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Enhanced Stability of Pt-Cu Single Atom Alloy Catalysts: In Situ Characterization of the Pt/Cu(111) Surface in an Ambient Pressure of CO Juan Pablo Simonovis, Adrian Hunt, Robert M. Palomino, Sanjaya D. Senanayake, and Iradwikanari Waluyo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00078 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Enhanced Stability of Pt-Cu Single Atom Alloy Catalysts: In Situ Characterization of the Pt/Cu(111) Surface in an Ambient Pressure of CO

Juan Pablo Simonovis,1 Adrian Hunt,1 Robert M. Palomino,2 Sanjaya D. Senanayake,2 Iradwikanari Waluyo1*

1

National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, NY 11973 2

Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973

*Corresponding author: [email protected]

Abstract The interaction between a catalyst and reactants often induce changes in the surface structure and composition of the catalyst, which, in turn, affect its reactivity. Therefore, it is important to study such changes using in situ techniques under well-controlled conditions. We have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to study the surface stability of a Pt/Cu(111) single atom alloy (SAA) in an ambient pressure of CO. By directly probing the Pt atoms, we found that CO causes a slight surface segregation of Pt atoms at room temperature. In addition, while the Pt/Cu(111) surface demonstrates poor thermal stability in UHV, where surface Pt starts to diffuse to the subsurface layer above 400 K, the presence of adsorbed CO enhances the thermal stability of surface Pt atoms. However, we also found that temperatures above 450 K cause a restructuring of the subsurface layer, which consequently strengthens the CO binding to the surface Pt sites, likely due to the presence of neighboring subsurface Pt atoms.

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Introduction The focus on single atom-catalyzed reactivity is becoming important for numerous conversion processes, and such sites may be the essential ingredients for activating challenging reactants.1-3 However, the true nature of such monodispersed single metal atom active sites and our ability to exploit their unique properties in catalytic pathways remains poorly understood. Careful characterization of such moieties under reaction conditions is necessary to elucidate the fundamental steps that can couple the role of active sites with mechanistic steps essential for chemistry. In addition, bimetallic alloys have long been known to display unique activity, selectivity, and stability compared to their monometallic components due to ligand/electronic and geometric/strain effects.4 The interaction between two metal atoms have been reported to yield enhanced catalytic properties.5 Understanding the properties and reactivity of bimetallics is important in designing a more economical catalyst that retains the active sites of a precious metal such as Pt or Pd by combining it with a less expensive and more abundant metal such as a 3d transition metal. For example, alloys of Pt with 3d transition metals such as Cu, Ni, and Fe have been identified to be promising alternatives to pure Pt catalysts that may even resist the poisoning effects of CO.6-8 Recently, Sykes and co-workers found that a low coverage (< 0.05 monolayer (ML)) of Pt or Pd deposited on Cu(111) forms highly dispersed isolated atoms on the surface called single atom alloy (SAA).9-11 Through a combination of scanning tunneling microscopy (STM) and temperature-programmed desorption (TPD), they found that at a coverage of as low as 0.01 ML, Pt SAA on Cu(111) is active in the dissociation of H2 and the spillover of H to Cu sites.12 This means that Pt-Cu SAA is potentially an active hydrogenation catalyst at a significantly reduced


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cost due to the minimized use of Pt. Indeed, Pt-Cu SAA nanoparticles were found to be active in the selective hydrogenation of 1,3-butadiene to butenes.13 Although Pt-Cu SAA have been extensively studied with experimental and theoretical methods,9, 12-16 there is still a lack of understanding of how this alloy behaves under reaction conditions, necessitating a systematic in situ spectroscopic study of the model system in ambient pressure conditions. In order to evaluate the effectiveness of such alloys as catalysts, their stability under reaction conditions must be taken into consideration, especially when the presence of gases could cause segregation that would result in changes in the surface composition and structure, potentially leading to the deactivation of the catalyst.17 Recently, polarization-dependent reflection absorption infrared spectroscopy (PD-RAIRS) was utilized to study an analogous system, Pd/Cu(111) SAA, in an ambient pressure of CO.18 CO is one of the most common reactants in various heterogeneous catalytic reactions, e.g. CO oxidation, watergas shift, and Fischer Tropsch reaction. However, CO is also known to readily poison Pt-based catalysts due to the strong Pt-CO interaction. In situ X-ray absorption spectroscopy (XAS) and pair distribution function (PDF) were used to study oxide-supported PtCu nanoparticles in CO, and it was found that Pt segregates to the surface.19 CuPt near surface alloy (NSA), where Cu is present in the subsurface layer, was found to have reduced Pt-CO binding, and it was proposed as an active water-gas shift catalyst.20 However, a combined experimental and theoretical study by Andersson et al. showed that ambient pressure of CO causes the subsurface Cu in CuPt NSA to segregate to the surface.21 These studies demonstrate that reactant-induced mass transport of metal atoms within catalysts was indeed significant. In this case, CO actually binds to Pt much more strongly when Cu is also present on the surface than it does on Pt(111).21 On Pt/Cu(111)


