Uranyl-Anchored MCM-41 as a Highly Efficient Photocatalyst in the Oxidative Destructionof Short Chain Linear Alkanes: An in situ FTIR Study

Vidya Krishna,†,‡ Valmik S. Kamble,§ Narendra M. Gupta,§,¶ and Parasuraman Selvam*,†,‡

National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology-Madras,Chennai 600 036, India, Solid State and Catalysis Laboratory, Department of Chemistry, Indian Institute ofTechnology-Bombay, Powai, Mumbai 400 076, India, Applied Chemistry DiVision, Bhabha Atomic Research Centre,Trombay, Mumbai 400 085, India and Catalysis DiVision, National Chemical Laboratory, Pune 411 008, India

ReceiVed: April 1, 2008; ReVised Manuscript ReceiVed: July 10, 2008

Uranyl ions anchored in the mesopores of MCM-41 molecular sieve were found to be a highly efficientheterogeneous photocatalyst in the complete degradation of short chain linear alkanes such as methane, ethane,propane, and butane, carried out under ambient conditions of light irradiation. In addition to the formation ofcarbon dioxide and water, a negligible amount of methane was detected during the photooxidation of ethane,propane, and butane. Further, small amounts of ethane were also obtained during photooxidation of butane,suggesting quenching of *UO2

2+ by a CsC bond cleavage, in addition to a hydrogen atom abstraction. Anin situ Fourier transform IR spectroscopy analysis was employed in order to monitor the photooxidation ofmethane and ethane over UO2

2+/MCM-41, where formic acid, formaldehyde, and formate species were thetransient species identified from methane, and acetic acid, acetaldehyde, and acetate species were theintermediates obtained from ethane. Appropriate reaction pathways were proposed based on the formation ofthese species (CsH cleavage) and also from the negligible quantitites of methane and ethane obtained duringphotooxidation of higher alkanes (CsC cleavage).

1. Introduction

Methane is considered as a hydrocarbon air pollutant obtainedfrom natural gas, fueled vehicles, and gas power plants and isknown to be a major contributor to global warming, due to itspotential as a greenhouse gas.1,2 In general, short chain linearalkanes like methane, ethane, propane, etc., are one of the leastreactive of all volatile organic compounds (VOCs) owing totheir inert C-H bonds, thus making their oxidation a highlyenergy intensive process.3 Further, the presence of alkanes inthe environment is found to have an adverse effect on our health,and prolonged exposures can cause damage to the centralnervous system.1 With these aspects in view, many attemptshave been made to completely oxidize alkanes to carbon dioxideusing conventional heterogeneous catalysts such as noble metals(Pt, Pd, Au, Rh),4-6 transition metal oxides,7 perovskites,8 dopedoxides,9 uranium oxide based catalysts,10 and so forth. However,disadvantages, such as low activity, poisoning of catalysts bychlorine and sulfur containing compounds, and requirement oftemperatures up to 873 K for complete oxidation, have limitedtheir use.

Over the past two decades, some efforts have been devotedto investigate the photoassisted catalytic oxidation of methaneand other alkanes.11-18 In these experiments, supported metaloxides and n-type solid oxide semiconductors were employedfor the oxidation of alkanes in presence of UV irradiationyielding small amounts of partial or deep oxidation productssuch as methanal, methanol, and carbon oxides at elevatedtemperatures of ∼493 K. Therefore, the employment of a

catalyst that is able to exhibit high activity under ambientconditions of temperature, pressure, light irradiation, and in thepresence of O2 as oxidant (λ > 380 nm) may offer a practicaland economic means for applications related to air cleaning ascompared to catalytic oxidation and photoassisted catalyticoxidation. In this regard, the uranyl ion (UO2

2+) is a potentiallysuitable candidate due to its distinctive photoabsorption, excita-tion, and emission characteristics as compared to any otherinorganic ion,19,20 its lowest excited eigenvalue (*UO2

2+) beinglong-lived, strongly oxidizing (E° ) 2.6 V), luminescent, andunreactive toward molecular O2, its ability to photooxidize avariety of substrates,21,22 and last, its absorption in solid andsolution state exhibiting weak bands in the visible regionbetween 380-500 nm, with stronger bands toward the ultravioletregion,19-22 thus signifying its potential use in applicationsrelated to air cleaning under ambient sunlight.

