Temperature-Controlled CO Adsorption Configurations on (2Ã1)Ni-O

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Temperature-Controlled CO Adsorption Configurations on (2×1)Ni-O/Ni(110) Surfaces Min Yu, Yunjun Cao, Shandong Qi, Zhengfeng Ren, Shi-shen Yan, Mingchun Xu, and Shujun Hu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12024 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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The Journal of Physical Chemistry

Temperature-Controlled CO Adsorption Configurations on (2×1)Ni-O/Ni(110) Surfaces Min Yu, Yunjun Cao, Shandong Qi, Zhengfeng Ren, Shishen Yan, Mingchun Xu* and Shujun Hu*

Abstract CO adsorptions and configuration evolutions with temperatures on (2×1)Ni-O row structures on Ni(110) surfaces were studied by using ultrahigh vacuum - Fourier transform infrared spectroscopy (UHV-FTIRS), thermal desorption spectroscopy (TDS) and density functional theory (DFT) calculations. Two kinds of adsorption configurations were identified. In the top configuration at low temperature (90 K), the carbon atom of one CO binds to one Ni atom in the Ni-O rows with a tilt angle of about 53°, and the vibration frequency of CO is 2100-2119 cm-1 depending on CO coverage. In the bridge configuration near room temperature (280 K), the carbon atom of one CO binds to two Ni atoms from the neighboring Ni-O rows, and the vibration frequency is 2030-2039 cm-1. By annealing the sample prepared at 90 K to 280 K, the top adsorption configuration gradually evolves into the ordered bridge adsorption configuration via a disordered state. In the disordered state, the top and bridge configurations are distributed in disorder, induce strong transverse distortion of the Ni-O rows along [110] direction, and thus lead to the significant CO frequency shifts. The distortion disappears after the complete desorption of CO above 300 K. The high stability of Ni-O rows may be the key factor to prevent CO oxidation on such surfaces.

School of Physics, Shandong University, 27 Shanda Nanlu, Jinan, Shandong, 250100, P. R. China *E-mail: M. Xu, [email protected] *E-mail: S. Hu, [email protected]

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Introduction The nickel oxides have exhibited outstanding catalytic power in CO oxidation,1 CO methanation2 and CO2-CH4 reforming reactions.3 Such powerfully catalytic properties are closely correlated to the multivalence nature of nickel ions, which is similar to that of the organic nickel complexes.4-6 The prepared NiO1x structures on different substrates like Al2O3,7 Ni,8 and Au1 serve as the ideal systems for investigating the relationships between the Ni oxidized states and their catalytic properties. Among them, the NiO1±x/Ni system exhibits excellent catalytic performance in CO activation7-12 and it can prevent the aggregation of carbons when C-contained molecules react on the surface.3 Mu et al. prepared a NiO1-x/Pt/Ni sandwich structure on Pt(111) surfaces and successfully realized 100% CO oxidation at room temperature. The comparison of reactivity of such sandwich structures with the Ni/Pt(111) and NiO1-x/Pt(111) surfaces indicated that the combination of NiO1-x and Ni is important.10 Zhao et al. found that the interfaces between the surface Ni oxide overlayers and the metallic Ni nanostructures altered geometric and electronic structures of the Ni nanoparticles, making them apt for CO activation under light irradiation.11 To address the role of Ni oxidized states in the catalytic properties, many efforts have been devoted to understanding of the interaction between CO and NiO1x/Ni systems, including the NiO/Ni heterostructures, the O-reconstructed Ni surfaces and the Ni-O-rows on Ni substrates.9, 12-16

