Article pubs.acs.org/JPCC
Faceting of Rhodium(553) in Realistic Reaction Mixtures of Carbon Monoxide and Oxygen C. Zhang,† E. Lundgren,† P.-A. Carlsson,‡ O. Balmes,¶ A. Hellman,‡ L. R. Merte,† M. Shipilin,† W. Onderwaater,§,∥ and J. Gustafson*,† †
Synchrotron Radiation Research, Lund University, Box 118, SE-221 00 Lund, Sweden Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden ¶ MAX IV laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden § ESRF-The European Synchrotron, 71, Avenue des Martyrs, Grenoble, France ∥ Huygens-Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands ‡
ABSTRACT: We have investigated the gas composition-dependent faceting of a Rh(553) surface during catalytic CO oxidation under semirealistic reaction conditions using surface X-ray diffraction. We find that under stoichiometric CO and O2 conditions, the Rh(553) surface maintains its surface orientation without facet formation. In oxygen excess, the CO oxidation reaction becomes mass-transfer limited by the CO diffusion, and the surface is observed to expose (331) or (111)̅ facets in coexistence with larger (111) terraces. The observed facet formation has previously been observed for pure O2 exposures of the Rh(553) surface, but at significantly lower O2 partial pressures. Surprisingly, in CO excess, which results in a CO-poisoned surface with low activity, we instead find coexisting (110) and (111) facets. The reasons for and possible implications of the observed facetings are discussed.
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INTRODUCTION A fundamental understanding of heterogeneous catalysis is one of the major aims of modern surface science. By studying flat single-crystal surfaces and carefully controlling gas exposures under ultrahigh vacuum (UHV) conditions, detailed information has been gained about the interaction between different gases and metal surfaces, but also about possible reaction paths between the adsorbed species.1,2 This kind of model system is, however, very different from industrial catalysis, which typically involves nanometer sized particles of an active material dispersed onto a porous oxide support operating under atmospheric, or higher, pressures. Hence, significant work has been done to bridge these so-called materials and pressure gaps.3,4 In particular, much attention has been focused on CO oxidation (CO + 1/2O2 ⇒ CO2) over noble metals (e.g., Pt, Pd, Rh, and Ir) under semirealistic conditions, and the bridging of the pressure gap has resulted in a debate concerning the active phase. Several studies have found that a thin surface oxide may form in conjunction with a switch from low to high catalytic activity, suggesting that an oxidic phase is responsible for the high activity,5−20 while other studies have suggested that the active surface is metallic21−26 also under realistic conditions. So far, most studies of this kind have been performed over lowindex single-crystal surfaces, although a significant number of reports suggest that atomic steps on the surface may be of crucial importance for the catalytic activity, see for instance refs 27−32. A straightforward way to investigate the effect of such features is to study vicinal surfaces with high, well-controlled densities of © 2015 American Chemical Society
steps. Although such surfaces are well-defined under UHV, exposure to gases has been found to alter surface structures significantly.33−35 For instance, Rh(553) has been found to undergo step-bunching upon O2 exposure, forming (331) and (111) facets at pressure of about 10−5 mbar. At this point, the (331) facets are covered by a one-dimensional oxide, while the (111) facets expose a (2 × 1) chemisorbed oxygen structure. At O2 pressures around 10−3 mbar, the step-bunching completes into (111) and (111̅) facets, both covered by a (9 × 9) surface oxide.33 We report on the faceting of a Rh(553) surface under semirealistic CO oxidation reaction conditions. We find that under stoichiometric and slightly CO-rich conditions, the Rh(553) surface orientation is maintained, while conditions of excess of CO and O2 induce step bunching, changing the local orientation of the surface. In the case of oxygen excess, we can relate the facet formation to the formation of one- (1D) and twodimensional (2D) oxides identical to observations in pure O2 at significantly lower O2 partial pressures. In the case of CO excess, the facet formation has to our knowledge not been observed previously. Preliminary density functional theory calculations indicate that they occur in order to avoid CO adsorption on unfavourably high coverages at high CO partial pressures. The results are compared to the previous study of faceting of Rh(553) Received: February 24, 2015 Revised: April 27, 2015 Published: May 4, 2015 11646
DOI: 10.1021/acs.jpcc.5b01841 J. Phys. Chem. C 2015, 119, 11646−11652
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The Journal of Physical Chemistry C in pure O2 at 10−5−10−3 mbar.33 The implications of the observed faceting is discussed.
