The Preparation of Supported NiO and Co₃O₄ Nanoparticles by the Nitric Oxide Controlled Thermal Decomposition of Nitrates

Abstract

Metal (oxide) nanoparticles smaller than about 20 nm have received widespread interest recently because of their envisioned applications in electronics, optics, and magnetic storage devices.[1] They are currently used as catalysts for the production of fuels and chemicals and the reduction of environmental pollution.[2] High surface-to-volume ratios are important for these particles since catalytic processes take place at the metal (oxide) surface; therefore supports such as SiO 2 and Al 2 O 3 are generally used to obtain small and thermally stable particles. Furthermore, the use of inert matrices allows the design of materials for specific applications, such as drug-delivery systems. [3] Small particles on a support material can be obtained by deposition from the vapor or liquid phase, [4] and the most widely used method is based on impregnation of a porous support with a precursor-containing solution, followed by drying. Subsequent thermal treatment in air converts the precursor into the desired metal oxide or metal if followed by high-temperature reduction. Particles with diameters of 1-3 nm can be deposited from organic precursor complexes, but their limited solubility allows only moderate loadings ( 10 wt %) by single-step impregnations;[5] therefore inorganic salts are typically used to achieve higher metal oxide loadings. Nitrates, in contrast to chlorides and sulfates, are the most commonly used salts, because they can be fully converted into the corresponding oxides. However, supported metal oxides prepared from nitrates generally display relatively large particle sizes. [5][6][7] Herein we present a new method that allows the preparation of uniform and small metal oxide particles based on impregnation with aqueous metal nitrate solutions. We describe nickel on silica as an example, but also show the relevance of this method for other systems. Moreover, the significance of these nanoparticles for catalysis is illustrated by the activity of Co/SiO 2 in the Fischer-Tropsch synthesis of hydrocarbons. 3 ½NiðOH 2 Þ 6 ðNO 3 Þ 2 ðaqÞ T¼120 C

A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer–Tropsch Catalyst

“…This shows the beneficial impact of NC on the Co 3 O 4 crystallite size, which is in line with previous findings [5,6,31].…”

“…[1][2][3][4] A recent example of improving the catalytic activity using this methodology is provided in a previous contribution from our laboratory, where a cobalt-on-silica catalyst was synthesized by an impregnation and drying step, followed by calcination of cobalt nitrate precursor in a flow of NO/He [5,6]. This method yielded a surfaceweighted cobalt particle size of *5 nm with a narrow particle size distribution.…”

“…After drying, the mixed nitrate samples were calcined in a flow of 1 vol% NO/He [5,24]. Please note that this calcination method is key to obtain cobalt particles with a narrow size distribution for the unpromoted Co/SiO 2 catalyst.…”

Copper Nitrate Redispersion To Arrive at Highly Active Silica-Supported Copper Catalysts

“…24, 29 We have previously reported that agglomeration of cobalt and nickel nitrate can effectively be prevented by replacing traditional air calcination with thermal treatment in 1% (v/v) NO/He flow, resulting in high NiO and Co 3 O 4 dispersions. 30,31 For silicasupported copper nitrate, it was reported that the drying step is of vital importance to the particle size distribution. 5 Drying at elevated temperatures resulted in agglomeration during the hydrolysis of copper nitrate hydrate to copper hydroxynitrate.…”

Effect of Calcination Conditions on the Dispersion of Cobalt Over Re, Ru and Rh Promoted Co/γ-Al2O3 Catalysts

“…metals and supports used, the size of the supporting particles, dispersion of the active metal on the surface of the support [10,11,12]. The selectivity is related to the properties of the catalyst used but also to a high degree dependent of process parameters used like pressure, temperature, gas composition and gas flow.…”