Electronic properties and stability of M2O3 (M = Al,Ga, In) and alloy (MxGa1-x)2O3 in α and β phases: A theoretical study

Ga₂O₃ is clearly emerging as an important wide band-gap semiconductor. Band-gap engineering is now highly demanded for expanding its applications. Alloying with the same group of metal oxides is a straightforward and effective way. In this work, by using hybrid density functional theory calculations, the structural, electronic properties, and phase stability of group IIIA (Al, Ga, In) metal oxides and their ternary alloys (M_xGa_1-x)₂O₃ (M = Al, In) in the corundum and monoclinic phases are systematically investigated. The lattice constants, elastic constants, modulus, formation energies, band-gaps, band-gap deformation potentials, band-edge alignments, band-gap bowing, and ternary alloy formation energies are obtained. The basic relations between the geometric structure and electronic properties are discussed. It is found that the cation ordered structure is the most stable alloy structure in the monoclinic phase, rather than the random alloy structure as is commonly thought. A phase stability diagram of the (M_xGa_1-x)₂O₃ alloys is established, showing that the stable phase of the alloy changes from the monoclinic phase to the corundum phase when the incorporation of Al₂O₃ (In₂O₃) is greater than 69% (76%). These results can be used to understand the relative experimental data and shed some light on the synthesis and device design efforts of Ga₂O₃.     

Electronic interactions decreasing the activation barrier for the hydrogen electro-oxidation reaction

A unified model for electrochemical electron transfer reactions which explicitly accounts for the electronic structure of the electrode recently proposed by us is applied to the hydrogen oxidation reaction at different metal electrocatalysts. We focus on the changes produced in the transition state (saddle point) as a consequence of the interactions with d-bands. We discuss different empirical correlations between properties of the metal and catalytic activity proposed in the past. We show which role is played by the band structure of the different metals and its interaction with the molecule for decreasing the activation barrier. Finally, we demonstrate why some metals are better electrocatalysts for the hydrogen electro-oxidation reaction than others.     

Synchrotron X-ray microtomography of Raney-type nickel catalysts prepared by gas atomisation: Effect of microstructure on catalytic performance

Synchrotron X-ray microtomography (SXRM) was used to examine the microstructure of various Ni–Al alloys prepared by gas atomisation. The resulting Raney-type nickel catalysts that were activated by chemical treatment with a concentrated sodium hydroxide solution (20 wt% in water) were also studied. The main objective of this work is to correlate the microstructure of various Ni–Al alloys prepared by gas atomisation and the catalytic performance of the resulting Raney-type nickel catalysts. It appears that a NiAl₃/Ni₂Al₃ ratio around 2.3 in the precursor alloy prepared by gas atomisation favours the formation of a dendritic network in the atomised spherical particle. The spherical shape of the particle and the dendritic network are still present after the leaching process in the Raney-type nickel catalysts. After activation, the interdendritic space forms a macroporous network that is directly linked to the catalytic performance. Parameters of the precursor alloy, i.e. particle size, phase composition, are chosen to obtain an optimal catalytic performance. In this way, an activity is obtained that is at least a factor of 2 higher than that of alloys prepared by the commercial cast-and-crush method. Inductively coupled plasma optical emission spectroscopy (ICP-OES) and BET measurements were used for bulk analysis and determination of the surface area, respectively. Hydrogenation of nitrobenzene and butyraldehyde were used as test reactions. A model that directly correlates the microstructure of a precursor alloy processed by gas atomisation and the catalytic performance of the resulting catalyst is proposed.     

The electronic structure effect in heterogeneous catalysis

Using a combination of density functional theory calculations and X-ray emission and absorption spectroscopy for nitrogen on Cu and Ni surfaces, a detailed picture is given of the chemisorption bond. It is suggested that the adsorption bond strength and hence the activity of transition metal surfaces as catalysts for chemical reactions can be related to certain characteristics of the surface electronic structure.     

Advances in chemical synthesis and application of metal-metalloid amorphous alloy nanoparticulate catalysts

This paper reviews the advances in the chemical synthesis and application of metal-metalloid amorphous alloy nanoparticles consisting of transition metal (M) and metalloid elements (B, P). After a brief introduction on the history of amorphous alloy catalysts, the paper focuses on the properties and characterization of amorphous alloy catalysts, and recent developments in the solution-phase synthesis of amorphous alloy nanoparticles. This paper further outlines the applications of amorphous alloys, with special emphasis on the problems and strategies for the application of amorphous alloy nanoparticles in catalytic reactions.     

