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Pt(1 1 1) surface of Pt-Fe catalyst
Applied Surface Science 476 (2019) 387–390
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Selective adsorption of trace H2O over O2 on Pt/Fe/Pt(1 1 1) surface of Pt-Fe catalyst ⁎
Teng Maa, , Rui Caoa, Xue Baoa, Er'bing Huaa, Mu Yanga, Yaqin Wanga,b, a b
T
⁎
Institute of Science, Shenyang Agricultural University, Shenyang 110866, Liaoning, China School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, Liaoning, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pt-Fe model catalyst Electronic structure Selective adsorption Trace H2O O2 High resolution electron energy loss spectroscopy
The selective adsorption of trace water over O2 is reported on Pt/Fe/Pt(1 1 1) surface of Pt-Fe model catalysts by using high resolution electron energy loss spectroscopy (HREELS). The Pt/Fe/Pt(1 1 1) surface has a sandwich structure of the topmost Pt atomic layer, subsurface Fe layer and underlying Pt(1 1 1) bulk, and presents the lower density of states (DOS) at 0–2.0 eV below the Fermi level with reference to Pt(1 1 1). On the Pt/Fe/Pt (1 1 1) surface the chemisorption of both O2 and H2O would be weakened with different extents, which would change the relative order of their adsorption strength. At 105 K and exposure to O2 with 0.1% v/v water, chemisorbed water species are dominated on the Pt/Fe/Pt(1 1 1) surface, while chemisorbed O2 species are dominated on Pt(1 1 1) surface. The selective adsorption of trace water on Pt/Fe/Pt(1 1 1) surface could provide a hint that the well-tuned electronic structure of bimetallic catalysts might change the relative amount of surface species significantly and produce an influential effect on the CO reaction route at low temperatures.
1. Introduction Water, even a trace amount of water, could give a significant influence on many surface chemical processes when molecular oxygen is also one of reactants, e.g. atmospheric corrosion of metals and heterogeneous catalysis [1,2]. In heterogeneous catalysis, CO oxidation has a particular importance and interest in fundamental research as a model reaction and practical applications of removing CO in the exhaust gases or H2 source gas for fuel cell systems. When there are trace amounts of water vapor in catalytic reactor or small coverage of adsorbed water molecules on catalyst surfaces, the maximum promotion effect of water on CO oxidation could be more than 10 times [3–5]. Surface science studies of CO oxidation could be effective to get a better understanding of fundamental problems such as the adsorption/ desorption of reactants/products and surface reaction routes by working on well-defined model catalysts under ultra high vacuum (UHV) or near-ambient conditions [4,6]. The interactions between water and different Pt model catalysts, in the presence/absence of oxygen or other coadsorbed species, have also been studied intensively for decades [7–13]. For example, on the classic Pt(1 1 1) surface, H2O chemisorbs by PteOH2 bonds around 85–120 K and desorbs around 150–170 K, but when atomic O is preadsorbed they would react to form hydroxyls around 150 K (which is a typical way of getting hydroxyls under UHV). The water-derived hydroxyls are also considered as one of ⁎
active species to oxidize CO molecules in CO oxidation at low temperatures or in H2 streams. Until now, most adsorption studies involving both water and oxygen on Pt surfaces were of molecular H2O and atomic O, rarely of molecular H2O and molecular O (O2) [5,14]. However, a recent report indicated that H2O interacts with O2 preadsorbed on Pt(1 1 1) to form [O2·(H2O)n] complex by hydrogen bond network and the complex could react with CO to give a new CO2 production channel below 200 K [5]. With the improved complexity, some Pt-Fe (or Ni, Co, etc) model catalysts have also well developed for CO oxidation or oxygen reduction reaction [15–18]. The Pt-Fe model catalysts have variant structures including Pt/Fe/Pt(1 1 1) alloys, metal-oxide interfaces and supported nanoparticles [19–22]. For Pt/Fe/Pt(1 1 1) with 0.5 ML subsurface Fe, O2 adsorption is weakened so significantly that CO oxidation could be observed only at 100–300 K [23]. Considering H2O adsorption is generally weak and less influenced by the surface structural and electronic properties [24,25], we are wondering that if the O2 adsorption strength has become comparable to the H2O adsorption strength on Pt/Fe/Pt (1 1 1) surface at cryogenic temperatures, i.e. around 100 K. The coadsorption experiments of O2 and H2O on Pt/Fe/Pt(1 1 1) surface would become a good platform of solving the problem. For identifying chemisorbed species on surface, high resolution electron energy loss spectroscopy (HREELS) is a powerful tool because of its high energy resolution and surface sensitivity [26]. Herein, we
Corresponding authors at: Institute of Science, Shenyang Agricultural University, Shenyang 110866, Liaoning, China (Y. Wang). E-mail addresses: [email protected] (T. Ma), [email protected] (Y. Wang).
