Adsorption and Reactions on TiO2: Comparison of N,N

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Adsorption and Reactions on TiO2: Comparison of N,N‑Dimethylformamide and Dimethylamine Jong-Liang Lin,* Yu-Cheng Lin, Bo-Chiuan Lin, Po-Chih Lai, Tzu-En Chien, Szu-Hui Li, and Yu-Feng Lin* Department of Chemistry, National Cheng Kung University, No. 1 Ta Hsueh Road, Tainan 70101, Taiwan, ROC ABSTRACT: Fourier transform infrared spectroscopy has been employed to study the adsorption and reactions of N,N-dimethylformamide (DMF) and dimethylamine (DMA) on powdered TiO2. DMF can be adsorbed in molecular form with the carbonyl interacting with the surface Lewis site (Ti4+) or in dissociative form of OCN(CH3)2. Theoretical adsorption study of rutile (110) points out that the C* and O atoms of OC*N(CH3)2 are bonded at a two-foldcoordinated O site and a five-fold-coordinated Ti site, respectively. The thermal products of DMF/TiO2 are found to be CO and DMA. Photochemical reaction of DMF on TiO2 in O2 generates CO2, HCOO, and NCO. O2 participates in the reaction with its oxygen atoms incorporated into the three products. DMA can be adsorbed in molecular form and imine species on TiO2. Photoirradiation of DMA/TiO2 in O2 generates CO2, HCOO, NCO, and imine species. Interestingly, DMF and OCN(CH3)2 are produced after postirradiation thermal treatment of DMA on TiO2, possibly from the reaction between residual DMA and HCOO photoproduct.

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INTRODUCTION Semiconductor-mediated photocatalysis is a promising approach to transform harmful chemicals to environmentally innocuous species. Among them, TiO2 has attracted much attention due to its chemical stability, economical availability, and band structure allowing feasible photodegradation for a large number of organic compounds.1−4 N,N-dimethylformamide (DMF) can react as an electrophilic or nucleophilic reagent and can be the source of various key intermediates participating in diversified reactions, as shown in Table 1,5,6 due to its versatile dissociation pathways under different reaction conditions. For instance, the rupture of the C(O)−H bond can furnish H. In addition, the breakage of the C(O)−N bond can generate −CHO and −N(CH3)2 for formylation and amination reactions, respectively. DMF and dimethylamine (DMA) have been widely used in chemical industry, such as in production of pesticides, dyes, synthetic leathers, and so on, and have long been recognized as toxic materials.7,8 With the aim of destroying DMF, several methods have been explored and reported, for example, wet air oxidation conducted at ∼200 °C and ∼60 bar with noble metals as catalysts supported on TiO2 or ZrO29 and photocatalytic degradation using TiO2 thin film.7 In the latter study, Chang et al. suggested the presence of carbonylic acids, aldehydes, amines, carbonate, and nitrate on TiO2 during the photocatalytic DMF decomposition on the basis of relatively rough postirradiation infrared analysis.7 In the case of DMA, Madix et al. reported the primarily reversible adsorption on rutile TiO2 (110), with a small fraction of the adsorbed molecules decomposing into CO, N 2 , and H 2 . 10 Helali’s group investigated photocatalytic degradation of DMA in TiO2 © XXXX American Chemical Society

aqueous suspensions and found the formation of CH3NH2, NH4+, NO3−, and HCOOH but focused on adsorption isotherm and kinetics.8 Lee’s group compared the difference in the reaction processes for DMA in aqueous solutions with suspended TiO2 or Pt/TiO2 in the presence of air or N2 under photoirradiation.11 In the systems of TiO2/air and Pt/TiO2/air, they reported the formation of CH3NH2, NH3, NO2−, and NO3−, while under the deaerated condition, NO2− and NO3− were not found. Additional N(CH3)3 was generated in Pt/TiO2 system with N2.11 Kachina et al. studied the gas-phase photodegradation of DMA over TiO2, showing the reaction products of CO2, H2O, NH3, and formamide.12 However, in these previous photocatalysis studies, adsorption structure and surface degradative processes of DMF and DMA on TiO2 were not emphasized. According to the diversified chemical roles of DMF (Table 1), it is considered to be an interesting subject to study the interaction between DMF and TiO2 surface. DMF possesses several structural characteristics, such as HCO (aldehydic H), −OCH−N (peptide linkage), and N(CH3)2 (methylamino), which can participate in the surface reactions thermally or photochemically. The adsorption and reaction of DMA on TiO2, which also has the functional −N(CH3)2, is worth being explored as well to fully understand the reaction process of DMF. In this article, we present the adsorption and thermal and photochemical reactions of DMF and DMA on TiO2 in a gas−solid system equipped with an in situ Fourier transform infrared spectrometer. Received: May 7, 2014 Revised: August 16, 2014

