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Elucidating N2O formation during cyclic NSR process using CO as a reductant Jun Wang, Xiuting Wang, Jinxin Zhu, Jianqiang Wang, and Meiqing Shen Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 29 May 2015 Downloaded from http://pubs.acs.org on May 30, 2015

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Environmental Science & Technology

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Elucidating N2O formation during cyclic NSR process using CO as a

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reductant

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Jun Wanga, Xiuting Wanga, Jinxin Zhua, Jianqiang Wanga,*, Meiqing Shena,b,c,* a

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Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China

b

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Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin

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300072, PR China c

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*

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China

To whom Correspondence should be addressed

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E-mail: [email protected]

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Tel.: (+86) 22-27892301

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E-mail: [email protected]

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Tel.: (+86) 22-27407002

ACS Paragon Plus Environment

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Abstract

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N2O formation pathway and effect of H2O on N2O formation during NSR process using CO as a

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reductant were investigated over Pt-BaO/Al2O3 catalyst. The NSR activity measurements and

17

transient in-situ DRIFTS experiments were performed to evaluate N2O evolution and elucidate

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N2O formation mechanism. N2O is formed in the lean, rich and delay2 phase. In the lean phase,

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N2O formation is related to the reactions between surface isocyanate and gaseous NO/O2, and

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NO is more responsible for N2O formation than O2. Moreover, N2O production decreases with

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H2O due to the hydrolysis of isocyanate species. In the rich phase, the amount of N2O formation

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also decreases in the presence of H2O at higher temperature due to high reduction ability of H2

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generated from the WGS reaction. During the delay2 phase, N2O is mainly formed by nitrite

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species reacting with Pt0-CO. Furthermore, the presence of H2O decreases the stability of nitrites,

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and results in more N2O production at low temperature.

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Keywords: N2O formation; NSR; Pt-BaO/Al2O3; CO; isocyanate; WGS reaction

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1. Introduction

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NOx storage and reduction (NSR) represents a promising aftertreatment technology for lean

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burn and diesel engines.1, 2 NSR catalysts operate under cyclic lean/rich conditions. During lean

30

conditions NOx is stored on the catalyst, while during rich conditions the stored NOx is released

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and reduced into N2 and possibly undesired byproducts, such as NH3 and N2O.3 N2O is a

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greenhouse gas and has 298 times the Global Warming Potential (GWP) of CO2, the U.S.

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Environmental Protection Agency (EPA) finalized standards that capped tailpipe N2O emission

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at 0.010 grams per mile for light-duty vehicles in model years 2012 through 2016.4

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N2O formation over NSR catalysts when using CO as a reductant has been investigated by


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several groups. It was reported that N2O selectivity depended on temperature and lean/rich cycle

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timing.5 N2O peaks were observed during lean-to-rich and rich-to-lean transitions.5–8 Isocyanate

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was an important intermediate species during NSR process when CO was used as a reductant.9–11

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During the reaction of NO and CO over noble metal catalysts, the isocyanate intermediate

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species was first identified by Unland.12,13 Subsequently, Solymosi et al.14,15 demonstrated that

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isocyanate formed on the Pt sites and then migrated rapidly to the support, and its reactivity was

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determined by the support. Bell et al.16 proposed that the accumulation of NCO and CN groups

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on the support adjacent to Pt crystallites may led to the deactivation of catalyst. Bártová et al.7

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suggested that N2O peak appeared after rich phase inception due to NOx partially reduced over

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platinum-group-metal (PGM) sites, and N2O peak appeared at the rich-to-lean transition as a

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result of reactions between surface-deposited reductive species (NH3, CO and/or isocyanate) and

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residual stored NOx. N2O peak increased with CO feed concentration increased, Dasari et al.17

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concluded that it was attributed to an increase in the surface concentration of isocyanate/cyanate

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species resulting from the increase in CO concentration.

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However, N2O formation pathway during NSR process using CO as a reductant is still

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unclear. In this paper, we aim to achieve a better understanding of N2O formation mechanism at

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low to intermediate temperature over Pt-BaO/Al2O3 using CO as a reductant. H2O could alleviate

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CO poisoning of Pt at low temperature,18 isocyanate could be readily hydrolyzed,11 and the gas-

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water shift reaction may occur in rich phase. The effect of H2O on N2O formation during NSR

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process was also investigated with a consideration of these factors. Cycling activity

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measurements were carried out to evaluate N2O evolution, and transient in-situ DRIFTS

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experiments were used to study the dynamic changes of surface intermediate species at 150 and

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250 ˚C. Moreover, activity measurements and DRIFTS experiments were performed using the


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same catalyst material and gas compositions during lean/rich cycling for particularly intuitive

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comparison.

