Role of the Surface Lewis Acid and Base Sites in the Adsorption of

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Role of the Surface Lewis Acid and Base Sites in the Adsorption of CO2 on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study Kaustava Bhattacharyya,†,‡ Alon Danon,†,‡ Baiju K.Vijayan,†,§,∥ Kimberly A. Gray,†,§ Peter C. Stair,†,‡ and Eric Weitz*,†,‡ †

Institute for Catalysis in Energy Processes, ‡Department of Chemistry, §Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: An understanding of the adsorption of CO2, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO2 on oxidized, reduced, and platinized TiO2 nanotubes (Ti-NTs) was studied using infrared spectroscopy. The Ti-NTs were characterized with TEM and XRD, and XPS was used to determine the oxidation state as a function of oxidation, reduction, and platinization. The XPS data demonstrate that upon oxidation, surface O atoms become more electronegative, producing sites that can be characterized as strong Lewis bases, and the corresponding Ti becomes more electropositive producing sites that can be characterized as strong Lewis acids. Reduction of the Ti-NTs produces Ti3+ species, a very weak Lewis acid, along with a splitting of the Ti4+ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti3+ is not observed on reduction of the platinized Ti-NTs, presumably because Pt acts as an electron sink. Exposure of the treated Ti-NTs to CO2 leads to the formation of differing amounts of bidentate and monodentate carbonates, as well as bicarbonates, where the preference for formation of a given species is rationalized in terms of surface Lewis acidity and or Lewis basicity and the availability of hydrogen. Our data suggest that one source of hydrogen is water that remains adsorbed to the Ti-NTs even after heating to 350 °C and that reduced platinized NTs can activate H2. Carboxylates, which involve CO2− moieties and are similar to what would be expected for adsorbed CO2−, a postulated intermediate in CO2 photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti3+/O vacancy formation is observed.

I. INTRODUCTION One-dimensional nanostructures have attracted significant attention from the materials chemistry community.1 Progress has been made in the controlled synthesis of inorganic (metal oxides, nitrides, carbides, sulfides, selenides, etc.) nanotubular structures.2 In particular, TiO2-based nanotubular structures have been extensively studied owing to their large aspect ratio, high surface area, improved catalytic and/or photocatalytic efficiency,3,4 and potential biomedical5 and H2 storage applications.6,7 These titania based nanostructures are also of significant interest for electrochemical lithium insertion and extraction in lithium ion batteries because of their large specific surface area and high density of surface defects.8 In the late 1990s, Kasuga et al. first reported the hydrothermal synthesis of a nanotubular structure of titania.9,10 Subsequently, researchers showed that these structures consisted of hydrated dititanate (H2 Ti 2 O 5), trititanate (H2Ti3O7),11−13 and H2Ti4O9·xH2O stoichimetries,14 as well as their Na salts. Titanate nanotubes combine many of the useful properties of conventional TiO2 nanoparticles (a wide © XXXX American Chemical Society

band gap semiconductor and a photocatalyst) with the properties of layered titanates (e.g., the H can undergo ion exchange with cations). Titanate nanotubes are of great interest for catalytic applications since their high cation-exchange capacity provides the possibility of achieving an augmented loading of active catalytic metal ions that are highly dispersed and evenly distributed. The open mesoporous morphology of these nanotubes and their high specific surface area can facilitate the transport of reagents to reactive sites. The semiconducting nature of TiO2 nanotubes can result in strong electronic interactions between the support and the catalyst, thus improving catalytic performance in redox reactions.3 The photocatalytic conversion of CO2 and water vapor into hydrocarbon fuels using light as an energy source is a very alluring prospect since it simultaneously achieves the goals of a renewable energy source that is carbon neutral. Since CO2 does Received: March 26, 2013 Revised: April 2, 2013

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dx.doi.org/10.1021/jp402979m | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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not absorb radiation at wavelengths longer than ∼200 nm, an efficient process of this type would require suitable photosensitizers. Both metal complexes and semiconductors have been utilized to absorb visible/UV radiation and transfer energy to CO2. Previous studies by the Gray group have shown titania nanotubes (Ti-NTs) and Pt-NT (Pt-Ti-NTs) to be interesting photocatalytically active materials for the reduction of CO2.15,16 The photocatalytic reduction of carbon dioxide is a complex process involving multiple steps. Minimally, carbon dioxide must adsorb on the surface of a photocatalyst, the photocatalyst must generate a reductant, typically thought to be electrons, which must interact with the adsorbed carbon dioxide. The reaction should then yield molecules with less negative heats of formation than carbon dioxide. However, the process is even more complex since new species are typically formed as a result of the interaction of carbon dioxide with the surface of a photocatalyst, and the production of desirable products, such as methanol or methane requires interaction with multiple photogenerated electrons: 6 for methanol and 8 for methane. Clearly, these electrons must be channeled to the appropriate reaction intermediates for the reaction to progress in a desired fashion. Additionally, hydrogen must be available in a utilizable form to produce a hydrocarbon from carbon dioxide. Though progress is being made,17 almost all the molecular level mechanistic details as to how these chemical transformations take place are currently unknown. Thus, our approach to understanding the mechanism for the photocatalytic reduction of carbon dioxide is to focus on specific aspects of the overall process. Adsorption is the first process that takes place. So, in order to understand the mechanism for CO2 reduction by TiNTs and Pt-Ti-NTs, it is imperative to understand the details of the interaction of CO2 with the relevant photocatalysts. There have been several prior studies of the adsorption of CO2 on TiO218,19 and a number of other metal oxide surfaces. There is an excellent review of the surface chemistry of titania by Hadjiivanov et al.20 More recently there has been an interesting report of CO2 adsorption on oxide nanoparticle surfaces by the Grassian group showing that the nature of the surface species can vary as a function of conditions.21 They studied the adsorption of CO2 on metal oxide surfaces in the presence of water and showed that when the relative humidity exceeded 40%, the nature of the adsorbed species changed from carboxylates on a dry surface to solvated carbonates. Acidic and basic sites are present on titania, and they can be critical in determining the details of the interaction between the surface and the reagents, reaction intermediates, and products. The photoreduction of CO2 is likely to be more efficient if the interaction of the photocatalyst surface with CO2 produces surface species that are optimal for transformation into the desired intermediates on the pathway to useful chemical feedstocks. Prior studies have shown that surface preparation and reaction conditions can alter the thermal surface chemistry of CO2.22−25 The aim of the present study is to develop an understanding of the adsorption of CO2 on the surface of Ti-NT and platinized Ti-NT that have been subjected to reductive and/or oxidative pretreatments. We focus on the factors that govern the reactive adsorption of CO2 and how pretreatment affects this chemistry. To the best of our knowledge, this study is the first that attempts to rationalize the reactive adsorption properties of CO2 on the TiO2 nanotube surfaces and the surface of platinized titania nanotubes in terms of the concepts of surface acidity and basicity. We find that differences in the

Lewis acidity and basicity of the Ti and O on the Ti-NT surface can be correlated with formation of, and preference for, carbonates and bicarbonates. However, reduced Ti-nanotubes form surface bicarbonates without the loss of adsorbed water or Ti−OH groups. Thus, the source of the hydrogen needed for this reaction is still an open question. Hopefully, the results of this study will provide new insights into how the chemistry of adsorbates on metal oxides can be viewed and manipulated. We also note that the formation of significant surface bound carboxylates (CO2−) is only observed in the presence of the Ti3+, which is accompanied by the formation of O− vacancies. These carbonxylates are compositionally the same as CO2−, a species widely postulated as a critical intermediate in the photoreduction of carbon dioxide.

