeO2 and NiO/ZrO2 catalyst activity in complete methane oxidation

The catalytic oxidation of methane was examined over 5% (w/w) NiO- on CeO2 or ZrO2. The particle size of single components was varied independently to investigate the effect of metal and support particle size on activity for complete methane oxidation. Catalyst activity is most affected by NiO particle size, however the support particle size also plays an active role. Results show that the particle size of ceria has a greater effect on activity than that of zirconia. This is attributed to ceria's greater oxygen transport capacity and contribution to the reaction mechanism. Changes in ceria particle size significantly affected the Arrhenius parameters, suggesting ceria forms part of the active site on a ceria-supported NiO catalyst. In contrast, zirconia particle size did not result in changes in activation energy, but affected only the number of catalytic sites as seen by changes in the pre-exponential factor. Ceria was also found to reduce deactivation of the catalyst due to increased oxygen transport to the active sites during reaction.

Effect of catalyst preparation and support composition on hydrodesulfurization of dibenzothiophene and Maya crude oil

Hydrodesulfurization (HDS) of dibenzothiophene (DBT) was carried out in a batch reactor at 3.3 MPa and 340 °C. Sulfided catalysts showed a wide range of activity variation as a function of support composition, which established that sulfided active phases strongly depend on the nature and composition of support. The incorporation of MgO, SiO 2, TiO 2 and ZrO 2 with γ-Al 2O 3 alters the nature of active phase interactions on the support surface. Therefore, these oxides play a structural promoting role to the support contribution and its interaction towards the active metal geometry. Different preparation methods of catalysts were carried out by using different additives during the impregnation. It is demonstrated that co-impregnation method found better activity than sequential impregnation. The urea additive prepared catalyst provides promising activity compared with other chelating agents. An Al 2O 3–B 2O 3 mixed oxide supported catalyst was tested at high pressure with real feed (Maya heavy oil diluted with hydrodesulfurized diesel). The mixed oxide supported catalyst showed rapid deactivation with time-on-stream than the alumina supported catalyst that may be due to an enhancement in acidic sites. The activity results demonstrated that the number of catalytic sites is a combination of support composition, additive, impregnation pH, drying and calcinations of catalysts.

Kinetics of CO2 Gasification of Alkaline Black Liquor from Wheat Straw. Influence of CO and CO2 Concentrations on the Gasification Rate

Interest in the use of nonwoody biomass as a raw material for pulp and paper production is growing worldwide. Moreover, new technologies are being developed to solve the problems caused by the silica present in alkaline black liquor (ABL) from straw in heat recovery systems. Because, among the various possible technologies, gasification is becoming attractive as a new alternative recovery system for ABL, in-depth studies of ABL gasification are therefore required. The present work is focused on the study of CO2 gasification of a char obtained from the ABL pyrolysis. Specifically, the influence of [CO] and [CO2] on the gasification rate at different gasification temperatures has been studied. The process may be described by a Langmuir−Hinshelwood equation for a specific range of operating conditions. The data obtained suggest that this operational range could be broader if the number of catalytic sites is considered as a function of [CO] and the final pyrolysis temperature rather than as constant.

Kinetic and mechanistic characterization of the polyhydroxybutyrate synthase from Ralstonia eutropha.

Purified Ralstonia eutropha polyhydroxybutyrate (PHB) synthase from recombinant cells can exist as monomer and dimer. The polymerization reaction catalyzed by this enzyme displays a lag phase, which causes difficulties for kinetic and mechanistic characterization of the enzymatic polymerization reaction. In this study, we developed a method to eliminate the lag phase of PHB synthase by physical means, i.e., adding multihydroxyl compounds to the enzyme solution. This method allows us to recognize the nature of the lag phase as a physical rather than a chemical process. With such lag-phase-free-enzyme, the kinetic properties of the enzyme were investigated. The results indicate that 3-hydroxybutyryl-CoA (3HBCoA) is the optimal substrate for the enzyme. A slower catalytic rate and lower binding ability account for a lower reactivity of 3-hydroxyvaleryl-CoA (3HVCoA) compared to that of 3HBCoA. The change of hydroxyl group from the beta to the gamma position causes dramatic decreases in the binding ability of 4-hydroxybutyryl-CoA (4HBCoA). By using a dilution strategy and size exclusion chromatographic technique, the active form of the enzyme was identified to be the dimeric form. The number of catalytic sites in the dimeric form of the enzyme was examined by comparing the molecular weight of polyhydroxybutyrate as a function of substrate-to-enzyme ratio. The results suggest that the dimeric enzyme has only one catalytic site. A revised model of polymerization reaction catalyzed by R. eutropha PHB synthase is described.

Ammonia-induced alterations in the activities of synaptosomal cholinesterases of rat brain under in vitro and in vivo conditions

Effects of in vivo and in vitro pathophysiological concentrations of ammonium acetate were studied on the activities of acetyl and pseudocholinesterases in the synaptosomes isolated from cerebral cortex, cerebellum and brain stem of rat brain. Administration of subacute and acute doses of ammonium acetate elevated the activities of both the cholinesterases in synaptosomes of all the above three regions. A linear relationship ( r = 0.98) was observed between brain-ammonia levels and magnitude of elevation in the activities of cerebral cholinesterases. Kinetic analysis revealed that this elevation was due to a change in the V m but not in the K m of the enzymes. Incubation of synaptosomes isolated from normal animals with 1, 5 and 10 mM ammonium acetate marginally elevated the activities of these enzymes and had no effects on purified enzyme. It is suggested that the changes in the activities might be due to altered architecture of the membranes which exposes more number of catalytic sites.

