The reactions of SO2 and SO3 with catalytic surfaces containing iron oxides, alone and in combination with various sodium salts, have been investigated. The SO3 concentration gradients above the surfaces were measured using a microprobe technique, showing that, as much as 2000 ppm SO3 can exist in a slowly flowing gas stream immediately next to a surface. Radioactive sulfur was employed as a tracer to determine the roles of SO2 and SO3 in reactions with these surfaces leading to formation of alkali iron trisulfates. These measurements indicated that on a molar basis, SO3 is 30,000 times more reactive than SO2 + O2 in forming complex sulfates with a mixture of sodium sulfate and iron oxide.

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