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Effect of oxygen vacancy sites on CO2 adsorption dynamics: The case of rutile (1 × 1)-TiO2 (1 1 0)
Chemical Physics Letters 422 (2006) 461–465 www.elsevier.com/locate/cplett
Effect of oxygen vacancy sites on CO2 adsorption dynamics: The case of rutile (1 · 1)-TiO2 (1 1 0) S. Funk, B. Hokkanen, E. Johnson, U. Burghaus
*
Department of Chemistry, Biochemistry, and Molecular Biology, North Dakota State University, 1231 Albrecht Avenue/Ladd Hall, Fargo, ND 58105-5516, USA Received 8 February 2006; in final form 1 March 2006 Available online 9 March 2006
Abstract The adsorption of CO2 has been studied by molecular beam scattering on slightly and strongly reduced TiO2(1 1 0). The defect density has qualitatively been characterized by CO2 TDS. Two peaks are present in TDS, which can be assigned to CO2 adsorption on pristine and oxygen vacancy sites. Interestingly, the initial reactivity, S0, towards CO2 adsorption decreases with increasing defect density and increases with increasing oxygen preexposure. The latter is related to the vanishing of the defect structure in TDS. In contrast, hydrogen preexposure leads to a decrease in S0 as well as the intensity of both TDS peaks decrease in sympathy. 2006 Elsevier B.V. All rights reserved.
1. Introduction Few [1–6] molecular beam scattering studies have been conducted on metal oxides. Furthermore, TiO2 developed into a prototype system for surface science studies on model catalysts [7]. Surface defects are the active sites for O2 and H2O dissociation [8–10] and metal nano-clusters nucleation [11]. Therefore, a further characterization of surface defects is important to gain a mechanistic understanding of adsorption processes and surface reactions. Ultra-high vacuum annealing above 600 K leads to the most well characterized defects; oxygen vacancy sites form [12]. The adsorption of O2 leads to ‘healing out’ of these defects. As discussed by Hendersson [13], Yates [14], and others, CO2 is an ideal probe molecule to quantify the effect of oxygen vacancy sites on TiO2(1 1 0) by thermal desorption spectroscopy (TDS). Furthermore, CO2 is the carbon feedstock for surface reactions such as methanol synthesis. In theoretical studies [15], it has been suggested that oxygen *
Corresponding author. Fax: +1 701 231 8831. E-mail addresses: [email protected], uwe.burghaus@ndsu. nodak.edu (U. Burghaus). URL: www.chem.ndsu.nodak.edu (U. Burghaus). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.03.007
vacancy sites lead to bent CO2 adsorbates which open up lower energy pathways for subsequent hydrogenation reactions. Furthermore, according to recent theoretical studies [16], the interaction of defect sites with metal nano clusters can lead to charging of these clusters which affects surface reaction mechanisms. A single CO2 TDS peak (hereafter, a1 peak) appears on the oxidized surface which was assigned to adsorption on fivefold coordinated Ti4+ sites [13]. Annealing at temperatures above 600 K resulted in an additional high temperature TDS peak (a2 peak), assigned to adsorption on Ti3+ oxygen vacancy sites [14]. Similar effects have recently been revealed for CO2/ZnO(0 0 0 1) [17], suggesting that CO2 may be an interesting probe of surface defects for a plethora of metal oxides. Annealing above 1000 K has been avoided in this study since it can induce the (2 · 1) TiO2(1 1 0) reconstruction. Even the nearly perfect surface consists, according to scanning tunneling microscopy (STM) results, [12] of 10% oxygen vacancy sites which depends on the history of the sample preparation, i.e., the defect density, C, should be estimated in any study about TiO2(1 1 0). Molecular beam scattering experiments with CO2 have been conducted for ZnO(0 0 0 1)AZn, Pt(1 1 1), Pd(1 1 1),
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and Cu(1 1 0) surfaces [18–20]. For these systems, molecular, precursor mediated, and non-activated adsorption has been observed. In this study, we characterize the macroscopic effect of surface defects on the reactivity of the system by means of molecular beam scattering and TDS with the goal to address the structure–activity relationship. Whereas annealing the surfaces leads to an increase in C, preadsorption of O2 decreases C. In contrast, hydrogen coadsorption at low temperatures leads to simple site blocking effects [21]. 2. Experimental procedures The measurements have been conducted by a supersonic molecular beam system [18]. The statistical error in S0 is below ±0.03. The S(H) and TDS data have been smoothed while conserving the shape of the curves. All adsorption probabilities have been obtained for normal impact on the surface. A saturation coverage of 1 ML of CO2 has been assumed for Ts 85 K, as is common. All TDS curves are obtained for surfaces saturated with CO2 by dosing the gas for 7 min with the beam system; a heating rate of 1.5 K/s has been used. The reading of the thermocouple has been calibrated in situ by TDS measurements of n-butane (multilayer desorption peak at 108 K). H2 has been dissociated on a hot tungsten filament before adsorption. The scattering chamber has been backfilled with H2 at a pressure of 1 · 10 6 mbar through a dosing capillary, which increases the H2 flux by a factor of 10 as compared to backfilling by a leak valve. O2 has been dosed by backfilling. Cold traps have been used to avoid H2O contamination. A combined TDS-adsorption probability measurement required less than 15 min; the base pressure of the scattering chamber is in the low 10 10 mbar range. The cleanliness of the samples have been characterized by Auger electron spectroscopy and the crystallographic order by low energy electron diffraction and TDS.
