Probing the Activity of Different Oxygen Species in the CO Oxidation

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Probing the activity of different oxygen species in the CO oxidation over RuO(110) by combining transient Reflection-Absorption Infrared Spectroscopy with Kinetic Monte Carlo Simulations 2

Franziska Hess, Christian Sack, Daniel Langsdorf, and Herbert Over ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02838 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Probing the activity of different oxygen species in the CO oxidation over RuO2(110) by combining transient Reflection-Absorption Infrared Spectroscopy with Kinetic Monte Carlo Simulations

Franziska Hessa,b, Christian Sacka, Daniel Langsdorfa and Herbert Overa*

a

Dept. of Physical Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany

b

Dept. of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue,

Cambridge MA 02139, USA *Corresponding author, Tel: +49 641-99-34550, E-mail: [email protected]

Abstract Transient spectroscopic surface chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to gain a wealth of molecular information on the kinetics of a catalytic surface reaction as exemplified with the CO oxidation reaction over RuO2(110). This approach surpasses the common problem that in the steady state reactions the prevailing species detectable by operando surface-sensitive spectroscopy are frequently spectator species, thereby obscuring the reactive surface species. We designed our experiment to be sensitive toward the relative activity of different oxygen species by saturating the surface with loosely-bound oxygen, leaving only single vacancies where CO can adsorb and recombine with oxygen. With in-situ reflection-absorption infrared spectroscopy (RAIRS) in combination with ab-initio based KMC simulations we followed the time evolution towards steady state (transient experiment). In this way, we were able to resolve a longstanding controversy about the active oxygen species in the CO oxidation over RuO2(110), evidencing that both surface O species (Obr and Oot) are equally active, although their adsorption energies differ by more than 150 kJ/mol.

Keywords: transient kinetic experiment, infrared spectroscopy: RAIRS, kinetic Monte Carlo simulations, ruthenium dioxide, CO oxidation

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1 Introduction In heterogeneous catalysis research, kinetic and in-situ spectroscopic data of a catalytic reaction are commonly acquired under steady state conditions for fixed composition of the reaction mixture and fixed reaction temperature. In favorable cases, in-situ spectroscopy enables the identification of the reaction intermediates that in turn helps to disentangle the reaction mechanism. This straightforward approach faces, however, a severe problem in the (frequently prevailing) presence of spectator species on the catalyst surface. A prime example for this problem is the Pd-catalyzed CO hydrogenation. Transient diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) experiments have revealed that inactive CO spectator species on the catalyst surface overwhelmingly contribute to the infrared spectra under steady state conditions, thus obscuring the actual reaction intermediates. A periodic modulation technique of DRIFTS combined with phase sensitive detection has shown to improve the sensitivity to reaction intermediates on the catalyst surface by many orders of magnitude. 1 A similar approach can be pursued in surface chemistry for studying model catalytic systems, employing molecular beam techniques. 2 A powerful alternative to periodic modulation technique is to change abruptly a single reaction parameter and then follow with in-situ spectroscopy how the catalytic system restores steady state conditions. 3 For instance, with steady-state isotopic transient kinetic analysis (SSITKA), 4 where the catalytic reaction is chemically in steady state, but isotopically in unsteady state, valuable information on the active species can be extracted. Relaxation methods can readily be implemented also in surface chemistry experiments without demanding molecular beam technique, but often using infrared spectroscopy. When faced with the question which oxygen species out of two possible candidates is more active in an oxidation reaction catalyzed by an oxide surface, one has to rely on a less direct approach, because oxygen adsorbed on an oxide surface is not detectable by infrared spectroscopic methods. This problem is often encountered in catalytic oxidation reactions, but equally applies to other common intermediates, such as atomically adsorbed hydrogen on metal surfaces in catalytic hydrogenation reactions. We propose here a new experimental strategy that relies on poisoning the catalyst surface with oxygen prior to the actual reaction and then follow the reactants (in our case CO) by a time-resolved surface-sensitive technique as oxygen is slowly replaced by the other reaction intermediate (adsorbed CO). Our spectroscopic method needs to be able to distinguish between CO adsorbed in different local surface environment, which is accomplished by the use of time-resolved Reflection Absorption Infrared Spectroscopy (RAIRS). Since RAIR spectra of CO adsorbed on surface are quite complex, the strength of this relaxation technique in surface chemistry only comes to full effect when it is combined with spatially resolved kinetic Monte Carlo (KMC) simulations, thereby providing for a wealth of new kinetic information. KMC methods rely critically on the chosen parameter set including the activation energies for the elementary reaction steps and diffusion processes, as well as on the adsorption energies of reaction intermediates. Two research groups published ab-initio based KMC simulations on the CO oxidation -2-


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reaction over RuO2(110) employing very different parameter sets where even the rate determining steps, i.e., the active surface oxygen species differed. 5-9 Nonetheless the final surface configurations under steady state reaction conditions turned out to be virtually indistinguishable so that steady state in-situ RAIRS experiments are not able to differentiate between these parameter sets. KMC simulations in the transient regime do show very different surface compositions (Figure 1c and d). Based on this finding, we previously proposed10 a surface-sensitive relaxation experiment to probe the relative activities of different oxygen species on the RuO2(110) surface. In this work, we present in-situ RAIRS experiments of the CO oxidation reaction over a RuO2(110) model catalyst in the transient regime combined with KMC simulations. The experiment is designed to be sensitive toward the relative activities of different surface oxygen species by starting from an oxygen saturated RuO2(110) surface and subsequently exposing this surface to an O2:CO = 1:1 reaction mixture (10-7 mbar) at 288 K. Under these conditions, the steady state is reached after three hours, a time period long enough to follow in detail the transition with infrared spectroscopy. Since all the appearing COrelated absorption bands in RAIRS can be unambiguously assigned to specific CO surface configurations1114 (cf.

