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Proton-Transfer-Connected Elementary Surface Reaction Network for Low-Temperature CO Oxidation Catalyzed by Metal-Oxide Nanocatalysts Yuekang Jin, Guanghui Sun, Feng Xiong, Zhengming Wang, 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, P. R. China ABSTRACT: Low-temperature CO oxidation catalyzed by metaloxide nanocatalysts is a hot topic, but the mechanism is strongly debated. Via controlled preparation of isotope-labeled surface adsorbates on an FeO(111)/Pt(111) inverse model catalyst, we herein demonstrate that elementary surface reactions for CO oxidation with similar activation energies exist both on the Pt surface and at the FeO− Pt interface when surface hydroxyl groups and water are present. Water and hydroxyl groups can enhance the dissociation probability of molecularly adsorbed O2 to produce oxygen adatoms. The proton transfer from surface hydroxyl groups to adjacent oxygen adatoms connects the elementary surface reactions on the Pt surface and at the FeO−Pt interface to constitute a surface reaction network. These results for the first time provide a view of the proton-transfer-connected elementary surface reaction network for the unanimous understanding of low-temperature CO oxidation catalyzed by metal-oxide nanocatalysts and reveal the dual roles of surface hydroxyl groups to promote CO oxidation and bridge various surface reaction pathways.
1. INTRODUCTION
The reaction mechanism of CO oxidation catalyzed by FeOx−Pt catalysts was also experimentally explored employing model catalysts of FeO(111) monolayer film or islands on Pt(111). Under ultrahigh vacuum (UHV) conditions, CO adsorbed on the Pt(111) surface was observed to be facilely oxidized by O2 at the Pt−FeO interface at RT,8 and such a surface reaction was further visualized with scanning tunneling microscopy (STM);12 meanwhile, solid experimental evidence was also reported for an oxygen-vacancy-controlled interfacial oxidation mechanism of CO adsorbed at the Pt(111) surface by hydroxyl groups on the FeO surface at the Pt−FeO interface to produce CO2 above 300 K.13−17 Under CO oxidation reaction at atmospheric pressure, FeO(111) monolayer film or islands on Pt(111) were found to restructure into FeO2 film or islands, but the resulting FeO2 film and Pt−FeO2 interface could only efficiently catalyze CO oxidation at 450 K while CO oxidation on the Pt surface produced CO2 at low temperatures.18−20 These contradictory results demonstrate the complex nature of the reaction mechanism of low-temperature CO oxidation catalyzed by metal-oxide nanocatalysts that likely involve various surface species and active sites.21 In this article, via controlled preparation of isotope-labeled surface adsorbates on an FeO(111)/Pt(111) inverse model catalyst, we unambiguously demonstrate the presence of a proton-transfer-connected elementary surface reaction network for low-temperature CO oxidation catalyzed by metal-oxide nanocatalysts and the
CO oxidation is of particular importance and interest in heterogeneous catalysis. On one hand, it is one of the major reactions in environmental catalysis;1 on the other hand, it has been long used as the prototype reaction for fundamental understanding of heterogeneous catalysis.2 CO oxidations catalyzed on metal surfaces (such as Pt and Pd) or metal oxide surfaces (such as CeO2) generally occur efficiently at temperatures above 400 K and respectively follow the LH and Mvk mechanisms.3,4 Recently, significant progress has been achieved on efficient CO oxidation at 300 K and below catalyzed by metal-oxide nanocatalysts from the pioneering oxide supported Au nanoparticles5 to oxide/hydroxide supported other noble metal nanoparticles, clusters, and single atoms6 and to noble metal supported oxide and hydroxide nanostructures.7 Interestingly, catalytic performances of metaloxide nanocatalysts in low-temperature CO oxidation generally depend sensitively on the presence of H2 or H2O in the reaction stream, and this brings strong debates on the active structure and reaction mechanism, particularly on the role of surface hydroxyl groups. For example, the Pt−FeO interface,8 the Pt−Fe(OH)x interface,9 and the PtFe−FeOx interface10 were proposed respectively as the active structures to catalyze low-temperature preferential oxidation (PROX) of CO in excess H2, CO oxidation in the presence of water, and CO oxidation with a water content below 4 ppm. Corresponding reaction mechanisms without or with the participation of surface hydroxyl groups were also proposed on the basis of DFT calculations.8,9,11 © XXXX American Chemical Society
Received: October 26, 2016 Published: November 7, 2016 A
DOI: 10.1021/acs.jpcc.6b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C versatile and vital role of surface hydroxyl groups in the network.