Fig. 1. The initial adsorption probability as a function of impact energy, Ei, for CO2/TiO2(1 1 0). The inset shows CO2 TDS of the two different crystals.
The coverage dependent adsorption probability, S(H), of CO2 shown in Fig. 2 depends only weakly on coverage until the surface saturates and S drops naturally to zero. Besides differences in S0, both samples led to identical curve shapes. The Monte Carlo simulation (MCS) [23,24] (solid lines in Fig. 2) used to fit the data includes two fit parameters; S0 and Soccu. The adsorption probability in the extrinsic precursor state, Soccu, decreases linearly from 0.40 to 0.18 within Ei = 0.33–1.29 eV. On samples cleaned with the initial preparation procedure, further defects have been induced by annealing at 900 K for 1 h in vacuum [22]. This procedure leads to distinctly asymmetric CO2 TDS curves (see the thick solid curves in Fig. 3) and a 0.1 decrease in S0. The large intensity of the a2 structure with respect to the a1 peak indicates a large density of oxygen vacancy sites, C, on the vacuum annealed surfaces [14]. However, note that it is very
3. Presentation of the results Two crystals have been studied (I: Princeton Scientific, II: MaTeck) which have been cleaned initially by a procedure consisting of 20 cycles of Ar+ sputtering (600 V, 5 lA, 30 min, Ts = 300 K) followed by annealing in oxygen (1 · 10 6 Torr, Ts 600 K, 10 min) and in vacuum (Ts = 750 K, 5 min). The intensity of the CO2 TDS defect peak (a2 peak) was for sample I distinctly larger than for sample II (see the inset of Fig. 1), indicating a larger C of the former crystal. The initial adsorption probability, S0, of CO2 decreases linearly with increasing impact energy, Ei, and is systematically smaller by 0.2 for sample I (Fig. 1). Thus, S0 on vacuum annealed (or sputtered without annealing) surfaces is smaller than on more strongly oxidized surfaces (see Ref. [22] for details).
Fig. 2. S(H) of CO2 for the second sample of TiO2(1 1 0), parametric in Ei and at 85 K. The lines are the result of Monte Carlo simulations.
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Fig. 4. S0 of CO2 on oxygen (Ei = 0.56 eV) or H predosed (Ei = 0.12 eV) surfaces. (Sample II).
Fig. 3. Shown are CO2 TDS curves as a function of O2 preexposure: (A) at 85 K and (B) at 250 K. The solid spheres in panel A are the result of a simulation. (Sample II).
large (>20 L) O2 exposures. Although cold traps have been used in the gas line, the latter effect is most likely associated with H2O contamination of the O2 gas; H2O can stabilize CO2 (see Ref. [13]). The solid spheres in Fig. 3A are the result of integrating the Polanyi–Wigner equation. S0, as a function of O2 and hydrogen preexposure and parametric in O2 adsorption temperature, is shown in Fig. 4. Oxygen preexposures lead, in all cases, to a small (0.1) but well reproducible increase in S0. This increase is slightly larger for atomically adsorbed than for molecularly adsorbed oxygen. In contrast, preexposed atomic hydrogen leads to a distinct exponential decrease in S0 with increasing H exposure. 4. Discussion
difficult to obtain identical C on the samples throughout the experiments. The effect of preadsorbed atomic oxygen and hydrogen on the adsorption of CO2 has been studied for the defected surface by TDS (Fig. 3) and adsorption probability measurements (Fig. 4). First oxygen or atomic hydrogen (Tads = 85 K) has been preadsorbed. Afterwards, the remaining clean areas of the surface have been saturated with CO2 (at Tads = 85 K) by the molecular beam system, recording simultaneously an adsorption transient. Subsequently a CO2 TDS curve has been detected. This measuring procedure has been repeated for different preexposures of the coadsorbates and in the case of oxygen for molecularly (Tads = 85 K) and atomically (Tads = 250 K) adsorbed oxygen [13]. At 0.4–0.6 L of O2 the a2 TDS peak disappears (Fig. 3) independent of the O2 adsorption temperature. (H preexposure leads to a decrease in both CO2 TDS peaks; see Ref. [22] for details.) For atomically adsorbed oxygen, the intensity of the a1 TDS peak is conserved, even for large (20 L) O2 preexposures. For molecularly adsorbed oxygen, an increase of the a1 TDS peak has been observed for very
