Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

7A/2001S2-003, 2003G-320

Mechanism of N+NO reaction on Rh(111) surface
studied with energy-dispersive NEXAFS
Ikuyo NAKAI, Hiroshi KONDOH, Toru SHIMADA, Masanari NAGASAKA,*Reona YOKOTA, Tetsuo KATAYAMA, and Toshiaki OHTAThe University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

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Users' Report

In the region III, a completely different reaction occurs.
Two possible paths are proposed; chemisorbed NO reacts
with N and generated N2O immediately dissociates intoN2 and O (NOa+Na→N2Oa→N2↑+Oa), or a ‘hot’ N atom
formed by the dissociation of NO reacts with an adsorbed
N atom (NOa+Na→N2↑+Oa).
References[1] F. Zaera et al., Chem. Phys. Lett. 332, 209 (2000), etc.
[2] K. Amemiya et al., Jpn. J. Appl. Phys. 40, L718
(2001).
* ohta@chem.s.u-tokyo.ac.jp

0.7
0.20

0.5
0.4

0.12

0.3
0.08

0.2

0.04
0.00

θNO [ML]θN [ML]

0.6

NON

0.16

0.1
0500

1000

1500

2000

0.0

Time [s]Fig. 1: Time evolution of the coverages of N and NO-9during the N+NO reaction at 120 K (PNO=5x10 Torr)II

6.0 IIII

5.0-3-1

Rate constant [10 s ]Introduction
Catalytic reduction of NO into molecular nitrogen on
rhodium surfaces in the presence of CO is widely used tocontrol air pollution. The most important step in the total
reaction is the conversion of atomic N into molecular N2,
but its mechanism is not clear yet. As the N-O bondscission occurs above 400 K, most of the previous studyof the reaction mechanism was conducted at hightemperatures [1]. We prepraed atomic-N covered Rh(111)surfaces and studied the reactivity with NO to understandthe mechanism of the N+NO reaction at low temperatures.
Experimental
The experiments were carried out at BL-7A with anUHV system. First, an atomic-nitrogen overlayer wasprepared on a clean Rh(111) surface; molecularly adsorbed NO was dissociated at 400 K, and generate atomic
oxygen was removed by the reaction with H2. Then the Ncovered Rh(111) surface was exposed to gaseous NO of aconstant pressure at a constant surface temperature. The
clean-off process of N with NO was monitored with N Kedge energy-dispersive NEXAFS, with which a NEXAFS
spectrum is obtained in several seconds [2]. From theobtained series of spectra, the time evolution of coverages
of N and NO was extracted by curve-fitting analysis.
Results and discussion
A typical result is shown in Fig. 1. An induction period precedes the reaction. The reaction rate was estimated from the slope of the reduction of N, and its temperature dependence was plotted in Fig. 2. The temperature dependence is classified into three regions.
The most interesting is that in the region II, the reaction
rate decreases with increasing reaction temperature,
which is contrary to the case of usual thermally activatedreactions. This indicates that a weakly bound adspecies inequilibrium with the gas-phase NO is involved in the reaction as a precursor. In this region, the total reaction isexpressed as N+NO→N2O↑, but the chemisorbed NO
molecules bound to the Rh substrate do not directly reactwith N atoms.
In the region I, the reaction rate decreases as the surface temperature decreases. The consumption of the precursor by the reaction becomes slower than its accumulation due to the low temperature. In this region, NO dimer,(NO)2 formed above an NO chemisorption layer was observed during the reaction. The observed NO dimer in thesecond layer corresponds to the precursor suggested in
the region II. It easily reacts with atomic N bound to theRh surface to form N2O below 370 K.

4.0
3.0
2.0
1.0
0.0
100

200

300

400500Temperature [K]Fig. 2: Temperature dependence of the reaction rate at the-9
initial stage of the N+NO reaction. (PNO=5x10 Torr)