Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd

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On the Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd(111) Rafal Jan Wrobel, Stefan Becker, and Helmut Weiss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508952f • Publication Date (Web): 16 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

On the Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd(111) Rafal J. Wrobel*1,2, Stefan Becker1, Helmut Weiss1 1

Otto-von-Guericke Universität, Chemisches Institut, Universitätsplatz 2,

39106 Magdeburg, Germany 2

West Pomeranian University of Technology, Institute of Chemical and Environment Engineering, Pulaskiego

10, 70-322 Szczecin, Poland

* corresponding author: Address: ul. Pulaskiego 10; 70-322 Szczecin tel. +48 91 449 41 32 e-mail: [email protected]

Abstract The formation of palladium subsurface oxygen i.e. Pd(111) saturated with oxygen below the surface has been demonstrated. This compound is much more stable than palladium surface oxide or bulk palladium oxide which easily react with CO at elevated temperatures. The catalytic activity of an atomically clean Pd(111) surface and of Pd(111) affected by subsurface oxygen (coverage Θ 0.00 ÷ 0.46 ML) was studied during the CO oxidation reaction. The measurements were performed for a temperature range 353 K ÷ 523 K, partial pressures of reactants in the range of 10-7 ÷ 10-5 mbar, and different subsurface oxygen content. A distinct, previously not observed feature in the dependency of the reaction rate vs. temperature was found. Furthermore it was found that subsurface oxygen reduces the catalytic activity of the Pd(111) surface significantly, what is attributed to reduction of the sticking coefficient of CO and oxygen. In the temperature range applied for investigation of catalytic CO oxidation the rate of the formation of subsurface oxygen is negligible. However the formation of this phase starts to be pronounced above ~ 600 K. The presence of subsurface oxygen was confirmed by the KLL Auger transition excited by monochromatic X-ray radiation not by electron beam. It was observed that the classical Auger measurement with electron beam destroys the subsurface oxygen compound. The combination of the CO titration with Auger peak intensity measurements allows the quantitative analysis of the subsurface oxygen. The problem of subsurface oxygen detection and its impact on the adsorption of CO and oxygen on Pd(111) are discussed. It was suggested that subsurface oxygen and surface oxide can be manufactured simultaneously. Then obtained activity in CO oxidation reaction will be a result of two competitive effects i.e. promoting from surface oxide and inhibiting ACS Paragon Plus Environment


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from subsurface oxide. This might be a source of discrepancy in results of surface oxide influence on CO oxidation reaction measured by different researchers.

Keywords carbon monoxide, CO titration, palladium, XPS, AES, QMS, sticking coefficient

Introduction Due to the wide applications in catalysis the properties of palladium are of great importance. Palladium is used e.g. to catalyze many organic reactions1-3, but nowadays the largest application of palladium (about 66% of total world production in 2007) is in automotive catalytic converters4. Palladium in form of nanoparticles is one of the key components in technically applied three-way catalysts (TWC) for abatement of exhaust gases from automobiles5. Nowadays many of the basic phenomena which are responsible for the mode of operation of commercial TWC are well understood. Nevertheless it is virtually impossible to comprehend all feedbacked processes on the atomic level in such complex devices like technically used TWC. That is why a lot of scientists, even today, follow the advice given by Langmiur in 1922: “Let us confine our attention mainly to reactions on plane surfaces. If the principles in this case are well understood, it should then be possible to extend the theory to the case of porous bodies"6. Based on this approach single crystal surfaces of the platinum group metals can be used in order to mimic simplified catalytically active systems. Such model systems are commonly applied for elucidation of the catalytic CO oxidation, which is one of the main reactions occurring in commercial TWC7-9. The catalytic CO oxidation on monocrystal surfaces of platinum group metals has attracted lot of scientific attention and it is one of the most recognized reactions. Therefore it can be considered in heterogeneous catalysis as equivalent of the drosophila in biology. The already classical studies of Engel and Ertl have shown that the catalytic CO oxidation on platinum group metals (e.g. Pd and Pt) follows the Langmuir-Hinshelwood mechanism10,11. The interaction of oxygen with Pd(111) was also studied and it was found that the interaction of oxygen with this surface, even at 573 K (pO2 = 2.0 ⋅ 10−7 mbar ), leads to creation of surface oxide which is often called subsurface oxygen12. Due to the overlapping of the O1s and the Pd 3p3/2 peak in the XPspectra surface oxide is hard to detect by XPS. Also with other analyzing techniques it is not a

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trivial task to detect it, especially in small amounts12-16. This is probably the main reason of very slow development of the awareness of the impact of surface oxide on the catalytic activity of palladium surfaces. Probably the first report about surface oxide was made in 1977 in the work of Conrad et al. about the interaction of NO and O2 with Pd(111) in a pressure regime around 7 ⋅ 10−6 mbar 17. The early work of Ertl et al. from 1969 gives the results of CO oxidation rate on Pd(110) without mentioning about the possibility of a surface oxide formation18. The work of Ladas et al. from 1989 and 1993 shows that the surface oxide can be formed during CO oxidation on Pd(110) and does affect the reaction rate19,20. In 1994 Ertl mentioned briefly the inhibitory effect of surface oxide on palladium catalytic activity, but limits the possibility of formation only to high oxygen pressures21. In 2000 it was found that surface oxide can be formed on Pd(111) even in a low pressure regime12. Since 1994 many studies related with the catalytic activity of palladium in the CO oxidation on monocrystals have appeared, where the influence of the role of surface oxide was not studied7,9,22,23. Recently many works on surface oxide formation on palladium surfaces have appeared. The research is focused rather on Pd(100)24-31 surface than on Pd(110)32,33 or Pd(111)34-43. Generally palladium surface oxidizes easily in oxygen partial pressures ranging from high vacuum to atmospheric pressure. It was found that Pd(110) surface oxidizes at temperatures 100 K lower than Pd(111) surface33. For Pd(111) surface, three25, four39 or even six40 different types of oxides are reported, however Pd5O4 was found to be most stable40. There are contradictive

