Adsorption and Diffusion of CO on Clean and CO2-Precovered ZnO

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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Adsorption and Diffusion of CO on Clean and CO-Precovered ZnO(1010) Hong Shi, Hao Yuan, Shiqi Ruan, Wenyuan Wang, Zhe Li, Zhenyu Li, and Xiang Shao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00039 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

ഥ0) Adsorption and Diffusion of CO on Clean and CO2-Precovered ZnO(10૚ Hong Shi1, Hao Yuan2, Shiqi Ruan1, Wenyuan Wang1, Zhe Li1, Zhenyu Li2,3,* and Xiang Shao1,3,* 1

Department of Chemical Physics, CAS Key Laboratory of Urban Pollutant Conversion, University of Science and Technology of China, Hefei 230026, China 2

3

HFNL, University of Science and Technology of China, Hefei 230026, China

Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China To whom correspondence should be addressed: [email protected] ; [email protected]

Abstract: The interaction of CO on the clean and a CO2-precovered ZnO(101ത0) surface has been investigated with low-temperature scanning tunneling microscopy (LT-STM) in combination with density-functional theory (DFT) calculations. On the clean surface, CO binds weakly to the surface Zn ions and diffuses readily along the [12ത10] direction even at liquid nitrogen temperature. In contrast, the presence of CO2 significantly enhanced the binding strength of the CO molecules in the vicinity and in turn suppressed their diffusion. These findings are believed to provide new insights of the atomistic interactions of distinct reagents on the ZnO surface, and hence shed light onto the roles of ZnO in the syngas-involved catalytic reactions.

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Introduction:

The enthusiasm in lucubrating the adsorption and activation of CO on solid surfaces not only springs from the fundamental interests using CO as a surface probe molecule, but principally also arises from the fact that CO is frequently involved in various important industrial processes, such as CO selective oxidation,1 water-gas shift reaction (CO+H2O→CO2+H2),2 methanol synthesis (CO+2H2→CH3OH)3 and so on. Conventional focuses were mostly restricted to CO and its interactions with co-adsorbates on multifarious transition metal surfaces,4 wherein a wide range of interaction strength has been revealed from extremely weak binding on Au(111) until strong and dissociative adsorption on Fe(111)5,6. Recently, more and more investigations7-13 were conducted around CO adsorption on oxide surfaces owing to the fact that a Mars-van-Krevelen mechanism14, 15 usually governs the CO oxidation on the surface of a metal/oxide catalyst. ZnO in particular, which is deemed to be a technically important material and especially the industrial ternary catalyst (Cu/ZnO/Al2O3) for methanol synthesis from syngas (CO/CO2/H2)16, has provoked an urgent interest in disclosing its interactions with CO molecules at the atomic scale. Distinctly orientated ZnO single crystals have been explored with both theoretical and experimental approaches.17-29 The polar surfaces usually suffers unavoidable surface roughening due to depolarization effect which leads to a number of controversial results.21 In contrast, the nonpolar surface such as ZnO(101ത0) can preserve the atomic scale flatness thus provides an ideal circumstance to study the CO adsorption/reaction. On this surface, large efforts with various spectroscopies and theoretical calculations have been applied and concluded that CO interacts weakly with the unsaturated Zn2+ cations and tilts away from the surface normal.17-19 Recently, Christof Wöll and coworkers, based on their 2


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temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS) measurements, reported that in the presence of a saturated CO2 grids the adsorption energy of CO can be significantly increased.23, 27 Nonetheless, the single molecular investigation of the interactions of CO with the clean as well as the pre-covered ZnO surfaces is still missing, which strongly demands the inputs of the spatially resolved characterizations.

Here in this work, we have conducted a systematic study of CO adsorption on both a clean and a CO2-precovered ZnO(101ത0) surfaces with low temperature scanning tunneling microscope (LT-STM) combined with density functional theory (DFT) simulations. The ZnO(101ത 0) surface is chosen in consideration of its dominating distribution in nano-crystallites as well as its achievable flatness under UHV preparations.21 By integrating the high resolution STM images and DFT calculations, for the first time we can directly observe the precise binding and the directional diffusion of CO on the ZnO(101ത0) surface at single molecular level. Furthermore, our results clearly demonstrate that the pre-adsorbed CO2 molecules can efficiently improve the binding strength of the neighboring CO molecules and suppress their diffusion.