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Table 1. Roles and Diversified Reactions Involving N,N-Dimethlyformamidea

a

role

reaction type

source of carbon monoxide (−C(O)−) source of formyl (−CHO) source of oxygen (−O−) source of formate (−C(O)O−) source of dimethylamino (−N(CH3)2) source of −C(O)N(CH3)2 source of CHN(CH3)2 and CHOH source of −OCHHN(CH3)2− source of cyano (−CN) electron transfer reducing and dehydrating agents precursor for H− or H·

carbonylation formylation formation of ethers and anhydrides formation of carboxylic acid amination aminocarbonylation formation of enamines cycloaddition cyanation

Refs 7 and 8.

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EXPERIMENTAL SECTION The chemical reagents used in this study were HCON(CH3)2 (99.8%, Aldrich) and HN(CH3)2 in H2O (40 wt %, Aldrich). TiO2 powder (Degussa P25, ∼50 m2/g) supported on a tungsten fine mesh was prepared first according to the similar procedure previously reported.13,14 In brief, TiO2 powder was well-dispersed in water/acetone solution to form a uniform dispersion system, which was then sprayed onto the entire area of the tungsten mesh. The mass of TiO2 deposited on the tungsten mesh was ∼0.07 g. The TiO2/W was then mounted inside the infrared cell for thermal treatment, which included the steps of resistive heating to 450 °C in a vacuum and 350 °C in the presence of O2. As the temperature of the TiO2/W sample was decreased to 35 °C, the cell was evacuated. An infrared spectrum was taken as reference background. The temperature of the TiO2 sample was measured by a K-type thermocouple spotwelded on the tungsten fine mesh. For the surface studies of DMF/TiO2 and DMA/TiO2, DMF or DMA vapor was directly introduced to the infrared cell, via the gas manifold for the adsorption on TiO2. In the study of thermal decomposition of DMF/TiO2 under vacuum, a saturation coverage of DMF on the surface was prepared by several cycles of exposing the TiO2 to DMF vapor, followed by evacuation until the maximum spectral absorbance was obtained. According to the molecular size of DMF, a assumption of closest packing on the surface, and van der Waals interaction between adsorbates, it is roughly estimated that the surface density of DMF on TiO2 at saturation coverage is ∼2.5 × 1014 molecules/cm2. Since aqueous DMA was used in the present study, it was difficult to estimate the surface concentration due to the coadsorption of water and DMA, but DMA would be adsorbed at submonolayer coverage, less than 3.1 × 1014 molecules/cm2. The IR cell with two CaF2 windows for IR transmission down to ∼1000 cm−1 was connected to a gas manifold, which was pumped by a turbomolecular pump to maintain a base pressure of ∼1 × 10−7 Torr. The pressure was monitored with a Baratron capacitance manometer and an ion gauge. In the photochemistry study, both the UV and IR beams were set 45° to the normal of the TiO2 sample. The light source used was a combination of a Hg arc lamp (Oriel Corp.) operated at 500 W, a water filter, and a band-pass filter with a bandwidth of ∼100 nm centered at ∼325 nm for DMA/TiO2 or at ∼400 nm for DMF/TiO2. Transmission IR spectra were obtained with a 4 cm−1 resolution on a Bruker FTIR spectrometer (Vector 22) with a MCT detector.

