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Cite this: Phys. Chem. Chem. Phys., 2012, 14, 6688–6697 www.rsc.org/pccp

PERSPECTIVE

CO oxidation over ruthenium: identification of the catalytically active phases at near-atmospheric pressures Feng Gao*a and D. Wayne Goodmanb Received 12th January 2012, Accepted 14th March 2012 DOI: 10.1039/c2cp40121e CO oxidation was carried out over Ru(0001) and RuO2(110) thin film grown on Ru(0001) at various O2/CO ratios near atmospheric pressures. Reaction kinetics, coupled with in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements, were used to identify the catalytically relevant phases under different reaction conditions. Under stoichiometric and reducing conditions at all reaction temperatures, as well as net-oxidizing reaction conditions below B475 K, a reduced metallic phase with chemisorbed oxygen is the thermodynamically stable and catalytically active phase. On this surface CO oxidation occurs at surface defect sites, for example step edges. Only under net-oxidizing reaction conditions and above B475 K is the RuO2 thin film grown on metallic Ru stable and active. However, RuO2 is not active itself without the existence of the metal substrate, suggesting the importance of a strong metal–substrate interaction (SMSI).

1. Introduction Among Pt-group metals, the anomalous behavior for Ru in CO oxidation has been recognized for decades.1–5 Briefly, metallic Ru is very unreactive under or near ultrahigh vacuum (UHV) conditions,1–3 yet becomes highly active at near-atmospheric pressures.4,5 This phenomenon is frequently used as an example to demonstrate that there is a ‘‘pressure gap’’ between surface science at UHV and heterogeneous catalysis at elevated pressures. It has to be noted that all the low activity findings for Ru in vacuum are based on studies over Ru(0001).2,3 Oxygen coverage cannot exceed 0.5 ML when this specific surface is exposed to molecular oxygen in vacuum. In this case, chemisorbed oxygen interacts strongly with the metal surface and thus cannot react with chemisorbed CO.6 However, this generally accepted notion, i.e., the low activity for Ru in vacuum, needs to be corrected. Recently a study by Yates et al.7 has revealed facile CO2 formation even at subambient temperatures using CO/O2 as reactants on a vicinal single crystal Ru(109) model catalyst in UHV. This is an important finding demonstrating that metallic Ru can be as active as other Pt-group metals8,9 in vacuum. Using Ru(0001) as the model catalyst, Peden and Goodman found that the active surface for CO oxidation under nearatmospheric pressure conditions is a Ru(0001)-(1 1)-O phase a

Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA. E-mail: [email protected]; Fax: +1 509 371 6066; Tel: +1 509 371 7164 b Department of Chemistry, Texas A & M University, P.O. Box 30012, College Station, TX 77842-3012, USA

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(yO = 1 ML). The structure was confirmed by ex situ AES and LEED analysis.1 This finding appears to easily explain the high activity since oxygen binding energy certainly decreases when its coverage increases.6 This notion, however, has been challenged recently.10 First, CO binds too weakly on the Ru(0001)-(1 1)-O terrace (o20 kJ mol 1).11 The CO coverage is expected to be too small to account for the rather high turnover frequencies under reaction conditions assuming the Langmuir– Hinshelwood mechanism applies. The activation barrier is too high for the Eley–Rideal mechanism to apply either, based on DFT calculations.12,13 DFT calculation also suggested that reaction cannot proceed at point defects of the Ru(0001)-(11)-O terrace (i.e., CO adsorption and reaction at missing oxygen sites), again due to the high activation barrier.13 Very recent DFT calculation has suggested that the reaction actually occurs on the monatomic step sites on Ru(0001)-(1 1)-O.14 We will show below that this finding is fully consistent with our experimental results. In recent years researchers, especially Over and co-workers, proposed that RuO2(110), which grows epitaxially on Ru(0001), is the phase that bridges the pressure gap, i.e., the phase that is active both at UHV and elevated pressures.15–31 RuO2 with other orientations, e.g. (100) and (101) phases, were also suggested by the same authors to be active in CO oxidation.16,30,32 Coordinatively unsaturated Ru sites (cus-Ru) have been suggested to be the catalytically active sites. The structure of RuO2(110) was precisely determined with LEED, STM and DFT calculations.15,17,20 In UHV, CO oxidation indeed occurs on this surface as determined with an array of techniques including TPD, HREELS, molecular beam,15,16,19,21,23,26 as well as theoretical calculations.33–35 However, in order for the ‘‘pressure gap’’ to be bridged by RuO2, one must prove that this phase is active and stable This journal is

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under elevated pressure conditions. Information gathered from the recent literature indicates that this is questionable: (1) For RuO2(110) films grown on Ru(0001), a ‘‘magic’’ film thickness of 1–2 nm was found to be the most active. However, Over and co-workers found that above 670 K the RuO2 film grows thicker and roughens, forming various facets, transforming the surface into a catalytically inactive RuO2 oxide in vacuum.29 Clearly one cannot argue that a ‘‘rough’’ surface is unreactive; rather it is conceivable that a ‘‘thick’’ film is unreactive. At near-atmospheric pressures, these authors also found sudden deactivation of RuO2 under (and only under) net-oxidizing conditions.28 Although they ascribed this to carbonate formation, this argument is not convincing since carbonate formation should also occur under reducing conditions upon gas phase CO2 formation yet deactivation had never been found. Again this deactivation behavior is better ascribed by thicker RuO2 oxide formation under net-oxidizing conditions. Interestingly, recent studies by Rosenthal et al. found that commercial RuO2 powder, which is immediately active in CO oxidation, always contains some reduced metal. However, when it is calcined in O2 to ensure complete oxidation, the resulting RuO2 is completely inactive in CO oxidation regardless of the CO/O2 ratio. Activity can only be achieved after an induction (reduction) period.36,37 On RuO2(100), Over and co-workers discovered that while the (1 1) phase is very active, a c(2 2) phase (derived from the (1 1) phase; the structure has not been identified yet) is inactive since CO does not adsorb on this phase, even though this phase is most likely Ru terminated.29,32 Nevertheless, it is simply incorrect to claim that thick RuO2 films grown on Ru metal and fully oxidized RuO2 powder are

