High Turnover Frequency COâNO Reactions over Rh Overlayer

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

High Turnover Frequency CO–NO Reactions over Rh Overlayer Catalysts: A Comparative Study with Rh Nanoparticles Hiroshi Yoshida, Kenichi Koizumi, Mauro Boero, Masahiro Ehara, Satoshi Misumi, Akinori Matsumoto, Yusuke Kuzuhara, Tetsuya Sato, Junya Ohyama, and Masato Machida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00383 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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The Journal of Physical Chemistry

High Turnover Frequency CO–NO Reactions over Rh Overlayer Catalysts: A Comparative Study Using Rh Nanoparticles

Hiroshi Yoshida,†,‡ Kenichi Koizumi,§,‡ Mauro Boero,# Masahiro Ehara,§,‡ Satoshi Misumi,† Akinori Matsumoto,† Yusuke Kuzuhara,† Tetsuya Sato,ǁ Junya Ohyama,†,‡ Masato Machida*†,‡

†

Department of Applied Chemistry and Biochemistry, Graduate School of Science and

Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan

‡

Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo, Kyoto 615-8245, Japan

§

Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

#

University of Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg UMR 7504, 23 rue du Loess, F-67034 Strasbourg, France

ǁ

Technical Division, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto, 860-8555, Japan

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ABSTRACT A comparative turnover frequency (TOF) study for structure-sensitive CO–NO reactions between overlayer (thin-film) and nanoparticle Rh catalysts was performed using a combined experimental and theoretical approach. Two types of honeycomb catalysts were prepared: one by the arc-plasma deposition of a 3-nm Rh overlayer having a (111) preferential orientation atop a 20-µm Fe–Cr–Al metal foil and the other by a conventional wet coating of Rh/ZrO2 powders comprised of Rh nanoparticles onto a cordierite honeycomb. The reaction rate of the overlayer was found to be more superior to the nanoparticles, despite a smaller surface area, as the TOF was 14-fold greater on the overlayer than on the nanoparticle. In situ infrared spectroscopy suggested that a hollow-site NO formed onto the overlayer in contrast to the bridge- and on-top-NO adsorptions on the nanoparticles. The energy barriers for surface reactions of adsorbed NO molecules were analyzed using a density functional theory based molecular dynamics approach for an Rh(111) slab and an Rh55 cluster to model the overlayer and the nanoparticle. The nanoparticle was more favorable to the dissociation of bridge NO compared with the hollow NO on the overlayer despite needing to overcome much greater barriers for N–N recombination, suggesting that the surface of the nanoparticles was predominately covered by N and O atoms, where N–N recombination was the rate-limiting step. Conversely, the Rh(111) overlayer, which offered not only moderate or comparable energy barriers for NO dissociation and N–N recombination but also a lower energy barrier migration for N atoms on the surface, enabled a high-TOF reaction. The proposed mechanism was rationalized by comparing the results with the empirical kinetics of CO–NO reactions.

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1. Introduction Noxious gases emitted from gasoline-fueled automobiles are traditionally purified using a threeway catalyst (TWC) containing nanoparticles of rhodium (Rh), palladium (Pd), and platinum (Pt) as active components.1-5 Of the several chemical reactions that occur in TWC, the equimolar reaction between CO and NO (CO + NO → CO2 + 1/2N2) plays an essential role in the NO reduction efficiency. With Rh found to be much more efficient in achieving this reaction compared with Pd and Pt,3 elucidating the actual surface reactions have been a central part of both experimental and theoretical investigations in this area.6-28 One striking feature of this reaction, which still remains unclear, is the Rh particle size effect on the turnover frequency (TOF). Oh et al.9, 12 performed a comparative study on the CO–NO reaction rates on single crystal Rh and supported nanoparticle Rh with different particle sizes and found a positive correlation between the TOF on the Rh particle size, which is in contrast to a similar TOF for the CO–O2 that was independent of Rh particle size. These different behaviors are referred to as “structure-sensitive” and “structure-insensitive” reactions. Figure 1 compares typical examples using experimental kinetic data measured from our study (see Supporting Information, Table S1). Notably, the TOF only for the CO–NO reaction occurs two orders of magnitude as much when the Rh particle size increases from 1 nm up to several 10 thousand nm. The largest possible particle size corresponds to a single crystal Rh, the TOF of which is estimated to exceed the size of a nanoparticle by a hundred-fold. As a substitute to a single crystal, a large two-dimensional Rh layer (with a thickness of a few nanometers) may be more functional to achieving a higher TOF reaction. To verify this hypothesis, we have recently prepared a nanometric Rh thin film, henceforth referred to as an Rh overlayer catalyst, that covers the surface of metal foils using pulsed cathodic arc-plasma (AP) deposition.29-31 Interestingly, a honeycomb structure comprising the metal foil catalysts was found to be very active for TWC reactions under realistic conditions, despite their 3



