Steady-state Catalytic Activity Research Articles

Supported molybdenum oxides for the aldol condensation reaction of acetaldehyde

The (retro-)aldol condensation reaction is an important chemical transformation in the upgrading of biomass-derived compounds into fuels and valuable specialty chemicals. In this study, we found that supported molybdenum oxide (MoOx) catalysts were active and selective for the aldol condensation of acetaldehyde to crotonaldehyde under steady-state reactor conditions. Through a combination of transmission electron microscopy (TEM), ultraviolet–visible (UV–VIS) diffuse reflectance spectroscopy, Fourier transform infrared (FTIR) spectroscopy of adsorbed pyridine, and steady-state reactor testing, we determined that highly dispersed MoOx has a strong interaction with a γ-Al2O3 support resulting in optimal catalyst performance at low weight loadings. In contrast, MoOx particles supported on SiO2 have a weaker interaction with the support, resulting in a monotonic relationship between Mo loading and aldol condensation activity. The Lewis acid site density and strength are important parameters for predicting aldol condensation activity across all samples. The concentration of weak acid sites had a poor correlation with aldol condensation activity, most likely because these sites are too weak to activate acetaldehyde for the reaction. Medium and strong acid sites both had good correlations to aldol condensation activity. Results from X-ray absorption near edge structure (XANES) and acetaldehyde temperature programmed desorption (TPD) indicated that partially reduced MoOx was more active for aldol condensation, but pretreatment in reducing or oxidizing environments had no significant effect on steady-state catalytic activity. Characterization of spent catalyst samples through temperature programmed oxidation (TPO) and thermogravimetric analysis (TGA) revealed that catalysts with high densities of strong acid sites tended to form more carbonaceous deposits on the surface over the course of the reaction.

Enhancing the Low-T NH3-SCR Activity of a Commercial Fe-Zeolite Catalyst by NH4NO3 Dosing: an Experimental and Modeling Study

The potential improvement of DeNOx efficiency enabled by injecting calibrated amounts of NH4NO3 (AN) aqueous solution into lean engine exhausts prior to NH3-SCR converters (Enhanced-SCR) has been investigated by dynamic and steady-state catalytic activity runs over a commercial Fe-zeolite monolith catalyst. The study was performed varying GHSV (35,000–100,000 h−1), temperature (150–550 °C), as well as AN and NO2 feed concentrations. The results demonstrate that already at low temperature (180 °C) the DeNOx activity of the investigated catalyst at Standard-SCR conditions is greatly boosted by the addition of AN: for example, the injection of 250 ppm of NH4NO3 incremented the NO conversion from ∼15 to ∼85 % at GHSV = 50,000 h−1, with feed NO = NH3 = 500 ppm. Furthermore, the dosed NH4NO3 was completely converted, with no undesired release of N2O. We show also that the Enhanced-SCR pathway proceeds through a first step wherein AN rapidly oxidizes NO to NO2, which then goes on to react with NH3 according to the Fast-SCR reaction. Based on this sequential reaction scheme, a simple kinetic model has been developed and successfully fitted to all the collected data.

Ag–Pt core–shell nanocomposites for enhanced methanol oxidation

Ag–Pt core–shell nanocomposites composed of Ag nanoparticle core and ultra-thin Pt nanorods shell have been successfully synthesized via a simple wet-chemical route at room temperature. The influence of reacting temperature on the final product was studied, and the results showed that Pt dendrites instead of Pt nanorods were formed at elevated temperature. The as-synthesized Ag–Pt core–shell nanocomposites were applied as catalyst for the methanol electro-oxidation reaction, and the results indicate that the Ag–Pt core–rods nanocomposites have the best steady-state catalytic activity and high catalytic activity.

