Pt Catalysts in Low-Temperature CO Oxidation

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The Active Phase of FeO/Pt Catalysts in LowTemperature CO Oxidation and PROX Reaction Hao Chen, Yun Liu, Fan Yang, Mingming Wei, Xinfei Zhao, Yanxiao Ning, Qingfei Liu, Yi Zhang, Qiang Fu, and Xinhe Bao J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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The Journal of Physical Chemistry

The Active Phase of FeOx/Pt Catalysts in Low-Temperature CO Oxidation and PROX Reaction Hao Chen†,‡,§, Yun Liu†,‡,§, Fan Yang†,* , Mingming Wei†,‡, Xinfei Zhao†,‡, Yanxiao Ning†, Qingfei Liu†,‡, Yi Zhang†,‡, Qiang Fu†, Xinhe Bao†,* †State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Collaborative

Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China. ‡University of Chinese Academy of Sciences, Beijing 100049, China.

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Abstract

The interface between metal and reducible oxide has attracted increasing interest in catalysis. The FeOx-Pt interface has been a typical example, which showed remarkable activity for the preferential oxidation of CO (PROX) at low temperatures. However, model catalytic studies under vacuum conditions or in high pressure O-rich environment at 450 K have reported two different active phases with iron in two different valence states, invoking a possible pressure gap. To identify the active phase for low-temperature CO oxidation and PROX, it is necessary to investigate the stability and activity of FeO/Pt(111) under the realistic reaction conditions. We thus conducted an in-situ study on FeO/Pt(111) from ultra-high vacuum to the atmospheric pressure of reactant gases. Our study shows FeO islands were easily oxidized in 1 Torr O2 to form the tri-layer FeO2 islands. However, the presence of 2 Torr CO could prevent the oxidation of FeO islands and lead to CO oxidation at the FeO/Pt(111) interface. The FeO/Pt(111) surface exhibits an excellent activity for CO2 production with an initial reaction rate measured to be ~ 1 × 1014 molecules∗cm-2∗s-1 at 300 K. FeO islands supported on Pt(111) were further investigated in the PROX gas, i.e. the mixture of 98.5% H2, 1% CO and 0.5% O2, at elevated pressures up to 1 bar. Our results thus bridged the pressure gap and identified the bilayer FeO islands on Pt(111) as the active phase for PROX under the realistic reaction conditions.


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1. Introduction CO oxidation ( 2CO + O2 → CO2 ), as a prototypical reaction1, has been widely studied in heterogeneous catalysis and electrocatalysis. The reaction is center to applications in emission control and hydrogen fuel cells. Due to its simplicity, CO oxidation has also been generally used to probe the fundamental concepts of catalysis in model systems2. The mechanistic details of CO oxidation, however, are not as simple as reflected by the reaction equation. Rather, the reaction mechanism varies with the reaction conditions3-4, as well as the active site structure5. On Pt group metals surfaces, the strong chemisorption of CO would block the dissociative adsorption of O2 and poison metal catalysts for low-temperature oxidation reactions6-7. Thus, CO oxidation on Pt-group metal catalysts was usually carried out at above the desorption temperature of CO3. FeOx/Pt catalysts have been found capable of annihilating the CO poisoning problem and demonstrated a remarkable activity for the preferential oxidation of CO (PROX) in excess H2 at room temperature or below8-9. Through model catalytic studies, FeO islands confined on Pt(111) and exposing coordinatively unsaturated ferrous (CUF) centers at the interface, have been shown as the active structure for CO oxidation at 300K, as demonstrated by the titration experiments in vacuum8. The activity of the FeO-Pt interface under vacuum conditions was also confirmed by Kudernatsch et al. in their in-situ STM study10. Yet, the stability and activity of the FeO-Pt interface has not been systematically examined under the realistic reaction conditions, where the pressure gap has been a typical challenge for surface science studies. Meanwhile, Sun et al. studied the FeO film on Pt(111) under high pressures and found that the formation of FeO2 on Pt(111), i.e. an O-Fe-O trilayer, could enhance CO oxidation on Pt(111) at 450 K11-13. Since interfacial oxygen in the FeO2 trilayer is bonded with Pt, the electron transfer from Pt caused all iron ions in FeO2 have a formal Fe3+ (ferric) oxidation state, which is confirmed by both density



