Photon Factory Activity Report 2005 #23Part B (2006)
Surface and Interface

9C,10B/2004G092

Elucidation of the coordination structure around copper ion in CuMFI associatedwith N2 adsorption at room temperature
Atsushi ITADANI, Masashi TANAKA, Toshinori MORI, Eiji TABUCHI, Haruka SHINOGI,
Yasushige KURODA*, Mahiko NAGAO
Okayama Univ., Tsushima-naka, Okayama 700-8530, Japana1.52

1.0

evac. at 873 K
exposure toN2 gas at r.t.
reevac.
at r.t.

Absorption

Results and discussion
Figure1 shows the EXAFS and XANES spectra forCuMFI under various conditions. For the 873 K-treatedsample, a EXAFS band, which is ascribed to backscattering from the nearest neighboring oxygen atom, isobserved at 1.52 Å (no phase-shift correction). The
coordination number of oxygen atom around copper ion(NCu-O) and the distance between copper ion and oxygen
atom (rCu-O) were estimated to be respectively 2.7 and 1.98
Å by the analysis of the EXAFS datum employing theleast-square method and by using Cu2O as a reference
sample. The copper ion in the sample takes the twocoordinated structure (distorted linear structure) or thethree-coordinated structure (trigonal pyramidal structure).
Taking account of our previous EXAFS data, for thepresent sample, the three-coordinated copper ion seems tobe formed in higher rate. In the XANES spectrum, the+band due to the 1s–4p transition of Cu is split into twobands: the bands at 8.983 keV (1s–4pπ transition) and
8.993 keV (1s–4pσ transition). The appearance of thesebands indicates the occurrence of the reduction of theexchanged divalent copper ions into the monovalent onesby evacuating the sample at 873 K and also suggests thepresence of copper ions differing in the coordination+environment; the formed Cu species take a linear (two)or a planar (three) coordination structure. Noteworthy isthat the 8.983 keV-band is drastically reduced in terms ofintensity when the sample is exposed to an N2 gas of
about 13 kPa at r.t., and the intensity of the band isrecovered after reevacuating the sample at r.t. Change in
the intensity of the observed band in XANES spectrum
has been interpreted as adsorption of N2 on the three+coordinated Cu species. When the N2 molecule is+adsorbed on such a site, the Cu species take the fourcoordinated arrangement analogous to the structure of the
distorted tetrahedral arrangement, and by bonding with N2
molecule, the 4pπ energy level of copper ion changes,
resulting in the shift of the band toward the side withhigher energy. A strong interaction between copper ion
and N2 molecule was also seen in the EXAFS spectra for
the CuMFI adsorbing N2; the band at around 1.5 Å
increases in width and its intensity decreases, comparedwith the properties of the band for the 873 K-treatedsample. By analysis of the EXAFS datum using Cu2O and[Cu(NH3)2]+ as reference samples, a three-coordinated+Cu species in the sample was found to adsorb an N2
molecule [NCu-O = 2.7, rCu-O = 1.97; NCu-N = 1.1, rCu-N = 1.91];+the Cu species has the pseudo-tetrahedral coordination
environment. In the present XAFS experiment, detailed
coordination environment around copper ion before andafter N2 adsorption at r.t. was clarified for the first time.

Magnitude

Introduction
N2 molecule is accepted to behave as a supercritical gas
at around room temperature (r.t.) and is adsorbed on solid
surfaces, only at low temperatures. We found that acopper ion-exchanged MFI (CuMFI) adsorbs N2 molecule,even at r.t.[1]. It has so far been shown that the+monovalent copper ion (Cu ) formed by heat treatment invacuo acts as adsorption site. However, the coordination
environment of the effective site for N2 adsorption and thestate of the adsorbed N2 species on the sample are still notcompletely clarified. One of the solutions of these
problems is to evaluate the adsorption site on CuMFI
having high levels of adsorption capability for N2
adsorption at r.t. Recently, we have demonstrated that theadsorption property of N2 associated with CuMFI differs
depending on the types of counter ion in the exchangesolution and the Si/Al ratio of MFI. In this contribution,
we report on the coordination structure around copper ion
adsorbing an N2 molecule, which was evaluated by usinga XAFS technique. The targeted CuMFI sample (Si/Al:19.8) was prepared via ion exchange using an aqueous
solution of copper propionate at r.t. (ion-exchange level:109%). Such a preparation condition, determined by us, isthe optimum conditions obtainable the sample having themost efficient property for N2 adsorption at r.t.

8.983
0.2bevac. at873 K

exposure toN2 gas at r.t.
8.993

reevac.
at r.t.

0 1 2 3 4 5 6 8.96 8.98 9.00 9.02
Distance / ÅPhoton energy / keV
Figure1: (a) EXAFS and (b) XANES spectra for CuMFI.

Reference[1] Y. Kuroda et al., J. Phys. Chem. B 103, 2155 (1999).
*kuroda@cc.okayama-u.ac.jp

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