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Structure sensitivity of low-temperature NO decomposition on Au surfaces
Journal of Catalysis 304 (2013) 112–122
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Structure sensitivity of low-temperature NO decomposition on Au surfaces Zongfang Wu a,b,c, Lingshun Xu a,b,c, Wenhua Zhang b,d,⇑, Yunsheng Ma c, Qing Yuan a,b,c, Yuekang Jin a,b,c, Jinlong Yang a,c, Weixin Huang a,b,c,⇑ a
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, China Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China d Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China b c
a r t i c l e
i n f o
Article history: Received 3 February 2013 Revised 17 April 2013 Accepted 21 April 2013 Available online 23 May 2013 Keywords: NO decomposition Au Single-crystal surface Structure sensitivity Structure–activity relation
a b s t r a c t We have comparatively studied adsorption and decomposition of NO on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces by means of TDS, XPS, and DFT theoretical calculation. The lowest-coordinated Au atoms on both surfaces are 7-coordinated, but the surface chemistry of NO differs very much on these two surfaces. An a-NO species dominates on the Au(9 9 7) surface, while besides the similar a-NO species, another less stable and more abundant b-NO species also appear on the Au(1 1 0)-(1 2) surface. Part of a-NO species decomposes into O adatom and N2O upon heating, but the less stable b-NO species exhibits a much higher decomposition reactivity than a-NO species and facilely decomposes into O adatom and N2O on the Au(1 1 0)-(1 2) surface during the NO exposure at 105 K. The accompanying DFT theoretical calculation results demonstrate that chemisorbed (NO)2 dimer species dominate the surface chemistry of NO on the Au surfaces. a-NO species is the most stable (NO)2 dimer species that chemisorbs on the 7-coordinated ridge Au atoms of both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces via the N atoms and exhibits a high activation barrier for the decomposition reaction. b-NO species corresponds to less stable (NO)2 dimer species that chemisorbs on the trench Au atoms of the Au(1 1 0)-(1 2) surface via both N and O atoms and exhibits a low activation barrier for the decomposition reaction. These comprehensive experimental and theoretical calculation results reveal at the molecular level the origin of structure sensitivity and low-temperature catalytic activity of supported Au nanocatalysts in NO decomposition reaction. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Since the first report of Haruta et al. [1], Au nanocatalysis has developed as one of the most exciting research topics in heterogeneous catalysis. Fundamental understanding of complicated heterogeneous catalytic reactions is of great importance and interest but remains challenging [2,3]. Surface science studies of well-defined model catalysts have been successfully developed as an effective strategy [4]. Catalytic reactions catalyzed by supported Au nanoparticles reported by far are structure sensitive, and the catalytic performance of supported Au nanoparticles sensitively depends on their size. This has intrigued extensive relevant fundamental studies employing well-defined Au model catalysts [5–7], but the molecular-level understanding still remains ambiguous. Low-coordinated Au atoms on the surface of supported Au nanoparticles are generally considered to play an essential role in the catalytic activ⇑ Corresponding authors at: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China. E-mail addresses: [email protected] (W. Huang), [email protected] (W. Zhang). 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.04.013
ity and can be modeled with stepped Au single-crystal surfaces. Previous studies have shown that the reactivity of stepped Au single-crystal surfaces depends not only on the geometry and density of surface steps but also on the surface structure of terrace [8–13]. The abatement of NOx (NO and NO2) is an important topic in environmental catalysis. Supported Au nanocatalysts were reported to show excellent catalytic activity toward low-temperature NOx reduction [14–16]. Thus, the adsorption and surface reaction of NOx on various Au single-crystal surfaces have been much investigated both experimentally and theoretically [17– 35]. NO does not adsorb on Au(1 1 1) at the temperature as low as 95 K [17,18], and the calculated adsorption energy of NO on Au(1 1 1) is 20 kJ/mol [19,20], but both experimental and DFT calculation results demonstrated that NO facilely reacted with atomic O on Au(1 1 1) to form NO2(a) [18–20]. On reconstructed Au(1 0 0), NO molecularly adsorbs at 170 K with a desorption activation energy of 57 kJ/mol, resulting in the lift of the hex reconstruction of Au(1 0 0) surface [21]. The enhanced adsorption ability of lowcoordinated Au atoms toward NO has also been demonstrated by DFT theoretical calculations [22–24]. On stepped Au(3 1 0) surface consisting of (1 0 0) terraces and (1 1 0) steps, the formation of
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N2O and O(a) was observed upon NO adsorption even at 80 K [25]. It was proposed that N2O was produced by the reaction of adsorbed NO(a) with atomic N(a) formed by NO decomposition; however, atomic N(a) was not experimentally indentified on the surface. Very recently, Bukhtiyariv et al. [26] employed in situ Xray photoelectron spectroscopy (XPS) to study the decomposition of NO on flat Au(1 1 1) and stepped Au(3 1 0) and (5 3 3) surfaces at elevated NO pressure up to 7 Pa and temperatures between 300 and 500 K. NO does not chemisorb on Au(1 1 1), but evidently dissociates on stepped Au(3 1 0) and (5 3 3) surfaces. These results demonstrate that the Au step sites are responsible for the adsorption and dissociation of NO. Interestingly, NO dissociation on Au(3 1 0) exclusively forms N(a) on the surface whose coverage decreases with the increase in the sample temperature; however, NO dissociation on Au(5 3 3) initially forms N2O(a) on the surface that is replaced by more stable N(a) with the increase of NO exposure and/or sample temperature. However, the underlying mechanism was not identified. Chau et al. [27] employed pulsed field desorption mass spectrometry to study NO adsorption on the Au field emitter tip under NO gas flow condition at 300 K and observed + the desorption of NO+, ðNOÞþ 2 , and N2O from the stepped surface region between the central (1 1 1) pole and the peripheral (0 0 1) plane. They thus concluded that N2O was formed via a (NO)2 dimer mechanism. DFT calculations demonstrated that NO could dissociate to form N2O via the (NO)2 dimer even on the Au(1 1 1) surface [28], but the adsorption energies of NO and (NO)2 dimer on Au(1 1 1) were too small to allow the experimental observation of these species [17–20,28]. On stepped Au(3 2 1) surface, DFT calculations proposed that the presence of hydrogen was a necessary condition for NO dissociation [29]. These previous results demonstrate that NO decomposition on Au surfaces is highly structure sensitive, but the molecular-level understanding lacks. In the present study, we have comparatively investigated the interaction of NO with two stepped Au surfaces, Au(9 9 7) and Au(1 1 0)-(1 2), by means of TDS, XPS, and DFT theoretical calculation. The lowest-coordinated Au atoms on both surfaces are 7-coordinated, but interestingly and unexpectedly, the surface chemistry of NO differs very much on these two surfaces. The accompanying DFT theoretical calculation results reveal that the surface structure-dependent stability and decomposition reactivity of various types of (NO)2 dimer are responsible for the different adsorption and decomposition behaviors of NO on Au surfaces.
