High-Coverage CO Adsorption and Dissociation on Ir(111), Ir(100

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

High Coverage CO Adsorption and Dissociation on Ir(111), Ir(100) and Ir(110) from Computations Chunli Liu, Ling Zhu, Pengju Ren, Xiao-Dong Wen, Yongwang Li, and Haijun Jiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11307 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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The Journal of Physical Chemistry High Coverage CO Adsorption and Dissociation on Ir(111), Ir(100) and Ir(110) from Computations Chunli Liu,a,b,c Ling Zhu,a,b,c Pengju Ren,a,b Xiaodong Wen,a,b Yong-Wang Li,a,b Haijun Jiaoa,d*

a) State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China; b) National Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing, 101400, China; c) University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing, 100049, PR China; d) Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany. E-mail: [email protected] ABSTRACT: CO adsorption on the perfect Ir(111) as well as the unreconstructed Ir(100) and Ir(110) surfaces at different coverage has been computed at the level of periodic density functional theory. General agreement between theory and experiment is found in top adsorption configuration at low coverage (vibrational frequencies) and adsorption patterns at given coverage (LEED) as well as saturation coverage (thermal adsorption). At the lowest coverage, the computed CO adsorption energy on Ir(111) agrees very well with the experiment [1.95 vs. 1.91±0.09 eV], while large differences are found on Ir(100) [2.22 vs. 2.04±0.05 and 1.47 eV] and Ir(110) [2.36 vs. 1.61 eV]. The same trend is also found in CO desorption temperature at the lowest coverage, i.e.; best agreement on Ir(111) [590 vs. 560 K] and large differences on Ir(100) [620 vs. 572 K] and Ir(110) [700 vs. 620 K], while well agreement at high coverage on all surfaces. At the lowest coverage, CO dissociation has very high barrier and is endothermic, in agreement with the experiment. Our results provide the basis for exploring CO activation and reaction on the Ir surfaces under real conditions.

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INTRODUCTION Transition metal-based catalysts play important roles in catalysis and platinum group metals are especially promising for many important technological and scientific applications in automotive, chemical and petroleum industries.1 Like other late transition metals, Ir-based catalysts show wide potential applications, e.g.; alkane dehydrogenation to olefins,27 CO preferential and selective oxidation,8 NOx decomposition and reduction.9 In addition, Ir oxyhydroxides (IrOx) have higher activity in oxygen evolution reaction,10 and well-defined homogeneous iridium complex has been used in methanol carbonylation for acetic acid synthesis, and this is the so-called Cativa process.11,12 Using low energy electron diffraction and Auger electron spectroscopy, Grant studied the Ir(111) and (100) surfaces. Alike Au and Pt, the Ir(111) surface is most stable.13,14 Instead of the normal expected periodicity of surface atoms from the termination of the bulk structure, the Ir(100) surface has a reconstructed (5×1) structure and the clean metastable (1×1) phase is kinetically stable up to 800 K, and adsorption of small molecules (CO, NO and H2) could lift the reconstruction.1518 The Ir(110) surface has a reconstructed (1×2) missing-row-type structure with the (111) micro face,19 which obviously differs from the Au(110) and Pt(110) surfaces, while the normal (1×1) structure could be stabilized by small amounts of carbon impurities.20 In addition to metallic surface structure and stability, CO adsorption on these surfaces has been studied using different techniques, i.e.; low-energy electron diffraction (LEED),20–29 Auger electron spectroscopy (AES),20,22,24,26,27 temperature-programmed desorption (TPD) and thermal desorption,20,22–26,30 UV-photoelectron spectroscopy (UPS),23,31–35 X-ray photoelectron spectroscopy (XPS),22,26,32,35 high-resolution electron energy loss spectroscopy (HREELS),36,37 infrared reflection-absorption spectroscopy (IRAS).28,30,36–42 All these provided useful information about CO adsorption on the Ir(111), Ir(100), and Ir(110) surfaces as well as on polycrystalline iridium films. The basic adsorption properties of CO on the Ir surfaces have been listed in Table S1. There are several reports about CO adsorption on Ir(111) using different techniques.22–25,30,32–36,40–42 Generally, two adsorption forms were observed,22–25,34,35,40,42 i.e.; a well-ordered ( 3 3)R30° over-layer structure at 0.33 ML coverage and a (2 32 3)R30° over layer structure at 0.58 ML saturation coverage. Thermal desorption studies showed the coverage dependent

