Introducing Time Resolution to Detect Ce3+ Catalytically Active Sites

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Letter 3+

Introducing Time Resolution to Detect Ce Catalytically Active Sites at the Pt/ CeO Interface through Ambient Pressure X-Ray Photoelectron Spectroscopy 2

Luca Artiglia, Fabrizio Orlando, Kanak Roy, René Kopelent, Olga E. Safonova, Maarten Nachtegaal, Thomas Huthwelker, and Jeroen Anton van Bokhoven J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02314 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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The Journal of Physical Chemistry Letters

Introducing Time Resolution to Detect Ce3+ Catalytically Active Sites at the Pt/CeO2 Interface through Ambient Pressure X-Ray Photoelectron Spectroscopy Luca Artiglia,†,* Fabrizio Orlando, † Kanak Roy, ‡ René Kopelent, † Olga Safonova, † Maarten Nachtegaal, † Thomas Huthwelker, † Jeroen A. van Bokhoven†,‡,*

†

Paul Scherrer Institute, CH-5232, Villigen (Switzerland).

‡

Institute for Chemical and Bioengineering, ETH Zurich, CH-8093 Zurich (Switzerland).

ABSTRACT X-ray photoelectron spectroscopy has been employed for the qualitative and quantitative characterization of both model and real catalytic surfaces. Recent progress in the detection of photoelectrons has enabled the acquisition of spectra at pressures up to a few tens of millibars. Although reducing the pressure gap represents a remarkable advantage for catalysis, active sites may be short-lived or hidden in the majority of spectator species. Timeresolved experiments, conducted under transient conditions, are a suitable strategy for discriminating between active sites and spectators. In the present work we characterized the surface of a Pt/CeO2 powder catalyst at 1.0 mbar of a reacting mixture of carbon monoxide

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and oxygen, and, by means of time resolution, identified short-lived active species. We replaced oxygen with nitrogen in the reaction mixture while fast-detecting the core level peaks of cerium. The results indicate that active Ce3+ sites form transiently at the surface when the oxygen is switched off. The analysis of the depth profile shows that Ce3+ ions are located at the ceria surface. The same experiment, performed on platinum-free ceria, reveals negligible reduction, indicating that platinum boosts the formation of Ce3+ active sites at the interface.


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Heterogeneous catalysts, consisting of metal nanoparticles deposited on high surface area oxides, are important in the chemical industry, for the detection and control of emissions, in environmental chemistry, and in the conversion of energy.1,2,3 It is well known that the activity of supported catalysts depends on the size distribution of the nanoparticles and on their morphology as well as on the support.4,5 Despite the possibility of determining the structure of a functioning catalyst, the nature and role of the active species involved in catalytic reactions is still a matter of debate. This is due mainly to the low concentration of active sites, their short lifetime, and the requirement to distinguish them from inactive spectators.6,7 Operando time-resolved characterization tools are an important aid to acquiring snapshots of a catalytic reaction, and enable the detection of differences in the relative population of the active sites.8,9 The time-resolved approach is better achieved under transient conditions, i.e. changing the gas or liquid feed concentration in an attempt to detect a modification of the population of active sites. Assuming a Langmuir–Hinshelwood mechanism, reactants adsorb on the catalytic surface, and they react and subsequently desorb, thus freeing the surface for the next cycle. The catalytically active site undergoes chemical and structural changes. The structure that is measured on a functioning catalyst depends on the rate-limiting step and the coverage of the adsorbates. The domain of transient measurements identifies changes occurring after a modification of the gas composition. Those species reacting at a rate similar to that of the rate-limiting step are relevant to the catalytic process.6 In the present study, we coupled ambient pressure X-ray photoelectron spectroscopy (APXPS) and synchrotron radiation10,11,12 to detect the population of Ce3+ active sites on Pt/CeO2 catalysts during the steady-state oxidation of carbon monoxide, and under transient conditions. A main advantage of XPS is that, due to inelastic scattering along their path, photoelectrons are emitted from the outer layers of the sample (only a few nanometers),13 and