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2. Experimental

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2.1. Catalyst preparation

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Pt-BaO/γ-Al2O3 (1/15/100 w/w) catalyst was prepared by the incipient wetness impregnation

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method, using aqueous solutions of Ba(CH3COO)2 and Pt(NO3)2. γ-Al2O3 support oxide was

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supplied by BASF. The impregnation was carried out in sequential manner: the alumina support

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was first impregnated with the Ba acetate solution and then with the Pt-containing solution. After

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each impregnation step, the catalyst was dried at 100 ˚C in air overnight. At last, the catalyst was

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calcined in air at 500 ˚C for 5 h. The BET surface area and Pt dispersion of catalyst are shown in

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Supporting Information (SI) Section 1.

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2.2. Activity measurements

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In NSR activity tests, 0.25 g Pt-BaO/Al2O3 catalyst was mixed with 0.75 g quartz sand. The

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total flow rate is 1 L/min, with the space velocity of 60,000 h-1. Before each experiment, the

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sample was oxidized in 10% O2/N2 balance at 350 ˚C for 30 min, and then reduced in 5% H2/N2

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balance at 450 ˚C for 20 min. The reactor was then cooled in N2 to the target test temperature.

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The gas exiting the reactor was maintained at 140 ˚C to avoid condensation and NH3 hold-up. A

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MKS MultiGas 2030 FT-IR analyzer was used to monitor NO, NO2, N2O, NH3, CO, CO2 and

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H2O concentrations at the reactor outlet.

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Experiments were performed at 150 and 250 ˚C, and the NSR performance was evaluated in

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a lean/delay1/rich/delay2 (10 min/2 min/5 min/2 min) cycle. The lean phase gas contained 600

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ppm NO, 10% O2, and a balance of N2; the rich phase contained 5.2% CO balanced by N2; and


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the two delay phase gas contained pure N2. The effect of H2O during cycling was also

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investigated, approximately 2% H2O was added to the gas flow when required. All reported

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values and plotted data were obtained after steady cycle-to-cycle (usually 5–6 cycles are required

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before stabilization) performance was attained.

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The water-gas shift reaction was also measured over Pt-BaO/Al2O3. A quadruple mass

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spectrometer (Hiden, HPR-20 QIC) was used to detect H2, N2 (and CO), CO2 and Ar. Ar was

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used as a tracer in the mass spectrometer for calibration purposes. The WGS reaction experiment

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was carried out by increasing temperature from 100 ˚C to 400 ˚C at a rate of 10 ˚C/min, using a

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feed comprising 5% CO, 3.5% H2O, 1% Ar and a balance of N2.

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2.3. In-situ DRIFTS study

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In-situ DRIFTS experiments were performed on a FTIR spectrometer (Thermo Scientific,

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6700 Nicolet) equipped with MCT detector at a resolution of 2 cm–1 and 4 scans for each

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spectrum. The total flow rate is 150 mL/min. Prior to recording DRIFT spectra, Pt-BaO/Al2O3

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catalyst was pretreated as described in Section 2.2. Background spectra were collected after the

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sample was cooled in N2 to the target test temperature. To match with activity measurements,

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DRIFT spectra were also collected during 10 min (lean) – 2 min (delay1) – 5 min (rich) – 2 min

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(delay2) cycling under the same compositions of feed gas.

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3. Results and discussion

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3.1. N2O formation mechanism in the absence of H2O

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3.1.1. N2O evolution during the NSR activity tests

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Outlet gas concentrations profiles at 150 and 250 ˚C during a stable NSR cycle are shown in

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Figure 1. At 150 ˚C (Figure 1a), a N2O peak (3.76 µmol/gcat) is found at the initial lean phase and


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the N2O intensity decreases with time, where most of continuous N2O tail (~3 ppm) comes from

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our nitric oxide cylinder. The NO and NO2 breakthroughs are immediately observed after

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gaseous NO/O2 is introduced, and the outlet concentrations of both NO and NO2 increase with

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time and tend to be constant levels. When the feed gas is switched into rich condition, NO

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release (34.6 µmol/gcat) and N2O emission (0.26 µmol/gcat) are immediately observed. During the

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delay2 phase, the N2O signal is detected 25 s after the switch from rich to delay2 phase, with

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2.82 µmol/gcat of N2O formed.