terminated by this single inert RuO2(100)-c(2 2) phase. For fully oxidized RuO2 powder, this is certainly not the case since XRD measurements discovered exposure of {110}, {100}, {101} and {111} facets.36 (2) RuO2 is thermodynamically stable in vacuum and in O2;15,34 however, it is facilely reduced by CO. The reduction can be classified as ‘‘mildly’’ or ‘‘heavily’’ whereas in the former case, only the surface bridging oxygen reacts whereas the rutile structure is maintained.24 However reduction does not stop at this stage at temperatures higher than B400 K where the surface is continuously (i.e., heavily) reduced to metallic Ru. Even in 1 10 5 mbar of CO at 418 K, SXRD and XPS measurements clearly demonstrate the disappearance of the oxide phase and the formation of the metallic phase, albeit rather slowly.29 It is fully conceivable that at higher temperatures and near-atmospheric pressures reduction can be much faster. Especially, the generation of a thin metal layer on top of RuO2 should be facile even if further reduction is more difficult since the latter process requires oxygen diffusion to the surface. The fact that RuO2 reduction occurs in CO/O2 mixtures (as will be shown below) raises a critical question that needs to be answered: on what surface does CO oxidation occur under near-atmospheric conditions, on the reduced metal layer, on RuO2, or at the oxide–metal interface? In the following we present our results of CO oxidation at elevated pressures over both clean Ru(0001) and pre-formed RuO2(110) on Ru(0001) under a variety of reaction conditions. We use reaction kinetics measurements coupled with polarizationmodulation infrared reflection absorption spectroscopy (PM-IRAS), which allows detailed in situ analysis of surface

Feng Gao received undergraduate and graduate education at Tianjin University, China in the 1990s in Chemical Engineering. He joined the University of Wisconsin-Milwaukee in 2000 as a graduate student and received PhD in Physical Chemistry in 2004 under the supervision of Prof. Wilfred T. Tysoe. From 2007 to 2009, he was a postdoc at Texas A & M University under the supervision of Prof. D. Wayne Feng Gao Goodman. After a brief stay at Washington State University as a research faculty, he is currently a staff scientist at Pacific Northwest National Laboratory (PNNL), conducting research in basic and environmental heterogeneous catalysis.

Wayne Goodman joined the faculty of the Chemistry Department at Texas A & M in 1988 where is currently Distinguished Professor and the Robert A. Welch Chair. Previously he was Head of the Surface Science Division at Sandia National Laboratories. He is the recipient of the Ipatieff Award of the American Chemical Society in 1983, the Colloid and Surface Chemistry Award of the American Chemical Society D. Wayne Goodman in 1993, the Yarwood Medal of the British Vacuum Society in 1994, a Humboldt Research Award in 1995, a Distinguished Research Award of Texas A & M University in 1997, the Giuseppe Parravano Award in 2001, the Adamson Award for Distinguished Service in the Advancement of Surface Chemistry of the American Chemical Society in 2002, the Gabor A. Somorjai Award of the American Chemical Society in 2005 and elected Fellow of the American Chemical Society in 2009. He is the author of over 540 publications/book chapters and is an active member/officer of a number of professional societies. He has served as an Associate Editor for the Journal of Catalysis, and currently serves on the Advisory Boards of Surface Science, Langmuir, Catalysis Letters, and the Journal of Physics: Condensed Matter.

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CO species at elevated pressures, aiming to unravel the complex structure–activity relationship. Some of these results have been published in 2009.11 However with the appearance of the new literature in the past few years, we feel it is necessary to update our understanding, and to clarify misunderstandings introduced by others in order to shed more light on this complex system.

2. Experimental section The experimental apparatus used for the polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) and reaction kinetics measurements has been described in detail previously.38,39 Briefly, the apparatus consists of: (1) a UHV section equipped with Auger spectroscopy (AES), low energy electron diffraction (LEED), and UTI 100 mass spectrometer and (2) a high-pressure reaction and infrared cell. Ru(0001) was cleaned using repeated ion sputtering, reacting with 1 10 5 Torr of O2 at 900 K to remove surface carbon, and flashing to 1600 K via electron bombardment to remove any volatile species. The sample cleanliness was verified by AES. RuO2(110) was formed on Ru(0001) using a procedure described by Over and co-workers, i.e. by reacting B1.5 106 L (1 L = 1 10 6 Torr s) of O2 with Ru(0001) at 700 K to form the RuO2(110) phase (1–2 nm thick) coexisted with the (1 1)-O phase.15 Following sample preparation, CO + O2 mixtures were introduced into the reactor to carry out the oxidation reaction. Reaction rates were derived from pressure change of the reactants monitored with a Baratron gauge39 and surface CO species were monitored simultaneously by PM-IRAS, carried out using a Bruker Equinox 55 FTIR spectrometer coupled with a polarization modulator to subtract infrared signals arising from gas-phase absorption. This method allows in situ measurements of surface species within a wide pressure range from UHV to atmospheric pressures.

