Catalysts rely on the active sites on their surface; these can be tiny platinum particles, acid centers, iron crystal surfaces, liquid vanadium oxide islands, finely dispersed nickel metal, etc. The accelerated reactions occur at these sites. If these sites are no longer available to the gas, the catalyst ceases to function. Typical detrimental substances to active sites include sulfates, dust, siloxanes (vitrification), chlorine, phosphorus, soot, etc. The purity of the feed gas is therefore crucial, and this is why fine purification processes are used. In damage analysis, the depletion is identified, and in cases of reversible deactivation, regeneration can be worthwhile.