DOI : 10.2240/azojomo0253

Characterization of Mo/SiO2 and Mo/Al2O3 and Their Photocatalytic Reactivity for the Selective Oxidation of CO with O2 in the Presence of H2

Dec 12 2007

Takashi Kamegawa, Rumi Takeuchi, Masaya Matsuoka and Masakazu Anpo

This is an AZo Open Access Rewards System (AZo-OARS) article distributed under the terms of the AZo–OARS https://www.azom.com/oars.asp which permits unrestricted use provided the original work is properly cited but is limited to non-commercial distribution and reproduction.

AZojomo (ISSN 1833-122X) Volume 3 December 2007

Topics Covered

Results and Discussion

Abstract

Photocatalytic selective oxidation of CO into CO₂ with O₂ in the presence of H₂ has been performed on Mo/SiO₂ and Mo/Al₂O₃ catalysts at 293 K. This reaction proceeded efficiently on Mo/SiO₂, while no reaction proceeded on Mo/Al₂O₃. After UV irradiation for 180 min, CO conversion and selectivity reached ~100% and 99%, respectively, on Mo/SiO₂. Photoluminescence and FT-IR investigations revealed that photoexcited tetrahedral Mo⁶⁺-oxides species ((Mo⁵⁺-O^-)^*) on SiO₂ reacts selectively with gaseous CO and is reduced to Mo⁴⁺ carbonyl species, while Mo⁴⁺ carbonyl species are efficiently oxidized to the original Mo⁶⁺-oxide species by gaseous O₂ in the dark. Such redox cycles of Mo⁶⁺-oxide species play a significant role in the selective oxidation of CO in the presence of H₂.

Keywords

selective oxidation of CO in H₂, Mo/SiO₂, Mo/Al₂O₃, TiO₂ (P-25), redox cycles, photoluminescence, PROX.

Introduction

Recently, the preferential oxidation (PROX) of CO in H₂-rich gas has attracted new interest due to its application in fuel cell technology. For instance, polymer electrolyte fuel cells (PEFC) are attractive for small scale and automotive applications, although PEFC requires high purity H₂ as fuel with CO concentrations below 10 ppm since the anode Pt catalyst is easily deactivated by CO contamination [1, 2]. PROX reaction is one of the most straightforward method to reduce the CO contamination in H₂-rich gas. For now, noble metals (Pt, Rh, etc.) supported catalysts have been tested and proposed for thermal PROX reaction [3-5]. On the other hand, highly dispersed transition metal oxides such as V [6], Cr [7, 8] and Mo [9-11] incorporated within the framework of zeolites or supported on various oxides show unique photocatalytic activity for various reactions such as decomposition of NOx, oxidation of CO and partial oxidation of hydrocarbons, under not only UV and/or visible light irradiation. However, to the best of our knowledge, no attempt has been made to apply photocatalysis for the PROX reactions. In the present work, highly dispersed Mo⁶⁺-oxide species supported on silica (Mo/SiO₂) and Al₂O₃ (Mo/Al₂O₃) were prepared by an impregnation method and characterized by various spectroscopic techniques. Furthermore, the photocatalytic selective oxidation of CO into CO₂ with O₂ in the presence of H₂ was investigated and compared with that of TiO₂ (P-25) photocatalyst.

Experimental

Mo/SiO₂ and Mo/Al₂O₃ (0.60 wt% molybdenum as metal; 62.5 μmol of Mo/g-cat) were prepared by an impregnation method of SiO₂ (aerosil 300; Degussa) or Al₂O₃ (Aluminium Oxide C; Degussa), respectively, using an aqueous solution of (NH₄)₆Mo₇O₂₄∙4H₂O. After impregnation, the catalyst was dried at 373 K for 12 h and calcined at 773 K for 8 h in air. TiO₂ (P-25; Degussa) was calcined at 723 K for 8 h in air. Prior to the photocatalytic reactions and spectroscopic measurements, the catalysts were calcined in O₂ (> 2.66 kPa) at 773 K (at 723 K for TiO₂) for 1 h, and then degassed at 473 K for 1 h. Diffuse reflectance UV-vis spectra were recorded at 298 K with a Shimadzu UV-2200A double-beam digital spectrophotometer. The XAFS spectra were obtained at the BL-19B2 facility of the SPring-8 at the Japan Synchrotron Radiation Research Institute (JASRI). The synchrotron radiation from 8.0 GeV electron storage ring was monochromatized by a Si(1 1 1) monochromator. The Mo K-edge spectra were measured in the transmission mode at room temperature. Curve fitting analysis of the EXAFS spectra was conducted on k³χ (k) in k-space (k range = 3-12 Å^-1) with a REX2000J program (Rigaku). The FT-IR spectra were recorded at room temperature with a FT-IR spectrometer (JASCO FT-IR 660 Plus) with self-supporting pellets of the samples in the transmission mode. The photoluminescence spectra were measured at 298 K with a SPEX Fluorolog-3 spectrofluorometer. Photocatalytic reactions were carried out in a closed system using quartz reactor (reaction volume: 101 cm³) under UV irradiation at 293 K using a 100 W high-pressure mercury lamp (Toshiba SHL-100UVQ-2) through water filter. In the present investigation, gas concentrations are defined as ppm, that is, the concentration of gas with its partial pressure of 1 atm is equivalent to the concentration of 10⁶ ppm. The initial concentration of gasses was CO 920 ppm, O₂ 1815 ppm and H₂ 5950 ppm. The amount of the catalyst used for photocatalytic reactions was fixed at 50 mg. The reaction products were analyzed by on-line gas chromatography.