2. Experimental section and computational detail All experiments were performed in a Leybold stainless steel ultrahigh vacuum (UHV) chamber with a base pressure of 1.2 1010 mbar [36]. The UHV chamber was equipped with facilities for XPS, UPS, LEED, and differentially-pumped TDS measurements. The Au(9 9 7) and Au(1 1 0) single crystals purchased from MaTeck were mounted on the sample holder by two Ta wires spot-welded to the backside of the sample. The sample temperature could be controlled between 100 and 1273 K and was measured by a chromel–alumel thermocouple spot-welded to the backside of the sample. Prior to the experiments, the Au samples were cleaned by repeated cycles of Ar ion sputtering and annealing until LEED gave a sharp diffraction pattern, and no contaminants could be detected by XPS. NO (>99.9%, Nanjing ShangYuan Industry Factory) was used as received without any further purification, and the purity was further checked by quadrupole mass spectrometer (QMS) prior to experiments. The base pressure of the chamber during the course of NO exposure was controlled to be below 5 1010 torr; therefore, a line-of-sight stainless steel doser (diameter: 8 mm) positioned 2 mm in front of the Au surface was used for relatively
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large NO exposures. The exposures of NO reported herein were corrected with the enhancement effect of the doser (1000) [37]. All exposures were reported in Langmuir (1 L = 1.0 106 torr s) without corrections for the gauge sensitivity. During the TDS experiments, the Au sample was positioned 1 mm away from the collecting tube of a differentially-pumped QMS and heated to 650 K with a heating rate of 3.0 K/s. The signals with m/e = 30 (NO), 46 (NO2), 18 (H2O), 28 (CO and N2), and 44 (N2O and CO2) were monitored. XP spectra were recorded by a hemispherical energy analyzer (PHBIOS 100 MCD, SPECS GmbH) with a pass energy of 20 eV using Al Ka radiation (hm = 1486.6 eV). All the calculations were performed by density functional theory implemented in DMol3 package [38,39]. The double-numeric quality basis set with polarization functions (DNP), DFT semicore pseudopotential (DSPP), and PBE functional [40] were used for all the calculations. The Au(1 1 0)-(1 2) surface was simulated by seven layers gold atoms with the bottom three layers fixed, and the Au(9 9 7) surface was simulated by stepped four layers gold atoms with the bottom two stepped layers fixed. The supercells used for Au(1 1 0)-(1 2) and Au(9 9 7) were (8.394 Å 8.394 Å 26.9032 Å) and (11.871 Å 21.605 Å 24.535 Å), respectively, and the vacuum layer was larger than 15 Å. For the energy calculations, the real space cutoff was set as 4.5 Å, and the (5 5 1) and (4 2 1) k-points samplings were used for Au(1 1 0)-(1 2) and Au(9 9 7), respectively. The transition states of elementary steps were searched by synchronous transit method with conjugated gradient refinements [41]. The core level shift of N 1s and O 1s was calculated by comparing the relative total energy of substation of different nitrogen (oxygen) to oxygen (fluorine) in the same supercell. The isolated adsorbed NO molecule was used as the reference.
3. Results and discussion Fig. 1 shows LEED patterns and schematic structural illustrations of clean Au(9 9 7) and Au(1 1 0) surfaces. Clean Au(9 9 7) vicinal surface exhibits a LEED pattern with splitting spots forming a hexagonal symmetry, similar to the LEED patterns of Pt(9 9 7) and Cu(9 9 7) [42,43]. The (9 9 7) vicinal surface is created by cutting a Au(1 1 1) crystal at an angle of approximately 7° offset to obtain 0 direction separated by atomic height steps along the dense ½1 1 (1 1 1) terraces and is composed of close-packed (1 1 1) terraces and a mono-atomic step with {1 1 1} microfacet. The coordination numbers of atoms on the step and terrace of Au(9 9 7) surface are 7 and 9, respectively. Clean Au(1 1 0) exhibits a sharp (1 2) LEED pattern, a characteristic of reconstructed Au(1 1 0)-(1 2) surface that can be described by a missing-row model with paired rows in the second layer and buckled rows in the third layer. The Au(1 1 0)-(1 2) surface gives rise to on top of row atoms, side of row atoms, and trench atoms whose coordination numbers are 7, 9, and 11, respectively.
Fig. 1. LEED pattern (upper) and schematic structural illustration (bottom) of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces. Ep = 85 eV.
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Fig. 2. NO, N2, and N2O TDS spectra following the saturation exposure (100 L) of NO on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces at 105 K.
Although the lowest-coordinated Au atoms are 7-coordinated on both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces, unexpectedly and very interestingly, the chemisorption and surface reaction of NO differ greatly on these two surfaces. Fig. 2 compares NO, N2O, and N2 TDS spectra following the saturation exposure of NO on both surfaces at 105 K. It is noteworthy that the fragmentation contribution of N2O to the signals of NO and N2 in mass spectroscopy, respectively, amounting to 31% and 11% [44], was subtracted
in all presented NO and N2 TDS spectra in the paper. The as-measured TDS profiles corresponding to Fig. 2 are shown in Fig. S1. The desorption traces of fragments at m/z = 12 and 14 (Fig. S2) support our assignment of the desorption peak of m/z = 28 to either N2 formed as the decomposition product of NO(a) or CO(a) formed due to the chemisorption of residual CO in the UHV chamber. Following the saturating NO exposure at the same temperature, a single NO desorption peak (denoted as a peak) centering at 144 K was observed for the Au(9 9 7) surface; However, for the Au(1 1 0)-(1 2) surface, besides a similar a desorption peak, another stronger b desorption peak centering at 123 K with a shoulder at 116 K was also observed. More striking differences come from the desorption features of N2O and N2 formed as the products of NO decomposition. The N2O and N2 desorption peaks are very obvious for the Au(1 1 0)-(1 2) surface but neglectable for the Au(9 9 7) surface; moreover, the N2O and N2 desorption peaks for the Au(1 1 0)-(1 2) surface are clearly associated with the b-NO desorption peak. These results demonstrate an unexpected structure sensitivity of NO chemisorption and decomposition on Au surfaces because the lowest-coordinated Au atoms on Au(1 1 0)-(1 2) and Au(9 9 7) surfaces are same. It can be thus concluded that surface sites other than the 7-coordinated Au atoms on the Au(1 1 0)-(1 2) surface are most catalytically active in decomposing NO at the saturating NO(a) coverage. LEED observations show that the reconstructed Au(1 1 0)-(1 2) surface is not lifted by NO chemisorption under our experimental conditions. Fig. 3A, B, and C, respectively, show NO, N2O, and N2 TDS spectra following various NO exposures on Au(9 9 7) at 105 K, and Fig. 3D
Fig. 3. (A) NO, (B) N2O, and (C) N2 TDS spectra following various exposures of NO on Au(9 9 7) surface at 105 K: (a) 0 L; (b) 1 L; (c) 5 L; (d) 10 L; (e) 20 L; (f) 50 L; (g) 100 L. (D) Integrated NO, N2O, N2 TDS peak areas and N2O/NO, N2/NO peak area ratios as a function of NO exposure.