adsorption,22–25,30 i.e.; a single peak at low coverage and a second peak at high coverage as well as a third shoulder at very high coverage. IRAS40,42 showed only one stretching band above 2000 cm–1 at all coverage at the temperature range between 90 and 350 K, indicating that each molecule feels a similar substrate potential and the adsorption sites may be regarded as equivalent. There are several reported CO adsorption energies on Ir(111) at different coverage (Table S1), e.g.; 1.91±0.09 eV at 0.05 ML and 1.61±0.04 eV at 0.6 ML.42 There are also studies of CO adsorption on Ir(100) and Ir(110). On Ir(100), there is a c(2×2) pattern at 0.50 ML28,29,37,38 and a ( 2 2)R45° pattern at high coverage.38 The CO desorption energy on Ir(100) is 2.04±0.05 eV at 0.002 ML,17 while a much small adsorption energy is reported (1.47 eV).28 On Ir(110), CO adsorption has a (2×1) pattern at 303 K and a c(2×2) pattern for the dissociated C and O at 523-773 K.27 The CO desorption energy on (110) is 1.61 eV20,26 and 1.52±0.09 eV.36 On the basis of these experimental results, we carried out systematic density functional theory computation into the coverage dependent structures and energies of CO adsorption on the perfect Ir(111) as well as the unreconstructed Ir(100) and Ir(110) surfaces. In addition, we compared our results with the available experimental data and found well agreement between theory and experiment. In turn, our results can be used to discuss the experimental results under real conditions and provide the basis for understanding CO activation on less active metals.43 COMPUTATIONAL METHODS AND MODELS Methods: All calculations were performed with the plane-wave based periodic density functional theory method implemented in the Vienna Ab Initio Simulation Package (VASP).44,45 The electron-ion interaction is described with the projector augmented wave method (PAW).46 The exchange and correlation energies are described within generalized gradient approximation using Perdew-Burke-Ernzerhof formulation (GGA-PBE).47 The plane wave cutoff energy was specified by 400 eV, ~2~


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The Journal of Physical Chemistry

and the Methfessel-Paxton48 electron smearing method with σ = 0.10 eV was used to ensure energies with errors due to smearing of less than 1 meV per unit cell. Geometry optimization was converged until all forces acting on the atoms were smaller than 0.02 eV/Å, whereas the energy threshold-defining self-consistency of the electron density was set to 10–4 eV. The vacuum layer between the periodically repeated slabs was set as 20 Å to avoid interactions among slabs. The optimized lattice parameter (a = 3.877 Å) is in excellent agreement (> 98%) with the available experimental result (3.834 Å49) and other computed data of 3.88,50 3.86,51 3.882,52 3.899,53 3.900, 3.905 and 3.908 Å.54 Models: For CO adsorption on the Ir(111), Ir(100), Ir(110) surfaces with consistent surface size, we used a four-layer p(4×4) model for Ir(111) and Ir(100) with totally 64 Ir atoms, where the top two layers including surface species were allowed to relax and the bottom two layers were fixed in their bulk positions. For Ir(110), we used a five-layer p(4×3) model with totally 60 Ir atoms, where the top three layers including surface species were allowed to relax and the bottom two layers were fixed in their bulk positions. The k-point mesh of 3×3×1 was used. The adsorption energy, Eads, is defined as Eads = E(CO/slab) – [E(slab) + E(CO)]; where E(CO/slab) is the total energy of the slab with one CO molecule, E(slab) is the total energy of the bare slab and E(CO) is the total energy of a free CO molecule in gas phase. Thus, the more negative the Eads, the stronger of the adsorption. For CO dissociation, all transition state structures were located using the climbing image nudged elastic band (CI-NEB) method.55 For each optimized stationary point, we carried out vibrational frequency analysis at the same level of theory to determine its nature (either minimum or saddle point) and zero-point energy (ZPE), and ensured that each transition state has only one imaginary frequency along the reaction coordinate. All reported relative energies for CO adsorption and dissociation include the correction of ZPE. The comparison of the adsorption energies with and without ZPE as well as with and without dispersion (D3 of Grimme et al.56) corrections is given in the Supporting Information (Tables S2-S4). Thermal Desorption: On the basis of the computed adsorption energies at different coverage; we estimated CO desorption by using the Redhead equation,57 assuming the exponential pre-exponential factor to be coverage independent. Thermal desorption is described on the basis of Arrhenius equation (Eq. 1); Where, 𝑟(𝜃) is the desorption rate [mol/(cm2 s)] as a function of surface coverage (𝜃, ML); A is the pre-exponential factor; Ea is the desorption Arrhenius activation energy [kJ/mol] as a function of 𝜃; R is the gas constant [J/(K mol)]; n is desorption order and T is the absolute temperature. 𝑑𝜃

