Visible light photocatalysis in transition metal incorporated titania-silica aerogels

Applied Catalysis B: Environmental 48 (2004) 151–154

Visible light photocatalysis in transition metal incorporated titania-silica aerogels J. Wang, S. Uma, K.J. Klabunde∗ Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, KS 66506, USA Received 28 August 2003; received in revised form 3 October 2003; accepted 8 October 2003

Abstract Transition metal salts were added to aerogels of TiO2 –SiO2 mixtures. The resulting M–TiO2 –SiO2 (M = transition metal, in general present in the form of oxides) materials had very high surface areas (600–1000 m2 /g) and pore volumes (1–4 cm3 /g), and contained highly dispersed transition metal oxides. Several samples were active photocatalysts for acetaldehyde oxidation to carbon dioxide using visible light; in particular chromium, cobalt, and nickel. These intimate aerogel mixtures were only active when SiO2 was present (i.e. M–TiO2 aerogels were not). High oxidation states of the transition metal ions (e.g. Cr6+ , Co3+ ) appear to be important in bringing about visible light photocatalyst activity. © 2003 Elsevier B.V. All rights reserved. Keywords: Photocatalysts; Titania–silica aerogels; Transition metal incorporation; Photocatalysis; UV and Visible light

1. Introduction Titania based UV photocatalysts that operate at room temperature have become important tools in environmental remediation [1]. With TiO2 the light energy used must exceed the band gap of 3.2 eV, thus, UV light where λ < 387.5 nm is required. In attempts to devise photocatalysts that can operate at lower energies, especially in visible wavelengths, a number of formulations have been investigated [1–11]. For example, Cr ion doped titania loaded MCM-41 (TiO2 /Cr-MCM-41) has exhibited reasonable photocatalytic activity with visible light for aqueous formic acid conversion [11]. Also, Cr ion doping by ion beam methods has met with some success [12–14]. Titania–silica aerogels possess very high surface areas (600–1000 m2 /g) and large pore volumes (1–4 cm3 /g), and thereby have attracted considerable interest. A number of studies have shown that titania-silica intimate mixtures exhibit enhanced UV photocatalytic activity compared with pure titania [15–19]. With this background, we undertook a project to combine the features of intimately mixed aerogels of TiO2 –SiO2 with ∗

Corresponding author. Tel.: +1-785-532-6665; fax: +1-785-532-6666. E-mail address: [email protected] (K.J. Klabunde). 0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2003.10.006

the incorporation of transition metal ions. The goal was to discover new visible light photocatalyst formations, especially with transition metal ions other than chromium. Indeed, several transition metal ions allow a band gap shift to the visible range when impregnated on large band gap semiconductors [3–8,20,21]. Herein we report results for TiO2 –SiO2 aerogels included with ions of V, Cr, Mn, Fe, and Ni. For comparison, formulations with only TiO2 were also prepared. What differentiates our work from others is that a modified aerogel procedure (MAP) was used to ensure rapid gelation and good mixing [22], and the fact that we compared the visible light photoactivities for the gas phase photooxidation of acetaldehyde to carbon dioxide (since gas phase processes were of primary interest to us, and solvent effects could be avoided).

2. Experimental 2.1. Synthesis of photocatalysts Synthesis methods were focused on the pre-hydrolysis of Si(OC2 H5 )4 as in the preparation of titania–silica mixed oxides [23–25]. The pre-hydrolysis of Si(OC2 H5 )4 has been shown as an essential step in order to adjust the sol–gel

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reactivities of titanium and silicon alkoxides. Three precursors are involved: transition metal salt, Ti(n-OC4 H9 )4 , and Si(OC2 H5 )4 . Compositions of final products were 0.5–5% transition metal oxide, 20–35% titania, and 60–80% silica. Example preparative procedure for the synthesis of transition metal incorporated titania–silica aerogel: for a typical composition of 5 wt.% transition metal oxide, 20 wt.% titania and 75 wt.% silica, the first step was the dissolution of 9.3 ml of Si(OC2 H5 ) (Alfa-Aesar) in 20 ml of methanol under stirring. This was followed by the addition of a mixture of 0.1 ml HCl (catalyst), and 0.8 ml of water in 5 ml of methanol for hydrolysis. The hydrolysis of Si(OC2 H5 )4 was allowed to proceed for 15–30 min. Next, 5 wt.% of transition metal (Mn), salt, Manganese(II) acetylacetonate (Aldrich) dissolved in 10 ml of methanol was added under stirring. The mixture was stirred thoroughly and the reaction was completed by the addition of 2.8 ml of Ti(n-OC4 H9 )4 (Alfa Aesar) in 10–15 ml of methanol. They were covered and aged for 12–16 h at room temperature resulting in the formation of clear transparent gels. The rate of gelation might also be increased by the addition of 4 ml of ammonium hydroxide (catalyst) solution in methanol (1 ml of NH4 OH dissolved in 4 ml of ethanol) in certain preparations (e.g. Cr/TiO2 /SiO2 ), where a clear gel formation could not be seen even after 16 h. Aerogels were made by high temperature supercritical drying of the solvent (methanol) in a standard 1 l autoclave (Parr). The autoclave with the gel was first flushed with nitrogen gas for 10 min. Then the autoclave was filled with nitrogen gas in such a way that an initial pressure of about 100 psi remains in the autoclave at room temperature. The autoclave temperature was then slowly increased up to a desired value (510 ◦ F) at a rate of 1 K/min and maintained at that temperature for about 10 min. The final pressure was about 1000 psi. After completion of the procedure, the pressure was quickly released by venting of solvent vapor. The sample was again flushed with nitrogen for 10 min and allowed to cool down in nitrogen. After drying the aerogels, they were calcined in air at 500 ◦ C for about 3 h. Textural characterization of the samples was performed on a Nova 1200 gas sorption analyzer (Quantachrome Corp.). X-ray powder diffraction experiments were conducted on a Scintag-XDS-2000 spectrometer with Cu K␣ radiation. Scans were made in the range 2θ range 20–70◦ with a scanning rate of 1◦ /min. After a final heat treatment under air at 500 ◦ C, the samples possessed surface areas of approximately 600 m2 /g, pore volumes of 1.1–2.2 cm2 /g, and average pore openings of 9–10 nm. Powder XRD indicated only the presence of nanocrystalline TiO2 (anatase, about 11 nm crystallite sizes as calculated by the Scherrer equation) [26] and did not show the presence of any bulk phases arising from the transition metal oxide or silicon dioxide. When no SiO2 was present, the M–TiO2 samples were 100 m2 /g with 10 nm anatase crystallite size.