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functional calculations (DFT) and X-ray photoelectron spectroscopy (XPS)14-16. Pan et al. later found that reducing the surface coverage of FeO2 to submonolayer could further boost the activity for CO oxidation at 450 K, and hence they suggested the FeO2-Pt interface as the active phase for CO oxidation17. Note that these studies were conducted in an O2-rich environment and at temperatures above the CO desorption temperature. To identify the active phase of the FeOx/Pt catalyst for low-temperature CO oxidation or PROX reactions, it is thus of utmost importance to investigate the stability and reactivity of FeO/Pt(111) under the realistic catalytic conditions. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) have emerged as suitable tools for in-situ structural studies of model catalysts over a large pressure span. Over the past two decades, these two techniques have been increasingly employed to identify the active phase18-21 during surface catalytic reactions at near ambient pressures (NAP). NAP-STM, in particular, provides an opportunity to study the local structural dynamics at the metal-oxide interface22-24. In this study, we combined NAP-STM and XPS to investigate the stability and reactivity of FeO islands on Pt(111) from ultrahigh vacuum (UHV) to the atmospheric pressure of PROX reaction at room temperature. The FeO/Pt(111) system was monitored by in-situ STM under elevated pressures of O2, the mixture gas of CO/O2 or the PROX mixture gas (H2/CO/O2). The reaction kinetics was also measured while visualizing surface morphologies of the working catalyst. FeO islands supported on Pt(111) were found stable and active during CO oxidation or PROX reaction at 300 K. Our studies suggest the dynamic nature of surface oxide nanostructures in response to the different reaction environments and thus invoke the importance of in-situ model catalytic studies under catalytically relevant conditions.


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2. Experimental Section The experiments were carried out in two combined ultrahigh vacuum (UHV) systems. The first system is equipped with low temperature scanning tunneling microscope (LT-STM, Createc, base pressure < 4× 10-11 mbar), X-ray photoelectron spectroscopy (XPS) and cleaning facilities. The second system is equipped with near-ambient-pressure (NAP) STM (Specs, base pressure < 2× 10-10 mbar), Mass spectroscopy (Hiden) and cleaning facilities. The Pt(111) single crystal (Mateck) was cleaned by cycles of Ar ion sputtering (2 keV, 10 µA) and annealing at 1200 K. FeO nanoislands were deposited onto the Pt(111) surface by evaporating Fe atoms in 1.3 × 10-7 mbar O2 at the Pt substrate temperature between 150-300 K. The as-deposited FeO nanoislands were then annealed to 600 K, leading to the formation of well-ordered FeO nanostructures. The size of FeO islands could be controlled by varying the deposition and annealing temperatures. Reactant gases were introduced into the micro-reactor of NAP-STM by leak valves and the pressure of the reactor was measured by a calibrated MKS-616 Baratron gauge. The reactant gas or mixture gases were all purified by liquid nitrogen. The mixture gases were mixed in a 1-litre glass bulb with the partial pressure monitored by a MKS-616 Baratron gauge in line. The leaking rate of the microreactor in NAP-STM was measured from the pressure change without the single crystal sample in the reactor and examined every time before the reaction study. The leaking rate was found constant under the same reactant gases and with the same initial pressure, and generally much smaller than the reaction rates found in our reaction studies. The STM images were processed with SPIP software, and all the images obtained in situ were specified in the images, while others were scanned in UHV or evacuated to UHV after NAP experiments. 3. Results and Discussion FeO nanostructures were prepared by depositing Fe in 1.0×10-7 Torr O2 onto Pt(111)at 300 K and then annealed at 600 K in an ultrahigh vacuum (UHV). The as-prepared FeO islands (Fig. 5 ACS Paragon Plus Environment


