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Surface Structures of Model Metal Catalysts in Reactant Gasses Franklin (Feng) Tao, Walter T. Ralston, Huimin Liu, and Gabor A. Somorjai J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06950 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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The Journal of Physical Chemistry

Surface Structures of Model Metal Catalysts in Reactant Gasses Franklin (Feng) Taoa*, Walter T. Ralstonb,c, Huimin Liua, Gabor A. Somorjaib,c* a

Departments of Chemical and Petroleum Engineering and Chemistry, University of Kansas, Lawrence, KS 66045 b

Department of Chemistry, University of California, Berkeley, CA 94720

c

Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720

Abstract Atomic scale knowledge of the surface structure of a metal catalyst is essential for fundamentally understanding the catalytic reactions performed on it. A correlation between the true atomic surface structure of a metal catalyst under reaction conditions and the corresponding catalytic performance is the key in pursuing mechanistic insight at a molecular level. Here the surface structures of model, metal catalysts in both ultrahigh vacuum (UHV) and gaseous environments of CO at a wide range of pressures is discussed. The complexity of observed surface structures in CO is illustrated, driving the necessity for visualization of the catalytic metals under realistic reaction conditions. Technical barriers for visualization of metal surfaces in situ at high temperature and high pressure are discussed.

*: To whom all correspondence should be addressed. Email [email protected] (F.T.) and [email protected] (G.A.S)

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Introduction Heterogeneous catalysis contributes to more than half of the production processes of the chemical and petroleum industries.1,2 One of the main achievements of fundamental surface science and catalysis studies on model catalysts in the last few decades is that a catalytic event can be understood at the microscopic scale with a “unit” defined as an active site.1,2 Such a catalytic site consists of one to several atoms of the topmost surface layer of a catalyst. The composition, geometric packing of atoms, and the electronic structure of a catalytic site are essential in determining its catalytic performance. As heterogeneous catalysis is a surface process occurring on the topmost layer of a catalyst surface, surface sensitive microscopy techniques are key for successfully identifying the active site or sites. In terms of fundamental studies of catalyst surfaces, scanning tunneling microscopy (STM) has been the main technique in characterizing the surface structures of model metal catalysts at an atomic scale and has provided unique insights into understanding the catalytic reactions which occur on the topmost surface layers.3–9 Since the birth of the STM technique,10–13 STM has been used in visualizing surfaces of model catalysts, including metals and oxides, for almost four decades. During the 1980’s and 90’s, most STM studies were performed in ultrahigh vacuum (UHV) below 300K for acquisition of highresolution images. During this time, very few STM studies were performed at elevated pressures or temperatures. Theoretically, STM can image the surface of a catalyst in the gas phase. In STM studies the distance between an atom of the surface and the tip is typically in the range of 1-10Å.14 When the STM system is exposed to a gas, the gas phase molecules fill in the tunneling gap between the surface and the tip. These free, gas phase molecules can scatter the electrons tunneling between the tip and sample. Since the molecular density of the gas molecules between the tip and surface should remain constant for all locations across the surface, we can consider that the electron-molecule scattering is uniform and does not modify the contrast obtained from the surface.15 From this point of view, the STM contrast of two distinct locations on the surface still represents an authentic difference in electronic and geometric structures of the two different locations. Due to the contribution of gas phase reactants to the surface chemical potential, studies have demonstrated that the surface exposed to gas phase reactants can be different from that in UHV, as shown schematically in Figure 1.1,2 These studies further proved that the surface structure of a catalyst in the presence of gaseous reactants cannot simply be extrapolated from the interpretation obtained in UHV. In the following sections, we will briefly review the structures of model metal catalysts in the presence of gas phase reactants, underscoring the need for structural studies under catalytically relevant conditions.