4.1. Adsorption dynamics of CO2 The decrease in S0 with Ei (Fig. 1) is commonly observed for weakly adsorbing adsorbates and indicates molecular and non-activated adsorption. Furthermore, S0 is independent of the adsorption temperature, Ts, as expected [3,4] for non-activated molecular adsorption [22]. The shape of S(H, Ei) curves (Fig. 2) is in agreement with precursormediated adsorption, as predicted by the Kisliuk model [25]. Furthermore, a crossover from Kisliuk-like to Langmuirian-like S(H, Ts) curve shapes is observed [22] with increasing Ts which simply indicates a competition of thermal desorption and adsorption (via the molecular beam) of CO2, as commonly observed for physisorption systems [3,4]. The MCS [23,24] (solid lines in Fig. 2) adopt the idea of the Kisliuk model. According to this MCS the trapping probability into the precursor state decreases by a factor of approximately two with increasing Ei indicating less efficient gas-surface energy transfer processes at large kinetic energies of the gas phase species, as expected. Thus, on a
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quick glance, the adsorption dynamics of CO2 on TiO2(1 1 0) obeys well established standard models which have also been revealed for other metal oxide surfaces. However, interestingly, S0 depends distinctly on the density of defects. 4.2. Effect of oxygen vacancy sites on adsorption dynamics of CO2 The result that the a2 peak intensity increases with respect to the one of the a1 peak by annealing the samples at 900 K in vacuum [22] as well as the quenching of the a2 peak with increasing oxygen preexposure (Fig. 3) is in agreement with earlier reports [13,14] and the assignment of the a1 and a2 peaks to Ti4+ (pristine) sites and Ti3+ oxygen vacancy (defect) sites. A large a2 to a1 peak intensity ratio indicates a large density of oxygen vacancy sites. Although sample I and II have been cleaned by literally identical procedures, their C are quite different. Sample I has (inset Fig. 1) a larger density of oxygen vacancy sites and a smaller S0 (Fig. 1). However, S0 is a macroscopic property. Therefore, the adsorption probability on different sites cannot directly and easily be distinguished. The variations in S0 seen for the different samples are consistent with a recent STM study [12] which emphasized that the very details of the sample cleaning procedures and the total project time affects the density of surface defects. Furthermore, these results appear plausible in regard of the bulk diffusion mechanism, by which the density of surface defects is controlled, which has been proposed before [26]. The density of surface defects affects S0 (Fig. 1) but does not significantly affect the shape of the adsorption probability curves [22]. Therefore, the extrinsic precursor dynamics of CO2 on TiO2(1 1 0) is little affected by oxygen vacancy sites. This is not necessarily expected since measurements on other systems [27] revealed distinct variations of the shape of S(H) curves with C. However, the decrease in S0 with increasing C indicates variations of the lattice force constants near defect sites. 4.3. Effect of preadsorbates – ‘healing’ of surface defects Preadsorbates and defects can lead to conceptually similar effects [23]. Therefore, we collected kinetics (TDS) and dynamics (adsorption probabilities) data for CO2 adsorption on TiO2(1 1 0) preadsorbed with hydrogen or oxygen. Furthermore, it is well known that oxygen can ‘heal’ defects [13]; offering the possibility to provide another test of our conclusions. Oxygen adsorption leads to quenching of the a2 TDS peak (Fig. 3), i.e., the oxygen vacancy sites, indeed, heal out and this TDS peak is indeed related to oxygen vacancy sites. In contrast, H preadsorption reduced the a1 and a2 TDS peak intensities approximately by the same amounts [22]. We may expect an increase in S0 with increasing density of oxygen vacancy sites [23] since new adsorption sites for
