Article pubs.acs.org/JPCC
In Situ Characterizations of Nanostructured SnOx/Pt(111) Surfaces Using Ambient-Pressure XPS (APXPS) and High-Pressure Scanning Tunneling Microscopy (HPSTM) Stephanus Axnanda,*,† Zhongwei Zhu,‡,§ Weiping Zhou,∥ Baohua Mao,† Rui Chang,†,⊥ Sana Rani,† Ethan Crumlin,† Gabor Somorjai,‡,§ and Zhi Liu*,†,⊥ †
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Chemistry, University of California, Berkeley, California 94720, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States ⊥ State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: We have conducted in situ measurement of “inverse catalysts” of SnOx nanostructures supported on Pt(111) using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and high-pressure scanning tunneling microscopy (HPSTM) techniques under CO exposure at room temperature and 450 K. Nanostructures of SnOx were prepared by depositing Sn on Pt(111) precovered by O2 layers at liquid nitrogen temperature. APXPS data show that the prepared SnOx nanoparticles are highly reduced, with Sn2+ being the dominant oxide species. The relative Sn2+concentration, compared to Sn4+ and Sn0, in the SnOx nanoparticles decreases slightly with increasing Sn coverage. In situ study of SnOx/Pt(111) inverse catalyst shows that for lower coverage of SnOx (0.25 monolayers (ML)), the amount of Sn2+ decreased steadily, while Sn0 amount steadily increased with negligible Sn4+ amount, as the surface was heated under CO exposure at 450 K. Meanwhile, for the higher coverage (1.0 ML), the decrease of Sn2+ is followed by sharp increase in the amount of Sn4+ and Sn0. HPSTM images show that small islands of SnOx are randomly formed on the substrate, with the size and density increasing with SnOx coverage. HPSTM images show morphology differences between low and high coverages of SnOx on Pt(111) under both UHV and CO exposure conditions.
1. INTRODUCTION SnOx promoted platinum-based electrocatalysts were found to have high activity toward ethanol oxidation reaction.1−7 The complexity and heterogeneity of this electrocatalyst make it challenging to enhance our fundamental understanding of this system. To overcome these obstacles, model systems have been used to represent electrocatalysts and heterogeneous catalysts to obtain fundamental insight of their catalytic activities.8−10 A model system where metal oxide nanoparticles deposited on metal single crystals, so-called “inverse” catalyst system, is often used to represent the more complex catalysts having metal oxide and metal components.1,9,11−16 These “inverse” catalysts can then be studied to correlate the effects of metal−metal oxide interface sites to the electrocatalytic activities and also to understand the role of the metal oxide component.1,9,11−16 Metal oxides are found to be an active component in heterogeneous catalysts instead of solely acting as high surface area support.9,11 Morphology and © 2014 American Chemical Society
structure of metal oxides is important in determining the activity of the oxides. It is proposed that the low-coordination sites that are available on small nanoparticles play an important role in the increased catalytic activity.1,10 In a previous study on SnOx/Pt inverse catalyst, we investigated ethanol oxidation reaction (EOR) on SnOx nanoparticles that were grown on Pt(111) using reactive layer assisted deposition (RLAD).1,10 This study shows that SnOx /Pt(111) inverse catalysts are catalytically more active for EOR in acidic media compared to bare Pt(111).1,10 It was also found that small coverage of SnOx, 0.3−0.4 monolayers (ML), has the highest activity, with a predominately Sn2+ oxidation state. It was suggested that the SnOx−Pt interface sites combined with the reduced Sn2+ species play an active role in the increased Received: September 17, 2013 Revised: December 23, 2013 Published: January 3, 2014 1935
dx.doi.org/10.1021/jp409272j | J. Phys. Chem. C 2014, 118, 1935−1943
The Journal of Physical Chemistry C
Article
of the Sn 3d5/2 were recorded at photon energies of 800 eV. The BE’s were referenced to the adventitious carbon, C 1s peak at 284.5 eV. Shirley backgrounds and asymmetric line shape fitting were used to fit the Sn 3d XPS spectra. All Sn 3d peaks were fitted with fwhm of 1.1 eV. Peak fitting for peak position and quantification was carried out using the software CasaXPS.18 “Lorentzian Asymmetric” line shape was used to fit Sn 3d spectra.18 CO (Air Gas, UHP) and O2 (Air Gas, UHP) were introduced through separate leak valves. The STM experiments were performed in a home-built instrument that was described in detail in previous reports.19 STM samples were prepared with procedures identical to those in the RLAD method. All the STM images were recorded at room temperature with commercial Pt/Ir tips coated by Au, at +0.1−0.2 V sample bias and 0.1−0.2 nA tunneling current. Pt(111) disc (MacTek) was 2 mm thick with a 1 cm diameter, and was oriented to better than ±1 degree. To clean Pt(111), multiple cycles of sputtering−oxidation−flashing process were applied. Sputtering was done in Ar gas at 1 × 10−5 Torr for 30 minutes followed by oxidation in 1 × 10−6 Torr oxygen at 850 K for 20 minutes. The Pt sample was then flashed at 1000 K for 10 s. Surface cleanliness was monitored by XPS. SnOx nanostructures were prepared following RLAD technique. RLAD technique involves a physisorbed multilayer of one reactant (oxygen) being initially deposited on the substrate surface followed by physical deposition of the second layer, the metal, at low temperature. During this initial deposition, the deposited metal reacts with the multilayer reactant. The temperature of the substrate is then raised causing the nonreacted adsorbed molecules to be desorbed, leaving an ensemble of nanoparticles of the deposited metal on the surface. Many metal and oxides nanoparticles have been prepared following this RLAD method.20−24 The RLAD preparation starts by evaporating Sn on clean Pt(111) that is precovered by 30 layers (L) O2 at 130 K. After Sn evaporation, the sample is then heated under 10−6 Torr of oxygen to 600 K. From previous work,10 annealing to 600 K will result in deposited SnOx with a dominant Sn2+ oxidation state, a Sn 3d peak that is quite close to that of the Sn2+ peak reported in other works.25,26 The sample is cooled to room temperature at a rate of 1 K/sec after annealing. Sn (Alfa-Aesar) was vapordeposited by resistively heating a ceramic tube containing an ingot of tin metal. Determination of the Sn coverage was based on the Pt 4f XPS peak intensity attenuation and calculated inelastic electron mean free paths.27,28 The measured Sn 3d5/2, O 1s, and Pt 4f7/2 peak intensities were corrected for differences in XPS sensitivity factors and then used to calculate the Sn/O and Sn/Pt total atomic ratios.
