Nitrous oxide decomposition active site on Ni–MgO catalysts characterized by
X-ray absorption fine structure spectroscopy
Yasuo Izumi,* Takehiro Shimizu, Takaaki Kobayashi and Ken-ichi Aika
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science andEngineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan.
E-mail: yizumi@chemenv.titech.ac.jp
Received (in Cambridge, UK) 6th April 2000, Accepted 8th May 2000

A contracted Ni2+ coordination site distorted in the directionof Ni…Mg and Ni…Ni was found to be responsible forcatalytic N2O decomposition when Ni2+ is diluted in a MgOmatrix (Ni2+/Mg2+ atomic ratio ≈ 1.0) based on a local
structure study by nickel K-edge X-ray absorption finestructure spectroscopy.
Nitrous oxide is one of the gases which affects the environment.
Catalysts for nitrous oxide decomposition have not been welldeveloped compared with various catalysts for nitric oxide
decomposition.1 Nickel–MgO2 and cobalt–MgO3,4 are reported
to form solid solutions. Metal ion-exchanged zeolites have beenreported to be unstable to the effects of formed water during NO
or N2O decomposition reactions. A thermally and water-stable
Co–MgO solid solution has been applied to N2O decomposition.4 The formation of an fcc structure-based solid solution wasdemonstrated by EXAFS (extended X-ray absorption finestructure) within the range of Co atom% [i.e. Co/(Co + Mg) 3100] of 0–5. However, a severe decrease of catalyst surface area
and activity was found when the cobalt content exceeded ca. 10atom% owing to the formation of the Co3O4 spinel phase.4
Herein, the structure of Ni–MgO catalysts was investigated byEXAFS, XANES (X-ray absorption near-edge structure) andXRD (X-ray diffraction). Solid solution formation and thecorrelation between the local coordination structure of Ni ionsand N2O decomposition reactivity are discussed.
Powders were prepared by the impregnation of nickel nitrate
into MgO and were heated in air at 1173 K for 24 h and in H2
at 573 K for 1 h. With increase of nickel concentration, the BET
surface area monotonously decreased from 50 m2 g21 (MgO) to14 m2 g21 (Ni–MgO, Ni 33.6 atom%) and then remained
constant upon further increase of nickel content. The decomposition of N2O (0.1%) in helium (balance to 101.3 kPa) wasmonitored at 873 K in a flow system (flow rate 100 ml min21,0.2 g catalyst used). The steady-state formation rate of N2 (andO2) per unit BET surface area increased linearly from 2 mmol N2h21 m22 (MgO) to 51 mmol h21 m22 (Ni–MgO, Ni 50.0atom%), and then gradually decreased to 22 mmol h21 m22(NiO). The presence of nickel at the catalyst surface is essentialfor increased N2O decomposition, but an excess of nickel ( > 50atom%) was detrimental to the performance of the catalyst.

Normalized Ni K-edge XANES spectra for Ni–MgO catalysts and NiO are shown in Fig. 1A.5 The rising edge wasalways within 8339.4–8339.8 eV, demonstrating the valence
state of nickel was +2 in the range of Ni 1.5–100 atom%. As thenickel content increases, the two peaks at 8342 and 8355 eV
gradually decreased. The peaks at 8362 and 8389 eV for Ni 1.5
atom% [Fig. 1A(a)] gradually shifted, and finally reached 8365
and 8398 eV, respectively, for NiO [Fig. 1A(f)]. The peakintensity at 8389–8398 eV gradually decreased.
These experimental spectra were compared to XANES data
generated by ab initio calculations. A code FEFF8 was applied
to fcc-based model clusters within 6.0 Å from the absorbing Ni
atom in self-consistent field (SCF) and multiple scattering
calculation modes.6,7 The imaginary optical potential used in
the calculations of the exchange-correlation potential was fixed
to 1.5 eV. The XANES spectrum was generated for an isolated
Ni2+ model which contains only one Ni2+ ion at the center of a
MgO cluster matrix (Fig. 1B, solid line). The peaks at 8342,8349 (main peak) and 8389 eV in the XANES of Fig. 1A(a) (Ni1.5 atom%) were reproduced in the spectra by FEFF8, but thecalculated peak at 8359 eV did not correspond to any peaks inFig. 1A. The gradual decrease in the peaks at 8342 and 8389 eVwith increase of Ni2+ content was reproduced by FEFF8 (Fig.

Scheme 1 Ni–MgO models. Model A: Ni 25 atom% unit cell model. Model
B: Ni 50 atom% unit cell model. Half of the unit cell is drawn for B. All
XANES calculations using FEFF8 were performed for complete (undistorted) fcc coordinates.

