Guard bed containing lead compounds upstream of a bed of copper-containing catalyst to prevent its contamination by chlorine and sulphur contaminants

A process for performing a catalytic reaction involving hydrogen selected from the list consisting of hydrogenation reactions, methanol synthesis, methanol decomposition, the shift reaction and the reverse shift reaction, comprising passing a process gas containing carbon monoxide and steam through a guard bed of a particulate composition containing a) at least one lead compound, other than lead oxide, that reacts with hydrogen chloride and b) a support therefor, and then passing said process gas at a temperature in the range of 150-300° C. through a bed of copper-containing catalyst.