2. EXPERIMENTAL SECTION All experiments were performed in a stainless-steel UHV system equipped with a polarization-modulated infrared reflection−absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), differentially pumped quadrupole mass spectrometer (QMS), and e-beam evaporator. The base pressure was 1.2 × 10−10 mbar. A 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 100 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) surface was cleaned by repeated cycles of Ar+ sputtering and annealing until LEED gave a sharp (1 × 1) diffraction pattern and no contaminants could be detected by XPS. FeO(111)/Pt(111) inverse model surfaces were prepared by the evaporation of submonolayer high-purity iron (99.995%, Alfa Aesar China Co., Ltd.) onto clean Pt(111) at room temperature followed by oxidation in 1 × 10−6 mbar 16O2 at 1000 K for 30 min. The formation of FeO(111) structure was confirmed by LEED. CO (>99.99%), D2 (>99.999%), and 16O2 (>99.999%) were purchased from Nanjing ShangYuan Industry Factory and used as received. 18O2 (18O > 97%) was purchased from Sigma-Aldrich. D218O (D > 98%, 18O > 98%, Huayi Isotopes Co.) and D2O (D > 99.9%, Sigma-Aldrich) were purified by repeated freeze−pump−thaw cycles and exposed through a line-of-sight stainless-steel doser positioned at ∼6 cm in front of the sample. The purities of all reactants were checked by QMS prior to experiments. All exposures were reported in Langmuir (1 L = 1.0 × 10−6 Torr·s) without corrections for the gauge sensitivity. During the thermal desorption spectroscopy (TDS) experiments, the sample was positioned ∼1 mm away from the collecting tube of the differentially pumped QMS and was heated at a heating rate of 3 K/s. XPS spectra were recorded using Mg Kα radiation (hν = 1253.6 eV) with a pass energy of 20 eV.
Figure 1. TDS spectra of (A) various O2 exposures on Pt(111) and 0.40 ML FeO(111)/Pt(111), (B) 0.02 L D2O exposure on 2 L O2covered Pt(111), (C) 0.02 L D2O exposure on 2 L O2-covered 0.40 ML FeO(111)/Pt(111), and (D) 50 L O2 exposure on 18OD-covered 0.40 ML FeO(111)/Pt(111). TDS spectra of corresponding individual adsorptions are included.
molecularly adsorbed O2 on 0.40 ML FeO(111)/Pt(111) is similar to that on Pt(111) in both desorption temperature and peak shape. The recombinative desorption peak of oxygen adatoms on 0.40 ML FeO(111)/Pt(111) occurs at higher temperature than that on Pt(111), demonstrating the stronger interaction of oxygen adatoms with the 0.40 ML FeO(111)/ Pt(111) surface. Previous DFT calculation results showed that the coordination-unsaturated Fe site at the FeO(111)−Pt(111) interface exhibits a stronger adsorption ability than the Pt(111) site.11 Different from the recombinative desorption of oxygen adatoms on Pt(111) with a typical second-order desorption kinetics, the recombinative desorption of oxygen adatoms on 0.40 ML FeO(111)/Pt(111) exhibits a zero-order desorption kinetics that indicates the islanding of oxygen adatoms on the surface. We propose that the islanding process likely originates from the strongly bound oxygen adatoms at the FeO(111)− Pt(111) interface and occurs during the heating process of TDS experiments. Co-adsorption of O2 and D2O on Pt(111) was studied. Agreeing with previous results,24 D2O molecularly adsorbs on Pt(111), giving rise to a desorption peak at 170 K, and the presence of preadsorbed oxygen species can induce the dissociation of D2O to form surface hydroxyl groups that recombine to produce D2O at 190 K (Figure 1B). Comparing the O 2 TDS spectrum of corresponding individual O 2 adsorption, the O2 desorption peaks of both molecularadsorbed O2 and the oxygen adatoms weaken upon the subsequent D 2 O coadsorption, but the ratio of the recombinative O2 desorption peak to the total O2 desorption peak increases from 0.2 for 2 L O2 adsorption to 0.27 for 2 L O2 and 0.02 L D2O coadsorption. In the corresponding XPS spectra (Figure 2B), the O 1s feature of molecularly adsorbed O2 weakens upon the subsequent water exposure. Meanwhile,