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EXPERIMENTAL SECTION The experiments were performed at the ID03 surface diffraction beamline36 at the European Synchrotron Radiation Facility (ESRF) using a photon energy of 18 keV. For the actual measurements, a 13 mL reactor part of the chamber is sealed off, where the sample can be exposed to a gas flow while its surface structure is probed by surface X-ray diffraction (SXRD). The flow is controlled by mass flow controllers (Bronkhorst) calibrated such that their controllable range is 2−50 mLn/min. (1 mLn is the amount of gas corresponding to 1 mL at standard pressure and temperature.) The pressure in the reactor37 is usually measured and controlled by a back-pressure controller (Bronkhorst) for the range 100−1200 mbar. In the first part of the present study, we wanted to be able to compare our measurements with high-pressure X-ray photoelectron spectroscopy (HPXPS) data and needed to lower the pressure below 1 mbar. We therefore mounted a bypass around the pressure controller. Because the reactor volume is minimized (to have as good a flow reactor as possible), the pressure gauge could not be mounted close enough to measure the pressure in the reactor accurately, but we estimate the pressure to be in the range of 0.1−1 mbar when the flow was in the range of 2−12 mLn/min. The Rh(553) surface consists of (111) terraces about five atomic rows in width separated by monatomic steps with (111̅) microfacets (Figure 1A). The crystal basis used to derive the reciprocal space lattice is a monoclinic basis set, which for the (553) coordinates gives a1 = (−a0/2, a0/2, 0), a2 = (−3a0/2, 0, 5a0/2), and a3 = (5a0, 5a0, 3a0), expressed as a function of the cubic Rh lattice with a1 and a2 lying in the surface plane and of length 2.687 and 11.079 Å, respectively, and a3 out-of-plane with length 29.188 Å. In this basis, α = β = 90° and γ = 111.333°. (a0 = 3.80 Å is the bulk Rh lattice constant.) The surface was cleaned in a standard manner by cycles of Ar+ sputtering and annealing to about 800 °C until the SXRD measurements showed the results expected for the clean Rh(553) surface. Vertically from the Bragg reflection at (0, −4, 7) reciprocal lattice units (RLU), we see a strong crystal truncation rod (CTR), and there is another at K = −5 RLU originating from the (0, −5, −6) Bragg reflection. The CTRs are always perpendicular to the surface orientation, which in this case confirms the overall (553) orientation of the clean surface. In addition, we see one rod leaning about 12° along the (111) direction, which reveals the presence of the (111) oriented terraces that are intrinsic for the (553) surface (Figure 1A). Figure 1B,C shows a map of the Rh[110̅ ] zone and map of the surface (0, K, 1) plane sampled in the measurements, in relation to the bulk Bragg reflections. Figure 1D,E show a mesh scan in the H = 0 plane, which is typically used throughout this study. The catalytic activity is monitored by a mass spectrometer (MKS Microvision 2) located in the UHV part of the setup, with a controlled leak in the 10−6 mbar range from the reactor. The CO and O2 mass spectrometer signals were calibrated against the known partial pressures in the reactor at low activity, while the CO2 signal was calibrated such that its increase agrees with the drops in CO and O2 upon catalytic activation.
Figure 1. (A) Model of the Rh(553) surface with the different gasinduced facets found in this study indicated. (B) Map of the Rh[11̅0] zone with planes corresponding to relevant facets marked, as well as a graphical construction of the reciprocal space unit cell. (C) Map of the surface (0, K, 1) plane sampled in the measurements, in relation to the bulk Bragg reflections. The blue box indicates the region of the interest, which is the mesh scan area. (D, E) SXRD mesh scan in the H = 0 plane of the clean Rh(553) surface displayed as a surface plot (D) where we can directly see the CTRs and measure their orientation or as lineplots (E) where it is easier to see weak CTRs. Between the (553) oriented CTRs at K = −4 and −5, we find a weak (111) oriented CTR corresponding to the (111) terraces between the steps.
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RESULTS The gas composition in the reactor is shown in the top panel of Figure 2, as well as SXRD data panels I−IV revealing the surface
structure, in different mixtures of CO and O2 at a sample temperature of about 250 °C and a total pressure in the 11647
DOI: 10.1021/acs.jpcc.5b01841 J. Phys. Chem. C 2015, 119, 11646−11652
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The Journal of Physical Chemistry C
Figure 2. Gas composition in the reactor during the measurements, as measured by mass spectrometry. In situ SXRD mesh scans in the H = 0 plane of Rh(553) in different CO/O2 mixtures in the 0.1−1 mbar range at a sample temperature of about 250 °C. Under CO-rich conditions we find (110) and (111) facets if there is a large excess of CO (I) or an unreconstructed (553) surface closer to the stoichiometric mixture (II). In O2 excess we find (331) and (111) facets in slight excess (III) and (111̅) and (111) facets in large excess of O2 (IV).
presence of (110) facets in coexistence with larger (111) facets (see Figure 3I). When the CO:O2 ratio is decreased to 5:2, the (110) rod disappears from the SXRD data Figure 2II, while the catalytic activity increases significantly. The activity is probably so high that it is limited by the diffusion of reactants to the surface rather than by the intrinsic activity of the surface, a situation referred to as the mass-transfer limit (MTL). Under the present conditions we have an excess of CO in the gas flow, and the reaction is limited by the amount of O2 present. In the MTL, the surface is mainly exposed to the excess reactant (CO in this case) and the usually inert product (CO2).
