SILVER CATALYZED ETHYLENE EPOXIDATION: NOVEL APPROACHES TO MECHANISTIC STUDIES (SURFACE ENHANCED RAMAN SPECTROSCOPY)
Abstract
The silver-catalyzed production of ethylene oxide is an important process in the chemical industry. Though much effort had been invested in mechanistic studies of this reaction, determination of intermediates and surface pathways has been elusive. New techniques involving simultaneous surface and gas phase analysis are believed to offer the greatest potential in acquiring new information about this reaction system. For this investigation, a medium pressure reactor system was constructed and subsequently interfaced with a Texas Instruments PM550 process control computer. The reaction of ethylene and oxygen over silver wires, foils and treated colloids, was studied over a range of pressures (0.03-10 torr) and temperatures (200-315(DEGREES)C). Kinetic results were easy to analyze due to the well-defined behavior of the reactor. Gas phase compositions were measured using a mass spectrometer; the surface was studied under reaction conditions using surface enhanced Raman spectroscopy (SERS). This study represents one the the first attempts to employ SERS in ths study of heterogeneous gas phase catalytic systems. An in situ Raman surface analysis of silver under ethylene oxidation reaction conditions revealed the presence of atomically adsorbed oxygen (440 cm('-1)) and a 'carbon-containing' species (1300-1600 cm('-1)) believed to be primarily amorphous carbon. The formation of adsorbate islands on the surface was also observed. The presence of adsorbed chlorine on the silver surface was monitored using SERS (255 cm('-1)), following the injection of 1,2-dichloroethane into the reactor system. The presence of the aforementioned species was confirmed using ex situ Auger electron spectroscopy. Under the conditions mentioned previously, selectivity to ethylene oxide was observed to be less than 10%. The apparent activation energy for both CO(,2) and C(,2)H(,4)O production at medium pressures was determined to be (TURN)100kJ/mol, suggesting a common surface intermediate. A high surface area (2000 cm('2)), unsupported silver catalyst was also developed and tested in order to facilitate medium pressure kinetic and mechanistic studies at lower reactor residence times. A tentative reaction mechanism is proposed, stemming in part from the observations mentioned.
Degree
Ph.D.
Subject Area
Chemical engineering
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