TiO2-based heterogeneous photocatalytic oxidation (PCO) is cost-effective and can operate at room temperature and atmospheric pressure. Hence, it has been widely investigated to decompose gaseous pollutants such as volatile organic compounds (VOCs) [1, 2, 3, 4]. Formaldehyde (HCHO) is a major indoor pollutant, and efficient removal of it in air has attracted great attention [5, 6, 7]. However, some key problems, such as catalyst deactivation [8, 9] and inhibition effect of other molecules (e.g., H2O, NO) [10, 11], hinder the practical application of PCO. Various approaches have been employed to overcome these drawbacks such as combination of PCO with microwave [12], ozone [13, 14], and adsorption [15, 16].
Vacuum ultraviolet (VUV, λ < 200 nm) with high-energy photons can dissociate oxygen and water molecules into reactive oxygen species (e.g., O(1D), O(3P), and O3) and hydroxyl radicals (•OH), which can readily react with various VOCs in gas phase [17, 18, 19]. The photocatalytic efficiency in removing indoor VOCs remarkably increases under UV254+185 nm irradiation (UV with λmax = 254 nm and minor VUV λ = 185 nm) compared with that achieved under common UV254 nm or UV365 nm irradiation [17, 18, 19, 20]. Furthermore, catalyst deactivation is reduced by effective decomposition of nonvolatile intermediates on the catalyst surface [18, 19]. Therefore, photocatalysis under UV254+185 nm irradiation is a facile and effective PCO technique for indoor air purification. However, hazardous contaminant O3 is also generated via dissociation of O2 with VUV [17, 18]. In general, removal of O3 byproduct, generated during photocatalysis under UV254+185 nm irradiation, has been attempted using two routes: photocatalysis followed by ozone destruction catalysis [18, 20], and in situ catalytic destruction using modified photocatalysts, e.g., TiO2 modified with noble metals [21]. We previously found that Pd-modified TiO2 films could simultaneously increase the formaldehyde degradation and O3 elimination under UV254+185 nm irradiation [21]. Additionally, the redox processes of Pd species under UV254+185 nm irradiation were critical for the simultaneous decomposition of formaldehyde and O3. The electronic states of the noble metal are likely to remarkably differ when noble metal-modified photocatalysts are applied in UV254 nm and UV254+185 nm irradiation photocatalytic systems owing to the presence of VUV-ph otochemically generated reactive species. Therefore, it is necessary to identify the electronic states of metal species when noble metal-modified photocatalysts are applied in the UV254+185 nm irradiation photocatalytic system.
Lowering of catalyst deactivation was reported for the photocatalytic decomposition of formaldehyde [21], toluene [17, 20], and benzene [18] under UV254+185 nm irradiation. Gaseous reaction intermediates were measured to compare the photocatalytic degradation performance under UV254 nm and UV254+185 nm irradiation [18, 22], thus to indirectly elucidate the decrease in catalyst deactivation under UV254+185 nm irradiation compared to UV254 nm irradiation. It is well known that accumulation of reaction intermediates on the catalyst surface is a key factor resulting in catalyst deactivation. Hence, quantification of accumulated intermediates could provide direct evidence for assessing degree of catalyst deactivation. However, little information on surface properties of photocatalysts has been reported to directly explain the low degree of catalyst deactivation under UV254+185 nm irradiation.
In this work, we investigated the performance of Pt- modified TiO2 films (Pt-TiO2) to photocatalytically remove formaldehyde and O3 under UV254+185 nm irradiation. To illustrate the mechanism of reduction of catalyst deactivation and elucidate the role of deposited Pt nanoparticles on decomposing O3, a comparative X-ray photoelectron spectroscopy (XPS) study was conducted to assess the accumulation of organic species on the catalyst surface and changes of the Pt valence states in the Pt-TiO2 films under three irradiation systems: UV254 nm, O3+UV254 nm, and UV254+185 nm.
