Diatomic Steps in Pt(997) Surfaces Are Better Catalysts than

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Diatomic Steps in Pt(997) Surfaces Are Better Catalysts than Monatomic Steps for the CO Oxidation Reaction near Atmospheric Pressure O. Balmes,† G. Prevot,‡ X. Torrelles,§ E. Lundgren,∥ and S. Ferrer*,⊥ †

Max IV Laboratory, Lund University, P. O. Box 118, SE-22100 Lund, Sweden Sorbonne Universités, UPMC Univ Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, F-75005 Paris, France § Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, 08193 Bellaterra, Cerdanyola del Vallès, Spain ∥ Department of Synchrotron Radiation Research, Institute of Physics, Lund University, P. O. Box 118, SE-22100 Lund, Sweden ⊥ ALBA Light Source, Carrer de la Llum 2-26, BP 1413, 08290 Cerdanyola del Vallès, Spain ‡

ABSTRACT: The oxidation of CO to CO2 has been investigated on Pt(997) single-crystal model catalysts with a flow reactor near ambient pressure with X-ray diffraction. Depending on the temperature and gas composition, the surface may consist of monatomic steps separated by 9 atom wide terraces or diatomic steps separated by 18 atom wide terraces. The diatomic stepped surface has been found to be significantly more active for CO2 production. The kinetics of the single- to double-step transition under reaction conditions has been investigated, as well as the dependence of the transition temperature with the gas composition.

KEYWORDS: surface chemistry, catalysis, CO oxidation, atomic steps, vicinal surface reactivity has also been evidenced for initially flat surfaces. For example, previous research on the oxidation of CO on Pt(110),4 used as a model catalyst, evidenced a higher CO2 production rate when the surface was rough, as determined from STM and X-ray diffraction measurements, than when it was flat. In that case, roughening was ascribed to the increased density of step atoms and to surface oxidation. CO oxidation has also been studied at ambient pressure on stepped surfaces. Experiments performed on the Pt(977) vicinal surface5 demonstrate that the reactivity for CO2 production and the density of steps were maxima when the reactants (CO and O2) had the stoichiometric proportions of 2:1, establishing a direct link among steps and reactivity. Outside the stoichiometry, the surface was not stable since it had the tendency to develop large (111) facets. In order to disentangle the relative role of the steps and facets for vicinal surfaces, we have investigated the same reaction using a Pt(997) surface as a model catalyst. Both (977) and (997) surfaces have terraces of (111) orientation with nearly the same width but differ in the atomic structure of the steps. In the first case the local atomic coordination is that of a (100) facet and in the second case that of a (111) facet.6

1. INTRODUCTION In heterogeneous catalysis, metal nanoparticles (NPs) of noble metals are among the most commonly used catalysts for oxidation and reduction reactions of gas molecules such as CO, NO, hydrocarbons, and alcohols. NPs have a large fraction of their atoms at the surface, and their morphology is usually polyhedral, consisting of flat microfacets of low crystalline index as (111), (100), and (110) separated by more or less rounded edges depending on thermodynamic conditions.1 Among the different atoms on the surface of a NP, those at the edges are particular, since they have lower atomic coordination than atoms in flat facets and are more reactive.2 In fact, step atoms have been considered as probable active sites of surface chemical reactions for decades.3 Whereas NP edges are sharp at very low temperatures and in UHV, they become rounded when the temperature and gas pressure are increased to reach reaction conditions. Rounding of NP implies shrinking of the low index facets and appearance of high Miller index surfaces also called vicinals. They consist of arrays of atomic steps separated by small terraces: see, e.g. Jeong et al.1 and references therein. The precise determination of the role of step atoms in the activity of the NP is thus difficult, since the existence of vicinals of different orientations in NPs averages up step atoms with different atomic neighborhoods. This difficulty can be overcome by employing macroscopic single crystals with welldefined crystalline orientations. The role of steps on catalytic © XXXX American Chemical Society

Received: November 10, 2015 Revised: January 8, 2016

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conditions was also investigated, and it is discussed below. We found that, under some assumptions, the kinetics is consistent with a limiting case of the Langmuir−Hinshelwood mechanism, where irreversible adsorption of oxygen atoms at the steps occurs which establishes a link between the UHV results and those presented in what follows, which are a basis for discussing the intermediate steps of the reaction.