reports about the influence of palladium surface oxide on CO

oxidation reaction. Some researchers have found the hyperactive influence of surface oxide2427,43

even 2-3 orders of magnitude30. It is believed that when surface oxide is present CO

oxidation reaction follows Mars-Van Krevelen mechanism in which oxide is continually consumed and reformed24. The other researchers reported that the palladium surface covered primarily with oxygen and the low CO coverage is the most active in CO oxidation reaction34,35. There is also agreement that surface oxide can be reduced to metallic palladium by CO even in conditions as mild as pCO=5·10-9 Torr at 450 K42. Therefore one can expect that keeping the palladium surface at UHV conditions all palladium surface oxide can be reacted off by residual CO. In works Ref.37, 38 it is reported that during thermal decomposition of surface oxide at 717 K the diffusion of oxygen in the palladium bulk occur. The substantial oxygen signal assigned to the dissolved species even at 923 K was detected. Dissolved oxygen cannot react with surface



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CO before oxygen segregates to the surface. The idea of subsurface oxygen is supported by results obtained by Vattuone et al44,45. In their investigations with rotationally aligned oxygen molecules they found that cartwheels oxygen may end up in sub-surface octahedral interstitials sites with O atoms sitting (1.60 ± 0.10) Å below the surface. The subsurface site is most probably metastable, though long living. It was also pointed out that result contradicts density functional theory (DFT) predictions45. To the best of our knowledge there is so far no investigation concerning the distinct influence of this special type of oxygen on the catalytic CO oxidation on the Pd(111) surface. Researchers usually investigate either bare palladium surface or palladium covered by the surface or bulk oxide. In the present study we demonstrate the strong influence of oxygen located under the surface of Pd(111). We cannot say if this palladium-oxygen compound is ordered or disordered (i.e. solid solution) and further we will refer to it as subsurface oxygen. The general scope of our scientific research are well-defined catalytic model systems consisting of a platinum metal surface covered with cerium oxide nanoislands46-48. Analogous model systems for palladium-ceria are also possible, however what differs platinum from palladium is an easy possibility of oxide formation. It seems possible that the subsurface oxygen can be created on Pd(111) in reaction conditions of the CO oxidation reaction even at low pressures12. Therefore the conditions in which it may be created and how it affects the catalytic activity have to be checked in details. Only with this knowledge, further investigations of the impact of cerium oxide layers on the CO oxidation reaction on Pd(111) are meaningful. Moreover the subsurface oxygen can be formed during the preparation of the oxide/metal systems. The problem of subsurface oxygen detection in systems consisting of palladium and an oxidic support is even more complicated. In such systems the oxygen from the oxidic support will obscure the subsurface oxygen signal in any spectroscopic method. Summarising - the scientific significance of the subsurface oxygen on Pd(111) are the following points: (i) the subsurface oxygen affects the catalytic activity of palladium; (ii) the subsurface oxygen is more easily created under real i.e. normal pressure conditions; (iii) a commercial heterogeneous catalyst usually consists of palladium nanoparticles on oxidic support what constitutes an experimental challenge for subsurface oxygen detection. The goal of this paper is to show the influence of subsurface oxygen on the catalytic activity of Pd(111) in the CO oxidation reaction. Depending on the reaction conditions the subsurface oxygen may be created during this reaction. The method of qualitative and quantitative subsurface oxygen analysis on Pd(111) with a commercial XPS setup will be shown. Furthermore the influence of the subsurface oxygen on the CO and oxygen adsorption


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will be discussed. Wherever it was not possible to find any literature data for Pd(111), the experimental results are compared with the ones for Pd(110).