II. EXPERIMENTAL SECTION A. Synthesis of Ti-NT and Pt-Ti-NT and Their in Situ Pretreatments. Titania nanotubes were prepared by a modified hydrothermal method that has been previously reported.9,10,26 In a typical experiment, 2 g of anatase titania powder (purity 99%, Sigma Aldrich Chemicals, USA) was stirred with 50 mL of 10 M NaOH solution (purity 97%, BDH Chemicals, USA) in a 125 mL closed Teflon cup. The Teflon cup was kept in an oven for 48 h at 120 °C, and the resulting precipitate was washed with 1 M HCl (purity 38%, EMD Chemicals, USA) followed by several washings with deionized water to attain a pH between 6 and 7. The Ti-NT powder thus formed was dried overnight in an oven held at 110 °C. The Ptdispersed Ti-NTs were prepared by mixing the resultant powder with an aqueous solution of hexa-chloroplatinic acid at a 0.5 mol % concentration. Dispersed and nondispersed titania samples were calcined under a hydrogen atmosphere (80 mL min−1) at 400 °C for 1 h. The Pt loaded Ti-NT samples will be referred to as Pt-Ti-NT. For the FTIR experiments the Ti-NT samples were initially heated in situ under an O2 atmosphere at 350 °C [O2 ≈ 2 Torr] to clean the surface and were then allowed to cool to room temperature under vacuum. The samples in which the only pretreatment is in situ exposure to oxygen at elevated temperatures will be referred to as Ti-NTO2. After this pretreatment, some of the TiO2 nanotubes (TiNT) were treated in an H2 atmosphere at 350 °C [H2 ≈ 2 Torr] for 3 h and then allowed to cool to room temperature under vacuum. These Ti-NTs will be referred to as Ti-NT-O2H2. The Ti-NTs that were initially exposed to hexachloroplatinic acid and then heated under O2 atmosphere at 350 °C [O2 ≈ 2 Torr] for 3 h in situ and allowed to cool to room temperature under vacuum will be referred to as Pt-TiNT-O2. Other Ti-NT samples were heated in vacuum to 350 °C and then allowed to cool to room temperature, at which point they was exposed to different pressures of CO2. This sample is referred to as Ti-NT-350. Pt-Ti-NT (precalcined first in a H2 atmosphere to disperse the Pt-metal) were initially heated under an O2 atmosphere at 350 °C [O2 ≈ 2 Torr] in situ, and then allowed to cool to room temperature under vacuum. This sample was then exposed to a H2 atmosphere at 350 °C [H2 ≈ 2 Torr] for 3 h. The Pt-Ti-NT sample was then allowed to cool to room temperature under vacuum. This sample is referred to as Pt-Ti-NT-O2-H2. Samples used to obtain powder X-ray diffraction (XRD), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS) data were subjected to the same treatments with the same heating rate as for the FTIR samples, except that the treatments were carried out ex situ. B



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Figure 1. TEM profiles of (a) Ti-NT, (b) Ti-NT-O2, and (c) Ti-NT-O2-H2. STEM profiles of Pt-Ti-NT-O2-H2 (d,e). (f) EDAX profile of Pt-Ti-NTO2-H2.

B. Characterization of the Ti-NTs. The morphology of the titania nanotubes was probed by TEM (STEMJEOL2100F), with an accelerating voltage of 200 kV. The EDX images were obtained with the same instrument. To establish crystallinity and phase purity of the Ti-NT and the Pt-Ti-NT samples, XRD patterns were recorded on a Rigaku Domex diffractometer using a continuous scan of Cu Kα radiation and a scintillation type detector for 2θ from 5° to 90°. XPS (An Omicron ESCA-2000-125 based spectrometer using an Al Kα radiation source (1486.6 eV, 30 mA × 8 kV)) was employed to obtain data on the oxidation states of the ions in these samples. The C 1s response at 284.6 eV was used as an internal reference for the absolute binding energy. The reducibility of the catalysts was investigated using temperature-programmed reduction (TPR) in an Altamira reactor. In the TPR experiments, ∼25 mg of the as-prepared catalyst was heated in a flow of 10% H2/Ar gas mixture. A heating rate of 7 °C/min was applied with a thermal conductivity detector (TCD) gain of 75 mA. C. Adsorption of CO2 Probed With in Situ FT-IR Spectroscopy. In situ FTIR spectra were recorded with a BioRad Excalibur FTS-3000 infrared spectrometer equipped with a mercury cadmium telluride (MCT) detector. Each spectrum was obtained by averaging 40 scans at a resolution of 2−4 cm−1. The samples were contained in a fabricated infrared cell, which was designed to study highly scattering powder samples in a transmission mode that consists of a stainless steel cube with two CaF2 windows and has been previously described in detail.27,28 For this study, samples were pressed onto a photoetched tungsten grid held between two nickel jaws. The grid is resistively heated to a temperature measured by a Chromel-Alumel thermocouple attached to its center. The cell was pumped down to a base pressure of 1 × 10−7 Torr. Unless otherwise stated, background spectra are of the samples cooled to ambient temperature, under vacuum, af ter pretreatment. A

Baratron capacitance manometer was used to monitor the pressure of CO2.

III. RESULTS A. TEM, STEM, and EDX. Figure 1 shows TEM and STEM images of the different Ti-NTs along with the EDX results for the Pt-Ti-NT. Different pretreatments do not alter the tubular structure of the Ti-NTs. STEM and TEM images show that all the Ti-NTs are multiwalled in nature and have an average diameter of 8−12 nm with a wall thickness of ∼2−3 nm and can be up to several micrometers in length. The majority of the tubes are open at both ends. Though there appears to be slight changes in the average dimensions of the tubes for different pretreatments, for any pretreatment the average dimensions still remain within the cited range. The nanotubes appear to have asymmetric walls, typically with five layers on one side and three layers on the other. This indicates that the tubes are likely formed by scrolling conjoined multilayer nanosheets.29 The interlayer spacing of the nanotube walls is ∼0.8 nm, close to the value reported in the literature.30,31 The EDX results show a Pt signal for the Pt-dispersed samples. B. X-ray Diffraction (XRD). The X-ray diffraction data for the TiO2 nanotubes is shown in Figure 2. The peak at a 2θ value of 10° in the XRD spectra typically corresponds to the reflection from the wall of the titania nanotubes, i.e., the titania nanotubes are formed by rolling of a titania layer formed in the hydrothermal process.32 On heating to 350 °C the XRD peak pattern shifts from a 2θ value of 10.9° to 13.3° due to dehydration of the tube walls. This removal of water leads to formation of anatase titania from hydrogen titanate (initially formed when the Ti-NTs were synthesized). The dehydration process leads to a change in the d-space between the adjacent layers from 0.8118 to 0.6658 nm.32 However, it is clear from the XRD data in Figure 2 that, independent of pretreatment for the Ti-NTs, there is no evidence in the XRD results for a phase C



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Figure 2. XRD profiles of (a) Ti-NT-350, (b) Ti-NT-O2, (c) Ti-NTO2-H2, and (d) Pt-Ti-NT-O2-H2..

other than anatase TiO2 (JCPDS 21-1272). There is, however, a small peak shift in the scattering angle for the (101) plane for the Ti-NT-O2-H2 pretreatment, which suggest there may be a slight lowering of the symmetry of this material. There is no change in this peak position for the Pt-Ti-NT-O2-H2, and we are able to observe the Pt(100) and (200) peaks, which again confirm the presence of the Pt in this system. C. X-ray Photoelectron Spectroscopy (XPS). Figure 3A displays the XPS Ti 2p spectra of the Ti-NT samples after the different pretreatments. The Ti-NT samples display two strong responses, shown in Figure 3A, at about 458.0 and 464.0 eV, that are ascribed to Ti 2p3/2 and Ti 2p1/2 states in TiO2.33 In the Ti-NT-O2 (Figure 3A) sample, there is a shift of both Ti peaks toward higher binding energies by 0.6 eV relative to the Ti-NT sample. This increment in the binding energy indicates a decrease in electron density around the Ti4+ ion, indicating that oxidative pretreatment produces relatively more acidic surface sites. In addition, the Ti 2p peaks become somewhat more symmetric suggesting that after oxidation there are fewer nonstoichiometric sites. The Ti-NT-O2-H2 samples are shown in the Figure 3A(iii) and display very interesting behavior. The broad shoulder at 454 eV can be assigned to the formation of Ti3+ sites.34 To maintain the electrical neutrality of the system, the formation of two Ti3+ sites on the surface can lead to an Ovacancy. This is described in detail in section IV.D.2 along with appropriate references to the literature. Thus, H2 treatment leads to reduction of some Ti4+ sites to Ti3+ sites. Additionally, the Ti4+ 2p peak is split into two peaks, with the major peak at 457.2 eV and similar amplitude thought less intense response at 459.1 eV. The splitting in the Ti4+ response indicates the presence of different distinct charge sites in the sample, which can be referred to as Ti(4‑δ) and Ti(4+α), where α indicates a small decrease in the electron density around the Ti4+ site and δ indicates a small increase in the electron density around the Ti4+ site. Clearly electric neutrality must be maintained. However, the amplitude of the peaks due to Ti(4‑δ) and Ti(4+α) are not equal. This implies that α and δ are not equal or the amplitudes of the peaks resulting from the same number of Ti(4‑δ) and Ti(4+α) sites are not equal, or another process is operative that maintains electrical neutrality by providing any residual charge difference between these two sets of sites. However, we have no evidence for another process that

