NO Reaction Pathways on Rutile TiO2(110): The Influence of Surface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

NO Reaction Pathways on Rutile TiO(110): The Influence of Surface Defects and Reconstructions Yunjun Cao, Min Yu, Shandong Qi, Zhengfeng Ren, Shi-shen Yan, Shujun Hu, and Mingchun Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06135 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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The Journal of Physical Chemistry

NO reaction pathways on rutile TiO2(110): the influence of surface defects and reconstructions

Yunjun Cao, Min Yu, Shandong Qi, Zhengfeng Ren, Shishen Yan, Shujun Hu* and Mingchun Xu*

School of Physics, Shandong University, 27 Shanda Nanlu, Jinan, Shandong, 250100, P. R. China

Corresponding author: *E-mail: S. Hu, [email protected]; *E-mail: M. Xu, [email protected].

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Abstract TiO2 exhibits excellent catalytic performance in degrading NO to N2O or N2. However, up to now, the detailed reaction pathways of NO on TiO2 surfaces are still debatable. In this paper, we studied NO adsorption and reactions on differently treated rutile TiO2(110) surfaces by using polarization/azimuth-resolved infrared reflection absorption spectroscopy (IRRAS). It is found that the surface defects (the oxygen vacancies (Vo)) and reconstructions on TiO2(110) have strong effect on the reaction pathways of NO → N2O conversion. The simplest pathway occurs on the defect-free oxidized TiO2(110) surface that two NO molecules adsorbed on adjacent surface Ti (Ti5c) sites first couple to the cis-(NO)2/Ti&Ti dimer though N-N bond and then convert to N2O species. On the moderately reduced TiO2(110)-(1×1) surface, due to the presence of surface Vo and the resulting polaron, two NO molecules adsorbed respectively on Vo sites and adjacent Ti5c sites couple to the trans-(NO)2/Ti&Vo dimer and then convert to N2O before the cis-(NO)2/Ti&Ti dimers occur. On the highly reduced quasi-TiO2(110)-(1×2) surface, however, the Ti2O3 row fragments hamper the conversion of trans-(NO)2/Ti&Vo → N2O and thus hamper the subsequent cis-(NO)2/Ti&Ti formation without polaron. In this case, the conversion of both the trans-(NO)2/Ti&Vo dimer and the isolated NO monomer to N2O is likely to be triggered by the gas NO impingement. The structure-reactivity relationship we proposed is helpful in understanding the catalytic mechanism of NO degradation on TiO2 surfaces.

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Introduction With the ever growing number of fossil-fuel automobiles and the development of industrial activities, the greatly elevated concentration of nitrogen oxides (NOx, such as NO and NO2) in the atmosphere causes the permanent damage to human lung tissues and the frequent formation of acid rain. How to remove or convert the nitrogen oxides to nontoxic species becomes one of the key issues in environment protection fields.1-2 As one of the environment friendly catalysts, TiO2 exhibits excellent catalytic performance in degrading NO to N2O or N2.3-5 In order to explore the NO degradation reaction (i.e., the NO → N2O conversion) mechanism at the atomic/molecular level, the model catalysts with atomically well-defined structures such as single crystal TiO2 surfaces instead of actual TiO2 powder catalysts have been applied.6-7 As early as 1994, based on temperature-programmed desorption (TPD) measurements, Lu et al.8 reported that the NO reduction product, N2O, formed only on defective surfaces and its amount was proportional to the density of surface oxygen vacancies (Vo) on reduced rutile TiO2(110) single crystal surfaces. Later, in 2000, Sorescu et al.9 theoretically investigated the formation of the cis-(NO)2 dimer in bidentate adsorption configuration during the NO → N2O conversion on oxidized TiO2(110) single crystal surfaces. Afterwards, in 2013, based on ultrahigh vacuum Fourier transform infrared spectroscopy (UHV-FTIRS) experiments on powdered TiO2 and large embedded cluster calculations, Stodt et al.10 proposed a monodentate (NO)2 dimer configuration at low NO dosage before the bidentate (NO)2 dimer

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formation. Meanwhile, by using TPD method, Kim et al.11 reported that the NO molecules adsorbed on TiO2(110) surfaces easily convert to N2O at the temperature as low as 50 K, and proposed that the surface charge rather than the Vo site plays the dominant role in NO reduction process. However, up to now, the knowledge of NO reaction pathways on TiO2 surfaces and the corresponding influence factors is still insufficient.7 It is known that the complicated reaction processes of NO on TiO2 surfaces are undoubtedly correlative to the high reactivity of NO on account of its unpaired 2π* electron.12-13 However, such correlativity is evidently affected by the treatment conditions for catalysts.14 The different treatment conditions may cause different surface defects or reconstructions, and thus affect the reaction process of NO. This provides us an idea to explore the reaction mechanism of NO degradation reactions, that is, by identifying each reaction pathway of NO→N2O conversion on the differently treated TiO2(110) surfaces to establish the structure-reactivity relationship. To achieve this aim, the clear identification of adsorbates, including reactants, intermediate species and reaction products on different TiO2(110) surfaces is vital. Infrared spectroscopy technique is a suitable method to identify the surface adsorbate species by detecting the vibrational absorption signals.15-16 However, on single crystal insulator or semiconductor surfaces, such as TiO2(110), it is a formidable task to detect the very weak absorption signals of adsorbates until the coming forth of the high sensitivity of UHV-FTIRS system.17 In virtue of the high signal/noise ratio of the UHV-FTIRS system, not only the adsorbate species can be