activity. However, the exact morphology of the prepared SnOx on Pt(111) following the RLAD was not directly confirmed. We then followed this investigation by observing CO adsorption and desorption on SnOx /Pt(111) inverse catalyst using temperature-programmed desorption (TPD) technique, reported in previous work.10 SnOx/Pt(111) surface is known to be active toward low-temperature CO oxidation reaction (COR).10 In contrast, neither SnOx nor Pt has comparable activity toward COR. The mechanism of the increased activity of SnOx/Pt toward COR is still in question. The activity of the inverse catalyst surface, SnOx/Pt(111) nanoparticles, for CO oxidation was found to be dependent on the nanoparticle size. We observed CO2 formation on low-coverage SnOx/Pt surfaces (< 0.25 ML) during the desorption of adsorbed CO by TPD experiment, where the CO-covered SnOx/Pt(111) is annealed from room temperature to 600 K where the maximum CO2 formation occurs at 450 K. It is also observed that small (< 0.25 ML) SnOx nanoparticles have the highest concentration of reduced Sn2+.10 For higher coverage, 1 ML SnOx/Pt(111), it was found that heating the surface in the presence of adsorbed CO does not lead to CO2 formation, but rather the appearance of a non-CO oxidation reaction path including surface disproportionation reaction and thermal-induced reduction in which Sn2+ is converted to Sn4+ and Sn0 surface species. It was suggested that both the presence of Sn2+−Pt interface sites and unique chemical properties of the SnOx nanoparticles play critical roles in determining the activity of SnOx/Pt catalysts for CO oxidation.10 However, this previous work was done ex situ utilizing XPS to observe the change of the SnOx/Pt(111) before and after the reaction.10 In the present study, we utilized synchrotron-based in situ APXPS to follow the oxidation state change of SnO x nanoparticles on the SnOx/Pt(111) surface by studying CO adsorption and desorption from the surface while annealing the surface at 450 K under CO exposure at 0.2 Torr. HPSTM study was also utilized to observe the morphology of prepared SnOx nanoparticles and how they change in the presence of CO gas. It was found that for the as-deposited 0.25 ML coverage SnOx nanoparticles, a majority of the Sn species were Sn2+. By annealing at 450 K under 0.2 Torr CO gas, the intensity of Sn2+ is slowly decreased while the intensity of Sn0 increases. When the amount of SnOx nanoparticles coverage is increased to 1 ML, upon annealing at 450 K and exposing to CO at 0.2 Torr the decrease in the amount of Sn2+ is followed by the rapid increase of both Sn4+ and Sn0. HPSTM images confirmed that the prepared SnOx are in the form of nanoparticle clusters. These in situ results supported the finding that unique chemical properties of the SnOx nanoparticles and SnOx nanoclusters’ morphology play critical roles in determining the activity of SnOx/Pt catalysts for CO2 formation annealed at 450 K under CO gas exposure.
3. RESULTS AND DISCUSSION 3.1. Ambient-Pressure XPS Characterizations of SnOx/ Pt(111) under CO Exposure at 450 K. SnOx layers with different coverages were prepared by following reactive layer assisted deposition method (RLAD) as explained in previous publications.1,10 From our previous reactivity study of SnOx/ Pt(111) toward CO2 formation during CO TPD in UHV, it was found that SnOx at submonolayer coverages showed the formation of CO2 which reached a maximum at 450 K. In contrast, higher SnOx coverages, one monolayer and above, did not show any CO2 formation.10 In this study, two coverages of SnOx on Pt(111) were prepared separately at 0.25 and 1 ML, which represent the low and high Sn coverage, respectively.
2. EXPERIMENTAL SECTION APXPS experiments were performed on Beamline 9.3.2 at the Advanced Light Source. This beamline operates within the soft X-ray light range, at photon energies of 30−850 eV. The main chamber is equipped to operate at gas pressures
