Luận án Low temperature catalytic oxidation of volatile organic compounds (vocs) over catalysts of CuO-Co₃O₄ on supports

which a chemical reaction is monitored while the temperature increases linearly in 
time. Whilst several forms of these techniques are currently in use, they are all 
applicable to real catalysts and single crystals, and have the advantage that they are 
experimentally simple and inexpensive in comparison to many other spectroscopies. 
Although interpretation on a qualitative basis is rather straightforward, obtaining 
reaction parameters such as activation energies or pre-exponential factors from TP 
methods is a complicated matter. The instrumentation for TP investigations is 
relatively simple; the set-ups for the temperature programmed desorption (TPD) 
studies of catalysts are shown in Fig. 2.8 
Figure 2.8. Experimental for temperature programmed reduction, oxidation and 
desorption. 
Application in thesis 
CO pulse analysis was applied to measure the metal dispersion, metal surface 
area, and activated particle diameter that was conducted in the Autochem II 2920 
(School of Chemical engineering, Hanoi University of Science and Technology): 100 
mg catalyst was pre-treated by Helium flow of 50 ml/min. The temperature was 
risen to 300oC with a rate of 10oC/min and remained for 60 minutes. After that, the 
catalysts were reduced by 5% H2/Argon flow at a temperature range from 30oC to 
300oC followed by cooling at room temperature. The CO pulse process was 
conducted with CO/He flow of 50 ml/min; this process ended when CO was not 
TCD
Temperature
Program
H
2
A
r
O
2
H
e
Reactor
Mass Spectrometer
42 | P a g e 
captured on the catalyst. The results were recorded every 0.5 seconds by TCD 
detector. The metal dispersion (D) is the percentage of metal atoms accessible to the 
probe molecule, defined as: 
𝐷 =
𝑉𝑎𝑑𝑠 × 𝑆𝐹
22.411
× 𝑀𝑤 × 100 Eq. 2.3 
where VAds is the amount of chemisorbed carbon monoxide, MW it the molecular 
weight of the metal, and SF is the stoichiometric factor; 
The average particle size can be estimated using this formula: 
𝑆𝐴𝑀𝑒𝑡𝑎𝑙 =
𝐴
𝑉 × 𝜌
 Eq. 2.4 
where A is the area of the particle, V is the volume of the particle; and ρ is the 
density of the metal. 
O2-TPD profiles were measured with the AutoChem II 2920: 100mg catalyst 
was pre-treated by He flow while temperature increased to 400oC at the rate of 
20oC/min. Oxygen was chemisorbed over the catalyst at this temperature for 90 
minutes. Afterward, He flow was used to clean oxygen for 30 minutes, the 
desorption was implemented by using He flow at the temperature range from 50oC 
to 700oC with the rate of 10oC/min and the temperature was kept for 30 minutes. 
The results were recorded every 0.5 second by TCD detector. 
CH4-TPD profiles were measured with the AutoChem II 2920: 100mg 
catalyst was pre-treated by He flow while temperature increased to 400oC at the rate 
of 10oC/min, then keep the temperature stable for 30 minutes. Then the temperature 
is cooled to 100oC, and CH4 was chemisorbed over the catalyst at that temperature 
for 90 minutes. Afterward, CH4 was stopped and remained CH4 was exhausted by 
He flow in a period of 30 minutes. The desorption was implemented by using He 
flow at the range temperature from 100oC to 700oC with the rate of 10oC/min, and 
the temperature was kept for 30 minutes. The results were recorded every 0.5 
seconds by TCD detector. 
43 | P a g e 
2.3. Adsorption and catalytic activity measurement 
2.3.1. Adsorption and nitrogen desorption measurement 
The adsorption and desorption of the catalysts are evaluated in the micro-
reactor systems, which is shown in Fig. 2. 9. 
1. N2 cylinder, 2. N2 mass flow controller, 3. N2 mass flow controller, 4. Toluene 
generator, 5. Reactor, 6. Oven, 7. Temperature controller, 8. Gas Chromatography 
with TCD detector, 9. Computer, V. Valve. 
Figure 2.9. Adsorption and desorption experiment systems. 
Nitrogen was obtained from the cylinder and divided into two parts: one was 
sent directly into the reactor; the other was sent to pass through a toluene generator 
to carry out pre-calibrated toluene to form waste gas containing toluene. All parts of 
nitrogen were metered and controlled through mass flow controllers (MFC). The 
mass flow controllers could be closed when gas was not used. 
