# Characteristics of Carbon Monoxide Oxidization in Rich Hydrogen by Mesoporous Silica with TiO2

Authors: Akira Nishimura; Yutaka Yamano; Tomokazu Hisada; Masafumi Hirota; Eric Hu
Journal: International Journal of Photoenergy
 (2010)

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## Abstract

Hydrogen (H2) is normally used as the fuel to power polymer electrolyte fuel cell (PEFC). However, the power generation performance of PEFC is harmed by the carbon monoxide (CO) in the H2 that is often produced frommethane (CH4). The purpose of this study is to investigate the experimental conditions in order to improve the CO oxidization performance of mesoporous silica loaded with TiO2. The impact of loading ratio of TiO2 and initial concentration ratio of O2 to CO on CO oxidization performance is investigated. As a result, the optimum loading ratio of TiO2 and initial concentration ratio of O2 to CO were 20 wt% and 4 vol%, respectively, under the experimental conditions. Under this optimumexperimental condition, the CO in rich H2 in the reactor can be completely eliminated from initial 12000 ppmV after UV light illumination of 72 hours.


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## Body

## 1. Introduction

Polymer electrolyte fuel cell (PEFC) has been developed vigorously in the world since it is an attractive and clean power generation technology. H2 is normally used as the fuel to power PEFC. However, the reduction of PEFC power generation performance has been observed due to the existence of CO in the H2 produced from CH4, CH3OH, and gasoline.CH4 is normally the feedstock to produce H2 with Ni or Ru as catalyst at the high temperature range of 873 K–973 K through the following reaction:(1)CH4+H2O⟶CO+3H2.After this reaction, there is about 10 vol% of CO in the products. The CO concentration can be reduced down to about 1 vol% by the following so-called shift reaction:(2)CO+H2O⟶CO2+H2.After the shift reaction, the concentration of CO needs to be further reduced down to 10 ppmV by the following selective oxidization reaction:(3)CO+12O2⟶CO2.In the H2 purification processes mentioned above, precious metal catalysts and thermal energy are used, and the processes are costly. An alternative process, that is, using the TiO2 photocatalyst combined with adsorbent to oxidize CO is being developed recently due to its potential cost and energy saving.TiO2 can oxidize CO under illumination of ultraviolet (UV) ray (available in sunlight) through the following reaction scheme [1, 2].Photocatalytic reaction:(4)TiO2+hv(<380nm)⟶h++e-.Oxidization of CO:(5)CO+h+⟶CO+.Reduction ofO2:(6)O2+e-⟶O2-,(7)O2-+h+⟶2O,(8)O+e-⟶O-.
From the products of the reactions (5) and (8), the following combined reaction occurs:(9)CO++O-⟶CO2.
Therefore, the total reaction scheme can be written as follows:(10)CO+12O2⟶CO2,
where hv is the energy of UV ray. h+ and e- represent the hole and electron produced by photocatalytic reaction, respectively.The oxidization process with TiO2 has the following merits. (1) There is a lot of TiO2 reserve in the earth compared with precious metal catalyst. The amount of Ti is the 9th largest among the elements consisting the earth crust [3]. (2) Cost is lower than using precious metal catalyst. (3) Energy consumption is less and the control of the reaction process is easier since high thermal energy is not necessary. (4) Solar energy can be used for the reaction. (5) TiO2 is stable in both acid and alkali environments.Literature survey shows that TiO2 photocatalyst combined with adsorbent such as activated carbon, zeolite, and silica (SiO2) was mainly used in environmental purification technologies such as NOx removal [4–6], decomposition of acetaldehyde [7], dimethylsulfide [8], 2-propoanol [9], degradation of organophosphate and phosphonoglycine [10], and CO2 reforming into fuel like CH4 and CH3OH [11, 12]. The CO oxidization by photocatalyst combined with FeOx, AL2O3, or CeOx and precious metal catalyst like Pt or Au was reported [13–15]. Furthermore, the CO oxidization characteristics of Pt loaded on zeolite without photocatalyst were also reported [16, 17]. Although there are reports on the CO oxidization characteristics of Mo/SiO2 or Cr/SiO2 [18, 19], there is no report on the CO oxidization characteristics of TiO2 combined with adsorbent except our previous study [20]. Our previous study investigated the effect of different loading methods of TiO2 to silica on CO oxidization performance. Comparing two types of TiO2 particle combined with silica, that is, the silica gel particles coated with TiO2 film and mesoporous silica particles loaded with TiO2, the amount of oxidized CO per unit mass of TiO2 for the mesoporous silica particles loaded with TiO2 was larger than that for silica gel particles coated with TiO2 film. Therefore, it revealed that loading was a more effective way to make use of TiO2 for CO oxidization. However, it also reported that the investigation on optimum experimental condition was necessary to promote the CO oxidization performance more.The purpose of this study is to investigate the experimental conditions in order to improve the CO oxidization performance of mesoporous silica loaded with TiO2. Since UV ray can penetrate the mesoporous silica, the particles of mesoporous silica loaded with TiO2 can be used in a bed-type reactor. The effect of loading ratio of TiO2 and initial concentration ratio of O2 to CO on CO oxidization performance was investigated to decide the optimum experimental conditions. The loading ratio of TiO2 is changed minutely by 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt%, and 80 wt%. The initial concentration of O2 is also changed closely by 0.5 vol%, 1 vol%, 2 vol%, 4 vol%, 6 vol%, 8 vol%, and 10 vol%. Moreover, the best CO oxidization performance of the mesoporous silica with TiO2 was evaluated under the optimum experimental conditions.


