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In Situ NAP-XPS and Mass Spectrometry Study of the Oxidation of Propylene over Palladium Vasily V Kaichev, Andrey Aleksandrovich Saraev, Andrey V. Matveev, Yury Vladimirovich Dubinin, Axel Knop-Gericke, and Valerii I. Bukhtiyarov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11129 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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

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The Journal of Physical Chemistry

In situ NAP-XPS and Mass Spectrometry Study of the Oxidation of Propylene over Palladium Vasily V. Kaichev,*,†,‡ Andrey A. Saraev,†,‡ Andrey V. Matveev, †,‡ Yury V. Dubinin,† Axel Knop-Gericke,§ Valerii I. Bukhtiyarov†,‡ †

Boreskov Institute of Catalysis, Lavrentieva Ave. 5, 630090 Novosibirsk, Russia ‡

§

Novosibirsk State University, Pirogova Str. 2, 630090 Novosibirsk, Russia

Department of Inorganic Chemistry, Fritz Haber Institute, Faradayweg 4-6, D-14195 Berlin, Germany

Keywords: Palladium, Propylene oxidation, Reaction mechanism, Catalyst deactivation, Temperature hysteresis, In situ study ABSTRACT: The oxidation of propylene over a Pd(551) single crystal has been studied in the mbar pressure range using near-ambient pressure X-ray photoelectron spectroscopy and mass spectrometry. It has been shown that irrespective of the O2:C3H6 molar ratio in the range 1-100, the total oxidation of propylene to CO2 and water and the partial oxidation of propylene to CO and H2 occur when the catalyst is heated above the light-off temperature; increasing the partial pressure of O2 leads to decreasing the catalytic activity. The selectivity toward CO2 is at least two times higher than selectivity toward CO indicating that the total oxidation is the main reaction route. The normal hysteresis with higher light-off temperature than extinction temperature is observed in the oxidation of propylene between 100 and 300 °C. According to NAP-XPS, the main reason of appearing the hysteresis is a competition between two surface processes: carbonization and oxidation of palladium. At low temperatures, the adsorption and following decomposition of propylene dominate, which results in accumulation of carbonaceous deposits blocking the palladium surface. Increasing the catalyst temperature leads to burning the carbonaceous deposits that initiates the following oxidation of propylene. The highest conversion of propylene is observed when both free surface sites and adsorbed oxygen atoms exist in a large amount on the catalyst surface. As the partial pressure of O2 increases, the catalyst surface gets

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covered by clusters of surface 2D palladium oxide, which is accompanied by a decrease in the catalytic activity. The mechanism of the oxidation of propylene over palladium is discussed. 1. INTRODUCTION Palladium is one of the most active materials for the catalytic oxidation of hydrocarbons and CO.1-3 As a result, Pd-based catalysts are widely used in automotive catalytic converters.4-5 Also palladium is expected to be employed as an active component in catalytic combustion chambers of gas turbine engines using natural gas as a fuel.6-7 The next problem which can be resolved successfully with palladium is the development of effective catalytic systems for reducing volatile organic compounds (VOCs) emissions. Due to practical importance, the mechanism of the total oxidation over Pd-based catalysts was studied many times; however, most of the studies were devoted to investigations of the oxidation of CO and methane. The catalytic oxidation of other hydrocarbons was not studied as often and, moreover, these investigations were usually performed ex situ (see Ref. [8] and refs therein) or under UHV conditions.9-12 Consequently, the studies could not provide direct information about the chemical state of palladium under real reaction conditions and reasons of catalyst deactivation in the VOCs oxidation. Herein we present the first results of our in situ NAP-XPS study of the oxidation of propylene over a Pd(551) single-crystal carried out in the mbar pressure range. Because the stepped surface of Pd(551) consists of many regular defects (the surface contains three-atomic terraces of the (110) plane separated by the (111) steps), it is a good model for the surface of metal nanoparticles in supported catalysts. It is generally accepted that stepped surface of metal nanoparticles plays an important role in their catalytic activity.13-14 The catalytic performance under the same conditions was studied using temperature-programmed reaction spectroscopy (TPRS). We choose propylene as a model hydrocarbon for the VOCs oxidation. We studied the oxidation of propylene under oxygen-rich conditions encompassing the range of interest for practical catalytic combustors. A special attention was given to the study of the propylene combustion in the low-temperature range because it can provide lower emissions of nitrogen oxides in the oxidation of hydrocarbons by air in different industrial applications. The use of NAP-XPS and TPRS allowed us to study the catalytic performance and the chemical state of palladium under reaction conditions and to determine the main reasons of catalyst deactivation. 2. EXPERIMENTAL SECTION The experiments were performed at the Innovative Station for In Situ Spectroscopy (ISISS) beamline at the synchrotron radiation facility BESSY II (Berlin, Germany). The experimental station was described in detail elsewhere.15 In short, this station was equipped with an electron energy analyzer (PHOIBOS-150, SPECS Surface Nano Analysis GmbH), a gas cell, and a 2 ACS Paragon Plus Environment