The experiment adsorption and desorption process were performed as 
following: 
Adsorption process: 
Step 1: Operate GC Thermo Focus (Italia) (8) with the factors: 
44 | P a g e 
Table 2.5. Operating factors of GC 
Operating factors’ GC 
Detector TCD 
Oven 
Block temperature (oC) 180 Oven run time (min) 8,5 
Trans temperature (oC) 180 Initial Temperature (oC) 60 
Flow of N2 (ml/phút) 35 Initial Time (min) 1 
Flow of He (ml/phút) 20 Ramp (oC/min) 60 
Final temperature (oC) 210 
 Final Time (min) 5 
Step 2. Toluene was loaded into generator (4). 
Step 3. 0.2gram catalyst was placed into reactor (5) with diameter of 
1/8 inches and put into oven (6). 
Step 4. The oven (6) was turned on and remained at 1800C during 
experiment, it was controlled by temperature controller (7). 
Step 5. Open and set the MFC (2) with a flow of 9.5 ml/min, 
corresponding to initial toluene concentration of 9000 ppm 
Step 6. V1, V6 and V7 were opened and others were closed to use N2 
flow (1) to clear the catalyst for 15 minutes 
Step 7. V1, V6 and V7 were closed and others were opened to measure 
initial toluene concentration by GC. 
Step 8. When the initial toluene concentration was stable, V5 was 
closed and others were opened. The toluene concentration at outlet was 
measured regularly. 
Step 9. When the outlet toluene was similar to the initial concentration, 
the adsorption experiment was stopped. 
Desorption process: When adsorption process finished, the desorption was 
conducted as follow steps: 
 Step 1. Open and set MFC (2) with a flow of 9.5 ml/min 
45 | P a g e 
Step 2. V1, V6 and V7 were opened and others were closed, this 
allows to use N2 flow (1) to desorb toluene from the catalyst. The 
toluene concentration at outlet was measure for a period time by GC. 
Step 3. When the outlet toluene was zero, the desorption was stopped. 
For the adsorption process, the adsorption capacity was calculated by 
throughout curve as the equation: 
𝐴𝐴𝑑 =
𝑄𝐴𝑑 × 92 × 10
−6
22.4 × 𝑚𝐶
∫ (𝐶𝑇𝑜𝑙
𝑜 − 𝐶𝑇𝑜𝑙,𝑡
𝑖 ) × 𝑑𝑡
𝑡𝑒
0
 Eq. 2.5 
where AAd is adsorption capacity (g/g), QAd is the total flow rate of the adsorption 
process (ml/min), CoTol is the inlet toluene concentration (ppm), CiTol,t is the outlet 
toluene concentration at ti in the adsorption process (ppm), te is equilibrium time 
(min), and mC is weight of the catalyst (g). 
For desorption process, the desorption amount was also calculated basing on 
throughout curve as the following equation 
𝐴𝐷𝑒 =
𝑄𝐷𝑒 × 92 × 10
−6
22.4 × 𝑚𝐶
∫ 𝐶𝑇𝑜𝑙,𝑡
𝑖 × 𝑑𝑡
𝑡𝑑
0
 Eq. 2.6 
where ADe is desorption amount (g/g), QDe is the total flow rate N2 of the desorption 
process (ml/min), CiTol,t is the outlet toluene concentration at ti in the desorption 
process (ppm), td is the desorption time by N2 (min), and mC is weight of the catalyst 
(g). 
2.3.2. Catalytic activity measurement for complete oxidation of toluene 
To evaluate the toluene oxidation, two experiment techniques are applied in 
the study: 
The toluene oxidation over catalyst in desorption process: 
Adsorption technique can treat toluene completely. However, this technique 
needs further steps with high temperature, such as desorption, incineration to 
recycle the adsorbents, thus the adsorbed toluene will be released during the 
46 | P a g e 
desorption process. Therefore, if it is not necessary to recover wasted toluene, it is 
necessary to remove adsorbed toluene by oxidation. Theory, VOCs firstly will 
adsorb onto porous materials (AC, MCM-41, silica gel), then VOCs will be 
desorbed by high temperature and a desorbed gas flow. In order to remove toluene 
in the desorbed flow, the desorbed flow need to contain enough oxygen while the 
adsorbents contain activated centers of catalysts. With the presence of the catalysts 
and oxygen, desorbed toluene will be immediately oxidized to produce CO2 and 
water. 