2.2. Photocatalysis studies The experimental set up for the photocatalytic oxidation of acetaldehyde includes a light source, a static reactor, and a circulating water bath. A typical experiment was carried out as follows. The required amount of the powdered sample (about 20–40 mg of M/TiO2 /SiO2 ; M = transition metal oxide) was uniformly spread over an Al-mesh (2.5 cm×2.5 cm) and then pressed with a hydraulic press. The sample was later placed in the air filled reactor. The temperatures for all the experiments were consistently carried out at 298 K. The reactor was closed and stirred continuously after the introduction of 100 ␮l of liquid acetaldehyde. The samples were illuminated with a 1000 W high pressure Hg lamp. A combination two filters, VIS-NIR long pass filter (400 nm) and a colored glass filter (>420 nm) was utilized for the purpose of allowing only visible radiation. For UV illumination most of the IR and visible light was cut off (320 nm < λ < 400 nm). Light intensities measured for visible light (>420 nm) and UV light (320 nm < λ < 400 nm) were 7.1 and 8.4 mW/cm2 , respectively as measured with a Power Max 500D laser power meter from Molectron Detector, Inc. Gaseous samples (35 ␮l) were extracted and analyzed by GC–MS (Gas Chromatograph equipped with a mass selective detector GCMS-QP5000 from Shimadzu) to follow the concentration of the reactant acetaldehyde and the product carbon dioxide.

3. Results and discussion 3.1. M–TiO2 samples (no SiO2 ) Even though the transition metal doped titania aerogels in the absence of silica absorb in the visible range (Co/TiO2 is shown in Fig. 1e), they did not exhibit any photocatalytic activity under visible light (>420 nm). Anpo [1] also reported that V(III), Mn(II), and Cr(III) doped titania prepared by

Fig. 1. Comparison of absorption spectra of (a) TiO2 /SiO2 , (b) Co/TiO2 /SiO2 , (c) Mn/TiO2 /SiO2 , (d) V/TiO2 /SiO2 , and (e) Co /TiO2 .

J. Wang et al. / Applied Catalysis B: Environmental 48 (2004) 151–154

impregnation or a sol–gel process did not exhibit any photocatalytic activity under visible light for decomposition of NO.

light off reloaded 100 µl CH3CHO light off

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Fig. 2. Photocatalytic properties of Co/TiO2 /SiO2 : CH3 CHO consumption and CO2 evolution as a function of irradiation time during the photodegradation of CH3 CHO under visible light (>420 nm).

ln(C0/C)

0.8

Interestingly, in the presence of silica, all the above transition metal doped titania-silica aerogels also have absorption in the visible light range (see Fig. 1b–d). Their absorption edges are clearly shifted (to 450–500 nm) with decreasing band gaps. Incorporation of cobalt resulted in a shoulder near 400 nm, with a broad peak around 600 nm. While the absorption at 400 nm indicated the presence of Co3+ , the absorption around 600 nm can be attributed to the presence of both Co2+ and Co3+ [27]. The strong absorptions seen for Mn–TiO2 –SiO2 , indicate the d–d transitions of Mn3+ and Mn4+ , since the Mn2+ transitions are normally very weak [28]. For vanadium (V5+ ), d–d charge localized transitions occur at 400–600 nm [29] (Fig. 1d). However, a simple direct correlation of the band gaps of the transition metal oxides with the observed visible absorption spectra is not possible because of the presence of various hetero-junctions (M–Ti, M–Si, and Ti–Si). As typical examples, the degradation of acetaldehyde and evolution of CO2 on Co–TiO2 –SiO2 is shown in Fig. 2. Note in Fig. 2 that the initial acetaldehyde concentration was lower than after a second injection. This is probably caused by the chemisorption of the aldehyde on the dry photocatalyst at the beginning of the experiment. After the second injection, more surface bound aldehyde was probably present, and thereby less aldehyde was adsorbed. Once the light was turned on again, rapid conversion of the aldehyde to CO2 took place. For comparison, all of the M–TiO2 –SiO2 samples were studied and compared with a typical standard, Degusa P-25 TiO2 particles. The standard showed no activity under visible light, but under UV light the standard was comparable 5