0.1−1 mbar range. Proposed schematic models are shown in Figure 3. Figure 2I shows the result from a CO:O2 ratio of 10:2. Under these conditions, the surface is most probably poisoned by adsorbed CO, which hinders dissociative O2 adsorption.26 This is reflected in the mass spectrometry measurements, which show low catalytic activity. In the SXRD data, we can at this point observe the CTRs from the (553) and (111) facets, as for the clean surface (Figure 1), but also a broad rod feature between K = −3 and −4, leaning about 22° in the other direction as compared to the (111) facet. This feature indicates the 11648
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Figure 3. Models for each gas composition corresponding to Figure 2.
Consequently, we still expect the surface to be dominated by adsorbed CO, but probably with a lower coverage due to the lower CO partial pressure and rapid reaction of adsorbed CO to CO2.35 The increase in catalytic activity results in an increase in sample temperature of approximately 10 °C due to the exothermicity of the reaction. In Figure 2III, the CO:O2 ratio has been reduced further to 3.5:2. Because each O2 molecule can oxidize two CO molecules, this gives a slight oxygen excess and the catalytic activity is now limited by the amount of CO that is supplied to the surface. The surface is then mainly exposed to O2 and CO2. When exposed to 5 × 10−6 mbar of pure O2, the Rh(553) has previously been found to rearrange into coexisting (331) facets, where the step edge exhibits a 1D oxide, and (111) facets with chemisorbed oxygen atoms.33 In the present case, the O2 partial pressure is much higher, but because of the presence of both CO and CO2 under reaction conditions, the resulting structure is similar, as shown in Figure 2III. Close to the (553) and (111) rods, around K = −4, there is a broadening toward the right, and around K = −2, there is a rod leaning about 9.7° relative to the (553) direction, indicating the formation of (331) facets. Finally, Figure 2IV shows the results from a CO:O2 ratio of 2:2, i.e., a large excess of O2. Although the mass spectrometry shows a lower activity, this is most probably only due to the lower CO partial pressure, and the reaction is still mass-transfer limited. The (331) facets are now almost gone, and instead a new rod oriented in the (111) direction, between K = −3.5 and K = −4, has appeared. This new rod is not a Rh(111) CTR, however, but corresponds to a superstructure on the (111) facets, namely the (9 × 9) trilayer surface oxide characterized in previous studies.38 Therefore, in a large excess of O2, it is deduced that the (553) surface is transformed completely into a combination of (111) and (111̅) facets.33 The faceting of the (553) surface was even more pronounced in measurements conducted using higher pressures of CO and O2. These measurements were made using a constant total pressure of 300 mbar, a constant CO partial pressure of 100 mbar, and a variable O2 partial pressure balanced with Ar. Figure 4A shows the diffraction pattern obtained under conditions of slight O2 excess (60 mbar O2, 100 mbar CO), after first cycling between oxygen-rich and CO-rich conditions. The measurement clearly reveals coexisting (111) facets, covered by a trilayer 2D surface oxide, and (331) facets with its 1D oxide. There is still a trace of the CO-induced (110) facet, but this is believed to be a residual feature from heavy faceting induced during the initial treatment at this pressure. After the conditions are switched back to being CO-rich (45 mbar O2 and 100 mbar CO), the (331) and
Figure 4. SXRD mesh scans in the H = 0 plane of Rh(553) after severe faceting in a total pressure of 300 mbar at about 250 °C. (A) 100 mbar CO, 60 mbar O2 and 140 mbar Ar (slight O 2 excess) results in (331) and (111) facets. (B) 100 mbar CO, 45 mbar O2 and 155 mbar Ar (slight CO excess) results in (101) and (111) facets. 11649
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The Journal of Physical Chemistry C surface oxide rods disappear, while the signal from (110) facets increases in intensity (Figure 4B). In this experiment, we do not observe the reappearance of the (553) rods.
