Design Of Improved Catalysts Research Articles

Correlation of Active Site Precursors and Olefin Metathesis Activity in W-Incorporated Silicates

SiO2-supported tungsten catalysts (WO3/SiO2) are currently used in industry to make propene via metathesis of butene and ethylene. Reliable information on structure–activity relationships, aimed at improved catalyst design, continues to be an active area of investigation. On the basis of ex situ XPS and solid-state NMR spectroscopy of calcined W-incorporated mesoporous silicates (W-KIT-6), complemented by computational studies and activity evaluation in a fixed-bed reactor, we demonstrate that W–O–Si species are the active site precursors for 2-butene + ethylene metathesis to produce propene at 450 °C. This conclusion is supported by evidence that the population of W–O–Si species on the calcined W-KIT-6 catalysts is linearly correlated with 2-butene conversion, regardless of W loading. We further show that for a given W loading, the accessible W–O–Si species and thereby the propene yield, can be enhanced by delaying the tungsten source addition during catalyst synthesis. Our results are consistent with previously reported in situ mechanistic studies using complementary analytical tools.

Potential-induced nanoclustering of metallic catalysts during electrochemical CO2 reduction

In catalysis science stability is as crucial as activity and selectivity. Understanding the degradation pathways occurring during operation and developing mitigation strategies will eventually improve catalyst design, thus facilitating the translation of basic science to technological applications. Herein, we reveal the unique and general degradation mechanism of metallic nanocatalysts during electrochemical CO2 reduction, exemplified by different sized copper nanocubes. We follow their morphological evolution during operation and correlate it with the electrocatalytic performance. In contrast with the most common coalescence and dissolution/precipitation mechanisms, we find a potential-driven nanoclustering to be the predominant degradation pathway. Grand-potential density functional theory calculations confirm the role of the negative potential applied to reduce CO2 as the main driving force for the clustering. This study offers a novel outlook on future investigations of stability and degradation reaction mechanisms of nanocatalysts in electrochemical CO2 reduction and, more generally, in electroreduction reactions.

Mechanism and rate constant of proline-catalysed asymmetric aldol reaction of acetone and p-nitrobenzaldehyde in solution medium: Density-functional theory computation

In search of new ways to improve catalyst design, the current research focused on using quantum mechanical descriptors to investigate the effect of proline as a catalyst for mechanism and rate of asymmetric aldol reaction. A plausible mechanism of reaction between acetone and 4-nitrobenzaldehyde in acetone medium was developed using highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies calculated via density functional theory (DFT) at the 6-31G∗/B3LYP level of theory. New mechanistic steps were proposed and found to follow, with expansion, the previously reported iminium-enamine route of typical class 1 aldolase enzymes. From the elementary steps, the first step which involves a bimolecular collision of acetone and proline was considered as the rate-determining step, having the highest activation energy of 59.07 kJ mol−1. The mechanism was used to develop the rate law from which the overall rate constant was calculated and found to be 4.04×10-8dm3mol-1s-1. The new mechanistic insights and the explicit computation of the rate constant further improve the kinetic knowledge of the reaction.

Bulky β-Diketones Enabling New Lewis Acidic Ligand Platforms.

The synthesis of a sterically encumbered β-diketone ligand (Aracac) substituted with 2,6-(2,4,6-Me3C6H2)2C6H3 is described. Coordination complexes of the type M(Aracac)2Cl(solv) (M = Ti, V, Cr; solv = THF, CH3CN) were prepared by the reaction of Aracac with MCl3 (M = V, Cr) or with TiCl4 to generate Ti(Aracac)2Cl2, followed by reduction. These complexes show a trend of alternating the cis/trans geometric preference with increasing dn electron count (n = 0, 1, 2, 3), which is rationalized in part by the unusual ability of β-diketonates to behave as either a weak π donor or a π acceptor in the cis and trans geometries, respectively. In this way, the bis-β-diketonate platform can accommodate the varying electronic demands of the coordinated metal ion. These results demonstrate the ability to limit the coordination of β-diketonates on metal complexes for the first time, providing a chemically robust and coordinatively versatile platform for mechanistic investigations, metal functionalization, and improved catalyst design.

Deactivation of a Cobalt Catalyst for Water Reduction through Valence Tautomerism

The activity of the water reduction catalyst [CoIII (L1 )(pyr)2 ]PF6 (1), where (L1 )2- is a bis-amido pyridine ligand and pyr is pyrrolidine, is investigated. Catalyst 1 has an overpotential of 0.54 V and a high observed TOF of 23 min-1 , albeit for a relatively short time. Considering the significant activity of 1 and aiming to improve catalyst design, a detailed structural and electronic study is performed to understand the mechanisms of deactivation. Experimental and theoretical evidence support that the metal-reduced [CoI (L1 )]- is in tautomeric equilibrium with the ligand-reduced [CoII (L1. )]- species. While [CoI (L1 )]- favors formation of a CoIII -H- relevant for catalysis, the [CoII (L1. )]- species leads to ligand protonation, structural distortions, and, ultimatley, catalyst deactivation.

