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2007, 111, 1076-1078 Published on Web 12/23/2006
Eliminating Traces of Carbon Monoxide Photocatalytically from Hydrogen with a Single-Site, Non-noble Metal Catalyst Takashi Kamegawa,† Jun Morishima,† Masaya Matsuoka,*,† John Meurig Thomas,‡ and Masakazu Anpo*,† Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan, and Department of Materials Science and Metallugy, UniVersity of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3QZ, U.K. ReceiVed: October 30, 2006; In Final Form: December 11, 2006
The photocatalytic preferential oxidation of CO with O2 in the presence of H2 was seen to proceed efficiently on Cr6+-MCM-41 with high CO conversion and selectivity under visible light (λ > 420 nm) or solar light irradiation at 293 K. Photoluminescence and FT-IR investigations show that the redox cycles of Cr-oxide species were found to play significant roles in the selective oxidation reaction of CO in the presence of H2.
Introduction With the central role that the H2-O2 fuel cell occupies in the current trend toward developing a hydrogen-based economy,1,2 the quest to minimize3 or altogether to eliminate4 the use of precious metal catalysts for the electro-oxidation and electroreduction involved is gaining momentum. Although significant progress in developing nonprecious metal (composite) catalysts for fuel cells has been registered recently,4 at present platinum remains the catalyst of choice in the overwhelming majority of hydrogen-based fuel cells. The performance of platinum for this purpose, however, is diminished by trace amounts of carbon monoxide because the adsorption of the latter poisons the platinum surface.5,6 The popular proton exchange membrane (PEM) fuel cells, also designated polymer electrolyte fuel cells (PEFC), demand a supply of H2 stringently free of CO impurity, and the favored method of achieving this is to catalytically oxidize the CO over either a transition metal oxide or, preferably, a supported Ptrich catalyst, known generally as PROX (preferential oxidation of CO in the presence of H2).7-9 Recent studies have shown10 that supported Pt-containing, bimetallic nanoparticles such as Pt5Fe2 and PtFe2 supported on silica are superior to traditional variants of the Pt/SiO2 catalysts for effecting the PROX process. But in view both of the continuing high cost and ultimate scarcity of Pt, it is highly desirable to design a PROX catalyst for static fuel cell installations that not only is cheap and readily preparable but also functions at ambient temperatures. Such a designed catalyst is the subject of this communication. Previously, it has been shown11-18 that transition metal oxides derived from V, Ti, Mo, and Cr may be dispersed as welldefined isolated entities at the surfaces of mesoporous silicas of various kinds, like the so-called MCM-41 family that consists of ordered pores in a hexagonal array or the internal surfaces of commercially available silicas (such as Davison 923). These * Corresponding authors. E-mail:
[email protected]. † Osaka Prefecture University,. ‡ University of Cambridge.
[email protected].;
10.1021/jp067121r CCC: $37.00
silicas have pore diameters that fall in the range from ca. 30 to 40 Å. A wide variety of spectroscopic techniques such as UVvis, XAFS, photoluminescence, and FT-IR13,16,19 confirms that, when appropriately prepared, Ti4+, Mo6+, and Cr6+ ions take up distorted tetrahedral sites in which each ion is surrounded by oxygen atoms. It has also been shown20 that the Ti4+ ion in MCM-41 is a good photocatalyst for the decomposition of NO to N2 and O2 and that isolated Mo6+ ions, likewise dispersed,16,19 will facilitate the conversion of CO to CO2 in the presence of H2 under UV irradiated at 293 K. Here, we show that Cr6+ ions function as a photocatalytic system under visible (or solar light) irradiation, thereby affording a clean, convenient (nonprecious metal) catalyst for the PROX system. Experimental Section Cr6+-MCM-41 (0.7 wt %) was synthesized in accordance with previous literature.21 Prior to spectroscopic measurements and photocatalytic reactions, the catalyst was calcined in O2 at 773 K and then degassed at 473 K. Photoluminescence was measured at 298 K with a Spex Fluorog-3 spectrophotometer. The XAFS (XANES and EXAFS) spectra were obtained at the BL-01B1 facility of SPring-8 at the Japan Synchrotron Radiation Research Institute (JASRI). The Cr K-edge absorption spectra were recorded in fluorescence mode with a Si(111) two-crystal monochromator at room temperature. The FT-IR spectra were recorded at room temperature with an FT-IR spectrometer (JASCO FT-IR 660 Plus) with self-supporting pellets of the samples in transmission mode at 4 cm-1 resolution. Photocatalytic reactions were carried out under a closed system using a quartz reactor (reaction volume: 101 cm3) with a flat bottom under visible light (λ > 420 nm) using a 500 W Xe arc lamp through water and colored filters (HOYA; L-42) or solar light at 293 K. Results and Discussion Fuller details of the characterization of the Cr6+-MCM-41 photocatalyst are given in the accompanying Figures, where it is seen that the absence of Cr-O-Cr shells in the FT EXAFS spectrum confirms the single-site nature of the active center. © 2007 American Chemical Society
Letters
Figure 1. (A-C) XANES and (a-c) Fourier transform of EXAFS spectra of (A and a) CrO3, (B and b) Cr2O3, and (C and c) Cr6+-MCM41.