碳纳米管、碳化钨在直接法合成过氧化氢中的应用

选用新型纳米材料——碳纳米管（CNTs）作为催化剂载体,用沉积沉淀法制备负载型钯-铂合金催化剂。同时选用过渡金属碳化物材料（碳化钨）为催化剂,其具有类似于Pt的表面电子结构,有望替代贵金属催化剂。探讨了Pd-Pt/CNTs及碳化钨在氢氧直接合成过氧化氢反应中的催化性能。     

Strained thin copper films as model catalysts in the materials gap

Thin copper films on silicon constitute model systems to investigate the influence of lattice strain on activity in heterogeneous catalysis. Thin copper films on silicon were investigated by ultraviolet photoelectron spectroscopy (UPS) to reveal the effect of strain in the copper films on the electronic structure of the surface. For cleaned and adsorbate-free surfaces, no effect of strain on the electronic structure was detected by UPS. Conversely, an oxygen-containing film exhibited a distinct effect of strain induced by cyclic heating and cooling on the electronic structure. Comparison with studies on a Cu single crystal under methanol oxidation reaction conditions revealed a characteristic hysteresis behavior in both the adsorbate structure and the catalytic properties of the metal surface. Hence, copper model systems that are suitable to unravel the correlation between strain and catalytic activity need to take the disordered microstructure of ‘‘real’’ copper catalysts into account. The present experiments reveal the correlation between surface restructuring and catalysis on the one side and the influence of lattice strain on either restructuring or the electronic structure of the surface on the other side.     

Monodisperse Metal Nanoparticle Catalysts: Synthesis, Characterizations, and Molecular Studies Under Reaction Conditions

We aim to develop novel catalysts that exhibit high activity, selectivity and stability under real catalytic conditions. In the recent decades, the fast development of nanoscience and nanotechnology has allowed synthesis of nanoparticles with well-defined size, shape and composition using colloidal methods. Utilization of mesoporous oxide supports effectively prevents the nanoparticles from aggregating at high temperatures and high pressures. Nanoparticles of less than 2?nm sizes were found to show unique activity and selectivity during reactions, which was due to the special surface electronic structure and atomic arrangements that are present at small particle surfaces. While oxide support materials are employed to stabilize metal nanoparticles under working conditions, the supports are also known to strongly interact with the metals through encapsulation, adsorbate spillover, and charge transfer. These factors change the catalytic performance of the metal catalysts as well as the conductivity of oxides. The employment of new in situ techniques, mainly high-pressure scanning tunneling microscopy (HPSTM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) allows the determination of the surface structure and chemical states under reaction conditions. HPSTM has identified the importance of both adsorbate mobility to catalytic turnovers and the metal substrate reconstruction driven by gaseous reactants such as CO and O₂. APXPS is able to monitor both reacting species at catalyst surfaces and the oxidation state of the catalyst while it is being exposed to gases. The surface composition of bimetallic nanoparticles depends on whether the catalysts are under oxidizing or reducing conditions, which is further correlated with the catalysis by the bimetallic catalytic systems. The product selectivity in multipath reactions correlates with the size and shape of monodisperse metal nanoparticle catalysts in structure sensitive reactions.     

非晶态合金催化剂的制备与改性研究进展

非晶态合金催化剂作为一种新型的催化材料,具有较高的催化活性和选择性,且易于制备,受到众多化学工作者的关注。介绍了非晶态合金催化剂的特性、制备方法,以及通过掺杂过渡金属、稀土元素和类金属作为第3或第4组分改性非晶态合金催化剂的研究进展,最后展望了非晶态合金催化剂在工业上的应用前景。     

The morphological and surface properties and their relationship with oxygen reduction activity for platinum-iron electrocatalysts

A series of iron-platinum bimetallic and pure platinum catalysts supported on carbon were prepared and characterized by means of H₂---O₂ titration, X-ray diffraction, TEM, XPS and cyclic voltammetry. Their catalytic activities for the electrochemical reduction of oxygen in 100% phosphoric acid were related to their physical properties. When the bimetallic catalysts were heated at 750°C, a disordered alloy phase was formed. An almost completely ordered superlattice structure, Pt₃Fe, was achieved when heated at 900°C. The mass activity showed a maximum value as a function of Pt particle size for both pure platinum and platinum—iron catalysts, and no noticeable enhancement in the activity was found by alloying platinum with iron. The specific activity increased as the particle size was increased for pure platinum catalysts, whereas it was nearly independent of the particle size for Pt—Fe alloy catalysts. The specific activities of Pt alloys were much higher than those of pure platinum over the tested range of particle size. Therefore, it is expected that a Pt alloy catalyst having a superior mass activity can be made only if metal particles are kept extremely small; ie appreciately smaller than 3 nm. The increase in the specific activity is caused by increased chemisorption strength of hydrogen and simultaneous decrease in the oxygen chemisorption strength in the cathode.     