Figure 2), the time evolution of RAIR spectra towards steady state can be directly compared to the

time evolution of KMC-simulated surface configurations. With this comparison, we are able to clearly differentiate between the two proposed parameter sets for KMC simulations. It turns out that both surface O species (Obr and Oot) are equally active in the CO oxidation reaction, although their adsorption energies differ by more than 150 kJ/mol.

2 Mechanism of the CO oxidation over RuO2(110) The stoichiometric RuO2(110) surface (cf. Figure 1a) exposes two kinds of coordinatively unsaturated sites: the one-fold undercoordinated Ru site (Rucus, drawn in red) and the bridging oxygen (Obr) that connects two Ru atoms with six-fold coordination. Occasionally these Obr atoms can be partly consumed during the catalytic CO oxidation reaction so that two-fold undercoordinated Ru atoms (Rubr, drawn in purple) become exposed. For the KMC simulations the surface geometry is reduced to the undercoordinated sites present on the reduced RuO2(110) surface (Rucus and Rubr). The bridge and cus sites are arranged in one-dimensional arrays along the [001] direction, resulting in a coarse-grained representation of the surface as depicted in Figure 1b. To facilitate visualization of surface configurations as determined by the KMC simulations the rows of Rucus and Rubr atoms are represented by a coarsegrained top view of thin red and thick purple lines, respectively (cf. Figure 1b), while adsorbates are indicated as small disks.

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Figure 1: (a) Ball and stick model of the RuO2(110) surface. Green balls represent O atoms, Ru atoms in bulk are colored blue while the Ru atoms on the surface are colored red (one-fold undercoordinated Rucus) and purple(Rubr). (b) Coarse-grained representation of the stoichiometric RuO2(110) surface, reduced to the active sites. (c) and (d) Transient KMC simulations at p(CO) = 2 p(O2) = 2∙10-7 mbar and T = 325 K with parameter sets by Reuter/Kiejna 8 and Wendt/Seitsonen6 adapted from Ref. 10.

The Rucus sites are the gateway for most of the incident molecules from the gas phase to be accommodated on the surface. For instance, exposing the stoichiometric RuO2(110) surface (cf. Figure 1a) to molecular oxygen will lead to a surface where part of the Rucus sites are occupied by on-top sitting oxygen atoms (Oot), which are formed upon O2 dissociation. The Oot species represent again undercoordinated surface oxygen atoms (this time two-fold undercoordinated). If the stoichiometric RuO2(110) surface is exposed to CO at temperatures below 250 K, the reaction with undercoordinated surface O atoms (Obr and Oot) is suppressed and on-top CO (COot) is stabilized. Upon reaction of on-top CO with Obr, a vacant bridge site is formed into which another CO can readily adsorb, forming a bridging CObr. 15, 16 CObr exists in two different adsorption geometries, depending on the coverage. 17 If only every second bridge site is occupied by CO and the others are free, those CObr adopt a symmetric geometry, where the C atom is coordinated to two Rubr atoms (COIIbr ), as schematically shown in Figure 2 (inset in leftmost panel). When every bridge site vacancy is occupied by CO, the geometry of CObr is asymmetric and only one bond to Rubr is formed (COIbr ), as shown in Figure 2 (inset in central panel). This distinction is important for the assignment of the RAIRS bands in Section 3.1. 12, 14 Since further reduction of deeper layers of RuO2(110) (i.e., beyond the replacement or consumption of bridging O by reactant molecules) is not considered, the periodic lattice for the KMC simulations is chosen to consist of alternating rows of Rucus and Rubr (cf. Figure 1b), which both can be occupied by reactants

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from the gas phase, preferentially in on-top position over Rucus (red balls) and bridging position above neighboring Rubr sites (purple balls). It has been a matter of debate whether the CO oxidation over RuO2(110) proceeds via a onedimensional or a two-dimensional Langmuir-Hinshelwood (LH) mechanism. 5, 10, 18 These mechanisms are distinguished not by different types of elementary reaction steps, but rather by their associated activation energies. In the two-dimensional mechanism (cf. Figure 1d) originally proposed by experiments and supported by DFT calculations, 6, 7 the COot + Oot and COot + Obr recombination steps have similar barriers and reaction rates. At peak activity, this leads to the coexistence of CObr/COot and Obr/Oot-covered domains in KMC simulations. 19 Under oxidizing conditions, where most of the bridge sites are occupied by Obr, the COot + Oot and COot + Obr recombination reactions contribute almost equally to the overall formation of CO2, suggesting similar activity in the CO oxidation of the Obr and Oot species despite different adsorption energies. 7 In the one-dimensional LH mechanism (cf. Figure 1c), 5, 8 on the other hand, the COot + Oot recombination has a significantly lower barrier than the COot + Obr recombination that results in lower CO oxidation activity of Obr compared to Oot and a significantly lower tendency of Obr to be replaced by COot. In the steady state under oxidizing and stoichiometric conditions, this causes most of the product CO2 molecules to be formed through the Oot + COot recombination. 7, 9 Other elementary steps, such as the COot + Obr recombination, contribute at least two orders of magnitude less to the CO2 formation at 350 K. Quite notably, under reducing conditions both proposed mechanisms behave identically in the steady state: all bridge sites are occupied by CO, and CO2 is almost exclusively formed through the Oot + CObr reaction, the step with lowest barrier among the recombination steps.

