经过氧化预处理的Ru/Al_2O_3催化剂的表征

CHARACTERIZATION OF Ru/Al_2O_3 CATA-LYSTS PRETREATED WITH O_2

对由Ru_3(CO)_(12)和RuCl_2制备的Ru/Al_2O_3催化剂进行的氧化预处理,显著地改变了催化剂在CO加氢反应中的选择性.在未经过氧化预处理或氧化预处理温度低于200℃时,主产物是C_2以上的烃类;而当氧化预处理温度在300℃或300℃以上时,主产物是甲烷.用H_2化学吸附法、TEM和XRD对y—Al_2O_3 载体上Ru粒子表征的结果表明:在H_2还原之后,载体上的Ru是一些粒径为10(?)左右的超微粒子或者这种超微粒子的聚集体,而在经过300℃或者更高温度下的氧化预处理及随后的还原之后,这些超微粒子或其聚集体转变成大的单晶.Ru粒子的这一微观形态的变化是引起催化剂选择性显著改变的根本原因.这—结果表明,对于负载型金属催化剂,不仅载体上金属粒子的分散度,而且这些金属粒子的微观形态也是决定催化剂在某些反应中的选择性的一个重要因素.

In CO hydrogenation over Ru/ Al2O3catalysts prepared from Ru3(CO) 12( denoted as Ru/ A12O3( A) ) and from RuCl3(denoted as Ru/ A12O3( B) ) , oxidation-reduction pretreatment affected their activity and selectivity remarkably. The activity decreased monotonically as oxidation temperature increased for both 2.5% Ru / A12O3(A) and (B); after oxidation at 500℃ the activity decreased to 1 / 10 of that pretreated only by reduction. As for selectivity, it can be seen that the product distribution was changed greatly when the catalysts were pretreated by oxidation at temperatures around 300℃ .The weight percentage of methane was about 50% after oxygen treatment at temperature lower than 200℃ , but increased to about 80% when the treatment temperature was higher than 300℃ .The same changes were observed in the case of 4.0% Ru / A12O3 (A) and 5.0% Ru / A12O3(B). Characterization of Ru metal particles by TEM, XRD and H2adsorption showed that, after reduction, the particle size of Ru on 2.5%and 4.0% Ru/ A12O3(A) was 10A° estimated by H2adsorption.These particles are too fine to be detected by TEM and XRD. On the other hand, it is evident that the Ru particles on 2.5% Ru / A12O3(B) were rather large after reduction, although the size estimated by TEM (71A°) was different from that by H2adsorption (44A°).However, it is important to indicate that these particles could not be detected by XRD. If particles with diameters of 44-71A° are present as crystallites, their XRD could be observed readily.There were two reasons to explain why Ru particles on 2.5% Ru/ A12O3(B) did not give XRD peaks: first, Ru particles are amorphous and , second, they are aggregates of small particles. The first reason seems to be not reasonable, because Ru crystallites were readily formed after reduction at 450℃ in the case of 5.0%Ru/ A12O3(B), which indicated that the reduction at 450℃ is sufficient to form Ru crystallites. It may be suggested that Ru particles are probably aggregates of small particles. The preoxidation causted significant increase in particle size, but the XRD pattern was not changed by the oxidation treatment at temperature below 200℃, XRD peaks of Ru(20 = 44.1 ° ) appeared after oxygen treatment at temperatures higher than 300℃, which indicates that oxidation at these temperatures with subse-quent reduction changed the Ru particles to large crystallites. After preoxidation treatment at 500℃, the average particle size of Ru on 2.5% Ru/ A12O3(A) was estimated to be 110A° , and that of Ru on 2.5% Ru / A12O3 (B) was 120A°. By use of these catalysts, it was observed that the relationship between activity and dispersion is consistent with results reported previously[4,5,6]. But we believe that the significant change in selectivity is , presumably, caused by the change of the microstructure of Ru particles, because small particles of Ru on Ru / Al2O3and their aggregates produced higher hydrocarbons in CO hydrogenation, while the large single crystallites produced mainly methane. It strongly suggested that selectivity of catalysts is determined by the primary particle size but not by the apparent dispersion. This is in contrast to the previous reports which claimed that the product distribution over Ru / Al2O3was indepedent of the dispersion of Ru. In those studies, Ru/ Al2O3was prepared from RuCl3and treated only by H2.We found by XRD, TEM and H2adsorption that Ru particles prepared under these conditions were polycrystallites (aggregates of small particles), so we presumed that this is the reason why the selectivity was indepedent of the dispersion of Ru in the previous studies. These results demonstrated that the morphology of metal is an important factor determining the selectivity of supported metal catalysts.