ARTICLE pubs.acs.org/JPCC
Hydroxyls-Involved Interfacial CO Oxidation Catalyzed by FeOx(111) Monolayer Islands Supported on Pt(111) and the Unique Role of Oxygen Vacancy Lingshun Xu,†,‡,§ Zongfang Wu,†,‡,§ Yulin Zhang,†,‡,§ Bohao Chen,§ Zhiquan Jiang,† Yunsheng Ma,§ and Weixin Huang†,‡,§,* †
Hefei National Laboratory for Physical Sciences at the Microscale,‡CAS Key Laboratory of Materials for Energy Conversion, and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, People's Republic of China
§
ABSTRACT: We have comprehensively investigated the reactivity of hydroxyls on FeOx(111) monolayer islands with different amounts of oxygen vacancy concentrations grown on Pt(111) by means of X-ray photoelectron spectroscopy, temperature-programmed desorption/reaction spectroscopy, and low energy electron diffraction. Hydroxyls on FeOx(111) monolayer islands are capable of oxidizing CO(a) on Pt(111) at the FeOx(111)-Pt(111) interface at low temperatures and such an interfacial oxidation of CO by hydroxyls to produce CO2 is not suppressed by either excess CO(a) or excess H(a) on FeOx(111)/Pt(111) inverse model catalyst surface. However, the reactivity of hydroxyls is controlled by the oxygen vacancy concentration in FeOx(111) monolayer islands. With the increase of oxygen vacancy concentration, reaction pathways of hydroxyls on FeOx(111) monolayer islands to produce H2O are thermodynamically suppressed, which thus opens other hydroxyls-involved reaction pathways including the interfacial oxidation of CO to produce CO2. These results greatly deepen the fundamental understanding of the reaction mechanism and catalytically active structure for low temperature WGS and PROX reactions catalyzed by oxide supported Pt nanocatalysts.
1. INTRODUCTION The polymer-electrolyte-membrane fuel cell (PEMFC) utilizing hydrogen as a fuel has been attracting much attention in the application to electric vehicles or residential power-generations because of its many attractive features such as high power density, rapid start-up, and high efficiency.1 However, the presence of small amounts of CO in the hydrogen supply produced from the steam reforming of various hydrocarbons seriously degrades the performance of the Pt electrode in PEMFC.2 A practical strategy to supply clean hydrogen is to reduce the CO concentration in the hydrogen stream to the acceptable level (normally below 10 ppm) via the combination of the water-gas shift (WGS) reaction (CO + H2O f CO2 + H2) and the preferential oxidation (PROX) of CO in excess H2.3 The WGS reaction is exothermic and thermodynamically limited, and therefore the high CO conversion can only be obtained at low temperatures. Meanwhile, the PEMFC currently operates at 353 K,1 it is thus desirable to produce the clean hydrogen fuel at 353 K or below 353 K for an economic operation. Therefore, it is critical to develop WGS and PROX catalytic processes at low temperatures. Oxide supported platinum nanoparticles are among the promising efficient commercial catalysts for both WGS4 6 and PROX7 9 reactions at low temperatures. Understanding the catalytic reaction mechanism at a molecular level is very important for the improvement and design of highly efficient catalysts, but still remains as a great challenge. r 2011 American Chemical Society
In the low temperature WGS and PROX reactions catalyzed by Pt/oxide nanocatalysts, CO molecules are generally believed to chemisorb on the Pt surface, but there is no agreement on the oxidation mechanism of COads, a key elementary step. An associative mechanism involving the interfacial reaction of COads on Pt with hydroxyls on the oxide surface has been long proposed to produce CO2 via surface intermediates such as formate or carboxylate in the low temperature WGS reaction.10 12 Recently, the similar mechanism has also been proposed to occur in the low temperature PROX reaction.13,14 The activity of Pt/oxide nanocatalysts in the PROX reaction at low temperatures has been reported to be enhanced by Fe or FeOx additives.15 17 A recent in situ DRIFT study14 indicated that the enhancement effect resulted from the oxidation of CO by hydroxyls. However, because of the complex nature of heterogeneous catalytic reactions, the unambiguous experimental evidence still lacks for the above associative oxidation mechanism. Surface science studies of well-defined model catalysts have been proven to be an effective approach to fundamentally understand the reaction mechanism at the molecular level.18,19 Well-defined iron oxide thin films grown on Pt(111) including FeO(111) monolayer film, Fe3O4(111), R-Fe2O3(0001), Received: May 3, 2011 Revised: June 7, 2011 Published: June 16, 2011 14290