3 Experimental Details 3.1 Preparation of catalyst surface and RAIRS measurements The Ru(0001) single crystal was cleaned by Ar sputtering in a UHV chamber. After annealing to 1000 K in 3∙10-7 mbar O2, the surface was exposed to 10-4 mbar O2 at 680 K for 150 min (≈ 7∙105 L(angmuir)). This preparation procedure is optimized to produce well-ordered, smooth films with large single-crystalline terraces suitable for our catalytic experiments.20, 21 The crystallinity of the RuO2(110) film was verified by Low Energy Electron Diffraction (LEED) and the surface cleanliness was checked by XPS. LEED patterns before and after oxidation are shown in Figure S 5 in the supporting material. The starting RuO2(110) surface for our catalytic experiments was prepared by annealing the asprepared RuO2 surface to 700 K and subsequently cooling down to 288 K under 4∙10-7 mbar O2. This results in a surface that is almost entirely covered by loosely-bound oxygen (Oot), leaving only few undercoordinated Rucus sites exposed. The starting state of the catalyst surface and the reaction conditions were optimized to probe specifically the selectivity between the COot + Oot and COot + Obr recombination steps in the induction period of the transient experiment. The transient experiment is initiated by abruptly co-dosing 1∙10-7 L O2 and 1∙10-7 L CO at 288 K. The choice of starting configuration and reaction parameters is justified in more detail in Section 5.1. -5-


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The (in-situ) RAIRS measurements were conducted using a Bruker Vertex 70v IR spectrometer with a liquid N2-cooled MCT detector at a resolution of 4 cm-1 in the same UHV chamber where the sample was prepared, thus avoiding exposure of the clean oxide surface to humidity and other atmospheric contaminations. The background pressure in the reaction chamber was below 2∙10-10 mbar. The IR beam was coupled into the chamber through IR-transparent CaF2 windows and reflected at the sample surface in a small angle of 6° towards the detector. RAIRS spectra are obtained by dividing the measured raw spectrum by a reference spectrum that was pre-recorded before CO was dosed in the transient series, thus representing the Obr/Oot-covered surface.

3.2 Assignment of RAIRS bands In the RAIRS measurements, CO is employed as a probe molecule in the CO oxidation over RuO2(110), besides being a reactant. Due to dipole-dipole interactions between adsorbed CO and other surface species, the CO stretch band experiences a frequency shift depending on the chemical environment. This results in unique bands which have previously been assigned in RAIRS and HREELS experiments. 11-14, 22 Figure 2 summarizes the assignments of CO stretching bands in the spectral range between 1850 cm-1 and 2155 cm-1 according to Ref. 14 and references therein.

Figure 2: Assignment of CO stretch frequencies on the RuO2(110) surface. Smudged areas indicate unclear boundaries. The figure compiles data taken from Refs. [11-14, 22]. Vibrational frequencies are grouped in three spectral regions (separated by thin lines), depending on the majority species on the bridge site (shown in the top inset): symmetric COIIbr (left), asymmetric COIbr (center) and Obr (right).

The spectral range between 1850 cm-1 and 2155 cm-1 can be roughly divided into three regions, depending on the majority species on the bridge site (shown as inset in Figure 2): The symmetric CObr (COIIbr ) region between 1850 and 1980 cm-1, the asymmetric CObr (COIbr ) region between 1980 and 2100 cm-1 and the Obr range between 2100 and 2155 cm-1 where CO solely binds in on-top position to the Rucus sites. The positions of the absorption bands also depend on total coverages of O and CO, with higher

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coverages leading to higher wavenumbers and neighboring O causing a stronger blue-shift than adjacent CO. An ordered overlayer of symmetric CObr (COIIbr ) with empty Rucus sites gives a single band at 1866 cm-1. Disordered overlayers, where symmetric COIIbr and Obr coexist, absorb IR from 1866 up to 1890 cm-1. When additional Oot is adsorbed on the cus sites, the (COIIbr ) absorption band shifts further up to 1925 cm-1 (cf. Section S 1 in the supporting information). When most or all of the bridge sites are occupied by CO, the CObr adopt an asymmetric adsorption geometry (COIbr ) with one-fold coordination to Rubr that results in a characteristic vibration of COIbr between 1990 and 2005 cm-1.

When COot coadsorbs with COIbr , the

vibrations of both CO species couple, resulting in a single band (totally symmetric vibration) at 2000 cm-1 that shifts to higher frequencies of up to 2086 cm-1 with increasing COot coverage. The position of this band (2086 cm-1) saturates at a COot-coverage of 0.64. 14 Adsorbed COot on the stoichiometric surface (i.e., next to Obr), results in an absorption band between 2100 cm-1 and 2123 cm-1, depending on COot coverage. Coadsorption of COot and Oot next to Obr can blue-shift the band up to 2155 cm-1. Absorption bands above 2145 cm-1 are characteristic for densely covered Obr/Oot overlayers with COot molecules adsorbed into isolated vacancies in these oxygen overlayers. This feature typically appears when CO is exposed to a surface previously saturated with oxygen. 14

4 Computational Details 4.1 Density Functional Theory calculations Density Functional Theory (DFT) calculations were conducted using the PBE-functional23 of the Generalized Gradient Approximation (GGA) family. The calculations were performed using the Vienna ab-

initio Simulation Package (VASP), version 5.3.5. 24, 25 The RuO2(110) surface is described in a symmetric slab model with five oxide trilayers separated by 25 Å of vacuum. All oxide layers were relaxed during the geometry optimization. The plane-wave cut-off energy was 500 eV with 6×12×1 k-points in the (1×1) super cells. For larger super cells the number of k-points was adapted to keep the k-point density in reciprocal space constant. This approach ensures that the adsorption energies are converged within 0.2 meV, and total energies are converged within 14 meV per atom.