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Fig. 4. (A) NO, (B) N2O, and (C) N2 TDS spectra following various exposures of NO on Au(1 1 0)-(1 2) surface at 105 K: (a) 0 L; (b) 1 L; (c) 2 L; (d) 5 L; (e) 10 L; (f) 20 L; (g) 50 L; (h) 100 L. (D) Integrated NO, N2O, N2 TDS peak areas and N2O/NO, N2/NO peak area ratios as a function of NO exposure.
shows the integrated NO, N2O, NO desorption peak areas and the N2O/NO, N2/NO desorption peak area ratios. At the lowest NO exposure (1 L), a single a-NO desorption peak appears at 144 K; meanwhile, a N2O desorption peak and a very tiny N2 desorption peak appear at 133 K. Previous studies have clearly demonstrated that NO does not adsorb on Au(1 1 1) at the temperature as low as 95 K [17,18]. Thus, the a-NO desorption peak can be reasonably assigned to NO species chemisorbed on the (1 1 1) step of Au(9 9 7) surface whose Au atoms are 7-coordinated. This agrees with the general observation that low-coordinated Au atoms exhibit enhanced chemisorption ability. The TDS results also demonstrate that some chemisorbed a-NO species undergoes decomposition reactions upon heating to give rise to the desorption traces of N2O and N2. With the increase of NO exposure, the NO desorption peak grows and a weak shoulder develops, but its peak maxima do not shift; the N2O desorption peak grows and gradually develops into a main peak with two shoulders; the N2 desorption peak initially slightly grows, but then weakens. As shown in Fig. 3D, the chemisorption of NO on Au(9 9 7) at 105 K saturates after an exposure of 50 L NO. We herein employ the N2O/NO and N2/NO desorption peak area ratios to approximate the decomposition reactivity of chemisorbed NO species. It could be seen from Fig. 3D that both N2O/NO and N2/NO desorption peak area ratios decrease rapidly with the increase of NO exposure and thus the surface coverage of chemisorbed NO species on Au(9 9 7). Thus, NO molecules chemisorbed on Au(9 9 7) at a low coverage are more prone to decompose than those at high coverages.
Fig. 4A, B, and C, respectively, show NO, N2O, and N2 TDS spectra following various NO exposures on Au(1 1 0)-(1 2) at 105 K, and Fig. 4D shows the integrated NO, N2O, NO desorption peak areas and the N2O/NO, N2/NO desorption peak area ratios. At the lowest NO exposure (1 L), a single a-NO desorption peak appears at 128 K and can be assigned to NO species chemisorbed on the 7coordinated on top of row Au atoms; meanwhile, weak N2O and N2 desorption peaks also appear at 128 K. With the increase of NO exposure, the a-NO desorption peak grows and reaches its saturation after an exposure of 5 L NO. Both N2O and N2 desorption peaks grow and broaden with two peak maxima at 121 and 135 K. When the NO exposure exceeds 5 L, an additional NO desorption peak (b peak) emerges at 116 K. Obvious N2O and N2 desorption peaks were observed to accompany the b-NO desorption peak. With the further increase of NO exposure, the b-NO desorption peak grows and approaches the saturation after an exposure of 100 L NO; accordingly, the b-N2O and N2 desorption peaks also grow. As shown in Fig. 4D, the dependence of N2O/NO and N2/NO desorption peak area ratios on the NO exposure varies oppositely before and after the exposure of 5 L NO. For NO exposures below 5 L where only the a-NO desorption peak appears, the N2O/NO and N2/NO desorption peak area ratios decrease with the increase of NO exposure. Thus, the NO molecules chemisorbed on the 7-coordinated Au atoms of Au(1 1 0)-(1 2) and Au(9 9 7) surfaces exhibit a similar trend toward decomposition reaction in which the chemisorbed NO molecules at a low coverage are more prone to decompose than those at high coverages. However, for NO
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exposures above 5 L where the a-NO desorption peak saturates, but the b-NO desorption peak appears and grows, the N2O/NO and N2/NO desorption peak area ratios increase with the increase of NO exposure. This demonstrates that the b-NO molecules chemisorbed on the Au(1 1 0)-(1 2) surface are more prone to decompose than the a-NO molecules, and their decomposition probability increases with the increase in their surface coverage. No obvious oxygen desorption was observed in the TDS experiments although the formation of N2O and N2 is very clear on the Au(1 1 0)-(1 2) surface. This could be due to the facile reaction of the resulted O(a) adatoms on the Au surface with both CO in the residual gas of the UHV chamber [45] and NO to form NO2(a) on the surface [18–20]. The formation of NO2(a) on the Au(1 1 0)(1 2) after 100 L NO exposure could be inferred by comparing its NO TDS spectrum with that following 10 L NO2 exposure on Au(1 1 0)-(1 2) (Fig. S3), in which the NO2 desorption peaks were observed at 205 and 312 K. Other indications for the formation of NO2(a) include the upshift of the desorption temperatures of aNO, a-N2O, and CO desorption features with the growth of the bNO desorption peak. It has been demonstrated that NO2(a) on Au surfaces could stabilize the co-adsorbed species [46]. The adsorption of NO on Au(9 9 7) and Au(1 1 0)-(1 2) was also studied by XPS. No signal could be observed by XPS for NO adsorption on Au(9 9 7). This could be due to the low coverage of NO(a) on Au(9 9 7). NO(a) chemisorbed on the 7-coordinated (1 1 1) step atoms of Au(9 9 7) at most only amounts to 0.125 ML (monolayer). A clear O 1s XPS peak was observed at 531.9 eV after an exposure of 5 L NO on Au(1 1 0)-(1 2) (Fig. 5A), and a very weak N 1s XPS peak could be identified at 401.5 eV (Fig. 5B). These features could be reasonably associated with the a-NO desorption peak in the corresponding TDS spectra. With the increase of NO exposure, both O 1s and N 1s features grow, and the O 1s feature slightly shifts to 531.6 eV; meanwhile, new O 1s and N 1s features, respectively, appear at 534.1 and 405.3 eV. Fig. 6 shows the O 1s and N 1s XPS spectra after a 100 L NO exposure on Au(1 1 0)-(1 2) at 105 K followed by annealing at elevated temperatures. After the exposure at 105 K, two O 1s features at 531.6 and 534.1 eV and two N 1s features at 401.6 and 405.3 eV were observed. After the annealing at 125 K, the O 1s feature at 534.1 eV and the N 1s feature at 405.3 eV completely disappear, and the N 1s feature at 401.5 eV weakens and shifts to 402 eV, but the O 1s feature at 531.6 eV only slightly weakens. These observed changes in the XPS spectra correspond to the b-NO, b-N2O and b-N2 desorption peaks in the TDS spectra. The O 1s feature at 531.6 eV and the N 1s feature at 402 eV weaken after the annealing at 170 K and completely disap-
