Mechanistic Studies of Thermal Decomposition of Nickel-Gallium Layered Double Hydroxides (Ni-Ga LDHs)

Layered double hydroxides (LDHs) represent one of the most technologically promising catalysts due to their low cost and relative ease of preparation. The large number of possible combinations available due to the numerous synthetic methods, as well as a broad variety of metals that could be used, make the LDHs suitable for a variety of catalytic applications. The purpose of this project is to explore the structure and the properties of nickel-gallium layered double hydroxides (Ni-Ga LDHs), and to investigate their potential application in catalytic dry reforming of methane. Therefore, the ultimate goal would be to explore the feasibility of using Nickel-Gallium LDHs as stable catalysts to facilitate the transformation of carbon dioxide and methane into syngas, which is a useful mixture of carbon monoxide and hydrogen. Syngas production currently relies on coal gasification, a harmful process for the environment. The choice to use nickel and gallium as the major components was based on the fact that nickel and gallium are highly selective and efficient to activate methane and carbon dioxide respectively.

A series of nickel-gallium layered double hydroxides (LDHs) with three different nickel-gallium molar ratios (3:1, 1:1, 1:3) were synthetized by hydrothermal method and sol-gel method. The hydrothermal method requires prolonged heating at a relatively high temperature (above 100 ⁰C) in autoclaves, while the sol-gel method requires overnight stirring at a moderate temperature (around 50-60 ⁰C).

A variety of characterization methods, including thermogravimetric-differential thermal analysis (TG/DTA), mass spectrometer (MS), and Fourier transform infrared spectroscopy (FTIR), were applied to investigate physical structures of the prepared nickel-gallium LDHs, as well as the decomposition mechanisms. Mechanistic studies revealed that the decomposition processes underwent three separate steps, namely dehydration (around 200 ⁰C), followed by dehydroxylation (around 300 ⁰C), and then decarbonation (around 400 ⁰C). Furthermore, different heating rates (2.5, 3.5, 5, 6.5, and 7.5 ⁰C/min) were applied to calculate the activation energies for different steps regarding carbon dioxide and water, and elucidate kinetics models. The average value of activation energy ranged from 60 to 90 kJ/mol for water, and from 130 to 200 kJ/mol for carbon dioxide. As-prepared samples were also calcined in air at three different temperatures (200, 260, and 355 ⁰C), and then characterized by thermogravimetric-differential thermal analysis (TG/DTA). Results suggested the presence of water in both the interlayer region and in the layer surface, as well as in the layer itself.