Experimental The experimental studies were performed in a commercial multipurpose (XPS, LEED, STM, UPS, AES) UHV surface analysis system (SPECS) which is operating at a base pressure in the low 10−10 mbar range. This UHV system consists of two chambers. The preparation chamber gives the possibility of cleaning (Ar+ bombardment and annealing up to 1400 K) and controls of the surface composition by combined Auger electron spectroscopy and low energy electron diffraction (AES, LEED; SPECS, ErLEED 150). The analysis chamber is equipped with monochromatic and non-monochromatic X-ray photoelectron spectroscopy (XPS; SPECS, Phoibos-150) and scanning tunnelling microscopy (STM; Omicron NanoTechnology, VT-AFM/STM). In the studies presented herein a Pd(111) single crystal with a diameter of 7.25 mm and a thickness of 2 mm was used. The crystal was mounted on a Mo sample holder equipped with a chromel-alumel thermocouple spot-welded to the crystals surface. The atomically clean Pd(111) surface was prepared by several cycles of Ar+ bombardment and oxygen treatment followed by annealing at 1173 K in UHV, like described in details in Ref.14. After this procedure the quality of the surface was confirmed by a sharp triangular LEED pattern. The absence of impurities from the clean Pd(111) surface was confirmed by XPS, AES and CO titration method, what means: The clean surface was saturated with CO (at room temperature ΘCO/max = 0.5 for the Pd(111)49,50) and the corresponding intensity of the C1s XP-peaks was measured. The common impurities block the adsorption places for CO therefore any deviation of C1s intensity from the maximal value may indicate the presence of impurities on the prepared surfaces12. The XP-spectra presented in this work (pass energy 10 eV and step size 0.1 eV) were recorded in averaging multiscan mode, what improves the signal-to-noise ratio significantly. A 150 mm hemispherical energy analyzer (Phoibos-150, SPECS) which allows simultaneous photoelectron detection on 9 channels has been used. The work function of the analyzer was calibrated with a clean gold foil by the measurement of the corresponding binding energy of 84.0 eV for the Au 4f7/2 signal. For excitation an Al/Ag twin anode X-ray source with X-ray monochromator (XR-50M, SPECS) and the Kα radiation of Al (1486.6 eV) has been used. For XPS data evaluation the CasaXPS software package was used51.



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The catalytic CO oxidation reaction on the clean Pd(111) single crystal surface was monitored by measurement of CO, CO2 and O2 partial pressures by quadrupole mass spectrometry (QMS; MKS Instruments, VacCheck). The partial pressure readings of QMS were calibrated by Bayard-Alpert nude ionization gauge Granville-Philips Co. series 274. The gas sensitivity factors for the gauge were taken into account (1.05, 1.42 and 1.01, respectively). The total pressure in front of the turbo molecular pump was measured with a Compact Full Range Gauge Pfeifer series PKR 251 (reproducibility ± 5%).

Reaction rate measurements The investigations of the CO oxidation reaction on the Pd(111) surface were performed in the analysis chamber under steady flow conditions. Due to the low pressure range (below 10-5 mbar) there are virtually no collisions between the molecules in the gas, and the experimental setup can be considered as a flow reactor with ideal mixing. For constant partial pressures of the reactants and constant pumping speed the molar fraction of CO2 is proportional to the reaction rate. In this way it is possible to measure the variation of reaction rate but in arbitrary unit only46,48. However having volume pumping speed and partial pressure of CO2 one can easily calculate the reaction rate expressed in [moleculesCO2 · s-1·cm-2] under steady flow conditions •

rCO 2

V ⋅ pout ⋅ N A ⋅ X CO 2 = 24.8 ⋅ S Pd (111)

(Eq. 1)

Where: •

rCO2 – reaction rate,

V – volume pumping speed of turbo molecular pump [dm3·s-1], pout – pressure on the reactor

outlet i.e. the pressure in front of the turbo molecular pump [bar], NA – Avogadro number, XCO2 – molar fraction of CO2 in the gas phase, SPd(111) – surface area of Pd(111) crystal available for the surface reaction [m2]

The pumping speed corresponds to the flow of the gasses through the vacuum chamber. The volume flow can be converted to the flow expressed in [mol/s] by using of ideal gas law. One can also relate this value to the surface of the crystal. Finally, including CO2 molar fraction we obtain Eq. 1. The pumping speed of the turbo molecular pump is pressure independent below 10-3 mbar (all measurements were done in this range), but depends on the mass of the molecule. In the pump specifications there were no data given for the investigated reactants and product i.e. CO, O2 and CO2 (28, 32, 44 a.u., respectively). However, data were available for gases of similar mass, namely nitrogen (28 a.u.) and argon (40 a.u.) with pumping speeds of 59 and 60 ACS Paragon Plus Environment

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dm3·s-1, respectively. Thus the pumping speed for nitrogen was assumed for all gases in the investigated reaction. The molar fraction of CO2 was calculated from the partial pressures of gases measured in the reaction chamber with QMS. The composition of the gas mixture is constant since on the path to the turbo molecular pump there are no sources which could change the gas ratio, e.g. hot filaments. The surface area of Pd(111) (SPd(111)) available for the surface reaction is the area of the precleaned palladium surface. The rim of the crystal and the back side do not participate in the reaction due to surface contamination14 and hindered gas adsorption access. Reaction sticking coefficient

The measurement of reaction rate in physically meaningful units enables measurement of the reaction sticking coefficient. This represents the probability with which the impinging molecules of oxygen or CO undergo adsorption and reaction on the surface. The sticking coefficient of the reaction is given by Eq. 2: r = SCO

rCO 2 Z CO

SOr 2 =

rCO 2 2 ⋅ Z O2

(Eq. 2)

Where: SrCO, SrO2 – reaction sticking coefficients of CO and oxygen, respectively, rCO2 – reaction rate (Eq. 1), ZCO, ZO2 – number of collisions of CO or O2 with the surface, respectively. The factor 2 results from reaction stoichiometry.