Previous UHV research on the Pt(997) surface brought to evidence7,8 that minute quantities of adsorbed oxygen transformed the surface from monatomic to diatomic steps and caused a doubling of the terrace dimensions, as depicted in Figure 1. A fundamentally interesting question related to the

2. EXPERIMENTAL DETAILS The Pt(997) sample was a disk of diameter 10 mm and thickness 3 mm obtained after cutting a single-crystalline rod. Previous to the experiment, it was annealed to 1000 °C in air to recrystallize and to eliminate carbon impurities. Afterward, it was mounted on a thermally insulated ceramic heater, which allowed reaching temperatures up to 900 °C, as measured with a thermocouple in contact with the crystal. The crystal and heater were installed in a flow reactor previously described9 connected to a UHV chamber, allowing sputtering/annealing cycles for surface preparation. Balmes et al.5 have described in more detail the experimental setup. The flow reactor was installed on a diffractometer of the ID3 beamline at ESRF. Xrays of 18 keV impinged the surface of the crystal at grazing angles (about 1°). The crystal lattice was described with a base adapted to the surface: A1 = (−7/2,−7/2,9), A2 = (1/2,−1/2,0) and A3 = (9,9,7) in units of the conventional lattice fcc parameter a0 = 0.392 nm. The L axis of the corresponding reciprocal space coordinates (H,K,L) is normal to the surface plane. Diffracted intensity from the regular surface is found for even values of H + K.

Figure 1. H scans at K = 1 and L = 11 from a (997) surface exhibiting diffraction peaks at H = 27 from single monatomic steps (top scan) and at H = 26 from double steps and terrace dimensions (bottom). The glitches at H = 27.8 are artifacts.

effect of the pressure gap between UHV conditions and atmospheric (or near atmospheric) pressures was to find out if this doubling process occurs under reaction conditions at ambient pressures. As will be shown in what follows, the step doubling process indeed occurs, and most interestingly it is associated with a change in chemical activity. Double steps produce about 3 times as much CO2 than single steps. In addition, the single-/double-step configuration can be reversibly manipulated by modifying the reaction conditions. The kinetics of the single-/double-step transition under reaction

Figure 2. (a) Temporal evolution of the sample temperature (black) and intensity (red) of the double step peak at H = 26. The inset shows the jump in temperature at t = 406 s. (b) CO2 partial pressure (black) and double-step intensity (red). (c) Partial pressures of O2 (black) and CO (red). (d) Evolution of the sample temperature (black) and of the single-step intensity peak at H = 27 (blue). (e) Evolution of the partial pressure of CO2 (black) and of the single-step peak intensity. (f) Evolution of the partial pressure of O2 (black) and of CO (red). 1286

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Figure 3. Open symbols indicating diffracted intensities of single (blue) and double (red) steps acquired at times 1−4 indicated in panels a and d of Figure 2. Curves 3 and 4 have been shifted vertically for clarity. The continuous lines are fits to the data. The table shows the integrated intensities normalized to that of scan 1 and full widths in H units obtained from the fits. The standard errors of the fitted parameters are 2% and 6% for the single- and double-step scans, respectively.

