Surface characterization and isotopic transient kinetic studies of ethylene epoxidation over silver

Daniel Chul Kim, Purdue University

Abstract

Transient mass spectrometry was used to measure the response of the steady state reaction to an isotopic oxygen switch. Quantitative analysis of the data shows that a large pool of irreversibly adsorbed atomic oxygen ($\theta$ = 0.8-1.0) was the reactive species. This species also exchanged with an inactive, subsurface oxygen pool, 4-6 monolayers for low purity powder and 0.2-0.5 monolayers for high purity powder. The low purity catalyst, with a chlorine coverage of roughly 0.2 monolayers, had a slightly higher product rate due to its higher surface area but half the turnover frequency of the high purity catalyst. A maximum was seen in the rate constants with changing reactant conditions. The active oxygen pool scaled roughly with surface area for the different catalysts and increased with higher oxygen-to-ethylene ratios. The amount of subsurface oxygen increased with increasing oxygen pressure, correlating to increased selectivity. Isotopic ethylene experiments showed a small, reversible active pool of ethylene for ethylene oxide production while for carbon dioxide production, both fast and slow pools were involved. SEM, BET, oxygen adsorption and hydrogen titration experiments were used to characterize the catalysts. Two species associated with very rapid and slower adsorption processes were found by hydrogen titration experiments with maximum surface oxygen to silver stoichiometry of 1:1. An untitratable (subsurface) oxygen species, although unaffected by hydrogen at 673K, demonstrated facile exchange with surface oxygen. XPS showed two oxygen species after reaction (530eV and 532eV), while a third oxygen species appeared after long dosage of oxygen at high temperatures (529eV). Graphitic and carboxyl carbon species were also present after the reaction. Amounts of chlorine and sodium on the low purity powders were two times greater than on the high purity silver powders. Steady state experiments revealed activation energies (22-33 kcal/mol) and reaction orders (oxygen = 0.5-1.0, ethylene = 0.0-0.5) for the different catalysts. A Langmuir-Hinshelwood expression was found to describe our reactions reasonably well. Selectivities were roughly constant with temperature but increased from 44% to 52% when tripling the oxygen partial pressures for the low purity powder.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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