Adsorption of chlorine on ZnO(0001)–Zn and coadsorption with HCOOH

Surface Science 458 (2000) 71–79 www.elsevier.nl/locate/susc

Adsorption of chlorine on ZnO(0001)–Zn and coadsorption with HCOOH Ann W. Grant, Andrew Jamieson, Charles T. Campbell * Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA Received 10 December 1999; accepted for publication 2 March 2000

Abstract The adsorption of Cl on the zinc-terminated ZnO(0001) surface at 300 K was studied with low-energy ion2 scattering spectroscopy (ISS ), X-ray photoelectron spectroscopy ( XPS), angle-resolved XPS (ARXPS ), and workfunction and band-bending measurements. The surface saturated with ~0.30 chlorine adatoms [Cl(a)] per zinc site, sitting above the zinc layer in no apparent registry with the substrate. Combined work-function and band-bending measurements indicate anionic Cl(a). Using formic acid to produce surface formate, the effect of Cl(a) on formate decomposition was studied also. On the chlorine-free ZnO(0001) surface, formate decomposes through two different pathways: dehydration and dehydrogenation. Pre-adsorbed chlorine suppresses the amount of adsorbed formate produced, and enhances the selectivity for its dehydrogenation. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Chlorine; Ion scattering spectroscopy; Surface electronic phenomena (work function, surface potential, surface states, etc.); X-ray photoelectron spectroscopy

1. Introduction Chlorine is a common impurity in oxide-supported metal catalysts. It is known to influence the dispersion of supported metal particles [1,2]. For instance, in low-temperature water gas shift catalysts, primarily Cu/ZnO, it is found that chlorine, in the form of HCl gas, acts to increase the sintering rates of the copper particles [2]. Chlorine, as a strong electron acceptor, can act also as a bonding modifier [3,4] by either strengthening or weakening the adsorbate–substrate bond. Chlorine is added to some catalysts as a modifier to increase selectivity, for instance in ethylene epoxidation on alumina-supported silver, where the addition of a * Corresponding author. Fax: +1-206-685-8665. E-mail address: [email protected] (C.T. Campbell )

few pm of ethylene dichloride suppresses the unwanted production of CO [5]. Therefore, the 2 interactions of chlorine with oxide surfaces are of fundamental interest. Here, we examine the reactivity of Cl with well-defined ZnO(0001)–Zn 2 surfaces. There are a few prior studies of chlorine adsorption on single-crystal ZnO surfaces examining structural and electronic properties. Changes in the conductivity of several metal oxide surfaces in response to exposure to Cl gas have been studied 2 [6 ]. ZnO was a main focus of study since it was well known to readily change in conductivity with adsorbed gases, and a potential candidate for Cl 2 gas sensors. Hopkins and Taylor [7] focused on the structural properties of Cl/ZnO(0001) by utilizing Auger electron spectroscopy (AES ), low-energy

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electron diffraction (LEED) and work-function measurements. They reported that Cl dissociates 2 upon adsorption at 300 K up to a saturation coverage of ~0.25 chlorine atoms per zinc surface atom. The Cl(a) was stable until ~800 K, where the Cl/O AES ratio decreases. Some chlorine was still present on the surface after heating to 1100 K. They found a work-function change of ~1 eV after saturation (retarding potential method ). Their LEED results showed that after first saturating the (1×1) surface, there was an overall increase in background intensity, but no discernible LEED pattern. After annealing for 20 min at ~700 K, a diffuse (2E3×2E3) pattern emerged. With several cycles of Cl saturation and annealing to between 2 950 and 1100 K, the surface reconstructed, and the chlorine could not be removed with sputtering. In addition, G. Thornton and C.A. Muryn (personal communication), while looking at Cl(a) on ZnO(0001) at 100 K, were not able to find any post-edge structure in surface extended X-ray absorption fine structure (SEXAFS), even though they found some edge structure which indicated to them that the Cl(a) was disordered on the surface or highly mobile. Not surprisingly, the oxygenterminated surface of ZnO showed far less reactivity to Cl [8]. 2 As part of an ongoing study of Cu/ZnO model catalysts, the effect of chlorine on both clean ZnO(0001)–Zn and on ultrathin films of copper grown on ZnO(0001)–Zn was investigated. The latter is described in a separate paper [9]. Here, we study the structural and electronic properties of the chlorine adlayer. In addition, we briefly look at how it affects formic acid decomposition reactions on ZnO(0001)–Zn. Cl(a) is found to act as both a site blocker and a bonding modifier, both by raising the temperature at which formate decomposition occurs and by altering the selectivity of formate decomposition towards more dehydrogenation rather than dehydration.