Acute and early effects of aldosterone on Na-K-ATPase activity in Madin-Darby canine kidney epithelial cells

The time course and mechanism of early effects of aldosterone on renal Na-K-adenosinetriphosphatase (Na-K-ATPase) activity and number of units were studied in MDCK cells. Aldosterone induced a time- and dose-dependent stimulation of Na-K-ATPase activity. The stimulatory effect of aldosterone on activity and number of pump units increased progressively and was inhibited by spironolactone. In presence of cycloheximide, the stimulatory effect of aldosterone on activity and number of catalytic sites persisted to the same extent until 30 min and decreased by 20% after 60 min. In these cells, dimethylamiloride addition during preincubation abolished the aldosterone-induced stimulation in Na-K-ATPase activity up to 60 min. In contrast, furosemide addition did not alter the effect of aldosterone on Na-K-ATPase activity. The present study demonstrates an early effect of aldosterone on Na-K-ATPase activity that can be separated into the following two successive periods: 1) increase in pump number due to insertion of presynthetized units secondary to Na entry through an amiloride-sensitive apical pathway; and 2) an increase in pump number by de novo protein synthesis.

Sites of thyroid hormone action on Na?K-ATPase along the rabbit nephron

Although Ismail-Beigi and Edelman demonstrated in 1971 that thyroid hormones control the activity of Na-K-ATPase in the mammalian kidney, the actual site of this regulation inside the organ was not located. We therefore decided to study the relationship between thyroid hormones and Na-K-ATPase activity in individual nephron segments obtained by microdissection of collagenase-treated rabbit kidneys. For this purpose, the changes in the activity and number of catalytic sites of Na-K-ATPase in response to thyroidectomy or triiodothyronine administration were examined. Eight to 12 days after thyroidectomy, Na-K-ATPase activity had dropped by 40 to 80% in the convoluted and straight portions of the proximal tubules, and in the cortical and outer medullary collecting tubules, but not in the thick ascending limbs of Henle's loops or distal convoluted tubules. The apparent number of catalytic sites for Na-K-ATPase, as measured by specific binding of 3H-ouabain, decreased in parallel with Na-K-ATPase activity, and therefore this enzyme's specific activity was not altered. Fourty eight hours after injection of thyroidectomized animals with a single dose of either 100 or 500 micrograms/kg triiodothyronine, Na-K-ATPase activity in target segments was restored to the level measured in control animals. These effects of thyroid hormone were specific for Na-K-ATPase, since the activity of adenylate cyclase, another marker of the basolateral membrane, was not altered by thyroidectomy. The results obtained indicate that triiodothyronine controls Na-K-ATPase activity in specific nephron segments, by altering the number of this enzyme's catalytic sites.

Na+ movement in a single turnover of the Na pump.

Ouabain-sensitive 22Na efflux from right-side-out membrane vesicles prepared from dog kidney has been examined with a time resolution of 30 msec. The vesicles are preloaded with 22Na and caged ATP [P3-1-(2-nitro)phenylethyl adenosine triphosphate], so that transport by the Na pump can be initiated by light. After a brief illumination, which releases less ATP than the number of catalytic sites, a burst of 22Na extrusion is observed corresponding to a single turnover of the Na pump. By the use of a rapid filtration apparatus, with which a continuous record of the rate of efflux is obtained, it has been possible to resolve the efflux burst in the time range of 20-1500 msec. The rate of efflux rises rapidly, but not instantaneously, to a peak and then decays, with a time constant of approximately equal to 6 sec-1 at 15 degrees C. The time course of Na efflux is unaffected by extracellular K+, as predicted by models of the Na pump in which Na is released early in the cycle. Unphotolyzed caged ATP is found to bind to the catalytic site of Na,K-ATPase, in competition with ATP that is produced in the flash, and the possibility has not been excluded that dissociation of unphotolyzed caged ATP and binding of ATP are involved in the Na efflux time course. It seems most likely that binding of ATP and translocation of 22Na are involved in the increase in the 22Na efflux rate in the single turnover and that the release of transported 22Na from extracellular pump sites limits the slow decay.

Studies on the catalysis of molten metal. 13. Kinetics of dehydrogenation of tetralin over molten tellurium catalyst

In a study of the thermal decomposition of tetralin, the rates of pyrolysis and of catalysed decomposition over molten tellurium or in the presence of tellurium vapour have been measured. All rate processes examined have been found to obey the first-order irreversible consecutive rate law and to proceed through the following reaction pathway: ▪ Both molten tellurium and tellurium vapour have been found to reduce the activation energy of the step A → B and enhance the frequency of the step B → C. Although the apparent activities of the two catalysts were found to be similar, estimates of the intrinsic activities indicate that the molten tellurium surface is more effective. It is suggested that the number of catalytic sites on the molten tellurium surface is larger than in the tellurium vapour.

Modification of the selectivity of catalytic consecutive reactions by poisoning

The intrinsic selectivity of a heterogeneous catalyst may be modified by addition of poisons when a different number of catalytic sites are occupied by various reacting species. We extend here a model of Herington and Rideal to account for modification of intrinsic selectivity of catalytic first order consecutive reactions. A study of the interactions between the catalytic selectivity, poisoning and diffusional limitations reveals the existence of several interesting and surprising behavioral features. For example, the intrinsic selectivity is predicted to be a monotonic function of θ (the fraction of poisoned sites). However, when diffusional resistances exist, the graph of the observed selectivity versus θ may attain one or more local extremal values.