3. Results and discussion 3.1 Comparison between Ru(0001) and RuO2(110) under stoichiometric (O2/CO = 1/2) reaction conditions Fig. 1 presents Arrhenius plots over Ru(0001) and freshly prepared RuO2(110) thin film under stoichiometric conditions (O2/CO = 1/2, Ptotal = 12 Torr). For comparison, kinetic data over Pt(110) and Pd(111) are also displayed.40 Two kinetic regimes are apparent over Pt and Pd, i.e., a linear regime (in Arrhenius form) at lower temperatures, which has been well-documented to be the CO-inhibited regime; and a mass-transfer limited regime at higher temperatures where the CO2 turnover frequency (TOF) is constant. On Ru(0001), a linear Arrhenius regime is found below 525 K while CO2 formation ‘‘rolls over’’ above 525 K. We emphasize that this rollover behavior is not due to mass-transfer limitation, but rather to a decrease in the residence time of CO(a) at higher temperatures. We note that under the reaction conditions, the catalytically active phase on Ru(0001) is Ru(0001)-(1 1)-O.1,41,42 It is also noticed that in the rollover regime CO TOF increases slightly with increasing temperature, presumably due to the population of subsurface oxygen.43 On freshly prepared 6690


Fig. 1 Arrhenius plots of CO2 formation over Pt(110) (’), Pd(111) (m), Ru(0001) (K) and RuO2(110) (.) under stoichiometric reaction conditions (Ptotal = 12 Torr, O2/CO = 1/2). Reaction was carried out in a 61.6 litre batch reactor. CO conversion was maintained below 10% to obtain differential rates. Kinetic data for Pt and Pd samples were taken from ref. 40. RuO2 thin film was grown on Ru(0001) using a method provided by ref. 15.

RuO2(110), linear Arrhenius behavior is observed below 700 K. Before further analyzing these data, it is worth recalling the classical work carried out by Cant et al. over silica supported Pt-group metals in CO oxidation under stoichiometric conditions. At 400–450 K, these authors found, on a per metal atom basis, that Ru is 1 to 2 orders of magnitude more active than Pt, Pd and Rh catalysts.4 At first glance, the kinetic data shown in Fig. 1 seem to suggest that both metallic Ru and RuO2 could be the active phase on the supported catalyst. However we emphasize, as will be shown below in more detail that the actual active phase in this case is not RuO2. Different research groups have calculated the binding energies of CO on RuO2 where consistent results were found. In summary, they found CO binding energies of B120 kJ mol 1 on cus-Ru sites on stoichiometric RuO2, B155 kJ mol 1 on cus-Ru sites on mildly reduced RuO2, and B178 kJ mol 1 on bridge-Ru sites on mildly reduced RuO2.33,44 We emphasize that the CO binding energy on Pd and Pt surfaces under elevated-pressure CO oxidation reaction conditions is also B120 kJ mol 1.45 On Pt-group metals other than Ru, it has been well-documented that at elevated pressures and within the CO-inhibited regime, the metal surfaces are almost completely covered with CO(a). In this case, CO desorption is the kinetically limiting step: CO oxidation only occurs when CO desorbs and creates open sites for O2 adsorption and dissociation. As such, reaction kinetics displays +1 order in O2 pressure and 1 order in CO pressure. Moreover, the measured apparent activation energy of B100 kJ mol 1 is rather close to the CO binding energy.45,46 We emphasize here, were RuO2 the dominant active phase, the reaction kinetics must follow the exact same trend for two reasons: (i) the CO binding energy on RuO2 is as large or even larger than on Pd and Pt surfaces at near-atmospheric pressures. Especially, both the binding energies of O(a) and O2(a) are substantially smaller than CO(a) on cus-Ru sites;17,33–35 a CO-inhibited situation would certainly occur at elevated CO pressures were RuO2 the stable and active phase; (ii) statistically it is more difficult to create open sites for O2 dissociation (must be two open sites next to each other) on This journal is

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CO fully covered RuO2 since the cus-Ru sites are one dimensional (cus-Ru rows are separated by lattice oxygen rows)30 while on other Pt-group metals the O2 dissociation sites are two dimensional (i.e., the open terrace). However, the kinetic data shown in the open literature are completely different from this ‘‘should be’’ picture. First, on any Ru catalysts under stoichiometric reaction conditions, the apparent activation energy of 80 10 kJ mol 1 1,4,22,24,25,28,29,36 is measurably smaller than the CO binding energies on RuO2.15,17,33–35 Second, the weak pressure dependence on both O2 and CO in reaction kinetics differs totally from other Pt-group metals45,46 but resembles metallic Ru1. All these experimental findings point to one fact: RuO2 is fully-covered by CO under stoichiometric reaction conditions and thus displays no activity. At least a portion however is reduced to metallic Ru by CO and the majority of CO oxidation is carried out on the reduced metal surface. In the following this picture is further demonstrated by reaction kinetics measurements, ex situ AES and in situ PM-IRAS. Fig. 2(a) shows kinetic data over a pre-formed RuO2(110) thin film in repeated cases. The experimental procedure is the following: first the freshly prepared RuO2(110) was exposed to 12 Torr of stoichiometric reactants and heated step-wise from 425 to 650 K. The sample was held at each target temperature for 10 min to obtain a reliable pressure drop for reaction rate (TOF) calculations. Following this, the sample was cooled and the reactant/product was pumped out. The sample was quickly annealed to 600 K to remove chemisorbed CO before Auger analysis. This is necessary because (1) the C and Mo Auger transitions overlap and (2) high-energy electron beam induces CO dissociation during AES measurements. Thereafter fresh reactant was introduced for the second reaction and characterization cycle. In this fashion, the experiment was repeated for 5 runs. Note that the experimental data for RuO2 shown in Fig. 1(.) are from this first run. It is clearly demonstrated in Fig. 2(a) that at each reaction temperature below 600 K, CO2 formation rate increases substantially in the second run and maximizes and stabilizes in subsequent runs. Displayed in Fig. 2(b) are selected Auger spectra of the RuO2 sample at different stages. Before the CO oxidation reaction, the as-formed RuO2(110) sample has an oxygen coverage of yO E33 ML (obtained by comparing with the O/Ru Auger signal ratio of the Ru(0001)-(2 1)-O surface at yO = 0.5 ML). It is worth noting that the oxygen coverage of at least 4 ML (two O–Ru–O trilayers) is needed to form RuO2.47 However, after the second run, the oxygen coverage drops to B1.8 ML demonstrating the disappearance of RuO2 via reduction. At this stage, the sample surface might be described as a mixture of a transition oxide phase and metallic phase with chemisorbed oxygen. At the end of the fifth run, the oxygen coverage of B0.84 ML clearly demonstrates the remaining phase being dominantly metallic Ru with chemisorbed oxygen. Fig. 2(c) displays the corresponding selected PM-IRAS spectra collected at 400, 500 and 600 K. We emphasize that it is far from trivial to make proper assignments of the CO bands on Ru surfaces since multiple factors including CO coverage (which affects dipole–dipole coupling), temperature (which affects CO coverage), oxygen coverage and nature, Ru oxidation state, sample morphology and supports could all This journal is