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At 250 oC (Figure 1b), 5.78 µmol/gcat of N2O is formed in lean phase, NOx breakthrough

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occurs about 20 s. In rich phase, the amounts of NO release and N2O emission reach 128.1

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µmol/gcat and 3.67 µmol/gcat, respectively. During the delay2 phase, 0.15 µmol/gcat of N2O is

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formed at the first 30 s (1020–1050 s).

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Compared with results obtained at 150 ˚C, amount of N2O formation increases during lean

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and rich phase but significantly reduces during delay2 phase at 250 ˚C. Based on the above trend

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of N2O formation during the NSR process, we conduct a set of experiments to find out how N2O

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forms in each phase.

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3.1.2. In-situ DRIFTS study of N2O formation pathway

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In-situ DRIFTS experiments were performed to understand which intermediates related to

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the N2O formation in the NSR process. Figure 2 shows the DRIFTS spectra of the surface states

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of Pt-BaO/Al2O3 in the absence of H2O during the fifth 10 min (lean) – 2 min (delay1) – 5 min

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(rich) – 2 min (delay2) cycle at 150 and 250 ˚C.

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As shown in Figures 2a and 2d, at the beginning of lean phase, several distinct bands are

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observed, which are ascribed to the residual species from the previous cycle. Band located at


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1233 cm–1 can be assigned to bridged bidentate nitrites on barium; bands at 1312 and 1539–1551

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cm–1 are attributed to bidentate nitrates on barium; bands at 1315–1319, and 1416 cm–1 are

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characteristics of ionic nitrates; band at 1471 cm–1 can be assigned to monodentate nitrates.10, 19-

128

21

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associated with the bridged and linear carbonyls of metallic platinum (Pt0-CO) are found at

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1757–1770 and 2061–2070 cm–1, respectively; bands around 2168–2171 and 2232–2241 cm–1

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are attributed to -NCO species on barium and alumina, respectively.24-28 At 150 ˚C (Figure 2a),

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NOx is mainly stored in the form of nitrites, the intensities of the bands for the bidentate nitrites

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and nitrates increase substantially with increasing the NO/O2 exposure time, while the residual

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Ba-NCO and bidentate carbonates bands increase in intensity slightly, reach a maximum and

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then decrease gradually, and Al-NCO band shows no obvious change. The intensity of the band

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for Pt0-CO is depleted rapidly. Compared to 150 ˚C, similar changes of these bands are found at

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250 ˚C (Figure 2d) with a few exceptions. The bands of Ba nitrites are not detected, moreover,

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the intensity of Ba-NCO species continuously decline.

Bands at 1557–1575 cm–1 are assigned to chelating bidentate carbonates on barium.22, 23 Bands

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After gaseous NO/O2 is introduced, both Ba-NCO and Pt0-CO species decrease.

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Simultaneously, N2O is formed (Figure 1). Thus, it is likely that N2O formation in the lean phase

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is related to the reaction between surface residual reductants (Ba-NCO and Pt0-CO species) and

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gaseous NO/O2. To further investigate the roles of NO and O2 in N2O formation, activity tests

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were performed with different compositions of feed gas: 600 ppm NO/N2; 600 ppm NO/10%

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O2/N2; 10% O2/N2 in the lean phase after a steady lean-rich cycle. And the outlet N2O

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concentrations profiles are shown in Figure 3. The amounts of N2O formation under these

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conditions at 150 and 250 ˚C follow the order: NO > NO + O2 > O2. It indicates that NO is more

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responsible for N2O formation than O2 in the lean phase.


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At 150 ˚C, the intensity for the band of Ba-NCO increases first and then decreases (Figure

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2a), it may be interpreted that some Pt0-CO react with NO leading to -NCO formation during the

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initial lean phase. Solymosi et al.14, 29 and Szailer et al.27 proposed that isocyanate was formed on

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the Pt sites and spilled over to the storage and support components (Eqs. 1 and 2):

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Pt- N+ Pt- CO ↔ Pt- NCO+ Pt *

(1)

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Pt- NCO+ M → M- NCO+ Pt*

(2)

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where M was abbreviated for the barium storage component or alumina support.