Owing to the high photochemical sensitivity of uranyl com-pounds and their fairly strong absorption in the visible region ofthe solar spectrum, the photocatalytic oxidation of hydrocarbonshave been studied in the liquid phase using aqueous uranyl ions.23,24

Wang et al.23 investigated the photooxidation of alkanes such ascyclopentane, cyclohexane, and so forth, by molecular oxygen inaqueous solution, which resulted in the formation of partialoxidation products, viz., cyclopentanone and cyclohexanone, pos-sibly due to the formation of photoinactive U(IV) species. On theother hand, in our earlier investigation we were able to demonstratefor the first time that uranyl ions anchored onto mesoporous silicatesbehave as highly efficient photocatalysts in the complete degrada-tion of methane, under ambient conditions of sunlight irradiation,25

and that U(VI) is the active and reversible participant in thephotooxidation process,26 responsible for the complete oxidationof methane.

In the present study, the photocatalytic activity of uranyl-anchored mesoporous MCM-41 was compared for the photo-

* To whom correspondence should be addressed. E-mail: selvam@iitm.ac.in. Phone: + 91-44-2257-4235. Fax: + 91-44-2257-4202/0509.

† Indian Institute of Technology-Madras, Chennai.‡ Indian Institute of Technology-Bombay, Mumbai.§ Bhabha Atomic Research Centre, Mumbai.¶ National Chemical Laboratory, Pune.

J. Phys. Chem. C 2008, 112, 15832–1584315832

10.1021/jp802779e CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/13/2008

oxidative destruction of short chain linear alkanes (C1-C4) underidentical conditions. With the help of in situ Fourier transformIR (FTIR) spectroscopy, the transient species formed duringthe photooxidation of methane and ethane were identified andsuitable reaction mechanisms were proposed and extended forpropane and butane as well.

2. Experimental Section

2.1. Catalyst Preparation. Uranyl-anchored MCM-41 (UO22+/

MCM-41) was prepared as reported earlier.27-29 As-synthesizedMCM-41 was contacted with uranyl nitrate solution (pH 5, 0.005M) and was stirred for 12 h. The uranyl-exchanged MCM-41

was washed with distilled water several times, filtered, and driedin an air oven at ∼363 K for 10-12 h. The UO2

2+-exchangedsample on subsequent calcination in N2 for 2 h and in air for6 h at 823 K was designated as UO2

2+/MCM-41 in the text.For comparison of the photocatalytic activity, we have also usedbulk TiO2.

2.2. Catalyst Characterization. The UO22+/MCM-41 con-

tained ∼ 9.8 wt % of uranium by ICP-AES analysis. The XRDpatterns of the catalyst exhibited reflections characteristic ofhexagonal MCM-41 structure.27 At higher 2θ values, weakreflections appeared at d ) 21.35, 26.03, 34.03, 45.83, and51.38°, indicating the presence of R-U3O8 crystallites encap-sulated in the mesopores of MCM-41.27,28 The BET surface areaand pore diameter of this sample was found to be ∼600 m2g-1

and 28 Å, respectively; the corresponding values in the case ofparent MCM-41 subjected to similar pretreatment being 1000m2g-1 and 30 Å, respectively. DRUV-vis, FTIR, and fluores-cence studies revealed that the uranyl groups were stronglybound with the defect sites (tSi-O-) of the MCM-41 matrixand only ∼10-15% of the uranyl ions were transformed intoR-U3O8 crystallites. These features have been described else-where in detail.27-29

2.3. Photocatalytic Activity. The photocatalytic oxidationof alkanes was carried out under conditions appropriate forthe treatment of VOCs: in the concentration range of 0.1-2.3vol % alkanes in air at room temperature (35 °C). For thispurpose, about 100 mg of catalyst was dispersed in a quartzreaction cell of 15 mm diameter, 150 mm length, and wassealed with a silicon rubber septum. The reactants wereintroduced into the cell and the final pressure of the cell waskept at around 1.2 bar. The reaction products were analyzedon a Chemito model-8510 gas chromatograph, equipped witha thermal conductivity detector (120 °C), Porapak-Q (30 °C)or Sperocarb (50 °C) column. The photocatalytic activity wasevaluated under a 400 W medium pressure mercury lamp(model 3040, type PM 135, serial no. H184, Applied

Figure 1. Time dependent variation in CO2 yield during photocatalytic oxidation of alkanes over UO22+/MCM-41, under 400 W medium pressure

mercury lamp. (9) methane, (b) ethane, (2) propane, and (1) butane, at ambient temperature (35 °C). (a-d) Concentrations of alkanes in air,varying at 0.1, 0.5, 1.0, and 1.5 vol %, respectively.