Experimental and calculation methods:

The STM experiments were carried out with a commercial UHV-LT-STM system from Createc Co. (Germany), which has a base pressure of 7.0 × 10-11 mbar in the STM chamber when operating at 77K. The sample preparation chamber, equipped with low energy electron diffraction optics (LEED, Specs) and quadrupole mass spectrometer (QMS, Pfeiffer), has a base pressure of 1.0 × 10-10 mbar. The ZnO(101ത0) 3



single crystal was purchased from Princeton Scientific Corp. Cycles of sputtering and annealing processes of the sample were implemented in the preparation chamber until a sharp (1×1) LEED pattern can be observed. One or two times annealing in the oxygen atmosphere were usually performed for removing the contaminations on the sample. After finishing the last annealing step, the single crystal was immediately transferred into the cold STM stage, which sits inside a liquid nitrogen (LN2) shield. CO gas (Linde Co., 99.999% purity) was directly introduced onto the sample through a capillary tube, while the sample was always kept inside the cold STM stage for preventing the contamination from residual gas in the UHV chamber. All the STM measurements were conducted at liquid nitrogen temperature. The images were acquired in constant-current mode using an electrochemically etched gold tip.

The First-principle calculations were performed based on the density functional theory as implemented in the Vienna ab ignition simulation package (VASP).30, 31 The electron-ion interaction were described with the project-augmented wave (PAW) method.32, 33 The generalized gradient approximation within Perdew-Burke-Ernzerhof function (GGA-PBE) was used to describe the exchange-correlation interaction.34 The plane-wave basis set was adopted with an energy cutoff of 500 eV. The criteria of convergence for energy and force were set to 10-5 eV and 0.01 eV/ Å, respectively. Vacuum layer is larger than 15 Å in all the calculations. The calculated lattice parameters of bulk ZnO are a=b=3.317 Å and c=5.314 Å, which are comparable with the experimental and other calculated values.29 A (4×4) ZnO(101ത0) surface with four ZnO layers was built and the last two ZnO layers were fixed. The adsorption energy was calculated as follows: Eads = Eadsorbate/ZnO – Eadsorbate - EZnO, Where Eadsorbate/ZnO, Eadsorbate, and EZnO represent

4


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energies of adsorbate on ZnO(10-10) surface, adsorbate alone, and bare ZnO(101ത 0) surface slab, respectively.

Results and discussion: ത 0) with atomic flatness can be routinely prepared by following the The clean surface of ZnO(101 cleaning procedures mentioned in the experimental methods. Figure 1 shows the STM images of the as-prepared clean surface without any contaminations, wherein the sharp step edges along the [0001] and [12ത10] directions can be clearly recognized. In the high-resolution images shown in Figure 1b, the rows of Zn ions can be readily observed once the tip is brought to the surface close enough. With the tip well-conditioned, true atomic resolution of the Zn lattice can be achieved, as shown in the inset of Figure 1b. One may notice that there are bright bulges distributed randomly on the terrace. We assign them as the subsurface defects whole top surface has a continuous lattice, as already addressed in our previous study. 29

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Figure 1. STM images of cleaned ZnO(101ത0) surface. (a) U=2.5 V, I=100 pA. Image size: 100 nm × 100 nm. (b) U=2.5 V, I=200 pA. Image size: 20 nm × 20 nm. The surface zinc atom chains along [12ത10] direction were clearly presented. The red arrow points to a bright bulge which is assigned to subsurface defects. Inset is the atomic resolution acquired at U=2.8 V, I=3 nA, wherein the round protrusions are assigned as the surface zinc atoms. Image size: 5 nm × 5 nm.

After dosing about 2.0 L (Langmuir, 1 × 10−6 Torr·s) CO molecules at 77 K, as shown in Figure 2a, a new species with a depression feature was observed which distributed randomly on the surface. These species were assigned to CO molecules as further confirmed by in-situ dosing experiments (see Figure S1 in the supporting information). It is well acknowledged that the STM topography of an isolated CO molecule on metal surfaces can sensitively depend on the termination of the tip,

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and it has been also

observed in the current case of ZnO surface. As illustrated in Figure 2b, CO exhibits as a normal pit in the top part of the image when the tip is metallic, while presents a specific sombrero feature when the tip switched to a CO-termination after picking up a CO molecules as marked by the yellow arrow. With the improved resolution, the surface Zn rows was clearly resolved under a positive sample bias, making vivid contrast to the weak resolution of the upper part of Figure 2b. The explicit adsorption site for an isolated CO can be safely assigned to surface Zn site from the zoomed-in STM image inserted in Figure 2b in combination with the corresponding height profiles in Figure 2c, as well as using CO2 as a marker to reproduce the substrate lattice (see Figure S2). This assignment is further supported by our DFT simulations presented by the optimized side- and top-view models in Figure 2d. The carbon atom of the CO molecule binds to the surface Zn2+ cation with a tilted angle of α=27.9° against the surface normal, 6