2. Experimental All experiments were performed in an ultrahigh vacuum apparatus ( UHV ) described previously

[10,11] with a base pressure of ~2×10−10 Torr during experiments and ~6×10−11 Torr normally. This system is capable of performing X-ray photoelectron spectroscopy ( XPS), ion-scattering spectroscopy (ISS ), LEED, and temperature-programmed desorption ( TPD) using a quadrupole mass spectrometer interfaced to a personal computer for multiplexing masses. All of the XPS spectra were taken with Al Ka radiation (1486.6 eV ). The binding energy of the spectrometer was calibrated with the Zn(2p ) 3/2 peak from the clean ZnO(0001) surface, which was set to 1021.7 eV [25]. The ISS spectra were taken with a primary-ion energy of 700 eV, and a helium pressure of 4×10−7 Torr as read by the ion gage (the ion current is ~20 nA cm−2). The ion beam was incident at 45° from the surface normal and the detection angle was normal to the surface, giving a scattering angle of 135°. Workfunction changes were measured by following the onset of secondary-electron distribution in XPS. During these experiments, the sample was negatively biased (−15.7), and a very low pass energy (8.95 eV ) for high resolution was used with a very low X-ray flux (20 W, 13 V ). All TPD spectra were taken with a heating rate of ~5 K s−1. The zinc-terminated ZnO crystal was the same as described in Ref. [12]. The details of the preparation, mounting and cleaning are described in that paper. In addition, the temperature of the ZnO(0001)–Zn crystal during TPD experiments was calibrated to the known formate decomposition temperature as described previously [12]. This was necessary as the thermocouple was mounted to the molybdenum sample holder, and not the ZnO(0001) itself. Chlorine was dosed on to the sample through a dosing tube connected to the gas reservoir, containing a 3.5% mixture of Cl in N , by a leak 2 2 valve. This directed doser pinhole was 1.5 cm from the surface and gives an enhancement factor of ~10 over a background dose [13]. However, it was found during the initial experiments that a pre-conditioning dose of the Cl /N mixture was 2 2 needed in order to saturate the walls of the dosing tube with Cl , or very little of the Cl came out of 2 2 the doser in the Cl /N mixture. Thus, the doser 2 2

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assembly was conditioned to new Cl exposures 2 prior to each new experiment. Still, the Cl :N 2 2 ratio in the mixture used was probably less than that in the gas reservoir. The formic acid used in this experiment was 98% pure (Sigma-Aldrich) with water as the major impurity. It was further purified by several freeze– pump–thaw cycles. The dosing method was similar to that used with the Cl /N mixture described 2 2 above. All formic acid exposures were ~3 L [1 Langmuir (L)=10−6 Torr s], and this exposure has been found to saturate the surface [12]. During dosing, there was a system pressure rise of ~2×10−9 Torr. The purity of the formic acid was verified with mass spectrometry. The HCOOH doser assembly was conditioned at the beginning of each day, and prior to each experiment.

