ZhongT Jiang

There is a growing body of literature that examines the catalytic capacity of metal oxides in acting as halogen fixation agents during thermal recycling of halogenated polymers. Franklinite (ZnFe2O4) represents one of the most abundant metal oxides in electric arc furnace dust (EAFD). EAFD are emitted as unwanted by-product from crude steel manufacturing operations. A great deal of experimental work has established that EAFD captures the chlorine and bromine contents in polyvinyl chloride (PVC) and brominated flame retardants. However, the specific underlying mechanisms for the interaction of HCl/Br and other halogenated C1-C6 cuts reactions with Franklinite is still not well understood. Density functional theory (DFT) calculations were carried out in the present work to investigate the chemical interplay between of HCl/Br and selected halogenated hydrocarbons (namely as chloroethene, 1-chloro-1-propene, chloroethane, 2-chloropropane, chlorobenzene, 2-chlorophenol and their brominated counterparts) with ZnFe2O4, as a model compound for metal oxides in EAFD. A detailed kinetic analysis points out that, the adsorption mechanism of HCl/Br on a clean ZnFe2O4 surface is based on a dissociative chemisorption pathway that encompasses halogen−hydrogen bond cleavage and forming oxyhalide structures via modest activation barriers. In that course of interaction, we have demonstrated that conversion of the oxyhalide structure into zinc halides occurs via further dissociative adsorption of HCl/Br molecules by the same surface Zn atom, followed by the release of a H2O molecule via an intramolecular hydrogen transfer. The results also indicated that the catalytic removal of halogen atoms from alkanes and olefins generally proceed via two pathways; direct elimination, or dissociative adsorption followed by hydrogen transfer to the surface. Both channels assume comparable reaction rate constants in reaction networks that ensure to produce acetylene from vinyl chloride/bromide.

Thermal recycling is currently deployed as a main stream strategy in the safe disposal of materials laden with bromine, most notably the polymeric fraction in electronic and electrical waste (e-waste). Thus, it is essential to comprehend the combustion chemistry underpinning the reaction of brominated constituents with common polymeric entities. In this contribution, we systematically report reaction and activation energies for interaction of bromine atom, bromophenol and benzene with polyethylene (PE) as a model compound for polymeric materials in e-waste. Via periodic density functional theory (DFT) calculations, we map out potential energy surfaces for a large array of plausible reactions. HBr signifies the far most abundant bromine-bearing species from thermal decomposition of brominated flame retardants (BFRs) that exist in appreciable loads in e-waste. Herein, we illustrate that HBr could readily form via H abstraction from PE by a gas phase Br atom through a most activation barrier of 16 kcal/mol. Creation of an apparent radical site in the PE skeleton via this reaction frees up an active radical. Reaction of a 2-bromophenol molecule with the radical site in PE ensues in two competing channels, abstraction of the hydroxyl’s H or the aromatic Br. The latter reaction requires a lower activation barrier by about 5 kcal/mol. From another angle, we illustrate a plausible bromination mechanism that is initiated by the adsorption of a phenoxy radical over a brominated PE. Conversion of a phenoxy radical into a 4-bromophenol molecule demands a sizable activation barrier of 56 kcal/mol. Transfer of bromine atom bounded to PE to the phenoxy radical signifies a bottleneck for this bromination route. Alternatively, the radical site in a gas phase phenyl radical could abstract a Br-PE via a trivial barrier of 16 kcal/mol forming abromobenzene molecule. Formation of phenyl radicals could be brought by migration of an aromatic H from a benzene molecule to a radical site in the PE. Modelled reactions herein are useful to understand the chemistry of bromine transformation during combustion of brominated polymers in general.

Catalytic co-pyrolysis of halogenated compounds with electric arc furnace dust (EAFD) constitutes an effective disposal strategy regarding energy recovery and environmental safeguard. However, despite many detailed experimental investigations over the last few years; the specific underlying mechanism of the reactions between the halogen laden materials with EAFD remain largely poorly understood. In this contribution, systematic theoretical thermo-kinetic investigations were performed using the accurate density functional theory calculations to understand, on a precise atomic scale, the reaction mechanisms of major products from thermal decomposition of polyvinyl chloride (PVC) and brominated flame retardants (BFRs) with nanostructures (clusters and surfaces) of hematite (α-Fe2O3), zincite (ZnO) and magnetite (Fe3O4). The detailed kinetic analysis indicates that the dissociative adsorption of hydrogen halides molecules, the major halogen fragments from thermal degradation of halogen laden materials, over those metal oxide structures affords oxyhalides structures via modest activation barriers. Transformation of oxyhalides into metal halides occurs through two subsequent steps, further dissociative adsorption of hydrogen halides over the same structures followed by the release of H2O molecule. In the course of the interaction of halogenated alkanes and alkenes with the selected metal oxide structures, the opening channel in the dissociative addition route requires lower activation barriers in reference to the direct HCl/Br elimination pathways. However, sizable activation barriers are encountered in the subsequent β C-H bond elimination step. The obtained accessible reaction barriers for reactions of halogenated alkanes and alkenes with the title metal oxides demonstrate that the latter serve as active catalysts in producing clean olefins streams from halogenated alkanes.