COMPUTATIONAL METHOD

The optimized chemisorption structure of OCN(CH3)2 on TiO2, with rutile (110) as a model surface, was calculated in the framework of density functional theory by using DMol3 package, in which generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) formulation was employed. A double-numeric quality basis set with polarization functional, a Monkhorst-Pack k-point set at 4 × 3 × 2 and a supercell of 24 [TiO2] units with dimensions 13.00 × 5.92 × 9.04 Å3 were used in this study. All slabs were separated by a vacuum space of 10.0 Å. The Ti and O atoms in the first and second layers of the slabs were allowed to be varied in position for the optimized structure calculation.

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RESULTS AND DISCUSSION Adsorption and Thermal Reactions of N,N-Dimethylformamide and Dimethylamine on TiO2. At first, a TiO2 (35 °C) with DMF adsorption at saturation coverage was prepared. The DMF/TiO2 surface was then heated to the preset temperatures for 1 min in a vacuum for infrared measurement to detect surface species. Figure 1 shows the temperature-dependent infrared spectra of DMF on TiO2. In the 35 °C spectrum, the most intense peak is located at 1653 cm−1, mainly from carbonyl absorption, with relatively small bands at 1112, 1259, 1383, 1397, 1436, 1496, 1565, 2839, 2893, 2933, and 2962 cm−1. These positions are listed in Table 2 and compared to the DMF infrared absorptions in liquid state reported previously.15 A close match in the peak frequencies, except the 1565 cm−1 band, is observed, indicating the presence of DMF molecules on TiO2 at 35 °C. Moreover, the interaction between adsorbed DMF and surface is revealed by the downshifted 1653 cm−1 peak (a difference of 24 cm−1 as compared to that of DMF liquid molecules), involving both the carbonyl group and surface Ti4+ Lewis site. The negative peaks shown in the range of 3600−3800 cm−1 reflect that adsorbed DMF molecules also interact with isolated surface OH groups. The small band at 1565 cm−1 is due to DMF dissociative chemisorption form and is discussed more later. Temperature increase to 100 °C induces several spectral changes, such as the peak shape at ∼1382 cm−1 and enhanced absorptions at 2843 and 3254 cm−1. The latter band suggests the presence of other surface species containing NHx. No peaks were observed in the range 1850−2750 cm−1 in the 35 °C spectrum. From 100 to 200 °C, the intensities at ∼1380 and 1565 cm−1 grow continuously. The peaks at ∼2843 and 3255 cm−1 are hardly B

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Figure 2. Infrared spectrum taken after DMF adsorption on TiO2 followed by surface heating to 200 °C for 1 h and temperature decrease to 40 °C with the cell closed. The spectrum in the range of 1050−1850 cm−1 is multiplied by a factor of 0.3.

cm−1 and an additional peak appearing at 2119 cm−1, which is attributed to CO on TiO2 from gas phase readsorption.16 Dissociation of DMF on TiO2 forming CO by bond breakage of C(O)−H and C−N(CH3)2 provides a clue to identify the species with the ∼3255 cm−1 band, that is, formation of dimethylamine. This point has been confirmed by DMA adsorption on TiO2. Figure 3 shows the infrared spectra taken

Figure 1. Infrared spectra taken at the temperatures indicated after saturation adsorption of N,N-dimethylformamide on TiO2 at 35 °C followed by evacuation. All traces in the ranges of 2650−3350 cm−1 and 3600−3800 cm−1 are multiplied by a factor of 2.

Table 2. Comparison of the Infrared Frequencies (cm−1) of N,N-Dimethylformamide modea

liq

a

2998 2956 2930

νas(CH3) νas(CH3) νs(CH3)

2857 1677

ν(CH) ν(CO) + δ(NCH) + ν(CN)

1507 1460 1439 1406 1388 1257 1152 1093

ν(CN) + δ(CH3) δ(CH3) δ(CH3) δ(CH3) + ν(CN) δ(CH3) + δ(NCH) ν(CN) + δ(CH3) ρ(CH3) ρ(CH3)

on TiO2 (35 °C) 2962 2933 2893 2839 1653 1565 1496 1436 1397 1383 1259 1112

Ref 15. Figure 3. Infrared spectra taken at the temperatures indicated after adsorption of dimethylamine on TiO2 at 35 °C followed by evacuation.

