International Journal of Photoenergy

Volume 2012 (2012), Article ID 174862, 8 pages

doi:10.1155/2012/174862

Research Article

Simultaneous Elimination of Formaldehyde and Ozone Byproduct Using Noble Metal Modified TiO₂ Films in the Gaseous VUV Photocatalysis

Pingfeng Fu,¹ Pengyi Zhang,² and Jia Li²

¹School of Civil and Environment Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, China

²State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China

Received 30 January 2012; Accepted 16 March 2012

Academic Editor: Baibiao Huang

Copyright © 2012 Pingfeng Fu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Simultaneous removal of low concentration formaldehyde (HCHO) and ozone byproduct was investigated in the gaseous VUV (vacuum ultraviolet) photocatalysis by using noble metal modified TiO₂ films. Noble metal (Pt, Au, or Pd) nanoparticles were deposited on TiO₂ films with ultrafine particle size and uniform distribution. Under 35 h VUV irradiation, the HCHO gas (ca. 420 ppbv) was dynamically degraded to a level of 10~45 ppbv without catalyst deactivation, and over 50% O₃ byproduct was in situ decomposed in the reactor. However, under the same conditions, the outlet HCHO concentration remained at 125~178 ppbv in the O₃ + UV254 nm photocatalysis process and 190~260 ppbv in the UV254 nm photocatalysis process. And the catalyst deactivation also appeared under UV254 nm irradiation. Metallic Pt or Au could simultaneously increase the elimination of HCHO and ozone, but the PdO oxide seemed to inhibit the HCHO oxidation in the UV254 nm photocatalysis. Deposition of metallic Pt or Au reduces the recombination of h⁺/e⁻ pairs and thus increases the HCHO oxidation and O₃ reduction reactions. In addition, adsorbed O₃ may be partly decomposed by photogenerated electrons trapped on metallic Pt or Au nanoparticles under UV irradiation.

1. Introduction

The heterogeneous photocatalytic oxidation (PCO) can eliminate various kinds of volatile organic compounds (VOCs) with the potential to improve indoor air quality (IAQ) [1–4]. However, the low degradation rate [2, 5] and possible photocatalyst deactivation [6, 7] limit its practical application. Vacuum ultraviolet (VUV) with high-energy photon can dissociate oxygen and water molecules in the gas phase to reactive oxygen species such as O(¹D), O(³P), and hydroxyl radicals () [8, 9]. Recently, several authors reported that VUV photocatalysis (i.e., TiO₂ photocatalyst combined with 185 nm VUV) showed higher decomposition rates of VOCs than the common UV254 nm photocatalysis [8–12]. Furthermore, the catalyst deactivation is alleviated by effective decomposition of nonvolatile oxidation intermediates on catalyst surface [8–10]. However, ozone byproduct at a ppm level is also formed in the gaseous VUV photocatalysis [9, 10]. As ozone is also a hazardous contaminant in indoor environment, the elimination of ozone byproduct is necessary for the safe use of VUV photocatalytic technique.

Ozone-decomposing catalysts (e.g., MnO₂) have been employed to remove ozone byproduct in a separate unit following the photocatalytic process [8, 10]. However, these catalysts usually lose activity while the humidity is high [13, 14]. Besides the thermal decomposition process, the semiconductor photocatalysis has also been considered to remove gaseous ozone [15, 16]. Modification of TiO₂ with noble metals (e.g., Pt, Ag, Au) can remarkably increase the decomposition rate of ozone by trapping photogenerated electrons with metal nanoparticles [17–19]. Therefore, to eliminate ozone byproduct in this case, it is an effective way to in situ photocatalytically decompose O₃ via increasing the reactivity of used photocatalysts. The aim of this study is to evaluate the feasibility of simultaneous elimination of formaldehyde and ozone byproduct in the VUV photocatalysis by using noble metal (Pt, Au, or Pd) modified TiO₂ film photocatalysts.

2. Experimental

Noble metal nanoparticles (NPs) modified TiO₂ films, that is, Pt-TiO₂, Au-TiO₂, or Pd-TiO₂ films supported on Ti mesh, were prepared via a low-temperature electrostatic self-assembly method as described in our previous paper [20]. The morphologies of the photocatalysts were observed using an ultra-high-resolution field-emission scanning electron microscope (FESEM, S-5500, Hitachi). X-ray diffraction (XRD) analysis was carried out with a Rigaku D/max-RB using Cu Kα radiation. To analyze the crystalline structure of loaded noble metal nanoparticles, the amount of deposited Pt, Au, or Pd nanoparticles reached ca. 4% (wt).

