La_2O_3的负载方式对La_2O_3助Pd/SiO_2催化剂上的CO吸附和加氢反应性能的影响

EFFECTS OF La_2O_3 LOADING MANNER ON THE CHEMISORPTION AND HYDROGENATION OF CO ON THE La_2O_3 PROMOTED Pd/SiO_2

利用in-situ FTIR,TPSR,TEM和CO加氢反应的活性测试,研究了助剂La_2O_3的不同负载方式对Pd/SiO_2催化剂的CO吸附和加氢反应性能的影响。结果表明,La_2O_3作为助剂加入Pd/SiO_2中,能提高CO的解离和加氢能力,引起CO的吸附容量降低,这可能是由于部分还原的La_2Ox斑块修饰了钯粒子的表面所致。因此,La_2O_3助剂对Pd/SiO_2催化剂的吸附和反应性能的影响与La_2O_3的负载方式密切相关。这意味着催化剂的制备方法对La_2O_3和表面钯粒子间的相互作用程度有直接影响。

In present work, the influence of La2O3 as a promoter of Pd/SiO2 catalyst and its loading manner on the chemisorption and hydrogena-tion of CO were investigated by in-situ FTIR combined with CO chemisorption, TPSR and activity measurements. Four catalysts were prepared by impregnation with PdCl2 and/or La(NO3)3 aqueous solutions. The first one is 6 % Pd/SiO2 ( abbrev. Pd/SiO2),the second one is 5 % La·6 % Pd/SiO2(abbrev.(La·Pd)/SiO2) which was prepared by co-impregnation. The third and fonrth catalysts are Pd ( La/SiO2 ) and La ( Pd/SiO2 ) which were prepared by two step impregnation with different sequences. Spectra in Fig. 3 for CO adsorbed on each catalyst give three types of CO species i.e. twin CO, linear CO,bridge CO on Pd (100) and Pd (111) respectively. Their intensities and frequencies not only depend on the CO pressure, but also on the catalysts. At lower CO pressure, the bridge bonded CO are the dominant species. Further increase of CO pressure not only caused the rapid growth in L and B, but also change in their relative intensity. The twin CO at 2129 cm-1 appears only at CO pressure≥100 Torr. The band at 2170 cm-1 stem from gaseous CO. The L and G bands on all four catalysts are quite similar. However, the B bands on various catalysts are somewhat different from each other. The B band on Pd/SiO2 at saturated coverage is a sharp peak, and the ratio of L to B band is about unit. The addition of La2O3 by two step impregnation cause the B band broadening with a simultaneous increase in the ratio of L to B band, meanwhile the B shifted to a lower frequency. An additional feature found from spectra of CO on Pd (La/SiO2) and La ( Pd/SiO ) is a broad band extending from 1700 to 1870 cm-1, which is probably due to the dissociation and reaction of CO. This broad band on Pd (La/ SiO2 ) is quite clear. By contrast, the spectra of CO on ( La · Pd ) / SiO2 are quite similar to those on Pd/SiO2. Capacities of CO adsorption on each catalyst were determined by-flow chemisorption method. Capacities of CO adsorption decrease in the following order: Pd/SiO2>(La · Pd)/SiO2>La ( Pd/SiO2 ) >Pd ( La/SiO2 ) . The activities of methation for each catalyst were evaluated using TPSR and ramping the catalyst temperature in a flowing synthesis gas. The activities of methanation increase in the following order. Pd/SiO2< ( La · Pd ) /SiO2<La ( Pd/SiO2 ) <Pd ( La/SiO2 ) The reason of the different behaviors of the catalysts studied in this work has been interpreted in terms of the modification of the interaction between La2O3 and Pd particles by different impregnation methods.