The difference between sticking coefficient and reaction sticking coefficient is that sticking coefficient gives probability of adsorption of gas molecule, while reaction sticking coefficient gives probability of adsorption followed by reaction of adsorbed molecule. Thus the sticking coefficient should have a value greater or equal than the reaction sticking coefficient, because adsorbed molecule may desorb before surface reaction occurs. However, changes in the value of reaction sticking coefficient, measured in constant temperature and gas partial pressure, usually indicate the changes in value of sticking coefficient. The number of collisions Z can be obtained from kinetic theory of gases, Eq. 3:

Z=

p 2π ⋅ m ⋅ k ⋅ T

(Eq. 3)

Where: Z – number of collisions [molecules·s-1·m-2], k – Boltzman constant [J/K], m – mass of a single molecule hitting the surface [kg], p – gas pressure [Pa], T – absolute temperature [K],



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Results and Discussion Catalytic activity of clean Pd(111) and Pd(111) affected by subsurface oxygen

During the CO oxidation reaction subsurface oxygen may be formed, which in turn influences the catalytic activity. The reaction rate was measured for the atomically clean Pd(111) and for Pd(111) with different amounts of subsurface oxygen. In Figs. 1 and 2 the variation of reaction rate for constant reactant partial pressures is presented vs. temperature. In Fig. 3 is presented the dependency of reaction rate, for fixed temperature and partial pressures of oxygen, on CO partial pressure. Fig. 4 shows the analogous dependency like Fig. 3 but with variation of oxygen partial pressure under isothermal conditions. The reader can check the accuracy of the experimental setup following very similar experiment with Pt(111) where these very time-consuming measurement were repeated several times52. Visible error bars (Fig. 1-5) correspond to confidence level of ∼68% with k = 1.

Temperature dependencies

Fig. 1 presents the variation of the reaction rate, determined by continuous measurements of the CO2 mass spectrometer signal, as a function of temperature. The measurements were done during the ongoing reaction, with constant temperature and for fixed composition of the reactants (pCO = 2.1 ⋅ 10−6 mbar and pO2 = 7.6 ⋅ 10−6 mbar ) in the gas phase. The different contents of subsurface oxygen, which are stable in the applied reaction conditions, were obtained by oxygen treatment at different temperatures above 600 K (see next subchapter). For the clean Pd(111) (Fig. 1 a) the highest variation of the reaction rate was observed. With the increase of the amount of subsurface oxygen on Pd(111) the variation of the reaction rate decreased drastically (Fig. 1 b,c and Fig. 2 b). Generally one can divide all three presented graphs in Fig. 1 into four different subsections (denoted by roman numbers). Region I: At lower temperatures there is no significant surface reaction noticeable. In this

temperature range the surface is saturated with CO (ΘCO/max = 0.5). Such surface, due to the Langmuir-Hinshelwood mechanism, is not reactive for the CO oxidation reaction, because

there are no adjacent adsorption places which are necessary for dissociative adsorption of molecular oxygen11. Furthermore the reaction at such temperatures is very slow due to kinetic reasons. All detected CO2 can basically be attributed to the background CO2 pressure which is ACS Paragon Plus Environment

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produced e.g. on the hot filaments inside the UHV chamber. Therefore in this range the reaction on the surface of Pd(111) is negligibly slow. Region II: In the second region one can observe an increase of the reaction rate with rising

temperature. In this temperature range occurs a significant desorption of CO from the Pd(111) surface (compare the TDS curve presented in Ref.53). This means that under these conditions the number of adsorption places, necessary for dissociative adsorption of the molecular oxygen, increases, leading to an increase of the reaction rate on the Pd(111) surface. Of course, the reaction rate increases due to kinetic reasons as well, which results in a superposition of those two effects. In Figs. 1 a and b, i.e. for Pd(111) surfaces not saturated with subsurface oxygen, one can notice a pronounced shoulder which has not been reported before by other researchers9,18. Probably the resolution of the temperature measurement (~50 K) in their experiments was not enough to observe this feature (in the here presented study the resolution is ~ 4 K, i.e. much higher). The different reaction conditions applied by the others and/or the different crystallographic orientation may be also the source of the discrepancy. In case of the sample fully saturated with subsurface oxygen (Fig. 1c) and in CO-rich atmosphere (Fig. 2 a) this shoulder disappears. For the first case (Fig. 1c), one can only speculate that the saturation with subsurface oxygen changes the CO desorption characteristics (i.e. sticking coefficient and surface binding energy): As Fig. 4 shows, subsurface oxygen strongly reduces the CO reaction sticking coefficient, in agreement with the work of Ladas et al.19,20. In order to explain the origin of the feature in region II, detailed TDS measurements are required, which can be currently not performed in the present experimental setup. Region III: Above ~ 450 K most of the CO has already desorbed or reacted off from the

surface. Therefore it does not block the surface, and the kinetics of the catalytic CO oxidation is the rate limiting step. Therefore an exponential slope is observed. Region IV: The maximum in the curve determines the beginning of region IV. Up to here the

reaction rate grows exponentially with temperature, till, depending on the ratio of the reactants partial pressures, the reaction is limited by one of its rates of adsorption18. Due to the decrease of sticking coefficients with temperature the curve decays at constant partial pressures of the reactants. This reasoning is supported by work of other authors. Nakao et al. have obtained very similar shape and maximum of the reaction rate curve for Pd(110) and Pd(111). They showed that the maximum of the reaction rate shifts with higher fluxes of reactants towards higher temperatures what confirms that the rate limiting step is adsorption of the reactants in region IV9. Ertl and Rau have shown that at 480 K a maximum in the



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reaction rate can be noticed for Pd(110). The LEED studies of oxygen on Pd(110) in the temperature range corresponding to region IV shows that oxygen coverage decays with temperature and above 773 K a superlattice of adsorbed oxygen could not be detected18.