CO2 form. Preadsorbates should lead, in the simplest case, to a decrease in S0, since they simply block adsorption sites. However, the experimental evidence contradicts, at least in part, these simple hypotheses. S0 decreases with increasing H preadsorption (as expected) but increases with increasing O2 preadsorption (Fig. 4). The latter effect is consistent with the observed decrease in S0 with C (Fig. 1) but is rather unexpected. However, in regard of the crystallographic structure of TiO2(1 1 0), it appears plausible that the adsorption of CO2 on oxygen vacancy sites is sterically hindered, which would reduce the probability of this process [23]. Thus, S0 would decrease with C, as consistent with the results obtained on the two different samples (Fig. 1), and S0 should increase when oxygen vacancy sites are filled by preadsorbed oxygen, as observed (Fig. 4). However, oxygen does not only heal out oxygen vacancy sites; it leads also to blocking of additional Ti sites [28]. (O2 from the gas-phase dissociates on defect sites. One oxygen atom fills the oxygen vacancy, the other adsorbs on an adjacent Ti site.) In the former case, S0 should increase due to the decrease in C but this effect competes with a decrease in S0 expected due to the blocking of Ti sites. These competing processes may explain the rather small effect of preadsorbed oxygen on S0. Furthermore, molecularly adsorbed O2 induces a slightly smaller increase in S0 than atomically adsorbed oxygen (Fig. 4) which is in agreement with the proposal that one oxygen vacancy site anchors up to three adsorbed O2 molecules [28]. Therefore, the site blocking effect of atomically adsorbed oxygen should be smaller than that for molecularly adsorbed oxygen; hence, the expected increase in S0 with O2 exposure (above Tads = 150 K) is larger. The O2ACO2 coadsorption experiments allow a precise determination of the CO2 binding energy on pristine sites. The shape and position of the a1 TDS peak (at 140 K) can be well reproduced (dotted line in Fig. 3A) by assuming Ed = 43–10 H kJ/mol for the heat of adsorption and m = 4 · 1013 1/s for the preexponential factor. Thus, overall repulsive interaction of CO2 is present. The binding energy on defect sites (a2 peak at 180 K) can, based on the peak position, be estimated to be 48 kJ/mol. The decrease in S0 with H pre-exposure (Fig. 4) indicates simply a physical site blocking and little effect of surface defects on the adsorption of hydrogen for TiO2(1 1 0), which is consistent with the CO2 TDS data on a H precovered TiO2(1 1 0) surface [22]. Furthermore, a He atom scattering study [21] revealed the formation of an ordered H overlayer on TiO2(1 1 0) without etching of the surface by hydrogen that has been observed for other [21] metal oxides. 5. Summary We correlated qualitatively the density of oxygen vacancy, sites, C, on TiO2(1 1 0) with the reactivity of CO2 adsorption. The adsorption of CO2 obeys nonactivated and precursor mediated dynamics. The TDS
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results verify that oxygen adsorption leads to a reduction of the oxygen vacancy concentration. Interestingly, S0 of CO2 decreases with increasing C and increases with increasing oxygen preexposure. H pre-adsorption induces a physical site blocking with S0 decreasing with increasing H coverage. Although defects are the active sites for bond breaking processes (O2, H2O adsorption), they can, as shown here, reduce the reactivity for molecular adsorption processes. However, the overall effect of surface defects on the adsorption dynamics of CO2 is surprisingly small. Acknowledgment Acknowledgment is made to ‘The Donors of the ACS Petroleum Research Fund’ for financial support. References [1] C. Becker, C.R. Henry, Surface Science 352–354 (1996) 457. [2] D. Brinkley, M. Dietrich, T. Engel, P. Farrall, G. Ganter, A. Schafer, A. Szuchmacher, Surface Science 395 (1998) 292. [3] T. Becker, C. Boas, U. Burghaus, C. Wo¨ll, Physical Review B 61 (2000) 4538. [4] M. Kunat, U. Burghaus, Surface Science 544 (2003) 170. [5] Z. Dohnalek, G. Kimmel, S. Joyce, P. Ayotte, R. Smith, B. Kay, Journal of Physical Chemistry B 105 (2001) 3747. [6] J. Libuda, H.J. Freund, Surface Science Reports 57 (2005) 157. [7] P.A. Cox, Transition Metal Oxides, Clarendon Press, Oxford, 1995. [8] M.A. Henderson, Surface Science 319 (1994) 315.