Figure 3. (A) XPS spectra for Ti 2p for the TiO2-nanotubes that have been subjected to different pretreatments: (i) Ti-NT-350, (ii) Ti-NTO2, and (iii) Ti-NT-O2-H2. (B) XPS spectra for O 1s for the TiO2 nanotubes only: (i) Ti-NT-350, (ii) Ti-NT-O2, and (iii) Ti-NT-O2H2..

maintains charge neutrality nor are we able to postulate a plausible additional process. In addition, since the two Ti sites must be different in that one has a greater local electron density than for the +4 sites and the other must have lower local electron density than +4; these two sets of sites are expected to have a different electronic environment. Given that there is a difference in the sites, possibly before and certainly after charge transfer, it is probably not surprising that there is not a direct correspondence between the amplitude and the absolute magnitude of the difference between +4 and the actual oxidation state for these sites. As such, we believe that it is plausible that α and δ are equal in magnitude, but that is not a conclusion we can definitively reach. Thus, H2 treatment produces Ti3+, a very weak Lewis acid (VWLA) site and another weak LA site in the form of Ti(4‑δ) as well as a strong Lewis acid sites in the form of Ti(4+α). Analogous phenomena have been reported after the reduction of Ti-NT with CO.34 It should be noted that the appearance of Ti(4‑δ) indicates nonstoichiometric sites that suggest a higher density of defect sites in this sample than with the other pretreated Ti-NTs.35−38 D



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Figure 3B displays the O 1s XPS responses for the Ti-NT and the pretreated Ti-NT samples. The O1s response for the Ti-NT is at 533.2 eV. The Ti-NT-O2 sample displays a 1 eV lower binding energy for the O 1s response. This change in the O1s response can be correlated with the change in the Ti4+ response and is explained by the fact that on oxidation (O2 treatment) electron density shifts toward the O, thereby creating stronger Lewis base sites on the surface O for most of the NTs we have studied. This electron density comes from the Ti4+ sites creating more acidic Ti4+α sites, which are the only two principle Ti sites in this sample. The oxygen peak appears to be more symmetric after O2 treatment, suggesting that oxidation removes some oxygen vacancies producing a more stoichiometric material. The O1s peak of the Ti-NT-O2-H2 material splits, but in contrast to the Ti-NT-O2 sample, this peak does not shift significantly toward lower binding energy. This behavior is indicative of the formation of the different electronic environments for Ti and their corresponding O peaks. There are three prominent O1s peaks. The peak at 528.5 eV is best assigned as resulting from O atoms associated with Ti3+ sites. As discussed, these sites are also associated O-vacancies.39 The other two responses are best assigned to O atoms associated with the Ti(4‑δ) and Ti(4+α) responses. Figure 4A displays the Ti 2p XPS peak for the Pt-Ti-NT samples. The Ti 2p3/2 (Figure 4A) of the Pt-Ti-NT is observed at 458.1 eV, a minimal shift in binding energy from that of the unplatinized Ti-NT. However, the peak is more symmetric than the unplatinized Ti-NTs suggesting that most of the Ti is present as Ti4+, consistent with earlier reports for Pt-TiNT.40,41 The Pt-Ti-NT sample shows no change in the binding energy of the Ti upon treatment with O2 (Figure 4A(ii)) indicating that Ti4+ is the dominant Ti species and suggesting there is no significant change in the electronic environment around the Ti4+ species in this sample when it is oxidized. However, when O2 pretreatment is followed by H2 treatment, a shoulder is observed at 456.7 eV, and the main Ti4+ response shifts approximately 0.3 to 458.4 eV. As previously mentioned, this peak splitting is likely due to the formation of the different Ti electronic environments, (Ti (4+α) and Ti(4‑δ)). Interestingly, when Pt is present, there is no observed formation of Ti3+ on reductive pretreatment. This is consistent with Pt acting as an electron sink and thus inhibiting the reduction of Ti4+ to Ti3+. In the TPR, we find a nonzero oxidation state of Pt (as shown in Supporting Information), indicating that the reduction process affects the oxidation state of Pt. This result is consistent with the XPS data on the reduced platinized Ti-NTs which shows that the Pt acts as an electron sink inhibiting the formation of Ti3+.42 Figure 4B shows that the O1s response for the Pt-Ti-NT-O2 samples has a negative binding energy shift of ∼0.7 eV when compared to the response of the Ti-NT. This shift suggests that the oxygen in the Pt-Ti-NT is a more electronegative oxygen than the oxygen in the Ti-NT sample. The binding energy of O in Pt-Ti-NT upon treatment with O2 increases by 0.4 eV. The presence of a single shifted O1s response suggests that Pt is not oxidized, but there is lower electron density around the lattice oxygen. There is no measurable change in the position of the Ti response for the oxidized sample. Thus, there is no evidence for the formation of new acidic Ti sites. The Pt-Ti-NT, after treatment with both O2 and H2 (Figure 4B(iii)), displays a pronounced shoulder at 531.3 eV along with one at 532.6 eV,

Figure 4. (A) XPS spectra for Ti 2p for the Pt-Ti-NT nanotubes only: (i) Pt-Ti-NT, (ii) Pt-Ti-NT-O2, and (iii) Pt-Ti-NT-O2-H2. (B) XPS spectra for O 1s for the Pt-Ti-NT nanotubes only: (i) Pt-Ti-NT, (ii) Pt-Ti-NT-O2, and (iii) Pt-Ti-NT-O2-H2.

thereby providing further support for the formation of different Ti sites: Ti (4+α) and Ti(4‑δ). Interestingly, Pt peaks (expected at ∼71 and 74.3 eV for Pt (0)) are not observed in the XPS spectrum for any of the Pt-TiNT samples. However, the STEM, XRD, and EDX results all indicate that Pt is present in these samples and, as discussed earlier, the TPR results for the Pt-NT samples are also consistent with the presence of Pt. One obvious possibility for the absence of the Pt 4f peaks is that the Pt concentration employed is below the detection limit of XPS. However, since XPS is primarily sensitive to the environment at the surface, another possibility, which we consider more likely, is that at least some of the Pt has percolated into the walls of the Pt-NT samples due to a more favorable electronic environment and thus is not detected with XPS. The most significant XPS results are summarized as follows: (1) Upon oxidation, the O sites become more electronegative (a strong Lewis basic site (SLB)), and the corresponding Ti becomes more electropositive (a strong Lewis acid site (SLA)). (2) Reduction produces a Ti3+oxidation state (very weak Lewis acid site (VWLA)) along with Ti4+α and Ti4‑δ sites, which correlate with the corresponding O sites. (3) Ti3+ is not observed on reduction of the Pt-Ti-NT sample. This E



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Figure 5. (A) In situ FT-IR spectra for the region [2200−2500 cm−1] subsequent to adsorption of CO2 on Ti-NT-O2 (a) shows the baseline, (b) shows the spectrum after exposure to 2 Torr of CO2 for 1 min, (c) is the spectrum after exposure to 2 Torr of CO2 for 5 min. (B) In situ FT-IR spectra for the region [1200−1800 cm−1] for adsorption of CO2 on the Ti-NT-O2 sample: (a) the baseline, (b) the spectrum taken after the evacuation, and (c) after exposing the Ti-NT-O2 catalyst to 10 Torr of CO2. (C) In situ FT-IR spectra for the region [2900−3400 cm−1] for adsorption of CO2 on the Ti-NT-O2 (a) baseline, (b) after exposing the Ti-NT-O2 catalyst to 10 Torr of CO2, and (c) after exposing the Ti-NT-O2 catalyst to 2 Torr of CO2. (D) In situ FT-IR spectra of the NR (Ti-NT heated at 650 °C to collapse the nanotube structure ex situ and then heated with 2 Torr of O2 at 350 °C and consequently exposed to 4 Torr of water vapor. In each case, absorbance is in arbitrary units.

