Factors affecting CO oxidation over nanosized Fe2O3

Article abstract of DOI:10.1016/j.materresbull.2006.07.009

Nanocrystallite iron oxide powders with different crystallite sizes were prepared by co-precipitation route. The prepared powders with crystallite size 75, 100 and 150 nm together with commercial iron oxide (250 nm) were tested for the catalytic oxidation of CO to CO₂. The influence of different factors as crystallite size, catalytic temperature and weight of catalyst on the rate of catalytic reaction was investigated using advanced quadrupole mass gas analyzer system. It can be reported that the rate of conversion of CO to CO₂ increased by increasing catalytic temperature and decreasing crystallite size of the prepared powders. The experimental results show that nanocrystallite iron oxide powders with crystallite size 75 nm can be recommended as a promising catalyst for CO oxidation at 500 °C where 98% of CO is converted to CO₂. The mechanism of the catalytic oxidation reaction was investigated by comparing the CO catalytic oxidation data in the absence and presence of oxygen. The reaction which was found to be first order with respect to CO is probably proceeded by adsorption mechanism where the reactants are adsorbed on the surface of the catalyst with breaking O{single bond}O bonds, then CO pick up the dissociated O atom forming CO₂.

Full text of DOI:10.1016/j.materresbull.2006.07.009

Materials Research Bulletin 42 (2007) 731–741
www.elsevier.com/locate/matresbu
Factors affecting CO oxidation over nanosized Fe2O3
b
a
b
K.S. Abdel Halim a, , M.H. Khedr , M.I. Nasr , A.M. El-Mansy
*
a Central Metallurgical Research & Development Institute (CMRDI), P.O. Box 87, Helwan, Cairo, Egypt
b Materials Chemistry Department, Faculty of Science, Beni Suef University, Egypt
Received 29 May 2006; received in revised form 9 July 2006; accepted 20 July 2006
Available online 1 September 2006
Abstract
Nanocrystallite iron oxide powders with different crystallite sizes were prepared by co-precipitation route. The prepared
powders with crystallite size 75, 100 and 150 nm together with commercial iron oxide (250 nm) were tested for the catalytic
oxidation of CO to CO2. The influence of different factors as crystallite size, catalytic temperature and weight of catalyst on the rate
of catalytic reaction was investigated using advanced quadrupole mass gas analyzer system. It can be reported that the rate of
conversion of CO to CO2 increased by increasing catalytic temperature and decreasing crystallite size of the prepared powders. The
experimental results show that nanocrystallite iron oxide powders with crystallite size 75 nm can be recommended as a promising
catalyst for CO oxidation at 500 8C where 98% of CO is converted to CO2. The mechanism of the catalytic oxidation reaction was
investigated by comparing the CO catalytic oxidation data in the absence and presence of oxygen. The reaction which was found to
be first order with respect to CO is probably proceeded by adsorption mechanism where the reactants are adsorbed on the surface of
the catalyst with breaking O–O bonds, then CO pick up the dissociated O atom forming CO2.
# 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructure; A. Oxides; B. Chemical synthesis; D. Catalytic properties; D. Microstructure
1. Introduction
CO is a very harmful gas where it strongly binds to the iron atom in blood hemoglobin and so the hemoglobin
becomes incapable of releasing oxygen. A sufficient exposure to carbon monoxide can reduce the amount of oxygen
taken up by the brain to the point that the victim becomes unconscious, and can suffer brain damage or even death from
anoxia. Catalytic oxidation of CO to CO2 is a good solution to solve such serious environmental problem since CO2 is
much less harmful compared to carbon monoxide.
Currently, the oxidation of CO to CO2 is catalyzed over noble metals as platinum and gold. The cost of noble metals
does not make it economically feasible to be used in the catalytic oxidation of CO. This had lead to searching for other
catalysts which are cheaper and more abundant. Nanosized materials are characterized by unique, essential and
important applications especially in the field of catalysis. An important class of the materials that are rich in oxygen
and yet contain O atoms with no O–O bonds are the metal oxide such as iron oxide, Fe2O3 [1–3].
The catalytic oxidation of CO over nanoparticle materials was recently investigated [4–11]. Li et al. [4], studied the
removal of CO by iron oxide nanoparticles where Fe2O3 nanoparticles were characterized both as a carbon monoxide
* Corresponding author. Tel.: +20 2 5010642; fax: +20 2 5010639.
E-mail address: khaledsaad@cmrdi.sci.eg (K.S. Abdel Halim).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.07.009