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During the lean phase, there may be two paths for Pt0-CO consumption that some participate

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in -NCO formation and the others are oxidized to CO2. Masdrag et al.30 even found that CO did

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not allow any significant NOx reduction in lean mixture, but it was fully converted to CO2 in the

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200–400 ˚C temperature range. Thereby, it infers that Ba-NCO plays a major role in N2O

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formation in the lean phase. Therefore, the following N2O formation pathways in lean phase can

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be summarized as Eqs. 3–7:

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Ba ( NCO ) 2 + 8 NO → 5 N 2 O + BaCO 3 + CO 2

(3)

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Ba ( NCO ) 2 + 3 NO → 5 2 N 2 + BaCO 3 + CO 2

(4)

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Ba ( NCO )2 + 2 O 2 → N 2 O + BaCO 3 + CO 2

(5)

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Ba ( NCO )2 + 3 2 O 2 → N 2 + BaCO 3 + CO 2

(6)

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Ba ( NCO )2 + 4 O2 → Ba(NO3 ) 2 + 2 CO2

(7)

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DiGiulio et al.10 had clarified that the reactions of isocyanate with NO and O2 were metal

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catalyzed pathways. As shown in Figure 3, the so-called comproportionation between -NCO and

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NO results in more N2O formation, where the valences of nitrogen element are –3 and +2,

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respectively. However, the amount of N2O formed by reacting -NCO with O2 decreases. Perhaps,


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oxygen has higher oxidation ability that can oxidize -NCO to high valence of N-containing

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species (e.g. nitrate). Increasing the temperature further suppresses N2O formation also supports

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this hypothesis (Figure 3b). The formed CO2 can react with BaO to form barium carbonate. And

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then carbonates can be partly replaced by nitrates as NOx storage.31 The dynamic change of

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carbonate species is in agreement with DRIFT result.

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Upon switching to the rich period with CO admission (Figures 2b and 2e), the band

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intensities for nitrites and nitrates decrease gradually, whereas barium carbonates (around 1557–

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1575 cm–1) accumulate on the catalyst, especially at 250 ˚C. Meanwhile, the bands of -NCO on

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barium and alumina as well as Pt0-CO species appear after 20 s of CO contact, and then these

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bands increase in intensity with CO exposure.

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According to the literatures,16, 27, 29, 32, 33 the following reactions can be proposed to take place

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on the Pt surface during the rich period (Eqs. 1, 8–15). Firstly, CO reacts with the adsorbed

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oxygen atom that is formed in the lean phase, thereby reduces Pt to Pt0 (Eq. 8). CO is then

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associatively adsorbed on the Pt surface (Eq. 9). In parallel, the NO released from the stored NOx

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can either dissociatively or associatively adsorbs on Pt (Eqs. 10 and 11), and the combination of

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these adsorbed species forms N2, N2O, -NCO and CO2 (Eqs. 1, 12–15).

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CO+ Pt- O → CO 2 + Pt*

(8)

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CO+ Pt * ↔ Pt- CO

(9)

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NO+ Pt * ↔ Pt- NO

(10)

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Pt- NO+ Pt * ↔ Pt- N+ Pt- O

(11)

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Pt- N+ Pt- N → N 2 + 2 Pt*

(12)

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Pt- NO+ Pt- NO → N 2 O+ Pt- O+ Pt*

(13)


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Pt- NO+ Pt- N ↔ N 2 O+ 2 Pt *

(14)

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Pt- N+ Pt- CO ↔ Pt- NCO+ Pt *

(1)

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Pt- CO+ Pt- O → CO 2 + 2 Pt *

(15)

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These formed -NCO species are readily spilled over to Ba and Al sites. In addition, according to

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the reduction by CO of nitrates stored on Ba/Al2O3, Forzatti et al.9 and DiGiulio et al.10

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suggested that small amount of -NCO could form directly on barium and alumina.