𝑟(𝜃) = ― 𝑑𝑡 = 𝐴𝜃𝑛𝑒𝑥𝑝( ―

𝐸𝑎(𝜃) 𝑅𝑇

Eq. 1

)

In the calculation process, the curve of –dθ/dT  T is generally called a TPD curve58 with a constant heating rate, dT/dt = β. On the surface, desorption of each adsorbent conforms to the model in Eq. 2, 𝑑𝜃

𝐴

― 𝑑𝑇 = (𝛽)𝜃𝑛𝑒𝑥𝑝( ―

𝐸𝑎(𝜃) 𝑅𝑇

Eq. 2

)

The pre-exponential factor A, which is an empirical value, is normally assumed as 11014 s1 (the results of A = 11012 s1 and 11013 s1 are shown in the Supporting information (Tables S5-S6 and Figures S1-S2); β is heating rate; n is desorption order, which is a first-order desorption process for CO on the catalysis surface,22 and Ea is expressed in Eq. 3: 𝐸𝑎(𝜃) = 𝑅𝑇 + 𝑈(𝜃)

Eq. 3

Consider the translational energy of CO, Eq. 3 can be expressed as Eq. 4: 5

𝐸𝑎(𝜃) = 2𝑅𝑇 + 𝑈(𝜃)

Eq. 4

Analogously, the diagram of –dθ/dT  T can be expressed as Eq. 5, which is the simulated TPD: 5

𝑑𝜃

𝐴

― 𝑑𝑇 = (𝛽)𝜃𝑒𝑥𝑝( ―

𝑈(𝜃) + 2𝑅𝑇 𝑅𝑇

)

Eq. 5 ~3~


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RESULTS AND DICUSSION (1) CO adsorption at lowest coverage: At first, we computed CO adsorption on the Ir(111), Ir(100) and Ir(110) surfaces at the lowest coverage (Figure 1). The Ir(111) surface has top (T), bridge (B), 3-fold hexagonal close-packed hollow (hcp-3H) and 3-fold face centered cubic hollow (fcc-3H) sites. The Ir(100) surface has top (T), bridge (B) and 4-fold hollow (4H) sites. The Ir(110) has top (T), short-bridge (S-B), long-bridge (L-B), pseudo 3-fold hollow (3H) and 4-fold hollow (4H) sites.

Figure 1. Top and side views of Ir(111), Ir(100) and Ir(110) surfaces [top (T), bridge (B), hcp hollow (hcp-3H), fcc hollow (fcc-3H), short bridge (S-B), long bridge (L-B), 4-fold hollow (4H) and pseudo 3-fold hollow (3H) sites; blue, white and green for the first, second and third layer Ir atoms, respectively]

There are four stable adsorption sites on Ir(111). Apart from the most stable top site (1.95 eV, 0.06 ML), the bridge, hcp-3H and fcc-3H sites have close adsorption energies with differences of less than 0.1 eV (Table S7), indicating their competitive adsorption at high coverage. Nearly the same trend is reported by Gajdoš et al.,50 and the adsorption energy at the top site and 0.25 ML (1.98 and 1.94 eV) agrees with our result, while the RPBE computed value is much lower (1.64 eV). Using PW91 and RPBE on the basis of ultra-soft pseudopotential at 0.25 ML, CO adsorption energy is 2.13 eV and 1.78 eV,51 respectively, and 2.11 eV and 1.85 eV by using DACAPO code,59 respectively. It is noted that our computed CO adsorption energy (1.95 eV) at the Ir(111) top site and 0.06 ML coverage agrees perfectly with the experimentally determined most stable CO adsorption ~4~


