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Synergistic Promotion of NO-CO Reaction Cycle by Gold and Nickel Elucidated using Well-Defined Model Bimetallic Catalyst Surface Atsushi Beniya, Yasuhiro Ikuta, Noritake Isomura, Hirohito Hirata, and Yoshihide Watanabe ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02714 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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Synergistic Promotion of NO-CO Reaction Cycle by Gold and Nickel Elucidated using Well-Defined Model Bimetallic Catalyst Surface Atsushi Beniya,† Yasuhiro Ikuta,† Noritake Isomura,† Hirohito Hirata,‡ and Yoshihide Watanabe*,† †Toyota Central R&D Labs, Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan ‡

Toyota Motor Corporation, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan

*Corresponding author: [email protected]

ABSTRACT: A bimetallic catalyst of Au and Ni significantly increased the catalytic activity of the NO-CO reaction compared to monometallic Au and Ni catalysts. Unraveling the roles of Au and Ni atoms in the each of NO-CO reaction steps occurring on the Au-Ni catalyst surface is crucial to reveal the origin of the increased activity. For this purpose, a well-defined Au/Ni(111) model catalyst was prepared, on which CO and NO adsorption, their co-adsorption, NO dissociation, CO 2 formation, and N 2 formation were investigated using infrared reflection absorption spectroscopy, temperature programmed desorption/reaction, and density functional theory calculations. In the reaction process, the catalyst surface would be dominantly covered by

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N and O atoms, which would be removed from the surface by N 2 formation and CO 2 formation. O atoms preferentially occupy the Ni hollow sites by segregating N atoms to the adsorption sites made up of Au and Ni atoms. Thus, only the N 2 formation step was affected by the Au atoms. The activation energy for the N 2 formation step, which was assigned as a rate limiting step, was significantly lowered by the Au atoms, and this effect will contribute to the decrease of the activation energy of the overall NO-CO reaction. These results suggest that, by utilizing the adsorption site preferences among the co-adsorbates on the bimetallic surface, the activation energy of a rate limiting step would be significantly decreased, and could be useful in the development of advanced NOx reduction catalysts.

KEYWORDS: NO reduction, CO oxidation, Au catalysis, alloy catalysis, automotive catalysis

1. INTRODUCTION Exhaust gas purification catalysts are necessary to remove CO, hydrocarbons, and NOx simultaneously from the exhaust of internal combustion engines, known as a three-way catalyst (TWC). NO reduction by CO (NO-CO reaction) is one of the primary TWC reactions.1-4 Exhaust gas purification catalysts contain platinum group metals (PGM), and reactions occur on the PGM surfaces. The total PGM use for catalysts is increasing to meet the more stringent regulations and increasing vehicle production. Given the limited global PGM resources, catalysts with decreased amounts of PGM or alternative technologies are required.


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Figure 1. Catalytic activity for NO reduction by CO over Au-Ni/SiO 2 , Au/SiO 2 , and Ni/SiO 2 . Reaction conditions: 3000 ppm NO, 3000 ppm CO, N 2 balance. Flow rate: 10 L/min. Space velocity: 158000 h-1. Metal loading of catalysts: Au–Ni/SiO 2 ; Au 1.92 wt% and Ni 0.61 wt%, Au/SiO 2 ; Au 3.74 wt%, Ni/SiO 2 ; Ni 1.14 wt%. Total metal loading of all catalysts: 1.94× 10-4 mol/g-cat. [Reproduced/Adapted] with permission from ref. 14. Copyright 2014 Springer.

One of the possibilities to reduce the PGM use is to develop bimetallic catalysts (without PGM), which often show electronic and chemical properties that are distinctly different from those of the original metals;5-7 bimetallic catalysts containing Au have received a lot of attention in recent years.8-9 Among them, bimetallic Au-Ni catalysts show several interesting catalytic properties, such as low temperature CO oxidation activity,10,11 high efficiency in oxidative esterification of aldehydes with alcohols,12 and high activity and robustness in the steamreforming reaction.13 In particular, our group found that bimetallic Au-Ni catalysts supported on SiO 2 showed superior activity for the NO-CO reaction compared to Au and Ni catalysts, as shown in Figure 1.14 Because metallic Ni atoms are necessary for the NO-CO reaction, this activity enhancement was assigned to the increased reducibility of NiO by Au atoms. Note that


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light-off temperature of Au-Ni catalyst (~270 °C) is lower than that of the Ni catalyst (~400 °C). Figure 2 shows typical NO-CO reaction cycle; CO and NO adsorb on the catalyst surface, then NO molecules dissociate to N and O atoms, followed by CO 2 and N 2 formation steps.15 The NOCO reaction cycle starts to proceed above the light-off temperature, thus, the temperature lowering observed in Figure 1 suggests that the Au atoms modify the reaction step(s) in the NOCO reaction cycle. Therefore, investigating the reaction steps should unveil the underlying reaction mechanisms.

Figure 2. NO-CO reaction cycle composed of CO and NO adsorption, NO dissociation, CO 2 formation, and N 2 formation.

