FT-IR Spectroscopy in Ultrahigh-Vacuum Conditions: A Surface Science Approach to Understanding Reactions on Catalytic Oxide Powders

The defect density on a rutile (r-TiO₂) surface, which is critical for its catalytic activity, has been determined using ultrahigh-vacuum Fourier transform infrared (FT-IR) spectroscopy equipment dedicated to the spectroscopic characterization of oxides, single crystals, and powders. IR data achieved for powder particles can be compared directly for the first time with a single crystal reference system. By combining these experimental results and the density functional theory calculations, the accordant catalytic activity of the oxide surface can be deduced.

The fundamental advantage of using Fourier transform infrared (FT-IR) spectroscopy in vacuum is that the absorption of atmospheric moisture and other gas species (CO₂) is avoided. Vacuum spectrometers provide a better stability and reproducibility in comparison to dry air or nitrogen-purged spectrometers. Furthermore, their ability to operate under ultrahigh-vacuum (UHV) conditions allows IR characterization of samples that have to be prepared and stored under UHV conditions without sample-transfer in air, and, therefore, enables the close combination of the FT-IR technique with other UHV experimental methods.

The defect density on an oxide surface is crucial for its catalytic activity. Understanding the role of the defects in particular is the precondition for investigating numerous catalytic reactions in detail. Recently, the oxygen defects on titanium dioxide (TiO₂) have been considered using scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations (1–8). Unfortunately, many standard experimental techniques in this research field can be applied only to single crystalline oxides in a straightforward fashion, but not to powders or nanoparticles, which are technologically the most important forms of oxide materials. In contrast, FT-IR spectroscopy is not limited to only the single crystalline phase, but can also be applied to powders, nanoparticles, and so forth (9). Therefore, UHV-FT-IR spectroscopy was applied to both the well-understood single crystal reference system and the powder particles, to determine the density of O vacancies and to demonstrate the role of O vacancies for the surface chemistry of formaldehyde on TiO₂. For this purpose, CO was used as a probe molecule to identify the defect sites on a rutile TiO₂ (r-TiO₂) surface (10).

Experimental

A newly established UHV-FT-IR apparatus dedicated to the spectroscopic characterization of oxides, single crystals, and powders has been used in this work. It combines a Bruker Vertex 80v vacuum FT-IR spectrometer with a Prevac UHV system consisting of load-lock, preparation, distribution, measurement, magazine, and analysis chambers. The UHV system enables additional characterizations using combined measurement techniques, such as X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and thermal desorption spectroscopy. The FT-IR experiments on the r-TiO₂ single crystal and powder samples were performed in the UHV chamber dedicated for IR measurements, which was internally adapted in the sample compartment of the spectrometer. The adaptation is achieved by vacuum-compliant window flanges that are mounted directly to the window fitting of the UHV chamber. The spectrometer can be demounted from and attached to the UHV-IR chamber by sliding it backwards and forwards along its supporting frame. The system's entire optical path is evacuated to avoid absorption of atmospheric moisture and other background signals from the gas phase (CO₂), thus providing the possibility to record IR data with the highest sensibility and stability. This is an essential prerequisite for monitoring molecular species adsorbed on oxide surfaces, because the optical properties of oxides result in a decreased signal-to-noise ratio (S/N) by one to two orders of magnitude relative to metals. A spectrometer internal motorized off-axis paraboloidal mirror in front of the spectrometer sample compartment together with another motorized 90° off-axis ellipsoidal mirror adjusted to the detector can be driven in two positions for transmission (powders) and IR reflection absorption spectroscopy (IRRAS) measurements at grazing incidence with an incident angle of 80° to surface normal (for flat substrates such as single crystals) (11).

The r-TiO₂ (110) single crystalline surface was cleaned with several cycles of an Ar ion sputtering and heating procedure up to 800 K in an O₂ atmosphere. The polycrystalline r-TiO₂ powder particles (Sachtleben, 100 m²/g) were pressed against a gold-coated stainless steel grid (0.5 cm × 0.5 cm) and fixed on a sample holder. The grid was then heated at 700 K in the UHV chamber to remove contaminants such as water and OH groups. The low-vacancy (perfect) surface of the powder sample was achieved by exposing the powder to a small amount of oxygen in UHV at 110 K. The reduction was carried out by controlled sputtering or over-annealing at an elevated temperature (900 K) to produce O vacancies. To identify the defects, CO was adsorbed onto a perfect and reduced r-TiO₂ surface. The r-TiO₂ single-crystal measurements were performed using IRRAS and the data for powder samples were recorded in transmission geometry. All spectra were measured with comparable parameters, 1024 scans, and a resolution of 4 cm^-1.

Results and Discussion

Figure 1 presents a surface structure model of r-TiO₂ (10). As supported by calculations and previous works, the exposed CO molecules will populate only the adsorption sites labeled 1 and 2 in Figure 1 (and sites located further away from the O vacancy), but will not bind to the Ti atoms exposed within or next to the O vacancy (labeled 0) (12).

Figure 1: Ball-and-stick model of the r-TiO2 surface. The CO molecules adsorbed at different Ti sites (labeled 0–5) are sketched. Adapted from reference 10.