CO oxidation over Au/TiO2 catalyst: Pretreatment effects, catalyst deactivation, and carbonates production

A commercially available Au/TiO2 catalyst was subjected to a variety of thermal treatments in order to understand how variations in catalyst pretreatment procedures might affect CO oxidation catalysis. Catalytic activity was found to be inversely correlated to the temperature of the pretreatment. Infrared spectroscopy of adsorbed CO experiments, followed by a Temkin analysis of the data, indicated that the thermal treatments caused essentially no changes to the electronics of the Au particles; this, and a series of catalysis control experiments, and previous transmission electron microscopy (TEM) studies ruled out particle growth as a contributing factor to the activity loss. Fourier transform infrared (FTIR) spectroscopy showed that pretreating the catalyst results in water desorption from the surface, but the observable water loss was similar for all the treatments and could not be correlated with catalytic activity. A Michaelis–Menten kinetic treatment indicated that the main reason for deactivation is a loss in the number of active sites with little changes in their intrinsic activity. In situ FTIR experiments during CO oxidation showed extensive buildup of carbonate-like surface species when the pretreated catalysts were contacted with the feed gas. A semi-quantitative infrared spectroscopy method was developed for comparing the amount of carbonates present on each catalyst; results from these experiments showed a strong correlation between the steady-state catalytic activity and amount of surface carbonates generated during the initial moments of catalysis. Further, this experimental protocol was used to show that the carbonates reside on the titania support rather than on the Au, as there was no evidence that they poison Au–CO binding sites. The role of the carbonates in the reaction scheme, their potential role in catalyst deactivation, and the role of surface hydroxyls and water are discussed.

Shell structure-enhanced electrocatalytic performance of Au–Pt core–shell catalyst

This paper describes the preparation of Pt nanorods by directly growing them on the surface of Au. Electrochemical measurements were performed to investigate their catalytic performance towards methanol oxidation. It was found from cyclic voltammetry that the Pt nanorods supported on Au nanoparticles had excellent steady-state catalytic activity and high catalytic activity for methanol electrooxidation. They also exhibited a slightly slower current decay over time.

Transient studies of low-temperature dry reforming of methane over Ni-CaO/ZrO2-La2O3

The low temperature reforming of methane by carbon dioxide is studied over a calcium oxide promoted Ni catalyst supported on a tetragonal zirconia stabilized by lanthana, which presents an improved stability compared to the non-promoted catalyst. Steady-state catalytic activity measurements, diffuse reflectance infrared Fourier transform spectroscopic analysis and isotopic temporal analysis of products experiments reveal the occurrence of a bifunctional mechanism on the promoted catalyst: methane is activated on the Ni particles, carbon dioxide interacts with the calcium oxide to form carbonates which scavenge carbon from nickel at the Ni-O-Ca interphase, thus restoring Ni particles to the original state. This is assumed to hinder the formation of deactivating coke, which explains the improved catalytic stability of the promoted catalyst. The main route for the carbon deposit formation is found to be the methane cracking in spite of the low temperature reaction.

Effects of Reaction Variables on Fischer–Tropsch Synthesis with Co-Precipitated K/FeCuAlO x Catalysts

The effects of reduction temperature and reaction temperature, pressure and space velocity on iron-based K/FeCuAlO x Fischer–Tropsch catalysts prepared by co-precipitation were investigated. The catalyst reduced at 150 °C deactivated quickly due to an abundance of unreduced iron species. With increasing reduction temperature, the iron oxide’s phase transformed from hematite (α-Fe2O3) to magnetite (Fe3O4) and finally to metallic iron (α-Fe). The induction period to reach steady-state catalytic activity was reduced at increased reduction temperatures due to in situ reduction by syngas during reaction. CO conversion increased with increasing reaction temperature, and selectivity to C5+ decreased with increasing reaction pressure and space velocity. At reaction temperatures up to of 300 °C, CO2 formation by the water–gas shift reaction was linearly correlated with the extent of CO conversion, and CO2 formation was slightly suppressed at ≥350 °C by a reverse water–gas shift reaction. The effects of reduction temperature and reaction temperature, pressure and space velocity on iron-based K/FeCuAlOx Fischer–Tropsch catalysts prepared by co-precipitation were investigated. The catalyst reduced at 150 °C deactivated quickly due to an abundance of unreduced iron species. With increasing reduction temperature, the iron oxide’s phase transformed from hematite (α-Fe2O3) to magnetite (Fe3O4) and finally to metallic iron (α-Fe). The induction period to reach steady-state catalytic activity was reduced at increased reduction temperatures due to in situ reduction by syngas during reaction. CO conversion increased with increasing reaction temperature, and selectivity to C5+ decreased with increasing reaction pressure and space velocity. At reaction temperatures up to of 300 °C, CO2 formation by the water–gas shift reaction was linearly correlated with the extent of CO conversion, and CO2 formation was slightly suppressed at ≥350 °C by a reverse water–gas shift reaction.

