Atomic-Scale Structural Evolution of Rh(110) during Catalysis

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Atomic-scale structural evolution of Rh(110) during catalysis Luan Nguyen, Lacheng Liu, Solomon Assefa, Christopher Wolverton, William F. Schneider, and Franklin (Feng) Tao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02006 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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Atomic-scale structural evolution of Rh(110) during catalysis

Luan Nguyen¶§ǁ, Lacheng Liu¶§ǁ, Solomon Assefa ʃǁ, Christopher Wolverton†, William F. Schneiderʃ*, Franklin (Feng) Tao¶§*

¶

Department of Chemical and Engineering Petroleum, University of Kansas, Lawrence, KS, USA 66045 § Department of Chemistry, University of Kansas, Lawrence, KS, USA 66045 ʃ Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, USA 46556 † Department of Materials Science and Engineering, Northwestern University, Evanston, IL, USA 60208

ǁ These authors made equal contribution. * To whom all correspondence should be addressed to [email protected] and [email protected].

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Abstract We report direct observation at the atomic-scale of the pressure- and temperature-dependent evolution of a model Rh(110) catalyst surface during transient and steady-state CO oxidation, using high pressure scanning tunneling microscopy (HP-STM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) correlated against density functional theory (DFT) calculations. Rh(110) is susceptible to the well-known missing row (MR) reconstruction. O2 dosing produces a MR structure and an O coverage of ½ monolayer (ML), the latter limited by the kinetics of O2 dissociation. In contrast, CO dosing retains the (1×1) structure and a CO coverage of 1 ML. We show that CO dosing titrates O from the (2×1) structure and that the final surface state is a strong function of temperature. Adsorbed CO accelerates and O inhibits the (2×1) to (1×1) transition, an effect that can be traced to the influence of the adsorbates on the energy landscape for moving metal atoms from filled to empty rows. During simultaneous dosing of CO and O2, we observed steady-state CO oxidation as well as a transition to the (1×1) structure at temperatures more modest than in the titration experiments. This difference may reflect surface heating generated during CO oxidation. At more elevated temperatures the metallic surface transforms to a surface oxide, also active for CO oxidation. These results demonstrate how operando experiment exploration in terms of correlation between surface structure dominated by reaction conditions and activity of a catalytic material and first-principles models can be integrated to disentangle the underlying thermodynamic and kinetic factors that influence the dependence of catalytic activity on surface structure at nano and atomic scales.

Keywords: Operando, XPS, STM, density functional theory, Rhodium, CO oxidation, restructuring


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1. Introduction Technical heterogeneous catalysts are commonly comprised of supported metal or oxide particles that, as prepared and applied, are nanoparticles with a range of sizes, shapes, exposed facets, and even surface compositions.1-2 For a high-surface area catalyst, access to information about the arrangement of adsorbates and catalyst atoms on the surface during catalysis is extremely challenging.1-2 Even for a model catalyst with a well-defined surface structure, it is difficult to resolve the evolution of the surface structure at the atomic scale and at the conditions relevant to catalysis. Thus, single-crystals are commonly used as models of high surface area nanoparticle catalysts, in particular because their well-defined structures are amenable to analysis with surfacesensitive techniques. Direct observation of the catalytic surface under reaction conditions is the most direct way to correlate surface chemistry and structure with performance. It has become clear that a static picture of the metal surface is incomplete.3-8 Reactants can restructure surfaces,3-4, 6, 9-13 thus modifying performance or even altering mechanism.1-2 The equilibrium structure of an adsorbatecovered surface is a function of the chemical potential of the adsorbing gases and of system temperature and pressure, all of which could thus influence observed structure.14 In situ observations have demonstrated that pressure-driven increases in the chemical potential of CO can induce surface segregation in bimetallic catalysts,7 and nano-cluster formation on a flat Pt(100)hex surface,15 vicinal surface Pt(557)3 and Cu(111).6 Notably, all these transitions only occur at pressures beyond those accessible by conventional surface analytical techniques in UHV.3-4, 15-17 Surface restructuring of a metal catalyst is also sensitive to temperature. It is challenging to observe the temperature-dependence of surface structure when a catalyst is buried beneath a gas phase. Spectroscopic access to the solid-gas interface is limited, particularly when the catalyst is


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at a high temperature and gas phase at a relatively high pressure. Advances in in situ technology and instrumentation, particularly scanning tunneling microscopy (STM), have made it possible to access surface structures and chemistry under relatively high temperature and pressure conditions.5, 18-19 The (110) facet of face-centered cubic (fcc) metals is particularly interesting due to its susceptibility to surface reconstructions that impact surface reaction rates. A common transformation is the so-called “missing row” (MR) reconstruction that exposes (111) microfacets (Figure 1) and appears either spontaneously20 or in the presence of adsorbates21 on transition metals.6,

22-25

Density functional theory (DFT) and cluster expansion models ascribe these

spontaneous reconstructions to ordering interactions between adjacent filled and empty rows of metal atoms that are particular to the late 5d row of transition metals.26 While the late 3d and 4d metals do not spontaneously exhibit the MR reconstruction, adsorbates including hydrogen (H),2729

oxygen (O),30 nitrogen (N)31 and alkali metals29,

32

have all been observed to induce the

reconstruction.

Figure 1. Top and side views of bulk-terminated (1×1) and missing-row (1×2) structures of a (110) face-centered-cubic facet.