dx.doi.org/10.1021/jp204106u | J. Phys. Chem. C 2011, 115, 14290–14299
The Journal of Physical Chemistry C KxFe22O34(111) thin films compose a nice series of model surfaces for iron oxides and have been successfully employed as model surfaces for the fundamental study of the industrial catalytic dehydrogenation process of ethylbenzene to styrene catalyzed by the iron oxide.20 24 Recently, FeO(111) monolayer structures on Pt(111) have been reported to exhibit unexpected catalytic activity in the oxidation of CO. Sun et al. reported that the FeO(111) monolayer film on Pt(111) can catalyze CO oxidation at a temperature at which Pt is inactive.25,26 Fu et al. proved that the interface between FeO(111) monolayer islands and the Pt(111) substrate is the active site for the oxidation of CO in low temperature PROX reaction.27 These surface science studies of FeO(111) monolayer structures on Pt(111) provide the compelling evidence that the ferrous iron oxide is the active Fe species in practical Pt-FeOx-based nanocatalysts for PROX and WGS reactions at low temperatures. The oxygen-terminated FeO(111) monolayer structure is very inert under ultrahigh vacuum (UHV) conditions.20 We first reported that the exposure of atomic hydrogen at room temperature (RT) could efficiently form hydroxyls and simultaneously create oxygen vacancies on FeO(111) monolayer film28 and further developed a concept of oxygen vacancy-controlled reactivity of hydroxyls on FeO(111) monolayer film.29 In our previous communication,30 we also successfully prepared hydroxyls on FeO(111) monolayer islands on Pt(111) and observed that they can easily undergo the interfacial reaction with COads on Pt(111) to produce CO2 at room temperature, providing direct experimental evidence for the associative mechanism of PROX and WGS reactions catalyzed by Pt/oxide nanocatalysts at low temperatures. In this work, we prepared four types of FeO(111) monolayer islands on Pt(111) with different oxygen vacancy concentrations (FeOx(111)/Pt(111) inverse model catalyst). Hydroxyls were prepared on various FeOx(111)/ Pt(111) inverse model catalysts and their reactivities were investigated. Oxygen vacancies in FeO(111) monolayer islands strongly affect the interfacial reactivity of hydroxyls. With the increase of oxygen vacancy concentration in FeO(111) monolayer islands, the water formation from hydroxyls gets inhibited, opening other hydroxyls-participated reaction channels including the interfacial oxidation with CO. These results not only deepen the fundamental understanding of the oxidation of CO catalyzed by Pt/oxide nanocatalysts, but also extend the concept of oxygen vacancy-controlled reactivity of hydroxyls on oxide surfaces that we recently proposed.29
2. EXPERIMENTAL SECTION All experiments were performed in a Leybold stainless-steel UHV chamber with a base pressure of 1 2 10 10 mbar.29 32 The UHV chamber was equipped with facilities for X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and differential-pumped thermal desorption spectroscopy (TDS)/ temperature-programmed reaction spectroscopy (TPRS). The UHV chamber was also equipped with a QUAD-EV-S mini e-beam evaporator and a MGC75 thermal gas cracker both purchased from the Mantis Deposition Ltd. The Pt(111) single crystal purchased from MaTeck was mounted on the sample holder by two Ta wires spot-welded to the back side of the sample. The sample temperature could be controlled between 130 and 1473 K and was measured by a chromel-alumel thermocouple spot-welded to the backside of the sample. Prior to the experiments, the Pt(111) sample was cleaned by repeated
ARTICLE
Figure 1. LEED patterns of various FeOx/Pt(111) inverse model catalyst surfaces with the oxidation time in 1 10 6 mbar O2 at 850 K for 5 min (FeO0.58(111)/Pt(111)), 10 min (FeO0.82(111)/Pt(111)), 15 min (FeO0.92(111)/Pt(111)) and 30 min (FeO(111)/Pt(111)). Ep = 85 eV.