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catalyst and as a carbon monoxide oxidant. It was found that the small particle size and the FeOOH content
presented in Fe2O3 contribute to its high performance as a CO catalyst, which was indicated by a reaction rate of
19 sꢀ1 mꢀ2 at 300 8C. The effect of catalytic activity of Au/Fe2O3 at low temperatures on a CO oxidation reaction
was investigated by Tripathi et al. [6]. Adsorption and changes in enthalpy were determined for the interaction of
CO, O2 or CO + O2 (2:1) pulses over Au/Fe2O3, Fe2O3 and polycrystalline gold catalysts between 28 and 179 8C.
The results demonstrate that the oxidation of CO on both Fe2O3 and Au/Fe2O3 occurs by means of similar redox
mechanisms involving the removal and replenishment of lattice oxygen, where the presence of gold promotes these
processes.
In the present study, catalytic oxidation of CO over nanocrystallite Fe2O3 was investigated. The influence of
different factors as crystallite size, catalytic temperature and mass of catalyst on the rate of catalytic reaction was
investigated using advanced quadrupole mass gas analyzer system. Furthermore, the experimental data were used to
study the mechanism of the catalytic oxidation reaction.
2. Experimental
Iron oxide samples with different crystallite sizes were prepared by wet technique using highly purified FeCl3 and
NH4OH solution. The precipitate was dried at 120 8C for 24 h. The produced powders were fired in a muffle furnace
fitted with an on/off temperature controller. The temperature sensor was 13% Pt/Pt–Rh thermocouple. The maximum
deviation of the thermocouple reading from the set point was 20 at 1300 8C. Samples were placed in high alumina
refractory holders then introduced in the hottest zone of the furnace. After proper soaking for a period of 3 h at 600 8C,
the powder was left to cool gradually to room temperature to avoid cracking due to thermal shocks. The samples were
thoroughly mixed in a ball mill at different times 24, 2 and 0 h to control the crystallite size at 75, 100 and 150 nm,
respectively.
Phase identification and crystallite size of the products were determined at room temperature by X-ray diffraction
˚
(XRD, Bruker axs D8, Germany) with Cu Ka (l = 1.5406 A) radiation and secondary monochromator in the range 2u
from 108 to 808. Crystallite size is automatically calculated from X-ray diffraction data using the diffraction peaks and
a computer software TOPAS 2 which applied Scherer’s formula [12].
The experimental apparatus of CO catalytic oxidation used pure CO gas obtained by passing CO2 over coal in a tube
furnace at 1100 8C. The produced CO is purified by soda lime to absorb any traces of CO2 then passed over silica gel to
remove moisture. Certain weight (0.3, 1, 2 and 3 g) of the prepared iron oxide samples were held between two pieces
of quartz wool in a tubular horizontal flow reactor (length 120 cm, i.d. 1.5 cm). The reactor was heated in a tube
furnace up to the required catalytic oxidation temperature. The sample was kept at that temperature for about 30 min
before the onset of the experiment to refresh the catalyst. CO/O2/N2 gas mixture was passed over the catalyst at a
constant rate. The starting composition of the gas mixture is 24% CO, 38% O2 and 38% N2. Monitoring of the
individual gas concentration in the gases mixture during measuring the catalytic activity of the samples is performed
using gas analyzer (QMS sample analysis HPR 20, Hiden-analytical, Warrington, UK).
3. Results and discussion
3.1. Crystal structure
Three types of nanocrystallite iron oxide powders were prepared in different crystal size values, namely 75, 100 and
150 nm for ball milling for 24, 2 and 0 h, respectively. A commercial iron oxide powder (ADWIC, 97%) with crystal
size 250 nm is used in the present study to test the catalytic behavior of the commercial samples. X-ray diffraction
patterns of the prepared powders are summarized in Fig. 1. The most abundant phase identified in all prepared samples
was Fe2O3. The narrow and high intensity peaks of samples show that the prepared powders are highly crystallized.
Fig. 2 shows photomicrographs of Fe2O3 powders with different crystallite size indicating two types of aggregates
with different grain sizes. One type of grains consists of dense matrix (light gray) constituting the majority of the
sample. The second type consists of smaller grains (dark gray), which is more likely to be maghamite and also in
nanosize range but more agglomerated.
TEM images for Fe2O3 samples with crystal size 75 and 100 nm are shown in Fig. 3. It was observed that iron oxide
powder was most probably formed in agglomerated and separated iron oxide nanosize.