Fig. 1 (A) Ni K-edge XANES for Ni–MgO for Ni atom% of 1.5(a), 4.4(b), 9.5(c), 33.6(d) and 50.0(e) as well as for NiO(f). (B), (C) Ni K-edge XANES
generated by FEFF8 for Ni2+ isolated in MgO (B, —) or at surface of MgO (B, -----), models A and B and for NiO (C).

DOI: 10.1039/b002773l

Chem. Commun., 2000, 1053–1054
This journal is © The Royal Society of Chemistry 2000

1053

Table 1 Ni K-edge EXAFS curve fitting results for Ni–MgO catalysts and NiO
Ni–ONi…Mg

Ni…Ni

Entry

Ni atom%NR/Å

D(s2)/1023 Å2 NR/ÅD(s2)/1023 Å2 N

a1.5

b4.4

c9.5

d33.6

e

50.0

4.7
(±1.2)
4.9(±1.2)
6.0(±0.8)
5.1(±0.9)
4.8(±1.1)
5.2(6)2.105(±0.005)
2.086(±0.005)
2.087(±0.004)
2.088(±0.005)
2.086(±0.003)2.093(2.089)

0.17(±0.93)
0(±1.1)1.1(±0.8)
1.0(±1.0)
5.3(±1.9)
1.0

2.971(±0.003)
2.95(±0.02)
2.92(±0.02)
2.918(±0.007)2.91(±0.01)

2.6(±1.4)
20.4(±1.7)
0.2(±1.3)
0(±1.2)21.3(±2.2)

12.0(±2.8)
11.2(±1.4)
10.0(±1.2)
7.6(±1.8)
7.8(±2.0)

fNiO
NiOgb
a R = ∫|k3c(obs.) 2 k3c(fit)|2dk/∫|k3c(obs.)|2dk. b According to X-ray crystallography.
f1B and C). The peak at 8359 eV in Fig. 1B shifted to 8362 eVfor models A and B (Scheme 1) and then to 8365 eV for the NiOmodel (Fig. 1C).
Based on these trends (peak position and intensity), the local
structure of Ni2+ in Ni–MgO (Ni 1.5 atom%) corresponds tomodel A. The experimental spectra for Ni–MgO (Ni 4.4 and 9.5atom%) were similar to theoretical XANES for model B. The
spectra for Ni 33.6 and 50.0 atom% [Fig. 1A(d), (e)] were moresimilar to theoretical XANES for model B than that for NiO(Fig. 1C). Each peak in the theoretical XANES spectra for thesurface isolated Ni2+ model was smeared out (Fig. 1B, dottedline), and did not resemble any spectra in Fig. 1A. Hence,impregnated nickel appears to be distributed statistically inside
the MgO matrix and less appears at the MgO surface. Althoughimaginary optical potential values and Debye–Waller factors
were varied, the shoulder peak at 8355 eV in Fig. 1A was not
reproduced in theoretical XANES spectra.8Ni K-edge EXAFS spectra for Ni–MgO catalysts wereanalyzed.9 The model parameters of Ni-O, Ni…Mg, andNi…Ni bonds were generated by FEFF8 in SCF mode. The
application of SCF FEFF8 parameters to this system wasvalidated by the curve fit for NiO (Table 1 entry f) which wasin accord with crystallographic data (entry g).10 The coordination number had an error of 12–26% for Ni–O and Ni…Mg andan error of 30–80% for Ni…Ni due to the fit procedure andFEFF theoretical parameters. For Ni–MgO catalysts, the best fit
results and fit errors based on 10–20 relatively good fits (Rf <2.0%) for each spectrum are listed in Table 1. Obtained Ni–Oand Ni…Mg distances for Ni 1.5 atom% indicate the same orslightly smaller ( < 0.007 Å) coordination distance compared tothe pure MgO framework (a = 4.2112 Å). Ni–MgO, (Ni 1.5atom%) corresponds to either model A (Scheme 1) or an
isolated single Ni2+ atom model in bulk MgO, in both cases
there is no second shell Ni…Ni bond.
With an increase of Ni content above 4.4 atom%, NNi–O andrNi–O remained almost constant while the NNi…Ni/NNi…Mg ratio
gradually increased (Table 1, entries b–e). Model B (Scheme 1)
has eight Ni…Mg and four Ni…Ni bonds, corresponding toTable 1, entries d and e. XANES generated for a model with one
to two Ni2+ at next-nearest sites (intermediate between models
A and B) was similar to XANES generated for model B (fournext-nearest Ni2+), consistent with obtained local coordinationby EXAFS (Table 1, entries b and c). The value of a according
to XRD monotonously decreased from 4.211 ± 0.002 Å (Ni 4.4atom%) to 4.190 ± 0.001 Å (Ni 50.0 atom%).11 When the Ni2+
ion concentration is nearly one per unit cell (Ni–MgO, Ni 4.4atom%), the Ni2+ site is locally contracted in an undistorted cellwith a ≈ 4.17 Å. With two Ni2+ ions per unit cell (Ni–MgO, Ni
33.6 and 50.0 atom%), the Ni2+ sites are locally contracted anddistorted in the direction of Ni…Mg and Ni…Ni (2.87–2.92 ±0.02 Å), smaller than A2rNi–O [2.950–2.953 (±0.007 Å)] by
0.03–0.08 Å.