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SAA, however, CO was found to bind on Pt sites more weakly than on larger Pt clusters, thereby potentially circumventing the CO poisoning problems that often plague Pt-based catalysts.14 In this study, we used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to compare the stability of Pt/Cu(111) SAA in UHV and ambient pressure conditions, particularly how its surface structure and composition can be influenced by the presence of CO. Since APXPS is a highly surface sensitive, elementally specific, and chemically sensitive technique, we can directly probe the Pt atoms to clearly identify different types of Pt on the surface/subsurface layers and monitor their evolution under different experimental conditions.

Experimental Details AP-XPS experiments were performed at the 23-ID-2 (CSX-2) beamline at the National Synchrotron Light Source II (NSLS-II), Brookhaven National Laboratory. The ellipticallypolarized undulator source provides soft x-ray photons with an energy range of 250 to 2000 eV with a resolving power of up to 104 E/∆E. The AP-XPS endstation has a base pressure of 2 x 10-9 Torr, and it is equipped with a Specs Phoibos 150 NAP analyzer with three differential pumping stages. The main measurement chamber and the first pumping stage of the analyzer are separated by a 300 µm aperture and the sample surface was placed ~600 µm away from the aperture. The beamline and the main chamber are separated by a 100 nm thick silicon nitride window. More details about the beamline and endstation can be found in a previous publication.22 Cu(111) was cleaned using repeated cycles of Ar+ sputtering (1 kV, 5 x 10-5 Torr Ar) and annealing to 850 K. Sample heating was done using a ceramic button heater, and the temperature was read by a K-type thermocouple spot-welded to a Ta foil placed between the heater and the sample. The cleanliness of the surface was judged by XPS and the amount of C and O


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contamination was below the detection limit. Pt was evaporated onto the surface using a Specs EBE-4 e-beam evaporator at a sample temperature of 370-380 K with an evaporation rate of 0.025 monolayer equivalent (MLE)/min. The evaporation rate was calibrated using a quartz crystal microbalance and the coverage was confirmed with XPS. CO (Matheson, research purity, 99.999%) was introduced into the chamber using a variable leak valve. The pressure was read by a combined hot cathode ion gauge and Pirani gauge at lower pressures (5 mTorr and below), and by a capacitance manometer at higher pressures (above 5 mTorr). The Cu 3p1/2 peak, which does not completely overlap with the Pt 4f peak, was used for energy calibration, and the absolute energy was confirmed by measuring the Au 4f spectrum of a gold foil mounted on the sample holder.

Results and Discussion A. Room Temperature CO Adsorption on the As-Deposited Pt/Cu(111) SAA Surface Figure 1 (a) shows the Cu 3p spectrum of the clean Cu(111) surface before Pt deposition as well as the Cu 3p and Pt 4f spectra after the deposition of 0.05 MLE Pt, taken in UHV and in 10 mTorr CO. This Pt coverage was chosen because it is low enough to be within the limit of SAA formation from the STM studies of Sykes and co-workers,9 and it is high enough to be easily observable by XPS, especially since the Pt 4f spectrum partially overlaps with Cu 3p. In order to clearly observe and fit the Pt 4f peaks, the spectrum of clean Cu(111) was subtracted from the spectra of Pt/Cu(111) and the results are shown at the top of Figure 1(a). It is apparent that each spin orbit peak of Pt 4f shows an asymmetrical peak with a lower binding energy component that is shifted to higher energy upon CO exposure. Figures 1(b) and (c) show the C 1s and O 1s spectra of the 0.05 MLE Pt/Cu(111) surface in 1 mTorr of CO, measured with photon