The gas-composition related faceting observed in the present investigation should be of importance for the understanding of the structure−activity relationship in industrial catalysts because similar energetics determine the presence of different facets on catalytically active nanoparticles. Previously it has been observed that pure oxygen will induce changes of the relative amount of facets and therefore also the shape of a Rh or Pd nanoparticle.42,43 Our study shows that this will occur not only under semirealistic reaction conditions for the CO oxidation reaction in excess of oxygen but also for conditions with excess of CO.
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DISCUSSION In the present report, we have studied the faceting of a Rh(553) surface under semirealistic conditions for catalytic CO oxidation. We have shown that, at low pressures (0.1−1 mbar), the surface maintains its (553) orientation under nearly stoichiometric (slightly CO-rich) conditions. We have also shown that the surface forms (110) facets under CO-rich conditions and (331) or (111)̅ facets under oxygen-rich conditions, all in coexistence with extended (111) facets. Under oxygen-rich conditions, the formation of (331) and (111) facets, which is indicative of the formation of a 1D oxide, first in coexistence with chemisorbed oxygen and later in coexistence with a 2D oxide, imitates closely the behavior of the Rh(553) surface when exposed to between 10−6 and 10−3 mbar of pure O2.33 Considering that the pressures used are significantly different, it is not obvious that this should be the case. It has, however, been observed previously that the MTL with O2 excess leads to a situation in which the surface behaves as in a lower pressure of pure O2.7,26 At this point, it is interesting to compare the present measurements with the previous HPXPS measurements from the Rh(100) surface, which were conducted under similar conditions.26 In contrast to the present study, where the surface oxide formation can be clearly seen (Figure 2III), the HPXPS study did not show any signs of surface oxide formation. While the Rh(553) and the Rh(100) surfaces obviously are different, similar trilayer surface oxides are formed on the two surfaces in the same pressure range (10−5−10−3 mbar) in pure O2.33,39 Therefore, we would expect the same structures in the HPXPS and the present SXRD experiments. The most likely reason for this discrepancy lies in the different designs of the reactors used. Most importantly, the gas flows differently in the different reactors, which affects how fast the gas over the sample is exchanged and thus to what degree the CO partial pressure is reduced near the surface when the reaction is mass-transfer limited. A full understanding of this would need extensive theoretical considerations.40 The formation of (110) facets under CO-rich conditions has, to our knowledge, not been observed previously. To elucidate the mechanism behind this faceting, we have initiated density functional theory (DFT) calculations of the surface energies and adsorption properties of CO on different Rh facets. The complete results of this study will be published elsewhere. Preliminarily, the calculations indicate that when the CO partial pressure is increased from 0.01 to 0.1 mbar, at 250 °C, the CO coverage on Rh(553) increases from 5 to 6 molecules per unit cell, while the coverages on Rh(111) and Rh(110) are unchanged. This leads to a significant increase of the surface energy of the (553) facet, and it becomes thermodynamically favorable to reform the surface into (110) and (111) facets. Within the accuracy of these calculations and our measurements, this would explain why we find faceting at the higher, but not at the lower, CO flows for the lower-pressure regime (panels I and II of Figure 2, respectively). It would also explain why the faceting occurs already at a slight CO excess at the higher pressure. On the other hand, we cannot rule out that the faceting is induced by atomic C or graphite that is formed on the surface as a result of CO dissociation, which is known to happen at stepped Rh surfaces.41
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CONCLUSION We have investigated the faceting of a Rh(553) surface during catalytic CO oxidation under semirealistic conditions. We find that the surface maintains its (553) orientation under lowpressure (0.1−1 mbar) stoichiometric conditions. In an excess of CO we show that the surface exposes (110) facets, while O2 excess leads to the formation of (331) or (111̅) facets, all coexisting with enlarged (111) terraces. While the faceting found in O2 excess agrees perfectly with what have previously been reported for pure O2 exposure, the CO-induced facets are, to our knowledge, new. Preliminary DFT results indicate that the CO-induced faceting is caused by an increased CO coverage from 5 to 6 molecules per Rh(553) surface unit cell, which increases the surface energy of the (553) facet and makes the transformation into (111) and (110) facets favorable. The present study demonstrates the gas compositiondependent faceting of a vicinal surface under semirealistic reaction conditions and highlights the structural dynamics of an active surface or nanoparticle-based catalyst under reaction conditions.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is done within the Röntgen-Ångström Cluster. The authors thank the Swedish Research Council, the Swedish Foundation for Strategic Research (SSF), and the Crafoord Foundation. Support by the ESRF staff is gratefully acknowledged.
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DOI: 10.1021/acs.jpcc.5b01841 J. Phys. Chem. C 2015, 119, 11646−11652
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DOI: 10.1021/acs.jpcc.5b01841 J. Phys. Chem. C 2015, 119, 11646−11652