On NiO/Ni heterostructures, CO adsorptions were studied early by using infrared reflection absorption spectroscopy (IRRAS),13-14 and the CO vibration frequencies were found to vary slightly depending on the surface structure [On NiO(100)/Ni(100), NiO(111)/Ni(111) and NiO(100)/Ni(110) surfaces, the CO vibration frequencies are 2159 cm-1, 2151 cm-1 and 2148 cm-1, respectively.]. Such frequency ranges indicate that CO is adsorbed on the surface Ni2+ cation via C atom. Recently, by using scanning tunnel microscopy (STM) and theoretical calculations, Knudsen et al. found that O-terminated octopolar NiO(111) film surface on Ni(111) substrates shows high activity for CO oxidation at 100 K, which is related to the spontaneous polarization of NiO thin films.8 On p(2×2)O-reconstructed Ni(100) surfaces, the carbonate formation was found upon CO exposure at 250 K. While on p(2×2) O-reconstructed Ni(111) surfaces, 10 L CO exposure does not induce the carbonate formation at the same 250 K.9 2


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Compared with the NiO/Ni heterostructures and the O-reconstructed Ni surfaces, the Ni-O rows on Ni(110) substrates construct a more open surface where alternate Ni and O atoms form Ni-O rows along [001] direction and between the Ni-O rows are the original Ni(110) surfaces.17-18 Depending on oxygen coverages, these ordered Ni-O rows distribute mainly in (n×1) (n=2, 3) reconstructed phases. Due to the high stability, the (21)Ni-O/Ni(110) surface can be easily prepared in wide temperature and pressure ranges, therefore, it is suitable as a model to investigate CO interaction with Ni-O row structures on Ni(110) surfaces, and thus has received the most attention.19-23 The clarification of CO adsorption states on (21)Ni-O/Ni(110) and their relationships is vital for understanding the interaction between CO and the substrates. However, to the best of our knowledge, the experimental studies of CO adsorption behaviors on ordered (21)Ni-O/Ni(110) surfaces are still lacking, except for a few early temperature-programmed desorption (TPD)15-16 and high-resolution electron-energy-loss spectroscopy (HREELS)

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studies. In these studies,15-16

three CO desorption bands were found, and only the lowest 180-190 K band was clearly assigned to CO desorption from Ni sites in Ni-O rows, the other adsorption states related to 240-275 K and 300-340 K bands are unclear.15-16 Therefore, the combined convincing experimental and theoretical studies on CO adsorptions on (21)Ni-O/Ni(110) surfaces are necessary. In this paper, we studied CO adsorptions and temperature-dependent configuration evolutions on (21)Ni-O/Ni(110) surfaces by using ultrahigh vacuum - Fourier transform infrared spectroscopy (UHV-FTIRS), thermal desorption spectroscopy (TDS) and density functional theory (DFT) calculations. Two kinds of CO adsorption configurations dependent on the annealing temperatures were identified. At 90 K, CO molecules adsorb in an ordered top configuration. Near room temperature, CO molecules adsorb in an ordered bridge configuration. Annealing the sample prepared at 90 K, the top adsorption state of CO transforms via a disordered state to the ordered bridge adsorption state near room temperature. In the disordered state during transformation, the top and bridge configurations coexist and strongly distort the Ni-O rows, causing the obvious frequency shifts of CO. Our studies thoroughly clarified CO adsorption states and their evolutions with temperatures on (21)Ni-O/Ni(110) surfaces for the first time, and will be helpful for understanding CO adsorptions and kinetics on Ni oxide surfaces. 3