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system of electron lenses with three differential pumping stages. The high brilliance of the synchrotron radiation combined with a short travel-length of the photoelectrons through a “high pressure” zone in the gas cell allowed us to obtain high-quality core-level spectra under flow conditions at pressure up to 1 mbar. The station was also equipped by a MKS type 121A baratron (MKS Instruments Inc.), a quadrupole mass spectrometer (Prizma, Pfeiffer Vacuum GmbH) connected through a leak valve to the gas cell, and an extractor type ion source operated at 10-4 mbar of argon, which was used for cleaning the surface of samples under study. A Pd(551) single crystal (diameter 6 mm, thickness 1 mm) was used as a catalyst. The single crystal was mounted on a sapphire sample holder between a SiC plate and a stainless steel plate, which had a hole of 5 mm in diameter for measuring the core-level spectra of the catalyst surface. The sample was heated from the rear via the SiC plate with a NIR semiconductor laser (λ = 808 nm). This system allowed us to heat the crystal up to 1000 °C in vacuum. The sample temperature was monitored with a K-type thermocouple spot-welded directly to the crystal edge. The sample was cleaned by repeated cycles of Ar-ion sputtering (1.5 keV, 5 µA) at RT for 30 min, followed by heating in vacuum up to 600 °C and an oxygen treatment (600 °C , 10-7 mbar O2) for 20 min to remove carbon and subsequent annealing in vacuum for 5 min at 600 °C to remove oxygen. High-purity gases (O2: 99.998%, C3H6: 99.99%) were used without further purification. The flow rates of O2 and C3H6 were regulated separately with leak-valves. The C1s, Pd3d, and overlapping Pd3p/O1s core-level spectra were measured in situ with photon energy of 720 eV. Photoelectrons were collected in the normal direction with respect to the sample surface. The positions of the photoelectron peaks on the binding energy scale were referenced to the Fermi edge of palladium in the metallic state. Curve fitting was done by the CasaXPS software. A Shirley-type background was subtracted from each spectrum. The line shape used in the fit of the C1s and O1s spectra was the product of Lorentzian and Gaussian functions. An asymmetric so-called “LF line shape” was used for approximation of different components in the Pd3d5/2 and Pd3p3/2 spectra.12 Taking into account previous studies16-18 we used four components for fitting the Pd3d5/2 spectra and four components for fitting the O1s spectra. In all cases intensity of the peak, its Full Width at Half Maximum (FWHM) and peak position were allowed to vary within a reasonable range (Table 1). To separate out the O1s photoemission, the Pd3p and O1s overlapped spectra were fitted and then the Pd3p peaks were subtracted from the raw Pd3p/O1s spectra.



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Table 1. Characteristics of the XPS peaks used for fitting Pd3d5/2 and O1s core-level spectra Pd3d5/2 core-level spectra XPS peak

Assignment

BE, eV

FWHM, eV

Metal

335.00 ± 0.05

0.80 ± 0.05

PdCx

Surface palladium carbide

335.45 ± 0.10

0.75 ± 0.05

PdOx1

2D surface oxide

335.55 ± 0.05

0.90 ± 0.05

PdOx2

2D surface oxide

335.25 ± 0.05

0.90 ± 0.05

Pd

0

O1s core-level spectra XPS peak

Assignment

BE, eV

FWHM, eV

O(I)

2D surface oxide

529.00 ± 0.10

1.00 ± 0.05

O(II)

2D surface oxide

529.55 ± 0.05

1.00 ± 0.05

Oads

Chemosorbed oxygen

530.35 ± 0.05

1.15 ± 0.05

OH-group/CO

Adsorbed species

531.30 ± 0.05

1.15 ± 0.05

3. RESULTS AND DISCUSSION In our previous study12 it was shown that a normal hysteresis with higher light-off temperature than extinction temperature is observed in the oxidation of propylene over Pd(551) under UHV conditions. We attributed this phenomenon to blocking the catalyst surface by carbonaceous deposits which prevent the adsorption of reactants at lower temperature. However, recently, Bychkov with coauthors have studied the self-sustained reaction-rate oscillations in the oxidation of methane over Pd/Al2O3 catalysts by thermogravimetry and mass spectrometry at ambient pressure and shown that the oscillations are originated due to periodic oxidation and reduction of palladium and, crucially important, the maximum activity was observed on carbonized metallic Pd at least at 300-350 °C under methane-rich conditions.19 These opposite conclusions provoked our further study of the oxidation of propylene over palladium at higher pressures at the same manner. We repeated the TPRS-XPS study at near ambient pressure conditions, which allowed us to compare our results with the data obtained by thermogravimetry. The main results of TPRS study are shown in Figure 1. In these experiments the massspectrometric analysis was performed for the gas phase during heating and cooling of the Pd(551) single crystal with a constant rate between 100 and 500 °C. The rate was approximately 1 °C/s. To elucidate the influence of the partial pressure of oxygen on the catalytic performance, three types of the experiments were carried out in the flow regime. Before starting the measurements, the flow of propylene was set to a certain level which provided the partial 4 ACS Paragon Plus Environment

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pressure of 5×10-4 mbar. Then, the flow of oxygen was also set to certain levels which provided the total pressure of 10-3, 5.5×10-2, and 5×10-1 mbar. Consequently, the O2:C3H6 molar ratio (R) was about 1, 10, and 100, respectively. In all these cases CO and CO2 were observed as products indicating that both partial and total oxidation of propylene occurred over palladium. H2 and H2O were also detected but due to a high level of background signals the TPR spectra of hydrogen and water were not representative. Other products were not detected by mass spectrometry. In full agreement with our previous study12 in all cases normal hysteresis with the light-off temperature higher than the extinction temperature was observed (Fig. 1). It should be noted that CO and CO2 are the products of other reactions and, as a result, the temperature dependences of CO and CO2 yield are different both with heating and with cooling. Consequently, the hysteresis can be characterized by the light-off temperature and the extinction temperature for both reactions separately. Because the conversion of propylene did not achieve 100% in our experiments, we defined the light-off and extinction temperatures of the total oxidation and the partial oxidation in points where the yield of CO2 and CO achieved 50% of a maximal value, respectively. The results are summarized in Table 2.