pear after the annealing at 325 K, corresponding to the a-NO and NO2 desorption peaks in the TDS spectra. XPS studies of NOx adsorption on Au surfaces are few, and only three relevant works have been found. We have recently studied the adsorption of NO2 on Au(9 9 7) and identified the N 1s and O 1s binding energy of NO2(a) on Au(9 9 7), respectively, at 401.3– 402 and 531 eV [35]. Vinod et al. studied the adsorption of NO and N2O on Au(3 1 0) at 80 K by XPS and TDS [25]. N2O(a) on Au(3 1 0) formed by the adsorption of N2O at 80 K exhibited two N 1s peaks at 401.5 and 405.5 eV that were, respectively, assigned to end-N (NNO) and middle-N (NNO) of chemisorbed N2O(a) species [25]. On Ag(1 1 1), the N2O(a) formed after NO adsorption at 90 K gave two N 1s features at 401.8 and 405.5 eV and an O 1s signals at 534.1 eV [47]. An exposure of NO (5 106 mbar, 30 min) on Au(3 1 0) at 80 K resulted in two N 1s features at 401.5 and 403.5 eV that were, respectively, assigned to N2O(a) formed by NO decomposition and NO(a) [25]. The assignment of the N 1s feature at 403.5 eV to chemisorbed NO(a) on Au(3 1 0) is questionable since this value is much higher than the N 1s binding energy of chemisorbed NO(a) on Ag(1 1 1) (401.8 eV) [47] and Cu(1 1 1) (401 eV) [48] and the N 1s binding energy of chemisorbed NO2(a) on Au(9 9 7) (401.3–402 eV) [35]. Bukhtiyariv et al. recently employed in situ XPS to study the decomposition of NO on Au(1 1 1), Au(3 1 0), and Au(5 3 3) surfaces at elevated NO pressure up to 7 Pa and temperatures between 300 and 500 K [26]. They identified the N 1s binding energy of chemisorbed N(a) on Au(3 1 0) and Au(5 3 3) to be 399.4 eV. They also observed a N 1s feature at 402.7 eV after NO exposure on Au(5 3 3) at 300 K that was assigned to N2O(a). It is noteworthy that, in the Vinod et al.’s work [25] and Bukhtiyariv et al.’s work [26], the employed NO pressures were high and might result in surface reactions other than NO decomposition and thus form N-contained surface species other than N(a), NO(a), and N2O(a). Comparing the TDS and XPS results of NO adsorption on Au(1 1 0)-(1 2), we assigned the O 1s feature at 531.9 eV and the N 1s feature at 401.5 eV to chemisorbed NO species, the O 1s feature at 534.1 eV and the N 1s features at 401.5 and 405.3 eV to chemisorbed N2O(a) species, and the O 1s feature at 531.6 eV and the N 1s feature at 402 eV to chemisorbed NO2(a) species. These XPS results clearly demonstrate that the bNO species is accompanied by N2O(a) during the exposure of NO on Au(1 1 0)-(1 2) at 105 K. N2O was reported to weakly chemisorb on clean Au(3 1 0) surface, but the interaction could be much enhanced with the coexistence of oxygen adatoms on the surface [25]. Thus, the b-NO species is much more facile to decompose into N2O(a) than the a-NO species on Au(1 1 0)-(1 2), whereas the
Fig. 5. (A) O 1s and (B) N 1s XPS spectra following indicated NO exposures on Au(1 1 0)-(1 2) surface at 105 K.
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Fig. 6. (A) O 1s and (B) N 1s XPS spectra after 100 L NO exposure on Au(1 1 0)-(1 2) surface at 105 K followed by annealing at indicated temperatures.
a-NO species binds the surface more strongly than the b-NO species. Above experimental results of adsorption and decomposition of NO on Au(1 1 0)-(1 2) and Au(9 9 7) can be summarized as follows: (1) Low-coordinated Au atoms exhibit an enhanced reactivity toward NO adsorption. NO does not chemisorb on Au(1 1 1) at 95 K [17,18], but chemisorb on the (1 1 1) step of Au(9 9 7) surface and on Au(1 1 0)-(1 2) that expose low-coordinated Au atoms at 105 K. (2) Chemisorbed NO species on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces can decompose to form N2O and N2 at low temperatures, and its decomposition reactivity depends on both the chemisorption site and the surface coverage. The a-NO species chemisorbed on the 7-coordinated Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces exhibits similar decomposition behaviors. However, on Au(1 1 0)-(1 2), the a-NO(a) species that binds the surface more strongly exhibits a much poorer decomposition reactivity than the b-NO species. This exemplifies an important concept in heterogeneous catalysis that the most strong chemisorption site on the catalysts surface sometimes is not most catalytically active. Moreover, the decomposition reactivity of a-NO species on both Au(9 9 7) and Au(1 1 0)-(1 2) decreases with the increase in its coverage, whereas the decomposition activity of b-NO on Au(1 1 0)-(1 2) increases with the increase in its coverage. The structure sensitivity of low-temperature NO decomposition catalyzed by Au surfaces can thus be concluded. These experimental results are very interesting, particularly concerning the distinctly different decomposition reactivities between the a- and b-NO species. The decomposition of NO on transitional metal surfaces has been extensively studied, and two mechanisms can lead to the N2O formation: one is the direct N– O bond breaking followed by a reaction of NO(a) with N(a) [49]; the other is the (NO)2 dimer mechanism in which N2O is formed by the N–O bond breaking of (NO)2 dimer [50,51]. The N2O formation mechanism by NO decomposition on Au surfaces remains controversial. Vinod et al. [25] proposed that the formation of N2O upon NO adsorption on Au(3 1 0) at 80 K was due to the reaction of adsorbed NO(a) with atomic N(a) formed by NO(a) decomposition. However, Chau et al. [17] proposed that N2O was formed via the (NO)2 dimer mechanism on Au field emitter tips under NO gas flow conditions at 300 K because of the observation of + the desorption of NO+, ðNOÞþ 2 , and N2O . The distinctly different decomposition reactivities between the a- and b-NO species on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces offer an opportunity to clarify the NO decomposition mechanism on Au surfaces. Thus, we