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a quantification of the amount of Ce3+ at the surface can be estimated after deconvolution of the spectra.27 Nevertheless, time-resolved XPS experiments performed on real catalysts are challenging due to the necessity of acquiring well resolved spectra in the milliseconds to seconds. Although ultrafast measurements were achieved on single crystals,14 measuring a real catalyst under relevant conditions is still a challenge due to the low concentration of Ce3+, carbon contamination of the samples and due to ambient pressure (in the mbar range), both of which contribute to the attenuation of the photoelectrons. Ceria is a key material that finds application as a support or as an active support for metal and oxide nanoparticles in several fields 15 such as catalysis,16,17 fuel cells,18,19 biochemistry/biomimesis,20,21,22 and photocatalysis.23,24 The low amount of energy required to form oxygen vacancies and stabilize Ce3+ sites at the surface strongly affects the reactivity.25,26,27,28,29,30,31,32,33,34,35A prototypical catalytic reaction to determine the performance of ceria-based materials is the low-temperature oxidation of carbon monoxide, which is important to lower automotive emissions.36 Metal nanoparticles, such as platinum, gold, and palladium, supported on ceria, and other reducible oxides show the best performance.37,38,39,40 In the case of a ceria support, the adsorption and activation of oxygen preferentially occur on the support, whereas carbon monoxide is adsorbed and supplied by the metal. Due to the rapid reoxidation of ceria and, thus, the small amount under catalytically relevant conditions, Ce3+ generated in the catalytic cycle is difficult to detect under steady state conditions.41 Studies employing time-resolved techniques are required to gather more information about the evolution of the active sites and their role in the reaction mechanism.6 Figure 1 shows the Ce 3d spectrum of the Pt(3.0%)/CeO2 sample, acquired at 2200 eV photon energy after activation in hydrogen. The spectra were collected at room temperature while maintaining a background pressure of 1.0 mbar hydrogen in the reaction cell. We


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acquired several scans of the Ce 3d on the same spot (not shown) before measuring the depth profile. No change in the line shape as a result of beam damage occurred in more than 60 minutes measurement time. After subtraction of a Shirley background,42 the spectrum in Figure 1 was separated into five doublets (labeled as v and u in agreement with the literature), corresponding to the spin-orbit split 3d5/2 and 3d3/2 core holes.43,44,454647 Voigt profiles enabled simulation of the peaks. The spin-orbit splitting is about 18.5 eV for Ce4+-related peaks and 18.1 eV for Ce3+;45 the intensity ratio between the 5/2 and 3/2 peaks was set at 1.5. The position and full width at half maximum of the components were fixed based on the values obtained on reference samples for Ce4+ (commercial polyhedral ceria nanoparticles) and for Ce3+ (Ce(NO3)3 powder). Three peaks and their corresponding satellites are related to stoichiometric ceria and, thus, to Ce4+ ions, labeled v, vII, and vIII. The one with the highest binding energy, vIII (898.5 eV) and its uIII satellite (917.0 eV), corresponds to the Ce 3d9 4f0 O 2p6 final state. The multiplets at lower binding energy v (882.7 eV) and u, vII and uII, are due to shake-down processes and correspond to the Ce 3d9 4f2 O 2p4 and the Ce 3d9 4f1 O 2p5 final states, respectively.43,45 The two doublets v0 (880.7 eV) and u0, vI (884.9 eV) and uI are associated with Ce3+ and correspond to the Ce 3d9 4f1 O 2p6 and Ce 3d9 4f2 O 2p5 final states, respectively.43,45 The deconvolution of the spectrum provides strong evidence that Ce3+ ions form after activation of the catalyst in hydrogen, because both the vI and v0 peaks are required to obtain a good correlation between the experimental points and the fitting spectrum.