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Rhodium is representative of the 4d group. In UHV, an adsorbate-free Rh(110) facet exhibits the non-MR (1⨉1) ordering. Molecular oxygen dissociatively adsorbs above 100 K.33 Oxygen readily induces surface restructuring and formation of a variety of MR Rh(110) overlayer structures at temperatures between 200 and 500℃.34-35 Previous UHV studies showed that this MR Rh(1⨉2) surface (with and without adsorbates) is metastable and can revert back to a (1×1) structure upon heating to 153 - 216℃.34,

36-37

Exposure of Rh(110)-(1×1) and Rh(110)-(1×2)

surfaces to CO below room temperature is reported to produce a variety of CO overlayer structures at coverages up to 1 monolayer (ML).38 A mechanistic study of CO oxidation at 160 K on the c(2 × 8) surface structure reveals a reaction mechanism different than the one at room temperature.39 Here we report atomic scale observation of a Rh(110) surface under conditions relevant to catalytic CO oxidation using coordinated ambient-pressure X-ray photoelectron spectroscopy (AP-XPS),19 high-pressure scanning tunneling microscopy (HP-STM)18 and DFT computational analysis. We show that the observed surface structure is a function of temperature and adsorbate dosing conditions and is controlled by a combination of kinetic and thermodynamic factors. Oxygen dosed as atomic oxygen produces a kinetically controlled (1×2) surface structure that is preserved when O is titrated by excess CO at low temperature but evolves into a lower energy (1×1) structure at higher temperatures or during continuous dosing of CO and O2. Experiments and calculations show that adsorbed CO promotes and adsorbed O retards the (1×2) to (1×1) transition relative to the adsorbate-free surface. The results illustrate a system in which the catalytically relevant surface structure depends sensitively on the conditions under which the system is observed.

2. Experimental and Computational Methods 2.1 Experimental 5 ACS Paragon Plus Environment

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2.1.1 Sample Preparation A Rh(110) single crystal (9 mm in diameter and 1 mm thickness) purchased from MaTeck GmbH was used for in-situ HP-STM and AP-XPS experiments. The Rh(110) surface was cleaned using well-established literature protocols.40-41 In brief, the Rh(110) single crystal was first sputtered in Ar at 3 ⨉ 10-8 Torr (emission current of 10 mA, and ion energy of 1 keV) for 20 minutes. The sample was then annealed at 827 ℃ in UHV for 10 minutes followed by oxidation at 527 ℃ in 2 ⨉ 10-7 Torr O2 for 5 minutes and reduction at 527 ℃ in 2 ⨉ 10-7 Torr H2 for 10 minutes. This cleaning cycle was repeated many times until a clean surface was obtained. The cleanliness of the surface was checked by STM and XPS.

2.1.2 HP-STM HP-STM was used to monitor surface structure during exposure to CO, O2, and gas mixtures. Instrumental details have been reported previously.18 The integration of a STM head with a reaction cell distinguishes this HP-STM from other UHV STM systems. The reaction cell volume is about 15 ml. In a typical experiment, the sample is inserted into the reaction cell and gases flowed through the cell and over the sample. The effluent gas is analyzed with a quadrupole mass spectrometer. The sample is heated with a laser source located between the reaction cell and STM chamber. Temperature was recorded using a K-type thermocouple spot-welded to the back of the sample. This combination of instruments allows surface structure to be visualized and reaction rates to be measured simultaneously.

2.1.3 AP-XPS AP-XPS was used to monitor sample surface chemistry during exposure to CO, O2, and gas mixtures using a flow reaction cell. The AP-XPS is equipped with a monochromatic Al Kα source 6 ACS Paragon Plus Environment

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(1486.74 eV). The sample was irradiated by Al Kα X-rays through a Si3N4 window mounted on the reaction cell. The excited photoelectrons were collected and analyzed with a differentially pumped hemispherical energy analyzer. The cell can be controllably heated and temperature was monitored with a K-type thermocouple. Spectra were collected in the presence of flowing gases. Based on literature42 the saturation coverage of CO on Rh(1×1) is 1.0 monolayer (ML), which is defined as one CO molecule for each surface Rh atom. We exposed a cleaned Rh(110)(1⨉1) crystal to 8×10-8 Torr CO for 4 minutes followed by vacuum purge to UHV and used a previously reported calibration42 to determine CO coverages at different pressures and temperatures. The ratio of the CO O1s to Rh3d XPS peak areas measured at the same condition as reference42 was used as a constant factor, pressure of CO was calculated by (

=

. The absolute coverage of CO (θCO) at other

)=

×

, where

, and

are integrated XPS

peak areas of O1s of the chemisorbed CO and Rh3d in gas with CO at partial pressure of CO. Spectra were deconvoluted with CasaXPS. A linear background was subtracted from all Rh 3d and O 1s XPS spectra followed by fitting using an asymmetric Doniac-Sunjic function for Rh 3d.

2.2 Computational DFT energies were computed using the Vienna ab-initio Simulation Package (VASP) version 5.2.12.43-46 The projector augmented wave (PAW)47-48 and PW91 generalized gradient approximation (GGA)49-50 were used to describe core electron states and electron exchange and correlation, respectively. Plane waves were included to a 400 eV cutoff. Electronic energies were converged to 1 × 10-5 eV and ionic positions relaxed until forces were less than 3 × 10-2 eV Å-1. Slab models included eight Rh layers and 13 Å of vacuum unless indicated otherwise. Supercells were orthorhombic, lattice constants were set to the computed bulk value of 3.8438 Å, the bottom 7 ACS Paragon Plus Environment

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two metal layers frozen at the bulk positions, and the first Brillouin zone sampled using 16/n kpoints in each lateral direction, where n is the number of Rh atoms in each direction.