is also assigned to a bidentate carbonate.49 A peak at 1197 cm−1 has also been previously observed on a γ-Al2O3 surface and assigned to a polydentate carbonate.44−51 The absorption at 1226 cm−1, is due to a surface bound bicarbonate.50,54 Strong absorptions are also observed at 1424 and 1553 cm−1 and are assigned to bicarbonates.50−54 There are interesting changes in the absorption spectrum on evacuation. After evacuation, the band at 1250 cm−1 is still present, while the absorption peak at 1277 cm−1 shifts to 1289 cm−1, a position that has been assigned to a bidentate carbonate.52 The band at 1289 cm−1 may have been present on initial exposure to CO2 but was not initially visible because it was obscured by the strong band centered at 1277 cm−1: deconvolution (not shown) of the 1277 cm−1 band before evacuation indicates that a peak due to an absorption at 1289 cm−1 is present. However, it is clear that the absorption at 1277 cm−1 has decreased on evacuation ,while the absorption at 1289 cm−1 has grown. An absorption at 1378 cm−1, which is assigned to a BDC, is now observed. This band was most likely also

observation is consistent with Pt-nanoparticles acting as an electron sink, thus inhibiting the reduction of Ti4+ to Ti3+. D. In Situ FT-IR Studies. 1. Ti-NT-O2. Figure 5 displays spectra of the Ti-NT-O2 in the 2200−2450 cm−1 region obtained subsequent to exposure to 10 Torr of CO2. This region contains the absorption for gas phase CO2 at 2349 cm−1, along with an additional adsorption at 2331 cm−1, which has a strong shoulder at 2380 cm−1. Its assignment will be discussed in Section IV.B. Figure 5B shows that carbonates, bicarbonates, and polydentate carbonates form as a result of reactive adsorption of CO2 on the Ti-NT-O2 surface. Absorptions that can be assigned to bidentate carbonates are at 1250, 1277, 1573, and 1688 cm−1.43−51 There is an absorption from ∼1320 to ∼1400 cm−1 that appears to be composed of two overlapping absorptions, which are centered at ∼1364 and ∼1375 cm−1. An absorption at 1364 cm−1 has been previously reported on both a Cr2O3 surface and a rutile TiO2 surface and is assigned to a monodentate carbonate.52,53 The absorption at 1375 cm−1 F



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present upon initial exposure to CO2 but was convoluted with, and obscured by, the strong 1364 cm−1 absorption of a more labile bidentate species, as deconvolution of the 1364 cm−1 absorption before evacuation indicated that a peak due to absorption at 1378 cm−1 is present. Additionally, the absorptions at 1220, 1424, and 1553 cm−1, assigned to bicarbonate species, are no longer present after evacuation. Clearly there is more than one type of BDC and/or binding site, as different BDCs behave differently on evacuation. Additional information and a further discussion of the stability of species on evacuation is provided in the Supporting Information. There are a number of interesting observations with regard to absorptions in the 2900−3400 cm−1 region, which are shown in the Figure 5C. A broad absorption at 3250 cm−1, which is attributable to surface hydroxyls on TiO2, grows as a function of the pressure of gas phase CO2. In addition, negative going peaks are observed at 2960 and 2927 cm−1. These peaks are at the same position as absorptions that appear when water is adsorbed by the NT samples. This was verified by the following procedure. The Ti-NTs were heated to 650 °C, a temperature at which the Ti-NTs have been shown to collapse.15 The resulting collapsed nanotubes are then pretreated with O2 and exposed to ∼4 Torr of water (Figure 5D). Exposure to water leads to the appearance of absorptions at 2960 and 2927 cm−1, which mirror the negative going peaks alluded to above. Two conclusions can be drawn from these observations. First, water remains adsorbed on the multiwalled structures of the Ti-NT after heating to 350 °C, and second, exposure to CO2 initiates a process in which the species responsible for the absorptions at 2960 and 2927 cm−1 are depleted leading to the appearance of negative peaks. Before discussing the process that leads to depletion of these absorptions, it is appropriate to discuss the assignments for the 2960 and 2927 cm−1 absorptions. Absorptions at or close to 2960 and 2927 cm−1 have been reported in a number of publications, but there is no prior clear consensus reached on their assignment. Assignments have included impurities, hydroxyapatite on titania, and hydroxyl groups on titanate, with the latter becoming a more common recent assignment.55 Since it is clear that these absorptions appear on exposure to water, we conclude they are not due to impurities. On the basis of our results and literature reports, the assignment of these absorptions to water interacting with a titanate phase of the nanotubes appears to be the most plausible explanation. Since the nanotubes are initially all titanate phase and are dehydrated to form anatase phase nanotubes, it is not implausible that exposure to water reverses the process, and our experiments provide support for this explanation. In this scenario, the absorptions at 2960 and 2927 cm−1 are due to OH groups on titanate and an OH stretch for water incorporated into the titanate phase. These observations are being explored in more detail. For brevity, we refer to this water as adsorbed water throughout the text. 2. Ti-NT-O2-H2. As shown in Figure 6A, and as seen with the Ti-NT-O2, exposure of Ti-NT-O2-H2 to CO2 leads to the formation, at 2380 cm−1, of a shoulder on the peak due to the gas phase adsorption of CO2. Figure 6B shows the formation of other new absorptions on exposure to CO2. On the basis of literature reports, these absorption can be assigned to carbonates and bicarbonates. The two strong peaks at 1202 and 1228 cm−1 are assigned to bicarbonates,56 while the other strong peaks, at 1262, 1344, 1367, and 1379 cm −1 , are assigned to monodentate

Figure 6. (A) In situ FT-IR spectra for the region [1200−1800 cm−1] subsequent to adsorption of CO2 on Ti-NT-O2-H2 after exposure to 10 Torr of CO2 for 1 min. (B) In situ FT-IR spectra for the region [1200−1800 cm−1] for adsorption of CO2 in NT-O2: (a) the baseline (shown in red color), (b) spectra after exposure to 10 Torr of CO2 for 1 min (marked as 10 Torr), and (c) spectra after evacuation subsequent to exposure to 20 Torr of CO2 (marked as evacuation). The inset has the corresponding spectra for the region [2900−3400 cm−1]: (a) the baseline and (b) spectra after exposure to 10 Torr of CO2.

carbonates.21,51,52,57 The peak at 1583 cm−1 has been assigned to a bidentate carbonate.50,51,57 The strong peak at 1281 cm−1 has been assigned to a bridging carbonate based on observation of absorptions on Ga2O3.58 A broad small absorption at 1409 cm−1, previously seen on Fe2O3, has been assigned to the ν3(OCO) mode of a bicarbonate.21 The peak at 1546 cm−1 has been seen on TiO2 rutile surfaces and assigned to a bicarbonate absorption.21 Interestingly, we find a very broad peak at 1686 cm−1 due to the formation of the CO2− carboxylate species. There is also a shoulder at the 1180 cm−1, which is assigned to the νs mode of a bent carboxylate.57 Interestingly, these carboxylates are only observed for the Ti-NT-O2-H2 samples. Ti3+ is also only seen for the Ti-NT-O2-H2 samples. As will be discussed, Ti3+ sites are always accompanied by O vacancies. This leads us to hypothesize that the formation of carboxylates on this Ti-NT sample results from the interaction of CO2 with Ti3+/O vacancy sites. This hypothesis is consistent with prior reports in the literature. Rasko et al. reported the observation of G



