engleski

Investigation of the catalytic and adsorptive properties of Cu/ZSM-5 catalyst for NOx abatement

Introduction Catalysts made by the exchange of copper onto zeolites such as ZSM-5 are active for a number of environmentally important reactions, including the decomposition of nitrogen oxides [1], the selective catalytic reduction of NOx by hydrocarbons [2] as well as with ammonia [3]. Understanding the nature of the redox properties of these catalysts may lead to better insight as to improvements that can be made for these materials. Ion-exchanged Cu-ZSM-5 was examined using both temperature programmed reaction methods (TPR-H2, TPO, TPD-MS) and other characterization methods as XRD, TG-DSC, HRTEM, SEM, N2 adsorption at liquid nitrogen temperature. To identify various Cu sites an experimental approach combining Cu+ luminescence, and Cu+2 EPR has been successfully employed. These spectroscopic methods are highly sensitive to the co-ordination of the Cu sites in the zeolite matrices. The aim of this preliminary study is to show differences in the character of the Cu sites active in the NO direct decomposition by investigating the Cu placing in Cu/ZSM-5 of various total Cu content (1.93 and 3, 65 wt%), and its correlation with the Cu ions red-ox properties and catalytic activity in direct NOx decomposition. Experimental The commercial H/ZSM-5 zeolites (KM-902, Si/Al=28 ; KM-903, Si/Al=40) were supplied by Degussa. Cu-ZSM-5 zeolites were prepared by Cu ion exchange of the Na-ZSM-5 form with diluted solutions of copper acetate at RT. Details of the ion exchange can be found elsewhere [1, 4]. After washing, the Cu-ZSM-5 samples (1.92 and 3.65 wt% of Cu) were dried overnight at 100oC. Further, the samples were thermally activated at 500oC under air flow during 1h. Catalytic reaction studies were performed in a stainless-steel tubular fixed-bed reactor (6.5mm I.D. and 150 mm length) operating at atmospheric pressure. The reactant gas was a standard gas mixture of 4% NO in helium. The reaction was performed at the temperatures from 350 to 500oC and at different space times (0.896-5.373 s). The catalyst samples were calcined in a helium flow at 500oC for 2h before experiment. Results and discussion The good crystallinity and purity of Cu-ZSM-5 samples was confirmed by XRD method. There are no visible other phases as CuO or SiO2 for Cu loading 1.92 and 3.65 wt%. The morphology of the samples was characterized by SEM and the zeolite samples consist of regular cuboid-shaped crystals without amorphous phase. Changing the Cu loadings and preparation procedures it is possible to create different combinations of Cu species in the Cu-ZSM-5 catalysts. In the calcined metal exchange zeolites, the active sites can be located principally in their internal channels and someone in the external surface of the zeolite crystallites. The copper species can be constituted as isolated Cu+2 cations and /or as (Cu-O-Cu)+2 oxocations or CuOx clusters. Crystalline or amorphous extra-framework metallic oxides deposited on the internal or on the external surface of the catalysts can also be present. Then, the characterization techniques to be applied will must allow the identification of one particular physical or chemical property of those metallic species. The reduction of our Cu/ZSM-5 catalysts occurs in two well-defined steps, the two peaks having maximum at temperatures between 240 and 305 &ordm ; C and between 420 and 600 &ordm ; C, respectively. The low-temperature peak is attributed to the reduction of Cu2+ to Cu+ and the peak at higher temperature to the reduction of Cu+ to Cu0. After the TPR analysis, the presence of elemental copper was confirmed by the reddish color of the samples. TPR results also reveal that irrespective of the Si/Al ratio of the zeolite, the reduction temperatures of Cu2+ and Cu+ increase as the copper content decreases. For samples with similar copper content, the reduction temperatures are higher for that with the highest Si/Al ratio. This behavior might be explained by enhanced interaction between the cationic copper species and the zeolite structure. Well dispersed CuO nanocrystals can also be formed at 3.65wt% Cu loadings during calcination from hydroxylated species generated during the ion exchange, resulting in a pH decrease from 6.4 to 6.2. TPO-MS profiles of Cu/ZSM-5 with different Cu loadings and after pretreatment in 5%H2/Ar at 500oC showed two O2 consumption peaks at ca 170-200oC and 300-350oC. The low-temperature peak was ascribed to the oxidation of Cuo to Cu+ or Cu+2, and the high-temperature peak was ascribed to the oxidation of Cu+ to Cu+2. EPR spectroscopy have been used to characterize the Cu+2 species existed in Cu/ZSM-5 samples (isolated Cu+2 ions). All Cu/ZSM-5 samples calcined in air at 500oC exhibit an axial EPR spectrum of Cu+2 ions at liquid helium temperature with well resolved hyperfine structure with g║ =2.37, g┴ = 2.12. The analysis of literature data from hydrated Cu-ZSM-5 zeolites [5] indicates that these spectra are typical for isolated Cu+2 ions stabilized in octahedral crystal field with small tetragonal distortion. Since EPR is only active for isolated Cu+2 ions and is inactive for CuO, binuclear species [Cu-O-Cu]+2, or Cu+1 ions, we used also photoluminescence spectroscopy to characterize Cu+1 ion. We tested the catalytic activity of Cu-ZSM-5 samples in direct decomposition of NO to N2 and O2. It can be seen that the activity of Cu-ZSM-5 samples with Si/Al = 40 increases with the Cu content up to 2 wt% and afterwards at higher Cu content the NO conversion into N2 decreases. The results reveal the optimal copper content at which the Cu/ZSM-5 catalyst exhibits maximum activity. We have also tested the influence of the activation thermal treatment temperature (500 – 800oC) on the physico-chemical and catalytic properties of the zeolite catalyst. The activity of catalysts decreased with the increase in calcination temperature. Activity of Cu-ZSM-5 catalyst increases with the increase in Si/Al ratio which leads to the conclusion that the active sites are the isolated copper sites. The results indicate that while the Cu2+/Cu1+ interconversion is reversible, reduction of the ionic Cu species to a metallic state is not fully reversible, and suggests that such conditions should be avoided in catalytic systems where Cu-ZSM-5 is employed to maintain high activity. Acknowledgement The financial support for this research provided by the Ministry of Scientific Research and Information Technology, Poland - Grant 3T09B 110 28 [2005-2007] is gratefully acknowledged. References [1] V.Tomasic, Z.Gomzi, S.Zrncevic, Appl.Catal. B: 18 (1998) 233. [2] J.Dedecek, L.Capek, B.Wichterlova, Appl.Catal.A: 307 (2006) 156. [3] H.Sjovall, L.Olsson, E.Fridell, R.J.Blint, Appl.Catal. B: 64 (2006) 180. [4] V.Tomasic, S.Zrncevic, Z.Gomzi, Catal.Today 90 (2004) 77. [5] S.A.Yashnik, Z.R.Ismagilov, V.F.Anufrienko, Catal.Today 110 (2005) 310.

Cu/ZSM-5 catalyst; TPR-H2; TPO; TPD-MS

nije evidentirano