In situ environmental transmission electron microscope investigation of NiGa nanoparticle synthesis

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 [1].

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 [2]. 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 [3].

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 crystalline.

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 [4]. 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 [2].

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.