溶胶-凝胶法制备的铑基CO加氢制C2含氧化合物催化剂

用XRD,FT-IR,吸附CO的程序升温脱附(TPD),吸附CO的程序升温表面加氢反应(TPSR)等技术,并结合高压下CO加氢反应研究了用溶胶-凝胶法制备的铑基催化剂.结果表明,用溶胶-凝胶方法制备的催化剂中,Rh以极高的分散状态(主要以单原子形式)存在.与文献报道的浸渍法催化剂不同,CeO2的加入使甲醇选择性显著提高,但降低了催化剂的活性.CeO2对Rh的促进作用可能主要是CeO2与Rh产生了电子效应,即CeO2向Rh转移了电子.

STUDY ON SOL-GEL DERIVED Rh-BASED CATALYSTS FOR SYNTHESIS OF C2 OXYGENATED COMPOUNDS THROUGH CO HYDROGENATION

A series of Rh/CeO 2/SiO 2 catalysts prepared by sol gel route were characterized by XRD, FT IR, temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR) of adsorbed CO with H 2 and studied in CO hydrogenation at 2 0 MPa for synthesis of C 2 oxygenated compounds. XRD results showed that at as high as 16% loading of CeO 2, its average particle size was only 3 3 nm. In FT IR, CO was mainly adsorbed in the form of geminal dicarbonyl twin bands, indicating that Rh was dispersed at atomic level. Bridge form CO was also detected, but it was very weak in intensity compared with the twin bands. With increasing CeO 2 loading, the intensity of all the CO bands decreased drastically and the wavenumber of bridge CO band shifted downwards, indicating that CeO 2 might donate electrons to Rh. On 2%Rh/CeO 2(I), twin bands were very weak in intensity, so CO was mainly adsorbed in linear and bridge forms. A band at 1?705 cm -1 was also found and assigned to a tilted CO species in which the end carbon of the molecule is bonded to Rh and the end oxygen to CeO 2. TPD and TPSR results implied that either hydrogen promoted CO dissociation or adsorbed CO was directly hydrogenated before dissociation. In CO hydrogenation, 2%Rh/SiO 2 mainly catalyzed the formation of acetaldehyde. Addition of CeO 2 increased the selectivity for both ethanol and acetaldehyde. In contrast to the literature results on catalysts prepared by impregnation method, CeO 2 decreased the catalytic activity, but increased appreciably the selectivity for methanol. These differences were attributed to the atomic dispersion of Rh in the sol gel derived catalysts.