https://doi.org/10.1016/j.apsusc.2019.01.101 Received 16 December 2018; Received in revised form 5 January 2019; Accepted 12 January 2019 Available online 17 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 476 (2019) 387–390
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report on a HREELS study of the coadsorption of O2 and trace H2O on Pt/Fe/Pt(1 1 1) at 105 K. The results indicate that even a trace amount of water in source gases might change the relative amount of O2 and H2O adsorbed on catalyst surfaces. 2. Experimental section The experimental work was taken in one Omicron multiprobe system with three vacuum chambers of sample preparation, spectroscopy analysis and microscope, at the base pressure of 1.0–3.0 × 10−10 mbar. The spectroscopy chamber had two parts, the upper part was the combination of spherical energy analyzer and different sources to achieve the functions of X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), and Ion Scattering Spectroscopy (ISS), the lower part was high resolution energy loss spectroscopy (HREELS, LK-ELS5000). The HREELS spectra were taken at 60 degree angle of emission with respect to the surface normal in the specular direction. The electron beam energy of HREELS was 7.287 eV. There was one Omicron VT STM in the microscope chamber. All STM images are acquired at room temperature with a constant current mode of a tungsten tip. The Pt/Fe/Pt(1 1 1) surface of Pt-Fe model catalyst, had been prepared on one Pt(1 1 1) crystal (ϕ10 mm × 2 mm). The Pt(1 1 1) crystal was cleaned by repeated cycles of oxidation (5.0 × 10−8 mbar O2) at 800 K, 2.0 kV Ar+ sputtering, and UHV annealing at 1000 K until the STM and XPS did not detect any impurities. The Fe source consisted of two pairs of 0.5 mm diameter tungsten wires wrapped with a coil of Fe wire (0.1 mm diameter, purity 99.998%, Alfa Aesar). High purity gases of Ar, He, CO and O2 were introduced into the vacuum chambers by allmetal leak valves, and the dose of various gases was calculated by Langmuir (1 L = 1.33 × 10−6 mbar·S). Another cylinder of O2 gas mixed with 0.1% H2O vapor had also been used to study coadsorption of oxygen and water.
Fig. 2. The He II UPS spectra (hv = 40.8 eV) of Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surfaces.
are only Pt atoms on the topmost layer of Pt/Fe/Pt(1 1 1) surface. Combing the ISS spectrum and STM images, we could conclude that the topmost layer of Pt/Fe/Pt(1 1 1) is similar to that of Pt(1 1 1). Moreover, through the attenuation of Fe 2p peak intensity in the XPS spectra, about 0.5 monolayer (ML) Fe atoms should exist in the subsurface layer of Pt/Fe/Pt(1 1 1) [20]. The theoretical DFT results also show that the subsurface layer is the most favorable position for Fe atoms in the Pt/ Fe/Pt(1 1 1) [19]. Concisely, the Pt/Fe/Pt(1 1 1) surface seems to be one sandwich structure of a Pt topmost layer, subsurface Fe layer and Pt (1 1 1) bulk substrate. Furthermore, the Pt/Fe/Pt(1 1 1) structure could keep stable in UHV/H2 or even shortly in O2 (1.0 × 10−6 mbar O2, less than 400 K) through cycled tests of oxidation and reduction. The topmost layer of Pt/Fe/Pt(1 1 1) is similar to Pt(1 1 1), but Fe atoms in the subsurface layer would impose their influence on surface electronic properties. He II UPS spectra in Fig. 2 presents that the Pt/Fe/Pt(1 1 1) surface has lower density of states (DOS) at 0–2.0 eV below the Fermi level than Pt(1 1 1) surface. So that the actual difference of Pt/Fe/Pt (1 1 1) and Pt(1 1 1) is their surface electronic structures, which could influence the adsorption strength of O2, H2O and other small molecules.
3. Results and discussions 3.1. Structure of Pt/Fe/Pt(1 1 1) surface
3.2. Coadsorption of O2 and H2O
The Pt/Fe/Pt(1 1 1) surface was built by the reported procedures of depositing 1.0 monolayer (ML) Fe on Pt(1 1 1) surface and the following UHV annealing at 850 K [19]. The structural properties of Pt/Fe/Pt (1 1 1) surface have been presented in Fig. 1. The STM image in Fig. 1a shows that there are flat terraces on Pt/Fe/Pt(1 1 1) surface and the terraces have an ordered 6-fold symmetry at atomic resolution. Through the sensitive ISS spectrum in Fig. 1b, it is confirmed that there
Before coadsorption of O2 and H2O begins, we should have a look at O2 adsorption on Pt/Fe/Pt(1 1 1) surface at the low temperatures. Fig. 3 shows that Pt/Fe/Pt(1 1 1) was exposed to 10 L and 20 L pure O2 gas at 105 K, respectively. Both the two O2 exposures have produced three peaks located at 56 meV, 83 meV, 103 meV. Because the atomic oxygen
Fig. 1. The structural characteristics of Pt/Fe/Pt(1 1 1) surface: (a) STM image, 100 × 100 nm2, −0.229 V, 2.75 nA (inset 6.0 × 6.0 nm2, 2.0 nV, 16 nA); (b) ISS spectra (inset, the schematic drawing of Pt/Fe/Pt(1 1 1) structure). 388
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Fig. 5. HREELS spectra of Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surface exposed to O2 with 0.1% water at 105 K and annealed at 150–160 K: 100 L for Pt/Fe/Pt(1 1 1), 30 L for Pt(1 1 1).
