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Promotion Effects and Mechanism of Alkali Metals and Alkaline Earth Subscriber access provided by RES CENTER OF ECO ENVIR SCI Article Promotion Effects and Mechanism of Alkali Metals and Alkaline Earth Metals on Cobalt#Cerium Composite Oxide Catalysts for N2O Decomposition Li Xue, Hong He, Chang Liu, Changbin Zhang, and Bo Zhang Environ. Sci. Technol., 2009, 43 (3), 890-895 • DOI: 10.1021/es801867y • Publication Date (Web): 05 January 2009 Downloaded from http://pubs.acs.org on January 31, 2009 More About This Article Additional resources and features associated with this article are available within the HTML version: • Supporting Information • Access to high resolution figures • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Environ. Sci. Technol. 2009, 43, 890–895 Promotion Effects and Mechanism such as Fe-ZSM-5 are more active in the selective catalytic reduction (SCR) of N2O by hydrocarbons than in the - ° of Alkali Metals and Alkaline Earth decomposition of N2O in a temperature range of 300 400 C (3). In recent years, it has been found that various mixed Metals on Cobalt-Cerium oxide catalysts, such as calcined hydrotalcite and spinel oxide, showed relatively high activities. Composite Oxide Catalysts for N2O One of the most active oxide catalysts is a mixed oxide containing cobalt spinel. Calcined hydrotalcites containing Decomposition cobalt, such as Co-Al-HT (9-12) and Co-Rh-Al-HT (9, 11), have been reported to be very efficient for the decomposition 2+ LI XUE, HONG HE,* CHANG LIU, of N2O. Yan et al. (13, 14) found that when the Co in Co3O4 + + + CHANGBIN ZHANG, AND BO ZHANG was partially replaced by Ni2 ,Zn2 ,orMg2 , the catalytic Research Center for Eco-Environmental Sciences, Chinese activity of cobalt spinel could be greatly improved. In our Academy of Sciences, 18 Shuangqing Road, previous work on the influence of preparation methods on Beijing 100085, PR China the cobalt-cerium composite oxide catalyst, a remarkable promotion effect of residual K was observed (15). The greatly 2+ Received July 22, 2008. Revised manuscript received improved redox ability of active site Co induced by residual November 21, 2008. Accepted November 21, 2008. K was supposed to be the main reason for the high catalytic activity of the catalyst. The presence of ceria in this catalyst was very important, because it could also improve the cobalt catalyst’s activity even in the presence of residual K (15, 16). A series of alkali metal- and alkaline earth metal-doped However, the promotion effect of alkali metals was much cobalt-cerium composite oxide catalysts were prepared by greater than that of ceria. Therefore, a detailed study of effect of alkali and alkaline earth metals on cobalt-cerium com- the citrate method and tested for the decomposition of N2O. Strong promotion effects of alkali and alkaline earth metals posite catalysts is very necessary. The promotion effects of alkali or alkaline earth metals on the activity of the catalyst were obtained in the order Li < < < < < < on the catalytic decomposition of N2O have also been Na K Rb Cs and Mg Ca Sr, Ba. The promotion observed by other researchers. For instance, Farris et al. (17) effects of alkaline earth metals were much weaker than the found in a scale-up process that an appropriate amount of effects of alkali metals. To investigate the origin of the promotion residual Na is very important for the catalytic activity of Co-Al - - effect, X-ray diffraction, Brunauer Emmett Teller surface hydrotalcite in N2O decomposition. Ohnishi et al. (18) area measurement, X-ray photoelectron spectroscopy, reported that alkali and alkaline earth metals could promote temperature-programmed desorption, and hydrogen temperature- the activity of a Co3O4 catalyst for N2O decomposition. The programmed reduction methods were used to characterize alkaline earth metals showed much greater promotion effects the alkali metal-doped catalyst. The analytical results indicated than alkali metals in their study. that alkali metals improved the redox ability of active site Alkali and alkaline earth metal compounds could serve Co2+ by acting as electronic promoters. Catalytic decomposition as textural or electronic promoters for catalysts in various catalytic processes. Haber et al. (19) reported that doping of N O proceeds through an oxidation-reduction mechanism 2 with alkali metals could influence the dispersion of rhodium with participation of electrons from Co2+, thus the increase in the on Al2O3, and then influence its catalytic activity for the redox ability of Co2+ should lead to an increase in the decomposition of N2O. Research by Konsolakis and Yentekakis activity of the catalyst. on the SCR of NO by propene over Pt/Al2O3 (20-22) showed that alkali and alkaline earth metals could promote the 1. Introduction adsorption of NO (electron-acceptor) on the catalyst surface s Catalytic decomposition