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carboxylates upon illumination of Rh/TiO2 samples. They attribute the formation of these carboxylates to the electronic and defect structure of the TiO2 where CO2 is activated to form a carboxylate by the transfer of an electron from the Ti3+ to the adsorbed CO 2 , yielding a partially negatively charged species.57−60 This form of activated CO2 is also reported by Busca et al. on both TiO2 and ZrO2 surfaces in the absence of light.18 They propose that CO2− is formed by the transfer of electron density from the Ti3+ or Zr3+ to CO2. However, for the first time, we are able to directly correlate the observation of Ti3+sites/O vacancies (which are accompanied by O vacancies), using XPS, with carboxylate formation on Ti-NT-O2-H2. On evacuation, the strong absorptions at 1275 and 1372 cm−1, which are due to monodentate bicarbonates, shift by a few wavenumbers. After evacuation, there is a band at 1578 cm−1 that appears to be slightly shifted from, and narrower than, the 1583 cm−1 absorption present before evacuation. We assign both of these bands to bidentate carbonates. However, the absorptions at 1262 and 1367 cm−1, which are assigned to monodentate bicarbonates, are present postevacuation only as shoulders. The peaks at 1202 and 1228 cm−1, which have been assigned to the bicarbonates, along with a peak due to a monodentate carbonate at 1262 cm−1 disappear on evacuation, as does the peak at 1407 cm−1 assigned to bicarbonate. This behavior suggests there are at least two types of monodentate carbonates and/or binding sites that exhibit different degrees of stability (bond strength) on evacuation. The spectrum of the 2900−3400 cm−1 region, shown in the inset of Figure 6B, shows the growth of a peak at 3250 cm−1 due to Ti−OH groups. However, interestingly the negative bands, due to the loss of the adsorbed water molecules that were seen for the Ti-NT-O2 sample, are absent. Furthermore, there is evidence for formation of molecular water (though at very low intensity) in this region when the sample is exposed to higher pressures of CO2. 3. Ti-NT-350. As expected, absorption due to the ν3 mode of gas phase CO2 is seen at 2349 cm−1, and as with the other materials, there is also an absorption at 2380 cm−1 due to adsorbed CO2. Upon exposure to CO2, the Ti-NT-350 shows minimal formation of bicarbonates and carbonates. A weak absorption at 1331 cm−1 (not shown here) is assigned to a bidentate carbonate,51,52 while a weak absorption at 1503 cm−1 (not shown here) is assigned to a monodentate carbonate. Low intensity peaks at 1575 and 1664 cm−1 are assignable to bicarbonates.50,51 However, as seen in Figure 7A, there is a very prominent Ti−OH hydroxyl absorption at 3250 cm−1, and there is loss of adsorbed water molecules, as evidenced by the decrease in absorptions at both 2960 and 2920 cm−1. Since minimal bicarbonates are formed on this Ti-NT, there must be a different loss mechanism for the hydroxyls. A possible mechanism for loss of hydroxyls is shown in Scheme 4 and involves the displacement of the surface bound OH by adsorbed CO2. 4. PtA-Ti-NT-O2 (Hexa-chloroplatinic Acid-NT-O2). PtA-TiNT-O2 behaves very similarly to the other Ti-NTs studied with regard to the positions of absorptions in the 2200−2400 cm−1 region, where gas phase CO2 adsorbs. The 1200 to 1800 cm−1 region is shown in the Figure 8. Strong absorptions that, based on the literature, can be assigned to bidentate carbonates are observed at 1250, 1288, 1377, and 1578 cm−1 and dominate the spectrum.51,52,57 The absorptions at 1221 and 1412 cm−1 and the shoulders at 1424 and 1550 cm−1 are assignable to

Figure 7. In situ FT-IR spectra for the region [2900−3400 cm−1] subsequent to adsorption of CO2 on Ti-NT-350: (a) spectrum of the baseline, (b) spectrum after evacuation subsequent to exposure of 20 Torr of CO2, (c) spectrum after exposure to 10 Torr of CO2 for 1 min, and (d) spectrum after exposure to 20 Torr of CO2.

Figure 8. In situ FT-IR spectra for the region [1200−1800 cm−1] for adsorption of CO2 on PtA-Ti-NT-O2 (hexa-chloroplatinic acid dispersed on Ti-NT-O2) shows (a) the baseline, (b) the spectra after exposure to 10 Torr of CO2 for 1 min, and (c) the spectra after evacuation subsequent to exposure to 20 Torr of CO2. The inset has the corresponding spectra for the region [2900−3400 cm−1]: (a) the baseline and (b) spectra after exposure to 10 Torr of CO2..

bicarbonates.21,49,51 The absorptions at 1359 and 1384 cm−1 are assignable to monodentate carbonates. A small peak at 1175 cm−1 (not shown) is indicative of the formation of a polydentate carbonate. Thus, while the formation of bidentate carbonates dominates, there is a multiplicity of species formed on PtA-Ti-NT-O2. Interestingly, evacuation has little effect on the intensity or position of almost all of these absorptions, indicating that the corresponding species are relatively strongly bound to surface sites. The hydroxyl region shows the formation of additional Ti−OH (3250 cm−1) groups along with the loss of adsorbed water, which is manifested by a decrease in absorptions at 2960 and 2927 cm−1. A blow up of this region is shown in Figure S-4 in the Supporting Information. 5. Pt-Ti-NT-O2-H2. Pt-Ti-NT-O2-H2 behaves very similarly to the other Ti-NT samples studied with regard to the positions of absorptions in the 2200−2400 cm−1 region, where gas phase H



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CO2 adsorbs. The 2200−2400 cm−1 region shows the ν3 absorption of gas phase CO2 at 2349 cm−1 along with a 2380 cm−1 absorption due to adsorbed CO2. As shown in the Figure 9, as with the other Ti-NTs, new absorptions are observed in the 1200−1800 cm−1 region upon

Figure 10. In situ FT-IR spectra for the region [1200−1800 cm−1] of the Pt-Ti-NT-O2-H2: (a) CO2 exposure at 10 Torr, (b) after exposure to 20 Torr of CO2 and evacuation of the Pt-Ti-NT-O2-H2 sample heated again at 100 °C under vacuum, and (c) baseline for the initial sample before exposure to CO2.

Figure 9. In situ FT-IR spectra for the region [1200−1800 cm−1] subsequent to adsorption of CO2 on Pt-Ti-NT-O2-H2: (a) the baseline, (b) spectra after exposure to 10 Torr of CO2 for 1 min, and (c) spectra after evacuation after the exposure to CO2. The inset has the corresponding spectra for the region [2900−3400 cm−1]: (a) the baseline and (b) spectra after exposure to 10 Torr of CO2.

bicarbonate that is sufficiently stable as to be present even after heating under vacuum. There is also a peak found at 1265 cm−1, which is still present postheating and is due to BDCs.21 The formation of the negative peaks indicates that some of the bicarbonates that were previously present are lost from the surface upon heating at 100 °C.

exposure of Pt-NT-O2-H2 to CO2. A strong sharp absorption is seen at 1226 cm−1, with shoulders at 1408 and 1550 cm−1. There is a very strong absorption at 1427 cm−1. On the basis of literature reports, these absorptions are assigned to bicarbonates.21,50,52 The shoulder at 1448 cm−1 is also indicative of a bicarbonate.49 Absorptions at 1280, 1304, 1376, and 1580 cm−1 are assignable to bidentate carbonates.51,52,57 The shoulders at ∼1384 and ∼1448 cm−1 are indicative of monodentate carbonates.51,55,60 The weak shoulder at 1647 cm−1 has been previously assigned to bridging carbonates.56 Once again, evacuation reveals information about the relative stability of these carbonates and bicarbonates. Bicarbonates (1226, 1427, and 1550 cm−1) remain even after the evacuation. Some of the absorptions due to bidentate carbonates, such as those at 1580 cm−1 and the shoulder at 1647 cm−1, disappear on evacuation, and a new absorption appears at 1550 cm−1, which suggests that these species may be converted into bicarbonates. Other absorptions due to BDCs (1280, 1304, and 1376 cm−1) are also present after evacuation. As shown in the inset of Figure 9, there is growth of the Ti− OH hydroxyl absorption on exposure to CO2. However, no decrease in the absorptions at 2960 and 2927 cm−1, which are due to adsorbed water, is observed Bicarbonates are typically relatively unstable species on titania.21 However, in this study some bicarbonates were stable on evacuation. Thus, we conducted additional experiments that involved heating adsorbed bicarbonates under vacuum to obtain more information about their stability. The spectrum shown in trace a in Figure 10, which is for Pt-Ti-NT-O2-H2 that are exposed to 10 Torr CO2, is identical to trace b in Figure 9. Trace b in Figure 10 shows a spectrum of the same sample after being heated to 100 °C under vacuum. Some bicarbonates that initially form are quite stable and are still present after evacuation and heating. The peak at 1233 cm−1 is attributed to