Fig. 3. HREELS spectra of Pt/Fe/Pt(1 1 1) surface exposed to pure O2 at 110 K.
has only one peak at 56 meV and occurs around 150 K or above [19], we could expect that there is molecular oxygen (O2) adsorbed on Pt/Fe/ Pt(1 1 1) surfaces at 105 K. While on the Pt(1 1 1) surface, the adsorbed O2 has three or four vibration modes at 47, 58, 89, 108 meV according to the coverage [27]. In the HREELS spectra of Pt/Fe/Pt(1 1 1) surface, the peak at 56 meV could be attributed to PteO stretching modes, while the peaks at 83 meV, 103 meV could be attributed to the OeO stretching modes of two molecular oxygen species at hollow and bridge sites respectively. Unlike sequential exposures of gases, we referred to the reported method [28] and exposed the target surface directly to a O2 gas source freshly mixed with 0.1% v/v water vapor, designated as O2 (0.1% H2O). Fig. 4 shows that Pt/Fe/Pt(1 1 1) is exposed to 10 L O2 (0.1% H2O) at 105 K, with Pt(1 1 1) as a reference. A distinguish difference occurs in HREELS spectra: there are three peaks of 59, 81,106 meV on Pt(1 1 1), while more peaks have appeared at 28, 42, 53, 84, 103, 122 meV on Pt/ Fe/Pt(1 1 1). Because the HREELS spectra of adsorbed water is slightly more complex, we would have a short review on the vibration modes of water. Generally, the reported vibration modes of water adsorbed on Pt (1 1 1) maybe be divided into six types: frustrated translation around 16, 33 meV; the PteOH2 stretching at 56, 65 meV, or 33 meV; the frustrated rotation (or libration) around 65, 84 meV; bending modes (PteHeOH) around 115, 129 meV; the scissoring modes around 200 meV; OH stretching modes around 420–460 meV [29,30]. However, at small H2O coverage, the peaks at 200 meV (scissoring modes), 243 and 420–460 meV (OH stretching modes) are not observable to probe adsorbed H2O because of their weak intensity. So that, at small H2O coverage, it would be possible to observe the following peaks only at 16, 33, 56, 65, 84, 115, 129 meV and the peaks at 16, 33, 129 meV
could be used to distinguish H2O from O2 adsorption. Compared with the reported water adsorption, there is no observed characteristics of water adsorbed on Pt(1 1 1) after the exposure of O2 (0.1% H2O), but only the characteristics of molecular O2. However, after the same exposure of O2 (0.1% H2O) to Pt/Fe/Pt(1 1 1), three obvious peaks at 28, 42, 122 meV are observed and could be attributed to the frustrated translation and bending modes of H2O. The exposure of O2 (0.1% H2O) have also present another three peaks at 53, 84, 103 meV on Pt/Fe/Pt (1 1 1), which are close to the O2 peaks at 56, 84,103 meV. However, the lower position of 53 meV peak and higher intensity at 103 eV peak would imply that the peaks should be attributed mainly to H2O molecules adsorbed on Pt/Fe/Pt(1 1 1). From the above results, after the same exposure of O2 (0.1% H2O), H2O molecules are observed only on Pt/Fe/Pt(1 1 1) surface while O2 molecules are observed only on Pt (1 1 1) surface. However, we believe that H2O and O2 molecules should have been present on both Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surfaces. To verify our speculation of the HREELS results, we also took another experiment of annealing Pt/Fe/Pt(1 1 1) surface exposed to O2 (0.1% H2O), also with Pt(1 1 1) as a reference, as shown in Fig. 5. On Pt (1 1 1) surface, after the exposure of 30 L O2 (0.1% H2O) at 105 K, the temperature has been increased to and kept at 160 K for 120 s. After annealing Pt(1 1 1) at 160 K, there are 5 observable peaks in HREELS spectra, in which one side peak is at 45 meV and the other five peaks are at 31, 58, 88, 110, 125 meV respectively. By comparing with the reported OH/O mixture through annealing atomic O and H2O at 155 K (six peaks at 30, 44, 59, 90, 107, 127 meV), all of results are close and comparable [29,31]. Furthermore, the peak at around 58 meV could be attributed mainly to PteO stretching mode of atomic O and partially to PteOH stretching mode of hydroxyl at its asymmetric lower energy side; the peaks at around 31, 45 meV are attributed to the hindered translation modes of hydroxyl; the peaks at around 88, 110, 125 meV are attributed to the bending modes of hydroxyls. The characteristics of HREELS spectra on Pt(1 1 1) surface displays that OH species coexist with atomic oxygen after annealing at 160 K in our case. While on Pt/ Fe/Pt(1 1 1), after the exposure of 100 L O2 (0.1% H2O) at 105 K, the temperature has been increased to and kept at 150 K for 120 s. The four main peaks of 55, 85, 104, 122 meV are present in the HREELS spectra of Pt/Fe/Pt(1 1 1), which means that OH species have also formed on Pt/Fe/Pt(1 1 1) surface at a slightly lower 150 K. Around 150–160 K, the adsorbed H2O or O2 molecules would experience partial desorption, while the O2 molecules would dissociate partially into atomic O and react with adsorbed H2O to form hydroxyls. In short, on both Pt/Fe/Pt (1 1 1) and Pt(1 1 1) surfaces, the OH formation might be run by the same pattern in our experiment:
Fig. 4. HREELS spectra of Pt/Fe/Pt(1 1 1) and Pt(1 1 1) exposed to O2 with 0.1% water at 105 K.