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while keeping the entire molecule in the (112ത0) plane. In the model one may also notice the off-plane lift of the Zn ion upon binding with the CO molecule, which almost restores to the ideal position of the truncated surface. Moreover, the binding energy of an isolated CO is calculated to be -0.377 eV, clearly depicting the weak interaction of CO with the surface. All these results are consistent with previous theoretical calculations.22, 36

Figure 2. (a) STM image of CO molecules adsorbed on the ZnO(101ത0) surface obtained by an normal Au tip. 14 nm × 14 nm, U=3.1 V, I=200 pA. (b) Tip-changing due to the capturing of a CO molecule at the apex during downward scanning. 12 nm × 12 nm. Inset shows a high resolution image obtained with a CO-tip with several yellow lines indicating the surface Zn chains. 5.3 nm × 3.6 nm. , U=3.1 V, I=200 pA. (c) Height profiles along the lines in the inset in (b). (d) Simulated side-(up) and top- view (down) ball-models of an isolated CO on ZnO(101ത0). The oxygen and carbon atoms of CO are colored in blue and brown, while the oxygen and zinc atoms of ZnO substrate are colored in red and gray, respectively.

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In Figure 2a one may also descry some “breakoff” features as have been highlighted by the white circles, evidencing the hopping events of the CO molecules during the tip scanning. The surface diffusion of adsorbates had been extensively studied by STM on various solids, 37-43 whereas those of CO on oxide surfaces were rarely reported, which is however of important relevance for understanding the surface reaction mechanisms on supported catalysts. Therefore, we have conducted an elaborated STM study of the diffusion dynamics of CO on the ZnO(101ത0) surface at varying temperatures. Figures 3a shows a STM image of low coverage of CO on ZnO(101ത0) acquired with a large tip-substrate distance (U=2.7 V, I=25 pA), which displays a negligible tip effect on the molecular diffusion events. In contrast, when reducing the tip-surface distance by increasing the tunneling current to 600 pA a tip-assisted CO migration can be clearly recognized, as shown in Figure S3. Figure 3b shows the specially colored difference image generated from Figure 3a subtracting a subsequent snapshot of the same area taken with an interval of 500 seconds. In this compiled image, one can immediately finger out that all 18 CO molecules in the imaged area have migrated away from their original positions. The orientations of these paired red (final positon) and blue (initial position) dots, as marked by the ovals, strongly evince that CO molecules diffuse exclusively along the [12ത10] direction. At the first glance, this one-dimensional (1D) hopping event is understandable considering the geometric anisotropy of the (101ത0) surface, wherein the lattice constant is 3.25 Å in [12ത10] but as large as 5.2 Å in [0001ത] direction. More visualized STM movies, which were collected at distinct tunneling parameters, temperatures, as well as with different tips, are presented in the

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supporting information. These evidences demonstrate that the CO diffusion is a spontaneous event as a function of temperature, instead of any artificial effects of the tip scanning.

Customarily, the diffusion of surface species can be well investigated by time-lapsed STM measurements from which the hopping rates of the diffusion species can be estimated.

44, 45

For the

directional CO diffusion along the [12ത10] direction of ZnO(101ത0) surface, it is feasible to fit with a 1D hopping model: D = D*·exp(-Ea/kT),46 wherein D* is the pre-factor, Ea is the activation energy, k is the Boltzman constant and T is the temperature. The diffusion coefficient D is temperature dependent and can be experimentally determined by the mean square displacement of the CO molecules after time t ( D= /2t ).47 We have thus measured the CO diffusion at varied temperatures ranging from 30 K to 100 K. It is found that CO completely freezes on the surface below ~60 K but moves too quickly to unambiguously trace their displacements at above 90 K. Figure 3c shows the plots of CO displacements as a function of time collected at 72 K, 77 K and 82 K, respectively, by which the diffusion coefficients can be deduced from the slopes. Further fitting D and T to the Arrhenius plot as shown in Figure 3d, we can estimate the activation energy Ea to be (-0.085 ± 0.02) eV. This deductive activation energy is reasonable in comparison with the thermal energy of the observation temperature. Its ratio to the calculated adsorption energy (-0.377 eV) is about 23%, falling in the range (10~30%) for most adsorbates on surfaces.48 A pre-factor D* = 7.62 × 10(-11.5 ± 0.7) cm2·s-1 is obtained from the interception of the Arrhenius plot, with which an attempt frequency of 0.8 × 10 5·s-1 can be estimated by dividing D* by the square of the interval between the nearest hopping sites along the [12ത10] direction.46 Such attempt frequency is close to those of other small adsorbates on similar oxide surfaces such as TiO2(110),39,40 but 9



quite much lower than CO molecules on coinage metal surfaces such as Cu (110),37 possibly due to the drastically different surface properties between oxides and metals.

