Chemistry Of Catalysts Research Articles

The Role of Chemistry and Morphology of Carbon in Oxygen Reduction Reaction. Structure-to-Property Studies

Development and optimization of electrocatalysts framework is hindered by the complex nature of the materials, a partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. The lack of this knowledge hinders the rational design of performance optimization strategies. In Pt-based catalysis, carbon-based support is used to ensure electronic conductivity between the electrocatalyst and the current collector. Surface chemistry and structural morphology of support, catalyst and other components of catalyst layers are critical for mass transport properties, water and heat management and subsequently electrocatalytic activity and durability. As an alternative to expensive Pt-based electrocatalysts, metal-nitrogen-carbon (MNC) and nitrogen-carbon composites based on the graphene network have been studied extensively in recent years. There is still a partial understanding of the reaction mechanisms and precise chemistry of the active site or sites. There are two main hypotheses, one claiming that nitrogen functionalities on carbon-based support are directly responsible for their ORR activity and the second suggesting that nitrogen groups serve as the coordinating environment for metal where the ORR happens. Candidate structures participating in ORR consist of multitudes of nitrogen defect motifs in the carbon matrix of different degrees of graphitization with metal incorporated as metal particles as well as those linked to nitrogen defects in a variety of possible architectures. In this report, the role of carbon chemistry and morphology in durability of Pt-based electrocatalysts and activity of MNC electrocatalysts is studied by multi-technique approach combining surface chemical analysis, morphological image processing and electrochemical testing. Presented research combines the strength of several analytical and computational material sciences methods. Surface chemistry of individual building blocks is characterized by XPS. Morphology of membrane electrode assemblies (MEAs) tested under different accelerated stress test (AST) conditions is characterized by FIB-SEM. Figure 1 shows 3D volumes reconstructed from low surface area carbon (LSAC)-supported Pt electrocatalysts at the beginning of life and after two AST protocols. Increase of pores at higher upper potential limit is evident. Morphological 3D metrics such as texture parameters, roughness, porosity, connectivity as measured by Euler number and other were extracted from 3D FIB-SEM datasets. For Pt-LSAC catalysts, area porosity stays the same while connectivity decreases, while for high surface area supported catalyst, the porosity increases along with loss in connectivity between particles. Correlation of structure and catalytic activity and durability of different families of electrocatalysts for oxygen reduction is achieved through the application of principal component analysis (PCA) to spectroscopic, macroscopic and microscopic data combined with results of electrochemical measurements on set of samples with range of electrochemical activities. Figure 1

Shape- and size-dependent catalysis activities of iron-terephthalic acid metal-organic frameworks

Precise control of the size and morphology of metal-organic frameworks (MOFs) presents an important direction for extending these inorganic-organic materials to many more advanced applications. However, because of the limit of the crystal-growth rule and mechanism, good-control of the size and morphology of MOFs remains challenging. In this contribution, an iron-terephthalic acid metal-organic framework with different shapes (octahedron, spindle and bipyramidal hexagonal) was easily and reproducibly synthesized via a solvothermal method. Sodium acetate and glycerol were used as modulators. Mechanism studies showed that the crystal nucleation rate and orientational growth both play important roles in determining the final shape of the MOFs. Further investigations showed that the as prepared MOFs exhibit shape-dependent catalytic activities, which means that MOFs can be designed to perform different catalytic functions. This investigation not only provides an effective guideline for the precise control of the size and morphology of metal-organic frameworks, but also extends MOFs to much more advanced applications in terms of catalyst chemistry.

ChemInform Abstract: The Yin and Yang in the Development of Catalytic Processes: Catalysis Research and Reaction Engineering

AbstractReview: [interplay of catalyst chemistry and reaction engineering for processes such as the sulfuric acid production, ammonia synthesis, methanol synthesis, fluid catalytic cracking, and direct epoxidation of propylene at BASF; 106 refs.