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identified, but on single crystal oxide surfaces in particular, the geometrical configurations of adsorbate species can be determined also18-20 through the polarized infrared reflection absorption spectroscopy (IRRAS) measurement mode and the corresponding surface selection rule on oxide surfaces.21 Using the UHV-FTIRS system, we have studied NO adsorption on reduced TiO2(110)-(1×1) surfaces at the very initial stage and observed two kinds of NO monomer adsorptions at very low NO dosage. Combined with density functional theory (DFT) calculations, we gave a clear scenario of polaron involved NO adsorption.22 In this paper, we reported the systematic studies of NO → N2O conversion on single crystal TiO2(110) surfaces by using UHV-FTIRS and DFT calculations. Through contrastive studies on oxidized TiO2(110)-(1×1) surfaces, moderately reduced TiO2(110)-(1×1) surfaces and highly reduced quasi-TiO2(110)-(1×2) surfaces, the complete structure-reactivity relationship for NO on TiO2(110) surfaces was given for the first time. Two new NO reaction pathways on differently reduced TiO2(110) surfaces were proposed, and the crucial role of surface Vo (or polarons) and surface reconstructions in NO reduction reactions was clarified, which will be helpful in understanding the NO→N2O conversion mechanism on TiO2(110) surfaces.

Experimental and theoretical methods The experiments were carried out in a UHV-FTIRS system combining a vacuum FTIR spectrometer (Bruker, VERTEX 80V) and a multichamber UHV system (PREVAC) with the base pressure better than 6 × 10-11 mbar, as described in detail

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previously.20 The clean rutile TiO2(110) surface (10×10×2 mm, Mateck) was prepared by cycles of Ar+ sputtering and annealing at 800 K in UHV conditions until a clear (1×1) low energy electron diffraction (LEED) pattern was obtained and no any impurities can be detected by Auger electron spectroscopy (AES) on surfaces. The oxidized TiO2(110)-(1×1) surface was prepared by annealing the clean surface in O2 atmosphere (5×10-7 mbar) at 700 K for 10 min and then cooling in the O2 atmosphere to room temperature, while the reduced TiO2(110)-(1×1) surface was prepared by UHV annealing the clean surface at 800 K for 5 min. After hundreds of cycles of sputtering and UHV annealing treatments, the highly reduced quasi-TiO2(110)-(1×2) surface was obtained detected by the LEED pattern. The IR measurements were performed in IRRAS mode with a fixed incidence angle of 80o, and the p- and s-polarized incidence light was along both [110] and [001] crystallographic directions respectively. The optical path inside the IR spectrometer and the space between the UHV chamber and the spectrometer were evacuated in order to avoid any unwanted IR adsorption from gas phase species. The IRRA spectra were recorded at 90 K with 4096 scans at 4 cm-1 resolution. High purity NO (99.9%), CO (99.99%) and O2(99.999%) were dosed via backfilling in experiments, where exposures are quoted in Langmuir (1 L=1.33×10-6 mbar·s). The vibration direction of adsorbate species was determined by the polarization- and azimuth-resolved IRRAS results combined with the surface selection rule of IRRAS on oxide surfaces.21 First-principles calculations were performed using the Vienna ab-initio simulation package (VASP)23 with a cut-off energy of 500 eV for the basis set. Γ-point

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was used for Brillouin zone sampling. The projector-augmented wave method (PAW)24 with the PBE type25 exchange–correlation potentials was adopted. To correctly model the localization of excess electrons in rutile TiO2,26 the DFT+U method with UTi:3d = 4.2 eV27 was employed. Furthermore, a van der Waals dispersion correction based on the D2 method was included in the calculation.28 The finite-displacement approach was used to determine the vibrational frequencies without scaling. To model the TiO2(110) surface, the experimentally determined lattice parameters of bulk rutile TiO2 (a = 4.594 Å and c/a = 2.959)29 were used to build a slab, including four tri-layers and a vacuum layer with a thickness of 15 Å, which models the TiO2(110) surface. A supercell with a p(4×2) geometry along 001 and [110] directions, respectively, was employed to perform the first-principles calculations. The atomic coordination of top three tri-layers was optimized until the forces are less than 0.02 eV/Å, while the bottom tri-layer were fixed at bulk positions and were terminated by pseudo-hydrogen atoms.30-31 To manipulate the location of polaron, a preliminary lattice-distortion approach was employed,32 where the bonds between the targeted Ti cation and its neighboring O anion were initially stretched by ~0.1 Å before the structural optimization. Detailed crystal structures of the simulated supercell, such as the adsorbing configuration, the oxygen defects, etc., will be described along with the corresponding results.