Pressure dependencies

The data presented in Fig. 3 show the variation of reaction rate versus partial pressure of CO. The reaction sticking coefficient is presented as well. The measurements at 463 K reveal a maximum in reaction rate followed by an asymptotic decay. This can be explained by the increase of CO coverage with CO partial pressure and hence blocking of the adsorption sites for oxygen by CO molecules. This effect does apparently not occur at 523 K, while with increase of CO partial pressure the reaction rate increases till a plateau is reached. Further reaction rate enhancement is not observed because the oxygen adsorption is now the rate limiting step. For both temperatures the presence of subsurface oxygen reduces the reaction sticking coefficient of oxygen. The dependencies presented in Fig. 4 show the variation of reaction rate versus partial pressure of oxygen. In contrast to the CO pressure dependencies, one cannot observe a blocking effect of oxygen on the CO adsorption process. Moreover one can even conclude that with increase of oxygen partial pressure the CO reaction sticking coefficient increases as well. This can be explained by the increased surface oxygen concentration and thus higher probability of adsorbed CO to undergo the oxidation reaction before it desorbs. Please note, that a Pd(111) surface saturated with oxygen (ΘO/max = 0.25) does allow CO adsorption. Thus an inhibiting effect, like in case of Fig. 3 for oxygen on the CO-covered surface (ΘCO/max = 0.5), does not occur and there is no maximum in the curves. The increase of CO reaction sticking coefficient is especially distinct in case of subsurface oxygen affected sample (compare the slope of curves in Fig. 4a and Fig. 4b).

Formation of subsurface oxygen and its impact on the reaction

According to Leisenberger et al., the surface oxide can be formed even in mild conditions like 523 K at 40 L of oxygen exposure at pO2= 2.0 ⋅ 10−7 mbar 12. According to Weissman-Wenocur et al.54 the surface can be saturated completely with subsurface oxygen at higher oxygen exposures like 100 L. It is not clear if Ref.12,54 describe the subsurface oxygen or surface oxide. However, in Ref.39,40 it is shown that thermal decomposition at temperatures above 717


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K leads to diffusion of oxygen into the palladium bulk. We will prove later that our palladium-oxygen compound cannot be surface oxide and thus must be subsurface oxygen. Our Pd(111) sample was subjected to a constant oxygen exposure of 240 L at pO2= 1.0 ⋅10−6 mbar at various temperatures (493 K to 873 K) in order to produce different amounts of subsurface oxygen. Every exposure lasted 4 min at the given temperature. Different amounts of subsurface oxygen were created in this way and the influence of the subsurface oxygen on the catalytic activity of the Pd(111) surface was determined. Fig. 5 presents the catalytic activity measured at 493 K vs. annealing temperature during oxygen exposure. It can be noticed that exposure to oxygen does not change catalytic activity up to 600 K substantially. Above this temperature the oxygen exposure leads to a drastic decrease of catalytic activity. Finally above ~ 800 K the surface is apparently saturated with subsurface oxygen and further decrease of catalytic activity is not observed. The catalytic activity of the Pd(111) totally affected by subsurface oxygen decreases to ~ 30 % of the initial clean Pd(111) surface activity. In Fig. 6 a comparison of the catalytic activity for different compositions of the reactants in the gas phase is presented for the bare as well as for the subsurface oxygen covered Pd(111) surface. It can be noticed that decrease of the catalytic activity occurs for both oxygen-rich and oxygen-lean atmospheres. The data presented in Fig. 3 and Fig. 4 clearly show that the reaction sticking coefficients of both oxygen and CO are reduced significantly in case of the subsurface oxygen saturated sample. This is in agreement with the work of Leisenberger et al. who have shown that the sticking probability of oxygen is lower at a Pd(111) surface affected by subsurface oxygen12. Similarly Ladas et al. suggest a lowered sticking probability of CO as an effect of subsurface oxygen present on Pd(110)19,20. Therefore the decrease of the catalytic activity at 493 K can be explained generally by the reduction of the sticking coefficient of the reactants. The presented saturation temperature (>800 K) is comparable with the temperature given by Weissman-Wenocur et al.54.