TABLE 1: Bond Dissociation Energies of CsH and CsCBond in Alkanes32,33

hydrocarbon bond energy, D (kJ.mol-1)

D (CH3sH) 438.9 ( 0.4D (C2H5sH) 423.0 ( 1.6D (n-C3H7sH) 423.3 ( 2.1D (i-C3H7sH) 409.1 ( 2.0D (n-C4H9sH) 425.4 ( 2.1D (sec-C4H9sH) 411.2 ( 2.0D (CH3sCH3) 377.4 ( 0.8D (CH3sC2H5) 370.3 ( 2.1D (CH3sn-C3H7) 372.0 ( 2.9D (C2H5sC2H5) 363.2 ( 2.5

TABLE 2: Yield of Methane and Ethane duringPhotooxidation of Higher Alkanes, Involving a CsC BondCleavage

reaction productsb

alkanea methane (%) ethane (%)

ethane 0.75propane 0.85butane 2.9 2.23

a ∼1.5 vol % alkane in air. b Products obtained at the end of1.5 h irradiation under 400 W Hg lamp at RT.

Uranyl-Anchored MCM-41 J. Phys. Chem. C, Vol. 112, No. 40, 2008 15833

Photophysics Ltd., UK). The mercury lamp was housed in awater jacket, which was placed inside a metal box to avoiddirect contact with the radiation. The average photon flux ofthe mercury lamp sunlight measured by uranyl oxalateactinometer,30 was found to be ∼6.5 × 1014 photon ·s-1 · cm-2.

2.4. FTIR Cell for in situ Measurements. FTIR studieswere performed on a JASCO model-610 spectrophotometer forin situ monitoring of the surface species and gaseous productsformed over uranyl containing mesoporous samples during theprogress of the photo-oxidation reaction.31 An ultraviolet lamp(tungsten-halogen lamp) was placed in a way that the light wasdirected vertically over the catalyst wafer. An arrangement wasmade for pretreatment of the catalyst by simultaneous pumpingand heating at a controlled temperature of up to ∼573 K. Thecell was also equipped with a flowsthrough or a pulsesmodeintroduction of a gas stream, and for subsequent digitalmeasurement of gas pressure in the range of 0 to 760 Torr. TheCaF2 disks of 4 cm diameter, placed in-between two pairs ofwater-cooled circular flanges at the two sides of the IR cell,served as windows for mid-IR region measurements. A self-

supported catalyst wafer was activated under vacuum at ∼473K prior to its exposure to the organics at room temperature.The organic vapors were prepared using dry air (2 vol % inair). The spectra were displayed after compensating for IRspectrum of the unexposed pellet. The reaction was monitoredafter introduction of organics plus air, with respect to irradiationtime.

3. Results and Discussion

3.1. Photocatalytic Oxidation of Alkanes over UO22+/

MCM-41. Figure 1a-d represent time dependent variations inCO2 yield obtained from the photooxidation of methane, ethane,propane, and butane over UO2

2+/MCM-41 for varying concen-trations in air (0.1-1.5 vol %) under a 400 W medium pressuremercury lamp. In all cases, carbon dioxide and water wereobtained, although no attempt was made for analysis of the latter.Further, negligible amounts of methane were detected duringthe photooxidation of ethane, propane, and butane, in additionto which, a small amount of ethane was also observed duringthe photooxidation of butane, indicating that for higher alkanes,a C-C bond cleavage occurs in addition to a C-H bondcleavage, which quenches the *UO2

2+. This seems feasible sincethe C-C bond is considerably weaker than the C-H bonds (seeTable 1).32,33 Shown in Table 2 are the yields (%) of methaneand ethane obtained during photooxidation of higher alkanes,over the uranyl photocatalyst. Similar experiments carried outover uranyl-free MCM-41 did not result in the formation ofcarbon dioxide or any other products. No reaction occurredduring the room temperature exposure of these alcohols overUO2

2+/MCM-41 under identical test conditions and in absenceof irradiation. Further, experiments conducted using bulk

Figure 2. Time dependent variations in CO2 yield during oxidation of methane in air over UO22+/MCM-41 and TiO2 under sunlight at ambient

temperature (35 °C). (a-d) Curves represent activity data for methane concentrations in air varying at 0.15, 0.5, 1.25, and 2.3 vol %, respectively.

Figure 3. FTIR spectrum of UO22+/MCM-41 with methane prior irradiation in the stretching and deformation regions.

TABLE 3: Assignment of the FTIR Bands of MethaneAdsorbed on UO2

2+/MCM-41 in Dry Air at RoomTemperature in the Absence of Irradiation

frequency (cm-1)

vibration mode this work methane gas adsorbed methane

ν(C-H) ∼3015 3019a 3008b,c

δ(C-H) ∼1302 1306a 1305b, 1308c

a Reference 36. b Reference 36. c Reference 37.