TiO2 films coated on a Ti wire net (40 mesh) (TiO2/Ti) were fabricated via a sol-gel dip coating method [23]. Pt nanoparticles (NPs) were deposited on the TiO2/Ti wire net using a low-temperature electrostatic self-assembly method, as described elsewhere [21, 24]. H2PtCl6 solution (100 mL, 0.3 mmol/L) containing polyvinyl alcohol (PVA, 300 mg) was cooled in an ice water bath, to which a fresh NaBH4 solution (4 mL, 0.07 mol/L) was rapidly added under sonication to obtain a reduced Pt colloidal solution. A piece of TiO2/Ti wire net (L×W = 80 mm × 80 mm) was then immersed into the fresh Pt colloidal solution. The negatively charged Pt NPs were spontaneously adsorbed onto the positively charged TiO2 surface via electrostatic interactions at pH ≈ 4. The self-assembly time was 30 min. The TiO2/Ti wire net was removed from the solution and washed with boiling water to remove adsorbed chloride ions. The dried TiO2/Ti wire net was then annealed in air at 300 °C for 1.5 h to fabricate the Pt-TiO2 film.
The morphology of the samples was observed using an ultra-high-resolution field-emission scanning electron microscope (FESEM, S-5500, Hitachi). X-ray diffraction (XRD) analysis was carried out on a Rigaku D/max-RB using Cu Kα radiation (λ = 0.15418 nm), operating at 40 kV and 100 mA. The surface properties of the Pt-TiO2 films were investigated by XPS (PHI-5300, ESCA) at a pass energy of 50 eV, using Al Kα as an exciting X-ray source. The base pressure in the measurement chamber was 4.53 × 10−7 Pa. The spectra were calibrated with respect to the C 1s line of graphitic carbon at 284.8 eV. Prior to individual elemental scans, a survey scan was conducted to detect all of the present elements. XPSPEAK software (version 4.1) with a Gaussian-Lorentzian mixed function and Shirley background subtraction was employed to deconvolute the XPS spectra. The peak positions were reproducible as well as the fixed Lorentz/Gaussian ratio of 1:4 and FWHM of the peaks.
The experimental setup is the same as that used in our previous work [21]. All UV lamps were purchased from Guangdong Cnlight Company. Two UV254+185 nm lamps (2 × 3 W), i.e., ozone-producing low-pressure mercury lamps with λmax at 254 nm and a minor emission (ca. 5%) at λ = 185 nm, were placed in the center of the flow-through reactor with an effective volume of 0.628 L. Two pieces of Pt-TiO2/Ti wire nets were fixed at both sides of the lamp, respectively. The flow rate was fixed at 1.8 L/min. The inlet concentration of HCHO was set at ca. 0.52 mg/m3. After the inlet and outlet HCHO concentrations reached equilibrium, the UV254+185 nm lamps were turned on. All photocatalytic experiments ran for 35 h. The UV lamps were then turned off to allow the system to equilibrate again. For the O3-enhanced UV254 nm (O3+UV254 nm) photocatalysis setup, two low-pressure mercury lamps (λmax = 254 nm, 2 × 3 W), which do not produce O3, were used. A mixture of HCHO and O3 gases was introduced into the reactor. The concentration of added O3 (ca. 22.5 mg/m3) is comparable with that generated by the two UV254+185 nm lamps. The subsequent steps were the same as those employed in the UV254+185 nm irradiation photocatalytic system. Photocatalysis under UV254 nm irradiation was carried out in the same manner as photocatalysis under O3+UV254 nm irradiation except that no O3 was added.
The O3 concentration was monitored with an online O3 analyzer (Model 49i, Thermo Electron). The formaldehyde concentration was analyzed by the MBTH method (GB/T18204.28, China). A KI-coated annular denuder was used to remove O3 for possible interference with formaldehyde analysis.
