BoneChar Mediated Dechlorination of Trichloroethyleneby Green Rust
Jing AiHui MaDominique J. ToblerMarco C. MangayayamChangyong LuFrans van den BergWeizhao YinHans Christian Bruun Hansen
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Abstract:
Biochars
function as electron transfer mediators and thus catalyze
redox transformations of environmental pollutants. A previous study
has shown that bone char (BC) has high catalytic activity for reduction
of chlorinated ethylenes using layered Fe(II)–Fe(III) hydroxide
(green rust) as reductant. In the present study, we studied the rate
of trichloroethylene (TCE) reduction by green rust in the presence
of BCs obtained at pyrolysis temperatures (PTs) from 450 to 1050 °C.
The reactivity increased with PT, yielding a maximum pseudo-first-order
rate constant (k) of 2.0 h–1 in
the presence of BC pyrolyzed at 950 °C, while no reaction was
seen for BC pyrolyzed at 450 °C. TCE sorption, specific surface
area, extent of graphitization, carbon content, and aromaticity of
the BCs also increased with PT. The electron-accepting capacity (EAC)
of BC peaked at PT of 850 °C, and EAC was linearly correlated
with the sum of concentrations of quinoid, quaternary N, and pyridine-N-oxide
groups measured by XPS. Moreover, no TCE reduction was seen with graphene
nanoparticles and graphitized carbon black, which have high degrees
of graphitization but low EAC values. Further analyses showed that
TCE reduction rates are well correlated with the EAC and the C/H ratio
(proxy of electrical conductivity) of the BCs, strongly indicating
that both electron-accepting functional groups and electron-conducting
domains are crucial for the BC catalytic reactivity. The present study
delineates conditions for designing redox-reactive biochars to be
used for remediation of sites contaminated with chlorinated solvents.Keywords:
Reactivity
Carbon fibers
Carbide-derived carbon
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Some volatile chlorinated hydrocarbons such as VC, 1,1-DCE, TCE, PCE, gamma-HCH, were dechlorinated by synthesized supported nanoscale Pd/Fe. The dechlorination reactions of PCE, TCE, 1,1-DCE, VC, gamma-HCH follow the pseudo-frist order kinetics equations with the k(obs) of 2.79 h(-1), 2.35 h(-1), 1.12 h(-1), 2.14 h(-1) and 4.02 h(-1) respectively. Little or no medial products were detected and the main end products were C2H6 and C2H4 during the dechlorination of VC, 1,1-DCE, TCE, PCE. The total carbon ratio of C2H6 and C2H4 were 70% and 10% respectively during the dechlorination of TCE. The supported nanoscale Pd/Fe particles after exposed to air for 24 h were used for 8 cycle experiments and the results indicate that the particles have favorable stability. The reactivity has no obvious decrease after 200 hours' successive dechlorination experiments of gamma-HCH which indicates that the particles represent good durability. The reaction activation energy of all the chlorinated hydrocarbons are bigger than 29 kJ x mol(-1) which shows that the surface-chemical reaction rather than diffusion is the rate-limiting step in the metal-mediated dechlorination process. A consistence between the experimental data and simulated curves indicates that the muti-step reaction pathways proposed offer a better explanation of the reaction mechanisms than sequential hydrogenolysis reactions in the transformation of chlorinated hydrocarbons by supported nanoscale Pd/Fe.
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Reductive Dechlorination
Reactivity
Carbon fibers
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o-Dichlorobenzene (o-DCB) was dechlorinated by Pd/Fe powder in water through catalytic reduction. The dechlorination reaction is believed to take place on the surface site of the catalyst via a pseudo-first-order reaction. The final reduction product of o-DCB is benzene. The dechlorination rate increases with the increase of bulk loading of palladium due to the increase of both the surface loading of palladium and the total surface area. Dechlorination efficiency accounts for 90% at Pd/Fe mass ratio 0.02% and metal to solution ratio about 53.3g-L-1 in 120 minutes. Dechlorination is affected by the reaction temperature, pH, Pd/Fe ratio and the addition of Pd/Fe. Ea is found to be 102.5kJ.mol-1 in the temperature ra,nge of 287-313K.
Dichlorobenzene
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Abstract In alkaline medium, it seems that both metal‐free and iron‐containing carbon‐based catalysts, such as nitrogen‐doped nanocarbon materials, FeO x ‐doped carbon, and Fe/N/C catalysts, are active for the oxygen reduction reaction (ORR). However, the order of activity of these different active compositions has not been clearly determined. Herein, we synthesized nitrogen‐doped carbon black (NCB), Fe 3 O 4 /CB, Fe 3 O 4 /NCB, and FeN 4 /CB. Through the systematic study of the ORR catalytic activity of these four catalysts in alkaline solution, we confirmed the difference in the catalytic activity and catalytic mechanism for nitrogen, iron oxides, and Fe–N complexes, respectively. In metal‐free NCB, nitrogen can improve the ORR catalytic activity with a four‐electron pathway. Fe 3 O 4 /CB catalyst did not exhibit improved activity over that of NCB owing to the poor conductivity and spinel structure of Fe 3 O 4 . However, FeN 4 coordination compounds as the active sites showed excellent ORR catalytic activity.
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VOX/TiO2 catalysts with different vanadium loading were prepared in order to study the influence of vanadium species on the effect of water in the simultaneous NO reduction through NH3-SCR and o-DCB oxidation reactions. The presence of isolated, polymeric and crystalline species and their redox and acid properties were evaluated by N2-Adsorption, XRD, Raman, H2-TPR, XPS and NH3-TPD. Water has a bimodal and reversible effect in both NO reduction and o-DCB oxidation depending on vanadium species and temperature. In SCR, water has a detrimental effect at low temperature due to competitive adsorption with NO and NH3, while at high temperature it promotes an increase of NO conversion associated to the suppression of side-reactions, which increase the selectivity towards N2. In o-DCB oxidation, the effect of water is the sum of two contributions: one positive, related to the removal of surface adsorbed detrimental species; and one negative, associated to the competitive adsorption with o-DCB. Thus, at high temperature water acts as inhibitor, while at low temperature water has a promotional effect in the highly dispersed vanadium catalysts due to their tendency to suffer deactivation, mainly by carbonaceous materials. The presence of water also favors total oxidation and decreases the formation of chlorinated by products.
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