An in Situ FTIR Study - American Chemical Society

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Photoinduced Reactions of Surface-Bound Species on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FTIR Study Weiqiang Wu,† Kaustava Bhattacharyya,† Kimberly Gray,‡ and Eric Weitz*,† †

Institute for Catalysis in Energy Processes and Center for Catalysis and Surface Science and Department of Chemistry, ‡Institute for Catalysis in Energy Processes and Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Photoinduced conversion of surface-bound species on titania nanotubes that were first oxidized and then reduced (Ti−NT−O2−H2) and on platinized titania nanotubes subjected to oxidation and reduction (Pt−Ti−NT− O2−H2) has been investigated by means of in situ FTIR spectroscopy. Bidentate and monodentate carbonates as well as bicarbonates and carboxylates are formed subsequent to exposure of both Ti−NT−O2−H2 and Pt−Ti−NT−O2−H2 to CO2. Formic acid was only observed on Pt−Ti−NT−O2−H2. UV illumination of the nanotubes led to an increase in the number of surface-bound species as a result of the further reaction with gas-phase CO2 with a greater increase in surface species on Ti−NT−O2−H2 than on Pt−Ti−NT−O2−H2. The underlying basis of the photoinduced increase in adsorbed species is discussed for both types of nanotubes. Photoinduced reactions of surface species also take place and are remarkably different on the two types of nanotubes. UV illumination of Ti−NT− O2−H2 converts bidentate carbonates and bicarbonates to monodentate carbonates and carboxylates. There are less, and different, photoinduced reactions of surface species on Pt−Ti−NT−O2−H2: bicarbonates and monodentate carbonates convert to bidentate carbonates on the platinized titania nanotubes, and there is no obvious reaction involving carboxylates and formic acid upon irradiation of the platinized nanotubes. These differences in reactive behavior are discussed in the context of platinum acting as an efficient trap for photoelectrons which mitigates against reduction of Ti4+ to Ti3+, stabilizes holes, and alters the surface photochemistry taking place on the two different types of nanotubes. Photoinduced holes play an important role in photochemistry via oxidation of “structural water” and concomitant production of undercoordinated titania sites. photocatalysts with a variety of interesting properties.19,20 Additionally, ion exchange can be used to uniformly disperse catalytically active metal ions through the Ti−NTs, and the high surface area of these open mesoporous NTs facilitates transport of reactants to reactive sites. These NTs can have strong support interactions with other semiconductors. For example, loading Ti−NTs with CdSe nanoparticles leads to remarkably improved catalytic performance for redox reactions,19 and doped Ti−NTs have been used extensively for photocatalytic conversion of CO2 to hydrocarbons.21−23 Results in this area are summarized in two excellent reviews dealing with photocatalytic conversion of CO2 using Ti− NTs.6,24 Photocatalytic reduction of CO2 (PCR) is a complicated multistep process where CO2 adsorbed on a photocatalyst interacts with photogenerated electrons, and possibly hydrogen, to yield an assortment of products including CO, methane, methanol, and possibly C2 and higher hydrocarbons.25,26 However, it is clear that adsorption of CO2 is the first step in

1. INTRODUCTION Utilization of renewable resources is a prerequisite for a sustainable society. One readily available renewable carbon source is carbon dioxide (CO2). Use of sunlight to convert CO2 and water vapor into hydrocarbon fuels represents an attractive prospect.1−5 It would effectively provide a means to store solar energy in the form of liquid-phase chemicals which are compatible with the present energy infrastructure.6 Additionally, such a process is potentially a carbon neutral and renewable source of energy. However, increasing the low conversion efficiency of CO2 photoreduction is a major challenge for scientists and engineers working in the field, as is the desirability of being able to employ a photocatalyst that can efficiently utilize a significant portion of the solar spectrum. TiO2-based nanotubular structures have attracted attention and been extensively investigated owing to their enhanced photocatalytic efficiency relative to titania powders.7−12 Hydrothermal synthesis of a nanotubular structure of titania was first reported in the late 1990s by Kasuga et al.13,14 These structures contain hydrated dititanate (H2Ti2O5), trititanate (H2Ti3O7),15−17and H2Ti4O9·xH2O18 as well as their respective sodium salts. Titanate nanotubes (referred to as Ti−NTs) have been reported to be wide band gap semiconductors and © 2013 American Chemical Society

Received: June 14, 2013 Revised: August 27, 2013 Published: October 1, 2013 20643

dx.doi.org/10.1021/jp405902a | J. Phys. Chem. C 2013, 117, 20643−20655

The Journal of Physical Chemistry C

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Figure 1. In situ FTIR spectra of Ti−NT−O2−H2 in the presence of ∼10 Torr CO2 at different irradiation times: (A) carbonate region (1000−1800 cm−1); (B) CO2 region (2100−2500 cm−1); (C) CO2 overtone region (3500−3800 cm−1); (D) OH stretch region (2800−3500 cm−1).