Unraveling the Dynamic Nature of a CuO/CeO2 Catalyst for CO Oxidation in Operando: A Combined Study of XANES (Fluorescence) and DRIFTS

The redox chemistry and CO oxidation (2CO + O2 → 2CO2) activity of catalysts generated by the dispersion of CuO on CeO2 nanorods were investigated using a multitechnique approach. Combined measurements of time-resolved X-ray absorption near-edge spectroscopy (XANES) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in one setup were made possible with the development of a novel reaction cell in which fluorescence mode detection was applied to collect the XANES spectra. This is the first reported example using XANES in a similar technique combination. With the assistance of parallel time-resolved X-ray diffraction (XRD) measurements under operando conditions, we successfully probed the redox behavior of CuO/CeO2 under CO reduction, constant-flow (steady-state) CO oxidation and during CO/O2 cycling reactions. A strong copper ↔ ceria synergistic effect was observed in the CuO/CeO2 catalyst. Surface Cu(I) species were found to exhibit a strong correlation with the catalyst activity for the CO oxidation reaction. By analysis of phase transformations as well as changes in oxidation state during the nonsteady states in the CO/O2 cycling reaction, we collected information on the relative transformation rates of key species. Elementary steps in the mechanism for the CO oxidation reaction are proposed based on the understandings gained from the XANES/DRIFTS operando studies.

(Invited) Impact of Surface Chemistry on the Electrochemical Performance of Perovskite Cathodes

Surface chemistry and reactivity of solid oxide fuel cell (SOFC) and electrolyzer (SOEC) electrodes play a key role for transport and exchange processes, such as the oxygen reduction reaction in SOFC cathodes, and drive electrochemical performance and durability. In order to identify drivers and problems, it is necessary to probe electron and ion transfer and transport processes at electrolyte/electrode interfaces and surfaces and build broad fundamental understanding. This knowledge can then be used to drive the innovation of novel, more powerful electrode materials and cell designs.We used a combination of spatially resolved scanning photoelectron microscopy and electrochemical measurements to study in situ the surface chemistry of cathode perovskite catalysts and electrolyte in electrochemical model cells during high temperature operation. The electrodes were made of screen-printed (La1-xSrx)MeO3 with Mn, Fe, Co, Ni as transition metal Me and x = 0 – 0.4 and covered a wide range in ionic and electric conductivity from pure ion to mixed conduction. The electrolyte was a thin yttrium-stabilized zirconia sheet. We studied the surface chemistry of oxide catalyst and electrolyte at temperatures between 400 and 700°C, oxygen pressure (10-7bar – 1mbar) and also in humidity and/or hydrocarbon containing environment and demonstrated systematic changes in the perovskite surface termination with oxygen chemical potential. Oxidizing environment was found to promote transition metal termination and presence of various oxygen surface species, while reducing environment drove segregation of strontium to the surface and suppressed the level of surface oxygen.We also showed that this simple segregation behavior was dramatically changed in presence of impurities. Silica segregated to the surface, forming a glassy, insulating surface layer that slowed down the oxygen incorporation reaction and produced a strong degradation of the cell performance. While the degradation of Ni/NiO electrodes could be reversed by an applied cell voltage, no such remedy was observed for perovskite electrodes.In our in-situ set up, oxygen in- or excorporation could be driven by an applied electric field across the cell. Oxygen ion flux and electric field caused dynamic changes of catalyst and electrolyte surface chemistry, including redox reaction, changes in surface segregation and long range surface diffusion at the electrode surface. The electrochemical response of the model cells was interpreted in terms of reactions and reaction steps that matched the spectroscopic observations, thus constructing new understanding of the processes in operating electrodes.