The powder Au-Ni catalyst (Figure 1) composed of bimetallic nanoparticles where it was confirmed that Au and Ni atoms coexist in a single nanoparticle.14 But, due to the structural and chemical inhomogeneity of the powder Au-Ni catalyst, the ability to gain insight into the reaction steps that occur on the catalyst surfaces is limited. Chen et al. demonstrated that single crystalline bimetallic surfaces are a good model system of supported catalysts when the single crystalline bimetallic surfaces are stable in reaction processes.16-17 Here, Besenbacher et al. established that a surface science approach using a model catalyst, i.e., a Au/Ni(111) surface alloy, provided an understanding of the elementary reaction steps in the steam-reforming


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reaction using bimetallic Au-Ni catalyst supported on MgAl 2 O 4 and SiO 2 .13, 18-19 Even though Au and Ni are bulk immiscible, it is possible to form a surface alloy with Au atoms substituted into the topmost surface layer of the Ni substrate to maintain the two-dimensional structure of the original Ni surface.13, 18-19 Because Au atoms that are alloyed into the surface layer help to lower the Ni surface energy, Au atoms prefer to reside at the surface layer of the Au-Ni catalyst.13, 18-22 This preferential residence of Au atoms at the surface is consistent with the elemental mapping result of Au and Ni atoms in a single Au-Ni nanoparticle of the powder AuNi catalyst.14 Thus, the Au/Ni(111) surface would be a good model system to investigate the reaction mechanism on the powder Au-Ni catalyst surface. We prepared the well-defined Au/Ni(111) model catalyst and investigated the NO-CO reaction steps shown in Figure 2, i.e., CO and NO adsorption, their co-adsorption, NO dissociation, CO 2 formation, and N 2 formation, to elucidate the effect of Au atoms on the NO-CO reaction by the powder Au-Ni catalyst, based on previous works.13, 18-22 The basis for this work is a surface science approach to gain insight about the catalytic activity of the powder Au-Ni catalyst. We therefore start in Section 2 by giving a description of the model surface preparation and experimental methods used. The reaction steps were analyzed using infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption/reaction (TPD/TPR), and density functional theory (DFT) calculations. In Section 3, we show the experimental results and discuss in detail the effect of the Au alloying on the NO-CO reaction steps (Figure 2). We connect these results to the NO-CO reaction cycle on the powder Au-Ni catalyst.

2. EXPERIMENTAL METHODS


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Experiments were performed in an ultrahigh vacuum chamber with a base pressure below 1 × 10−8 Pa.23 The Ni(111) substrate was 6 × 10 mm and oriented to within 0.1° (Surface Preparation Laboratory). A clean Ni(111) surface was carefully prepared by several cycles of Ar ion sputtering and annealing under vacuum at 1000 K. The temperature was monitored by a Chromel-Alumel (K-type) thermocouple that was spot-welded to the side of the Ni(111) surface.24 The Au/Ni(111) system forms the surface alloy when the Au coverage is below 0.3 ML (1 ML = 1.9 x 1015 atoms/cm2), where Au atoms randomly substitute for the Ni atoms at the topmost surface layer without disturbing the Ni lattice.18 We prepared the model Au/Ni(111) surface by depositing mass-selected Au+ ions onto the Ni(111) surface at 300 K. The Au+ ions were produced by a DC magnetron sputtering source (Angstrom Sciences, ONYX-1) and mass selected using a quadrupole mass filter (Extrel, MEXM-4000).23 The total coverage of the deposited Au, determined from the integrated Au+ neutralization current on the sample, was ~0.1 ML. After the Au deposition, the sample was annealed at 773 K 25(see section 1 in Supporting Information). IRAS measurements were performed using a Fourier transform infrared spectrometer (Bruker IFS66v/S) with a mercury-cadmium-telluride detector. The incident beam was passed through a KRS-5 polarizer to remove the unwanted s-polarized component. All of the spectra were recorded at 4 cm−1 resolution as the average of 200 scans. In the TPD/TPR measurements, the desorbing molecules were detected using a quadrupole mass spectrometer (QMS, Canon Anelva Corp., M-201QA-TDM), and the ionization region was enclosed in a small Ta envelope. The crystal surface was placed in front of the small opening (3


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mm diameter) of this cup with a distance of 1 mm for the TPD/TPR measurements. The heating rate was set to 3.5 K/s. 13

CO (Cambridge Isotope Laboratories, Inc., isotopic purity = 99%), NO (Takachiho Chemical

Industrial Co., Ltd., purity >99.9 %), and O 2 (Takachiho Chemical Industrial Co., Ltd., purity >99.9 %) gases were introduced through a pulse gas dosing system or variable leak valve onto the sample surface, and the purity of each gas was checked using mass spectrometry.

3. RESULTS and DISCUSSION 3.1 CO and NO adsorption CO adsorption on Ni(111) has been extensively investigated.24, 26-31 Figure 3(a) shows the IRAS spectra of 13CO on Ni(111); the CO adsorption and IRAS measurements were performed at 88 K. Peaks for on-top and hollow CO were observed at ~2000 cm-1 and ~1850 cm-1, respectively.24 Figure 3(b) shows the IRAS spectra of 13CO on Au/Ni(111). On-top and hollow CO molecules binding to Ni atoms were observed, but CO molecules binding to Au were not observed (a frequency of about ~2060 cm-1 is typical on the Au surface).8 The integrated intensity of the hollow and on-top peaks is shown in Figures 3(c) and 3(d), respectively, as a function of exposure. The peak intensity of the hollow CO on the Au/Ni(111) surface is smaller than that on the Ni(111) surface, which is a result of the disappearance of the available Ni adsorption sites.25 The peak intensity of the on-top CO started to appear at >10 shots on Ni(111) (Figure 3(d)), and the appearance of the on-top CO peak was attributed to CO island formation.24 In contrast, the on-top CO peak was observed from the initial stage of the adsorption on Au/Ni(111). The Au atoms interacted repulsively with the CO molecule,25, 32-35 suggesting that the lateral motion of the adsorbed CO molecules is limited. Thus, one possible explanation


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for the above result is that the CO molecules formed an island at the initial stage of adsorption. However, the hollow CO peaks were observed at the same frequency as that on Ni(111) (Figure 3(a) and 3(b)). Because the hollow CO frequency is sensitive to the island formation,24 the same frequency of hollow CO indicates that the local coverage is not high enough to produce on-top CO on Au/Ni(111). At >10 shots, on-top CO peak intensity on Au/Ni(111) increases with similar gradient with that on Ni(111) indicating that the CO island formation occurred only at high coverage region.