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non-porosity, small surface area, and less amounts of Rh loaded. The honeycomb structure displayed

outstanding

light-off

performances

for

simulated

exhausts

(CO−C3H6−NO−O2−H2O−N2) at a practical gaseous hourly space velocity of 1.2 × 105 h−1 due to an extremely high TOF; the value of which, for a CO–NO reaction, was more than ten times greater than that of the conventional honeycombs prepared through the wet coating of powder catalyst containing Rh nanoparticles.29 As the apparent catalytic performance of these structures is determined by the TOF as well as the number of surface Rh atoms, a high TOF would more than compensate for the small surface area of the metal foil catalysts. In other words, conventional nanoparticle (powder) catalysts would require larger surface areas and thus a larger number of active sites to achieve the required performance. Conversely, the Rh overlayer catalyst has the ability to achieve a higher performance because of a higher TOF despite the small surface area. Moreover, the overlayer catalysts use much less Rh compared with that of the powder-coated catalysts. These advances have been widely recognized as future research trends because of the limited Rh supply and its increasing demands in other applications. Notwithstanding, the reason for the high TOF in the Rh overlayer structure still remains unclear. In this present study, the catalytic CO–NO reaction over metal overlayers has been studied with the intent to elucidate the mechanisms of high TOF surface reactions via combined experimental and computational approaches. Here, the catalytic performance and kinetic behavior of two catalyst types, one with an overlayer and the other with a nanoparticle-type structure, were evaluated as monolithic honeycomb. Among the three precious metals, Rh, Pd, and Pt, only Rh has been found to achieve significantly high TOFs in the overlayer form compared with the nanoparticle form. On the basis of the structural characterization results and reaction kinetic analysis, complementary first-principles molecular dynamics (FPMD) simulations32 within the density functional theory (DFT) framework33-34 were used jointly with the constrained dynamics method35 to analyze NO 4


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surface adsorption and reactions. This includes a microscopic picture of the reaction pathways that induce NO dissociation and N–N recombination. To date, such approaches have already been extensively used to determine reaction mechanisms on surfaces,36-37 most especially for heterogeneous catalysts. Reaction barriers for catalytic processes have even been refined by transition path minimization in the total energy space using the nudged elastic band (NEB) approach.38-39 The latter has been used primarily to provide any initial information on the reaction pathways through FPMD simulations. These processes have allowed us to clarify the main mechanisms for NO reduction processes and the reasons behind the higher activities of Rh overlayer compared with Rh nanoparticles.

2. Methods and Materials 2.1 Experimental methods 2.1.1 Catalyst preparation A heat-resistant Fe–Cr–Al based metal (SUS) foil comprising 75 at.% Fe, 20 at.% Cr, and 5 at.% Al (Nippon Steel & Sumikin Materials, Japan), with a thickness of approximately 20 µm, was used as a substrate for the overlayer catalyst. To examine the effects of surface roughness, the SUS foil was mirror-polished on a Buehler MetaServ 250 grinder-polisher using diamond suspension solution with particle sizes between 1 and 3 μm. A 0.05 μm polishing paste was subsequently used until the degree of surface roughness (Ra) was less than 4 nm. Unless otherwise mentioned, the assupplied SUS foil was used without any further polishing. The Rh overlayer was formed on the SUS foil using pulsed cathodic AP deposition, with the method described in our previous report.2931, 40

The AP deposition setup consists of a vacuum chamber connected to a turbo-molecular

pumping system, an arc discharge source (ARL-300, Ulvac) fitted with a Rh metal cathode (f10 mm, 99.9%, Furuya Metals, Co. Ltd.), and a stage to place substrates on.41 Below 10−3 Pa, 5



1000 shots of AP pulses were made in a 0.2 ms time period, with a current amplitude of 2 kA used to generate on an Rh metal target with a frequency of 1 Hz. The AP pulses were irradiated on both sides of the SUS foil to allow of Rh deposits on the surface at ambient temperature. The Rh deposition amount was controlled using a quartz crystal microbalance (STM-2, Inficon) until the thickness was approximately 3 nm. Flat and corrugated foils (10 mm × 40 mm and 10 mm × 55 mm, respectively) after Rh deposition with both sides were wrapped together in a monolithic honeycomb shape (f 8 mm × 10 mm, 900 cells in−2, see Supporting Information, Figure S1). Both Pd and Pt overlayers were formed on SUS foils in the same manner. The as-prepared metal overlayer catalysts are referred to as Rh/SUS, Pd/SUS, and/or Pt/SUS in the following sections. Nanoparticle catalysts, used as reference materials, were prepared by wet coating of a slurry containing Rh/ZrO2, Pd/Al2O3, or Pt/Al2O3 (50 mg each) onto cordierite honeycomb substrates (f10 mm×10 mm, 600 cells in−2, see Supporting Information, Figure S1). Each of the powdered catalysts with a 0.4 wt% metal loading were prepared using the impregnation method, where ZrO2 (JRC-ZRO-3, Catalysis Society of Japan, BET surface area (SBET) = 61 m2 g−1), g-Al2O3 (JRCALO-8, Catalysis Society of Japan, SBET = 150 m2 g−1), and SiO2 (JRC-SIO-9A, Catalysis Society of Japan, SBET = 343 m2 g−1) were mixed with aqueous solutions of Rh(NO3)3, Pd(NO3)2, or Pt(NO2)2(NH3)2 (Tanaka Kikinzoku) and then dried and calcined in air at 600 °C for 3 h. The reference honeycomb catalysts were prepared by dipping a f10 mm × 10 mm sized cordierite honeycomb substrate (600 cells in−2, NGK Insulators, Ltd.) into a slurry, which was prepared by ball-milling of the catalyst powder, an inorganic binder and water. After drying and calcination at 600 °C for 3 h, as-prepared honeycomb catalysts were reduced in a 5% H2/He atmosphere at 200 °C for 1 h prior to any catalytic reactions.