(Invited) Impact of Surface Chemistry on the Electrochemical Performance of Perovskite Cathodes

Surface chemistry and reactivity of electrodes in electrochemical devices play a key role for electrochemical performance and cell durability. In this work, we use spatially resolved scanning photoelectron microscopy and electrochemical measurements to study in situ the surface chemistry of perovskite catalysts in SOFC cathode model cells during high temperature operation at 500-550°C in oxygen containing environment (10-6mbar) at OCV or under oxygen flux. Oxygen ion flux and electric field cause dynamic changes of catalyst and electrolyte surface chemistry, including redox reaction, surface segregation and long range surface diffusion. In LSM cathodes, oxidizing environment was found to promote transition metal-rich cathode surface termination with high surface oxygen content and a variety of oxygen surface species, while reducing environment produced surface segregation of strontium and low level of surface oxygen. The electrochemical response provided the rate-determining reaction steps of the operating model cell and could directly be compared to the spectroscopic observations.

ChemInform Abstract: In‐situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

AbstractReview: 106 refs.

Crystal‐Plane‐Controlled Surface Chemistry and Catalytic Performance of Surfactant‐Free Cu2O Nanocrystals

Surfactant-free Cu2 O nanocrystals, including cubes exposing {100} crystal planes, octahedra exposing {111} crystal planes, and rhombic dodecahedra exposing {110} crystal planes, were used as model catalysts to study the effect of the crystal plane on the surface chemistry and catalytic performance for CO oxidation of Cu2 O nanocrystals. The catalytic performance follows the order of octahedra rhombic dodecahedra>cubes; this suggests that Cu2 O(111) is most active in catalyzing CO oxidation among Cu2 O (111), (110), and (100) surfaces. CO temperature-programmed reduction results demonstrate that Cu2 O octahedra are the most easily reduced of the Cu2 O cubes, octahedra, and rhombic dodecahedra. Diffuse reflectance FTIR spectra show that CO chemisorption on Cu2 O nanocrystals depends on their shape and the chemisorption temperature. CO chemisorption is strongest on rhombic dodecahedra at 30°C, but at 150°C on octahedra. Both the reducibility and chemisorption ability of various Cu2 O nanocrystals toward CO are consistent with their catalytic performance in CO oxidation. The observed surface chemistry and catalytic performance in CO oxidation of various Cu2 O nanocrystals can be well correlated with their exposed crystal plane and surface composition/structure. Cu2 O octahedra expose the {111} crystal plane with coordinated, unsaturated Cu(I) sites, and thus, are most active in chemisorbing CO and catalyzing CO oxidation. These results nicely demonstrate the crystal-plane-controlled surface chemistry and catalytic performance of oxide catalysts.

In-Situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

Elucidation of complex heterogeneous catalytic mechanisms at the molecular level is a challenging task due to the complex electronic structure and the topology of catalyst surfaces. Heterogeneous catalyst surfaces are often quite dynamic and readily undergo significant alterations under working conditions. Thus, monitoring the surface chemistry of heterogeneous catalysts under industrially relevant conditions such as elevated temperatures and pressures requires dedicated in situ spectroscopy methods. Due to their photons-in, photons-out nature, vibrational spectroscopic techniques offer a very powerful and a versatile experimental tool box, allowing real-time investigation of working catalyst surfaces at elevated pressures. Infrared reflection absorption spectroscopy (IRAS or IRRAS), polarization modulation-IRAS and sum frequency generation techniques reveal valuable surface chemical information at the molecular level, particularly when they are applied to atomically well-defined planar model catalyst surfaces such as single crystals or ultrathin films. In this review article, recent state of the art applications of in situ surface vibrational spectroscopy will be presented with a particular focus on elevated pressure adsorption of probe molecules (e.g. CO, NO, O2, H2, CH3OH) on monometallic and bimetallic transition metal surfaces (e.g. Pt, Pd, Rh, Ru, Au, Co, PdZn, AuPd, CuPt, etc.). Furthermore, case studies involving elevated pressure carbon monoxide oxidation, CO hydrogenation, Fischer–Tropsch, methanol decomposition/partial oxidation and methanol steam reforming reactions on single crystal platinum group metal surfaces will be provided. These examples will be exploited in order to demonstrate the capabilities, opportunities and the existing challenges associated with the in situ vibrational spectroscopic analysis of heterogeneous catalytic reactions on model catalyst surfaces at elevated pressures.