15834 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Krishna et al.

uranium oxide, R-U3O8 also did not result in the formation ofcarbon dioxide. These results thus provide evidence of thecrucial role played by the highly dispersed UO2

2+ in thephotocatalytic oxidation process.25,26

It can be seen from Figure 1a-d that the extent of conversionto carbon dioxide depends on the initial concentration of alkanesin air with the reaction going to completion earlier for lowerconcentrations. For instance, the photooxidation of 0.1 vol %butane over UO2

2+/MCM-41 resulted in ∼100% conversion tocarbon dioxide in 30 min, whereas for 1.5 vol %, the butane tocarbon dioxide conversion was ∼65%. A similar trend was

observed for methane, ethane, and propane as well. Hence, therequired irradiation time for reaching saturation increasedprogressively with increasing concentration of alkanes. Eventhough the overall CO2 yield was higher in the experimentsconducted with higher alkane concentrations in air, the productyield in terms of percentage conversion was lower.

Further, from Figure 1 a-d, it could also be seen that thephotocatalytic activity was found to increase as the numberof carbon atoms or chain length of the alkane increased. Thisis understandable, considering that the dissociation energyrequired to break a C-H bond decreases in the followingorder: methane > ethane > propane > butane (see Table1).32-34 It is thus possible that the C-H bond controls theease of destruction of these molecules. The abstraction ofan H-atom from a C-H bond by *UO2

2+ results in theformation of an alkyl radical with the simultaneous reductionof U(VI) to U(V).20,26 The order of stability of alkyl radicalsformed decreases in the order 3° > 2° > 1° > methyl,32 andhence opposing factors, viz., the energy required to break aC-H bond and the stability of the alkyl radicals probablyexplains the gradual increase in formation of carbon dioxidefrom methane to butane. Second, this could also be due tothe fact that higher linear alkanes are also involved in C-Cbond breaking (Table 1) apart from C-H cleavage. Hence,the energy expended in the formation of methane and ethaneas additional reaction products possibly reflects in the gradualincrease in CO2 formation from methane to butane.

To show the effectiveness of the uranyl ion as a photocatalyst,a comparative study on the photooxidation of methane wascarried out over TiO2 and UO2

2+/MCM-41 under sunlight and

Figure 4. Subtracted spectra of UO22+/MCM-41 and methane under irradiation in the stretching and deformation region. The spectra of UO2

2+/MCM-41 and methane in the absence of irradiation is taken as the reference/background.

TABLE 4: Assignment of FTIR bands generated during thephotocatalytic oxidation of methane over UO2

2+/MCM-41

frequency (cm-1)

species vibration mode this work literature

formic acid ν(O-H) ∼3305 3300-2500a

ν(CdO) ∼1675 1760a

δ(C-OH) ∼1454 1440-1395a

formaldehyde ν(C-H) ∼2950 2977b

∼2896 2913b

ν(CdO) ∼1675 1650b

δ(CH2) ∼1428 1428b

formate νas(COO) ∼1572 1554c

δas(C-H) ∼1327 1381c

νs(COO) ∼1304 1358c

methoxy δas(C-H) ∼1454 1450d

carbon dioxide νa(CdO) ∼2350 2349a

carbon monoxide ν(CdO) ∼2175, ∼2112 2127a

water δ(O-H) ∼1623 1596a

a Reference 38. b Reference 40. c Reference 41. d Reference 26.

Uranyl-Anchored MCM-41 J. Phys. Chem. C, Vol. 112, No. 40, 2008 15835

at ambient temperature. As seen from the figure, UO22+/MCM-

41 was found to be approximately at least six times moreeffective in terms of the time-dependent rate of oxidation ofmethane as compared to bulk TiO2, depending on the initialconcentration of methane feed. For example, the CH4 f CO2

conversion over UO22+/MCM-41 and TiO2 for a methane

concentration feed of 0.15 vol % at the end of 2 h solarirradiation was found to be ∼80 and ∼12%, respectively. Asimilar trend was observed for the other concentrations ofmethane (curves b-d, Figure 2). From the results presented inFigure 2, we conclude that the more efficient and fast response

photocatalytic behavior exhibited by the uranyl-anchored me-soporous silicates vis-a-vis TiO2 may arise not only because ofthe visible region absorption of the former, but also because ofthe two distinct mechanistic routes followed. It is well documentedthat for conventional semiconductor oxides like TiO2, photocata-lyzed oxidation proceeds via a electron-hole mechanism, whereelectron hole pairs are generated upon UV-induced band gapexcitation. The electrons and holes interact with reactant moleculesto form molecular or ionic radicals in excited-state and undergomolecular transformation, thereby leading to product formation.