The used VUV lamp (3 W) was an ozone-producing low-pressure mercury lamp with at 254 nm and a minor emission (ca. 5%) at 185 nm. Two VUV lamps were located at the center of the flow photoreactor with an effective volume of 0.628 L. Two pieces of Pt (Au, Pd)-TiO₂/Ti (or TiO₂/Ti) mesh were, respectively, fixed at both sides of the lamp. The UV254 nm intensity at the photocatalyst surface was 3.2 mW/cm². The effective residence time of HCHO gas was 21 s in the reactor with the gas-solid contact time of 0.34 s. Because the indoor formaldehyde concentration usually lies at a ppbv level, typically less than 300 ppbv [21], the inlet concentration of HCHO was set at ca. 420 ppbv. After the HCHO concentration reached dynamical equilibrium (2 h), two VUV lamps were turned on to start the 35 h VUV photocatalysis. To carry out the O₃ enhanced UV254 nm (O₃ + UV254 nm) photocatalysis, two UV254 nm lamps (3 W) were used, and the HCHO gas mixed with O₃ was introduced. The concentration (ca. 22.5 mg/m³) of introduced O₃ is close to that of ozone generated by two VUV lamps. The other procedures were as same as the VUV photocatalytic experiments. The 35 h UV254 nm photocatalysis was carried out with the same procedure except no ozone is mixed with HCHO gas. The O₃ concentration was monitored with an on-line O₃ analyzer (Model 49i, Thermo Electron). The formaldehyde concentration was analyzed by MBTH method (GB/T18204.28, China). Because the ozone may disturb the formaldehyde analysis with MBTH method, a KI (potassium iodide) coated annular denuder was used to scrub ozone while sampling.

In this study, the removal rate of HCHO and O₃ was calculated according to and , and the reaction rate of HCHO oxidation was calculated with :

where the was the equilibrium concentration of HCHO before irradiation, the was the steady-state concentration of HCHO under UV irradiation; represented the outlet concentration of VUV generated ozone (without photocatalysts) with a value of 22.5 mg/m³, and the was the steady-state concentration of ozone under UV irradiation; was the flow rate of HCHO gas (m³/min), and was the effective volume of the photoreactor (m³).

3. Results and Discussion

3.1. Characterization of Pt (Au, Pd)-TiO₂ Film Photocatalysts

Figure 1 shows the ultra-high-resolution FESEM images of Pt, Au, and Pd modified TiO₂ films. Noble metal (Pt, Au, Pd) nanoparticles (NPs) are physically separated and uniformly dispersed on TiO₂ films with an interparticle spacing of 5~20 nm. The average particle size of loaded Pt, Au, and Pd NPs is 1.9 nm, 4.2 nm, and 3.9 nm, respectively. Each TiO₂ particle is coated with nearly 2~3 metal NPs. The surface density of noble metal NPs reaches (5~10) × 10¹¹ particles per cm². It is obvious that noble metal NPs can be well deposited on TiO₂ films in the electrostatic self-assembly process.

(a)

(b)

(c)

(d)

Figure 1: FESEM images of deposited Pt (a), Au (b), and Pd (c) nanoparticles on TiO₂ films. (d) The size distribution histogram of deposited Pt, Au, and Pd nanoparticles.

The XRD analysis (Figure 2) reveals that TiO₂ films have mixed crystalline phases with 62.5% anatase and 37.5% rutile. As shown in Figure 2, the diffraction peaks at , 46.4°, 67.5°, and 81.3° can be attributed to the (111), (200), (220), and (311) reflections of metallic Pt (JCPDS number 4-0802). For the Au-TiO₂ films, the diffraction peaks at , 44.4°, 64.7°, 77.5°, and 81.9° can be attributed to the (111), (200), (220), (311), and (222) reflections of metallic Au (JCPDS number 65-8601). For the Pd-TiO₂ films, the peaks at , 41.9°, 60.3°, 60.9°, and 71.5° can be attributed to the (101), (110), (103), (200), and (202) reflections of palladium oxide (PdO, JCPDS number 41-1107), respectively. After annealed in air at 300°C for 1.5 h, Pt and Au NPs are still in metallic form, while Pd NPs have been oxidized to palladium oxide (PdO).