This technology is used to evaluate the oxidized toluene over catalysts in 
desorption of adsorbed toluene in the catalyst by a flow of oxygen during the 
desorption process after the prior adsorption of toluene. The experiment system was 
designed as Fig. 2.10 
1. O2 cylinder, 2. N2 cylinder, 3. O2 mass flow controller, 4. N2 mass flow 
controller, 5 Toluene generators, 6. Reactor, 7. Oven, 8. Temperature controller, 9. 
Gas Chromatography with TCD detector, 10. Computer, V. Valves 
Figure 2.10. The toluene adsorption – desorption oxidation experiment systems. 
The experiment toluene oxidation over catalyst in desorption process were 
performed as followings: 
Adsorption process: 
Step 1: Operate GC Thermo Focus (Italia) (8) with the factors in Tab.2.5 
47 | P a g e 
Step 2. Toluene was loaded into generator (4). 
Step 3. 0.2gram catalyst was placed into reactor (5) with diameter of 
1/8 inches and put into oven (6). 
Step 4. The oven was turned on and remained at 1800C during 
experiment, it was controlled by temperature controller (7). 
Step 5. Open and set MFC (4) with a flow of 9.5 ml/min, corresponding to 
initial toluene concentration of 9000 ppm 
Step 6. V1, V6 and V7 were opened and others were closed to use N2 
flow (2) to clean the catalyst for 15 minutes 
Step 7. V1, V6 and V7 were closed and others were opened to measure 
initial toluene concentration by GC. 
Step 8. When the initial toluene concentration was stable, V5 was 
closed and others were opened. The toluene concentration at outlet was 
measured regularly. 
Step 9. When the outlet toluene was similar to initial concentration, the 
adsorption was stopped. 
Oxidation during the desorption process: When adsorption process finished, 
the desorption by oxygen was conducted as follow steps: 
 Step 1. Open and set MFC (3) with a O2 flow of 9.5 ml/min 
Step 2. V1, V6 and V7 were opened and others were closed, this 
allows to use O2 flow (1) to desorb prior adsorbed toluene from the 
catalyst. The toluene concentration at outlet was measure for a period 
time by GC. 
Step 3. When the outlet toluene was zero, the desorption by oxygen 
was stopped. 
It was assumed that the different amount of toluene, which were generated 
from desorption processes by nitrogen and oxygen, was oxidized at the reaction 
temperature. So, toluene conversion was calculated as the following equation: 
48 | P a g e 
𝜂𝑇𝑜𝑙 =
𝐴𝐷𝑒
𝑁2 − 𝐴𝐷𝑒
𝑂2
𝐴𝐷𝑒
𝑁2
× 100% Eq. 2.7 
Where ɳTol it the toluene conversion (%), AN2De is the toluene desorption amount by 
using nitrogen flow (g/g), and AO2De the toluene desorption amount by using oxygen 
flow (g/g). 
The rate of conversion from toluene to CO2 can be calculated as: 
𝛾𝐶𝑂2 =
𝑌𝐶𝑂2
7 × (𝐴𝐷𝑒
𝑁2 − 𝐴𝐷𝑒
𝑂2 )
× 100% Eq. 2.8 
where γCO2 is the rate of conversion from toluene to CO2 (%), YCO2 is the amount of 
CO2 produced by oxidation in the desorption process (g/g), AN2De is the toluene 
desorption amount by using nitrogen flow (mol/g), and AO2De is the toluene 
desorption amount by using oxygen flow (mol/g). 
Amount of CO2 by oxidation in the desorption process can be calculated as: 
𝑌𝐶𝑂2 =
𝑄𝑂𝑥 × 44 × 10
−6
22.4 × m𝐶
∫ 𝐶𝐶𝑂2,𝑡
𝑖 × 𝑑𝑡
𝑡𝑑
0
 Eq. 2.9 
where YCO2 is the amount of CO2 in the desorption process by O2 (g/g), QOx is the 
total flow rate of O2 of the desorption process (ml/min), CiCO2,t is the outlet CO2 
concentration at t in the desorption oxidation (ppm), mC is weight of the catalyst (g), 
and td is desorption time by O2 (min). 
The complete oxidation technique: 
In the complete oxidation technique, toluene in the flow was directly 
oxidized by the catalyst and oxygen in the reactant flow. The experiment system for 
complete oxidation of toluene was organized as in Fig. 2.11 
49 | P a g e 
1. O2 cylinder, 2. O2 mass flow controller, 3. Toluene generator, 4. Reactor, 5. 
Oven, 6. Temperature controller, 7. Gas Chromatography with TCD detector, 8. 