Oxygen Evolution at Manganite Perovskite Ruddlesden-Popper Type Particles: Trends of Activity on Structure, Valence and Covalence.

An improved understanding of the correlation between the electronic properties of Mn-O bonds, activity and stability of electro-catalysts for the oxygen evolution reaction (OER) is of great importance for an improved catalyst design. Here, an in-depth study of the relation between lattice structure, electronic properties and catalyst performance of the perovskite Ca1−xPrxMnO3 and the first-order RP-system Ca2−xPrxMnO4 at doping levels of x = 0, 0.25 and 0.5 is presented. Lattice structure is determined by X-ray powder diffraction and Rietveld refinement. X-ray absorption spectroscopy of Mn-L and O-K edges gives access to Mn valence and covalency of the Mn-O bond. Oxygen evolution activity and stability is measured by rotating ring disc electrode studies. We demonstrate that the highest activity and stability coincidences for systems with a Mn-valence state of +3.7, though also requiring that the covalency of the Mn-O bond has a relative minimum. This observation points to an oxygen evolution mechanism with high redox activity of Mn. Covalency should be large enough for facile electron transfer from adsorbed oxygen species to the MnO6 network; however, it should not be hampered by oxidation of the lattice oxygen, which might cause a crossover to material degradation. Since valence and covalency changes are not entirely independent, the introduction of the energy position of the eg↑ pre-edge peak in the O-K spectra as a new descriptor for oxygen evolution is suggested, leading to a volcano-like representation of the OER activity.

Revised Oxygen Evolution Reaction Activity Trends for First-Row Transition-Metal (Oxy)hydroxides in Alkaline Media.

First-row transition-metal oxides and (oxy)hydroxides catalyze the oxygen evolution reaction (OER) in alkaline media. Understanding the intrinsic catalytic activity provides insight into improved catalyst design. Experimental and computationally predicted activity trends, however, have varied substantially. Here we describe a new OER activity trend for nominally oxyhydroxide thin films of Ni(Fe)O(x)H(y) > Co(Fe)O(x)H(y) > FeO(x)H(y)-AuO(x) > FeO(x)H(y) > CoO(x)H(y) > NiO(x)H(y) > MnO(x)H(y). This intrinsic trend has been previously obscured by electrolyte impurities, potential-dependent electrical conductivity, and difficulty in correcting for surface-area or mass-loading differences. A quartz-crystal microbalance was used to monitor mass in situ and X-ray photoelectron spectroscopy to measure composition and impurity levels. These new results provide a basis for comparison to theory and help guide the design of improved catalyst systems.

Unexpected ligand lability in condition of water oxidation catalysis

In the search for rational design of improved water oxidation catalysts, enhanced catalytic activities were reported for single site Ru catalysis with Ru-iodide coordination. As these complexes are not initially capable of proton coupled electron transfer (PCET) and RuO formation, a proposal was put forward on the generation of catalytically active 7-coordinate Ru species. We tested this hypothesis by EPR and X-ray spectroscopy and found that [RuII(bpy)(tpy)I]+ only serves as a precursor for the formation of [RuIV(bpy)(tpy)O]2+. Upon oxidation with excess of CeIV the RuI bond quickly dissociates with the formation of [RuIII(bpy)(tpy)H2O]3+ and [RuIV(bpy)(tpy)O]2+ complexes. The catalytic steady state was composed of 95% [RuIV(bpy)(tpy)O]2+ species. Thus, introducing the RuI bond into initial catalysts does not serve to improve catalyst design. This manuscript also shows how EXAFS can directly probe transition metal–halogen interaction for in situ catalysis.

Probing the nature of gold–carbon bonding in gold–alkynyl complexes

Homogeneous catalysis by gold involves organogold complexes as precatalysts and reaction intermediates. Fundamental knowledge of the gold–carbon bonding is critical to understanding the catalytic mechanisms. However, limited spectroscopic information is available about organogolds that are relevant to gold catalysts. Here we report an investigation of the gold–carbon bonding in gold(I)–alkynyl complexes using photoelectron spectroscopy and theoretical calculations. We find that the gold–carbon bond in the ClAu–CCH− complex represents one of the strongest gold–ligand bonds—even stronger than the known gold–carbon multiple bonds, revealing an inverse correlation between bond strength and bond order. The gold–carbon bond in LAuCCH− is found to depend on the ancillary ligands and becomes stronger for more electronegative ligands. The strong gold–carbon bond underlies the catalytic aptness of gold complexes for the facile formation of terminal alkynyl–gold intermediates and activation of the carbon–carbon triple bond.