Spectroscopic detection of subsurface oxygen

Due to the overlapping of the O1s and the Pd 3p3/2 peak in the XP spectra, the subsurface oxygen is hard to detect by XPS12. An alternative possibility seems to be Auger spectroscopy (AES). In our AES experiments we were not able to detect any oxygen signal for samples with relatively high subsurface oxygen content. Similar problems encountered E.H. Voogt et al.16. These authors attribute the lack of oxygen Auger signal to the interaction of subsurface oxygen with the electron beam during the AES measurements. The high energetic electrons (3



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keV) cause the removal of subsurface oxygen on Pd(111) so fast that the detection of subsurface oxygen is not possible16. The solution for this problem is the using of the KLL Auger transition, but with K(1s) hole creation by X-rays instead of high energetic electrons. The X-ray excitation disturbs much less the investigated surface than the high-energy electron beam. However, removal of subsurface oxygen by secondary electrons cannot be excluded during XPS measurements. Therefore measurements with Al Kα radiation from an X-ray monochromator, which cause much smaller X-ray exposure, were utilized in the present study. The oxygen KLL transition also occurs in case of adsorbed common vacuum contaminants like CO or water which can obscure the subsurface oxygen signal. Due to the fact that subsurface oxygen is thermally relatively stable53, while adsorbed CO and water are not, one can circumvent this problem by performing the measurements at elevated temperatures. Moreover, palladium surface oxides are not stable at temperatures higher than 450 K in the presence CO44. Residual CO is always present in UHV conditions thus removal of surface oxides requires just heating of the surface. In Fig. 7 are presented the KLL Auger transitions obtained by monochromatic X-ray excitation. The insets correspond to the C1s XP electrons from the respective sample. Fig. 7a corresponds to the clean Pd(111) surface, i.e. without subsurface oxygen, saturated with CO (exposure of 24 L) at room temperature. The CO coverage of 0.5 ML gives rise to a distinct KLL oxygen as well as a pronounced C1s XP signal. Fig. 7b corresponds to the same Pd(111) surface, but measured at 523 K. The contaminants, like CO and water, can not be present at the surface at this temperature, and only a flat background is observed in the spectra. Fig. 7c corresponds to the sample saturated with subsurface oxygen at 873 K (see Fig. 1c) and with CO adsorbed at room temperature. The KLL signal is much more intensive than the one presented in Fig 7a, while the C1S signal is comparable. Due to the fact that CO and oxygen coadsorbed at the surface react easily under these conditions12,15, atomic oxygen has to be located in the subsurface region and the CO is adsorbed on top of the surface. Fig 7d corresponds to the same Pd(111) surface, but measured at 523 K. From the inset one can conclude that the detected KLL signal is now due only to the subsurface oxygen. In order to quantify the amount of subsurface oxygen the intensities of the KLL Auger signals (Figs. 7a,d) were compared. Based on the assumption that the intensity of the peak shown in Fig. 7a is connected exclusively with CO (Θ = 0.5 ML), the amount of oxygen in Fig. 7d is assigned to 0.46 ML. Voogt et al. have obtained similar maximal coverage of subsurface oxygen possible on Pd(111) (~ 0.5 ML)16.


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Impact of subsurface oxygen on the properties of Pd(111)

The above presented results show that subsurface oxygen strongly influences the catalytic CO oxidation on Pd(111) and can reduce the catalytic activity down to ~ 30 % of the unaffected surface (Fig. 1). The question is: Why? In the previous chapter it was shown that oxygen in the oxidized sample occupies vacancies below the topmost surface atoms, which is in agreement with literature data, e.g.34, 55. Previously only few authors have considered how the subsurface oxygen may modify the surface electronic structure of a metal during experimental studies concerning e.g. work function measurements12,56. Oxygen, as an electronegative element, will lower the electron density of the topmost layer of the palladium surface when it is located in the subsurface region. The binding of atomic oxygen occurs through the surface electrons and atomic oxygen is known to be an acceptor of those electrons. Therefore one may expect that the subsurface oxygen reduces the electron density of the surface. In case of the CO molecule, binding to the surface occurs via a bonding-backbonding mechanism by donoring the carbon atom with surface electrons. This in turn should result in such effects like: lower sticking coefficients, lower adsorption energies, higher binding energies of the C1s and O1s XPS signals. The differences in the electron donation - backdonation between a CO molecule and the surface can also be observed in case of the clean Pd(111) surface, where the bridged and on top adsorption positions differ in electron donoring and this results in different

C1s binding energies (285.7 and 286.2 eV, respectively14,57). Leisenberger et al. observed only small changes of the C1s binding energy and no significant influence on the CO sticking coefficient on Pd(111) by subsurface oxygen. It has to be underlined that in such work the C1s signal was considered only as a single component. However, these observations were made for a sample only slightly affected by subsurface oxygen (40 L O2 at 523 K). At such conditions (see Fig. 5) the catalytic activity of Pd(111) is virtually not affected12, and a much stronger impact on the CO adsorption at higher oxygen saturation levels cannot be excluded from their experiments. Indeed, a strong influence on the CO binding energy is required in order to explain the disappearance of the characteristic feature (shoulder) in region II (see Fig. 1) and reduction of the CO sticking coefficient (see Fig. 4). In the work of Goschnick et al. it was shown that the CO sticking coefficient on an oxygen pre-covered Pd(110) surface is strongly reduced as compared with the CO covered Pd(110) surface58. The change of electronic properties of the surrounding palladium atoms was suggested by the authors, i.e. the same effect as it is to be expected in case of subsurface oxygen. The works of Ladas et al. show the inhibitory effect of the subsurface oxygen and attribute it to the reduction of the



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sticking coefficient as well as the adsorption energy for CO. They suggested as well that the presence of subsurface oxygen reduces the sticking probability of oxygen19,20. The work of Leisenberger et al. also supports that the subsurface oxygen lowers the sticking coefficient of oxygen. This conclusion was drawn for samples saturated with subsurface oxygen (8000 ML at 973 K), much higher than it was in the above mentioned study of CO (40 L O2 at 523 K). Summing up, the decrease of sticking coefficients of both oxygen and CO explains the reduction of catalytic activity observed in the present study, which is in agreement with literature data.