O2(ad) → 2Oad, Oad + H2Oad → 2O Had. 389
(1)
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The formation of hydroxyls shows that, at 105 K, both O2 and H2O molecules should exist on the Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surfaces after the exposure of 100 L O2 (0.1% H2O). However, at 105 K, the relative amount of adsorbed O2 and H2O is varied so significantly on different surfaces that only one of them could be displayed clearly in the corresponding HREELS spectra. When exposure conditions are the same, the relative amount of O2 and H2O should result from the influence of their adsorption strength. Besides, the adsorption strength of O2 and H2O are weakened on Pt/Fe/Pt(1 1 1) and other Pt skin surfaces, compared to Pt(1 1 1). The reasonable explanation should be that the adsorption strength of O2 is weakened much more than that of H2O by surface electronic structure so that the order of their relative amount are reversed on Pt/Fe/Pt(1 1 1). In another word, the O2 adsorption has become weaker than H2O on Pt/Fe/Pt(1 1 1), which could be possible according to the bonding mechanism of chemical adsorption. On the Pt surfaces or nanoparticles, the Pt 5d states that are responsible for PteO2 bonding of O2 adsorption reside mainly within 2 eV below the Fermi level, while the Pt 5d states for PteOH2 bonding of H2O adsorption reside locally around 4–5 eV [13,25]. We have known that on the Pt/ Fe/Pt(1 1 1) surface a significantly decrease of DOS occurs at 0–2.0 eV below the Fermi level, while negligible changes of DOS occur around 4–5 eV by UPS spectra. So that, H2O adsorption on the Pt/Fe/Pt(1 1 1) surface would be just slightly weaker and comparable to Pt(1 1 1), while O2 adsorption would be weakened significantly and even become weaker than the H2O adsorption on the Pt/Fe/Pt(1 1 1) surface. In another word, the selective adsorption of H2O on Pt/Fe/Pt(1 1 1) has been achieved by tuning its electronic structure to reverse the relative adsorption strength of H2O and O2.
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4. Conclusions (1) Pt/Fe/Pt(1 1 1) and Pt(1 1 1) share the similar topmost layer, but there is lower density of electronic states at 0–2.0 eV below Fermi level on Pt/Fe/Pt(1 1 1) surface because of its subsurface Fe atoms (about 0.5 ML), which would weaken the adsorption of small molecules. (2) O2 and H2O adsorption are weakened with sharply different extents on the Pt/Fe/Pt(1 1 1) surface. At 105 K and exposure of O2 with 0.1% v/v water onto Pt/Fe/Pt(1 1 1) and Pt(1 1 1), chemisorbed water species are dominated on the Pt/Fe/Pt(1 1 1) surface, while chemisorbed O2 species are dominated on Pt(1 1 1) surface. Our experiments conclude that adsorption of O2 and H2O have been tuned to alter their relative order of adsorption strength and achieve the selective adsorption of trace H2O over O2 when H2O adsorption on Pt/Fe/Pt(1 1 1) is much less influenced by surface electronic properties than O2. The selective adsorption phenomena of trace H2O over O2 might change CO reaction routes on Pt-Fe catalysts at low temperatures. Acknowledgements This work was supported by Mountain Tianzhu Scholar Program of Shenyang Agricultural University, China and National Natural Science Foundation of China (No. 2140113). Dr. Ma thanks Prof. X. Bao from Dalian Institute of Chemical Physics, Chinese academy of Sciences, China, for his help and suggestions during the experimental process. References [1] T. Ma, Z. Wang, W. Han, A review of atmospheric corrosion of aluminum and
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Full Length Article
Selective adsorption of trace H2O over O2 on Pt/Fe/Pt(1 1 1) surface of Pt-Fe catalyst ⁎
Teng Maa, , Rui Caoa, Xue Baoa, Er'bing Huaa, Mu Yanga, Yaqin Wanga,b, a b
T
⁎
Institute of Science, Shenyang Agricultural University, Shenyang 110866, Liaoning, China School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, Liaoning, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Pt-Fe model catalyst Electronic structure Selective adsorption Trace H2O O2 High resolution electron energy loss spectroscopy
