"Decoking" of a "coked" zeolite catalyst in a glow discharge

'Decoking' of a 'coked' zeolite catalyst in a glow discharge in oxygen is investigated. The 'decoking' process involves reactions of atomic oxygen (O atoms) with 'coke' and yields gases such as CO, CO₂ as well as other gaseous products that could be easily pumped out.Three different modes of discharge were investigated including a static mode, a flowing-gas mode, and a periodic-purge mode where the oxygen and other gaseous products of the discharge were replaced by fresh O₂ gas after short but regular intervals of time. In some cases, additional heating was also used to provide base temperatures of the order of 100 °C to facilitate penetration of oxygen atoms into the inner layers and cages of the zeolite catalyst.This paper presents some results of spectroscopic analytical techniques used to monitor the atomization of oxygen, oxidation of 'coke', and to confirm the process of 'decoking'. More specifically, radiation emission on the 3 s ⁵S– 3p ⁵P transitions of O around 777.2–777.5 nm were selected for monitoring the atomization of O₂. On the other hand, X-ray photo-electron spectroscopy (XPS) was used to determine the amount of residual carbon and extent of 'decoking'. Furthermore, evolution of CO and CO₂ gases as a function of time was systematically monitored in real time. For CO, the 451.1 nm band head belonging to the B¹ - A¹ bands of the Angstrom system of the CO spectrum was used, while for CO₂, the band head at 353.4 nm belonging to the CO₂⁺ spectrum was used. The rates of evolution of CO and CO₂ were related to the rate of 'decoking' of the catalyst. It is noted that in the periodic-purge mode, about 63% of the total yield of CO from a given sample of the catalyst appears in the first 3-min exposure to discharge whereas it takes up to 15 min to remove nearly 94% of the removable carbon under our experimental conditions.     

"Green" pyrotechnics: a chemists' challenge

Fireworks are probably the application of chemistry which resonates best with the general public. However, fireworks and (civil and military) pyrotechnic applications cause environmental pollution and thus have given rise to the development of new, environmentally friendly pyrotechnic compounds and formulations. Nitrogen-rich energetic materials, such as the derivatives of tetrazoles and tetrazines, are about to revolutionize traditional pyrotechnic compositions. This Review summarizes the sources of pollution in current formulations and recent efforts toward "green" pyrotechnics.     

Biologically active molecules with a "light switch"

Biologically active compounds which are light-responsive offer experimental possibilities which are otherwise very difficult to achieve. Since light can be manipulated very precisely, for example, with lasers and microscopes rapid jumps in concentration of the active form of molecules are possible with exact control of the area, time, and dosage. The development of such strategies started in the 1970s. This review summarizes new developments of the last five years and deals with "small molecules", proteins, and nucleic acids which can either be irreversibly activated with light (these compounds are referred to as "caged compounds") or reversibly switched between an active and an inactive state.     

让我们给"物质的量"命个名

在我们的化学教学和化学写作中 ,恐怕还找不到一个比“物质的量”更为别扭的术语。而偏偏这个术语在化学教学和化学实验中要经常使用。例如在中学化学教材中 ,常见有诸如“5 0 0mL 0 .1mol/LNaOH溶液中NaOH的物质的量为 0 .0 5mol”、“H2SO4 的物质的量的浓度为 0 .5mol/L…”等[1] 叙述。在高考的试卷中也只好按照教材的口径说 :“分别用等物质的量的这些物质氧化足量的KI…”等[2 ] 。本来mole的概念在中学化学中就是一个教学难点 ,加上这么个别扭的术语 ,更是雪上加霜。对于广大的化学研究工作者也不例外 ,他们不能使用已经熟悉和…     

给超细金纳米粒子做“外科手术”

正近年来在纳米科学研究中,金属纳米团簇(metal nanoclusters,超细的金属纳米粒子,尺寸约1-3 nm)这一领域逐渐兴起并引起了研究者们的广泛关注~(1,2)。与尺寸更大且多分散的纳米粒子相比,纳米团簇取得了原子尺度上的精确,具有确定的分子式和X射线单晶结构~(3,4)。因此,研究纳米团簇使得人们可以从根本上理解一些纳米结构与性能的关系~(1-3),以及跟踪纳米世界里的某些     