Figure 2: XRD patterns of as-prepared TiO₂ films (a), Pt-TiO₂ (b), Au-TiO₂ (c), and Pd-TiO₂ (d) nanocomposite films scratched from Ti wire net.

3.2. VUV Photocatalytic Degradation of Formaldehyde and Simultaneous Removal of Ozone Byproduct

As shown in Figure 3, the HCHO can be rapidly decomposed under VUV irradiation even without the photocatalyst, but ozone byproduct with high concentration (ca. 22.4 mg/m³) appears. Abundant reactive oxygen species such as hydroxyl radical, O(¹D), and O(³P) are formed under 185 nm VUV irradiation. So the VUV photolysis (VUV photochemical process) of HCHO can effectively occur. While TiO₂ or noble metal modified TiO₂ photocatalysts are presented, the reaction rate of HCHO decomposition increases from 1.16 to above 1.4 mg/m³·min. The outlet concentration of residual HCHO lowers enough to 10~45 ppbv (Table 1). These steady-state HCHO concentrations are much lower than the WHO guideline level of indoor formaldehyde (80 ppbv).

Table 1: The outlet concentration and removal rates of HCHO and O₃, and reaction rates of HCHO in the 35 h VUV, O₃ + UV254 nm, and UV254 nm photocatalysis, respectively.

Kinds of photocatalysis
Photocatalysts
Formaldehyde
Ozone

Concentration

(ppbv)
Removal rate

(%)
Reaction rate

(mg/m³·min)
Concentration

(mg/m³)
Removal rate

(%)

VUV photocatalysis
VUV photolysis
122.4
72.9
1.16
22.4
0

TiO₂
36.1
91.4
1.34
19.2
14.3

Pd-TiO₂
20.1
95.3
1.43
14.2
36.6

Pt-TiO₂
9.5
97.7
1.47
10.7
52.2

Au-TiO₂
27.6
93.6
1.41
15.1
32.6

photocatalysis
TiO₂
178.8
57.4
0.85
13.8
37.8

Pd-TiO₂
150.1
65.7
0.96
9.3
58.1

Pt-TiO₂
125.1
68.6
1.01
6.2
72.1

Au-TiO₂
130.6
68.8
1.02
10.8
51.4

 photocatalysis
TiO₂
263.4
37.8
0.57

Pd-TiO₂
251.3
37.6
0.56

Pt-TiO₂
190.6
56.7
0.89

Au-TiO₂
218.1
45.2
0.63

(a)

(b)

Figure 3: Time courses for the outlet concentration of HCHO (a) and ozone byproduct (b) with irradiation time in the 35 h VUV photocatalysis. The VUV photolysis is the VUV photochemical process without the photocatalyst.

In the VUV photocatalysis, the HCHO can be decomposed both in gas phase via VUV photochemical process and on the photocatalyst surface via UV254 nm excited photocatalysis [8–10]. So the presence of photocatalysts under VUV irradiation can remarkably increase the oxidation rate of VOCs. However, in the VUV photocatalysis of HCHO, only minor improvement of HCHO removal is observed by modifying TiO₂ films with noble metals. This may be attributed to that most of HCHO molecules are decomposed in gas phase not on the photocatalyst surface.

However, the removal ratio of ozone byproduct increases from 14.3% (TiO₂) to 32~52% (Au, Pd, or Pt-TiO₂) with an enhancement of 2.3~3.6-folds (Table 1). At the same time, the outlet O₃ concentration can keep almost at a same level for a long steady-state period for noble metal modified TiO₂, but it gradually rises in the case of pure TiO₂. The result suggests that ozone byproduct can be in  situ photocatalytically decomposed in the photoreactor, and modification of TiO₂ films with noble metals can both enhance the photodegradation of O₃ and alleviate the catalyst deactivation. The results reveal that simultaneous removal of low concentration HCHO and O₃ byproduct is surly feasible in the VUV photocatalysis when TiO₂ films are modified with Pt, Au, or Pd NPs.