Figure 3. IRAS spectra of 13CO on (a) Ni(111) and (b) Au/Ni(111) as a function of exposure. CO dosing and all IRAS measurements were performed at 88 K. The integrated absorbance of (c) hollow CO and (d) on-top CO on Ni(111) (squares) and Au/Ni(111) (filled circles) as a function of exposure.


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Figure 4. IRAS spectra of

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CO on Ni(111) (lower) and on Au/Ni(111) (upper) after annealing the CO

saturated surface at 290 K. The CO adsorption and IRAS measurements were performed at 88 K.

An alternative interpretation is that the on-top CO was stabilized by the Au atoms, as judged by investigating the thermal stability of the CO molecules. Figure 4 shows IRAS spectra after annealing the CO saturated surface at 290 K. Only hollow CO molecules were observed on the Ni(111) surface.24, 26-27 In contrast, the on-top CO peak was observed at 2009 cm-1 on Au/Ni(111) beside that for the hollow CO molecules at 1862 cm-1. Therefore, the on-top CO would be stabilized by the presence of Au (on-top CO, which may adsorb at the next nearestneighbor Ni site of Au, became more stable by 23 meV, see section 2 in Supporting Information). After the annealing, the peak intensity of the hollow CO was less than that on Ni(111), indicating that Au decreased the number of CO adsorption sites (Ni hollow sites), as previously reported.25 This is consistent with the result of the DFT calculation that the adsorption energy at the hollow site largely decreased due to the presence of Au (section 3 in Supporting Information). Thus, the Au atom stabilized the on-top CO and decreased the number of Ni hollow sites. Because the degree of stabilization of the on-top CO was very small (only 23 meV), the dominant effect of


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the Au atoms on the CO adsorption would be the decreased number of CO adsorption sites. The adsorption energy of CO on Au/Ni(111) was estimated using TPD (which corresponds to the average adsorption energy of the adsorbed CO molecules) and decreased with increasing Au coverage.25 Next, we investigated the NO adsorption on Ni(111) and Au/Ni(111). Figure 5(a) shows the IRAS spectra of NO on Ni(111) as a function of exposure. Hollow NO peaks were observed at ~1550 cm-1. At the saturation coverage, hollow NO molecules that form a c(4 × 2)-2NO structure (θ NO = 0.5 ML) were observed at 1596 cm-1.36, 37 The NO molecules adsorbed only on the hollow sites of Ni(111), unlike that observed for CO/Ni(111).36, 38-40 Figure 5(b) shows the IRAS spectra of NO on Au/Ni(111) as a function of exposure. The hollow NO peak was observed at ~1550 cm-1, but the peak intensity was smaller than that on Ni(111). Thus, the Au atoms decreased the number of adsorption sites, which is consistent with the result of the DFT calculations (section 3 in Supporting Information). A new NO peak was observed at 1799 cm-1, which was assigned to the on-top NO.36, 41-42 The peak for on-top NO on the Au surface was observed at 1810 cm-1,41-42 and was observed at 1800–1880 cm-1 on Ni(111).36 TPD measurements were performed to determine whether the on-top NO was bound to Au or Ni atoms. Figure 5(c) shows the NO TPD spectra for Ni(111) (black) and Au/Ni(111) (gray). The hollow NO desorption peak was observed at ~400 K, both on the Ni(111) and on Au/Ni(111). Desorption peak observed at ~300 K on Au/Ni(111) corresponds to the desorption of the on-top NO. Because NO desorbed from the Au surface at 150 K,41-42 we assigned the adsorption site of the on-top NO to the Ni atom. As expected from the CO adsorption result, the Au atom decreased the number of hollow NO adsorption sites and induced the appearance of the on-top NO on Ni. Because the on-top NO desorption was observed at a lower temperature than that of the hollow NO, the average


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adsorption energy would be decreased by the Au atoms. Therefore, the Au atoms decreased the number of NO adsorption sites and the adsorption energy. Note that the experimental temperature is lower than the reaction temperature of the powder Au-Ni catalyst (Figure 1). Catalytic reactions proceed along the most stable adsorption states, which can be elucidated by low-temperature experiments. Thus, the adsorption states observed experimentally would be the dominant adsorption state at the reaction temperature.

Figure 5. IRAS spectra of NO on (a) Ni(111) and on (b) Au/Ni(111) as a function of exposure. NO dosing and all IRAS measurements were performed at 88 K. (c) TPD spectra of saturated NO on Ni(111) (black) and on Au/Ni(111) (gray). The Ni(111) and Au/Ni(111) surfaces were saturated by NO at 88 K followed by TPD measurements. The heating rate was set to 3.5 K/s. The QMS sensitivity was not calibrated, and so the intensity of each spectrum was normalized.

3.2 Co-adsorption of CO and NO


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The adsorption states of CO24, 26-31 and of NO36, 38-40 on Ni(111) surfaces have been reported extensively in the literature. However, only a few studies have examined the co-adsorption of NO and CO.43-44 Basic knowledge of NO-CO co-adsorption states is an important step toward the understanding of the NO-CO reaction mechanism.15 Co-adsorbed NO induced the siteconversion of CO from hollow to on-top sites.43-44 In our previous study, we found that the siteconversion of CO from hollow to on-top sites occurred because of local interactions between CO and NO, which decreased the adsorption energy of CO from 1.1 eV at the hollow site to 0.6 eV at the on-top site.44 Because of the CO destabilization, adsorbed CO molecules were easily displaced by post-adsorbed NO above 300 K.44 These results are consistent with those performed at an ambient pressure for a silica-supported Ni catalyst.45

Figure 6. IRAS spectra of the co-adsorbed NO and

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CO on Au/Ni(111) as a function of NO exposure on

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the CO saturated surface. Adsorption and IRAS measurements were performed at 300 K.