The selective adsorption of trace water over O2 is reported on Pt/Fe/Pt(1 1 1) surface of Pt-Fe model catalysts by using high resolution electron energy loss spectroscopy (HREELS). The Pt/Fe/Pt(1 1 1) surface has a sandwich structure of the topmost Pt atomic layer, subsurface Fe layer and underlying Pt(1 1 1) bulk, and presents the lower density of states (DOS) at 0–2.0 eV below the Fermi level with reference to Pt(1 1 1). On the Pt/Fe/Pt (1 1 1) surface the chemisorption of both O2 and H2O would be weakened with different extents, which would change the relative order of their adsorption strength. At 105 K and exposure to O2 with 0.1% v/v water, chemisorbed water species are dominated on the Pt/Fe/Pt(1 1 1) surface, while chemisorbed O2 species are dominated on Pt(1 1 1) surface. The selective adsorption of trace water on Pt/Fe/Pt(1 1 1) surface could provide a hint that the well-tuned electronic structure of bimetallic catalysts might change the relative amount of surface species significantly and produce an influential effect on the CO reaction route at low temperatures.
1. Introduction Water, even a trace amount of water, could give a significant influence on many surface chemical processes when molecular oxygen is also one of reactants, e.g. atmospheric corrosion of metals and heterogeneous catalysis [1,2]. In heterogeneous catalysis, CO oxidation has a particular importance and interest in fundamental research as a model reaction and practical applications of removing CO in the exhaust gases or H2 source gas for fuel cell systems. When there are trace amounts of water vapor in catalytic reactor or small coverage of adsorbed water molecules on catalyst surfaces, the maximum promotion effect of water on CO oxidation could be more than 10 times [3–5]. Surface science studies of CO oxidation could be effective to get a better understanding of fundamental problems such as the adsorption/ desorption of reactants/products and surface reaction routes by working on well-defined model catalysts under ultra high vacuum (UHV) or near-ambient conditions [4,6]. The interactions between water and different Pt model catalysts, in the presence/absence of oxygen or other coadsorbed species, have also been studied intensively for decades [7–13]. For example, on the classic Pt(1 1 1) surface, H2O chemisorbs by PteOH2 bonds around 85–120 K and desorbs around 150–170 K, but when atomic O is preadsorbed they would react to form hydroxyls around 150 K (which is a typical way of getting hydroxyls under UHV). The water-derived hydroxyls are also considered as one of ⁎
active species to oxidize CO molecules in CO oxidation at low temperatures or in H2 streams. Until now, most adsorption studies involving both water and oxygen on Pt surfaces were of molecular H2O and atomic O, rarely of molecular H2O and molecular O (O2) [5,14]. However, a recent report indicated that H2O interacts with O2 preadsorbed on Pt(1 1 1) to form [O2·(H2O)n] complex by hydrogen bond network and the complex could react with CO to give a new CO2 production channel below 200 K [5]. With the improved complexity, some Pt-Fe (or Ni, Co, etc) model catalysts have also well developed for CO oxidation or oxygen reduction reaction [15–18]. The Pt-Fe model catalysts have variant structures including Pt/Fe/Pt(1 1 1) alloys, metal-oxide interfaces and supported nanoparticles [19–22]. For Pt/Fe/Pt(1 1 1) with 0.5 ML subsurface Fe, O2 adsorption is weakened so significantly that CO oxidation could be observed only at 100–300 K [23]. Considering H2O adsorption is generally weak and less influenced by the surface structural and electronic properties [24,25], we are wondering that if the O2 adsorption strength has become comparable to the H2O adsorption strength on Pt/Fe/Pt (1 1 1) surface at cryogenic temperatures, i.e. around 100 K. The coadsorption experiments of O2 and H2O on Pt/Fe/Pt(1 1 1) surface would become a good platform of solving the problem. For identifying chemisorbed species on surface, high resolution electron energy loss spectroscopy (HREELS) is a powerful tool because of its high energy resolution and surface sensitivity [26]. Herein, we
Corresponding authors at: Institute of Science, Shenyang Agricultural University, Shenyang 110866, Liaoning, China (Y. Wang). E-mail addresses: [email protected] (T. Ma), [email protected] (Y. Wang).