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1a~b) are of monolayer thickness and expose coordinatively unsaturated ferrous (CUF) sites at the step edges. The atomic structure of FeO islands on Pt(111) has been well characterized as a polar bilayer with the Fe layer in contact with Pt and the O layer exposed as the topmost plane. Exposing the prepared FeO/Pt(111) surface in 4×10-6 Torr O2 at 400 K for 60 min led to further oxidation to form FeO2 islands15, i.e. the O-Fe-O trilayer on Pt(111) (Fig.1c~d). FeO2 domains exhibit a higher apparent height (~2.2 Å) than that of the FeO island, ~ 1.1 Å, and thus can be easily distinguished in the scanning tunneling microscopy (STM) topography. The moiré pattern of FeO2 also shows a periodicity different from that of FeO islands and displays a larger surface corrugation (Fig. 1). The formation of FeO2 trilayer structure is thermodynamically favorable at high oxygen pressures and remains stable in UHV upon its formation. The transformation of FeO islands to FeO2 islands could also be distinguished in by the Fe 2p spectra of X-ray photoelectron spectroscopy (XPS). Fig. 1 shows the FeO/Pt(111) surface gave the binding energy (BE) of Fe2p3/2 at 709.3 eV, which is characteristic for Fe2+. In comparison, the FeO2/Pt(111) surface gave an Fe2p3/2 peak at 710.1 eV25, which is close to the value (710.9 eV) observed for Fe2O3 on Au(111)16 and indicates the formation of Fe3+. Consistently, DFT calculations have suggested that all iron ions in FeO2 have a formal Fe3+ (ferric) oxidation state due to the charge transfer from the Pt substrate to the FeO2 overlayer14-15. STM images on the atomic structure of FeO2 domains show a hexagonal lattice, which aligns well with the FeO lattice and exhibits a lattice spacing same as that of FeO(111). Besides the (1 × 1)-FeO2 structure, STM on the FeO2 domains have also shown a (√3×√3)R30°-FeO2 structure15, 26, which exhibits a lattice spacing of 5.4 Å and rotates by 30° with respect to the FeO(111) lattice (Fig.2a, b). The (√3×√3)R30°-FeO2 structure has been suggested to form by the buckling of the FeO2 trilayer. With some Fe ions moving outwards, the outmost O layer displays


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the (√3×√3)R30° structure over the FeO lattice15. Merte et al. suggested recently that the (√3×√3)R30° structure observed in STM might be due to H adatoms occupying partially the top sites of the topmost O layer26. In our experiments, the (√3×√3) R30° pattern on FeO2 was usually observed when the sample was not moved to the cryostat of LT-STM immediately after the preparation. The (√3×√3) R30° structure would disappear upon scanning under high sample bias, for instance at + 4.5V, and display again the (1 × 1) structure of the FeO2 lattice (Fig. 2c). Thus, our experiments suggest the formation of the (√3×√3)R30° structure might originate from the background adsorption, consistent with the proposal by Merte et al26. The oxidation of FeO nanostructures on Pt(111) was then studied at 300 K in the elevated pressures of O2 (Fig. 3). Sun et al. has reported that the oxidation of a full layer FeO film on Pt(111) requires an oxygen chemical potential higher than vacuum conditions and 300 K, where the authors exposed the FeO/Pt(111) surface 20 mbar O2 at 470 K to induce the formation of FeO2 layer12, 15. We found the oxidation of FeO islands on Pt(111) was much easier and could be achieved under vacuum conditions. At P(O2) = 1.0×10-8 Torr, we observed the formation of oxygen dislocation lines at 300 K, which has been attributed as a precursor state for further oxidation of the FeO nanoislands26-27. As the O2 pressure was raised to 0.02 Torr, we observed the formation of FeO2 domains around the periphery of FeO islands. Fig. 3 also suggested that, at submonolayer coverage, the step edges of FeO islands provided a fast channel for oxygen penetration into the interface between FeO and Pt(111). At P(O2) above 0.4 Torr, we observed the formation of FeO2 domains all over the oxide island surface at 300 K. Due to the limited diffusivity of oxygen at the interface between FeO and Pt(111), FeO2 domains formed at 300 K did not display a long range order. Consistently, near-ambient-pressure (NAP) XPS of the FeO/Pt(111) surface in 0.05 Torr O2 shows a blue shift of the Fe2p3/2 BE to 710.1 eV, indicating