2.1.2 Characterization 6


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The crystal structure of the as-prepared catalysts was determined using X-ray diffraction (XRD), taken with monochromatic Cu Ka radiation at 40 kV and 200 mA (RINT-TTR III, Rigaku) in a symmetric 2q-q scan mode. Surface analysis was performed using a Thermo-Scientific Ka X-ray photoelectron spectrophotometer (XPS) with monochromatic Al Ka radiation at 1486.6 eV. A charge correction was made using the C1s binding energy at 285 eV. Transmission electron micrographs (TEM) were acquired using an FEI TECNAI F20 operating at 200 kV. The surface topography and actual surface area of metal foil catalysts were measured using a VK-Z250 laser scanning confocal microscope (Keyence). In situ Fourier-transform infrared (FTIR) spectra of adsorbed NO on Rh were acquired, in transmission mode, using a Nicolet iS50 FTIR spectrometer (Thermo Fisher Scientific) fitted with a temperature-controllable reaction cell. For this purpose, a 3 nm Rh overlayer was created by the pulsed AP deposition on both sides of a 0.2 mm thick mica sheet (Okabe Mica, Japan), which allowed for a 56% transmittance at 2000 cm−1. To use as a reference for the Rh nanoparticle catalyst, 0.4 wt% Rh/SiO2 powder (Rh particle size: 1.4 nm) was ground with a mortar and pressed into a 20 mm disc using a laboratory disc press. The sample was placed into the temperature-controllable reaction cell and heat treated at 200 °C for 30 min under a dry He supply (50 mL min−1). Each background spectrum was collected under constant He flow at each temperature measured. The sample was subsequently exposed to 1% NO/He for 10 min before each spectrum was collected. Spectra were referenced to the signal for He flow before exposure to NO.

2.1.3 Catalytic reactions To measure the catalytic reaction rates of the honeycomb catalysts, tests were performed using a Horiba SIGU/MEXA reaction analysis system, which consists of a gas supply unit, an infrared image furnace (with four ellipsoidal reflectors) and a gas analysis unit. The catalytic activity was 7



evaluated by heating the catalyst bed from 100 °C to 500 °C at a rate of 10 °C min−1 in the presence of a 0.1% NO/0.1% CO gas mixture (N2 balance, 1.0 L min−1) at atmospheric pressure. The gas hourly space velocity (GHSV) was maintained at 1.2 × 105 h−1 for the SUS foil and 0.76 × 105 h−1 for the cordierite, respectively. Gas analysis was performed using nondispersive infrared CO/CO2 detectors, a flame ionization hydrocarbon detector, and a chemiluminescence NOx (NO + NO2) detector. The TOF was calculated from the steady-state NO conversion measurements (below 20%), which allowed for a rough approximation for the differential reactor. The number of Rh surface sites on the SUS foil catalysts was calculated using actual surface areas, which measured by a confocal laser scanning microscope, and atomic density of the Rh(111), Pd(111), or Pt(111). The number of surface metal sites on the powdered catalyst was determined using CO chemisorption experiments at 50 °C, in pulse-injection mode, if the stoichiometry was assumed to be between the chemisorbed CO and the surface metal atom at a ratio of 2:1 for the Rh or 1:1 for Pd and Pt. For kinetic analysis under steady-state conditions, a 2 mm × 30 mm metal foil catalyst was evaluated in a fixed reactor. The powdered catalysts (i.e., 0.4 wt% Rh/ZrO2 and 0.4 wt% Pd/Al2O3, 50 mg) were used as reference catalysts. A steady-state was confirmed through constant conversions versus time on stream, which was kept below 20%. To determine the apparent activation energy, a CO–NO reaction (0.1% NO, 0.1% CO, He balance, 0.1 L min−1) was carried out at temperatures between 230 °C and 290 °C. The effects of CO/NO partial pressures on CO/NO conversion rates were measured at 245 °C for Rh or 280 °C for Pd. The CO partial pressure in the gas feed was varied from 0.05 to 0.20 kPa, with the NO partial pressure kept at 0.20 kPa (He balance, 0.1 L min−1). When the CO partial pressure was kept at 0.20 kPa, the NO partial pressure was then varied from 0.05 to 0.20 kPa.

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2.2 Theoretical methods Spin-unrestricted DFT33-34 calculations (implemented in the CP2k code42-43) were performed for all electronic structure calculations, geometry optimizations and reaction path samplings. For the auxiliary FPMD simulations32 used to estimate the reaction mechanisms, the CPMD code was used.44 The Perdew–Burke–Ernzerhof (PBE) functional45 was then used to determine both exchange and correlation interactions. To describe core–valence interactions, norm-conserving Hartwigsen–Goedecker–Hutter pseudopotentials were used for all atoms.46-48 All valence electrons were expanded on a triple-z Gaussian basis set, with an energy cutoff of 500 Ry for the auxiliary plane-wave basis set. For the FPMD simulations, the cutoff energy for the plane-wave basis set was set to 90 Ry. The Rh(111) surface was modeled as a slab of four atomic layers with a 4×4 exposed surface. The bottom layer was kept fixed at their crystallographic positions. A Rh55 cluster with a cuboctahedral symmetry was used to simulate the Rh nanoparticles. This model has an approximately 1.0 nm quasi-spherical shape that contains close-packed (111)-like facets and (100)like facets to facilitate a comparison with the (111) surface. Here, none of the atoms in the cluster were fixed during geometry optimizations or FPMD simulations. The FPMD simulations were performed in the canonical (NVT) ensemble using the Nosé–Hoover chain scheme49-51 as a means to control the temperature. An integration time step for the dynamical simulations was set to 0.1 fs and the reaction pathways were sampled via the constrained dynamics using a distance constraint according to the blue moon ensemble technique.52-53 To calculate the energy barriers for a reaction step, we used the NEB method.38-39 The reaction pathways used as an input for the NEB calculations were obtained through the auxiliary blue moon ensemble FPMD simulation. When the maximum force and the root mean squared displacement were smaller than 0.03 and 0.001 a.u., we determined that convergence was achieved.