performed DFT calculation studies of adsorption and decomposition of NO on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces. The adsorption of NO on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces was firstly investigated. On both surfaces, the most preferable site for NO adsorption was found to be the most coordinationunsaturated (7-coordinated) Au atoms along the ridge. Fig. 7 shows the adsorption structures of NO(a) on the top and bridge sites of 7coordinated Au atoms, and Table 1 summarizes the corresponding adsorption energy and geometric structure. The adsorption energy Eads is defined as follows: Eads = (Esurf + nENO Etot)/n, where n is number of NO unit (for NO(a), n = 1; and for (NO)2 dimer, n = 2), Esurf is the energy of clean Au(1 1 0) or Au(9 9 7) surface, ENO is the energy of NO gas phase, and Etot is the energy of adsorbed system. The adsorption energies of NO(a) on the top and bridge sites of 7coordinated Au atoms vary between 0.50 and 0.60 eV. These values agree with previous reported adsorption energies of NO(a) on stepped Au surfaces such as Au(1 1 0)-(1 2), Au(2 1 1), Au(3 2 1), and Au(3 2 2) [23,24,29]. The similar adsorption energies of NO(a) on the top and bridge sites of 7-coordinated Au atoms imply that NO(a) on Au ridge can migrate easily. The adsorption energies of atop NO(a) on 9-coordinated Au atoms of Au(9 9 7) (the (1 1 1) terrace) and Au(1 1 0)-(1 2) (the side of row atoms) were calculated to be 0.37 and 0.33 eV, respectively. These values agree with previous reported adsorption energy of atop NO(a) on Au(1 1 1) [19,20,28]. These calculation results clearly demonstrate that low-coordinated Au atoms exhibit enhanced chemisorption ability toward NO. However, comparing with gas-phase NO (Table 1), the N–O bond distance of NO(a) on both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is only a little bit elongated, indicating that the adsorption of NO on the Au surfaces does not weaken the N–O bond much. As the result, the activation barriers for the direct decomposition of atop NO(a) on Au(9 9 7) and Au(1 1 0)-(1 2) into N(a) and O(a) were calculated to be 3.68 and 4.68 eV, respectively. Therefore, the formation of N2O observed experimentally after NO adsorption on Au(9 9 7) and Au(1 1 0)-(1 2) at 105 K under our experimental conditions is not likely resulted from the reaction of adsorbed NO(a) with atomic N(a) formed by NO(a) decomposition. DFT calculation results demonstrate that two NO(a) molecules adsorbed on neighboring ridge Au sites of Au(9 9 7) and Au(1 1 0)(1 2) surfaces facilely form adsorbed (NO)2 dimer without barrier for the flat potential surface along the ridge gold atoms. The formed (NO)2 dimer on the ridge Au atoms of Au(9 9 7) surface (denoted as (NO)2 shown in Fig. 8A) exhibits an adsorption energy of 0.75 eV (Table 1), and the formed (NO)2 dimer on ridge Au atoms of Au(1 1 0)-(1 2) surface (denoted as (NO)2-I shown in Fig. 9A)
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Fig. 7. Top views of NO(a) adsorbed on top site (A) and bridge site (B) of Au(1 1 0)-(1 2) ridge atoms and on top site (C) and bridge site (D) of Au(9 9 7) ridge atoms. The insets show the corresponding side views. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Adsorption energy and structural parameters of various surface species, surface intermediates and transition states for NO adsorption and decomposition on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces shown in Figs. 10 and 11. The calculated structural parameters of free NO, (NO)2 and N2O are also included for the comparison. Species
Eads (eV)
dAu–N (Å) dAu–O (Å)
Gas-phase NO Gas-phase (NO)2 Gas-phase N2O Au(9 9 7)
Bridged-NO(a) Atop NO(a) TS0 in Fig. 10
0.57 0.59
(NO)2
0.75
(NO)2
0.54
(NO)2
0.38
dN–N (Å)
1.164 1.167 1.186
2.007 1.161
2.320(2.325) 2.159 2.093/2.089/2.496 2.157 2.216/2.213
1.181 1.170 1.975 1.180/1.179
1.956
1.245/1.246
1.430
1.296/1.316
1.241
1.167/1.170
2.101
1.184/1.187
1.810
1.310/1.295
1.332
1.766/1.269
1.185
(NO)2-I
0.71
2.316/2.317 2.579/2.573 – 2.310(2.324)/2.209 – 2.955/3.022 2.831/2/905 2.744/2.731 – 2.206/2.359 – 2.189(2.22)/2.386 2.29 2.134 2.076(2.099) 1.986 2.221(2.215)
(NO)2-II
0.59
2.575(2.667)/2.540(2.680)
1.174/1.175
1.978
(NO)2
0.50
1.247/1.247
1.424
(NO)2
0.36
2.300/2.306 2.535/2.531 – 2.272(2.261)/2.217 3.366/2.820 2.675/2.945 2.918/2.077 4.315/4.326 5.375/5.561 3.145/2.764 – 2.339/2.185(2.181)
1.425/1.306
1.239
1.167/1.175
1.939
1.211/1.224
1.637
1.167/1.165
2.026
1.284/1.681
1.199
TS1 in Fig. 10 TS2 in Fig. 10 TS3 in Fig. 10 TS4 in Fig. 10 Au(1 1 0)
dN–O (Å)
Bridged-NO(a) Atop NO(a) TS0 in Fig. 11
0.56 0.57
TS1 in Fig. 11 TS2 in Fig. 11 TS3 in Fig. 11 TS4 in Fig. 11
exhibits an adsorption energy of 0.71 eV (Table 1). Note that the adsorption energy of (NO)2 dimer is referred to each NO molecule. Thus, (NO)2 dimer adsorbed on the ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is more stable than corresponding NO(a). Comparing with free (NO)2 dimer molecule (Table 1), these chemisorbed structures of (NO)2 on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces do not activate (NO)2 much. Meanwhile, the N–O bond distance of (NO)2 dimer adsorbed on the ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is very close to that of corresponding NO(a). These results demonstrate that the (NO)2 dimer
1.183 1.171 2.128 1.178
1.977
adsorbed on the ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces via the N atoms does not promote the activation of the N–O bond. Besides the (NO)2 dimer species, other types of (NO)2 dimer were found to form on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces. On the Au(9 9 7) surface, (NO)2 dimer with its O atom bonded with the ridge Au atoms and its N atom bonded with the terrace Au atoms (denoted as (NO)2 shown in Fig. 8B) exhibits an adsorption energy of 0.53 eV. It is noteworthy that adsorbed (NO)2 dimer compete with (NO)2 for the ridge Au atoms on Au(9 9 7). On the
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119
Fig. 8. Top views of (NO)2 dimer adsorbed on Au(9 9 7) surface. The insets show the corresponding side views. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Top views of (NO)2 dimer adsorbed on Au(1 1 0)-(1 2) surface. The insets show the corresponding side views. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Au(1 1 0)-(1 2) surface, two additional types of adsorbed (NO)2 dimer were identified: (NO)2-II (Fig. 9B) with both N atoms bonded with the bridge site of edge Au atoms of the trench exhibiting an adsorption energy of 0.59 eV, and (NO)2 (Fig. 9C) with both N atoms and O atoms bonded with the top site of edge Au atoms of the trench exhibiting an adsorption energy of 0.50 eV. The (NO)2 dimer species on both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is thermodynamically much more stable than other types of (NO)2 dimer species; however, as demonstrated from their geometric parameters summarized in Table 1, it is much less activated than other types of less stable (NO)2 dimer species. The N–O bond distance of (NO)2 dimer on Au(9 9 7) is greatly elongated to 1.246 Å, and the N–N bond is greatly shortened to 1.430 Å. Similarly, the N–O bond distance of (NO)2 dimer on Au(1 1 0)-(1 2) is greatly elongated to 1.247 Å, and the N–N bond is greatly shortened to 1.424 Å. These theoretical calculation results also exemplify the concept that the most strongly chemisorbed surface species is not always most strongly activated. We