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1077
Figure 3. Reaction time profiles of the photocatalytic oxidation of CO with O2 in the presence of H2 on Cr6+-MCM-41 under (A) visible light irradiation (λ > 420 nm) and (B) solar light irradiation. (Initial amount of gasses for CO: 3.8 µmol, O2: 7.5 µmol, and for H2: 24.6 µmol.)
The absorption and emission spectra are attributed to the following charge-transfer processes on the CrdO moieties of the tetrahedral monochromate species (CrO42-) involving an electron transfer from O2- to Cr6+ ions and a reverse radiative decay from the charge-transfer excited triplet state:25,26 hV
hV′
[Cr6+dO2-]98 [Cr5+-O-]*98 [Cr6+dO2-]
Figure 2. Photoluminescence spectra of Cr6+-MCM-41 measured: (a) in vacuum, (b) after visible light irradiation in the presence of CO for 0.5 h and its subsequent evacuation, and (c) after the addition of O2 on (b) at 298 K and its subsequent evacuation. (Measurement temperature: 298 K; Excitation: λex ) 500 nm.)
The XANES spectrum of Cr6+-MCM-41 showed an intense and characteristic pre-edge peak, indicating that Cr6+-MCM-41 contains a Cr6+-oxide species in tetrahedral coordination, as can be seen in Figure 1C.21,22 Curve-fitting analysis of the Cr-O peaks (Figure 1c) revealed that the Cr6+-oxide species exists in a highly distorted tetrahedral coordination with two shorter CrdO double bonds [bond length (R) ) 1.59 Å, coordination number of CN ) 2.0, Debye-Waller factor of σ2 ) 0.004 Å2] and two longer Cr-O single bonds (R ) 1.85 Å, CN ) 2.1, σ2 ) 0.007 Å2). The UV-vis spectrum of Cr6+-MCM-41 exhibits three distinct absorption bands at around 240, 350, and 460 nm due to the ligand to metal charge-transfer transition (LMCT: from O2- to Cr6+) of the tetrahedrally coordinated Cr6+-oxide species.17,18,23 Considering the low loading of Cr (0.7 wt %) and high surface area of the support, it can be concluded that the tetrahedrally coordinated monochromate species, or isolated Cr6+-oxide species, exist as the dominant species.24 As shown in Figure 2, the Cr6+-MCM-41 exhibited a photoluminescence spectrum at around 550-800 nm upon excitation at around 240, 350, and 460 nm at 298 K, in good coincidence with the photoluminescence spectrum for the tetrahedrally coordinated Cr6+-oxide species.