https://doi.org/10.1016/j.apsusc.2019.01.101 Received 16 December 2018; Received in revised form 5 January 2019; Accepted 12 January 2019 Available online 17 January 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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report on a HREELS study of the coadsorption of O2 and trace H2O on Pt/Fe/Pt(1 1 1) at 105 K. The results indicate that even a trace amount of water in source gases might change the relative amount of O2 and H2O adsorbed on catalyst surfaces. 2. Experimental section The experimental work was taken in one Omicron multiprobe system with three vacuum chambers of sample preparation, spectroscopy analysis and microscope, at the base pressure of 1.0–3.0 × 10−10 mbar. The spectroscopy chamber had two parts, the upper part was the combination of spherical energy analyzer and different sources to achieve the functions of X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), and Ion Scattering Spectroscopy (ISS), the lower part was high resolution energy loss spectroscopy (HREELS, LK-ELS5000). The HREELS spectra were taken at 60 degree angle of emission with respect to the surface normal in the specular direction. The electron beam energy of HREELS was 7.287 eV. There was one Omicron VT STM in the microscope chamber. All STM images are acquired at room temperature with a constant current mode of a tungsten tip. The Pt/Fe/Pt(1 1 1) surface of Pt-Fe model catalyst, had been prepared on one Pt(1 1 1) crystal (ϕ10 mm × 2 mm). The Pt(1 1 1) crystal was cleaned by repeated cycles of oxidation (5.0 × 10−8 mbar O2) at 800 K, 2.0 kV Ar+ sputtering, and UHV annealing at 1000 K until the STM and XPS did not detect any impurities. The Fe source consisted of two pairs of 0.5 mm diameter tungsten wires wrapped with a coil of Fe wire (0.1 mm diameter, purity 99.998%, Alfa Aesar). High purity gases of Ar, He, CO and O2 were introduced into the vacuum chambers by allmetal leak valves, and the dose of various gases was calculated by Langmuir (1 L = 1.33 × 10−6 mbar·S). Another cylinder of O2 gas mixed with 0.1% H2O vapor had also been used to study coadsorption of oxygen and water.
Fig. 2. The He II UPS spectra (hv = 40.8 eV) of Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surfaces.
are only Pt atoms on the topmost layer of Pt/Fe/Pt(1 1 1) surface. Combing the ISS spectrum and STM images, we could conclude that the topmost layer of Pt/Fe/Pt(1 1 1) is similar to that of Pt(1 1 1). Moreover, through the attenuation of Fe 2p peak intensity in the XPS spectra, about 0.5 monolayer (ML) Fe atoms should exist in the subsurface layer of Pt/Fe/Pt(1 1 1) [20]. The theoretical DFT results also show that the subsurface layer is the most favorable position for Fe atoms in the Pt/ Fe/Pt(1 1 1) [19]. Concisely, the Pt/Fe/Pt(1 1 1) surface seems to be one sandwich structure of a Pt topmost layer, subsurface Fe layer and Pt (1 1 1) bulk substrate. Furthermore, the Pt/Fe/Pt(1 1 1) structure could keep stable in UHV/H2 or even shortly in O2 (1.0 × 10−6 mbar O2, less than 400 K) through cycled tests of oxidation and reduction. The topmost layer of Pt/Fe/Pt(1 1 1) is similar to Pt(1 1 1), but Fe atoms in the subsurface layer would impose their influence on surface electronic properties. He II UPS spectra in Fig. 2 presents that the Pt/Fe/Pt(1 1 1) surface has lower density of states (DOS) at 0–2.0 eV below the Fermi level than Pt(1 1 1) surface. So that the actual difference of Pt/Fe/Pt (1 1 1) and Pt(1 1 1) is their surface electronic structures, which could influence the adsorption strength of O2, H2O and other small molecules.
3. Results and discussions 3.1. Structure of Pt/Fe/Pt(1 1 1) surface
3.2. Coadsorption of O2 and H2O
The Pt/Fe/Pt(1 1 1) surface was built by the reported procedures of depositing 1.0 monolayer (ML) Fe on Pt(1 1 1) surface and the following UHV annealing at 850 K [19]. The structural properties of Pt/Fe/Pt (1 1 1) surface have been presented in Fig. 1. The STM image in Fig. 1a shows that there are flat terraces on Pt/Fe/Pt(1 1 1) surface and the terraces have an ordered 6-fold symmetry at atomic resolution. Through the sensitive ISS spectrum in Fig. 1b, it is confirmed that there
Before coadsorption of O2 and H2O begins, we should have a look at O2 adsorption on Pt/Fe/Pt(1 1 1) surface at the low temperatures. Fig. 3 shows that Pt/Fe/Pt(1 1 1) was exposed to 10 L and 20 L pure O2 gas at 105 K, respectively. Both the two O2 exposures have produced three peaks located at 56 meV, 83 meV, 103 meV. Because the atomic oxygen
Fig. 1. The structural characteristics of Pt/Fe/Pt(1 1 1) surface: (a) STM image, 100 × 100 nm2, −0.229 V, 2.75 nA (inset 6.0 × 6.0 nm2, 2.0 nV, 16 nA); (b) ISS spectra (inset, the schematic drawing of Pt/Fe/Pt(1 1 1) structure). 388
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Fig. 5. HREELS spectra of Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surface exposed to O2 with 0.1% water at 105 K and annealed at 150–160 K: 100 L for Pt/Fe/Pt(1 1 1), 30 L for Pt(1 1 1).
