Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10783-10791
Inverse Catalysts for CO Oxidation: Enhanced Oxide−Metal Interactions in MgO/Au(111), CeO2/Au(111), and TiO2/Au(111) Robert M. Palomino,† Ramón A. Gutiérrez,‡ Zongyuan Liu,† Samuel Tenney,§ David C. Grinter,† Ethan Crumlin,†,∥ Iradwikanari Waluyo,⊥ Pedro J. Ramírez,‡,# José A. Rodriguez,*,† and Sanjaya D. Senanayake*,† †
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, United States
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ABSTRACT: Au(111) does not bind CO and O2 well. The deposition of small nanoparticles of MgO, CeO2, and TiO2 on Au(111) produces excellent catalysts for CO oxidation at room temperature. In an inverse oxide/metal configuration there is a strong enhancement of the oxide−metal interactions, and the inverse catalysts are more active than conventional Au/ MgO(001), Au/CeO2(111), and Au/TiO2(110) catalysts. An identical trend was seen after comparing the CO oxidation activity of TiO2/Au and Au/TiO2 powder catalysts. In the model systems, the activity increased following the sequence: MgO/Au(111) < CeO2/Au(111) < TiO2/Au(111). Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to elucidate the role of the titania−gold interface in inverse TiO2/Au(111) model catalysts during CO oxidation. Stable surface intermediates such as CO(ads), CO32−(ads), and OH(ads) were identified under reaction conditions. CO32−(ads) and OH(ads) behaved as spectators. The concentration of CO(ad) initially increased and then decreased with increasing TiO2 coverage, demonstrating a clear role of the Ti−Au interface and the size of the TiO2 nanostructures in the catalytic process. Overall, our results show an enhancement in the strength of the oxide−metal interactions when working with inverse oxide/metal configurations, a phenomenon that can be utilized for the design of efficient catalysts useful for green and sustainable chemistry. KEYWORDS: Low-temperature CO oxidation, Gold, Titania, Ceria, Magnesium oxide, Ambient pressure X-ray photoelectron spectroscopy, Inverse oxide/metal catalysts
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INTRODUCTION In our modern world, catalysis plays a significant role in the production of more than 80% of the chemicals and materials used in industrial operations. Heterogeneous catalysts are frequently used in processes aimed toward green and sustainable chemistry. Most heterogeneous catalysts contain a combination of metals and oxides.1−3 They are usually prepared by dispersing a small amount of metal on an oxide support.1,3 In these systems, the oxide phase can act as a simple template for the scattering of the metal phase or it can be a direct participant in the catalytic process.2 When the oxide is involved in the catalysis, there is a motivation to revamp the traditional configuration of industrial catalysts to exploit the intrinsic properties of metal oxides and obtain a superior performance.4−7 Due to their limited size and high density of defects, oxide nanoparticles can have special electronic and chemical properties.8 Catalysts generated by the deposition of oxide particles on a metal substrate have been studied for a long time in fundamental studies.9,10 Recent studies using high-resolution © 2017 American Chemical Society
transmission electron microscopy (HRTEM) have observed overlayers of ZnO on top of the Cu particles present in copper/ zinc oxide catalysts used for the industrial synthesis of CH3OH.11,12 In these industrial catalysts, the active phase probably has an inverse oxide/metal configuration.11,12 The same is valid in Cu/MoOx, Rh/TiO2, Pt/CeOx, and Pt/TiO2 catalysts usually activated by pretreatment (reduction) in hydrogen.9,11−15 Oxides that have a low surface free energy exhibit a tendency to cover metals upon partial reduction. Thus, an oxide/metal configuration maybe more common in industrial heterogeneous catalysts than expected. In the area of green chemistry and the control of environmental pollution, inverse oxide/metal catalysts have shown an excellent performance for the oxidation of CO,7,10,16−21 the conversion of CO2 into alcohols,22−25 the Received: August 9, 2017 Revised: September 12, 2017 Published: September 26, 2017 10783
DOI: 10.1021/acssuschemeng.7b02744 ACS Sustainable Chem. Eng. 2017, 5, 10783−10791
Research Article
ACS Sustainable Chemistry & Engineering
preparation capabilities, details of which can be found elsewhere.44 The O 1s region was probed with a photon energy of 650 eV and the C 1s, Au 4f, and Ti 2p regions with photon energy of 490 eV. The Au 4f7/2 photoemission line was used for binding energy calibration. The Au(111) single crystal was cleaned via Ar+-sputtering (2 × 10−5 Torr, 30 min) and annealing (823 K, 20 min) cycles until the C 1s spectrum was clean. The MgO/Au(111), CeO2/Au(111), and TiO2/ Au(111) surfaces were prepared following procedures reported in the literature.37,39,42 The coverage of the oxide on the Au(111) surface was determined by means of ISS and/or XPS monitoring the attenuation of the corresponding signals for the gold substrate. Studies with High-Surface Area Catalysts. Experiments for the oxidation of CO were also carried out using TiO2/Au and Au/TiO2 high-surface area powder catalysts. Using atomic layer deposition (ALD), small particles of TiO2 were deposited on unsupported Au nanocrystals with a size of about 20 nm.45 The details for the methodology followed in the ALD process are described elsewhere.7 After applying TiO2 ALD, 20−30% of the surfaces of the Au nanocrystals were covered by titania. TiO2 islands (1.5−2 nm in size) grew on corner and defect sites of the Au nanocrystals.7 The Au/TiO2 powder catalysts were prepared by gold deposition−precipitation (DP) on an anatase titania support.31,46 Loadings of 1 and 2 atom % of Au were dispersed on the titania. This led to catalysts with average Au particle sizes of 1.8 ± 0.6 and 3.1 ± 0.5 nm, respectively, as measured by TEM. The catalytic tests for CO oxidation were performed in a flow reactor (gas feed: 1%CO/1%O2/98%N2, 20 mL/min, ∼100,000 h−1) under a steady-state mode between 225 and 500 K.
production of hydrogen through the water−gas shift or the photocatalytic splitting of water,26,27 and the reduction of nitrogen oxides (DeNOx).5 They can help to minimize the impact of chemical processes on the environment, and their efficiency and use of nonexpensive oxide components are ideal for sustainable chemistry. Furthermore, inverse oxide/metal catalysts offer a convenient way to study the effects of the metal−oxide interface on the mechanism of many reactions important for the chemical industry.23−27 Are properties of the metal−oxide interface similar in inverse oxide/metal and conventional metal/oxide catalysts? This is an important issue for fundamental studies and practical applications. In this article, we compare the performance for CO oxidation of catalysts in inverse and conventional configurations which combine Au with TiO2, CeO2, and MgO. Au(111) does not catalyze the oxidation of CO but becomes catalytically active upon the addition of oxide nanoparticles. Bulk TiO2, CeO2, and MgO also do not catalyze the oxidation of CO at low temperatures (
