Silver catalyst characterization

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

The experimental apparatus has been described in detail elsewhere (11,12,22). In previous communications we have also described the porous silver catalyst film deposition and characterization procedure (11,12). Ten different reactor-cells were used in the present investigation. The cells differed in the silver catalyst surface area as shown in Table I. Catalysts 2 through 5 had been also used in a previous study (17). The reactor-cells also differed in the zirconia electrolyte thickness which could not be measured accurately. The electrolyte thickness varies roughly between 150 and 300 ym. 

Characterization of the Adsorbed Layer of a Silver Catalyst in the Oxidation of Ethylene from Its Transient Adsorption Behavior... 

The experimental apparatus and the silver catalyst preparation and characterization procedure is described in detail elsewhere (10). The porous catalyst film had a superficial surface area of 2 cm2 and could adsorb approximately (2 +. 5) 10-b moles O2 as determined by oxygen chemisorption followed by titration with ethylene (10). The reactor had a volume of 30 cm3and over the range of flowrates used behaved as a well mixed reactor (10, 11). Further experimental details are given in references (10) and (11). 

Early production was based on the chlorohydrin process (Box 2), characterized by high yields but also encumbered by technical and environmental problems due to the use of chlorine. The introduction, in the 1940-1950s, of the oxidation of ethylene on silver catalysts with lower costs and better environmental standards, led to its phasing out, virtually completed by the end of the 1960s. 

Table 2 Characterization data of gold and silver catalysts prepared with carbohydrates...

Nowadays, the availabihty of fiber-optic probes offers several possibilities for performing in situ catalyst characterization. As quartz glass is transparent to Raman laser and UV-vis radiation, these spectroscopies are particularly versatile. Thus, the respective UV-vis probe or Raman laser optics can be focused from outside or directly immersed in the reaction solution (in the latter case, a Raman probe is used). In contrast, infrared radiation is strongly adsorbed by quartz hence, special infrared transmissive fibers are required. Such silver halogenide fibers have proven suitable and are now well established [35-37] despite their fiagihty and degradation over time. 

Limited structural information is available for silver(I) carboxylates, despite their extensive use as catalysts in the manufacture of urethane polymers. This is in part due to their frequent insoluble and light-sensitive nature making chemical characterization of the complexes difficult. Dimeric structures have been reported for the perfluorobutyrate249 and trifluoroacetate complexes.250 In each case two-fold symmetry was crystallographically imposed. The Ag—O bond lengths were 223-224 pm, and in the more accurate determination of the trifluoroacetate, the Ag—Ag separation was found to be 297 pm. A dimeric structure was also found for the silver(I) complex of 3-hydroxy-4-phenyl-2,2,3-trimethylhexane carboxylate.251 In the asymmetric crystal unit the Ag---Ag separations were 277.8 and 283.4 pm. 

Ag/Al203 catalysts with various contents have been characterized using TG, DTA, TPD and NH3 and CO2 adsorption calorimetry [72]. The results were discussed in terms of silver loading and interactions with the alumina surface. An increase of the total amount of basic sites was globally observed. 

Assemblies of screens or grids of solids are often employed for oxidation reactions, particularly those using platinum or silver as catalysts. These reactions are frequently characterized by a high rate constant, so that mass transfer of reactants from fluid to solid surface can be a significant part of the total resistance. Data on mass transfer from single screens has been reported by Gay and Maughan. Their correlation is of the form... 

The aim of this work is to explore the applicability of the sol-gel method for the preparation of Ag/Si02 and Cu/Si02 catalysts and to see whether such a method can yield silver and copper species stabilized by the carrier. Characterization of the catalyst structure by several physical and chemical techniques, including N2 adsorption-desorption isotherms, mercury porosimetry measurements, X-ray diffraction and transmission electron microscopy, has been used to correlate the microstructure of Ag/Si02 and Cu/Si02 catalysts with their catalytic performance. 

A considerable amount of research has been concerned with the nature of the electrophiles that are involved in Friedel-Crafts acylation reactions. We will summarize the main points. Acyl halides and carboxylic acid anhydrides have been known, for many years, to form stable complexes with a variety of acid catalysts. A well-defined product is formed between acetyl fluoride and boron trifluoride at low temperatures. Analytical and conductivity data characterized the material as acetylium tetrafluoroborate, and this was further confirmed by IR measurements. In the system acetyl chloride-aluminum chloride the acetylium ion can be differentiated from the donor-acceptor complex involving the carbonyl group by means of their IR carbonyl stetching frequencies. A number of other acyl fluorides have been shown to form well-defined acylium salts by interaction with a number of metal fluorides. Acylium salts can also be prepared from acyl chlorides by means of metathetical reactions involving anhydrous salts such as silver hexafluoroantimonate. As well as characterization by means of IR spectroscopy, acylium salts have been studied in non-nucleophilic solvents by NMR spectroscopy. The NMR data for the ben-... 

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