Computer, V. Valves 
Figure 2.11. The complete oxidation of toluene experiment systems. 
The complete oxidation process was implemented as: 
Step 1: Operate GC Thermo Focus (Italia) (8) with the factors in 
Tab.2.5 
Step 2. Toluene was loaded into generator (3). 
Step 3. 0.2gram catalyst was placed into reactor (4) with diameter of 
1/8 inches and put into oven (5). 
Step 4. Open and set MFC (2) with a O2 flow of 9.5 ml/min, corresponding 
to initial toluene concentration of 9000 ppm 
Step 5. The oven was turned on with a program of temperature 
(Increasing from room temperature to 450oC with the rate of 
2.5oC/min). 
Step 6. V4 and V6 were closed others were opened to analyze initial 
toluene concentration. 
Step 7. When the initial toluene concentration was stable, V5 was 
closed and others were opened during experiment. 
50 | P a g e 
In this case, toluene conversion was calculated as: 
𝜂𝑇𝑜𝑙 =
𝐶𝑇𝑜𝑙,𝑇
𝑜 − 𝐶𝑇𝑜𝑙,𝑇
𝑖
𝐶𝑇𝑜𝑙,𝑇
𝑜 × 100% Eq. 2.10 
where ɳTol is the toluene conversion (%), CoTol,T is the inlet toluene concentration at 
temperature T (ppm), CiTol,T is the outlet toluene concentration at temperature T 
(ppm). 
The rate of conversion from toluene to CO2 was calculated as: 
𝛾𝐶𝑂2 =
𝐶𝐶𝑂2,𝑇
𝑖
7 × (𝐶𝑇𝑜𝑙,𝑇
𝑜 − 𝐶𝑇𝑜𝑙,𝑇
𝑖 )
× 100% Eq. 2.11 
where γCO2 is the rate of conversion from toluene to CO2 (%), CiCO2,T is the outlet 
CO2 concentration at temperature T (ppm), CoTol,T is the inlet toluene concentration 
at temperature T (ppm), and CiTol,T is the outlet toluene concentration at temperature 
T (ppm). 
2.3.3. Catalytic activity measurement for complete oxidation of methane 
The methane oxidation experiment is described in Fig. 2.12. 0.2gram catalyst 
was exanimated at a range of temperature of 150-450oC (heating rate of 2.5oC/min) 
with a total mixed flow rate of 75 ml/min (N2: O2: CH4 = 60.75: 13.5: 0.75) which 
was controlled by mass flow controllers, corresponding to initial methane 
concentration of 1000 ppm. 
51 | P a g e 
1. N2 cylinder, 2. CH4 cylinder, 3. O2 cylinder, 4. N2 mass flow controller, 5. CH4 
mass flow controller, 6. O2 mass flow controller, 7. Reactor, 8. Oven, 9. 
Temperature controller, 10. Gas Chromatography with TCD detector, 11. 
Computer, V. Valve 
Figure 2.12. Total methane oxidation experiment systems. 
In this case, the methane conversion was calculated as: 
𝜂𝐶𝐻4 =
𝐶𝐶𝐻4,𝑇
𝑜 − 𝐶𝐶𝐻4,𝑇
𝑖
𝐶𝐶𝐻4,𝑇
𝑜 × 100% Eq. 2.12 
where ɳCH4 is the methane conversion (%), CoCH4,T is the inlet methane 
concentration at temperature T (ppm), and CiCH4,T is the outlet methane 
concentration at temperature T (ppm). 
52 | P a g e 
CHAPTER 3. RESULTS AND DISCUSSIONS 
3.1. Characterizations of supports and catalysts 
3.1.1. Thermal analysis 
Because of the thermal instability of activated carbon, thermal analysis in 
(static air) of the catalysts on AC bases are measured to ensure these catalysts are 
not decomposed in high-temperature environment. The thermal analysis of activated 
carbon and some representative catalysts on AC were shown in Fig. 3.1. 
a. TG curves 
b. DSC curves 
Figure 3.1. Thermal analysis in static air of catalyst on AC. 