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At low temperature (e.g. 150 ˚C), rather dense CO adsorption layers can be formed on Pt

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sites,34 which inhibits the access of NO onto Pt surface and thereby decreases the dissociative or

200

associative adsorption of NO (Eqs. 10 and 11), and NO reduction is inhibited by CO. Thus, it is

201

difficult for the recombination of NOad and Nad forming N2O (Eqs. 13 and 14). At higher

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temperature (e.g. 250 ˚C), a decrease in CO adsorption makes Pt accessible to NO and enhances

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the coverage of NOad and Nad. As a result, the amounts of N2O and -NCO formation increase

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(Figures 1b and 2e).

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After switching to the delay2 phase (a period of N2 purge) at 150 ˚C (Figure 2c), the

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intensities for bands of nitrites and Pt0-CO decrease gradually, while the bands of carbonates

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increase. The band of Ba-NCO species decreases in intensity first, and then increases (at ~30 s,

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corresponding to total time of 1050 s in Figure 1a). And the intensity of Al-NCO band shows no

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significant change. At 250 ˚C (Figure 2f), the intensity of band for nitrates decrease, and the

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trends of band intensities for Pt0-CO and carbonates are similar to those at 150 ˚C. However, the

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intensity of the bands for -NCO slowly increase, reach its maximum (at~40 s, corresponding to

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1060 s in Figure 1b), and then decrease. In order to clearly observe the changes of the bands

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intensity during the delay2 phase, the spectra are displayed under common scale (see SI Figure


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S1). As previously mentioned, 2.82 µmol/gcat of N2O is observed in the delay2 phase at 150 ˚C,

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whereas only trace amount (0.15 µmol/gcat) of N2O is found in this period at 250 ˚C.

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Based on above species evolutions, the following reactions may occur. At low temperature

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(e.g. 150 ˚C), nitrites react with Pt0-CO producing -NCO and N2O (Eqs. 16 and 17). -NCO may

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also react with nitrites forming N2O (Eqs. 18).

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Ba ( NO2 ) 2 + 6 Pt- CO → Ba ( NCO ) 2 + 4 CO2 + 6 Pt*

(16)

220

Ba ( NO2 ) 2 + 2 Pt- CO → BaO+ N 2O+ 2 CO2 + 2 Pt*

(17)

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2 Ba ( NO2 )2 + Ba ( NCO )2 → 3BaO+ 3 N 2O+ 2 CO2

(18)

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At higher temperature (e.g. 250 ˚C), nitrates are the main adsorbed NOx species. The bands of

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nitrites are supposed to be overlapped by nitrates bands, and nitrates may be reduced by either

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Pt0-CO or -NCO but without N2O formation (Eqs. 19 and 20).

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Ba ( NO3 )2 + 8 Pt- CO → Ba ( NCO )2 + 6 CO2 + 8 Pt*

(19)

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3Ba ( NO3 )2 + 5 Ba ( NCO )2 → 8 BaO+ 8 N 2 +10 CO2

(20)

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Some BaCO3 are produced by CO2 reacting with BaO during these processes.

228

Alternatively, through Pt catalyzed pathway,10 the reaction of -NCO with NO to form N2O

229

(Eq. 3) cannot be ruled out due to NO and -NCO species exist simultaneously, and the intensity

230

of Pt0-CO band decreases gradually leading to more reduced Pt.

231

3.2. The effect of H2O on N2O formation

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3.2.1. NSR activity tests in the presence of H2O

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Figure 4 shows a steady state lean-rich cycle during NSR activity tests using CO as the

234

reducing agent with 2% H2O at 150 and 250 ˚C. At 150 ˚C (Figure 4a), the NO breakthrough is


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immediately observed and NO2 is observed after 60s. 1.45 µmol/gcat of N2O is formed in the lean

236

phase. Upon switching to the rich phase, 49.3 µmol/gcat of NO release and 0.18 µmol/gcat of N2O

237

emission are found. During the delay2 phase, a significantly high N2O peak (8.76 µmol/gcat) is

238

observed.

239

At 250 ˚C (Figure 4b), 17.1 µmol/gcat of NH3 is formed during the lean phase, NO and NO2

240

breakthroughs start at 40 and 60s, respectively. But N2O is not formed in the lean phase in the

241

presence of H2O. During the rich phase, the amount of NO release and N2O emission are 55.4

242

µmol/gcat and 1.10 µmol/gcat, respectively. 110.4 µmol/gcat of NH3 is subsequently formed.