Table 2. Light-off temperatures and extinction temperatures of the total and partial oxidation of propylene O2:C3H6 molar ratio R=1 R = 10 R = 100

Light-off temperature, °C partial oxidation total oxidation 273 290 212 213 188 192

Extinction temperature, °C partial oxidation total oxidation 173 169 155 153 135 136

The yield of CO2 was approximately two times higher than the yield of CO irrespective of the O2:C3H6 molar ratio. It means that the total oxidation of propylene dominates over palladium at least at near-ambient pressure conditions. In contrast, in our UHV study, where the partial pressure of propylene was 1×10-7 mbar, the selectivity toward CO2 changed significantly with the O2:C3H6 molar ratio.12 For example, after heating Pd(551) in the propylene/oxygen mixture at R = 1, CO was the main product; at R = 10 only strong CO2 peak was observed in the TPRS spectra while the yield of CO was unnoticeable. This finding indicates that the oxidation of propylene over palladium depends on the pressure: the product distributions observed in UHV and at higher pressures are different. For instance, the selective oxidation of olefins to CO is observed in UHV conditions only.10, 12



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Figure 1. CO (a) and CO2 (b) partial pressures measured by the mass spectrometer during heating and cooling Pd(551) in the mixture of propylene and oxygen at the O2:C3H6 molar ratio of 1, 10, and 100, respectively. To examine the changes in chemical state of the catalyst under reaction conditions, the C1s, Pd3d, and overlapping Pd3p/O1s core-level spectra of the palladium surface were obtained at fixed temperatures during stepwise heating from 100 to 500 °C and subsequent cooling in the same manner. The spectra were acquired at the set temperature for approximately 30 min; the heating/cooling rate between the XPS measurements was 1 °C/s. Figure 2 shows the typical C1s, Pd3d5/2, and O1s spectra obtained under oxygen-lean conditions (R = 1). One can see that several intense peaks observed in the C1s spectrum at 100 °C indicating that the catalyst surface is covered by carbonaceous deposits. The spectrum slightly changes after heating to 250 °C whereas at higher temperature no peaks were observed in the C1s spectra. In contrast, the C1s spectrum obtained after cooling from 500 to 250 °C contains weak peaks at 283.7, 284.4, and 285.5 eV.


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Figure 2. Normalized C1s, Pd3d5/2, and O1s core-level spectra obtained during stepwise heating from 100 to 500 °C and following cooling of Pd(551) in the mixture of propylene and oxygen at R = 1. All the spectra were normalized on the total intensity of the Pd3d spectra. The C1s spectra obtained at 100 °C and after the followed heating to 250 °C contain two intense peaks at 283.7 and 284.4 eV, and weak peaks at 285.5, 286.3, and 287.2 eV. According to the literature data,17, 20-21 the first two peaks correspond to the carbon species adsorbed on the surface and the carbon species dissolved in the Pd bulk, respectively. Indeed, in our previous study12 we discussed that propylene adsorbs on palladium at 100-250 °C to form molecularly adsorbed propylene which can transform to propylidyne and then to ethylidyne and CHx species; ethylidyne can also further decompose to the CHx species. This process can be accelerated by defects22 existing on the Pd(551) surface in a large amount. Even at room temperature these CHx species can completely dehydrogenate on palladium to produce atomic carbon species which can dissolve in the near-surface region of palladium to form a surface PdCx phase. This effect was discovered by Ziemecki et al.23 who found that the interstitial PdCx phase can be formed upon interaction of palladium with various carbon-containing molecules. As showed by Yudanov et al.,24 the migration of carbon atoms from the surface hollow sites into the octahedral subsurface sites of palladium is almost an isoenergetic process. According to literature,20,

23, 25-26

the

maximum carbon content of this PdCx phase varying from 0.12 to 0.15. In the presence of oxygen, the adsorbed carbon species can oxidize to CO. Consequently, the weak peaks at 285.5 and 286.3 eV can be attributed to CO molecules adsorbed in hollow/bridge and on-top positions, respectively.20 The peak at 287.2 eV may be attributed to carbon atoms doubly bound to oxygen 7 ACS Paragon Plus Environment


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in ketones (i.e., C=O).27 Certainly, other carbon-contained adsorbed species such as C-OH and CxHyO may present on the palladium surface because several peaks are observed in the O1s spectra. Unfortunately, it is hard to unscramble these O1s spectra obtained at 100 and 250 °C when a high concentration of carbonaceous species is observed. We can speculate only that the peak at 531.1 eV may be attributed to hydroxyl group and adsorbed CO,28 whereas the peak at 530.2 eV may be partially assigned to chemisorbed oxygen.16 The formation of the PdCx phase is confirmed by the shape of the Pd3d5/2 core-level spectra. Indeed, the Pd3d5/2 spectra obtained at 100 and 250 °C during heating in the reactant mixture consist of two peaks at 335.0 and 335.4 eV (Fig. 2). Both these values of the Pd3d5/2 binding energy are typical of the palladium in the metallic state. The first peak corresponds to the atoms in the palladium bulk; the second peak can be assigned to the electronically altered PdxCy surface state.17, 25-26 At low temperatures the peak at 335.4 eV is much higher in intensity than the peak at 335.0 eV; increasing the temperature leads to an increase of the peak at 335.0 eV whereas the peak at 335.4 eV decreases. It means that carbon atoms intensively dissolve in the palladium bulk at least at temperatures between 100 and 250 °C. At higher temperatures, when all C1s peaks disappear, the Pd3d5/2 peak at 335.4 eV also disappears and new peaks at 335.6 and 336.3 eV are observed. According to literature,16, 29-31 these peaks can be attributed to a surface palladium oxide. For example, the surface 2D oxide, Pd5O4, formed over Pd(111) during the oxidation of methane is characterized by two Pd3d5/2 peaks at 335.5 and 336.25 eV corresponding to the Pd atoms with two O neighbors and the Pd atoms with four O neighbors, respectively.16 Similar Pd3d5/2 peaks were observed by Zemlyanov et al.31 in the spectra of an oxidized Pd(110) surface. Unfortunately, the structure and exact stoichiometry of the surface 2D palladium oxide over Pd(110) remain unknown. Two different palladium components of the 2D oxide correspond to two different coordination numbers of palladium atoms with oxygen. Because the Pd(551) surface contains three-atomic terraces of the (110) plane separated by the (111) steps, both structures of the surface 2D palladium oxide may be formed in our case. Lundgren et al.30 showed that in the Pd5O4 structure over Pd(111) half of the oxygen atoms are bonded only to the in-plane Pd atoms (3-fold O), whereas the second half – those located above Pd atoms in the subsurface layer – are also coordinated to subsurface Pd atoms (4-fold O). As a result, two O1s peaks, approximately at 529.0 and 529.55 eV, are observed which can be assigned to 3-fold and 4-fold coordinated O atoms within the Pd5O4 surface oxide structure.16 Therefore, the O1s peaks at 529.0 and 529.5 observed in our case at 300, 400, and 500 °C during heating in the reactant mixture and following cooling to 250 °C (Fig. 2) can be attributed to the oxygen species in the 2D surface oxide. The presence of oxygen species dissolved in the near8 ACS Paragon Plus Environment