then searched for the decomposition pathways of various types of (NO)2 dimer species on the Au(9 9 7) and Au(1 1 0)(1 2) surfaces. Fig. 10 presents the results on the Au(9 9 7) surface. The (NO)2 dimer firstly converts to an intermediate species bonded with the surface via the O atom (denoted as (NO)2 dimer) with an activation energy of 0.62 eV; then, the (NO)2 dimer facilely decomposes to O adatom and N2O with an activation energy of 0.17 eV. The (NO)2 dimer exhibits an adsorption energy of 0.38 eV, two N–O bond distances of 1.296 and 1.316 Å, and the N–N bond distance of 1.241 Å (Table 1). The decomposition of (NO)2 dimer was found to follow various pathways: one is that the (NO)2 dimer firstly converts to (NO)2 dimer with an activation energy of 1.24 eV followed by the decomposition pathway of (NO)2 dimer; another is that the (NO)2 dimer firstly converts to (NO)2 dimer with an activation energy of 1.34 eV followed by the decomposition pathway of (NO)2 dimer. These calculation results demonstrate that the (NO)2 dimer species with a small adsorption energy on the Au(9 9 7) surface exhibits a higher decomposition
reactivity to form N2O than the (NO)2 dimer species with a large adsorption energy. This is reasonable because comparing the (NO)2 dimer species, the (NO)2 dimer species exhibits a weakened N–O bond and a strengthened N–N bond (Table 1). On the Au(1 1 0)-(1 2) surface (Fig. 11), the (NO)2 dimer firstly converts to an intermediate species bonded with the surface via the O atom (denoted as (NO)2 dimer) with an activation energy of 0.49 eV; then, the (NO)2 dimer facilely decomposes to O adatom and N2O with an activation energy of 0.11 eV. The (NO)2 dimer exhibits an adsorption energy of 0.36 eV, two N–O bond distances of 1.425 and 1.306 Å, and the N–N bond distance of 1.239 Å (Table 1). The decomposition of the most stable (NO)2-I dimer adsorbed on the ridge Au atoms could follow various pathways: one is that the (NO)2-I dimer firstly converts to (NO)2 dimer with an activation energy of 0.70 eV followed by the decomposition pathway of (NO)2 dimer; another is that the (NO)2-I dimer firstly converts to (NO)2 dimer with an activation energy of 0.95 eV followed by the decomposition pathway of (NO)2 dimer. Similar to the case of Au(9 9 7) surface, less stable (NO)2 dimer species adsorbed on the Au(1 1 0)-(1 2) surface exhibits a higher decomposition reactivity to form N2O. Above theoretical calculation results can be summarized as follows: (1) Low-coordinated Au atoms exhibit an enhanced ability to adsorb NO. NO(a) adsorbed on the 7-coordinated Au atoms is less stable than the (NO)2 dimer species adsorbed on the same sites, but exhibits a similar stability to other types of (NO)2 dimer species. (2) (NO)2 dimer adsorbed on the 7-coordinated Au atoms is more stable than NO(a) adsorbed on the same sites, but the activation energy of (NO)2 dimer decomposition is much lower than that of NO(a) decomposition. Thus, the active surface species for the decomposition of NO on Au surfaces is (NO)2 dimer. (3) The reactivity of (NO)2 dimer species on Au surfaces to decompose into O adatom and N2O sensitively depends on its structure, and multireaction pathways coexist for the (NO)2 dimer decomposition reaction. Particularly, the (NO)2 dimer species with a smaller adsorption energy exhibits a higher decomposition reactivity to form
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Fig. 10. Calculated decomposition reaction pathways of NO(a) and (NO)2 on Au(9 9 7) surface. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. The structural parameters of surface species and transition states are summarized in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
N2O. The (NO)2 dimer bonded with the Au(9 9 7) and Au(1 1 0)(1 2) surfaces via its O atom served as the intermediate species shown in Figs. 10 and 11 exhibits the smallest activation barrier for the decomposition reaction but meanwhile the lowest adsorption energy. These results agree with previous theoretical calculation results of (NO)2 dimer decomposition on Au(1 1 1) [28] and Ag(1 1 1) [51] in which (NO)2 dimer bonded with the surface via the O atom exhibits the lowest barrier for the decomposition reaction. However, there is a major difference regarding the stability of various (NO)2 dimer species on Ag(1 1 1) and Au surfaces. The (NO)2 dimer species bonded with Ag(1 1 1) via the O atom was calculated to be slightly more stable than the (NO)2 dimer species bonded via the N atom [51], while the (NO)2 dimer species bonded the surface via the O atom on Au(1 1 1) was calculated to be much less stable than the (NO)2 dimer bonded via the N atom [28], agreeing with our theoretical calculation results of Au(9 9 7) and Au(1 1 0)(1 2) surfaces. These theoretical calculation results suggest that the formation of chemisorbed (NO)2 dimer species and their decomposition should dominate the surface chemistry of NO on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces under our experimental conditions. Adsorbed (NO)2 dimer species will decompose into NO(g) as soon as it desorb from the surface and thus is indistinguishable from NO(a) in the TDS spectrum. No XPS result has been reported for the chemisorbed (NO)2 dimer species on metal surfaces. We have thus calculated the N 1s and O 1s binding energy shifts between chemisorbed NO and (NO)2 on both Au(1 1 0)-(1 2) and
Au(9 9 7) surfaces. The results show that the N 1s binding energy of chemisorbed (NO)2 is 0.2 eV higher than that of chemisorbed NO, while its O 1s binding energy is 0.3 eV lower than that of chemisorbed NO. Thus the experimentally observed O 1s feature at 531.9 eV and the N 1s feature at 401.5 eV could also be assigned to chemisorbed (NO)2 on Au surfaces. Therefore, although the presence of NO(a) cannot be excluded, on the basis of theoretical calculation results, we propose that the a-NO desorption peak in the TDS spectra largely arises from most stable (NO)2 dimer species chemisorbed on the 7-coordinated ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces (Figs. 7A, B and 8A), while the bNO desorption peak in the TDS spectra largely arises from less stable (NO)2-II and (NO)2 dimer species chemisorbed on the trench Au atoms of Au(1 1 0)-(1 2) surface (Fig. 8B and C). On the Au(9 9 7) surface, less stable (NO)2 dimer (Fig. 7C) needs to compete with more stable (NO)2 for the ridge Au atoms; thus, its formation is suppressed but still indicated by the weak low-temperature shoulder peak shown in Fig. 3A. The experimentally observed different decomposition reactivities of a- and b-NO species can be well explained by the calculated stability-dependent decomposition reactivity of various types of (NO)2 dimer species. The most stable (NO)2 dimer species chemisorbed on the 7-coordinated ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces via the N atoms exhibits a high activation barrier (1.0 eV), and thus, only a small portion of a-NO species undergoes the decomposition reaction when heated (Fig. 3). The less stable (NO)2-II and (NO)2 dimer species chemisorbed on the trench Au atoms of Au(1 1 0)-(1 2) sur-