Results and discussions 1. NO adsorption and reaction on oxidized TiO2(110)-(1×1) surfaces

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The polarization- and azimuth-resolved IRRAS results of NO adsorption on the defect-free oxidized TiO2(110) surface with different NO dosages at 90 K are shown in Figures 1a-1d. At very low dosage (0.02 L), one negative band at 1875 cm-1 and one positive band at 1748 cm-1 have already occurred for the p-polarized light along [100] direction shown in Figure 1b. Correspondingly, only one negative 1875 cm-1 band appears for the p-polarized IR beams along the [11 0] direction in Figure 1a and one negative 1748 cm-1 band appears for the s-polarized IR beams along the [110] direction in Figure 1c, but no signal is observed for the s-polarized IR beams along the [001] direction in Figure 1d. The typical absorption bands of NO adsorption and reactions are all listed in Table 1 for the p-polarized IR beams along the [110] and [001] directions. In our experiments, the incident angle (80°) is higher than the Brewster angle of TiO2 (67°). The surface selection rule of IRRAS on oxide surfaces lets us know that on oxide surfaces, when the incident angle is higher than the Brewster angle of the substrate, the sign of IRRA band in s-polarized spectra shows negative, i.e., the reflectivity of s-polarized light increases on the absorbate-covered surface in comparison with that on the clean substrate. 21 While for p-polarized light, the sign of IRRA band shows positive for the molecular vibration parallel to the in-plane (x) electric field component and negative for the molecular vibration parallel to the out-of-plane (z) electric field component.21 According to the selection rule21 and the polarization/azimuth-resolved anisotropic IRRAS results, we can exactly clarify the vibration modes and the

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corresponding configurations of (NO)2 dimers on oxidized TiO2(110) surfaces from the incident geometries of IR beam in Figures 1e and 1f: the negative 1875 cm-1 band in Figures 1a and 1b corresponds to the out-of-plane vibration; the 1748 cm-1 band with opposite signs in Figures 1b and 1c corresponds to the in-plane vibration along [001] direction. These results confirm the presence of the cis-(NO)2 dimer,33-34 which adsorbs on the oxidized TiO2(110) surface in bidentate configuration along [001] direction. In other words, two NO molecules adsorbing on adjacent Ti5c sites couple though N-N bond,9 as shown in Figures 1g and 1h, denoted as cis-(NO)2/Ti&Ti dimer. For such configuration, the negative band at 1875 cm-1 and the positive band at 1748 cm-1 were assigned to the symmetric and asymmetric vibrations of cis-(NO)2 dimer respectively.33-34 On the other hand, the component of the stretching vibration along [110] direction is absent, thus none IR absorption signal was detected for s-polarized light along 001 direction, as shown in Figure 1d. In Figures 1a and 1b, with increasing NO dosage, the 1875 cm-1 and 1748 cm-1 bands decrease in intensity and finally disappear. Meanwhile, a negative band at 2243 cm-1 occurs for the p-polarized light along both [110] and [001] directions, which corresponds to the N2O molecule on Ti5c sites with vertical configuration.10,34 Such results indicate that the cis-(NO)2/Ti&Ti dimer has converted to N2O species with increasing NO dosage. At NO dosage of 40 L, besides the negative 2243 cm-1 band, a new positive 2233 cm-1 band occurs for p-polarized light along [110] direction. For the s-polarized light along [001] direction, the 2233 cm-1 band exhibits negative value in Figure 1d. According to the IRRAS selection rule on oxide surfaces, both 2233

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cm-1 bands correspond to the vibration of N2O along [110] direction. DFT calculations reveal that the horizontal N2O molecule along [110] direction is caught above two neighboring bridge oxygen (Obr) as shown in Figure S1d. Such N2O horizontal adsorption on Obr sites does not occur until the vertically adsorbed N2O on Ti5c nearly reaches saturation, which is similar to the case of CO2 adsorption on TiO2(110) surfaces in our previous reports.20 It should be noted that the 1875 cm-1 and 1748 cm-1 bands always occur in pairs at each NO dosage, and none isolated IR band of NO monomers (~1870 cm-1) was observed all the time. Such results indicate that the lifetime of the isolated NO monomer on oxidized TiO2(110) surfaces is too short to be detected in our experiments. Once the gas NO molecule adsorbs on the oxidized TiO2(110) surface, it instantaneously migrates along Ti5c rows and couples with another mobile NO forming a cis-(NO)2/Ti&Ti dimer. As the density of the dimers increasing, the dimers gradually convert to N2O molecules and vertically adsorb on Ti5c sites. Meanwhile, the released oxygen atoms adsorb on Ti5c sites, which are proven to be stable below 393 K by previous STM studies.35 Additionally, in our previous study of Au clusters grown on TiO2(110),36 we found that the Au atoms at the interface were oxidized to positive charge state after NO → N2O conversion, also confirming the existence of oxidative oxygen atoms on TiO2(110) surfaces. Based on above results, we present the whole reaction pathway of NO on oxidized TiO2(110) surfaces as follows: NO + NO → cis-(NO)2/Ti&Ti,

(1.1)

cis-(NO)2/Ti&Ti → N2O/Ti + Oa,

(1.2)

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N2O/Ti → N2O/Obr.