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energy of 1.91±0.09 eV at 0.05 ML by Boyle et al..42 As listed in Table S1, CO adsorption energies determined under different conditions are quite different; and they are also different at the same coverage; and this makes a general comparison difficult. In addition, we included van der Waals dispersion correction on the basis of the PBE optimized geometry; and it is found that the computed CO adsorption energy (2.14 eV) becomes higher than that without D3 correction by 0.19 eV (Table S2). It shows clearly that CO adsorption is strongly underestimated by RPBE, while slightly overestimated by dispersion correction. On the (1x1) phase of Ir(100) at 0.06 ML (Table S7), CO adsorption at the top site is most stable (2.22 eV), tightly followed by that at the bridge site (2.11 eV), and that at the 4H site is least stable (1.76 eV). The same order is reported by Titmuss et al.,29 by using a c(2×2) phase on the basis of PW91 and ultra-soft pseudopotential (2.65, 2.29 and1.50 eV, respectively). Using ultra-soft scalar-relativistic pseudo-potential and PBE, Erikat et al.,53 identified the top adsorption as most stable and the computed adsorption energy (2.37 eV) is close to our value. Experimentally, there are two sets of data available.17,28 At a total coverage of 0.002 ML, Ali et al.,17 found a coverage independent isosteric heat of adsorption on the (1x1) surface of 2.04±0.05 eV. On the (1x5) surface, the lowest isosteric heat of adsorption is about 1.56 eV, which is the most realistic value in the limit of zero coverage from a defect free surface. Quite differently, Anic et al.,28 reported a strong (1.47 eV) and a weak (1.15 eV) adsorption of CO for both (1x1) and (1x5) phases, especially for the strong adsorption for the (1x1) phase (2.04±0.05 vs. 1.47 eV), while those for the (1x5) phase are close (1.56 vs. 1.47 eV). Although our computed value is much more close to the reported value by Ali et al.,17 than that by Anic et al.,28 a difference of 0.18 eV is still found. Including van der Waals dispersion correction gives CO adsorption energy of 2.47 eV, higher than that without correction by 0.25 eV (Table S3). On Ir(110) at 0.08 ML coverage (Table S7), CO adsorption at the top site is most stable (2.36 eV), followed by that at the short bridge site (1.98 eV), and that at the long bridge site is least stable (1.60 eV). No stable adsorption configurations on the 3H and 4H sites can be located. It is noted that the experimentally estimated adsorption energy (1.61 eV20,26) in the limit of zero coverage is lower than our computed value by 0.75 eV. Indeed, Boyle et al.,42 suggested that LEED measurements are less well suited for isosteric heat of adsorption since this method is not entirely nonintrusive and is limited to certain specific coverage. Including van der Waals dispersion correction overestimates CO adsorption energy by 0.23 eV (2.60 vs. 2.36 eV, Table S4). (2) High Coverage CO Adsorption: In addition to CO adsorption at the lowest coverage, we calculated all possible CO adsorption configurations on the Ir(111), Ir(100) and Ir(110) surfaces at different coverages to find the most stable configuration at individual coverage, where we started CO adsorption at top sites in remote positions. In order to get the saturation coverage, we used stepwise adsorption energy, ΔEads = E(CO)n+1/slab – [E(CO)n/slab + ECO], where a position ΔEads for (n+1) adsorbed CO molecules indicates the saturation adsorption with nCO molecules. As listed in the Supporting Information (Tables S2-S4), van der Waals dispersion correction gives always higher adsorption energies from low to high coverage. On the Ir(111) surface (Figure 2), the first four adsorbed CO molecules (nCO = 1-4, up to 0.25 ML) are located on the top site and have very close stepwise adsorption energies (1.95, 1.92, 1.89 and 1.86 eV), indicating their negligible lateral repulsive interaction. The average adsorption energy is 1.91 eV for the adsorption of the first four CO molecules. Subsequently, lateral repulsion among the adsorbed CO molecules becomes obvious at increasing coverage. At nCO = 5-9 (up to 0.56 ML), adsorption configurations having adjacent CO molecules appear and the top adsorbed CO molecules become titled. At nCO = 10 (0.625 ML), the first hcp-3H adsorption site appears and the top adsorbed CO molecules are titled. At nCO = 12 (0.75 ML), there are four top, four hcp-3H and four fcc-3H adsorbed CO molecules, and this adsorption configuration is consistent with the experimental pattern of (2 32 3)R30°.22 At nCO = 13 (0.81 ML), the stepwise adsorption energies is 0.39 eV, which is the saturation coverage. However, it is noted that the experimental saturation coverage is 0.58 ML at 150-300 K,22,34 0.68 ML at room temperature23,24 and 0.70 ML at 90-350 K.30 ~5~


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Figure 2. Stepwise CO adsorption structures and energies (ΔEads) on the Ir(111) (blue, white and green for the first, second and third layer Ir atoms, respectively, as well as gray for C and red for O atoms)