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Figure 1. Photoemission spectrum and deconvolution of Ce 3d after activation of the catalyst in hydrogen at 150°C.

To quantify the Ce3+/Ce4+ ratio as a function of depth, we acquired the Ce 3d spectra at increasing photon energy (2200-5000 eV). The ratios between Ce3+- and Ce4+-related peaks were constrained during the whole experiment. After deconvolution, the Ce3+/Ce4+ ratio was calculated as

u 0 + v0 + u I + v I , where ui and vi are peak areas. Figure 2 shows an u + v + u II + v II + u III + v III

increase in the ratio at the lowest photon energy, which corresponds to the lowest kinetic energy of the photoelectrons and, thus, to the lowest probed depth. This means that a higher concentration of Ce3+ sites is found near the surface as a consequence of hydrogen dissociation on platinum nanoparticles and spillover of the atoms to ceria, which causes its reduction to Ce3+.48 Our results, compared with other experiments (XPS, nuclear magnetic resonance and temperature programmed reaction) and density functional theory calculations


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showing that the spillover of hydrogen atoms is limited to about 4 Å around the nanoparticles of platinum, demonstrate that the activation of the catalyst in hydrogen leads to the formation of Ce3+ defects around the perimeter of the platinum nanoparticles.49 A similar trend was observed recently in the Pt(2.0%)/CeO2 nanocubes system.50 The same experiment was repeated on the platinum-free ceria support. After pretreatment in hydrogen, only the spectrum acquired at hv=2200 eV showed a small concentration of Ce3+. Figure 2 shows that the value of Ce3+/Ce4+ on pure ceria is 8 times lower than Ce3+/Ce4+ on Pt/CeO2, validating the hypothesis that platinum promotes ceria reduction at the surface.49 We employed a simple model to fit the experimental data and obtain a quantification of the depth of reduction of ceria on Pt/CeO2. An exponential distribution reproduced the attenuation of photoelectrons in a semi-infinite specimen, our ceria support, with a uniform overlayer of thickness t, representing the reduced ceria layer.50,51,52 The ratio between the overlayer (O) and the support (S) electron intensities (I) is given by:

I O FO ρ Oσ O λOo [1 − exp( −t / λOo cos θ )] = ⋅ IS FS ρ S σ S λ Ss exp(−t / λSo cos θ )

(1),

where F is a factor that accounts for the X-ray flux, the detection efficiency of the instrument for the photo-generated electrons, the effective analyzed area, and the angular asymmetry factor. ρ is the atomic density of the analyzed species, σ is the differential cross section, λ is the inelastic mean free path, t is the thickness of the overlayer and θ is the electron emission angle with respect to the surface normal of the sample.51,52 In our case, (1) can be simplified because photoelectrons originated from the same shell, and we assumed λ to be the same in the Ce3+ and Ce4+ oxides.50 The fitting with an exponential distribution is plotted in Figure 2 (inset) and gives a value of t of 1.00±0.06 nm. This result further confirms that Ce3+ sites are



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located in the outer layers of the ceria support and that, under these conditions, oxygen does not diffuse from the bulk to the surface.

Figure 2. Ce3+/Ce4+ intensity ratio in Ce 3d XPS of Pt/CeO2 (●) and clean CeO2 (●) obtained by changing the excitation energy from 2200 to 5000 eV. Inset: simulation of the intensity ratio vs. photoelectron kinetic energy (depth profile) with an exponential distribution.

