Effect of preadsorbed sulfur compounds on chemisorption of CO and CO 2 on iron catalysts

Abstract

The effects of the chemisorption of H 2S, SO 2, CS 2, COS, C 2H 5SH, and thiophene on the nitrogen isotherm at −195 °C, the carbon monoxide chemisorption at −195 °C, and the carbon dioxide chemisorption at −78 °C were determined on synthetic ammonia catalysts promoted with MgO, MgOK 2O, Al 2O 3, and Al 2O 3K 2O. The nitrogen isotherm gives a measure of the surface area; the CO chemisorption, a measure of the free metallic iron surface; and the CO 2 chemisorption, a measure of the alkali surface. Two types of poisoning are considered: I, poisoning at room temperature by the entire sulfur molecule, and II, poisoning by the residual sulfur left after hydrocracking of the organic part of the sulfur-bearing molecule. Poisoning of type II is comparable to the type experienced in ammonia synthesis and Fischer-Tropsch synthesis. In general, the poisoning did not affect the surface area, but it did decrease both the metallic iron and alkali surfaces. In all cases of Type I poisoning, initial chemisorption of 1 mole of sulfur compound initially decreased the chemisorbed CO + CO 2 by about 1 mole. Type II poisoning was less effective by a factor of 3 or 4 in decreasing chemisorbed CO than Type I poisoning. The nature and degree of Type II poisoning depends only on the amount of sulfur deposited and is relatively independent of the compound from which it was formed. Hydrogen sulfide cracked partially to hydrogen and some adsorbed complex even at room temperature, and, therefore, only Type II poisoning was studied. For catalysts containing alkali, the iron surface decreases to a minimum as sulfur is added in Type II poisoning, rises to a maximum (somewhat lower than the initial free iron surface), and then gradually decreases. This phenomenon may be the result either of the onset of the formation of polysulfide or surface pyrite or of the nucleation of sulfide crystals. The alkali surface, as measured by carbon dioxide chemisorption, was also decreased by poisoning with sulfur compounds, both at room temperature (Type I) and after the Type I chemisorbed compounds on the surface have been treated with hydrogen at 450 °C (Type II). With CS 2 and COS, Type I chemisorption produces a greater effect on the alkali surface than Type II. With C 2H 5SH and SO 2, the decrease in alkali surface is the same whether the C 2H 5SH and SO 2 are held in Type I or in Type II chemisorption.