Results and Discussion 1. Identification of adsorbates on surface of model catalyst It is well known that the surface structure of adsorbates on a metal catalyst can be resolved when the surface is at cryogenic temperatures. However, it is quite challenging to identify an individual adsorbate of reactants such as hydrogen, ethylene, or benzene at room temperature. This most likely results from the high mobility of adsorbates at room temperature and the low contribution of adsorbates in the tunneling events between the sample surface and tip.16–20 In some cases, individual


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adsorbates such as CO molecules can be readily visualized due to its relatively strong chemisorption on metal surfaces;16–20 these adsorbates can be directly imaged with STM. For example, CO molecules form an overlayer on surfaces of most metal catalysts and can be readily identified.21 Thus, here we chose CO as a reactant gas to review how the existence of gas phase reactants at various metal surfaces can influence their respective surface structures.

2. (111) Surface of precious metal model catalysts Precious metals are one of the main types of catalysts used in the hydrocracking of hydrocarbons and other reactions in the petroleum and refining industry. Understanding the adsorption of CO on precious metals is technically important since the H2 source of catalytic hydrocracking typically contains trace levels of CO, and the accumulation of CO on the metal catalyst surface can readily poison the catalyst. The adsorption of CO on Pt(111), Rh(111), and Pd(111)21–25 has been studied with STM and other characterization techniques. Coverages of chemisorbed CO increase with the increase of CO pressure.21,22 The specific binding configurations of adsorbed CO molecules depends on the type of metal. For example, the adsorbed CO on Pt(111) take non-specific binding positions when the CO pressure is higher than 1x106 Torr.22 The repulsion of adjacent chemisorbed CO on Pt(111) plays a more important role than the binding of a CO molecule to a specific Pt site on Pt(111). As shown in Figure 2a, each CO is surrounded by six neighboring CO molecules (Figure 2a, yellow panel) giving the CO adsorbate layer a 2-D hexagonal structure. This hexagonal CO adsorbate layer overlaps the hexagonal pattern of Pt on Pt(111) (Figure 2a, blue panel) forming a Moiré pattern (Figures 2b, 2c). The superstructure of the Moiré pattern appears as a brighter area, where the chemisorbed CO molecules can be resolved since they appear as individual spots in the brighter area. Different from CO on Pt(111),22 adsorption of CO molecules on Rh(111)21,24 does not obviously form a Moiré pattern. As shown in Figure 3a, the CO adsorbate on Rh(111) exhibits a periodic structure with a unit cell of c(2x2).24 Compared to Pt(111), CO on Rh(111) binds to specific Rh sites, schematically shown in Figure 3b. As CO molecules do not arrange in a hexagonal pattern, no Moiré pattern on Rh(111) is observed. The difference in molecular arrangement of the CO adlayer between Pt(111) and Rh(111) probably results from the difference in binding strength of CO molecules on the two surfaces – 120kJ/mol for CO on Pt(111) and 139kJ/mol for CO on Rh(111).26 When the adsorption of CO on a metal surface is relatively weak, the adsorbed CO have a higher possibility to adopt non-specific binding to the metal surface. Similar to Rh(111), no Moiré pattern is observed for CO on Pd(111), consistent with the stronger binding of CO to Pd(111) of 143kJ/mol.26

3. Molecule – dependent adsorption in the gas phase For the same surface, the structure of the adsorbate layer strongly depends on the type of gas adsorbing and on the gas present in the gas phase. For example, H2 dissociates on Pt(111) to form atomic hydrogen at room temperature. Adlayers of hydrogen atoms on Pt(111) do not exhibit any specific structure. Due to the weak adsorption of atomic hydrogen on Pt(111), the hydrogen atoms have a high mobility and are thus invisible in STM images (Figure 4a). Similar to adsorbed hydrogen atoms, ethylene molecules adsorbed on Pt(111) through weak π binding do not form a specific adsorbate



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structure (Figure 4b).17,20 The high mobility of ethylene molecules makes the observation of the Pt(111) surface challenging; however, introducing a small amount of CO to the ethylene allows for the immediate visualization of the surface. When CO was added to the mixture of ethylene and H2, a clear absorption pattern is visible, confirming that adsorption of CO on Pt(111) is much stronger than both ethylene and H2.17,20 This well rationalizes the poisoning effect of trace amounts of CO in the H2 gas during hydrocracking in the petroleum industry.