3. RESULTS 3.1. Reactivity of Single and Double Steps. The sample was prepared by performing several sputter/anneal cycles to ∼800 °C followed by slow cooling (about three cycles were needed to achieve low surface roughness). The resulting surface exhibited a well-ordered step array, as evidenced with diffraction scans. Figure 1 shows some representative scans of the diffracted intensity. The top curve in blue that displays an intense peak at H = 27.0 and a weaker one at H = 25.0 corresponds to a wellordered surface with monatomic steps as schematized. Bulk Bragg peaks are found at (H,K,L) = (27,1,21) and (25,1,−4), which correspond respectively to the Bragg reflections (1,−1,3) and (0,−2,2) in usual cubic coordinates. The L = 11 coordinate of the scans shown is thus approximately at the center of L = 21 and −4 and therefore is sensitive to the surface structure. The bottom curve has its main peak at H = 26 that arises from step and terrace doubling as shown in the figure and a weak peak at H = 27 that corresponds to diffraction from the remaining single steps. Starting with a well-ordered single-step surface in UHV, the gas reactor chamber was filled with a total pressure of 200 mbar of gas consisting of 190 mbar of Ar, 6.6 mbar of CO, and 3.3 mbar of O2. Defining R as P(CO)/P(O2), R = 2.0 corresponds to the stoichiometric proportion of the 2CO + O2 → 2CO2 reaction. Once the reactant mixture was established, the sample temperature was changed by increasing or decreasing the current of the sample heater. Simultaneously, the diffracted peak intensities from single or double steps were monitored and also the gas composition by leaking out the reactor, through an adjustable valve, to the RGA (residual gas analyzer). Figure 2 summarizes the main results. In panel a the temporal evolution of the temperature above 225 °C is displayed in black. The temperature increased linearly with time up to t = 407 s (vertical dashed line), where a jump of ∼2° occurred, as shown in detail in the inset. The temperature was still ramped to 248 °C, kept stable for 12 min, and then decreased. During this heating/cooling cycle, the intensity of the double steps at H = 26 evolves as shown in the figure (red curve). Two sudden jumps indicated by the dashed lines in the figure occur at T = 242 °C in the heating cycle and at T = 221 °C in the cooling cycle. In panel b, the double- step intensity is reproduced to facilitate the comparison with the black curve that corresponds

to the production of CO2. The reaction rate that was already significant before t = 407 s exhibits a sudden jump that coincides with the appearance of the double steps. The CO2 partial pressure changes from 4.7 × 10−9 mbar to 7.8 × 10−9 mbar in less than 1 min, indicating that the reaction rate suddenly increased by a factor of 1.6. As the number of double steps per area unit is half that of single steps, one may estimate that the reactivity of the double steps relative to the single steps is 2 × 1.6 = 3.2. In panel c of Figure 2 the temporal evolution of the reactants is displayed. As expected, they essentially mirror the CO2 production curve in panel b. Panels d−f depict the results of a similar heating/cooling cycle done after that displayed in panels a−c (the measurements of the intensities at H = 26, 27 were carried out sequentially, since the acceptance of the X-ray detector was not large enough to cope with both signals simultaneously). The blue curve corresponds to the peak intensity of the single steps at H = 27. The intensity of single steps increases from T = 228 to 246 °C, where it suddenly decreases (dashed line) and then keeps decreasing to a lower rate until a constant value at T = 248 °C. At t = 5533 s the single-step peak intensity starts to grow and it displays a jump (less pronounced than that at t ≈ 4500 s) at t = 5600 s, indicated by the dashed line, while T changes from 197 to 210 °C. Then, after the intensity reaches a maximum at t = 5648 and T = 212 °C, it decreases continuously. Panel e illustrates that the CO2 production displays pronounced changes (dashed lines) that coincide with the abrupt changes in the single-step intensity. Panel f shows that the CO 2 production is accompanied by a consumption of reactants. Our results resemble these recently published by Lundgren and co-workers,10,11 who investigated CO oxidation on Pt and Pd catalysts and monitored the CO and CO2 distribution just above the catalyst surface with laser spectroscopy. Upon increasing the temperature, they also found a sudden increase in the reactivity, designed as ignition of the reaction as the CO desorbs from the surface, allowing for the dissociative adsorption of oxygen. The gas phase was at this point associated with an almost pure CO2 atmosphere of the gas above the surface, whereas below the ignition the gas in contact with the surface was rich in CO (O2 was not measured), resulting in CO poisoning of the surface. The ignition is 1287

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Figure 4. (a) Double-step peak intensity (red) and CO2 partial pressure (black) vs sample temperature in a heating/cooling cycle. (b) Temporal evolution of the single- to double-step and double- to single-step transitions (red and black curves, respectively). The continuous lines are the fits using the model mentioned in the text.