The HCHO reaction rate (RHCHO, mg/(m3·min)) and O3 decomposition rate (RO3, mg/(m3·min)) were calculated as follows:
RHCHO = [Q × ([HCHO]initial − [HCHO]steady)]/V (1)
RO3 = [Q × ([O3]VUV − [O3]steady)]/V (2)
where [HCHO]initial is the equilibrium concentration of HCHO before irradiation; [HCHO]steady is the steady-state outlet concentration of HCHO following the 35-h photocatalytic reaction; [O3]VUV is the outlet concentration of the VUV-generated O3 (in the absence of photocatalysts) or the added O3 (22.5 mg/m3); [O3]steady is the steady-state concentration of O3 in the O3 + UV254nm and UV254+185 nm irradiation photocatalytic systems; Q (m3/min) is the flow rate of HCHO gas; and V (m3) is the effective volume of the reactor.
An ultra-high-resolution FESEM image of the Pt-TiO2 film is shown in Fig. 1. Pt NPs (white dots) were uniformly dispersed on the TiO2 surface. The average size of the Pt NPs was 1.9 nm, indicating the presence of a large amount of surface unsaturated atoms in Pt NPs. The surface density of the Pt NPs was 1.43 × 1012 NPs/cm2, which was almost one order of magnitude higher than that of deposited Pd NPs in our previous work [21]. As shown in Fig. 1 inset, approximately 4‒8 Pt NPs were deposited onto each TiO2 particle. The uniform dispersion of ultrafine Pt NPs is beneficial for increasing the amount of reactive sites and metal-TiO2 contact area.
XRD patterns (Fig. 2) suggest that the as-synthesized TiO2 film has a well-organized crystal structure, comprising 62.5% anatase and 37.5% rutile. Following the self-assembly process, the resulting crystalline structure of TiO2 showed no notable changes. The presence of Pt NPs in the Pt-TiO2 film is indicated by the diffraction peaks at 2θ = 39.9°, 46.4°, 67.5°, and 81.3° (Fig. 2) that can be attributed to the (111), (200), (220), and (311) reflections of metallic Pt (JCPDS 4-0802), respectively. The broad diffraction peaks strongly indicate the nanocrystalline nature of the deposited Pt NPs. The XRD result reveals that the metallic form of the deposited Pt NPs was maintained following annealing in air at 300 °C.
The performance of HCHO degradation and O3 elimination using pure TiO2 and Pt-TiO2 photocatalysts was assessed under UV254 nm, O3+UV254 nm, and UV254+185 nm irradiation. Figure 3 presents the HCHO reaction rate as a function of irradiation time over TiO2 and Pt-TiO2 photocatalysts under the three irradiation processes. The degradation rate of HCHO decreased in the order of UV254+185 nm > O3+UV254 nm > UV254 nm for both photocatalysts studied. As HCHO can be easily degraded in gas phase via photochemical processes under 185-nm VUV irradiation, the HCHO degradation rate under UV254+185 nm irradiation was the highest among the three processes. Deactivation of the pure TiO2 and Pt-TiO2 did not take place under UV254+185 nm irradiation, as indicated by the absence of a decreased HCHO reaction rate during the 35-h photocatalytic process. However, reduced HCHO reaction rates were noted in both O3+UV254 nm and UV254nm irradiation systems at reaction time above 25 h, resulting in catalyst deactivation. Nevertheless, a lower degree of catalyst deactivation could be observed in the O3+UV254 nm irradiation system relative to the UV254 nm irradiation system owing to the addition of O3.
In the UV254 nm photocatalytic system, the steady-state HCHO reaction rate over Pt-TiO2 was ca. 1.3-fold higher than that over pure TiO2, thereby indicating the positive effect of Pt NPs in improving the photocatalytic activity of Pt-TiO2. In addition, the HCHO reaction rate decreased much faster in the case of pure TiO2, showing that modification with Pt NPs was beneficial to reducing catalyst deactivation. Furthermore, as shown in Fig. 3, under O3+UV254 nm and UV254+185 nm irradiation, the HCHO reaction rate obtained over the Pt-TiO2 films was higher than that over pure TiO2. These results indicate that deposition of Pt NPs on TiO2 is beneficial to increasing the photocatalytic degradation efficiency of HCHO in all three processes.