WGS Catalysis and In Situ Studies of CoO1–x, PtCon/Co3O4, and PtmCom′/CoO1–x Nanorod Catalysts

Water-gas shift (WGS) reactions on Co3O4 nanorods and Co3O4 nanorods anchoring singly dispersed Pt atoms were explored through building correlation of catalytic performance to surface chemistry of catalysts during catalysis using X-ray absorption spectroscopy, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and environmental TEM. The active phase of pure Co3O4 during WGS is nonstoichiometric cobalt monoxide with about 20% oxygen vacancies, CoO0.80. The apparent activation energy (Ea) in the temperature range of 180-240 °C is 91.0 ± 10.5 kJ mol(-1). Co3O4 nanorods anchoring Pt atoms (Pt/Co3O4) are active for WGS with a low Ea of 50.1 ± 5.0 kJ mol(-1) in the temperature range of 150-200 °C. The active surface of this catalyst is singly dispersed Pt1Co(n) nanoclusters anchored on Co3O4 (Pt1/Co3O4), evidenced by in situ studies of extended X-ray absorption fine structure spectroscopy. In the temperature range of 200-300 °C, catalytic in situ studies suggested the formation of Pt(m)Co(m') nanoclusters along with the reduction of Co3O4 substrate to CoO(1-x). The new catalyst, Pt(m)Co(m')/CoO(1-x) is active for WGS with a very low Ea of 24.8 ± 3.1 kJ mol(-1) in the temperature range of 300-350 °C. The high activity could result from a synergy of Pt(m)Co(m') nanoclusters and surface oxygen vacancies of CoO(1-x).

Catalytic Performance and in Situ Surface Chemistry of Pure α-MnO2 Nanorods in Selective Reduction of NO and N2O with CO

Search of catalysts made of nonprecious metal for reduction of nitric oxide is important for having a sustainable environment. Catalytic performances of α-MnO2 nanorods in reduction of nitric oxide and nitrous oxide by carbon monoxide were investigated. Surface chemistry of α-MnO2 catalyst during catalysis was tracked with ambient pressure X-ray photoelectron spectroscopy. Correlation between catalytic performance and the corresponding in situ surface chemistry during catalysis revealed that Mn3+ ions and oxygen vacancies are active catalytic sites. Bulk phase of α-MnO2 nanorods below the catalyst surface is restructured to Mn3O4 in catalysis. Kinetics studies suggested that the reduction of nitric oxide with CO is performed through the formation of the intermediate N2O with a followed dissociation to N2. This study suggested restructuring of transition metal oxides can tune catalytic performance and even develop catalysts.

Metal-based methods for protein inactivation

Metal complexes are efficient catalysts for protein inactivation. Because of their special reactivity, metal-based molecules can achieve results that are difficult, if not impossible to obtain with more traditional, organic-based inhibitors. This article summarizes the chemistry of hydrolytic and oxidative catalysts for protein inactivation.

Atomic-resolution environmental TEM for quantitativein-situmicroscopy in materials science