accompanied by a sudden increase of the sample temperature, as depicted in the inset of panel a in Figure 1. Figure 3 shows reciprocal space H scans at the times 1−4 indicated in the panels a and d of Figure 2. The intensities and widths of the scans have been evaluated by fitting the main peaks to Lorentzian functions, and the resulting integrated intensities and widths are shown in the table. The widths of the double-step peaks are sensibly larger than these for the single steps, probably reflecting their reduced density within the diffracting area due to the increase of their spacing. Interestingly, the integrated intensities are also somewhat larger for double steps, which suggests a better ordering of the double-stepped surface. Returning to panel d of Figure 2, the decreasing single-step intensity beyond t = 5600 s is accompanied by a significant broadening of the peak at H = 27. At the end of the experiment, at t = 6000 s and T = 189 °C, the single-step intensity reached 0.53 and the width 0.60, indicating that disordering and roughening of the surface occurred at low temperatures. In fact, in several experiments it was observed that after lowering the temperature to room temperature, in the presence of the reactants, the surface was exhibiting (111) microfacets coexisting with the monatomic steps. The faceting was evidenced with characteristic peaks between H = 26 and H = 27 in scans such as those in Figure 3. 3.2. Reaction Kinetics. As mentioned above, there is thermal reversibility of the single-/double-step transition. However, the transition occurs at higher temperature upon heating than upon cooling. To visualize this hysteresis, panel a in Figure 4 displays the peak at H = 26 as a function of T and also the corresponding CO2 pressure. Upon heating, the double-step concentration increases rapidly at T = 242 °C, whereas upon cooling, it decreases abruptly at T = 222 °C. The CO2 partial pressure follows closely the double-step evolution upon heating, but it separates in the cooling cycle. The hysteresis curve suggests a first-order transition. The separation between the black and red curves upon cooling is presently not understood. To get some additional insight into the nature of the catalytic reaction, the kinetics of the single- to double-step and doubleto single-step transitions was also investigated while slowly heating or cooling the crystal under the reaction conditions. Panel b in Figure 4 shows single (S) to double (D) (red data) and D to S (black data) representative transformations. In many cases the S to D transformation displayed better quality data, in terms of statistics, than the D to S transitions. Due to this, we mostly concentrated on the former to attempt to fit the

data with kinetic models. Several models were investigated as first- and second-order kinetics as in Pearl et al.12,13 Specifically, we tried the expression Δ(t ) = ((1 − exp( −t /τ ))n

(1)

with n = 1, 2 and the expression n ⎛ 1 ⎞ ⎟ Δ(t ) = ⎜1 − ⎝ t /τ + 1 ⎠

(2)

with n = 1, 2. Here Δ(t) represents the density of double steps which is assumed to be proportional to the normalized intensity of the diffracted peak and τ is an adjustable parameter. The quality of the fit was evaluated calculating the square root of the total quadratic deviation (Er) of the model and the data. For the first-order kinetics (eq 1) with n = 1, Er = 0.30; trying n = 2 as suggested by Pearl et al.12,13 resulted in a Er value of 0.53. Equation 2 with n = 1 gave Er = 0.24, and with n = 2, Er = 0.20. We also tested the expression proposed by Khare et al.14 on their Montecarlo simulation: Δ(t ) = 1 − exp( − t /τ )

(3)

which resulted in Er = 0.29. Alternatively, we modeled the concentration of double steps as Δ(t ) = (1 − exp(−t /τ ))0.5

(4)

resulting in Er = 0.13 and τ = 28 s, which is the fit displayed in the figure. Additionally, the exponents 0.3−0.7 were also investigated, producing a worse fit. Consequently, the last expression with the exponent 0.5 produces the best fit and, to our knowledge, it has not been considered previously in the literature. It results in a very rapid initial growth of the concentration of double steps since the slope dΔ/dt =

1 exp(t /τ ) 2Δτ

(5) −1

is initially very large. As dΔ/dt is proportional to Δ , the kinetics is formally of order −1. The above expression, with reversed temporal evolution (i.e., changing t to −t), gave also good fits for the transition D to S, as shown in panel b of Figure 4 (black curve). Initially, within first order in t, our model gives Δ(t) ≈ (t/τ)1/2, which is characteristic of growth limited by the attachment and detachment from step edges.15 It is considered to be well established that the CO oxidation obeys a Langmuir−Hinshelwood kinetics, which normally supposes constant surface concentration of the reactants (for a fixed temperature) due to the balance of adsorption and 1288