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the formation of FeO2 islands on Pt(111)25. Note that, these FeO2 islands remained unchanged in UHV after the evacuation of O2 (Fig.3d). Despite the facile oxidation of FeO islands in ambient O2, the presence of CO could inhibit the oxidation of FeO islands. Fig. 4 shows a FeO/Pt(111) surface exposed to the mixture gas of CO/O2 with a stoichiometric 2:1 molar ratio. In-situ studies show the surface remains the form of FeO islands at elevated pressures up to 3 Torr. STM images also showed the Pt surfaces surrounding FeO islands were saturated with a Moiré structure, which was magnified in Fig. 4c. The same Moiré structure has been observed previously, when exposing Pt(111) to 1 bar CO at room temperature28-29, and analyzed as a (√19 × √19)R23.4°-13 CO structure based on the combined STM and DFT studies. Vestergaard et al. have also reported that the same COsaturated Moiré structure could be observed during vacuum exposure of CO at 170 K29 and suggested that increasing the partial pressure of gas phase equals to lowering the temperature of metal substrates. The saturation of Pt surfaces by CO prevents the adsorption of O2 on Pt(111), i.e. the well-known problem for Pt group metals in CO oxidation at low temperatures. Previous study30 has also suggested a critical CO coverage at 0.44 ML, above which the dissociative adsorption of O2 on Pt(111) is inhibited. Under 3 Torr CO partial pressure, the (√19 × √19)R23.4°-13 CO structure results in a surface CO coverage at 0.68 ML and thus poisoned the Pt(111) surface from CO oxidation. Meanwhile, the exposure of 3 Torr CO/O2 (2:1) mixture gases led to CO oxidation on the FeO/Pt(111) surface. CO oxidation and the production of CO2 could be detected by leaking the gas from the reaction cell of AP-STM into a mass spectrometer attached to the chamber. To prevent the influence of background CO or CO2 in the chamber, isotope

13

CO was used to

prepare the 3 Torr 13CO/O2 (2:1) mixture gas. Fig. 4g showed clearly the production of 13CO2 in


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the mass spectrum. Since CO oxidation ( 2CO + O2 → CO2 ) results in the decrease of gas molar volume, the reactivity could be quantitatively measured by the continuous decrease of total pressure in the Baratron gauge, which has been a regular practice in the quantitative activity measurements on single crystals31. We found that the total pressure of the mixture gases decreases by 0.05 Torr during the first hour of the reaction on the FeO/Pt(111) surface, whereas the pressure change on the Pt(111) surface was not appreciable. Thus, the reaction rate of CO2 production on the FeO/Pt(111) surface could be derived at ~1 × 1014 molecules∗cm-2∗s-1. Previous studies have shown that O2 could be activated at the edge sites of FeO islands and react with neighboring CO molecules to form CO28, 10. Thus the presence of surface CO not only prevents the adsorption of O2 on Pt(111), but also inhibits the penetration of oxygen into the interface between FeO and Pt(111) by reacting with oxygen to form CO2 and leave the surface. Pan et al. showed that the oxidation of FeO islands would proceed at higher temperatures (450 K) and in excess of O2 (10 mbar CO + 50 mbar O2)17. Since CO starts to desorb at 400 K on Pt(111), the oxidation of FeO islands on a Pt(111) surface not poisoned by CO is consistent with our studies and again suggested the importance of CO in reversing the oxidation kinetics. To investigate the active phase of FeOx/Pt catalysts during the PROX reaction, the FeO/Pt(111) surface was further exposed to the mixture gas of H2, CO and O2, whose molar ratio for H2: CO: O2 was fixed at 98.5:1:0.5 and identical to that of the PROX gases used in powder catalyst studies8, 25. The partial pressure of each gas component was kept below its saturation vapor pressure at 80 K so that the molar ratio of the mixture gas could be preserved during the gas purification by liquid N2. Fig. 5 shows the surface of FeO/Pt(111) being exposed to the PROX mixture gases at elevated pressures at 300 K. In-situ study under 1.2~64 Torr of the PROX gas showed no signs of FeO2 formation on the FeO islands. Same as the case with CO/O2



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mixture gas, the surface of Pt(111) was saturated by the (√19 × √19)R23.4°-13 CO phase, due to the strong chemisorption of CO on Pt. Under 760 Torr of PROX gas, i.e. the realistic catalytic condition of PROX, in-situ STM studies became difficult due to the disturbance of tunneling junctions by diffusive species on the substrate, which could be from the increased concentration of impurities from the background. Thus, we looked into the FeO/Pt(111) surface after an exposure of 1 bar PROX gas for 30 min. Fig. 5f shows FeO islands remained the form of FeO bilayer, with an apparent height of 0.1 nm. There were some adsorbates at the step edges of FeO islands, which could be attributed to contamination from the background due to the exposure of high-pressure gases. For FeO islands, the surface plane of FeO was known to be chemically inert, whereas CUF sites at the edges of FeO islands were active for O2 and other reactive gases. From Fig. 5f, no FeO2 domains were formed during the PROX reaction, since FeO2 domains have been shown as the thermodynamically stable phase in UHV and could survive the evacuation of reactant gases. Consistently, XPS measurements of the FeO/Pt(111) surface in response to different reactant gases confirmed our above results. Fig. 6 shows Fe 2p3/2 BE of the as-prepared FeO/Pt(111) sample remained at 709.3eV after the exposure at 300 K to 760 Torr PROX gas (CO:O2:H2=1:0.5:98.5) for 30 min or to 760 Torr CO/O2 mixture gas (CO:O2:He=1:20:79) for 30 min. The Fe 2p3/2 peak showed no observable change in either peak position or peak intensity before and after the exposure of mixture gases. Only an exposure of pure O2 gas at 760 Torr could oxidize the FeO islands into the FeO2 phase, which gave the Fe 2p3/2 BE at 710.1 eV. The completely oxidized FeO2 islands on Pt(111) were not stable after the exposure to 760 Torr PROX gas and would be reduced back to FeO with Fe 2p3/2 BE at 709.3 eV25. But owing to the very low reactivity of FeO2 with CO or H2 at 300 K, Fig. 6 indicates that the reduction to FeO is


