1. Computational Methodology
Catalytic effectiveness of franklinite (110) surface toward recycling of the halogenated waste steam
Introduction
Results
Oday H. Ahmed1, 2, Mohammednoor Altarawneh1, Mohammad Al-Harahsheh3, Bogdan. Z. Dlugogorski1 and Zhong-Tao Jiang1
The first step reaction of HBr/Cl molecules characterises strong molecular physisorption states of the two molecules evidenced by sizable binding energies of 6.5 and 15.4 kcal mol-1 for the two surface-molecule adducts M1(Br) and M1(Cl); respectively.
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
1School of Engineering and Information Technology, Murdoch University, Murdoch, WA 6150, Australia
2 Department of Physics, College of Education, Al- Iraqia University, Baghdad, Iraq
3 Department of Chemical Engineering, Jordan University of Science and Technology, Irbid 22110, Jordan
Dissociation of HBr/Cl molecules demands modest activation barriers of 32.7 and 37.3 kcal mol-1, correspondingly (in reference to their physisorbed states).
TS /32.6
Br/Cl +
However, the specific underlying mechanisms for the interaction of HBr/Cl and other halogenated C1-C6 cuts reactions with Franklinite is still not well understood.
The catalytic capacity of franklinite toward uptake of gas phase HBr/Cl becomes evident when contrasting the low energy barriers (i.e and 37.3 kcal mol-1) with the energy requirements for the uncatalysed bond fission of Br/Cl-H bonds at 87.1 versus kcal mol-1 M4 M3
Br/Cl
Br/Cl
TS /27.0
Br/Cl
Density functional theory (DFT) calculations were carried out in the present work to investigate the chemical interplay between of HBr/Cl and selected halogenated hydrocarbons (namely as chloroethane, 2-chloropropane, and their brominated counterparts) with ZnFe2O4, as a model compound for metal oxides in EAFD.
Conversion of ZnFe2O4 into FeBr/Cl(n=2-3) occurs through two subsequent steps, further dissociative adsorption of HBr/Cl molecules on O-Zn linkages and the release of water molecules.
Decomposition of the selected halogenated compounds over ZnFe2O4 surface can take place through the direct scission of C-Br/Cl bond in the adsorbed radicals, followed by ß- hydride elimination
The direct scission of C-Br/Cl bond is found to occur via a barrierless interaction
The second step signifies the formation of bromine/chlorine-free olefins by activating the carbon-hydrogen bond characterised by the transfer of a hydrogen atom to a surface oxygen via a β-hydride elimination step +
M /-6.1
M /-42.0 M2
Br/Cl
Br/Cl M5
HBr
2.38Å
Br/Cl
Br/Cl
HCl
2.30Å
Oxygen atoms
Hydrogen atoms
Iron atoms
HCl/Br atoms
Zinc atoms -
TS /37.3
Br/Cl
Computational Methodology
M /-58.2
M /2.0
Br/Cl
Br/Cl
Side view for the first four layers of franklinite (110) surface
HBr
1.46Å
HCl
1.39Å -
All structural and energetic calculations were performed using the Vienna ab initio simulation package (VASP) based on density functional theory (DFT) under the generalised gradient approximation (GGA) with the exchange-correlation functional proposed by Perdew and Wang (PW91). + M1 M6
Br/Cl
Br/Cl
HBr
1.43Å
HCl
1.29Å
Top view
M /-9.3
M /-15.4
Br/Cl
Br/Cl
Side view
M /-5.2
M /-2.5
Br/Cl
Br/Cl
M /-27.0
M /-21.0
Br/Cl
Br/Cl
TS5
TS4
Br/Cl
Br/Cl
11.2/15.1
29.2/15.0
M /-22.5
M /-41.3
Br/Cl
Br/Cl
Arrhenius plots for reactions of franklinite (110) surface with HCl molecules
Reaction of Chloro/bromoethane with franklinite (110) surface
Reaction of Chloro/bromoethane with franklinite (110) surface
Arrhenius plots for reactions of franklinite (110) surface HBr molecules
Acknowledgement
This study has been supported by the National Computational Infrastructure (NCI), Australia and the Pawsey Supercomputing Centre in Perth, as well as funds from the Australian Research Council (ARC). O.A thanks the higher committee for education development in Iraq (HCED) for the award of a postgraduate scholarship.