We investigated the co-adsorption of NO and CO on the Au/Ni(111) surface. Figure 6 shows IRAS spectra of co-adsorbed CO and NO on Au/Ni(111). The Au/Ni(111) surface was first


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saturated with 13CO followed by NO adsorption. Adsorption and IRAS measurements were performed at 300 K. With increasing NO exposure, the peak intensities of the on-top CO (~2040 cm-1) and hollow CO (~1900 cm-1) decreased, and those of the hollow NO (1600–1489 cm-1) increased. At an NO exposure of 4.2 L, a new peak at 1874 cm-1 was observed. This peak was due to the on-top NO molecules interacting with the chemisorbed O atoms produced by NO dissociation.36 Above 8.2 L, only the NO peaks were observed. Therefore, as previously reported on Ni(111),44 the CO saturated surface was completely displaced by post-dosed NO molecules. When NO molecules impinged on the CO covered surface, they adsorbed on the hollow sites by inducing the site-conversion of hollow CO to on-top CO. Because of the site-conversion, CO molecules were destabilized, which resulted in CO desorption. By repeating the process, the CO molecules that covered the surface were easily displaced by NO. This result suggests that NO molecules and their derivatives, such as N and O atoms, would be dominant adsorbates on the Au-Ni catalyst surface at NO-CO reaction conditions.

3.3 NO dissociation In section 3.1, NO molecules were shown to prefer to adsorb at the Ni sites on the Au/Ni(111) surface. Ni is on the traditional borderline in the periodic table between purely molecular and purely dissociative adsorption.39 One previous study reported that NO molecules dissociate above 300 K on Ni(111) at low coverage.38 With increasing coverage, the NO molecules began to desorb around 400 K because NO dissociation requires the existence of empty sites.40, 44 We also investigated the temperature dependence of the IRAS spectra for adsorbed NO at low coverage to gain insight into the NO dissociation on Ni(111) and Au/Ni(111).


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Dissociation temperature of NO (~0.15 ML) on the Ni(111) surface was investigated by measuring IRAS spectra as a function of the annealing temperature, as shown in Figure 7(a). The peak for hollow NO molecules was observed at ~1500 cm-1. With increasing temperature, the peak intensity of the hollow NO decreased. The integrated intensity of the hollow NO peak was plotted as a function of temperature in Figure 7(c). At this low coverage, NO desorption was not observed in TPD,44 and all of the adsorbed NO molecules dissociated on the surface. Thus, the observed intensity decreasing in the IRAS spectrum was due to the NO dissociation.

Figure 7. IRAS spectra of NO adsorbed on (a) Ni(111) and on (b) Au/Ni(111) as a function of the annealing temperature. NO molecules (~0.15 ML) were adsorbed at 88 K, and then the surfaces were annealed at the indicated temperatures. The IRAS measurements were performed at 88 K. (c) The normalized integrated intensity of the adsorbed NO molecules as a function of the annealing temperature.


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NO molecules began to dissociate at 300 K, as previously reported.38 Figure 7(b) shows the IRAS spectra of ~0.15 ML NO on Au/Ni(111) as a function of the annealing temperature. As shown in Figure 5(b), NO molecules adsorbed only on the Ni hollow sites at low coverage. The temperature dependence of the integrated intensity is shown in Figure 7(c). The peak intensity started to decrease at around 300 K, which was the same behavior as that observed on Ni(111). On the Au/Ni(111) surface, NO molecules dissociated on the Ni hollow sites, indicating that Au atoms would not affect the rate constant of the NO dissociation step. However, the NO dissociation step required at least the presence of an adjacent empty site for N or O atoms. The NO dissociation rate would be decreased by the presence of Au atoms because Au atoms decrease the number of adsorption sites for O atoms46 and NO molecules.

3.4 CO 2 formation O atoms produced by the NO dissociation would react with CO molecules, and they would be consecutively removed from the surface as CO 2 molecules (Figure 2). We investigate the CO 2 formation from chemisorbed CO and O on Ni(111) and Au/Ni(111). First, the co-adsorption states of CO and O were studied using IRAS. Figure 8 shows the IRAS spectra of saturated 13CO on the Ni(111) and Au/Ni(111) surfaces pre-adsorbed by O (~0.15 ML). To adsorb O, the clean Ni(111) and Au/Ni(111) surfaces were exposed to O 2 molecules at 88 K followed by annealing at 500 K. After the O adsorption, the surfaces were saturated by 13CO at 88 K. On the Ni(111) surface, a new peak was observed at 2049 cm-1 in addition to the on-top CO peak (2023 cm-1) and hollow CO peak (1800–1900 cm-1). The IRAS spectrum corresponded reasonably well with the previous report, and the new peak was assigned to the on-top CO


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(designated as CO*) adsorbed on the Ni atoms, which are the nearest neighbors to the O atoms adsorbed on the hollow sites.47 The peak intensity of the hollow CO was about half of that on the clean Ni(111) surface (Figure 3(a)) because the co-adsorbed O atoms blocked the adsorption sites for the hollow CO molecules. The small peak at 2077 cm-1 would be adsorbed CO molecules on NiO.48 Note that when oxygen atoms adsorb (chemisorb) on the Ni surface, surface Ni atoms become slightly cationic. On the other hand, the oxidation state of Ni atoms in Ni oxide (NiO) is +2. This difference can be identified by the peak position of adsorbed CO (2049 and 2077 cm-1), as shown in Figure 8.

Figure 8. IRAS spectra of saturated 13CO on the O pre-adsorbed Ni(111) and Au/Ni(111) surfaces. The O preadsorbed (~0.15 ML) surfaces were prepared by exposing the clean surfaces to O 2 molecules at 88 K, followed by annealing at 500 K. The surfaces were then saturated by 13CO at 88 K. IRAS measurements were performed at 88 K.