4.2 Kinetic Monte Carlo Simulations An accurate and reasonably fast way to follow the time evolution of a system of chemically reacting molecules on the catalyst’s surface is established by Kinetic Monte Carlo simulations (KMC) that account for fluctuations, correlations and the spatial distribution of the reaction intermediates on the surface. In KMC simulations, the RuO2(110) surface is represented by a periodic lattice consisting of a twodimensional array of on-top sites (Rucus) and bridge sites (connecting two Rubr) (cf. Figure 1b). These metal sites can either be vacant or accommodate the reactants/intermediates during the simulation, depending on the applied reaction conditions.

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The KMC simulation approach is described in detail in Ref. 26. The KMC simulations presented here take explicitly into account the interaction between the adsorbates via a cluster expansion (all parameters are reported in Section S2 in the ESI), the diffusion of the intermediates on the surface, adsorption/desorption of the reactants/intermediates including different site demands, and the activation barriers for elementary reaction steps. Within the transition state theory, the kinetics of elementary steps are determined by the activation energy and the frequency factor. The activation barriers of elementary steps in the reverse direction result directly from the detailed balance constraint, ensuring the overall thermodynamic consistency in KMC simulations. The adsorption energies are expressed in terms of a cluster expansion exclusively determined from density functional theory (DFT) studies, while for activation energies two different sets of parameters are studied in order to compare simulations for different mechanisms with the experimental results (cf. Section 5.3). The adsorption processes are treated within the kinetic gas theory, assuming a sticking coefficient of unity. The size of the simulation lattice can be a critical issue if microstructure formation occurs during the simulations. In case of island formation, the lattice must be large enough to make sure that individual islands do not interact with images of themselves due to periodic boundary conditions. Moreover, for transient simulations the lattice size should be statistically representative because we do not average over time as usually done in steady-state simulations, which means that each simulation run must be representative, and little variation should occur between individual simulations. A lattice size of 50 × 50 sites using periodic boundary conditions was found to be sufficient for all simulations presented in this work. The figures show sections of 24 × 24 sites extracted from the 50 × 50 configurations. In order to compare the KMC simulations to the experimental RAIRS results, only the lattice configurations as a function of time and the assignment of vibrational bands to configurational motifs (cf. Figure 2) are required.

5 Results 5.1 Design of transient experiment We want to probe the elementary steps, COot + Oot and COot + Obr, with similar sensitivities in the initial stage of the reaction. In the simple mean-field picture, the reaction rate of an elementary step is given by

= ∙ ∙ , i.e., the rate is proportional to the rate constant , as well as the coverages of the reactants . In a lattice-based model, it is more appropriate to consider the probability of reactants being present on adjacent sites, rather than the coverages. In this sense, corresponds to the probability of adjacent sites being occupied by reactants 1 and 2.

= ∙ , -8-


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so that for the COot + Oot and COot + Obr recombination steps

= ∙ and = ∙ . Our experiment is designed in such a way that in the initial state every CO molecule on the surface has exactly the same number of neighboring Obr as Oot, so that the probability terms in the initial rates of the competing elementary steps and are equal. The rates differ then only by the rate constants. This situation is experimentally realized by saturating the surface by Oot prior to CO exposure. Because O2 always needs two neighboring sites for dissociative adsorption, full saturation of the surface is not possible. To a first approximation (neglecting diffusion) single vacancies (defects) at a coverage of about 13 % always remain,27, 28 where CO can adsorb. Every cus vacancy is surrounded by two cus sites and two bridge sites, all of which are occupied by oxygen (Obr/Oot), so that the joint probabilities and only depend on the CO coverage. This simplifies the initial rate equations to

= ∙ and = ∙ , so that the initial reaction rates of the two elementary reactions depend only on the respective rate constants and , which corresponds to the initial vacancy concentration and is equal in both equations. The reaction temperature is set to 288 K in order to realize a large difference between and

. The combination of surface poisoning by Oot and T = 288 K results in a long induction period of several hours during which the surface slowly returns from its poisoned state to an active steady state. Because the time-resolution of the RAIRS measurements is on the scale of minutes (93 s with the present setup), the temporal evolution of the reaction system can be readily captured. Covering the entire surface with oxygen has the additional benefit of passivating the surface to a great extent against contamination, such as water, that could accumulate on the surface during the experiment owing to suppressed desorption at 288 K.

