A zirconium-based mixed oxide or zirconium-based mixed hydroxide which is capable of (a) at least 90% v/v conversion of naphthalene at atmospheric pressure at a temperature in the range 600-700° C. using a residence time of about 0.3 seconds, and/or (b) providing an initial heat of adsorption of ammonia of greater than 150 kJ/mol when measured by ammonia flowing gas microcalorimetry. Also, a method for purifying gas produced from the gasification of carbonaceous materials, comprising the step of bringing the gas into contact with such mixed oxides or mixed hydroxides.