Figure 3. (a) STM image of CO adsorption on ZnO(101ത0) surface acquired at U=2.7 V, I=25 pA. Image size: 20 nm × 20 nm. (b) Color-code difference image generated by subtracted (a) from and another STM image in the same area with a time lapse of 500 s. Red and yellow circles mark the two opposite migration directions, respectively. (c) Mean square displacements versus the time at three different temperatures. The calculated diffusion coefficients are D72K = (4.91 ± 0.80) × 10-18cm2·s-1, D77K = (1.82 ± 0.21) × 10-17cm2·s-1, and D82K = (3.13 ± 0.18) × 10-17cm2·s-1, respectively. (d) Arrhenius plot of the diffusion coefficient versus the measured temperatures.

In addition to the isolated CO molecules, we also paid attention to the possible CO aggregates on the ZnO(101ത0) surface in order to learn about the intermolecular interactions between the CO molecules, 10


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which can play important roles in the surface reactions under practical conditions. As shown in Figure 4a, we have considered four types of linear aggregates of CO in our DFT calculations and the corresponding adsorption energies are listed in Table 1. For all the considered circumstances, a reduced binding energy is resulted in comparison with the CO monomer. It is worth pointing out that the CO dimer along the [0001] direction, marked by the dashed rectangle in Figure 4a, has very close adsorption energy as the CO monomer (-0.370 eV VS -0.377 eV). Therefore, its occurrence is evidenced by the STM image shown in Figure 4b (highlighted by the dashed rectangle) whereas other proposed structures were never observed at such low coverage. Figure 4c shows the side-view ball-stick model of this specific CO-dimer-0001. One can find that the two CO molecules stay in the (12ത10) plane but the tilting angle of the C-O bond increases to 32.8o. With such a more inclined configuration, the dipole-dipole repulsion between the neighboring CO molecules can be significantly reduced hence the stability increased.

Figure 4. (a) Four simulated CO aggregates on ZnO(101ത0) surface. (b) STM image of the CO dimer orientated along [0001]. 8.8 nm × 5.8 nm, U=2.7 V, I=100 pA. (c) Side-view ball-stick model of

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CO-dimer-0001 as indicated by the dashed rectangle in (a). (d) Full coverage of CO with (1×1) structure obtained at high exposure. 6.4 nm × 3.8 nm. White oval circulates the defect with two CO molecules missing.

Further extending the CO dimers into infinite chains along the two representative low-index directions leads to even weakening of the CO bindings, as seen in Table 1. Therefore, the chain-like structures were never observed for the low coverages of CO, drastically different from the readily formed CO2 chains on the same surface.29 Keep dosing large amount of CO onto the ZnO(101ത0) surface finally resulted in a dense and ordered (1×1) adsorption structure at LN2 temperature, as shown in Figure 4d. Interestingly, here the contrast of the CO molecules has transformed into positive compared with the substrate, as revealed by the defects site highlighted by the white oval. In the image sequence of the in-situ dosing experiment shown in Figure S1, one can find the sudden switch of contrast at full monolayer coverage. Since we never observed CO as protrusions under low coverage conditions, we tend to attribute such contrast change to the drastically electronic change of the surface at close to monolayer coverage.

Configurations

Ead (eV)

△E (eV)

CO-dimer-0001 CO-dimer-12ത10 CO-chain-0001

-0.370 -0.343 -0.357

0.007 0.034 0.020

12


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Table

1.

The

binding

CO-chain-12ത10 CO-full layer

-0.284 -0.268

0.093 0.109

molecule in dimer and

energies (Ead) of a CO chain structures and the

differences (△E ) compared with a CO monomer on the ZnO(101ത0) surface.