5.2 Transient RAIRS experiment at 288 K The transient RAIRS measurement was conducted at T = 288 K with p(O2) = p(CO) = 10-7 mbar, i.e., under slightly oxidizing reaction conditions. The stoichiometric surface was initially restored and saturated with oxygen by annealing the surface in 4∙10-7 mbar O2 to 700 K and cooling down to 288 K in O2 (p(O2) = 4∙10-7 mbar). Subsequently, CO was co-dosed to start the reaction. For a 1:1 mixture of O2 and CO it took 195 min to reach steady state. The obtained spectra (each averaged over 93 s) are shown as a 2D-contour plot in Figure 3a (left). Spectra at notable points in time with marked bands are depicted in

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the Figure 3a (right), while specific surface configurations together with vibrational frequencies are summarized in Figure 3b. In the beginning (at 0 min, before CO is dosed), the surface is covered by Oot with few single vacancies where further O2 cannot adsorb dissociatively. This configuration is chosen as the reference in our RAIRS measurements, resulting in a flat line in the infrared range between 1800 and 2200 cm-1 (Figure 3, first spectrum). When CO is dosed, all the vacancies in the Oot/Obr overlayer are rapidly filled by CO (Figure 3a, spectrum after 10 min). This results in the instantaneous appearance of a signal at 2147 cm-1. Immediately afterwards, a second band at 2087 cm-1 appears, which is assigned to CObr, coadsorbed with COot at high local coverage. The band at 2087 cm-1 clearly indicates that COot is adsorbed next to CObr, rather than Obr with an absorption band at >2100 cm-1. The simultaneous occurrence of bands at 2147 cm-1 (O-covered) and 2087 cm-1 (CO-covered) shows that the surface consists of patches that are predominantly COcovered and areas that are predominantly O-covered with CO sitting in vacancies of the O-overlayer. The following RAIR spectra are thus discussed in terms of two separate domains occupying the surface simultaneously.

Figure 3: Transient RAIRS measurement at T = 288 K with p(O2) = p(CO) = 10-7 mbar. Top: Assignment of bands to surface configurations. Bottom left: contour plot of spectra as a function of time with lowest intensity drawn in white and highest intensity drawn in black. Bottom right: Spectra at selected points in time with high-intensity features indicated.

High background appears between these main absorption features between 2087 and 2147cm-1, but no individual bands are discernible so that mixed configurations of Obr, CObr, Oot and COot are expected to contribute to this spectral range. We assign this high background to the boundary of the CObr/COot -10-


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domain, which we interpret as an advancing reaction front where Oot and Obr are consumed by the reaction with CO and successively replaced by COot and CObr. This interpretation is supported by the slow growth of the band at 2087 cm-1 over time. A strong signal in the range between 2100 and 2123 cm-1, indicative of a significant number of COot adsorbed next to Obr, is absent. Between 10 min and 90 min (Figure 3) the 2087 cm-1 band grows continuously and a small side band at 2100 cm-1 develops (spectrum at 90 minutes). The 2147 cm-1 band slightly shifts to lower frequencies (2140 cm-1), evidencing a shrinkage of the dense Oot/Obr islands, which are slowly replaced by CObr/COot (resulting in growth of the 2087 cm-1 band). In this time frame a new double feature at 1912 and 1925 cm-1 starts to grow, which is assigned to symmetric (COIIbr ) in the presence of Obr and some Oot (experimental data for band assignment is provided in Section S 1). We thus observe the formation of a new, third domain, which is predominantly oxygen-covered, coadsorbed with unreactive COIIbr . Reaction between COIIbr and Oot/Obr does not occur in this temperature range. This CO species thus constitutes a spectator. Between 90 and 130 min the spectra undergo an abrupt change. The band at 2140 cm-1 rapidly shifts to lower frequencies (clearly visible in the contour plot in Figure 3) until it merges with the band at 2087 cm-1, showing that the residual dense Oot/Obr islands are being consumed. A new strong band at 2075 cm-1 appears, overlapping with the band at 2087 cm-1, resulting in apparent dramatic growth of the latter. The new band at 2075 cm-1 originates from areas where CObr is coadsorbed with COot at a lower local coverage (≈ 0.5) than in areas absorbing at 2087 cm-1. The double-band at 1912/1925 cm-1, due to a spectator COIIbr species in proximity with Obr and Oot, increases as well, suggesting the growth of the new predominantly oxygen-covered domains. In the final phase before reaching steady state (spectra at 130 min to 195 min in Figure 3a), the band at 2087 cm-1 vanishes, leaving only the band at 2075 cm-1 (CO-covered domains), while the intensity of 1912/1925 cm-1 double band (O-covered domains) keeps increasing. With no further changes taking place over the course of 30 minutes we assume that steady state of the reaction has been achieved. In steady state, the surface exposes two types of domains, which absorb at 1912/1925 cm-1 (Ocovered) and 2075 cm-1 (CO-covered), respectively. The most intensive band is at 1912/1925 cm-1 which is ascribed to surface areas where (COIIbr ) is adsorbed in the neighborhood of Obr and Oot. This configuration is inert at the given temperature, i.e., the (COIIbr ) does not recombine with Oot to form CO2 below 450 K, as verified by our additional experiments in Section S 1. This COIIbr species that dominates the spectrum is thus considered as a spectator species. The band with the second highest intensity is observed at 2075 cm-1, which corresponds to closed CObr rows coadsorbed with COot at a coverage of about 0.5 ML, where COot and single vacancies are arranged in alternating fashion. Since O2 requires two neighboring sites for dissociative adsorption, O2 adsorption and recombination with CO cannot occur in these dense CObr/COot domains, thus also establishing a spectator species.

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How is this surface still able to produce CO2? In the steady state, the domains with high local CObr/COot coverage (2075 cm-1) are posed to coexist with domains where the bridges are occupied by COIIbr and Obr, and the cus sites are partially capped by Oot, (1912/ 1925 cm-1). This implies the existence of domain boundaries, where CO and O can adsorb next to each other and recombine to form CO2. Furthermore, in the O-covered domains, there are free cus sites next to Obr and Oot where COot can readily adsorb and react with either Obr or Oot. Due to fast reaction the active intermediates in the steady state, i.e., adsorbed COot next to Obr or Oot in domain boundaries and cannot be observed spectroscopically.

