A DFT Structural Investigation of New Bimetallic PtSnx Surface Alloys

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A DFT Structural Investigation of New Bimetallic PtSn Surface Alloys Formed on the Pt(110) Surface and Their Interaction with Carbon Monoxide Jian Zheng, Michael Busch, Luca Artiglia, Tomáš Skála, Jan Rossmeisl, and Stefano Agnoli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06638 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

A DFT Structural Investigation of New Bimetallic PtSnx Surface Alloys Formed on the Pt(110) Surface and Their Interaction with Carbon Monoxide Jian Zheng,1,# Michael Busch,2,§ Luca Artiglia,1,† Tomáš Skála3, Jan Rossmeisl2,4 and Stefano Agnoli1* 1

Department of Chemical Sciences and INSTM UNIT, University of Padova, Via Marzolo, 1, I-

35131, Padova, Italy 2

Center for Atomic-scale Materials Design, Department of Physics, Technical University of

Denmark, DK-2800 Kongens Lyngby, Denmark 3

Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma

Science, V Holešovičkách 2, CZ-18000 Prague, Czech Republic 4

Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken

5, DK-2100 Copenhagen, Denmark

ACS Paragon Plus Environment

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ABSTRACT Two surface alloys with p(3×1) and p(6×1) periodicity have been identified after the deposition of metallic Sn on the (1×2)-Pt(110) surface. These two structures have been characterized by low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and photoemission spectroscopy. Based on the experimental results and density functional theory (DFT) calculations, we propose atomic models for these surface alloys, which both consist of a highly corrugated row structure with a very similar surface motif. CO temperature programmed desorption (TPD) experiments indicate that CO desorbs from the PtSnx surfaces at about 415-425 K compared to 495 K on the clean Pt(110). The energetics and geometry of the CO chemisorption sites have been studied by DFT calculations, obtaining an adsorption energy of 0.7-0.86 eV on the p(3×1) and 0.9-1.05 eV on the p(6×1). Overall our theoretical and experimental results indicate that the introduction of Sn strongly reduces the CO adsorption energy on the (110) oriented PtSnx surfaces.

1. INTRODUCTION CO2 emitted from the combustion of fossil fuels is a major contributor to global warming. Hence it is necessary to develop alternative energy carriers that allow for CO2-neutral energy production and storage. 1 Among several solutions that could satisfy the energy demand, fuel cells (FCs) have drawn much attention due to their high conversion efficiency, 2 limited environmental impact,3 and because they offer a solution to distributed energy requirements. 4 Thus, the development of efficient proton exchange membrane FCs (PEMFCs) has been the focus of a huge amount of investigations. Platinum (Pt) based electrocatalysts (ECs), which are most frequently used in PEMFCs, contribute significantly to the total cell cost due to the high price of the raw materials. Reducing or replacing Pt-based ECs, particularly at cathode catalyst


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layer,5,6 7 8,9 without compromising the device performances, is a suitable strategy to meet the cost requirement for PEMFCs commercialization. A viable route to achieve this goal is the introduction of other metals into the Pt electrode to form alloys. Such approach has also the additional benefit of enhancing the EC performance of the electrode in terms of efficiency and/or durability, by exploiting the geometric and electronic effects of bifunctional materials.10,11,12 Pt-Sn alloys have been investigated as alternative electrodes for PEMFCs, due to their excellent performance for CO oxidation reaction (COOR), 13,14,15 methanol oxidation reaction (MOR), 16,17 ethanol oxidation reaction (EOR)16, 18 and oxygen reduction reaction (ORR).19,20 In particular, Pt-Sn alloys15, 21 , 22 , 23 have attracted great attention due to their inhibition of CO poisoning. The promoting effect of alloyed Sn may be explained by two factors: bifunctional and ligand (electronic). The former takes place when the additional metal, alloyed with Pt, can promote the formation of –OHad on the surface, which, reacting with COad on Pt atoms, can produce water and CO2 . 14,15,24 The latter is traced back to a change in the electronic properties of Pt atoms, due to the introduction of Sn, which induces an inhibition of CO absorption.16,25,26,27,28 A very valuable method to help the understanding of EC processes is the model catalysts approach: by exploiting ultra-high vacuum (UHV) technology and surface science techniques, single-phase surfaces can be designed at an atomic scale and their properties can be investigated to determine structure–property relationships. The first step of this method is to prepare and characterize the structural and electronic properties of the model catalysts. Such a study has been already reported in the literature for some PtSnx surface alloys grown on the Pt(111)29,30,31 and Pt(100)32,33 surfaces, whereas it is still missing in the case of Pt(110), which can be more reactive because of its less close-packed structure. The connection of this information to the


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electrochemical functional properties can provide new guidelines for the design of improved catalysts. As a part of a comprehensive investigation of the PtSnx/Pt(110) system, 34,35 we report here a surface science based study of two surface alloys, namely the p(3×1)-PtSnx/Pt(110) and p(6×1)PtSnx/Pt(110). Preliminary LEED and STM measurements were already reported,35 while herein, the set of experimental data have been enlarged with advanced photoemission measurements and supported by theoretical calculations based on density functional theory (DFT), in order to extract plausible structural models. In addition, the chemical activity toward CO of two novel surfaces was investigated both experimentally using temperature programmed desorption (TPD) experiments and theoretically by calculating the density of states in the valence band region and the energetics and geometry of CO chemisorption.