3. Results and Discussion 3.1 Rh(110) under UHV Adsorbate-free Rh(110) was prepared as described above. A (1×2) surface structure can be prepared and maintained at room temperature40 but has been found to convert irreversibly to (1×1) upon heating to approximately 150oC in UHV.37, 51 Observed atom-resolved STM images are shown in Figures 2b-d. Consistent with prior observations,40 the annealed surface shows characteristic (110) rows separated by 3.8 Å in the [001] direction, consistent with a (1×1)

Figure 2: UHV constant current STM images of Rh(110)-(1×1) ((b) through (d)) and Rh(110)-(1×2)-O ((f) through (h)) and their respective structural models: ((a) and (e)). (a) Model of Rh(110)-(1×1). (b-d) STM images of Rh(110)-(1×1) at different scale; ((b)=20 × 20 nm, (c) = 5 × 5 nm, (d) = 1.2 × 1.2 nm). (e) Model of Rh(110)(1×2)-O. Green rectangle represents exposed fcc(111) micro-facet. (f-h) STM images of Rh(110)-(1×2)-O of varying resolution; ((f) = 50 × 50 nm, (g) = 30 × 25 nm, (h) = 3 × 3 nm).


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ordering. Consistent with these results, we compute the (1×1) termination to be 2.6 meV Å–2 (0.042 J m–2) lower in energy than the (1×2) within the GGA approximation.

3.2 Oxygen adsorption on Rh(110) We exposed the (1×1) Rh(110) sample to 2 ⨉ 10-8 Torr O2 at 500 ℃ for 5 minutes, a condition previously reported to produce a MR reconstruction and 0.5 ML of adsorbed O.40 After calibration as described in the Experimental section, XPS analysis confirmed the presence of 0.5 ML O. No oxide layer was formed as Rh 3d signal shows metallic state. Figure 2f-2h show representative STM images. As shown in Figure 2h, the inter-row spacing along [001] is doubled to 7.6 Å, consistent with a MR structure. Adsorbed oxygen atoms appear as depressions adjacent to the bright Rh rows and occupy alternating 3-fold sites with p2mg symmetry as evidenced by zig-zig shaped dark contrasts. Additional (1⨉n) structures can be obtained by changing the oxygen coverage or annealing temperature.35, 51-52 To explore the site preferences and energetics of O adsorption on the (1×1) and (1×2) surfaces, we used DFT models to compute the potential energy surfaces for adsorbed O, shown in Figures 3a and b, respectively. After relaxing the (1×1) and (1×2) slabs, we rastered a single O adsorbate along a 11 × 3 grid and allowed it to relax along the surface normal while keeping all metal atoms fixed. Results are reported as an overlaid color map referenced to the clean surface and ½

. In general, O adsorbates avoid atop sites and prefer the bridge regions along the [110]

rows. The O potential energy surface (PES) is relatively flat on the (1×1) surface. In contrast, the energy landscape is strongly corrugated on the (1×2) surface; O adsorbates avoid the MR “valleys” and strongly prefer three-fold sites within the microfacets exposed by the MR reconstruction.


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Figure 3. Binding energy contour surface for O ((a) and (b)) and CO adsorption ((c) and (d)) on Rh(110)-(1 × 1) and Rh(110)-(1 × 2) surfaces. Dashed boxes represent the super cell used while the raster area is (1/4) of the area. We used these computational results to guide the construction of higher-coverage O configurations incorporating both bridge and three-fold sites on the (1×1) and (1×2) surfaces. Calculations were performed in 2×2 supercells and included nominal coverages from ¼ to 1 ML. Results are summarized in Figure 4a, reported as formation energies referenced to gas-phase O2 and Rh bulk:

where

is the DFT energy of a slab with nads adsorbates, A is the slab surface area,

the O2 reference energy, a = ½, and

is

is the surface energy of a bulk-terminated slab. This

construction returns the relaxed surface energy at zero coverage plus the changes to this surface energy with adsorption. Results for the (1×1) surface in Figure 4 are reported as diamonds and the lowest-energy configurations at each coverage connected by a dashed line. At ¼ and ½ ML we find O to prefer 10 ACS Paragon Plus Environment

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to adsorb at bridge sites ordered such that no two O share the same Rh. At ¾ ML Rh-sharing becomes unavoidable, and O begin to occupy bridge sites on alternating sides of the [110] row. At 1 ML, all O occupy these alternating bridge sites. Corresponding results for the (1×2) surface are shown as filled circles and lowest-energy configurations connected by a solid line. Within the MR, O continues to prefer to occupy sites along the [110] rows, first filling bridge sites at ¼ ML and then occupying the three-fold sites in an alternating zig-zag O pattern. At 1 ML we find a structure in which all these sites are occupied to be lowest in energy. Figure 4a shows that the slight energetic preference for the (1×1) over the (1×2) structure at zero ML disappears at ¼ ML; the small energy penalty for creating the MR is just offset by the increased O binding energy. At ½ ML this difference becomes the most pronounced. The lowenergy (2⨉1)-O structure is identical to that reported previously40 and observed in the STM images above. One might imagine that the experimentally observed coverage, ½ ML O could be related to a minimum in the surface energy The (1×1) and (1×2) surface energies continue to decrease at coverages beyond ½ ML, albeit at a slightly reduced rate. The slopes of these formation energies determine the differential adsorption energies. These results are reported in Figure 4b. Adsorption energies are computed to be > 1 eV/adsorbate exothermic up to 1 ML O. We conclude that oxygen adsorption does favor the MR over non-MR at ¼ ML O and above, and the experimentally observed ½ ML-O coverage obtained following O2 dosing is determined by the kinetics of O2 dissociation rather than the coverage-dependent energetics of O adsorption. Similar kinetic coverage control has been demonstrated previously for O2 dosing to Pt(111).53