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have profound effects on the CO vibrational frequencies. In order to do this we first examine representative nCO data (listed in Table 1) from the open literature on Ru(0001),48 O/Ru(0001),49,50 O/Ru(109),7 RuO2(110)30 and Ru/SiO25,22 samples. If we limit ourselves to the assignment of linear (atop) CO bands, the following summary can be obtained: (1) nCO at B2050 cm 1 is safely assigned to CO adsorption on reduced metallic Ru; (2) nCO higher than B2050 cm 1 but below 2080 cm 1 is assigned to CO adsorption on oxygen-covered metallic Ru where oxygen coverage is not dense (e.g., r0.5 ML on Ru(0001)); (3) nCO higher than B2080 cm 1 could be assigned to CO on dense chemisorbed

Fig. 2 (a) Arrhenius plots of CO2 formation over RuO2(110) during repeated runs using the same sample under stoichiometric reaction conditions (Ptotal = 12 Torr, O2/CO = 1/2). Reaction was carried out in a 61.6 litre batch reactor. CO conversion was maintained below 10% to obtain differential rates. Each rate point is the average of 10-min of reaction at each temperature, derived from the pressure change of the gas phase. Different symbols represent data obtained in different runs: 1st (’), 2nd (K), 3rd (m), 4th (.) and 5th (E). RuO2 thin film was grown on Ru(0001) using a method provided by ref. 15. (b) Auger spectra of the as-formed RuO2(110) sample (bottom), after the second run (middle), and after the fifth run (top). All spectra were taken after briefly annealing the sample to 600 K in UHV to remove volatile species. The corresponding oxygen coverage is marked adjacent to each spectrum, calculated based on O/Ru Auger ratio of a Ru(0001)(2 1)-O surface which has oxygen coverage of 0.5 ML. (c) Selected PM-IRAS spectra for the experiment shown in Fig. 2(a) at 400 K (left panel), 500 K (middle panel) and 600 K (right panel). Each spectrum is a superimposition of 10-min of scans at 4 cm 1 (totally B600 scans). Reaction runs are marked adjacent to each spectrum.


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oxygen covered metallic Ru, or RuO2, or transition oxides; (4) on supported Ru catalysts, the 2130–2140/2060–2080 cm 1 bands are assigned to carbonyl species Rux+(CO)y where both x and y can vary. Based on the assignments shown above, the 2067 cm 1 band (400 K), 2078 cm 1 band (500 K) and 2046 cm 1 band (600 K) in Fig. 2(c) demonstrate the reduction of RuO2 to metallic Ru, fully consistent with the AES data shown in Fig. 2(b). The B2130 and B2086 cm 1 bands are more difficult to assign. Based on work by Over et al., both bands can be assigned to CO adsorbed on RuO2, especially the 2130 cm 1 band.30 Yet additional assignments (i.e., overlapping bands) should be added: (1) the 2130/2086 cm 1 features also contain some contribution from carbonyl species. This is specifically apparent for spectra acquired at 500 K during the 4th and 5th runs (Fig. 2(c), middle panel). During these two runs the RuO2 sample has already been heavily reduced yet clear blue-shift was noticed for these two bands (at 2133/2089 cm 1) as compared with the first 3 runs. It is certainly more logical to assign these two relatively weak features to carbonyl species (for chemisorbed linear CO, blue shift of nCO with both CO and oxygen coverage decrease is not expected). On planar terraces metal carbonyl species should not form. However for heavily reduced RuO2, the surface has been demonstrated to be very rough20,36 and the increased population of edge and corner Ru atoms (where carbonyl species form) is fully expected. Also the rather well-resolved 2021 cm 1 feature (assigned to CO adsorbed on reduced Ru) also strongly suggests the existence of surface carbonyl species, as is often seen in the case of supported Ru that this band always accompanies surface carbonyl species.5,22 (2) The 2086 cm 1 band (which red shifts with increasing temperature) also contains contribution from CO on dense chemisorbed oxygen covered metallic Ru. Clearly, as shown from the spectra collected at 600 K, this feature maintains even after the RuO2 has been fully reduced. We note that on chemisorbed oxygen covered Ru(109) step sites,7 CO chemisorbed in the step edge has nCO of 2084 cm 1. The reaction kinetics and spectroscopies shown in Fig. 2 deliver very important information regarding the activity of the RuO2 phase. It is clearly seen, especially by comparing

2048 2056, 2080 2090, 2068, 2050 2123 2086 2084, 2130, 2130, 2135, 2134,

1

2080 2076 1849 (b)a

a

2068, 1858 (b)

2080, 2010 2060, 2010 2080, 2030 2078 (sh)b, 2051, 2010

Bridging CO.