## 2. Experiment

### 2.1. Preparation Method of Mesoporous Silica Loaded with TiO2

Table1 lists the physical properties of mesoporous silica loaded with TiO2, which has the following characteristics(i)
Average pore size is in nanoscale. Since the molecular diameter of CO and O2 is relatively close to the average pore diameter, the high adsorption performance is expected.(ii)
TiO2 particle is located inside of pores of the mesoporous silica. Light can pass through mesoporous silica, and gases can also get into the pores of mesoporous silica through the diffusion, therefore, good photocatalytic reaction as well as good adsorption performance can be expected.Table 1
Physical properties of mesoporous silica loaded with TiO2.
Mesoporous silicaloaded with TiO2 particlesParticle size (mm)0.03–0.25 (primary particle)2.0–5.6 (agglomerated particle)Specific surface area (m2/g)219–1038Average pore diameter (nm)2.7–3.1Pore volume (mL/g)0.63–0.94Ratio of loaded TiO2 (wt%)1, 10, 15, 20, 30, 60, 80Figure1 shows the preparation method of mesoporous silica particles loaded with TiO2 in our laboratory, which is developed by referring to the literatures [21–23]. P25 (Degussa, P25, JAPAN AEROSIL Corp., LTD.) powder was selected as TiO2 source to load. Because of the primary particle size of P25 which is ranged between 20 nm and 30 nm, P25 is suitable for being inserted into the mesoporous silica particle whose size is ranged between 30 nm and 100 nm generally. P25 plays the role of the core for forming mesoporous silica. The pores of mesoporous silica are formed in or around the particles of P25 as illustrated in Figure 2 [23]. Figure 3 shows TEM image of prepared mesoporous silica particles loaded with TiO2 to understand the structure illustrated in Figure 2. The ratio of loaded TiO2 to mesoporous silica was controlled by the amount of P25 added to the mixture solution of ion-exchange water, CH3(CH2)15N(CH3)3Br (purity of 99 wt%, Nacalai Tesque Corp.), (C2H5O)4Si (purity of 95 wt%, Nacalai Tesque Corp.), and NH3 (purity of 28 wt%, Nacalai Tesque Corp.). The amount of P25 particle added to mixture solution was 0.05 g, 0.54 g, 0.86 g, 1.18 g, 2.10 g, 7.30 g,and 19.5 g for the ratio of 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt%, and 80 wt%, respectively. Here, the ratio of loaded TiO2 is named after preparation condition since it is very difficult to measure the weight of TiO2 in mesoporous silica particle directly after preparation process. Particle size of agglomerated mesoporous silica particles loaded with TiO2 was sieved into the range between 2.0 mm and 5.6 mm after burning.Figure 1
Preparation method of mesoporous silica loaded with TiO2.Figure 2
Schematic drawing of mesoporous silica loaded with TiO2.Figure 3
TEM image of mesoporous silica loaded with TiO2 particlesfor the ratio of loaded TiO2 of 15 wt%.


### 2.2. Experimental Apparatus and Procedure

Figure4 illustrates the experimental apparatus which consists of a reactor, a gas mixing chamber, a mass flow controller (MODEL 3660, KOFLOC), a dew point meter (HMT337, VAISALA), a regulator, and a gas cylinder. The reactor, which is a batch type, consists of stainless steel pipe (450 mm (L.) × 60.5 mm (O.D.) × 2.5 mm (t.); reaction space 50 mm (L.) × 55.5 mm (I.D.)) which includes two acrylic cylinders to cover both ends of UV lamp in it, gas supply and exhaust pipe, valves, gas sampling tap, and UV lamp (FL15BLB, TOSHIBA Co., 436 mm (L.) × 25.5 mm (D.)) located at the center of stainless steel pipe. The reaction space for charging gas and filling TiO2 particles is 9.22 × 104 mm3. The central wavelength and mean light intensity of UV light is 352 nm and 4.34 mW/cm2, respectively. This light intensity is almost the same as the UV intensity in solar radiation at daytime in the summer of Japan.Figure 4
Experimental apparatus.In the experiment, O2 (purity of 99.9999 vol%) and the premixed gas of H2 and CO (H2: 99 vol%, CO: 1 vol%) were mixed in the gas mixing chamber before being supplied to the reactor. By adjusting the flow rate and the pressure of the gases, the initial concentration of O2 to CO could be controlled. This remixed gas was charged into the reactor, and the concentration and pressure of gases were confirmed before starting the experiment. The ratios of gasses were charged as CO : O2 = 1 : 0.5, 1 : 1, 1 : 2, 1 : 4, 1 : 6, 1 : 8, and 1 : 10 (balanced by H2). Although 1 mol CO reacts with 0.5 mol O2 theoretically as shown in the reaction of (10), it is necessary to confirm the practical optimum initial concentration ratio of O2 to CO in rich H2 environment.The total pressure in the reactor was set at 0.1 MPa. The gas temperature in the reactor was kept at about 300 K during the experiment. Before the mixed gas for CO oxidization was supplied, mesoporous silica particles loaded with TiO2 were filled into the reactor by 50 vol% of full reactor volume size.The experiment was started when illumination of UV light was applied. The gas in reactor was sampled hourly during the experiment. The gas excluding H2O vapor samples was analyzed by TCD gas chromatograph (VARIAN micro-GC CP-4900, GL Science Corp.) equipped with double columns of Molsieve 5A and PoraPLOT Q. The minimum resolution of the gas chromatograph was 1 ppmV. The concentration of H2O vapor in the experimental apparatus was measured by the dew point meter whose minimum resolution was 1 ppmV.