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[9] M.A. Henderson, Surface Science Reports 46 (2002) 1. [10] L.Q. Wang, D.R. Baer, M.H. Engelhard, A.N. Shultz, Surface Science 237–250 (1995) 344. [11] B.K. Min, W.T. Wallace, D.W. Goodman, Surface Science 600 (2006) L7. [12] S. Wendt, R. Schaub, J. Matthiesen, E.K. Vestergaard, E. Wahlstro¨m, M.D. Rasmussen, P. Thostrup, L.M. Molina, E. Lægsgaard, I. Stensgaard, F. Besenbacher, Surface Science 598 (2005) 226. [13] M.A. Henderson, Surface Science 400 (1998) 203. [14] T.L. Thompson, O. Diwald, J.T. Yates, Journal of Physical Chemistry 107 (2003) 11700. [15] S.A. French et al., Angewandte Chemie 113 (2001) 4569. [16] G. Pacchioni, A.M. Ferrari, P.S. Bagus, Surface Science 350 (1996) 159. [17] J. Wang, U. Burghaus, Chemical Physics Letters 403 (2005) 42– 46. [18] J. Wang, U. Burghaus, Journal of Chemical Physics 122 (2005) 044705. [19] C.L. Kao, A. Carlsson, R.J. Madix, Surface Science 497 (2002) 356. [20] S. Funk, B. Hokkanen, J. Wang, U. Burghaus, G.H. Bozzolo, J.E. Garces, Surface Science 600 (2006) 583. [21] M. Kunat, U. Burghaus, C. Wo¨ll, Physical Chemistry Chemical Physics 6 (2004) 4203–4207. [22] S. Funk, U. Burghaus, Journal of Chemical Physics (submitted for publication). [23] J. Stephan, U. Burghaus, Surface Science 507–510 (2002) 736. [24] U. Burghaus, Surface Review and Letters 8 (2001) 353. [25] P. Kisliuk, Journal of Physics and Chemistry of Solids 5 (1958) 78. [26] M.A. Henderson, Surface Science 419 (1999) 174. [27] U. Burghaus, Surface Review and Letters 10 (2003) 7. [28] M.A. Henderson et al., Journal of Physical Chemistry B 103 (1999) 5328.
Effect of oxygen vacancy sites on CO2 adsorption dynamics: The case of rutile (1 · 1)-TiO2 (1 1 0) S. Funk, B. Hokkanen, E. Johnson, U. Burghaus
*
Department of Chemistry, Biochemistry, and Molecular Biology, North Dakota State University, 1231 Albrecht Avenue/Ladd Hall, Fargo, ND 58105-5516, USA Received 8 February 2006; in final form 1 March 2006 Available online 9 March 2006
Abstract The adsorption of CO2 has been studied by molecular beam scattering on slightly and strongly reduced TiO2(1 1 0). The defect density has qualitatively been characterized by CO2 TDS. Two peaks are present in TDS, which can be assigned to CO2 adsorption on pristine and oxygen vacancy sites. Interestingly, the initial reactivity, S0, towards CO2 adsorption decreases with increasing defect density and increases with increasing oxygen preexposure. The latter is related to the vanishing of the defect structure in TDS. In contrast, hydrogen preexposure leads to a decrease in S0 as well as the intensity of both TDS peaks decrease in sympathy. 2006 Elsevier B.V. All rights reserved.
1. Introduction Few [1–6] molecular beam scattering studies have been conducted on metal oxides. Furthermore, TiO2 developed into a prototype system for surface science studies on model catalysts [7]. Surface defects are the active sites for O2 and H2O dissociation [8–10] and metal nano-clusters nucleation [11]. Therefore, a further characterization of surface defects is important to gain a mechanistic understanding of adsorption processes and surface reactions. Ultra-high vacuum annealing above 600 K leads to the most well characterized defects; oxygen vacancy sites form [12]. The adsorption of O2 leads to ‘healing out’ of these defects. As discussed by Hendersson [13], Yates [14], and others, CO2 is an ideal probe molecule to quantify the effect of oxygen vacancy sites on TiO2(1 1 0) by thermal desorption spectroscopy (TDS). Furthermore, CO2 is the carbon feedstock for surface reactions such as methanol synthesis. In theoretical studies [15], it has been suggested that oxygen *
Corresponding author. Fax: +1 701 231 8831. E-mail addresses: [email protected], uwe.burghaus@ndsu. nodak.edu (U. Burghaus). URL: www.chem.ndsu.nodak.edu (U. Burghaus). 0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.03.007
vacancy sites lead to bent CO2 adsorbates which open up lower energy pathways for subsequent hydrogenation reactions. Furthermore, according to recent theoretical studies [16], the interaction of defect sites with metal nano clusters can lead to charging of these clusters which affects surface reaction mechanisms. A single CO2 TDS peak (hereafter, a1 peak) appears on the oxidized surface which was assigned to adsorption on fivefold coordinated Ti4+ sites [13]. Annealing at temperatures above 600 K resulted in an additional high temperature TDS peak (a2 peak), assigned to adsorption on Ti3+ oxygen vacancy sites [14]. Similar effects have recently been revealed for CO2/ZnO(0 0 0 1) [17], suggesting that CO2 may be an interesting probe of surface defects for a plethora of metal oxides. Annealing above 1000 K has been avoided in this study since it can induce the (2 · 1) TiO2(1 1 0) reconstruction. Even the nearly perfect surface consists, according to scanning tunneling microscopy (STM) results, [12] of 10% oxygen vacancy sites which depends on the history of the sample preparation, i.e., the defect density, C, should be estimated in any study about TiO2(1 1 0). Molecular beam scattering experiments with CO2 have been conducted for ZnO(0 0 0 1)AZn, Pt(1 1 1), Pd(1 1 1),