Z. Wu et al. / Journal of Catalysis 304 (2013) 112–122
121
Fig. 11. Calculated decomposition reaction pathways of NO(a) and (NO)2 on Au(1 1 0)-(1 2) surface. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. The structural parameters of surface species and transition states are summarized in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
face exhibit a low activation barrier (0.5 eV), and thus, the b-NO species can facilely undergo the decomposition reaction to form N2O(a) on the surface during the adsorption process at 105 K (Figs. 4 and 5). The catalytic performances of various supported Au nanocatalysts were examined in NO reduction by CO, H2, and hydrocarbons [14]. Supported Au nanocatalysts were active at low reaction temperatures, their activity sensitively depends on the catalyst structure, and the reduction product was dominantly N2O at low reaction temperatures but N2 at high reaction temperatures. Our comprehensive experimental and theoretical calculation results provide some deep insights into the fundamental understanding. Firstly, various types of chemisorbed (NO)2 dimer species form and coexist with NO(a) on Au surfaces. Secondly, chemisorbed (NO)2 dimer species on Au surfaces are more facile to decompose than NO(a); thus, chemisorbed (NO)2 dimer species, instead of NO(a), are the likely active surface species for low-temperature NO reduction catalyzed by supported Au nanocatalysts. Thirdly, N2O(a) and O(a) are the primary decomposition products of chemisorbed (NO)2 dimer species on Au surfaces. This agrees with previous report that the reduction product of NO reduction catalyzed by supported Au nanocatalysts was dominantly N2O at low reaction temperatures [14]. Fourthly, the decomposition reac-
tivity of chemisorbed (NO)2 dimer species sensitively depends on its chemisorption configuration and thus the local structure of Au surface. This explains the structure sensitivity of NO reduction catalyzed by supported Au nanocatalysts [14]. A noteworthy observation is that the decomposition reactivities of various types of chemisorbed (NO)2 dimer species to N2O vary in a trend contrast to their adsorption energies on Au surfaces. Comparing with other supported metal nanocatalysts, a unique advantage of supported Au nanocatalysts is its exceptional high activity at low reaction temperatures. The observation that the decomposition reactivities of various types of chemisorbed (NO)2 dimer species to N2O vary in a trend contrast to their adsorption energies on Au surfaces sheds light on the nature of the exceptional high activity of supported Au nanocatalysts at low reaction temperatures. During the catalytic reaction, the coverage of a surface species on the catalyst surface is determined by the gas-phase pressure (liquid-phase concentration), the reaction temperature, and its adsorption energy on the catalyst surface. At high reaction temperatures, the surface species with the largest adsorption energy dominates on the catalyst surface, and its reactivity determines the observed catalytic activity; with the decrease in reaction temperature, its surface coverage increases, meanwhile, other types of surface species with smaller adsorption energies that do not compete
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the surface sites with the most stable surface species will also appear on the catalyst surface, and their coverages also increase; thus, the reactivities of all types of surface species present on the catalyst surface contribute to the observed catalytic activity. For a catalyst surface on which the surface species with small adsorption energies exhibit high reactivities, or better reactivities than the surface species with the largest adsorption energy, it will exhibit a high low-temperature catalytic activity or a better catalytic activity at low temperatures than at high temperatures. NO decomposition into O adatom and N2O on Au surfaces reported herein is a nice example. The activation energy of (NO)2 dimer species with the largest adsorption energy is much higher than that of other types of (NO)2 dimer species with small adsorption energies; thus, the Au surface is more active in catalyzing the NO decomposition into O adatom and N2O at low temperatures where (NO)2 dimer species with small adsorption energies are present on the surface than at high temperatures where only the (NO)2 dimer species with the largest adsorption energy is present on the surface. CO oxidation catalyzed by supported Au nanocatalysts is another example in which CO oxidation involves different types of surface species and proceeds with different reaction mechanisms at different regions of reaction temperatures [52–56]. We believe that above concept is also applicable for other heterogeneous catalytic reactions catalyzed by supported Au nanocatalysts with an exceptional high activity at low reaction temperatures.
References
4. Conclusions
[26]
By a combined experimental and theoretical calculation study of NO adsorption and decomposition on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces, we have successfully understood the structure sensitivity of low-temperature NO decomposition on Au surfaces at the molecular level. (NO)2 dimer species, instead of NO(a), is the active surface species for the NO decomposition into O adatom and N2O. Depending on the Au surface structure, various types of (NO)2 dimer species form. The (NO)2 dimer species bonded with the lowest-coordinated Au atoms of the surface via the N atom is thermodynamically more stable than other types of (NO)2 dimer species but also exhibits a much larger activation energy for the decomposition reaction than other types of (NO)2 dimer species. Therefore, the Au surface exhibits a high activity in catalyzing the NO decomposition reaction at low temperatures where less stable, but highly reactive (NO)2 dimer species are present on the surface and facilely undergo the decomposition reaction. These conclusions learned from the well-defined model catalyst study provide deep insights into the fundamental understanding of structure sensitivity and exceptional high low-temperature catalytic activity of supported Au nanocatalysts.