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energies of 460 eV and 710 eV, respectively. At this pressure, the peaks for adsorbed CO on Cu(111), which may overlap with those for CO on Pt sites, are not observed. Both the C 1s and O 1s spectra for the adsorbed CO on Pt only show a single peak, with a binding energy (286.6 and 532.65 eV, respectively) consistent with the literature values for atop CO adsorption on Pt(111) (Table 1).23-24 The absence of any obvious bridge site-bound CO is consistent with the single atom nature of the Pt sites. Above 0.05 ML Pt coverage, STM images showed the growth of 1D Pt chains.12 While it is possible that a small amount of Pt chains could be present on the surface studied in this present work, we did not observe CO adsorbed on such sites by XPS. The effect of both lower and higher Pt coverage is a potential future follow-up experiment. The Pt 4f7/2 peaks of the as-deposited sample, starting from UHV and in up to 100 mTorr of CO at room temperature, are shown in Figure 2. The Pt 4f7/2 spectrum can be fitted with three components. The lower binding energy component at 70.95 eV (blue), which is predominant in UHV, is assigned to free surface Pt, which dramatically decreases in intensity upon CO adsorption, accompanied by the appearance of the 72.25 eV component. The latter component is thus assigned to CO-bound surface Pt. Based on previously reported STM data,9 the surface Pt atoms here are those substituted in the surface layer of Cu. In UHV at room temperature, a small amount of CO from the chamber background was already adsorbed on the Pt sites, and it desorbed when the sample was heated to 350 K. The remaining component at 71.4 eV (green) is assigned to subsurface Pt, which is not shifted by CO adsorption. From separate Pt depositions, we determined that the surface Pt consistently amounts to 68 ± 3 % of the total Pt, which means that 0.05 MLE of deposited Pt corresponds to ~0.034 ML surface Pt initially. For simplicity, we will continue to refer to the surface as 0.05 MLE Pt/Cu(111) due to the dynamic nature of the surface and subsurface Pt atoms in different


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conditions. STM studies found that at an alloying temperature of 315 K, almost all Pt atoms on Cu(111) terraces are substituted in the surface layer, and subsurface Pt was only observed at higher alloying temperatures.9 Pt deposition temperature of 380 K did not seem to result in the formation of subsurface Pt in the terraces either.12 A medium energy ion scattering (MEIS) study by Dastoor et al. of 0.5 ML Pt deposited on Cu(111) showed that the intermixing of Pt and Cu in the first two layers occurred on step edges even at 200 K.25 Therefore, it is very likely that the subsurface Pt atoms we observe in our spectra are located at the step edges instead of the terraces. The Pt 4f7/2 binding energy values are 0.3-0.5 eV higher than the reported literature values for Pt(111), as summarized in Table 1. A positive shift of the Pt 4f binding energy has been reported in the literature for various Pt alloys, and it can be attributed to compressive strain and/or electronic effects. 26-30 Figure 3 shows the fraction of each Pt component as a function of CO pressure from 1 x 10-6 Torr to 1 x 10-1 Torr at 300 K. The initial fractions of surface and subsurface Pt did not change in 1 x 10-6 Torr CO. With increasing CO pressure, the fraction of CO-bound surface Pt increased from 58% to 76%, accompanied by a decrease in the fractions of both free surface Pt and subsurface Pt. Above 1 mTorr, all surface Pt sites are saturated by CO, as indicated by the absence of free surface Pt. The continuous decrease in subsurface Pt from 32% at 1 x 10-6 Torr to 23% at 1 x 10-1 Torr indicates that the presence of CO causes the segregation of subsurface Pt to the surface layer. Figures S1 and S2 in the Supplementary Information show the desorption of CO from the surface after gas-phase CO evacuation at room temperature and after subsequently heating the sample to 350 K and 380 K. We found a ~22% decrease in adsorbed CO approximately four minutes after the gas was pumped down; this drop increased to 30% after 30 minutes in UHV. After the sample was heated to 350 K, most of the CO molecules desorbed


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from the surface, leaving only 14% of the initial amount of adsorbed CO. This CO desorption temperature from Pt/Cu(111) SAA is in exact agreement with the TPD data of Liu et al.,14 and it is lower than the 410-435 K desorption temperature of CO from Pt(111),31 confirming the weaker binding of CO on the single Pt sites. Recently, Zhao et al. performed density functional theory (DFT) calculations to determine CO binding strengths on various Pt-Cu alloys.32 They determined the binding energy of CO on Pt SAA to be -1.26 eV, slightly weaker than that on Pt(111), which was calculated to be -1.40 eV. However, CO still binds more strongly to Pt SAA than to Cu(111) (binding energy of -0.52 eV from experiments33 and -0.62 eV from DFT34). In a bimetallic alloy, the adsorbate-induced surface segregation of the component that interacts more strongly with the adsorbate has been extensively reported,19, 35-38 which is consistent with our results.