such a process. Thus, an understanding of the photochemical behavior of surface-bound species, formed as a result of the adsorption of CO2, is a necessary component in the elucidation of the mechanism(s) of CO2 photoreduction. However, prior studies have focused mainly on the design and synthesis of novel photocatalysts and detection of possible photoproducts rather than the mechanistic details of the overall photoconversion process. In this article we focus on photoinduced reactions of the species that form upon adsorption of CO2 on reduced Ti−NTs and reduced platinized Ti−NTs. These species include carbonates and bicarbonates, which have been observed on other metal oxide systems including TiO2,27,28 Al2O3,28 and Ge2O329 and doped metal oxides such as Cu/TiO230,31 (Pt, Rh, Ir)/TiO227,28 and Rh/(Al2O3, SiO2, MgO).27,28 Upon illumination with UV light the absolute and relative amounts of carbonates and bicarbonates on these metal oxides can change dramatically.27,28,30 In the present work we report on the photoinduced conversion of surface-bound species on reduced Ti−NTs and reduced platinized Ti−NTs using in situ FTIR spectroscopy. We observe both the thermal and the photochemical formation of carboxylates, surface-bound species that contain the CO2− moiety which has been widely postulated as a critical intermediate in the photoreduction of CO2.27,28,30

Photoinduced reaction pathways for the surface-bound species are discussed in light of the reaction kinetics and Lewis acid and base character of surface sites. Both photoinduced electrons and holes play a role in the observed chemistry, and the presence of Pt on the Ti−NTs significantly alters both the thermal and photochemical processes in a way that can be explained by Pt acting as an electron “sink” or electron trap.

2. EXPERIMENTAL PROCEDURES 2.1. Synthesis and in Situ Pretreatments. Ti−NTs were prepared by modification of a published hydrothermal method.13,14,32 All materials were used without further purification. Typically, 2 g of anatase titania powder (99% purity, Sigma Aldrich Chemicals, USA) was stirred with 50 mL of 10 M NaOH solution (97% purity, BDH Chemicals, USA) in a closed 125 mL Teflon cup which was kept in an oven for 48 h at 120 °C. The resulting precipitate was washed with 1 M HCl (38% purity, EMD Chemicals, USA) followed by several washings with deionized water to attain a pH between 6 and 7. This powder was dried overnight in an oven at 110 °C. Platinized Ti−NTs were prepared by mixing the resultant powder with an aqueous 0.5 mol % solution of hexachloroplatinic acid (99.9%, Alfa Aesar, USA). Platinized and nonplatinized titania samples were calcined under a hydrogen 20644



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Figure 2. Relative changes in CO2 intensity (I(0) − I(t)) at different illumination times: (A) Ti−NT−O2−H2 and (B) Pt−Ti−NT−O2−H2.

atmosphere (80 mL min−1) at 400 °C for 1 h. For in situ pretreatment, samples were sequentially (a) heated to 350 °C (3 °C/min) under vacuum, (b) oxidized with 10 Torr O2 for 2 h at 350 °C, (c) subject to vacuum for 0.5 h, and (d) reduced in 10 Torr H2 for another 2 h followed by degassing for 0.5 h and then cooling to room temperature for in situ FTIR studies. 2.2. Characterization. Samples used in this work were the same as those used by Bhattacharyya et al.33 Their morphology, crystallinity, and oxidation state were characterized in that study using TEM, XRD, and XPS, respectively. Details of this work can be found in ref 33. 2.3. In Situ FT-IR Spectroscopy. In situ FTIR spectra were recorded with a Nicolet 6700 (Thermo Scientific) infrared spectrometer equipped with a mercury cadmium telluride (MCT) detector. Each spectrum was obtained by averaging 128 scans at 4 cm−1 resolution. Samples were mounted in a customfabricated IR cell which was specifically designed to study highly scattering powder samples in transmission mode. The IR cell and preparation of Ti−NTs powder samples, which are supported on a highly transmissive tungsten mesh (∼4 cm2 area), have been described previously.34,35 In brief, the Ti−NTs powder was pressed onto a photoetched tungsten grid. This sample was then mounted inside the IR cell, which is connected to a turbo pumped gas manifold with a base pressure of ∼5 × 10−7 Torr. The cell is sealed with three CaF2 windows, two of which are used for IR transmission down to ∼1000 cm−1, while the third is used as an inlet for an uncollimated 100 W UV beam (UVP, B-100), which overlapped with the IR beam on the sample, with both beams at approximately 45° to the sample normal. The sample can be resistively heated to a set temperature which is measured by a K-type Chromel-Alumel thermocouple attached to the center of the grid. After in situ pretreatment and cooling to room temperature, ultra-highpurity CO2 (>99.999%) is introduced into the cell with the pressure monitored with a capacitance manometer.