cycles of Ar+ sputtering and annealing until LEED gave a sharp (1 1) diffraction pattern and no contaminants could be detected by XPS. FeOx(111) monolayer islands were prepared on Pt(111) by the evaporation of submonolayer high-purity iron (99.995%, Alfa Aesar China Co., Ltd.) onto Pt(111) at RT followed by the oxidation in 1 10 6 mbar O2 at 850 K. Totally four types of FeOx(111) monolayer islands on Pt(111), i.e., FeOx(111)/ Pt(111) inverse model catalyst, were prepared by the evaporation of the same amount of iron onto Pt(111) at RT followed by different oxidation time. CO (>99.99%, Nanjing ShangYuan Industry Factory) and D2 (>99.8%, Nanjing ShangYuan Industry Factory) were used as received and their purity was further checked by quadrupole mass spectrometer (QMS) prior to experiments. The exposure of D2 was accomplished by a switched-off MGC75 thermal gas cracker ended with an Ir capillary (diameter: 2 mm) that was positioned ∼8 cm in front of the sample. The thermal gas cracker in operation could generate gas phase atomic deuterium (D(g)) with a cracking efficiency of ∼60%. Other gases were dosed by backfilling. All exposures were reported in Langmuir (1 L = 1.0 10 6 Torr 3 s) without corrections for the gauge sensitivity. During the TDS/ TPRS experiments, the sample was positioned ∼1 mm away from a collecting tube of a differential-pumped QMS and the heating rate was 8 K/s. XPS spectra were recorded using Mg KR radiation (hν = 1253.6 eV) with a pass energy of 50 eV. After each experiment, the surface was reoxidized in 5 10 8 mbar O2 at 850 K for 5 min to restore the original surface.
3. RESULTS AND DISCUSSION 3.1. Characterization of FeOx/Pt(111) Inverse Model Catalysts. We prepared four types of FeOx/Pt(111) inverse model
catalysts by evaporating the same amount of iron onto Pt(111) at RT followed by oxidizing in 1 10 6 mbar O2 at 850 K for 5, 10, 15, and 30 min. Their LEED patterns (Figure 1) are the same with that of FeO(111) monolayer film on Pt(111),20 proving the 14291
dx.doi.org/10.1021/jp204106u |J. Phys. Chem. C 2011, 115, 14290–14299
The Journal of Physical Chemistry C
ARTICLE
Figure 2. (A) Fe 2p and (B) O 1s of various FeOx(111)/Pt(111) inverse model catalyst surfaces prepared by oxidation in 1 10 6 mbar O2 at 850 K for 5 min (FeO0.58(111)/Pt(111)), 10 min (FeO0.82(111)/Pt(111)), 15 min (FeO0.92(111)/Pt(111)) and 30 min (FeO(111)/Pt(111)). (C) The O 1s/Fe 2p XPS intensity of various FeOx(111)/Pt(111) inverse model catalyst surfaces as a function of oxidation time normalized to that of FeO(111)/ Pt(111).
Figure 3. (A) CO-TDS spectra after exposure of 2 L CO and (B) D2-TDS spectra after exposure of 100 L D2 at 130 K to clean Pt(111) surface (a) and FeO0.58(111)/Pt(111) (b), FeO0.82(111)/Pt(111) (c), FeO0.92(111)/Pt(111) (d), and FeO(111)/Pt(111) (e) inverse model catalyst surfaces at 130 K. (C) The fraction of bare Pt(111) surface on various FeOx(111)/Pt(111) inverse model catalyst surfaces calculated by normalizing the integrated CO and D2 desorption peak areas from various FeOx(111)/Pt(111) inverse model catalyst surfaces to those from clean Pt(111) surface.