Figure 4 shows the O3 decomposition performance under O3+UV254 nm and UV254+185 nm irradiation. Despite the very low decrease in the O3 decomposition rates observed over both pure TiO2 and Pt-TiO2 films in the early stages of the process, in general, the reaction rate was relatively constant throughout the 35-h photocatalytic process, indicating that both TiO2 and Pt-TiO2 did not deactivate under O3+UV254 nm or UV254+185 nm irradiation. The O3 decomposition rate over the Pt-TiO2 films was much higher than that over pure TiO2, resulting in a 3.1-3.4-fold enhancement regardless of whether the O3 was introduced or generated in situ. The results clearly indicate that deposition of Pt NPs on TiO2 remarkably enhances the O3 decomposition. The O3 decomposition rate over Pt-TiO2 was 1.4-fold higher relative to that over previously reported Pd-TiO2 films under UV254+185 nm irradiation [21].
The chemical states of carbon (oxygen)-containing functional groups of Pt-TiO2 films were characterized by high- resolution XPS. XPS spectra of C 1s and O 1s of the Pt-TiO2 films before and after the 35-h photocatalytic process are shown in Fig. 5, and the corresponding relative content and binding energy (BE) of the functional groups are listed in Table 1. The O 1s peak at 529.8‒530.2 eV can be assigned to lattice oxygen of the Ti-O bond [25]. The as-prepared Pt-TiO2 films displayed graphitic carbon (C 1s at 284.8 eV), C-O of alcoholic hydroxyls (C1s at 285.9 eV), and surface hydroxyl groups (O 1s at 532.1 eV) features [26, 27]. The alcoholic hydroxyl groups likely correspond to residual PVA on the TiO2 surface after the annealing treatment. For the Pt-TiO2 films, subjected to a 35-h UV254 nm irradiation photocatalytic process, the broad C 1s peak can be deconvoluted into three bands. The C 1s peaks at 287.1 and 288.5 eV correspond to carbon from carbonyl groups of aldehydes (37.9%) and carboxyl groups (10.6%), respectively [26, 28]. The deconvoluted O 1s peaks at 531.8 and 534.3 eV are assigned to carbonyl oxygen of aldehydes (32.1%) and oxygen atoms in carboxyl groups (12.9%), respectively [26]. Both the C1s and O 1s XPS spectra indicate the presence of carbonyl and carboxyl groups on the Pt-TiO2 surface. Previous studies have shown that the oxidation intermediates, generated during the photocatalytic decomposition of formaldehyde, are CO and formic acid; the latter tends to be strongly adsorbed on the catalyst surface [29, 30]. Therefore, the carbonyl and carboxyl groups in this case can be assigned to corresponding groups in formaldehyde and formic acid adsorbed on Pt-TiO2, respectively. It is clear that a large amount of formaldehyde and formic acid molecules have been accumulated on the Pt-TiO2 films during the UV254 nm photocatalytic process. This result is consistent with the fast deactivation of Pt-TiO2 under UV254 nm irradiation as the reaction time extends over 25 h (Fig. 3(b)).
Both C 1s and O 1s peaks of the resulting Pt-TiO2 films, following O3+UV254 nm photocatalytic process for 35 h, were narrower relative to those produced following UV254 nm photocatalytic process. The C 1s peak at 287.8 eV corresponds to carbonyl groups (8.3%) of adsorbed formaldehyde. The O 1s peaks at 531.1 and 534.7 eV can be assigned to carbonyl oxygen (21.6%) of formaldehyde and oxygen atoms (5.7%) in carboxyl groups of formic acid, respectively [26, 28]. The fitting results in Table 1 reveal that the accumulation of formaldehyde and formic acid on Pt-TiO2 decreased in the presence of O3. This may be attributed to the higher decomposition rates of formaldehyde and formic acid under O3+UV254 nm irradiation when compared with those under UV254 nm irradiation. The C 1s and O1s XPS findings are consistent with the observed higher HCHO reaction rate under O3+UV254 nm irradiation relative to that under UV254 nm irradiation. Therefore, the lower degree of catalyst deactivation under O3+UV254 nm irradiation is ascribed to lower amounts of adsorbed formaldehyde and formic acid on Pt-TiO2. Addition of O3 has been reported to enhance the degradation of hydrophobic VOCs, and inhibits the deactivation of photocatalysts in a TiO2/UV/O3 reaction system [31, 32].