UV irradiation of Ti−NT−O2−H2 exposed to ∼10 Torr CO2. The initial reactive adsorption of CO2 leads to formation of bidentate carbonates, monodentate carbonates, and bicarbonates as well as carboxylates. IR bands at 1580 and 1320 cm−1 in Figure 1A are attributed to the νas(CO3) and νs(CO3) modes, respectively, of a bidentate carbonate species.29,36 The band at 1059 cm−1 is assigned to another bidentate carbonate.37 These two bidentate carbonates can be easily discriminated by their different stabilities upon evacuation. As can be seen in the sixth trace in Figure 1A, taken after 4 h of UV irradiation and evacuation, the bidentate carbonate with absorption bands at 1589 and 1310 cm−1 is more stable than the bidentate carbonate with an absorption around 1059 cm−1, since the former bidentate carbonate retained significant intensity on evacuation while the latter one completely disappears. The strong peak at 1432 cm−1 has been previously reported on both a Cr2O3 surface and a rutile TiO2 surface and is assigned to the νs(CO3) mode of a monodentate carbonate.29,38 The intensity of this absorption band did not change significantly on evacuation. In addition, there are two absorption bands located at 1222 and 1402 cm−1, which have been reported to be δ(OH) and νas(CO3) modes of two different bicarbonates.38 These two different bicarbonates can again be discriminated based on their different stabilities on evacuation. As seen in Figure 1A, after 1 h of UV illumination, multiple new peaks, including those at 1248, 1377, 1627, and 1670 cm−1, appear. On the basis of literature reports, absorptions at 1377 and 1627 cm−1 are assigned to a monodentate carbonate and bicarbonate, respectively.36,38 A broad band at 1670 cm−l, present at a very low intensity before illumination, is best assigned to the νas(CO2) mode of a surface-bound carboxylate.29,39−42 There is also an absorption at 1248 cm−1 which is assigned to the νs(CO2) mode of a carboxylate. Rasko et al. previously reported the observation of carboxylates upon illumination of Rh/TiO2 samples.27,28 They attributed formation of these carboxylates to the electronic defect structure of TiO2 where CO2 is activated to form a carboxylate by the transfer of electron density from the Ti3+ to the adsorbed CO2, yielding a partially negatively charged species. A previous study

3. RESULTS 3.1. Identification of Surface-Bound Species on Ti− NT−O2−H2. Figure 1 shows IR absorption spectra after 0−4 h 20645



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Figure 3. Intensities of surface-bound species on Ti−NT−O2−H2 including bidentate carbonate (□), bicarbonate (Δ), monodentate carbonate (○), and carboxylate (▽). (A) Absolute intensity of each species. (B) Relative intensity of each species (relative yield). (C) Stacked IR spectra with an isosbestic point at 1450 cm−1.

At the same time, absorptions due to Ti−OHs increase, strongly suggesting that upon illumination loss of structural water is associated with formation of additional Ti−OH moieties. The mechanism for this process will be discussed in more detail in section 3.3. These results also indicate that the pretreatments of the sample, including heating to 350 °C, do not remove all of the structural water. This is an interesting result and implies that at least some water incorporated into the titanate phase is bound strongly enough to remain after heating under vacuum at 350 °C. 3.2. Photoinduced Reactions of Surface Species on Ti−NT−O2−H2. Various surface species are formed from reactive adsorption of CO2 on active sites of Ti−NT−O2−H2. It has been reported that CO2 interacts with the Ti−NTs surface to generate “adsorbed CO2” with an absorption at ∼2380 cm−1.39,45 However, this absorption was not observed in the IR spectra in the present experiments taken either before or after illumination. It is not implausible that any “adsorbed CO2” that is formed promptly converts to other surface species, implying the reduced nanotubes are more reactive than the titania employed in the references cited above. It should be pointed out that the total intensities of surface species on Ti− NT−O2−H2 and Pt−Ti−NT−O2−H2 increased with CO2 exposure time in an apparently constant ratio until saturation of surface binding sites was achieved. The time to saturation of the active sites was approximately 20 min under experimental conditions for Ti−NT−O2−H2 and ∼5 min or less for the platinized NTs. Though the platinized NTs took less time to saturate under illumination there was a smaller increase in the number of surface species on illumination. After that time the concentration of surface species did not change in the absence of illumination. Therefore, all Ti−NTs were first exposed to CO2 for at least 30 min in order to achieve saturation of active surface sites before the surface was illuminated. Dramatic changes occurred when Ti−NT−O2−H2 were illuminated with UV light in the presence of CO2. As shown in Figure 1B and 1C the IR intensity of gas-phase CO2 decreases with a corresponding increase in the intensities of surface species (Figure 1A). The data in Figure 3A for Ti−NT−O2−H2 show that the absolute intensities of the monodentate carbonates and carboxylates increase while those of bidentate

of Ti−NTs provided evidence that carboxylates are formed at O vacancies with electron donation from the associated Ti3+ sites.33 The carboxylate observed in the current work and shown in Figure 1A is relatively unstable toward evacuation as the absorption bands at 1248 and 1670 cm−1 disappeared completely on evacuation. The features in the 2100−2500 cm−1 region, shown in Figure 1B, arise from the ν3 absorption of gas-phase CO2, as do the broad absorptions centered at ∼3620 and ∼3726 cm−1 seen in Figure 1C. These higher frequency absorptions are due to relatively weak overtone bands of gas-phase CO2. As shown in Figure 2A, upon illumination of the Ti−NT−O2−H2 samples, the intensity of the absorptions of gas-phase CO2 decreased. In the initial 3 h, the amount of gas-phase CO2 decreased by ∼25% while the sum of the intensity of all IR absorptions of the surface species increased by ∼29%. Thus, additional gas-phase CO2 is gradually converted to surface species on illumination. After 4 h there is no further significant change in either the amount of gas-phase CO2 or the intensity of the surface IR absorptions. We estimate that the error limits on the percentage yield and change in percentages of the surface-bound species are between 10% and 15% of the quoted numbers. There are several interesting observations with regard to the absorptions in the 2900−3500 cm−1 region shown in Figure 1D. The broad positive absorption centered at ∼3250 cm−1, which grows in with time, is attributable to surface hydroxyls (Ti−OHs). In addition, “negative going” peaks that grow in with time are present at ∼2850 and ∼2920 cm−1. These latter absorptions are assigned to water that remains in the Ti−NTs even after the nanotubes are heated to 350 °C.33,43 We previously referred to this water as “structural water”, and more details on the behavior of this water and its assignment can be found in another study.33 The “negative absorption bands” appearing upon UV illumination are indicative of depletion of this structural water.44 Since a double-beam spectrum is obtained by subtracting the background from the “sample” spectrum, “negative absorptions” can appear when an absorption that is present in the background spectrum is smaller in the “sample spectrum”. Depletion of the absorption is normally due to treatment of the sample after the background is obtained; in this case, treatment is UV irradiation. 20646