Results and Discussion

The photocatalytic selective oxidation of CO into CO₂ with O₂ in the presence of H₂ was investigated on Mo/SiO₂ and Mo/Al₂O₃. As shown in Figure 1, UV irradiation of Mo/SiO₂ led to the efficient oxidation of CO into CO₂, accompanying the stoichiometric formation and consumption of CO₂ and O₂, respectively. Concentration of CO gas reached below the detection limit of GC analysis (less than 8 ppm) after UV light irradiation for 180 min, while the amount of H₂ remained constant. The turnover number for the reaction defined as the ratio of the amount of CO₂ to the amount of Mo⁶⁺-oxide species included in the catalyst exceeded unity, indicating that the reaction proceeded photocatalytically. The amount of CO₂ produced as well as the amount of O₂ and H₂ consumed reached 920 ppm, 460 ppm and 7.26 ppm, respectively. From these values, the CO conversion and selectivity were calculated to be ~100% and 99%, respectively, by using the following equation:

CO selectivity (%) = {(CO_{2 (t=180min)}) / [(H_{2 initial} - H_{2 (t=180min)}) + CO_{2 (t=180min)}]} x 100

(H_{2 initial}: the initial amount of H₂, CO_2(t=180min), H_{2 (t=180min)}: the amount of CO₂ produced and the amount of H₂ in the gas phase after the UV irradiation of 180 min).

Figure 1. Reaction time profiles of the selective photocatalytic oxidation of CO with O₂ in the presence of H₂ on Mo/SiO₂ under UV light irradiation at 293 K.

Moreover, as shown in Figure 2, the time profiles of the amounts of CO, O₂ and CO₂ (depicted as solid line) show good coincidence with that of the reaction performed in the presence of H₂ (Figure 1). These results indicate that the CO oxidation reaction is hardly affected by the presence of H₂. In contrast to the Mo/SiO₂, it was found that Mo/Al₂O₃ does not exhibit any photocatalytic activity for this reaction. Moreover, photocatalytic oxidation of CO with O₂ in the presence of H₂ was also performed on the TiO₂ (P-25). CO oxidation reaction proceeds on TiO₂, however, CO conversion (81%) as well as CO selectivity (89%) of TiO₂ after UV irradiation of 360 min were lower than those of Mo/SiO₂. Then, the photocatalytic oxidation of H₂ with O₂ were performed on Mo/SiO₂ and TiO₂. As shown in Figure 2, the amounts of H₂ and O₂ clearly decreased on TiO₂ after UV light irradiation for 180 min (depicted as open symbol), however, the amounts of H₂ and O₂ hardly changed on Mo/SiO₂ (depicted as dotted line). These results indicate that H₂ is hardly oxidized by O₂ on Mo/SiO₂ under UV irradiation.

Figure 2. Reaction time profiles of the photocatalytic oxidation of CO with O₂ on Mo/SiO₂ (solid line) as well as the photocatalytic oxidation of H₂ with O₂ on Mo/SiO₂ (dotted line) and TiO₂ (open symbol) at 293 K.

In order to elucidate the active sites for the photocatalytic reactions and their local structures, Mo/SiO₂ and Mo/Al₂O₃ were characterized by various spectroscopic methods. The local structure of Mo⁶⁺-oxide species on SiO₂ and Al₂O₃ were investigated by Mo K-edge XAFS measurements. XANES spectra of Mo/SiO_{2 and Mo/Al2O3 exhibit well-defined preedge peaks due to the 1s-4d transition of}Mo atoms, indicating that Mo⁶⁺-oxide species exist in a tetrahedral symmetry for both samples (Figure 3 (A), (B)). The Fourier transformed EXAFS (FT-EXAFS) spectra of Mo/SiO₂ and Mo/Al₂O₃ showed peaks due to the existence of neighboring oxygen atoms (Mo-O) at around 0.8-2.0 Å, while the additional peak due to the Mo-O-Mo bond was not observed between 3.0-4.0 Å. These results suggest that Mo⁶⁺-oxide species are highly dispersed on SiO₂ and Al₂O₃ [12, 13]. Moreover, the results of curve fitting analysis of Mo-O bonds suggest that Mo⁶⁺-oxide species on SiO₂ and Al₂O₃ have two short Mo=O double bonds and two long Mo-O single bonds, while Mo⁶⁺-oxide species on Al₂O₃ has longer M=O double bonds and shorter Mo-O single bond than that on SiO₂, as shown in Table 1. The long Mo-O single bond of Mo⁶⁺-oxide species on SiO₂ can be ascribed to the small ionic character of framework metal-oxygen bond (Si-O) of SiO₂ as compared to that (Al-O) of Al₂O₃ [14]. The long Mo-O single bond also indicates the weak interaction between Mo⁶⁺-oxide species and SiO₂ [14].

