Trends In Catalytic Activity Research Articles

Understanding Trends in the Electrocatalytic Activity of Metals and Enzymes for CO2 Reduction to CO.

We develop a model based on density functional theory calculations to describe trends in catalytic activity for CO2 electroreduction to CO in terms of the adsorption energy of the reaction intermediates, CO and COOH. The model is applied to metal surfaces as well as the active site in the CODH enzymes and shows that the strong scaling between adsorbed CO and adsorbed COOH on metal surfaces is responsible for the persistent overpotential. The active site of the CODH enzyme is not subject to these scaling relations and optimizes the relative binding energies of these adsorbates, allowing for an essentially reversible process with a low overpotential.

2,6-Dibenzhydryl-N-(2-aryliminoacenaphthylenylidene)-4-chlorobenzenamino-palladium dichlorides: Synthesis, characterization, and use as catalysts in the Heck-reaction

A series of 2,6-dibenzhydryl-N-(2-aryliminoacenaphthylenylidene)-4- chlorobenzenaminopalladium(II) chloride complexes (C1–C6) were prepared and fully characterized by FT-IR and NMR spectroscopy, and by elemental analysis. The molecular structures of representative complexes C2 and C6 were confirmed as square planar at the palladium center by single crystal X-Ray diffraction. All palladium complexes exhibited good activity in the Heck cross-coupling reaction with conversions greater than 90%. Catalytic activity trends were attributed to differences in the solubility of the pre-catalysts; palladium complexes bearing more soluble alkyl-substituents performed with higher catalytic activities.

Effects of Heat Treatment on the Catalytic Activity and Methanol Tolerance of Carbon-Supported Platinum Alloys

This work studies the effect of heat treatment of carbon-dispersed platinum and platinum alloys on its methanol tolerance and catalytic activity as gas diffusion electrodes for oxygen reduction reaction (ORR) in acid medium. The catalyst powders were subjected to heat treatments at three different temperatures for a fixed period at controlled atmospheres. Differences in catalyst morphology were characterized using X-ray diffraction, energy dispersive X-ray analysis, and transmission electron microscope techniques. The electrochemical characteristics and activity of the electrocatalysts were evaluated for ORR and methanol tolerance using cyclic voltammetry in the form of gas diffusion electrodes. The optimum heat treatment temperature is found to be strongly dependent on the individual catalyst. The maximum ORR activity and better methanol tolerance for ORR was observed in Pt-Fe/C and Pt-Cu/C catalysts subjected to heat treatment at 350 °C. A trend of catalytic activity for ORR was obtained: Pt-Cu/C (350 °C) > Pt-Fe/C (350 °C) > Pt-Ni/C (350 °C) > Pt-Co/C (250 °C) > Pt/C (350 °C), showing that Pt-Cu/C-type catalysts had a higher catalytic activity with reasonable methanol tolerance.

Catalytic Activity Trends of Oxygen Reduction Reaction for Nonaqueous Li-Air Batteries

We report the intrinsic oxygen reduction reaction (ORR) activity of polycrystalline palladium, platinum, ruthenium, gold, and glassy carbon surfaces in 0.1 M LiClO(4) 1,2-dimethoxyethane via rotating disk electrode measurements. The nonaqueous Li(+)-ORR activity of these surfaces primarily correlates to oxygen adsorption energy, forming a "volcano-type" trend. The activity trend found on the polycrystalline surfaces was in good agreement with the trend in the discharge voltage of Li-O(2) cells catalyzed by nanoparticle catalysts. Our findings provide insights into Li(+)-ORR mechanisms in nonaqueous media and design of efficient air electrodes for Li-air battery applications.

Trend of catalytic activity of CO oxidation on Rh and Ru nanoparticles: Role of surface oxide

Recent studies suggest that surface oxides on transition metal nanoparticles play an important role in determining the catalytic activity of CO oxidation. In this work, we investigated the influence of surface modification of Rh and Ru nanoparticles on the catalytic activity of CO oxidation using UV-ozone surface treatment. Monodisperse Rh and Ru nanoparticles were synthesized by polyol reduction using poly(vinylpyrrolidone) (PVP) as a capping agent. The size of the nanoparticles was controlled by varying the concentration of the Rh and Ru precursors or using the seeded-growth method. The changes that occurred during UV-ozone surface treatment were characterized with X-ray photoelectron spectroscopy, which showed that the oxidation state increased after surface treatment. The catalytic activity and activation energy of Rh and Ru nanoparticle arrays were measured before and after the chemical modification. Our reaction studies indicate that the turnover rate of CO oxidation on Rh nanoparticles increases by a factor of three after UV-ozone treatment due to the formation of catalytically active Rh oxide. In contrast, the catalytic activity of Ru nanoparticles decreases after UV-ozone treatment, suggesting that the Ru bulk oxide formed during UV-ozone treatment is catalytically inactive. The results suggest an intriguing way to tune catalytic activity via engineering of the nanoscale surface oxide.