3. Results and discussion

3.1. Cl adsorption on ZnO(0001)–Zn 2 Exposing the clean (chlorine-free) ZnO(0001)– Zn surface at room temperature to Cl led to an 2 uptake curve that saturated at a normalized chlorine XPS intensity, I[Cl(2p)]/I[Zn(2p )]0, of 3/2 0.0048, where I[Zn(2p )]0 is the integrated XPS 3/2 intensity from the clean Zn(2p ) peak. A dose of 3/2 ~40 L was sufficient to reach >80% saturation. At low exposures, the initial slope of this curve is proportional to the initial sticking coefficient, S . 0 By assuming that the Cl :N ratio coming out of 2 2 the doser is the same as in the gas reservoir (3.5% Cl ) and that the enhancement factor due to the 2 pinhole doser is 10 [13], it was found from this slope that S is ~0.06. This S is low compared 0 0 with literature values for the initial sticking coefficient of Cl on metals [such as Pt(111), Cu(100) 2 and Au(111)], which ranges from 0.5 to 1 for adsorption at 300 K [14–16 ]. However, all of these studies used either an AgCl electrochemical cell or pure Cl (g) as chlorine sources. Our sticking 2 coefficient may be lower due to a lower percentage of Cl in the Cl /N mixture coming out of the 2 2 2 doser than in the gas reservoir (see above). Therefore, this initial sticking coefficient should be

Fig. 1. Variations in the zinc and chlorine ISS integrated intensities with increasing chlorine coverage as measured by the XPS intensity ratio of Cl(2p) to chlorine-free Zn(2p ) XPS areas. 3/2 Notice also that the top axis is defined in units of monolayers (ML), or the saturation coverage of Cl(a).

taken as a lower limit. However, Cl may be less 2 reactive with oxide surfaces than with typical metals, since the reaction of Cl is not as enthalpi2 cally favorable as with Cu(s). ( The standard enthalpy change for the reaction ZnO(s)+ Cl (g)ZnCl +1/2O (g) is −16.0 kcal mol−1, 2 2 2 whereas the standard enthalpy of formation of CuCl (s) is −52.6 kcal mol−1 [20].) 2 The variations in the integrated areas of the chlorine ISS signal and the zinc ISS signals with Cl(a) coverage (as measured by the I[Cl(2p)]/I[Zn(2p )]0 XPS integrated intensity 3/2 ratio) are shown in Fig. 1. There is a linear increase in the chlorine ISS signal with chlorine coverage and a linear decrease in the zinc ISS intensity to almost zero at saturation. This would be expected if the chlorine forms a simple, atomically adsorbed but nearly close-packed overlayer, since ISS probes to a depth of only ~0.1 nm [17]. The point at which a linear best fit of the zinc ISS data crosses

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the I[Cl(2p)]/I[Zn(2p )]0 axis (at 0.0048) is 3/2 defined as the saturation coverage of Cl(a), which we shall define as one monolayer (1 ML). Chlorine coverages (h ) in ML are shown on the top axis Cl of Fig. 1. The number of chlorine atoms on the surface at saturation coverage was estimated to be ~0.30 chlorine atoms per zinc atom from the observed attenuation of the Zn(2p ) XPS signal. 3/2 I[Zn(2p )]/I[Zn(2p )]0 is 0.88 at saturation. 3/2 3/2 Assuming that the inelastic mean free path of ˚ through ZnO Zn(2p ) photoelectrons, l , is 8 A 3/2 Zn [8,16 ], the effective thickness of the chlorine film, ˚ , using I[Zn(2p )]/ t, was calculated to be 1.02 A 3/2 I[Zn(2p )]0=exp(−t/l ) [18]. Similarly, the 3/2 Zn observed XPS ratio, I[Cl(2p)]/I[Zn(2p )]0= 3/2 0.0048, was converted to a chlorine thickness, t, assuming an inelastic mean free path for Cl(2p) ˚ [19] and bulk sensitivity photoelectrons of 30 A factors of 0.73 for Cl(2p) and 4.8 for Zn(2p ), 3/2 following [18]. An effective thickness for Cl(a) of ˚ was found. These two chlorine thickness 1.5 A values show relatively good agreement. From ˚ ) and the known packing dentheir average (1.3 A sity of elemental chlorine (as pure liquid, 2.65×1022 Cl atoms cm−3 [20]), the surface atomic density of 1 ML of Cl(a) is found to be 3.5×1014 Cl atoms cm−2. Knowing that the number of zinc sites on the surface of the ZnO(0001)–Zn face is 1.1×1015 Zn atoms cm−2 [10,12], this corresponds to ~0.30 chlorine atoms per zinc site on the surface at 1 ML. This packing density is about half what one would predict (8.8×1014 Cl atoms cm−2) assuming a hexagonally close-packed layer of spheres with radius ˚ , the anionic radius of Cl− [20]. This sug1.81 A gests that there may be repulsive lateral interactions or kinetic effects that limit the amount of Cl(a). We found with mass spectrometry that neither atomic chlorine, HCl, Cl nor ZnCl desorbs upon 2 annealing the chlorine-saturated surface to ~750 K, although the Cl(2p) XPS intensity and the chlorine ISS intensity both decreased by ~35%. This would be consistent with the loss of chlorine atoms into the bulk or into thick, threedimensional islands of ZnCl . x The chlorine-free ZnO(0001) surface shows