5.3. Parameter choice in KMC simulations For the CO oxidation over RuO2(110), the activation energies of the elementary recombination steps were investigated in several experimental and computational studies.5-8, 19 These studies can be sorted into two groups which are distinguished by the difference of activation energies for the COot + Oot and COot + Obr elementary reaction steps. In the group of Reuter/Kiejna (both DFT-based), the COot + Oot recombination has a significantly lower activation energy than the COot + Obr recombination, resulting in preferential reaction of COot with Oot (1D mechanism). In the parameter sets of Wendt/Seitsonen (experimental and DFT-based), the two barriers are approximately equal (2D mechanism), leading to the same activity of Obr and Oot in the oxidation of CO over RuO2(110). For comparison with actual experimental data, the preliminary KMC simulation results based on the original parameter sets of Kiejna and Seitsonen are, however, not considered as reliable because both parameter sets do not contain lateral interactions between the reactants adsorbed on the surface. Lateral interactions have been shown to strongly influence the reaction kinetics of the CO oxidation over RuO2(110), for instance, reducing the apparent activation energy from over 280 kJ/mol to 90 kJ/mol with a small COot-COot interaction of 10 kJ/mol.19 The lateral interactions also affect the coverages of intermediates, to which RAIRS is particularly sensitive. Unfortunately, our previous experiment-based parameter set includes only a COot-COot interaction, 19 but not a full set of interaction energies (O-O, CO-O, CO-CO). Instead of using one of these simple parameter sets, we derived here new parameter sets with full lateral interactions from DFT calculations. This new set of adsorption energies properly accounts for the interactions between all the reactants in their individual neighborhoods. It is based on a cluster expansion and was derived in the same way as our parameter set for the HCl oxidation. 26 The full set of interaction parameters is given in the ESI in Section S 2. The activation energies for the initial configuration (single COot in Obr/Oot matrix) are given in Table 1; these values are virtually identical to those already published.8, 19 The barriers of the other recombination steps (CObr + Oot and CObr + Obr) were chosen as equal for the 1D and 2D mechanisms. The literature agrees on the CObr + Oot recombination having the lowest barrier in the set. Since CObr is not present in the starting configuration of the KMC simulations (oxygensaturated surface), the CObr + Oot reaction cannot occur, thus having practically no influence on the initial transient behavior. This is also the case for the CObr + Obr recombination, which has the highest barrier in -12-


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ACS Catalysis

the set (2.0 eV at zero-coverage) and is not expected to contribute significantly to the overall transformation of CO to CO2 at 288 K.

Table 1: Full-coverage activation energies for the elementary CO + O recombination reactions in the two scenarios (1D and 2D). Activation energies are given for a single COot in an Obr/Oot matrix, which is the starting configuration in our experiment and simulations. Only for the CObr + Oot recombination the value is given for full coverage of COIIbr and Oot. Zero-coverage activation energies are given in Table S 6 in the ESI.

Full-coverage activation energy / eV reaction

1D

2D

COot + Oot

0.78

0.96

COot + Obr

0.96

0.96

CObr + Oot

0.81

0.81

5.4. Transient KMC simulations The starting configuration of the simulations is the oxygen-rich Oot/Obr overlayer prepared by dosing O2 at room temperature, resulting in an Oot coverage of 87 %.27 Temperature and partial pressures were chosen as in the experiment (T = 288 K, p(O2) = p(CO) = 10-7 mbar). The reaction starts from isolated vacancies where CO can readily adsorb. Due to reactions between adsorbed CO and O on the catalyst surface, a reaction front is expected to propagate from such defects in the Obr/Oot overlayer and its shape depends on the choice of activation energies as sketched by the black shading in Figure 4a. In the 1D mechanism the reaction front is expected to propagate exclusively along the cus rows because the COot + Oot reaction is much faster than the COot + Obr reaction. In the 2D mechanism, the two activation energies are equal, so that the reaction propagates evenly along the cus rows and perpendicular to the cus rows. The reaction front thus assumes an approximately circular shape. We first consider the 1D scenario (Figure 4b), where the reaction between Oot and COot proceeds faster than the recombination of COot and Obr. In the initial stage of the simulation, CO adsorption sets in instantaneously at 10-7 mbar, and the Oot coverage starts to decrease immediately due to the low barrier of COot/Oot recombination. After 3 min (Figure 4b, left) the simulation clearly shows COot accumulation next to Obr sites and no isolated COot in oxygen matrix that would correspond to the experimentally observed band at 2147 cm-1. The RAIR spectrum of the simulation snapshot at 3 min should instead display one or several bands between 2100 and 2125 cm-1 only, indicating COot at varying coverages adsorbed next to Obr. Over time more and more Oot is consumed, resulting in a steep increase of the averaged COot coverage. The first COot/Obr recombination takes place at 15 min, resulting in the formation -13-


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of bridge vacancies which are quickly filled by CO. Once a few defects in the Obr rows have been formed, the CObr rows start to grow over time. Since COot is adsorbed next to CObr at a coverage of 0.5, an additional band at 2070 cm-1 should appear after 20 min (Figure 4b, center). As most of the Oot have already been consumed, further replacement of the Obr by CObr proceeds faster. However, it takes about 90 minutes (Figure 4b, right) until the Obr coverage falls below 10 %. The persistence of COot adsorbed next to Obr should result in a clearly detectable band between 2100-2123 cm-1. In the steady state all bridge sites are covered by CObr and about 50 % of the cus sites are occupied by COot, while Oot and Obr are both minority species with an average coverage of 1 %. In the steady state, a single band at 2070 cm-1 is expected, indicative of COot being adsorbed next to CObr.