IV. DISCUSSION A. Nature of Surface Sites. As expected, the XPS data indicate that the O atoms in the Ti-NTs act as Lewis base sites and the Ti atoms as Lewis acid sites. The terms Lewis acid and Lewis base are being used here based on their broad definition. Though G. N. Lewis defined a Lewis acid as an electron pair acceptor and a Lewis base as an electron pair donor, the definition of a Lewis acid was generalized by Mulliken to involve vacant orbitals that can accept electron density with an analogous broadening of the definition of a Lewis base. XPS data can be interpreted as providing information about the oxidation state of surface atoms, which also contains information about the electron density for these atoms. We then draw a correlation between electron density and Lewis acid−base character based on a generalization of Lewis acid− base concepts. This approach has been taken before.61 On the basis of XPS data, pretreatment with O2 and H2 leads to changes in the Lewis basicity or acidity of the O atom and Ti atoms surface sites, respectively. As will be discussed below, these changes favor the formation of specific adsorbed surface species, specifically bidentate or monodentate carbonates or bicarbonates, and carboxylates that are observed to form on exposure of the titania to CO2. Though other species such as polydentate carbonates and polycarbonates are observed for some pretreatments, these are relatively minor species under our experimental conditions. Thus, we focus on obtaining an understanding of the factors that control changes in the propensity for formation of the major species we observe as a function of surface pretreatment. In doing this, the question I



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Scheme 1

the water can readily dissociate on the O vacancies to form OH groups on TiO2 that is not immersed in an aqueous phase.64 The surface concentration of the hydroxyls groups (−OH) on hydrophilic Degussa P25 has been reported to be 3.3 −OH/ nm2 for the conditions described in ref 65. Without trying to be more specific because of potential uncertainties as to what constitutes a monolayer for a given material, it is clear that this coverage corresponds to less than a monolayer. Both physisorbed and chemisorbed −OH groups have been reported to be present on the surface of the TiO2.66,67 Upon calcination to 350 °C, the physisorbed −OH groups are lost but the chemisorbed −OH remain.66,67 In our current work, we are more concerned with the Lewis acid and basic sites on the TiNT surface, which are Ti and O sites, and not the Brönsted acid base sites as represented by the −OH/H groups. Therefore, our representation of the calcined Ti-NT surface is shown with a termination of Ti and O atoms, but this is not intended to depict the coordination geometry for either Ti or O on this surface. Wherever we hypothesize the involvement of an −OH, we have depicted it as a Ti−OH group. We represent the calcined TiO2 surface as O−Ti−O as part of our effort to correlate the oxidation states of Ti and O, as determined from XPS data, with the acid−base character of these sites and to understand the dominant aspects of the surface chemistry in terms of acid−base concepts. B. Adsorbed CO2. A detailed analysis of the 2200−2400 cm−1 region reveals that the full width half maxima (fwhm) for the absorption bands of the various adsorbed CO2 moieties do not vary significantly for the different Ti-NTs. A deconvolution of the peaks (not shown) in this region shows that the absorption at 2380 cm−1 is present on all the Ti-NTs. This high frequency absorption has been reported for CO2 interacting with an F doped alumina and was assigned to a strongly bound chemisorbed species on Lewis acid sites.68 The 2380 cm−1 band has also been previously assigned as a ν1 + 2ν2 Fermi resonance doublet, where its observation would suggest a lowering of molecular symmetry on adsorption.18 However, on the basis of gas phase frequencies, the expected position for the v1 + 2v2 combination band is between ∼2600 to 2700 cm−1. Though

arises as to whether the amplitude of the various absorption bands can be directly related to the concentration of the respective surface species. Though there is no data, we are aware of reports on the oscillator strength of the relevant adsorbed species, it does seem reasonable to assume that the oscillator strengths for the species we observe are similar since we are focusing on a region in the infrared that involves similar types of vibrational modes. In any case, it is clear that we are seeing changes in the mix of species that are present on the surface. Thus, minimally, our data can be interpreted in terms of the reason(s) for a given species being more or less dominant following one type of pretreatment versus another type of pretreatment. As seen in the XPS data, oxidation leads (Ti-NT-O2) to formation of a stronger Lewis base O atom site accompanied by a stronger Lewis acid Ti atom site. Reduction of the unplatinized NTs (Ti-NT-O2-H2) by H2 leads to formation of Ti3+ along with Ti(4‑δ) sites. We also see formation of a comparable amount of Ti(4+α), which helps effect charge compensation within the lattice. Ti3+ is a very weak Lewis acid site as compared to Ti4+. Interestingly, as will be discussed in more detail below, Ti3+ does not form in significant concentration on the Pt-Ti-NTs following reduction (Pt-TiNT-O2-H2). This difference in formation of Ti3+ on platinized versus unplatinized Ti-NT samples is consistent with the concept that nanoscale Pt particles can act as electron sinks.41 In light of this conclusion, it is not surprising that excess electron density would migrate to the Pt-nanoparticles rather than residing on the Ti atoms. The termination of the surface of Ti-NTs is an interesting issue that has received some attention in the literature. Ti-NTs are expected to have a more complex surface structure than a plane of a single phase crystal, and though it would be very interesting to determine the distribution of the types of coordination for the Ti and the O atoms in the NTs, this is currently unknown. Though Ti-NTs can have a Ti, an O, or an H as the terminal atoms on the surface, it is clear that there are OH groups present on the surface of titania that has been exposed to an aqueous phase.62,63 It is also widely accepted that J



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Scheme 2

of the acid−base character of surface sites that can result in the stabilization of a particular product formed as a result of the reaction adsorption of CO2. They are not intended to indicate a specific configuration of reacting species or preferred reaction geometry. We note that as shown in Scheme 1 the strongest Lewis acid Ti site is accompanied by the strongest Lewis basic O atom site. This geometry favors coordination of the C in CO2 to the basic O atom site and coordination of an O in CO2 to the strong Ti Lewis acid site. The question of which atom is coordinated first is discussed in ref 70, with the conclusion that it is energetically more favorable for the C atom in CO2 to coordinate to the basic O atom site on the surface, followed by coordination of a CO2 O atom to the acidic Ti site. Thus, the presence of a strong Lewis acid site and a strong Lewis base site in close proximity provide conditions that favor formation of BDCs. Though in the discussion that follows we focus on the three major species that are seen on any of the Ti-NTs (MDC, BDC, and BC), we note that formation of polydentate carbonates (PDC) is favored by the same acid−base environment that favors formation of BDCs, i.e., a SLB in conjunction with a SLA. Weaker Lewis acid surface sites (Ti3+and Ti(4‑δ)) accompany a SLB oxygen site. Coordination of the carbon of CO2 to the strong Lewis base oxygen site leads to a relatively more negative oxygen on the bound CO2. This in turn leads to a lower propensity for the oxygen to coordinate with the more electron rich Ti sites. Thus, in this case, there is less of an energetic driving force for coordination of the O atom of the incipient carbonate to the Ti surface site, which would lead to a preference for the formation of MDC relative to a BDC. Scheme 2 illustrates the conditions that favor the formation of MDCs. BCs are favored (see Schemes 3 and 4) when the surface has a SLB along with a SLA or a WLA site. However, there must be a source of H. The requisite H can come from either the hydroxyl group present on the surface of the TiNTs/Pt-Ti-NTs or from adsorbed H2O. As will be discussed in more detail in next section, there is evidence that water can

interactions with surfaces can lead to lowering of vibrational frequencies, an interaction that would lower the frequency for this combination band by at least 200 cm−1 and, at the same time, not significantly affect the position of the v3 absorption appears unlikely. However, it is possible that adsorption at certain surface sites is accompanied by electron transfer, which leads to CO2 having a partial charge, thus making the CO2 moiety look more like CO2δ‑, where δ is between 0 and 1. The asymmetric stretching mode of CO2− has a much lower frequency for bent CO2 than for linear CO2.62 In addition, calculations69 indicate that the frequency of the bending mode could increase somewhat in CO2δ‑. Thus, it is conceivable that the absorption at 2380 cm−1 is the ν3 + v2 combination band for bent CO2δ‑. Clearly, this alternative explanation is speculative, and at this point, we feel that it is best to view the 2380 cm−1 absorption as resulting from the interaction of carbon dioxide with a strong Lewis acid site. Though to our knowledge, the 2380 cm−1 absorption has not been studied theoretically, it can be rationalized that if this absorption is a shifted ν3 mode, this would imply an increase in bond order, which could result from a loss of electron density from the filled antibonding orbitals of CO2. This would be consistent with the presence of a strongly electronegative ion such as F−. Strongly electronegative O atom sites on the TiNTs could act in a similar manner. C. Correlation between Species Formed on Different Ti-NTs and Available Surface Sites. The general order for the Lewis basicity of the predominant surface species formed on the Ti-NTs is BDC > MDC > BC. Thus, we would expect that stronger Lewis acid sites would favor formation of BDC and the weaker Lewis acid sites would favor formation of MDC and BC, provided that there was a source of hydrogen for formation of BC. Scheme 1 shows a mechanism for the formation of the BDC (bidentate carbonates) on the Ti-NT surface, which is expected to be favored by the strongest Lewis acid site, which is Ti(4+δ). As discussed above, this scheme and subsequent schemes are intended as schematic representations K