Descriptor‐Based Analysis Applied to HCN Synthesis from NH3 and CH4

The design of solid metal catalysts using theoretical methods has been a long-standing goal in heterogeneous catalysis. Recent developments in methodology and computer technology as well as the establishment of a descriptor-based approach for the analysis of reaction mechanisms and trends across the periodic table allow for the fast screening for new catalytic materials and have lead to first examples of computational discoveries of new materials. The underlying principles of the descriptor-based approach are the existence of relations between the surface electronic structure, adsorption energies and activation barriers that result in volcano-shaped activity plots as function of simple descriptors, such as atomic binding energies or the d-band center. Linear scaling relations have been established between the adsorption energies of hydrogen-containing molecules such as CH{sub x}, NH{sub x}, OH{sub x} and SH{sub x} and the C, N O and S adsorption energies on transition-metal surfaces. Transition-state energies have also been shown to scale linearly with adsorption energies in a similar fashion. Recently, a single transition state scaling relation has been identified for a large number of C-C, C-O, C-N, N-O, N-N, and O-O coupling reactions. The scaling relations provide a powerful tool for the investigation of reaction mechanisms and the prediction of potential energy surfaces. They limit the number of independent variables to a few, typically adsorption energies of key atoms. Using this information as input to a microkinetic model provides an understanding of trends in catalytic activity across the transition metals. In most cases a volcano-shaped relation between activity and the key variables, the descriptors, is observed. In the present paper we will provide an example of the approach outlined above and show how one can obtain an understanding of activity/selectivity trends of a reaction with just a few new calculations.

Optimizing the Electrocatalytic Activity of Surface Confined Co Macrocyclics for the Electrooxidation of Thiocyanate at pH 4

AbstractWe have studied the trends in catalytic activity of several Co macrocyclics confined on the surface graphite electrodes for the oxidation of thiocyanate. A plot of log i (at constant E) versus the formal potential of the catalyst gives a volcano correlation, indicating that the Co(II/I) redox potential needs to be tuned, in order to achieve maximum reactivity. Graphite electrodes modified with Co phthalocyanine at pH 4 exhibit linear amperometric response for thiocyanate concentration in the range 10−7 and 10−3 M. Theoretical calculations show that electrocatalytic activity (as log i at constant E) plotted versus the energy of the LUMO of the Co complex also gives a volcano correlation.

Understanding Trends in Catalytic Activity: The Effect of Adsorbate–Adsorbate Interactions for CO Oxidation Over Transition Metals

Using high temperature CO oxidation as the example, trends in the reactivity of transition metals are discussed on the basis of density functional theory (DFT) calculations. Volcano type relations between the catalytic rate and adsorption energies of important intermediates are introduced and the effect of adsorbate–adsorbate interaction on the trends is discussed. We find that adsorbate–adsorbate interactions significantly increase the activity of strong binding metals (left side of the volcano) but the interactions do not change the relative activity of different metals and have a very small influence on the position of the top of the volcano, that is, on which metal is the best catalyst.

The influence of reducing and sulfiding conditions on the properties of unsupported MoS 2-based catalysts