strong angular variation due to photoelectron diffraction in the Zn(2p ) and the O(1s) XPS 3/2 intensities [17], as shown in the solid curves of Figs. 2a and b. The major forward focusing peaks occur at 0°, 36° and 55°, and 0°, 18° and 33°, respectively, as shown by the arrows. These reproduce earlier results at this azimuth [17], and are at positions expected for bulk termination. No new forward scattering peaks developed after chlorine saturation in either the Zn(2p ) or the O(1s) 3/2 angular distribution, as shown by the dotted curves. The peaks were attenuated overall, however, as expected from the presence of a saturation coverage of an adsorbate. This is evidence that the Cl(a) is isotropically (or amorphously) distributed on the ZnO(0001) surface, and does not prefer a specific site on the surface, consistent with the disorder seen in LEED. The variation of the Cl(2p) XPS peak along this same azimuth is shown in Fig. 2b, at a saturation chlorine coverage. No forward scattering peaks are seen above the noise. This is not surprising, as scattering atoms above the emitting atom are required for forward focusing peaks. This implies that Cl(a) sits above the topmost zinc plane, as was also clear from ISS. In addition, this makes it obvious that Cl(a) does not agglomerate into ordered three-dimensional islands. Adsorption on semiconductors is often accompanied by band bending, which is monitored by following the shift in substrate core levels with respect to the Fermi level (E =spectrometer referF ence). The band-bending changes with increasing chlorine coverage on the ZnO(0001) surface at room temperature are shown in Fig. 3. Here, the band bending is monitored by the shift in binding energy (BE ) of the Zn(2p ) core level. The O(1s) 3/2 peak shift was similar. At low chlorine coverages (<0.2 ML), the binding energy of these peaks decreases rapidly, reflecting an upward band bending, associated with long-range electron transfer from the ZnO depletion layer to the adsorbed chlorine. This shifting saturates at ~0.8 eV at >0.2 ML, and thereafter remains constant. The Cl(2p) binding energy undergoes a similar shift with increasing Cl(a) coverage, which we attribute to this same band bending. Thus, the chlorine core levels track those of the substrate surface atoms,

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Fig. 3. Changes with Cl(a) coverage (compared with the clean surface value, E0) in the work function (DW) and the binding energy of the Zn(2p ) peak relative to the Fermi level (E ), 3/2 F during Cl adsorption at room temperature on to ZnO(0001). 2 The latter value gives the chlorine-induced band bending, with the negative change in binding energy (BE) indicating upward band bending. The sum of these two curves, also shown, gives the chlorine-induced downward shift in the valence-band maximum ( VBM ) relative to the vacuum level (E ). The slope of vac this difference curve is directly proportional to the local surface dipole moment of the ClMZnO complex.