formation of FeO(111) monolayer structure in all FeOx/Pt(111) inverse model catalysts. Figure 2A,B, respectively, shows the Fe 2p and O 1s XPS spectra of clean FeOx(111)/Pt(111) inverse model catalysts, in which the Fe 2p3/2 and O 1s binding energies are 710.0 and 529.3 eV, respectively. These values are also in line with those in FeO.20 The intensities of Fe 2p3/2 and O 1s peaks increase with the oxidation time, indicating the increasing FeO coverage on Pt(111). This could be attributed to the oxygeninduced segregation and the subsequent oxidation of Fe that diffuses into the subsurface region of Pt(111) during the course of deposition.33 For perfect FeO(111) monolayer islands on Pt(111), their O:Fe stoichiometric should depend on the type of ion (Fe(II) cation or O anion) exposed on Pt-FeO(111) boundary and the size distribution of FeO(111) monolayer islands. However, these microscopic structures of FeOx(111) monolayer islands in the current study could not be determined by available techniques. It is well-established that oxidizing a Pt(111) surface fully covered by Fe in 1 10 6 mbar O2 at 850 K for 30 min can prepare a stoichiometric FeO(111) monolayer
film on Pt(111),20 thus we assumed that FeOx in FeOx/Pt(111) inverse model catalyst prepared with the oxidation time of 30 min is stoichiometric FeO. The ratio between O 1s XPS peak intensity and Fe 2p XPS peak intensity of other three FeOx(111)/Pt(111) inverse model catalysts was then normalized to that of FeO(111)/Pt(111) inverse model catalyst (Figure 2C). FeOx(111) monolayer islands in FeOx(111)/Pt(111) inverse model catalysts prepared with the oxidation time of 5, 10, and 15 min are with the stoichiometric of FeO0.58, FeO0.82 and FeO0.92, respectively. These results imply that the oxygen vacancy concentration in FeO(111) monolayer islands on Pt(111) increases with the decrease of the oxidation time, which is plausible. A unique advantage of the FeO(111) monolayer structure is that oxygen vacancies in the FeO(111) monolayer structure can keep stable even at elevated temperatures because of the lack of the fast diffusion process of lattice oxygen from the bulk to the surface that usually occurs in a thick oxide structure. It has been reported that CO34 and H228 do not chemisorb on the stoichiometric FeO(111) monolayer film on Pt(111), 14292
dx.doi.org/10.1021/jp204106u |J. Phys. Chem. C 2011, 115, 14290–14299
The Journal of Physical Chemistry C
Figure 4. O 1s XPS spectra of clean inverse model catalyst surfaces, clean inverse model catalyst surfaces exposed to 10 L D at 130 K, clean inverse model catalyst surfaces exposed to 10 L D at 130 K followed by flash to 600 K.
therefore, we employed the chemisorption of CO and D2 to titrate the fraction of bare Pt(111) surface on various FeOx(111)/ Pt(111) inverse model catalyst surfaces. Figure 3A shows COTDS spectra following the saturating exposure (2 L) of CO at 130 K. Besides the desorption feature from Pt(111) surface, a small CO desorption peak appears at ∼235 K which could be assigned to CO(a) chemisorbed on Fe(II) sites in FeOx(111) monolayer islands.34 These Fe(II) sites expose on the oxygen vacancy sites on FeOx(111) (x < 1) monolayer islands and also likely on the Pt(111)-FeOx(111) (x e 1) boundary. The CO desorption feature from Pt(111) surface weakens with the increase of x value in FeOx, indicating that the bare Pt(111) surface fraction on FeOx(111)/Pt(111) inverse model catalyst surfaces decreases with the increase of oxidation time, in agreement with XPS results. Figure 3B shows D2-TDS spectra following the saturating exposure (100 L) of D2 at 130 K. The D2 desorption feature from FeOx(111)/Pt(111) inverse model catalyst surfaces could be assigned to D(a) chemisorbed on bare Pt(111) since D2 is not likely to dissociate on exposed Fe(II) sites. Similar to the CO-TDS results, the D2 desorption peak also weakens with the increase of x value in FeOx. The fraction of bare Pt(111) surface on FeOx(111)/Pt(111) inverse model catalyst surfaces was estimated by normalizing the integrated area of CO
ARTICLE