Light-enhanced catalysis by pyridoxal phosphate-dependent aspartate aminotransferase.

The mechanisms of pyridoxal 5'-phosphate (PLP)-dependent enzymes require substrates to form covalent "external aldimine" intermediates, which absorb light strongly between 410 and 430 nm. Aspartate aminotransferase (AAT) is a prototypical PLP-dependent enzyme that catalyzes the reversible interconversion of aspartate and α-ketoglutarate with oxalacetate and glutamate. From kinetic isotope effects studies, it is known that deprotonation of the aspartate external aldimine C(α)-H bond to give a carbanionic quinonoid intermediate is partially rate limiting in the thermal AAT reaction. We show that excitation of the 430-nm external aldimine absorption band increases the steady-state catalytic activity of AAT, which is attributed to the photoenhancement of C(α)-H deprotonation on the basis of studies with Schiff bases in solution. Blue light (250 mW) illumination gives an observed 2.3-fold rate enhancement for WT AAT activity, a 530-fold enhancement for the inactive K258A mutant, and a 58600-fold enhancement for the PLP-Asp Schiff base in water. These different levels of enhancement correlate with the intrinsic reactivities of the C(α)-H bond in the different environments, with the less reactive Schiff bases exhibiting greater enhancement. Time-resolved spectroscopy, ranging from femtoseconds to minutes, was used to investigate the nature of the photoactivation of C(α)-H bond cleavage in PLP-amino acid Schiff bases both in water and bound to AAT. Unlike the thermal pathway, the photoactivation pathway involves a triplet state with a C(α)-H pK(a) that is estimated to be between 11 and 19 units lower than the ground state for the PLP-Val Schiff base in water.

Hydrolytic decomposition of CF 4 over alumina-based binary metal oxide catalysts: high catalytic activity of gallia-alumina catalyst

The hydrolytic decomposition of CF 4 has been conducted on gallia promoted alumina (Ga-Al oxide) catalysts at 803–903 K and 0.1 MPa. Steady state activity on 20% Ga-Al oxide was 15 times of that on Ce10%-AlPO 4 catalyst, on which the highest activity has been reported. The catalytic activity was further improved by incorporation of sulfate anion in Ga-Al oxide and by applying sol–gel method in preparation of the catalyst. XRD spectra of Ga-Al oxides showed a shift of diffraction peaks assigned to γ-alumina toward lower angles, indicating the formation of gallia-alumina solid solution. In situ FT-IR of pyridine adsorption spectra of Ga-Al oxides showed peaks solely attributable to Lewis acid (L-acid) sites at 1445–1455, ca. 1495, 1577, ca. 1594, and ca. 1622 cm −1. The steady state catalytic activities increased with increasing peak areas at 1446 or 1622 cm −1 of Ga-Al oxides with various Ga%, suggesting participation of Lewis acid sites into the reaction. It is demonstrate from surface area measurements of Ga-Al oxide catalyst before and after the reaction that not only higher catalytic activity but also higher catalyst stability were observed on Ga-Al oxide, Ga-Al oxide with sulfate, and Ga-Al oxide prepared by sol–gel method than on their parent oxides of alumina, Ga-Al oxide, and Ga-Al oxide prepared by incipient wetness method, respectively.

