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Surface Reactions on Pt during NOx Storage-Reduction Studied by Polarization-Modulation Infrared Reflection-Absorption Spectroscopy Nobutaka Maeda, Atsushi Urakawa,* and Alfons Baiker* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, H€ onggerberg, HCI, CH-8093 Zurich, Switzerland
ABSTRACT NOx storage-reduction (NSR) cycles on a model catalyst, consisting of Pt particles on a BaO thin film deposited on a polished Al substrate, have been investigated by means of in situ polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). The method affords simultaneous time-resolved detection of bulk Ba species and species in gas-phase and on the Pt surface. The study revealed the special role of atop NO adsorption sites on platinum for NO oxidation leading to efficient NOx storage on the Ba component, and a gradual change in adsorbate composition resulting in a transition from N2Oto NO2-formation reaction on Pt during lean periods. SECTION Surfaces, Interfaces, Catalysis
focusing on properties of the Ba phase10-15 and catalyst supports,16,17 very little is known so far about surface species on Pt particles during NSR. Here, we present an in situ PM-IRRAS study using a model catalyst consisting of Pt particles on a BaO thin film, providing detailed insight into the surface reaction on Pt during NSR by simultaneous detection of surface and gas-phase species evolutions. The model catalyst was prepared by impregnating 20 wt % Pt on a 20 nm thick BaO film deposited on a polished aluminum substrate (10 10 1 mm3, surface mean square roughness of 1.87 nm) by physical vapor deposition (PVD). The detailed preparation procedure is described in the Supporting Information. Scanning electron microscopy (SEM) (Figure 1A) of the model catalyst after a number of NSR cycles showed large (ca. 2 μm) and small ( 99.9999%) at 373 K. The formation of atop CO (2087 cm-1) and bridged CO (1877 cm-1) adsorbed on Pt particles confirmed that surface Pt sites were available for gas adsorption. In a next step, the evolution of surface and gas-phase species on the model catalyst under NSR conditions was investigated. Figure 2 shows PM-IRRA surface spectra during
R
ecent developments of in situ spectroscopic techniques have facilitated mechanistic and kinetic studies of heterogeneously catalyzed reactions under working conditions. Vibrational spectroscopy combined with online gas-phase analysis (gas chromatography/mass spectrometry) can offer a deeper understanding of reaction steps and the overall mechanisms.1-3 Polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) is a highly surface-sensitive technique that can be operated under pressure and is thus suited for in situ studies.4 The unique feature of PM-IRRAS is its capability to monitor both surface and gasphase species simultaneously but separately using a single technique.5,6 NOx storage-reduction (NSR) is one of the most promising technologies to achieve problematic abatement of NOx under oxygen-rich (with respect to hydrocarbon fuel) conditions as encountered in lean-burn and diesel engines.7,8 A typical NSR catalyst consists of Pt and Ba species finely dispersed on γ-Al2O3. NSR utilizes an unsteady-state operation by switching between fuel-lean (oxygen-rich) to fuel-rich atmospheres. During lean phases, barium species, e.g., Ba carbonate, oxide, and hydroxide, depending on the reductant employed, capture NOx as barium nitrites and nitrates. On the other hand, intermittent fuel-rich pulses induce decomposition of the nitrites and nitrates and further reduce the released NOx to nitrogen and ammonia on Pt particles. During the lean-rich NSR cycles, transformations of barium species occur, and, as a result, barium oxide, peroxide, nitrite, nitrate, hydroxide, and carbonate can coexist.9 Various gas-phase components evolve in concert with changes of the barium phase.7,8 This complex nature of the NSR catalysis makes mechanistic studies extremely challenging and intriguing. Despite a number of reports on the elucidation of NSR mechanisms, particularly
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Received Date: September 23, 2009 Accepted Date: October 30, 2009 Published on Web Date: November 06, 2009
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Figure 1. (A) SEM image of the model Pt-Ba NSR catalyst after the reaction, (B) EDX spectrum of the highlighted region (rectangular box), and (C) PM-IRRA surface spectrum of adsorbed CO on the model catalyst.
rich (3.3% H2) to lean (1000 ppm NO þ 3.3% O2) cycles. The spectra of 10 NSR cycles were averaged into one cycle to increase the signal-to-noise (S/N) ratio.5,6 During lean periods, three bands emerged; adsorbed NO in atop (1778 cm-1) and bridged (1600 cm-1) configurations on the Pt surface,18,19 and the asymmetric stretching mode of nitrate in Ba(NO3)2 (1360 cm-1).20,21 The vibrational frequencies of the atop- and bridged-NO bands were higher than those of NO adsorbed on terrace sites such as Pt(111), but they are in a similar range as those on stepped surfaces such as Pt(112),19 indicating a more defective surface and a small particle size of Pt. At the beginning of lean phases, atop and bridged NO predominantly covered the Pt surface, as most clearly seen at 473 K (Figure 2A). The coverage decreased and, in exchange, the band of bulk Ba(NO3)2 increased during the course of lean phases. The reaction temperature had a significant influence on the relative concentrations of these species (Figures 2A-C); higher NO coverage and less Ba(NO3)2
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formation were observed at lower temperatures. Note that the NO2 concentration observed in the gas-phase, which appeared almost constant during lean phases (ca. 150-250 s in Figure 3A), was higher at lower temperatures, although NO oxidation activity was expectedly lower. Assuming the widely accepted two-step NOx storage mechanism,7,9 i.e., NO oxidation to NO2 on Pt followed by storage of NO2 on BaO and/or Ba(OH)2 (or BaCO3 when hydrocarbon or CO is reductant), the temperature dependence clearly indicates the enhanced rate of the second process, thus lowering the released NO2 concentration by successful NO2 capturing on the surface of the Ba component and further penetration of nitrates into the bulk. Concentration profiles shown in Figure 3 extracted from PM-IRRA surface and gas-phase spectra gave further information about surface reactions occurring on Pt particles. During the first 30-40 s of lean phases, a large amount of N2O was formed, which was more prominent at lower temperatures (Figure 3B). This period coincided with that where the NO
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Figure 2. Time-resolved PM-IRRA surface spectra during rich (3.3% H2 in He, t=62.5 s) to lean (1000 ppm NO þ 3.3% O2 in He, t=187.5 s) cycles on Pt/20 nm BaO film at (A) 473 K, (B) 523 K, (C) 573 K, and (D) 573 K in the absence of oxygen in lean phases. The last spectrum recorded during rich periods was taken as the reference for normalization.