(a)

(b) Fig. 2. (a) The top panel shows variations in the integrated intensities of the Zn(2p ) XPS peak with polar angle from the 3/2 surface normal, along the 1: 100 azimuth of the ZnO(0001)– Zn surface at room temperature. The solid line shows the peak intensities on the clean surface, and the broken line shows the peak intensities on the chlorine-saturated surface (1.0 ML) also at room temperature. These curves have been normalized to the XPS intensity at 0° for clarity. The bottom panel is the same for the O(1s) peak. (b) The variations in the Cl(2p) XPS area with polar angle from the surface normal.

as expected for strong binding to the substrate. Since ClO species have a Cl(2p) binding energy x of ~201 eV [25], these results suggest that no such anions are forming at high concentrations. As seen also in Fig. 3, the work function increased rapidly by ~1 eV within the first 0.08 ML. After this initial sharp increase in slope, the work function increases steadily until it reaches a final change of ~2.4 eV. The sum of the band-bending and work-function curves in semiconductors gives the shift of the position of the valence-band maximum ( VBM ) relative to the vacuum level [21,23,29,31]. This value is shown also in Fig. 3 versus h . Its slope Cl can be interpreted in the same way as the slope of work-function changes on metal surfaces, and it directly reflects the local surface dipole of the adsorbate–substrate complex [21]. The magnitude of the slope of this curve can be analyzed with the Helmholtz equation [21,23,29] to obtain the dipole moment of the adsorbate–substrate complex. Its positive value here indicates a dipole moment for

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the chlorine–substrate complex with its negative end pointing away from the solid, as expected for anionic Cl(a). Within the first ~0.1 ML, the largest slope is seen. This initial slope of ~6.2 eV ML−1 gives a dipole moment of ~5 D for the complex. Taking ˚ [22,24], we a typical ZnMCl bond length of 2.3 A can estimate the extent of short-range charge transfer from surface zinc atoms to chlorine. Assuming that the chlorine sits atop a zinc cation, this gives ~0.44e− per chlorine. On the other hand, assuming a three-fold hollow site, this gives ~1.2e−. Above this coverage, the constant slope indicates that there is no further change in ionicity with increasing Cl(a) coverage. This constant slope of ~1.1 eV ML−1 gives a local surface dipole of ~0.8 D and a charge transfer of −0.08e− per chlorine atom if the chlorine is at atop sites. This is close to the expected charge transfer based on estimated electronegativity differences between ZnO and Cl (2.37 and 3.1, respectively) of 0.12e− [23]. In a semi-empirical quantum mechanical study of the bonding of chlorine to ZnO(0001), Rodriguez [22] found that the charge transfer upon chlorine adsorption led to a charge of −0.76e on the chlorine, which is close to the initial short-range charge transfer observed. Note that both the long-range charge transfer (i.e., band bending) and the short-range surface dipole indicate transfer of electron density from the ZnO to the chlorine, but the magnitude per chlorine adatom is tiny for the long-range component. The magnitude of the saturation work-function change seen in Fig. 3 is about twice that reported by Hopkins and Taylor [7]. These may differ because we reached a higher saturation coverage, or because of different extents of band bending (not reported in [7]). The latter would arise if the Fermi level were pinned at different energies on the starting surfaces, due to defects or impurities. 3.2. HCOOH adsorption on Cl/ZnO(0001) Adsorbed formic acid dissociates on the chlorine-free zinc-terminated ZnO(0001) surface almost immediately at 300 K, forming an adsorbed formate and an adsorbed hydrogen [26,27]. This adsorbed formate undergoes two competing

decomposition steps upon heating, dehydrogenation (1) and dehydration (2): HCOO(a)CO (g)+H(a) 2 HCOO(a)CO(g)+OH(a).

