In Situ IR Characterization of CO Interacting with Rh Nanoparticles Obtained by Calcination and Reduction of Hydrotalcite-Type Precursors

Supported Rh nanoparticles obtained by reduction in hydrogen of severely calcined Rh/Mg/Al hydrotalcite-type (HT) phases have been characterized by FT-IR spectroscopy of adsorbed CO [both at room temperature (r.t.) and nominal liquid nitrogen temperature] and Transmission Electron Microscopy (TEM). The effect of reducing temperature has been investigated, showing that Rh crystal size increases from 1.4？nm to 1.8？nm when the reduction temperature increases from 750°C to 950°C. The crystal growth favours the formation of bridged CO species and linear monocarbonyl species with respect to gem-dicarbonyl species; when CO adsorbs at r.t., CO disproportionation occurs on Rh and it accompanies the formation of (CO)2. The role of interlayer anions in the HT precursors to affect the properties of the final materials has been also investigated considering samples prepared from silicate-instead of carbonate-containing precursors. In this case, formation of (CO)2 and CO disproportionation do not occur, and this evidence is discussed in terms of support effect. 1. Introduction Hydrotalcites or layered double hydroxides (LDH’s) belong to a large class of natural and synthetic anionic clays. Although they are less diffuse in nature than cationic clays, they can be easily synthesized [1]. Hydrotalcites-type (HT) compounds have the general formula: The cations are present in a layer with brucite-type structure [Mg(OH)2], in which part of Mg2+ is replaced by Al3+ and, as a consequence, the positive additional charge of the cations is balanced by the insertion of anions between layers. Usually carbonates are used as anions, even if samples with silicates have shown higher thermal and textural stability [2]. The mixed oxides obtained after calcination of HT phases at temperature above 500°C are very useful for a wide range of applications like antiacids, anion exchanger, adsorbents, catalysts, and catalyst supports because of their undeniable advantages as high surface area and structural stability [3]. A further increase of the calcination temperature above 750°C gives rise to the segregation of the stoichiometric spinel phase and the formation of less defective MgO-type phase, giving rise to stable supports or catalysts for high temperature processes. In particular, the modification of the catalytic properties is possible by the substitution of part of Mg with other bivalent cation as Ni, Pt, Pd, Co, and/or part of Al ions with Rh, Fe, Cr obtaining a high variety of formulation [3, 4]. The structure and surface properties of Mg-Al HT phases and of the resulting mixed