(1.3)

The ball-stick schematics of the above reaction pathway are shown in Figure S1 (Supporting Information). 2. NO adsorption and reaction on moderately reduced TiO2(110)-(1×1) surfaces After cycles of sputtering and UHV annealing treatments, 5% - 10% of surface bridging oxygen atoms are removed, that is, the density of surface oxygen vacancies (Vo) is 5% - 10% on the moderately reduced TiO2(110) surface.37-38 To clarify the role of the surface Vo in NO reduction, NO adsorption and reaction on reduced TiO2(110) surfaces at 90 K was studied. The corresponding p-polarized IRRAS results are shown in Figures 2a-2b. Compared to oxidized TiO2(110) surfaces, several new absorption bands occur on the reduced TiO2(110) surface, indicating the occurrence of new adsorbed species. At NO dosage of 0.01 L, two negative bands at 1624 cm-1 and 1750 cm-1 appear for p-polarized light along both [110] and [001] directions, respectively, which correspond to the out-of-plane vibration of N-O stretching. Combined with the NO/CO co-adsorption experiments and DFT calculations, we have ascribed these two bands to the isolated NO monomers adsorbed on Vo sites (denoted as NO/Vo(pol)) and Ti5c sites near Vo (denoted as NO/Ti(pol)) respectively.22 Here, the “pol” in the bracket denotes one polaron being involved in the adsorption. In such adsorption configurations, the subsurface polaron electron, introduced by the surface Vo, transfers to the Ti:3d-NO:2p hybrid orbital on NO, leading to the large redshifts of vibration frequencies of NO.22 In contrast to that on oxidized TiO2(110) surfaces, the polarons on reduced TiO2(110) surfaces enhance the binding energy of NO/Ti(pol)

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and weaken the mobility along Ti5c rows, thus stabilizing the isolated NO monomers at very low coverage. Increasing NO dosage to 0.04 L, for p-polarized light along [110] direction shown in Figure 2a, both 1624(−) and 1750(−) cm-1 bands are weakened, meanwhile, a pair of bands at 1783(−) cm-1 and 1622(+) cm-1 emerge (the subscript +/− denote the positive/negative band here and hereinafter). For p-polarized light along [001] direction at the same time, only one 1783(−) cm-1 band was observed, as shown in Figure 2b. During such evolution, since none desorption occurs, all the NO related to 1624(−) cm-1 and 1750(−) cm-1 bands should evolve to the NO related to new 1783(−) and 1622(+) cm-1 bands with increasing NO coverage. Additionally, if the clean reduced TiO2(110) surface was first exposed to 2 L O2 prior to NO adsorption, all above bands will not appear (data not shown here), which further confirms that all these bands are associated with the surface Vo or the polarons. Next we try to clarify the corresponding NO species corresponding to the out-of-plane 1783(−) cm-1 vibration and in-plane 1622(+) cm-1 vibration that occurred at 0.04 L NO dosage. Firstly, considering that the two bands develop synchronously in both adsorption and desorption processes (see Supporting Information, Figure S2), it is reasonable to speculate that they originate from the same adsorption configuration. Secondly, since all the NO related to 1624(−) and 1750(−) cm-1 bands evolve to the NO related to new 1783(−) and 1622(+) cm-1 bands as NO coverage increasing, it is also reasonable to speculate that the subsequently adsorbed NO/Vo or NO/Ti respectively couple to the adjacent NO/Ti(pol) or NO/Vo(pol), forming the (NO)2

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dimer around the Vo at higher NO dosage (0.04 L). To verify our hypothesis, the configuration that one NO adsorbs on one Vo site and the other adsorbs on the nearest neighboring Ti5c site was simulated by DFT calculations. The optimized structure is shown in Figures 3a and 3b, where one NO binds to Vo sites via N atom and tilts to the adjacent NO binding to Ti5c sites, forming an irregular trans-(NO)2 dimer, denoted as trans-(NO)2/Ti&Vo dimer. The corresponding calculated frequencies are 1819 and 1621 cm-1 for the symmetric and asymmetric vibration modes, respectively, consistent with the 1875 cm-1 and 1625 cm-1 in experiments. Furthermore, the calculated symmetric vibration mode is along out-of-plane direction and the asymmetric vibration mode is along [110] direction, also in accordance with the vibration modes inferred from IRRA spectra. The good agreement between the calculations and the IRRAS results supports our hypothesis, that is, two adjacent NO/Vo and NO/Ti with only one polaron couple to a trans-(NO)2/Ti&Vo dimer. The calculated charge transfer density isosurface of the trans-(NO)2/Ti&Vo dimer is shown in Figure 3c, where the yellow and the green isosurfaces denote the electron accumulation and loss respectively. For bare reduced TiO2(110) surfaces, two polarons are respectively located at two subsurface Ti6c sites near the Vo. While for the TiO2(110) surface with trans-(NO)2/Ti&Vo dimer formation, one polaron (solid blue circle) still retains in its initial subsurface Ti6c site, whereas the other one (dashed green circle) has transferred to the Ti:3d-NO:2p hybrid orbital. It is worthwhile to note that only one polaron electron involves in the trans-(NO)2/Ti&Vo dimer

