Chemical selforganization of composite catalysts during catalytic reactions

Recent progress in the study of selforganization phenomena in catalytic reactions on multi-component surfaces is reviewed. As chemically more complex systems a Rh(1 1 1) surface with ultra-thin vanadium oxide layers (θ_V < 0.5 MLE) and a bimetallic Rh(1 1 1)/Ni surface, both subjected to the H₂ + O₂ reaction, were chosen. Applying spatially resolving methods in situ, it is shown that under reaction conditions a reversible redistribution of the components of the catalyst occurs. The redistribution processes are essentially driven by the different chemical affinities of the components to reacting species.     

Chemisorption on supported-metal catalysts

A Gree-function formalism is developed to describe the electronic and chemisorption properties of a supported-metal composite substrate. Within the framework of the tight-binding approximation, the metal catalyst is represented by a finite chain of d-orbitals, while the semi-infinite semiconductor support is characterized by a linear chain of alternating s- and p-orbitals. The Anderson-Newns model is used to calculate the chemisorption energy and adatom charge transfer for hydrogen chemisorption on the Ni/ZnO composite system.     

Ward-Takahashi Identity on the Light Front

. The Ward-Takahashi identity, reflecting local gauge invariance, is perturbatively verified for a boson model in light-front field theory. A careful integration over the light-front energy, corresponding to exactly taking into account pair terms, which are the contributions of the zero longitudinal-momentum mode, is crucial to obtain this result. Furthermore, the one-loop boson form factors are calculated for arbitrary off-shell momenta. Received July 1, 1997; accepted for publication October 10, 1997     

Finite-Temperature Field Theory on the Light Front

The formulation of statistical physics using light-front quantization, instead of conventional equal-time boundary conditions, has important advantages for describing relativistic statistical systems, such as heavy ion collisions. We develop light-front field theory at finite temperature and density with special attention to quantum chromodynamics. First, we construct the most general form of the statistical operator allowed by the Poincaré algebra. In light-front quantization, the Green’s functions of a quark in a medium can be defined in terms of just two-component spinors and do not lead to doublers in the transverse directions. Since the theory is non-local along the light cone, we use causality arguments to construct a solution to the related zero-mode problem. A seminal property of light-front Green’s functions is that they are related to parton densities in coordinate space. Namely, the diagonal and off-diagonal parton distributions measured in hard scattering experiments can be interpreted as light-front density matrices.     

Effect of Microscopic Noise on Front Propagation

We study the effect of the noise due to microscopic fluctuations on the position of a one dimensional front propagating from a stable to an unstable region in the linearly marginal stability case. By simulating a very simple system for which the effective number N of particles can be as large as N=10¹⁵⁰, we measure the N dependence of the diffusion constant D_N of the front and the shift of its velocity v_N. Our results indicate that D_N(log N)^–3. They also confirm our recent claim that the shift of velocity scales like v_min–v_NK(log N)^–2 and indicate that the numerical value of K is very close to the analytical expression K_approx obtained in our previous work using a simple cut-off approximation.     

Oxidation of dodecane on vanadium-molybdenum catalysts

The catalytic oxidation of dodecane with air oxygen on individual and mixed vanadium-molybdenum (1 ? x)V₂O₅ · xMoO₃ oxide is studied over a temperature range of 250 to 400°C. It is shown that oxidation of dodecane to organic acids at 250°C produces undecylic acid C₁₁H₂₆COOH and carbon oxides, as products of the subsequent oxidation of formic acid. The most effective catalyst is a mixed oxide containing 75 mol % MoO₃ and 25 mol % V₂O₅. At 275–300°C, this catalyst provides the maximum yield of acids and a relatively low fraction of complete-oxidation products. Above 250°C, a mixture of acids and carbon oxides is formed, the yield of which increases with the temperature up to 300°C. As the temperature is increased still further, the yield of acids decreases due to their subsequent oxidation. The catalytic oxidation is accompanied by changes in the phase composition, morphology, and degree of crystallinity of the mixed-oxide catalysts. A possible mechanism of the catalytic process is considered.     

曲率与运动界面发展

对界面传播速度依赖于曲率的界面发展问题进行研究,传播速度包括法向和切向,并且,在界面传播过程中全变差的变化仅依赖在曲率为零处的法向速度对曲率的导数,切向速度对全变差的变化没有影响.     

Light Front Hamiltonian Approach

The semiphenomenological way to describe nonperturbative quantum chromodynamics using light front gluon zero mode is proposed and applied to quark-antiquark bound state.     