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Figure 4. Deconvolution of IR spectra of absorbed CO2 on Ti−NT−O2−H2 at different illumination times in the presence of ∼10 Torr CO2: (A) 0, (B) 1, (C) 2, and (D) 3 h.

relative intensities of the surface species presented in section 4.2 supports this assumption. We note that bidentate carbonates and bicarbonates are the major species before illumination with monodentate carbonates and carboxylates present at lower concentrations. With increasing duration of illumination, the relative concentration of bidentate carbonates and bicarbonates gradually decreases while the relative concentration of the monodentate carbonates and carboxylates steadily increases, suggesting reactive conversion of the bidentate carbonates to monodentate carbonates and bicarbonates to carboxylates. As shown in Figure 3C, an isosbestic point, which is characteristic of a direct reaction of bidentate carbonates to form monodentate carbonates, is visible at 1450 cm−1. Furthermore, the existence of an isosbestic point implies direct conversion of one surface species to another: specifically bidentate carbonate to monodentate carbonate. Clearly, the increase in the amount of surface carboxylates requires a precursor. Given that bidentate carbonates convert to monodentate carbonates on UV illumination, the correlation between the decrease in bicarbonates and the increase in carboxylates that takes place on UV illumination strongly suggests that bicarbonates convert to carboxylates. As seen in Figure 1D, on UV irradiation, the decrease in absorptions at 2850 and 2920 cm−1, due to loss of structural water, is accompanied by a gradual increasing in the Ti−OH absorption at 3250 cm−1.43,46 This is in agreement with the conclusion made by Sakai et al. that photoinduced hydrophilic conversion of water to Ti−OHs proceeded upon UV illumination.46 3.3. Identification of Surface-Bound Species on Pt− Ti−NT−O2−H2. As shown in Figure 5A, exposure of Pt−Ti− NT−O2−H2 to CO2 also leads to formation of surface species

carbonates and bicarbonates decrease, indicating that UV illumination leads to formation of more monodentate carbonates and carboxylates and loss of bidentate carbonates and bicarbonates. Thus, photoinduced reactions of surface species can take place as a result of UV illumination, and conversion can be monitored by in situ FTIR spectroscopy. Figure 3A and 3B displays the absolute intensities and relative yields, respectively, of the surface-bound species on Ti−NT−O2−H2 at different illumination times. The following procedure was implemented to provide a semiquantitative description of the photoinduced conversion of surface species: (a) IR spectra at different illumination times were deconvoluted to obtain the amplitude of the absorptions of each species in the spectrum in Figure 4; (b) respective deconvoluted absorption peaks were then integrated to obtain relative intensities (or relative yields) for the different surface-bound species. The intensities shown are for all surface species of the same chemical composition. The semiquantitative aspect of this procedure involves the assumption that the absorption coefficients of each of the relevant species we observe are the same. This, of course, is only an approximation; there is almost certainly a difference in the absorption coefficients of the different surface-bound species. However, the absolute absorption coefficients of these surface species are not known, and we believe that the assumption of similar absorption coefficients is reasonable in that we are comparing intensities for similar vibrational modes. Such an assumption allows for establishment of a semiquantitative relationship between the intensities of the absorptions of surface species and their concentrations, both before and after UV irradiation. Data on the changes in the 20647



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Figure 5. In situ IR spectra of Pt−Ti−NT−O2−H2 with ∼10 Torr CO2 at different illumination times: (A) carbonate region (1000−1800 cm−1); (B) CO2 region (2100−2500 cm−1); (C) CO2 overtone region (3500−3800 cm−1); (D) OH stretch region (2600−3500 cm−1).