乙炔选择性加氢：催化剂结构敏感性分析及调控

Pd催化乙炔选择性加氢是石脑油蒸汽裂解和煤基电石乙炔路径制备聚合级乙烯的关键。传统的Pd基催化剂使用成本较高且选择性和稳定性较差。本文综述了近年来乙炔选择性加氢催化剂结构敏感性及其调控方面的相关研究进展,重点介绍了包括活性金属粒径、纳米颗粒形貌和电子结构等对乙炔选择性加氢反应性能的重要作用,进而阐明了催化剂结构调控的目标与方向。进一步归纳总结了针对该反应特性的催化剂结构定向调控的研究进展,主要包括合金和金属间化合物催化剂、单原子及其合金催化剂的设计。通过合理地调控催化剂结构,优化关键物种的吸脱附和反应动力学行为,能够显著提高乙炔加氢催化剂的选择性及稳定性。在未来的研究中,如何针对该反应特性,构筑高效、稳定和低成本的催化剂将会是该体系催化剂研究的重点与难点。     

Model oxide-supported metal catalysts: energetics,particle thicknesses,chemisorption and catalytic properties

Many industrially important catalysts consist of late transition metal particles supported on the surfaces of oxide materials. Our studies of such systems using model catalysts consisting of metal films vapor deposited onto the surfaces of single-crystalline oxides are reviewed here. Systems studied include Cu on ZnO, Pt on ZnO, Au on TiO₂ and Cu, Ag and Pb on MgO. A unique adsorption microcalorimeter was developed to measure directly the energetic stability of the metal atoms on the oxide surfaces and the adhesion energy at the metal/oxide interface, which clarify the structural and chemisorption properties of the ultrathin metal particles. The structure of the oxide surface and the metal particles was elucidated by low-energy electron diffraction (LEED), low-energy ion scattering spectroscopy (ISS), angular-resolved X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD). The electronic character of the metal particles was revealed by XPS, Auger electron spectroscopy (AES), band-bending and work function measurements. Sintering rates were measured by temperature-programmed ion scattering spectroscopy (TPISS). The chemisorption properties of these particles and their catalytic reactivity were monitored by mass spectroscopy and temperature-programmed desorption (TPD).     

Model catalysis by metals and alloys: From single-crystal surfaces to well-defined nano-particles

During the last decades surface science has played an important role for modelling surface chemistry on metallic surfaces and understanding catalytic processes for simple reactions. Progresses are now made possible by the recent development of specific tools for characterising at the nanoscale level model materials and following the kinetics of reactions on these materials of small area in dedicated reactors.It needs the preparation, and characterisation at the atomic level, of well-defined materials. They can be either single-crystal surfaces having a well-defined orientation or in shape of model supported nano-particles. Most has been done in the framework of monometallic materials, but more difficult is the preparation of well-defined alloy surfaces and surface alloys, and the elaboration of alloy nano-particles well-defined in size and composition.Kinetic studies in dedicated reactors show how the catalytic behaviour of model samples may depend on the specific sites present at surface. For example, surface sites having a low coordination number (like steps, kinks, edges and corners) are very efficient for bond-breaking. In bi-metallics, ad-layers of a given metal on a foreign substrate may show new and original structures having very specific catalytic properties. Thus, works on catalysis at the atomic scale proposes new active/selective sites, and it is now a challenge to design new industrial catalysts on the basis of these fundamental works.In order to move near the conditions for real catalysis one has now to bridge the “pressure gap”, i.e. to make in situ (during reaction under pressure of reactants) characterisation of both the surface itself and ad-species. This needs the development of specifics tools able to work in such conditions. This is a challenge for today and future works in the field of model catalysis.     