Recently, by means of resonant X-ray emission spectroscopy (RXES), the kinetics of formation and consumption of Ce3+ species on a Pt (1.5%)/CeO2 catalyst was monitored under transient conditions by rapidly switching the oxygen supply on and off in a carbon monoxide/oxygen mixture.41 The results show that the rate of carbon monoxide conversion in the presence of oxygen is quantitatively correlated with the initial rate of formation of Ce3+ sites when oxygen is removed from the reaction mixture. This indicates that the formation of


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Ce3+ is related to the rate-determining step of carbon monoxide oxidation. Active Ce3+ species are observable only under transient conditions, because they re-oxidize promptly to Ce4+ in the presence of oxygen. Not all the Ce3+, formed during the activation of the catalyst in hydrogen, was reoxidized in the presence of oxygen, suggesting that other Ce3+ sites (spectator sites) exist. Due to their weak tendency to reoxidize, they are inactive.41 Unfortunately, RXES, a bulk sensitive technique, does not reveal the location of spectator sites. Here we introduce a new approach, based on APXPS, to study the Ce3+ species formed under transient conditions. We acquired fast scan of the Ce 3d core levels while switching the oxygen on and off in a carbon monoxide/oxygen reaction mixture. As explained in the Experimental Methods section, we collected good intensity and well-resolved Ce 3d5/2 spectra in about 20 seconds. After activation of the sample in hydrogen at 150°C, we started the experiment by exposing the catalyst to a mixture of carbon monoxide and oxygen at 100oC. In that environment a constant amount of Ce3+ was detected. At time zero, we switched from a carbon monoxide and oxygen mixture to a carbon monoxide and nitrogen one, while following the peaks in fast acquisition mode. It was assumed that active Ce3+ sites would form as a consequence of surface oxygen consumption.41 We carried out the experiment at 2200 and 3000 eV, corresponding to 2.1 and 3.0 nm inelastic mean free path, respectively. The temperature of the sample was set at 100°C, to avoid charging during the measurements. Figure 3a shows the sum of two subsequent Ce 3d5/2 spectra53 over 200 seconds and at a photon energy of 2200 eV. A low binding energy shoulder increases as a function of the acquisition time. Thus, each spectrum was deconvoluted into four peaks, whose position, full width at half maximum, Voigt shape, and ratio between Ce3+- and Ce4+related peaks were constrained. Figure 3b shows the integrated areas for each component as a function of time. The curves associated to Ce3+ (Figure 3b, bottom), and Ce4+ (Figure 3b, top) show the same behavior: the former increase whereas the latter are nearly constant. The



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Ce3+/Ce4+ ratio, calculated by adding the v0 and vI peak areas and dividing by the sum of the v and vII peak areas, is plotted as a function of the acquisition time (Figure 3 c red markers). Each value of Ce3+/Ce4+ was normalized to the minimum to better display the signal behavior as a function of time. There is a significant increase between 80 and 160 seconds, which corresponds well to the behavior of the peaks (Figure 3 a and b) related to Ce3+. The delay in the detection of active Ce3+ sites could be due to the uptake of small amount of oxygen from the pores on the walls of the experimental chamber and tubes. This event takes place after the substitution of oxygen with nitrogen in the reaction mixture and, according the residual gas analysis, is supposed to last approximately 60 seconds. The catalyst is extremely sensitive to the presence of oxygen, which continues to be adsorbed and activated. The same experiment, performed at 3000 eV, displays only weak positive oscillations of the Ce3+/Ce4+ ratio (green markers). For comparison, we show the behavior of the sample in a carbon monoxidenitrogen background (hν=3000eV) prior to the influx of oxygen in the reaction mixture (black markers); this is to be considered as a reference. The green and black markers show similar trends. The noticeable difference between the data points acquired at 2200 eV and 3000 eV, after replacing the oxygen by nitrogen in the reaction mixture, proves that the catalytically involved Ce3+ sites are at the surface. To summarize, we combined the results acquired under transient reaction conditions with the depth profile discussed above and other works reported in the literature.39,41 Catalytically active Ce3+ sites that participate in the oxidation of carbon monoxide are formed during the pre-treatment of the catalyst in hydrogen, and are confined to the interface between the support and the nanoparticles of platinum.49,50 During the oxidation of carbon monoxide, the oxygen is activated on the support (Ce3+ sites), and reacts with the carbon monoxide adsorbed on the metal, yielding carbon dioxide, an oxygen vacancy and two Ce3+ ions. The oxygen vacancy is rapidly healed by the hopping of an oxygen atom from neighbor sites, so that Ce3+ sites extend to the whole surface of the ceria


