Chemistry of NO x on TiO 2 Surfaces Studied by Ambient Pressure XPS: Products, Effect of UV Irradiation, Water and Coadsorbed K +

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1 SUPPORTING INFORMATION Chemistry of NO x on TiO 2 Surfaces Studied by Ambient Pressure XPS: Products, Effect of UV Irradiation, Water and Coadsorbed K + Olivier Rosseler,, Mohamad Sleiman, V. Nahuel Montesinos,,π, Andrey Shavorskiy, Valerie Keller, Nicolas Keller, Marta I. Litter,,, Hendrik Bluhm, Miquel Salmeron,±, Hugo Destaillats *. Lawrence Berkeley National Laboratory, Environmental Energy Technology Division, Material Sciences Division, Chemical Sciences Division and Advanced Light Source, Berkeley, CA, USA. Université de Strasbourg, Institut de Chimie et Procédés pour l'energie, l'environnement et la Santé (ICPEES),,, CNRS, Strasbourg, France.Comisión Nacional de Energía Atómica, Gerncia Química, San Martín, Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires, Argentina π. Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina. Instituto de Investigación e Ingeniería Ambiental, Universidad de Gral. San Martín, Argentina ±. University of California Berkeley, Materials Science and Engineering Department, Berkeley, CA, USA * Corresponding author HDestaillats@lbl.gov 1. Peak fitting 1.1. Methodology Table S1 details the peak parameters obtained after fitting the various gas phase and surface spectra described in the article and in these Supporting Information. All the surface peaks are fitted to symmetric line-shapes comprised of 30% Gaussian and 70% Lorentzian functions, with a Shirley background for O1s, C1s, and Ti2p spectra, and a linear background for N1s spectra. The best fit for gas phase peaks was obtained using a Gaussian to Lorentzian ratio of 60/40. Gas phase N1s spectra were fitted with four peaks originating from two paramagnetic species, with equal FWHM originally. This constraint was then relaxed. The position of the peaks fits well with previously published results for NO 2 and NO in the gas phase. 1,2 Surface N1s spectra were fitted with five peaks, accounting for four species, based on experimental evidence under increasing NO 2 pressure, and on the literature. 3 6 Assuming that the peak around ev can be attributed to NO 2, which is weakly adsorbed, it stands to reason that the molecule would keep its paramagnetic properties, and the contribution of the singlet state was therefore taken into account with a satellite peak at 0.7 ev higher binding energy, with a 3:1 intensity S1

2 ratio. 2,7 First, the binding energy of these peaks was fixed and their FWHM was kept equal, all relative to the NO - 3 peak. Then their binding energy was relaxed, before finally lifting the constraint on the FWHM. The same procedure was applied to fit the C1s region with five peaks corresponding to C=C, CH x, C-OH, C=O and COOH, using binding energy and FWHM references from Beamson and Briggs 8, and Mysak et al. 9 Finally, the O1s region was fitted, taking into account the oxygen species identified in the N1s and C1s regions. A relative sensitivity factor was determined for the O1s and N1s spectra from the gas phase signal of pure NO 2. This factor accounts for differences in X-ray flux at 735 ev (O1s) and 600 ev (N1s), as well as differences in the photoemission cross-section of O1s and N1s. 10 In these conditions, the sensitivity factor of O1s to N1s was found to be This parameter was kept constant when fitting O1s XPS spectra featuring peaks of NO x,chem and NO 2,phys. NO - 3 and NO - 2 in N1s were assumed to both contribute to a larger NO x,chem peak in O1s. The binding energy difference between NO x,chem and NO 2,phys was also kept constant at 2.75 ev. 5 Then, the C-OH and C=O contributions in O1s were fitted. It was assumed that COOH in C1s contributes equally to C-OH and C=O in O1s. The binding energy difference between these peaks in the O1s region was kept constant at 1.1 ev. Careful examination of the spectra obtained under NO 2 -free atmosphere allowed us to estimate the O1s to C1s ratio to The intensity of the C-OH and C=O peaks in the O1s region was initially fixed, before eventually being relaxed. The presence of hydroxyl groups on the surface of TiO 2 was identified on the clean surface, under Ultra High Vaccum (UHV), and its binding energy difference with the lattice oxygen peak was kept constant at ev, as well as its surface area (10% of that of the lattice oxygen), in agreements with the literature. 11 A peak at ~533.5 ev, characteristic of water adsorbed on TiO 2, was also detected. Its binding energy difference with lattice oxygen was fixed at +2.9 ev. The binding energy of all these peaks was kept constant, relative to lattice oxygen. Their FWHM was initially fixed equal at first and eventually relaxed. S2