K.S. Abdel Halim et al. / Materials Research Bulletin 42 (2007) 731–741
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Fig. 1. XRD patterns of nanocrystallite iron oxide powders with different crystal sizes: (a) 75 nm, (b) 100 nm and (c) 150 nm.
3.2. Effect of crystal size on the CO oxidation
Catalytic oxidation tests were carried out in the simulated reactor to study the effect of crystal size of the prepared
iron oxide powders on the removal of CO by catalytic oxidation reaction. The oxidation tests were investigated
isothermally through the oxidation of CO to CO2 as a function of crystal size of the prepared samples in the
temperature range 200–500 8C. Before the catalytic tests, iron oxide was heated for 30 min in ambient atmosphere in
order to activate the catalyst by removing any adsorbed gases, then CO, O2 and N2 were allowed to pass over the
powdered oxide at the desired temperature. The degree of conversion of CO to CO2 is estimated from the following
equation:
COi ꢀ COt
COconversion ð%Þ ¼
ꢁ 100
COi

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Fig. 2. Photomicrograph of iron oxide powders: (a) 75 nm and (b) 150 nm (ꢁ1000).
where COi is the initial CO concentration and COt is the CO concentration at time t. The efficiency of using Fe2O3
powder as a catalyst for CO oxidation can be defined by the maximum degree of CO conversion extent. Fig. 4
shows the effect of crystallite size of iron oxide powders on the maximum value of CO conversion (efficiency%). It
can be noticed that crystallite size of the prepared powders plays a considerable role in the removal of CO by
catalytic oxidation reactions. The maximum value of CO conversion increased with decreasing of crystallite size
of the prepared samples. The complete oxidation of CO to CO2 was observed upon using iron oxide powders with
75 nm crystallite size at 500 8C. However, the typical behavior of CO oxidation reaction as a function of time is
shown in Fig. 5 for iron oxide samples tested at 300 8C. It was observed that for all test samples, as the time of
oxidation reaction increases, the CO conversion extent increases till the maximum value and then remains
constant. The catalytic oxidation curves are very close to each other at the early stages of reaction, till 35%
conversion extent. This can be attributed to the homogeneity of the prepared samples and the presence of more
active sites on the surface of the catalyst which enhance and control the catalytic activity of the samples. At
moderate and final stages of oxidation reactions, the effect of crystallite size of iron oxide samples on the CO
conversion becomes significant and CO conversion extent increased by lowering crystallite size of the catalyst
(40–70% conversion extent). The commercial iron oxide samples with crystallite size 250 nm show very low
response toward CO oxidation (ꢂ20% conversion extent) which reports the effective using of nanocrystallite iron
oxide samples prepared by co-precipitation method.

K.S. Abdel Halim et al. / Materials Research Bulletin 42 (2007) 731–741
735
Fig. 3. TEM micrographs of iron oxide powders: (a) 75 nm and (b) 100 nm.
Fig. 4. Effect of crystal size of nanocrystallite iron oxide on the catalytic oxidation of CO.