Figure 5. Growth of (a) transient species and (b) gaseous products formed with irradiation time during photooxidation of methane over UO22+/

MCM-41.

Figure 6. FTIR spectrum of UO22+/MCM-41 with ethane prior irradiation in the stretching and deformation regions.

TABLE 5: Assignment of the FTIR Bands of EthaneAdsorbed on UO2

2+/MCM-41 in Dry Air at RoomTemperature in the Absence of Irradiation

frequency (cm-1)

vibration mode this work gaseousa adsorbed ethane

ν(C-H) ∼3002 ∼2994 2974-2977b, 3055c

∼2971 ∼2963 2941-2946b, 3022c

∼2930 ∼2954 2920-2925b, 2973c

∼2892 ∼2899 2875-2879b, 2969c

δ(C-H) ∼1463 ∼1469 1463b, 1457c

a Reference 36. b Reference 44. c Reference 45.

TABLE 6: Assignment of FTIR Bands Generated duringthe Photocatalytic Oxidation of Ethane over UO2

2+/MCM-41

frequency (cm-1)

species vibration mode this work literature

acetic acid ν(OsH) ∼3205 2937a

ν(CdO) ∼1695 1740-1720b

acetaldehyde ν(CdO) ∼1695 1715c, 1730b

acetate νas(COO) ∼1596 1583d

carbon dioxide νa(CdO) ∼2347 2349b

carbon monoxide ν(CdO) ∼2176, ∼ 2119 2127b

water δ(OsH) ∼1623 1596b

a Reference 36. b Reference 38. c Reference 40. d Reference 41.

15836 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Krishna et al.

The quantum efficiency of this process depends upon variousrecombination modes of these species, the details of which aredescribed in literature.35 In the case of uranyl ions, photoexcitationcauses an electron transfer from a methane molecule to photoex-cited UO2

2+ (*UO22+) leading to the formation of a U(V)-molecule

radical cation complex.20,22 The deprotonation within the radicalpair gives rise to the formation of a methyl radical, resultingsubsequently in its decomposition/oxidation. The efficiency of thisoverall electron transfer process between *UO2

2+ and methanemolecule is expected to be close to 100%. This explains the

Figure 7. Subtracted spectra of UO22+/MCM-41 and ethane under irradiation in the stretching and deformation region. The spectra of UO2

2+/MCM-41 and ethane in the absence of irradiation is taken as the reference/background.

Figure 8. Growth of (a) transient species and (b) gaseous products formed with irradiation time during photooxidation of ethane over UO22+/

MCM-41.

Uranyl-Anchored MCM-41 J. Phys. Chem. C, Vol. 112, No. 40, 2008 15837

instantaneous and high photocatalytic activity of uranyl-anchoredcatalysts as compared to conventional TiO2

3.2. In situ FTIR Study of Photocatalytic Oxidation ofMethane over UO2

2+/MCM-41. Shown in Figure 3 is theFTIR spectrum of methane (2 vol % in air) adsorbed on UO2

2+/

MCM-41 with the catalyst taken as the background/referencein the absence of irradiation. The positive bands observed arethus due to methane gas adsorbed on UO2

2+/MCM-41, whereas,negative bands indicate consumption of species on adsorption.From Figure 3, it can be seen that the Q, R, S bands due to

SCHEME 1: Reaction Mechanism of Photooxidation of Methane over UO22+/MCM-41

15838 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Krishna et al.

rotational structure of methane gas were observed in thestretching (∼3019 cm-1) and deformation regions (∼1306 cm-1)due to C-H bond.36,37 Band assignments and vibration frequen-

cies from this work and other cited literature are presented inTable 3. In the absence of irradiation, no additional bands apartfrom those due to methane were observed.

SCHEME 2: Reaction Mechanism of Photooxidation of Ethane over UO22+/MCM-41 Involving C-H bond Cleavage

Uranyl-Anchored MCM-41 J. Phys. Chem. C, Vol. 112, No. 40, 2008 15839

Under irradiation, at varying durations, a number of changeswere observed in the spectrum of methane adsorbed on UO2

2+/MCM-41. Figure 4 present spectra of methane and UO2

2+/MCM-41 in the presence of irradiation, subtracted from thosein the absence of irradiation, as a function of time. Hence, thebands observed are due to transient species/gaseous productsgenerated during photooxidation of methane over the uranyl-photocatalyst. The following are the highlights of these changesobserved.