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surface regions which are also characterized by the O1s peak at 529.0 eV16 cannot be excluded as well. Indeed, the Pd5O4 surface oxide on Pd(111) prepared by treatment in 5×10-6 mbar O2 at 300 °C for 600 s is characterized by two O1s peaks with the same intensity.30 Gabasch et al.16 showed that the peak at 529.0 eV is slightly bigger (by factor 1.35) than the peak at 529.55 eV. In our case the ratio between the peaks at 529.0 and 529.5 eV can achieve 4.5. A weak additional peak at 530.2 eV can be attributed to chemosorbed oxygen species.16 No 3D PdO clusters characterized by the Pd3d5/2 peak at 336.6 eV were detected under the used conditions. Similar core-level spectra were observed at higher partial pressure of O2 (See Figures S1 and S2 in Supporting Information). The in situ NAP-XPS data indicate that the competition between processes of formation and removal of carbonaceous deposits on the catalyst surface determines the temperature hysteresis in the catalytic activity. At low temperatures near 100-200 °C the palladium surface is covered by CHx species mainly (Fig. 2). Due to the absence of free sites, these species cannot dehydrogenate and react with adsorbed oxygen which also exists on the surface. Moreover, atomic carbon species appearing as a result of the dehydrogenation of CHx can diffuse into the subsurface palladium layer and avoid reacting with oxygen. The oxidation rate of adsorbed CO is also negligible; it may be due to a low rate of the surface diffusion. A similar behavior was observed in the oxidation of methanol over Pd(111).20 Increase in the temperature leads to expansion of the surface diffusion which initiates burning of the carbonaceous deposits. As a result, no carbon-containing species are detected by XPS above the light-off temperature and the surface is mainly covered by chemisorbed oxygen and clusters of the surface 2D palladium oxide. Indeed, after heating above 250 °C the characteristic peaks at 529.0, 529.5, and 530.2 eV are observed in the O1s spectra (Fig. 2). These oxygen species are very active toward adsorbed CO and carbon atoms, and prevent the formation of carbonaceous deposits. As a result, a high rate of the oxidation of propylene observed during cooling even below the light-off temperature. Our hypothesis that the temperature hysteresis is due to competition between formation and removal of carbonaceous deposits is additionally supported by a decrease of the light-off temperature and the extinction temperature with increasing the partial pressure of O2. For example, the maximal yield of CO at R = 1, 10, and 100 is reached at 295, 225, and 205 °C, respectively (Fig. 1). It should be stressed that a high concentration of the oxygen species also suppresses the oxidation of propylene. We found that an increase in the partial pressure of O2 in the reactant mixture leads to a decrease in the conversion of propylene. For example, the maximal conversion of propylene measured during heating of the Pd(551) single crystal reaches 91%, 64%, and 47% at R = 1, 10, and 100, respectively (Fig. 3a). According to NAP-XPS, this effect is due to 9 ACS Paragon Plus Environment


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blocking the catalyst by clusters of the surface 2D palladium oxide. Indeed, the Pd3d5/2 peaks at 335.6 and 336.3 eV increase in intensity with R whereas the Pd3d5/2 peak at 335.0 eV corresponded to the Pd bulk decreases (Fig. 3b). The amount of the surface 2D palladium oxide measured during the heat to 300, 250, and 250 °C corresponding to approximately maximal conversion of propylene are 9%, 18%, and 25% at R = 1, 10, and 100, respectively. A similar trend is observed during cooling (Fig. 3c): the amount of the surface 2D palladium oxide at 250 °C is 14%, 17%, and 24% at R = 1, 10, and 100, respectively.

Figure 3. Propylene conversion during heating and cooling Pd(551) in the mixture of propylene and oxygen at the O2:C3H6 molar ratio of 1, 10, and 100 (a), and the Pd3d5/2 spectra measured at 300, 250, and 250 °C during heating (b) and following cooling to 250 °C (c) at the same conditions. Finally, we can discuss the origin of the most active state of palladium for the oxidation of hydrocarbons. Several points of view can be found in the literature. In early works, most of the authors considered only one of two possible states of palladium under reaction conditions: palladium in the metallic state or bulk PdO. Indeed, classic thermodynamics precludes coexisting of Pd and PdO crystallites simultaneously at steady state.3 Afterwards, some authors suggested that other metastable phases, such as “subsurface oxide” or “surface oxide”, can form under reaction conditions and affect the rate of catalytic reactions. The surface phase diagram built on the basis of the free energies from density-functional theory calculations as a function of p,T by using the oxygen chemical potential showing stability regions of the different palladium oxide structures can be found elsewhere.32 These findings allowed developing several models of active 10 ACS Paragon Plus Environment