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Figure 4. (a) Various O overlayer formation energies on (1×1) (filled circles) and (1×2) (fill diamonds) surface structures vs. coverage. Dotted and solid lines indicate corresponding energy hulls. Insets illustrate lowest-energy configurations at each coverage, vertically ordered by energy. (b) Differential O binding energies relative to ½ O2 on Rh(1×1) (dotted line) and Rh(1 × 2) (solid line).


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3.3 CO adsorption on Rh(110) As shown in Figure 5b, exposure of a prepared clean Rh(110)-1×1 to a CO pressure of either 8⨉108

or 0.08 Torr at room temperature yields the same total CO coverage, as determined by the method

described in the Experimental section. Figure 5d shows the observed O 1s spectrum for Rh(110) in CO at 8⨉10-8 Torr. A primary peak at 531.0 eV corresponds to bridge-bound CO and accounts for more than 90% of total CO; a minority feature at 532.1 eV corresponds to lower-coordination, atop-bound CO.38, 54 These relative proportions are independent of total CO pressure. To probe the structure of the CO-covered state, we performed HP-STM on a surface exposed to 0.08 Torr CO. In Figure 5a, the Rh atoms were imaged as bright spots while adsorbed CO molecules were imaged as depression. As shown in Figure 5a, the surface retains the (1×1) structure.


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Figure 5. In-situ AP-XPS and HP-STM studies of Rh(110)-(1 × 1) under 8 × 10-8 to 0.08 Torr CO pressure at 25°C. (a) STM image of Rh(110)-(1 × 1) under PCO = 0.08 Torr and 25°C. The black areas could result from Rh vacancies formed during preparation or oxygen atoms chemisorbed on surface since annealing in oxygen is one of the steps of cleaning Rh(110). (b) CO coverage as a function of pressure calculated through analysis of O 1s and Rh 3d photoemission intensity of APXPS. (c) Candidate adsorption sites from deconvoluted AP-XPS spectra (red = atop, blue = pseudo three-fold). (d) O 1s photoemission feature of adsorbed CO on Rh(110)-(1 × 1) at PCO = 0.08 Torr. (e) Representative model of Rh(110)-(1 × 1) with CO adsorbate occupying pseudo 3-fold sites. The CO adsorption results (Figure 5) stand in contrast to the effect of atomic oxygen (Figure 2f-2h) on surface structure. To understand the difference, we again turn to DFT. Figures 3c and d show the computed energy landscapes for a CO rastered above the (1×1) and (1×2) surfaces such that the x and y coordinates of C are constrained and all other CO coordinates relaxed. As with O, CO prefers sites along the [110] rows on the (1×1) surface; the energy is minimized at a 3-fold site, but bridge and atop sites are < 0.3 eV higher in energy. Regions between the rows are relatively higher in energy, but only by < 0.5 eV. In general, CO exhibits weaker site preferences than does O. The same general observations hold for the (1×2) surface; the low-energy three-fold regions on either side of the [110] rows increase in size and drop in energy and the valleys between rows repel CO, but total energy differences between these regions remain ~ 0.5 eV. Further, the differences between the (1×1) over the (1×2) surfaces is less pronounced for CO than O. Figure 6(a) and (b) show DFT-computed surface formation energies as a function of CO coverage. The results of the potential energy scans were used to guide the selection of candidate adsorption configurations; lowest-energy configurations at each coverage are shown schematically in the figure. On the (1×1) surface, CO slightly prefer atop sites over bridge at the lowest coverage but move to three-fold-like sites on alternating sides of alternating [110] rows by ½ ML, forming a zigzag pattern. At 1 ML, the same zigzag CO arrangement is repeated on all [110] rows. A


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similar zigzag-like pattern is computed at ½ ML on the (2⨉1) surface, and at higher coverages CO begin to populate sites within the vacant [110] rows. As shown in the figure, CO binding energies on the (1×1) surface are nearly coverage-independent. While the (1×2) is slightly preferred over the (1×1) surface structure at ½ ML, the (1×1) is clearly favored at 1 ML. The final zigzag like arrangement of three-fold CO at 1 ML is consistent with the experimentally observed HP-STM (Figure 5a) and AP-XPS (Figure 5d) observations and the retention of the (1×1) structure in the presence of CO. Minority atop CO observed in the AP-XPS is likely attributed to defects on the (1×1) surface.