3.3. Comparison of HCHO Degradation in Long-Term VUV Photocatalysis, O₃ + UV254 nm Photocatalysis and UV254 nm Photocatalysis

Figures 4(a) and 5 show the outlet concentrations of HCHO versus irradiation time in the O₃ + UV254 nm and UV254 nm photocatalysis, respectively. As shown in Figures 4(a) and 5 and Table 1, the outlet concentration of HCHO reaches 125~178 ppbv in the O₃ + UV254 nm photocatalysis and 190~260 ppbv in the UV254 nm photocatalysis, while it is just 10~45 ppbv in the VUV photocatalysis when the same photocatalyst is used. The reaction rate of HCHO oxidation in the VUV photocatalysis is ca. 1.4 times and 2 times higher than that in the O₃ + UV254 nm and UV254 nm photocatalysis, respectively. As listed in Table 1, the HCHO degradation rate in the VUV photolysis (without photocatalyst) has been much higher than that in the O₃ + UV254 nm or UV254 nm photocatalysis. Obviously, the VUV photochemical process in gas phase makes great contribution in the oxidation of HCHO molecules in the VUV photocatalysis.

(a)

(b)

Figure 4: Time courses for the outlet concentration of formaldehyde (a) and ozone (b) with irradiation time in the 35 h O₃ + UV254 nm photocatalysis.

Figure 5: Time courses for the outlet concentration of formaldehyde with irradiation time in the 35 h UV254 nm photocatalysis.

Additionally, the stability of the outlet HCHO concentration in the long-term VUV photocatalysis is also very different from that in the O₃ + UV254 nm or UV254 nm photocatalysis. The HCHO concentration gradually rises up, while the reaction time exceeds 25 h in the O₃ + UV254 nm or UV254 nm photocatalysis (Figures 4(a) and 5), while it remains at a very low level in the VUV photocatalysis (Figure 3). Due to the lower degradation rate of HCHO in the O₃ + UV254 nm or UV254 nm photocatalysis, the reaction intermediates combined with adsorbed HCHO molecules may persistently accumulate on the catalyst surfaces, which leads to observed catalyst deactivation. Nevertheless, the degradation behaviour under VUV irradiation suggests that no catalyst deactivation occurs in the VUV photocatalysis regardless of modification of TiO₂. Due to abundant reactive species generated in gas phase, the oxidation of reaction intermediates and HCHO should been significantly enhanced under VUV irradiation. Therefore, the carbon accumulation on the catalyst could be alleviated even the HCHO gas is continuously introduced with very short residence time. The results in this study are highly consistent with the previously observed long lifetime of the photocatalysts under VUV irradiation [8–10].

While the HCHO concentration is at a typical indoor level, the film-diffusional resistance at the interface of gas phase and catalyst film becomes very large, leading to remarkable decrease of HCHO degradation rate in the UV254 nm photocatalysis [22]. Until now, it is still a great challenge for UV254 nm photocatalysis using semiconductor films to effectively decompose low concentration HCHO, while the gas residence time becomes short enough to several seconds [22–24]. In this study, the results indicate that the VUV photocatalysis cannot only rapidly decompose HCHO vapor with low concentration but also avoid the deactivation of the photocatalysts. Therefore, it can be anticipated that the VUV photocatalysis is an alternative method to efficiently decompose indoor VOCs.

3.4. Enhanced Effects of Deposited Noble Metals on the Decomposition of HCHO and Ozone

As shown in Table 1, deposition of noble metals (Pt or Au) on TiO₂ surface actually increases the decomposition rate of HCHO in the VUV, O₃ + UV254 nm or UV254 nm photocatalysis. For the ozone decomposition, all of these three noble metal NPs (Pt, Au, or Pd) can have evident positive role. Especially, the removal ratio of HCHO increases from 57.4% (TiO₂) to ca. 68% (Pt, or Au-TiO₂) and that of O₃ augments from 37.8% (TiO₂) to 72.1% (Pt-TiO₂) in the O₃ + UV254 nm photocatalysis. In the UV254 nm photocatalysis, the reaction rate of HCHO oxidation increases from 0.57 mg/m³·min (TiO₂) to 0.89 mg/m³·min (Pt-TiO₂). However, deposition of Pd NPs in form of PdO seems to be deleterious to HCHO oxidation in the UV254 nm photocatalysis (Figure 5).

These results indicate that deposited metallic NPs (Pt, Au) can enhance the photocatalytic decomposition of both HCHO and O₃ molecules. While the PdO NPs appears, photocatalytic oxidation of HCHO is inhibited, but the ozone decomposition surely enhanced. The O₃ removal behaviours (Figures 3 and 4) also reveal that the ozone, in situ generated in the VUV photoreactor or exteriorly added, can be effectively decomposed similarly. In the tested noble NPs (Au, Pt, or Pd), the metallic Pt NPs modified TiO₂ exhibits the highest activity in the HCHO oxidation and O₃ reduction reactions. In addition, it can find that noble metal deposition has much more evident effect in O₃ removal than in the HCHO degradation.