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species, which can predict even nonlinear effects in the oxidation of hydrocarbons over palladium. However, it should be noted that some of these models are conflicting. Thus, Zhang et al.33 studied the self-sustained reaction rate oscillations in the oxidation of methane over Pd foil and concluded, basing on ex situ XRD analysis, that the oscillations are originated due to switching back and forth from the reduced low-active state to the oxidized (PdO) high-active state. In contract, recently, Bychkov et al.,19 while studying the self-sustained reaction-rate oscillations in the oxidation of methane over supported Pd/Al2O3 catalysts by thermogravimetry and mass spectrometry, concluded that indeed the oscillations appears due to periodic oxidation and reduction of palladium; however, the maximum activity is observed on carbonized metallic Pd at least at 300-350 °C under methane-rich conditions at atmospheric pressure. However, in our experiments at 300 °C no carbon species were detected by NAP-XPS (Fig. 2) in spite of a high rate of the oxidation of propylene (Fig. 1). The PdCx phase was detected at lower temperatures only when the propylene conversion was extremely low. Unfortunately, the obtained in situ data cannot allow us to compare the catalytic properties of the PdCx phase and clean metallic Pd. However, according to previous XANES studies the electronic state in these cases is almost the same. Indeed, in spite of high sensitive of XANES to the chemical state of palladium, McCaulley25 showed that the Pd K-edge XANES spectra of PdC0.13 is nearly identical to that of metallic Pd. It means that the main distinction between the PdCx phase and metallic Pd is a small difference in the Pd-Pd distances. The lattice constants for Pd and PdC0.13 are 0.3890 and 0.3996 nm, respectively.25 From this point of view, the adsorption properties and catalytic performance of the PdCx phase and clean metallic Pd cannot differ significantly. Indeed, according to our data (Fig. 2) the sharp increase of the CO and CO2 production is due to transition from the low-active state, when palladium is covered by the carbonaceous deposits, to high-active state, when palladium is covered by chemosorbed oxygen species and clusters of the surface 2D palladium oxide. We can speculate that removal of the carbonaceous deposits and adsorption of oxygen initiates an increase of the oxidation rate of propylene. Simultaneously, a decomposition of the PdCx phase occurs leading to the appearance of the extra yield of CO which can be observed as a sharp peak during heating Pd(551) in the mixtures with R = 1 (Fig. 1). Another hypothesis was set up by Martin et al.,34 who studied the oxidation of methane on Pd(100) by NAP-XPS in the mbar pressure range at temperatures between 250 and 500 °C. They supposed that the highest activity was achieved when a two-layer oriented film of PdO(101) was formed on the palladium surface. Again, this finding disagrees with our XPS data indicating that the formation of the surface palladium oxide during the oxidation of propylene leads to blocking the palladium surface and decreases the conversion of propylene (Fig. 3). This controversial 11 ACS Paragon Plus Environment


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conclusion was made by Martin et al.34 perhaps because the NAP-XPS study was performed under near stoichiometric conditions only; the authors did not analyze the rate of methane oxidation at different O2:CH4 molar ratios. Of course, the different adsorption properties of methane and propylene and different surface structures of single-crystals under study cannot be excluded as well. Beside, Gabasch et al.,16 studying the temperature hysteresis in the oxidation of methane over Pd(111) by NAP-XPS under oxygen-rich conditions, found that the formation of bulk PdO “seeds” on top of the 2D Pd5O4 surface oxide covered the Pd(111) single crystal induces a strongly enhanced CH4 oxidation activity, highlighting that highly dispersed PdO is the most active phase, even if formed on extended Pd bulk catalysts. However, no features of bulk PdO “seeds” were detected in our in situ experiments (Figures 2 and 3b). Hence, we can conclude that the highest activity in the oxidation of propylene is demonstrated by palladium in the metallic state; however, the chemistry and structure of upper layers of the catalyst can be changed under the reaction conditions. As a result, the mechanism of the oxidation of propylene is complicated, and the dominant reaction routes depend on the reaction temperature and the partial pressures of reactants. The reaction starts from the decomposition of propylene to carbon and hydrogen atoms occurring on the palladium in the metallic state with a high rate. In brief, this process could be described by equation (1). The carbon atoms can dissolve in the Pd bulk to form the surface PdCx phase (2). Both these processes occur with an appreciable rate above 100 °C17, 20 and cannot limit the oxidation of propylene in the temperature range between 150 and 250 °C (Fig. 3). The low rate of the oxidation of propylene at temperatures below the light-off temperature is because of blocking the palladium surface by carbonaceous deposits which cannot decompose to carbon and hydrogen atoms due to absence of free adsorption cites. The hydrogen atoms can associate and desorb (3), which leads to the creation of the free adsorption sites. Further, oxygen can adsorb to produce chemisorbed oxygen species (4) which have high reactivity toward adsorbed carbon and hydrogen atoms. As a result, adsorbed CO and hydroxyl groups form in accordance with equations (5) and (6). Hydroxyl groups can react to produce water (7); CO can desorb as a product (8) or be further oxidized to CO2 (9). Dissolved carbon can diffuse onto the surface (10) and react with oxygen species again. We do not consider the dissolution of hydrogen atoms in the Pd bulk and formation of the PdH2 phase because palladium hydride is unstable and decomposes at approximately 80 °C.7 These steps describe a Langmuir-Hinshelwood reaction mechanism of the oxidation of propylene on the metallic palladium.

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2‫ܪ‬௔ௗ௦ → ‫ܪ‬ଶ ↑

(3)

ܱଶ → 2ܱ௔ௗ௦

(4)

ܱୟୢୱ + ‫ܥ‬௔ௗ௦ → ‫ܱܥ‬௔ௗ௦

(5)

ܱୟୢୱ + ‫ܪ‬௔ௗ௦ → ܱ‫ܪ‬௔ௗ௦

(6)

2ܱ‫ܪ‬ୟୢୱ → ܱ௔ௗ௦ + ‫ܪ‬ଶ ܱ ↑

(7)

‫ܱܥ‬ୟୢୱ → ‫↑ ܱܥ‬

(8)

‫ܱܥ‬ୟୢୱ + ܱ௔ௗ௦ → ‫ܱܥ‬ଶ ↑

(9)

‫ܥ‬ୢ୧ୱ → ‫ܥ‬௔ௗ௦

(10)

One can see that desorption and oxidation of adsorbed CO are competitive processes, and selectivity toward CO (and correspondently toward CO2) depends on the concentration of adsorbed oxygen species. This explains why, after being heated to certain temperature, CO is observed at first as the main product, whereas CO2 starts to produce at a higher temperature. Thus, at R = 1 the yield of CO increases monotonically between 200 and 295 °C while a sharp increase of the CO2 yield is observed at approximately 290 °C (Fig. 1). In fact, this finding indicates that at low temperatures near the light-off temperature there is the competition between two surface processes: carbonization and oxidation of palladium. Accumulation of the carbonaceous deposits leads to the catalyst deactivation. After burning of the carbonaceous deposits the concentration of adsorbed oxygen species increases that leads to a shift of the reaction toward CO2 production. Recently, a similar effect was observed in the oxidation of methane over Ni foil that led to the oscillatory behavior appearing.35 It is very important to note that chemisorbed oxygen species can transform to the surface palladium oxide (11). These PdOx species can also react with carbonaceous species to produce CO (12) and CO2 (13). Since the selectivity toward CO weakly depends on the partial pressure of O2, the formation of CO and CO2 may proceed through different reaction routes. Besides, the reaction mechanism changes after full oxidation of the palladium surface. The oxidation of hydrocarbons over metal oxide-based catalysts usually proceeds through the redox mechanism, where the oxidized catalyst surface oxidizes the reactant and is reoxidized by gas-phase oxygen.36