## 2.1. Preparation Method of Mesoporous Silica Loaded with TiO2

Table1 lists the physical properties of mesoporous silica loaded with TiO2, which has the following characteristics(i)
Average pore size is in nanoscale. Since the molecular diameter of CO and O2 is relatively close to the average pore diameter, the high adsorption performance is expected.(ii)
TiO2 particle is located inside of pores of the mesoporous silica. Light can pass through mesoporous silica, and gases can also get into the pores of mesoporous silica through the diffusion, therefore, good photocatalytic reaction as well as good adsorption performance can be expected.Table 1
Physical properties of mesoporous silica loaded with TiO2.
Mesoporous silicaloaded with TiO2 particlesParticle size (mm)0.03–0.25 (primary particle)2.0–5.6 (agglomerated particle)Specific surface area (m2/g)219–1038Average pore diameter (nm)2.7–3.1Pore volume (mL/g)0.63–0.94Ratio of loaded TiO2 (wt%)1, 10, 15, 20, 30, 60, 80Figure1 shows the preparation method of mesoporous silica particles loaded with TiO2 in our laboratory, which is developed by referring to the literatures [21–23]. P25 (Degussa, P25, JAPAN AEROSIL Corp., LTD.) powder was selected as TiO2 source to load. Because of the primary particle size of P25 which is ranged between 20 nm and 30 nm, P25 is suitable for being inserted into the mesoporous silica particle whose size is ranged between 30 nm and 100 nm generally. P25 plays the role of the core for forming mesoporous silica. The pores of mesoporous silica are formed in or around the particles of P25 as illustrated in Figure 2 [23]. Figure 3 shows TEM image of prepared mesoporous silica particles loaded with TiO2 to understand the structure illustrated in Figure 2. The ratio of loaded TiO2 to mesoporous silica was controlled by the amount of P25 added to the mixture solution of ion-exchange water, CH3(CH2)15N(CH3)3Br (purity of 99 wt%, Nacalai Tesque Corp.), (C2H5O)4Si (purity of 95 wt%, Nacalai Tesque Corp.), and NH3 (purity of 28 wt%, Nacalai Tesque Corp.). The amount of P25 particle added to mixture solution was 0.05 g, 0.54 g, 0.86 g, 1.18 g, 2.10 g, 7.30 g,and 19.5 g for the ratio of 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt%, and 80 wt%, respectively. Here, the ratio of loaded TiO2 is named after preparation condition since it is very difficult to measure the weight of TiO2 in mesoporous silica particle directly after preparation process. Particle size of agglomerated mesoporous silica particles loaded with TiO2 was sieved into the range between 2.0 mm and 5.6 mm after burning.Figure 1
Preparation method of mesoporous silica loaded with TiO2.Figure 2
Schematic drawing of mesoporous silica loaded with TiO2.Figure 3
TEM image of mesoporous silica loaded with TiO2 particlesfor the ratio of loaded TiO2 of 15 wt%.