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not complete after 9 hours of reaction. Overall, our results demonstrate that the bilayer structure of FeO nanoislands is the active phase in the H2-rich PROX gas and the presence of CO or H2 could increase the stability of FeO bilayer structure against the oxidation by O2. Our studies on the FeOx/Pt(111) model system suggested FeO islands that are the highly active and stable under vacuum conditions, remain their stability and activity under the realistic conditions of PROX reaction at 300 K. The presence of CO and H2 gases enhanced the stability of FeO islands. Despite the pressure gap often invoked in surface science studies, our studies from UHV to realistic reaction conditions indicated a consistency observed in CO oxidation at the FeO/Pt(111) interface. Indeed, as long as the active phase and key reaction steps are accessible under vacuum conditions, surface science studies in UHV could provide atomic scale insight extremely useful for understanding the reaction mechanism and kinetics of powder catalysts32-33. The pressure gap in surface science studies demands a careful and case-by-case evaluation by gradually spanning the study from UHV to NAP conditions. In a forthcoming paper, we would compare the atomic-scale studies of CO oxidation at the FeO/Pt(111) interface from UHV to NAP conditions, as well as the reaction kinetics and energetics.

4. Conclusion The stability of FeO nanostructures on Pt(111) was investigated in O2, the mixture of CO/O2 and under the realistic condition of PROX reaction. In 1 Torr O2, FeO islands are vulnerable to further oxidation to form the O-Fe-O trilayer islands at 300 K. The formed FeO2 islands are stable in UHV and exhibit a buckled (√3× √3)R30° surface reconstruction, which was resolved with unprecedented resolution in this study. Despite their facile oxidation in O2, FeO islands supported on Pt(111) remain stable in 3 Torr CO/O2 (2:1) mixture gas or in 1 bar PROX gas, i.e. 10 mbar CO + 5 mbar O2 + 985 mbar H2. The presence of CO could block the adsorption of O2 11 ACS Paragon Plus Environment


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on Pt(111) and react with activated oxygen at the FeO/Pt(111) interface, thus enhancing the stability of FeO islands. The activity of CO2 production over the FeO/Pt(111) surface was measured to be ~ 1 × 1014 molecules∗cm-2∗s-1 at 300 K. The presence of H2 further promotes the stability of FeO islands under the realistic reaction condition of PROX. The highly active FeO islands on Pt(111) were thus confirmed as the active phase for FeOx/Pt catalysts during PROX reaction at 300K.


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Author information Corresponding Author: *[email protected] (F.Y.). *[email protected] (X.B.) Author Contributions F.Y. and X.B. conceived and supervised the project. H.C., Y.L. and X.Z. performed NAP-STM experiments. Y.L., Y.N., Q.L., and Y.Z. performed LT-STM experiments. M.W. and Q.F. carried out the XPS experiments. H.C., Y.L, F.Y. and X.B. wrote the paper. Author Contributions §H.C. and Y.L.: Both contributed equally to this work. Notes The authors declare no competing financial interest. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21303195 and No. 21473191), Ministry of Science and Technology of China (No. 2013CB933100), Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB17020200) and the Thousand Talent Program for Young Scientists.