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measurements were obtained either by increasing the CO/O2 pressure ratio at a nearly constant temperature or by decreasing the temperature at a fixed pressure ratio. Both experiments gave the same results. The transition D to S occurring while the sample was cooled takes place at higher temperatures when the oxygen content is reduced. Also, at a given temperature, COrich mixtures favor single steps, whereas decreasing CO content toward oxygen-rich environments favor double steps. The temporal evolution of the concentration of steps was also studied while the proportions of the reactants were varied. For these experiments only, the argon partial pressure was set to 0, while the total pressure was kept to 200 mbar. Panel a of Figure 6 displays the evolution of the intensity from double steps as the gas mixture was continuously varied from R = 57/33 = 1.7 to R = 56/34 = 1.6, i.e. while the reaction was run under oxygen-rich conditions (R < 2), increasing the relative oxygen content at T ≈ 290 °C. Only 75 s after having finished the scan of the gas mixture, the intensity of the double steps increased. The continuous curve is a fit with our model with τ = 16 s. Panel b shows the decrease of the double-step intensity when the gas mixture was varied from R = 59.5/30.5 = 1.95 to R = 60.5/29.5 = 2.03. At variance with panel a, the intensity continuously decreases. The sample temperature was 324 °C, resulting in the fitted value τ = 15 s, as shown by the continuous curve. The values of τ of Figure 5 are smaller than those in panel b of Figure 4 because the temperature is higher. A crude estimation of the activation energy Ea of the doubling process may be done by assuming that τ is proportional to exp(Ea/kT). Combining the values of τ at elevated and lower temperatures to eliminate the proportionality factor leads to Ea ≈ 0.20 eV. This value has to be compared with the kink energy (linked to the diffusivity along the steps) of 0.17 eV from Giesen et al.,16 obtained from STM data from steps on Pt(111) surfaces. The work of Niu et al.8 reports activation energies for the single/double transitions which depend on the coverage of oxygen at the steps. They found that, for oxygen coverages of 2.4 × 10−3 monolayers, Ea = 0.85 eV and, for coverages of 9.6 × 10−3 monolayers, Ea = 0.52 eV. Hoogers et al.17 on Rh(332) found Ea = 1.1 eV.

desorption processes. If this condition is not fulfilled because one species has a very small desorption rate in comparison to the other, then within this limiting scenario, the reaction rate is inversely proportional to the gas-phase concentration of the irreversibly adsorbed reactant, which consequently results in a −1 order. On the basis of the complete coincidence of the curves in Figure 2b in the vicinity of the right side of the dashed line, we may assume that, within this short time interval, the CO2 production rate is proportional to the concentration of double steps. One may also assume, on the basis of the results of Pearl et al.,12,13 that, within this narrow coverage range, the concentration Δ of double steps is proportional to the oxygen coverage, which is also proportional to the concentration of oxygen in the gas phase. If all these conditions were fulfilled, then the order −1 found in dΔ/dt would be a consequence of the limiting case of the Langmuir−Hinshelwood kinetics mentioned above: i.e., of the irreversible adsorption of oxygen at the steps. 3.3. Effects of Changing the Proportions of Reactants. We also investigated the evolution of the double to single transition as a function of the CO to O2 pressure ratio. A summary of the results is shown in Figure 5. The experimental

Figure 5. Double-step to single-step transition temperature vs CO to O2 pressure ratio. The vertical line indicates the stoichiometric proportion, and the borderline is a guide to the eye.

Figure 6. (a) Temporal evolution of the diffracted intensity from double steps when the mixture of reactants was progressively enriched in oxygen. At 75 s after the stoichiometry R = 1.6 = 56/34 was established, the intensity exhibited a rapid increase. (b) Temporal evolution of the intensity from double steps while the proportion of oxygen was decreased, indicated in the top X axis. 1289