On the Au/Ni(111) surface, the on-top CO peaks were observed at the same position as those on Ni(111). The intensity of the 2023 cm-1 peak is higher than that for Ni(111), which is consistent with the results described in section 3.1. Two hollow CO peaks were observed at 1862


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and 1827 cm-1. The peak positions of the hollow CO are sensitive to the coverage due to intermolecular interactions.24 This result indicates that the lateral distance between the hollow CO on O/Au/Ni(111) is larger than that on O/Ni(111) because both the Au and co-adsorbed O atoms blocked the adsorption sites of the hollow CO molecules.

Figure 9. TPR spectra of CO 2 from O/Ni(111) (upper) and O/Au/Ni(111) (lower) under a

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CO pressure. O

pre-adsorbed (~0.15 ML) surfaces were prepared by exposing the clean surfaces to O 2 molecules at 88 K followed by annealing at 500 K. Then, the TPR measurements were performed at P(CO) = 1 ×10-4 Pa. The heating rate was 3.5 K/s. The QMS sensitivity was not calibrated, and so the intensity of each spectrum was normalized. The fitted results by the Arrhenius equation (CO 2 desorption rate = C exp(−E/ k B T)) at the leading edge region are shown as red curves. The E were estimated as 0.39 eV on Ni(111) and 0.33 eV on Au/Ni(111).

Next, we analyzed the CO 2 formation using TPR. However, as reported previously, CO 2 desorption was not observed from the co-adsorbed layer of CO and O both on Ni(111)47-49 and on Au/Ni(111).50-51 The absence of CO 2 desorption would be due to the CO molecules desorbed from the surface before they reacted with O in the TPR process, indicating that the activation energy for CO desorption was smaller than that for CO 2 formation. Thus, we performed TPR


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measurements under CO pressure to maintain the CO coverage above the CO desorption temperature. First, O atoms (~0.15 ML) were adsorbed by exposing the clean surfaces to O 2 molecules at 88 K followed by annealing at 500 K. Then, CO 2 TPR measurements were performed under a 13CO pressure of 1×10-4 Pa. Figure 9 shows the CO 2 TPR spectra from the O pre-adsorbed Ni(111) and Au/Ni(111) surfaces. From both surfaces, the CO 2 molecules started to desorb at ~350 K, indicating that the CO 2 formation reaction occurred according to the same mechanism on both surfaces: CO and O adsorbed on the Ni sites reacted. By fitting the leading edge region using the Arrhenius equation (CO 2 desorption rate = C exp(−E/ k B T) where E is apparent activation energy, k B is the Boltzmann constant, T is the temperature, and C is constant),52 the apparent activation energy (E) was estimated to be ~0.36 eV on both surfaces (0.39 eV on Ni(111) and 0.33 eV on Au/Ni(111), the fitted results are shown as red curves in Figure 9). The obtained apparent activation energy (~0.36 eV) could be derived from several elementary steps as discussed below.53 The CO oxidation kinetics was considered to determine the activation energy for the CO 2 formation step (i.e., CO + O → CO 2 ). It is generally accepted that the CO oxidation reaction followed a Langmuir-Hinshelwood mechanism,53 CO (g) → CO (a)

(R1)

CO (a) → CO (g)

(R2)

CO (a) + O (a) → CO2 (g)

(R4)

O2 (g) → 2O (a)

(R3)

where (g) and (a) represent the gas and adsorbate, respectively. The elementary steps of (R1) and (R2) represent the adsorption and desorption of CO. (R3) is the dissociative adsorption of O 2 (O 2 molecules dissociatively adsorb on Ni(111) and Au/Ni(111) above 300 K).50 The O 2 desorption is negligible because O atoms do not desorb from the surface but diffuse into the substrate above


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600 K.54 (R4) is CO 2 formation, followed by desorption. At steady state, the reaction rate (r) becomes, 53 𝑟𝑟 =

𝑑𝑑𝑃𝑃CO2 𝑘𝑘2 𝑘𝑘3 𝑃𝑃O2 𝑘𝑘2 𝑘𝑘3 = 𝑘𝑘4 𝜃𝜃CO 𝜃𝜃O = �1 − � 𝑑𝑑𝑑𝑑 𝑘𝑘2 + 𝑘𝑘1 𝑃𝑃CO − 𝑘𝑘3 𝑃𝑃O2 𝑘𝑘4 α

α = 𝑘𝑘1

𝑃𝑃CO − 𝑘𝑘3 𝑃𝑃O2

(1)

(2)

where θ x (X = CO or O) is the adsorbate coverage, and k i is the rate constant for each of the elementary steps (R1)~(R4). k i relates to the activation energy (E i ) and pre-exponential factor (ν i ) via k i =ν i exp(−E i /k B T). It is difficult to directly evaluate the rate constant for CO 2 formation (k 4 ) from the experiment using equations (1) and (2). In this experiment, the O atoms were preadsorbed. This controlled procedure largely simplifies the reaction rate equation. Using the steady state approximation for adsorbed CO from (R1), (R2), and (R3), the CO coverage and reaction rate become 𝜃𝜃CO =

𝑟𝑟 =

𝑘𝑘1 𝑃𝑃CO 𝑘𝑘2 + 𝑘𝑘4 θO

(3)

𝑘𝑘4 𝑘𝑘 𝑃𝑃 𝜃𝜃 𝑘𝑘2 + 𝑘𝑘4 𝜃𝜃O 1 CO O

(4),

respectively. Here, because the activation energy for CO desorption would be smaller than that of CO 2 formation, as described above, CO desorption would be much faster than the CO 2 formation, i.e., k 2 >> k 4 θ O , which further simplifies equation (4) as 𝑟𝑟 ≈