Fig. 3. HREELS spectra of Pt/Fe/Pt(1 1 1) surface exposed to pure O2 at 110 K.
has only one peak at 56 meV and occurs around 150 K or above [19], we could expect that there is molecular oxygen (O2) adsorbed on Pt/Fe/ Pt(1 1 1) surfaces at 105 K. While on the Pt(1 1 1) surface, the adsorbed O2 has three or four vibration modes at 47, 58, 89, 108 meV according to the coverage [27]. In the HREELS spectra of Pt/Fe/Pt(1 1 1) surface, the peak at 56 meV could be attributed to PteO stretching modes, while the peaks at 83 meV, 103 meV could be attributed to the OeO stretching modes of two molecular oxygen species at hollow and bridge sites respectively. Unlike sequential exposures of gases, we referred to the reported method [28] and exposed the target surface directly to a O2 gas source freshly mixed with 0.1% v/v water vapor, designated as O2 (0.1% H2O). Fig. 4 shows that Pt/Fe/Pt(1 1 1) is exposed to 10 L O2 (0.1% H2O) at 105 K, with Pt(1 1 1) as a reference. A distinguish difference occurs in HREELS spectra: there are three peaks of 59, 81,106 meV on Pt(1 1 1), while more peaks have appeared at 28, 42, 53, 84, 103, 122 meV on Pt/ Fe/Pt(1 1 1). Because the HREELS spectra of adsorbed water is slightly more complex, we would have a short review on the vibration modes of water. Generally, the reported vibration modes of water adsorbed on Pt (1 1 1) maybe be divided into six types: frustrated translation around 16, 33 meV; the PteOH2 stretching at 56, 65 meV, or 33 meV; the frustrated rotation (or libration) around 65, 84 meV; bending modes (PteHeOH) around 115, 129 meV; the scissoring modes around 200 meV; OH stretching modes around 420–460 meV [29,30]. However, at small H2O coverage, the peaks at 200 meV (scissoring modes), 243 and 420–460 meV (OH stretching modes) are not observable to probe adsorbed H2O because of their weak intensity. So that, at small H2O coverage, it would be possible to observe the following peaks only at 16, 33, 56, 65, 84, 115, 129 meV and the peaks at 16, 33, 129 meV
could be used to distinguish H2O from O2 adsorption. Compared with the reported water adsorption, there is no observed characteristics of water adsorbed on Pt(1 1 1) after the exposure of O2 (0.1% H2O), but only the characteristics of molecular O2. However, after the same exposure of O2 (0.1% H2O) to Pt/Fe/Pt(1 1 1), three obvious peaks at 28, 42, 122 meV are observed and could be attributed to the frustrated translation and bending modes of H2O. The exposure of O2 (0.1% H2O) have also present another three peaks at 53, 84, 103 meV on Pt/Fe/Pt (1 1 1), which are close to the O2 peaks at 56, 84,103 meV. However, the lower position of 53 meV peak and higher intensity at 103 eV peak would imply that the peaks should be attributed mainly to H2O molecules adsorbed on Pt/Fe/Pt(1 1 1). From the above results, after the same exposure of O2 (0.1% H2O), H2O molecules are observed only on Pt/Fe/Pt(1 1 1) surface while O2 molecules are observed only on Pt (1 1 1) surface. However, we believe that H2O and O2 molecules should have been present on both Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surfaces. To verify our speculation of the HREELS results, we also took another experiment of annealing Pt/Fe/Pt(1 1 1) surface exposed to O2 (0.1% H2O), also with Pt(1 1 1) as a reference, as shown in Fig. 5. On Pt (1 1 1) surface, after the exposure of 30 L O2 (0.1% H2O) at 105 K, the temperature has been increased to and kept at 160 K for 120 s. After annealing Pt(1 1 1) at 160 K, there are 5 observable peaks in HREELS spectra, in which one side peak is at 45 meV and the other five peaks are at 31, 58, 88, 110, 125 meV respectively. By comparing with the reported OH/O mixture through annealing atomic O and H2O at 155 K (six peaks at 30, 44, 59, 90, 107, 127 meV), all of results are close and comparable [29,31]. Furthermore, the peak at around 58 meV could be attributed mainly to PteO stretching mode of atomic O and partially to PteOH stretching mode of hydroxyl at its asymmetric lower energy side; the peaks at around 31, 45 meV are attributed to the hindered translation modes of hydroxyl; the peaks at around 88, 110, 125 meV are attributed to the bending modes of hydroxyls. The characteristics of HREELS spectra on Pt(1 1 1) surface displays that OH species coexist with atomic oxygen after annealing at 160 K in our case. While on Pt/ Fe/Pt(1 1 1), after the exposure of 100 L O2 (0.1% H2O) at 105 K, the temperature has been increased to and kept at 150 K for 120 s. The four main peaks of 55, 85, 104, 122 meV are present in the HREELS spectra of Pt/Fe/Pt(1 1 1), which means that OH species have also formed on Pt/Fe/Pt(1 1 1) surface at a slightly lower 150 K. Around 150–160 K, the adsorbed H2O or O2 molecules would experience partial desorption, while the O2 molecules would dissociate partially into atomic O and react with adsorbed H2O to form hydroxyls. In short, on both Pt/Fe/Pt (1 1 1) and Pt(1 1 1) surfaces, the OH formation might be run by the same pattern in our experiment:
Fig. 4. HREELS spectra of Pt/Fe/Pt(1 1 1) and Pt(1 1 1) exposed to O2 with 0.1% water at 105 K.