(1) (2)

Here, H(a) refers to a surface hydrogen atom (possibly bonded to bridging oxygens of the lattice), which rapidly desorbs as H . The OH(a) 2 desorbs as H O. These products of formate decom2 position from the chlorine-free surface, CO , H , 2 2 CO and H O, are observed as TPD peaks desorb2 ing nearly simultaneously at ~575 K. The bottom curves a in Fig. 4 show the signals for the ions CO+ (m/e=44), H+ (m/e=2) and 2 2 CO+ (m/e=28) in the TPD spectrum from the chlorine-free ZnO(0001)–Zn surface after roomtemperature exposure of ~3 L HCOOH (taking into consideration the enhancement factor of ~10 from the pinhole doser). This dose has been shown to saturate the formate species [12]. The nearly simultaneous peaks for m/e=2, 28 and 44 at ~575 K are due to formate decomposition via reactions (1) and (2), and are very similar to those reported previously [12,27]. The small peaks at ~380–400 K in all three masses are attributable to molecular desorption of chemisorbed formic acid, not seen at slightly lower doses, but seen with much greater intensity if given multilayer doses at 150 K [12,27]. The slightly larger m/e=2 peak seen at ~380–400 K can be attributed also to the acid hydrogen from the adsorbed formic acid [12], which decomposes to produce adsorbed formate, HCOO(a) and H(a). The H O (m/e=18) 2 TPD peak (not shown) was quite similar to the CO (m/e=28) peak, as expected, but was noisier due to a higher background water pressure. The relative yield of the m/e 28:44 peaks, after correcting for the cracking pattern of CO at m/e=28 2 and for relative sensitivities, gives a CO:CO yield 2 ratio of 70:30, similar to a previous report [12]. This shows that the dehydration reaction (2) is favored over dehydrogenation on the chlorinefree surface. After ~0.95 ML of Cl(a) was pre-adsorbed on the surface, the same formic acid exposure gave curves d of Fig. 4. The formate decomposition peaks (the sum of the properly scaled CO+CO 2

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Fig. 4. TPD spectra of ZnO(0001)–Zn recorded after different pretreatments and then a saturation dose of HCOOH (~3 L, taking the enhancement factor of the doser into account) at room temperature: (a) chlorine-free ZnO(0001), (b) a Cl(a) coverage of 0.6 ML, (c) the same surface as (b) after annealing the surface to ~700 K, (d ) chlorine-saturated ZnO(0001), (e) the same surface as (d ) after annealing to 700 K. The three panels show the major products of formate decomposition: CO , H and CO. Baselines have been offset for clarity. 2 2

intensities) are attenuated by ~85%. This is attributed to a site-blocking phenomenon, where nearly of all the available Zn2+ sites are covered with Cl(a), thereby limiting the amount of adsorbed formate. The attenuation is not the full 95%, perhaps due to compression of the chlorine atoms

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into higher-density islands by the formate. Even with this coverage of Cl(a), CO (m/e=44) is still 2 formed with 34% of its initial yield. This peak appears at ~20 K higher temperature (600 K ). The other peaks are much more severely attenuated. Clearly, the selectivity of formate decomposition has changed. Primarily dehydrogenation, not dehydration, occurs on the nearly chlorine-saturated surface. On this surface no H was seen, and 2 more H O desorbed than expected based on the 2 CO yield. This suggests that the H(a) produced in formate dehydrogenation can more easily abstract a lattice oxygen to make H O when chlorine is 2 present. This seems reasonable, since chlorine atoms compete with oxygen atoms for the zinc. After annealing the surface with 0.95 ML of Cl(a) to ~700 K, curves e of Fig. 4 show the reappearance of the formate decomposition peaks. The desorption temperatures are ~15 K higher than for the clean surface, indicating the Cl(a) is still exerting an electronic effect on formate decomposition. Relative to the chlorine-free surface, the total CO+CO yield is 35–40%. Control experi2 ments with 1 ML of Cl(a) showed that annealing to ~750 K caused the Cl(2p) XPS intensity and the chlorine ISS intensity to decrease by ~35% (see above), consistent with this value. Pre-adsorption of 0.60 ML of chlorine yields changes in formic acid TPD similar to those seen with 0.95 ML. The formate decomposition peaks (sum of CO+CO ) were attenuated by ~50% 2 relative to the clean surface, as shown in curves b of Fig. 4. This is in agreement with that expected for simple site blocking by chlorine, whereby the fractional coverage of formate obtainable decreases as (1−h /hsat ). This decrease was Cl Cl entirely due to the loss of CO. The CO (m/e= 2 44) was not attenuated, but the H (m/e=2) was 2 attenuated by ~75% from the chlorine-free surface value. The CO yield, however, was attenuated by ~80% (after accounting for the m/e=28 intensity that is due to the cracking pattern of CO ). Again, 2 the temperature of the formate decomposition peaks increased by ~30 K. This increase in peak temperatures indicates that the activation energy is higher in the presence of Cl(a), and that Cl(a) acts as a bonding modifier. In addition, the H O 2 peak is attenuated less than expected from the