3 Table S1 Peak fitting parameters Gas phase N1s Peak NO 2 singlet NO 2 triplet NO singlet NO triplet Relative BE FWHM 0.5~ ~ ~ ~0.8 Surface N1s Peak NO 3 - NO 2,phys NO 2,phys Singlet Triplet Relative BE 0-0.9~ ~ ~ ~-7.1 FWHM 1.1~ ~ ~ ~ ~2.6 Surface C1s Peak C=C CH x C-OH C=O COOH Relative BE FWHM 1.1~ ~ Surface O1s Peak Lattice O OH H 2 O chem NO x,chem. NO 2,phys C=O C-O Relative BE FWHM 1.1~ ~ ~ ~ ~ ~ ~1.6 NO 2 - NO N1s peak fittings are presented in the main paper in Figure 1 and Figure 3a for the surface and gas phase spectra, respectively Carbon signal Upon NO 2 introduction, a uniform carbon contamination was observed on the previously clean surface (Figure S1a). This contamination might come from the NO 2 cylinder or NO 2 gas line, despite its conditioning prior to the experiment. It most likely was due to the displacement of traces of carbon from the surfaces of the chamber. The carbon signal was found to be constant throughout every experiment and every condition. Figure S1b gives an example of the time evolution of the various C1s components under 0.05 Torr of NO 2 and UV light for 100 minutes and shows their stability. This carbon contamination is therefore unlikely to have played any significant role in the reactions described in the main article. S3

4 Figure S1. Time evolution of the C1s peaks under 0.05 Torr of NO 2 and UV irradiation 1.3. Oxygen signal The surface O1s signal deconvolution was performed using previously published results. 5,11 Figure 1b (in the main article) represents the O1s signal under 0.05 Torr of NO 2. As reported by Haubrich et al. 5, the introduction of NO 2 displaces trace molecules adsorbed on the walls of the chamber. It accounts for the presence of a peak at ~533.5 ev in the O1s region, characteristic of water molecules adsorbed on TiO 2 surface. 11 The anatase single crystal is partially covered with chemisorbed water molecules at 0.05 Torr of NO 2. At this pressure also, another peak identified as gas phase NO 2 appears at ev with a FWHM of ~1.1 ev Titanium signal Figure S2 shows the Ti2p spectra at different stages of the experiment. After sputtering (Figure S2a) the single crystal shows numerous defects, including oxygen vacancies (Ti 3+ ). After annealing in O 2 (10-5 Torr) at 773K, the defects were partially healed (Figure S2b). However, the introduction of NO 2 (10-6 Torr and up) at 293K fully oxidized the surface (Figure S2c). S4

5 Figure S2 Ti2p spectra: (a) after Ar + sputtering, (b) after annealing in O 2, (c) after NO 2 introduction (shown also in Figure 1d, in the main article). S5

6 1.5. Assignment of gas-phase N1s APXPS signals While the sample is grounded and the binding energies for surface species are accurate, the gas phase is not. A few studies report experimental 1,2 and theoretical 3 binding energies for nitrogen gas phase species. Using these results and the Mulliken charge of different nitrogen species, we were able to plot the position of each peak, relative to N 2 (Figure S3). The Mulliken charges reported here are from the NIST database, obtained by DFT (B3LYP 6-31G*). 12 An average straight line passing through the experimental results can be drawn. A linear correlation between the binding energy of the nitrogen atom and its Mulliken charge was assumed. The peak position of NO 2 triplet and NO triplet found in this study are in good agreement with this linear correlation. The negative nitrogen mass balance observed when the TiO 2 single crystal is exposed to 0.05 Torr of NO 2 and 0.05 Torr of H 2 O under UV light is a sign that nitrogenated gas phase products are generated. Previously published results indicate that these compounds are most likely NO, HONO and/or N 2 O. There expected binding energy relative to N 2 are +0.4 ev for NO, +1.2 to +1.4 for HONO, whereas N 2 O should have two features, at -0.6 ev for NNO and +2.8 ev for NNO. Unfortunately, if produced, these compounds are not seen in the gas phase, probably due to their low concentration or reactivity under UV light. Figure S3 Relative binding energy of gas phase nitrogenated species vs Mulliken charge of the N atom this work [1] [2] [3] S6

