In this contribution, we investigated the preparation of Ni/SiO 2 catalysts with aqueous [Ni(OH 2 ) 6 ](NO 3 ) 2 solutions via the impregnation and drying method using ordered mesoporous silica SBA-15 (mesopore diameter of 9 nm) as model support to study each step in the preparation: impregnation, drying, calcination, and reduction. After impregnation, not all the mesopores of SBA-15 appeared filled with precursor solution. Consecutive drying led to formation of 9 nm Ni 3 (NO 3 ) 2 (OH) 4 crystallites exclusively within the mesopores. During air calcination, severe sintering and redistribution took place, resulting in a low NiO dispersion, including large NiO crystals outside of the mesopores and rodlike NiO particles inside the mesopores. The degree of sintering depended on the concentration of Ni 3 (NO 3 ) 2 (OH) 4 decomposition products (NO 2 , N 2 O, O 2 and H 2 O), and in particular NO 2 and O 2 were found to promote sintering and redistribution. Therefore, maintaining low concentrations of the latter components during the thermal nitrate decomposition is advocated, which was achieved by carrying out the treatment in the presence of H 2 . The latter treatment prevented formation of NO 2 /O 2 as decomposition products, moderated the decomposition rate of Ni 3 (NO 3 ) 2 (OH) 4 into NiO as observed from in situ XRD experiments, and led to NiO particles of 3 nm on average at a loading of 20 wt % Ni/SiO 2 .

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

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An explanation is put forward for the beneficial effect of thermal decomposition of supported Ni 3 (NO 3 ) 2 (OH) 4 in NO/He flow (0.1-1 vol%) that enables preparation of well-dispersed (3-5 nm particles) 24 wt% Ni-catalysts via impregnation and drying using aqueous [Ni(OH 2 ) 6 ](NO 3 ) 2 precursor solution. Moreover, combining electron tomography, XRD and N 2 -physisorption with SBA-15 support yielded a clear picture of the impact of air, He and NO/He gas atmospheres on NiO shape and distribution. TGA/MS indicated that NO 2 , N 2 O, H 2 O products evolved more gradually in NO/He. In situ XRD and DSC revealed that NO lowers the nitrate decomposition rate and appears less endothermic than in air supposedly due to exothermic scavenging of oxygen by NO, which is supported by MS results. The Ni/SiO 2 catalyst prepared via the NO-method displayed a higher activity in the hydrogenation of soybean oil as the required hydrogenation time decreased by 30% compared to the traditionally air calcined catalyst.

By the combination of electron tomography with image segmentation, the properties of 299 NiO crystallites contained in 6 SBA-15 pores were studied. A statistical analysis of the particle size showed that crystallites between 2 and 6 nm were present with a distribution maximum at 3 and 4 nm, for the number-weighted and volume-weighted curves, respectively. Interparticle distances between nearest neighbors were 1-3 nm with very few isolated crystallites. In the examined pores, a local loading twice the applied average of 24 wt % NiO was found. This suggests that a very high local loading combined with a high dispersion is achievable.