## 2.2. Experimental Apparatus and Procedure

Figure4 illustrates the experimental apparatus which consists of a reactor, a gas mixing chamber, a mass flow controller (MODEL 3660, KOFLOC), a dew point meter (HMT337, VAISALA), a regulator, and a gas cylinder. The reactor, which is a batch type, consists of stainless steel pipe (450 mm (L.) × 60.5 mm (O.D.) × 2.5 mm (t.); reaction space 50 mm (L.) × 55.5 mm (I.D.)) which includes two acrylic cylinders to cover both ends of UV lamp in it, gas supply and exhaust pipe, valves, gas sampling tap, and UV lamp (FL15BLB, TOSHIBA Co., 436 mm (L.) × 25.5 mm (D.)) located at the center of stainless steel pipe. The reaction space for charging gas and filling TiO2 particles is 9.22 × 104 mm3. The central wavelength and mean light intensity of UV light is 352 nm and 4.34 mW/cm2, respectively. This light intensity is almost the same as the UV intensity in solar radiation at daytime in the summer of Japan.Figure 4
Experimental apparatus.In the experiment, O2 (purity of 99.9999 vol%) and the premixed gas of H2 and CO (H2: 99 vol%, CO: 1 vol%) were mixed in the gas mixing chamber before being supplied to the reactor. By adjusting the flow rate and the pressure of the gases, the initial concentration of O2 to CO could be controlled. This remixed gas was charged into the reactor, and the concentration and pressure of gases were confirmed before starting the experiment. The ratios of gasses were charged as CO : O2 = 1 : 0.5, 1 : 1, 1 : 2, 1 : 4, 1 : 6, 1 : 8, and 1 : 10 (balanced by H2). Although 1 mol CO reacts with 0.5 mol O2 theoretically as shown in the reaction of (10), it is necessary to confirm the practical optimum initial concentration ratio of O2 to CO in rich H2 environment.The total pressure in the reactor was set at 0.1 MPa. The gas temperature in the reactor was kept at about 300 K during the experiment. Before the mixed gas for CO oxidization was supplied, mesoporous silica particles loaded with TiO2 were filled into the reactor by 50 vol% of full reactor volume size.The experiment was started when illumination of UV light was applied. The gas in reactor was sampled hourly during the experiment. The gas excluding H2O vapor samples was analyzed by TCD gas chromatograph (VARIAN micro-GC CP-4900, GL Science Corp.) equipped with double columns of Molsieve 5A and PoraPLOT Q. The minimum resolution of the gas chromatograph was 1 ppmV. The concentration of H2O vapor in the experimental apparatus was measured by the dew point meter whose minimum resolution was 1 ppmV.


## 3. Results and Discussion

### 3.1. Effect of Loading Ratio of TiO2 on CO Oxidization Performance

Figures5 and 6 show the concentration change of CO2 and CO with UV light illumination time for the different loading ratios of TiO2. From these figures, it can be seen that the concentration of CO for each loading ratio of TiO2 is decreased with the increase of UV light illumination time, while the concentration of CO2 is increased. Although the amount of CO reduced does not match the amount of CO2 produced which is predicted by (9), the reason of it is that the CO adsorption performance of prepared mesoporous silica loaded with TiO2 is different among different loading ratios. The increase ratio of CO2 with time is estimated by the following regression line which is derived according to the tendency of data plot: (11)[CO2]=Rinc-CO2t,
where [CO2] is the concentration of CO2 at each time (ppmV), Rinc-CO2 stands for the increase ratio of CO2 (ppmV/h), and t is the time for UV light illumination (h). According to Figure 5, Rinc-CO2 is 159 ppmV/h, 412 ppmV/h, 493 ppmV/h, 706 ppmV/h, 542 ppmV/h, 527 ppmV/h, and 259 ppmV/h for loading ratio of TiO2 of 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt%, and 80 wt%, respectively. Rinc-CO2 is larger with the increase in loading ratio of TiO2 up to 20 wt%, while it becomes smaller with the increase in loading ratio of TiO2 from 30 wt%. Although the amount of TiO2 loaded in mesoporous silica is increased with the increase in loading ratio of TiO2, which alludes to that the CO oxidization performance is promoted with the increase in the loading ratio of TiO2, the optimum loading ratio of TiO2 is in the middle ratio.Figure 5
Change of concentration of CO2 with UV light illumination time for different loading ratios of TiO2.Figure 6