3.4 Rh(110)-(2×1)-O in CO To probe the effect of CO exposure on the MR (1×2) structure, we began with a Rh(110) sample prepared initially in the ½ ML Rh(1×1)-O structure, Rh(110)-(1×2)-O. This surface was exposed to CO and changes monitored using the HP-STM and AP-XPS. Figure 7a shows a sequence of HP-STM images taken after exposure of the O-precovered surface to 8×10-8 Torr at 25˚C. Blue and pink regions in the images represent areas of adsorbed O and CO, respectively. The O fraction decreases and CO fraction increases with time. The surface is almost completely CO-covered within 31 minutes (Figure 7a6). Evident as well in the HP-STM is that the surface maintains the original (1⨉2) structure. An ex-situ study by Leibsle et al55 observed similar etching of c(2 × 6) and (1 × 2) orderings of O on Rh(110) by impinging CO molecules; notably, they were unable to discriminate between bare and CO-covered Rh(110) as the final reacted surface state. In our experiments, the surface was continuously exposed to CO during STM image collection and APXPS interrogation. Therefore, it is reasonable for the reacted surface to be covered with CO molecules. In addition, the CO coverage (Figure 7b) and the O1s photoemission feature from 15 ACS Paragon Plus Environment

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chemisorbed CO (Figure 7b) monitored with AP-XPS confirmed the loss of adsorbed oxygen atoms (529.9 eV) and the formation of a CO adlayer with a final total CO coverage of 0.85 ML. The O 1s binding energies of the two photoemission peaks in Figure 7c correspond to CO molecules chemisorbed on Rh surface. The observation of two new O1s photoemission peaks in Figure 7c clearly shows the replacement of chemisorbed oxygen atoms by CO molecules. Deconvolution of the 531.0 and 532.1 eV features in the XPS yields 0.5 ML 3-fold CO and 0.35 ML atop CO. Upon increasing the CO pressure to 0.08 Torr, the total CO coverage increases to 0.95 ML including 0.45 ML bridge and 0.5 ML atop CO; the coverage of CO at 0.08 Torr is similar to that on CO at 8×10-8 Torr as shown in Figure 7b. Mass spectrometry confirms the production of CO2 during the O etching by CO in CO gas. Isotope labeling experiment further confirmed that CO2 is produced by reaction of CO with surface oxygen atoms. An 18O-labelled adsorbate layer, Rh(110)(1×2)-18O was pre-prepared by annealing Rh(110) in

18

O2 gas before exposure to C16O. Mass

spectrometry detected C16O18O based on the observation of signal at m/e=46 upon the isotopelabelled catalyst, Rh(110)-(1×2)-18O was exposed to C16O. HP-STM (data not shown) confirmed that the (1⨉2) ordering is retained at 0.08 Torr CO at 25℃. As shown in Figure 7a, the (1×2) structure remains even when chemisorbed oxygen is completely replaced with CO molecules. The formation of the Rh(110)-(1⨉2)-CO after the etching of surface oxygen atoms with CO is consistent with the computational prediction for a 1 ML CO on a (1×2) constrained surface which possess a 50:50 mixture of three-fold CO along the ridges and atop CO in the valleys (Figure 7d). Based on the computational results, Rh(110)-(2⨉1)-CO is metastable to the (1×1) CO-covered surface. We infer that kinetic factors limit the transformation of Rh(110)- (1×2)-CO to Rh(110)- (1×1)-CO. 16 ACS Paragon Plus Environment

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Figure 6. (a) Various CO overlayer formation energies on (1×1) (filled circles) and (1×2) (fill diamonds) surface structures vs. coverage. Dotted and solid lines indicate corresponding energy hulls. Insets illustrate lowest-energy configurations at each coverage, vertically ordered by energy. (b) Differential CO binding energies on Rh(1×1) (dotted line) and Rh(1 × 2) (solid line).


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Figure 7. In-situ observation of CO adsorption on Rh(110)-(1×2)-O at 25°C and PCO = 8 × 10-8 Torr. (a) Sequential STM images of the same area with color contrast added; blue and pink represent adsorbed O and CO species respectively. (b) O and CO coverage calculated from the quantitative analysis of O 1s and Rh 3d photoemission intensities. (c) O 1s photoemission of Rh(1×2)-O surface at UHV and PCO = 8 × 10-8 Torr (c1 and c2 respectively). (d) Adsorption models depicting (1×2)-O replacement with CO on multiple adsorption sites.

To test this prediction, we performed CO dosing experiments to Rh(110)-(2×1)-CO at 55˚C to explore whether the kinetic barriers to recovery of the (1×1) surface could be overcome. Figure 8 shows the time evolution of a Rh(110)-(1×2)-O surface at 55ºC exposed to CO of 0.08 Torr. The loss of the (1×2) regions and emergence of the (1×1) areas at 55oC are evident within the first two minutes. Transformations at the step edges are clearly evidenced by the decrease in inter-row spacing from 7.6 to 3.8 Å (Figure 8c2). As marked with blue ovals in Figures 8a2, 8b2, and 8c2 the 1⨉2 transformation to 1⨉1 is further increased in four minutes. These results suggest that kinetic limitations to the (1×2) to (1×1) transition are overcome at 55˚C in CO gas though this transition is impossible in UHV at this temperature in literature.41 37 Note that CO molecules are 18 ACS Paragon Plus Environment

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still chemisorbed on the surface at 55oC in CO gas, as evidenced by AP-XPS data shown in the following section.

Figure 8. Time lapse in-situ HP-STM images of the same area at PCO = 0.08 Torr and T = 55°C. (a1), (b1) and (c1) represent STM images of the same area in 2 minutes intervals. (a2), (b2) and (c2) represent zoomed in images of area marked by blue ovals. The inter-row distances in the [001] direction on two distinct regions of the sample are marked in (c2).