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attenuation of the CO, and we attribute the extra H O to the reaction between lattice oxygen and 2 surface hydrogen atoms (produced by formate dehydrogenation). After correcting for the cracking pattern and sensitivities, the relative yield of CO:CO at 0.6 ML Cl(a) was 30:70, compared 2 with 70:30 for clean ZnO(0001)–Zn [12]. Clearly, the selectivity in formate decomposition is shifted dramatically towards more dehydrogenation rather than dehydration, just as on the chlorinesaturated surface. From the surface with 0.60 ML of Cl(a) that was pre-annealed to ~700 K, as shown in curves c of Fig. 4, the total intensity of the formate decomposition peaks increased, and the desorption temperatures also moved closer to their chlorinefree temperatures. The total amount of CO+CO desorbing from this surface is ~65% 2 that of the chlorine-free surface, compared with ~50% before annealing. The total amount of formate decomposing on the surface increased after annealing at both coverages of Cl(a), consistent with the loss of chlorine signal in XPS and ISS. The relative loss of chlorine, however, was smaller at 0.6 ML chlorine. No chlorine-containing ions were seen in TPD from a surface predosed with ~0.5 ML of chlorine and then HCOOH, which implies that the hydrogen atom from HCOOH does not greatly facilitate the removal of atomic chlorine from the surface (for example, as HCl ). The increase in formate dehydrogenation selectivity due to Cl(a) may be related to: (1) the dipole moment of Cl(a) causing a destabilization of the transition state for dehydration relative to that for dehydrogenation of nearby formates; (2) possible selective blocking of defect sites by chlorine, assuming defects are more active in dehydration; (3) the Cl(a) making the surface effectively more basic since basic oxides favor dehydrogenation over dehydration in formate decomposition [28]; or (4) other changes in the electronic character of the surface. Vohs and Barteau [27] found that shifts in the selectivity for formate dehydration increased with the extent of surface reduction in the vicinity of the adsorbate. The dehydrogenation pathway producing CO was favored on the more 2 highly oxidized surfaces. Adding chlorine atoms

can be thought of as increasing the extent of oxidation of the ZnO. In addition, the adsorption of HCl on hydroxylated ZnO surfaces has been shown to displace the OH groups, leading to a depression of room-temperature adsorption of CO [30]. The decrease in the total formate decom2 position, however, proves that Cl(a) also blocks sites for formate production from HCOOH on ZnO(0001)–Zn.

4. Conclusions When Cl adsorbs on the ZnO(0001)–Zn sub2 strate, it dissociates and forms a true overlayer that saturates at ~0.30 chlorine atoms per zinc site. No long-range order of the surface was found with ARXPS. The measured work-function change, corrected for band bending, gave a local surface dipole for the adsorbate–substrate complex of ~5 D at the lowest coverages, and ~0.8 D above 0.2 ML Cl(a), which indicates that Cl(a) is initially anionic but that depolarization occurs as the coverage increases. The presence of Cl(a) suppresses the amount of adsorbed formate produced upon exposure of the surface to formic acid. It also alters the selectivity for formate decomposition reactions on ZnO(0001)–Zn, enhancing dehydrogenation at the expense of dehydration.

Acknowledgements Financial support for this research by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences is gratefully acknowledged. A.J. thanks the Research Experience for Undergraduates Program of the National Science Foundation for financial support.

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