The C 1s and O 1s XPS spectra of Pt-TiO2 film subjected to UV254+185 nm irradiation photocatalytic process for 35 h are shown in Fig. 5. The deconvoluted C 1s peak at 286.1 eV can be assigned to carbonyl groups (13.2%) of adsorbed formaldehyde. The deconvoluted O 1s peak at 531.2 eV corresponds to carbonyl oxygen (16.9%) of formaldehyde [26, 28]. It can be inferred that only a small amount of formaldehyde was accumulated on the Pt-TiO2 surface after the 35-h UV254+185 nm photocatalytic process. Thus, formaldehyde was effectively decomposed to final products (e.g., CO2 and H2O) without accumulation of detectable oxidation intermediates.
The relative atomic content of C, O, Ti, and Pt on the surface of Pt-TiO2 films before and after 35-h photocatalytic process was calculated from the corresponding peak areas of the XPS spectra (Table 2). The C content significantly increased from 33.21% (before photocatalytic reaction) to 39.77% and 38.89% after the Pt-TiO2 films were subjected to 35-h UV254 nm and O3+UV254 nm photocatalytic processes, respectively. A corresponding increase in the relative O content was also observed, as listed in Table 2. The significant increase in C and O contents is directly related to the accumulation of organic species on Pt-TiO2 after the 35-h UV254 nm and O3+UV254 nm photocatalytic processes. However, only a slight increase in the C and O relative atomic content was observed after the 35-h UV254+185 nm photocatalytic process, which is indicative of the lowest amount of accumulated organic species among the three types of photocatalytic systems.
XPS spectra of the Pt 4f core level region of the Pt-TiO2 films before and after the 35-h photocatalytic process are presented in Fig. 6. Several peaks that correspond to multiple oxidation states of Pt were observed. Pt 4f7/2 and 4f5/2 peaks were deconvoluted into two sets of spin-orbit doublet. The BE, relative content, and peak FWHM of the different Pt species in the Pt-TiO2 before and after the 35-h photocatalytic processes are listed in Table 3. The BE of Pt 4f7/2 in the as-prepared Pt-TiO2 at 71.2 and 72.4 eV can be assigned to metallic Pt0 (81.7%) and Pt2+ (18.3%), respectively [33, 34, 35]. The Pt 4f7/2 peak, observed at 72.4 eV, has a lower BE than that associated with PtO species (i.e., 73.4 eV); thus the oxidized Pt2+ species were assigned to PtOads (Pt2+ with adsorbed oxygen) [34]. However, no oxidized Pt species appeared in the XRD analysis (Fig. 2) owing to the relatively small amount of PtOads in Pt NPs. Because of the large amount of unsaturated surface atoms, as featured by the Pt NPs, it is highly likely that Pt NPs partially were oxidized to PtOads during annealing in air at 300 °C.
Following the 35-h UV254 nm photocatalytic process, the Pt-TiO2 films displayed Pt 4f7/2 peaks at 71.1 and 72.1 eV, which can be assigned to metallic Pt0 (79.9%) and PtOads (20.1%), respectively [33, 34, 35, 36]. This shows that both metallic Pt0 and PtOads are stable under UV254 nm irradiation. In contrast, Pt 4f7/2 peaks of the Pt-TiO2 films, following a 35-h O3+UV254 nm photocatalytic process, were shifted to 72.3 and 74.2 eV, which correspond to PtOads (76.9%) and Pt4+ species (23.1%), respectively [33]. Thus, oxidation of the metallic Pt0 to Pt species with higher oxidation states occurred during the O3+UV254 nm photocatalytic process.