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3. Results and Discussion 3.1 Activity comparisons between overlayer and nanoparticle catalysts Three metal overlayer catalysts (Rh/SUS, Pd/SUS, and Pt/SUS) were prepared using the pulsed cathodic AP deposition technique to compare their catalytic activity. After 1000 shots, the surface of the SUS foil was fully covered with an overlayer, having a thickness of approximately 3 nm for Rh and Pd and 1 nm for Pt (see Supporting Information, Figures S2 and S3), in the metallic state. The light-off characteristics of the CO–NO reaction over the metal honeycomb structures are shown in Figure 2 and compared with those of the reference cordierite honeycombs coated with the powdered catalysts (Rh/ZrO2, Pd/Al2O3, and Pt/Al2O3) containing metal nanoparticles. The light-off temperature sequence observed for the metal honeycombs is Rh < Pd < Pt, which corresponds well with those of the cordierite honeycombs. Note that there is an activity difference between metal and cordierite honeycombs where Rh/SUS demonstrates a light-off temperature (400 °C) than Rh/SUS. This is not the case for Pd/SUS as the light-off temperature is higher than that of the cordierite honeycombs (Pd/Al2O3). Table 1 compares the TOF for NO conversion in the CO–NO reaction of these honeycomb catalysts at 280 °C. It should be noted that the TOF of Rh/SUS (186 min−1) is 14-fold greater than Rh/ZrO2 (13.1 min−1). This may be the reason for the high catalytic performance of Rh/SUS, despite its small surface area and Rh loading. It is well known that the CO–NO reaction on Rh catalysts are structurally sensitive, with the TOF value rising with increased Rh particle size.12 Oh and coworkers found the TOF of single crystal surfaces two orders of magnitude greater in comparison with Rh nanoparticles.9 The present results imply that the two-dimensional structure of Rh is an important factor as discussed further in detail. Although the Pt catalyst was found to 10


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demonstrate a similar behavior, its TOF at 280 °C was found to be negligibly small. Pd/SUS and Pd/Al2O3 showed similar TOFs at around the 6 min−1 mark. The catalytic performance of Pd should therefore increase with increasing amount of surface Pd. Here, the superiority of the overlayer compared with the supported nanoparticles is therefore dependent on the metal elements, with Rh being the most promising in comparison with the other metals. We also compared the catalytic activity of the CO–NO, CO–O2, and C3H6–O2 reactions over the Rh/SUS honeycomb and the reference cordierite honeycomb (Rh/ZrO2) (see the Supporting Information, Figure S4). Rh/SUS was found to be more superior to the reference Rh/ZrO2 catalyst for the CO–NO reaction, while it was less active for the other two reactions. This suggests that the superiority of the overlayer structure is dependent on the type of catalytic reactions that occur. We therefore focused our attention on the reasons behind the higher TOF CO–NO reactions obtained for the Rh overlayer compared with the Rh nanoparticles.

3.2 Structure of Rh overlayer Because the surface roughness of SUS foils may have a drastic effect on the catalytic activity of the Rh overlayer, we examined this idea using SUS foils both with and without a mirror polish treatment before the AP deposition of Rh. As shown in Figures 3a and b, the untreated foil was found to have a surface roughness of about 500 nm, with the rolling process leading to a contoured surface texture due to deformation. After mirror polishing, the surface roughness decreased significantly to 4 nm, although the actual surface area of the SUS foil was found almost the same, albeit with an error of about 1%. When Rh was deposited onto the surface using 100 shots of AP pulses, the catalytic activity for the CO–NO reaction was evaluated using a flat strip 2 mm × 30 mm in size, as shown in Figure 3c. Clearly, the Rh overlayer as deposited onto the polished foil showed a much higher catalytic conversion efficiency than the unpolished foil. 11



Surface roughness can therefore be said to negatively affect the catalytic activity, probably because, on a smoother surface, the two-dimensional Rh layers are more easily covered. This plays a key role in achieving higher TOF reactions. This notion is also supported by the Rh surface coverage, which was found to be approximately 78% for the polished SUS but only 42% for the unpolished SUS. Even if this difference in surface coverage is taken into consideration, we then calculate the TOFs for the CO–NO reaction to be different, this being 373 min−1 with polishing and 163 min−1 without polishing at 300 °C. Figure 4 compares the XRD patterns of the Rh overlayer as deposited onto the mirror-polished and unpolished SUS foils. Polycrystalline Rh foils were also analyzed for reference. When the number of AP pulses was increased to 2000 shots, the surface of the SUS foil was found to be fully covered with the Rh overlayer at thickness of approximately 6 nm. The XRD patterns of the Rh reference foil shows three strong peaks that are assigned to 111, 200, and 220 reflections with a relative intensity ratio of I111:I200:I220 = 100:50:40, which is in close agreement with the theoretical intensity ratio for the face-centered-cubic Rh. Conversely, two Rh overlayer samples showed the 200 and 220 peaks to be much less intense than the 111 peak, yielding a ratio of I111:I200:I220 = 100:20:0. The polished and unpolished samples exhibited no significant differences in the peak intensity ratio. This result suggests that the (111) orientation of the as-deposited Rh overlayer was more prominent on the SUS foil surface. On the other hand, a TEM photograph of the supported Rh nanoparticles (Rh/ZrO2) showed a hemispherical shape with no specific orientation (the Supporting Information, Figure S5). One plausible reason for the high TOF for the CO–NO reactions may therefore be associated with certain preferential orientations for the overlayer structure. On the basis of this result, we attempted to analyze the reaction pathway using DFT-based NEB calculations to work out the energy barriers for the dissociation of the adsorbed NO and subsequent N2 formation on each of the surface sites. 12