Figure 3. XANES (A-D) and Fourier transforms of EXAFS (a-d) spectra of: (A, a) Mo/SiO₂, (B, b) Mo/Al₂O₃, (C, c) Na₂MoO₄ and (D, d) MoO₃

Table 1. Results of the curve fitting analyses of the Mo K-edge EXAFS data for Mo/SiO₂ and Mo/Al₂O₃.

The diffuse reflectance UV-vis spectra of Mo/SiO₂ and Mo/Al₂O₃ exhibited broad absorption bands at around 220-240 and 260-280 nm (data not shown) which have been assigned to the ligand to metal charge transfer absorption bands of the tetrahedrally coordinated Mo⁶⁺-oxide species [15]. Furthermore, these catalysts do not exhibit any absorption band of MoO₃ in the wavelengths region above 350 nm, showing that these Mo⁶⁺-oxide species exist in a highly dispersed state as supported by the Mo K-edge XAFS results.

The reaction mechanism of the selective oxidation of CO in the presence of H₂ was also investigated in detail. Under UV irradiation at around 300 nm, Mo/SiO₂ exhibited photoluminescence at around 450 nm at 298 K attributed to the radiative decay process from the charge transfer excited triplet state of the isolated tetrahedral Mo⁶⁺-oxide species ((Mo⁵⁺-O^-)^*) [16, 17]. The absorption and emission processes can be represented by the following equation (Eq. 1).

Figure 4 (A-C) show the effect of the addition of CO, O₂ and H₂ on the photoluminescence spectra of Mo/SiO₂. Photoluminescence is efficiently quenched by the addition of CO, O₂ and H₂, accompanying the decrease in the photoluminescence lifetime, indicating that the Mo⁶⁺-oxide species dynamically interact with CO, O₂ and H₂ in its charge transfer excited triplet state. The evacuation of the added quencher molecules after the quenching of the photoluminescence led to almost a complete recovery of the original photoluminescence yield for O₂ and H₂, while only a partial recovery of the photoluminescence yield could be observed for CO, indicating that an irreversible reaction proceeds between CO and the charge transfer excited triplet state of Mo⁶⁺-oxide species ((Mo⁵⁺-O^-)^*). On the other hand, Mo/Al₂O₃ exhibited quite weak photoluminescence (data not shown) as compared to that of Mo/SiO₂. From the results of XAFS measurements, the weak photoluminescence intensity of Mo/Al₂O₃ can be explained by the efficient thermal deactivation from photoexcited Mo⁶⁺-oxide species to support, which is enhanced by the strong interaction between Mo⁶⁺-oxide species and Al₂O₃ as suggested by XAFS results. The efficient thermal deactivation process of photoexcited state of Mo⁶⁺-oxide species on Al₂O₃ dramatically decrease catalytic reaction rate on Mo/Al₂O₃.

Figure 4. Effect of the addition of CO (A), O₂ (B) and H₂ (C) on the photoluminescence spectrum of the Mo/SiO₂ at 298 K (λ_ex=300 nm).

A: 1) 0.0; 2) 2.96; 3) 12.8; 4) 110.6 ppm; 5) degassed for 1 h; 6) addition of O₂ and then degassed for 1 h after 5), B: 1) 0.0; 2) 7.90; 3) 38.5; 4) 131.3 ppm; 5) degassed for 1 h, C: 1) 0.0; 2) 952.6; 3) 4240; 4) 13700 ppm; 5) degassed for 1 h.