Apart from the agreement in CO adsorption energy at the lowest coverage, we computed the average adsorption energy at 0.31 and 0.38 ML for the comparison with the experiment (0.33 ML). The computed average CO adsorption energy (1.86 and 1.84 eV, respectively) is higher than the estimated value of 1.69,24 1.56,22,25 and 1.48 eV 23 at 0.33 ML. Further, we computed the CO average adsorption energy at 0.63 ML for comparing with the experimental value at 0.60 ML. Indeed, the computed CO average adsorption energy agrees very well with the estimated value (1.64 vs. 1.61 eV42), but differs from the reported 1.39 eV at 0.67 ML.23 On the Ir(100) surface (Figure 3), the lateral repulsive interaction is not obvious because of the rather long distance between iridium atoms (2.74 Å). At nCO = 1-8, there are only top adsorbed CO molecules with very close stepwise adsorption energies (about 2.20 eV), indicating their coverage independence as found experimentally up to 0.5 ML.17 At nCO = 8, the adsorption structure agrees well with the experimental LEED pattern which shows a c(2×2) structure at 300 K.28,29,37,38 At nCO = 9-12, bridge adsorption configurations appear and they have close stepwise adsorption energies (1.66, 1.62, 1.46, 1.68 eV). At nCO = 13-16, top and bridge adsorption configurations appear and they have close adsorption energies (1.04, 1.02, 1.03, 1.08 eV). At nCO = 17 (Figure S3), CO stepwise adsorption energy becomes positive, revealing an 1 ML saturation coverage. This is consistent with the saturation coverage of 0.94 ML or 1 ML obtained by experiment.31


On the Ir(110) surface (Figure 4), the repulsion among CO molecules is negligible due to the rather long atomic distances ~6~


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(2.74 and 3.88 Å). There are only top adsorption configurations at all coverage. At nCO = 1-6, all CO molecules are upright. At nCO = 6, the adsorption structure agrees with the experimental LEED pattern which shows a (2×1) structure.20,21,26,27 At nCO = 12 (1.00 ML), the stepwise adsorption energy is 1.40 eV. At nCO = 13 and 14, the stepwise adsorption energy distinctly reduces (0.93 and 0.36 eV ) due to the obvious repulsion between two molecules. At nCO = 15 (Figure S4), the stepwise adsorption energy becomes positive (0.58 eV), indicating that Ir(110) has CO saturation coverage of 1.17 ML; which is consistent with the experiment.20,26,36,39


In addition to the adsorption configurations and energies, we computed CO stretching frequencies at high coverage for comparison (Table 1). Since CIr bonds have very low stretching frequencies (Tables S8-S10), we did not include these values. At low coverage, there is only CO stretching band for CO at the top sites on all surfaces, in agreement with experimental results.28,30,36– 42

At high coverage, CO adsorptions at the bridge and hollow sites have much low stretching frequencies; however, there are no

such experimental data available for comparison. Experimentally, CO stretching frequencies are coverage dependent, and they are shifted from lower wavenumber at low coverage to higher wavenumbers at high coverage on all three surfaces. This trend is also found computationally. Table 1. Computed CO stretching frequencies (C–O, cm1) on Ir(111), Ir(100) and Ir(110) at different coverage (a for hcp-3H site, b for bridge site, c for 3H site). The average value between the lowest and highest wavenumbers are given in curly brace (1 L = 10–6 torrsec–1) nCO (ML) 1 (0.06) 2 (0.13) 3 (0.19) 4 (0.25) 5 (0.31) 6 (0.38) 7 (0.44) 8 (0.50) 9 (0.56)

Ir(111) 2007 1999; 2014 {2007} 1999-2020 {2010} 2000-2025 {2012} 1990-2033 {2012} 1986-2038 {2012} 1989-2048 {2012} 1977-2051 {2014} 1976-2058 {2012}

nCO (ML) 1 (0.06) 2 (0.13) 3 (0.19) 4 (0.25) 5 (0.31) 6 (0.38) 7 (0.44) 8 (0.50) 9 (0.56)

10 (0.63)

1981-2067 {2024} 1691 [a] 1980-2075 {2027} 1693 [a] 2049-2066 {2057} 1740-1813 [c] 1999-2073 {2036} 1770 [a]

10 (0.63)

11 (0.69) 12 (0.75) 13 (0.81)

11 (0.69) 12 (0.75) 13 (0.81)

Ir(100) 1990 1987;1998 {1992} 1985-2011 {1998} 1984-2018 {2001} 1981-2023 {2002} 1982-2031 {2006} 1981-2037 {2009} 1979-2042 {2010} 1984-2042 {2013} 1821-1849 [b] 1979-2046 {2013} 1819-1846 [b] 1981-2050 {2015} 1831-1827 [b] 1995-2048 {2021} 1825-1862 [b] 2004-2048 {2026} 1833-1911 [b] ~7~


nCO (ML) 1 (0.08) 2 (0.17) 3 (0.25) 4 (0.33) 5 (0.42) 6 (0.50) 7 (0.58) 8 (0.67) 9 (0.75)