Change of concentration of CO with UV light illumination time for different loading ratios of TiO2.To compare the oxidization rate of CO, Figure7 shows the change of residual ratio of CO with UV light illumination time for different loading ratios of TiO2. The residual ratio of CO is defined as(12)Rres-CO=[CO][CO]0×100,
where Rres-CO stands for the residual ratio of CO (%), [CO] is the concentration of CO at each time (ppmV), and [CO]0 is the initial concentration of CO at the beginning of the experiment (ppmV). Regression line is derived according to the tendency of data plot:(13)Rres-CO=100e-αt,
where α is the coefficient of CO removal (1/h). According to Figure 7, the α which indicates the oxidization rate of CO is 9.40 × 10−3, 8.30 × 10−2, 1.31 × 10−1, 1.11 × 10−1, 1.42 × 10−1, 9.81 × 10−2, and 2.54 × 10−2, for loading ratio of TiO2 of 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt% and 80 wt%, respectively. α is larger with the increase in loading ratio of TiO2 up to 30 wt%, after that, it is smaller with the increase in loading ratio of TiO2.Figure 7
Residual ratio of CO for different loading ratio of TiO2.Figure8 shows the comparison of selection ratio of CO oxidization among different loading ratios of TiO2. The selection ratio of CO oxidization is calculated by the following equation:(14)Rsel-CO=[CO2]-[CO2]0([CO2]-[CO2]0)+([H2O]-[H2O]0)×100,
where Rsel-CO stands for the selection ratio of CO oxidization (%), [CO2] is the concentration of CO2 at each time (ppmV), [CO2]0 is the initial concentration of CO2 at the beginning of the experiment (ppmV), [H2O] is the concentration of H2O vapor at each time (ppmV), and [H2O]0 is the initial concentration of H2O vapor at the beginning of the experiment (ppmV). In this study, the selection ratio of CO oxidization means the ratio of amount of CO2 to total oxide.Figure 8
Comparison of selection ratio of CO oxidization among different loading ratio of TiO2.From this figure, it is known that the middle loading ratios are better compared with the lower and higher loading ratio conditions. Above all, the best selection ratio of CO oxidization is obtained for the loading ratio of TiO2 of 20 wt%. Considering the results including Rinc-CO2 and α, it can be said that mesoporous silica loaded with TiO2 has the best CO oxidization performance in the middle loading ratio, that is, around 20 wt%. According to our previous study [20], the amount of TiO2, that is, the number of TiO2 particle in mesoporous silica is increased with the increase in loading ratio of TiO2. However, the adsorption performance of mesoporous silica loaded with TiO2 is dropped with the increase in loading ratio of TiO2 due to the pore diameter expansion and the weakening of the honeycomb shape of mesoporous silica by increased loaded TiO2. Therefore, it can be thought that the best match loading condition between high photocatalytic reaction performance and high adsorption performance is obtained in the middle loading ratio for the mesoporous silica loaded with TiO2.To evaluate the CO oxidization performance of mesoporous silica loaded with TiO2 from diverse view points, the summation of the performance comparison factor F which is calculated by (15) is introduced:(15)F=Indexeach-IndexaveIndexave,
where Indexeach and Indexave stand for the value of evaluation index on CO oxidization performance such as Rinc-CO2, α, and Rsel-CO under each loading ratio of TiO2, and the average value of evaluation index on CO oxidization performance among all loading ratios of TiO2, respectively. Here, the data after UV light illumination of 6 hours are used to calculate F for α and Rsel-CO.Table2 lists F and the summation of F for each loading ratio of TiO2. From this table, it reveals that the loading ratio of TiO2 of 20 wt% is the best loading condition. Although the middle loading ratio of TiO2 was clarified to be suitable for CO oxidization in our previous study [20], the current study confirms that the loading ratio of TiO2 of 20 wt% is the optimum loading ratio for the promotion of the CO oxidization performance of mesoporous silica loaded with TiO2.Table 2
Comparison ofF and the summation of F for each loading ratio of TiO2.
F(—)∑F (—)Loading ratio of TiO2 (wt%)Rinc-CO2αRsel-CO1−0.60−0.87−0.29−1.76100.0270.110.0450.18150.230.75−0.130.85200.760.480.671.91300.350.890.291.53600.310.310.110.7380−0.35−0.66−0.25−1.26