B. Thermal Stability of Pt/Cu(111) SAA in UHV To investigate the thermal stability of Pt/Cu(111) SAA, we monitored the evolution of the different Pt 4f7/2 components during heating in UHV as shown in Figure 4(a) with the fraction of each Pt component plotted as a function of temperature in Figure 4(b). Upon heating to 400 K, no significant change is observed, indicating that the SAA remains stable at this temperature. However, starting at 450 K, we clearly observe the diffusion of the surface Pt atoms to the subsurface layer, as shown by the decrease in the fraction of surface Pt from 68% of the initial total amount at 300 K to 50% at 450 K, and eventually to 20% at 600 K. The diffusion of surface Pt, however, is not accompanied by a proportional increase in the fraction of the subsurface Pt, as the latter only increases from 32% to 47%, and the total amount of Pt at 600 K decreases to 65% of the initial amount. At 250 eV photon energy used for these measurements, the kinetic


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energy for Pt 4f electrons is ~180 eV, which corresponds to an inelastic mean free path (IMFP) of ~0.5 nm or a sampling depth of ~1.5 nm. Therefore, the decrease in the amount of total Pt indicates that, by heating the sample, not only are the surface Pt atoms diffusing to the subsurface, but the subsurface Pt atoms also start to migrate deeper into the bulk, beyond the escape depth of the photoelectrons at this kinetic energy. To confirm this, we performed a depth profiling study by increasing the photon energy to 710 eV, corresponding to a kinetic energy of 640 eV (IMFP of ~1.1 nm and sampling depth of 3.3 nm). Figure S3 clearly shows that upon heating, higher fractions of both the total Pt and subsurface Pt are observed with the higher photon energy than with the lower photon energy. The heating-induced subsurface diffusion of Pt is consistent with the observation of Sykes and co-workers, where alloying at 550 K resulted in 20% surface Pt on terraces as opposed to 70% at 450 K and 99% at 315 K.9 STM images also showed the presence of subsurface Pt on terraces at higher alloying temperatures. Although the surface free energy of Pt is calculated to be higher than that of Cu, which indicates that Cu is more likely to segregate to the surface, the calculated surface segregation energy for Pt solute in Cu host shows no tendency for either surface segregation or bulk/subsurface diffusion, likely due to the larger radius of Pt compared to that of Cu.39-41 Experimental data have shown that > 0.1 ML Pt films deposited on Cu(111) are stable up to 500 K, and bulk diffusion only occurs above this temperature, resulting in the formation of Pt-Cu surface alloys.42-44 Our XPS data show that Pt/Cu(111) SAA exhibits a similar thermal-induced diffusion behavior, but it has a lower onset temperature for bulk diffusion.

C. Thermal Stability of Pt/Cu(111) SAA in CO


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Thermal activation accelerates mass transport and weakens bonds between reactants and surface sites. In addition, the presence of ambient pressures of reactants during heating adds an extra complexity as collision rates and bonding of adsorbates may influence the properties of surface active sites. We have thus probed the behavior of the Pt/Cu(111) SAA under an ambient pressure of CO with annealing. Figure 5 shows the evolution of the Pt 4f7/2 peaks during the stepwise heating of the Pt/Cu(111) SAA under 10 mTorr of CO. Contrary to the results observed under UHV conditions, heating in a CO environment largely prevents the subsurface diffusion of Pt at up to 500 K, where 68% of the initial total Pt amount remains on the surface as opposed to 32% in UHV. Figure S4 shows that at a lower CO pressure (0.1 mTorr), the Pt/Cu(111) SAA is stable up to 450 K. The presence of CO not only stabilizes surface Pt atoms, but also seems to inhibit the diffusion of subsurface Pt atoms deeper into the bulk. Figure 4(b) shows that heating to 500 K in UHV causes the decrease in the total amount of Pt in the probed depth to 80% of the total initial amount. However, in 10 mTorr CO, as shown in Figure 5(b), there is no change in the total amount of Pt from 300 K to 500 K, indicating that the existing subsurface Pt atoms are stable upon heating. We only start to observe an appreciable decrease in surface Pt accompanied by an increase in subsurface Pt at 550 K. While CO desorbs from the as-deposited Pt/Cu(111) SAA at ~350 K in UHV (Figure S1), at 10 mTorr pressure, CO still adsorbs on surface Pt sites at up to 550 K since the higher pressure shifts the equilibrium to the adsorbed state. This adsorbed CO likely protects surface Pt atoms, effectively preventing the migration of Pt from the surface to the subsurface layer. Although strongly adsorbed CO, which desorbs only at relatively high temperatures, could potentially poison Pt sites, Liu et al. demonstrated that the presence of gas-phase CO did not deactivate PtCu SAA nanoparticles in H2 dissociation or acetylene hydrogenation reactions,