4. Surface structure-dependent behavior of Pt model catalysts in CO Industrial precious metal catalysts always have small sizes, typically in the 1-5 nm range for the purpose of obtaining a higher dispersion – or having more exposed surface atoms versus bulk atoms.1,2 Mathematically, we can readily understand that the fraction of surface atoms among all atoms of a metal nanoparticle increases when the nanoparticle size decreases. As reported in the literature,27,28 the atomic fraction of surface atoms of a nanoparticle with a size of 2nm is about 50%. One obvious consequence of decreasing nanoparticle size is that the average coordination number of all metal atoms of the metal nanoparticle also decreases. The fraction of metal atoms at edges and corners with low coordination numbers, CN(M-M), increases with smaller nanoparticle size. These highly undercoordinated metal atoms at edges and corners of a small metal nanoparticle could alter surface adsorption and the relevant catalytic performance quite drastically from a metal nanoparticle with larger size. To understand how the under-coordinated surface atoms of small metal nanoparticles used as industrial catalysts (Figure 5a and 5b) will respond in various reactant gas environments, a Pt(557) stepped surface (Figures 5c and 5d) was chosen as a model catalyst to represent the highly undercoordinated surface atoms of a small nanoparticle as both have a high density of under-coordinated Pt atoms. CO was chosen as a probe molecule as it is an important reactant in many industrial catalytic reactions. Surprisingly, compared to the straight, parallel step edges of clean Pt(557) (Figures 6a and 6b),7 the Pt(557) under CO gas exhibits distinctly different surface structures – even as low as 5x10-8 Torr CO as shown in Figure 6c. The clean Pt(557) surface in UHV has highly regular, parallel packed step edges whereas when the surface is exposed to 5x10-8 Torr CO, the step edges become curvy (Figure 6c). The formation of curvy step edges is driven by the high mobility of Pt atoms at the step edges (marked with a red line in Figure 6a). As discussed above, the strong repulsion of CO molecules on Pt(111) was suggested based on the observed Moiré pattern in CO gas (Figure 2). On the Pt(111) terrace, CO can slightly “spillover” on the flat terrace, driven by the repulsion of neighboring CO molecules, such that an equilibrium structure of CO “spillover” repulsion interactions and preferential binding is obtained. Compared to the weak adsorption of CO on Pt(111) atoms with a coordination number of 9, CN (PtPt)=9, 1/6 of the atoms of the Pt(557) surface have a coordination number of 7. The binding energy of CO on the Pt step edges of Pt(557) with CN(Pt-Pt)=7 is stronger than that on Pt atoms of Pt(111), CN(PtPt)=9. The strong binding of CO at the step edges of the Pt(557) surface makes the CO “spillover” impossible. In other words, the Pt(557) cannot adopt the non-specific binding model like Pt(111) to release repulsion of neighboring CO molecules. How the strong repulsion of adjacent adsorbed CO is minimized on the Pt(557) surface is key for understanding the dramatic change in surface structure of Pt(557) when the CO pressure is increased from 5x10-8 to 1 Torr. As the coordination number CN(Pt-Pt) for Pt(557) step edges is 7, the Pt-Pt bonds are much more easily broken than the higher coordinated