3.5. Proposed Mechanism of Positive Role of Deposited Noble Metallic NPs

In this work, the thermal decomposition of O₃ on noble metal modified TiO₂ films is proved to be little (less than 5%) at room temperature in the dark. So the observed O₃ decomposition should be ascribed to heterogeneous photodegradation. Therefore, we propose the mechanism of enhanced electron-hole separation caused by deposited noble metallic Pt or Au NPs and O₃ decomposition routes (Figure 6). As presented in Figure 6, locally formed metallic Pt or Au NPs act as the electron trapping center. When the photogenerated electrons are trapped by metallic Pt or Au, electron-hole pairs are efficiently separated, producing more on the TiO₂ surface [20, 25–28]. Thus, the photocatalytic oxidation of HCHO is significantly enhanced.

Figure 6: Proposed mechanism of enhanced electron-hole separation caused by deposited noble metallic Au or Pt nanoparticles and O₃ decomposition routes on the Pt or Au modified TiO₂.

For the photocatalytic decomposition of O₃, the previous ESR measurements revealed that ozonide radical anion , surface radicals, and hydroperoxyl radical could be produced at the gaseous O₃/TiO₂ interface under UV irradiation [29]. Therefore, we propose the photocatalytic decomposition of O₃ on exposed TiO₂ surface of the Pt-TiO₂ (or Au-TiO₂) as follows [15, 29, 30]:

The adsorbed O₃ is reduced to radicals by capturing the photogenerated electrons . Under UV irradiation, the unstable radicals rapidly split to form O₂ and radicals . Then, the adsorbed water molecules are oxidized by or radicals to form radicals and . Alternatively, adsorbed O₂ will also capture electrons, leading to formation of and radicals and . Obviously, enhanced separation of pairs allows more photogenerated electrons to be captured by adsorbed O₃ molecules.

As shown in Table 1, the O₃ removal rate is improved over 2 times with metallic Pt or Au modified TiO₂. The enhanced separation efficiency by deposited Pt or Au NPs could improve the photocatalytic decomposition of adsorbed O₃ on exposed TiO₂. But it should be noted that the exposed TiO₂ surface area, available for O₃ decomposition, is also remarkably reduced owing to covering Pt or Au NPs layer. Since the ESR tests have proved that adsorbed O₃ can capture electrons to form radicals on CeO₂ [31] or TiO₂ [29] surface even in the dark. We propose here that adsorbed O₃ may also interact with electron-rich surface sites on Pt or Au NPs to form radicals by considering the fact that photogenerated electrons have been readily trapped by metallic Pt or Au NPs. With the help of UV irradiation, the split of radicals on Pt or Au NPs is enhanced . The similar phenomenon had been observed in the ESR test of gaseous O₃/TiO₂ interface [29]. Then, the chain radical reactions proceeded on Pt or Au NPs may be similar to those on UV-irradiated TiO₂   and .

4. Conclusions

In this work, we investigated the feasibility of simultaneous elimination of gaseous formaldehyde and ozone byproduct in the VUV photocatalysis process. The gaseous HCHO with the inlet concentration of ca. 420 ppbv can be readily degraded to meet the WHO guideline level of indoor formaldehyde in a short solid-gas contact time (0.34 s). However, the O₃ + UV254 nm or UV254 nm photocatalysis shows much lower removal ratio of HCHO with observable catalyst deactivation. By modifying TiO₂ film with noble metallic Pt or Au, both HCHO removal and ozone decomposition are significantly improved. Especially, over 50% ozone byproduct can be in  situ decomposed by using Pt-TiO₂ films. However, deposition of PdO oxide seems to inhibit the oxidation of HCHO in the UV254 nm photocatalysis but enhances the O₃ decomposition in the VUV or O₃ + UV254 nm photocatalysis. Loaded metallic Pt or Au particles reduce the recombination of pairs and thus increase the HCHO oxidation and O₃ photocatalytical reduction on exposed TiO₂ surface. In addition, adsorbed O₃ may also be decomposed on electron-rich surface sites of metallic Pt or Au under UV irradiation.

Acknowledgments

The authors gratefully acknowledge the financial support from National High Technology Research and Development Program of China (2012AA062701), National Nature Science Foundation of China (50908132), and Tsinghua University Initiative Scientific Research Program.

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