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Figure 9. Tracked surface structure at atom scale with a couple of minutes for understanding mechanism of restructuring of Rh(110)-(1×2) to Rh(110)-(1×1) in gas of CO at 0.08 Torr at 55°C. (a1) Image collected at t0. (a2) Image collected at t0 + 2 minutes. (a3) Image collected at t0 +4 minutes. In (a1), (a2), and (a3), the triangles are the landmark of the same area. (b1) Highresolution image of one area of Rh(110) CO at 0.08 Torr at 55°C collected at t0; (b2) Highresolution image of one area of Rh(110) CO at 0.08 Torr at 55°C collected at t0+2 min; (b1) and (b2) show the transition of ridge. (c1) High-resolution image of one area of Rh(110) CO at 0.08 Torr at 55°C collected at t0, which has one added Rh adatom; (c2) High-resolution image of one area of Rh(110) CO at 0.08 Torr at 55°C collected at t0 +2 min; (c1) and (c2) show the spillover of Rh adatom.


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Careful analysis of the images in Figure 8a1 uncovers three types of surface restructuring events. As shown in Figures 9a1 and 9a2, breaking one row of ridge Rh atoms and transiting one atom to the valley between two rows of ridge was observed in areas marked with “1” in the first two minutes. A dramatic change was observed in the second two minutes (Figure 9a3), as more Rh atoms are translated to the valley and thus new short rows of Rh atoms are formed between two original rows of ridges. Figure 9a3 can be considered as a transition surface phase in the formation of Rh(110)-(1⨉1) from Rh(110)-(1⨉2). Another type of restructuring performed under this condition is the translation of short segment of a row of Rh atoms to valley area. The insets of Figures 9b1 and 9b2 schematically illustrate the local restructuring. In Figure 9b1, two blue arrows mark the two adjacent rows of ridges; a black box is marked on row 2; this black box marks a normal row of ridge of Rh(100)-(1⨉2). Atoms in the black box translate to the valley between rows 1 and 2. In two minutes, a bright region immediately next to row 1 is observed in Figure 9b2; the empty space left upon this transition appears as a dark area of row 2 marked with a dashed black box (Figure 9b2). In addition to these two restructurings, spillover of single Rh atom in Figure 9c1 and 9c2 was observed. The spot with highest contrast marked with a green circle in Figure 9c1 is a Rh adatom above a row of four Rh atoms marked with a blue box. The Rh adatom formed from spilling over from the position above the segment of four Rh atoms (marked with a blue box in Figure 9c) was added to the end of the row of four atoms along [1-10] direction to make a segment of five atoms marked with a blue box in Figure 9c2. This spillover is evidenced by both the decrease of contrast of the spot marked with the green circle and the increase of the length of the segment marked with a blue box in Figure 9c2.


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Figure 10. In-situ AP-XPS studies of Rh(1 × 2)-O as a function of temperature at PCO = 0.08 Torr. (a) Calculated surface coverage for CO and O adsorbates. (b) and (c) C 1s and O 1s photoemission spectra of adsorbates stacked vertically as a function of temperature. Scatter points and black line at the bottom of (b) and (c) indicate respective spectra at UHV conditions. The surface chemistry of a model Rh(110)-(1×2) catalyst in 0.08 Torr CO was further monitored at temperatures up to 200℃ using AP-XPS. As shown in Figure 10a, the CO coverage on Rh(110)(1×2) in 0.08 Torr CO at 25ºC is 0.95 ML and about equal number of adsorbed CO molecules are bound on atop and bridge sites. A maximum CO coverage of about 1 ML was measured at 55ºC in the AP-XPS. The high coverage suggests a significant restructuring of the Rh(110)-(1×2) surface at 55oC in CO. Notably, the coverage of CO in bridge-bound configuration accounts for this increase of the overall coverage. This is consistent with the preferential bridge binding of CO on Rh(110)-(1×1) predicted by DFT (Figure 3). The coverage of both atop and 3-fold CO on Rh(110)-(1⨉2) in 0.08 Torr CO decreases slightly as temperature is raised from 55˚C to 100ºC. The CO coverage decreases more dramatically between 120 and 200oC, and at 200oC the adsorbed 22 ACS Paragon Plus Environment

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CO is completely replaced by adsorbed carbon (Figures 10a and 10b). While this carbon could be assigned to hydrocarbon, CO dissociation is a more likely source as no hydrocarbon was introduced into the system. CO has been shown to dissociate at low coordinate step edge sites on Rh(110).56 The kinetics for the transformation of adsorbate-free (1×2) Rh(110) to (1×1) under UHV have been studied with STM36 and synchrotron-based XPS.37 Sequential STM imaging revealed the loss of the (1×2) and appearance of the (1×1) structure to begin at about 105℃ and to be completed by 132℃.36 XPS measurements of the rate of restructuring between 153oC and 200℃ showed an activation barrier for the transformation to be 91.6 kJ/mol.41 Based on this reported activation barrier, the rate constant k for this transformation can be expressed as /

=

. Taking the prefactor to be approximately constant over the range 55oC-153oC, the

unitless ratio of the rate constants at 55oC to 153oC is ratio of rate constants at 55oC to 153oC, and

=

=

. By introducing

is calculated to be 2.6×10-4.

is equal to

, the where

are formation rate of (1×1) at 55℃ and 153℃, respectively. For the Rh(110)-(1⨉2)

surface at 153oC in UHV, the

is 0.35 ML within 1500 seconds. With this, the portion of the

formed (1⨉1) to be transformed to (1×2) at 55oC within 1500 seconds is estimated to be ( × )(

℃)

= 0.35 × 2.6 × 10

(

) = 9.1 × 10

.