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adsorption, in contrast to the case of isolated NO monomers that each NO on the Vo site or on Ti5c site captures one polaron electron.22 As continuously increasing NO dosage to 1 L, all the Vo-related bands almost disappear, instead, the 1872(−) cm-1 and 1752(+) cm-1 bands occur in Figure 2b and the 1872(−) cm-1 band also occurs in Figure 2a. These bands are similar to those on oxidized TiO2(110) surfaces, revealing the occurrence of the cis-(NO)2/Ti&Ti dimers. Further increasing NO dosage to 40 L, the 1872(−) cm-1 and 1752(+) cm-1 bands disappear, while the 2243(−) cm-1 and 2233(+) cm-1 bands are enhanced simultaneously, indicating that all NO species have converted to N2O molecules adsorbed on Ti5c and Obr sites. Obviously, in this case, the evolution of NO at high dosage is similar to that on oxidized TiO2(110) surfaces. Accordingly, we propose the reaction process on reduced TiO2(110) surfaces as follows. At very low coverage, the isolated NO adsorbs either on Vo sites or on nearby Ti5c sites as monomer, thus there exist two possible reaction processes. (i) For the pre-adsorbed NO/Vo(pol), with increasing NO coverage, the subsequently adsorbed NO on Ti5c sites can easily migrate near to the pre-adsorbed NO/Vo(pol) and couple to a trans-(NO)2/Ti&Vo dimer. (ii) For the pre-adsorbed NO/Ti(pol), the subsequently adsorbed NO easily occupies the empty Vo site and simultaneously captures another polaron forming a new NO/Vo(pol); almost at the same time the pre-adsorbed NO/Ti(pol) releases a polaron and transfers toward the new NO/Vo(pol) forming a trans-(NO)2/Ti&Vo also. Further increasing NO coverage, the trans-(NO)2/Ti&Vo dimer converts to N2O on the Ti5c site and the released oxygen atom heals the Vo.

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Since the polarons are introduced by the Vo, the healing of the Vo means that the polaron electrons are captured again by the oxygen filling the Vo. Such results have also been proved by previous electron energy loss spectroscopy (EELS) studies.39 In this case, the subsequent NO adsorption and reaction process on the reduced TiO2(110) surfaces will be similar to that on oxidized TiO2(110) surfaces. According to above description, we present the whole NO reaction pathway on moderately reduced TiO2(110)-(1×1) surfaces as follows: (i) NO → NO/Vo(pol),

(2.1)

NO/Vo(pol) + NO/Ti → trans-(NO)2/Vo&Ti(pol);

(2.2)

(ii) NO → NO/Ti(pol),

(2.3)

NO/Ti(pol) + NO/Vo(pol) → NO/Ti + NO/Vo(pol) → trans-(NO)2/Vo&Ti(pol),

(2.4)

trans-(NO)2/Vo&Ti(pol) → N2O/Ti + Obr,

(2.5)

NO + NO → cis-(NO)2/Ti&Ti → N2O/Ti + Oa,

(2.6)

N2O/Ti → N2O/Obr.

(2.7)

The ball-stick schematics of above reaction pathway are shown in Figure S3 (Supporting Information). Furthermore, we also studied the effect of UV irradiation on the Vo-related NO species to explore the role of surface Vo in NO photoreduction reaction.5 The corresponding IRRAS results are shown in Figure 4. At 0.01 L NO dosage, the 1750 cm-1 band (NO/Ti(pol)) was completely eliminated under 5 min UV irradiation, but the 1624 cm-1 band (NO/Vo(pol)) survived and even was slightly enhanced. At 0.04 L

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NO dosage, both 1783 cm-1 and 1622 cm-1 bands (trans-(NO)2/Ti&Vo dimer) were eliminated under 5 min UV irradiation. The appearance of the weak 2240 cm-1 band after UV irradiation indicates that partial missing NO monomers or dimers converted to N2O.33 It is known that UV irradiation can easily trigger the (NO)2 → N2O conversion on TiO2(110) surfaces.9 Our present results show that the NO/Ti(pol) and the trans-(NO)2/Vo&Ti dimer can be removed by UV irradiation, and the NO/Vo(pol) can surprisingly survive. For the isolated NO/Ti(pol), we propose that UV irradiation can decouple the polaron from NO/Ti(pol) and thus promote NO/Ti mobility along Ti5c rows. This is consistent with the theoretical predicated “roll-over” diffusion scenario of NO on Ti5c rows of TiO2(110) surfaces.40 When one NO/Ti meets another NO/Ti, under UV irradiation, they can immediately convert to N2O probably via the intermediate cis-(NO)2/Ti&Ti dimer. However, for the NO/Vo(pol) species, its relatively higher binding energy may suppress the effect of UV irradiation. Additionally, during the NO/Ti diffusion under UV irradiation, some NO will be trapped by the empty Vo sites and thus increase the amount of NO/Vo(pol). 3. NO adsorption and reaction on highly reduced quasi-TiO2(110)-(1×2) surfaces After hundreds of cycles of sputter/annealing treatments followed by UHV annealing at 900 K, the TiO2(110) surface has become highly reduced. The high deficiency of surface oxygen may reconstruct the surface structure, which was probed by electron diffraction methods shown in Figures 5a-5f. In contrast to the clear (1×1)