Figure 4: (a) Sketch of the growth of CObr/COot domains (black) in the two mechanisms, starting from a defect in the Obr/Oot overlayer. In the 1D mechanism (1D) Oot is consumed and replaced by CO faster than Obr, resulting in the formation of arrays of COot adsorbed next to Obr. In the 2D mechanism (2D) Obr and Oot are replaced by CO equally fast, leading to the growth of CObr/COot-patches extended in both directions. (b) and (c) Results of transient KMC simulations for scenarios 1D (b) and 2D (c) at different elapsing times starting from Obr and Oot coverages of 1 and 0.87 with T = 288 K and p(O2) = p(CO) = 10-7 mbar. The first image in both rows displays the state after initial CO adsorption (at 3 min), the second image shows the propagation of the reaction front after 20 min and the third image in each row shows the surface close to steady state when only 10 % of the Obr are left on the surface. Absorption bands for majority species are given in cm-1.

In the 2D scenario (Figure 4c) the COot/Oot and COot/Obr recombination reactions proceed with similar rates. At the beginning of the reaction, the cus vacancies are filled by COot, resulting in isolated COot molecules in a dense Obr/Oot matrix. The first few recombination reactions occur between COot and Oot as well as between COot and Obr, resulting in short arrays of COot adsorbed next to Obr. Some Obr vacancies are formed as well and immediately filled by CO, thus forming the first CObr already after 3 min (Figure 4c, left). The majority species at this point, however, is COot in a dense Oot/Obr matrix, giving an absorption band at 2145-2153 cm-1. Starting from these COot defects, loosely packed CObr/COot islands with a COot -14-


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coverage of 0.5 start to form at 20 min (Figure 4c, center), leading to an absorption band at 2070 cm-1. These islands grow evenly in both directions without forming pronounced COot/Obr arrays as observed in the 1D scenario. At this point the isolated COot species in a Obr/Oot matrix (giving a band at 2145-2153 cm1)

still exists. A few isolated COot molecules persist on the surface for up to 70 min. Afterwards, only dense

oxygen islands (without isolated COot) as well as CObr/COot domains coexist. At the domain boundaries, typically COot is adsorbed next to Obr, giving adsorption bands in the range between 2100 and 2123 cm-1. The residual Oot/Obr islands are consumed by CO over time. However, it takes quite long (115 min) until 90 % of the Obr are replaced by CObr (Figure 4c, right). The steady state configuration is similar to that of the 1D scenario, with all bridge sites occupied by CObr and approximately half the cus sites occupied by COot.

6. Discussion In order to decide which KMC scenario best describes the experimental data, we focus on the first 90 min of the reaction where the differences in the surface configurations between the KMC parameter sets are most pronounced (cf. middle column of Figure 3). In the experiment, the absorption spectra do not vary very much in the time window between 10 and 90 min, giving only two major bands at 2087 cm-1 and 2140-2147 cm-1 and high intensity in between. The spectra are reproduced together with the simulated configurations at 20 min in Figure 5 a and b. In the 1D scenario (Figure 5a) the majority species in the initial phase of the reaction is COot adsorbed at high coverage next to Obr, for which a dominant absorption band between 2100 cm-1 and 2123 cm-1 is expected. The experimental spectra, however show only weak bands at 2100 cm-1 and 2123 cm-1 (marked by arrows), which are barely noticeable compared to the main bands at 2087 cm-1 and 2140-2147 cm-1. Neither of the experimentally observed strong bands at 2147 cm-1 and 2087 cm-1 can be explained by the simulation due to the absence of isolated COot in dense Obr/Oot matrix and the absence of CObr early in the reaction. In the 2D scenario (Figure 5b) the majority species are isolated COot in Obr/Oot matrix (21452155 cm-1) and COot next to CObr at a local COot coverage of 0.5-1.0 (2070-2086 cm-1). In addition, there are domain boundaries where the CObr/COot-covered islands touch the Obr/Oot-covered areas. The COot and CObr molecules adsorbed at these boundaries will presumably absorb IR light somewhere in the range between 2080 and 2120 cm-1. The heterogeneity of the domain boundaries may lead to a broad band or high background in this range. Quite notably, COot in Obr/Oot matrix persists at low coverages up to 70 minutes, in good agreement with the experiment, where this species starts to vanish only after 90 minutes have passed. A smaller band indicating COot adsorbed next to Obr with lower abundance should appear around 2123 cm-1. As indicated in the comparison with the spectra in Figure 5 a and b, scenario 1D can clearly be ruled out because it does not explain the experimentally observed bands at 2087 cm-1 and the one at 2140-2147 cm-1. Instead, the strongest bands in the induction phase should appear between 2100 cm-1 and 2123 cm-1. Only the 2D scenario (Figure 5b) is able to account for the bands at 2087 cm-1 and 2140-15-


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2147 cm-1 during the first 70 minutes of the reactions, as well as the high background in between these two bands, which can be traced to the boundaries between the CObr/COot and Obr/Oot islands and the (sparse) occurrence of COot adsorbed next to Obr.

Figure 5: (a) and (b) Comparison between simulations and experiment (spectra at 10 and 90 minutes, simulation snapshots at 20 minutes) for (a) 1D and (b) 2D models.

