124 Reductive Aggregation and Oxidative Redispersion of Silver Species as a Crucial Step in De-NO x Catalysis Nitrogen oxides (NO x : NO and NO 2 ) are major air pollutants that cause photochemical smog formation and acid rain. The emission of various nitrogen oxides into our atmosphere occurs on a massive scale. Worldwide, over 30 million tons of NO x are vented into the earth's atmosphere each year. On the other hand, the demand for diesel engines in vehicles, which is the one of the major sources of NO x emissions, is expected to increase because of the strong efforts to reduce CO 2 emissions. However, the full worldwide expansion of such engines is hindered by the difficulties in effective NO x and particulate removal. Three major catalytic techniques are widely proposed and used for the removal of NO x emissions from lean-burn and diesel-operated vehicles: NO x storage, urea-selective catalytic reduction (SCR), and HC-SCR. Among these techniques, HC-SCR is the most efficient and simplest method, because small quantities of unburnt fuel in diesel engine exhaust streams can be used to reduce the pollutant NO x to N 2 . Numerous catalysts have been tried and tested for HC-SCR. It is now well established that silver supported on alumina (Ag/Al 2 O 3 ) is one of the most active and selective catalysts for HC-SCR. However, the temperature window for NO x reduction is narrow and they are not sufficiently active in the low temperature region (150-300 °C), which is typical of diesel engines. Until recently, these drawbacks have prevented the commercialization of Ag/Al 2 O 3 catalysts for HC-SCR. However, the recent discovery that the addition of a small amount of hydrogen to the feed of the HC-SCR can dramatically improve the performance of Ag/Al 2 O 3 catalysts has stimulated interest in HC-SCR for practical use [1]. Recent attention has been focused on the mechanistic causes of the hydrogen effect, even though a comprehensive explanation of the hydrogen effect has yet to be obtained. We performed detailed mechanistic studies, including the identification of surface intermediates and their dynamic changes [1-3]. A typical example of the H 2 -promoted HC-SCR catalysis of Ag/Al 2 O 3 is shown in Fig. 1. The addition of 0.5% H 2 into a NO+C 3 H 8 +O 2 gas mixture boosted the NO conversion rate at temperatures lower than 400 °C. Interestingly, the promotion effect of H 2 on NO conversion is reversible. The NO conversion rate decreased after the removal of H 2 . This reversible change in NO conversion rate is in agreement with the time-dependence of the intensity of the band (350 nm) assignable to the Ag n δ + cluster. This finding suggests that the formation of the Ag n δ + cluster is essential for the “hydrogen effect.” The structure of the Ag n δ + cluster was analyzed by in situ EXAFS at 573 K (Ag K-edge Quick XAFS Fig. 1. Effect of H 2 addition on NO conversion in HC-SCR using Ag/Al 2 O 3 . Fig. 2. Fourier transforms of Ag K -edge in situ EXAFS spectra measured at 573 K. 125 measurements performed in the transmission mode at beamline BL01B1 ) [2]. A wafer form of Ag/Al 2 O 3 was placed in a quartz in situ cell [4] under a gas flow. Before the reaction, the Ag/Al 2 O 3 wafer shows only a large Ag-O contribution, indicating that Ag + ions are highly dispersed on the alumina surface. Results shown in Fig. 2 show that the reduction of Ag + ions by 0.5% H 2 results in their aggregation to yield silver clusters. Finally, an Ag-Ag shell having a coordination number of 3.2 and a bond distance of 2.83 Å was observed. These values are smaller than those for bulk Ag metal particles (12 Ag atoms at 2.89 Å), and we assign the Ag-Ag contribution to the Ag 4 2+ cluster. The cluster is redispersed to Ag+ ions upon reoxidation at 573 K. When the flowing gas was switched from H 2 to O 2 , the coordination number for the Ag-Ag shell decreased, whereas that for the Ag-O shell increased. Taking into account the FTIR result that the acidic protons formed by the H 2 reduction of Ag/Al 2 O 3 are consumed in the reoxidation reaction with O 2 and the ESR result showing the formation of O 2 - (superoxide) radicals under a similar condition, the catalytic cycle in Fig. 3 is presented [2]. Combined with kinetic results [3], the total mechanism of the hydrogen effect is shown in Fig. 4. The reaction consists of the following steps: (1) The H 2 dissociation on the Ag site, (2) the spillover of H atoms to form protons, (3) the aggregation of isolated silver to form reduced Ag n δ + , (4) the reduction of O 2 with Ag n δ + and H + to yield O 2 - and H 2 O, (5) the partial oxidation of hydrocarbons by O 2 - to yield an acetate intermediate, and (6) the oxidation of NO to the NO 2 intermediate. This conclusion provides, for the first time, an explanation of H 2 -promoted HC-SCR on the molecular level, which will be useful for the rational design of more efficient catalysts. The present results improve our ability to tailor the structure and catalytic behavior of silver clusters by controlling the reactant gas composition. Fig. 4. Proposed mechanism of H 2 promoted HC-SCR. Ken-ichi Shimizu Department of Applied Chemistry, Nagoya University E-mail: kshimizu@apchem.nagoya-u.ac.jp References [1] K. Shimizu and A. Satsuma: Phys. Chem. Chem. Phys. 8 (2006) 2677. [2] K. Shimizu, M. Tsuzuki, K. Kato, S. Yokota, K. Okumura and A. Satsuma: J. Phys. Chem. C 111 (2007) 950. [3] K. Shimizu et al. : J. Catal. 239 (2006) 402. [4] K. Okumura et al. : J. Phys. Chem. B 109 (2005) 12380. Fig. 3. Changes in structure of Ag species in Ag/Al 2 O 3 upon H 2 reduction and subsequent O 2 oxidation.
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