School of Physics - Theses
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ItemHeterogenous catalysis in a non-equilibrium atmospheric plasma( 2017)The long-established Haber-Bosch process for the manufacture of ammonia is not suitable for small scale production. An alternative to the Haber process is plasma catalysis, which, despite showing early signs of promise, has not yet been proven to be a viable alternative route for ammonia fabrication. This is at least in part due to our lack of understanding of the complex mechanisms underlying the plasma-catalyst interactions. Gaining such an understanding is a prerequisite step for exploiting the potential of plasma catalysis for ammonia production. In this study, by controlling the surface functionality of carbon material such as nanodiamonds and diamond-like carbon, the significant role of surface reactivity has been investigated along with a kinetic modelling in a non-equilibrium atmospheric-pressure N2-H2 plasma. The experiments used a packed-bed dielectric barrier discharge reactor, with functionalized-nanodiamond and diamond-like-carbon coatings on α-Al2O3 spheres used as catalysts. An extensive zero-dimensional model of the plasma kinetic processes, including surface interactions, was developed, and used to simulate the reactions occurring in the reactor. The results provide improved our understanding of the crucial role of surface chemical functionality in plasma catalysis and also the influence of catalytic materials on plasma characteristics. Oxygenated nanodiamonds were found to increase the production yield of ammonia, while hydrogenated nanodiamonds decreased the yield. Neither type of nanodiamond affected the plasma properties significantly. Using diffuse-reflectance FT-IR and XPS, the role of different functional groups on the catalyst surface was investigated. Evidence is presented that the carbonyl group is associated with an efficient surface adsorption and desorption of hydrogen in ammonia synthesis on the surface of the nanodiamonds, and an increased production of ammonia. Conformal diamond-like-carbon coatings, deposited by plasma-enhanced chemical vapour deposition, led to a plasma with a significantly higher electron density, and increased the production of ammonia. Moreover, the voltage at which a discharge could be sustained was significantly reduced. The detailed plasma kinetic model was applied to understand the mechanisms of ammonia synthesis in a low electron energy N2–H2 atmospheric-pressure discharge. The model considers the vibrational kinetics, including excited N2 (X, ν>0) and H2 (X, ν>0) species, and surface reactions such as those occurring by the Eley–Rideal and Langmuir–Hinshelwood mechanisms and dissociative adsorption of molecules. The predictions of the model were compared to the measured ammonia concentration produced in the packed-bed dielectric barrier discharge reactor as a function of process parameters such as input gas composition and applied voltage. The predictions of the model were found to agree reasonably well with the experimental observations. The model was used to provide a detailed understanding of the important species and reactions in ammonia formation. The dominant mechanisms differ considerably from those previously suggested for atmospheric-pressure plasmas. In particular, surface-adsorbed atomic hydrogen was found to be of much greater importance than surface-adsorbed atomic nitrogen, which has previously been postulated to be the main precursor. The surface-absorbed atomic species, were found to be predominantly generated through the dissociative adsorption of molecules. NH radicals were also found to be important, as was the surface reactivity of the catalyst material. Important differences were also identified between the important mechanisms at atmospheric pressure, and those that have been identified in typical low-pressure plasma processes. For example, under the plasma conditions considered in our work (reduced electric field in the range 30 to 50 Td, electron density of the order 10^8 cm^-3), the influence of ions was found not to be significant. Instead, the reactions between radicals and vibrationally-excited molecules are more important. A maximum production yield of 2.3 [% H_2 conv.] was obtained in the experiments presented, demonstrating the potential of plasma catalysis for small-scale ammonia production using affordable carbon coatings under atmospheric-pressure and close to ambient temperature. Although the yield is modest, it should be recalled that it is at present not possible to generate significant amounts of ammonia molecules under ambient conditions using conventional thermal equilibrium processes. There are several possible avenues for further improvement of the process. In particular, the diamond-like carbon coating allowed a plasma discharge to be initiated and sustained at a significantly lower input voltage than an uncoated catalyst. This indicates that the approach has promise as a practical method to produce ammonia on demand with an inexpensive and portable system.