coverage on Pt was high. The increased ratio of atop to bridged NO during N2O formation (Figure 3E) also indicated high surface coverage of NO on Pt because NO molecules occupy more atop Pt sites at high coverage.22 After complete depletion of N2O formation, NO oxidation was stabilized at a concentration of ca. 50 ppm. It is also known that NO oxidation does not occur at low O coverage, i.e., high NO coverage.23 Our results confirm that there is a transformation from a Pt surface where NO prevails to one where NO and O are coadsorbed, leading to a transition from prevalent formation of N2O24 to formation of NO2. An identical NSR experiment at 573 K in the absence of oxygen provided a deeper insight into the surface reactions. NO oxidation to NO2 was suppressed without oxygen in the feed (Figure 2A), and the NOx storage occurred to a minor extent (Figure 2D, and Figure S1-C in the Supporting Information). Interestingly, the ratio of atop to bridged NO on Pt during lean phases drastically changed, as emerges by comparing Figure 2A and D; more prominent bridged adsorption occurred in the presence of oxygen, while atop adsorption was more prominent in the absence of oxygen. The ratio shown in Figure 3E clearly indicates the enhanced atop configuration without oxygen. Furthermore, the comparison of the adsorption mode of NO at the same temperature (573 K, red and green lines in Figure 3C,D) reveals that only the concentration of atop NO was influenced by the
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presence/absence of oxygen, while that of bridged NO remained at the same level during stable NOx storage (after ca. 120 s). Considering the fact that NO oxidation to NO2 is the main cause for the difference in the gas-phase species between the two experiments, it seems that the atop adsorption sites are mainly responsible for NO oxidation. The constant concentration of atop NO and NO2 in the gas phase during the stable NOx storage period in contrast to steadily increasing bridged NO strongly supports the above conjecture and also indicates that the bridged NO acts rather as a “spectator” during the storage process. Moreover, the formation of adsorbed NO (Figure 3C,D) and gaseous N2O (Figure 3B) were delayed for 15-20 s in the absence of oxygen. Instead, NH3 was produced via NO hydrogenation (Supporting Information, Figure S2), which proceeded rapidly and did not allow the detection of adsorbed NO on the Pt surface. In conclusion, simultaneous detections of species in the gas-phase and on the Pt surface as well as bulk Ba species by in situ PM-IRRAS allowed us to gain further insight into the NSR mechanism. The model catalyst prepared by the combined PVD-wet method proved to be suitable for studying chemical species involved during both NOx storage and reduction processes. Relevant surface and bulk species involved in the NSR process could be identified, and their role in the NSR process elucidated. A striking finding is that atop NO adsorption sites on Pt play a key role in NO oxidation to NO2.
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adsorption, and NSR conditions) and concentration profiles of H2O(g), NH3(g), and bulk Ba(NO3)2 during NSR. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail address:
[email protected] (A.U.);
[email protected] (A.B.).
ACKNOWLEDGMENT We thank Dr. Frank Krumeich and Dr. Karsten Kunze for performing the SEM and EDXS analyses at the EMEZ (Electron Microscopy ETH Z€ urich). Dr. Wolfgang Kleist and Dr. Daniel M. Meier are acknowledged for their technical support in the sample preparations and XPS measurements.
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Figure 3. Concentration profiles, expressed by the band area, of gas (g) and surface species (a) during rich (3.3% H2 in He, t = 62.5 s) to lean (1000 ppm NO þ 3.3% O2 in He, t = 187.5 s) cycles on Pt/ 20 nm BaO film at 473, 523, and 573 K. The following regions were used for integration of respective bands: 1583-1605 cm-1 for NO2, 2160-2270 cm-1 for N2O, 1700-1860 cm-1 for atop NO, and 1556-1655 cm-1 for bridged NO.
The studies also confirm that there is a transition from N2O to NO2 formation during lean phases, caused by changes in composition of surface NO and oxygen; at the beginning, adsorbed NO predominantly occupies the Pt surface, resulting mainly in the formation of N2O. Afterward, NO and oxygen are well-mixed on the Pt surface, which gives rise to the steady formation of NO2 and initiates NOx storage as Ba(NO3)2. Bulk Ba(NO3)2 formation rate is greatly enhanced by increasing temperature. Other important insight was gained by comparing surface and gas-phase species on the same time scale; the consumption of atop NO was found to correlate with NO2 formation, while the concentration of bridged NO did not change upon admission of oxygen.
SUPPORTING INFORMATION AVAILABLE Experimental details (the model catalyst preparation, XPS, SEM-EDX, CO
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