Here

(

) ( × )

is

defined to be the fraction of the areas of clean (1⨉1) patches formed from clean (1⨉2) at 55oC in UHV. Furthermore, the rate of the transformation of surface phase from clean (1⨉2) to clean (1⨉1) at 55oC in UHV is 6 × 10−8 ML per second (=

. ×

).

Based on this rate derived from literature kinetics,37 the portion of Rh(110)-(1⨉1) at 55oC in UHV within ten minutes should be only 3.6 ⨉ 10-5 ML, too small to be observed. However, a 23 ACS Paragon Plus Environment

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restructuring from Rh(110)(1⨉2) to Rh(110)(1⨉1) at 55oC in 0.08 Torr CO was clearly resolved in our experiments (Figures 8 and 9). Within ten minutes, approximately 40% of surface is transformed to Rh(110)-1(⨉1). The rate of the observed restructuring at 55ºC, calculated by dividing the fraction of the formed Rh(110)-(1⨉1) by time of this transition, is about 6.7×10-4 ML per second in CO gas, which is much larger than the average rate of 6×10-8 ML per second in UHV. Thus, CO “catalyzes” the transformation of surface phase (1⨉2) to (1⨉1).

Figure 11. DFT calculated relative energies of intermediate structures for (1 × 2) to (1 × 1) transformation of a clean (green), CO (blue) and O (red) covered surfaces. Red boxes depict the super-cell used in the calculations. We again turn to DFT to contrast the energetics of surface reorganization of clean Rh(110)1×2 (green line in Figure 11), presence of O adsorbates (red line in Figure 11), presence of CO 24 ACS Paragon Plus Environment

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adsorbates (blue line in Figure 11). Motivated by the experimental observations, we considered three steps for each of the three surfaces: the breaking of a [110] row by displacement of a single atom to an adjacent valley, further growth of the new row at the expense of an adjacent broken row, and finally completion of the newly translated row which thus creates a step defect on the (110) facet. The results for the adsorbate-free surface are shown in green in Figure 11. The first step to break two Rh-Rh bonds along the [110] chain and create an adjacent lone atom in the MR costs approximately 1 eV/site, a value similar to the apparent activation energy for adsorbate-free re-organization in UHV inferred from reference;37 additional displacements involve modest energy changes; the final (110)-1⨉1 plus step structure is slightly lower in energy than the initial MR structure, consistent with the results of Figure 4. For Rh(110)-1×2, incorporation of O into the displacing row pushes the energy landscape significantly upward, reflecting the strong preference to retain the uninterrupted zigzag O structure (red line in Figure 11); further, the final translated row structure is 0.5 eV/site higher in energy than the starting structure. In contrast, for Rh(110)1×2-CO (blue line in Figure 11), the adsorbed CO lowers the energy cost to displace Rh atoms; this difference can be attributed to the weaker adsorption of CO than atomic O and the small energy cost to move CO from three-fold to atop sites; in fact, in the presence of CO, the displacement landscape becomes nearly flat (blue lines in Figure 11), consistent with the observed accelerating effect of adsorbed CO on the (2⨉1) to (1⨉1) transition in Figures 8 and 9.

3.5 CO oxidation on Rh(110) Surface chemistry and structure of Rh(110)-(1⨉2)-O during CO oxidation in a mixture of 0.08 Torr CO and 0.02 Torr O2 in the temperature range of 25oC-200oC were tracked with HPSTM and AP-XPS. As shown in Figure 12, the catalyst surface is largely restructured from (1⨉2) 25 ACS Paragon Plus Environment

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to (1⨉1) in a mixture of 0.08 Torr CO and 0.02 Torr O2 at 25oC although Rh(1⨉2)-CO does not restructure in pure CO under pressure of 0.08 Torr at 25℃ as analyzed in Section 3.4. Figures 12a and 12b present STM images of the same area of (1⨉2) collected at to and to+25 minutes. The restructuring was confirmed by the measured inter-row distance of 3.8 Å. Within 25 minutes, approximately 40% of the surface is restructured to (1⨉1). These simultaneous measurements of partial pressure of CO2 in the reaction cell using the on-line mass spectrometer installed on our HP-STM system18, 57-58 confirmed that Rh(110) was catalyzing CO oxidation to form CO2 at 25oC.

Figure 12. In-situ STM images of Rh(1 × 2)-O in a gas environment of mixture of CO and O2 at Ptot = 0.1 Torr in a 4:1 (CO:O2) mixture and T = 25°C. (a) STM image at t = to with white box representing largely unreconstructed Rh(110)-(1 × 2)-CO surface with 7.6 Å inter-row distance. (b) Time lapse image of the same region marked in (a) at t = to+25 minutes. (c) Enlarged image of region in blue box in (b). Green arrows mark landmarks used to identify the same region in the STM image taken at different time. As shown in Figure 13a, the CO coverage in the mixture of 0.08 Torr CO and 0.02 Torr O2 is slightly higher than Rh(110)-(1⨉2) in pure CO at 25oC (Figure 10b). Notably, as analyzed in Section 3.4, pure CO at 0.08 Torr alone does not induce the (1⨉2) to (1⨉1) transition at 25oC although it does at 55oC, but the mixture of 0.02 Torr O2 and 0.08 Torr CO does it at 25oC. Thus, we hypothesized that the restructuring is driven by catalytic CO oxidation. CO oxidation is 26 ACS Paragon Plus Environment