On the basis of a detailed examination of pure CO adsorption, it would be of practical importance to further consider the impacts of CO2 co-adsorption, whose abundance can be remarkable during methanol synthesis considering the unavoidable water-gas-shift reaction. In Figure 5a, we present six simulated co-adsorption configurations upfront labeled by (1)-(3) for CO with an isolated CO2 and (4)-(6) for CO with dimerized CO2 molecules, respectively. The corresponding binding energies of CO for each configuration are attached at the bottom right of each model. One may immediately recognize that the most favorable CO-CO2 co-adsorption configuration in Figure 5a-(2), which gives a significantly increased binding energy of -0.404 eV. This preferential site can be correlated with the relatively more positive nature of this specific Zn ion around an adsorbed CO2 molecule (see Figure S4 for the charge distribution around a surface CO2). In contrast, a markedly weakened adsorption is found for the configuration in Figure 5a-(3) having a coplanar arrangement of CO and CO2 in (12ത10), possibly due to 13



the repulsive dipole-dipole interactions considering the polarity of the adsorbed CO2. Lengthening the isolated CO2 to a dimer increases the positive charges at the Zn ions around the CO2 chains, hence increases the binding energies of CO in all the configurations shown in Figure 5a-(6), as long as CO is not in the co-planar sites of the CO2 dimer. Expectedly, this trend would grow along with the extension of the CO2 chains.

Figure 5. (a) Top-view ball-stick models for six different configurations of co-adsorbed CO and CO2 on ZnO(101ത0) surface with corresponding binding energy of CO stuck aside . (b) STM image of CO adsorption on a CO2-pre-adsorbed ZnO(101ത0) surface. 20 nm × 20 nm, U=2.7 V，I=100 pA. Inset in (b) is the zoom-in image of a single CO2 molecule. (c) Difference image between (b) and a subsequent image acquired on the same area after 500s. Only three CO molecules show recognizable displacements and were marked by the white ellipses.

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Figure 5b shows the STM result of CO-CO2 co-adsorption, wherein CO2 shows as dim protrusions (inset in Figure 5b) while CO as depressions. The assignments of these surface species are based on our systematic study of independent CO2 or CO adsorptions on ZnO(101ത0). One can also find more direct evidences from Figure S5, wherein the sequential adsorption data of CO2 and CO at the same surface (not the exact same area) have been presented. As what had been proposed above, almost all the CO molecules reside nearby a CO2 species. Only few CO molecules stay away from any CO2 species due to the random distribution. Figure 5c shows the difference image generated by Figure 5b subtracting a subsequent image of the same area. One can find that out of 22 CO molecules only that three isolated CO molecules (marked by white ovals) show some displacements after an imaging interval of 500 seconds. A short movie consisting of the complete image sequence can be found in supporting information, which confirms that all CO2-pinned CO molecules except the three “free” CO molecules stayed still all the time. Such visualized evidence clearly demonstrates that the pre-adsorbed CO2 species totally suppressed the migration of CO at the experimental temperature due to the enhanced surface binding.

This stabilization effect of CO2 could be highly instructive for understanding the role of ZnO in the reactions of syngas transformation. Due to the weak interaction with the ZnO surface, reaction of pure phase of CO with H2 can hardly be expected since the latter is also a weak adsorbate on the same surface.27 However, in the presence of CO2, which is the general situation for many syngas feedstock, CO molecules are obviously more activated and their lifetimes on the ZnO surface can be significantly 15



increased. Moreover, the adsorbed CO2 species also establish a suitable arena for both reactants and raise their encountering probability for reaction. Both factors finally lead to a remarkable increase of the reaction probability. As a matter of fact, such co-adsorption effect may also be valid for other co-adsorbed molecules such as H2O, which concomitantly operates in the transformation of CO into higher-grade organic molecules.

Conclusion:

In conclusion, we have studied the adsorption behavior of CO on the clean and CO2-preadsorbed ZnO(101ത0) surface utilizing LT-STM combined with DFT calculations. The binding configuration of CO to the clean surface is evidently imaged by high resolution STM. Novel one-dimensional diffusion of CO along the [12ത10] direction was systematically studied under varied temperatures and an activation energy of (0.085 ± 0.02) eV was evolved. In contrast, the one-dimensional diffusion of CO is significantly suppressed on the CO2-preadsorbed ZnO(101ത0) surface. DFT calculations point out that it is the increased positive charge at the Zn ions around the CO2 species that strengthens the binding of CO and raises the diffusion barrier. These findings not only broaden our knowledge of the CO interactions with the oxide surfaces, but also provide an important input for deepened understanding of the role of ZnO in catalyzing syngas transformation.

Supporting information: 16


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Additional STM images and calculation results, and videos of the 1D diffusion of CO at varied conditions.

Acknowledgement: We are grateful for the financial support of NSFC (91545128, 21333001) and MOST (2014CB932700). X. S. thanks the financial support of the Thousand Talent Program for Young Outstanding Scientists of the Chinese government.

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TOC Graphic

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Adsorption and diffusion of CO on the clean and a CO2-precovered ZnO(10-10) surface was investigated with scanning tunneling microscopy in combination with DFT calculations 50x39mm (300 x 300 DPI)


Adsorption and Diffusion of CO on Clean and CO2-Precovered ZnO

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