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Figure 1. The structures of FeO and FeO2 nanoislands on Pt(111). (a) Large scale STM images of FeO islands on Pt(111). (b) The atomic structure of an FeO island with CUF-terminated step edges. Scanning parameters: It=5.2 nA, Vs=10 mV. (c) Large scale STM images of FeO2 islands on Pt(111). (d) The atomic structure of an FeO2 island with highly corrugated surface domains. Scanning parameters: It=1.3 nA, Vs=150 mV. (e) Line profiles across the surface terraces of FeO and FeO2 islands. (f) Fe 2p spectra of the FeO/Pt(111) and FeO2/Pt(111) surfaces measured by XPS.


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Figure 2. Surface structure of the FeO2 islands. (a, b) STM images of the FeO2 island display (√3 ×√3)R30° surface structure. (c, d) STM images of the FeO2 NP after high bias scan display a (1×1) surface structure relative to pristine FeO lattice. The corresponding lattice spacing of the (√3×√3)R30° and (1×1) structures are 0.54nm (b) and 0.31nm (d), respectively. Scanning parameters: (a, b) Vs= 48mV; It= 2 nA. (c, d) Vs=280 mV; It=0.36 nA.



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Figure 3. STM images on the oxidation of FeO islands on Pt(111) in elevated pressures of O2 at 300K. (a) The FeO surface showing oxygen dislocations after the exposure of 1.0×10-8 Torr O2 for 60 min. The Pt substrate was covered by O adatoms, which display the (2×2)-O structure in the STM image. Scanning parameters: It=0.77 nA, Vs=210 mV. (b) The partially oxidized FeO islands on Pt(111) after an exposure of 0.02 Torr O2 for 110 min. (c) In-situ STM image of FeO2 islands on Pt(111), formed by exposing the FeO/Pt(111) surface in 0.4 Torr O2 for 30 min.(d)


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STM image of the FeO2/Pt surface obtained after the evacuation of 1 Torr O2 and pumping to UHV.

Figure 4. STM images of the FeO/Pt(111) surface at 300K and in elevated pressures of CO/O2 mixture gas, which was prepared in the stoichiometric ratio (CO:O2=2:1). Large scale STM images of the FeO/Pt(111) surface under 0.01 Torr CO/O2 mixture gas were displayed (a). The FeO island marked by the dotted square is magnified in (b). During the exposure of CO/O2 mixture gas, the Pt(111) surface was saturated by a (√19 × √19)R23.4°-13 CO phase, whose structure was resolved by STM in (c) and illustrated by the model in (d) with color representations of : Pt-blue and CO-white and purple . In-situ STM image of the FeO/Pt(111) surface under 3 Torr CO/O2 mixture gas was displayed in (e). After the exposure to 3 Torr CO/O2 mixture gas for 4 hours, the FeO/Pt(111) surface was found to remain the FeO phase, as shown in (f). (g) Mass spectra of the gas products leaked from the reaction cell into the mass 21 ACS Paragon Plus Environment


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spectrometer after the reaction on the FeO/Pt(111) surface at 300 K. The CO/O2 mixture gas in the reaction cell consists of 2 Torr Vs=-34

mV.

(c)

13

CO and 1 Torr O2. Scanning parameters: (b) It=1.42 nA, It=1.78

nA,

Vs=33

mV.


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Figure 5. STM images of the FeO/Pt(111) surface at 300K and in elevated pressures of PROX gas, which was prepared with a gas volume ratio at CO:O2:H2=1:0.5:98.5. In-situ STM images of the FeO/Pt(111) surface under 1.2 Torr PROX gas were displayed in large scale (a) and atomic resolution (b). The FeO/Pt(111) surface after the exposure of 9.3 Torr PROX gas for 3 hours was shown in (c), which was taken after evacuating the reactor cell to UHV due to the accumulation of diffusive contamination in the reactor cell. Inset displays the atomic structure of the dotted square in (c), which shows the FeO(111) structure. In-situ STM images of the FeO/Pt(111) surface under 64 Torr PROX gas were displayed in (d). The FeO/Pt(111) surface after the treatment in 760 Torr PROX gas for 30 min was displayed in (e), and with the line profile (f), showing the height of an FeO island. Scanning parameters: (b) It=1.33 nA, Vs=78 mV. Inset in (c) It=0.47 nA, Vs=436 mV.



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Figure 6. Fe2p3/2 XPS spectra of the 0.25 ML FeO/Pt(111) surface after the exposure of various reactant gases with the same total pressure at 760 Torr. The sequence of gas exposure was carried out from bottom to top, as labeled in the figure.


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TOC Graphic


Pt Catalysts in Low-Temperature CO Oxidation

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