1220, 1409, 1424, 1481, and 1623 cm−1 are assigned to bicarbonates.30,36,47 It is clear that there are multiple bicarbonate species since upon evacuation the absorptions at 1203 and 1220 cm−1, of at least one bicarbonate, disappear completely while the absorptions at 1409, 1424, and 1481 cm−1 still exhibit some intensity and are thus assigned to a more stable bicarbonate(s). Weak absorptions at 1250 and 1670 cm−1 are assigned to a carboxylate.29,39−41 Upon evacuation, the carboxylate bands completely disappear. There is another absorption band at 1725 cm−1, which is not found with Ti− NT−O2−H2, which is assigned to adsorbed formic acid.48,49 This absorption of adsorbed formic acid also disappears on evacuation. As seen in Figure 5B and 5C, the absorption bands in the 2100−2500 cm−1 region and at 3620 and 3726 cm−1

but with a pattern of absorptions that is distinct from that seen with Ti−NT−O2−H2. On the basis of literature reports, these absorptions can be assigned to carbonates, bicarbonates, carboxylates, and formic acid. Specifically, the strong absorptions that grow in at 1278, 1378, and 1554 cm−1 are, respectively, assigned to the νs(CO3), νas(CO3), and ν(CO) modes of one bidentate carbonate.30,38 The absorption at 1364 cm−1 is assigned to the νas(CO3) mode of the another bidentate carbonate.38 The fact that there are two distinct bidentate carbonates is clear based on changes in the intensity of the above absorptions on evacuation. As seen in the lowest trace of Figure 4A, the latter bidentate carbonate is much less stable on evacuation. The absorption at 1321 cm−1 is assignable to the νas(CO3) mode of a monodentate carbonate.38 Absorptions at 1203, 20648



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Figure 6. Intensities of surface-bound species: bidentate carbonate (□), bicarbonate (Δ), monodentate carbonate (○), carboxylate (▽), and formic acid (◊) on Pt−Ti−NT−O2−H2. (A) Absolute intensity of each adsorbed species. (B) Relative intensity of each adsorbed species (relative yield). (C) Stacked IR spectra for Pt−NT−O2−H2.

carboxylate to form bidentate carbonate. It is worth noting that there is no isosbestic point in the IR spectra shown in Figure 6C that corresponds to the isosbestic point seen in Figure 3C. The lack of an isosbestic point and the increase in bidentate carbonate and the decrease in bicarbonate and monodentate carbonate clearly indicate that different surface chemistry is occurring on Pt−Ti−NT−O2−H2 upon illumination.

which are assigned to absorptions of gas-phase CO2 decrease on irradiation. Interestingly, as seen in Figure 5D, depletion of the structural water at 2927 and 2960 cm−1 and formation of the surface hydroxyls is more obvious on the platinized Ti−NTs than on Ti−NT−O2−H2. Platinum has been demonstrated to be an “electron sink”.50,51 Its presence as nanoparticles on Ti−NTs inhibits recombination of electrons with the holes that are formed on supra band gap UV illumination of the Ti−NTs. It is well known that holes can oxidize water.46 As such, it would be expected that these holes can oxidize structural water to H2O+, which can dissociate into H+ and OH (reactions 1 and 2). hν + Ti − NTs → h+ + e−

(1)

h+ + H 2O → H 2O+ → H+ + OH

(2)

4. DISCUSSION 4.1. Differences in Adsorbed Species on Ti−NT−O2− H2 and Pt−Ti−NT−O2−H2 in the Dark. Reactive adsorption of CO2 leads to the same major surface-bound species on both Ti−NT−O2−H2 and Pt−Ti−NT−O2−H2, but the relative amounts differ. Insights into the formation mechanisms and preference of these surface-bound species on the Ti−NTs are aided by a consideration of Lewis acid−base concepts.33,52,53 In the generalized formulation, Lewis acids and bases are, respectively, defined to be electron acceptors and electron donors. The Ti and O atoms in Ti−NT−O2−H2 and Pt−Ti− NT−O2−H2 are considered to be Lewis acid and Lewis base sites, respectively. It has been demonstrated that the acidity and basicity of Ti−NTs can be altered by oxidative or reductive pretreatment of the NTs.33 Oxidation of Ti−NTs with O2 leads to formation of O atom sites which have been characterized as a strong Lewis base. Formation of these sites is accompanied by Ti4+ sites which have been characterized as a strong Lewis acid, while treatment of the Ti−NTs with H2 leads to production of O vacancies and reduction of some of Ti4+ sites to Ti3+ sites, which are weak Lewis acid sites.54,55 Reductive removal of each O atom is expected to be accompanied by production of two Ti3+ sites to maintain electrical neutrality.54,56−58 As discussed in section 2.2 the same nanotubes are used in this study as in ref 33, where characterization of the Ti3+ sites is described. The order of Lewis basicity for the surface-bound species found on both platinized and nonplatinized Ti−NTs after exposure to CO2 is bidentate carbonate > monodentate carbonate > bicarbonate. In the context of Lewis acid−base concepts, a strong Lewis acid should be favored to react with a strong Lewis base and a weak Lewis acid is favored to react with a weak Lewis base. This trend has been observed in a prior

Some of these OHs bind to the surface of Ti−NTs to form Ti− OHs, making the surface more hydrophilic, and other OHs can react with hydrogen to reform water.46 Our experiments do not allow us to definitely determine the source of hydrogen, but it plausibly could come from dissociation on Pt of the H2 used to reduce the NTs or from neutralization of a proton by a photogenerated electron. 3.4. Photoinduced Reactions of Surface-Bound Species on Pt−Ti−NT−O2−H2. As was seen with Ti−NT−O2− H2, upon illumination of Pt−Ti−NT−O2−H2 the total intensities of all surface species grow during the initial hour of UV irradiation. However, the intensity and change in intensity of each species are very different from what was observed for Ti−NT−O2−H2. These IR spectra were analyzed to obtain insights into the conversion of the surface-bound species using the same procedure employed for spectra of Ti− NT−O2−H2 (Figure S1 of the Supporting Information). As shown in Figure 6A, UV illumination significantly affects the relative concentration of the bidentate carbonate and bicarbonate while the monodentate carbonates, carboxylates, and formic acid are only modestly affected. As shown in Figure 6B, the change in the relative concentration of bidentate carbonate correlates very well with the change in the concentration of bicarbonate and monodentate carbonate, implying there is reaction of monodentate carbonate and 20649