Ir(110) 1963 1958; 1971 {1964} 1961-1983 {1972} 1958-1991 {1974} 1967-2005 {1986} 1963-2012 {1987} 1962-2021 {1991} 1963-2030 {1996} 1960-2035 {1998}

10 (0.83)

1962-2053 {2007}

11 (0.92)

1963-2060 {2010}

12 (1.00)

1962-2068 {2015}

13 (1.08)

1963-2065 {2014} 1699 [c]

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1807-1860 [b] 14 (0.88) 15 (0.94) 16 (1.00) HREELS36 FTIR30

2025-2050 (7.9 L) 2028-2090

EELS37 IRAS28

2009-2047 {2028} 1840-1934 [b] 1996-2068; {2032} 1835-1894 [b] 1998-2077; {2038} 1836-1894 [b] 2027 (0.5 L); 2070 (16 L) 2061 (1 L); 2063 (50 L)

14 (1.17)

1968-2075 {2021} 1696-1704 [c]

HREELS36 IRAS39

2012-2070 (6.3 L) 2001-2086 (0.0150 L)

(3) CO Thermal desorption: Considering the relationship between desorption rate and actual reaction pressure and temperature, we simulated desorption temperature at different coverage (Figure 5) by using the Redhead equation,57 and the maximum desorption temperatures are listed in Table 2. On Ir(111), the computed CO desorption temperatures are in excellent agreement with the experimental results.22 At the lowest coverage (0.06 ML), for example, the computed maximum desorption temperature at 590 K is close to the experimental 560 K. With the increase of the coverage, CO desorption is shifted to lower temperature (560 K, 0.25 ML). The second desorption appears at 460 K agrees very well with the experimental value of 460 K. Most importantly, we found the peak at around 380 K at 0.50 ML, which was found by Lauterbach et al. in their experimental study.40 Table 2: Maximum desorption temperature of CO (T/K) on Ir(111), Ir(100) and Ir(110) Coverage

T (DFT)

T (expt.)

Ir(111) nCO = 8 (0.50 ML) nCO = 4 (0.25 ML) nCO = 1 (0.06 ML)

460 560 590

46022 52022 56022

470 620 620

46528 56528 57228

380

38026

560 670 700

51026 60026 62026

Ir(100) nCO = 12 (0.75 ML) nCO = 8 (0.50 ML) nCO = 1 (0.06 ML) Ir(110) nCO = 12 to 9 (1.00 ML) nCO = 9 to 6 (0.75ML) nCO = 6 (0.50 ML) nCO = 1 (0.08 ML)

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Figure 5. CO desorption temperature at the different coverage on the Ir(111), Ir(100) and Ir(110) surfaces.

On Ir(100), there are two desorption peaks (Figure 5). At the lowest coverage (0.06 ML), the computed maximum desorption temperature is 620 K, which is different from the reported maximum desorption temperature of 572 K at low coverage by Anic et al.,28 and desorption temperature hardly changes with the coverage. The second maximum desorption peak appears at about 470 K, which agrees very well with the reported value of 465 K at high coverage by Anic et al..28 The difference between computation and experiment at the low coverage should be related to the difference in adsorption energy. As discussed above, the adsorption energy by Anic et al. is much smaller than the computed value (1.47 vs. 2.22 eV), while our computed value is much close to the coverage independent isosteric heat of adsorption on the (1x1) surface of 2.04±0.05 eV by Ali et al..17 On Ir(110), there are three desorption peaks (Figure 5), and they agree with the experimental results.26 At the lowest cover~9~