Unsupported MoS 2 catalysts were obtained from the decomposition of ammonium tetrathiomolybdate (ATM) at variable temperatures (400–700 °C) and under different gas compositions, from pure H 2S to pure H 2. The catalysts were further studied in the non-promoted state or promoted by Ni and Co. Catalytic activity and selectivity were studied in the model reaction of thiophene hydrodesulfurization (HDS). Surface areas, crystalline phase and particle size distributions were determined by Brunauer–Emmet–Teller (BET), X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. A comparison of average values calculated from these techniques has enabled the understanding of the morphology of the solids. The catalysts were characterized before and after catalytic tests by X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS) and temperature-programmed reduction (TPR). Comparison of catalytic activity trends with the results of the characterizations show that over-stoichiometric sulfur, present in the fresh catalysts in the form of edge-located S 2 2 - species, plays a key role for the activity of unsupported MoS 2 and for its ability to be promoted. Direct hydrogenation (HYD) of thiophene to butane occurs presumably with the participation of –SH groups, produced from the opening of S–S bridges by hydrogen. Whatever the gas atmosphere, any treatment leading to the removal of over-stoichiometric sulfur leads to a decrease in HYD selectivity. Thus, very similar catalytic properties were observed for MoS 2 annealed at 700 °C in pure H 2, H 2S or N 2 gases. Ni and Co introduced by means of reflux with acetylacetonates, gave identical promotion trends for all the MoS 2 samples. The solids treated in pure H 2S could not take up promoter atoms at the edges, whereas for the H 2-reduced samples high promotion levels were achieved. The degree of stacking does not seem to have a significant impact on the thiophene HDS activity and selectivity of the unsupported MoS 2 catalysts.

Influence of rare earth promoters on the performance of Ni/Mg(Al)O catalysts for hydrogenation and steam reforming of toluene

CeO2-, La2O3-, and ZrO2-promoted Ni/Mg(Al)O catalysts synthesized by hydrotalcite-type precursors have been investigated with respect to catalytic activity and carbon formation in the hydrogenation and steam reforming of toluene as a model tar compound. X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area and H2-temperature programmed reduction (TPR) were used to observe the characteristics of the prepared catalysts. The carbon formation and its amount on the used catalysts were examined by transmission electron microscope (TEM), scanning electron microscope (SEM) and thermogravimetric (TG). The trend of catalytic activity as derived from the experimental results followed the order: Ni-Ce>Ni-Las>Ni-Zr>Ni. The catalyst modified with CeO2 exhibited the highest catalytic performance and had good carbon resistance in the hydrogenation and steam reforming of toluene. A toluene conversion of 96.8%, a CH4 yield of 45.2% and a CO yield of 50.4% have been achieved. The addition of promoters led to better dispersion of nickel species and higher interaction nickel-support, which were favorable for increasing the catalytic activity and effectively preventing carbon formation.

An understanding and implications of the coverage of surface free sites in heterogeneous catalysis

The crucial roles of the coverage of surface free sites in determining catalytic activity trend are quantitatively addressed with the help of density functional theory and microkinetics. First, by analyzing activity trends of NO oxidation catalyzed by Ru, Rh, Pd, Os, Ir, and Pt surfaces with full kinetic considerations, we identify that the activity trend is in general determined by the competition between the reaction barrier and the coverage of surface free sites. Second, since the dissociation of many important molecules, such as the dissociation of N(2), O(2), and CO, follows the same Bronsted-Evans-Polanyi relationship, the coverage of surface free sites is usually a decisive term that affects the overall activity. Third, an equation is derived for the coverage of surface free sites and it is found that the coverage of surface free sites contains not only all the key thermodynamic parameters but also all the kinetic properties in the catalytic system.

PtM/C Catalyst Prepared Using Reverse Micelle Method for Oxygen Reduction Reaction in PEM Fuel Cells

Synthesis of carbon-supported PtM/C catalysts (M = Co, Cr, or Fe) using a new preparation technique, a reverse micelle method, is reported. The catalysts were characterized by different surface techniques:? X-ray diffraction, scanning electron microscope, transmission electron microscope, and energy dispersive X-ray microanalysis. Surface characterization showed that Pt/M nanoparticles on catalysts were synthesized using the reverse micelle method. Pt/M nanoparticles were observed to be uniform spherical objects. The performance of the PtM/C catalysts was tested by the rotating disk electrode technique. A trend of catalytic activity for oxygen reduction reaction (ORR) was obtained:? PtCo/C(T, 500) PtCo/C(S) > PtCr/C(S) > PtFe/C(S) Pt/C > PtFe/C(T, 500) PtCr/C(T, 500), showing that PtCo/C-type catalysts had a higher catalytic activity for ORR.