### 3.2. Effect of Initial Ratio of O2 to CO on CO Oxidization Performance

Figures9 and 10 show the concentration change of CO2 and CO with UV light illumination time for the different initial concentrations of O2. From these figures, it can be seen that the concentration of CO for each initial concentration of O2 is decreased with the increase in UV light illumination time, while the concentration of CO2 is increased. According to Figure 9, Rinc-CO2 is 139 ppmV/h, 162 ppmV/h, 223 ppmV/h, 260 ppmV/h, 232 ppmV/h, 198 ppmV/h, and 239 ppmV/h for initial concentration of O2 of 0.5 vol%, 1 vol%, 2 vol%, 4 vol%, 6 vol%, 8 vol%, and 10 vol%, respectively. Figure 11 shows the change of residual ratio of CO with UV light illumination time for different initial concentrations of O2. α is 1.43×10-2, 2.88×10-2, 4.62×10-2, 5.23×10-2, 4.00×10-2, 3.97×10-2, and 4.43×10-2 for initial concentration of O2 of 0.5 vol%, 1 vol%, 2 vol%, 4 vol%, 6 vol%, 8 vol% and 10 vol%, respectively. From these results, it is confirmed that the initial concentration of O2exceeding the stoichiometric ratio, that is, 0.5 vol%, is necessary. In addition, Rinc-CO2 and α are increased with the initial concentration of O2 up to 4 vol% and decreased over 4 vol%. Since the experimental apparatus in this study is a batch type and forcible gas mixing is not carried out, it might be thought that excess amount of O2 is necessary for O2 to contact with CO near the surface of mesoporous silica particle loaded with TiO2. However, the excess amount of O2 is thought also to block the diffusions of CO to the surface and CO2 from the surface. Consequently, there is an optimum initial concentration of O2 existing.Figure 9
Change of concentration of CO2 with UV light illumination time for different initial concentrations of O2.Figure 10
Change of concentrations of CO with UV light illumination time for different initial concentrations of O2.Figure 11
Residual ratio of CO for different initial concentrations of O2.Figure12 shows the comparison of selection ratio of CO oxidization among different initial concentrations of O2. From this figure, it is known that the best selection ratio of CO oxidization is obtained for the initial concentration of O2 of 4 vol%, the same as the results of Rinc-CO2 and α as described above. With the lower initial concentration of O2, it seems that the CO oxidization performance is not good due to lack of gas supply to the reaction surface as mentioned above. On the other hand, CO oxidization performance declines at the higher initial concentration of O2. Since H2O that is a byproduct in this reaction is adsorbed by mesoporous silica more easily than CO, O2 and CO2 [24], the CO adsorption by mesoporous silica might be dropped under the higher initial concentration of O2. Therefore, the CO oxidization performance of mesoporous silica loaded with TiO2 also declines. Consequently, the optimum initial concentration of O2 is in the middle level of initial concentration of O2. Table 3 lists F and the summation of F for each initial concentration of O2. From this table, it is revealed that the initial concentration of O2 of 4 vol% is also the best initial concentration from diverse view points. Therefore, the optimum initial concentration of O2 to promote the CO oxidization performance of mesoporous silica loaded with TiO2 is decided at 4 vol%.Table 3
Comparison ofF and the summation of F for each initial concentration of O2.
F(-)∑F(-)Initial concentration of O2 (vol%)Rinc-CO2αRsel-CO0.5−0.33−0.62−0.37−1.321−0.22−0.24−0.32−0.7820.0740.220.230.5240.250.380.350.9860.120.0540.290.468−0.0460.046−0.17−0.17100.150.17−0.0130.31Figure 12
Comparison of selection ratio of CO oxidization among different initial concentrations of O2.