2. EXPERIMENTAL SECTION 2.1 Preparation of the PtSnx/Pt(110) surface alloys For all experiments, the substrate is a (1×2)-reconstructed Pt(110) surface, which was prepared by cycles of Ar+ sputtering and annealing at 973 K, followed by a short flash in oxygen at 10-7 mbar to eliminate residual carbon contamination. The cleanliness and crystallographic order of the final surface were checked by low-energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS). Sn was deposited from an e-beam evaporator in UHV conditions using a molybdenum crucible filled with Sn lumps (MaTecK 99.99%). The calibration of the Sn deposition rate was obtained by scanning tunneling microscopy (STM) measurements. In the following we will make reference to a Sn monolayer equivalent (MLE) defined as 1 Sn atom per Pt(110) surface unit


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cell, i.e., 1.8×1015 atoms/cm2. To obtain the surface alloys, UHV thermal treatments were carried out at 625 K and 723 K for the p(3×1) and the p(6×1) phases, respectively.35 After the annealing the surface structures were characterized by LEED. Synchrotron-based photoemission data (SRPES) were acquired at the Materials Science Beamline at the Elettra synchrotron (Trieste), whose experimental chamber is equipped with an electron energy analyzer (Specs Phoibos, 150 mm mean radius, with 9 channels), a fast entry, sputter gun, gas inlet, mass spectrometer, and sample heating and cooling. In the conditions used for acquiring the photoemission data (hν=125 eV and 60 eV), the photon flux on the sample was 3×1010 photons/s at a resolving power of 2000. All STM measurements were performed in an Omicron variable temperature (VT) STM system. The instrument consists of a UHV preparation chamber with a base pressure of 2×10-10 mbar containing equipment for sample sputtering, thermal annealing, metal evaporation, precise gas dosing. STM topographic images were acquired in constant current mode at 300 K using Pt– Ir electrochemically etched tips. Tip bias values (VT) and tunneling current (It) are reported for all images. In this paper, all the obtained STM images were obtained with positive bias. TPD experiments were carried out in a home-built system with the base vacuum at 2×10-10 mbar. The system is equipped with a quadrupole mass analyzer (Hiden HAL 301 PIC) to record the desorbed gas signal. The desorption spectra were recorded with a heating rate of 2 K/s. The sample, mounted on two parallel Ta wires, could be cooled down to 120 K with liquid nitrogen and heated up to about 1000 K by resistive heating. The desorption energies were calculated following the Redhead's approximation by the following equation: 36

=

− 3.64

Equation 1


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where Edes is the desorption energy, which is independent from the heating rate for a first-order desorption process. R is the ideal gas constant, 8.314 J/(K⋅mol); Tp is the temperature of the maximum of the desorption peak; β is the heating rate with unit K/s; ν is pre-exponential factor, which in general is assumed to be in the range 1013>ν/β >108.

2.1 Theoretical Calculations Closed-shell DFT calculations were performed using GPAW (version: 0.9.0.8965)37,38 code within the Atomic Simulation Environment (ASE) (version: 3.7.1.3184 ) 39 at the RPBE generalized gradient approximation (GGA) level of theory.

40

The core electrons were

approximated by projector augmented wave functions (PAW)

41

as implemented into

GPAW(0.9.0.8965). A finite difference grid basis set with a grid spacing of 0.15 Å was used for the valence electrons. Depending on the surface reconstruction, we employed k-point sets containing a 5×5×1 mesh for p(3×1) and Pt(110), whereas 9×3×1 mesh for p(6×1). In the case of the bulk structures, we chose a 5×5×5 mesh for Sn8, a 9×9×9 for Pt3Sn and 5×5×9 for Pt15Sn. The geometries were relaxed using a BFGS algorithm as implemented into ASE. The convergence of the electronic structure was assumed for energy differences below 0.0005 eV and convergence of the geometry for forces below 0.05 eV/Å. STM images of the fully converged surfaces for different potentials were simulated using ASE 42 , 43 , 44 with the Tersoff–Hamann approach.45,46 Employing this setup, a lattice constant of 4.01 Å for Pt, 4.10 Å for PtnSn (including Pt3Sn and Pt15Sn), and 6.74 Å for Sn8 (α-Sn8) were obtained. These results are in agreement with experimental values.47,48 All surfaces were modeled employing a 5 monolayer (ML) slab where the lattice spacing of the lowest ML was kept constant to the value of the reference systems with


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the closest stoichiometry (pure Pt or PtnSn) and the slabs were separated by a vacuum of at least 8 Å to avoid spurious interactions. The formation energies EF were calculated according to the general rational summarized in Scheme 1, using the equation 2: =

!/# − #$%

& ' ∙ ) * +,-. −

!/ +,-. 0 &

∆234 ∙ 56 & ∆237 ∙ 568

Equation 2 in which 56 and 568 is respectively the Sn atoms in the lowest layer and the second lowest layer at backside (i.e. lowest two layers). Actually, due to the limitations of the slab model, the Sn atoms can be placed either in the bulk (i.e. in the lowest three layers) or in the surface motif. This choice has a significant influence on the energies of formation of the slab since the cost for replacing a Pt in the backside of the slab are different from those for replacing a Pt in the bulk. This error is corrected by factors (∆234 and ∆237 ).

Scheme 1. Schematic drawing representing the methods used for the calculation of formation energies for the various reconstructions (light and dark gray balls represent Pt and Sn atoms, respectively)

In the above equation, m is either 3 (p(3×1)) or 6 (p(6×1)), and y corresponds to the number of replaced Pt atoms to form the reconstructed surface alloys 9: 1 ? /

A DFT Structural Investigation of New Bimetallic PtSnx Surface Alloys

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