detectable in the 250 °C spectrum. At this temperature, the main peaks appear at 1373, 1433, 1574, 1647, and 2943 cm−1, but only a small peak of 1647 cm−1 exists at 350 °C. The NHx stretching mode responsible for the observed ∼3255 cm−1 band signifies the decomposition of DMF on TiO2. To trace the possible reaction products generated in the gas phase, we carried out an additional experiment in which infrared spectra were obtained at a lower temperature after DMF adsorption and surface heating with the cell closed because the gas products formed could be readsorbed and then measured. Figure 2 shows the infrared spectrum obtained following DMF adsorption, surface annealing at 200 °C for 60 min, and temperature decreasing to 40 °C. As compared with the 200 °C spectrum of Figure 1, there is a similar peak at 3261

at the temperatures indicated following the adsorption of DMA and surface heating under vacuum. The main peaks observed in the 35 °C spectrum are compared with the DMA infrared absorptions in the solid state (Table 3).17,18 As shown, the DMA adsorption in intact form is reflected by its characteristic frequencies at 3257 cm−1 (ν(NH)) and 1469 cm−1 (δ(CH3)) as well as in the ranges of 2700−3100 cm−1 (ν(CH3) + combinations + overtones) and 1150−1300 cm−1 (ρ(CH3)). The characteristic NH stretching frequency 3257 cm−1 of DMA on TiO2 strongly supports the DMA formation in the reaction of DMF, with the presence of ∼3255 cm−1 band in Figures 1 C

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Table 3. Comparison of the Infrared Frequencies (cm−1) of Dimethylamine DMA solida

modea

3215

ν(NH)

2965

ν(CH3) +combinations +overtones

2943 2932 2905 2883 2852 2820 2785

1525 1472 1458 1451 1250 1176 a

on TiO2 (35 °C) 3257 3009 2967

2936 2905 2891 2841 2795 1648 1624 δ(NH) δ(CH3)

ρ(CH3)

1469 1432 1413 1247 1206

Refs 17 and 18.

and 2. In Figure 3, the 1624 cm−1 peak in the 35 °C spectrum is due to residual adsorbed water molecules, which can be removed by heating to 200 °C. The small 1648 cm−1 band overlapping with the water absorption is possibly originated from δ (NH) of adsorbed DMA. This mode has been assigned for the peaks between 1613 and 1640 cm−1 observed in the systems of DMA on H-mordenite, H-chabazite, and ionexchange resin.19,20 While heating the surface to 250 °C, the peak at 1645 cm−1 grows at the expense of DMA, evidenced by the smaller ν (NH) band. This trend continues up to 300 °C. At this temperature, the assignment of δ (NH) mode of DMA for the enhanced 1645 cm−1 peak under the consideration of the minute ν (NH) peak of 3257 cm−1 seems inappropriate. The band at the 1645 cm−1 is attributed to imine (ν(CN)) from decomposition of DMA. In the 300 °C spectrum, the other relatively strong bands of 1469, 2905, and 2936 cm−1 suggest that the imine species possesses CH3. Therefore, it is proposed that DMA thermally degrades on TiO2 at a temperature larger than 200 °C, forming imine with CH3− NC or CH3−NCH− structure. Imine formation has been observed, with the functional CN stretching frequency of 1654 cm−1 in the dissociative adsorption of DMA on Al2O3 at ∼280 °C.21 The 350 °C spectrum of Figure 3 shows only the presence of the imine without DMA. Formation of Chemisorbed OCN(CH3)2 from DMF Decomposition. As shown in Figure 1, the absorptions of ∼1375 and 1565 cm−1 increase at 200 °C after DMF adsorption on TiO2. These two peaks are likely from a functional group similar to carboxylates. For example, formate on TiO2 has two relatively strong ν (COO) bands at ∼1350 and 1550 cm−1.16 We have calculated the optimized structure of OCN(CH3)2, the species from loss of the carbonylic hydrogen of DMF, on a model rutile (110) surface and found the consistent infrared absorptions. The optimized OCN(CH3)2 adsorption form and structural parameters are shown in Figure 4a,b. For this surface species, the C and O atoms of the carbonyl group are bonded at a two-fold-coordinated O site

Figure 4. Theoretically calculated adsorption structure of OCN(CH3)2 on rutile (110) (a) and tabulated structural parameters (b). The calculated infrared absorptions (cm−1) of this chemisorption species are shown in panel c.