The interplay of aromatic molecules with 3d transition metals, such as Fe and Cu, and their oxide surfaces provide important fingerprints for environmental burdens associated with thermal recycling of e-waste. Previous DRIFTS and EPR measurements established a strong interaction of the phenol molecule with the metal oxides via the formation of phenolic and catecholic intermediates. In this contribution, we comparatively examined the adsorption of phenol molecule, as a representative model for oxygen-containing components on pure metallic surfaces and their partially oxidized surfaces through accurate density functional theory (DFT) studies. In particular, it is the aim of elucidating the specific underlying mechanism of these reactions as well as to unravel the catalytic effect of these different substrates. Simulated results show that, the phenol molecule undergoes partial hydrogenation to generate phenoxy-type adduct mediated by the metallic surfaces. Investigation found that the phenol physisorbed adapts on the pure Cu and Fe surfaces are very weak; with the binding energies of -2.4 and -2.1 kcal/mol compared to -3.1 and -5.5 kcal/mol for their partially oxidized surfaces, respectively. Molecular attributes based on charge transfer and geometrical features provide an insightful explanation into these energetic trends. Furthermore, the thermo-kinetic parameters established over the temperature region of 300 and 1000 K, exhibit a lower activation energy for phenol decomposition into phenoxy group over the oxide surfaces in reference to their pure surfaces (24 and 43 kcal/mol vs 38 and 47 kcal/mol). Clearly, vacancies on pure metallic surfaces substantially reduce the activation energies required in the fission of the phenolic’s O-H bonds.

In-situ synchrotron X-ray diffraction (SR-XRD) measurement, in the temperature range of 25 - 700 °C, was carried out to ascertain the crystalline phases transformation of magnetron sputtered Cr1-xAlxN thin films. The Rietveld refinement method, using TOPAS software, was employed to analyse the crystalline phase compositions, crystallite sizes, microstrain and residual stresses of these coatings. The main crystalline phases below 600 °C were a combination of c-CrN and c-AlN while at higher temperatures the phase composition also included α-Cr with small amounts of Al2O and Al2O3. The mechanical properties (hardness and elastic modulus) of the coatings were measured by Nanoindentation techniques at room temperature. A combination of refinement results and nanoindentation data indicated that Al-dopants effectively refines the microstructure and improves the hardness and elastic modulus of the coatings in as-deposited and annealed conditions. In addition, Al dopants slow down the growth of crystallite and stress release in the coating lattice at high temperature.

Lanthanide(Ln) oxides represent an array of materials which exhibit unique properties, such as, superior mechanical, thermal, optical and magnetic properties, derived by their unfilled semicore 4f orbitals. Two forms of cerium oxide(CeO2 and Ce2O3) for instance have been the subject of numerous studies aiming to elucidate chemical and physical characteristics of their bulk and thin film properties. Cerium oxides have been widely deployed as catalysts in the preparation of active metal nanoparticles, as electrolytes or anode support materials. On the theoretical side, density functional theory(DFT) investigation has elucidated structures and electronic properties by utilizing hybrid DFT methods. Studied compounds include the A-type hexagonal structure and CeO2 with the cubic fluorite structure (space group Fm-3m). This study presents a comprehensive DFT + U account into electronic structures, mechanical properties of C-type lanthanide sesquioxides and thermodynamic (redox) properties. The aim of this work is fourfold: (1) to evaluate the effects of the Hubbard U parameter on the electronic and structural properties of C-type lanthanide sesquioxides (Ln2O3), (2) to assess the mechanical stability of all C-type lanthanide sesquioxides, (3) to elucidate the thermodynamic feasibility of CeO2 to undergo a redox reaction at temperatures relevant to catalytic applications, and (4) to underpin the effect of adding Hf and Zr to CeOδ [ δ=2-1.5] on reduction energies. We find that a Ueff value of ~ 5 eV reproduces the analogous experimental band gap of Ce2O3. Bader’s charge distributions on the C-type Ln2O3 have verified the ionic bonding nature of these compounds. Our analysis for the reduction energy of CeO2, in a wide range of temperatures, demonstrates that transfer cerium oxide between the two + 4 and + 3 oxidation states exhibit a temperature independent behavior. Preliminary results indicate that CeO2 alloyed with Hf or Zr results in enhancing its redox characteristics by lowering reduction enthalpies.

Crude steel manufacturing operations are accompanied by emission of a large amount of hazardous electric arc furnace dust (EAFD) (around 20 kg of dust per each ton of produced steel). Global production of polyvinyl chloride (PVC) plastics is projected to grow to about 180 million tons in 2021. Combined treatment of both EAFD and PVC is a promising technique for extracting valuable metals from EAFD as it is a solution to minimize their environmental harmfulness. However, despite of many detailed experimental studies over the last few years; the specific underlying mechanism of the reactions between EAFD and PVC remain largely poorly understood. This contribution provides a systematic theoretical thermo-kinetic study of the initial reactions between (Fe3O4)(111) surface, as a representative model for metal oxides in EAFD with HCl and selected chlorinated hydrocarbons, as major products from thermal degradation of PVC. Breakage of the H-Cl bond over magnetite and the formation of iron chlorides is in line with experimental findings, pointing out to degradation of organic contaminants through their reaction with magnetite. Moreover, in analogy to the well-documented role of alumina and other metal oxides, the catalytic-assisted HCl removal demonstrated herein indicates that, iron oxides serve as active catalysts in producing clean olefins streams from chlorinated alkanes. Results from this study should be instrumental to understand, on a precise atomic scale, fixation of halogens on transitional metal oxides; a viable thermal recycling approach for polymeric materials laden with halogenated constituents.