Ordered mesoporous materials as model supports to study catalyst preparation

Catalysts are indispensable to modern-day society because of their prominent role in petroleum refining, chemical processing, and the reduction of environmental pollution. The catalytically active component often consists of small metal (oxide) particles that are supported on a carrier such as silica or alumina. These particles are generally introduced via impregnation of the support with a precursor-containing solution followed by drying. Subsequent thermal treatment in air converts the precursor into the desired metal oxide, or metal when followed by high-temperature reduction with hydrogen. The choice of precursor and the experimental conditions applied during the preparation largely affect the final dispersion of the active phase. This thesis describes the use of ordered mesoporous materials as model supports, in particular SBA-15. It is demonstrated that by combining ordered mesoporous supports with (3D)-TEM and bulk characterisation techniques such as XRD and Nitrogen physisiorption fundamental insight can be obtained into the effects of the individual steps in preparation on the final dispersion of the metal (oxide) particles and the implications thereof for catalysis. The research focussed on Ni and Co ex nitrate catalysts as these are amongst the most widely applied metals and their preparation using nitrate precursors is attractive, but generally disappointing as poor metal dispersions are obtained. The outcome of a comprehensive study carried out to identify the reasons for this are described and it is demonstrated that the poor dispersions are caused by severe sintering and redistribution during the air calcination step used to convert the nitrate precursor into the respective oxide. The factors that play an important role are discussed and a novel method is presented that comprises of decomposition in NO/He atmosphere, which allows preparation of supported nickel oxide and cobalt oxide particles of 3 to 5 nm at loadings of 24 to 37 wt% and 16 to 23 wt%, respectively. The validity of this method for different systems is demonstrated and an explanation for the role of NO is proposed. Moreover, the relevance of this new method for catalysis is proven using the hydrogenation of Soybean oil and the Fischer-Tropsch synthesis of syngas into hydrocarbons over silica supported Ni and Co catalysts, respectively. It is shown that the activity of the NO/He treated catalysts is superior compared to the catalysts treated with air calcination. Finally, the results of our study on the use of ordered mesoporous materials as host for homogeneous PCP and SCS-pincer Pd catalysts are presented. It is demonstrated that the complexes were successfully tethered to the support via a trialkoxysilane moiety without destruction of the ordered mesoporous support. The activity of the SBA-15 modified with PCP-pincer Pd-complex as Lewis acid in the aldol condensation between methyl isocyanoacetate and benzaldehyde is discussed together with recycling results that show the true heterogeneous nature of the catalyst in this reaction

Metal nitrate conversion method, patent application

A method for converting a supported metal nitrate into the corresponding supported metal comprises heating the metal nitrate to effect its decomposition under a gas mixture that contains nitric oxide and has an oxygen content o

Measuring location, size, distribution, and loading of NiO crystallites in individual SBA-15 pores by electron tomography

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

Unravelling the structural and chemical features influencing deformation-induced martensitic transformations in steels

A combination of electron backscattered diffraction and high-sensitivity electron probe microanalysis was used to correlate the changes in microstructural features upon deformation with local chemical composition in transformation-induced plasticity steels. A novel cleaning procedure was developed that allows complete monitoring of transformation and deformation processes in relation to the local crystal structure, microstructure and chemical composition. Here we show direct evidence that local variations in manganese content enable a gradual transformation of the retained austenite grains

Unravelling the structural and chemical features influencing deformation-induced martensitic transformations in steels

A combination of electron backscattered diffraction and high-sensitivity electron probe microanalysis was used to correlate the changes in microstructural features upon deformation with local chemical composition in transformation-induced plasticity steels. A novel cleaning procedure was developed that allows complete monitoring of transformation and deformation processes in relation to the local crystal structure, microstructure and chemical composition. Here we show direct evidence that local variations in manganese content enable a gradual transformation of the retained austenite grains

Combining confinement and NO calcination to arrive at highly dispersed supported nickel and cobalt oxide catalysts with a tunable particle size

Control over the size and size distribution of supported nanoparticles is key to their efficient use in catalysis. In the preparation of nanoparticles by impregnation using nitrate precursors, the support pore diameter can be used to influence the average crystallite size. However, the particle size distributions obtained via this method are generally broad and the dispersions relatively low. Higher dispersions and narrow particle size distributions are obtained via thermal decomposition of the metal nitrate precursor in 1% (v/v) NO in Ar instead of air. Here we will show that by combining the confinement effect of ordered mesoporous silica with a decomposition step of metal nitrates in NO, silica supported nickel and cobalt oxides with a tunable particle size (2–4 nm) can be obtained at high loadings (10–20 wt%)

Ordered mesoporous silica to study the preparation of Ni/Si2 ex nitrate catalysts: impregnation, drying, and thermal treatments

In this contribution, we investigated the preparation of Ni/SiO2 catalysts with aqueous [Ni(OH2)6](NO3)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 Ni3(NO3)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 Ni3(NO3)2(OH)4 decomposition products (NO2, N2O, O2 and H2O), and in particular NO2 and O2 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 H2. The latter treatment prevented formation of NO2/O2 as decomposition products, moderated the decomposition rate of Ni3(NO3)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/SiO2

How nitric oxide affects the decomposition of supported nickel nitrate to arrive at highly dispersed catalysts

An explanation is put forward for the beneficial effect of thermal decomposition of supported Ni3(NO3)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(OH2)6](NO3)2 precursor solution. Moreover, combining electron tomography, XRD and N2-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 NO2, N2O, H2O 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/SiO2 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