𝑘𝑘4 𝑘𝑘 𝑃𝑃 𝜃𝜃 𝑘𝑘2 1 CO O

(5)

At the leading edge region in the CO 2 TPR (Figure 9), only a small amount of CO 2 molecules were formed. Thus, the small amount of adsorbed oxygen atoms reacted, which indicates that the amount of adsorbed oxygen atoms (θ O ) would be almost constant.52 In addition, CO adsorption is


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Figure 10. Energy diagram for the CO 2 formation reaction on the Ni sites. The red, black, and gray spheres represent O, C, and Ni atoms, respectively.

a non-activated process, meaning k 1 is constant.31 Thus, k 1 P CO θ O in equation (5) would be considered as a constant. As a result, ln(𝑟𝑟) = −

𝐸𝐸4 − 𝐸𝐸2 + const. 𝑘𝑘B 𝑇𝑇

(6)

was obtained. Therefore, the apparent activation energy estimated from Figure 9 would be considered as the difference of the activation energy between CO 2 formation (E 4 ) and CO desorption (E 2 ); E 4 − E 2 = 0.36 eV. The energy diagram of the CO 2 formation process is illustrated in Figure 10. The adsorption energy (desorption activation energy) of CO (E 2 ) was estimated to be 1.1 eV from our previous reports.31, 44 CO and O adsorbed at the nearest neighbor sites to create a bond between them, at which the CO adsorption energy decreased to 0.73 eV (adsorption energy of CO*, designated as E 2 * in Figure 9).47 The activation energy for CO 2 formation (E 4 ) was estimated to be 1.09 or 1.46 eV, depending on the CO adsorption sites. This estimation is in good agreement with the result of recent DFT calculations (0.79–1.43 eV).49


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3.5 N 2 formation N atoms produced by NO dissociation would desorb from the surface as N 2 molecules (Figure 2). We investigated the N 2 desorption from Ni(111) and Au/Ni(111) using TPD. Figure 11 shows the N 2 desorption from Ni(111) and Au/Ni(111), where the surfaces were first saturated by NO at 88 K followed by TPD measurements. As previously reported, N 2 molecules start to desorb at 700 K, and the peak maximum was observed at 802 K on the Ni(111) surface. 55 On the Au/Ni(111) surface, the desorption peak maximum was slightly shifted to a low temperature (783 K), and a shoulder peak was observed at a similar temperature for Ni(111) (~800 K). Besides these desorption peaks, low temperature desorption was also observed on Au/Ni(111); N 2 molecules started to desorb at 550 K. Using the leading edge analysis method,52 the activation energy was estimated to be ~1.0 eV. It was estimated to be ~1.7 eV on the Ni(111) surface. Because the ~0.7 eV decrease of the activation energy was significantly large, N atoms would have a bond with Au atom; they would adsorb on the hollow sites next to an Au atom.


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Figure 11. TPD spectra of N 2 from NO/Ni(111) (upper) and NO/Au/Ni(111) (lower). The Ni(111) and Au/Ni(111) surface were saturated by NO at 88 K followed by TPD measurements. The heating rate was 3.5 K/s. The QMS sensitivity was not calibrated, and so the intensity of each spectrum was normalized. The N 2 desorption signal was measured by mass 14, where it was confirmed that NO, N 2 O, and NO 2 desorption was not observed in this temperature region.


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Figure 12. (a)−(d) Configurations of the co-adsorbed N and O atoms on the Au/Ni(111) surface. (a) Both N and O atoms adsorbed on the Ni hollow sites, (b) N atom adsorbed on the Ni hollow site and O atom adsorbed on the AuNi hollow site, (c) N atom adsorbed on the AuNi hollow site and O atom adsorbed on the Ni hollow site, and (d) Both N and O atoms adsorbed on the AuNi hollow sites. (e) Energy diagram of configurations (a)(d).

Au atoms did not affect the activation energy at the CO 2 formation step because the O atoms adsorbed on the Ni hollow sites on Au/Ni(111) (section 3.4).46 Here, we considered the reason why only the N atom has a bond to an Au atom when N and O atoms co-adsorbed. On the Au/Ni(111) surface, there are two kind of hollow sites: Ni hollow site and hollow site composed of one Au and two Ni atoms (hereafter we refer to this as the AuNi hollow site). Thus, considering configurations of two adsorbates (N and O atoms) on Au/Ni(111), four types of adsorption configurations would be possible, as shown in Figure 12(a)−(d): (a) both N and O atoms adsorbed on the Ni hollow sites, (b) N atoms adsorbed on the Ni hollow sites and O atoms adsorbed on the AuNi hollow sites, (c) N atoms adsorbed on the AuNi hollow sites and O atoms adsorbed on the Ni hollow sites, and (d) both N and O atoms adsorbed on the AuNi hollow sites.


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Of these configurations, configuration (a) is the most stable because of the adsorption only on the Ni hollow sites. To judge which configuration is more stable among (b)−(d), the adsorption energy difference with respect to configuration (a) was considered. The adsorption energy of configuration (a) (E(a)) could be represented as E(a) = E(N/Ni) + E(O/Ni), where E(N/Ni) and E(O/Ni) represent the adsorption energy of the N and O atoms on the Ni hollow site. In a similar fashion, the adsorption energies of configurations (b)−(d) could be represented as E(b) = E(N/Ni) + E(O/AuNi), E(c) = E(N/AuNi) + E(O/Ni ), and E(d) = E(N/AuNi) + E(O/AuNi), respectively. According to the interpolation principle,56 the adsorption energy at a site with several different metal neighbors to a first approximation is an appropriate average of the contributions from the individual components. In our case, adsorbate at hollow site binds to three surface atoms. On the AuNi hollow site, the adsorbate binds to two Ni atoms and one Au atom; thus, E(X/AuNi) is approximated by 2/3 E(X/Ni) + 1/3 E(X/Au), where X is the adsorbate (X = N or O).56 Using this principle, the binding energy differences of (b)−(d) with respect to (a) become

