Potassium and Water Coadsorption on TiO2(110): OH-Induced Anchoring of Potassium and the Generation of Single-Site Catalysts

BNL-113326-2016-JA David C. Grinter, Elena Rodríguez Remesal, Si Luo, Jaime Evans, Sanjaya D. Senanayake, Dario J. Stacchiola, Jesus Graciani, Javier Fernández Sanz, and José A. Rodriguez Submitted to Journal of Physical Chemistry Letters October 2016 Chemistry Department Brookhaven National Laboratory U.S. Department of Energy USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22) Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DESC0012704 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. Potassium and Water Coadsorption on TiO2(110): OH-Induced Anchoring of Potassium and the Generation of Single-Site Catalysts DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 1 Potassium and Water Co-Adsorption on TiO2(110): OH-induced Anchoring of Potassium and the Generation of Single-site Catalysts David C. Grinter,a Elena Rodríguez Remesal,b Si Luo,a,c Jaime Evans,d Sanjaya D. Senanayake,a Dario J. Stacchiola,a Jesus Graciani,b Javier Fernández Sanzb,* and José A. Rodriguez a,c* a Chemistry Department, Brookhaven National Laboratory, Upton, NY, 11973, United States b Departamento de Química Física. Universidad de Sevilla. 41012-Sevilla. SPAIN c Department of Chemistry, State University of New York (SUNY) at Stony Brook, Stony Brook, New York, 11794, United States dFacultad de Ciencias, Universidad Central de Venezuela, Caracas 1020 A, Venezuela 2 Abstract Potassium deposition on TiO2(110) results in reduction of the substrate and the formation of loosely bound potassium species which can move easily on the oxide surface to promote catalytic activity. The results of density functional calculations predict a large adsorption energy (~ 3.2 eV) with a small barrier (~ 0.25 eV) for diffusion on the oxide surface. In scanning tunneling microscopy images, the adsorbed alkali atoms lose their mobility when in contact with surface OH groups. Furthermore, K adatoms facilitate the dissociation of water on the titania surface. The K-(OH) species generated are good sites for the binding of gold clusters on the TiO2(110) surface producing Au/K/TiO2(110) systems with a high activity for the water-gas shift. TOC Graphic 3 Alkali metals act as promoters of heterogeneous catalysts in a very wide range of important chemical processes (ammonia synthesis, CO oxidation, NO reduction, alcohol synthesis from CO/CO2 hydrogenation, the water-gas shift reaction, Fischer-Tropsch synthesis, olefin epoxidation, etc.).1 The precise mechanisms behind the alkali promotional effects are not yet fully understood.2 In principle, an alkali can participate directly in a catalytic process or it can modify the chemical properties of the catalyst components.3 Most catalysts used in the chemical industry combine metals and oxides. The study of the co-adsorption of alkali metals and simple molecules on well-defined surfaces of transition metals has been a favored topic of research for almost a century since the pioneering work by Langmuir in 1923.4 To explain the effects of alkalis on the surface chemistry of metals, the following mechanisms have been proposed in the literature: (1) site-blocking effects, (2) through-the-metal electronic interactions, (3) through-the-space electronic interactions (e.g., electrostatic), (4) direct chemical interactions, and (5) alkali-induced surface reconstructions.4-7 In comparison, much less is known about the interaction of alkali metals with well-defined surfaces of oxides.8-17 Recently, it has been shown that the addition of sodium or potassium to water-gas shift (WGS, H2O + CO → H2 + CO2) catalysts which contain gold or platinum produces a substantial increase in catalytic activity by inducing the formation of single-site (Pt or Au)-O(OH)x-(Na or K) species.18-20 Active sites that contain only a metal center are formed as a consequence of interactions of the metal with (OH)x-(Na or K) groups. It is not clear how these groups are formed by the reaction of the alkali with water molecules. In this work, we investigate the behaviour of ultralow coverages of potassium in the K/TiO2(110) and {K+H2O}/TiO2(110) systems using a combination of scanning tunneling microscopy (STM), photoemission, and calculations based on density-functional theory (DFT). K atoms display a very high