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support.41 When the oxygen is removed from the reaction mixture, active Ce3+ sites cannot be healed and are detected by means of XPS. Furthermore, a “background” of Ce3+ sites is visible throughout the experiment and does not reoxidize to Ce4+ in the presence of oxygen. These sites formed after activation of the catalyst in hydrogen and are spectators. They do not participate in the activation of oxygen from the gas phase. According to the depth profile analysis, the geometrical location of spectator Ce3+ is considered to be in the first layers of the ceria support.

Figure 3. (a) XPS fast scans and deconvolution of Ce 3d from a Pt/CeO2 catalyst for 40 to 200 seconds after replacing oxygen with nitrogen in the reaction mixture: (b) area of each component of the Ce 3d5/2 spectrum reported as a function of time: (c) Ce3+/Ce4+ ratio, calculated from (a) and (b), reported as a function of time at hν=2200 (■) and 3000 eV (■,■).



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Due to their low concentration and the co-presence of spectator sites, it is difficult to detect catalytically active Ce3+ involved in the oxidation of carbon monoxide under steady state conditions. We acquired the cerium photoemission core levels in fast scan mode, while changing the composition of the reaction mixture. In the first three minutes after replacing the oxygen with nitrogen, there was a significant increase in the amount of Ce3+, which is assumed to be involved in the activation of oxygen. Combining the analysis of the depth profile and pulsed experiments, we showed that Ce3+ sites form preferentially at the Pt/CeO2 interface and in the topmost layers of ceria.54 Bulk oxygen does not play a role. Catalytically active Ce3+ forms at the surface of the ceria support, whereas a constant background of spectator ions is distributed beneath the surface. When oxygen is dosed, the active sites immediately re-oxidize to Ce4+. As a perspective, we plan to exploit the same experimental setup to further investigate the oxidation of carbon monoxide on ceria-based catalysts. For instance, it is known that hydroxyls promote the oxidation of carbon monoxide.55,56 Hydroxyls can form directly upon the activation of the catalyst, or exposing the catalyst to water. The experiments conducted under transient conditions could be repeated co-dosing different amounts of water in the reaction mixture. The evolution of the Ce 3d could be correlated to the O 1s core level, by which the amount of hydroxyls can be evaluated. The potential of such experimental equipment is strong. This technique could be applied to other samples and reactions to identify the active sites involved. Our results demonstrate that a combination of time resolution, achieved by fast scanning of a core level peak in different reactive mixtures, and depth resolution, obtained by changing the photon energy, can provide a unique overview of the nature and distribution of catalytically active sites at the surface. Such studies can be greatly improved by increasing the time resolution.


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EXPERIMENTAL METHODS Agglomerated polyhedral particles of cerium oxide with a high surface area (110 m2/g) were obtained commercially. Platinum nanoparticles, 1 to 2 nm in diameter, were supported on cerium oxide by means of incipient wetness impregnation by tetraammine platinum(II) nitrate (Aldrich, 99.995%) followed by drying (200°C, 4h), calcination in air (400°C, 4 h), and reduction in a 30% flow of H2 (300°C, 4 h).41 The powders (clean ceria standard and Pt/CeO2) were dispersed in ethanol and drop-casted on a gold foil (0.1 mm thickness). After evaporation of the solvent, a thin and homogeneous layer of powder covered the sample holder. The gold foil was fixed to the manipulator head by tantalum clips. The temperature was monitored by a Pt100 sensor, and the sample was heated by an IR laser projected on back side of the sample holder. The power vs. temperature calibration was performed prior to the experiment by adding a second Pt100 sensor connected to the center of the upper surface of the sample holder to obtain a more precise reading of the measured area. Calibration curves were calculated at different nitrogen pressure (0.1-10 mbar) in the experimental cell. In the geometry adopted during the experiments, photoelectrons were detected at an angle of 30° with respect to the direction of surface normal. After putting the sample into the experimental cell, it was activated by heating at 150°C for 60 minutes at 1.0 mbar hydrogen. The spectra were acquired by means of a differentially pumped Scienta R4000 HiPP-2 spectrometer connected to the newly developed endstation for the manipulation of solids in the ambient pressure range.57 The mobile endstation was operated at the PHOENIX beamline of the Swiss Light Source synchrotron. An elliptical undulator with tunable polarization was the photon source, and a monochromatic beam was created by a double-crystal Si(111) monochromator. Linearly polarized light was used during all the experiments. The available photon energy ranged from 2000 to 8000 eV.