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3.3 Analysis of NO adsorption on Rh overlayer and Rh nanoparticles To calculate the adsorption energies of an NO molecule on the Rh overlayer, we took the Rh(111) surface as a model for the overlayer catalyst, with calculations for NO adsorption performed for the on-top, two-fold bridge, and three-fold hollow hexagonal close-packed (hcp) sites, as shown in Figure 5a. Hereafter, the adsorption energy is indicated as Eads. The adsorbed NO molecule was found to be more stable on a three-fold hollow site (Eads = 2.34 eV) compared with the on-top (Eads = 1.96 eV) or bridge-type (Eads = 2.23 eV) adsorption sites. These results were comparable with those of Loffreda et al. who undertook a similar theoretical study.54 The Eads values for the Rh nanoparticle were also calculated using a cuboctahedral Rh55 cluster as shown in Figure 5b. Once more, the three-fold hollow site on the (111)-like facet showed the largest Eads value of 2.86 eV, although the differences between the other sites (i.e., on-top with an Eads of 2.68 eV and bridgetype with an Eads of 2.66 eV) were found to be rather small in comparison with the infinite Rh(111) surface model. These results are in good agreement with the work of Deushi et al., where an icosahedral Rh55 cluster was used.26 To complement the theoretical simulations, we performed in situ FTIR analysis on the chemisorbed NO on Rh overlays and Rh nanoparticles at different temperatures. For this purpose, a mica sheet covered with a 3 nm thick Rh film along with an Rh/SiO2 powdered catalyst, having an Rh particle size of 1.4 nm (see the Supporting Information, Table S1), were used as model samples for the Rh overlayer and the Rh nanoparticle, respectively. As NO molecules adsorbed onto Rh surfaces are known to dissociate rapidly even below ambient temperature,55-56 the measurements were performed at temperatures (T) below 0 °C. As shown in Figure 6, bands appearing at 1420, 1600–1650, 1750, and 1870 cm−1 are much less intense when the temperature is close to 0 °C. According to the literature,7, 19, 23, 57-59 bands that appear at higher wavenumbers 13



(i.e., 1750 and 1870 cm−1) are due to negatively charged and/or neutral NO molecules adsorbed at the on-top site, whereas those at 1600–1650 cm−1 are adsorbed on the bridge site. The band at 1420 cm−1 is attributed to NO adsorbed onto the three-fold hollow site. As this band appears only for the Rh overlayer (Figure 6a), the adsorption behaviors of NO must then be in accordance with the greatest Eads values, as implied in the simulation shown in Figure 5. In contrast, negligible NO adsorption was observed on the hollow site for the Rh nanoparticle (Figure 6b), where differences in Eads values were found to be rather small. Thus, NO adsorption on the hollow site is only characteristic of the two-dimensional Rh overlayer structure. This is in accordance with the recent near-ambient pressure XPS study reported by Ueda et al.,60 who investigated NO reduction by CO on a Rh(111) single crystal and revealed that hollow NO is a reactive species. To determine the dissociation of adsorbed NO molecules and the subsequent formation of N2 onto the Rh(111) surface, we computed their energy barriers since it is essential to understand the mechanism behind high TOF reactions. For the dynamical simulations, the NO molecule was initially located onto the most stable three-fold hollow site (Figure 7a, panel (i)) as evidenced experimentally by in situ FTIR (Figure 6). After the system was equilibrated initially for 1 ps, the distance between the N and O atoms was gradually elongated to cleave the N–O bond. The resulting N and O atoms were subsequently trapped inside the three-fold hollow site (Figure 7a, panel (ii)). Using the information provided by the optimized initial and final geometries, the dissociation pathway was investigated via the NEB method, with the resulting energy barrier determined at 1.65 eV. We then estimated the activation barrier for the hopping process of an N atom from one three-fold hollow site to the next. Such barriers, however, are sufficiently small (~0.4 eV) to be overcome by thermal fluctuations, even at room temperature. A pair of N atoms were then placed inside the vicinity of two neighboring three-fold hollow sites, together with additional O atoms, as illustrated in Figure 7a (iii). When dynamics simulation was constrained, the distance between the 14


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two N atoms gradually decreased. The results of the simulation show that an N2 molecule is formed in the first instance, which then starts to desorb from the Rh(111) surface over time. The complementary NEB calculation started from the same optimized initial and final geometries through the constrained dynamics that indicated the reaction barrier for N–N recombination as being 1.76 eV. The reaction barriers for the NO dissociation and N–N recombination process are slightly higher than those reported by Loffreda et al.,54 probably due to the different (albeit older) exchange-correlation function used (BP86).47 It should be noted that the tendency of the activation barrier toward N–N recombination is slightly higher than NO dissociation, which is comparable with all the reports shown here. Other possible surface reaction routes via NO + N or (NO)2 dimerization reactions were also simulated to obtain a comprehensive picture of each reaction. These results are shown in Figure S6 of the Supporting Information. All results were found to corroborate well with a reaction that occurs along the path shown in Figure 7a, as they were found to be energetically more favorable. The dissociation of adsorbed NO and following N2 formation were also simulated using the cuboctahedral Rh55 cluster. Figure 7b shows both the geometries and related energies of NO during surface reactions that occur from the bridge site (i), which were found to be the most abundant species, as shown in Figure 6b. When the N–O bond was stretched to 1.80 Å, the N atoms started to dissociate and move toward the adjacent bridge site (ii) by overcoming an energy barrier of 1.00 eV. Following the same procedure, the barriers for the dissociation of NO molecules adsorbed on the on-top sites of Rh55 were also calculated (see the Supporting Information, Figure S7). This was a necessary simulation to run in view of the fact that the FTIR results in Figure 6b implied that the on-top NO site was another major species present on the Rh nanoparticles. Here, the computed energy barrier for N–O dissociation turned out to be larger (1.39 eV) than that on the bridge site (1.00 eV). The next step was to inspect the N–N recombination for the two N and O atom pairs on 15