在Word 2000中创建"化学"工具栏

经常使用Ｗｏｒｄ的化学教师都会有一个这样的愿望 ,就是希望Ｗｏｒｄ能象ＷＰＳ 2 0 0 0那样有一个化学工具栏。其实在Ｗｏｒｄ中 ,我们完全可以创建一个比ＷＰＳ 2 0 0 0更具学科特色 (如 :“化学仪器”、“实验装置”等栏目 )个性化的化学工具栏。具体步骤如下 :1　创建“化学”工具栏　　 (1)启动Ｗｏｒｄ 2 0 0 0 ,用鼠标右键单击“工具”栏 ,在快捷菜单中选择“自定义”。(2 )在“自定义”对话框的“工具栏”选项卡中 ,选中“菜单栏”单击“新建”按钮。　　　　　　　　图　 1图 2　　 (3)在“新建工具栏”对话框中的“工具栏名称”…     

Calcite mesocrystals: "morphing" crystals by a polyelectrolyte

Crystallization of calcite from different concentrated calcium chloride solutions by the CO(2) vapor diffusion technique in the presence of polystyrenesulfonate (PSS) yields defined assemblies of nanoparticles with unusual morphology. From the typical calcite rhombohedra, the morphology can be systematically varied via rounded edges and truncated triangles to concavely bent lens-like shapes. Although these "crystals" are apparently well facetted as observed in light microscopy, electron microscopy analysis and BET isotherms reveal that the structures are highly porous and are composed of almost perfectly three-dimensionally aligned calcite nanocrystals, scaffolded to the final, partly curved structures. The formation of all mesostructures is discussed within the framework of a polymer-mediated structure-formation process, in which the polymer is acting in four different ways. The present model case also provides evidence for the importance of nonclassical mesoscopic processes in polymer-controlled crystallization in general.     

Sequential affinity chromatography miniaturized within a "lab-on-valve" system

An automatically renewable microcolumn, subjected to operation by programmable flow, is presented and for the first time used for separation and quantification of biomolecules on Sepharose Protein A beads, with absorbance measurement at 280 nm and a detection limit of 6 ng mouse IgG microL(-1).     

The "benzothiazine" chromophore of pheomelanins: a reassessment

The characteristic absorption and photochemical properties of pheomelanins are generally attributed to "benzothiazine" structural units derived biogenetically from 5-S-cysteinyldopa. This notion, however, conveys little or no information about the structural chromophores responsible for the photoreactivity of pheomelanins. At pH 7.4, natural and synthetic pheomelanins show a defined maximum around 305 nm, which is not affected by reductive treatment with sodium borohydride, and a monotonic decrease in the absorption in the range 350-550 nm. These features are not compatible with a significant proportion of structural units related to 2H-1,4-benzothiazine and 2H-1,4-benzothiazine-3-carboxylic acid, the early borohydride-reducible pheomelanin precursors featuring absorption maxima above 340 nm. Rather, these features would better accommodate a contribution by the nonreducible 3-oxo-3,4-dihydrobenzothiazine (lambdamax 299 nm) and benzothiazole (lambdamax 303 nm) structural motifs, which are generated in the later stages of pheomelanogenesis in vitro. This conclusion is supported by a detailed liquid chromatography/UV and mass spectrometry monitoring of the species formed in the oxidative conversion of 5-S-cysteinyldopa to pheomelanin, and would point to a critical reassessment of the commonly reported "benzothiazine" chromophore in terms of more specific and substantiated structural units, like those formed during the later stages of pheomelanin synthesis in vitro.     

Deformation of a "rigid" molecule in self-assembled nanostructures

A simple spirobifluorene molecule with pseudotetrahedral structure was investigated for its supposed conformational resilience upon adsorption. Through deposition at room temperature of this molecule on a Cu(111) surface and subsequent observation at 5 K with an ultrahigh vacuum scanning tunneling microscope, this "rigidity" upon physisorption is confirmed. However, an unexpected chemisorbed state was also found with the molecules arranged in trimers. The unique coexistence of physisorbed and chemisorbed states on the same substrate is thus demonstrated at the early stage of self-assembly.