243

Another considerable NH3 peak (22.9 µmol/gcat) is also found during the delay2 phase.

244

Compared to that without H2O (Figure 1), the amounts of N2O formed in lean and rich phase

245

decrease at 150 ˚C. However, more N2O is formed during delay2 phase in the presence of H2O.

246

The result of almost no N2O formation in lean and delay2 phase indicates that the effect of H2O

247

on N2O formation is more pronounced at 250 ˚C. To better understand this effect, the water-gas

248

shift reaction was measured.

249

As shown in Figure 5, WGS reaction occurs above 210 ˚C on Pt-BaO/Al2O3 sample, and H2

250

production increases monotonously with the temperature increasing. H2 concentration reaches

251

about 0.1% at 250 ˚C. Olympiou et al.35, 36 evidenced that formate (-COOH) was an important

252

intermediate during WGS reaction on Pt/Al2O3, and at least two kind of formate species residing

253

on the alumina surface. Dasari et al.37 summarized the following WGS reaction mechanism:

254

CO+ Pt * ↔ Pt- CO

(9)

255

H 2 O+ Pt * ↔ Pt- H 2 O

(21)

256

Pt- H 2 O+ Pt * ↔ Pt- H+ Pt- OH

(22)


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Pt- CO + Pt- OH → Pt- COOH+ Pt *

(23)

258

Pt- COOH → CO2 + Pt- H

(24)

259

Pt- H + Pt- H ↔ H 2 + 2 Pt *

(25)

260

They further suggested that NH3 was mainly produced by the reduction of NO by surface

261

hydrogen formed from the WGS reaction during the reduction of NO by CO in the presence of

262

excess water. In addition, less N2O is formed in the reduction of NOx by efficient H2 than CO at

263

250 ˚C in the rich phase.38

264

3.2.2. In-situ DRIFTS study in the presence of H2O

265

Figure 6 shows the DRIFT spectra recorded during NSR process with 2% H2O at 150 and

266

250 ˚C. Compared to the results in the absence of H2O (Figure 2), different evolutions of surface

267

species could be found. New bands appeared at 1328–1337 cm–1 are assigned to bridged

268

bidentate carbonate species.22, 23 Band at 1508 cm–1 is attributed to monodentate nitrate species.

269

It is difficult to assign the band at 1458 cm–1, since monodentate nitrite, monodentate nitrate and

270

ionic nitrate species on barium all have features in this area.19 During the lean phase at 150 ˚C

271

(Figure 6a), the initial band intensity of Ba-NCO is slightly higher than that in the absence of

272

H2O (Figure 2a), while the intensity of Pt0-CO band is much lower. Moreover, the time of -NCO

273

band disappeared is faster than that in the absence of H2O (2 min vs 5 min). It indicates that the

274

consumption rate of isocyanate species is faster in the presence of H2O which is due to the

275

hydrolysis of isocyanate has occurred at 150 ˚C.39 Although NH3 is a product of this hydrolysis

276

reaction,27, 40 its breakthrough is not detected during the lean phase (Figure 4a). It is most likely

277

that the rate of NH3 formation is slower than that reacting with gaseous NO/O2 at 150 ˚C.

278

Therefore, N2O production decreases during lean phase in the presence of H2O, which is caused

279

by less Ba-NCO species available to react with NO/O2 (Eqs. 3 and 5).


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280

At 250 ˚C (Figure 6d), few residual Ba-NCO and Pt0-CO species leave on the surface, and

281

isocyanate is more easily hydrolyzed at elevated temperature. A shoulder peak around 1584 cm–1

282

is assigned to residual formate.41 Then CO2 and atomic hydrogen can be formed from the

283

decomposition of formate (Eq. 27). NH3 is found to generate instead of N2O (Figure 4b). It is

284

inferred that in the lean phase NH3 is also mainly formed via the reduction of NOx by surface

285

hydrogen. Because isocyanate species are depleted in the first 10 s of lean phase. N2O is also not

286

formed via the reactions of generated NH3 with gaseous NO/O2 at 250 ˚C.