∆E(b) = E(b) − E(a) = 1/3 [E(O/Au) − E(O/Ni)]

(7)

∆E(c) = E(c) − E(a) = 1/3 [E(N/Au) − E(N/Ni)]

(8)

∆E(d) = E(d) − E(a) = 1/3 [E(O/Au) − E(O/Ni)] + 1/3 [E(N/Au) − E(N/Ni)] = ∆E(b) + ∆E(c) (9), respectively. It is clear that the configuration (d) is the most unstable. The relative stability of configuration (b) (or (c)) is approximated by the adsorption energy difference of the O (N) atom between on the Ni and Au surfaces. According to recent DFT calculations,57 the adsorption energies of the O and N atoms on various substrates have a good correlation, and the adsorption energy difference of the O atom is larger than that of the N atom, which means the value of equation (7) is larger than that of equation (8). Thus, configuration (c) would be more stable than (b). The relative stability of configurations (a)−(d) was summarized in Figure 12(e). This energetical consideration is in agreement with the experimental results. When N and O atoms co-


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adsorb on the Au/Ni(111) surface, they compete to occupy the Ni hollow sites, where the Ni hollow sites may be dominated by O atoms by segregating N atoms to the AuNi hollow sites.

3.6 NO-CO reaction mechanism on the Au-Ni catalyst Based on the above results obtained using the model Ni(111) and Au/Ni(111) surfaces, we will discuss the NO-CO reaction mechanism on the powder Ni and Au-Ni catalyst. Surface Ni atoms are easily oxidized, even at low temperatures of 100 K, in the presence of O 2 molecules,51 and the surface oxide is more stable than its own bulk oxide due to an adhesion energy at the interface between the surface oxide and metal substrate.58 Therefore, the surface of the Ni catalyst would be covered by its own oxides in the reaction process. On the NiO surface, NO molecules are unable to dissociate,55 meaning that the NO-CO reaction cycle (Figure 2) does not proceed on the NiO surface. Therefore, the presence of metallic Ni atoms is a prerequisite for the NO-CO catalytic reaction. The very low catalytic activity that was observed by the Ni catalysts (Figure 1) suggests that NiO dominantly covers the catalyst surface. The small amount of metallic Ni would be a result of O dissolution into the bulk Ni; O atoms start to diffuse from the surface NiO layer into the Ni substrates above 600 K.54 The addition of Au atoms to the Ni catalysts promotes the reduction of NiO and increases the number of metallic Ni atoms at the Au-Ni catalysts surface.14 This result is consistent with previous reports that Au atoms decrease the number of oxygen adsorption sites46 and prohibit NiO formation on the Ni surface.51 Because of the increase of the metallic Ni atoms, the Au-Ni catalyst would have a higher NO-CO reactivity than the Ni catalyst. However, only the presence of metallic Ni atoms cannot explain another important result that the Au-Ni catalyst started to catalyze the NO-CO reaction at lower


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temperatures than the Ni catalyst by ~130 K (the light-off temperature was observed at ~673 K for the Ni catalyst and ~543 K for the Au-Ni catalyst, Figure 1). Comparing the light-off temperatures observed for the powder catalyst with the onset temperature of the desorption and reaction steps on the model catalyst may be useful to consider the reaction mechanism. The onset temperature of CO desorption, NO desorption, NO dissociation, CO 2 formation, and N 2 formation on the model Ni(111) and Au/Ni(111) surfaces are summarized in Figure 13 (the light-off temperatures of the powder catalysts were also indicated). N 2 formation requires the highest temperature in these steps, indicates that the NOCO reaction cycle started to proceed above this onset temperature. Interestingly, the light-off temperatures for the powder Ni and Au-Ni catalyst corresponded reasonably well with the onset temperature of N 2 formation. This result suggests that the rate-limiting step (RLS) in the NO-CO reaction on the Ni and Au-Ni catalysts may be the N 2 formation step.

Figure 13. Onset temperatures for CO and NO desorption, NO dissociation, CO 2 formation, and N 2 formation on the model Ni(111) (blue) and Au/Ni(111) (red) surfaces. The reported values25 of CO desorption temperature on Ni(111) and Au/Ni(111) were used. The light-off temperatures of the NO-CO reaction for the powder Ni and Au-Ni catalysts (see Figure 1) are also indicated by the blue and red bars.


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CO and NO molecules would prefer to adsorb on the Ni sites on the Au-Ni catalyst surface. Adsorbed CO molecules are easily displaced by NO molecules due to the displacement reaction in the NO-CO reaction process where NO and CO molecules coexisted in the gas phase. The NO molecules dissociated to N and O atoms above 300 K where NO desorption is also operative. Thus, N and O atoms would dominantly cover the catalyst surface. These atoms would be removed from the surface by N 2 formation and CO 2 formation. Based on the energetic considerations, O atoms would preferentially occupy the Ni hollow sites by segregating N atoms to the AuNi hollow sites. Thus, only the N 2 formation step would be affected. The activation energy of the N 2 formation step, which may be the RLS of the NO-CO reaction, was largely lowered by the Au atoms. This effect will contribute to decrease the activation energy of the overall NO-CO reaction. The role of Au atoms in Au-Ni catalyst can be summarized as follows. First, Au atoms prefer to reside on the surface,20-21 and suppress the formation of NiO14, 51 which hinders the NO dissociation step.55 Due to the promotion of metallic Ni formation, the NO-CO reaction cycle proceeds on the Au-Ni catalyst surface. Second, because Au has a deeper d-band than Ni, binding energy to the Au atom is lower than that to the Ni atoms.56, 59 This effect would contribute to decrease the activation energy of the N 2 formation step and lower the light-off temperature of the NO-CO reaction. Third, due to the adsorption energy decrease, some adsorbates could not adsorb at sites adjacent to the Au atoms, known as the site-blocking effect. The reaction rates of surface reaction depend on the adsorbate coverages; thus, this effect contributes to decrease the reaction rates. However, the suppression of NiO formation would have a stronger effect on the reaction rate than the site-blocking effect because the NO-CO reaction cannot proceed on the NiO surface. Note that the site-blocking effect could also


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decrease the activation energy of the overall NO-CO reaction (see section 4 in Supporting Information), which also supports the lowering of the light-off temperature.