85
90
95
100
0 50 100 150 200 250 300
W
ei
g
h
t 
lo
ss
 (
%
)
Temperature (oC)
AC SS-AC7Cu3Co SS-AC5Cu5Co
SS-AC3Cu7Co WI-AC5Cu5Co
-15
-10
-5
0
5
0 50 100 150 200 250 300
D
S
C
 (
m
W
/m
g
)
Temperature (oC)
AC SS-AC7Cu3Co SS-AC5Cu5Co
SS-AC3Cu7Co WI-AC5Cu5Co
53 | P a g e 
As shown in Fig. 3.1, the mass of AC sample was not decreased by heating in 
static air at the temperature range from 50 to 270oC, while the mass decreased was 
recorded for the catalysts from 200oC. When temperature raises to over 200oC, the 
weight of SS-AC3Cu7Co, SS-AC5Cu5Co, and WI-AC5Cu5Co decreased more, by 
around 15%. The decrease of mass at high temperatures from 200oC in static air, 
together with endothermic effect in DSC curves (Fig. 3.1b) shows that the samples 
are burnt out due to the incineration of activated carbon at high temperature. 
The impregnation of oxides of Cu and Co can affect the heat resistance of 
activated carbon may be due to their high catalytic activity for oxidation, therefore 
the sample containing catalysts showed higher decrease of mass during the heating 
in static air. Samples SS-AC3Cu7Co, SS-AC5Cu5Co, and WI-AC5Cu5Co showed 
higher influence on the burning of AC, indicating that these catalysts may be more 
active for the complete oxidation of AC. Due to the burn of AC in the samples, the 
processes of adsorption, desorption, and oxidation using the catalysts on AC base 
cannot be implemented at a temperature of above 200oC to avoid the incineration of 
activated carbon. 
3.1.2. Physisorption 
The N2 physical adsorptions of the supports (AC, MCM-41, and silica gel) 
were described in Fig. 3.2. It is clear that the adsorption-desorption isotherm of 
activated carbon and MCM41 were classified as type IV (pore size from 2-50nm), 
while isotherm of silica gel exhibited isotherms of type VI (pore size > 50nm) 
basing on the IUPAC classification. 
54 | P a g e 
a. Isotherm linear plot of activated carbon (AC) 
b. Isotherm linear plot of Silica gel 
0
100
200
300
400
500
600
700
800
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Q
u
a
n
ti
ty
 a
d
so
rb
ed
 (
cm
3
/g
 s
tp
)
Relative pressure (P/Po)
Adsorption Desorption
0
100
200
300
400
500
600
700
800
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Q
u
a
n
ti
ty
 a
d
so
rb
ed
 (
cm
3
/g
 s
tp
)
Relative pressure (P/Po)
Adsorption Desorption
55 | P a g e 
c. Isotherm linear plot of Silica gel 
Figure 3.2. Isotherm linear plot of AC, silica gel and MCM-41 
Besides, these results are in agreement with the results of pore distribution 
obtained from the BJH desorption (Fig. 3.3). They also are matched to the previous 
published data for AC, silica gel and MCM-41. 
Figure 3.3. Pore distribution of AC, silica gel and MCM-41 
0
100
200
300
400
500
600
700
800
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Q
u
a
n
ti
ty
 a
d
so
rb
ed
 (
cm
3
/g
 s
tp
)
Relative pressure (P/Po)
Adsorption Desorption
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
10 100 1000
In
cr
em
en
ta
l 
P
o
re
 V
o
lu
m
e 
(c
m
³/
g
)
Average Width (Å)
MCM41 AC Silica gel
56 | P a g e 
The BET surface, pore volumes and average pore sizes of these sorbents are 
presented in Tab. 3.1. 
Table 3.1. The Surface characteristics of AC, silica gel and MCM-41 
No Supports 
BET 
(m2/g) 
Pore volume 
(m3/g) 
Average 
pore size 
(Ao) 
1 AC 1003 0.26 39.43 
2 Silica gel 295 1.04 96.2 
3 MCM41 1148 0.97 33.74 
 The surfaces’ areas of AC and MCM-41 were quite large, above 1000 m2/g, 
while the surface of silica gel is much lower (295 m2/g), but the biggest average 
pore size is recorded on silica gel (96.2Ao), followed by AC (39.43Ao) and MCM-
41 (33.74Ao). From the results, it can be predicted that AC and MCM41 can adsorb 
toluene on their surfaces more than silica gel does since surface area of silica gel is 
much smaller while its pore size is too big compared to kinetic diameter of toluene 
(6.7-8.7Ao [64]). 
The BET surfaces of catalysts based on AC and silica gel were described as 
Tab. 3.2. It is showed that loading metallic oxides on AC is the main reason to 
reduce the AC’s surface by 40-58%, while that has insignificant effects on the 
surface of silica gel with over 10% of reduction of surface area. The surface area of 
catalysts on AC decreased more when the composition of Co increased, thus,