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LEED spots of the moderately reduced TiO2(110) surface (Figure 5a), the obvious streaks interconnecting the LEED spots along [110] direction were observed for the highly reduced TiO2(110) surface (Figure 5d). Onishi et al. reported41-42 that after annealing the TiO2(110) surface at 880 K, only the clear (1×1) LEED spots were observed; after annealing at 1150 K, however, new diffraction spots formed between the LEED spots along [110] direction, denoting the occurrence of the (1×2) reconstructed structure. Coincidentally, a recent STM study observed the isolated micro (1×2) stripes composed of the Ti2O3 row fragments on the cycles of sputter/annealing (1000 K) treated TiO2(110) surface.43 Based on the reported results,41-42 the presence of the streaks between the LEED spots along [110] direction of our sample indicates the formation of the Ti2O3 row fragments, rather than the well defined (1×2) reconstructed structure, on our highly reduced TiO2(110) surface.43 The structural dependence on the annealing temperature also supports our speculation. In the following we donate such surface as the quasi-TiO2(110)-(1×2)

surface.

Such

three-dimensional

(3D)-like

surface

reconstructions were further detected by reflection high-energy electron diffraction (RHEED), as shown in Figures 5e and 5f. Compared to moderately reduced TiO2(110) surfaces, the RHEED streaks of the highly reduced quasi-TiO2(110)-(1×2) surface along [110] direction are much fainter, and along [001] direction the RHEED pattern exhibits the brighter spot-like streaks. Such results further confirm the occurrence of the 3D-like structures on the surface. The density of Vo on moderately and highly reduced TiO2(110) surfaces was

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detected by CO titration experiments at 90 K. The IRRA spectra of CO adsorbed on moderately reduced TiO2(110)-(1×1) surfaces are shown in Figure 6a. Two bands at 2184 cm-1 (Band I) and 2175 cm-1 (Band II) were observed, corresponding to CO adsorbed on the regular Ti5c sites and the next-nearest neighbor Ti5c sites adjacent to Vo respectively.21, 44 Based on our previous studies, the ~9 cm-1 redshift of Band II relative to Band I results from the polaron involvement at surface Ti5c sites underneath CO.44 Basically, the intensities of saturated Bands I and II are proportional to the numbers of regular exposed Ti5c sites and Vo sites. On the highly reduced quasi-TiO2(110)-(1×2) surface, due to the lack of exposed Ti5c sites, the Ti2O3 row structures are inert to CO adsorption, so the formation of Ti2O3 structures decrease the numbers of the surface Ti5c sites, including the regular Ti5c sites and the Ti5c sites next-nearest to Vo. However, on the highly reduced quasi-TiO2(110)-(1×2) surface, the area of Band I is reduced by nearly half while Band II almost keeps constant, shown in Figure 6b. This indicates that the number of surface regular Ti5c sites is significantly decreased while the number of Vo almost keeps constant. That is to say, the Vo density on the exposed TiO2(110) surface has significantly increased. Based on above knowledge of surface structures, we studied NO adsorption and reaction on the clean quasi-TiO2(110)-(1×2) surface, the corresponding IRRA spectra are shown in Figures 7a-7b. At 0.01 L NO dosage, both 1623(−) cm-1 and 1750(−) cm-1 bands appear for p-polarized light along [110] and [001] directions. Increasing NO dosage to 0.04 L, the 1783(−) cm-1 and 1620(+) cm-1 bands appear for p-polarized light along [110] direction in Figure 7a, while only one 1783(−) cm-1 band occurs

18


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along [001] direction in Figure 7b. Similar to the reduced TiO2(110)-(1×1) surfaces, these bands originate from the NO/Vo(pol), NO/Ti(pol) monomers and the trans-(NO)2/Vo&Ti dimer, respectively. Further increasing NO dosage, however, it is strangely found that most of trans-(NO)2/Vo&Ti dimers still exist even for 40 L NO dosage, whereas they are largely removed by UV irradiation. In addition, at 1 L dosage, a band at 1872 cm-1 occurs along [110] direction, but its counterpart 1750 cm-1 band for the (NO)2/Ti&Ti dimer as that on TiO2(110)-(1×1) surfaces does not appear. Such results suggest that the NO monomers should exist on Ti5c sites in this case. According to the IRRAS selection rule on oxide surfaces21 and the adsorption anisotropy, we propose that the NO molecule adsorbs as the tilted configuration along [001] direction, as shown in Figure S4. For this tilted configuration, the positive absorption signal of the in-plane vibration component just compensates with the negative out-of-plane vibration component, resulting in the absence of absorption feature along [001] direction. To rule out the effect of polarons on the NO monomer adsorption, we further studied NO adsorption on the oxidized quasi-TiO2(110)-(1×2) surfaces. Such oxidized surface was obtained by flashing the highly reduced quasi-TiO2(110)-(1×2) surface with saturated NO to 220 K under UHV conditions to desorb the NO and N2O. On such surface, all the Vo are healed by O atoms generated during NO→N2O conversion. As shown in Figures 8a-8b, with increasing NO dosage, except for the 2243 cm-1 band at higher dosage, only the 1872 cm-1 band is observed for the p-polarized IR light along [110] direction, meaning that the NO still adsorbs as monomer with the titled