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The section above demonstrates that under stoichiometric reaction conditions at near-atmospheric pressures, pre-formed RuO2 is easily reduced to metallic Ru and the latter is the real catalytically active phase for CO oxidation. However, as mentioned in the introduction, the open terrace on Ru(0001) does not appear to be active enough for two reasons: (1) at low coverages, oxygen binds too strongly with Ru resulting in low activity; (2) at an oxygen coverage of 1 ML (i.e. Ru(0001)-(1 1)-O), CO binds too weakly on the terrace sites and therefore cannot serve as the reactant. However, as shown in Fig. 1 (also from ref. 1), Ru(0001)-(1 1)-O is indeed very active in CO oxidation. In the following we show that the reaction actually occurs on step defect sites. Fig. 3(a) displays Arrhenius plots of stoichiometric reactants over Ru(0001)-(1 1)-O at various pressures from 0.01 to 16 Torr.

Surface

Spectra acquisition condition

Reference

Ru(0001) O/Ru(0001), yO = 0.33 ML O/Ru(0001), yO = 0.5 ML O/Ru(0001), yO E 1.0 ML (2CO + O)(2 2)/Ru(0001), yCO = 0.5 ML (CO + O)(2 2)/Ru(0001), yCO = 0.25 ML Stoichiometric RuO2(110), yCO = 0.6 ML Mildly reduced RuO2(110), yCO = 0.64 ML CO/O/Ru(109), O/Ru = 0.5 at terrace sites, O/Ru B1.3 at step sites Ru/SiO2

500 K, 2.5 Torr CO 500 K, 2.5 Torr CO + 0.05 Torr O2 500 K, 2.5 Torr CO + 0.1 Torr O2 85 K, saturation with CO 102 K 102 K 110 K 110 K O adsorption: 5 1015 O2 cm 2 at 500 K, followed by CO saturation at 85 K 373 K, CO/O2 = 2, PCO = 16 Torr 373 K, CO/O2 = 4, PCO = 32 Torr 373 K, CO/O2 = 0.5, PCO = 8 Torr 423 K, 150 NmL min 1 (1.7% CO, 0.43% O2), CO/O2 = 4 423 K, 150 NmL min 1 (1.7% CO, 3.4% O2), CO/O2 = 0.5

48

Ru/SiO2

2120, 2078, 2010 (sh)b a

3.2 Nature of the active sites and reactive species on metallic Ru

Representative CO vibrational frequencies on Ru(0001), O/Ru(0001), O/Ru(109), RuO2(110) and Ru/SiO2 samples

Table 1 nCO/cm

infrared spectra taken at 400 and 500 K from the 2nd to 4th runs (Fig. 2(c)) with the reaction kinetics data (Fig. 2(a)), that the disappearance of the RuO2 phase (accompanied by the generation of the metallic Ru phase) causes CO2 formation rate to increase for roughly an order of magnitude. This is convincing evidence to demonstrate that below B500 K and under stoichiometric near-atmospheric conditions, CO oxidation over Ru catalysts in any form is essentially irrelevant to RuO2. We will show below that this is due to the much reduced COinhibition (i.e., lower CO binding energy) on the metallic Ru phase. Only at very high temperatures (above B600 K) the activity of RuO2 (if this phase survives transiently before reduction) can be higher than metallic Ru due to a very simple reason: the much higher CO binding energy with RuO2. This allows, at very high temperatures (at which reaction activation barrier is easily surpassed), a higher CO resident time and thus higher reaction probability on RuO2 as compared with metallic Ru. Based on this, Over and co-workers made an argument that RuO2 is more active than non-oxidic Ru.31 We emphasize that this statement is incorrect below B600 K and insignificant since RuO2 is thermodynamically unstable under high-temperature stoichiometric conditions. Moreover, this is largely irrelevant with kinetics studies over supported Ru catalysts where reactions are typically carried out at much lower temperatures.4,25

b

49 50 30 7 5 22

Shoulder peak.


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The CO2 formation rate rolls over at high reaction temperatures at all pressures from a common line (data taken from ref. 1) indicating common saturated surface CO concentrations independent of the gas phase pressure as long as the sample temperature is sufficiently low.51 With increasing pressure, the CO reaction probability (Prx) at the rollover points (marked with A, B, C and D) increases slightly from B2 10 5 to B4 10 5 of the corresponding CO flux calculated using the Hertz–Knudsen equation.52 The rather constant CO reaction probability at vastly different gas pressures is not a coincidence but rather demonstrates that the reaction occurs at (density largely invariant) surface defect sites. It is important to note that at temperatures above the rollover points, CO conversion keeps almost constant (the slight increase with increasing temperature might be due to subsurface oxygen accumulation). This could only be rationalized by the fact that every CO molecule impinging at a surface defect site reacts to form CO2. This finding allows (1) the density of surface defect sites to be estimated as B10 5 ML; and (2) the isosteric heat of adsorption (binding energy) of the ‘‘reactive’’ CO species to be calculated using the Clausius–Clapeyron equation, where temperatures at the rollover points (A–D) and the corresponding CO pressures are used for the calculation. As displayed in Fig. 3(b), the calculated binding energy of the ‘‘reactive’’ CO species is B68 kJ mol 1. Very recent DFT calculation has revealed that the defect sites on Ru(0001)-(1 1)-O are monatomic step sites on which the reaction activation energy between CO and O is 0.63–0.68 eV, far below that on terrace sites (1.5 eV).14 It is interesting to note that calculated CO binding energy at step edges (0.69 eV) is identical to the value we get experimentally in this study. The experimental study by Yates and co-workers7 also elegantly demonstrated why the step sites are reactive: O/Ru ratio is high at the step sites (this results in low O(a) binding energy) yet these oxygen-covered sites are still able to accommodate reactive CO(a) (at 2084 cm 1) for subsequent combination reactions. 3.3