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(near step saturation in the case of Ni(977)12,13), the surface changes to double-step and terrace dimensions. The driving force of this morphological change might be due to a lower free energy of double-height-step Pt oxide in comparison to monatomic-step Pt oxide or alternatively to a lower energy of the doubled terrace after oxygen adsorption. The double-step oxide acts as a reservoir of oxygen to oxidize nearby CO molecules, and it is associated with higher reactivity. When a double-step oxide is cooled (Figure 2), the reverse scenario takes place. When the temperature is lowered, the concentration of oxygen at the steps diminishes to the point where the double-step oxide is unstable and the surface reverts to the single-step oxide due to CO readsorption. The transformation has hysteresis, since the temperature at which the transition occurs is lower upon cooling than upon heating. However, reversibility has to be significant in the process, since as mentioned above the kinetics of the single to double and double to single transitions are described by the same law, inverting the sense of the time. The single-step surface displays less reactivity. Ulterior cooling of the surface leads to roughening and eventually to faceting, which indicates that Pt surface atoms remain mobile. The experiments in Figures 5 and 6 support the above picture. Panel a of Figure 6 indicates that, when the relative oxygen proportion in the mixture of reactants is increased, the surface reverts to double steps. In panel b, the removal of oxygen leads to the transformation to a single-stepped surface. This indicates that the pictures emerging from UHV experiments appear to be valid under the reaction conditions and that critical step coverage transforms the single-stepped surfaces to double steps and vice versa. However, the experiments in Figure 6 exhibit an important difference. In panel a, the appearance of double steps is delayed 75 s after the maximum relative oxygen concentration was established (R = 1.6), whereas in panel b the surface transforms to single steps as soon as the gas proportion R equals 60.0/30.0 = 2.0. This is explained by a small temperature increase during the scan, induced by the reaction as depicted in the inset of panel a of Figure 2. It is also interesting to compare our results with those of Blomberg et al.10 and Zetterberg et al.11 mentioned above. Their studies were carried out at atmospheric pressures nearly the same as ours and clearly demonstrated that the ignition of the reaction is associated with a very rich CO2 atmosphere near the surface, the supply of reactants to the surface being hindered by their low diffusion. Under this regime, all CO molecules that reach the surface are oxidized. In addition, our findings indicate that under these conditions the steps are not blocked by CO, which stabilizes the double steps. When the temperature is lowered, the ignition regime turns off, the reaction rate decreases, and the atmosphere in contact with the sample is rich in reactants. Then, there is enough CO on the surface to block the reoxidation of step atoms, which leads to a decrease in the concentration of step oxide, and the surface reverts to the single-step state. The experiments reported in Figure 5 support this interpretation. Starting with a D surface and elevated temperature (ignition reaction on), lowering the temperature causes a decrease in the reaction rate (ignition reaction off) and a switch to the S surface. For CO-rich conditions the extinction of the reaction occurs at temperatures higher than those for lower CO proportions, indicating that CO destabilizes the D surface and the ignition reaction.

4. DISCUSSION Oxygen-induced step doubling on stepped surfaces was first reported by Comsa et al.7 on Pt(997). They observed by LEED and helium scattering experiments under a UHV environment that minute amounts of oxygen originated the doubling of the terrace dimensions. Later, Hoogers et al.17 obtained similar results on Rh(332), and they also investigated the kinetics of the single/double transformation. Niu et al.8 also using neutral helium atom diffraction in UHV observed the same phenomenon on Ni(977) and found that about 1−2% of a monolayer of oxygen was sufficient to transform the surface from single to double steps in the temperature range 115−200 °C. They also performed studies of the kinetics of the doubling transition. The same group published several years later detailed STM experiments under UHV,12,13 demonstrating that the single/double transformation was due to the coalescence of two adjacent single steps due to their meandering above room temperature. As soon as two adjacent steps touched and had a “point” of contact, a zipping process occurred, merging the two adjacent steps and forming terraces of doubled dimensions. The merging behavior was investigated under different conditions and temperatures. When the temperature was increased or some extra oxygen was added above 0.25 L exposure, the surface reverted to single steps. Wang et al.18 investigated the initial adsorption of oxygen on a Pt(332) surface and concluded from XPS data that initially oxygen binds at the Pt steps. The authors identified a step oxide as probable precursor of bulk oxidation. The step oxide reacted with CO more easily in comparison to oxygen atoms chemisorbed on the terraces. Bandlow et al.19 reached the same conclusions on Pt(322) and Pt(355). They also performed density functional theory calculations, concluding that below 0.1 adsorbed layer the oxygen atoms exclusively decorate the steps. The above results prove that, under UHV conditions, adsorption of oxygen at the steps is the primary cause of the doubling transformation. On this basis, we may assume that the same occurs at pressures of several mbar. A possible qualitative picture of the reaction emerges from our experiments. At low temperatures (room temperature and slightly above) the reaction rate is negligible and the single-stepped surface exposed to the CO and O2 stoichiometric mixture is mostly covered with CO. This is due to the fact that O2 dissociation at the surface is hindered by CO-adsorbed molecules, whereas CO adsorption is not blocked by oxygen-adsorbed atoms, leading to a very small concentration of adsorbed oxygen.20 In a previous study of CO adsorbed on Ni(111) at atmospheric pressures,21 it was found that, at gas pressures of few mbar, the CO adlayer was incommensurate and compressed, with the majority of the molecules sitting in low-symmetry sites. This scenario might also hold in our case at low temperatures. Possibly, diffusion of CO molecules toward the steps is not rate-limiting in the reaction kinetics due to the high concentration of adsorbed CO. When the temperature is raised, statistically the CO desorption increases and the readsorption decreases, allowing for oxygen to dissociate on the surface, forming CO2 more often. This process leads to the adsorption of oxygen at the steps and to the formation of a single-step oxide, as shown by Wang et al.18 Similar results have also been reported by Zhu et al.22 on Pt(557), which identified the formation of surface Pt oxide decorating the steps. Once the concentration of oxygen at the steps reaches a critical coverage 1290