287

When switched into rich condition, the stored NOx reduction by CO at 150 ˚C (Figure 6b) is

288

similar to that in the absence of H2O (Figure 2b). Differently, Al-NCO band is not formed in the

289

presence of H2O, similar result had been reported by DiGiulio et al.10. Ji et al.11 suggested that

290

Al-NCO was more reactive than Ba-NCO toward H2O. The intensity for Pt0-CO band (2063 cm–

291

1

292

H2O on Pt site.42 The band intensity of Ba-NCO (2168 cm–1) is also lower than that in the

293

absence of H2O. Additionally, the intensity of -NCO band increases first, reaches a maximum

294

and then decreases, which is due to the coexistence of isocyanate formation and consumption.

295

Isocyanate is formed by reacting of CO and NO, meanwhile, isocyanate is hydrolyzed. The

296

amount of N2O formation is not significantly affected by H2O at 150 ˚C. It may be due to H2O

297

reduces CO coverage on Pt sites and then occupies these sites so that little change of NO

298

adsorption is affected by H2O.

) is lower than that without H2O. It is likely that there is a competitive adsorption of CO and

299

The effect of H2O on surface species evolutions is more significant at 250 ˚C (Figure 6e).

300

The band intensity of Ba-NCO (2168 cm–1) is much lower than that in the absence of H2O.

301

Because the WGS reaction could further reduce the amount of Pt0-CO species and isocyanate is

302

rapidly hydrolyzed to NH3 at this temperature. Formate band at 1587 cm–1 markedly rises at 250


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˚C. The accumulation of surface formate on the catalyst is caused by slow decomposition of

304

formate (Eq. 27) as a rate limiting step of WGS reaction.43

305

During the delay2 phase, combining NSR activity tests (Figures 1a and 4a) and DRIFTS

306

study (Figures 2c and 6c) in the absence and presence of H2O at 150 ˚C, the amount of NO

307

released and nitrites consumption are higher with H2O. These results indicate that H2O reduces

308

the stability of nitrites that is in agreement with our NOx-TPD result.44 Thus, more nitrites could

309

be reduced by Pt0-CO or isocyanate results in more N2O production. At 250 ˚C (Figure 6f),

310

the decomposition of formate extends into the delay2 phase, and leads to the formation of large

311

amount of NH3 (Figure 4b). The reason for the amount of N2O formation decreases during the

312

delay2 phase is resemble with the rich phase, and that is NOx is effectively reduced by H2.

313

Acknowledgments

314

This work was financially supported by the National Natural Science Foundation of China

315

(Grant NO. 21476170).

316

References

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Figure Captions

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Figure 1. Outlet gas evolution in NSR process over Pt-BaO/Al2O3 at: (a) 150 ˚C and (b) 250 ˚C.

435

Inlet gas composition of the lean phase was 600 ppm NO, 10% O2 in N2; rich phase, 5.2% CO in

436

N2; two delay phase, only N2.

437

Figure 2. In-situ DRIFT spectra recorded during the fifth cycle over Pt-BaO/Al2O3 without H2O:

438

(a) lean phase, (b) rich phase and (c) delay2 phase at 150 ˚C; (d) lean phase, (e) rich phase and (f)

439

delay2 phase at 250 ˚C.

440

Figure 3. Outlet N2O concentrations under different compositions of feed gas: 600 ppm NO/N2;

441

600 ppm NO/10% O2/N2; 10% O2/N2 introduced to the lean phase after a steady lean/rich cycle

442

at: (a) 150 ˚C and (b) 250 ˚C.

443

Figure 4. Outlet gas evolution in NSR process over Pt-BaO/Al2O3 at: (a) 150 ˚C and (b) 250 ˚C

444

with 2% H2O. Inlet gas composition of the lean phase was 600 ppm NO, 10% O2 and 2% H2O in

445

N2; rich phase, 5.2% CO and 2% H2O in N2; two delay phase, 2% H2O in N2.

446

Figure 5. H2 concentration profiles during temperature-programmed WGS reaction with a flow

447

of 5.2% CO/3.5% H2O/N2 from 100 ˚C to 400 ˚C at a ramp rate of 10 ˚C/min on Pt-BaO/Al2O3.

448

Figure 6. In-situ DRIFT spectra recorded during the fifth lean-rich cycle over Pt-BaO/Al2O3

449

with 2% H2O: (a) lean phase, (b) rich phase and (c) delay2 phase at 150 ˚C; (d) lean phase, (e)

450

rich phase and (f) delay2 phase at 250 ˚C.


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Elucidating N2O Formation during the Cyclic NOx ... - ACS Publications

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