RuO2 under net reducing conditions

The results shown above demonstrate the high activity of step sites on metallic Ru. Previous studies have also revealed that upon heavy reduction, the metallic Ru surface generated by RuO2 reduction is highly defective.20,36 It is expected that such a metal surface is much more active than the Ru(0001) surface on which the defect density is merely B10 5 ML. In the following, a RuO2 sample generated on Ru(0001) (initial oxygen coverage B20 ML) is exposed to a reaction mixture of 40 Torr CO and 4 Torr O2 at 500 K. Reaction rate data are displayed in Fig. 4(a) as a function of reaction time, where each data point represents the rate averaged over a 5 minute period. Fig. 4(b) presents ex situ AES spectra at various stages of the reaction, acquired after stopping the reaction, pumping out reactants, evacuating the reactor to B10 9 Torr and flashing the sample to 600 K to remove on-surface CO species. The initial RuO2 sample (spectrum A) has an oxygen coverage of B20 ML. Following 120 minutes of reaction, the oxygen coverage drops to B1.7 ML (spectrum B) suggesting the disappearance of RuO2. At this stage, the catalyst surface is best described as a combination of a transition oxide phase and metallic phase. After reaction for 360 minutes, the catalyst surface exhibits an oxygen coverage This journal is

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Fig. 3 (a) Arrhenius plots of CO2 formation over Ru(0001)-(1 1)-O under stoichiometric conditions at four different CO initial pressures: 0.01 (’), 0.1 (K), 1.0 (m) and 16 (.) Torr. Reaction was carried out using a high-pressure cell (0.6 litre) as the batch reactor. The common Arrhenius plot (—K—) from which the rollover plots deviate at high temperatures is obtained from ref. 1. (b) Isosteric heat of adsorption of reactive CO species on Ru(0001)-(1 1)-O defect sites calculated using CO pressures and the corresponding rollover temperatures obtained from Fig. 3(a).

of B0.34 ML (spectrum C) indicating a chemisorbed oxygencovered metallic phase. Note that the points at which the corresponding Auger spectra were acquired (spectra (A), (B) and (C)) are designated in Fig. 4(a). Fig. 4(c) presents the corresponding in situ PM-IRAS spectra at different stages of reaction. Again, the 2052 cm 1 band demonstrates formation of metallic Ru (with low coverage of chemisorbed oxygen). The 2128 cm 1 band contains contributions from linear CO adsorbed on RuO2 (possibly also transition oxides) and surface carbonyl species. Specifically, at the end of the reaction where the overall oxygen coverage is only B0.34 ML (both RuO2 and transition oxides are absolutely removed at this point), the still detectable 2128 cm 1 feature is best assigned to a surface carbonyl species. As is discussed above, the surface roughness induced by RuO2 reduction makes carbonyl formation possible on a planar surface. The 2086 cm 1 band is assigned to the combination of carbonyl, linear CO on transition oxides and linear CO on dense chemisorbed oxygen covered metallic Ru at the beginning of the reaction. At the point where all oxide species are fully reduced, this is mainly carbonyl and linear CO on dense chemisorbed oxygen covered metallic Ru. In line with the discovery by Yates et al.,7 dense chemisorbed oxygen only occurs at surface defect (step) sites but not on open terraces at low overall oxygen coverages. The B2020 cm 1 band is assigned to linear CO on reduced Ru. Interestingly, on supported Ru catalysts, this band appears to always accompany the carbonyl species.5,22 Results shown in Fig. 4 further strengthen two important points discussed above for stoichiometric reaction conditions: (1) the reducibility of RuO2 during reaction; and (2) the high activity of surface defects sites. Previous studies by Peden et al. realized that on a defect-deficient Ru(0001) surface at oxygen coverage of B1/3 ML, CO2 formation rate is two orders of magnitude smaller1,48,53 than the maximum CO2 TOF shown in Fig. 4(a). The high activity of metallic Ru surface generated via RuO2 reduction is clearly due to the fact that this surface is Phys. Chem. Chem. Phys., 2012, 14, 6688–6697

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Fig. 4 (a) CO2 formation rate as a function of reaction time at 500 K over a pre-formed RuO2(110) sample under net-reducing reaction conditions (Ptotal = 44 Torr, O2/CO = 1/10). Each rate point is the average of 5 min of reaction at each temperature, derived from the pressure change of the gas phase. Reaction was carried out in a 61.6 litre batch reactor. CO conversion was maintained below 10% to obtain differential rates. (b) Auger spectra of the as-formed RuO2(110) sample (A), after 120 min of reaction (B) and after 360 min of reaction (C). All spectra were taken after briefly annealing the sample to 600 K in UHV to remove volatile species. The corresponding oxygen coverage is marked adjacent to each spectrum, calculated based on O/Ru Auger ratio of a Ru(0001)-(2 1)-O surface which has oxygen coverage of 0.5 ML. (c) The corresponding PM-IRAS spectra for the experiment shown in Fig. 4(a). Each spectrum is a superimposition of 5 min of scans at 4 cm 1 (totally B300 scans). Reaction time is marked adjacent to each spectrum.

defect-rich. Furthermore, between reaction time from 300 to 360 min, both CO2 TOF and PM-IRAS spectra maintain invariant suggesting even under such a heavily reducing environment a steady state is reached. 3.4

Ru(0001) and RuO2 under net oxidizing conditions

Only under net-oxidizing reaction conditions, is it possible for RuO2 to be the dominant and even the only phase. Fig. 5(a) displays reaction kinetics using clean Ru(0001) and a mixture of 8 Torr of CO and 40 Torr of O2 at 550 K. A reaction temperature of 550 K is chosen in order for (1) the RuO2 phase to form during reaction; and (2) the formation rate to be slow enough to follow. Note that at temperatures below B500 K, RuO2 does not form even with extended oxidation time since oxygen penetration into the subsurface is kinetically inhibited.54 As shown in the figure (each data point represents the reaction 6694