The IRRAS data shown in Figure 2 (left) demonstrate that on a fully oxidized (perfect) r-TiO₂ surface with a low density of defects only one absorption band at 2188 cm^-1 is visible in the CO-stretching regime related to CO adsorbed on perfect parts of the surface (labeled 2–5 in Figure 1), whereas on reduced r-TiO₂ a second band at 2178 cm^-1 appears, which is assigned to CO bound to Ti cations located on sites labeled 1 in Figure 1. To corroborate the assignment of the band at 2178 cm^-1, we blocked the sites close to the O defects by exposing the substrates to small amounts of O₂ at 110 K. Previous work has shown that under such conditions molecular O₂ adsorbs exclusively at oxygen vacancies (7,13,14). As expected, in the IR spectra recorded after such substrates with preadsorbed O₂ were exposed to CO at 110 K, the band at 2178 cm^-1 was found to be strongly reduced relative to the band at 2188 cm^-1. DFT calculations with the vienna ab initio simulation package (VASP) program support our deduction, that CO molecules only occupy the adsorption sites 1 and 2 labeled in Figure 1. On the basis of the intensity ratio of the two bands (at 2188 cm^-1 and 2178 cm^-1) and considering the maximum coverage of CO on r-TiO₂ at 110 K (0.5 monolayer) (15), the concentration of O vacancies can be estimated to an amount of around 10%.

Figure 2: Left: IRRAS data for CO adsorbed on perfect and reduced r-TiO2 single-crystal surface at 110 K. The absorbance amounts to 10-5 AU. Right: UHV-FT-IR data for CO adsorbed on (a) oxidized and (b) reduced r-TiO2 powder particles at 110 K recorded in transmission mode. Adapted from reference 10.

The same procedure was applied to the r-TiO₂ powder particles. The FT-IR spectra are shown in Figure 2 (right). On oxidized powder particles (Figure 2a, right) only one absorption band at 2184 cm^-1 related to CO bound to Ti cations distant to O vacancies or other defects was detected. On reduced powder particles (Figure 2b, right) a new band appears at 2174 cm^-1 related to CO adsorbed on site 1, which is in accordance with the IRRAS data for r-TiO₂ single crystal surface. Furthermore, the defect density on the powder particles can be calculated from a comparison of the relative band intensities and amounts to about 8%.

To demonstrate the potential of this established method for determining defects densities in a semiquantitative fashion, the reductive coupling of formaldehyde to ethylene catalyzed by TiO₂, an important C-C coupling reaction was investigated. IRRAS experiments on differently treated r-TiO₂ powder samples were carried out. After exposure of fully oxidized powder particles to formaldehyde at 400 K, no adsorbate-induced absorption bands could be detected (Figure 3b), indicating that CH₂O adsorbed at perfect Ti cation sites is not stable at 400 K. In contrast, UHV-IR results recorded for reduced r-TiO₂ powder particles after exposure to formaldehyde at the same temperature reveal a distinct IR band at 1040 cm^-1 (Figure 3c), which can be assigned on the basis of the previous data for single-crystal r-TiO₂ (110) surface to the C-O stretching vibration within a surface-bound C₂ diolate species (-OCH₂CH₂O-) formed by activation of formaldehyde at O vacancy sites. Upon heating to 600 K, the diolate-related IR band disappears (Figure 3d) as a result of the ethylene formation and the subsequently desorption from the particle surface (16). Only on the reduced powder was the intermediate of the catalyzed surface reaction identified and the yield of ethylene correlates well with the above estimated density of defects on the r-TiO₂ powder particles.

Figure 3: UHV-FT-IR data obtained for r-TiO2 powder samples. (a) Clean r-TiO2 powder. Powder was (b) oxidized and then exposed to formaldehyde (1 × 10-4 mbar) at 400 K; (c) reduced and then exposed to formaldehyde (1 × 10-4 mbar) at 400 K. (d) The sample form (c) was heated to 600 K before the spectrum was measured. All UHV-FT-IR spectra were collected in transmission mode at 110 K. Adapted from reference 10.

Conclusion

Combined experimental and computational investigation methods were used to estimate the defect density on a rutile surface, which indicates its catalytic activity. UHV-IR data from both single crystal and powder r-TiO₂ have been collected and compared with each other for absorption bands assignment. The design of the UHV-FT-IR apparatus enabled comparable measurement conditions in both reflection (for single crystal) and transmission (for powder) modes and demonstrated that it can be used in developing methods for the determination of defect densities on catalyst surfaces in a semiquantitative manner.

The reliability of this method for catalyst activity forecasting has been successfully demonstrated by the C-C coupling reaction catalyzed by TiO₂. The reduced catalyst powder with more defects, or rather O vacancies, was proven to exhibit higher activity. Furthermore, the product yield is directly related to the estimated density of defects.

Acknowledgment

We greatly thank Professor Dr. Christof Wöll (director of Institute of Functional Interfaces, KIT) and his coworkers for the measurements data and the helpful discussion, whom contributed a lot to the optimization of the UHV-FT-IR apparatus with their valuable feedback and suggestions from the application side.

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X. Stammer is a chemist at Bruker Optik GmbH, in Ettlingen, Germany.

S. Heißler is a lab leader with the Karlsruher Institut für Technologie (KIT) in Eggenstein-Leopoldshafen, Germany.

Direct correspondence to: Xia.Stammer@brukeroptics.de