Experimental and calculation methods The experiments were carried out in a UHV-FTIRS system,24 which combines a vacuum FTIR spectrometer (Bruker, Vertex 80v) and a multi-chamber UHV system (the base pressure better than 5×10-11 mbar) equipped with a low energy electron diffraction (LEED) / Auger electron spectroscopy (AES) (with a gain power of microchannel plates BDL 600IR-MCP) and a quadrupole mass spectrometer for TDS measurements. The optical path inside the IR spectrometer and the space between the UHV chamber and the spectrometer were evacuated in order to avoid any unwanted IR absorption from gas phase species. The IR measurements were performed in IRRAS mode with the unpolarized IR beam at a fixed incidence angle of 80o. The recorded reflected absorption signal is defined as A= log10(R0/R), where, R0 and R are the reflected signals from the clean and adsorbate covered (2×1)Ni-O/Ni(110) surfaces, respectively. During measurements, the MCT detector was cooled down by liquid nitrogen. All spectra are recorded with 512 scans and 2 cm-1 resolution without mention. The single crystal Ni(110) (dia.10 mm × thickness 1.5 mm, Mateck) surface was cleaned by repeated cycles of Ar+ sputtering (2.5 kV, 10 mA) and annealing at 1100 K under UHV conditions until negligible impurities were detected by AES and perfect (1×1) LEED patterns were obtained. The sample temperature can be changed from 90 K with liquid nitrogen cooling to 1200 K heated by electronic beam with a resistance wire located in the sample holder. The ordered (2×1)Ni-O/Ni(110) surfaces were prepared by exposure the clean Ni(110) surface to 2×10-8 mbar O2 at 550 K for 15 min. High purity O2 (99.999%) and CO (99.999%) were used in experiments, where exposures are quoted in Langmuir (1 L = 1.33 × 10-6 mbar·s). First-principles calculations were performed using the Vienna ab-initio simulation package (VASP)25 with a cut-off energy of 400 eV for the basis set. The projector-augmented wave method (PAW)26 with the PBE type27 exchange-correlation potentials was adopted. The 3p, 3d and 4s electrons of nickel, and the 2s and 2p electrons of carbon and oxygen were treated as the valence electrons. The rotationally invariant DFT+U approach28 was used to correct the 3d electron-electron interaction of Ni in the Ni-O rows. The value of UNi:3d= 5.3 eV as implemented 4


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in the current calculations was determined by fitting the magnetic moment and bulk modulus of NiO to the experimental value.29 The vibrational frequencies were derived from Hessian matrix calculated by the finite-displacement method. The electronic convergence criteria for the structure optimization and vibrational frequency calculations are 10-6 eV. Additionally, calculations with U = 0 were also performed but the calculated results are far away from the experimental values. To model the (2×1)Ni-O/Ni(110) surface, the experimentally determined lattice parameter of bulk Ni of a = 3.52 Å30 was used to build slabs including seven Ni layers and a vacuum layer with a thickness of 15 Å. One supercell with the p(4×4) geometry along [110] and [001] directions was employed. The atomic positions of the top four layers were optimized until the forces were less than 0.02 eV/Å, while the bottom three layers were fixed at bulk positions. Based on the geometry of the supercell, a 1×2×1 k-point mesh was used for Brillouin zone sampling. The adsorption stability of CO is evaluated by the binding energy, which is defined as: 𝐸b = 𝐸(2 × 1)NiO/Ni(110) + 𝐸CO ― 𝐸CO + (2 × 1)NiO/Ni(110).

(1)

Where E(2×1)Ni-O/Ni(110) is the total energy of a pristine (2×1)Ni-O/Ni(110) supercell, ECO is the energy of an isolated CO molecule, and ECO+(2×1)Ni-O/Ni(110) is the total energy of CO-adsorbed (2×1)Ni-O/Ni(110) supercell. Based on the definition, the higher value means the higher stability, and vice versa.

Results and discussion The structures of the prepared (21)Ni-O/Ni(110) surfaces were monitored by LEED. The clear and sharp (21) diffraction spots of the LEED pattern shown in Figure 1a demonstrate the high quality and high ordering of the prepared (2×1)Ni-O row structure surface. Furthermore, the absence of detectable change of the LEED patterns at different temperatures from 90 K to 500 K (not shown here) also indicates the high stability of such surfaces.

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Figure 1. LEED patterns of the clean (21)Ni-O/Ni(110) surface at 90 K (a), CO covered surfaces at 90 K (b), 265 K (c) and 283 K (d), respectively. The electronic energy is 63.5 eV.

