Organized mesoporous silico-nickelates (OMSiNi) and silico-lanthano-nickelates (OMSiLaNi): Crystallogenesis vs. morphogenesis and microporosity vs. pore anisotropy

Abstract

In this work organized mesoporous silico-nickelates (OMSiNi) and silico-lanthano-nickelates (OMSiLaNi) have been studied. The synthesis took place in one step using of poly-acrylic acid (Pac), complexed with cetyl-trimethyl-ammonium-bromide (C 16TAB) towards a mesostructured flexible backbone on which hydrolysis of tetra-ethyl-ortho-silicate (TEOS) and Ni(NO 3) 2, or Ni(NO 3) 2 + La(NO 3) 3, takes place at increasing pH values. The loading of nickel up to 10%, and nickel plus lanthanum up to 5% each, increases with the pH values (=5.5, 7.5, 9.5) where the materials were isolated and the same effect has the increase of the temperature of the preparation bath from RT to 323 K. The surface area of the OMSiNi and OMSiLaNi solids, containing the lower amounts of metals, was determined in the range ∼1100 and ∼970 m 2/g, respectively, and drops as the Ni and/or the Ni + La loading increases. The OMSiNi materials exhibits organized mesoporosity of MCM-41 type in nanometer scale (XRD) and remarkable morphologenesis of the Ni-doped silicate mass in micrometer scale (SEM). The introduction of La in the group OMSiLaNi results in gradual deterioration of organized porosity and destruction of morphogenesis. Both OMSiNi and OMSiLaNi solids contain NiO crystallites of 3–6 nm size (XRD) while the samples with high La content prepared at 323 K show extensive crystallogenesis of perfect NiO crystals of micrometer size (SEM). The pore anisotropy b was determined in the range 7 < b < 56,000 and is lower in the samples prepared at higher temperature and containing higher amounts of metal(s). The % microporosity of the solids is related to the log b by a relationship log b = log b 0 − k(% micro) where k is a parameter related inversely to the width of the micropore range of distribution. An explanation of this is proposed based on the assumption that the introduction of micropores, in conjunction with the width of their distribution, result in numerous interruptions of mesopore channels and as a result b decreases exponentially.