ܲ݀ + ܱ௔ௗ௦ → ܱܲ݀௫

(11)

‫ܥ‬௔ௗ௦ + ܱܲ݀௫ → ܲ݀ + ‫↑ ܱܥ‬

(12) 13 ACS Paragon Plus Environment


‫ܥ‬௔ௗ௦ + 2ܱܲ݀௫ → 2ܲ݀ + ‫ܱܥ‬ଶ ↑

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(13)

Therefore, our data clearly indicate that the activity of the surface 2D palladium oxide is lower than that of palladium in the metallic state. Indeed, the oxidation of palladium surface leads to the deactivation. Earlier, similar behavior was observed by NAP-XPS during the oxidation of CO over Pd(111).37 This effect is significant at moderate temperatures only; at high temperatures the palladium oxide is unstable. We have showed above that an increase in the concentration of the surface 2D palladium oxide is accompanied by a decrease in the conversion of propylene. Moreover, after heating above 430 °C under oxygen-rich conditions the conversion of propylene increases sharply (Fig. 3a), which indicates the thermal decomposition of palladium oxide.38 According to NAP-XPS, the amount of the surface 2D palladium oxide at 250 and 400 °C (R = 10) is near 17-18%, while at 500 °C it is approximately 11% (Fig. S1). A similar effect was observed in the oxidation of methane over Pd(111).16 Such dependence of catalytic activity is not unusual and there are several experimental works proving that transition metal oxides are less active in the catalytic oxidation of hydrocarbons than corresponding metals. For example, recently, we have studied in situ the self-sustained oscillations in the catalytic oxidation of propane and methane over Ni foils by NAP-XPS and XRD, and found that the oscillations originate due to periodical oxidation and reduction of nickel.39-40 In both cases Ni in the metallic state provided a high conversion of reactants while the formation of a thick NiO layer over the foil led to a decrease of the catalytic activity. The activity of Ni in the metallic state is approximately 10 times higher than the activity of NiO.39, 41 We can speculate that the activity of other transition metals may also be higher than activity of their oxides in the oxidation of hydrocarbons. However, to prove this idea, additional operando studies must be performed. Finally, it should be noted that dissolved oxygen can also effect the catalytic activity. For example, Wrobel et al.42 showed that subsurface oxygen reduces significantly the catalytic activity of the Pd(111) surface in the oxidation of CO due to reduction of the sticking coefficient of CO and O2. In contrast, in our case an increase of the oxygen partial pressure, which usually must be accompanied by the dissolution of oxygen in the near-surface region, leads to a decrease of the light-off and extinction temperatures of the total and partial oxidation of propylene (Table 2). It means that the dissolved or subsurface oxygen species at least do not decrease the activity of palladium in the oxidation of propylene. CONCLUSIONS The oxidation of propylene over a Pd(551) single crystal was studied in situ in the mbar pressure range using NAP-XPS and TPRS. In contrast to our previous UHV study we found that 14 ACS Paragon Plus Environment

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even in the oxygen-lean conditions the main reaction route is the total oxidation of propylene to CO2. The normal hysteresis with higher light-off temperature than extinction temperature is observed in the oxidation of propylene due to a competition between two surface processes: carbonization and oxidation of palladium. The highest activity was observed when palladium was in the metallic state. However, at low temperatures palladium is deactivated due to covering by carbonaceous deposits which block the surface and prevent catalytic reactions. After heating to a certain temperature, which depends on the O2:C3H6 molar ratio, the carbonaceous deposits are removed by oxygen and the oxidation of propylene occurs with a high rate. The further heat in the reactant mixture is accompanied by formation of surface palladium oxide species, which leads to the catalyst deactivation in a moderate temperature range. Above approximately 430 °C the palladium oxide species decompose and the increasing reaction rate is observed. During cooling a reverse scheme in the surface transformation occurs where the carbonaceous deposits form at temperatures below the light-off temperature. Such behavior under the reaction conditions indicates that both the carbon deposition and the palladium oxidation lead to the catalyst deactivation in the oxidation of propylene. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publication website at DOI:. Normalized C1s, Pd3d5/2, and O1s core-level spectra obtained during stepwise heating from 100 to 500 °C and following cooling of Pd(551) in the mixture of propylene and oxygen at R = 10 and 100 are given. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Budget Project #АААА-А17-117041710078-1for the Boreskov Institute of Catalysis SB RAS. Notes 15 ACS Paragon Plus Environment