CO adsorptions on (21)Ni-O/Ni(110) surfaces respectively at 90 K and 280 K were first studied by IRRAS. The IRRA spectra of adsorbed CO as a function of CO dosage are shown in Figure 2. In the case of very low dosage (0.05 L, ~0.05 ML. Here, 1 ML is defined as the density of the Ni sites in Ni-O rows on ordered (21)Ni-O/Ni(110) surfaces.) at 90 K, a vibration absorption band at 2110 cm-1 first appears, as shown in Figure 2a. As the CO dosage is increased, the band intensity increases rapidly with a slow blueshift. At 4.35 L (~0.5 ML), the band finally shifts to 2119 cm-1 and its intensity reaches saturation. Meanwhile, a weak band at 2154 cm-1 first appears at 0.15 L and slowly redshifts with increasing CO dosage. At 4.35 L, the band shifts to 2140 cm-1 with a slight increase in intensity. We assigned the weak 2154-2140 cm-1 band to the stretching vibration of CO adsorbed on the Ni2+ cations of a few totally oxidized NiO fraction on our prepared (21)Ni-O/Ni(110) surfaces according to the IRRAS study of CO adsorption on NiO films by Sanders et al..13 The IRRA spectra of CO adsorbed at 280 K are shown in Figure 2b. A single band at 2030 cm-1 first appears at 0.05 L (~0.05 ML) CO. By increasing the CO dosage, the band gradually blueshifts and increases in intensity. At 1.13 L (~0.25 ML), the band finally shifts to 2039 cm-1 and reaches saturation. Compared to the bands at 90 K, the bands at 280 K are obviously redshifted.

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Figure 2. IRRA spectra of adsorbed CO on ordered (2×1)Ni-O/Ni(110) surfaces as a function of CO dosage at (a) 90 K and (b) 280 K. The inset in (b) shows the LEED pattern of 1.15 L of CO adsorbed at 280 K.

Feigerle et al. have observed a vibration band at 2120 cm-1 for saturated CO adsorption on (31)Ni-O/Ni(110) by using HREELS.15 Probably due to the poor resolution of HREELS, the authors have not specified the definite adsorption site of CO. Additionally, on both NiO particles31 and NiO/SiO2,32 a weak vibration band around 2100 cm-1 was attributed to CO adsorbed on some Ni sites that less oxidized, but no further discussion about the specific details was performed. The sharp (21) diffraction spots in the LEED pattern shown in Figure 1a and the very narrow full width at half maximum (FWHM) (~7 cm-1) of the IR absorption bands shown in 7



Figure 2 demonstrate that the (21)Ni-O reconstructed structure is of high uniformity and spreads all over the whole Ni(110) surface. On such an ordered (21)Ni-O/Ni(110) surface, the Ni-O rows along [001] direction are bulgy relative to the Ni(110) surface. The original Ni(110) surfaces between adjacent bulgy Ni-O rows along [001] direction are called Ni troughs in the following (Refer to Figure S1 in SI). The Ni-O rows and Ni troughs distribute alternately along [110] direction.18 In this case, there are at least two kinds of Ni sites: one is in the bulgy Ni-O rows and the other in the Ni troughs. Furthermore, the former should be more oxidized than the latter since there are two O atoms bonded to each Ni site for the former. Considering that the CO vibration frequency is close to that on completely oxidized NiO, we assigned the 2110-2119 cm-1 band to CO vibration on Ni sites in Ni-O rows. In Ni-O rows, the Ni atoms are still less oxidized than the Ni2+ cations in NiO particles or films, which is consistent with the judgment that the vibration band around 2100 cm-1 originates from CO adsorbed on some less oxidized Ni sites.31-32 Now, the 2030-2039 cm-1 band cannot been assigned only depending on the experimental results. We cannot rule out the possibility that the lower frequency 2030-2039 cm-1 band comes from the vibration of CO adsorbed in Ni troughs. To clarify the detailed adsorption configurations of CO on Ni atoms in Ni-O rows (corresponding to the 2110-2119 cm-1 band), the first-principle calculations were performed. A series of CO adsorption configurations were designed at CO coverages from 1/8 to 1/2 ML on a supercell with a p(4×4) geometry along [110] and [001] directions, respectively. On Ni sites in Ni-O rows, the optimized adsorption configurations are shown in Figures 3a-c, in which, the CO molecules adsorb on top of Ni sites in Ni-O rows with a tilt angle θ of about 53°. In the case of 1/8 ML coverage (named 1CO-t) as shown in Figure 3a, the calculated binding energy of CO (Eb, the value is listed in Table 1) is 1.15 eV, and the calculated vibrational frequency (VF) is 2058 cm-1. This basically corresponds to the 2110 cm-1 band shown in Figure 2 at low CO dosage. For 1/4 ML coverage, the most stable adsorption configuration shown in Figure 3b (named 2CO-t-y) is a chain-like adsorption configuration along [110] direction. The calculated Eb is 1.16 eV and the VF is 2062 cm-1. Such chain-like CO adsorption mode was also observed on (21)Cu-O/Cu(110) surfaces.33-34 At 1/2 ML, the calculated VF shifts to 2068 cm-1 (Figure 3c), corresponding to the 2119 cm-1 band in IR spectra. Furthermore, the calculated frequency shift (10 cm-1, 2058 to 2068 cm-1) with respect to CO coverage is in very good agreement with that of IRRAS results (9 cm-1, 8


