选择催化还原(SCR)脱硝技术在中国燃煤锅炉上的应用(下)

针对燃煤电厂采用高尘选择催化还原(SCR)脱硝装置时可能遇到的问题进行分析,并对设计SCR反应器和选择适合的催化剂方面提出了建议。在选择催化剂时,针对中国煤,要考虑催化剂活性位潜在积灰以及飞灰中高CaO含量所引起的催化剂的失活。以台山电厂600MW机组和阳城电厂600MW机组及美国肯塔基州Elmer Smith电站150MW机组的SCR脱硝工程为例进行了说明。     

Sulfur bonding in MoS2 and Co-Mo-S structures

The structure and bonding in small MoS₂ structures with and without Co is studied theoretically using self-consistent density functional theory with a non-local exchange-correlation energy. The structures model the catalysts used extensively in hydrotreating. We study in detail the structure and binding energies as a function of the amount of sulfur. The calculations show that extensive reconstructions occur at the two types of MoS₂ edges where the sulfur dimerizes and occupies non-lattice positions. These structures are shown to be in good agreement with available experimental data. We also study the energy required to form sulfur vacancies, which are believed to be the active sites for many hydrotreating reactions. The presence of Co atoms at the edges is shown to lead to a significant lowering of the metal-sulfur binding energy. This imposes an increase in the concentration of active sites for the reactions and may thus explain the promoting effect of Co. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cylindrical poiseuille flow of a Fermi liquid

The integral equation for cylindrical Poiseuille flow of a general excitation gas is derived for diffuse scattering at the wall. For a degenerate Fermi gas or liquid the flow through a circular cylindrical tube is determined numerically by a variational method.     

Theoretical Insight into the Nature of Ammonia Adsorption on Vanadia-Based Catalysts for SCR Reaction

Density functional theory (DFT) calculations were carried out on monomeric and oligomeric vanadium oxide clusters to probe the factors leading to the formation of NH₄ species from the adsorption of ammonia. The interaction of ammonia with monomeric vanadium oxide clusters leads to the formation of hydrogen-bonded NH₃ species, with energy changes for ammonia adsorption near -50 kJ/mol. The interaction of ammonia with oligomeric vanadium oxide clusters leads to the formation of bidentate NH_4 species, where the ammonium cation is coordinated between two V=O groups on adjacent vanadium cations. The energy change for ammonia adsorption in this mode is near -100 kJ/mol. Adsorption of ammonia as NH₄ species was not observed when the oligomeric vanadium oxide clusters were reduced by addition of hydrogen atoms, i.e., in clusters where the formal oxidation state of the vanadium cations was 4+. Based on our findings, a model for the generation of Brönsted acidity through the interaction of vanadium oxide oligomers with the titanium oxide support is proposed.     

Importance of Co-Mo-S Type Structures in Hydrodesulfurization

The great current interest in hydrodesulfurization (HDS) and other hydrotreating reactions is related to the need for efficient upgrading of crude oil fractions or coal-derived liquids. The catalysts used for such reactions generally consist of molybdenum (or tungsten) supported on high surface area aluminas with cobalt or nickel added as promoters. Great efforts have been devoted to the understanding of the structural and chemical form in which the different elements are present in the active catalyst and to the establishment of correlations between such information and the various catalytic functions. This massive research effort has given valuable information on many aspects of such catalyst systems (for recent reviews of the extensive literature, see, e.g., Refs. 1–11). However, it has not been possible to reach general agreement on the types of structures present in the active catalysts and the origin of promotion     

Nano-Particles in Heterogeneous Catalysis

The introduction of in situ techniques has had a vast impact on research and development in the area of heterogeneous catalysis as emphasized in many reviews and monographs. Recently, the number of in situ techniques that can give information at the atomic scale has increased significantly and new possibilities exist for making the measurements under industrially relevant conditions. In order to fully exploit the results from the in situ and operando studies, it has also become increasingly gainful to combine the experimental studies with theoretical methodologies based on, for example, Density Functional Theory (DFT). This has allowed one to extract more detailed atomic-scale information from the measurements and it has also allowed the establishment of detailed structure-activity relationships. Furthermore, the interplay between in situ techniques and theory has helped bridging the pressure gap such that in situ information obtained at conditions far from industrial ones may be used in a more relevant manner. Here, we will illustrate how microscopy-, spectroscopy- and X-ray-based techniques in combination with experimental and theoretical surface science methods can aid industrial catalyst developments. We will do this by presenting examples of our current understanding and latest developments in the areas of heterogeneous nano-particle catalysts for methanol synthesis, steam reforming and hydrotreating.     

Changes in metal–sulfur bond energy in promoted and unpromoted molybdenum catalysts

Insight into the changes in the metal–sulfur bond energy in unpromoted and promoted (Co, Ni, or Fe) MoS₂ catalysts is obtained by temperature‐programmed reduction (TPR) studies of in situ sulfided catalysts. The experiments reveal a significant decrease in the metal–sulfur bond energy for the more active Co–Mo–S and Ni–Mo–S structures as compared to MoS₂. A similar decrease is not observed for the less active Fe–Mo–S structure. The sulfur coordination is directly measured by in situ EXAFS and the results support that one of the roles of the promoter atoms is to facilitate vacancy formation. The findings are in accord with recent density functional theory calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

函数的温度参数对人工神经网络的影响

对人工神经网络的激励函数作了一些研究。用遗传算法优化各神经元Ｓｉｇｍｏｉｄ函数的温度参数来解决‘异或’问题，取得良好效果。在大量试验基础上总结了一些规律。表明了激励函数对于网络学习性能的重要作用。     

Morphology and Atomic-Scale Structure of MoS2 Nanoclusters Synthesized with Different Sulfiding Agents

We use scanning tunneling microscopy to investigate the morphology and atomic-scale edge structure of MoS₂ nanoclusters synthesized on a gold single crystal as a model system for hydrodesulfurization catalysts using three different sulfur sources for sulfiding. Crystalline and triangularly shaped MoS₂ nanoclusters are predominantly produced from sulfiding with H₂S, dimethyl disulfide (DMDS) and dimethyl sulfide (DMS), but the detailed dispersion, stacking and distribution of active edge sites is sensitive to the selection of the sulfiding agent. The main effect of varying the sulfiding agent seems to be related to the variation in the reactivity of the sulfur, i.e. changes in the sulfur chemical potential. H₂S and DMDS both yield fully sulfided edge structures, but lower sulfur content is obtained on the edges of MoS₂ sulfided with DMS reflected by Mo-edges terminated by sulfur monomers. The present model studies demonstrate that MoS₂ morphology and hereby also the dispersion, type and amounts of active edge sites can be manipulated by control of sulfiding conditions and choice of sulfiding agent. The findings are in line with previous reports on activity variations on technical catalysts which have undergone different sulfidation procedures.