3. RESULTS AND DISCUSSION As described previously,16 an FeO(111)/Pt(111) inverse model catalyst consisting of 0.4 monolayer (ML) FeO(111) islands dispersed on Pt(111) was prepared by deposition of sub- (ML) Fe on a clean Pt(111) substrate at room temperature followed by oxidation in 1 × 10−6 Torr O2 at 1000 K for 30 min. FeO(111) islands on Pt(111) have been identified to be a Fe− O bilayer structure22 with both Fe and O terminations at the FeO(111)−Pt(111) interface.12 Figure 1A compares O2 TDS spectra from the Pt(111) surface and the 0.40 ML FeO(111)/Pt(111) inverse model surface. At 110 K, O2 adsorption on Pt(111) gives rise to a sharp O2 desorption peak at 130 K and a broad O2 desorption peak above 600 K respectively arising from the desorption of molecularly adsorbed O2 and the recombinative desorption of oxygen adatoms. XPS results show that oxygen adatoms are dominantly formed by the dissociation of molecularly adsorbed O2 during heating (Figure 2A). These agree with previous results.23 On 0.40 ML FeO(111)/Pt(111), both O2 desorption peaks attenuate, since O2 does not adsorb on the FeO surface under UHV conditions at 110 K. The desorption feature of B
DOI: 10.1021/acs.jpcc.6b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. O 1s XPS spectra of (A) 2 L O2 adsorption on Pt(111) and (B) 0.02 L D2O adsorption on 2 L O2-precovered Pt(111) at 110 K followed by annealing at indicated temperatures, and (C) integrated O 1s XPS spectra of various O components observed in parts A and B.
Figure 3. O 1s XPS spectra of (A) 0.02 L D2O adsorption on 0.40 ML FeO(111)/Pt(111) and (B) 0.02 L D2O adsorption on 2 L O2-precovered 0.40 ML FeO(111)/Pt(111) at 110 K followed by annealing at indicated temperatures, and (C) integrated O 1s XPS spectra of various O components observed in parts A and B.
reactivity of hydroxyl groups on the FeO(111) surface that we proposed previously. 14,15,26,27 Comparing the O 2 TDS spectrum of corresponding individual O2 adsorption, the desorption peak of molecularly adsorbed O2 gets significantly weakened while the recombinative desorption peak of oxygen adatoms exhibits similar intensities but its shape resumes back to that from bare Pt(111). Thus, the subsequent water adsorption is capable of substituting molecularly adsorbed O2 but meanwhile enhancing the dissociation probability of molecularly adsorbed O2. Moreover, the peak shape of the recombinative desorption of oxygen adatoms indicates that the substitution of molecularly adsorbed O2 by water adsorption should be so profound at the FeO−Pt interface that the remaining molecularly adsorbed O2 at the FeO−Pt interface is not enough to induce the islanding of oxygen adatoms on the 0.40 ML FeO(111)/Pt(111) surface. This might reflect the large difference between the adsorption strengths of water and O2 at the FeO−Pt interface, which can be attributed to the dissociation of water at the FeO−Pt interface. In order to further clarify the interaction of coadsorbed water and O2 at the FeO−Pt interface, the 0.40 ML FeO(111)/ Pt(111) surface with hydroxyl groups at the FeO−Pt interface was prepared by 0.02 L D218O adsorption at 110 K followed by flash at 200 K. The 0.40 ML FeO(111)/Pt(111) surface with 18 OD at the FeO−Pt interface was then exposed to 50 L O2 at 110 K. Similar to water adsorption on the O2-precovered 0.40 ML FeO(111)/Pt(111) surface, the coadsorbed oxygen completely switches the reaction pathway of the hydroxyl groups at the FeO−Pt interface from hydrogen production to water production (Figure 1D). Comparing the O2 TDS