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2110 to 2119 cm-1) for CO adsorption at 90 K.

Figure 3. Calculated geometries of CO-adsorbed (21)Ni-O/Ni(110) surfaces. (a)-(c) top adsorption structures with CO coverage of (a) 1/8 ML, (b) 1/4 ML, (c) 1/2 ML. (d)-(e) bridge adsorption structures with CO coverage of (d) 1/8 ML, (e) 1/4 ML. (f) the structure with neighboring top and bridge configurations. The upper row represents the side view and the lower the top view. Here, light yellow balls denote Ni atoms in the Ni(110) substrate, green balls denote Ni atoms in Ni-O rows, pink balls denote O atoms and grey balls C atoms.

The adsorption configuration of CO related to the 2030-2039 cm-1 band at 280 K was also explored by DFT calculations. A bridge adsorption configuration was designed, in which the carbon atom of one CO binds to two Ni atoms from the neighboring Ni-O rows, as shown in 9



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Figures 3d-e. In such a bridge adsorption configuration, the adsorbed CO is above the Ni-O rows. The configuration for 1/8 ML is shown in Figure 3d, named 1CO-br. The calculated Eb is 1.65 eV and the calculated VF is 1925 cm-1. As the CO coverage increases to 1/4 ML (Figure 3e, named 2CO-br), the VF blueshifts to 1931 cm-1. The calculated Eb of CO in both 1CO-br and 2CO-br cases are much higher than that of CO in the top configuration, and the VF are significantly redshifted. Furthermore, the dependence of frequency shift (6 cm-1) on the coverage is also consistent with the IR result (9 cm-1). Such results indicate that the 2030-2039 cm-1 band at 280 K comes from CO in the bridge adsorption configuration. Such bridge adsorption induces significant lattice distortion along [110] direction. Additionally, we also tried simulating the CO adsorption on the short bridge site of bottom Ni atoms in Ni troughs between bulgy Ni-O rows, the calculated Eb is as high as 2.12 eV, and the calculated VF of CO is as low as 1766 cm-1. The too low CO VF obviously excludes the possibility of CO adsorption in Ni troughs. Table 1. Calculated binding energies (Eb) and vibrational frequencies (VF) of CO on (21)Ni-O/Ni(110) with different CO coverages in different configurations. The corresponding experimental VF (Exp-VF) were also listed for comparison. The calculated VF are corrected by multiplying a scaling factor of 1.0079 based on the calculated VF of gas phase CO.

Conf.

