Catalytic oxidation of toluene over Pd/Co₃AlO catalysts derived from hydrotalcite-like compounds: Effects of preparation methods

A series of novel Pd/Co₃AlO catalysts derived from hydrotalcite-like compounds (HTlcs) have been prepared and investigated for total oxidation of toluene. The HTlcs phase Co-Al precursors were prepared by coprecipitation method and Pd active species were introduced by different approaches, i.e., impregnation (IMP), wet ion exchange (WIE) or directly at coprecipitation stage (COP). It is found that all hydrotalcite-derived Pd/Co₃AlO catalysts are much more active than the Pd/Co₃AlO catalyst prepared via traditional thermal combustion method (TCB) in toluene elimination. The activities of all synthesized catalysts obey the following sequence: Pd/Co₃AlO (COP)>Pd/Co₃AlO (WIE)≥Pd/Co₃AlO (IMP)>Pd/Co₃AlO (TCB). Excellent catalytic activities of the novel hydrotalicite-derived Pd/Co₃AlO catalysts could be contributed to their high surface area, small mean crystallized size of support and highly dispersed PdO particles. Besides, they are well positively associated with catalyst reducibilities and the amounts of oxygen vacancies. According to the XPS, TEM and TPR results, it was found that the strong synergistic effect between Co₃O₄ and PdO, but not the amount of the surface palladium species which are generally believed to be the active sites, is the main factors determining the catalytic activity in this work. The stability tests indicate that all Pd/Co₃AlO catalysts have no obvious deactivation, and some catalysts even show improved activities during the reaction process due to the emergence of metal Pd.