the cuboctahedral Rh55 cluster. In this case, the simulation protocol adopted consisted of a combined DFT-based NEB calculation and FPMD simulation. When the N atoms were initially located on the bridge sites, and the O atoms are on the (100) hollow sites (Figure 7b, iii), two N atoms were found to move to the hollow sites within the (111) surface of the cluster (Figure 7b, iv). This NEB approach indicates that the geometry is a local minimum of the potential energy surface, having a high stability of 1.59 eV. Due to this stabilization effect, the N–N recombination would then need to overcome a high reaction barrier at 2.18 eV. A similar geometric change was observed when the NO molecule was adsorbed onto the on-top and hollow sites of Rh55 (see the Supporting Information, Figure S7a, c, d). These alternative routes, however, were found to be less favorable and unlikely to be realized because of these high barriers. Here, the Rh55 cluster was modeled with a flexible structure and distorted geometries, with two N atoms significantly stabilized on the (100) hollow sites to produce larger reaction barriers for N–N recombination. We can therefore conclude that the N–N recombination on the Rh55 cluster was slower than the analogous process on the Rh(111) surface.

3.4 Possible mechanisms for high TOF reactions The present study revealed the superiority of the metal overlayer compared with the nanoparticle in terms of high TOFs, which was found strongly dependent on the types of metal elements and catalytic reactions used. The rate for CO–NO reaction over Rh was more than 14-fold greater on the overlayer than on the nanoparticle, whereas no such difference was found for the Pd. Even over the Rh overlayer, the TOFs of other reactions (i.e., CO–O2 and C3H6–O2) were found comparable with or less than those over the Rh nanoparticle. This is consistent with recent work by Kibis et al.61 who pointed out that oxidized Rh films are more active toward CO–O2 reaction. To rationalize the pronounced effect on the CO−NO reaction, the kinetic analysis of the catalytic reactions is able

16


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to bridge our experimental and theoretical results. Table 2 compares the apparent activation energies (Ea) and partial orders with respect to the NO and CO partial pressures for the conversion rates of NO and CO over the overlayer and nanoparticle (Supporting Information, Figures S8–S12). We found the activation energy to be slightly lower for the Rh overlayer (Rh/SUS, 112 kJ mol−1) compared with the Rh nanoparticles (Rh/ZrO2, 126 kJ mol−1). A similar trend was also observed for the Pd catalysts. The Rh overlayer shows that the CO–NO reaction was near zero order for both CO and NO. Contrarily, the Rh nanoparticle catalysts showed slightly different behaviors that provide more positive orders in both CO and NO. Nevertheless, both the Pd overlayer (Pd/SUS) and the Pd nanoparticle (Pd/Al2O3) showed negative reaction orders in CO, but larger positive orders in NO. According to previous studies,3, 16-18, 20 the following elementary steps are accepted for the CO– NO reaction over a metal surface: NOgas → NOads

(1)

COgas → COads

(2)

NOads → Nads + Oads

(3)

2Nads → N2gas

(4)

COads + Oads → CO2gas

(5)

NOads + Nads → N2Ogas

(6)

NOads + Nads → N2gas + Oads

(7)

where the subscript “ads” denotes the adsorbed species on the metal surface. The kinetic parameters in Table 2 suggest the steps that may be rate determining, depending on whether the overlayer, or the nanoparticle of the Rh or the Pd were used. Here, the negative CO reaction order for the Pd gave a clear indication that the surface coverage was dominated by adsorbed CO (2), leading to a lower activity for the NO reaction steps (1, 3, 6, and 7) on Pd. In addition, the positive orders in 17



NO imply that NO dissociation (3) is energetically less favorable on Pd compared with Rh. As these reaction orders show no noticeable differences between the Pd overlayer and the Pd nanoparticles, their reaction mechanism should be the same. This behavior was comparable with their similar TOFs (Table 1) and the structure-insensitive behavior of the CO–NO reaction over the Pd catalysts. Conversely, both CO and NO achieved nearly zero reaction order, which was characteristic of the Rh overlayer and similar to those reported for the Rh single crystals. According to Zhdanov,20 the surface is close to the adsorption–desorption equilibrium with respect to NO and CO at relatively low temperatures, with the reaction limited by NO dissociation. If the COads species were dominant on the surface, a higher NO pressure is more favorable for NO conversion. The latter is one plausible reason why the Rh nanoparticles exhibit a slight positive order in NO. Oh et al.9 concluded that weakly negative or moderately positive reaction orders in NO suggest that the NO dissociation reaction is not rate determining. They also demonstrated that when the NO dissociation rate is rapid, the catalyst surface is covered largely with both adsorbed N atoms and molecularly adsorbed NO.12 For a higher surface coverage of NO, the rate was found to exhibit a negative NO reaction order. If the surface coverage of adsorbed N atoms is high, however, the rate was found to exhibit a zero reaction order for NO. In addition, a similar low surface coverage for both NO and Nads may lead to similar quantities of N2 and N2O generation, although this reaction would be unlikely to occur over the present Rh catalysts. The DFT-based simulations concur to corroborate that the Rh nanoparticles possess the ability to promote the dissociation of bridge-adsorbed NO compared with the Rh overlayer, where it is not prone to favor N–N recombination (Figure 7). An important prediction of our work is that the surface of the Rh nanoparticles is predominately covered by Nads and Oads atoms, with nitrogen desorption found to be the rate-limiting step. Yet this may be inconsistent with the positive reaction 18


