Study of the Adsorption of Hydrogen, Ethane, Ethylene, and Acetylene on Iridium by Field Emission Microscopy

Abstract

A field emission microscope permitting precise control of iridium emitter temperature from 4 to 1700°K was used to study changes in emission patterns and work functions resulting from time and temperature dependent surface reactions in the adsorption of hydrogen, ethane, ethylene and acetylene on iridium. The qualitative character of the emission patterns indicates a rather uniform covering of the high index faces of iridium by all species studies in the temperature range 70–300°K. Hydrocarbon species, once chemisorbed, are substantially immobile at temperatures below 700°K; above 700°K an intensification of emission pattern, probably due to carbonization, occurs around the edges of the 111 planes. Iridium surfaces containing adsorbed species were flashed for controlled periods of time to controlled temperatures; characteristic changes in work function resulted which were both time and temperature dependent. These experiments indicate that hydrogen is readily desorbed by iridium above 400°K, and that a large portion of adsorbed ethane is readily desorbed at 100°K, but a residue from adsorbed ethane is not desorbed below 1000°K. For a given flash time, curves representing work function as a function of flash temperature for adsorbed ethylene and acetylene show characteristic differences below 450°K but are very similar above 450°K. Results are interpreted as indicating chemisorption of ethylene, dehydrogenation of chemisorbed ethylene to form chemisorbed acetylene and chemisorbed hydrogen, desorption of chemisorbed hydrogen, dehydrogenation of chemisorbed acetylene and finally crystallization of carbon residue.