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age (0.08 ML), the maximum desorption temperature is at 700 K, higher than the experimental value of 620 K. With the increase of the coverage, CO desorption is shifted to lower temperature (670 K, 0.50 ML). The second maximum desorption temperature appears at about 560 K, which is close to the experimental 510 K. It is very interesting to note that the third desorption temperature at 380 K and 1.00 ML agrees perfectly with the experimental result (380 K). It is noted that the simulated desorption temperature agrees well with the experimental at high coverage, however, at the low coverage, especially at the lowest coverage, the simulated temperature is higher than the experimental one. One reason for this difference may come from the lowest coverage, which can be easily computed, but cannot be simply obtained under the experimental conditions. (4) CO dissociation: In addition to CO adsorption, we computed CO dissociation at the lowest coverage on these surfaces (Figure 6). Since CO top adsorption represents the most stable configuration, where CO is least activated compared with those at the hollow sites, we computed CO dissociation at the least stable adsorption site; and the apparent dissociation barrier is calculated on the basis of the most stable adsorption site. Accordingly, the CO dissociation energy is the energy difference between the most stable CO molecular adsorption at the top site and the most stable dissociative co-adsorption of C and O in remote sites. To get the most stable C+O co-adsorption, we fixed the C atom at the most stable site and shifted the O atom to the other most stable sites. The detailed results are shown in the Supporting Information (Figures S5-S7). On the Ir(111) surface, CO adsorption at the top site has IrC distance of 1.850 Å and CO distance of 1.164 Å. At the hcp-3H site, the average IrC distance is 2.121 Å and the CO distance is 1.204 Å. The elongated CO distance at the hcp-3H site indicates stronger activation. At the transition state (TS1), the carbon atom is at the hcp-3H site with IrC distance of 1.931, 2.011 and 2.040 Å, and the oxygen atom moves to the nearest bridge site with IrO bond distances of 2.081 and 2.110 Å. The CO distance is 1.840 Å. After the dissociation, the C atom is at the hcp-3H site and the O atom is at the neighboring fcc-3H site. Finally, the O atom diffuses to the remote fcc-3H site (Figure S5), which is more stable than the neighboring co-adsorption by 0.34 eV. Totally, CO dissociation on Ir(111) has an effective barrier of 3.17 eV and is endothermic by 1.52 eV. This indicates that CO does not dissociate on Ir(111). Referring to gaseous CO, the apparent barrier is 1.22 eV, close to the computed 1.47 eV by Liu et al.,60 by using the constrained minimization method, and the reaction is exothermic by 0.43 eV. On the Ir(100) surface, CO adsorption at the top site has IrC distance of 1.889 Å and CO distance of 1.166 Å. At the 4H site, CO adsorption has average IrC distance of 2.280 Å and CO distance of 1.211 Å, indicating an activated CO bond. In the transition state (TS2), the C atom is at the 4H site with IrC distances of 2.010, 2.011, 2.107, 2.108 Å, and the O atom moves to the nearest bridge site with IrO distance of 2.055, 2.056 Å; and the CO distance is 1.897 Å. After the dissociation, the C atom is at the 4H site and the O atom is to the neighboring bridge site. Finally, the O atom diffuses to the remote bridge (Figure S6), which is more stable than the neighboring bridge site by 0.17 eV. Totally, CO dissociation on Ir(100) has an effective barrier of 2.29 eV and is endothermic by 0.58 eV. Referring to gaseous CO, the apparent barrier is 0.07 eV and the reaction is exothermic by 1.64 eV. Using a (2x2) surface unit cell model and PBE method, Erikat et al.,18 also computed CO dissociation by using similar method, i.e.; using the most strongly bound molecular state at top as the initial state and the most stable final state for the C atom at the hollow site and the O atom at the bridge site. However, they found a very high barrier of 3.01 eV and much endothermic reaction energy by 1.40 eV; and these are different from our results. Detailed analysis shows that in the transition state, the O atom is located at the bridge site in our calculation, while titled at the top site as found by Erikat et al.. In addition, our unit cell has much larger size than that used by Erikat et al.. These might be the reason for the differences. On the Ir(110) surface, CO adsorption at the top site has IrC distance of 1.859 Å and CO distance of 1.171 Å. At the least stable long bridge site, CO adsorption has average IrC distance of 2.125 Å and CO distance of 1.201 Å, indicating an activated CO bond. In the transition state (TS3), the C atom is located at the long bridge site with IrC distances of 2.000, 2.003, 2.037 Å, and the O atom moves to the nearest bridge site with IrO distance of 1.982 and 2.153 Å; and the CO distance is 1.863 Å. After ~ 10 ~


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the dissociation, the C atom is at the long bridge site and the O atom is to the near short bridge site. Finally, the O atom diffuses to the remote bridge site (Figure S7), which is more stable than the near bridge site by 0.30 eV. Totally, CO dissociation on Ir(110) has an effective barrier of 2.65 eV and is endothermic by 1.11 eV. Referring to gaseous CO, the apparent barrier is 0.29 eV and the reaction is exothermic by 1.25 eV. On all three surfaces, CO dissociation is very difficult, not only has very high barriers but also is very endothermic. On Ir(111), Comrie et al.,22 reported that heating the Ir crystal to 600 K in vacuum resulted in the complete CO desorption without any indication of thermal dissociation. At temperature over  650 K and pressure over p  1.3310–9 bar CO, significant CO dissociation was observed. On Ir(100), Anic et al.,28 proved that the c(22) structure remained stable up to 0.1 bar and at 700 K. Nieuwenhuys et al.,27 found that carbon is absent on the surface after flashing the sample of CO covered on the Ir(110) surface in ultrahigh vacuum to 673 K by using AES.