mobility on the titania substrate. The adsorption energy of the alkali is large but there is a low barrier for diffusion. This could facilitate a close contact between the alkali and adsorbates essential for catalytic promotion. Furthermore, the potassium adatoms facilitate the dissociation of water, forming (OH)-K units that have a very low mobility on the TiO2(110) substrate. When water is pre-dosed on titania, the species acts as a nucleation site for potassium atoms and the formed (OHt)-K units are proposed to be excellent sites for the binding of metals such as Au or Pt. An oxidized o-TiO2(110) surface is characterized by a low density of point defects, and the appearance in empty-states STM (Figure 1a) is dominated by bright rows running along the [001] direction, corresponding to Ti5c, with a few oxygen adatoms located on top of these rows (triangular highlight).12 4 Upon exposure to a very low coverage of potassium (1.35% ML) at 300 K, the surface displayed in Figure 1b is formed; it has a significant number of hydroxyl species in bridging (OHb) and terminal (OHt) positions (hexagonal highlight in Figure 1b, total coverage 2% ML) which are identified in STM by their locations between and on top of the Ti5c rows and their ability to be removed by voltage pulses from the STM tip. A number of tall (250-300 pm) species are also observed (highlighted with a circle) which interact heavily with the tip as evidenced by streaking in the images, we assign these as highly mobile potassium clusters. Their binding to the surface is so weak that after a single scan over a region of the surface they are no longer visible, even under relatively gentle tunnelling conditions. Further exposure to the same amount of potassium (total 2.7% ML) approximately double the hydroxyl concentration (5% ML) due to dissociation of water present in the background gases of the chamber (Figure 1c). The concentration of large K species is also increased (circular highlight), these were observed to be slightly more stable under the STM tip than at the lower coverage (it took a couple of STM scans under normal conditions to clear the area of adsorbates), this suggests some stabilization in the presence of additional OHb. The ratio between the concentration of OH and the calibrated K dose is of the order of 2:1, this is consistent with the formation of two hydroxyl groups per K atom deposited via reaction with background H2O within the chamber (p ~1×10-9 Torr).21 It is important to note that due to the facile movement of K by the STM tip, the K coverage observed in imaging is much lower than the calibrated dose. (0.24% ML vs. 2.7% ML). Figure 1. Sequential evaporation of K onto o-TiO2(110) at 300 K. (30 × 30 nm2, V = +1.74 V, I = 0.11 nA) XPS spectra of o-TiO2(110) upon adsorption of potassium are displayed in S.I. Figure S1. As observed in the STM images in Figure 1, there is reduction of the TiO2 surface upon K adsorption (Figure S1a), 5 concomitant with an increase in OHb species and partially oxidized K, consistent with previous studies of K/TiO2.22-24 Water is prevalent in many real catalytic situations in which alkali metals are present and has important effects in the reactivity of TiO2(110). The results of K/o-TiO2(110) in Figure 1 suggested that OH groups played a role in stabilizing the potassium species. To probe this further, we prepared a fully hydroxylated h-TiO2(110) surface, where all the oxygen vacancies are reacted with water to form a high concentration of OHb species12, prior to depositing potassium at 300 K, the results of which are displayed in Figure 2. One additional motivation for pre-hydroxylating these surfaces is from a practical point of view – the as-prepared TiO2(110) surfaces are very reactive to any background water in the UHV system and within a few hours a mixture of vacancies and hydroxyl species can be observed. In order to have a more controlled system consisting of only hydroxyls, we therefore chose to expose to water prior to potassium evaporation. 