In an energy system based around decentralized hydrogen production, methanol synthesis under lower
pressure conditions could be a way to store hydrogen on location. In the search of catalysts that might open up
new process, conditions studies based on density functional theory (DFT) calculations have predicted a nickel
gallium alloy to be active for this reaction . NiGa catalysts prepared by incipient wetness impregnation on a
high surface area silica support (Saint-Gobain NorPro), using a solution of nickel and gallium nitrates have
shown very promising results . This work presents detailed Environmental Transmission Electron
Microscope (ETEM) investigations of synthesis of NiGa nanoparticles on a thin film support.
Samples were prepared by dissolving Ni(NO3)2 and Ga(NO3)3 in a Ni:Ga ratio of 5:3 in millipore water. The
solution was subsequently dispersed on transmission electron microscope (TEM) sample grids. The sample grid
was then mounted in a TEM heating holder and inserted in a FEI Titan ETEM with imaging Cs corrector as well
as facilities for in situ gas reactions . The ETEM was operated at 300 kV. The synthesis was performed in
situ in a H2 flow of 2 Nml/min at a pressure of 130 Pa. The reaction was investigated from room temperature
(RT) to 660°C by subsequently obtaining bright field TEM images, diffraction patterns (DP), High Resolution
TEM (HRTEM) images, and Electron Energy Loss Spectroscopy (EELS) data.
Figure 1 shows bright field images of the sample during synthesis. The dispersed nitrate salts (A) starts to
decompose around 300°C (B). From 400°C to 660°C (C) NiGa nanoparticles are formed. The particle diameter
at 660C was between 5 nm and 20 nm. From HRTEM and DP it is observed that the nanoparticles are
Figure 2(A) shows a particle at 660°C with two overlapping crystal domains. The insets show the fast fourier
transform (FFT) of the overlapping crystals (FFT1) and single crystal area (FFT2), respectively. The FFT2
resembles the orthorhombic Ni5Ga3 viewed along the [1 1 -4] zone axis . Figure 2(B) shows EELS of a single
particle at 660°C. Both Ni and Ga edges are observed in the spectra. Quantification of Ni:Ga ratio is hampered
by the presence of the Ni L1 edge.
The ETEM experiments have been supported by complementary in situ X-Ray Diffraction (XRD)
measurements on synthesis of Ni5Ga3 catalyst on a high surface area silica support prepared by wet
impregnation . Although the in situ XRD was performed at significantly higher H2 flow (40 Nml/min) and
pressure (100 kPa) the complimentary data correlates with the main temperature dependence of phase and
structure and shows formation of the Ni5Ga3 phase for temperatures higher than 300°C.
|Conference||Microscopy Conference MC 2011|
|Period||28/08/2011 → 02/09/2011|
- NiGa alloys
- In situ XRD