1054

Chem. Commun., 2000, 1053–1054

R/ÅD(s2)/1023 Å2 Rfa
1.9

1.0(±0.8)
2.2(±0.9)
4.1(±1.5)
4.4(±1.3)
12.1(12)2.94(±0.01)
2.91(±0.04)2.91(±0.02)2.872(±0.007)2.954(2.954)22.2(±3.2)
0.3(±5.4)
5.8(±2.5)
7.0(±3.2)
2.8

0.92
1.7
1.4
0.71
1.2

Models with locally contracted Ni2+ sites (Table 1, entriesc–e) shows significantly different Ni…Mg and Ni…Ni distances (2.97–2.94 Å) in the whole range of Ni (0.02–100atom%) of ref 12. The lower calcination temperature of Ni–MgO samples (773 K)12 may be the reason for this discrepancy,
compared to the temperature of 1173 K used in this work and1473 K in ref 3. Solid solution formation from supported NiOparticles over MgO was suggested to proceed at > 1073 K.2
The reason for the dependence of N2O decomposition rates
on Ni2+ content in catalysts can be addressed based on thedetermined Ni2+ local structure. Contracted Ni2+ sites at thesurface are responsible for the dissociation of the N2–O bond.
As the nickel content increases, the number of contracted Ni2+
site increases and therefore the N2 formation rate per unit BET
surface area increases to Ni 50 atom%. The next elementarystep, O(ads) + O ? O2, relates to the activity decrease above Ni
50 atom%. An isotope study utilizing 18O labeling of thecatalyst surface and 16O(ads) resulted in exclusive 16O18O
formation, i.e. the step O(ads) + O(surf) ? O2. The Ni2+ local
coordination for a Ni content of 50.0 atom% (Table 1, entry e)corresponds to model B (Scheme 1). The rate of 16O18O
formation was in the order, Ni–MgO (Ni 50.0 atom%) > MgO> NiO, suggesting that surface oxygen bonded to Mg2+ is morereactive and diagonal distortion of Ni…Mg and Ni…Nidestabilizes lattice oxygen. The nitrous oxide adsorption wasweak and not detected by FTIR. Raman and IR spectroscopy
may give further insight of distorted lattice vibration around
doped Ni2+.13

Notes and references
1 M. Shelef, Chem. Rev., 1995, 95, 209.
2 F. Arena, B. A. Horrell, D. L. Cocke, A. Parmaliana and N. Giordano,J. Catal., 1991, 132, 58.
3 A. Kuzmin, N. Mironova, J. Purans and A. Sazanov, Phys. Status SolidiA, 1993, 135, 133.
4 Y. Izumi, K. Oshihara and K. Aika, Chem. Lett., 1998, 727.
5 Spectra were measured at the KEK-PF, 7C (99G259) using an Si(111)
double crystal monochromator in transmission mode at 290 K.
6 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev.
B, 1998, 58, 7565.
7 J. J. Rehr, J. Mustre de Leon, S. I. Zabinsky and R. C. Albers, J. Am.
Chem. Soc., 1991, 113, 5135.
8 Theoretical difficulties in incorporating the experimental effects ofbeamline energy resolution may be the reason for this result.
9 T. Yokoyama, H. Hamamatsu and T. Ohta, EXAFSH 2.1, The
University of Tokyo.
10 A. Corrias, G. Mountjoy, G. Piccaluga and S. Solinas, J. Phys. Chem. B,1999, 103, 10 081.
11 Based on (111), (200), (220), (311) and (222) reflections.
12 T. Yoshida, T. Tanaka, H. Yoshida, T. Funabiki and S. Yoshida, J. Phys.
Chem., 1996, 100, 2302.
13 J. M. Stencel, Raman Spectroscopy for Catalysis, Van Nostrand
Reinhold, New York, 1990.