摘要
本文应用XPS,在模拟工业催化剂和实际操作条件下,对低压甲醇合成铜基催化剂的表面形态进行了研究.结果表明,在还原及反应状态下,催化剂表面没有稳定的Cu^(2+)、Cu^+离子存在,仅Cu^0能被检测到;ZnO被还原,形成缺氧结构ZnOx(X≤1),表面出现氧空位.认为高度分散的Cu^0与其紧密接触的部分还原的ZnOx联合组成了甲醇合成的表面活性中心,也即表面上的Cu-Zn-O(“口”为氧空位)构成了最佳合成活性单元.提出反应原料气中适量CO_2的加入有助于金属Cu微晶的分散,认为CO_2的作用主要是使反应气氛中含有一定的氧化势,它与原料气中的CO+H_2一起,对金属Cu微晶起氧化还原作用.CO_2可将Cu^0氧化成Cu^(?+)(甚至Cu^+).但Cu^(?+)会马上被CO+H_2还原成Cu^0.通过这样的氧化还原循环,阻止了Cu微晶的聚集长大,延长了催化剂的使用寿命.
CuO-ZnO(atomic ratio Cu : Zn = 1 : 1 )and CuO-ZnO-Al2O3 ( atomic ratio Cu : Zn : Al= 6 : 3 : 1 ) catalysts for low pressure methanol synthesis were prepared by a conventional coprecipitation method. Surface characterization of those catalysts were performed using XPS combined with a reactor system shown in Fig. 1 under simulated industrial operating conditions. Specimens were prepared by pressing powdered catalysts into small pellets, evacuated in the pretreatment chamber at room temperature to 5 ×10-9 Torr, then transferred to the reactor, where the specimens were treated with reduction gases ( H2 + Ar, H2/Ar = 1/9 ) and/or reaction gases ( CO + H2 +CO2, CO : H2 : CO2 =3:6:1) at given temperatures for a specified times.After treatment, specimens were evacuated and transferred to the analysis chamber in which XPS experiments were performed. Cu2p, Zn2p, CuL3VV and ZnL3M23M45spectra were obtained. The XPS results of these simulated catalysts treated under various conditions are shown in Table 1. The Cu2p3/2 level shifted from 933.7 eV to 932.2 eV for a reduced catalyst and 932.1 eV for a reacted catalyst. These results show that the CuO on the catalyst was reduced. But Cu+ and Cu0 are hard to distinguish by the Cu2p spectra. However, they are more easily distinguished using the CuL3VV spectra.The kinetic energies of CuL3VV and ZnLMM spectra for both reduced and reacted samples are shown in Table 1 and Fig. 3 . By comparing with the CuL3VV spectra of Cu0, Cu2O, CuO (Fig. 2 ), it is shown that no stable Cu2+ and Cu+ ions were present on the specimens under both reducing and reaction conditions; only Cu0 species was detected on the surface. Meanwhile, the ZnO on the catalysts could be partially reduced with the formation of a oxygen-deficient structure ZnOx (X<1) (See Table 3). It is confirmed that highly dispersed Cu0 in intimate contact with ZnOx is one of the active components for methanol synthesis and the structure of the
active unit is Cu - Zn-O ( '□' is oxygen vacancy ) which was \ /
□
proporsed in a previous work [18].
According to the XPS measurements of the relative atomic ratio N Cu/Nzn, the dispersion of copper can be roughly estimated. The NCu/Nzn value initially is 1.35 on the untreated Cu-Zn-Al catalyst. After treatment in H2+Ar at 200℃ for 2 h, the Ncu/Nzn ratio decreased substantially to 0.92, and with subsequent treating in CO+H2 at 250℃ for 2 h, the ratio further decreased to 0.70. But, after treatment in H2+Ar, subsequent treating in CO+H2+CO2 at 250℃ for 2 h, increased the value of Ncu/NZn to 1.13 as shown in Table 3 . It is reasonable to propose that the main role of CO2 in the feed gas is to establish an oxidation potential for the syngas so as to accelerate the oxidation-reduction cycle between Cu0 and Cu8+ or Cu+ during methanol synthesis as well as to prevent Cu0 crystallites from aggregation and thus to maintain the catalyst activity and lifetime.
出处
《分子催化》
EI
CAS
CSCD
1989年第4期253-261,共9页
Journal of Molecular Catalysis(China)
基金
国家自然科学基金资助课题