of nitrous oxide (N2O) is an attractive by acting as electronic promoters. The N O bond was proposition from an environmental point of view because weakened at the same time, thus facilitating NO dissociation. - N2O is considered to be responsible for the depletion of the To clarify the role of alkali promoters in cobalt cerium ozone layer and for contributing to global warming (1-4). composite oxides (molar ratio of Ce/Co ) 0.05) (15, 16), a - Although N2O is not the major contributor to global warming, series of alkali and alkaline earth metal-doped cobalt cerium it is much more potent than either of the other two most composite oxides were prepared and tested for the decom- common anthropogenic greenhouse gases, CO2 and CH4.It position of N2O. A significant increase of catalytic activity has 310 and 21 times the global warming potential (GWP) of was found when these promoters were present on the - CO2 and CH4, respectively (3). catalysts. Methods of X-ray diffraction (XRD), Brunauer - N2O is produced by both natural and anthropogenic Emmett Teller (BET) surface area measurement, X-ray sources. To control the emission of N2O from chemical photoelectron spectroscopy (XPS), carbon dioxide temper- processes, catalysts for N2O decomposition have been widely ature-programmed desorption (CO2-TPD), oxygen temper- studied in the past decade; these catalysts include supported ature-programmed desorption (O2-TPD), and hydrogen metals, transition metal ion exchanged zeolites, and pure temperature-programmed reduction (H2-TPR) were used to and mixed oxides (1, 3, 5-14). Supported noble metal characterize these catalysts in order to investigate the origin catalysts, such as Rh and Au catalysts (5-7), show high of this significant promotion effect. activities in N2O decomposition reactions at 200-300 °C. However, the high cost of the noble metal limits their 2. Experimental Section application. Transition metal ion exchanged zeolite catalysts 2.1. Catalyst Preparation. All catalysts were prepared by the citrate method. Nitrates of cobalt, cerium and alkali, or * Corresponding author phone: +86-10-62849123; fax: +86-10- alkaline earth metals were used in appropriate quantities. 62923549; e-mail: [email protected]. The details of the method can be seen in the Supporting 890 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 3, 2009 10.1021/es801867y CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009 Information or in our previous paper (15). The catalyst is referred to as CC-M-x (M indicates the doping alkali or alkaline earth metal; x indicates the molar ratio of M/Co, so that x ) 0 for the undoped catalyst). 2.2. Catalytic Activity Measurement. The catalytic reac- tion was carried out in a fixed-bed quartz flow reactor containing approximately 0.5 g of catalyst in all the experi- ments. The reactor was heated by a temperature-controlled furnace. A thermocouple was placed on the outside of the reactor tube. Prior to the reaction all samples were pretreated for 30 min with 20% O2 in Ar at 400 °C to yield clean surfaces, followed by cooling to the reaction temperature in the same gas. A reaction mixture of N2O (1000 ppm) in Ar was then introduced into the reactor at a flow rate of 150 cm3 min-1, yielding a space time (W/F) of 0.2gscm-3. Analysis of the reaction products was carried out using a gas chromatograph (Agilent 6890N equipped with Porapak Q for the analysis of N2O, CO2, and molecular sieve 5A columns for the analysis of N2,O2 and NO). The reaction system was kept for1hat FIGURE 1. Catalytic activities of cobalt-cerium composite each reaction temperature to reach a steady state before oxide catalysts doped with optimal amounts of alkali metals for 3 -1 analysis of the product was performed. In all tests, N2 and N2O decomposition. Conditions: total flow, 150 cm min ; W/F -3 O2 were the only gaseous products that were observed. ) 0.2gscm ; gas composition, 1000 ppm N2O/Ar. 2.3. Catalyst Characterization. The details for the catalyst - characterization (X-rays diffraction (XRD), Brunauer used in the selective catalytic reduction (SCR) of NO by NH - x 3 Emmett Teller (BET), X-rays photoelectron spectroscopy (23).The presence of the poisoning element could block both (XPS)) can be seen in the Supporting Information. Brønsted acid and V5+ ) O sites of the catalysts, which play 2.4. O2-TPD, CO2-TPD and H2-TPR. The O2-TPD experi- a crucial role in the SCR mechanism. The opposite effects 3 -1 ments were performed in a flow of He (30 cm min ) over of the same metals in the SCR reaction and the decomposition ° -1 200 mg of catalyst using a heating rate of 30 C min . Prior reaction are understandable because the two reactions use to the TPD experiment, the catalysts were pretreated under different catalyst and they have totally different catalytic ° a flow of 2% N2O/Ar (or 10% O2/He) at 400 C for 1 h, followed mechanism.
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