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

7A/2001S2-003

New Reaction Path in Ammonia Formation on Rh(111) Surface
Masanari NAGASAKA, Hiroshi KONDOH, Kenta AMEMIYA, Ikuyo NAKAI,Toru SHIMADA, Reona YOKOTA, and Toshiaki OHTA*Department of Chemistry, Graduate School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Introduction
Rhodium is mainly used as a three-way catalyst toconvert nitrogen oxides from automotive exhausts toharmless nitrogen. Although the key step to promote thecatalytic reaction is the dissociation of NO and theformation of N2 gas, the ammonia formation from N andH is also important to understand the mechanism of therhodium catalyst. Moreover, ammonia formation is theprototypical reaction for the sequential hydrogenation.
For the ammonia formation, the N-preadsorbed Rh(111)is exposed to gaseous hydrogen at room temperature. In
the past, it was believed that the reaction is the simple
hydrogenation reaction: N + 2 H →NH2 and NH2 + H→NH3↑ [1]. However, our time-resolved NEXAFS
experiments observed a reverse reaction from NH2 toform N and 2 H. The mechanism of ammonia formation
on Rh(111) is so complicated that theoretical simulation isnecessary for full understanding.
Experimental
The experiments were performed at BL-7A under-10
ultrahigh vacuum condition (2.0 × 10 Torr). Timeresolved NEXAFS is a method to measure a NEXAFS
spectrum in several seconds [2].
The atomic nitrogen covered Rh(111) surface was firstprepared by exposing to 1.5 L gaseous NO at 150 K,
annealing at 400 K to form N and O from the dissociationof NO, and exposing to 1.4 L gaseous H2 at 400 K toremove O atoms. For the ammonia formation, the surface-7
was exposed to gaseous H2 (1.0 × 10 Torr) at 350 K.
Each N-K NEXAFS spectrum was taken in 10 sec during
H2 exposure to monitor the real-time changes in N andNH2 coverages. Note that the amount of NH3 wasobtained from the total coverage of N and NH2 because
NH3 is immediately desorbed in this temperature region.
Results and Discussion
Figure 1 shows a time evolution of N K-NEXAFS
spectra. As increasing exposure time, the peak due toatomic N (399.3 eV) disappears. The feature derived fromNH2 (408.0 eV) first increased and decreased later. Afterthe reaction, no adsorbates remained on the surface. To
obtain the coverage of each species, the spectra werefitted by the standard spectra of N and NH2. Figure 2
shows the real-time change of N, NH2, and NH3 during
the reaction. The NH3 curve means the total desorbed NH3species. The obtained changes make us gain an insight
into the reaction mechanism. At first, the N coverage

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rapidly decreases and NH2 grows until the ratio of N andNH2 becomes constant. After reaching the equilibrium,
both N and NH2 coverages decrease gradually to form
NH3 gas.
When the H2 gas was stopped during the reaction, thepeak due to N rapidly increased, while the structure due
to NH2 decreases. It clearly suggests that the new reaction
path, NH2 → N + 2 H, is opened. That is why the reaction
has the equilibrium due to this reverse reaction.
To interpret the experimental results and to understandthe reaction mechanism in more detail, atomic-levelkinetic simulation should be necessary. DFT-Monte Carlosimulations for this reaction are now underway.
References[1] R. M. van Hardeveld et al., J. Phys. Chem. B 101, 998(1997).
[2] K. Amemiya et al., Jpn. J. Appl. Phys. 40, L718
(2001).
* ohta@chem.s.u-tokyo.ac.jp

FIG. 1. N-K NEXAFS spectra at grazing incidence as a-7
function of time (350 K, H2: 1.0 × 10 Torr).

FIG. 2. Coverages of N, NH2, and NH3 as a function of time
obtained from Fig. 1. The NH3 desorbs from the surface.