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orders of NO (Table 2). On a real surface, however, the NO dissociation rates decreases with increasing surface coverage, which is much faster compared with the Langmuir model due to steric hindrance.17 In such a situation, the N–N recombination rate has also been known to depend strongly on the relative surface occupancy of adsorbed species. Considering that Oads can be removed immediately through a reaction with COads (5), this mitigates crowdedness on the surface. Such situations may explain the positive orders in NO and CO. The present study reveals the superiority of the Rh overlayer compared with the Rh nanoparticles in terms of high TOFs for the catalytic CO–NO reaction. The behavior was found closely related to the structure-sensitive surface reaction mechanism. First, the NO molecule is adsorbed onto three-fold hollow sites on the Rh overlayer, which then dissociates into Nads and Oads atoms. The Nads was found to easily migrate on the surface, overcoming relatively low barriers. This migration resulted in a pairing between N–N atoms, promoting recombination to form N2. In contrast to the surface of Rh nanoparticles, where Nads atoms were found to be stabilized after dissociation on the three-fold hollow site, a higher energy barrier needed to be overcome to form an N2 molecule (Figure 7). Here, a large two-dimensional surface of the Rh overlayer was found more favorable for N–N recombination. This is an essential reason for the structure-sensitive reaction. The higher TOF of the overlayer compared with the nanoparticle cannot be achieved by Pd alone, as the Pd surface is dominated by adsorbed CO. The rate of NO dissociation was also found to be low. A moderate or high surface area coverage of dissociated Nads atoms on the Rh overlayer thus plays a key role in the structure-sensitive behavior.

4. Conclusion The present study indicates that a higher TOF in the CO–NO reactions is readily achieved on metal overlayer compared with metal nanoparticle catalysts, which was characteristically more peculiar 19



for Rh but not so obvious for Pd. Although Pt is similar to Rh in terms of behavior, the catalytic activity was found much less efficient. The high TOF was also a reaction-dependent phenomenon and did not occur for either CO–O2 or C3H6–O2 reactions, even on the Rh overlayer. As the Rh overlayer exhibits a (111) preferential orientation, a possible adsorption reaction of NO molecules over the Rh(111) overlayer and the Rh55 nanoparticle were then studied using FPMD simulation, which clarified the reaction mechanisms via NO dissociation, migration of N atoms, and N–N recombination. On the Rh(111) overlayer, a three-fold hollow site provided a larger adsorption energy for the NO molecule compared with the on-top or bridge sites. The energy barriers for N– O dissociation and N–N recombination were moderate and comparable. In contrast, cuboctahedral Rh55 nanoparticles provided much different sites for NO adsorption with similar adsorption energies. The dissociation energy for the bridge site was found to be significantly smaller than on the hollow site for the Rh(111) overlayer. The flexible Rh55 structure stabilized the geometry drastically so that the two N atoms were trapped within the hollow sites. Hence, the recombination of N atoms from these hollow sites accompanied a larger energy barrier to produce N2 and reduce the catalytic reactivity. Conversely, the rigid Rh overlayer, which provided only moderate barriers for both NO dissociation and N–N recombination, was superior to the Rh nanoparticles. Furthermore, the atomic N produced by NO dissociation was found to migrate rather freely on the surface and recombined easily to produce an N2 molecule. The different energy barriers for N–N recombination depended on whether the overlayer or nanoparticle structure was used, which was also a possible reason for the structure-sensitive features of the CO–NO reactions over Rh.

ASSOCIATED CONTENT Supporting Information. 20


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Rh particle size effects on the TOF, photographs of honeycombs, thickness of the metal overlayer, XPS, catalytic activities, TEM, FPMD simulations, and kinetic analysis. This information is available free of charge via http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author * Masato Machida, Professor Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555 Japan, Fax: 096-342-3651 E-mail: [email protected]

Author Contributions The manuscript was written with contributions from all named authors. Each author has approved the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS

21



Part of this work was supported by the Ministry of Education Culture, Sports, Science and Technology (MEXT) program, “Elements Strategy Initiative to Form Core Research Center” (since 2012), which is run by MEXT, Japan.

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Table 1 TOFs for the CO–NO reaction of metal overlayer and nanoparticle catalysts.

Catalyst

Honeycomb substrate

Structure

Surface metal

NO conversion e

TOF

/µg

/µmol

/%

/min−1

Rh/SUS

Overlayer, a 3-nm thick

SUS foil

85.5

0.0504 c

23.2

186

Rh/ZrO2

Nanoparticle, b 6.3-nm size

Cordierite

200

0.340 d

11.0

13.1

Pd/SUS


SUS foil

72.0

0.0482 c

0.8

6.5

Cordierite

200

0.733 d

11.3

6.3

Pd/Al2O3 Nanoparticle, b 2.9-nm size

a

Metal loading

Pt/SUS


SUS foil

42.8

0.0474 c

0

0

Pt/Al2O3

Nanoparticle, b 7.4-nm size

Cordierite

200

0.174 d

0

0

Prepared by 1000 shots of AP pulses. The area of a foil is 10 mm × 190 mm. The surface coverage

is 100%. b

50 mg of catalyst powder with 0.4 wt% metal loading coated onto a cordierite honeycomb

substrate. The catalyst was reduced in 5% H2/He at 200 °C prior to catalytic reactions. c

Determined using the actual surface area of a foil (10 mm × 190 mm), surface coverage (100%)

and atomic density of the Rh(111), Pd(111), and Pt(111). The calculated fraction of exposed Rh per total Rh is about 6%. d

Determined using the metal loading (0.20 mg) and the metal dispersion (Rh:17.5%, Pd:38.5%,

Pt:15.1%). e

Steady-state NO conversion for the CO–NO reaction at 280 °C (0.1% NO, 0.1% CO, and N2

balance at 1.0 L min−1), GHSV = 1.2 × 105 h−1 for SUS or 0.76 × 105 h−1 for cordierite.