1. A broadband between 3500 and 3000 cm-1, centered at ∼3305 cm-1 with a shoulder at ∼3218 cm-1, was observed onirradiation, which was found to increase in intensity with time.This band is characteristic of O-H stretching vibration ofcarboxylic acid (in this case, formic acid), which are hydrogen-bonded with the U-OH/Si-OH bond of the photocatalyst.36,38,39

The two bands observed at ∼3305 and ∼ 3218 cm-1 could bedue to ν(O-H) of HCOOH hydrogen-bonded to U-OH andSi-OH sites with the assignments being ambiguous.

2. A distinct band was also observed at ∼1675 cm-1 due toν(CdO) of methanal and/or formic acid.38,40 Aliphatic aldehydesusually absorb in the region 1740-1720 cm-1, whereas saturatedcarboxylic acids show absorption at ∼1760 cm-1.36,38,39 How-ever, hydrogen bonding shifts the absorption to lower wave-numbers. For instance, salicylaldehyde shows an IR bandν(CdO) at 1666 cm-1, and salicylic acid absorbs at 1665cm-1.38 Hence, in the present study, the shift of the ν(CdO)band at ∼1675 cm-1 could be due to hydrogen bonding ofmethanal and/or formic acid with the photocatalyst (U-OH).

The corresponding δ(C-H) of methanal was observed at ∼1428cm-1,40 and the δ(C-OH) band of formic acid was seen at∼1454 cm-1.36,38,41 The band at ∼1454 cm-1 could also beattributed to the δas(C-H) of methoxy species (-OCH3).26,38

Further, the presence of methanal was validated by the weakbands of ν(C-H) at ∼2950 and ∼2896 cm-1.38

3. A weak shoulder at ∼1572 cm-1 was observed due toνas(COO-) of formate species with the corresponding δas(C-H)(∼1327 cm-1) and νs(COO-) (∼1304 cm-1) seen as abroadband between 1483-1273 cm-1.41 The formation of newbands generated on photooxidation of methane with theirvibration frequencies and assignments have been presented inTable 4.

4. IR bands were also observed at ∼2350 cm-1 correspondingto νas(CdO) of carbon dioxide molecules and at ∼2175 and∼2112 cm-1 due to ν(CdO) of carbon monoxide (Table4).36,38,39 The two bands of CO could be due to physisorbedcarbon monoxide on different sites, viz., the uranyl site and thesilanol site, as seen in an earlier study.42 Further, the formationof water was also observed in the δ(O-H) band at ∼1623 cm-1

(Table 4).36,38,39 The formation of carbon dioxide and watercorroborates our photocatalytic results.

5. The absorbance of IR bands due to formic acid, methanal,and formate species were plotted as function of irradiation time,as seen in Figure 5a. Figure 5b presents plot of intensities ofIR bands due to carbon dioxide and water, as a function of time.From Figure 5a, it can be seen that the intensity of formic acidwas found to be the highest, followed by methanal and formate

SCHEME 3: Reaction Mechanism of Photooxidation of Propane and Butane over UO22+/MCM-41 Involving a C-H

Bond Cleavage

15840 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Krishna et al.

species. The high intensity of formic acid indicates that itsformation from methane is greater than its consumption to formsubsequent products. On the other hand, the lower intensity ofmethanal suggests its conversion to formic acid by oxidation.In the formation of formic acid and methanal, hydrogen bondingwith the photocatalyst is suggested, as seen in the ν(O-H) bandof HCOOH hydrogen bonded with UO2

2+/MCM-41 between

3500 and 3300 cm-1. The presence of formate species over thecatalyst indicates a bonding with the isolated -OH species ofUO2

2+/MCM-41, giving rise to its depletion, as seen in theincreasing negative intensity at ∼3720 cm-1.43 The low intensityfor formate species observed in Figure 4a shows that itsconsumption to form products like carbon dioxide and water isgreater than its formation from methane.

