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Catalytic performance, Thermo-elastic, surfaces and coating properties of transition metal nitrides

Jaf, Zainab (2018) Catalytic performance, Thermo-elastic, surfaces and coating properties of transition metal nitrides. PhD thesis, Murdoch University.

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Among transition metal nitrides (TMNs), refractory molybdenum nitrides have been deployed in a wide array of strategic industrial applications due to their remarkable mechanical electrical and catalytic properties. This dissertation has a two-fold aims; first to comprehensively report electronic, thermo-mechanical and electronic characteristics of the bulk and surfaces of molybdenum nitride, and secondly to construct robust mechanistic pathways for prominent catalysed reactions. Most parts of this thesis utilise the density functional theory (DFT) framework to acquire these two objectives. I have carefully benchmarked the accuracy of theoretically-obtained results against experimental quantities pertinent to investigated systems, most notably, lattice parameters of bulk phases, bulk modulus, surface relaxations, and chemical conversion values.

Atomic charge distributions and density of states enabled to illustrate the bonding nature of investigated phases of molybdenum nitrides. For instance, we found that the MoN phase largely exhibits a metallic character with strong Mo-N ionic bonds. Based on estimated Gibbs free energies, the cubic phase of molybdenum nitrides incurs higher thermodynamic stability than the hexagonal phase, with no detected phase transition in the selected T–P ranges, as experimentally observed. The elastic stiffness coefficients of MoN in the hexagonal structure indicated higher mechanical stability in reference to the cubic structure. The optical conductivity of both phases near zero-photon energy coincides well with their metallic character inferred by their corresponding density of states curves.

Motivated by the fact that, the hexagonal structure δ-MoN is a potential replacement for cubic boron nitride (c-BN) and diamond (ultra-incompressible materials), we elect to computationally study the electronic properties, thermodynamic stability phase diagram, and vacancy formation energies of all plausible atomic terminations of NiAs and WC-type configurations of δ3-MoN and δ-WN hexagonal phases, correspondingly. Various low Miller indices of surface terminations of δ3-MoN and δ-WN have been considered. Ab initio atomistic thermodynamic analysis predicts that N-terminated (111) and (100) slabs to be the most energetically favourable surface terminations amongst the explored surfaces of δ3-MoN and δ-WN, respectively. Evidenced by plotted density of states, bulk and surfaces of δ3-MoN and δ-WN display a metallic character. In terms of surface relaxation and reconstructions, most investigated surfaces experience mainly downward displacements of their topmost layers. Most notably, the relaxed Mo-termination in (111) and (100) surfaces of δ3-MoN, demonstrate significant reconstructions resulted in the first layer to be solely truncated with nitrogen atoms instead of molybdenum in the un-relaxed initial geometry. Nevertheless, no significant surface reconstruction has been noticed in most of considered δ-WN configurations. Calculated Bader’s electronic charges reveal charge transfer from Mo/W atoms to N atoms, largely retaining the ionic bond nature in their bulk phases. Moreover, computed vacancy formation energy calculations indicated that creation of surface nitrogen vacancies are a highly endothermic process.

After considering bulk and surface properties, mechanistic and kinetics aspects for several hydrogen-transfer involved reactions have been investigated. The selective hydrogenation of acetylene (ethyne), present in hydrocarbon feed into ethylene (ethene) plays a critical importance in petroleum unit operations. The dissociative adsorption of hydrogen over the γ-Mo2N surface preferentially occurs over the vacant nitrogen sites. Our calculated overall uptake rate of hydrogen molecules by the γ-Mo2N surface correlates very well with analogous experimental findings. Likewise, accessible energy barriers for hydrogen migration from a low energy site (N-vacant) to a higher energy site (atop N) accounts for the experimentally observed facile mobility of hydrogen atoms over surfaces of Mo2N. The constructed detailed mechanisms for the partial and full hydrogenation of C2H2 into C2H4 and C2H6; respectively, highlight several thermodynamic and kinetic factors that underline the selective occurrence of partial rather than full hydrogenation of alkynes over transitional metal nitrides in general. For instance, we demonstrate that a higher barrier for the initial hydrogenation step in the full hydrogenation route in comparison to the analogous step in partial hydrogenation pathway (17.4 kcal mol-1 versus 10.8 kcal mol-1) to contribute significantly in the selective occurrence of partial hydrogenation of alkynes cuts.

It is well-known that catalytic removal of the S-content from thiophenic compounds is an essential step in efforts aiming to diminish the environmental burdens of transportation fuels. We therefore investigate the hydrodesulfurization HDS mechanisms of thiophene C4H4S over γ-Mo2N catalyst via DFT calculations. We found that the thiophene molecule preferentially adsorbs in a flat mode over 3-fold fcc nitrogen hollow sites. The HDS mechanism may potentially proceed either unimolecularly (direct desulfurization) or via H-assisted reactions (hydrogenation). Due to a sizable activation barrier required for the first C−S bond scission, we predict that the direct desulfurization to contribute insignificantly in the HDS mechanism. Migration of adsorbed hydrogen atoms from the γ-Mo2N surface to the thiophene ring significantly reduces activation barrier required in the C−S bond scission. Estimated conversion values predict a 50-70% consumption of thiophene at temperature as low as 700 K and at low values of gas hourly space velocities. Our computed conversion values were in a qualitative agreement with analogous limited experimental estimates.

I also probed the reduction mechanism of p-chloronitrobenzene (p-CNB) to p-chloroaniline (p-CAN) over the γ-Mo2N (111) surface. Our findings display that, p-CNB prefers to be adsorbed over two distinct adsorption sites namely, hollow fcc and N-hollow hcp sites with relatively sizable adsorption energies. We establish that, the activation of nitro group proceeds through direct pathway along with forming several reaction intermediates. Most of these intermediaries reside in a significant well-depth in reference to the entrance channel. Central to the constructed mechanism is H-transfer steps from fcc and hcp hollow sites to the NO/-NH groups through modest reaction barriers. Our computed rate constant for the conversion of p-CNB correlates very well with the experimental finding at the temperature window of the catalytic tests. Plotted species profiles via a simplified kinetics model confirm the experimentally reported high selectivity toward the formation of p-CAN at relatively low temperatures.

In this dissertation, not only binary TMNs, but also ternary TMNs have been investigated in the form of Cr−Mo−N thin films. Closed field unbalanced magnetron sputtering ion plating (CFUMSIP) was deployed to fabricate the aforementioned films with different Mo contents. XRD results confirmed the face centered cubic (fcc) structure of pure CrN film. The incorporation of molybdenum (Mo) in the CrN matrix, however, was confirmed by both XRD and XPS analyses. The CrMoN coatings demonstrate various polycrystalline phases including CrN, γ-Mo2N, Cr with oxides layers of MoO3, CrO3, and Cr2O3. Microstructure results show that the grain size of Cr-Mo-N coatings increases with the increase of Mo content due to the formation of MoN phase, when Mo atoms interact with N atoms around the grain boundaries of CrN phase. The optical results revealed that, the synthesised coatings exhibit low reflection magnitudes in the visible region of the solar spectrum, introducing them as good antireflection surfaces. Mo-containing samples improved solar absorptance of about 76% was recorded in the wavelength range of 200 – 800 nm. However, low thermal emittance of about 20% was obtained at 2500 – 15000 nm. Measured experimental and DFT computed values exhibited absorption coefficients (α) with a matching trend, across the wavelength range of 200 to 800 nm.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor(s): Altarawneh, Mohammednoor and Jiang, Zhong-Tao
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