Trends in catalytic activity for SOFC anode materials

Quantum mechanical calculations on the level of density-functional theory are used to calculate the stability of surface-adsorbed hydrogen atoms, oxygen atoms, and hydroxyl radicals for a variety of metals (Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Pt, Au) that may be used as electrode materials for solid oxide fuel cell (SOFC) anodes. The reaction energies along the hydrogen oxidation pathway were quantified for both, oxygen spillover and hydrogen spillover mechanisms at the three-phase boundary. The ab initio results are compared to previously-obtained experimental anode activities measured for nine different metal/stabilized zirconia anodes. The experimentally-observed variation of electrode activity with anode material is well-correlated with the calculated stability of surface-adsorbed atomic oxygen, but uncorrelated with the stability of surface-adsorbed hydrogen. This finding indicates that oxygen spillover, where adsorbed oxygen is a key intermediate, is the dominant reaction pathway under the conditions used in the experiments. In this way the activity is linked directly to the microscopic binding affinities of reaction intermediates, providing a new understanding of the anode reaction.

Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H‐ZSM‐5 Crystals by Time‐ and Space‐Resolved Optical and Fluorescence Microspectroscopy

A combination of in-situ optical and fluorescence microspectroscopy has been employed to investigate the oligomerization of styrene derivatives occurring in the micropores of coffin-shaped H-ZSM-5 zeolite crystals in a space- and time-resolved manner. The carbocationic intermediates in this reaction act as reporter molecules for catalytic activity, since they exhibit strong optical absorption and fluorescence. In this way, reactant selectivity and restricted transition-state selectivity for 14 substituted styrene molecules can be visualized and quantified. Based on a thorough analysis of the time- and space-resolved UV/Vis spectra, it has been revealed that two main parameters affect the reaction rates, namely, the carbocation stabilization effect and the diffusion hindrance. The stabilization effect was tested by comparison of the reaction rates for 4-methoxystyrene versus 4-methylstyrene and in the series 4-bromo-, 4-chloro and 4-fluorostyrene; in both cases less electronegative substituents were found to accelerate the reaction. As to the steric effect, bulkier chemical groups bring down the reaction rate, as evident from the observation that 4-methoxystyrene is more reactive than 4-ethoxystyrene due to differences in their diffusivity, while heavily substituted styrenes, such as 3,4-dichlorostyrene and 2,3,4,5,6-pentafluorostyrene, cannot enter the zeolite pore system and therefore do not display any reactivity. Furthermore, beta-methoxystyrene and trans-beta-methylstyrene show limited reactivity as well as restricted reaction-product formation due to steric constraints imposed by the H-ZSM-5 channel system. Finally, polarized-light optical microspectroscopy and fluorescence microscopy demonstrate that dimeric styrene compounds are predominantly formed and aligned within the straight channels at the edges of the crystals, whereas a large fraction of trimeric carbocations along with dimeric compounds are present in the straight channels of the main body of the H-ZSM-5 crystals. Our results reinforce the observation of a non-uniform catalytic behavior within zeolite crystals, with specific parts of the zeolite grains being less accessible and reactive towards reactant molecules. The prospects and potential of this combined in-situ approach for studying large zeolite crystals in the act will be discussed.

Cover Picture: Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H-ZSM-5 Crystals by Time- and Space-Resolved Optical and Fluorescence Microspectroscopy (Chem. Eur. J. 25/2007)

Cover Picture: Revealing Shape Selectivity and Catalytic Activity Trends Within the Pores of H-ZSM-5 Crystals by Time- and Space-Resolved Optical and Fluorescence Microspectroscopy (Chem. Eur. J. 25/2007)

Trends in catalytic NO decomposition over transition metal surfaces

The formation of NOx from combustion of fossil and renewable fuels continues to be a dominant environmental issue. We take one step towards rationalizing trends in catalytic activity of transition metal catalysts for NO decomposition by combining microkinetic modelling with density functional theory calculations. We show specifically why the key problem in using transition metal surfaces to catalyze direct NO decomposition is their significant relative overbinding of atomic oxygen compared to atomic nitrogen.

Immobilized Complexes of Metals with Amino Acid Ligands − A First Step toward the Development of New Biomimetic Catalysts

The natural amino acids histidine and glutamic acid were immobilized on the surface of functionalized mesoporous silica supports. Histidine and glutamic acid alone and combinations of both were covalently bound to the amino groups on the surface of the support by the formation of amide bonds. The immobilized amino acids were used as ligands for Fe(II). The obtained biomimetic iron complexes were tested as catalysts for cyclohexane oxidation at mild reaction conditions with hydrogenperoxide as oxidant. This stepwise preparation method allowed the synthesis of materials with varying nature and loading of immobilized amino acids. The observed trends in catalytic activity show that both the type and the surface density of immobilized amino acid have an influence on the catalytic performance during cyclohexane oxidation and also on the structure of the immobilized Fe complex.

