Promotion effect of ceria on Pd/Al2O3 catalyst for the hydrogenation of carbon monoxide to dimethyl ether

Hydrogenation of CO was carried out over Pd catalyst supported on Al2O3, MgO, silica alumina, and silica gel supports. Addition of CeO2 increased the activity of Pd/Al2O3 and the main product was dimethyl ether (DME). DME formation proceeded via dehydration of methanol on the acid sites of the supports. However, the yield of DME was much higher over Al2O3 than silica alumina. Therefore, Al2O3 has an inherently high activity for methanol production. Temperature programmed reduction experiments showed that CeO2 on Al2O3 was reduced with H-2, whereas CeO2 on MgO underwent little reduction as confirmed by a XAFS measurement. XAFS analysis indicated that Pd/CeO2/MgO contained Pd-0 plus Pd2+ owing to the presence of the reduction-resistant CeO2 on MgO and the Pd on CeO2/Al2O3 was in a more reduced state although Pd2+ still remained. The results of CO adsorption measurement and XAFS and TEM observation revealed that an addition of CeO2 increased the dispersion of Pd metal particles on Al2O3. The intermediate strength interactions between Pd, CeO2, and Al2O3 seems to account for the higher activity of the Pd/CeO2/Al2O3 catalyst for O-containing compounds formation than Pd/CeO2/MgO.