Following the 35-h UV254+185 nm photocatalytic process, the Pt-TiO2 displayed Pt 4f7/2 peaks at 72.4 and 74.5 eV, which were assigned to PtOads (57.4%) and Pt4+species (42.6%), respectively [33, 34]. Thus, under UV254+185 nm irradiation, metallic Pt0 was no longer apparent and more Pt species were oxidized to Pt4+. Because the light intensity of the UV254nm irradiation and concentration of O3 in the O3+UV254 nm photocatalytic system were comparable to those in the UV254+185 nm photocatalytic system, the VUV-photochemically generated atomic oxygen species (O(1D), O(3P)) and hydroxyl radicals (•OH) were likely to also contribute to the oxidation of the Pt species, thereby resulting in a large amount of Pt4+ species in the UV254+185 nm photocatalytic system.
As discussed from the above comparative XPS study for the Pt-TiO2 films, the Pt species were significantly oxidized in the O3+UV254 nm photocatalytic system (PtOads (76.9%) and Pt4+ species (23.1%)) whereas subtle changes were noted in the UV254 nm photocatalytic system. This clearly indicates that the added O3 should be involved in the oxidation of the Pt species. Falconer et al. [37] reported that Pt particles on TiO2 could serve as adsorption and reaction active sites for O3 decomposition. Thus, in the O3+UV254 nm photocatalytic system, Pt NPs can act as centers for capturing O3; the adsorbed O3 can then be readily reduced to ozonide radical anion (O3•−) by reaction with the photogenerated electrons from TiO2 (Eq. (3)) [21, 38, 39]. The unstable O3•− radicals rapidly split to form O2 and O•− radicals under UV irradiation (Eq. (4)), as confirmed by ESR measurements [40, 41]. The adsorbed water molecules on the Pt NPs can be oxidized by the O3•− and O•− radicals to form •OH radicals (Eqs. (5) and (6)), resulting in abundant •OH radicals on the surface of the Pt NPs. Owing to the strong oxidizing power of •OH radicals, the •OH radicals adsorbed on the Pt NPs are expected to oxidize metallic Pt0 to Pt species with higher oxidation states during the O3+UV254 nm photocatalytic process (Eqs. (7) and (8)) [21].
O3 + e− → O3•− (3)
O3•− + hν → O2+ O•− (4)
O•− + H2O → OH− + •OH (5)
O3•− + H2O → OH− + •OH + O2 (6)
Pt0 + 2•OH →Pt2+(PtOads) + 2OH− (7)
Pt2+(PtOads) + 2•OH → Pt4+ + 2OH− (8)
Pt species with high oxidation states have stronger electron affinities. Therefore, PtOads and Pt4+ species can act as electron trapping centers for the Pt-TiO2 nanocomposites in the O3+UV254 nm photocatalytic system. This will result in a more efficient photogenerated electron-hole pair separation, consequently resulting in a prolonged electron and hole lifetime. After trapping of the photogenerated electrons, the oxidized Pt species are reduced to Pt species with lower oxidation states (Eqs. (9) and (10)). The reduction of PtO2 to Pt0 in a reported photo-irradiated alcoholic suspension solution supports this assumption [42]. Thus, Pt species comprising both PtOads and Pt4+ species can steadily form on the TiO2 surface during the O3+UV254 nm photocatalytic process.