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Fig. 5. Variation of CO conversion extents with time for nanocrystallite iron oxide powders at 300 8C.
The apparent activation energy for CO oxidation was calculated using Arrhenius equation [4]:
ꢀ ꢁ
ꢀ
ꢁ
v
Ea
ln½ꢀlnð1 ꢀ xÞꢃ ¼ ln A þ ln
ꢀ
u
RT
where (x) is the carbon monoxide to carbon dioxide conversion rate, (A) the pre-exponential factor in sꢀ1, (u) the flow
rate in ml sꢀ1, (v) the total volume of the catalyst in cm3, (Ea) the apparent activation energy in kJ molꢀ1, (R) the gas
constant and (T) is the absolute temperature in Kelvin.
The value of activation energy can be calculated by plotting ln[ꢀln(1 ꢀ x)] versus 1/T as shown in Fig. 6 for iron
oxide samples with different crystallite sizes. It can be observed that the value of activation energy decreased with
increasing crystallite size which confirms again the increasing of CO conversion with decreasing of crystallite size.
3.3. Effect of temperature on the CO oxidation
The catalytic oxidation temperature is one of the most important factors that control the efficiency of catalytic
reaction. Fig. 7a–c shows CO conversion extents as a function of time at the temperature range 200–500 8C for iron
Fig. 6. Arrhenius plots for CO oxidation over iron oxide nanocatalyst at 200–500 8C.

K.S. Abdel Halim et al. / Materials Research Bulletin 42 (2007) 731–741
737
Fig. 7. Influence of catalytic oxidation temperature on the CO conversion for nanosized Fe2O3: (a) 75 nm, (b) 100 nm and (c) 150 nm.
oxide samples with crystallite size 75, 100 and 150 nm, respectively. As time proceeds, CO conversion increases and
the rate of conversion is high initially and slows down till the end of the reaction. At relatively lower temperature
(200 8C), the maximum CO oxidation extents were almost 20–25% for all samples which reflects the low sensitivity of
iron oxide as catalyst for conversion of CO to CO2 at lower temperatures. As temperature increases to 300 8C, the
maximum conversion extents jump to about 50% and iron oxide samples with lower crystallite size show good
response towards oxidation of CO. At relatively high temperatures 400 and 500 8C, the conversion extents increases to

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Fig. 8. Photomicrographs of nanocrystallite iron oxide powders with 100 nm: (a) pure Fe2O3, (b) Fe2O3 after oxidation at 300 8C and (c) Fe2O3 at
500 8C (ꢁ1000).
90 and 98%, respectively, for samples with 75 nm (Fig. 7a). It was observed that the response of all samples
toward CO oxidation at 400 8C is generally better than that obtained at 500 8C. This phenomenon could be
attributed to sintering processes which occurred for iron oxide samples at relatively high temperature (500 8C).
Fig. 8a–c shows photomicrographs of iron oxide powders with crystallite size of 100 nm before and after
oxidation reaction. It was observed that with increasing temperature, micropores decreased and macropores
increased whereas grain growth with partial coalescence takes place which decreases the number of active sites
available for catalytic process.
The results of catalytic oxidation tests were used to investigate the kinetics and mechanism of CO oxidation. The
order of CO oxidation was measured at 400 8C for a gas mixture containing 38% O2 and CO content ranged from 20 to
25% which resulted in a conversion extent between 70 and 90%. The extent of CO conversion to CO2 was calculated
and the corresponding data are shown in Fig. 9, from which a straight line is obtained indicating that the catalytic CO
oxidation is a first order reaction with respect to CO.
On the other hand, the conversion of CO in absence of O2 over the nano-Fe2O3 catalyst (75 nm) at 200–500 8C
was investigated to confirm the mechanism of the reaction. The CO oxidation reaches only a maximum value of
ꢄ27% at 500 8C while at lower temperature 300 and 400 8C it gives only 5 and 18%, respectively, as shown in
Fig. 10.
The value of activation energy was calculated by plotting ln[ꢀln(1 ꢀ x)] versus 1/T (Fig. 6). The obtained value of
Ea for CO oxidation in the absence of O2 is 34.6 kJ molꢀ1 which is greater than Ea for CO oxidation in the presence of
O2 reported to be 26.3 kJ molꢀ1. These values together with the low reduction extents in absence of O2 reveal that the
adsorption mechanism probably controls the oxidation of CO where the reactants are adsorbed with breaking O–O
bonds, then CO pick up the dissociated O atom forming CO2 [13].