(O1) and a five-fold-coordinated Ti site (Ti3), respectively, with the bond distances of 1.330 and 2.176 Å. Besides, the N−CO bond is shorter than the N−CH3 bond, ca. 1.36 Å with respect to 1.46 Å. Interestingly, the bond angles in the backbone of the surface structure OOCN(CH3)2 are close to 120°, near a planar skeleton. Because of the chemisorption of OCN(CH3)2, the surface O1 atom moves upward, leading to the decreased Ti1− O1−Ti2 angle at 91.8° and elongated O1−Ti1 and O1−Ti2 distances at ∼2.167 Å, in contrast with the original values of 98.8° and 1.949 Å. Figure 4c shows the theoretically calculated main infrared absorptions and the corresponding modes based on the vibrational animation for the surface structure of OOCN(CH3)2. The frequencies of 1374 and 1550 cm−1 are similar to those (∼1375 and 1565 cm−1) observed in Figure 1, which can be assigned mainly to the vibrational modes of the OOCN moiety in OOCN(CH3)2. Photocatalytic Reactions of N,N-Dimethylformamide and Dimethylamine on TiO2. Figure 5a shows the infrared spectra taken before and during photoirradiation of DMF adsorbed on TiO2 in the closed cell with 10 Torr of 16O2 initially. The infrared spectrum taken after the irradiation followed by cell evacuation is also shown. Photodecomposition of DMF occurs, as revealed by the diminishing peaks at 1651 and 2934 cm−1 and the formation of new bands at 1359, 1568, 2202, and 2359 cm−1. For the new bands, the first two are ascribed to formate, and the last one is ascribed to weakly adsorbed CO2, which can be removed under vacuum.16 The 2202 cm−1 band is identified as −NCO (isocyanate), which has been reported in the previous study of CH3CN photooxidation D

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Figure 6. Infrared spectra taken before and after the indicated photoirradiation times for dimethylamine/TiO2 in the closed cell with 10 Torr of 16O2 (a) or 18O2 (b) initially. The infrared spectra taken after photon exposure and cell evacuation are also shown. All traces in the range of 2050−2450 cm−1 are multiplied by a factor of 3.

Figure 5. Infrared spectra taken before and after the indicated photoirradiation times for N,N-dimethylformamide/TiO2 in the closed cell with 10 Torr of 16O2 (a) or 18O2 (b) initially. The infrared spectra taken after photon exposure and cell evacuation are also shown. All traces in the range of ∼1050−1850 cm−1 are multiplied by a factor of 0.5.

The latter one is from CO2 in the gas phase. NC16O is responsible for the 2204 cm−1 band. Formate contributes to the absorptions at 1359, 1381, and 1558 cm−1. The 1644 cm−1 could be due to stretching vibration of CN, CO, or N− CO (peptide linkage). Its identity can be resolved by oxygen isotopic study. Figure 6b shows the infrared results from the DMA/TiO2 photocatalytic study using 18O2. The absorption near 2331 cm−1 is due to 18O-containing CO2.24 The 2190 cm−1 band reveals the formation −NC18O, without incorporation of TiO2 lattice oxygens. The formate formed also contains 18O, leading to the broadened band at ∼1355 cm−1 with a shoulder of 1333 cm−1 and the red-shifted 1550 cm−1 band. The 1641 cm−1 band, only red-shifted by ∼3 cm−1 as compared with the 16O2 case, is assigned to CN stretching vibration (ν(CN)) of imine species, not to ν (CO), because the difference between ν (C16O) and ν (C18O) is ∼30 cm−1.24 The origin for the ∼1408 cm−1 band is unclear but could be associated with the imine species. Formation of DMF and OCN(CH3)2 from Thermal Treatment after Photoirradiation of DMA on TiO2. Figure 7a shows the infrared spectra taken under vacuum at the indicated temperatures following the photochemical reactions of DMA on TiO2 in O2. Heating the surface after photoirradiation on DMA/TiO2 causes the decrease in formate (1358, 1556 cm−1) and residual, unreacted DMA (1469 and 3257 cm−1). In contrast, the absorption at ∼1645 cm−1 becomes sharper and larger, while the temperature is raised to 200 °C or higher. The peak positions and relative intensities of the 300 °C spectrum in Figure 7a, except the small DMA band at 1469 cm−1, conspicuously resemble those absorptions observed at the same temperature in the thermal decomposition of DMF on TiO2, which are shown in Figure 7b for a direct comparison. At 300 °C of the DMF/TiO2 system, the surface is covered with chemisorbed DMF and OCN(CH3)2.