rate averaged over a 5 minute interval), the CO2 formation rate remains constant at TOF E 20 for the first 60 minute interval and gradually increases in the second 60 minute interval. The reaction rate maximizes at a TOF E 80 during the third 60 minutes of reaction. Fig. 5(b) presents selected PM-IRAS spectra at different stages of reaction. During the first 20 minutes of reaction, no surface CO is detected indicating the initial formation of Ru(0001)-(1 1)-O. On this surface the CO coverage is below the detection limit. As the reaction proceeds (for example at reaction time of 90 min), weak CO signals are evident at 2107, 2080 and 2040 cm 1. These features are believed to arise from oxygen penetration beneath the first oxygen layer, altering the surface Ru coordination and allowing CO to adsorb, or alternatively, by CO adsorption on Ru atoms moving outward from Ru(0001)-(1 1)-O. Higher frequency CO bands at 2130 and 2140 cm 1 are also apparent, and are assigned to linear CO on RuO2 (and/or transition oxides), or carbonyl species. At reaction times beyond 120 minutes, the dominant CO band is at 2094 cm 1. Post-reaction AES measurement reveals an oxygen coverage of B20 ML following 180 minutes of reaction confirming the formation of RuO2. Recent studies by Over et al., utilizing surface X-ray diffraction (SXRD), show that the phase generated under similar reaction conditions is RuO2(110).31 It is important to note that at 550 K and net oxidizing conditions, the RuO2(110) phase is characterized by the CO band at 2094 cm 1. The CO2 formation rate data displayed in Fig. 5(a) show that RuO2 exhibits a reactivity B4 times higher than Ru(0001)-(1 1)-O at 550 K under oxidizing reaction conditions. Similar results have also been found recently.31,47 This seems to suggest that RuO2 is more active than metallic Ru as argued by Over et al.31 However, we have discussed earlier that under stoichiometric and reducing reaction conditions this comparison is not meaningful since RuO2 never serves as the dominant active phase below B500 K and only transiently exists above B600 K. Only under net-oxidizing conditions where Ru(0001)-(1 1)-O can be kinetically stable for rather long reaction times and RuO2 is thermodynamically stable, is this comparison meaningful. Even in this case, the statement of RuO2 being more active is very misleading. This is because the Ru(0001)-(1 1)-O phase is defect-deficient where the active site concentration is on the order of 10 5 ML; on RuO2(110) the density of the active cus-Ru sites is higher than 10 1 ML. In this sense, on a per active site basis, Ru(0001)(1 1)-O is still much more active, fully consistent with the considerably reduced binding energy of the reactive CO species (B68 kJ mol 1 vs. B120 kJ mol 1 on RuO2(110)). In the following the activity of pre-formed RuO2(110) is tested in 8 Torr of CO and 40 Torr of O2. Below 475 K, rather unexpected kinetic behavior is found as shown in Fig. 6(a). In this case, the CO2 formation rate changes rather dramatically during repeated measurements (error bars represent root mean square standard deviation for 4 different measurements). Moreover, an unexpected phenomenon, i.e., CO2 formation rate decreases with increasing temperature, is found. Only from 475 to 550 K, as presented in Fig. 6(b), is normal Arrhenius behavior found on RuO2(110). Reaction kinetics under identical conditions over Pt and Pd surfaces are also shown in Fig. 6(b) for the purpose of direct comparison.40 This journal is

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Fig. 5 (a) CO2 formation rate as a function of reaction time at 500 K over Ru(0001) under net-oxidizing reaction conditions (Ptotal = 48 Torr, O2/CO = 5/1). Each rate point is the average of 5 min of reaction at each temperature, derived from the pressure change of the gas phase. Reaction was carried out in a 61.6 litre batch reactor. CO conversion was maintained below 10% to obtain differential rates. (b) The corresponding PM-IRAS spectra for the experiment shown in Fig. 5(a). Each spectrum is a superimposition of 5 min of scans at 4 cm 1 (totally B300 scans). Reaction time is marked adjacent to each spectrum.

For Pt and Pd samples, we have shown previously that the CO-inhibited regime is below B550 K, where the measured apparent activation energy of B100 kJ mol 1 is close to the CO binding energies with the surfaces. Above 550 K a mass transfer limited regime is reached where the CO2 TOF remains rather constant with increasing temperature. The arrival of the mass transfer limitation is because at such high temperatures the metal surfaces are no longer inhibited by chemisorbed CO; and CO2 formation becomes limited by the arrival of gas CO to the surface. For Pd at high temperatures, the CO2 TOF is even slightly lower than the mass transfer limitation due to the formation of less-active Pd oxide species.40 For RuO2(110), the linear Arrhenius behavior found between 475 and 550 K with an apparent activation energy of B100 kJ mol 1 is fully consistent with our expectation for a stable RuO2(110) phase within the CO-inhibited regime. On this surface, rollover also occurs at B550 K, however the CO2 TOF is somewhat lower than the mass transfer limitation line. The lower CO2 TOF on RuO2(110) compared with Pd and Pt is believed to be caused by the fewer numbers of active sites (cus-Ru) than Pd and Pt such that some CO molecules impinge on the surface but do not have the chance to react. For the latter all surface metal atoms are active. In situ PM-IRAS results are plotted in Fig. 6(c) to further elucidate the dramatic difference in reaction kinetics below and above 475 K on RuO2(110). Below 475 K, the dominant CO band is found at 2083 cm 1. Between 475 and 550 K, the dominant CO band is found at 4095 cm 1. We have shown in Fig. 5(b) that under net-oxidizing conditions the RuO2(110) phase is characterized by the 4095 cm 1 CO band. This immediately shows that below 475 K the sample surface is not RuO2(110). This means, even under net-oxidizing conditions the surface layer of the RuO2 thin film is reduced to metallic Ru and/or transition oxides at relatively low temperatures. The CO vibration at 2083 cm 1 is fully consistent with this reduction picture. The decrease in CO2 TOF with increasing temperature This journal is