and D2 desorption peaks from FeOx(111)/Pt(111) inverse model catalyst surfaces to the corresponding value from clean Pt(111). It can be seen from Figure 3C that the results estimated from CO-TDS spectra and from D2-TDS spectra agree quite well, demonstrating the fraction of bare Pt(111) surface is 0.40, 0.32, 0.20, and 0.16 monolayer (ML) on FeO0.58(111)/Pt(111), FeO0.82(111)/Pt(111), FeO0.92(111)/Pt(111), and FeO(111)/ Pt(111) inverse model catalyst surfaces, respectively. Therefore, four types of FeOx(111)/Pt(111) inverse model catalyst surfaces that we prepared are Pt(111) covered with 0.60 ML FeO0.58(111) monolayer islands, Pt(111) covered with 0.68 ML FeO0.82(111) monolayer islands, Pt(111) covered with 0.80 ML FeO0.92(111) monolayer islands, and Pt(111) covered with 0.84 ML FeO(111) monolayer islands. However, it has to be pointed out again that the microscopic structure of FeOx(111)/Pt(111) inverse model catalyst surfaces including the size distribution of FeOx(111) monolayer islands and the type of ion (Fe(II) cation or O anion) exposed on Pt-FeO(111) boundary is unknown. 3.2. Chemisorption of Atomic Deuterium on FeOx/Pt(111) Inverse Model Catalysts. We have previously reported that exposure of FeO(111) monolayer film on Pt(111) to atomic H(g) or D(g) can produce hydroxyls species (OH(a) or OD(a)) and their reactivities depend on the oxygen vacancy concentration in FeO(111) monolayer film.28,29 Figure 4 shows O 1s XPS spectra after the exposure of 10 L D(g) on various FeOx(111)/ Pt(111) inverse model catalyst surfaces at 130 K followed by flashing to 600 K. Ten L D(g) at 130 K was demonstrated to be a saturating exposure for FeO(111) monolayer film29 but not for clean Pt(111).35 The interaction of atomic D(g) with FeOx(111) monolayer islands depends on their structure. On FeO0.58(111)/ Pt(111) and FeO0.82(111)/Pt(111), the chemisorption of atomic D(g) at 130 K leads to the formation of hydroxyls on FeOx(111) monolayer islands, as evidenced by the O 1s XPS peak at 531.3 eV;28 however, on FeO0.92(111)/Pt(111) and FeO(111)/Pt(111), the chemisorption of atomic D(g) at 130 K leads to not only the formation of hydroxyls, but also the formation of chemisorbed water with an O 1s binding energy at 532.5 eV.29 Moreover, the fraction of chemisorbed water formed on FeO(111)/Pt(111) is much larger than that on FeO0.92(111)/Pt(111). Flashing FeOx(111)/Pt(111) inverse model catalysts after the exposure of atomic D(g) at 130 to 600 K removes all chemisorbed surface species. Figure 5A shows D2-TDS spectra after the exposure of 10 L D(g) on clean Pt(111) and FeOx(111)/Pt(111) inverse model catalyst surfaces. D(a) majorly chemisorbs on Pt(111) but also likely on Fe(II) sites in FeOx(111) monolayer islands, whereas OD(a) and D2O(a) only on FeOx(111) monolayer islands. Likely surface reactions on FeOx(111)/Pt(111) inverse model catalyst surfaces with chemisorbed D(a), OD(a), and D2O(a) are quite complex and summarized in Table 1. Surface reactions to produce D2 include the recombinative desorption of D(a) on Pt(111), the interfacial reaction between D(a) on Pt(111) and OD(a) on FeOx(111), the recombinative desorption of D(a) on Fe(II) sites, and the reaction between OD(a) on FeOx(111). The recombinative desorption of D(a) from clean Pt(111) occurs between 200 and 400 K (curve a in Figure 5A); the recombinative desorption of D(a) on Fe(II) sites in FeOx(111) monolayer film occurs below 200 K and the reaction of OD(a) to produce D2 occurs between 200 and 400 K on FeOx(111) monolayer films with low oxygen vacancy concentrations and above 450 K on FeOx(111) monolayer films with high oxygen 14293
dx.doi.org/10.1021/jp204106u |J. Phys. Chem. C 2011, 115, 14290–14299
The Journal of Physical Chemistry C
ARTICLE
Figure 5. (A) D2-TDS and (B) D2O-TDS spectra after exposure of 10 L D to clean Pt(111) (a) and FeO0.58(111)/Pt(111) (b), FeO0.82(111)/Pt(111) (c), FeO0.92(111)/Pt(111) (d), and FeO(111)/Pt(111) (e) inverse model catalyst surfaces at 130 K. The inset in (A) shows the zoom-in D2-TDS spectra above 400 K.
Table 1. Likely Surface Reactions on FeOx(111)/Pt(111) Inverse Model Catalyst Surfaces with Chemisorbed D(a), OD(a) and D2O(a)a Reaction sites Pt(111)
Surface reactions D(a) + D(a) f D2(g) (200 400 K)
Pt(111)-FeOx(111) interface
D(a) + OD(a) f D2(g) + Olattice
D(a) + OD(a) f D2O(g) + Ovacancy
FeOx(111) monolayer islands
(350 400 K) D(a) + D(a) f D2(g)
(200 300 K) OD(a) + OD(a) f D2O(g) + Ovacancy
(450 K c)
(