Tin-Platinum catalysts interactions on titania and silica

Pt-Sn was supported on titania and silica, and the resulting interactions between the components in prepared samples and the resulting interactions between the components before and after treatment with hydrogen were characterized by Mössbauer spectroscopy, X-ray diffraction, Rietveld refinement, high-resolution transmission electron microscopy (HRTEM) and catalytic tests data. Results show the presence of Pt and SnO₂ after calcinations, and Pt₃Sn, PtSn and PtSn₃ after reduction. Rietveld analysis shows that some Ti⁴⁺ are replaced by Sn⁴⁺ atoms in the titania structure. Finally, HRTEM and the practically absence of activity observed confirms that metallic platinum is encapsulated.     

EXAFS study on ultrafine Ni-Co-B amorphous catalysts

\chem{\chemvar{A}C_{60}} with , , . From the temperature dependent line shift of we have extracted information about the electron density distribution on the molecules. The data also support the existence of an like bonding type between the molecules. The magnetic phase transition observed at low temperature in and is monitored by an increase of the second moment as well as an increase of the spin lattice relaxation rates of the and alkali nuclei. The results are compared to NMR data of where no signs of a magnetic ordering are found. The magnetically ordered phases are discussed in terms of antiferromagnetic spin density wave. We also report on NMR measurements of orthorhombic one dimensional and rhombohedral two dimensional polymers of obtained under high pressure. By spinning the samples up to , we are able to identify two resonances at and for the orthorhombic polymer and six resonances at , , , , and for the rhombohedral one. The static distortion of the molecules induced by the transformation under pressure must be at the origin of the observed inequivalent carbons in both systems. The NMR lineshape simulations of the obtained spectra are compatible with the suggested polymeric structures where the molecules are connected by cycloadditions. Received: 10 October 1996/Accepted: 13 November 1996     

Nonperturbative True Muonium on the Light Front with TMSWIFT

The true muonium \({(\mu\bar{\mu})}\) bound state presents an interesting test of light-cone quantization techniques. In addition to exhibiting the standard problems of handling non-perturbative calculations, true muonium requires correct treatment of \({e\bar{e}}\) Fock-state contributions. Having previously produced a crude model of true muonium using the method of iterated resolvents, our current work has focused on the inclusion of the box diagrams to improve the cutoff-dependent issues of the model. Further, a parallel computer code, TMSWIFT, allowing for smaller numerical uncertainties, has been developed. This work focuses on the current state of these efforts to develop a model of true muonium that is testable at near-term experiments.     

Stability of Three-Body Bound States on the Light Front

We investigate the stability of the relativistic three-boson system with a zero-range force in the light-front form. In particular, we study the dependence of the system on an invariant cut-off. We discuss the conditions for the relativistic Thomas collapse. Finally, we fix the parameters of the model introducing a scale.Received February 2, 2003; accepted May 30, 2003 Published online September 24, 2003     

Mössbauer spectroscopy studies of catalysts: Some recent examples with emphasis on hydrodesulfurization catalysts

Mössbauer spectroscopy has become a very important technique for studying many catalyst systems. The technique may provide unique information about many problems such as (i) the nature and concentration of the phases present in the working state of a catalyst (also in the case where the catalysts contain multiphasic, micro-crystalline or amorphous structures), (ii) support interactions, (iii) preparation, activation and deactivation phenomena, (iv) particle size, (v) surface structure and composition, and (vi) active sites. The variety of information, which may be obtained, will be discussed on the background of recent studies reported in the literature. Special emphasis will be given to studies of hydrodesulfurization catalysts since Mössbauer spectroscopy studies have allowed one to disclose the nature of the active phase in these industrially very important catalysts.     

The initiation of superfluorescence

The early stages of a superfluorescent pulse are described with full account of both quantum and propagation effects. The pulse is triggered by zero point fluctuations of the atomic polarization field. If the number of atoms N is large these zero point fluctuations, and the electric field radiated at early times as well, have a gaussian distribution with width 4/N. An equivalent classical stochastic process is found in terms of which the later, intrinsically nonlinear part of the radiation problem can be analyzed.     

曲率相关的爆轰波在非结构四边形网格上的数值方法

假设爆轰波阵面的法向速度是曲率的线性函数,在非结构四边形网格上采用水平集方法模拟爆轰波阵面的运动过程.水平集方程的曲率无关项采用正格式离散,曲率项采用伽辽金等参有限元方法空间离散,时间离散采用半隐格式.在笛卡儿网格和随机网格上,含曲率的水平集方程的离散格式为强一阶精度,重新初始化方程的离散格式精度为近似一阶精度.曲率收缩的不光滑界面和多个爆轰波阵面相互作用的算例说明格式可有效地模拟爆轰波与曲率相关的运动.     

Front roughening in three-dimensional imbibition

We investigate the structure and dynamics of the interface between two immiscible liquids in a three-dimensional disordered porous medium. We apply a phase-field model that includes explicitly disorder and discuss both spontaneous and forced imbibition. The structure of the interface is dominated by a length scale ξ_× which arises from liquid conservation. We further show that disorder in the capillary and permeability act on different length scales and give rise to different scalings and structures of the interface properties. We conclude with a range of applications.