29



Page 30 of 38

Table 2 Kinetic parameters of the CO–NO reaction for the metal overlayer and nanoparticle catalysts. Ea

Reaction order for

Reaction order for

NO reduction rate

CO oxidation rate

a

Catalyst

−1

/kJ mol

NO

CO

NO −0.11 b

CO

Rh/SUS

Overlayer

112

0.058 b

0.096 b

Rh/ZrO2

Nanoparticle

126

0.18 b

0.20 b

Pd/SUS

Overlayer

90

0.41 c

−0.21 c

0.31 c

−0.14 c

Pd/Al2O3

Nanoparticle

105

0.55 c

−0.13 c

0.33 c

−0.18 c

0.062 b

0.10 b 0.28 b

a

Activation energy calculated from NO reduction rates at a temperature range of 230–290 °C.

b

Reaction temperature: 245 °C.

c

Reaction temperature: 280 °C.

30


Page 31 of 38

Figures

100

CO-NO TOF / min-1

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


10

1

CO-O2 0.1 1

100

10000

Rh particle size / nm Figure 1. TOFs for the CO–NO and CO−O2 reactions over Rh catalysts with different particle sizes (See Supporting Information, Table S1 for details).

31



100

100

Rh

80

Conversion / %

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

60

Rh/SUS

40

Rh/ZrO2

100

Pd

80

NO CO

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80

60

NO CO

40

Pd/Al2O3

NO CO

60 40

20

20

Pt

Pt/SUS

20

Pt/Al2O3

Pd/SUS

0

0 100

200

300

400

Temperature / °C

500

0

100

200

300

400

Temperature / °C

500

100

200

300

400

500

Temperature / °C

Figure 2. Light-off curves for the CO–NO reaction over the honeycomb catalysts. The metal honeycomb consisted of 1000-pulses for Rh/SUS, Pd/SUS, and Pt/SUS. The cordierite honeycomb coated with the powdered catalysts (0.4 wt% Rh/ZrO2, Pd/Al2O3, and Pt/Al2O3) after being reduced at 200 °C for 1 h in 5% H2/He.

32


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100

(a)

NO

(c)

80

Conversion / %

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


CO

60

Ra=4 nm 40

(b) 20 0 200

300

400

500

600

Temperature / ˚C Ra=500 nm

Figure 3. Confocal laser scanning microscope images of the Rh overlayers as deposited onto (a) mirror-polished and (b) unpolished SUS. Ra values (insert) indicate surface roughness. (c) Catalytic light-off curves of the CO–NO reaction over the Rh overlayer formed by 100 AP pulses on 2 nm × 30 mm SUS foil with and without mirror polishing treatment (0.1% NO, 0.1% CO, He balance, 0.1 L min−1).

33



*

Intensity / a.u.

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

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*

(a)

×800

(b)

×800 111 200

220

(c) 30

40

50

60

70

80

2q / degree

Figure 4. XRD patterns of the Rh overlayers deposited onto (a) mirror-polished and (b) unpolished SUS and (c) a polycrystalline Rh foil. Peaks marked with an asterisk are from the SUS foil.

34


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


(a) Rh(111) On-top Eads = 1.96 eV

Bridge Eads = 2.23 eV

Three-fold hollow Eads = 2.34 eV

(b) Rh55 cluster On-top Eads= 2.68 eV

Bridge Eads= 2.66 eV

Hollow (100) Eads= 2.64 eV

Hollow (111) Eads= 2.86 eV

Figure 5. Sites and energies used to simulate NO adsorption on (a) Rh(111) and (b) Rh55 cluster surfaces. N, O, and Rh atoms are represented by blue, red, and pink spheres, respectively.

35



(b)

(a) T/°C

T/°C

-90

-90 -80

-80

-70 -70

Intensity / a.u.

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

Page 36 of 38

-60

-60

-50

-50

-40 -40 -30 -30

1950

1850

1750

1650

1550

1450

1350

-20 1950

1850

Wavenumber / cm-1

1750

1650

1550

1450

1350

Wavenumber / cm-1

Figure 6. In situ FTIR spectra of NO adsorbed species on (a) Rh overlayer (Rh/SUS) and (b) Rh nanoparticles (Rh/SiO2) detected at different temperatures and measured in a stream of 1% NO balanced with He.

36


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


NO s → N s + O s

(a) Rh(111) Hollow

2N s + 2O s → N2 s + 2O s

Ea=1.65 eV

0.0 eV

Ea=1.76 eV

-0.48 eV

0.0 eV -1.60 eV

(i)

(ii)

(b) Rh55 cluster Bridge

(iii)

(iv)

Bridge

Ea=2.18 eV

Ea=1.00 eV 0.0 eV

-0.09 eV

0.0 eV

Hollow -0.81 eV -1.59 eV

(i)

(ii)

(iii)

(iv)

(v)

Figure 7. Formation energy profiles for the intermediates in the NO reduction on (a) Rh(111) and (b) Rh55 cluster surfaces. Note that (i and ii) is the NO dissociation step and (iii, iv, and v) is the N–N recombination step. N, O, and Rh atoms are represented by blue, red, and pink spheres, respectively.

37



TOC Graphic. CO-NO reaction 1000

Rh overlayer

100

TOF / min-1

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

10

1

Rh nanoparticle

0.1 1

100

10000

Rh particle size / nm

38


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High Turnover Frequency COâNO Reactions over Rh Overlayer

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