Effect of H2 On the Denox Performance of Ag/Al2O3 Catalyst by Simulated Diesel with OHC

Wednesday, November 10, 2010: 9:12 AM
150 D/E Room (Salt Palace Convention Center)
Pyung Soon Kim, School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, South Korea, Mun Kyu Kim, Chemical Engineering, Pohang University of Science and Technology, Pohang, South Korea, In-Sik Nam, Department of Chemical Engineering/School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang, South Korea, Byong K. Cho, Chemical & Environmental Sciences Lab, General Motors R&D Center, Warren, MI and Se H. Oh, Chemical Sciences & Materials Systems Lab, General Motors Global R&D Center, Warren, MI

             The selective catalytic reduction of NOx by hydrocarbons is an alternative technology for removing NOx from automotive engine under lean condition without concerning the drawbacks of the commercially available deNOx technologies including Urea/SCR and LNT.  However, the deNOx performance of HC/SCR technology, particularly its low temperature activity may not be appropriate to be practically applied to next generation energy efficient vehicles including diesel engine [1].  One way to improve the low temperature deNOx activity over Ag/Al2O3 catalyst may be the addition of hydrogen into the feed gas stream, regardless of hydrocarbons employed as a reductant [2-4]. 

Eränen et al. [3] reported that the NO to N2 conversion over 2 wt.% Ag/Al2O3 catalyst increased from 3 to 60% by addition of 1% H2 to the feed stream with octane at 250 oC, mainly due to the enhancement of hydrocarbon oxidation and formation of the adsorbed NOx species on the catalyst surface.  The addition of hydrogen also promoted the partial oxidation of C2H5OH to form enolic species at low temperature [4].  Moreover, the spectroscopic studies including UV-vis, EXAFS and ESR have been conducted to examine the chemical state of Ag formed on the surface of 2 wt.% Ag/Al2O3 catalyst upon the addition of 0.5% H2 into the feed gas stream for the reduction of NO by C3H8 [5].  However, the role of hydrogen, more specifically the alteration of Ag species in the presence of hydrogen is still unclear, due to ambiguity and difficulty of the application of in-situ spectroscopic techniques for characterizing Ag/Al2O3 [6].  It should be noted that H2 exists in the exhaust gas stream from diesel engine and its content varies from 0.03 to 1% with respect to the size and the operating condition of engine [7, 8].

             In the present study, the deNOx performance and the characteristics of Ag/Al2O3 catalyst by in-situ FTIR, O2-TPD and UV-vis upon the addition of H2 into feed have been systematically examined to identify the formation of key reaction intermediates improving the conversion of NO to N2 during the course of the SCR reaction by simulated diesel with OHC.  The Ag/Al2O3 catalyst was prepared by the incipient wetness method with AgNO3 aqueous solution.  The deNOx activity has been examined over a packed-bed flow reactor system under the feed stream consisting of 400 ppm NO, 6% O2, 0-1% H2, 2.5% H2O, 640 ppm ethanol, simulated diesel fuel (a mixture of 17 ppm dodecane and 15 ppm m-xylene) and He balance (GHSV: 60,000 h-1) [9].

             Fig. 1 shows the NOx to N2 conversion over Ag(3.8)/Al2O3 catalyst with respect to the concentration of H2 (0-1%).  As the hydrogen concentration increases, the deNOx performance of Ag(3.8)/Al2O3 catalyst is significantly enhanced in the low temperature region less than 300 oC.  Moreover, Ag(3.8)/Al2O3 catalyst achieves 55% of the NOx to N2 conversion even at 175 oC in the presence of 1% H2 in feed.  Note that H2 has been hardly involved in the direct formation of N2, NH3 and N2O, while it plays an important role for oxidations of NO to NO2 and HCs.  This reveals that H2 may accelerate the formation of the reaction intermediates including nitrates and enolic species on the catalyst surface [3, 4].

Richter et al. [2] proposed that H2 may reduce Ag species to Ag0 on the catalyst surface of Ag/Al2O3.  They speculated that the reactive O2- and/or OH- species formed by the reaction of H2 and O2 on the metallic Ag might enhance the oxidation of NO to NO2.  However, no direct experimental evidence for the formation of the reactive oxygen on the surface of Ag/Al2O3 catalyst has been reported.