Catalytic hydrodehalogenation of halon 1211 (CBrClF2) over γ-alumina-supported Ni, Pd and Pt catalysts

The hydrodehalogenation of halon 1211 over Ni, Pd and Pt supported on γ-alumina was studied. The effect of reaction temperature and hydrogen/halon 1211 input ratio was examined. Steady state catalytic activities of Pd and Pt are very similar, and much higher than that of Ni. Hydrodehalogenation over Pd favours the formation of CH2F2, while Pt and Ni produce primarily CH4 and CH3F, respectively. Product profiles suggest that the hydrodehalogenation of halon 1211 over Pd and Pt follows a reaction mechanism which is similar to hydrodehalogenation of CFC-12. Over Ni catalysts, the primary hydrodehalogenation reactions are those which first remove one halogen (Cl or Br) and then remove two halogens (one F and either Br or Cl). The higher mobility of hydrogen atom on the surface of Pt is consistent with the observation that only trace amounts of C2+ hydrocarbons were detected when Pt was used compared with Pd and Ni. Increasing selectivity to CH2F2 with time on stream over Pd and Pt is mainly ascribed to the transformation of support alumina to partially fluorinated alumina.

Hydrolysis of CF4 over alumina-based binary metal oxide catalysts

Carbon tetrafluoride (CF4), which is the most stable compound in perfluorocarbons (PFCs), was catalytically decomposed by the hydrolysis reaction at mild condition as 850–950K on alumina-based binary metal oxide catalysts. The catalytic activities for the hydrolysis reaction of CF4 with water on the selected metal-Al oxide catalysts were optimized in terms of H2O/CF4 ratio, loading amount of metal oxide to alumina and pre-treatment conditions of the catalyst. Then, the decomposition reaction of CF4 in the presence of water was examined at the optimized conditions over a variety of metal oxides supported on Al2O3. The steady state catalytic activities on the binary metal oxide catalysts with 5% metal oxide loading on alumina at the reaction temperatures between 853–953K were found to decrease in the order of Ga≈Zn≈Zr>Ni≈Cr>Sn≈(Al2O3)≈Ce≈In>B≈W>Mg≈Mn≈Cu>Ge≈Pb≈Li>Bi>Cd>Tl. More than 90% of the robust CF4 can be decomposed over alumina-based Ga, Zn, Ni, In, Sn, and Zr oxide catalysts. The catalytic activity maintained for more than 18h.Surface metal fluorides produced on binary metal oxide catalysts by the reaction with HF were hydrolyzed under temperature programmed mode and the products were analyzed by a mass spectrometer. It was clearly demonstrated from the experiments that the amount of released HF was well correlated with the corresponding catalytic activity. Further the amounts of L-acid sites measured by in situ FT-IR studies of adsorbed pyridine on the various binary metal oxides also followed the same order as the catalytic activity. These results suggest that breaking of CF bond in CF4 molecule by L-acid sites is a rate-limiting step in the hydrolysis reaction of CF4 and resulting surface fluoride is easily hydrolyzed with water. Other physical techniques such as XRD, weight and surface area of samples before and after the CF4 hydrolysis reaction were demonstrated that some of binary oxide catalysts were stable under the reaction conditions, especially Ga-Al and Ni-Al binary oxide catalysts.