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The flow-tube configuration of the experimental chamber is extremely versatile. We modified it by connecting the upper side (see Figure 4) to a gas line by means of an on-off valve and a 3.0 mm copper tube. The lower side was connected to a 27 m3/h root pump by a diaphragm valve. The gas line, made of a copper tube (6.0- mm OD) and brass adapters to avoid formation of nickel carbonyl, was connected to LabView-controlled mass flow controllers (one mass flow controller per gas), a pressure head, and a pump (70 L/s turbo + membrane pumping station). Both static and pulsed experiments were realized as follow: - The gas line was pumped down to low 10-3 mbar. - The on-off valve between the experimental cell and the gas line was opened (base pressure lower than 10-2 mbar). - The desired gas mixture was dosed through mass flow controllers, and the pressure in the experimental cell was tuned to 1.0 mbar by opening the diaphragm valve to the root pump. The experiments were performed in an excess of oxygen (1:4 molar ratio of carbon monoxide to oxygen). Previous studies demonstrate that the reactions conducted under stoichiometric conditions, with an excess of oxygen, and with an excess of carbon monoxide show very similar kinetics and a similar mechanism.39 - During pulsed experiments, we switched off the oxygen flow and replaced it with nitrogen. The purity of the gases, the composition of the reaction mixture and the switch from oxygen to nitrogen were controlled by a quadrupole mass spectrometer (QMS), located in the second differential pumping stage of the analyzer. Based on the QMS reading, it took 10 to 15 seconds to switch completely from a mixture of carbon monoxide and oxygen to carbon monoxide and nitrogen. During pulsed experiments, photoemission spectra of the Ce 3d5/2 peaks were acquired in fast acquisition mode with a 100 eV pass energy, a 0.1 eV kinetic energy step, and 50 ms dwell time. The acquisition of a single scan lasted about 20 seconds. During the same experiment,


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we switched the oxygen on and off several times to verify the reproducibility and quantify the error bars (Figure 3b). All the other spectra were acquired in swept mode, with a 100 eV pass energy, 0.1 kinetic energy step, and 100 ms dwell time. Due to the large band gap of ceria, charging was observed on all samples under vacuum conditions. To minimize it, reference spectra were also acquired in an inert gas background (0.1 mbar of nitrogen). All the spectra were aligned using the C 1s peak as a reference (a small carbon contamination was present on all the samples).

Figure 4. Sketch of the experimental chamber, showing the flow tube configuration.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected]. 15 ACS Paragon Plus Environment


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Astrid Waldner for help during beamtime and Mario Birrer for technical development of the solid chamber and the assistance during the preparation of the beamtime. Financial support of the Swiss National Science Foundation (project number 200021_140750) for R. K. is gratefully acknowledged.

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Photoemission spectrum and deconvolution of Ce 3d after activation of the catalyst in hydrogen at 150°C. 179x111mm (300 x 300 DPI)


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Introducing Time Resolution to Detect Ce3+ Catalytically Active Sites

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