Figure 6. Energy diagram of CO dissociation on the Ir surfaces and the top view of geometric structures for the initial state (IS), transition state (TS) and the most stable final state (FS) (blue, white and green for the first, second and third layer Ir atoms, respectively, as well as gray for C and red for O atoms)

CONCLUSION ~ 11 ~



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Systematic density functional theory computations have been carried out to study the adsorption and desorption of CO on the perfect Ir(111), unreconstructed Ir(100) and Ir(110) surfaces at different coverages as well as CO dissociation at the lowest coverage. At low coverage, CO prefers top adsorption configurations on the surfaces, and the CO stretching frequencies are shifted from low to high wavenumbers with the increase of CO coverage; and these are in agreement with the experimentally observed results. In contrast, three-fold hollow and bridge adsorption configurations can only be found at very high coverage; but there are no experimental data available for comparison. In addition, CO adsorption patterns at given coverage agree also with the LEED experiments. The saturation coverage is 0.75 ML on Ir(111), about 1 ML on Ir(100) and over 1 ML on Ir(110), in agreement with the experiment. Apart from these structural agreements between theory and experiment, both agreements and disagreements in adsorption energies between theory and experiment are found. It is also noted there are disagreements between different experimental studies. On Ir(111), the computed CO adsorption energies are in perfect agreement with the experimental values reported by Boyle et al. at the lowest coverage (1.95 eV/0.06 ML vs. 1.91±0.09 eV/0.05 ML) as well as at medium coverage (1.64 eV/0.63 ML vs. 1.61 eV/0.6 ML), while other reported experimental results differ from each other and also from other computed values. On Ir(100), the computed CO adsorption energy at 0.06 ML (2.22 eV) is close to the reported result (2.04±0.05 eV) by Ali et al., but differs strongly from the reported result (1.47 eV) by Anic et al.. On Ir(110), the computed CO adsorption energy at the lowest coverage differs strongly from the experimental result (2.36 vs. 1.61 eV). It is reported that the LEED experiment underestimated CO adsorption energy on Ir surfaces. On the basis of the computed adsorption energies, CO thermal adsorption according to the Redhead equation is simulated. On Ir(111), the simulated CO desorption temperatures agree perfectly with the experiment at the lowest coverage (590 vs. 560 K) and at high coverage (460 vs. 460 K). On Ir(100) and Ir(110), the same trends are found in experiment and computation. However, the simulated CO desorption temperatures at lowest coverage on Ir(100) and Ir(110) are higher than the experimental values (620 vs. 572 K and 700 vs. 620 K, respectively). In contrast, the simulated CO desorption temperature at high coverage on Ir(100) and Ir(110) are in perfect agreement with the experimental values (470 vs. 465 K and 380 vs. 380 K, respectively). On all three surfaces, CO dissociation has to overcome very high barrier and is very endothermic; in agreement with the experiment. Since CO does not prefer dissociation on Ir surfaces, CO hydrogenation will be our subsequent subject. Electronic Supporting information available: Experimental result in adsorption structures, energies at different coverages (Table S1); computed stepwise adsorption energies with and without van der Waals dispersion and ZPE correction (Tables S2-S4); simulated maximum CO desorption temperature (T/K) according to the Redhead equation (Tables S5-S6); computed adsorption energies and structural parameters as well as CO and IrC stretching frequencies at the lowest coverage (Table S7); computed distances (dCIr and dCO, Å) and CIr stretching frequencies (CIr, cm1) at the different coverages (Tables S8-S10); simulated temperature-programmed desorption according to the Redhead equation (Figures S1-S2); estimated saturation coverage (Figures S3-S4); the configuration and dissociation adsorption energy for the CO of the dissociated C and O atom at the different sites (Figures S5-S7) Acknowledgment: This work was supported by the National Natural Science Foundation of China (no. 21673268), and the Chinese Academy of Science and Synfuels CHINA. Co., Ltd. We also acknowledge general financial support from the BMBF and the state of Mecklenburg-Vorpommern.

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TOC Graphic

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High-Coverage CO Adsorption and Dissociation on Ir(111), Ir(100

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