Voltammetric studies of bidirectional catalytic electron transport in Escherichia coli succinate dehydrogenase: comparison with the enzyme from beef heart mitochondria

The succinate dehydrogenases (SDH: soluble, membrane-extrinsic subunits of succinate:quinone oxidoreductases) from Escherichia coli and beef heart mitochondria each adsorb at a pyrolytic graphite ‘edge’ electrode and catalyse the interconversion of succinate and fumarate according to the electrochemical potential that is applied. E. coli and beef heart mitochondrial SDH share only ca. 50% homology, yet the steady-state catalytic activities, when measured over a continuous potential range, display very similar catalytic operating potentials and energetic biases (the relative ability to catalyse succinate oxidation vs. fumarate reduction). Importantly, E. coli SDH also exhibits the interesting ‘tunnel-diode’ behaviour previously reported for the mitochondrial enzyme. Thus as the potential is lowered below ca. −60 mV (pH 7, 38°C) the rate of catalytic fumarate reduction decreases abruptly despite an increase in driving force. Since the homology relates primarily to residues associated with active site regions, the marked similarity in the voltammetry reaffirms our previous conclusions that the tunnel-diode behaviour is a characteristic property of the enzyme active site. Thus, succinate dehydrogenase is an excellent fumarate reductase, but its activity in this direction is limited to a very specific range of potential.

Reactivity and transformation routes of surface nitrates in the reaction of NOx reduction by C3H8 over cation-exchanged zeolites

FITRS combined with15N and18O isotope substitution and TPD have revealed that nitrates are the most abundant ad-NOx species on over-exchanged Cu- and Co-ZSM-5 zeolites. the transformation rates of such species under the action of a propane—oxygen mixture were found to be comparable and sufficiently high to afford steady-state catalytic activities. A scheme of the reaction mechanism including C, N-containing intermediates detected in work is suggested.

A highly active and highly selective oxide catalyst for the conversion of ethanol to acetone in the presence of water vapour

In the presence of water vapour and a suitable catalyst, ethanol is converted to acetone rather than to ethylene and acetaldehyde. In order to develop an appropriate catalyst for this reaction, steady-state catalytic activities and selectivities were studied for 24 oxide catalysts. It was found that the acetone selectivity is high on a catalyst having both surface acidity and basicity, suggesting that the acetone formation is an acid-base bifunctional catalytic reaction. Iron oxide is superior to the other oxides studied here in both conversion and acetone selectivity. The superiority is greatly enhanced by mixing iron oxide with zinc oxide. The preparation method for the iron–zinc mixed oxide catalyst was also studied and it was found that the optimal composition is Zn/(Fe + Zn)= 0.1–0.4 and that the optimal condition for calcination is 773 K for 3 h. The catalyst gave 100% ethanol conversion and 94% acetone selectivity at a reaction temperature of 713 K. It is initially a mixture of Fe2O3 and ZnO but is converted to a spinel type compound, ZnFe2O4, during the reaction. The optimal reaction temperature was determined to be 713 K, and at this temperature, the acetone yield decreased by 34% after a time of 24 h.

Conversion of isopropyl alcohol to acetone catalysed by Cr2O3 at 473 K: role of molecular oxygen

The conversion of isopropyl alcohol to acetone has been studied over Cr2O3 powder at 473 K in a flowing microcatalytic reactor. Appreciable steady-state catalytic activity was achieved only when oxygen was present in the reactant gas flow. The effect of mild pretreatment of Cr2O3 in O2–N2 at 473 K on the initial activity, and the observed dependence of the process on O2 pressure has been related to the oxidation–reduction cycle of the surface chromium species. Cr(3+n)+ on the surface of Cr2O3 have been considered to be the active species, where n= 1–3.

Kinetic and Infrared Spectroscopic Studies of Fe-Y Zeolites for the Selective Catalytic Reduction of Nitric Oxide by Ammonia

Reaction kinetics and infrared spectroscopic studies of the selective catalytic reduction of nitric oxide by ammonia were conducted at reactant concentrations near 500 ppm and temperatures from 473 to 623 K, over Fe-Y zeolites having Si/Al ratios of 2.4, 3.2, and 4.4. The presence of iron cations within the zeolite was necessary to sustain steady-state catalytic activity, which was enhanced by about two orders of magnitude in the presence of oxygen. The reaction was near first order in NO and near zero order in ammonia. Ammonia oxidation was found to become a significant competing reaction between 493 and 543 K over zeolites with Si/Al ratios equal to 2.4 and 3.2. Ammonia oxidation did not occur over the sample with a Si/Al ratio of 4.4. The primary surface species observed under reaction conditions by in situ IR spectroscopy were NH3 and NH+4 In the absence of oxygen, the rate-limiting step appears to be the reoxidation of iron.