Surface Science Studies on the Zirconia-Based Model Catalysts

Zirconia possesses ideal chemical and mechanical stability properties. It has been widely used in many technical applications such as gas sensors, protective coatings and heterogeneous catalysis. In particular, in heterogeneous catalysis, zirconia has been used in many catalytic reactions not only as the metal catalysts’ support but also as the pure catalyst; it can be also used as an additive to improve the catalytic performances of the catalysts. To gain fundamental understanding of the roles that zirconia plays in catalysis, significant surface science studies based on zirconia model catalysts have been performed. In this paper, we will present a short review of recent surface science studies on the zirconia-based model catalysts. These model catalysts include single crystalline yttria-stabilized zirconia surfaces, zirconia thin films which were grown on metal single crystal surfaces and zirconia-supported metal catalysts. Besides the focuses on the surface chemistry of model zirconia surfaces, the surface structures and adsorption/reaction properties of the zirconia-supported metal catalysts will be also addressed.     

Characterization of Pt-Cu-Fe ternary electrocatalysts supported on carbon black

Ternary Pt-Cu-Fe alloy catalysts, useful for low temperature fuel cells, were prepared from aqueous media, followed by heat treatment at 900 °C for various heating periods. Supported metal crystallites were characterized with various techniques including XRD, XPS, TEM and ICP-AES. XRD patterns indicate that the lattice structure of platinum changes from a face-centred cubic to a contracted facecentred tetragonal structure as it forms an alloy. As the heating period increases, the extent of formation of an ordered alloy increases and the formation is completed in 2.5 h, as confirmed by the intensity of superlattice diffraction lines. The presence of different oxidation states is confirmed by XPS and the amount of higher oxidation state is reduced by heat-treatment, but there is no evidence of development of a new photoelectron peak or shift in binding energy by alloy formation. For the electrochemical reduction reaction of oxygen in fuel cell operation, ordered alloys have shown improved catalytic activity compared to platinum alone. After the stability test in hot phosphoric acid, the ordered structure is preserved even though a significant amount of transition metal is dissolved, and some increase in particle size in the heat-treated catalysts is observed.     

Bimetallic Cobalt Based Catalysts

Highly dispersed, oxide- or zeolite-supported bimetallic catalysts are widely used in the catalytic industry, such as in catalytic reforming, nitrogen industry and gas-to-liquid technology. The paper highlights the nano-sized Co-based bimetallic system in terms of correlation between structure and reactivity/selectivity promoted by the second metal. Opposite to the bulk alloys nano-sized bimetallic catalysts are extremely sensitive to the structure, morphology, valence state of the supporting oxide material in which the nano-particles are embedded. In this case, one of the less reducible components, which strongly interact with the supports, may stabilize the second, more noble metals, and thus the latter can be stabilized in highly dispersed state. Conversely, addition of noble metal to the hardly reducible component may facilitate reduction, which causes the retardation of the deactivation process of some hydrocarbon reaction. The future trend is the application of bimetallic nano-particles although careful consideration and experimentation should be taken to elucidate the structure of such type of catalyst. The various effects of Co-based bimetallic particles, such as particle size, metal/support interface, morphology and electronic effects, on the activity/selectivity in given catalytic reactions will be discussed.     

Trends in catalytic NO decomposition over transition metal surfaces

The formation of NO_x 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.     

Electrocatalytic oxygen reduction on single-walled carbon nanotubes supported Pt alloys nanoparticles in acidic and alkaline conditions

The objective of this study is to improve the catalytic activity of platinum by alloying with transition metal (Pd) in gas diffusion electrodes (GDEs) by oxygen reduction reaction (ORR) at cathode site and comparison of the acidic and alkaline electrolytes. The high porosity of single-walled carbon nanotubes (SWCNTs) facilitates diffusion of the reactant and facilitates interaction with the Pt surface. It is also evident that SWCNTs enhance the stability of the electrocatalyst. Functionalized SWCNTs are used as a means to facilitate the uniform deposition of Pt on the SWCNT surface. The structure of SWCNTs is nearly perfect, even after functionalization, while other types of CNTs contain a significant concentration of structural defects in their walls. So catalysts supported on SWCNTs are studied in this research. The electrocatalytic properties of ORR were evaluated by cyclic voltammetry, polarization experiments, and chronoamperometry. The morphology and elemental composition of Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The catalytic activities of the bimetallic catalysts in GDEs have been shown to be not only dependent on the composition, but also on the nature of the electrolytes. The GDEs have shown a transition from the slow ORR kinetics in alkaline electrolyte to the fast ORR kinetics in the acidic electrolyte. The results also show that introduction of Pd as transition metal in the Pt alloys provides fast ORR kinetics in both acidic and alkaline electrolytes. The performance of GDEs with Pt–Pd alloy surfaces towards the ORR as a function of the alloy’s overall composition and their behavior in acidic electrolyte was also studied. These results show that the alloy’s overall composition and also the nature of the electrolytes have a large effect on the performance of GDEs for ORR.