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Scheme 3

Scheme 4

L



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Ti3+ sites is shown in Scheme 5. For each pair of Ti3+sites that are formed, electrical neutrality of the lattice can be maintained

remain adsorbed to the Ti-NTs even after pretreatment at 350 °C. In the case of the reduced Pt surface/Ti-NT surface, H atoms could be supplied as a result of activation of the H2 used in the reduction process by the surface Pt. D. Observed Species on Ti-NT and Pt-Ti-NT with Different Pretreatments. 1. Ti-NT-O2. As expected, the XPS results indicate that O atoms on the Ti-NT-O2 surface are strongly electronegative, producing a strong Lewis base site, while the Ti sites are Lewis acid sites with the acidity depending on the local electron density. The XPS data show that pretreatment with O2 (oxidation) leads to formation of stronger Lewis base O sites: The oxidizing oxygen can donate some of its electron density to the lattice O making the lattice O more negative, which we designate as Oγ‑. As seen in the XPS data, the greater electronegativity of O versus Ti makes the further transfer of electron density from the Ti sites to the O sites favorable leading to the formation of Ti sites that we designate as Ti(4+δ) and O sites that we designate as Oγ−. Clearly, the change in the charge at the Ti sites must be balanced by the change in charge at the O sites. This is in accord with the XPS results that show the O to be highly electronegative, displaying a shift in binding energy of 1 eV toward lower energy and the Ti to have a higher binding energy of ∼1 eV. On exposure to CO2, the strong oxygen Lewis base site associated with the Lewis acid Ti sites provides for a favorable situation for the formation of bidentate carbonates. Therefore, as expected from the discussion in section C, BDCs are expected to be, and are observed to be, the most favorable species on the oxidized Ti-NT samples. This is shown schematically in the Scheme 1. However, BCs are also observed in the FT-IR spectrum. Their formation requires a source of hydrogen and, as shown in the Scheme 3, they can be formed from the interaction of adsorbed carbon dioxide with the hydroxyl groups present on the TiO 2 as (Ti−OH). Alternatively, they can be produced as shown in Scheme 4, where adsorbed water on the Ti-NT sites can react with CO2 to solvate the CO2 producing carbonic acid, which in turn can interact with the TiO2 surface to form bicarbonates. The involvement of adsorbed water in the formation of BCs is consistent with the loss of adsorbed water that is observed on the oxidized NTs after exposure to CO2. This loss of adsorbed water leads to the appearance of negative absorptions in the 2960 and 2923 cm−1 region (see Figure 5 and associated discussion). The loss of adsorbed water is accompanied by an increase in intensity in the hydroxyl region, which is seen in Figure 5C and is depicted schematically in Scheme 4, where an OH from the solvating water molecule can interact with a Ti site leading to formation of Ti−OH groups. Scheme 4 shows three possible reaction steps starting with solvated carbon dioxide. Steps (a) and (b) are expected to occur sequentially. Step (c) will occur if the H+ is taken up in step (b), but the HCO3− ions do not react with the same under-coordinated Ti site. Then, as shown in the step (c), the HCO3− ion could react with a different under-coordinated site. 2. Ti-NT-O2-H2. The XPS data show that, on exposure to H2 (reduction), Ti3+ forms along with Ti(4‑δ) and Ti(4+α) sites. There is a corresponding O atom site associated with each type of Ti sites. The Ti3+ can be characterized as very weak Lewis acid sites. Therefore, as discussed in section IV.C, MDCs are favored on this surface relative to the BDCs. This is shown schematically in the Scheme 2. It is found that, although some BCs and BDCs are formed, primarily (MDC) are formed on these Ti-NTs. A mechanism for formation of MDCs involving

Scheme 5

by formation of an O vacancy. This process has been experimentally observed and explained theoretically by Pacchioni et al.71,72 The resulting O vacancy leads to formation of a surface bound O2β‑ (where β indicates a small quantity of charge), which can lead to severe lattice strain and, as a result, can be easily scavenged by the CO2 to form CO3β‑, which then interacts with the surface to form MDCs. The lack of strong Lewis acid sites from this pretreatment disfavors the formation of BDCs. Though we discuss this process as if formation of Ti3+ precedes formation of an O vacancy, our data does not allow us to determine whether vacancy formation precedes or follows Ti3+ formation. A possible alternative mechanism for the production of MDC by O vacancies is shown in Scheme 5. The oxygen vacancy [O-vacancy] can also be treated like a strong Lewis acid. It is possible that the oxygen lost to form the [Ovacancy] can interact with adsorbed CO272 to form a CO3β‑ moiety, where this process would lower the lattice strain. Now this SLB forms, in conjunction with the Ti3+, a VWLA. Therefore, as in our previous discussion, this should also lead to formation of MDCs, which is depicted in Scheme 5. We note that, upon illumination of M/TiO2 (M = Rh, Pt, Ir, Pd, and Ru), Rasko et al.59 hypothesized that electrons flowed primarily from the reduced TiO2 to the polycrystalline noble metal, which subsequently transferred electron density to the π* orbital of CO2 to form a partially negatively charged CO2−. This conclusion was based upon work functions of the noble metal covered TiO2 as compared to that of reduced nonmetallized TiO2. However, though we observe formation of carboxylates on the reduced Ti-NTs, we do not observe formation of carboxylates on the Pt-Ti-NT-O2-H2. Therefore, the process reported by Rasko et al.59 does not appear to be active for Pt deposited on structurally reduced TiO2 (NT). Nevertheless, it will be interesting to determine if carboxylates form on the Pt-Ti-NT-O2-H2 upon illumination. A mechanism for the formation of the carboxylates on Ti-NT-O2-H2 in the absence of illumination is presented in Scheme 6, where a bond forms between Ti−C, and the O of the CO2− occupies an adjacent O-vacancy. Another scheme that involves the energetically less favorable70 CO2 nucleophilic attack on Lewis acid sites, like Ti(4+δ), that could lead to formation of a MDC by abstraction of a lattice oxygen is shown in Scheme S-1 in the Supporting Information. For this pretreatment, we observe the formation of a small amount of BCs as well as the growth of adsorbed water and the formation of OH groups. However, interestingly, BC formation occurs without an obvious source of hydrogen. One possibility is that the source of hydrogen needed to form bicarbonates is M



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with depletion of structural water molecules. A plausible explanation for both of these observations is that CO2 initially displaces adsorbed water molecules before formation of surface bound moieties. Alternatively, a variant of Scheme 4 could be operative, where a small amount of CO2 is solvated, which leads to the formation of a small amount of bicarbonates. However, since we do not observe significant BCs, for the latter scheme to be operative, these BCs would have to convert to putatively more stable BDCs. Thus, our conclusion can be summarized by saying that the Ti-NT-350 °C surface is relatively unreactive, and it is more likely that exposure to CO2 results in displacement of water and resultant formation of OH groups rather than reactive adsorption of CO2. 4. PtA-Ti-NT-O2. The PtA-NT-O2 system was studied because its surface is very acidic since it has been directly treated with hexa-chloroplatinic acid. It would therefore be expected that the most basic surface species would dominate, and in fact, FTIR data demonstrates that BDCs, which are the most basic surface moiety, are the dominant surface species. However, since this type of pretreatment is not typical for catalytic surfaces, we did not study it in as much detail as the other pretreatments. As such, we do not include XPS or XRD data for the PtA-Ti-NT-O2 surface. 5. Pt-Ti-NT-O2-H2. The Pt-Ti-NT-O2-H2 surface has both Ti(4+α) and Ti(4‑δ) sites, but there is no detectable formation of Ti3+. The corresponding Lewis base O sites are also present. Since Ti(4+α) sites are present, it is not unexpected that BDCs form as a result of these sites, which are the most acidic of the sites we observe. However, essentially no MDCs are formed and the dominant species that is observed are BCs. Interestingly, we do not see a depletion of the absorption profile for adsorbed water, as was seen with the Ti-NT-O2 where adsorbed water molecules were proposed as the source of hydrogen for the BCs formed on Ti-NT-O2. However, we do see a significant increase in the Ti−OH infrared absorption. Therefore, the most likely source of the hydrogen needed to form BCs is activation of H2 on the Pt-nanoparticles. It is well documented that Pt-nanoparticles can activate H2.74 However,