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Research Article

ACS Catalysis

(2) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Science 2005, 307, 555−558. (3) Somorjai, G. A.; Li, Y. Introduction to Surface Chemistry and Catalysis; Wiley: Hoboken, NJ, 2010. (4) Hendriksen, B. L. M.; Frenken, J. W. M. Phys. Rev. Lett. 2002, 89, 046101. Ackermann, M. D.; Pedersen, T. M.; Hendriksen, B. L. M.; Robach, O.; Bobaru, S. C.; Popa, I.; Quiros, C.; Kim, C.; Hammer, B.; Ferrer, S.; Frenken, J. W. M. Phys. Rev. Lett. 2005, 95, 255505. (5) Balmes, O.; Prevot, G.; Torrelles, X.; Lundgren, E.; Ferrer, S. J. Catal. 2014, 309, 33−37. (6) van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1980, 92, 489−518. (7) Comsa, G.; Mechtersheimer, G.; Poelsema, B. Surf. Sci. 1982, 119, 159−171; Surf. Sci. 1982, 119, 172−183. (8) Niu, L.; Koleske, D. D.; Gaspar, D. J.; King, S. F.; Sibener, S. J. Surf. Sci. 1996, 356, 144−160. (9) van Rijn, R.; Ackermann, M. D.; Balmes, O.; Dufrane, T.; Geluk, A.; González, H.; Isern, H.; de Kuyper, E.; Petit, L.; Sole, V. A.; Wermeille, D.; Felici, R.; Frenken, J. W. M. Rev. Sci. Instrum. 2010, 81, 014101. (10) Blomberg, S.; Brackmann, Ch.; Gustafson, J.; Aldén, M.; Lundgren, M.; Zetterberg, J. ACS Catal. 2015, 5, 2028−2034. (11) Zetterberg, J.; Blomberg, S.; Gustafson, J.; Evertsson, J.; Zhou, J.; Adams, E. C.; Carlsson, P.-A.; Alden, M.; Lundgren, E. Nat. Commun. 2015, 6, 7076. (12) Pearl, T. P.; Sibener, S. J. J. Chem. Phys. 2001, 115, 1916−1927. (13) Pearl, T. P.; Sibener, S. J. J. Phys. Chem. B 2001, 105, 6300− 6306. (14) Khare, V.; Einstein, T. L.; Bartelt, T. L. Surf. Sci. 1995, 339, 353−362. (15) Ozcomert, J. S.; Pai, W. W.; Bartelt, N. C.; Reutt-Robey, J. E. Phys. Rev. Lett. 1994, 72, 258−261. (16) Giesen, M.; Icking-Konert, G. S.; Stapel, D.; Ibach, H. Surf. Sci. 1996, 366, 229−238. (17) Hoogers, G.; King, D. A. Surf. Sci. 1993, 286, 306−316. (18) Wang, J. G.; Li, W. X.; Borg, M.; Gustafson, J.; Mikkelsen, A.; Pedersen, T. M.; Lundgren, E.; Weissenrieder, J.; Klikovits, J.; Schmid, M.; Hammer, B.; Andersen, J. N. Phys. Rev. Lett. 2005, 95, 256102. (19) Bandlow, J.; Kaghazchi, P.; Jacob, T.; Papp, C.; Trankenschuh, B.; Streber, R.; Lorenz, M. P. A.; Fuhrmann, T.; Denecke, R.; Steinruck, H.-P. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 174107. (20) Engel, T.; Ertl, G. Advances in Catalysis; Academic Press: New York, 1979; pp 1−78. (21) Quirós, C.; Robach, O.; Isern, H.; Ordejón, P.; Ferrer, S. Surf. Sci. 2003, 522, 161−166. (22) Zhu, Z.; Tao, F.; Zheng, F.; Chang, R.; Li, Y.; Heinke, L.; Liu, Z.; Salmerón, M.; Somorjai, G. A. Nano Lett. 2012, 12, 1491−1497. (23) Prévot, G.; Croset, B. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 235410. (24) Prévot, G.; Steadman, P.; Ferrer, S. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 245409. (25) Prévot, G.; Barbier, L.; Steadman, P. Surf. Sci. 2010, 604, 1265− 1272. (26) Pearl, T. P.; Sibener, S. J. Surf. Sci. 2002, 496, L29−L34.