SCHEME 4: Reaction Mechanism of Photooxidation of Ethane, Propane, and Butane over UO22+/MCM-41 Involving a

C-C Bond Cleavage, Forming Methane

SCHEME 5: Reaction Mechanism of Photooxidation of Butane over UO22+/MCM-41 Involving a C-C Bond Cleavage,

Forming Ethane

Uranyl-Anchored MCM-41 J. Phys. Chem. C, Vol. 112, No. 40, 2008 15841

6. In situ experiments carried out over pristine MCM-41sample under identical conditions (not shown here), did notresult in the formation of new IR bands, thus signifying theimportance of UO2

2+ in a photocatalytic process.25,26

3.3. In situ FTIR Study of Photocatalytic Oxidation ofEthane over UO2

2+/MCM-41. The in situ FTIR analysis ofthe intermediate species obtained during the photooxidation ofethane were found to be analogous with those obtained fromthe photooxidation of methane. Figure 6 shows a representationof the IR spectrum of ethane adsorbed on the uranyl catalyst inthe absence of irradiation in the stretching and deformationregions, whereas Figure 7 shows the spectra of the IR bandsgenerated during photooxidation, obtained by subtracting spectraunder irradiation from those in the absence of irradiation. Theband assignments of gaseous ethane including those reportedin literature have been presented in Table 5. Table 6 lists theassignment of IR bands due to the intermediate species, viz.,ethanal, acetic acid, and acetate species, and gaseous productssuch as carbon dioxide, carbon monoxide, and water, obtainedduring the photooxidation of ethane. The growth of intermediatespecies, viz., acetic acid, ethanal and acetate species, as seen inFigure 8a, and gaseous products, viz., carbon dioxide and water,as seen in Figure 8b, were plotted as a function of irradiationtime and exhibited patterns similar to those obtained from theintermediate species of methane photooxidation (Figure 5a,b).From IR data in Figure 7, it is indicated that acetic acid andhence ethanal are hydrogen-bonded to the uranyl site (νO-H bandat ∼3205 cm-1 due to H-bonded acetic acid).36 Formation ofacetate species is suggested to take place through bonding withisolated -OH groups of uranyl photocatalyst, as in the case offormate species. In situ experiments carried out over uranyl-free MCM-41 sample under identical conditions did not resultin the formation of new IR bands, thus signifying the importanceof UO2

2+ in the photocatalytic process.25,26

3.4. Suggested Reaction Mechanism. C-H bond CleaWage.On the basis of the formation of formic acid, methanal, andformate species during the photooxidation of methane, theproposed pathway has been presented in Scheme 1. Formic acidand methanal are believed to be hydrogen bonded to UO2

2+/MCM-41, whereas formate species are bonded with the pho-tocatalyst through the depletion of their isolated -OH groups,as seen in independent reaction routes in Scheme 1.

1. In route 1, irradiation of adsorbed methane on UO22+/

MCM-41 resulted in hydrogen atom abstraction from methaneby *UO2

2+ forming methyl radical (1a), and simultaneousreduction of U(VI) to U(V).20,26

2. In the presence of oxygen, the methyl radical formedmethyl hydroperoxide (1b) hydrogen bonded to the uranylphotocatalyst with the reoxidation of UO2

+ to UO22+.

3. Irradiation of methyl hydroperoxide resulted in the cleavageof the O-O bond by *UO2

2+, giving rise to an OH radical.Further irradiation resulted in the abstraction of an H atom fromthe methyl group by *UO2

2+, which on expulsion of a watermolecule gave rise to methanal (1c).

4. Under irradiation, H-atom abstraction from methanal by*UO2

2+ formed formyl radical (1d), which in the presence ofoxygen, yielded formic acid (1e) hydrogen bonded to UO2

2+/MCM-41. Further oxidation of formic acid yielded carbonoxides and water.

5. In route 2, adsorbed methyl radical in the presence ofoxygen, [O] resulted in the formation of methoxy group (1f)bonded to the uranyl photocatalyst. Irradiation of the methoxygroup resulted in H-atom abstraction by *UO2

2+ forming a

carbon-centered radical. The carbon-centered radical in thepresence of oxygen formed methoxy hydroperoxide (1g).

6. Methoxy hydroperoxide, under irradiation, resulted in thecleavage of the O-O bond by *UO2

2+, giving rise to an OHradical. Further irradiation resulted in the abstraction of an Hatom from the methylene group by *UO2

2+ forming a carboncentered radical, which on expulsion of a water molecule, gaverise to formate species (1h). Formate species on further oxidationwith n moles of oxygen gave carbon dioxide, carbon monoxide,and water.