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Scheme 1. Possible Schematic Mechanisms for Formation of Carbonates and Bicarbonate on Ti−NT−O2−H2

Scheme 2. Possible Schematic Mechanisms for Conversion of Carbonate and Bicarbonate on Ti−NT−O2−H2 as a Result of UV Irradiation

This scheme is not intended as a unique or detailed picture of formation of surface species; rather we present a plausible schematic of a mechanism for formation that is consistent with data and Lewis acid and base concepts. 4.2. Changes in Surface Species on Illumination. UV illumination of Ti−NT−O2−H2 and Pt−Ti−NT−O2−H2 leads to significant changes in the concentrations of the various surface species. These changes result from generation of electron−hole pairs and their reactions. UV illumination excites electrons from the valence band to the conduction band with concomitant generation of holes. Most of the photoelectrons recombine with holes on a femtosecond time scale.59 However, some of the photoelectrons and holes interact with moieties on or in the nanotubes. On Ti−NT−O2−H2, some of the photoelectrons reduce strong Lewis acid Ti4+ sites to weak Lewis acid Ti3+ sites. This reduction results in the surface of Ti−NT−O2−H2 becoming less acidic.59 The change in surface acidity is consistent with reactions of some of the surface-bound

study in which the data reported is consistent with bidentate carbonates and bicarbonates being mainly formed on strong Lewis acid Ti4+ sites and weaker Lewis acid sites favoring formation of monodentate carbonates.33 Carboxylates are reported to be formed on weak Lewis acid Ti3+ sites which are associated with an O vacancy.33 X-ray photoelectron spectroscopy (XPS) has previously been performed on the same samples of Ti−NT−O2−H2 and Pt−Ti−NT−O2−H2 that were used in the current study. That work indicated that, in the dark, Ti4+ sites predominate over Ti3+ sites,33 consistent with the fact that, as shown in Figures 3 and 6, bidentate carbonates and bicarbonates, which are expected to form on strong Lewis acid Ti4+ sites, are major species. Monodentate carbonate and carboxylate which are expected to form on the weak Lewis acid Ti3+ sites are minor species. Possible pathways of formation of surface-bound species such as bidentate carbonates, bicarbonates, and monodentate carbonates as well as carboxylates are shown in Scheme 1. 20650



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Scheme 3. Possible Schematic Mechanism for Formation of Formic Acid and Conversion of Carbonate and Bicarbonate on Pt− Ti−NT−O2−H2 as a Result of UV Irradiation

species. Reduction of Ti4+ to Ti3+ sites is accompanied by conversion of more basic bidentate carbonates to the less basic monodentate carbonates. The occurrence of this reaction is supported by the good correlation between the change in the relative yields of bidentate carbonates and monodentate carbonates, which is shown in Figure 3B. Figure 3B is constructed from the data in Figure 3A by dividing the sum of the intensities of a specific mode of all adsorbed species of the same chemical composition and same bonding by the sum of the intensities of the C−O stretch in the OCO moiety of all adsorbed species. The relative yield of bidentate carbonates decreased from 0.59 to 0.47 of the total intensity, while that of monodentate carbonates increased from 0.07 to 0.18 after illumination of Ti−NT−O2−H2 for 2 h. A possible mechanism for this conversion, involving a C−O bond fission reaction, is shown schematically in Scheme 2. Again, the mechanism presented in Scheme 2 should not be taken as a presentation of a unique mechanism with regard to the steps or reaction geometries. We simply present a possible plausible mechanism for the processes under consideration. The holes also drive aspects of the observed chemistry that takes place on irradiation. Holes can oxidize structural water molecules, which are coordinated with Ti atoms on/in Ti− NT−O2−H2 to protons and OH (reactions 1 and 2). Loss of the structural water leads to formation of an undercoordinated site. As shown in Scheme 2, a plausible mechanism can be drawn which leads to conversion of bicarbonates to carboxylates involving interaction with an undercoordinated titanium site. Bicarbonates can interact with these undercoordinated sites and lose a hydrogen atom (which presumably combines with an OH or another H). Charge transfer from Ti3+ sites to the surface-bound CO2 moiety, shown in the rightmost