Our results, in comparison to those obtained under very similar conditions for Pt(977), show that the step orientation has a great influence on the surface reactivity. For Pt(977), the maximum reactivity was related to single steps and was obtained, independently of the temperature, for a stoichiometric CO/O2 mixture.6 No abrupt transition to high- or lowreactivity regimes was observed for this surface. As both surfaces have the same (111) terraces, oxygen and CO adsorption at their steps have to be different. It appears that oxygen adsorption is not inhibited by CO adsorption for (100) oriented steps, which could be related to the higher binding energy for oxygen on (100) steps in comparison to (111) steps.15 The step evolutions during the reaction are also very different for the two vicinals. No step-doubling transition was observed for Pt(977). However, on this surface, faceting was observed as soon as the gas mixture was not stoichiometric. It is likely for the (977) surface that the CO oxidation experiment was continuously in the Langmuir−Hinshelwood regime; that is, the complete desorption of CO never happened and there was no “ignition”. These differences might be related to the different interaction energies between steps on both surfaces. Previous work23−25 showed that, within an elastic deformation framework, the repulsive energy between (111) steps is significantly higher (a factor of 3.5) than that for (100) steps, which might explain the step doubling for the (997) surface in contrast to the faceting observed for Pt(977). Step fluctuations leading to faceting are easier on Pt(977) due to the lower energetic cost of terrace width fluctuations. On Pt(997), step doubling is initiated by a very local fluctuation of a single-step edge,26 which should have thus a moderate elastic cost. After pairing, from an elastic point of view, the increase of the elastic interaction between steps is balanced by the doubling of the interstep spacing.

5. CONCLUSIONS The main conclusions of this of this report are as follows. (1) Single- to double-step and double- to single-step transformations on Pt(997) under CO oxidation reaction conditions have been identified and investigated. The reactivity of double steps relative to that of single steps has been estimated to be about a factor of 3 higher. (2) The kinetics of the single- to double-step transition has been described with a rate law that is consistent with the Langmuir−Hinshelwood model in the limiting case of irreversible adsorption of oxygen at the steps. (3) At a fixed reaction temperature, CO-rich reaction conditions favor single steps and O2-rich conditions favor double steps.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.F.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.T. thanks the MICINN for financial support through project MAT2012-38213-CO2-02.

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REFERENCES

(1) Jeong, H. C.; Williams, E. D. Surf. Sci. Rep. 1999, 34, 171−294. 1291

DOI: 10.1021/acscatal.5b02526 ACS Catal. 2016, 6, 1285−1291