Scheme 2 presents the proposed pathway for the photooxi-dation of ethane, based on the formation of acetic acid, ethanal,and acetate species, and was found to follow a similar route asthat of methane. Acetic acid and ethanal are hydrogen bondedto UO2

2+/MCM-41, whereas acetate species are bonded withthe isolated -OH of the photocatalyst, as seen in independentreaction routes (Scheme 2). In line with the in situ FTIR analysesfor the photooxidation of methane and ethane over UO2

2+/MCM-41, Scheme 3 presents a possible general reactionmechanism for the photooxidation of propane and butane.Adsorbed propane/butane (3a) under irradiation, resulted in theformation of sec-propyl/sec-butyl radical (3b) with the con-comitant reduction of U(VI) to U(V),20,26 which in the presenceof oxygen formed sec-propyl hydroperoxide/sec-butyl hydrop-eroxide (3c). Under irradiation, sec-alkyl hydroperoxides re-sulted in the cleavage of O-O bond by *UO2

2+, giving rise toan OH radical. Further irradiation resulted in the abstraction ofan H atom from the methyne group by *UO2

2+ forming a carboncentered radical, which on expulsion of a water molecule, gaverise to acetone/butanone (3d). Further oxidation of acetone/butanone with n moles of oxygen resulted in the formation ofcarbon oxides and water.

C-C Bond CleaWage. The photocatalytic oxidation of thehigher alkanes, viz., ethane, propane, and butane, also resultedin small amounts of methane in addition to carbon dioxide andwater, as seen in Table 2. Scheme 4 gives a general reactionmechanism depicting C-C bond cleavage during the photo-oxidation of ethane, propane, and butane. It can be seen fromScheme 4 that the adsorbed alkanes (4a) on irradiation resultedin the formation of the adsorbed alkyl radical (4b) and methylcation due to C-C bond cleavage with the simultaneousreduction of U(VI) to U(V).20,26 In the presence of oxygen, thealkyl radicals formed alkyl peroxy radicals (4c), which in thepresence of alkane, resulted in its H-atom abstraction by *UO2

2+,forming methane, and a dialkyl peroxide (4d). In the presenceof irradiation and n moles of oxygen, the dialkyl peroxideunderwent further oxidation to give carbon dioxide and water.

Shown in Scheme 5 is the reaction pathway for the photo-oxidation of butane, which yielded small amounts of ethane, inaddition to methane, carbon dioxide, and water, as seen in Table2. Adsorbed butane (5a) on irradiation resulted in the formationof ethyl cation and adsorbed ethyl radical (5b) with thesimultaneous reduction of U(VI) to U(V).20,26 In the presenceof oxygen, adsorbed ethyl radical formed ethyl peroxy radical(5c). Further, under irradiation and in the presence of butanemolecule, H-atom abstraction from butane took place by*UO2

2+, resulting in the formation of ethyl sec-butyl peroxideand ethane (5d). Further oxidation of the dialkyl peroxide ledto the formation of carbon dioxide and water.

4. Conclusions

In conclusion, UO22+/MCM-41 was found to be a highly

efficient photocatalyst in the complete oxidation of short chainlinear alkanes like ethane, propane, and butane to carbon dioxide

15842 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Krishna et al.

and water, under ambient conditions. In addition, negligibleamounts of methane and ethane were detected in the photooxi-dation of higher alkanes, suggesting a C-C bond cleavage. Therates of conversions of alkanes to carbon dioxide was found toincrease in the order methane < ethane < propane < butane.The difference in photoactivity was attributed to the weakeningof the C-H bond with increasing carbon chain length, thestability of the alkyl radicals formed as a result of H-atomabstraction on photoexcitation of UO2

2+ ions, and the energyexpended in the cleavage of the carbon-carbon bond. In situFTIR spectroscopy analyses were carried out in order to identifythe transient species formed during the photooxidation ofmethane and ethane over the uranyl-anchored photocatalyst. Thephotooxidation of methane resulted in formic acid, methanal,and formate species over UO2

2+/MCM-41, as seen in the IRbands, whereas photooxidation of ethane resulted in the forma-tion of ethanal, acetic acid, acetate and species as the intermedi-ates. On the basis of the FTIR results, it was deduced that formicacid and ethanal were hydrogen bonded with UO2

2+/MCM-41,whereas formate species were bonded with the photocatalystthrough depletion of isolated hydroxyls. From the intensitiesof the intermediate species formed, formic acid was obtainedfrom oxidation of ethanal, whereas the lower intensity of formatespecies indicated its further oxidation to form carbon oxidesand water. Similar observations were made for ethanal, aceticacid, and acetate species. From these studies, based on C-Hand C-C bond cleavage, appropriate reaction pathways wereproposed for the photooxidation of methane and ethane and wereextended for propane and butane.

Acknowledgment. The authors acknowledge Professor K. D.Deodhar, Department of Chemistry, IIT-Bombay, for valuablesuggestions. This work was performed under BRNS ContractNo. 99/37/BRNS/1049.

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