Pt4+ + 2e− → Pt2+(PtOads) (9)
Pt2+(PtOads) + 2e− → Pt0 (10)
In this work, the O3 concentration and light intensity of the UV254 nm irradiation in the UV254+185 nm photocatalytic system were comparable to those in the O3+UV254 nm photocatalytic system. Therefore, the reactions given in Eqs. (3)-(6) are also expected to occur, leading to a large amount of •OH radicals on the Pt NPs in the UV254+185 nm photocatalytic system. Additionally, the VUV-photochemically generated •OH radicals must be present in gas phase and adsorb on active sites of the Pt-TiO2 surface. Therefore, higher amounts of •OH radicals are generated under UV254+185 nm irradiation relative to O3+UV254 nm irradiation, leading to enhanced oxidation of Pt species to Pt4+ species (Eq. (8)). The higher amount of Pt4+ species, as determined by Pt 4f XPS, supports the above discussion relating to reaction process under UV254+185 nm irradiation. The Pt species at higher oxidation states can be also reduced according to Eqs. (9) and (10) to form mixed Pt species in the UV254+185 nm photocatalytic system.
As mentioned above, the dispersed Pt species can effectively trap photogenerated electrons from the conduction band of TiO2, which helps to produce further •OH radicals on TiO2 (Eqs. (11)-(13)). The surface unsaturated Pt atoms on the Pt NPs are highly active, and can act as reactive sites for adsorption and degradation of HCHO. Therefore, a larger number of •OH radicals and reaction sites were observed on the Pt-TiO2 surface than on pure TiO2 in the O3+UV254 nm and UV254+185 nm photocatalytic systems, which resulted in a higher HCHO degradation rate, as observed in Fig. 3.
TiO2 + hν → h+ + e− (11)
OH−(ads) + h+ → •OH(ads) (12)
H2O(ads) + h+ → •OH(ads) + H+ (13)
Compared with pure TiO2, oxidized Pt species, acting as effective redox catalysts, can provide more reactive sites for capturing O3 molecules [38], thus enhancing O3 decomposition. The adsorbed O3 molecules on the Pt NPs can be decomposed via the reactions given in Eqs. (3)-(6) by trapping the electrons generated upon UV irradiation. Additionally, thermal catalysis decomposition of O3 should be effective on oxidized Pt species [38]. Therefore, the higher O3 decomposition rate over the Pt-TiO2 films relative to that over pure TiO2 under O3+UV254 nm and UV254+185 nm irradiation, as shown in Fig. 4, agrees with the reported study whereby Pt-loaded TiO2 accelerated the photocatalytic degradation of gaseous ozone [43].
XPS was used to assess the electronic states of Pt-TiO2 films employed for the photocatalytic degradation of formaldehyde under UV254 nm, O3+UV254 nm, and UV254+185 nm irradiation conditions. The formation of carbonyl and carboxyl groups, corresponding to formaldehyde and formic acid on the Pt-TiO2 films following photocatalytic reactions, was determined by deconvolution of the C1s and O1s XPS spectra. The content of the organic species on the Pt-TiO2 and degree of catalyst deactivation under the studied conditions decreased in the order of UV254 nm > O3+UV254 nm > UV254+185 nm. More specifically, catalyst deactivation was negligible under UV254+185 nm irradiation. The accumulation of organic species on Pt-TiO2, as determined by XPS, gives direct evidence to elucidate the degree of catalyst deactivation.
The deconvoluted Pt 4f XPS spectra revealed that metallic Pt NPs were stable under UV254 nm irradiation; however, they were oxidized to a mixture of PtOads and Pt4+ species under O3+UV254 nm and UV254+185 nm irradiation conditions. The •OH radicals adsorbed on Pt NPs, generated during the O3 decomposition process or dissociation of water under 185-nm VUV, are expected to be involved in the oxidation of Pt species. Pt species with high oxidation states can readily trap the photogenerated electrons to increase the photocatalytic activity of Pt-TiO2. Ultrafine Pt NPs with a strong electron affinity can provide numerous reactive sites for O3 decomposition. Therefore, formaldehyde degradation and O3 elimination can be simultaneously enhanced in the O3+UV254 nm and UV254+185 nm irradiation photocatalytic systems by modifying TiO2 with Pt.
2014, Vol. 35 Issue (2): 210-218
PDF (761KB)
2014, Vol. 35 Issue (2): 210-218
PDF (761KB)