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and Cu(1 1 0) surfaces [18–20]. For these systems, molecular, precursor mediated, and non-activated adsorption has been observed. In this study, we characterize the macroscopic effect of surface defects on the reactivity of the system by means of molecular beam scattering and TDS with the goal to address the structure–activity relationship. Whereas annealing the surfaces leads to an increase in C, preadsorption of O2 decreases C. In contrast, hydrogen coadsorption at low temperatures leads to simple site blocking effects [21]. 2. Experimental procedures The measurements have been conducted by a supersonic molecular beam system [18]. The statistical error in S0 is below ±0.03. The S(H) and TDS data have been smoothed while conserving the shape of the curves. All adsorption probabilities have been obtained for normal impact on the surface. A saturation coverage of 1 ML of CO2 has been assumed for Ts 85 K, as is common. All TDS curves are obtained for surfaces saturated with CO2 by dosing the gas for 7 min with the beam system; a heating rate of 1.5 K/s has been used. The reading of the thermocouple has been calibrated in situ by TDS measurements of n-butane (multilayer desorption peak at 108 K). H2 has been dissociated on a hot tungsten filament before adsorption. The scattering chamber has been backfilled with H2 at a pressure of 1 · 10 6 mbar through a dosing capillary, which increases the H2 flux by a factor of 10 as compared to backfilling by a leak valve. O2 has been dosed by backfilling. Cold traps have been used to avoid H2O contamination. A combined TDS-adsorption probability measurement required less than 15 min; the base pressure of the scattering chamber is in the low 10 10 mbar range. The cleanliness of the samples have been characterized by Auger electron spectroscopy and the crystallographic order by low energy electron diffraction and TDS.
Fig. 1. The initial adsorption probability as a function of impact energy, Ei, for CO2/TiO2(1 1 0). The inset shows CO2 TDS of the two different crystals.
The coverage dependent adsorption probability, S(H), of CO2 shown in Fig. 2 depends only weakly on coverage until the surface saturates and S drops naturally to zero. Besides differences in S0, both samples led to identical curve shapes. The Monte Carlo simulation (MCS) [23,24] (solid lines in Fig. 2) used to fit the data includes two fit parameters; S0 and Soccu. The adsorption probability in the extrinsic precursor state, Soccu, decreases linearly from 0.40 to 0.18 within Ei = 0.33–1.29 eV. On samples cleaned with the initial preparation procedure, further defects have been induced by annealing at 900 K for 1 h in vacuum [22]. This procedure leads to distinctly asymmetric CO2 TDS curves (see the thick solid curves in Fig. 3) and a 0.1 decrease in S0. The large intensity of the a2 structure with respect to the a1 peak indicates a large density of oxygen vacancy sites, C, on the vacuum annealed surfaces [14]. However, note that it is very
3. Presentation of the results Two crystals have been studied (I: Princeton Scientific, II: MaTeck) which have been cleaned initially by a procedure consisting of 20 cycles of Ar+ sputtering (600 V, 5 lA, 30 min, Ts = 300 K) followed by annealing in oxygen (1 · 10 6 Torr, Ts 600 K, 10 min) and in vacuum (Ts = 750 K, 5 min). The intensity of the CO2 TDS defect peak (a2 peak) was for sample I distinctly larger than for sample II (see the inset of Fig. 1), indicating a larger C of the former crystal. The initial adsorption probability, S0, of CO2 decreases linearly with increasing impact energy, Ei, and is systematically smaller by 0.2 for sample I (Fig. 1). Thus, S0 on vacuum annealed (or sputtered without annealing) surfaces is smaller than on more strongly oxidized surfaces (see Ref. [22] for details).
Fig. 2. S(H) of CO2 for the second sample of TiO2(1 1 0), parametric in Ei and at 85 K. The lines are the result of Monte Carlo simulations.
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Fig. 4. S0 of CO2 on oxygen (Ei = 0.56 eV) or H predosed (Ei = 0.12 eV) surfaces. (Sample II).