P25 TiO2 (UV), 0.009m-1

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3.2. M–TiO2 –SiO2

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Ni, 0.006m-1 0.6 Fe, 0.004m-1

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V, 0.002m-1

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Mn, 0.004m -1 0

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Fig. 3. Plot of ln(C0 /C) vs. irradiation time t of acetaldehyde consumption P25 was measured under UV light (>300 nm), all the others were measured under visible light (>420 nm). The transition metal with the respective rate constants for the (5 wt.%)transition metal oxide/(20 wt.%)TiO2 /(75 wt.%) SiO2 are also shown. The standard deviation for the rate constants is ±0.003.

with the M–TiO2 –SiO2 samples under visible light; Fig. 3 shows the plots of ln(C0 /C) versus time (t), where C0 and C denote the gas phase concentration of aldehyde at t = 0 and t = t, respectively. The ln(C0 /C) value increased linearly with t, indicating that the photocatalytic reactions followed a pseudo-first-order rate law. These apparent first-order rate constants listed in Fig. 3 under the experimental conditions are useful only to compare relative activities of the catalysts, since a complete kinetic treatment was not considered while calculating the rate constants [30]. Note from Fig. 3 that several transition metal ion incorporated TiO2 –SiO2 aerogel samples exhibited very good activity using filtered visible light, and compared well with the standard P25 TiO2 sample using UV light. Furthermore, the samples showed good photocatalytic stability. For example, the Co or Mn oxide systems could be reused without loss in activity and their UV-Vis absorption spectra did not change after such use. Even indoor lighting as the light source was sufficient to cause acetaldehyde photoxidation with the Co, Cr, and Mn oxide introduced samples. Some of the M–TiO2 –SiO2 (M = V, Cr, and Mn) samples were investigated for the decomposition of acetaldehyde under UV irradiation. Fig. 4 shows the plots of ln(C0 /C) versus time (t), where C0 and C denote gas phase concentration of acetaldehyde at t = 0 and t = t, respectively. These results indicate that transition metal oxide moities in high surface area TiO2 –SiO2 are efficient photocatalysts for the decomposition of acetaldehyde under UV light. The reaction rates under UV light irradiation for M–TiO2 –SiO2 for M = V and Mn are nearly five and 10 times, respectively grater (Fig. 4) than their corresponding rate constants under visible light irradiation (Fig. 3). Furthermore, the activity of these catalysts under UV light irradiation is double that of the standard Degussa P25 TiO2 catalyst. The present results indicate that

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V, 0.02m

1.2 1.0

-1

Mn, 0.019m

References

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Cr, 0.017m

0.8 ln(C0/C)

edged with gratitude. We also thank Dr. A. Bedilo for his experimental support.

0.6 0.4 0.2 0.0 0

10

20

30

40

50

60

70

Irradiation Time (min)

Fig. 4. Plot of ln(C0 /C) vs. irradiation time t of acetaldehyde consumption measured under UV light (>300 nm). The transition metal with the respective rate constants for the (5 wt.%)transition metal oxide/(20 wt.%)TiO2 /(75 wt.%) SiO2 are also shown. The standard deviation for the rate constants is ±0.002.

the extent of dispersion and local structure of the metal oxides play a significant role in the decompsition reaction of acetaldehyde under visible as well under UV light irradition. This report in conjunction with those referenced herein, show that visible light photocatalysts for gas phase oxidation reactions are accessible. Further, work will undoubtedly reveal even more active and stable formulations. Of particular interest for the work reported here are several things: (1) cobalt and nickel visible light photocatalysts can be prepared that are of comparable activity to analogous chromium systems; (2) gas phase photoxidation of acetaldehyde to CO2 is possible with visible light with these catalysts; (3) under the conditions employed, only the M–TiO2 –SiO2 formulations were active, not M–TiO2 . Silica seemed to be essential for the photooxidation of acetaldehyde using these M-TiO2 -SiO2 catalysts. In addition, we investigated M-SiO2 catalysts and found interesting photocatalysts under visible light irradiation for M = Cr and Co [31]. These findings are not clearly understood yet, and further experiments are underway; (4) the photocatalysts reported herein were only active under visible light after a 500 ◦ C heat treatment in air. This treatment caused color changes that suggest the formation of higher oxidation states, such as Co3+ and Cr6+ . Higher oxidation states are necessary for visible light activity, but this remains to be proven. Indeed, mechanism details of those visible light driven processes are still far from understood.

Acknowledgements The support of the U.S. Army Research Office (ARO) through a MURI contract (DAAD19-01-1-0619) is acknowl-

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