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was performed within the framework of Budget Project for the Boreskov Institute of Catalysis SB RAS. The authors are grateful to M. Hävecker, D. Teschner, and R. Blume for fruitful discussions and their support during the in situ experiments. We also thank HZB for allocation of synchrotron radiation beam time. REFERENCES 1. Aryafar, M.; Zaera, F., Kinetic Study of the Catalytic Oxidation of Alkanes over Nickel, Palladium, and Platinum Foils. Catal. Lett. 1997, 48, 173-183. 2. Cordonna, G. W.; Kosanovich, M.; Becker, E. R., Gas Turbine Emission Control: Platinum and Platinum-Palladium Catalysts for Carbon Monoxide and Hydrocarbon Oxidation. Platinum Met. Rev. 1989, 33, 46-54. 3. Ciuparu, D.; Lyubovsky, M. R.; Altman, E.; Pfefferle, L. D.; Datye, A., Catalytic Combustion of Methane over Palladium-Based Catalysts. Catal. Rev. 2002, 44, 593-649. 4. Sekiba, T.; Kimura, S.; Yamamoto, H.; Okada, A., Development of Automotive Palladium Three-Way Catalysts. Catal. Today 1994, 22, 113-126. 5. Wang, J.; Chen, H.; Hu, Z.; Yao, M.; Li, Y., A Review on the Pd-Based Three-Way Catalyst. Catal. Rev. 2015, 57, 79-144. 6. Dalla Betta, R. A., Catalytic Combustion Gas Turbine Systems: The Preferred Technology for Low Emissions Electric Power Production and CO-Generation. Catal. Today 1997, 35, 129-135. 7. Yashnik, S. A.; Chesalov, Y. A.; Ishchenko, A. V.; Kaichev, V. V.; Ismagilov, Z. R., Effect of Pt Addition on Sulfur Dioxide and Water Vapor Tolerance of Pd-Mn-Hexaaluminate Catalysts for High-Temperature Oxidation of Methane. Appl. Catal. B 2017, 204, 89-106. 8. Ferhat-Hamida, Z.; Barbier, J.; Labruquere, S.; Duprez, D., The Chemical State of Palladium in Alkene and Acetylene Oxidation: A Study by XRD, Electron Microscopy and TDDTG Analysis. Appl. Catal. B 2001, 29, 195-205. 9. Altman, E. I., The Reaction of Propene with Oxygen-Covered Pd(100). Surf. Sci. 2003, 547, 108-126. 10. Gabasch, H.; Knop-Gericke, A.; Schlögl, R.; Unterberger, W.; Hayek, K.; Klötzer, B., Ethene Oxidation on Pd(111): Kinetic Hysteresis Induced by Carbon Dissolution. Catal. Lett. 2007, 119, 191-198. 11. Hartmann, N.; Esch, F.; Imbihl, R., Steady State Kinetics of the Decomposition and Oxidation Of Methanol on Pd(110). Surf. Sci. 1993, 297, 175-185. 12. Matveev, A. V.; Kaichev, V. V.; Saraev, A. A.; Gorodetskii, V. V.; Knop-Gericke, A.; Bukhtiyarov, V. I.; Nieuwenhuys, B. E., Oxidation of Propylene over Pd(551): Temperature Hysteresis Induced by Carbon Deposition and Oxygen Adsorption. Catal. Today 2015, 244, 2935. 13. Hendriksen, B. L. M.; Ackermann, M. D.; van Rijn, R.; Stoltz, D.; Popa, I.; Balmes, O.; Resta, A.; Wermeille, D.; Felici, R.; Ferrer, S.; et al., The Role of Steps in Surface Catalysis and Reaction Oscillations. Nat. Chem. 2010, 2, 730. 14. Ferrer, D.; Blom, D. A.; Allard, L. F.; Mejia, S.; Perez-Tijerina, E.; Jose-Yacaman, M., Atomic Structure of Three-Layer Au/Pd Nanoparticles Revealed by Aberration-Corrected Scanning Transmission Electron Microscopy. J. Mater. Chem. 2008, 18, 2442-2446. 15. Knop-Gericke, A.; Kleimenov, E.; Hävecker, M.; Blume, R.; Teschner, D.; Zafeiratos, S.; Schlögl, R.; Bukhtiyarov, V. I.; Kaichev, V. V.; Prosvirin, I. P.; et al., Chapter 4, X-Ray 16 ACS Paragon Plus Environment