O2(ad) → 2Oad, Oad + H2Oad → 2O Had. 389
(1)
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The formation of hydroxyls shows that, at 105 K, both O2 and H2O molecules should exist on the Pt/Fe/Pt(1 1 1) and Pt(1 1 1) surfaces after the exposure of 100 L O2 (0.1% H2O). However, at 105 K, the relative amount of adsorbed O2 and H2O is varied so significantly on different surfaces that only one of them could be displayed clearly in the corresponding HREELS spectra. When exposure conditions are the same, the relative amount of O2 and H2O should result from the influence of their adsorption strength. Besides, the adsorption strength of O2 and H2O are weakened on Pt/Fe/Pt(1 1 1) and other Pt skin surfaces, compared to Pt(1 1 1). The reasonable explanation should be that the adsorption strength of O2 is weakened much more than that of H2O by surface electronic structure so that the order of their relative amount are reversed on Pt/Fe/Pt(1 1 1). In another word, the O2 adsorption has become weaker than H2O on Pt/Fe/Pt(1 1 1), which could be possible according to the bonding mechanism of chemical adsorption. On the Pt surfaces or nanoparticles, the Pt 5d states that are responsible for PteO2 bonding of O2 adsorption reside mainly within 2 eV below the Fermi level, while the Pt 5d states for PteOH2 bonding of H2O adsorption reside locally around 4–5 eV [13,25]. We have known that on the Pt/ Fe/Pt(1 1 1) surface a significantly decrease of DOS occurs at 0–2.0 eV below the Fermi level, while negligible changes of DOS occur around 4–5 eV by UPS spectra. So that, H2O adsorption on the Pt/Fe/Pt(1 1 1) surface would be just slightly weaker and comparable to Pt(1 1 1), while O2 adsorption would be weakened significantly and even become weaker than the H2O adsorption on the Pt/Fe/Pt(1 1 1) surface. In another word, the selective adsorption of H2O on Pt/Fe/Pt(1 1 1) has been achieved by tuning its electronic structure to reverse the relative adsorption strength of H2O and O2.
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4. Conclusions (1) Pt/Fe/Pt(1 1 1) and Pt(1 1 1) share the similar topmost layer, but there is lower density of electronic states at 0–2.0 eV below Fermi level on Pt/Fe/Pt(1 1 1) surface because of its subsurface Fe atoms (about 0.5 ML), which would weaken the adsorption of small molecules. (2) O2 and H2O adsorption are weakened with sharply different extents on the Pt/Fe/Pt(1 1 1) surface. At 105 K and exposure of O2 with 0.1% v/v water onto Pt/Fe/Pt(1 1 1) and Pt(1 1 1), chemisorbed water species are dominated on the Pt/Fe/Pt(1 1 1) surface, while chemisorbed O2 species are dominated on Pt(1 1 1) surface. Our experiments conclude that adsorption of O2 and H2O have been tuned to alter their relative order of adsorption strength and achieve the selective adsorption of trace H2O over O2 when H2O adsorption on Pt/Fe/Pt(1 1 1) is much less influenced by surface electronic properties than O2. The selective adsorption phenomena of trace H2O over O2 might change CO reaction routes on Pt-Fe catalysts at low temperatures. Acknowledgements This work was supported by Mountain Tianzhu Scholar Program of Shenyang Agricultural University, China and National Natural Science Foundation of China (No. 2140113). Dr. Ma thanks Prof. X. Bao from Dalian Institute of Chemical Physics, Chinese academy of Sciences, China, for his help and suggestions during the experimental process. References [1] T. Ma, Z. Wang, W. Han, A review of atmospheric corrosion of aluminum and
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