### 3.3. Evaluation on the Maximum CO Oxidization Performance of Mesoporous Silica Loaded with TiO2

The above described results are evaluated by UV light illumination of 6 hours. To evaluate the maximum CO oxidization performance of mesoporous silica loaded with TiO2, a longer time experiment was carried out under the optimum experimental condition as decided above.Figure13 shows the change of each gas concentration with UV light illumination time in the long time experiment with loading ratio of TiO2 of 20 wt% and initial concentration of O2 of 4 vol%. From this figure, the concentration of CO could decrease from 12000 ppmV down to 0 ppmV after UV light illumination time of 72 hours. In other words, although taking longer time, the CO was finally eliminated, which is comparable or superior to the results of the other CO oxidization processes [18, 19, 25, 26]. This proves that the proposed technology of TiO2 combined with silica is a promising alternative CO oxidization process. To promote the CO oxidization performance of mesoporous silica loaded with TiO2, that is, to promote the CO oxidization rate further, the investigation on the gas supply and adsorption control and UV light illumination intensity in reactor is thought to be the next subject to study.Figure 13
Change of each gas concentration with UV light illumination time in the long time experiment (loading ratio of TiO2 of 20 wt%, initial concentrations of O2 of 4 vol%).


## 3.1. Effect of Loading Ratio of TiO2 on CO Oxidization Performance

Figures5 and 6 show the concentration change of CO2 and CO with UV light illumination time for the different loading ratios of TiO2. From these figures, it can be seen that the concentration of CO for each loading ratio of TiO2 is decreased with the increase of UV light illumination time, while the concentration of CO2 is increased. Although the amount of CO reduced does not match the amount of CO2 produced which is predicted by (9), the reason of it is that the CO adsorption performance of prepared mesoporous silica loaded with TiO2 is different among different loading ratios. The increase ratio of CO2 with time is estimated by the following regression line which is derived according to the tendency of data plot: (11)[CO2]=Rinc-CO2t,
where [CO2] is the concentration of CO2 at each time (ppmV), Rinc-CO2 stands for the increase ratio of CO2 (ppmV/h), and t is the time for UV light illumination (h). According to Figure 5, Rinc-CO2 is 159 ppmV/h, 412 ppmV/h, 493 ppmV/h, 706 ppmV/h, 542 ppmV/h, 527 ppmV/h, and 259 ppmV/h for loading ratio of TiO2 of 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt%, and 80 wt%, respectively. Rinc-CO2 is larger with the increase in loading ratio of TiO2 up to 20 wt%, while it becomes smaller with the increase in loading ratio of TiO2 from 30 wt%. Although the amount of TiO2 loaded in mesoporous silica is increased with the increase in loading ratio of TiO2, which alludes to that the CO oxidization performance is promoted with the increase in the loading ratio of TiO2, the optimum loading ratio of TiO2 is in the middle ratio.Figure 5
Change of concentration of CO2 with UV light illumination time for different loading ratios of TiO2.Figure 6
Change of concentration of CO with UV light illumination time for different loading ratios of TiO2.To compare the oxidization rate of CO, Figure7 shows the change of residual ratio of CO with UV light illumination time for different loading ratios of TiO2. The residual ratio of CO is defined as(12)Rres-CO=[CO][CO]0×100,
where Rres-CO stands for the residual ratio of CO (%), [CO] is the concentration of CO at each time (ppmV), and [CO]0 is the initial concentration of CO at the beginning of the experiment (ppmV). Regression line is derived according to the tendency of data plot:(13)Rres-CO=100e-αt,
where α is the coefficient of CO removal (1/h). According to Figure 7, the α which indicates the oxidization rate of CO is 9.40 × 10−3, 8.30 × 10−2, 1.31 × 10−1, 1.11 × 10−1, 1.42 × 10−1, 9.81 × 10−2, and 2.54 × 10−2, for loading ratio of TiO2 of 1 wt%, 10 wt%, 15 wt%, 20 wt%, 30 wt%, 60 wt% and 80 wt%, respectively. α is larger with the increase in loading ratio of TiO2 up to 30 wt%, after that, it is smaller with the increase in loading ratio of TiO2.Figure 7
Residual ratio of CO for different loading ratio of TiO2.Figure8 shows the comparison of selection ratio of CO oxidization among different loading ratios of TiO2. The selection ratio of CO oxidization is calculated by the following equation:(14)Rsel-CO=[CO2]-[CO2]0([CO2]-[CO2]0)+([H2O]-[H2O]0)×100,
where Rsel-CO stands for the selection ratio of CO oxidization (%), [CO2] is the concentration of CO2 at each time (ppmV), [CO2]0 is the initial concentration of CO2 at the beginning of the experiment (ppmV), [H2O] is the concentration of H2O vapor at each time (ppmV), and [H2O]0 is the initial concentration of H2O vapor at the beginning of the experiment (ppmV). In this study, the selection ratio of CO oxidization means the ratio of amount of CO2 to total oxide.Figure 8
Comparison of selection ratio of CO oxidization among different loading ratio of TiO2.From this figure, it is known that the middle loading ratios are better compared with the lower and higher loading ratio conditions. Above all, the best selection ratio of CO oxidization is obtained for the loading ratio of TiO2 of 20 wt%. Considering the results including Rinc-CO2 and α, it can be said that mesoporous silica loaded with TiO2 has the best CO oxidization performance in the middle loading ratio, that is, around 20 wt%. According to our previous study [20], the amount of TiO2, that is, the number of TiO2 particle in mesoporous silica is increased with the increase in loading ratio of TiO2. However, the adsorption performance of mesoporous silica loaded with TiO2 is dropped with the increase in loading ratio of TiO2 due to the pore diameter expansion and the weakening of the honeycomb shape of mesoporous silica by increased loaded TiO2. Therefore, it can be thought that the best match loading condition between high photocatalytic reaction performance and high adsorption performance is obtained in the middle loading ratio for the mesoporous silica loaded with TiO2.To evaluate the CO oxidization performance of mesoporous silica loaded with TiO2 from diverse view points, the summation of the performance comparison factor F which is calculated by (15) is introduced:(15)F=Indexeach-IndexaveIndexave,
where Indexeach and Indexave stand for the value of evaluation index on CO oxidization performance such as Rinc-CO2, α, and Rsel-CO under each loading ratio of TiO2, and the average value of evaluation index on CO oxidization performance among all loading ratios of TiO2, respectively. Here, the data after UV light illumination of 6 hours are used to calculate F for α and Rsel-CO.Table2 lists F and the summation of F for each loading ratio of TiO2. From this table, it reveals that the loading ratio of TiO2 of 20 wt% is the best loading condition. Although the middle loading ratio of TiO2 was clarified to be suitable for CO oxidization in our previous study [20], the current study confirms that the loading ratio of TiO2 of 20 wt% is the optimum loading ratio for the promotion of the CO oxidization performance of mesoporous silica loaded with TiO2.Table 2
Comparison ofF and the summation of F for each loading ratio of TiO2.
F(—)∑F (—)Loading ratio of TiO2 (wt%)Rinc-CO2αRsel-CO1−0.60−0.87−0.29−1.76100.0270.110.0450.18150.230.75−0.130.85200.760.480.671.91300.350.890.291.53600.310.310.110.7380−0.35−0.66−0.25−1.26