Fig. 3. Shown are CO2 TDS curves as a function of O2 preexposure: (A) at 85 K and (B) at 250 K. The solid spheres in panel A are the result of a simulation. (Sample II).
large (>20 L) O2 exposures. Although cold traps have been used in the gas line, the latter effect is most likely associated with H2O contamination of the O2 gas; H2O can stabilize CO2 (see Ref. [13]). The solid spheres in Fig. 3A are the result of integrating the Polanyi–Wigner equation. S0, as a function of O2 and hydrogen preexposure and parametric in O2 adsorption temperature, is shown in Fig. 4. Oxygen preexposures lead, in all cases, to a small (0.1) but well reproducible increase in S0. This increase is slightly larger for atomically adsorbed than for molecularly adsorbed oxygen. In contrast, preexposed atomic hydrogen leads to a distinct exponential decrease in S0 with increasing H exposure. 4. Discussion
difficult to obtain identical C on the samples throughout the experiments. The effect of preadsorbed atomic oxygen and hydrogen on the adsorption of CO2 has been studied for the defected surface by TDS (Fig. 3) and adsorption probability measurements (Fig. 4). First oxygen or atomic hydrogen (Tads = 85 K) has been preadsorbed. Afterwards, the remaining clean areas of the surface have been saturated with CO2 (at Tads = 85 K) by the molecular beam system, recording simultaneously an adsorption transient. Subsequently a CO2 TDS curve has been detected. This measuring procedure has been repeated for different preexposures of the coadsorbates and in the case of oxygen for molecularly (Tads = 85 K) and atomically (Tads = 250 K) adsorbed oxygen [13]. At 0.4–0.6 L of O2 the a2 TDS peak disappears (Fig. 3) independent of the O2 adsorption temperature. (H preexposure leads to a decrease in both CO2 TDS peaks; see Ref. [22] for details.) For atomically adsorbed oxygen, the intensity of the a1 TDS peak is conserved, even for large (20 L) O2 preexposures. For molecularly adsorbed oxygen, an increase of the a1 TDS peak has been observed for very
4.1. Adsorption dynamics of CO2 The decrease in S0 with Ei (Fig. 1) is commonly observed for weakly adsorbing adsorbates and indicates molecular and non-activated adsorption. Furthermore, S0 is independent of the adsorption temperature, Ts, as expected [3,4] for non-activated molecular adsorption [22]. The shape of S(H, Ei) curves (Fig. 2) is in agreement with precursormediated adsorption, as predicted by the Kisliuk model [25]. Furthermore, a crossover from Kisliuk-like to Langmuirian-like S(H, Ts) curve shapes is observed [22] with increasing Ts which simply indicates a competition of thermal desorption and adsorption (via the molecular beam) of CO2, as commonly observed for physisorption systems [3,4]. The MCS [23,24] (solid lines in Fig. 2) adopt the idea of the Kisliuk model. According to this MCS the trapping probability into the precursor state decreases by a factor of approximately two with increasing Ei indicating less efficient gas-surface energy transfer processes at large kinetic energies of the gas phase species, as expected. Thus, on a
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quick glance, the adsorption dynamics of CO2 on TiO2(1 1 0) obeys well established standard models which have also been revealed for other metal oxide surfaces. However, interestingly, S0 depends distinctly on the density of defects. 4.2. Effect of oxygen vacancy sites on adsorption dynamics of CO2 The result that the a2 peak intensity increases with respect to the one of the a1 peak by annealing the samples at 900 K in vacuum [22] as well as the quenching of the a2 peak with increasing oxygen preexposure (Fig. 3) is in agreement with earlier reports [13,14] and the assignment of the a1 and a2 peaks to Ti4+ (pristine) sites and Ti3+ oxygen vacancy (defect) sites. A large a2 to a1 peak intensity ratio indicates a large density of oxygen vacancy sites. Although sample I and II have been cleaned by literally identical procedures, their C are quite different. Sample I has (inset Fig. 1) a larger density of oxygen vacancy sites and a smaller S0 (Fig. 1). However, S0 is a macroscopic property. Therefore, the adsorption probability on different sites cannot directly and easily be distinguished. The variations in S0 seen for the different samples are consistent with a recent STM study [12] which emphasized that the very details of the sample cleaning procedures and the total project time affects the density of surface defects. Furthermore, these results appear plausible in regard of the bulk diffusion mechanism, by which the density of surface defects is controlled, which has been proposed before [26]. The density of surface defects affects S0 (Fig. 1) but does not significantly affect the shape of the adsorption probability curves [22]. Therefore, the extrinsic precursor dynamics of CO2 on TiO2(1 1 0) is little affected by oxygen vacancy sites. This is not necessarily expected since measurements on other systems [27] revealed distinct variations of the shape of S(H) curves with C. However, the decrease in S0 with increasing C indicates variations of the lattice force constants near defect sites. 