Cu(111) by CO

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Potassium-Promoted Reduction of CuO/Cu(111) by CO Iradwikanari Waluyo, Kumudu Mudiyanselage, Fang Xu, Wei An, Ping Liu, Jorge Anibal Boscoboinik, Jose A. Rodriguez, and Dario J. Stacchiola J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07403 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

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Potassium-Promoted Reduction of Cu2O/Cu(111) by CO Iradwikanari Waluyo1,a, Kumudu Mudiyanselage1,b, Fang Xu1,c, Wei An1,d, Ping Liu1, J. Anibal Boscoboinik,2 José A. Rodriguez1, and Dario J. Stacchiola1* 1Chemistry 2Center

Department, Brookhaven National Laboratory, Upton, NY 11973

for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973

Abstract In situ X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS) and scanning tunneling microscopy (STM) were used to study the reduction of Potassium-modified Cu2O/Cu(111) by CO. By following the time evolution of the O 1s peak of Cu2O, we determined that the apparent activation energy for Cu2O reduction by 2 × 10-4 Torr CO is decreased by ~30% in the presence of K. On the K-modified surface, both XPS and IRRAS data show the formation of a surface species identified by IRRAS as carbonate (CO32-), likely forming a K+-CO32- complex, which is stable up to 500 K. STM images show that K+-CO32- complexes form chains around reduced Cu islands, thereby hindering the mass transfer of Cu atoms and preventing the reconstruction of the surface. Theoretical calculations show that the formation of carbonate on the K-modified ‘44’ Cu2O structure is thermodynamically favorable compared to the formation of CO2 on either the bare or K-modified surfaces. * Corresponding author: [email protected] Keywords: IRRAS, XPS, STM, alkali promoter, catalysis, copper Present address: aNational

Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973;

bSABIC-CRD cDepartment dCollege

at KAUST, Thuwal, Saudi Arabia;

of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138;

of Chemistry and Chemical Engineering, Shanghai University of Engineering Science,

Shanghai 201620 (P.R. China)

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Introduction The addition of alkali promoters has long been known to enhance the activity of supported transition metal catalysts for a variety of industrially important reactions such as ammonia synthesis,1 CO2 hydrogenation,2 benzyl alcohol oxidation,3 ethylene epoxidation,4 and preferential CO oxidation in the presence of H2 (PROX).5-7 On single crystal metal surfaces, the co-adsorption of alkali metals and surface species such as CO, CO2, H2O, and O has been extensively studied.89

The co-adsorption of alkali metals and CO, for example, results in the increased heat of

adsorption of both CO and the alkali metal10-11 as well as a red-shift of the CO IR stretch frequency.12-14 Various models have been proposed to explain the alkali-promoted CO stabilization on metal surfaces, generally involving the weakening of the internal C-O bond and strengthening of metal-CO bond.9, 15-22 In addition, although CO2 tends to adsorb weakly on transition metal surfaces, the highly electropositive alkali metals have been found to activate CO2 through electron donation, resulting in the formation of various negatively charged species, including carbonate (CO32-), oxalate (C2O42-), carboxylate (CO2-) and carbonite (CO22-), with carbonate being considered as the most stable.23-26 An infrared reflection absorption spectroscopy (IRRAS) study recently showed that at low temperatures, potassium multilayers promote the formation of formate from CO2 and H2O through the activation of CO2 and the dissociation of H2O.27 Alkalis are widely used in the industry for promoting catalysts.28-29 Despite extensive research, proposed mechanisms of alkali promotion for various catalytic reactions remain controversial. This is partly due to the lack of in-situ characterization studies on the promoting effect during reaction conditions. In ammonia synthesis, potassium atoms are considered to be localized at the active centers present on the surface to enhance the activity.30 In Fischer-Tropsch


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synthesis, the strong basicity of potassium promotes the adsorption of reactants (CO and H2) on the active sites, which effects activity and selectivity.31 The effect of alkali promoters on metal oxide films has not been studied in situ as extensively using surface science techniques due to the high complexity of these systems, as reported previously for ammonia synthesis.32 Since the identification of the role of promoters is essential for the enhancement of the activity, it is very important to carry in situ investigations of the role of promoters in model systems using surfaces science techniques, which provides molecular level mechanistic information. Cu can be easily oxidized to Cu2O under ultra-high vacuum (UHV) conditions and can be further oxidized to CuO at higher temperatures and O2 pressures.33 The interaction between CO and copper surfaces has been extensively studied because Cu-based catalysts are a well-known class of heterogeneous catalysts in CO conversion reactions such as CO oxidation (2 CO + O2  2 CO2),34-35 methanol synthesis (CO + 2 H2  CH3OH),36-37 and the water-gas shift reaction (CO + H2O  CO2 + H2).38 In the case of copper oxide (CuxOy) catalysts, their activity for CO oxidation depends on the oxidation state of Cu, with a decreasing activity and increasing activation energy observed with increasing oxidation state of Cu.39-40 CO is known to be able to reduce Cu2O to metallic Cu via the Mars-van Krevelen mechanism in which CO reacts with an oxygen atom from the oxide surface and forms CO2. The kinetics of this reaction can also be considered as a model for the initial rate of the CO + O step in the CO oxidation reaction in a highly reducing environment, thereby avoiding the decreased catalytic activity due to the oxidation of Cu+ to Cu2+ by O2. The reduction of Cu2O/Cu(111) by CO was studied using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) under UHV conditions.41 A two-stage process was observed: a slow initial