7 2. Gas phase contribution Figure 1c (in the main article) shows the presence of an additional peak in the O1s region, detected at 0.05 Torr of NO 2 and above, and attributed to gas phase NO 2. If a contribution for NO 2,gas can be identified in the O1s surface spectra, it stands to reason that NO 2,gas should also appear in the N1s surface spectra. Figure S4 shows the same gas phase N1s spectrum as in Figure 3a, but plotted on an absolute binding energy scale, instead of a relative one. When the sample is fully retracted, the main NO 2,gas peak appears at ev. Since the gas phase is not grounded, bringing the sample close to the collector to record the surface spectra can modify the binding energy of gas phase species. But it seems reasonable to assume that the NO 2,gas peak identified in the surface O1s region could contribute to the intensity of the peak at ev in the surface N1s region. In order to account for this possibility, the expected intensity of this NO 2,gas peak in the surface N1s region was estimated for each experimental point using the O/N ratio of 0.58 previously established (see Peak fitting above) and the intensity of the NO 2,gas peak measured in the O1s region. The error bars for the NO - 3 peak in Figure 2 reflect that potential contribution of gas phase NO 2 to the intensity of the peak at ev in the surface N1s region. Figure S4 Gas phase N1s signals under 0.05 Torr plotted on an absolute binding energy scale. 3. NOx surface spectra evolution with pressure A preliminary study was performed to define the optimum working pressure by recording the N1s photoemission intensity at different NO 2 pressures. With increasing NO 2 pressure, the N1s signal increased on the surface. A rather strong signal was obtained at 10-4 Torr of NO 2, but the nitrogen gas phase signal was too weak for pressures below 0.05 Torr of NO 2. These considerations lead us to work S7

8 under 0.05 Torr of NO 2 throughout the experiment, equivalent to 70 ppmv of NO 2 or 0.3 %RH for 0.05 Torr of H 2 O. In Figure S5, we illustrate peak intensity changes as a function of the NO 2 pressure for each of the four N1s signals. Figure S5 Surface N1s intensity for each signal as a function of NO 2 pressure. 4. Experimental methods The measurements were carried out at the beamline at the Advanced Light Source at the LBNL. This instrument is designed for in situ measurements of chemical reactions at pressures of up to 5 Torr, which allowed probing the surface of the catalyst under realistic regimes with synchrotron radiation. 13 Ti2p, O1s, N1s, C1s, and K2p XPS spectra were collected with photon energies of 665, 735, 600, 490, and 490 ev, respectively, to ensure an equal probing depth in all spectral regions. Each surface N1s data point is the average of two experimental values taken in the same conditions in two different places of the sample. A gold foil, in contact with the TiO 2 single crystal, was used as a binding energy reference, setting the BE of the Au 4f 7/2 signal at 84.0 ev. All the experiments involving TiO 2 were conducted using an anatase (101) single crystal, purchased from Surface Preparation Lab (Netherlands). Low Energy Electron Diffraction (LEED) was used to characterize the surface orientation of the crystal, and the diffraction pattern (Figure S6a) is consistent with the literature. 14,15 X-Ray diffraction shows one peak corresponding to the anatase (101) diffraction at 2 = 23.5, although the signal to noise ratio indicates poor crystallinity (Figure S6b). S8

9 a) Figure S6 a) LEED pattern at 138 ev, and b) X-Ray diffraction pattern of the clean Anatase (101) single crystal The (101) surface of the anatase single crystal was preconditioned by cycles of Ar + sputtering (1.5 kev, 10-5 Torr) followed by annealing under O 2 (10-5 Torr) at 773 K prior to introduction in the APXPS chamber. Spectral features changed in the dark over time due to the interaction of the high intensity X-ray beam with the surface. In order to avoid this effect, the sample was moved between each cycle of measurements (N1s, C1s, Ti2p, and O1s). When spectra were not recorded, the X-Ray beam was shut. Each experiment presented in this article was found to be reproducible. A LED-UV light source (365 nm, Figure S7) was placed outside the chamber, 15 cm away from the sample. The light emits a focused UV beam of 5 cm in diameter. The intensity received through a quartz window by the sample (0.6 mw.cm -2 ) was measured inside the APXPS chamber using a photodiode at the sample position. In addition to the focused beam, the UV light illuminated the whole chamber with a low intensity cone (10 µw.cm -2 ). The focused beam accounts for 2% of the irradiated volume, and 7% of the total UV intensity inside the chamber. S9