panel of Scheme 2, then leads to formation of a surface-bound carboxylate. Due to the presence of platinum the surface photochemistry of Pt−Ti−NT−O2−H2 is significantly different than that seen for the nonplatinized NTs. XPS data for Pt−Ti−NT−O2−H2 indicated that in the dark there is a low concentration of Ti3+ atoms after pretreatment,33 which means the oxidative and reductive pretreatments resulted in less conversion of Ti4+ to Ti3+ than with the nonplatinized NTs. In fact, in ref 33 the concentration of Ti3+ was low enough that it was not observable. Reports that platinum can act as an electron trap provide a plausible explanation for the relatively low concentration of Ti3+ on Pt−Ti−NT−O2−H2.50,51,60 This explanation also rationalizes the differences in the behavior of the nonplatinized and Pt−Ti−NT−O 2−H2 during UV illumination. Trapping of electrons by Pt inhibits electron hole recombination and leads to the chemistry initiated by the holes becoming more important on the platinized NTs. Trapping of photoelectrons by platinum would be expected to mitigate against the reduction of Ti4+ to Ti3+, and thus, moieties formed at Ti4+ sites would still be expected to be the dominant species after illumination. As discussed, there is evidence that bicarbonates and bidentate carbonates form at Ti4+ sites. On the other hand, the greater concentration of holes leads to more oxidization of structural water on UV irradiation of Pt−Ti−NT−O2−H2.46 As shown in Figure 5D, this is manifested by formation of additional surface Ti−OHs on UV irradiation. As a result of loss of more structural water, more undercoordinated sites are generated. Scheme 3 shows a plausible mechanism by which these undercoordinated sites facilitate conversion of monodentate carbonates and bicarbonates to the bidentate carbonates. In this mechanism (again, 20651



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not intended to be unique) an oxygen in a monodentate carbonate interacts with an undercoordinated site to yield a bidentate carbonate. The same process can take place with bicarbonates. However, interaction of an O with an undercoordinated surface site is then accompanied by loss of an H atom. After 2 h of UV illumination, the relative yield of bidentate carbonate increased from 0.56 to 0.63 while the relative yields of monodentate carbonate and bicarbonate decreased from 0.10 and 0.23 to 0.08 and 0.17. The sum of the decrease in the relative yields of monodentate carbonate and bicarbonate is 0.08, which is approximately equal to the increase in the relative yield of bidentate carbonate of 0.07. There is no significant change in the carboxylate and formic acid moieties. These latter moieties are expected to form on Ti3+ sites. Thus, the absence of a significant change in their concentration is consistent with minimal reduction of Ti4+ to Ti3+ sites due to the more efficient trapping of photoelectrons. Surface-bound species form in the dark as a result of reactive adsorption of CO2 on Ti−NT−O2−H2. When the system is illuminated, the amount of gas-phase CO2 decreases and there is a concomitant increase in the amount of surface-bound species. This behavior is consistent with an increase in surface binding (reactive) sites. We presented evidence that is consistent with these photogenerated surface sites resulting from oxidation of structural water by photogenerated holes and production of new Ti3+ sites by reduction of Ti4+ by photogenerated electrons. Additionally, the surface-bound species appear to be relatively stable in the dark, at least for the short times (∼20 min) over which we probed this process. This implies that the new adsorption sites do not require UV illumination to be stable. This would be expected for the undercoordinated Ti sites but is a more interesting result for Ti3+ sites. 4.3. Formation of Formic Acid on Pt−Ti−NT−O2−H2. Formic acid is observed subsequent to interaction of CO2 with Pt−Ti−NT−O2−H2 in the dark. Formic acid is also one of the possible photoreduction products of CO2.61,62 Of course, products other than formic acid have been observed during photochemistry taking place on titania and doped titania materials, but those processes are not the focus of our current work.61,62 A sequence of hypothetical reactions which includes reactions 1 and 2 as well as 3−6 shows how formic acid could be produce from CO2 as a result of UV irradiation.27,28,30 e− + CO2 (ads) → CO2−(ads)

(3)

e− + H+(ads) → H•(ads)

(4)

CO2−(ads) + H•(ads) → HCOO−(ads)

(5)

HCOO−(ads) + H+ → HCOOH(ads)

(6)