Coverage (ML)

Eb (eV)

VF (cm-1)

Exp-VF (cm-1)

1CO-t

1/8

1.15

2058

2110

2CO-t-x

1/4

1.03

2061

-

2CO-t-y

1/4

1.16

2062

-

2CO-t-zigzag

1/4

1.14

2058

-

2CO-t-x-oppo

1/4

1.02

2062

-

4CO-t

1/2

1.03

2068

2119

1CO-br

1/8

1.65

1925

2030

2CO-br

1/4

1.29

1931

2039

2CO-1t-1br

1/8

-

2055/1927

-

Our results suggest that the substrate temperature can control the adsorption configurations of CO on (21)Ni-O/Ni(110) surfaces. To further examine the relation between these two adsorption states with the temperature and the thermal stability of two kinds of CO adsorption configurations, the desorption behaviors of saturatedly adsorbed CO on (21)Ni-O/Ni(110) surfaces were studied by TDS. At the same time, the signal of CO2 desorption was also monitored to check the 10


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possibility of CO oxidation during the annealing. Figure 4 shows the thermal desorption spectra of CO and CO2 as a function of temperatures. In Figure 4a, CO is dosed at 280 K, corresponding to the bridge adsorption state at 1/4 ML. As the temperature increases, only one desorption band from 283 K till 308 K exists in CO thermal desorption spectrum, and the maximum desorption rate appears at around 289 K, corresponding to a desorption energy of 76.8 kJ/mol calculated by the first-order desorption kinetic analysis.35 In Figure 4b, CO is dosed at 90 K corresponding to the top adsorption state at 1/2 ML. As the temperature increases, three obvious desorption bands appear successively at around 183 K, 230 K and 294 K, corresponding to the desorption energies of 47.9 kJ/mol, 60.7 kJ/mol and 78.2 kJ/mol respectively.35 This is consistent with early TPD results.15-16 During the whole annealing, no CO2 was detected in CO2 thermal desorption spectrum, also consistent with the early TPD results by Feigerle et al..15 This indicates that CO oxidation or disproportionation reaction does not occur on ordered (2×1)Ni-O/Ni(110) surfaces under UHV conditions.

Figure 4. Thermal desorption spectra of CO and CO2 on the CO-covered (21)Ni-O/Ni(110) surfaces. CO was dosed at (a) 280 K and (b) 90 K.

To clarify the three CO desorption bands and their correlations, the IRRA spectra of CO adsorbed on (2×1)Ni-O/Ni(110) as a function of temperatures are shown in Figure 5. In Figure 5a, CO was saturatedly dosed at 280 K (~1/4 ML) and the IRRA spectra was recorded also at 280 K. As the temperature increases, the original 2039 cm-1 band decreases in intensity with a redshift. At 295 K, the band shifts to 2029 cm-1 with a weak intensity. Till 298 K, the band completely disappears. Evidently, with increasing the temperature, the bridge-adsorbed CO directly desorbs 11



from the substrate, no other adsorption configuration occurs before desorption. This corresponds to the single desorption band shown in Figure 4a. In Figure 5b, CO was saturatedly dosed at 90 K (~1/2 ML) and the IRRA spectra were also recorded at 90 K. Upon heating, the original 2119 cm-1 band has a dramatic decrease in intensity with a slow redshift. Above 180 K, two new bands appear at 2075 cm-1 and 2057 cm-1, respectively. Meanwhile, the 2119 cm-1 band finally shifts to 2098 cm-1 and disappears at about 220 K. By further increasing the temperature, the two new bands gradually redshift. The intensity of 2075 cm-1 band first increases from 180 K to 200 K, and then keeps constant till 250 K; afterwards, the intensity starts decrease and completely disappears below 286 K. The intensity of 2057 cm-1 band increases from 180 K to 265 K, then gradually decreases and completely disappears near room temperature. Obviously, the desorption temperature ranges of 2119-2098 cm-1, 2075-2048 cm-1 and 2057-2038 cm-1 vibration bands are well consistent with the three desorption bands centered at around 183 K, 230 K and 294 K shown in Figure 4b. It is noted that the calculated binding energies of the three vibration bands are larger than the corresponding values estimated from TDS bands by the first-order desorption kinetic analysis.35 This offset is a general phenomenon in DFT calculations for Ni-based materials at present.36-37 Additionally, during the whole annealing, no CO2 IRA band was observed on the IRRA spectra, which is consistent with the TDS results shown in Figure 4.