Figure 5 shows the Stern-Volmer plots for the quenching of the photoluminescence of Mo/SiO₂. The Stern-Volmer equation (Eq. 2) can be obtained for the quenching of the photoluminescence with the quencher molecules by applying steady-state treatment [18], as follows:

Φ₀/Φ = 1+τ₀ k_q [Q] (2)

where Φ₀ and Φ show the yields of the photoluminescence in both the absence and presence of quencher molecules, respectively, and τ₀, k_q and [Q] are the lifetimes of the charge transfer excited triplet state of the Mo⁶⁺-oxide species ((Mo⁵⁺-O^-)^*) in the absence of quencher molecules, the quenching rate constant and the concentration of the quencher molecules, respectively. The absolute quenching rate constants (k_q) for each gas determined by the slope of the Stern-Volmer plots increases in the following order: H₂ << O₂ < CO, indicating that CO interacts most efficiently with the charge transfer excited triplet state of Mo⁶⁺-oxide species ((Mo⁵⁺-O^-)^*), while the interaction is weak in the case of H₂, as shown in Figure 5.

Figure 5. Stern-Volmer plots of the Φ₀ / Φ values for the yields of the photoluminescence versus the concentration of the quencher such as CO (a), O₂ (b) and H₂ (c). (Lifetime of Mo/SiO₂ under vacuum: 86 μsec at 298 K.)

The reaction mechanism was also investigated by FT-IR measurements. As shown in Figure 6 (a), UV irradiation of Mo/SiO₂ in the presence of CO (13130 ppm) led to the appearance of FT-IR peaks due to the dicarbonyl (Mo⁴⁺(CO)₂; 2127, 2077 cm^-1) and monocarbonyl (Mo⁴⁺(CO); 2043 cm^-1) species, indicating that Mo⁶⁺-oxide species are photoreduced to Mo⁴⁺ carbonyl species [19]. However, the addition of O₂ on this system led to the complete disappearance of these FT-IR peaks (Figure 6(b)), indicating the regeneration of tetrahedrally coordinated Mo⁶⁺-oxide species by the addition of O₂. This redox cycle is also confirmed by the fact that the photoluminescence yield of the photoreduced sample is recoverd to the original level by the addition of O₂ and followed its evacuation (Figure 3 (A)). According to the above characterization, the catalytic reaction cycles in the photocatalytic oxidation of CO on the tetrahedrally coordinated Mo⁶⁺-oxide species on SiO₂ under UV irradiation could be considered as follows (Eq. 3-5).

That is, initially, tetrahedral Mo⁶⁺-oxide species ((Mo⁶⁺=O^2-)) on SiO₂ are photoexcited into its charge transfer excited triplet state ((Mo⁵⁺-O^-)^*) reacting selectively with CO to form CO₂ and photoreduced into Mo⁴⁺ carbonyl species. Then the Mo⁴⁺ carbonyl species are oxidized by O₂ and the original Mo⁶⁺-oxide species are generated. Thus, it can be concluded that the redox cycles of Mo⁶⁺-oxide species play a significant role in the selective oxidation reaction of CO in the presence of H₂. As mentioned above, the CO selectivity for this reaction on TiO₂ (89%) is lower than that of Mo/SiO₂ (99%). It has been reported that photoformed oxygen species (O^-, O₂^- and O₃^-) play an important role in the photocatalytic oxidation reaction on TiO₂ [20]. The strong and nonselective oxidation ability of these photoformed oxygen species may lead to the low CO selectivity of the reaction on TiO₂. A more detailed study of the mechanisms behind the selective photocatalytic oxidation of CO is now underway.

Figure 6. FT-IR spectra observed under UV light irradiation of Mo/SiO₂ in the presence of CO (a); and the effect of the addition of O₂ in the dark on the FT-IR spectra of CO adsorbed species formed on the pre-photoreduced Mo/SiO₂ with CO (b). All spectra were recorded as the difference in the spectrum before and after UV light irradiation.

Conclusions

UV-vis, photoluminescence and XAFS measurements revealed that Mo⁶⁺-oxide species are present in a highly dispersed state in a tetrahedral symmetry on SiO₂ and Al₂O₃. The selective photocatalytic oxidation of CO in the presence of H₂ proceeded efficiently on the tetrahedrally coordinated Mo⁶⁺-oxides species on SiO₂ with high CO conversion (~100%) and selectivity (99%) under UV light irradiation for 180 min. It was found that this reaction hardly proceeded on the tetrahedrally coordinated Mo⁶⁺-oxides species on Al₂O₃, since the strong interaction of Mo⁶⁺-oxide species with Al₂O₃ results in the enhanced thermal deactivation process of the photoexcited Mo⁶⁺-oxide species on Al₂O₃. Furthermore, in-situ spectroscopic investigations revealed that the reaction is closely related to the high reactivity of the charge transfer excited triplet state of Mo⁶⁺-oxide species ((Mo⁵⁺-O^-)^*) on SiO₂ to oxidize CO into CO₂, as well as the high reactivity of the photoreduced Mo⁴⁺-oxide species with O₂ to produce the original Mo⁶⁺-oxide species.

Acknowledgements

The authors wish to thank NSERC, Canada for financial support for this work.

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