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demonstrating the tolerance of the SAA to CO poisoning.14 At 600 K, we observe a more significant depletion of surface Pt atoms as CO can no longer adsorb to the Pt sites at this temperature, clearly demonstrating the role of adsorbed CO in stabilizing surface Pt atoms. In addition to stabilizing the Pt/Cu(111) SAA at high temperatures, CO is also able to draw Pt from the bulk/subsurface to the surface for a sample that has been annealed under UHV conditions. Figure 6(a) displays the fitted Pt 4f7/2 XPS spectra for a 0.05 MLE Pt/Cu(111) SAA under sequential experimental conditions: first, the as-deposited surface was exposed to 20 mTorr of CO at 300 K as an initial reference and to confirm the amount of surface Pt atoms, then CO was pumped down and the sample was heated to 500 K in UHV, followed by an exposure to CO at 0.1, 2, and 20 mTorr at 500 K. Figure 6(b) shows the fraction of Pt components at each of these conditions. As expected, we observe decreased total and surface Pt fractions along with a small increase in subsurface Pt as the diffusion of both surface and subsurface Pt atoms occur due to heating to 500 K in UHV. After the introduction of 0.1 mTorr CO at 500 K, we start to see the adsorption of CO on surface Pt atoms although the fractions of subsurface and free surface Pt atoms are unchanged. A further increase in the fraction of surface Pt, mainly in the form of CObound surface Pt, accompanied by a decrease in the subsurface Pt is observed at 2 mTorr CO. At 0.1 and 2 mTorr, the pressure is not high enough for CO to occupy all surface Pt sites at this temperature. Finally, in 20 mTorr CO, the fraction of surface Pt increases significantly, almost all CO-bound, nearly to the same initial fraction of the as-deposited surface, and the fraction of subsurface Pt dropped lower than the initial amount. The total detected amount of Pt, which decreases to ~78% of the initial amount after heating in UHV, grows to ~95% in 20 mTorr CO at 500 K, indicating that CO also draws out Pt atoms located deeper in the bulk. Therefore, under


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these conditions, CO is able to recover, almost quantitatively, all of the Pt atoms that initially migrated to the subsurface/bulk region due to heating in UHV. Andersson et al. previously reported the stability of a Cu-Pt surface alloy formed through the CO-induced self-organization of Cu/Pt(111) NSA.45 In a case in which the oxidation of the sample resulted in the formation of a CuOx film on the surface, they demonstrated that the Cu-Pt surface alloy could be recovered by exposing the sample to 100 mbar CO at room temperature. However, CO adsorbs strongly to this Cu-Pt surface alloy with a desorption temperature of as high as 580 K.21 For Pt/Cu(111) SAA, CO is bound more weakly to the surface, at least for the freshly deposited surface, so the fact that not only can it restore the surface Pt but also draw out Pt atoms from the bulk is quite interesting. This implies that the location of active surface sites in catalytic conditions is likely influenced by the behavior and concentration of the reactant.