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Fig. 6 (a) CO2 formation rate as a function of reaction temperature between 400 and 475 K over pre-formed RuO2(110) under net-oxidizing reaction conditions (Ptotal = 48 Torr, O2/CO = 5/1). Reaction was carried out in a 61.6 litre batch reactor. CO conversion was maintained below 10% to obtain differential rates. Error bars represent root mean square standard deviation for 4 different measurements. (b) Arrhenius plots of CO2 formation over Pt(110) (’), Pd(111) (K) and pre-formed RuO2(110) (m) under net-oxidizing reaction conditions (Ptotal = 48 Torr, O2/CO = 5/1). Reaction was carried out in a 61.6 litre batch reactor. CO conversion was maintained below 10% to obtain differential rates. For RuO2(110), reaction temperatures vary from 475 to 600 K. Error bars represent root mean square standard deviation for 4 different measurements. Kinetic data for Pt and Pd samples were taken from ref. 40. (c) Representative PM-IRAS spectra for the experiment shown in Fig. 6(a) and (b) over RuO2(110). Each spectrum is a superimposition of 10 min of scans at 4 cm 1 (totally B600 scans). Reaction temperature is marked adjacent to each spectrum.

shown in Fig. 6(a) presumably reflects the situation that higher reaction temperature favors formation of less active (i.e., more oxidized) phases. It is rather apparent that above 475 K, RuO2 reduction stops and only the oxide phase dominates. Most importantly, the expected kinetic behavior above 475 K for RuO2 is: (1) a linear Arrhenius regime on CO-inhibited surface; (2) an apparent activation energy of B100 kJ mol 1 similar to CO binding energy at cus-Ru sites; (3) a rollover that occurs when the surface CO coverage becomes undetectable with IRAS. It must be emphasized that under reaction conditions where RuO2 is thermodynamically stable (this is also confirmed via post-reaction AES analysis) as those shown in Fig. 6(b), RuO2 is less active than metallic Pt and Pd. This is another strong piece of evidence demonstrating that for supported Ru catalysts under stoichiometric conditions where Ru has been found to be Phys. Chem. Chem. Phys., 2012, 14, 6688–6697

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substantially more active than other Pt-group metals,4,5 the active phase is not RuO2 but rather metallic Ru phases. Very recently, Rosenthal et al. found the oscillatory behavior of CO oxidation over bulk Ru oxide under (and only under) oxidizing conditions.37 We realize in this study that under net-oxidizing conditions RuO2 is reducible at relatively low temperatures and stay oxidized at higher temperatures. This could provide a good explanation to the oscillation behavior found by Rosenthal et al. Presumably, RuO2 is first reduced to a more active state, and then the strongly exothermic nature of CO oxidation causes the catalyst temperature to rise so that the catalyst becomes reoxidized and less active. Interestingly, the reaction temperature at which oscillation was found (503 K) is very close to the temperature we found that RuO2 changes from reducible to stable (475 K). 3.5

The remaining puzzle

Previous studies by various researchers have reached what appears to be rather contradictory results that need clarification. Rosenthal et al.36 reported that fully oxidized RuO2 powder with the exposure of different facets including {110} and {100} (which certainly contain cus-Ru sites), is completely inactive in CO oxidation regardless of the CO/O2 ratio prior to the activation period (especially reduction). Very recently these authors reported that under net oxidizing conditions RuO2 can also be activated, presumably via the formation of certain defective structures.37 Over et al. also reported that thicker RuO2 grown on metallic Ru becomes much less active.28,29,32 We note that it is not convincing to assign all types of deactivation to the formation of a common inert RuO2(110)-c(2 2) structure. It appears that even under reaction conditions where RuO2 is thermodynamically stable, the active phase is not RuO2 alone but rather a thin RuO2 film on a metallic substrate. Recently, several research groups discovered CO oxidation activity of monolayer FeOx films grown on Pt.55–57 Although a detailed reaction mechanism has not been reached, strong metal–support interaction (SMSI) must play a significant role since FeOx itself is completely unreactive. In this case, as suggested by Sun et al., electron transfer could occur through the oxide film into the adsorbate therefore under favorable conditions such electron transfer induces reactivity between molecules.55 The fact that RuO2 is an excellent conductor (compared with most metal oxides) may allow SMSI to be effective over quite a few RuO2 trilayers.

4. Conclusions (1) The CO reaction probability maintains constant at B10 5 over a wide pressure range from 0.01 to 16 Torr on Ru(0001). These results demonstrate that the surface active sites are minority defect sites, rather than majority terrace sites. In accordance with recent theoretical calculations, the active sites are surface step sites. The binding energy of CO on these sites is B68 kJ mol 1, essentially identical to theoretical calculations. (2) Under reducing and stoichiometric reaction conditions, pre-formed RuO2 thin films on Ru(0001) are reduced to defect-rich metallic Ru which is both thermodynamically stable and catalytically highly active. This finding is fully consistent with studies using RuO2 powder catalysts. Even under net-oxidizing 6696


reaction conditions, pre-formed RuO2 thin films are reduced at temperatures below B475 K as evidenced by in situ PM-IRAS and high catalytic activity. (3) Only under net-oxidizing conditions and temperatures above B475 K is the RuO2 thin film thermodynamically stable and catalytically active as confirmed by reaction kinetics, in situ PM-IRAS, and ex situ AES measurements. This phase has lower activity than metallic Pd and Pt catalysts under identical reaction conditions, consistent with the high CO binding energies on this surface. This further proves that under reaction conditions where (supported) Ru shows much higher activity than other Pt-group metals, the active phase must be a reduced Ru metallic phase rather than an oxide. (4) In line with the recent discoveries that thicker RuO2 films on metallic Ru and fully oxidized RuO2 powder are completely unreactive in CO oxidation, we propose that besides coordinately unsaturated sites (cus-Ru), strong metal substrate interaction (SMSI) is also necessary for the RuO2 phase to be catalytically active in CO oxidation, both in vacuum and near-atmospheric pressures.

Acknowledgements We gratefully acknowledge the support for this work by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences (DE-FG02-95ER-14511), and the Robert A. Welch Foundation.

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