the O 1s peak intensity ratio of oxygen adatoms to molecularly adsorbed O2 increases from 0.21 for 2 L O2 adsorption to 0.28 for 2 L O2 and 0.02 L D2O coadsorption (Figure 2C). Thus, both TDS and XPS results demonstrate that the subsequent water adsorption can substitute molecularly adsorbed O2 on Pt(111) to some extent but meanwhile enhance the dissociation probability of molecular-adsorbed O2 to produce oxygen adatoms. Co-adsorption of O2 and D2O on 0.40 ML FeO(111)/ Pt(111) was also studied (Figure 1C and Figure 3). Agreeing with previous results,15−17,25 water molecularly adsorbs on Pt and FeO surfaces of the 0.40 ML FeO(111)/Pt(111) surface, producing a water desorption peak at 170 K, and water both molecularly adsorbs and dissociates into hydroxyl groups at the FeO−Pt interface, respectively producing water at 192 K and D2 above 400 K. On the O2-precovered 0.40 ML FeO(111)/ Pt(111) surface, the water desorption peak at 170 K significantly attenuates, which could be attributed to the preadsorbed oxygen-species-induced D2O dissociation on Pt(111) into surface hydroxyl groups. Another strong water desorption peak appears at 200 K, but no D2 desorption feature could be observed. The XPS results demonstrate that all surface hydroxyl groups disappear upon annealing at 300 K. Thus, the strong water desorption peak at 200 K should be contributed by reactions of hydroxyl groups on both Pt(111) and at the FeO(111)−Pt(111) interface. These observations suggest that the oxygen atoms on the surface completely switch the recombination reaction pathway of hydroxyl groups at the FeO−Pt interface from H2 production to water production, in consistence with the concept of oxygen-vacancy-controlled C
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The Journal of Physical Chemistry C spectrum of 50 L O2 adsorption, the O2 desorption peak of molecularly adsorbed O2 weakens due to the site-blocking effect of hydroxyl groups on the subsequent O2 adsorption, while both the intensity and shape of the recombinative O2 desorption peak of oxygen adatoms do not change much. This indicates the enhancement effect of hydroxyl groups at the FeO−Pt interface on the O2 dissociation and the islanding of oxygen adatoms. Very interestingly, the 18O16O and D216O desorption traces were observed to accompany the recombinative 16O2 and D218O peaks, respectively. These results clearly demonstrate the O-isotope exchange between 16O adatoms and 18 OD groups that can only occur via the proton transfer process. Such a proton transfer process between hydroxyl groups at the FeO−Pt interface and oxygen adatoms on the Pt surface can produce oxygen adatoms at the FeO−Pt interface and hydroxyl groups on the Pt surface, the former of which might result in the islanding of oxygen adatoms on the surface. The effects of coadsorbed water or hydroxyl groups on CO oxidation were then examined. On Pt(111) (Figure 4),
water exposure. These results demonstrate that the enhancement of CO2 production by the addition of water to O2 + CO on Pt(111) should result from the water-enhanced decomposition of molecularly adsorbed O2 to provide increased oxygen adatoms instead of the oxidation of CO by water or hydroxyl groups. On the 0.40 ML FeO(111)/Pt(111) surface, coadsorption of O2 and CO gives the α and β CO2 desorption peaks on the Pt surface and an additional β′ CO2 desorption peak at 200 K that should result from CO(a) + O(a) reaction at the FeO−Pt interface (Figure 5A). Thus, the FeO−Pt interface constitutes
Figure 4. TPRS spectra from Pt(111) surfaces exposed to 2 L O2 followed by 5 L CO and to 2 L O2 followed by 5 L CO and 0.02 L D2O, and from oxygen-adatom-covered Pt(111) surfaces exposed to 5 L CO and to 5 L CO followed by 0.02 L D2O.