Conclusions The formation of palladium subsurface oxygen i.e. Pd(111) saturated with oxygen below the surface has been demonstrated. This compound is much more stable than palladium surface oxide or bulk palladium oxide which easily reacts with CO at applied conditions. The KLL Auger transition of oxygen, excited by monochromatic X-ray radiation, unambiguously allows the identification of subsurface oxygen on Pd(111) after extended oxygen treatment at elevated temperatures. Its maximum coverage corresponds to about 0.5 ML, as determined from a quantitative comparison to CO titration experiments. It was shown that the subsurface oxygen inhibits the catalytic CO oxidation on Pd(111) significantly. In comparison with the atomically clean Pd(111) surface, ~ 0.5 ML of subsurface oxygen decreases the catalytic activity of the system down to ~ 30 %. The catalytic activity starts to be significantly affected by oxygen treatment at temperatures higher than ~ 600 K. Treatment above ~ 800 K leads to saturation of the subsurface region and further lowering of catalytic activity was not observed. The inhibition of the catalytic reaction seems to be the result of reduced sticking coefficients of the reactants. The obtained reactivity dependencies vs. temperature, both for clean and subsurface oxygen affected (but not saturated) Pd(111), reveal a distinct feature not observed earlier by other researchers. This feature was attributed to the desorption of CO and was not observable in case of samples saturated with subsurface oxygen. The disappearance of this feature suggests a significant influence of the subsurface oxygen on the CO adsorption on Pd(111). It is possible that subsurface oxygen and surface oxide can be manufactured simultaneously. Therefore the obtained activity in CO oxidation reaction will be a result of two competitive effects i.e. promoting from surface oxide and inhibiting from subsurface oxide. This might be a source of discrepancy in results of surface oxide influence on CO oxidation reaction measured by different researchers.


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(7) Watanabe, K.; Uetsuka, H.; Ohnuma, H.; Kunimori, K. Infrared Chemiluminescence Study of the Dynamics of Catalytic Oxidation of CO and HCOOH on Pd(111) and Polycrystalline Pd Surfaces. Appl. Surf. Sci. 1996, 99, 411-416. (8) Klötzer, B.; Hayek, K.; Konvicka, C.; Lundgren, E.; Varga, P. Oxygen-induced Surface Phase Transformation of Pd(111): Sticking, Adsorption and Desorption Kinetics. Surf. Sci. 2001, 482-485, 237-242. (9) Nakao, K.; Ito, S.; Tomishige, K.; Kunimori, K. Structure of Activated Complex of CO2 Formation in a CO + O2 Reaction on Pd(110) and Pd(111). J. Phys. Chem. B. 2005, 109, 17553-17559. (10) Engel, T.; Ertl, G. A Molecular Beam Investigation of the Catalytic Oxidation of CO on Pd(111). J. Chem. Phys. 1978, 69, 1267-1281. (11) Engel, T.; Ertl, G. Elementary Steps in the Catalytic Oxidation of Carbon Monoxide on Platinum Metals. Adv. Catal. 1979, 28, 1-78. (12) Leisenberger, F.P.; Koller, G.; Sock, M.; Surnev, S.; Ramsey, M.G.; Netzer, F.P.; Klötzer, B.; Hayek, K. Surface and Subsurface Oxygen on Pd(111). Surf. Sci. 2000, 445, 380393. (13) Légaré, P.; Hilaire, L.; Maire, G.; Krill, G.; Amamou, A. Interaction of Oxygen and Hydrogen with Palladium. Surf. Sci. 1981, 107, 533-546.



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STM Study Surf. Sci. 2007, 601, 1574−1581. (30) Chen, M.; Cai, Y.; Yan, Z.; Gath, K.; Axnanda, S.; Goodman, D.W. Highly Active Surfaces for CO Oxidation on Rh, Pd, and Pt Surf. Sci. 2007, 601, 5326−5331. (31) Fernandes, V.; Gustafson, J.; Svenum, I.-H.; Farstad, M.; Walle, L.; Blomberg, S.; Lundgren, E.; Borg, A. Reduction Behavior of Oxidized Pd(100) and Pd75Ag25(100) Surfaces Using CO Surf. Sci. 2014, 621, 31−39. (32) Lundgren, E.; Kresse, G.; Klein, C.; Borg, M.; Andersen, J.N.; Santis, M. De.; Gauthier, Y.; Konvicka, C.; Schmid, M.; Varga, P. Two-Dimensional Oxide on Pd(111). Phys. Rev. Lett. 2002, 88, 246103-1 – 246103-4.

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(36) Martin, N. M.; Van den Bossche, M.; Grnbeck, H.; Hakanoglu, C.; Zhang, F.; Li, T.; Gustafson, J.; Weaver, J. F.; Lundgren, E. CO Adsorption on Clean and Oxidized Pd(111) J. Phys. Chem. C 2014, 118, 1118−1128.