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terrace sites on Pt(111), with CN(Pt-Pt)=9. Figure 6d shows the new surface structure obtained upon increasing the CO pressure to 1 Torr; the step edges have almost disappeared but seem corrugated. On closer inspection (Figure 6e) triangular nanoclusters have formed on the step edges. Considering the lower coordination number and the more ‘open’ space around a step edge compared to flat Pt(111) sites, the Pt-Pt bonds at the step edges break and rearrange when the coverage of CO is increased. The breaking of Pt-Pt bonds at the step edges increases the surface energy of the Pt atoms, as the average coordination number of all Pt surface atoms decreases from 8.7 to 8.0; this would imply that the Pt(557) surface is less stable after reconstruction. However, the stability of the restructured surface has actually increased because the intermolecular repulsion of CO was largely decreased upon restructuring. This decrease of CO repulsion was realized by increasing the fraction of highly under-coordinated Pt atoms (the Pt atoms at step edges) and thus increasing the number of CO molecules adsorbed on undercoordinated sites. In addition, the CO molecules adsorbed on the Pt atoms at the edges of the triangular clusters (Figure 6g and 6h) will tilt toward the open surface side (away from the nanocluster). This fanout spatial effect helps to decrease the Pt(557) surface energy when a full adlayer of CO is present. The fan-out phenomenon is similar to airline passengers who are sitting on aisle seats; the extra space afforded by the aisle allows these passengers to tilt toward the aisle and increase the available space between themselves and their neighbor in the middle seat.29,30 More interestingly, the nanocluster-covered surface formed at elevated CO pressures changes when the gas environment of CO disappears. As shown in Figure 7, the surface of Pt(557) reverts to the curvy stepped surface after CO gas at 1.0 Torr is evacuated to 5x10-8 Torr.7 When the CO pressure is reduced, the coverage of CO on the Pt(557) surface decreases. This decreased coverage of CO causes the under-coordinated step edges to have a higher surface energy, as they no longer have CO adsorbates to stabilize them. To reduce the surface energy of the Pt atoms with lower CO coverage, these Pt atoms will aggregate to increase their coordination number. The observed sintering of adjacent triangular nanoclusters to form the curvy stepped surface (Figure 7a) confirms that the fraction of Pt atoms with CN(Pt-Pt)=7 decreased and therefor the overall surface energy decreased. Thus, the clusterlike surface formed in CO at “a relatively high pressure” (1 Torr) changes to a curvy step-like surface at significantly lower CO pressure (5x10-8 Torr). Subsequently, by increasing the pressure of CO again to 1 Torr, the curvy step-like surface will restructure into the triangular clusters. Such a reversible restructuring of the Pt(557) is clearly driven by CO pressure; in nature, it is driven by the high and low CO coverages originating from the differences in pressure. This study provided the first example of the pressure-dependent reversible change of the surface structure of a metal catalyst driven by pressure of the reactant; it gives solid evidence for the necessity of observing surface structures under reaction conditions. 5. Restructuring of Cu(111) in CO The Pt atoms at the step edges of Pt(557) exhibit a high mobility on the surface, since these Pt atoms have a low coordination number with surface Pt atoms, CN(Pt-Ptsurface)=4. These Pt atoms can be readily detached from the step edge; compared to these, a Pt atom of the (111) surface is fully coordinated with six neighboring Pt atoms, CN(Pt-Ptsurface)=6. Similar to the breaking of metallic Pt-Pt bonds with low CN(M-Msurface) at step edges, the much weaker bond strength of Cu-Cu on Cu(111) vs PtPt on Pt(111) allows for the breaking of Cu-Cu bonds by CO on the Cu(111) surface at Torr pressures. Different metals have quite different cohesive energies, which are a sign of how easy it is to break the M-M bond. Using the cohesive energies of different metals, we can evaluate the relative strength of a