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exothermic, and the released thermal energy could locally heat the surface. As demonstrated in sections 3.4, a temperature increase from 25oC to 55oC in 0.08 Torr CO gave rise to an obvious restructuring from Rh(110)-(1⨉2) to Rh(110)-(1⨉1). If we assume that the energy released from CO oxidation is dissipated into the Rh metal, a 30oC increase of temperature for a volume made of a surface area of 1 nm2 with a depth of 10 nm needs energy of 8.96×10-19 Joules according to the heat capacity of Rh59. Based on the turn-over rate reported in literature,60-61 0.02 CO2 molecules per nm2 per second, there are about 30 reaction events on 1 nm2 over 25 minutes which produces 30 CO2 molecules. The 30 reaction events release 1.4×10-17 Joules based on the calculated ΔH of the CO oxidation, -283 kJ/mol (using standard enthalpy of formation for gas phase CO, O2 and CO259). This energy released from the CO oxidation on each nm2 surface of our model catalyst is about 15 times more than the heat needed to heat up Rh(110)-(1⨉2)-CO from 25 to 55oC where a restructuring from Rh(110)-(1⨉2)-CO to Rh(110)-(1×1)-CO was observed (Figure 10). Therefore, from the energy release point of view, the observed restructuring could be driven by local heating of heat released from CO oxidation. Alternatively, the heating of Rh crystal can be understood from the heat released from dissociation of molecular O2 on Rh(110)-(1×2). As calculated in Figure 4, dissociation of ½ O2 to atomic oxygen releases 1.4 eV, 2.24×10-19 J. Based on the reaction rate, 0.02 CO2 molecules per nm2 per second reported,60 there are about 30 reaction events on 1 nm2 over 25 minutes which dissociates 15 O2 and thus releases 6.72×10-18 J; this energy is more than 8.96×10-19 J needed to heat the Rh single crystal from 25oC to 55oC.


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Figure 13. In-situ AP-XPS studies of CO oxidation on Rh(110)-(1 × 2)-O as a function of temperature at Ptot = 0.1 Torr for a 4:1 (CO:O2) mixture. (a) Calculated surface coverage for CO and O adsorbates. (b) and (c) C 1s and O 1s photoemission spectra of adsorbates stacked vertically as a function of temperature. Scatter points and black line at the bottom of (b) and (c) indicate respective spectra at UHV condition. Under the same catalytic condition as HP-STM, AP-XPS was used to track the adsorbate layer while the partial pressure of CO2 was simultaneously measured with a mass spectrometer. As shown in Figures 13a and 13b, the catalyst surface at 25oC is covered with CO molecules while it is in gas environment of a mixture of 0.08 Torr CO and 0.02 Torr O2 . CO coverage decreases with increasing reaction temperature. By comparing to the coverage of CO on nominal Rh(110)(1⨉2) in pure CO of 0.08 Torr at 70-130oC (Figure 10a), the coverage of CO on Rh(110) during CO oxidation in the same temperature range (Figure 13a) is obviously lower. This is because certain portion of surface sites are used for catalytic events when the catalyst is in the mixture of 28 ACS Paragon Plus Environment

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CO and O2 at 70-130oC. In particular, the decrease in CO coverage is due to a decrease in on-top and bridge site CO while coverage of atop CO remains relatively constant. Notably, AP-XPS studies of CO oxidation at 200℃ shows formation of Rh oxide even though the reaction condition is CO rich. As marked with red line in Figure 13c, the O 1s photoemission feature of the Rh surface oxide (529.4 eV in Figure 13c) is different from chemisorbed oxygen atoms (530.0 eV) of Rh(110)-1×2-O as its binding energy is 0.60 eV lower. Thus, the active surface at 200oC is in fact surface rhodium oxide instead of metallic Rh observed at low temperature 50-130oC.

Figure 14. Schematic summary of Rh(110)-(1 × 1) and (1 × 2) transformations as a function of total pressure, temperature, and transient (single pass) vs. continuous catalytic reaction conditions.


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4. Conclusions Surface chemistry and structure of Rh(110)-(1×2) in gas of CO at different pressure and different temperature and during CO oxidation at different temperature were studied with AP-XPS, HP-STM, and DFT calculations. We find here that the surface structure is sensitive to CO coverage and pressure, temperature, and whether gas molecules are introduced to surface in a transient, single-pass way or continuously. Sufficiently mild conditions leave the surface in a meta-stable state, while more aggressive conditions representative of catalytic turnover cause the surface to evolve towards lower-energy surface structures, as summarized in Figure 14. In-situ monitoring during catalytic CO oxidation revealed that the initial Rh(110)-(1⨉2) surface restructured to (1⨉1) at 25℃. This phase transformation observed during catalysis results from the heat released from exothermic CO oxidation. The active phase for catalytic CO oxidation evolves from metallic Rh(110) at low temperature (25-130oC) to surface rhodium oxide at temperatures >130oC.

Acknowledgments This work was supported by the National Science Foundation under NSF-CBET-1264798 and CBET-1264963. F.T acknowledges the partial support for L.C.L. from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under Grant No. DE-SC0014561. WFS and SA acknowledge computational support from the Notre Dame Center for Research Computing.


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ACS Catalysis

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Atomic-Scale Structural Evolution of Rh(110) during Catalysis

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