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stage or induction process in which CO reacts with chemisorbed oxygen atoms, creating Odeficient Cu2O phase, followed by a fast stage in which CO reacts with the lattice oxygen atoms of Cu2O.41 Ambient pressure STM experiments revealed that Cu2O reduction starts at defect sites and the mass transfer of released Cu from terraces to step-edges causes the formation of metallic phase fronts that propagate across the surface, resulting in the reconstruction of the step-edges and separation of the oxide and metallic phases.42 In this work, in situ XPS, IRRAS, and STM were used to study the effect of K on the reduction of Cu2O/Cu(111) by CO. We show that K promotes the reduction of Cu2O by CO and reduces the activation energy of the reaction by ~30%. The reduction of K/Cu2O/Cu(111) by CO results in the formation of a surface species that can be identified as carbonate (CO32-), which forms chains and islands around the reduced Cu islands. This is supported by results from theoretical calculations, which show that K stabilizes the formation of CO32- on the surface. Since CO needs to react with two O surface atoms in order to form CO32-, the favorable formation of CO32- due to the presence of K, therefore, accelerates the reduction of the surface.

Experimental Methods XPS measurements were performed at beamline X1A1 at the National Synchrotron Light Source (NSLS). The endstation was equipped with a differentially pumped hemispherical electron analyzer (SPECS™ Phoibos 150 NAP). A 300 m entrance aperture to the first differential pumping stage was located > 0.5 mm from the sample. The sample was heated using a ceramic button heater and the sample temperature was measured by a K-type thermocouple spot-welded to a Ta foil placed between the sample and the heater. XP spectra were obtained with a pass energy of 10 eV and photon energies of 689 and 523 eV for the O 1s and C 1s regions, respectively.


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IRRAS experiments were performed in a combined UHV surface analysis chamber and elevated-pressure reactor/IRRAS cell system. The elevated-pressure cell was coupled to a Bruker IFS 66v/s Fourier-transform infrared (FT-IR) spectrometer. The Cu(111) single crystal was held tightly using a Ta loop embedded in a groove machined around the crystal edge. The Ta loop was attached directly to the manipulator feed-through, which was used for both mechanical support and heating/cooling. The sample temperature was measured by a K-type thermocouple attached to the top edge of the crystal. IRRA spectra were collected at 4 cm−1 resolution using a grazing angle of approximately 85 to the surface normal.43 STM experiments were performed using a SPECS™ Aarhus HT-NAP STM, which contained a sealed ambient pressure cell housed in a UHV chamber. A commercial etched tungsten tip was used for all of the scanning under constant current mode. Images were scanned from bottom to top and each image took ~150 s to obtain. In all experiments, Cu(111) was cleaned by repeated cycles of Ar+ sputtering and 800-850 K annealing. The sample was then annealed to 550-650 K in 5 × 10-7 Torr O2 for 20 minutes, resulting in the formation of well-ordered Cu2O(111)-‘44’ film.44 K was evaporated using a K getter source from SAES. The coverage of K was calibrated using a quartz crystal microbalance (QCM) for XPS experiments and by Auger electron spectroscopy for IRRAS experiments. The CO stretch IR peak was also used to confirm the amount of K on the surface as its frequency was sensitive to K coverage. Since the deposition of K caused a partial reduction of Cu2O, the asdeposited surface was then re-oxidized at 500 K under 5 × 10-7 Torr O2 for 10 min. For the reduction experiments, CO was introduced to the chamber via a variable leak valve. CO pressures were read by a hot cathode/MicroPirani combination gauge (XPS), a Pirani gauge (IRRAS), and a baratron gauge (STM).



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Theoretical Methods Calculations were performed by using periodic density functional theory (DFT) as implemented in the Vienna ab-initio simulation package (VASP).45-46 Ion-core electron interactions were described using the projected augmented wave method (PAW),47-48 and PerdewWang functional (GGA-PW91) within the generalized gradient approximation (GGA)49-50 was used to describe exchange-correlation effects. The cutoff energy of the plane-wave basis set was 400 eV. To model the ‘44’ structure, we built a Cu2O-like O-Cu-O overlayer on top of a threelayer p(4×4) Cu(111) slab. Nine Cu ions and seven O ions were included in the oxide layer.51 Figure 1(a) shows the optimized ‘44’ structure where OU and OL represent lattice oxygen atoms in the upper and lower layers, respectively, and Oads is a chemisorbed oxygen atom. Here, x = 1.3 according to the ratio between Cu and O on the O-Cu-O layer. To model the K-deposited ‘44’ structure (Figure 1(b)), we added additional O to the center of the O-Cu-O hexagonal ring and K on top of each chemisorbed O atoms.52 Such an arrangement corresponds to our experimental procedures and calculated K binding orderings, i.e., -2.44eV

Cu(111) by CO

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