6 Figure 2. The structure and mobility of 0.6% ML K on h-TiO2(110). (a) 50 × 50 nm2 image acquired immediately after evaporation of K, (b) & (c) 17 × 20 nm2 images of the blue highlighted section in (a) acquired sequentially afterwards, demonstrating the tip-induced movement of K species. (V = +1.78 V, I = 0.10 nA) (d) height distribution of the K species. In the large area image in Figure 2a we see a uniform distribution of atomic-sized bright features across the surface, with no nucleation at step edges, implying a relatively strong interaction with the h- TiO2(110). Interestingly this is an opposite effect to that observed for atoms of less reactive metals such as Au, where the presence of OHb facilitates their diffusion across the surface, leading to nucleation at 7 the step edges.25 These bright features are assigned as K species and their concentration is consistent with the dose. (0.6% ML) The stability of the potassium is markedly higher than on the o-TiO2(110) with the majority of the atoms remaining nearly immobile over many tens of scans (hours) under similar tunnelling conditions. Figure 2b & 2c show expanded views of the region highlighted in Figure 2a and were obtained from sequential STM scans. The potassium atoms are highlighted with circles, and an OHb with a hexagon. Two of the atoms (white arrows) move upwards a short distance along the [001] direction. Despite the [001] being the slow scan direction for these images, the anisotropy of the TiO2(110) surface results in a much lower barrier for diffusion along [001] compared to [1-10].26 In addition to the movement of these two atoms, the much taller feature marked by purple circles also shows a clear shift, with a bright streak becoming visible in Figure 3c. The enhanced mobility of potassium species along [001] was also observed in an earlier work by Yurtsever et al. using NCAFM,13 although in that case the attractive interaction with the AFM tip is likely to be stronger than for the STM. The OHb concentration is measured to be 18(±1) % ML, a slight increase from before dosing K (16(±2) % ML), and importantly is significantly more than observed for K/o-TiO2(110). (5(±2) % ML) This correlates well with the increased stability for the potassium under STM scanning when comparing K/h-TiO2(110) to K/o-TiO2(110). A histogram of the height distribution for the potassium species obtained from STM imaging is displayed in Figure 2d. It is roughly trimodal with the majority of the K having a height of ~120 pm, which can be attributed to single atomic species. Taller features with heights in the ranges 150-200 pm and 220-270 pm are also observed, originating either from K species containing multiple atoms, or possibly KOH/KOx. For comparison, the heights of OHb under these imaging conditions is 50 (±10) pm, this is quite variable as it is dependent on the termination of the STM tip as well as the tunnelling current and bias voltage.27 The XPS spectra for potassium deposited on the hydroxylated surface also show reduction of the TiO2 and the generation of additional OHb species, along with the formation of partially oxidized K. (S.I. Figure S2) To precisely identify the binding sites of the potassium species, high resolution STM images were acquired, an example of which is displayed in Figure 3a. The round yellow features (hexagonal highlight) are OHb (see Figure 3b), these permit accurate positioning of the bridging O rows (white lines in Figure 3a). The potassium species have a green/blue colour and can be broadly separated into 14 the supercell model we used the optimized lattice parameters for the bulk a = 4.669Å, c = 3.025 Å. In structural optimizations the two lowest TiO2 trilayers were kept frozen while the rest of the atoms were allowed to fully relax their atomic positions. Reference single point calculations using the HSE06 hybrid exchange-correlation functional were performed with slabs 12 atomic layer thick. Acknowledgements The experimental studies at Brookhaven National Laboratory were supported by the US Department of Energy, Chemical Sciences Division (DE-SC0012704). The work performed at the University of Seville was funded by the Spanish Ministerio de Economía y Competitividad, grant CTQ2015-64669- P, Junta de Andalucía, grant P12-FQM-1595, and European FEDER. Computational resources were provided by the Barcelona Supercomputing Center/Centro Nacional de Supercomputación (Spain). Supporting Information  XPS spectra for K/o-TiO2(110)  XPS spectra for K/h-TiO2(110)  Calibration of K deposition via STM  Identification of OHb species via STM References (1) Koel, B.E.; Kim, J. 