19



configuration on such surfaces under the conditions without the presence of polarons. Accordingly, we proposed NO adsorption and reaction processes on the highly reduced quasi-TiO2(110)-(1×2) surfaces as follows: with increasing NO dosage, NO molecules first adsorb as NO/Vo(pol) or NO/Ti5c(pol), then couple to the trans-(NO)2/Vo&Ti dimers; afterwards, the later NO adsorbs on regular Ti5c sites as the tilted monomer configuration. In this case, the NO→N2O conversion efficiency is significantly lowered for both the trans-(NO)2/Vo&Ti dimers and the NO monomers without UV irradiation, and the intermediate cis-(NO)2/Ti&Ti dimer was not observed during the whole conversion process. We consider that the specific NO reaction pathway on quasi-TiO2(110)-(1×2) surfaces is caused by the spatial limitation of the randomly distributed Ti2O3 row fragments. The Ti2O3 row fragments divide the long Ti5c rows into short ones, and the isolated trans-(NO)2/Vo&Ti dimers further divide the short bare Ti5c rows into smaller areas, both inhibiting the formation of cis-(NO)2/Ti&Ti dimers. On the other hand, the distribution of NO/Ti monomers and trans-(NO)2/Vo&Ti dimers is rather sparse, which hinders the routine conversion to N2O species mentioned above. In this case, to explain the subsequent N2O formation, the gas NO impingement mechanism was proposed. For the NO monomer, when the gas NO from vacuum impinges the NO monomer, they may directly overcome the barrier and immediately convert to N2O species via Eley-Rideal mechanism.45-46 Similarly, for the (NO)2/Vo&Ti dimer, the conversion to N2O here is also activated by the gas NO impingement. In this case, the gas NO only provides the necessary energy by impingement for the (NO)2/Vo&Ti

20


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dimer to overcome the barrier for conversion to N2O, instead of directly joining the conversion. On the oxidized quasi-TiO2(110)-(1×2) surface, the Vo sites are all healed and some surface Ti5c sites are also covered by Oa atoms, but the Ti2O3 row fragment distribution remains the same to that on the reduced quasi-TiO2(110)-(1×2) surface. In spite of the absence of the trans-(NO)2/Vo&Ti dimer in this case, the adsorbed Oa atoms and the randomly distributed Ti2O3 row fragments still prevent the formation of cis-(NO)2/Ti&Ti dimer. To

sum

up,

the

NO

reaction

pathway

on

the

highly

reduced

quasi-TiO2(110)-(1×2) surface is presented as follows:

or

NO → NO/Vo(pol),

(3.1)

NO/Vo(pol) + NO/Ti → cis-(NO)2/Vo&Ti(pol);

(3.2)

NO → NO/Ti(pol),

(3.3)

NO/Ti(pol) + NO/Vo → cis-(NO)2/Vo&Ti(pol).

(3.4)

NO impingement

cis-(NO)2/Vo&Ti(pol) N2O/Ti + Obr, NO impingement

NO/Ti + NO N2O/Ti + Oa.

(3.5) (3.6)

And on the oxidized quasi-TiO2(110)-(1×2) surfaces, the pathway is as follows: NO → NO/Ti,

(3.7)

NO impingement

NO/Ti +NO N2O/Ti + Oa.

(3.8)

The ball-stick schematics of the above reaction pathway are shown in Figure S4 (Supporting Information).

21



Page 22 of 44

Conclusions In summary, NO adsorption and reactions on differently treated rutile TiO2(110) surfaces were studied by using UHV-FTIRS. The achieved structure-reactivity relationship of NO → N2O conversion on TiO2(110) surfaces demonstrates that the reaction pathway strongly depends on the surface defects (Vo) and reconstructions. (i) On the defect-free oxidized TiO2(110) surface, the NO molecules first form the cis-(NO)2/Ti&Ti dimer on adjacent Ti5c cations and then convert to N2O with increasing NO dosage. (ii) On the moderately reduced TiO2(110)-(1×1) surface, the NO molecules first adsorb as monomers on Vo or Ti5c sites due to polaron involvement, then transform to irregular trans-(NO)2/Ti&Vo dimers adsorbed on the adjacent Vo and Ti5c sites. Afterwards, the (NO)2→N2O conversion occurs healing the surface Vo. (iii) However, on the highly reduced quasi-TiO2(110)-(1×2) surfaces, the randomly

distributed

Ti2O3

row

fragments

hamper

the

conversion

of

trans-(NO)2/Ti&Vo to N2O, and thus the stable trans-(NO)2/Ti&Vo dimers hamper the formation of cis-(NO)2/Ti&Ti dimer, instead, the NO monomers adsorb on Ti5c sites with a tilted configuration without polarons. In this case, both the trans-(NO)2/Ti&Vo dimer and the isolated NO monomer conversion to N2O is probably triggered by the gas NO impingement. Our studies will be helpful in clarifying the catalytic mechanism of NO degradation on TiO2 surfaces.