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Figure 5. IRRA spectra of adsorbed CO on (21)Ni-O/Ni(110) surfaces as a function of annealing temperature. CO was dosed at (a) 280 K and (b) 90 K, respectively.

Since no more CO exposure during the annealing process, the 2075-2048 cm-1 and 2057-2038 cm-1 bands are evidently transformed from the original 2119 cm-1 band. Furthermore, such transformation only occurs at higher temperature (at least higher than 150 K). During the transformation, the 2119 cm-1 band redshifts to 2104 cm-1 and even to 2098 cm-1 before disappearance, and the final 2038 cm-1 band is shifted from the 2057 cm-1 band at the beginning. Therefore, the 2119-2098 cm-1 band has the same origin to the original 2119 cm-1 band, and the 2057-2038 cm-1 band has the same origin to the final 2038 cm-1 band. The significant frequency shifts are evidently induced by the surface structure change during the transformation. Considering the similar thermal stability and the similar vibration frequency, we suggest that the final 2038 cm-1 band has the same adsorption configuration to the 2039 cm-1 band prepared at 280 13



K shown in Figure 2b, i.e, the ordered bridge adsorption configuration. That is, during the annealing process, there exists a structure transformation from the top adsorption to the bridge adsorption. Here，the more stable bridging state is not found at low temperature (90 K) indicating that there exists an energy barrier for the structure transformation probably due to the displacement of the Ni cation in the bridging state. Only at higher temperature (~ 150 K) can the barrier energy be overcome and the bridge state appears. To further verify our judgment, LEED examinations were performed at different temperatures for the CO-covered (2×1)Ni-O/Ni(110) surface. In Figure 1b, CO adsorption at 90 K makes the (2×1) diffraction spots very hazy. Meanwhile, some indistinct (2×2) diffraction spots are faintly visible, which means that at saturate adsorption, every other Ni site in the Ni-O rows adsorbs one CO molecule, corresponding to the 1/2 ML CO coverage. This pattern corresponds to the top adsorption configuration with the VF at 2119 cm-1. When the temperature is increased above 200 K, the hazy (2×1) diffraction spots strangely transform into the clearer diffraction streaks along [110] direction, and the (1×1) spots also become clearer, as shown in Figure 1c. These results demonstrate that, besides desorption of partial CO molecules, the Ni-O rows become disordered along [110] direction with increasing temperature. At this temperature, the 2075-2048 cm-1 band and 2057-2038 band coexist in the IRRA spectra shown in Figure 5b. As the temperature is further increased to 283 K, the streaks along [110] direction are compressed and the (2×1) diffraction spots reappear. This indicates that the structure ordering shown in Figure 1d is higher than that in Figure 1c. At the temperature above 300 K, the diffraction streaks are completely removed and the clear (2×1) diffraction spots appear again in the LEED pattern, similar to that in Figure 1a. This indicates that after CO complete desorption, the lattice distortion has disappeared, and thus the substrate surface has recovered into the ordered (2×1)Ni-O/Ni(110) structure. The high stability of the Ni-O rows may be the reason why CO is hard to be oxidized on such surfaces under UHV conditions. Additionally, the surface with CO adsorbed at 280 K was also detected by LEED, and the pattern is shown in the inset of Figure 2b. This pattern is very similar to that in Figure 1d, indicating that both surfaces have the similar CO adsorption configurations, i.e., the ordered bridge adsorption configuration. This further confirms our speculation that the final 2038 cm-1 band during annealing for CO adsorbed at 90 K has the same bridge adsorption configuration to the 2039 cm-1 band for CO adsorbed at 280 K. 14