We have compiled our recent in-situ quantitative environmental transmission electron microscopy (ETEM) studies on typical gold nanoparticulate catalysts for the low-temperature oxidation of CO to describe the issues surrounding the application of ETEM, with a special regard to catalyst chemistry. Thanks to the recent development of high-resolution environmental transmission electron microscopes that can work robustly to accumulate observation data in controlled environments, we can deal with the electron irradiation effects and heterogeneity of real catalysts. We established a structural evolution diagram that summarizes the structure of catalysts under electron irradiation as a function of the electron current density ϕ and the electron dose, D. By extrapolating to ϕ = 0, D = 0, we could deduce the intrinsic catalysis structure (without electron irradiation) in various environments, including reaction environments. By numerically and statistically analyzing a substantial number of ETEM images of gold nanoparticles, we established a morphology phase diagram that summarizes how the majority of gold nanoparticles change their morphology systematically as a function of the partial pressures of CO and O(2). Similar diagrams will be helpful in elucidating the phenomena that directly correlate with the catalytic activity determined from ETEM observations. Using these quantitative analyses, we could analyze Cs-corrected ETEM images of the catalysts. The surfaces of gold nanoparticles were structurally reconstructed under reaction conditions, via interactions with CO molecules. CO molecules were observed on the surfaces of catalysts under reaction conditions using high-resolution ETEM. Finally, we discuss the potential of environmental transmission electron microscopy for quantitative in-situ microscopy at the atomic scale.

One-step waferscale synthesis of 3-D ZnO nanosuperstructures by designed catalysts for substantial improvement of solar water oxidation efficiency

We investigated a rational mechanism for one-step waferscale synthesis of ZnO three dimensional (3-D) nanosuperstructures using designed catalysts via chemical vapor deposition. By precisely engineering the morphology and chemistry of the catalysts, we obtained 3-D nanosuperstructures from one dimensional (1-D) nanowire and two-dimensional (2-D) network catalysts (porous gold) in one-step. Tuning the material chemistry along the lengths of the nanowire catalysts yielded 3-D ZnO nanosuperstructures with distinct morphology on each segment. The growth mechanism and roles of chemistry of catalysts in growth of ZnO nanosuperstructures were studied. Compared with nanowire arrays synthesized from commonly used zero-dimensional (0-D) dot catalysts, 3-D nanosuperstructures significantly enhanced water oxidation efficiency by 150%. This work may inspire a new general paradigm for synthesis of 3-D semiconductor nanostructures as electrode materials for various energy related applications.

Influence of Oxygen Evolution during Water Oxidation on the Surface of Perovskite Oxide Catalysts

Perovskites such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF82) can be highly active for the oxygen evolution reaction (OER) upon water oxidation in alkaline solution. Here we report that BSCF82 can quickly undergo amorphization of its surface at OER potentials, which is accompanied by reduced surface concentrations of Ba2+ and Sr2+ ions as well as increased pseudocapacitive and OER currents. Such quick amorphization during OER was also observed for perovskite catalysts with similar OER activities such as Ba0.5Sr0.5Co0.4Fe0.6O3−δ and SrCo0.8Fe0.2O3−δ. In contrast, perovskite oxides with lower OER activities than BSCF82 did not undergo this transformation when subjected to identical electrochemical conditions. These findings demonstrate that the active chemistry and structure of oxide catalysts during OER can significantly differ from those of the as-synthesized material and that understanding how the oxide surface may change and impact the OER activity is critical to the design of highly active and stable OER catal...

Catalytic Conversion of Carbon Dioxide to Methane on Ruthenium–Cobalt Bimetallic Nanocatalysts and Correlation between Surface Chemistry of Catalysts under Reaction Conditions and Catalytic Performances

Catalytic conversion of CO2 to CH4 on cobalt oxide and Ru-doped cobalt oxide nanorods was studied with in-house ambient pressure X-ray photoelectron spectroscopy (AP–XPS) using monochromated Al Kα. A correlation between catalytic performances of the two catalysts and their surface chemistry under reaction conditions was built. Active phases of the two catalysts are metallic cobalt and bimetallic Co–Ru, respectively. Light-off temperature of Co–Ru catalyst is lower than that of a cobalt catalyst. Selectivity to production of CH4 and activity on the Ru-doped cobalt oxide are obviously enhanced by formation of bimetallic Co–Ru ultrathin film in its surface region in contrast to that of cobalt catalyst in the temperature range of 200–340 °C.