Scheme 6

the H2 that has been used as a reductant. There are reports in the literature that exposure of multiwalled titania NTs to H2 leads to a large decrease in electrical resistance due to strong interactions of the H2 with the nanorods.4,73 However, there is no direct experimental evidence we are aware of that shows that multiwalled titania Ti-NTs dissociate H2 to form H. As such, we used D2 as a reductant to determine if it would be incorporated into bicarbonates. Though some new bands did appear in the IR spectrum (not shown), we did not see shifts in the bicarbonate absorptions that were consistent with deuterium incorporation. Thus, at this point, it is not clear what the source of hydrogen is for bicarbonate formation nor is it clear why new absorptions are seen on reduction with deuterium. However, the apparent new absorptions could be the result of shifts in the frequency of vibration of surface species. We note that the observation of new absorptions is at least superficially consistent with incorporation of deuterium in a surface species. Clearly, this observation is deserving of further study. 3. Ti-NT-350. The XPS data for these Ti-NTs shows the formation of Lewis base (O−) sites and also weak Lewis acid sites (Ti(4+δ)). A small amount of BDCs are observed on these Ti-NTs. The combination of the LB and WLA can lead to BDC formation since interaction of the carbonate oxygen with the Ti acid site is possible. The infrared spectra of these samples show an increase in the concentration of Ti-hydroxyl groups, along Scheme 7. Reactions Schemes for Pt-Ti-NT-O2-H2

N



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Scheme 8. Reactions Schemes for Pt-Ti-NT-O2-H2

surface species. The band at 1222 cm−1 is not seen after deuteration, which is consistent with the literature.75,76 As such, it is plausible that the 1222 cm−1 absorption shifts to 1191 cm−1 on deuteration. These observations are consistent with the conclusion that H2 is activated at the Pt sites for Pt-Ti-NT-O2H2. A plausible mechanism for the formation of BCs on Pt-TiNT-O2-H2 and the observed increase in Ti−OH moieties is shown in Schemes 7and 8.

we have previously discussed, it is not evident that Ti-NT-O2H2 can activate H2, which can be incorporated in bicarbonates. This is observed upon reduction with the D2; we did not see the expected shifts in the bicarbonate absorptions that were consistent with deuterium incorporation. Thus, at this point, it is not clear what source of hydrogen is in bicarbonate formation for Ti-NT-O2-H2. Yet for both the Pt-Ti-NT-O2-H2 and the TiNT-O2-H2 systems, we see bicarbonate formation without loss of adsorbed water or hydroxyl groups. This means there are two potential mechanisms for activation of H2 and it would be a priori reasonable to assume they might operate in parallel for the Pt-Ti-NT-O2-H2 system. However, interestingly, as discussed later in this section, the bicarbonates produced on the Pt-Ti-NT-O2-H2 surface are more stable than those produced on the Ti-NT-O2-H2 surface to the extent that evacuation and heating of the Pt-Ti-NT-O2-H2 surface has little effect on their intensity. This observation suggests that formation of the bicarbonates on the Pt-Ti-NT-O2-H2 surface and on the Ti-NT-O2-H2 surface take place by different routes or possibly at different sites. One plausible but speculative explanation is that activation of H2 on the Ti-NT-O2-H2 surface takes place at Ti3+ sites and/or bicarbonate forms at these sites, whereas the formation of the H that is incorporated in bicarbonates on the Pt-Ti-NT-O2-H2 surface occurs principally at Pt sites. To test this hypothesis, the oxidized Pt-Ti-NTs were reduced in D2. These IR data (not shown) display a red shift in absorptions that are assigned to bicarbonates. Specifically, we now observe absorption at 1191 cm−1, along with a shift in the band at 1426 to 1373 cm−1. These results are consistent with the previous literature assignments for deuterated bicarbonates on titania.75 We also observe the formation of what appears to be a new absorption at 1191 cm−1, but this apparent new absorption is most likely due to a shift in the absorption of bicarbonate that overlapped with the absorption of another

V. CONCLUSIONS There are no significant changes in the Ti-NT morphology as a result of oxidative and reductive pretreatments or platinization. Formation of bidentate carbonates (BDC) is favored by strong Lewis base sites in conjunction with strong Lewis acid sites. Monodentate carbonates (MDCs) are favored by strong Lewis base sites in conjunction with weak Lewis acid sites, such as Ti3+, where coordination of the Oδ‑ of CO2 with less acidic Ti sites is not expected to be as energetically favored as coordination with a stronger LA site. Bicarbonates are the least basic of the major surface species and their formation would be expected to be favored by weak Lewis base sites in conjunction with weak Lewis acid sites. However, bicarbonate formation also requires a source of hydrogen. In the systems under study, hydrogen incorporation appears to be the result of one of four pathways. The H can come from the titanol −OH groups on the TiO2 surface. Adsorbed water molecules can solvate CO2 to form H2CO3, which can lose a proton to produce bicarbonates. Both of these pathways are consistent with observations on the Ti-NT-O2 surface. Bicarbonates can also be formed from activation of hydrogen on the Ptnanoparticles and the reaction of the activated hydrogen with a carbonate precursor. However, bicarbonates also form on the Ti-NT-O2-H2 without Pt to activate the H2 and without the O



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loss of adsorbed water or loss of the titanol −OH groups. At this point, it is not clear what the source of hydrogen is in bicarbonate formation on this surface. Though Ti3+ sites are present on the Ti-NT-O2-H2 surface, no Ti3+ is observed for the same pretreatment of the Pt-Ti-NT. This is attributed to Pt acting as an electron sink. However, the Pt-nanoparticles provide sites for activation of hydrogen to produce H-adatoms, which can react with surface carbonates to produce bicarbonates. It is interesting to note that unlike some statements in the literature, in the systems we have studied, Ti3+ sites are not essential for formation of surface carbonates (BDCs and MDCs) or BCs. However, we note that carboxylates are only observed for the Ti-NT-O2-H2, which is the only Ti-NT on which we observe formation of Ti3+. Thus, our data are consistent with carboxylate formation at Ti3+sites/O vacancies. Other groups have also linked Ti3+ sites with the formation of carboxylates.18,57−60 Nevertheless, this observation deserves additional study before a firm conclusion can be reached with regard to the possible role of Ti3+ and accompanying O vacancies in the formation of carboxylates. What we have now is a correlation rather than direct evidence for carboxylate formation at Ti3+ sites. The importance of the potential correlation between Ti3+ sites/O vacancies and CO2− is that CO2− is widely believed to be a candidate for the initial intermediate in the photochemical reduction of carbon dioxide.77 However, it is virtually certain that a strong interaction between carbon dioxide and the surface of the photocatalyst will be needed for efficient transfer of a photogenerated electron. Thus, carbon dioxide strongly interacting with a surface, which likely means CO2 that is adsorbed on the surface, will not look like gas phase CO2. Carboxylates are surface species that are structurally similar to an isolated CO2−. Thus, carboxylates are a candidate for a moiety derived from adsorbed CO2 that could be crucial in the photoreduction of carbon dioxide. The photocatalytic reduction of CO2 to hydrocarbon fuels requires a source of hydrogen. Our results indicate that Ptnanoparticles can activate hydrogen. In the current study, this activation of hydrogen leads to formation of bicarbonates. It will be interesting to determine whether this activation process leads to different and more desirable products, such as hydrocarbons, when these Ti-NTs are used as catalysts for photoreduction processes.

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No. DE-FG02-03-ER15457). The XPS work was performed in the Keck-II facility of NUANCE Center at Northwestern University. NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University. We are thankful to Junling Lu for taking the TEM and the STEM images and the EDX for us.

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ASSOCIATED CONTENT

S Supporting Information *

Results of the TPR experiments, along with some EDAX, TEM data, and Scheme S6. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Present Address ∥

Centre for Materials for Electronics Technology (C-MET), DeitY, Thrissur, Kerala 680 581, India. Notes

The authors declare no competing financial interest.

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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 P



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