The effects of alkali metal hydroxide pretreatment on the hydrogenation activity of nickel-loaded Y zeolites

A range of NiKY and NiNaY zeolites prepared by ion exchange were treated with NaOH and KOH solutions of varying pH at room temperature and reduced in a hydrogen atmosphere at 723 K. By a combination of iodometric titrations and X-ray line broadening it is shown that reduction of the hydroxide treated samples generates a greater mass of nickel metal, which is in the form of smaller crystallites compared with the untreated zeolites. The pretreated samples exhibit a uniquely high steady state catalytic activity in the hydrogenation of benzene to cyclohexane.

Reduction of nitric oxide by carbon monoxide over silica-supported platinum-rhodium alloys, comparison with the CO + O2 reaction

The reduction of NO by CO was investigated over various silica-supported Pt-Rh alloy catalysts. Steady state catalytic activity as a function of temperature was measured over a range of gas phase compositions. The results were compared with those obtained earlier for the CO + O2 reaction over the same catalysts. The results can be interpreted on the basis of our earlier Pt-Rh single crystal work. Important factors influencing the performance of the Pt-Rh alloy catalysts are the specific differences in intrinsic properties of Pt and Rh, the experimental conditions of the reaction and the composition of the alloy catalyst. We conclude that at the relatively high temperatures required for the CO + NO reaction the composition of the gas phase may induce large variations of the surface composition.

Flow as a regulator of the activation of factor X by tissue factor

A novel enzyme reactor for phospholipid-dependent reactions was used to study the effects of flow on tissue factor-initiated coagulation. Microcapillaries were coated with a phospholipid bilayer containing tissue factor, a transmembrane protein that is an essential cofactor for a plasma procoagulant enzyme, factor VII. We show that, in contrast to static, closed systems, the steady-state catalytic activity is independent of enzyme concentration and the time to steady state becomes a function of the enzyme concentration.

Kinetics of carbon monoxide oxidation by oxygen or nitric oxide on rhodium(111) and rhodium(100) single crystals

The oxidation of CO by O/sub 2/ or NO over Rh(111) and Rh(100) single crystals has been studied in a high-pressure reaction-high-vacuum surface analysis apparatus. Steady-state catalytic activity as a function of temperature and partial pressures of CO, O/sub 2/, and NO has been measured. The CO-O/sub 2/ reaction was found to be insensitive to the structure of the surface as evidenced by the identical rates, activation energies, and partial pressure dependencies measured on the two-single-crystal surfaces. Both surfaces deactivated at high O/sub 2/ partial pressures to the formation of a near-surface oxide (probably Rh/sub 2/O/sub 3/) which is catalytically inactive for this reaction. The deactivation occurred at a slightly lower O/sub 2/ partial pressure on Rh(100), likely due to the relative ease of oxygen diffusion through this surface leading to more rapid bulk oxidation. Unlike the CO-O/sub 2/ reaction, CO oxidation by NO was very sensitive to the geometric structure of the surface. This behavior was evident in all kinetic parameters measured on the two surfaces. The kinetic data obtained on Rh(111) agree quantitatively with a surface chemistry model previously developed for this reaction. The model predicts that the rate is limited by the formation of N/sub 2/ frommore » the recombination of adsorbed N atoms. and that the surface, under steady-state reaction conditions, is covered largely with adsorbed N atoms and smaller amounts of adsorbed NO. On Rh(100) the data are consistent with a reaction rate limited by the formation of N/sub 2/ from the surface reaction of adsorbed NO and N atoms.« less