4.3. Effect of preadsorbates – ‘healing’ of surface defects Preadsorbates and defects can lead to conceptually similar effects [23]. Therefore, we collected kinetics (TDS) and dynamics (adsorption probabilities) data for CO2 adsorption on TiO2(1 1 0) preadsorbed with hydrogen or oxygen. Furthermore, it is well known that oxygen can ‘heal’ defects [13]; offering the possibility to provide another test of our conclusions. Oxygen adsorption leads to quenching of the a2 TDS peak (Fig. 3), i.e., the oxygen vacancy sites, indeed, heal out and this TDS peak is indeed related to oxygen vacancy sites. In contrast, H preadsorption reduced the a1 and a2 TDS peak intensities approximately by the same amounts [22]. We may expect an increase in S0 with increasing density of oxygen vacancy sites [23] since new adsorption sites for
CO2 form. Preadsorbates should lead, in the simplest case, to a decrease in S0, since they simply block adsorption sites. However, the experimental evidence contradicts, at least in part, these simple hypotheses. S0 decreases with increasing H preadsorption (as expected) but increases with increasing O2 preadsorption (Fig. 4). The latter effect is consistent with the observed decrease in S0 with C (Fig. 1) but is rather unexpected. However, in regard of the crystallographic structure of TiO2(1 1 0), it appears plausible that the adsorption of CO2 on oxygen vacancy sites is sterically hindered, which would reduce the probability of this process [23]. Thus, S0 would decrease with C, as consistent with the results obtained on the two different samples (Fig. 1), and S0 should increase when oxygen vacancy sites are filled by preadsorbed oxygen, as observed (Fig. 4). However, oxygen does not only heal out oxygen vacancy sites; it leads also to blocking of additional Ti sites [28]. (O2 from the gas-phase dissociates on defect sites. One oxygen atom fills the oxygen vacancy, the other adsorbs on an adjacent Ti site.) In the former case, S0 should increase due to the decrease in C but this effect competes with a decrease in S0 expected due to the blocking of Ti sites. These competing processes may explain the rather small effect of preadsorbed oxygen on S0. Furthermore, molecularly adsorbed O2 induces a slightly smaller increase in S0 than atomically adsorbed oxygen (Fig. 4) which is in agreement with the proposal that one oxygen vacancy site anchors up to three adsorbed O2 molecules [28]. Therefore, the site blocking effect of atomically adsorbed oxygen should be smaller than that for molecularly adsorbed oxygen; hence, the expected increase in S0 with O2 exposure (above Tads = 150 K) is larger. The O2ACO2 coadsorption experiments allow a precise determination of the CO2 binding energy on pristine sites. The shape and position of the a1 TDS peak (at 140 K) can be well reproduced (dotted line in Fig. 3A) by assuming Ed = 43–10 H kJ/mol for the heat of adsorption and m = 4 · 1013 1/s for the preexponential factor. Thus, overall repulsive interaction of CO2 is present. The binding energy on defect sites (a2 peak at 180 K) can, based on the peak position, be estimated to be 48 kJ/mol. The decrease in S0 with H pre-exposure (Fig. 4) indicates simply a physical site blocking and little effect of surface defects on the adsorption of hydrogen for TiO2(1 1 0), which is consistent with the CO2 TDS data on a H precovered TiO2(1 1 0) surface [22]. Furthermore, a He atom scattering study [21] revealed the formation of an ordered H overlayer on TiO2(1 1 0) without etching of the surface by hydrogen that has been observed for other [21] metal oxides. 5. Summary We correlated qualitatively the density of oxygen vacancy, sites, C, on TiO2(1 1 0) with the reactivity of CO2 adsorption. The adsorption of CO2 obeys nonactivated and precursor mediated dynamics. The TDS
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results verify that oxygen adsorption leads to a reduction of the oxygen vacancy concentration. Interestingly, S0 of CO2 decreases with increasing C and increases with increasing oxygen preexposure. H pre-adsorption induces a physical site blocking with S0 decreasing with increasing H coverage. Although defects are the active sites for bond breaking processes (O2, H2O adsorption), they can, as shown here, reduce the reactivity for molecular adsorption processes. However, the overall effect of surface defects on the adsorption dynamics of CO2 is surprisingly small. Acknowledgment Acknowledgment is made to ‘The Donors of the ACS Petroleum Research Fund’ for financial support. References [1] C. Becker, C.R. Henry, Surface Science 352–354 (1996) 457. [2] D. Brinkley, M. Dietrich, T. Engel, P. Farrall, G. Ganter, A. Schafer, A. Szuchmacher, Surface Science 395 (1998) 292. [3] T. Becker, C. Boas, U. Burghaus, C. Wo¨ll, Physical Review B 61 (2000) 4538. [4] M. Kunat, U. Burghaus, Surface Science 544 (2003) 170. [5] Z. Dohnalek, G. Kimmel, S. Joyce, P. Ayotte, R. Smith, B. Kay, Journal of Physical Chemistry B 105 (2001) 3747. [6] J. Libuda, H.J. Freund, Surface Science Reports 57 (2005) 157. [7] P.A. Cox, Transition Metal Oxides, Clarendon Press, Oxford, 1995. [8] M.A. Henderson, Surface Science 319 (1994) 315.
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