K.S. Abdel Halim et al. / Materials Research Bulletin 42 (2007) 731–741
739
Fig. 9. CO oxidation extent as a function of inlet CO for the calculation of the reaction order.
Fig. 10. CO conversion extent as a function of time at temperatures 200–500 8C in absence of O2.
3.4. Effect of catalyst weight
The effect of catalyst weight on the catalytic oxidation of CO was studied at different crystallite sizes and
temperatures. The experimental results are summarized in Table 1. It was found that catalyst weight has a significant
effect on the CO conversion.
Fig. 11 shows the effect of Fe2O3 weight (0.3, 1, 2 and 3 g) on the CO conversion at 200–500 8C for Fe2O3 with
75 nm crystallite size. At 200 8C, the change in weight is insignificant whereas at 300 8C, the change in weight from 1
Table 1
Values of activation energy at different weights of Fe2O3 nanosized samples with maximum conversion of carbon monoxide
Weight (g)
Crystallite size (nm)
75
100
150
250
Ea
(kJ molꢀ1
Maximum
Ea
(kJ molꢀ1
Maximum
conversion
of CO (%)
Ea
(kJ molꢀ1
Maximum
conversion
of CO (%)
Ea
(kJ molꢀ1
)
Maximum
conversion
of CO (%)
)
conversion
of CO (%)
)
)
0.3
1
13.71
11.01
26.3
82
70
98
95
10.48
14.1
60
65
73
64
12.8
9.26
8.97
9.77
65
74
64
60
9.97
6.4
50
65
80
75
2
15.76
15.26
13.17
10.77
3
23.8

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Fig. 11. Effect of Fe2O3 weight on the CO conversion at 200–500 8C for Fe2O3 with 75 nm crystallite size.
Fig. 12. The conversion of carbon monoxide as a function of time for different weights of Fe2O3 samples with crystallite size 75 nm at temperatures:
(a) 200 8C and (b) 400 8C.

K.S. Abdel Halim et al. / Materials Research Bulletin 42 (2007) 731–741
741
to 2 g shows a sharp increase in CO conversion from 62 to 90%, the same trend is shown at 500 8C with a change in
conversion from 70 to 95% which indicates that maximum amount of catalyst for maximum conversion is attained
with 2 g of iron oxide catalyst.
Fig. 12a and b shows CO conversion percent as a function of time for different weights of Fe2O3 with crystallite size
of 75 nm at temperature 200 and 400 8C, respectively, from which it is clear that as time proceeds CO conversion
increases for all weights and the difference in conversion extent increases as time proceeds at 200 8C. At 400 8C, all
weights show almost the same CO conversion at the initial stages up to 35% then an irregular change in rate is observed
till the end of the conversion. It can be noticed that 2 g Fe2O3 samples show high conversion while 3 g samples show
the maximum rate at the final stages.
Activation energies were calculated using Arrhenius equation and the values are given in Table 1. It is clear that the
activation energy is directly proportional to the percent conversion and the maximum activation energy
(26.3 kJ molꢀ1) is attained at 98% conversion for 2 g Fe2O3 samples with crystalline size of 75 nm while the minimum
activation energy (ꢄ8.2 kJ molꢀ1) was attained for 1 g Fe2O3 samples with crystal size of 150 nm.
4. Conclusion
Three types of nanocrystallite iron oxide powders were prepared by co-precipitation method in different crystal size
75, 100 and 150 nm. It was observed that the maximum value of CO conversion extent decreased with the increasing of
crystallite size of the prepared samples. The commercial iron oxide samples with crystallite size of 250 nm show very
low response toward CO oxidation (ꢂ20% conversion extent) which reflects the effective use of nanocrystallite iron
oxide samples prepared by co-precipitation method. The catalytic temperature plays a significant role in the oxidation
of CO to CO2. At relatively high temperatures of 400 and 500 8C, the conversion extents increase to 90 and 98%,
respectively, for samples with 75 nm. It was observed that the response of all samples toward CO oxidation at 400 8C is
better than that obtained at 500 8C which can be attributed to sintering processes at high temperature (500 8C).
The experimental data were used to study the mechanism of the catalytic oxidation reaction. The reaction was
found to be first order with respect to CO. The data of CO catalytic oxidation was compared in the absence and
presence of oxygen. It was found that the catalytic oxidation of CO over nanocrystallite Fe2O3 probably proceeded by
adsorption mechanism where the reactants are adsorbed on the surface of the catalyst with breaking O–O bonds, then
CO pick up the dissociated O atom forming CO2.
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