Positron Annihilation Studies in Silica Supported Nickel Carbonate Systems G. P. Babu and V. Manohar Hindustan LeVer Research Centre, Chakala, Andheri (E), Mumbai 400 099, India K. Sudarshan, P. K. Pujari, S. B. Manohar, and A. Goswami* Radiochemistry DiVision, Bhabha Atomic Research Centre, Mumbai, Trombay 400 085, India ReceiVed: NoVember 8, 2001; In Final Form: February 27, 2002 A number of silica supported nickel carbonate systems were studied by positron annihilation lifetime spectroscopy (PAS) and nitrogen gas adsorption method (BET) to evaluate the pore size distribution in the samples. Results show that the long-lived o-Ps component gives a pore size much smaller than the average pore radius obtained from BET analysis. This indicates that o-Ps represents the pores of smaller size in a sample with wide pore size distribution. Introduction Some of the amorphous solids such as silica, alumino-silicate, aluminum phosphate, metal carbonates, and crystalline solids such as zeolites, clays, aluminum phosphates, and silicalites are known to have characteristic porosity. Such porous solids have many important industrial applications in the form of adsorbents, catalysts, and catalyst supports. Characterization of the pore structure of such solid materials is of great importance to understand the diffusion of molecules (reactants/adsorbates) and accessibility of reactants to the active sites which are located in pores of certain dimension. In commercial scale hydrogena- tion of oils/fatty acids, a highly porous silica supported nickel catalyst is used. Performance of the nickel catalyst depends on metal dispersion and availability of metal crystallite in suitable pores to the substrate (oil/fatty acid) molecule. Various tech- niques such as small angle neutron scattering 1 and BET 2 have been used to evaluate the pore structures in these catalytic materials. In recent years, the positron annihilation spectroscopy technique has emerged as a powerful probe for microstructure characterization of porous materials. 3-6 It is an in situ probe and applicable to size spectroscopy in subnanometer to nano- meter scale. This is based on the affinity of ortho-Positronium (o-Ps) species to the pores/defects in a porous material. The o-Ps species formed in the material are stabilized in the pores which are regions of lower electron density. It subsequently undergoes either pick-off annihilation with an electron from the pore wall or three-photon (3γ) annihilation with intrinsic lifetime of 142 ns. The former process reduces its lifetime to a few nanoseconds, and the rate of annihilation depends on the size of the pore. In addition, the positron annihilation technique also offers possibility of probing the electronic structure surrounding the pores. This is possible because the o-Ps species are long- lived and, therefore, can undergo reactions with chemical species present on the surface of the pore walls. 7 Hence, positron annihilation lifetime spectroscopy is considered to be a potential tool to study the porous materials. However, understanding the behavior of positronium in a complex porous chemical matrix is far from complete. The Tao-Eldrup model 8,9 has been used over the years to correlate positron lifetime and pore size in many porous materials. 7,10 However, the Tao-Eldrup model apparently fails to estimate the lifetime when pore sizes are larger than 10 Å. The apparent anomaly is attributed to various reasons such as insufficient sampling of 3γ fraction in the lifetime measurement 11 and chemical effects such as the presence of water, acidity, etc. 12-14 in different matrixes. The apparent failure of the Tao-Eldrup model for larger pores has also been explained to be due to the use of only ground state wave function of Ps. In large pores, there can easily be thermally excited states of Ps that shorten its lifetime. Ciesielski et al. 15 extended the Tao-Eldrup model to take into account this effect. Recently, Dull et al. 16 and Ito et al. 17 have attempted to modify the Tao-Eldrup model to get a proper fit of the measured PAS data with pore sizes in different porous materials. It has been pointed out 16 that the calibration of PAS lifetime data with pore size in different matrixes is important for the method to be used as a routine tool. Therefore, it will be interesting to carry out parallel measurements on pore size distribution by conventional nitrogen adsorption (BET) method and PAS and to examine the data in the light of the existing models. In this paper, we present the results of PAS studies on porous, amorphous silica supported nickel carbonate systems. These samples are the catalyst precursors. They are reduced at (400- 450 °C) to convert them into supported nickel catalysts used in the hydrogenation of oils. In these amorphous systems, reduced nickel crystallite is attached/bonded to the porous network structure of Si-O-Ni linkages. 18,19 Evidence of such bonding comes from higher reduction temperature (400-500 °C) for nickel in these compounds compared to unsupported systems (300-320 °C) 20 as well as incomplete reduction of nickel in supported systems even at 450 °C. 21,22 To investigate the correlation between o-Ps lifetimes and pore size/pore size distribution, a series of compounds having a varying SiO 2 /Ni ratio and doped with Mg and Li separately were synthesized in a controlled manner with different pore sizes. 20 An attempt has been made to correlate the results of PAL investigations with BET data. Experimental Section The silica supported nickel carbonate samples were prepared by adding separate solutions of nickel sulfate and sodium * To whom correspondence should be addressed. E-mail: agoswami@ apsara.barc.ernet.in. Fax: 91-22-5505151. 6902 J. Phys. Chem. B 2002, 106, 6902-6906 10.1021/jp014099n CCC: $22.00 © 2002 American Chemical Society Published on Web 06/14/2002