on TiO2.22,23 Photocatalytic decomposition of DMF (HCON(CH3)2) in O2 to form −NCO can take place via two different pathways. Because DMF itself contains NCO moiety, dissociation of the H−CO and N−CH3 bonds is the first, reasonable route for −NCO formation. In this pathway, further photooxidation of the CH3 groups from breakage of N−CH3 bonds of DMF would result in another photocatalytic products of formate. In the second pathway, O2 is chemically attached to one of the CH3 groups of DMF, in conjunction with dissociation of the HC(O)−N bond and loss of the other CH3, finally leading to −NCO formation The actual pathway for DMF photodegradation on TiO2 generating −NCO can be identified using 18O2 in place of 16O2. For the second reaction pathway in the presence of 18O2, −NC18O is expected to be formed with a red-shifted vibrational mode with resect to −NC16O. Figure 5b shows the infrared result from the DMF/ TiO2 photocatalytic experiment using 18O2. Interestingly, the −NC16O peak (2202 cm−1) observed in Figure 5a is shifted to 2187 cm−1 in the 18O2 case due to the 18O incorporation to form −NC18O.23 Clearly, the CH3 groups of DMF must be the reactive sites to generate −NC18O in 18O2; that is, the carbon atom of −NC18O is from CH3 of DMF, not from the HCO group. In Figure 5b, the band at 2346 cm−1 and the shoulder at 1333 cm−1 also indicate that the 18O contributes to the CO2 and formate formation.16 Figure 6a shows the infrared spectra taken before, during, and after photoirradiation of DMA adsorbed on TiO2 in the closed cell with 10 Torr of 16O2 initially. DMA on TiO2 is subjected to photodecomposition, as shown by the decreasing DMA bands with illumination time and appearance of new bands at 1359, 1381, 1408, 1558, 1644, 2204, and 2349 cm−1. E

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NCO and HCOO results in CO2. Thermal decomposition of DMA on TiO2 generates imine species. Photocatalytic degradation of the DMA also produces imine species in addition to CO2, NCO, and HCOO. Interestingly, postirradiation thermal treatment of DMA on TiO2 forms DMF and OC(NH3)2, likely from the reaction between residual DMA and the photochemical product of formate.

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AUTHOR INFORMATION

Corresponding Authors

*J.-L.L.: E-mail: [email protected]. Phone: 886-62757575, ext. 65326. Fax: 886-6-2740552. *Y.-F.L.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Ministry of Science and Technology of Republic of China (Grant NSC 101-2738-M006-004).

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Figure 7. Comparison of the infrared spectra taken in the reactions of DMF and DMA on TiO2 at the indicated experimental conditions, showing the formation of chemisorbed DMF and OC(NH3)2 from postirradiation thermal treatment of DMA on TiO2. All of the traces in the range of ∼1050−1850 cm−1 are multiplied by a factor of 0.5.