D. Pt-CO Binding Strength on Post-Heated Pt/Cu(111) SAA Figure 7 shows the plot of the fraction of CO-bound and free surface Pt during an experimental sequence for CO desorption for the as-deposited Pt/Cu(111) SAA surface and after the sample was heated to 530 K in 10 mTorr CO then cooled down to 300 K. The fitted Pt 4f7/2 spectra for the post-heated surface are shown in Figure S5. For the as-deposited surface, as we have described earlier, some CO already starts to desorb upon evacuation at 300 K, and most of the remaining CO desorbs at 350 K. However, for the post-heated surface, we observe a higher onset in CO desorption temperature. The fraction of CO-bound surface Pt is unchanged upon evacuation at 300 K, showing that no CO desorption occurred. At 350 K, only half of the CO desorbs, and after heating to 380 K, ~ 30% of the adsorbed CO still remains on the surface. All CO desorbs at 400 K (not shown). While this desorption temperature is still lower than the CO


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desorption temperature from Pt(111),31 it shows that CO binds more strongly to the post-heated surface than to the as-deposited surface. This enhanced CO binding on the post-heated surface can likely be explained by the changes in the subsurface Pt peak. Upon a close examination of the subsurface Pt peak in Figures 4(a), 5(a), and 6(a), we found that its binding energy is shifted from the initial 71.4 eV at 300 K to 71.3 eV at 500 K, and its full width at half maximum (FWHM) decreases from 0.65 eV at 300 K to 0.48 eV at 500 K. These changes are observed whether the sample was heated in UHV or in CO, but they are more apparent when CO is present since the subsurface peak is more separated from the CO-bound surface peak. These changes are not caused by the presence of CO since they are not observed during the room temperature adsorption of CO (Figure 2). As we have previously mentioned, the subsurface Pt atoms in the as-deposited SAA surface are likely found on step-edges, while almost all of the Pt atoms on terraces are substituted in the surface layer.9, 25 At higher temperatures, Lucci et al. suggested based on STM images that the alloying mechanism becomes more complex as atomic exchanges between Cu and Pt atoms occur between step edges and terraces as well as within terraces, and they observed subsurface Pt atoms on terraces.9 Although we found that CO stabilizes surface Pt atoms during heating, this exchange may still occur in the subsurface layer, resulting in the dispersion of subsurface Pt atoms from step edges to terraces. The dispersion of Pt atoms in a Cu lattice has been found to minimize compressive strain, so it is thermodynamically favorable over the formation of Pt clusters.9, 46 Since a positive shift in the Pt 4f binding energy in alloys has been attributed to compressive strain,26 this is consistent with our observation of a negative binding energy shift for subsurface Pt upon heating, indicating less strain. The accompanying decrease in FWHM could be attributed to the more uniform environment of the dispersed subsurface Pt atoms.


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DFT studies have revealed that the presence of subsurface Pt atoms in the immediate vicinity of a surface Pt atom enhances the CO binding strength compared to when the Pt atom substituted in the surface layer is only surrounded by Cu atoms on the surface and subsurface layers.32 Although we cannot directly infer this from the XPS spectra, it is highly likely that the presence of dispersed subsurface Pt atoms in the terraces due to heating is the cause of the stronger CO binding on the post-heated surface compared to the as-deposited surface when only surface Pt atoms are substituted in the terraces.

Conclusion We have used AP-XPS to investigate the Pt/Cu(111) single atom alloy in an ambient pressure of CO to elucidate the properties of the surface in reaction conditions. Our study shows that CO binds more weakly on Pt/Cu(111) SAA compared to on Pt(111) or Pt-Cu surface alloys, yet it can still cause the segregation of Pt from the subsurface to the surface layer even at room temperature. In addition, while the SAA surface is thermally unstable in UHV above 400 K, the presence of CO during heating stabilizes both the surface and subsurface Pt up to 500 K as well as suppresses subsurface and bulk diffusion at higher temperatures. In the case where the loss of surface Pt has occurred, for example due to heating in UHV, and possibly could cause the deactivation of the catalyst, thermal treatment of the sample in CO to recover the surface Pt can be used to reverse this deactivation. However, this thermal stability as well as surface recovery in CO comes at the expense of slightly enhanced CO-Pt interaction, likely caused by the presence of neighboring subsurface Pt resulting from the thermal-induced restructuring of the subsurface layer. Whether the post-heated surface still resists the poisoning effects of CO remains to be seen, but these results demonstrate the importance of studying not only the changes in the surface


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of a catalyst under reaction conditions, but also the near-surface region, which may also influence the reactivity of the catalyst.

Acknowledgements This research used resources of the 23-ID-2 beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. JPS is supported through the NSLS-II Director’s Postdoctoral Program jointly with the BNL Chemistry Division. SDS is supported by a DOE Early Career Award.

