Carbonization of iron-treated Loy Yang coal
Jun-ichi Ozaki
a,
*
, Yoshiyuki Nishiyama
b
, John D. Cashion
c
, L. Joan Brown
c
a
Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan
b
Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai, 980-77 Japan
c
Department of Physics, Monash University, Clayton, Victoria, Australia 3168
Received 3 October 1997; received in revised form 31 July 1998; accepted 2 September 1998
Abstract
Carbonization of untreated and iron-treated Loy Yang coal was carried out at selected temperatures in the range of 200ЊC–700ЊC. The
electrical conductivity was higher in all the iron-treated samples, except for the sample prepared at 700ЊC, which was identical. Arrhenius
plots of the conductivity of carbons treated above 500ЊC exhibited considerable non-linearity but showed that the conductivity increase was
caused by a change in the pre-exponential factor rather than in the activation energy. Mo
¨
ssbauer measurements revealed that the three,
initially dispersed, paramagnetic iron species were transformed into magnetite and then principally,
a
- and
g
-iron between 400ЊC and 600ЊC.
The electrical conductivity depended strongly on the concentration of carboxyl groups and suggested a model in which conduction between
the
p
-electron reservoirs was via an electron hopping mechanism along hydrogen-bonded cross-links. The enhanced removal of the carboxyl
groups by the presence of iron aids the destruction of these poorly conducting links during carbonization. ᭧ 1999 Elsevier Science Ltd. All
rights reserved.
Keywords: Coal; Carbonization; Mo
¨
ssbauer spectroscopy; Electrical properties
1. Introduction
The electronic properties of carbons are known [1,2] to
change drastically with carbonization below 1000ЊC. For
example, these materials show an increase in electrical
conductivity by 10 orders of magnitude in this process.
This suggests that it is possible to obtain materials that
have desirable electronic properties by controlling their
preparation. The authors have been engaged in developing
ways to prepare electronically functional carbons, such as
semiconductors [3], photoconductors [4,5] and catalytic
electrodes [6]. We have proposed three ways to control
the preparation: (a) to select or design the starting mole-
cules, (b) to modify the carbonization process, or (c) post-
treatment of carbons. As an example of the second way, we
have introduced iron into the carbonization system expect-
ing catalytic modification of the carbonization process. The
effects of iron were: (a) to increase the electrical conductiv-
ity by promoting carbonization [7], (b) to enhance the
photoresponse of junctions between the iron-containing
carbons and n-type silicon [5], and (c) to give good electro-
des comparable to platinum electrodes [6]. The state of iron
in the carbons was found to be finely dispersed and some-
times in a low valence state [5,7].
The brown coals mined in Victoria, Australia are good
sources for industrial carbon materials, as they generally
contain relatively low concentrations of inorganics. New
uses were proposed which include activated carbons, mole-
cular sieving carbons, methane storage, metallurgical reduc-
tants and high density carbons [8–10], although brown coals
are still consumed locally for power generation. Cation
exchangeable functional groups, such as carboxyl groups,
can be used for the introduction of iron into brown coals and
gives highly dispersed supported metal systems.
We have studied the effect of exchanged iron on the
carbonization of a run-of-mine Victorian Loy Yang brown
coal with particular attention to the electrical conductivity
of the carbons prepared at temperatures below 1000ЊC.
Further, the chemical changes in the coal matrix and the
state of iron were studied in order to determine the processes
that occur.
2. Experimental
2.1. Sample preparation
The Loy Yang coal was acid-washed with 0.5 mol dm
Ϫ3
Fuel 78 (1999) 489–499
0016-2361/99/$ - see front matter ᭧ 1999 Elsevier Science Ltd. All rights reserved.
PII: S0016-2361(98)00176-8
* Corresponding author. Tel.: ϩ81-277-30-1352; Fax: ϩ81-277-30-
1353; e-mail: jozaki@chem.gumma-u.ac.jp