REFERENCES

(1) Fox, M. A.; Dulay, M. T. Heterogeneous Photocatalysis. Chem. Rev. 1933, 93, 341−357. (2) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Photocatalysis on TiO2 Surface:Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735−758. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (4) Fujishima, A.; Zhang, X.; Tyrk, D. A. Surf. Sci. Rep. 2008, 63, 515−582. (5) Muzart, J. N,N-Dimethylformamide:Much More Than a Solvent. Tetrahedron 2009, 65, 8313−8323. (6) Ding, S.; Jiao, N. N,N-Dimethylformamide:A Multipurpose Building Block. Angew. Chem. Int. Ed. 2012, 51, 9226−9237. (7) Chang, C.-P.; Chen, J. N.; Lu, M.-C.; Yang, H.-Y. Photocatalytic Oxidation of Gaseous DMF Using Thin Film TiO2 Photocatalyst. Chemosphere 2005, 58, 1071−1078. (8) Helali, S.; Puzenat, E.; Perol, N.; Safi, M.-J.; Guillard, C. Methylamine and Dimethylamine Photocatalytic Degradation-Adsorption Isotherms and Kinetics. Applied Catalysis A: General 2011, 402, 201−207. (9) Grosjean, N.; Descorme, C.; Besson, M. Catalytic Wet Air Oxidation of N,N-Dimethylformamide Aqueous Solutions:Deactivation of TiO2 and ZrO2-Supported Noble Metal Catalysts. Appl. Catal., B 2010, 97, 276−283. (10) Farfan-Arribas, E.; Madix, R. J. Characterization of the Acid-base Properties of the TiO2 (110) Surface by Adsorption of Amines. J. Phys. Chem. B 2003, 107, 3225−3233. (11) Lee, J.; Choi, W. Effect of Platinum Deposits on TiO2 on the Anoxic Photocatalytic Degradation Pathways of Alkylamines in Water: Dealkylation and N-Alkylation. Environ. Sci. Technol. 2004, 38, 4026− 4033. (12) Kachina, A.; Preis, S.; Kallas, J. Gas-Phase Photocatalytic Oxidation of Dimethylamine:the Reaction Pathway and Kinetics. Int. J. Photoenergy 2007, 2007, 1−4. (13) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Wide Temperature Range IR Spectroscopy Cell for Studies of Adsorption and Desorption on High Area Solids. Rev. Sci. Instrum. 1988, 59, 1321−1327. (14) Wang, J. C. S.; Linsebigler, A.; Lu, G.; Fan, J.; Yates, J. T., Jr. Photooxidation of CH3Cl on TiO2 (110) Single Crystal and Powdered TiO2 Surfaces. J. Phys. Chem. 1995, 99, 335−344. (15) Stakhandske, C. M. V.; Mink, J.; Sandstrom, M.; Papai, I.; Johansson, P. Vibrational Spectroscopic and Force Field Studies of N,N-dimethylthioformamide, N,N-dimethylformamide, Their Deuter-

As shown in Figure 6, DMA molecules adsorbed on TiO2 are not completely degraded photochemically in the experimental condition, as revealed by the presence of its characteristic 1469 cm−1 after 240 min photoirradiation. Therefore, the spectral similarity shown in Figure 7 suggests that the survival DMA molecules may react with the photocatalytic intermediates in the postirradiation thermal treatment, likely formate in this case, to form DMF and OCN(CH3)2. The reaction between carboxylic acids and amines in the presence of dehydrating agent indeed can produce amides.25 However, different pathways involving other surface species, such as imines, and TiO2 surface cannot be completely ruled out at the present time.

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SUMMARY Scheme 1 shows the surface reactions of DMF and DMA on powdered TiO2 we propose. Adsorption of DMF on TiO2 leads Scheme 1. Proposed Surface Reactions of N,NDimethylformamide and Dimethylamine on Powdered TiO2

to red-shifted carbonyl stretching mode, indicating an interaction between the CO group of DMF and the surface Lewis site. DMF can also lose the carbonylic H to form OCN(CH3)2 chemisorbed species. Thermal decomposition products of DMF on TiO2 are found to be CO and DMA. DMF on TiO2 is subjected to photocatalytic degradation forming NCO and HCOO. O2 participates in the reaction with its oxygen atoms incorporated into these products. The origin of carbon in NCO is from CH3 of DMF. Further oxidation of F

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp5044859 | J. Phys. Chem. C XXXX, XXX, XXX−XXX