Elsevier

Chemical Physics Letters

Volume 349, Issues 5–6, 7 December 2001, Pages 511-516
Chemical Physics Letters

Pulsed EPR study of the (NO)2–Na+ triplet state adsorption complex

https://doi.org/10.1016/S0009-2614(01)01250-7Get rights and content

Abstract

The (NO)2–Na+ triplet state complex stabilised in Na–A zeolites at low temperature was studied by pulsed EPR. The 3-pulse ESEEM spectra at 5 K were analysed by simulations in the time-domain, followed by Fourier transformation. The ESEEM was attributed to the interaction with Na+. The hyperfine coupling A(23Na) = (4.6, 4.6, 8.2) MHz and nuclear quadrupole Q(23Na) = (0.3, 0,3, −0.6) MHz tensors are of comparable magnitude as in the NO–Na+ complex determined earlier. The complexes are proposed to be either pairs of NO–Na+ or ON–Na+–NO.

Introduction

Studies of NO, nitric oxide, adsorption on zeolites have attracted attention because of its ability to form paramagnetic complexes with diamagnetic active sites on the surface [1], [2], [3], [4]. Some zeolites have high catalytic activity for NO decomposition [5]. It is therefore of great interest to investigate the structure of NO-surface complexes in these systems. Kasai et al. [6], [7] observed not only NO monomers, but also triplet state (NO)2 dimers in these systems.

It was concluded that the dimer was stabilised by the interaction with Na+ present in the NaA zeolite. The interaction could not be resolved in the EPR spectrum, however.

Recently, the interaction between NO and active sites in the Na–A zeolite has been probed using high field EPR and pulsed ENDOR [4]. An interaction with Na ions was observed, indicative of a NO–Na complex. A geometric model could be derived from the measured hyperfine and quadrupolar interactions with the Na nucleus. In the present work we have attempted to obtain similar information of the geometric and electronic structure for the NO–NO dimer. For this purpose pulsed EPR was used to obtain evidence for interaction with Na. Kasai's model with two adjacent NO–Na+ complexes, and a complex with two NO bonded to the same Na+ can both account for the experimental observations. An attempt to differentiate between the models on the basis of the hyperfine structure anisotropy is made.

Section snippets

Experimental

Samples of Na–A with adsorbed NO were prepared as described previously [1]. The pulse-EPR experiments have been recorded on a Bruker Elexsys E580 FT/CW X-band spectrometer equipped with a Bruker EN4118X-MD4-W1 dielectric resonator and with an Oxford CF935 cryostat for the low temperature measurements. All the spectra have been recorded at 5 K.

Three-pulse ESEEM. A sequence p1d1p1d2p1 has been used, where p1=16ns; the resulting stimulated echo, integrated by using a window of 100 ns, was

Pulsed EPR measurements

In Fig. 1 is shown the field-swept echo-detected EPR spectrum, from NO monomers and (NO)2 radical pairs in Na–A. An arrow indicates the field at which the 3-pulse ESEEM experiments have been performed. The field was chosen so that the signal is entirely from radical pairs. In Fig. 2a the FT 3-pulse spectra for different d1 values are shown. The experimental FT spectra were insensitive to the value of d1 indicating that no blind spots are present. For the simulations a choice of d1=200 ns was

Discussion

The main aim of the present study was to clarify the mechanism of pairwise bonding of NO molecules to the surface of the NaA zeolite to form radical pairs.

It was previously supposed that the NO molecules are stabilised on a pair of adjacent cations [3], [6], [7], [19] (Scheme 2a), in this case the distance between the radicals in the pair will reflect the distance between the cations on the surface. The molecules were concluded to lie approximately along a line.

The structure with the two NO

Acknowledgements

This work was made possible by the financial support of the Technical Research Council (TFR), the Swedish Royal Academy of Science and the Swedish Natural Science Research Council (NFR). D.B. acknowledges with gratitude support of the Foundation Blanceflor Boncompagni-Ludovisi, neé Bildt. A.V. acknowledges with gratitude support of the Russian Foundation for Basic Research under Grant 00-03-32441.

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