(37) Zemlyanov, D.; Aszalos-Kiss, B.; Kleimenov, E.; Teschner, D.; Zafeiratos, S.; Havecker, M.; Knop-Gericke, A.; Schlogl, R.; Gabasch, H.; Unterberger, W.; et al. In Situ XPS Study of Pd(111) Oxidation. Part 1: 2D Oxide Formation in 10-3 mbar O2 Surf. Sci. 2006, 600, 983994.



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(51) http://www.casaxps.com (52) Wrobel, R.J.; Becker, S.; Weiss, H. Second/Additional Bistability in a CO Oxidation Reaction on Pt(111): An Extension and Compilation, J. Phys. Chem. C 2012, 116, 22287– 22292. (53) Klötzer, B.; Unterberger, W.; Hayek, K. Adsorption and Hydrogenation of CO on Pd(111) and Rh(111) Modified by Subsurface Vanadium. Surf. Sci. 2003, 532-535, 142-147. (54) Weissman-Wenocur, D.L.; Shek, M.L.; Stefan, P.M.; Lindau, I.; Spicer, W.E. The Temperature Dependence of the Interaction of Oxygen with Pd(111); A Study by Photoemission and Auger Spectroscopy. Surf. Sci. 1983, 127, 513-525. (55) Ketteler, G.; Ogletree, D.F.; Bluhm, H.; Liu, H.; Hebenstreit, E.L.D.; Salmeron, M. In Situ Spectroscopic Study of the Oxidation and Reduction of Pd(111). J.Am.Chem.Soc. 2005, 127, 18269-18273.

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Figure captions Fig. 1. Reaction rate of the CO oxidation on Pd(111) vs. temperature (pressure conditions: pCO = 2.1 ⋅ 10−6 mbar and pO2 = 7.6 ⋅ 10−6 mbar ). Different regions of the curves are denoted with roman numbers and described in the text. (a) The atomically clean Pd(111) surface. (b) Subsurface oxygen affected Pd(111) (after 240 L exposure to oxygen at 713 K). (c) Pd(111) with subsurface oxygen saturated surface (after 240 L oxygen exposure at 873 K).

Fig. 2. Reaction rate of the CO oxidation on Pd(111) vs. temperature (pressure conditions: pCO = 7.6 ⋅ 10−6 mbar and pO2 = 2.1 ⋅ 10−6 mbar ). (a) The atomically clean Pd(111) surface. (b) Pd(111) with subsurface oxygen saturated surface (after 240 L oxygen exposure at 873 K). Fig. 3.

Reaction rate of the CO oxidation on Pd(111) vs. CO pressure at constant

temperature. The right axis presents calculated reaction sticking coefficients of oxygen. (a) The atomically clean Pd(111) surface. (b) Pd(111) with subsurface oxygen saturated surface (after 240 L exposure to oxygen at 873 K). Fig. 4. Reaction rate of the CO oxidation on Pd(111) vs. oxygen pressure at constant temperature. The right axis presents calculated reaction sticking coefficients of CO. (a) The atomically clean Pd(111) surface. (b) Pd(111) with subsurface oxygen saturated surface (saturated with 240 L oxygen at 873 K).

Fig. 5.

Reaction rate of the CO oxidation reaction at 493 K (pCO = 2.1 ⋅ 10−6 mbar and

pO2 = 7.6 ⋅ 10−6 mbar ) as a function of the annealing temperature of Pd(111) during exposure to 240 L of oxygen for every data point. The right axis presents calculated reaction sticking coefficient of CO. Fig. 6. Comparison of the impact of subsurface oxygen on the Pd(111) catalytic activity at different reaction conditions.


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Fig. 7. Auger KLL transitions of oxygen excited by monochromatic Al Kα-radiation for the Pd(111) sample without (a,b) and with (c,d) subsurface oxygen. The insets show the corresponding C1s XP signals. (a) The KLL Auger peak for maximal CO coverage at room temperature on the atomically clean Pd(111) surface after exposure to 24 L CO.. (b) The KLL signal of the atomically clean Pd(111) at 523 K. (c) The KLL signal for the same sample after 240 L of oxygen exposure at 873 K and subsequent exposure to 24 L of CO at room temperature. (d) The KLL signal at 523 K. The inset presents the corresponding C1s XP- peak at 523 K. The lack of the C1s XP peak combined with the presence of the KLL signal clearly indicates the presence of subsurface oxygen and absence of carbon, i.e. CO.



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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Pd(111)

Catalytic activity [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Pd(111) saturated with subsurface oxygen

150 pO2= 7.6x10-6 mbar

pO2= 2.1x10-6 mbar

pCO= 2.1x10-6 mbar

pCO= 7.6x10-6 mbar

100

50

0 0

493 K

523 K

9

10


1 1 1K 2 493 490K

13

14

1 5 1K 6 523

17

Fig. 6

Page 29 of 29

286 284 288 Binding 283

a

Intensity [a.u]

Energy [eV]

b

2 86 Binding 284 288

Intensity [a.u]

Energy [eV]

28 2 283

286 284 288 Binding 283

c

Energy [eV]

Intensity [a.u]

3000

Intensity [a.u]

KLL- transition

Intensity [a.u]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


286 Binding 284 2 82 283 288

d

960

Energy [eV]

970

980

Binding Energy [eV] ACS Paragon Plus Environment

990 Fig. 7

Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd

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