In the simulations, the steady state of the surface is dominated by closed CObr rows coadsorbed with COot and few Oot and Obr, irrespective of the chosen activation energies (Figure 6a). We note that this steady state should only result in a single strong band at 2070 cm-1 without the additional double band at 1912/1925 cm-1 observed in the experiment, which is assigned to alternating COIIbr /Obr rows with coadsorbed Oot. Our simulations overestimate the CO coverage on the surface, which can be traced to the well-documented CO overbinding in PBE calculations of transition metal surfaces. 29 In order to give an estimate of this overbinding error, the partial pressure of CO was reduced in the simulation until the experimentally observed steady state with bands at 2075 cm-1 and 1912/1925 cm-1 is obtained. It is reproduced by the simulation at p(CO) ≈ 2.2∙10-8 mbar and p(O2) = 1.0∙10-7 mbar, which reduces the chemical potential of CO by about μ"CO# $ % & ln (

.∙ )*+

)*,

- $ -0.04 eV. In this more oxygenated steady

state, most of the bridge sites are occupied by alternating CObr and Obr coadsorbed with Oot (Figure 6b and cf. Section S 3). Since the absolute error of the CO adsorption energy is quite small, we presume that the CO overbinding does not affect the qualitative mechanistic aspects discussed in the present study as the CO overbinding merely leads to higher total CO coverage and faster relaxation into the CO-covered steady state of the reaction (115 min in scenario 2D instead of 195 min in the experiment) by suppressing CO desorption which competes with CO + O recombination reactions on the surface. -16-


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Figure 6: (a) Steady state of the simulation at p(CO) = 10∙10-8 mbar. (b) Steady state at p(CO) = 2.2∙10-8 mbar with some areas that absorb IR at 1912/1925 cm-1 and 2070 cm-1 marked in blue and red, respectively.

As inferred from Figure 6b, the KMC simulation is capable of reproducing the role of COIIbr in Obr rows as a spectator species in the steady state. These COIIbr species are indeed static in the simulation, as CO + O recombination reactions occur only between COot and Obr, as well as between COot and Oot in these COIIbr /Obr domains. Analyzing this steady state configuration more closely reveals that most of the COot are indeed adsorbed in inactive configurations, often being flanked by CObr and single cus vacancies where O2 cannot adsorb dissociatively. The adsorbates present on the surface (and detectable by surface-sensitive techniques) thus mostly represent inactive spectators. In the steady state, most CO2 molecules are formed by CO molecules that adsorb into vacancies flanking Oot and/or Obr and recombine instantaneously, thus highlighting the need for transient techniques in heterogeneous catalysis. Altogether the experimental data combined with the KMC simulations strongly favor the 2D mechanism as originally proposed by Wendt and Seitsonen, 6 while clearly ruling out a 1D mechanism as proposed by Reuter/Kiejna, 5, 8 where the recombination of Oot with COot was proposed to be significantly faster than the recombination of COot with Obr.

7. Summary and Conclusion Previous ab-initio based KMC simulations based on two different parameter sets (with differing rate determining steps: both COot+Oot and COot+Obr versus only COot+Oot) for the CO oxidation over RuO2(110) lead to almost identical surface configurations and activities in steady state, which have not been distinguishable by in-situ spectroscopic methods so far.10 By switching from the steady state to the transient reaction regime we were able to provide unambiguous evidence that bridging oxygen (Obr) and on-top oxygen (Oot) are equally active in the CO oxidation over RuO2(110), although the adsorption energy of Oot is 150 kJ/mol lower than that of Obr. This chemically counterintuitive and surprising finding is relevant for any kind of catalytic oxidation reaction where species with different coordination (and thus

different adsorption energies) coexist on the surface. -17-


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In doing so, we followed the time evolution by reflection-absorption infrared spectroscopy (RAIRS) towards steady state when exposing an almost completely Obr/Oot-covered RuO2(110) surface to the reaction mixture (p(CO) = p(O2) = 10-7 mbar) at low temperatures (288 K). The RAIR spectra taken during the first 90 minutes of the reaction reveal an instantaneous formation of CObr/COot domains as well as the presence of COot within dense Obr/Oot islands. With comparative KMC simulations the occurrence of these surface species in the transient regime can be traced a mechanism, where the recombination reactions of COot + Oot and COot + Obr are almost equally fast. The presented transient approach overcomes a common problem of steady state catalytic experiments: Frequently, the prevailing species in operando surface-sensitive spectroscopy are frequently so-called spectator species which cover (block) most of the catalyst’s surface and have extended life time, but do not participate in the actual catalytic turnover from the reactants to the product. The reactive species, on the other hand, are often present only in too low concentrations on the catalyst’s surface to be detectable. In our designed transient experiment, the starting surface was prepared to be free of these steady-state spectators and the surface configurations that are “visited” on the way towards steady state can be followed both by operando spectroscopy (such as RAIRS) and KMC simulations. This unique combination provides a wealth of information about the underlying reaction kinetics as exemplified with the CO oxidation over RuO2(110). Transient spectroscopic surface chemistry experiments in combination with spatially resolved KMC simulations have great potential to disentangle reaction mechanisms in heterogeneous catalysis research.

Supporting Information Experimental assignment of the 1912/1925 cm-1 double band Cluster expansion parameters DFT-computed normal modes of vibration Steady state at reduced p(CO) Structural characterization of RuO2(110) surface

Acknowledgement The authors acknowledge funding by BMBF (HEXCHEM) and thank the Hessian High Performance Computing Network (HKHLR) and the computing centers of the University Giessen and Technical University Darmstadt for providing computational time and support on the Skylla and Lichtenberg high performance clusters.

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Graphical Abstract

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Probing the Activity of Different Oxygen Species in the CO Oxidation

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