## 3.2. Effect of Initial Ratio of O2 to CO on CO Oxidization Performance

Figures9 and 10 show the concentration change of CO2 and CO with UV light illumination time for the different initial concentrations of O2. From these figures, it can be seen that the concentration of CO for each initial concentration of O2 is decreased with the increase in UV light illumination time, while the concentration of CO2 is increased. According to Figure 9, Rinc-CO2 is 139 ppmV/h, 162 ppmV/h, 223 ppmV/h, 260 ppmV/h, 232 ppmV/h, 198 ppmV/h, and 239 ppmV/h for initial concentration of O2 of 0.5 vol%, 1 vol%, 2 vol%, 4 vol%, 6 vol%, 8 vol%, and 10 vol%, respectively. Figure 11 shows the change of residual ratio of CO with UV light illumination time for different initial concentrations of O2. α is 1.43×10-2, 2.88×10-2, 4.62×10-2, 5.23×10-2, 4.00×10-2, 3.97×10-2, and 4.43×10-2 for initial concentration of O2 of 0.5 vol%, 1 vol%, 2 vol%, 4 vol%, 6 vol%, 8 vol% and 10 vol%, respectively. From these results, it is confirmed that the initial concentration of O2exceeding the stoichiometric ratio, that is, 0.5 vol%, is necessary. In addition, Rinc-CO2 and α are increased with the initial concentration of O2 up to 4 vol% and decreased over 4 vol%. Since the experimental apparatus in this study is a batch type and forcible gas mixing is not carried out, it might be thought that excess amount of O2 is necessary for O2 to contact with CO near the surface of mesoporous silica particle loaded with TiO2. However, the excess amount of O2 is thought also to block the diffusions of CO to the surface and CO2 from the surface. Consequently, there is an optimum initial concentration of O2 existing.Figure 9
Change of concentration of CO2 with UV light illumination time for different initial concentrations of O2.Figure 10
Change of concentrations of CO with UV light illumination time for different initial concentrations of O2.Figure 11
Residual ratio of CO for different initial concentrations of O2.Figure12 shows the comparison of selection ratio of CO oxidization among different initial concentrations of O2. From this figure, it is known that the best selection ratio of CO oxidization is obtained for the initial concentration of O2 of 4 vol%, the same as the results of Rinc-CO2 and α as described above. With the lower initial concentration of O2, it seems that the CO oxidization performance is not good due to lack of gas supply to the reaction surface as mentioned above. On the other hand, CO oxidization performance declines at the higher initial concentration of O2. Since H2O that is a byproduct in this reaction is adsorbed by mesoporous silica more easily than CO, O2 and CO2 [24], the CO adsorption by mesoporous silica might be dropped under the higher initial concentration of O2. Therefore, the CO oxidization performance of mesoporous silica loaded with TiO2 also declines. Consequently, the optimum initial concentration of O2 is in the middle level of initial concentration of O2. Table 3 lists F and the summation of F for each initial concentration of O2. From this table, it is revealed that the initial concentration of O2 of 4 vol% is also the best initial concentration from diverse view points. Therefore, the optimum initial concentration of O2 to promote the CO oxidization performance of mesoporous silica loaded with TiO2 is decided at 4 vol%.Table 3
Comparison ofF and the summation of F for each initial concentration of O2.
F(-)∑F(-)Initial concentration of O2 (vol%)Rinc-CO2αRsel-CO0.5−0.33−0.62−0.37−1.321−0.22−0.24−0.32−0.7820.0740.220.230.5240.250.380.350.9860.120.0540.290.468−0.0460.046−0.17−0.17100.150.17−0.0130.31Figure 12
Comparison of selection ratio of CO oxidization among different initial concentrations of O2.


## 3.3. Evaluation on the Maximum CO Oxidization Performance of Mesoporous Silica Loaded with TiO2

The above described results are evaluated by UV light illumination of 6 hours. To evaluate the maximum CO oxidization performance of mesoporous silica loaded with TiO2, a longer time experiment was carried out under the optimum experimental condition as decided above.Figure13 shows the change of each gas concentration with UV light illumination time in the long time experiment with loading ratio of TiO2 of 20 wt% and initial concentration of O2 of 4 vol%. From this figure, the concentration of CO could decrease from 12000 ppmV down to 0 ppmV after UV light illumination time of 72 hours. In other words, although taking longer time, the CO was finally eliminated, which is comparable or superior to the results of the other CO oxidization processes [18, 19, 25, 26]. This proves that the proposed technology of TiO2 combined with silica is a promising alternative CO oxidization process. To promote the CO oxidization performance of mesoporous silica loaded with TiO2, that is, to promote the CO oxidization rate further, the investigation on the gas supply and adsorption control and UV light illumination intensity in reactor is thought to be the next subject to study.Figure 13
Change of each gas concentration with UV light illumination time in the long time experiment (loading ratio of TiO2 of 20 wt%, initial concentrations of O2 of 4 vol%).


## 4. Conclusions

Based on the above experimental results and discussion, the following conclusions can be drawn from this experimental study.The optimum loading ratio of TiO2 is around 20 wt% and the optimum initial concentration of O2 is 4 vol% from the viewpoint of best matching of reaction rate of CO oxidization and selection ratio of CO oxidization. The best match loading condition between high photocatalytic reaction performance and high adsorption performance is in the middle loading ratio for the mesoporous silica loaded with TiO2.The initial concentration of O2 in excess of the stoichiometric ratio is necessary to ensure enough gas supplied to the reaction surface. However, too much excess initial concentration of O2 would cause the block of gas diffusion to or from the surface which undermines the CO adsorption performance and would produce too much water that was more easily adsorbed by the mesoporous silica particle loaded with TiO2, resulting in the drop of the CO oxidization performance.The CO of 12000 ppmV in the rich H2 could be completely oxidized after UV light illumination time of 72 hours, which is comparable with the other CO removal methods.



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*Source: 294217-2010-12-22.xml*