Figure 5. TPRS spectra of (A) O2 and CO coadsorption, (B) 18OD and CO coadsorption, and (C) 18OD, O2, and CO coadsorption on 0.40 ML FeO(111)/Pt(111).
an active site to catalyze CO(a) + O(a) reaction with a lower activation barrier than the Pt surface and the introduction of FeO on the Pt surface can enhance the catalytic activity in lowtemperature CO oxidation. In addition, hydroxyl groups at the FeO−Pt interface can mediate the reaction of CO at the Pt surface with lattice oxygen in FeO to produce CO2 at 390 K (Figure 5B).13−17 With coadsorption of 2 L O2 and 5 L CO on the 0.40 ML FeO(111)/Pt(111) surface with 18OD at the FeO−Pt interface (Figure 5C), the α and β C16O2 desorption peaks from the Pt surface were observed but the β′ C16O2 desorption peak from the FeO−Pt interface was not. This indicates that the adsorption and decomposition of O2 at the FeO−Pt interface should be blocked by the presence of 18OD and that the 16O species should form only on the Pt surface. Therefore, under our experimental conditions, the 18O species is exclusively at the FeO−Pt interface while the 16O species is exclusively on the Pt surface. A C16O18O desorption peak (δ′) was also observed at 288 K and could be inferred by the location of 18O species to result from the COPt + OPt + 18 ODFeO−Pt reaction at the FeO−Pt interface. When the O2 exposure was increased to 5 L, the α and β C16O2 desorption peaks from the Pt surface and the δ′ C16O18O desorption peak from the FeO−Pt interface grow; meanwhile, additional C16O2 and C16O18O desorption peaks appear with similar desorption
coadsorption of O2 and CO at 110 K gives two CO2 desorption peaks at 130 (α peak) and 310 K (β peak), respectively, resulting from the CO(a) + O2(a) and CO(a) + O(a) reactions.23 With the addition of water to produce adsorbed water and hydroxyl groups, the α CO2 desorption peak weakens greatly while the β CO2 desorption peak increases greatly. This could be related to the decreased coverage of molecularly adsorbed O2 and increased coverage of oxygen adatoms induced by water coadsorption. Meanwhile, two additional CO2 desorption peaks were observed at 150 (γ peak) and 205 K (δ peak). The γ CO2 peak is accompanied by the desorption peak of molecularly adsorbed water and can be attributed to the water-promoted CO(a) + O(a) reaction, while the δ CO2 peak is attributed to an OH-promoted CO(a) + O(a) reaction. Both reaction pathways were previously proposed on the basis of DFT calculations.28 The addition of water not only opens up water/hydroxyl-groups-promoted CO(a) + O(a) reaction with smaller barriers than CO(a) + O(a) reaction but also significantly enhances the total CO2 production from O2 + CO reaction on Pt(111). However, when water is exposed to an O(a) + CO-covered Pt surface, the γ and δ CO2 peaks also appear but the β CO2 peak attenuates, and the total CO2 production remains similar to that without D
DOI: 10.1021/acs.jpcc.6b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 6. Schematic illustration of the proton migration-connected elementary surface reaction network for low-temperature CO oxidation catalyzed on an FeO(111)/Pt(111) inverse model catalyst surface. Black, green, gray, and blue balls respectively represent C, H, Fe, and Pt atoms, while yellow, red, and pink balls respectively represent O of CO, O2/H2O, and FeO.
Third, the proton transfer processes between the Pt surface and FeO−Pt interface bridge various surface reactions to constitute a surface reaction network to produce CO2. Particularly, the proton transfer from hydroxyl groups at the FeO−Pt interface to oxygen adatoms on the Pt surface simultaneously opens up the OHPt-promoted COPt + OPt and COPt + OFeO−Pt reactions with lower barriers to produce CO2, which leads to enhanced activity in catalyzing low-temperature CO oxidation. These results demonstrate that the complex mechanism of low-temperature CO oxidation on FeO−Pt nanocatalysts can be described by a proton-transfer-connected elementary surface reaction network. Therefore, the stability and coverage of hydroxyl groups on the catalyst surface greatly affect the activity in low-temperature CO oxidation, and the higher stability of hydroxyl groups at the FeO−Pt interface than on the Pt surface is also responsible for the enhanced catalytic activity by the introduction of FeO on the Pt surface.