The photoluminescence of Cr6+-MCM-41 is quenched in its intensity by the addition of CO, O2, and H2, indicating that the Cr6+-oxide species, in its charge-transfer excited triplet state, interacts with CO, O2, and H2 easily. Moreover, the absolute quenching rate constants (kq(l/mol·sec)) for each gas, which were determined by the Stern-Volmer plots,27 were found to increase in the following order: H2 (8.63 × 105) , CO (5.91 × 109) < O2 (1.12 × 1010). These results indicated that CO interacts very efficiently with the photoexcited Cr6+-oxide species as compared to H2. The photocatalytic preferential oxidation of CO with O2 in the presence of H2 was investigated on Cr6+-MCM-41 at 293 K. Visible light irradiation (λ > 420 nm) led to the efficient oxidation of CO into CO2, accompanied by the stoichiometric formation and consumption of CO2 and O2, respectively, as shown in Figure 3A. The concentration of the CO gas reached below the detection limit for GC (less than 8 ppm) after visible light irradiation for 150 min, while the amount of H2 remained almost constant. The turnover number for the reaction, defined as the ratio of the amount of CO2 to the amount of Cr6+-oxide species included in the catalyst, reached 1.4 after 150 min irradiation, indicating that the reaction proceeded photocatalytically. The amount of CO2 produced (CO2 (t ) 150 min)) and the amount of H2 consumed (∆H2 (t ) 150 min)) during the reaction were 3.8 and 0.12 µmol, respectively. On the basis of these results, CO conversion and CO selectivity were determined to be ∼100% and 97%, respectively, after visible light irradiation for 150 min. Here, the CO selectivity is calculated by the following equation:
CO selectivity (%) ) 100 × [CO2 (t ) 150 min)/ (CO2 (t ) 150 min) + ∆H2 (t)150 min))]
1078 J. Phys. Chem. C, Vol. 111, No. 3, 2007 SCHEME 1: Complete Reaction Cycle in the Photocatalytic Oxidation of CO with O2 in the Presence of H2 on Cr6+-MCM-41
Letters Conclusions In conclusion, it was found that Cr6+-MCM-41 acts as an efficient catalyst for the photocatalytic preferential oxidation of CO under visible light (λ > 420 nm) and also under natural solar light at 293 K with a high CO conversion and selectivity. The high and selective reactivity of the photoexcited Cr6+-oxide species with CO as well as the high reactivity of the photoreduced Cr4+-oxide species with O2 to produce the original Cr6+oxide species were found to play significant roles in the selective oxidation reaction of CO in the presence of H2. The present results demonstrated that Cr6+-MCM-41 can be applied for clean and cost-efficient photo-PROX reaction systems using solar light energy at ambient temperatures without the use of Pt or other precious noble metals. Acknowledgment. We thank Prof. F. Tanaka of Osaka Prefecture University for measurements of the decay curves.
Fieldwork experiments were also carried out to investigate the photocatalytic reactivity of Cr6+-MCM-41 for the preferential oxidation of CO with O2 in the presence of H2 under natural conditions of clean and safe solar light irradiation. The reaction time profiles of the photocatalytic oxidation of CO with O2 in the presence of H2 on Cr6+-MCM-41 under solar light are shown in Figure 3B. These data were observed from 11:00 to 14:30 of a sunny day with average solar light intensity of 78.5 mW/cm2 and irradiation area of 3 cm2, clearly showing that Cr6+-MCM-41 could operate efficiently as a photocatalyst for CO oxidation. After light irradiation for 3.5 h, CO conversion and selectivity reached ∼100% and 96%, respectively. The present results demonstrated that Cr6+-MCM-41 can be applied for clean and cost-efficient photo-PROX reaction systems using solar light energy at ambient temperatures without the use of Pt or other precious noble metals. FT-IR investigations were carried out in order to elucidate the reaction mechanism. Visible light irradiation of Cr6+-MCM41 in the presence of CO led to the appearance of a typical FT-IR band due to the monocarbonyl Cr4+ species [Cr4+(CO)] at 2201 cm-1 accompanied by the formation of CO2.28 This photoreduction process of Cr6+-oxide species is also suggested by the decrease in the photoluminescence intensity of Cr6+oxide species after the visible light irradiation of Cr6+-MCM41 in the presence of CO and its subsequent evacuation as shown in Figure 2. Moreover, the addition of O2 on this system at 298 K led to the complete disappearance of FT-IR band as well as the complete recovery of the original intensity of the photoluminescence, indicating the regeneration of the original Cr6+oxide species. The catalytic reaction cycles on Cr6+-MCM-41 under visible light (λ > 420 nm) or solar light irradiation can be proposed as in Scheme 1. That is, initially, the tetrahedral Cr6+-oxide species is photoexcited to its charge-transfer excited triplet state and reacts with CO to form CO2 and a photoreduced Cr4+-oxide species. Then the Cr4+-oxide species are efficiently oxidized by O2 and the original Cr6+-oxide species are generated. The high CO selectivity observed for the Cr6+-MCM-41 can be attributed to the high and selective reactivity of the photoexcited Cr6+-oxide species with CO, as indicated by the high quenching efficiency of CO as compared to H2. The redox cycles of Cr-oxide species were found to play significant roles in the selective oxidation reaction of CO in the presence of H2. A more detailed study of the mechanisms behind these selective photocatalytic reactions on Cr6+-MCM-41 is now underway.
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