To identify the amount of O2 species adsorbed on silver, Ag(3.8)/Al2O3 catalyst has been characterized by O2-TPD with or without H2.  The desorption profile of the oxygen species including O2- and O2 from Ag/Al2O3 catalyst was directly obtained by on-line MS.  As shown in Fig. 2, a large amount of the dissociated atomic oxygen, O2- was readily released from Ag(3.8)/Al2O3 catalyst under the (O2 + H2) flow in the temperature range from 150 to 500 oC.  This indicates that the significant amount of the reactive oxygen may be formed on the catalyst surface during the course of the present reaction with H2.

In order to determine the formation of the reactive oxygen adsorbed onto the surface of Ag/Al2O3 catalyst, in-situ FTIR technique has been employed under the sequential feed gas conditions.  Fig. 3A shows the FTIR spectra of Ag species under the feed of (O2, O2 + H2 and O3) flows at 200 oC for 30 min.  During the feed of the (O2 + H2) flow [Fig. 3A(b)], the broad bands appear in the range of 1000-1200 cm-1, mainly attributed to the O-O stretching vibration of Ag-Ox [10-12].  The bands observed at 1030 and 1078 cm-1 are assigned to the O3 physisorbed onto Ag and Ag+(O2)- species, respectively [10, 11].  Moreover, it is commonly recognized that the bands at 1053 and 1040-1065 cm-1 are mainly due to the formation of Ag-O2 and ozonides, respectively [12, 13].  The weak bands in the range from 1300 to 1400 cm-1 are attributed to the oxidation of aluminum by O3 [14].  This indicates that the ozone formation reaction through (O + O2 <-> O3) may occur over Ag/Al2O3 catalyst under the (O2 + H2) flow.  To further examine the formation of Ag-Ox complex on the catalyst surface, O3 was directly adsorbed onto Ag(3.8)/Al2O3 catalyst.  As shown in Fig. 3A(c), the bands for the formation of Ag-Ox and Al-Ox appear in the wide region of 1000-1400 cm-1.  This is quite consistent to the FTIR result for Ag/Al2O3 catalyst observed under the (O2 + H2) flow.

In addition, the formation of nitrate species on the catalyst surface has been also examined under the presence of NO in the feed as shown in Fig. 3B.  When 1% of H2 was added to the (NO + O2) flow [Fig. 3B(b)], the formation of nitrate species including monodentate nitrate at 1250 and 1550 cm-1, bidentate nitrate at 1295 and 1585 cm-1 [15] and bridging nitrate at 1614 cm-1 [16] becomes apparent, compared to that without H2 in feed [Fig. 3B(a)].  Large amounts of the nitrate species on the catalyst surface can be also observed under the (NO + O3) flow [Fig. 3B(c)].  It reveals that the reactive oxygen species including O2- and O3 as well as O2- are the critical reaction intermediates to form the nitrates eventually converting NO to N2.  They play an important role for the formation of the Ag-Ox complex including Ag-O2, Ag+(O2)- and Ag-O3 on the surface of Ag/Al2O3 catalyst upon the addition of H2 into the feed gas stream.  Note that the enhancement of the formation of enolic species has been also observed in the presence of H2.  It can be concluded that one of the primary roles of hydrogen for the enhancement of the low temperature deNOx activity of Ag/Al2O3 catalyst is the formation of large amounts of the reactive oxygen species, particularly onto Agndelta+ and Ag0.  They promote the formation of the reaction intermediates including nitrate and enolic species enhancing the NOx removal activity by simulated diesel with ethanol.

References

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Fig. 1. The deNOx activity over Ag(3.8)/Al2O3 catalyst with respect to the concentrations of hydrogen.

 

Fig. 2. O2-TPD profiles of Ag(3.8)/Al2O3 catalyst with or without hydrogen.

                     

Fig. 3. In-situ FTIR spectra of A;(a) O2 (b) O2 + H2 (c) O3 flow, B;(a) NO + O2 (b) NO + O2 + H2

(c) NO + O3 flow in He balance at 200 oC over Ag(3.8)/Al2O3 catalyst.


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