4. CONCLUSIONS The roles of Au and Ni atoms on the NO-CO reaction for a powder Au-Ni catalyst were obtained by investigating CO and NO adsorption, their co-adsorption, NO dissociation, CO 2 formation, and N 2 formation on the model Ni(111) and Au/Ni(111) surfaces. As previously reported, prohibiting the oxide formation or creating the metallic atoms is a prerequisite for the NO-CO reaction. CO and NO molecules would prefer to adsorb on the Ni sites. At the NO-CO reaction condition where NO and CO molecules coexist in the gas phase, the adsorbed CO molecules were easily displaced by NO molecules due to the displacement reaction. NO molecules dissociated to N and O atoms above 300 K where NO desorption is also operative. Thus, N and O atoms would dominantly cover the catalyst surface. N and O atoms would be removed from the surface by N 2 formation and CO 2 formation. Based on the energetic considerations, O atoms would preferentially occupy the Ni hollow sites by segregating N atoms to the AuNi sites. Thus, only the N 2 formation step would be affected by the Au atoms. The activation energy of the N 2 formation step, which may be the RLS, was largely lowered. This effect will contribute to decrease the activation energy of the overall NO-CO reaction.

ASSOCIATED CONTENT Supporting Information. Characterization of Au/Ni(111), adsorption states of CO on the Au/Ni(111) surface, DFT calculation results of CO and NO on the Ni(111), Au(111) and


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Au/Ni(111) surfaces, consideration of the NO-CO reaction kinetics. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Yoshihide Watanabe ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ABBREVIATIONS DFT, density functional theory; IRAS, infrared reflection absorption spectroscopy; PGM, platinum group metals; QMS, quadrupole mass spectrometer; RLS, rate-limiting step; TPD, temperature-programmed desorption; TPR, temperature-programmed reaction; TWC, three-way catalyst


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(42) McClure, S. M.; Kim, T. S.; Stiehl, J. D.; Tanaka, P. L.; Mullins, C. B. J. Phys. Chem. B 2004, 108, 17952-17958. (43) Chen, J. G.; Erley, W.; Ibach, H. Surf. Sci. 1990, 227, 79-89. (44) Beniya, A.; Isomura, N.; Hirata, H.; Watanabe, Y. Surf. Sci. 2013, 613, 28-34. (45) Morrow, B. A.; Sont, W. N.; Stonge, A. J. Catal. 1980, 62, 304-315. (46) Leon, C. C.; Lee, J. G.; Ceyer, S. T. J. Phys. Chem. C 2014, 118, 29043-29057. (47) Xu, Z.; Surnev, L.; Uram, K. J.; Yates, J. T. Surf. Sci. 1993, 292, 235-247. (48) Guo, X. C.; Yoshinobu, J.; Yates, J. T. J. Chem. Phys. 1990, 92, 4320-4326. (49) Peng, G. W.; Merte, L. R.; Knudsen, J.; Vang, R. T.; Laegsgaard, E.; Besenbacher, F.; Mavrikakis, M. J. Phys. Chem. C 2010, 114, 21579-21584. (50) Lahr, D. L.; Ceyer, S. T. J. Am. Chem. Soc. 2006, 128, 1800-1801. (51) Knudsen, J.; Merte, L. R.; Peng, G. W.; Vang, R. T.; Resta, A.; Laegsgaard, E.; Andersen, J. N.; Mavrikakis, M.; Besenbacher, F. ACS Nano 2010, 4, 4380-4387. (52) Miller, J. B.; Siddiqui, H. R.; Gates, S. M.; Russell, J. N.; Yates, J. T.; Tully, J. C.; Cardillo, M. J. J. Chem. Phys. 1987, 87, 6725-6732. (53) Xu, X. P.; Goodman, D. W. J. Phys. Chem. 1993, 97, 7711-7718. (54) Tyuliev, G. T.; Kostov, K. L. Phys. Rev. B 1999, 60, 2900-2907. (55) Yoshinobu, J.; Guo, X. C.; Yates, J. T. J. Chem. Phys. 1990, 92, 7700-7707.


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(56) Bligaard, T.; Nørskov, J. K. In Chemical Bonding at Surfaces and Interfaces,; Nilsson, A.; Pettersson, L. G. M.; Nørskov, J. K., Eds.; Elsevier: Amsterdam, 2008; p 255. (57) Montemore, M. M.; Medlin, J. W. J. Am. Chem. Soc. 2014, 136, 9272-9275. (58) Campbell, C. T. Phys. Rev. Lett. 2006, 96, 066106. (59) Hammer, B.; Nørskov, J. K. Adv. Catal. 2000, Volume 45, 71-129.

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Figure 1 36x43mm (150 x 150 DPI)


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Figure 3_revised 83x83mm (300 x 300 DPI)


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Figure 5 83x103mm (300 x 300 DPI)


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Figure 7 83x124mm (300 x 300 DPI)


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Figure 11 84x88mm (300 x 300 DPI)


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Figure 13 84x50mm (300 x 300 DPI)


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Synergistic Promotion of NO-CO Reaction Cycle by ... - ACS Publications

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