temperatures (∼200 K) but slightly different shapes. Judging by the locations of the 16O and 18O species, the C16O2 desorption peak at ∼200 K arises from OH-promoted CO + O reaction on the Pt surface (the δ peak), while the C16O18O desorption peak at ∼200 K arises from CO + 18O reaction at the FeO−Pt surface (the β′ peak). The observations of both reactions can be attributed to the occurrence of proton transfer from 18OD at the FeO−Pt interface to 16O adatoms on Pt that produces 18O adatoms at the FeO−Pt interface and 16OH on the Pt surface, as demonstrated by the above results. The appearance of the δ C16O2 and β′ C16O18O peaks at 5 L O2 exposure but not at 2 L O2 exposure suggests that the coverage of oxygen adatoms on Pt needs to be high enough to form oxygen adatoms on Pt adjacent to hydroxyl groups at the FeO−Pt interface to facilitate the occurrence of the proton transfer process. Controlled by the oxygen vacancy concentration, the D2 production resulting from hydroxyl groups decreases with the increasing O2 exposure while the water production increases. The above results of controlled experiments demonstrate important and versatile roles of coadsorbed water and hydroxyl groups on CO oxidation catalyzed by the 0.40 ML FeO(111)/ Pt(111) surface. First, coadsorbed water/hydroxyl groups can enhance the dissociation probability of molecularly adsorbed O2 into oxygen adatoms. Second, coadsorbed water/hydroxyl groups open up novel surface reaction pathways for CO oxidation. Considering the relative stability between hydroxyl groups and water and between oxygen adatoms and molecularly adsorbed O2, we mainly discuss surface reactions involving oxygen adatoms and hydroxyl groups that likely contribute to the catalytic activity of metal-oxide nanocatalysts in low-temperature CO oxidation. As schematically illustrated in Figure 6, without the presence of hydroxyl groups, CO adsorbed on the Pt surface (COPt) can react with oxygen adatoms either on the Pt surface (OPt) or at the FeO−Pt interface (OFeO−Pt) to produce CO2, and the former exhibits a larger barrier. The presence of hydroxyl groups on the Pt surface (OHPt) opens up the OHPt-promoted COPt + OPt reaction with a similar barrier to COPt + OFeO−Pt, while the presence of hydroxyl groups on the FeO−Pt interface (OHFeO−Pt) opens up the COPt + OPt + OHFeO−Pt and OHFeO−Pt-mediated COPt + OFeO reactions at the FeO−Pt interface. The COPt + OPt + OHFeO−Pt reaction exhibits a similar barrier to COPt + OPt, and the OHFeO−Pt-mediated COPt + OFeO reaction exhibits an increased barrier.
4. CONCLUSIONS In summary, we for the first time demonstrate a protontransfer-connected elementary surface reaction network for low-temperature CO oxidation catalyzed on FeO−Pt nanocatalysts when surface hydroxyl groups and water are present. Elementary surface reactions for CO oxidation with similar activation energies exist both on the Pt surface and at the FeO− Pt interface. The proton transfer from surface hydroxyl groups to adjacent oxygen adatoms connects various elementary surface reactions on the Pt surface and at the FeO−Pt interface to constitute a surface reaction network. Water and hydroxyl groups can enhance the dissociation probability of molecularly adsorbed O2 to produce oxygen adatoms. Such a concept provides a unanimous fundamental understanding of lowtemperature CO oxidation catalyzed by metal-oxide nanocatalysts.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 008655163600435. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Basic Research Program of China (2013CB933104), National Natural Science E
DOI: 10.1021/acs.jpcc.6b10789 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Foundation of China (21525313, U1332113), Chinese Academy of Sciences (KJZD-EW-M03), MOE Fundamental Research Funds for the Central Universities (WK2060030017), and Collaborative Innovation Center of Suzhou Nano Science and Technology.
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