Structural, Adsorption And Catalytic Properties
STRUCTURAL, ADSORPTION AND CATALYTIC PROPERTIES
OF Cu-, Pd- AND Cu-Pd/ERIONITES
L. P. Oleksenko, V. K. Yatsimirsky, I. V. Kuzmich, T. G. Verbezkaya
Kiev Shevchenko National University, Kiev, Ukraine
The interest for studying of adsorption, structural and acidic properties of Cu- and Pd-containing zeolites is connected with high catalytic activity of metal-containing zeolites in the various gaseous reaction oxidation. The Cu-, Pd- and bimetallic Cu-Pd-containing zeolites by ion exchange from [Cu(NH3)4]Cl2 and [Pd(NH3)4]Cl2 solutions were obtained. In the case of bimetallic zeolites the various order of introduction of Pd and Cu in zeolite during ion exchange was used. The catalytic activity in CO oxidation metalcontaining zeolites after reduction treatment in hydrogen flow have been studied.
It was established that the catalytic activity of Cu-erionites in CO oxidation is rather high both after their treatment in gaseous reaction mixture and after reduction treatment of metal-zeolites at hydrogen flow. It may be connected with presence of copper ions in square-planar coordination state in zeolite matrix. It was found that ESDR-spectra of Cu-erionites include the absorption bands at 18 000-20 000 sm-1 which correspond to copper ions in square-planar coordination state. It should be not iced that formation of such high-active centers may be reached as during catalytic reaction flowing as after reduction treatment of zeolites.
The investigation of acidic properties of Cu-erionites allow to fix three types of adsorbed ammonia molecules in TPD spectra at 200-260, 370-390 and 470-5500C which correspond to strong, medium and weak acidic centres in Cu-erionites. It was established that introduction of metal cations to the zeolites and their reduction in hydrogen flow causes to acidity decreasing of initial erionite. It is explained by decreasing of acidic center number due to cation exchange and dehydratation during hydrogen treatment. The correlation of catalytic activity of Cu-erionites and quantity of acidic centers in their matrix was found. The decreasing of acidic centres, both weak and strong in Cu-erionites leads to the inceasing of their catalytic activity in CO oxidation. It should be noted that the strength of acidic centers changed when erionite
include the copper ions. The energy of ammonia molecules desorption is increased for Cu-erionite in comparison with Na,K-erionite in the case of weak acidic centres - the maximum of temperature desorption shifted on 500C in high temperature region. The vertual dependence is odserved for the changing of energy desorption for strong acidic centers - the maximum of temperature desorption shifted on 400C in low temperature region. The catalytic activity of Pd-erionite is much higher than Cu-containing zeolite. The different character of energy activation changing of CO oxidation reaction for Cu-and Pd-erionites was found. It may connected with various mechanisms of reaction f low on Cu-and Pd-erionites. In the same time the critical temperature phenomena and hysteresis on dependence of CO oxidation rate from temperature were fixed for Cu- and Pd-erionite systems. The hysteresis of reaction rate "against-hand" for these systems can be explain by formation of active centers of the investigated metal-containing zeolites during reduction treatment in hydrogen flow and their using catalytic reaction.
It was shown that introduction of the second active metal ions in zeolite matrix in the case of bimetallic Cu-Pd and Pd-Cu/erionites lead to essentially increasing of their catalytic activity in comparison with Cu-erionite and Pd-erionite. In the case of Pd-Cu/erionite such increasing of catalytic activity occurs only for gaseous mixture (0,5%CO+20%O2). The catalytic activity which practically correspond to monometallic Pd-erionite was fixed for Pd-Cu/erionite in the cases when the mixtures enrich ed by CO were used. It was established that Cu-Pd/erionite has higher catalytic activity in CO oxidation as compared with Pd-Cu/erionite when the reaction gaseous mixtures with various CO content (0,5%, 1% and 2%CO+20%O2) were used.
Mutual influence ¦u and Pd ions in bimetallic Cu-Pd-erionites which leads to increasing of catalytic activity may be connected with presence of interaction between copper and palladium ions, formation of Cu-Pd clusters and their more strong interaction with zeolite matrix. It can lead to more weak adsorption of CO molecules on such Cu-Pd centres and it is favourable for catalysis.
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