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metal M-M bond. For example, Cu has a much lower cohesive energy, 336kJ/mol compared to Pt with 564 kJ/mol.31 Thus, a Cu-Cu bond in the CU(111) surface can be readily broken although a Pt-Pt bond of the Pt(111) surface cannot be broken in the Torr pressure range. In addition, the Cu(111) reconstruction in the presence of Torr pressures of CO gives a completely different surface structure than Pt(111). As shown in Figure 8, the Cu(111) surface reconstructs to nanoclusters in CO gas even at a pressure of 1 Torr.8 Conclusion The above work has clearly demonstrated the necessity of examining surfaces under catalytic/reaction conditions, since the reactants can readily drive restructuring for metal catalysts. Metals being important catalysts in chemical and petroleum engineering, it is imperative to be able to visualize and understand the surface structures of these materials at an atomic scale under working conditions. Knowledge of how metal surfaces change with reactant gas, reactant pressure, and temperature will help establish intrinsic correlations between authentic surface structures and the corresponding catalytic performance. In most cases, unfortunately, we have not yet been able to establish such a correlation due to the difficulty in accessing the atomic surface structure at realistic catalysis conditions, which are typically at elevated pressures (>1 bar) and temperatures (200 – 600°C or higher).32 This technical barrier has prevented us from accessing authentic surface structures at high spatial resolutions. Future challenge – Visualization of catalyst surface at high temperatures Temperature is one necessary factor for catalysis; most industrial catalytic reactions are performed at a temperature of 200 – 600°C or higher.32 Unfortunately, the majority of the fundamental STM studies of model single crystal metal surfaces performed were at UHV and cryogenic temperatures, and most in situ studies reported were performed at room temperature. The temperature factor was skipped in order to visualize surfaces at high spatial resolution, as higher surface temperatures contribute to higher thermal drift of the STM head giving lower spatial resolution. Future challenge – Visualization of catalyst particles on support Due to the slow response of the feedback loop in present STM, imaging a sharp change in height such as a nanoparticle on a flat support is difficult without the tip crashing into the sample surface. Typically this has limited STM samples to large, flat surfaces (>100nm) like polished single crystals consisting of domain sizes larger than a few hundred nanometers.7,30,33–43 In addition to pure metal single crystal surfaces, thin films with a thickness less than a few nanometers supported on a single crystal surface – such as CeO2 nanoclusters grown on Au(111)9 – have also been used to simulate the surface of a realistic catalyst. Most STM studies of catalyst surfaces reported in the literature were performed on flat model catalysts including both single crystals and thin nanoclusters deposited on a flat single crystal surfaces. As these flat model catalyst surfaces may not reflect the surface of a metal nanoparticle supported on an inert substrate under reaction conditions, it is necessary to make it our goal to visualize the surfaces of nanoparticle catalysts under the same reaction conditions (reactant gas mixture, pressure, and temperature) that an industrial nanoparticle catalyst would experience. To reach this goal, development of new scanning microscopy techniques which are robust at harsh catalytic conditions is necessary.


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Acknowledgements F.T. acknowledges financial support from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, under Grant No. DESC0014561. G.A.S acknowledges financial support from the Materials Sciences and Engineering Division, Basic Energy Sciences, Office of Science, U.S. Department of Energy, under Contract No. DE-AC02-05CH11231

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Figure 1. Schematic showing (a) clean surface of a well prepared metal crystal in UHV and (b) cartoon demonstrating the restructured surface of a model metal catalyst in the presence of gas phase reactant(s). The originally flat, clean surface could be restructured due to the contribution of gas phase reactants which change the chemical potential of the clean surface and thus result in reorganization of surface atoms at higher pressure in contrast to a surface in UHV or in gasses at a lower pressure.


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Figure 2. Surface of Pt(111) single crystal model catalyst under CO at Torr pressures. (a) Schematic showing the formation of a Moiré pattern due to the overlap between the hexagonal surface lattice of Pt atoms of the topmost layer of Pt(111) (blue panel) and the hexagonal lattice of the CO molecules adlayer (yellow panel). (b) STM image of Pt(111) in 0.01 Torr CO at room temperature. (c) STM image of Pt(111) in 720 Torr CO at room temperature. (d) Structural model of the Pt(111) surface with adsorbed CO molecules (marked with blue dots) in CO at 0.01 Torr; the large empty circles mark Pt atoms of the topmost layer of Pt(111). (e) Structural model of the Pt(111) surface with adsorbed CO molecules (marked with blue dots) in CO at 720 Torr. (b-e) Reproduced with permission from Ref. 22. Copyright 2004 American Chemical Society.



