Catalyst poisons lower the activity of the catalyst. CeO is used as an oxidation catalyst in catalytic converters. Dioxolane is formed by reaction of ethylene glycol with aqueous formaldehyde over an acid catalyst. Sulfuric acid is a typical catalyst for this reaction. It is the precursor to a hydroformylation catalyst, cobalt tetracarbonyl hydride. The chemical reaction used to produce the acetaldehyde used mercury sulfate as a catalyst. Unsaturated fats can be altered by reaction with hydrogen effected by a catalyst. MoS finds use as a hydrogenation catalyst for organic synthesis. The hydroxide ion may act as a base catalyst. Alkanes will react with steam in the presence of a nickel catalyst to give hydrogen. The reaction is slow in the absence of a catalyst. Hydrolysis of an epoxide in presence of an acid catalyst generates the glycol. In the absence of the catalyst, benzene is impervious to hydrogen. HBr requires refluxing with a sulfuric acid catalyst. Copper acts as a catalyst for these reactions.