10 Figure S7 Emission spectrum of the UV LED used in the experiments Water vapor (HPLC grade, Baker) was introduced into the chamber through a variable high precision leak valve after being degassed by multiple freeze pump thaw cycles. Nitrogen dioxide (Scott, purity 99.5%), and oxygen (Airgas, purity %) were introduced into the chamber directly from commercial cylinders. The evolution of the gas phase composition with time under UV irradiation was investigated under 0.05 Torr of NO 2 in the absence of water and 0.05 Torr of NO Torr of H 2 O. The gases were admitted in the chamber, with a base pressure 4x10-10 Torr, using a leak valve. Both the surface and gas phase compositions were monitored in the APXPS chamber. When measuring the composition of the surface, the sample was placed in the path of the X-Ray beam coming from the synchrotron (Figure S8a). Its position was adjusted to maximize the photoemission intensity. Gas phase spectra were taken prior to introducing the anatase sample in the path of the X-Ray and UV beams and again after 2 hours of UV irradiation of the TiO 2 to measure how surface reactions contributed to the gas phase composition. The same experiment was performed with the TiO 2 single crystal fully retracted (Figure S8b) when only the gas phase was irradiated with UV light for two hours to monitor the changes induced by gas phase reactions. Under UV irradiation, the final gas phase composition was found to be identical in both cases. However, when the sample was retracted (Figure S8b), the UV beam propagates to the wall of the chamber. The changes in gas phase composition could therefore be due to photolysis of purely gas phase NO 2, or of NO 2 adsorbed on the walls of the chamber. S10

11 98% of the chamber was irradiated by low intensity UV (10 µw.cm -2 ), while 2% of the molecules in the chamber received 0.6 mw.cm -2 from the intense UV beam focused on the sample position. Figure S9 presents the measured (red solid line) and expected results from different kinetic models for a batch reactor configuration: gas phase photolysis of the entire gas phase under low (green solid line) and high UV irradiance (blue dashed line), and photolysis of a monolayer of NO 2 adsorbed on the entire inner surface of the chamber under low UV irradiance (grey dotted line). NO 2 photolysis is a first order reaction, simulated NO 2,gas concentration over time under UV light were established with two apparent rate constants obtained with the low and high irradiance measured in the chamber, the NO 2,gas absorption cross section in the nm range (4.7x10-19 cm 2 ), and the quantum yield for NO 2 photolysis at 298 K (0.986). 16 The reported absorption cross section for adsorbed NO 2 is 1 to 2 orders of magnitude higher than in the gas phase, leading to a third simulated gas phase composition. 17,18 Assuming that the NO 2 triplet photoemission intensity scaled linearly with the NO 2 concentration, and since 98% of the volume of the chamber received a low dose of UV light, the evolution of the concentration of NO 2 over time under UV irradiation is unsurprisingly closer to the gas phase photolysis under low UV irradiance model. From the results presented in Figure S9 it was concluded that surface reactions on the walls of the chamber do not contribute significantly to the composition of the gas phase under UV. The pressure in the XPS chamber was typically in the 0.05 to 0.1 Torr range. To bridge the pressure gap between the chamber at p 0 and the hemispherical analyzer under ultra high vacuum, the system uses three differential pumping stages. Electromagnetic lenses were used to focus the electrons generated by the X-Ray beam that passes through the collector. 13 With increasing pressure in the chamber, the photoemission intensity decreased due to scattering of the photo-electrons by the gas phase. In order to compare results obtained at different pressures a normalization factor (Kp) was applied (S1). (S1) Where î P is the average baseline intensity of the considered element at pressure P, and î Pref is the average baseline intensity of the same element at the reference pressure. S11

12 Figure S8 Instrument schematics indicating the position in the sample when (a) recording surface species and (b) gas phase species Figure S9 Measured (red solid line) and simulated NO 2 triplet photoemission intensity for NO 2 photolysis triggered by low UV irradiance in the gas phase (green solid line), high UV irradiance in the gas phase (blue dashed line), and low UV irradiance on adsorbed NO 2 (grey dotted line) S12

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