Supporting Information CO desorption data (Pt 4f7/2 spectra and Pt fraction plot) from as-deposited 0.05 MLE Pt/Cu(111); depth profiling of the surface during heating in UHV; Pt 4f7/2 and Pt fraction during heating in 0.1 mTorr CO; Pt 4f7/2 spectra during CO desorption from the post-heated surface.


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Figure Captions

Figure 1. (a) Cu 3p and Pt 4f spectra of clean Cu(111), 0.05 MLE Pt/Cu(111) in UHV, and 0.05 MLE Pt/Cu(111) in 10 mTorr CO. The Pt 4f spectra from the subtraction of the clean Cu(111) spectrum from the Pt/Cu(111) spectra are shown at the top of the figure. (b) C 1s and (c) O 1s spectra of Pt/Cu(111) in 1 mTorr CO.

Figure 2. Pt 4f7/2 peak of 0.05 MLE Pt/Cu(111) in increasing pressure of CO at 300 K. The solid lines represent the fits while the dotted lines are the experimental spectra. Red: CO-bound surface Pt, green: subsurface Pt, blue: free surface Pt.

Figure 3. Plot of the fraction of each Pt component from the fitting of Figure 2 as a function of CO pressure (in log scale). The lines are only guides to the eye.

Figure 4. (a) Pt 4f7/2 peak of 0.05 MLE Pt/Cu(111) during heating in UHV. The solid lines represent the fits while the dotted lines are the experimental spectra. Red: CO-bound surface Pt, green: subsurface Pt, blue: free surface Pt. (b) Plot of the fraction of Pt components from the fitting of Figure 4(a) as a function of temperature. The Pt fractions are normalized to the initial total amount at 300 K. The total surface Pt (purple) at each temperature is obtained by summing the CO-bound surface Pt (red peak) and free surface Pt (blue peak). The lines are only guides to the eye.


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Figure 5. (a) Pt 4f7/2 peak of 0.05 MLE Pt/Cu(111) during heating in 10 mTorr CO. The solid lines represent the fits while the dotted lines are the experimental spectra. Red: CO-bound surface Pt, green: subsurface Pt, blue: free surface Pt. (b) Plot of the fraction of Pt components from the fitting of Figure 5(a) as a function of temperature. The Pt fractions are normalized to the initial total amount at 300 K. The total surface Pt (purple) at each temperature is obtained by summing the CO-bound surface Pt (red peak) and free surface Pt (blue peak). The lines are only guides to the eye.

Figure 6. (a) Pt 4f7/2 peak of 0.05 MLE Pt/Cu(111), in sequence from top to bottom: the asdeposited surface in 20 mTorr CO at 300 K; after CO was pumped down and the sample was heated to 500 K in UHV; after CO was introduced at 500 K at 0.1, 2, and 20 mTorr. (b) The fraction of each Pt component from the fitting in (a) at each indicated experimental condition. The total surface Pt (purple) is obtained by summing the CO-bound surface Pt (red) and free surface Pt (blue). The lines are only guides to the eye.

Figure 7. The plot of the fraction of free (blue) and CO-bound (red) surface Pt for the asdeposited (solid markers) and post-heated (open markers) Pt/Cu(111) SAA during a CO desorption experiment, starting with exposure to 1 mTorr CO at 300 K, followed by CO evacuation at 300 K and measurements in UHV for up to 30 min, and heating to 350 and 380 K in UHV.


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Pt 4f – bulk Pt 4f – free surface Pt 4f – atop CO Pt 4f – bridge CO C 1s – atop CO C 1s – bridge CO O 1s – atop CO O 1s – bridge CO

Pt(111) – ambient pressure (Ref 24) 71 70.6 72 71.3 286.8 286.1 532.9 531.2

Pt(111) – UHV (Ref 23) 70.9 70.5 71.91 71.23 286.7 286.0 532.7 531

Pt/Cu(111) SAA (this work) 71.3-71.4 (subsurface) 70.95 72.25 Not observed 286.6 Not observed 532.65 Not observed

Table 1. Comparison of the binding energies (in eV) of Pt 4f7/2, C 1s, and O 1s core levels for CO on Pt(111) in ambient pressure and UHV and on Pt/Cu(111) SAA.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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TOC Graphic


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Enhanced Stability of Pt-Cu Single-Atom Alloy ... - ACS Publications

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