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Above results indicate that as the temperature increases from 90 K to 280 K, the evolution from the top adsorption to the ordered bridge configuration must undergo a disordered state. Considering that the 2075-2048 cm-1 band has the similar vibrational frequencies and similar binding energies to those of the 2057-2038 cm-1 band, and they induce the similar distortions of Ni-O rows along [110] direction, we suggest that the 2075-2048 cm-1 band also comes from the bridge adsorption configuration, but the distribution is more disordered than the ordered bridge configuration. Therefore, the disordered state contains both the top adsorption configuration and the bridge adsorption configurations. The different distributions of the bridge adsorption configurations result in the disordered bridge adsorption domain and the quasi-ordered bridge adsorption domain. That is, the evolution starts with the occurrence of the top and bridge configurations. Due to the insufficiency of relaxation at lower temperature, the original top configuration and the new bridge configuration coexist and are distributed disordered initially. As the temperature increases, more top configurations transform into the bridge configurations until the top configurations disappear. By further increasing the temperature, the disorderedly distributed bridge configurations gradually relax to the ordered bridge configurations. The schematics of the evolution are shown in Figure S2 of SI. To simulate the frequency shift dependence on the Ni-O row distortion in the disordered state, we designed such an adsorption structure in the p(4×4) supercell that includes a top adsorption and a neighboring bridge adsorption, as shown in Figure 3f, named 2CO-1t-1br. The calculated CO VF of the top configuration is 2055 cm-1, and redshifted by 3 cm-1 relative to the 2058 cm-1 (the calculated VF of 1/8 ML top configuration shown in Table 1). Although the calculated shift is much less than the IR result (from 2119 cm-1 to 2098 cm-1) from the pure top configuration to the coexistence of the top and bridge configurations, the shift trends are consistent. Similarly, the calculated shift trend (from 1925 cm-1 to 1927 cm-1) of the bridge configuration is also consistent with that from 2038 cm-1 to 2057 cm-1 in experiments. Such results support our speculation that the top adsorption and the bridge adsorption coexist in the disordered state. Meanwhile, this also confirms the existence of the interaction between the top and bridge configurations via the lattice distortion of Ni-O rows. To accurately simulate the disordered state, the supercell much larger than the p(4×4) geometry is necessary which is beyond our present computing power. 15



Conclusions In summary, CO adsorptions and configuration evolutions on (21)Ni-O/Ni(110) surfaces have been studied by using UHV-FTIRS, TDS and DFT calculations. Two types of absorption configurations were identified dependent on the temperature. Near room temperature (280 K), CO adsorbs in the bridge configuration, in which the carbon atom of one CO binds to two Ni atoms from the neighboring bulgy Ni-O rows. The bridge-adsorbed CO exhibits the 2030 cm-1 - 2039 cm-1 vibration band depending on the CO coverage. At low temperature (90 K), CO prefers to adsorb in the top configuration, in which the carbon atom of one CO binds to one Ni atom in the bulgy Ni-O row. The top-adsorbed CO exhibits the 2110 cm-1 - 2119 cm-1 vibration band. Annealing the surface with the top-adsorbed CO from 90 K, the top adsorption configuration gradually transforms into the ordered bridge adsorption configuration at 280 K via a disordered state. In the disordered state, the top adsorption, the disordered and ordered bridge adsorptions coexist. The top and bridge adsorption configurations cause the transverse distortion of the Ni-O rows along [110] direction in different extents, and lead to different frequency shifts of the CO vibration. CO desorption above 300 K completely removes the distortion of the Ni-O rows. The high stability of the Ni-O rows may be the key factor to prevent CO oxidation on such surfaces under UHV conditions. Our studies are helpful in understanding the interaction of adsorbed CO with the Ni-O rows and the CO dynamics on (2×1)Ni-O/Ni(110) or other nickel oxide surfaces.

Notes The authors declare no competing financial interest.

Acknowledgment This work was supported by the National Science Foundation of China (Grant Nos. 21273132 and 11504203), the Shandong Provincial National Science Foundation, China (Grant No. ZR2018MA041) and the 111 project B13029. 16


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Supporting Information available: Additional data on the ball-stick schematics of the clean Ni(110) surface and the (2×1)Ni-O/Ni(110) surface and the schematics of CO adsorption configuration evolutions with temperatures are available in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC Graphic

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Temperature-Controlled CO Adsorption Configurations on (2Ã1)Ni-O

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