(401c) Effect of H2 On the Denox Performance of Ag/Al2O3 Catalyst by Simulated Diesel with OHC

             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 O^2- 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 O^2- 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, O^2- 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-O_x [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-O_x 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-O_x and Al-O_x 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 O^2- 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-O_x 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 Ag_n^delta+ 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.