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SPAIN c Department of Chemistry, State University of New York (SUNY) at Stony Brook, Stony Brook, New York, 11794, United States dFacultad de Ciencias, Universidad Central de Venezuela, Caracas 1020 A, Venezuela Contents  XPS spectra for K/o-TiO2(110)  XPS spectra for K/h-TiO2(110)  Calibration of K deposition via STM  Identification of OHb species via STM S 2 XPS spectra for K/o-TiO2(110) Figure S1. XPS spectra of clean (black line) and 0.3 ML potassium deposited (red line) o-TiO2(110) at 300 K. (a) Ti 2p, (b) O 1s, (c) C 1s and K 2p. (hν = 1253.6 eV) XPS spectra of o-TiO2(110) before (black) and after (red) adsorption of potassium are displayed in Figure S1. As observed in the STM images in Figure 1, there is significant reduction of the TiO2 surface (% Ti3+ goes from 5.1 to 16.7) upon K adsorption (Figure S1a), concomitant with an increase in OHb species in the O 1s region (Figure S1b; % OHb goes from 6.6 to 20.2), consistent with previous studies of K/TiO2.1-3 The corresponding K 2p and C 1s spectrum is shown in Figure S1c, the binding energy of the K 2p3/2 peak is found to be 294.2 eV, similar to that observed for K/Cr2O3(0001)4 and K/MgO(001)5, and slightly lower than the 294.4 eV reported for metallic K, suggesting a partial oxidation of the potassium.6 The presence of metallic potassium would usually be confirmed by significant asymmetry of the 2p region with large plasmon features at higher binding energies, which are not observed in our spectra.4,6 S 3 XPS spectra for K/h-TiO2(110) Figure S2. XPS spectra of clean (black line) and 0.3 ML potassium deposited (red line) h-TiO2(110) at 300 K. (a) Ti 2p, (b) O 1s, (c) C 1s and K 2p. (hν = 1253.6 eV) The XPS spectra for potassium deposited on the hydroxylated surface show a similar reduction of the TiO2. (Figure S2) The Ti3+ component (Figure S2a) increases from 9.6 % to 23 %, as the OHb component (Figure S2b) increases from 10.4 % to 17.2 %. The binding energy of the K 2p3/2 is 294.3 eV, consistent with that observed for K/o-TiO2(110), for partially oxidised potassium. (Figure S2c) S 4 Calibration of K deposition via STM Figure S3. Calibration of K evaporation rate on Cu2O/Cu(111). (13 × 13 nm2, V = +1.78 V, I = 0.10 nA) Figure S3 details the calibration of the potassium evaporation rate on Cu2O/Cu(111) using STM. The clean surface is displayed in Figure S1a, with images obtained after sequential 1 minute doses shown in Figure S3b-d. The potassium nucleates as individual atomic-sized species with no mobility (either thermally or tip-induced) observed on the timescale of the STM measurement, the density and corresponding coverage in MLE are displayed under each image. S 5 Identification of OHb species via STM Figure S4. Removal of OHb by 3V scans of the STM tip. The areas bordered with the dashed white lines in each image designate the high-bias scan regions. (a) 50 × 50 nm2, (b) 22 × 22 nm2. (V = +1.50 V, I = 0.10 nA) High (>3 V) bias scans with the STM tip have been demonstrated to selectively desorb OHb, yielding the pristine surface and leaving Ovac unaffected.7 In Figure S4a we demonstrate the effect of performing a +3 V scan over a 30 × 30 nm2 region of the surface marked by the dashed white line. The OHb have been removed: in the enlarged image shown in figure S4b the bright Ti5c rows can be clearly seen, along with the high density of OHb outside of the cleared area. Using this method, we were able to check for the presence of unreacted Ovac, and if necessary dose with H2O to react them in order to form the fully hydroxylated h-TiO2(110) surfaces prior to K deposition. S 6 References (1) Hardman, P. J.; Casanova, R.; Prabhakaran, K.; Muryn, C. A. Electronic Structure Effects of Potassium Adsorption on TiO2(100). Surf. Sci. 1992, 269, 677-681. (2) Heise, R.; Courths, R. A Photoemission Investigation of the Adsorption of Potassium on Perfect and Defective TiO2(110) Surfaces. Surf. Sci. 1995, 331, 1460-1466. (3) Muscat, J.; Harrison, N. M.; Thornton, G. First-Principles Study of Potassium Adsorption on TiO2 Surfaces. Phys. Rev. B 1999, 59, 15457-15463. (4) Wilde, M.; Beauport, I.; Stuhl, F.; Al-Shamery, K.; Freund, H. J. Adsorption of Potassium on Cr2O3(0001) at Ionic and Metallic Coverages and UV-Laser-Induced Desorption. Phys. Rev. B 1999, 59, 13401-13412. (5) Huang, H. H.; Jiang, X.; Siew, H. L.; Chin, W. S.; Xu, G. Q. 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