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Figure 3. STM image of the Rh(111) surface in 700 Torr CO at 25oC (a) and the (2×2) pattern observed in CO at 700 Torr (b). The average corrugation of these bright spots is 0.20±0.03Å. Scale bar in (a) is 5 Å. Reproduced with permission from Ref. 21. Copyright 2000 Elsevier.


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Figure 4. STM images of the Pt(111) surface (100 Å ×100 Å) under different pressures: (a) 20 mtorr H2, (b) 20 mtorr H2 and 20 mtorr ethylene, and (c) mixture of 20 mtorr H2, 20 mtorr ethylene, and 2.5 mtorr CO. The presence of CO induced the formation of a (√19 × √19)R23.4° structure on the surface. (d) (100 Å ×100 Å) STM image showing two CO adsorbate rotational domains of (√19 × √19)R23.4°. Reproduced with permission from Ref. 20. Copyright 2004 American Chemical Society.



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Figure 5. Schematic showing the similarly high fractions of metal atoms at edges and corners in an industrial nanoparticle catalyst and a stepped surface of a single crystal model catalyst. (a) TEM image of Pt nanoparticles supported on Al2O3 of an industrial catalyst. (b) Schematic of a cuboctahedron with high fractions of atoms which are highly under-coordinated. (c) Surface of a Pt(557) stepped single crystal model catalyst. (d) Coordination numbers of atoms on the (111) terrace (purple atoms) and step edges (bright blue atoms). Reproduced with permission from Ref. 7. Copyright 2010 AAAS.


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Figure 6. Pressure-dependent restructuring of Pt(557). (a) Structural model of Pt(557); the Pt atoms at step edges are shown in bright blue and terrace atoms in dark blue. (b) STM image of Pt(557) in UHV (1×10-10 Torr). (c) STM image of Pt(557) in CO gas at a pressure of 5×10-8 Torr. (d) STM image of Pt(557) in CO gas at a pressure of 0.1-1.0 Torr at 25oC. (e) Enlargement of Pt(557) image of (d) consisting of triangular clusters in 0.1-1.0 Torr CO at 25oC; the red spots are oxygen atoms of CO molecules. (f) Structure of the restructured Pt(557) in 0.1-1.0 Torr CO; the structure was optimized through DFT calculations; the fan-out effect is clearly observed; the bright blue balls are Pt atoms of the triangular cluster formed through restructuring in CO at 0.11.0 Torr. (g) and (h) Enlargement of the edges of the formed triangular cluster; the oxygen atoms of the CO molecules adsorbed at the edges of the triangular clusters tilt to the side towards the open surface so that the repulsion of adjacent CO molecules can be reduced, by which the surface energy of restructured Pt(557) with 100% coverage of CO is decreased. Reproduced with permission from Ref. 7. Copyright 2010 AAAS.



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Figure 7. Reversible restructuring of Pt(557) in CO at a low pressure (5×10-8 Torr) (a) and in CO at a relatively high pressure (1 Torr) (b). The red arrows represent reversible restructuring, showing that the cluster covered surface of Pt(557) formed in CO at a relatively high pressure of 1 Torr will change back to the curvy step-surface similar to the original surface. Reproduced with permission from Ref. 7. Copyright 2010 AAAS.


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Figure 8. Surface structures of Cu(111) in CO gas at pressure ranges of 0.1-10 Torr. (A) Cu(111) in UHV; the bottom inset is one atomically resolved image. (B) Cu(111) in 0.1 Torr of CO; clusters formed at step edges. (C) Cu(111) in 0.2 Torr CO. (D) Cu(111) in CO at 10 Torr; a high density of clusters with adsorbed CO molecules (expanded in the inset) completely covers the surface; the bright spots, due to CO on top sites, form (2×2)-3CO and c(4×2) unit cells. Reproduced with permission from Ref. 8. Copyright 2016 AAAS.



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Surface Structures of Model Metal Catalysts in Reactant Gases - The

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