Photo-degradation catalyst screening by high throughput experiments

A high throughput fluorescence photo-imaging method has been developed for the screening of photo-degradation catalysts. When employed to analyze the photo-degradation of 1,6-hexamethylenediamine (the probe molecule), the method produced results that are comparable to that when methyl orange was employed as a probe molecule and UV spectrometry as an analysis device. We found that the variation trends in catalytic activity over the same group of catalysts are the same in both cases. The results indicate that the fluorescence photo-imaging method is applicable to the screening of photo-degradation catalysts. For comparison, mesoporous catalysts of high specific surface area prepared by using P123 (HO(CH 2CH 2O) 20(CH 2CH(CH 3)O) 70(CH 2CH 2O) 20H) as a template and “regular” catalysts (those prepared without P123) were tested. The results showed that most of the former catalysts are superior to the latter counterparts. Our study also revealed that the addition of ZrO 2 to TiO 2 did not result in significant improvement in catalytic activities, whereas marked improvement was observed when Nb 2O 5 or WO 3 was added to TiO 2. When Nb 2O 5 and WO 3 were co-doped to TiO 2, the tertiary catalysts exhibited higher catalytic activities. The catalysts containing 20–30% of Nb 2O 5 and 10–20% of WO 3 (balanced with TiO 2) are superior to any of pure TiO 2, Nb 2O 5, WO 3, and the binary catalysts composed of any two of TiO 2, Nb 2O 5, and WO 3. Our study has demonstrated that besides the traditional TiO 2-domain catalysts, active photo-degradation catalysts can be prepared by combination of Nb 2O 5, WO 3, and ZrO 2.

Evolution of Catalytic Activity of Au−Ag Bimetallic Nanoparticles on Mesoporous Support for CO Oxidation

We report a novel Au-Ag alloy catalyst supported on mesoporous aluminosilicate Au-Ag@MCM prepared by a one-pot synthesis procedure, which is very active for low-temperature CO oxidation. The activity was highly dependent on the hydrogen pretreatment conditions. Reduction at 550-650 degrees C led to high activity at room temperature, whereas as-synthesized or calcined samples did not show any activity at the same temperature. Using various characterization techniques, such as XRD, UV-vis, XPS, and EXAFS, we elucidated the structure and surface composition change during calcination and the reduction process. The XRD patterns show that particle size increased only during the calcination process on those Ag-containing samples. XPS and EXAFS data demonstrate that calcination led to complete phase segregation of the Au-Ag alloy and the catalyst surface is greatly enriched with AgBr after the calcination process. However, subsequent reduction treatment removed Br- completely and the Au-Ag alloy was formed again. The surface composition of the reduced Au-Ag@MCM (nominal Au/Ag = 3/1) was more enriched with Ag, with the surface Au/Ag ratio being 0.75. ESR spectra show that superoxides are formed on the surface of the catalyst and its intensity change correlates well with the trend of catalytic activity. A DFT calculation shows that CO and O2 coadsorption on neighboring sites on the Au-Ag alloy was stronger than that on either Au or Ag. The strong synergism in the coadsorption of CO and O2 on the Au-Ag nanoparticle can thus explain the observed synergetic effect in catalysis.

Role of substitution in catalytic activity of La–Th–V–O mixed oxides for the reaction of methanol

Abstract A series of composite metal oxide catalysts of nominal composition La x Th 1− x V 2 O 7−δ ( x = 0.0–0.5) were synthesized by ceramic route and characterized using XRD, IR and temperature programmed reduction (TPR) measurements. The partial substitution at A-sites had profound effect on the catalytic behavior of thorium pyrovanadate for the dehydration of methanol, the extent of which depended upon the value of x . The following was the trend of catalytic activity of different samples as a function of the value of x : x up to 0.2 > 0.3 > 0.5 > 0 > ThO 2 . In addition, the substituted samples exhibited higher selectivity for the formation of dimethyl ether and also a better compositional stability towards the successive cycles of reduction/reoxidation. These characteristics of substituted samples are found to be related to existence of a phase, viz. La x Th 1− x V 2 O 7−δ , where a part of lattice Th sites are substituted by La. On the other hand, the other secondary phases, viz. LaTh(VO 4 ) 3−δ and Th(VO 3 ) 4 that were produced simultaneously during the sample preparation, play no important role. It is suggested that the nonstoichiometry generated in the pyrovanadate phase is responsible to the improvement in the catalytic activity and the stability of the substituted samples.