Though we have no direct evidence with regard to the concerted or stepwise nature of reaction 8, it is likely that reaction 8 takes place sequentially. First, gas-phase CO2 weakly interacts with Ti−OHs via an O atom. Then, a surface resident hydrogen atom attacks the carbon center to form a surfacebound formic acid. It is plausible that these hydrogen atoms are generated from dissociation of H2 used for reduction of Ti− NTs on the platinum nanoparticle surface.63−67 4.4. Ramification for Surface Photochemistry. CO2− has long been proposed as a critical intermediate in the photoreduction of CO2, and its formation is shown in reaction 3.27,28,30 Carboxylates, species containing a bound CO2− moiety, are observed on both Ti−NT−O2−H2 and Pt−Ti− NT−O2−H2. Though the yields of carboxylates are low on both Ti−NT−O2−H2 and Pt−Ti−NT−O2−H2 surfaces in the dark, the yields of carboxylates upon UV illumination are very different on these two Ti−NTs. The yield of carboxylates on Ti−NT−O2−H2 gradually increases with illumination time, while the yield on Pt−Ti−NT−O2−H2 is almost constant. As discussed in sections 4.1 and 4.2, the yield of carboxylates correlates with Ti3+ sites. However, we also consider the possibility that undercoordinated Ti sites can play a role in binding of CO2 and facilitating transfer of electron density to the bound CO2. UV illumination of the Ti−NT−O2−H2 surface led to formation of more Ti3+ sites and undercoordinated Ti sites, thus promoting production of a surfacebound CO2−-containing moiety. It should be noted that the Ti3+ sites generated on UV illumination are expected to be different than the Ti3+ sites generated by reductive pretreatment, since generation of Ti3+ by reductive pretreatment is associated with O vacancies. On the Pt−Ti−NT−O2−H2 surface, the platinum acts as an electron trap, inhibiting formation of Ti3+ sites. Therefore, the yield of carboxylates does not change significantly upon UV illumination of Pt−Ti−NT− O2−H2. Since there are more undercoordinated sites generated on Pt−Ti−NT−O2−H2, the lack of growth of carboxylates suggests that if undercoordinated sites are involved in carboxylate formation they are associated with Ti3+ sites. We note that it is possible that this constant level of carboxylates is actually the result of a photochemical steady state in which the bound carboxylates are converted to gas-phase products and more carboxylates are produced. However, since we do not see a significant decrease in the amount of gas-phase CO2 on irradiation, such a process, if it takes place, would not involve a significant “turnover” at surface Ti3+ sites. This is not surprising given that water, which enhances hydrocarbon product formation, has not been added to the system. Nevertheless, it is clear that more sites for adsorption of carbon dioxide are produced on illumination. If these sites are active for conversion of CO2 to hydrocarbons that are released into the gas phase, our data suggest this takes place as a result of a photochemical steady state that is established in the presence of UV light. Of course, this is what would be expected for a photocatalytic process. However, we also note that given the small yields for photochemistry with titania-based systems, it is not clear that our IR data are sufficiently sensitive to detect very small changes in the nature of surface species that occurs as a result of extended UV irradiation. Independent of this statement, UV-induced conversion of species formed as a result of adsorption of CO2 on Ti−NT−O2−H2 is more efficient than on Pt−Ti−NT−O2−H2. Therefore, Ti−NT− O2−H2 is more photoactive than Pt−Ti−NT−O2−H2 for at least these processes.

In the present experiments, the yield of formic acid is almost constant before and after UV illumination of Pt−Ti−NT−O2− H2, indicating that formic acid is not produced by photoreduction (or if it is, it is destroyed at the same rate as it forms). This observation suggests there is another mechanism for formation of formic acid in this system. A possible mechanism for formation of formic acid in this system is illustrated in Scheme 3. This scheme involves the following two reactions H 2(Pt) → 2H•

(7)

2H• + CO2 (ads) → HCOOH(ads)

(8) 20652


The Journal of Physical Chemistry C Finally, for many years there has been speculation about a phenomenon commonly referred to as “photoinduced adsorption”, in which adsorption of one or more species was apparently induced as a result of UV illumination. We have now demonstrated that this process can occur in appropriate systems and provided a rationale for its occurrence. Whether the photoinduced adsorption process we observe can be generalized to other systems remains to be determined.68−70

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 showing IR spectra of absorbed CO2 on the Pt-TiNT-O2-H2. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Award No. DE-FG02-03-ER15457).

5. CONCLUSION Photoinduced conversion of surface species on Ti−NT−O2− H2 and Pt−Ti−NT−O2−H2 has been studied using in situ FTIR spectroscopy. Initially, exposure of both types of nanotubes to ∼10 Torr of CO2 in the “dark” leads to formation of bidentate carbonates, monodentate carbonates, bicarbonates, and carboxylates, with bidentate carbonates and bicarbonates being the dominant species. However, after no more than between 20 and 25 min of exposure of these NTs to ∼10 Torr CO2, growth of surface species terminates and does so even more quickly for the platinized NTs. UV irradiation has a dramatic effect on the concentration and chemistry of the surface-bound species. First, UV illumination results in an increase in the concentration of surface species and a corresponding loss of gas-phase CO2. In addition, UV illumination of the reduced NTs leads to significant conversion of bidentate to monodentate carbonates as well as conversion of bicarbonates to carboxylates. However, there is only moderate conversion of bicarbonates and monodentate carbonates to the bidentate carbonates on the Pt−Ti−NT− O2−H2 surface, while the concentration of carboxylates and formic acid does not change significantly. On the basis of, in part, Lewis acid and base concepts, bidentate carbonates and bicarbonates are proposed to form on Ti4+ sites and monodentate carbonates and carboxylates are proposed to form on Ti3+ sites. Photoelectrons from UV illumination of Ti−NT−O2−H2 reduce Ti4+ to Ti3+ sites. This change in Ti oxidation state is accompanied by a change in the nature of the surface species, which is consistent with transformation of those species favored by Ti4+ sites to those favored by Ti3+ sites. Pt−Ti−NT−O2−H2 are much less photoactive than Ti−NT−O2−H2. For Pt−Ti−NT−O2−H2, Pt acts as an electron trap, inhibiting reduction of Ti4+ to Ti3+, with the conclusion that there is no significant conversion of Ti4+ to Ti3+ sites. However, trapping of electrons inhibits electron−hole recombination, leading to a greater concentration of holes for Pt−Ti−NT−O2−H2, which can react with structural water that is coordinated to Ti atoms in the Ti−NTs. This reaction produces undercoordinated Ti sites. These undercoordinated Ti sites can facilitate reaction of bicarbonates and monodentate carbonates to form the bidentate carbonates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 20653



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An in Situ FTIR Study - American Chemical Society

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