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Photoelectron Spectroscopy for Investigation of Heterogeneous Catalytic Processes. In Adv. Catal., Academic Press: 2009; Vol. 52, pp 213-272. 16. Gabasch, H.; Hayek, K.; Klötzer, B.; Unterberger, W.; Kleimenov, E.; Teschner, D.; Zafeiratos, S.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R.; et al., Methane Oxidation on Pd(111): In Situ XPS Identification of Active Phase. J. Phys. Chem. C 2007, 111, 7957-7962. 17. Gabasch, H.; Kleimenov, E.; Teschner, D.; Zafeiratos, S.; Hävecker, M.; Knop-Gericke, A.; Schlögl, R.; Zemlyanov, D.; Aszalos-Kiss, B.; Hayek, K.; et al., Carbon Incorporation during Ethene Oxidation on Pd(111) Studied by In situ X-Ray Photoelectron Spectroscopy at 2×103 mbar. J. Catal. 2006, 242, 340-348. 18. Zemlyanov, D.; Aszalos-Kiss, B.; Kleimenov, E.; Teschner, D.; Zafeiratos, S.; Hävecker, M.; Knop-Gericke, A.; Schlögl, 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, 983-994. 19. Bychkov, V. Y.; Tulenin, Y. P.; Slinko, M. M.; Khudorozhkov, A. K.; Bukhtiyarov, V. I.; Sokolov, S.; Korchak, V. N., Self-Oscillations of Methane Oxidation Rate over Pd/Al2O3 Catalysts: Role of Pd Particle Size. Catal. Commun. 2016, 77, 103-107. 20. Kaichev, V. V.; Miller, A. V.; Prosvirin, I. P.; Bukhtiyarov, V. I., In situ XPS and MS Study of Methanol Decomposition and Oxidation on Pd(111) Under Millibar Pressure Range. Surf. Sci. 2012, 606, 420-425. 21. Morkel, M.; Kaichev, V. V.; Rupprechter, G.; Freund, H. J.; Prosvirin, I. P.; Bukhtiyarov, V. I., Methanol Dehydrogenation and Formation of Carbonaceous Overlayers on Pd(111) Studied by High-Pressure SFG and XPS Spectroscopy. J. Phys. Chem. B 2004, 108, 1295512961. 22. Rupprechter, G.; Kaichev, V. V.; Unterhalt, H.; Morkel, M.; Bukhtiyarov, V. I., CO Dissociation and CO Hydrogenation on Smooth and Ion-Bombarded Pd(111): SFG and XPS Spectroscopy at mbar Pressures. Appl. Surf. Sci. 2004, 235, 26-31. 23. Ziemecki, S. B.; Jones, G. A.; Swartzfager, D. G.; Harlow, R. L.; Faber, J., Formation of Interstitial Palladium-Carbon Phase by Interaction of Ethylene, Acetylene, and Carbon Monoxide with Palladium. J. Am. Chem. Soc. 1985, 107, 4547-4548. 24. Yudanov, I. V.; Neyman, K. M.; Rosch, N., Density Functional Study of Pd Nanoparticles with Subsurface Impurities of Light Element Atoms. Phys. Chem. Chem. Phys. 2004, 6, 116-123. 25. McCaulley, J. A., Temperature Dependence of the Pd K-Edge Extended X-Ray Absorption Fine Structure of PdCx (x~0.13). Phys. Rev. B 1993, 47, 4873-4879. 26. Teschner, D.; Vass, E.; Hävecker, M.; Zafeiratos, S.; Schnörch, P.; Sauer, H.; KnopGericke, A.; Schlögl, R.; Chamam, M.; Wootsch, A.; et al., Alkyne Hydrogenation over Pd Catalysts: A New Paradigm. J. Catal. 2006, 242, 26-37. 27. Kundu, S.; Wang, Y.; Xia, W.; Muhler, M., Thermal Stability and Reducibility of Oxygen-Containing Functional Groups on Multiwalled Carbon Nanotube Surfaces: A Quantitative High-Resolution XPS and TPD/TPR Study. J. Phys. Chem. C 2008, 112, 1686916878. 28. Held, G.; Schuler, J.; Sklarek, W.; Steinrück, H. P., Determination of Adsorption Sites of Pure and Coadsorbed CO on Ni(111) by High Resolution X-Ray Photoelectron Spectroscopy. Surf. Sci. 1998, 398, 154-171. 29. Blomberg, S.; Hoffmann, M. J.; Gustafson, J.; Martin, N. M.; Fernandes, V. R.; Borg, A.; Liu, Z.; Chang, R.; Matera, S.; Reuter, K.; et al., In Situ X-Ray Photoelectron Spectroscopy of Model Catalysts: At the Edge of the Gap. Phys. Rev. Lett. 2013, 110, 117601. 30. Lundgren, E.; Kresse, G.; Klein, C.; Borg, M.; Andersen, J. N.; De Santis, M.; Gauthier, Y.; Konvicka, C.; Schmid, M.; Varga, P., Two-Dimensional Oxide on Pd(111). Phys. Rev. Lett. 2002, 88, 246103. 31. Zemlyanov, D.; Klötzer, B.; Gabasch, H.; Smeltz, A.; Ribeiro, F. H.; Zafeiratos, S.; Teschner, D.; Schnörch, P.; Vass, E.; Hävecker, M.; et al., Kinetics of Palladium Oxidation in the mbar Pressure Range: Ambient Pressure XPS Study. Top. Catal. 2013, 56, 885-895. 17 ACS Paragon Plus Environment


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32. 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. 33. Zhang, X.; Lee, C. S. M.; Mingos, D. M. P.; Hayward, D. O., Oscillatory Behaviour During the Oxidation of Methane over Palladium Metal Catalysts. Appl. Catal. A 2003, 240, 183-197. 34. Martin, N. M.; Van den Bossche, M.; Hellman, A.; Grönbeck, H.; Hakanoglu, C.; Gustafson, J.; Blomberg, S.; Johansson, N.; Liu, Z.; Axnanda, S.; et al., Intrinsic Ligand Effect Governing the Catalytic Activity of Pd Oxide Thin Films. ACS Catal. 2014, 4, 3330-3334. 35. Lashina, E. A.; Kaichev, V. V.; Saraev, A. A.; Vinokurov, Z. S.; Chumakova, N. A.; Chumakov, G. A.; Bukhtiyarov, V. I., Experimental Study and Mathematical Modeling of SelfSustained Kinetic Oscillations in Catalytic Oxidation of Methane over Nickel. J. Phys. Chem. A 2017, 121, 6874-6886. 36. Kaichev, V. V.; Chesalov, Y. A.; Saraev, A. A.; Klyushin, A. Y.; Knop-Gericke, A.; Andrushkevich, T. V.; Bukhtiyarov, V. I., Redox Mechanism for Selective Oxidation of Ethanol over Monolayer V2O5/TiO2 Catalysts. J. Catal. 2016, 338, 82-93. 37. Toyoshima, R.; Yoshida, M.; Monya, Y.; Kousa, Y.; Suzuki, K.; Abe, H.; Mun, B. S.; Mase, K.; Amemiya, K.; Kondoh, H., In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd(111) Surfaces. J. Phys. Chem. C 2012, 116, 18691-18697. 38. Hinojosa, J. A.; Weaver, J. F., Surface Structural Evolution during the Thermal Decomposition of a PdO(101) Thin Film. Surf. Sci. 2011, 605, 1797-1806. 39. Kaichev, V. V.; Teschner, D.; Saraev, A. A.; Kosolobov, S. S.; Gladky, A. Y.; Prosvirin, I. P.; Rudina, N. A.; Ayupov, A. B.; Blume, R.; Hävecker, M.; et al., Evolution of Self-Sustained Kinetic Oscillations in the Catalytic Oxidation of Propane over a Nickel Foil. J. Catal. 2016, 334, 23-33. 40. Saraev, A. A.; Vinokurov, Z. S.; Kaichev, V. V.; Shmakov, A. N.; Bukhtiyarov, V. I., The Origin of Self-Sustained Reaction-Rate Oscillations in the Oxidation of Methane over Nickel: An Operando XRD and Mass Spectrometry Study. Catal. Sci. Technol. 2017, 7, 16461649. 41. Saraev, A. A.; Kosolobov, S. S.; Kaichev, V. V.; Bukhtiyarov, V. I., Origin of Temperature Oscillations of Nickel Catalyst Occurring in Methane Oxidation. Kinet. Catal. 2015, 56, 598-604. 42. Wrobel, R. J.; Becker, S.; Weiss, H., Influence of Subsurface Oxygen in the Catalytic CO Oxidation on Pd(111). J. Phys. Chem. C 2015, 119, 5386-5394.


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