Coordination Chemistry of Gold Catalysts in Solution: A Detailed NMR Study

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh(3), JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S-tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOH≪hex-3-yne <MeCN≈OTf(-) ≪Me(2)S<2,6-lutidine<4-picoline<CF(3)CO(2)(-) ≈DMAP<TMTU<PPh(3) <OH(-) ≈Cl(-). Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)(2) and S-tolBINAP(AuOTf)(2) showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh(3) causes abstraction of one gold atom to give mononuclear complexes LLAuPPh(3)(+) and (Ph(3)P)(n)Au(+), but other N and S ligands give ordinary dicationic species LL(AuNu)(2)(2+). In reactions with different bases, LAu(+) provided new oxonium ions whose chemistry was also studied: (DTBPAu)(3)O(+), (L2Au)(2)OH(+), (L2Au)(3)O(+), (L3Au)(2)OH(+), and (IMesAu)(2)OH(+). Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand- or base-assisted interconversions between (L2Au)(2)OH(+), (L2Au)(3)O(+), and L2AuOH are described. Reactions of dppf(AuOTf)(2) and S-tolBINAP(AuOTf)(2) with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)(2)](3)O(2)(2+), L8(Au)(2)OH(+), and [L8(Au)(2)](3)O(2)(2+), but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe(+) with S(2-) was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined.

Highly chiral-selective growth of single-walled carbon nanotubes with a simple monometallic Co catalyst

We report on the growth of single-walled carbon nanotubes from a monometallic Co catalyst on an oxidized Si wafer support by the most simple growth recipe (vacuum annealing, growth by undiluted C${}_{2}$H${}_{2}$). Nevertheless, multiwavelength Raman spectroscopy and transmission electron spectroscopy show a remarkable selectivity for chiral indices and thus, e.g., high abundance with a single chirality representing 58% of all semiconducting tubes. In situ x-ray photoelectron spectroscopy monitors the catalyst chemistry during carbon nanotube growth and shows interfacial Co-Si interactions that may help to stabilize the nanoparticle/nanotube diameter. We outline a two-mechanism model explaining the selective growth.

Action of bimetallic nanocatalysts under reaction conditions and during catalysis: evolution of chemistry from high vacuum conditions to reaction conditions

Bimetallic catalysts are one of the main categories of metal catalysts due to the tunability of electronic and geometric structures through alloying a second metal. The integration of a second metal creates a vast number of possibilities for varying the surface structure and composition of metal catalysts toward designing new catalysts. It is well acknowledged that the surface composition, atomic arrangement, and electronic state of bimetallic catalysts could be different from those before a chemical reaction or catalysis based on ex situ studies. Thanks to advances in electron-based surface analytical techniques, the surface chemistry and structure of bimetallic nanoparticles can be characterized under reaction conditions and during catalysis using ambient pressure analytical techniques including ambient pressure XPS, ambient pressure STM, X-ray absorption spectroscopy and others. These ambient pressure studies revealed various restructurings in the composition and arrangement of atoms in the surface region of catalysts under reaction conditions or during catalysis compared to that before reaction. These restructurings are driven by thermodynamic and kinetic factors. The surface energy of the constituent metals and adsorption energy of reactant molecules or dissociated species on a metal component are two main factors from the point of view of thermodynamics. Correlations between the authentic surface structure and chemistry of catalysts during catalysis and simultaneous catalytic performance were built for understanding catalytic mechanisms of bimetallic catalysts toward designing new catalysts with high activity, selectivity, and durability.

Progress in Copper-Catalyzed Direct Arylation of Aromatic C—H Bonds

Copper-catalyzed direct arylation of aromatic C—H bonds becomes an important method for the synthesis of biaryl compounds and also a highlight on organic and catalyst chemistry. The recent progress in copper-catalyzed direct aryla- tion of aromatic C—H bonds is reviewed according to the different types of substrates or bond formation. Keywords copper catalysis; direct arylation; C—H activation; biaryls; progress 含联芳烃骨架的有机分子广泛存在于医药、农药、