Highly Enantio‐ and Diastereoselective Generation of Two Quaternary Centers in Spirocyclopropanation of Oxindole Derivatives.

Spirocyclopropanes: Only one out of eight possible stereoisomers was obtained in the asymmetric cascade cyclopropanation of alkylidene oxindoles with ethyl 2-chloroacetoacetate. Improved catalyst design ensured that spirocyclopropyl oxindoles featuring two quaternary centers were synthesized in high yield and high enantio- and diastereoselectivity (see scheme).

Growth of Ultrahigh Density Single-Walled Carbon Nanotube Forests by Improved Catalyst Design

We have grown vertically aligned single-walled carbon nanotube forests with an area density of 1.5 × 10(13) cm(-2), the highest yet achieved, by reducing the average diameter of the nanotubes. We use a nanolaminate Fe-Al(2)O(3) catalyst design consisting of three layers of Al(2)O(3), Fe, and Al(2)O(3), in which the lower Al(2)O(3) layer is densified by an oxygen plasma treatment to increase its diffusion barrier properties, to allow a thinner catalyst layer to be used. This high nanotube density is desirable for using carbon nanotubes as interconnects in integrated circuits.

Profiling Physicochemical Changes within Catalyst Bodies during Preparation: New Insights from Invasive and Noninvasive Microspectroscopic Studies

Cylindrical or spherical catalyst bodies with sizes ranging from tens of micrometers to a few millimeters have a wide variety of industrial applications. They are crucial in the oil refining industry and in the manufacture of bulk and fine chemicals. Their stability, activity, and selectivity are largely dependent on their preparation; thus, achieving the optimum catalyst requires a perfect understanding of the physicochemical processes occurring in a catalyst body during its synthesis. The ultimate goal of the catalyst researcher is to visualize these physicochemical processes as the catalyst is being prepared and without interfering with the system. In order to understand this chemistry and improve catalyst design, researchers need better, less invasive tools to observe this chemistry as it occurs, from the first stages in contact with a precursor all the way through its synthesis. In this Account, we provide an overview of the recent advances in the development of space- and time-resolved spectroscopic methods, from invasive techniques to noninvasive ones, to image the physicochemical processes taking place during the preparation of catalyst bodies. Although several preparation methods are available to produce catalyst bodies, the most common method used in industry is the incipient wetness impregnation. It is the most common method used in industry because it is simple and cost-effective. This method consists of three main steps each of which has an important role in the design of a catalytic material: pore volume impregnation, drying, and thermal treatment. During the impregnation step, the interface between the support surface and the precursor of the active phase at the solid-liquid interface is where the critical synthetic chemistry occurs. Gas-solid and solid-solid interfaces are critical during the drying and thermal treatment steps. Because of the length scale of these catalyst bodies, the interfacial chemistry that occurs during preparation is space-dependent. Different processes occurring in the core or in the outer rim of the catalytic solid are enhanced by several factors, such as the impregnation solution pH, the metal ion concentration, the presence of organic additives, and the temperature gradients inside the body. Invasive methods for studying the molecular nature of the metal-ion species during the preparation of catalyst bodies include Raman, UV-vis-NIR, and IR microspectroscopies. Noninvasive techniques include magnetic resonance imaging (MRI). Synchrotron-based techniques such as tomographic energy dispersive diffraction imaging (TEDDI) and X-ray microtomography for noninvasive characterization are also evaluated.

Highly Enantioselective Benzoin Condensation Reactions Involving a Bifunctional Protic Pentafluorophenyl-Substituted Triazolium Precatalyst

Improved catalyst design by incorporating a hydrogen bond donating substituent to improve enantiocontrol together with an acidifying pentafluorophenyl substituent to enhance catalyst efficiency results in a triazolium ion precatalyst that promotes the asymmetric archetypal benzoin condensation with excellent efficiency and unprecedented enantioselectivity.

Incorporation of Deuterium in Coke Formed on an Acetylene Hydrogenation Catalyst

Researchers have been attempting to characterize heterogeneous catalysts in situ in addition to correlating their structures with their activity and selectivity in spite of many challenges. Here, we review recent experimental and theoretical advances regarding alkyne selective hydrogenation by Pd-based catalysts, which are an important petrochemical reaction. The catalytic selectivity for the reaction of alkynes to alkenes is influenced by the composition and structure of the catalysts. Recent progress achieved through experimental studies and atomic simulations has provided useful insights into the origins of the selectivity. The important role of the subsurface species (H and C) was revealed by monitoring the catalyst surface and the related catalytic performance. The atomic structures of the Pd catalytic centers and their relationship with selectivity were established through atomic simulations. The combined knowledge gained from experimental and theoretical studies provides a fundamental understanding of catalytic mechanisms and reveals a path toward improved catalyst design.