Notes The authors declare no competing financial interest.

22


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Acknowledgements This work was supported by the National Science Foundation of China (Grant No. 21273132), Shandong Provincial National Science Foundation, China (Grant No. ZR2018MA041) and 111 project B13029. Supporting Information available: Additional data on the ball-stick schematics of NO reaction pathway and IRRA spectra of NO desorption processes on TiO2(110) surfaces are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

23



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TOC Graphic

27



Figure 1. a-d) IRRA spectra of NO adsorbed on oxidized rutile TiO2(110) surfaces at 90 K as a function of NO dosage. a) p-polarized IR beams along the [110] direction; b) p-polarized IR beams along the [001] direction; c) s-polarized IR beams along the [110] direction; d) s-polarized IR beams along the [001] direction. e-f) The illustration of incident geometry of IR beams coupled with different molecular vibration modes. g-h) The vibration modes of the bidentate cis-(NO)2/Ti&Ti dimer.

28


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Figure 2. P-polarized IRRA spectra of NO adsorbed on moderately reduced rutile TiO2(110) surfaces at 90 K as a function of NO dosage. The IR beams are a) along the [110] direction and b) along the [001] direction.

29



Figure 3. a-b) Top and side views of the calculated trans-(NO)2/Ti&Vo dimer configuration. The gray ball denotes Ti atom, the blue ball denotes N atom, the red and pink ball denotes O atom. c) Charge transfer density isosurface for the trans-(NO)2/Ti&Vo dimer formation. The yellow and green isosurfaces show the electron accumulation and loss, respectively, with the isovalue of 0.03 electron/Å3. The solid blue and dashed green circles indicate the initial locations of polarons before NO adsorption.

30


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Figure 4. P-polarized IRRA spectra of 0.01 L and 0.04 L NO dosages on moderately reduced TiO2(110) surfaces at 90 K with and without UV irradiation. The IR beams are along the [110] direction.

31



Figure 5. a) LEED patterns of moderately reduced TiO2(110) surfaces; b-c) RHEED patterns of moderately reduced TiO2(110) surfaces taken along [001] and [110] directions respectively. d) LEED patterns of highly reduced TiO2(110) surfaces; e-f) RHEED patterns of highly reduced TiO2(110) surfaces taken along [001] and [110] directions respectively.

32


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Figure 6. P-polarized IRRA spectra of CO adsorbed on a) moderately reduced and b) highly reduced TiO2(110) surfaces at 90 K. The IR beams are along the [11 0] direction. The dashed lines were obtained by fitting Gaussian functions to the spectra.

33



Figure 7. P-polarized IRRA spectra of NO adsorbed on highly reduced quasi-TiO2(110)-(1×2) surfaces at 90 K as a function of NO dosage. The IR beams are a) along the [11 0] direction and b) along the [001] direction. The gray IRRA spectrum was obtained for 40 L NO dosage under UV irradiation.

34


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Figure

8.

P-polarized IRRA spectra of NO

adsorbed on the

oxidized

quasi-TiO2(110)-(1×2) surface at 90 K as a function of NO dosage. The IR beams are a) along the [110] direction and b) along the [001] direction.

35



Page 36 of 44

Table 1 List of typical absorption bands in IRRA Spectra for NO adsorption and reactions on different TiO2(110) surfaces. Surfaces Oxidized

0.04

TiO2(110)-(1×1)

40

1748(+)b

1624(−)a,b

0.01 Moderately reduced TiO2(110)-(1×1)

Absorption bands of IRRA spectra (cm-1)

NO dosage (L)

0.04

1875(−)a,b

quasi-TiO2(110)-(1×2)

1

2243(−)a,b

1872(−)a,b

1752(+)

40

0.04

2233(+)a

1783(−)a,b b

1

Highly reduced

2243(−)a,b

1750(−)a,b

1622(+)a

1623(−)a,b

0.01

2233(+)a

1750(−)a,b

1620(+)a

1783(−)a,b 1872(−)a 2243(−)a,b

40

a

The p-polarized IR beams along the [110] direction.

b

The p-polarized IR beams along the [001] direction.

36


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1090x886mm (96 x 96 DPI)



152x202mm (300 x 300 DPI)


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177x296mm (96 x 96 DPI)



224x177mm (300 x 300 DPI)


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NO Reaction Pathways on Rutile TiO2(110): The Influence of Surface

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