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This work was financially supported by National Basic Research Program of China (2013CB933104, 2010CB923301), National Natural Science Foundation of China (20973161, 21103156), MOE Fundamental Research Funds for the Central Universities.
[48]
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2013.04.013.
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Structure sensitivity of low-temperature NO decomposition on Au surfaces Zongfang Wu a,b,c, Lingshun Xu a,b,c, Wenhua Zhang b,d,⇑, Yunsheng Ma c, Qing Yuan a,b,c, Yuekang Jin a,b,c, Jinlong Yang a,c, Weixin Huang a,b,c,⇑ a
Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China CAS Key Laboratory of Materials for Energy Conversion, University of Science and Technology of China, Hefei 230026, China Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China d Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China b c
a r t i c l e
i n f o
Article history: Received 3 February 2013 Revised 17 April 2013 Accepted 21 April 2013 Available online 23 May 2013 Keywords: NO decomposition Au Single-crystal surface Structure sensitivity Structure–activity relation
a b s t r a c t We have comparatively studied adsorption and decomposition of NO on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces by means of TDS, XPS, and DFT theoretical calculation. The lowest-coordinated Au atoms on both surfaces are 7-coordinated, but the surface chemistry of NO differs very much on these two surfaces. An a-NO species dominates on the Au(9 9 7) surface, while besides the similar a-NO species, another less stable and more abundant b-NO species also appear on the Au(1 1 0)-(1 2) surface. Part of a-NO species decomposes into O adatom and N2O upon heating, but the less stable b-NO species exhibits a much higher decomposition reactivity than a-NO species and facilely decomposes into O adatom and N2O on the Au(1 1 0)-(1 2) surface during the NO exposure at 105 K. The accompanying DFT theoretical calculation results demonstrate that chemisorbed (NO)2 dimer species dominate the surface chemistry of NO on the Au surfaces. a-NO species is the most stable (NO)2 dimer species that chemisorbs on the 7-coordinated ridge Au atoms of both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces via the N atoms and exhibits a high activation barrier for the decomposition reaction. b-NO species corresponds to less stable (NO)2 dimer species that chemisorbs on the trench Au atoms of the Au(1 1 0)-(1 2) surface via both N and O atoms and exhibits a low activation barrier for the decomposition reaction. These comprehensive experimental and theoretical calculation results reveal at the molecular level the origin of structure sensitivity and low-temperature catalytic activity of supported Au nanocatalysts in NO decomposition reaction. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction Since the first report of Haruta et al. [1], Au nanocatalysis has developed as one of the most exciting research topics in heterogeneous catalysis. Fundamental understanding of complicated heterogeneous catalytic reactions is of great importance and interest but remains challenging [2,3]. Surface science studies of well-defined model catalysts have been successfully developed as an effective strategy [4]. Catalytic reactions catalyzed by supported Au nanoparticles reported by far are structure sensitive, and the catalytic performance of supported Au nanoparticles sensitively depends on their size. This has intrigued extensive relevant fundamental studies employing well-defined Au model catalysts [5–7], but the molecular-level understanding still remains ambiguous. Low-coordinated Au atoms on the surface of supported Au nanoparticles are generally considered to play an essential role in the catalytic activ⇑ Corresponding authors at: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China. E-mail addresses: [email protected] (W. Huang), [email protected] (W. Zhang). 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.04.013
ity and can be modeled with stepped Au single-crystal surfaces. Previous studies have shown that the reactivity of stepped Au single-crystal surfaces depends not only on the geometry and density of surface steps but also on the surface structure of terrace [8–13]. The abatement of NOx (NO and NO2) is an important topic in environmental catalysis. Supported Au nanocatalysts were reported to show excellent catalytic activity toward low-temperature NOx reduction [14–16]. Thus, the adsorption and surface reaction of NOx on various Au single-crystal surfaces have been much investigated both experimentally and theoretically [17– 35]. NO does not adsorb on Au(1 1 1) at the temperature as low as 95 K [17,18], and the calculated adsorption energy of NO on Au(1 1 1) is 20 kJ/mol [19,20], but both experimental and DFT calculation results demonstrated that NO facilely reacted with atomic O on Au(1 1 1) to form NO2(a) [18–20]. On reconstructed Au(1 0 0), NO molecularly adsorbs at 170 K with a desorption activation energy of 57 kJ/mol, resulting in the lift of the hex reconstruction of Au(1 0 0) surface [21]. The enhanced adsorption ability of lowcoordinated Au atoms toward NO has also been demonstrated by DFT theoretical calculations [22–24]. On stepped Au(3 1 0) surface consisting of (1 0 0) terraces and (1 1 0) steps, the formation of
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N2O and O(a) was observed upon NO adsorption even at 80 K [25]. It was proposed that N2O was produced by the reaction of adsorbed NO(a) with atomic N(a) formed by NO decomposition; however, atomic N(a) was not experimentally indentified on the surface. Very recently, Bukhtiyariv et al. [26] employed in situ Xray photoelectron spectroscopy (XPS) to study the decomposition of NO on flat Au(1 1 1) and stepped Au(3 1 0) and (5 3 3) surfaces at elevated NO pressure up to 7 Pa and temperatures between 300 and 500 K. NO does not chemisorb on Au(1 1 1), but evidently dissociates on stepped Au(3 1 0) and (5 3 3) surfaces. These results demonstrate that the Au step sites are responsible for the adsorption and dissociation of NO. Interestingly, NO dissociation on Au(3 1 0) exclusively forms N(a) on the surface whose coverage decreases with the increase in the sample temperature; however, NO dissociation on Au(5 3 3) initially forms N2O(a) on the surface that is replaced by more stable N(a) with the increase of NO exposure and/or sample temperature. However, the underlying mechanism was not identified. Chau et al. [27] employed pulsed field desorption mass spectrometry to study NO adsorption on the Au field emitter tip under NO gas flow condition at 300 K and observed + the desorption of NO+, ðNOÞþ 2 , and N2O from the stepped surface region between the central (1 1 1) pole and the peripheral (0 0 1) plane. They thus concluded that N2O was formed via a (NO)2 dimer mechanism. DFT calculations demonstrated that NO could dissociate to form N2O via the (NO)2 dimer even on the Au(1 1 1) surface [28], but the adsorption energies of NO and (NO)2 dimer on Au(1 1 1) were too small to allow the experimental observation of these species [17–20,28]. On stepped Au(3 2 1) surface, DFT calculations proposed that the presence of hydrogen was a necessary condition for NO dissociation [29]. These previous results demonstrate that NO decomposition on Au surfaces is highly structure sensitive, but the molecular-level understanding lacks. In the present study, we have comparatively investigated the interaction of NO with two stepped Au surfaces, Au(9 9 7) and Au(1 1 0)-(1 2), by means of TDS, XPS, and DFT theoretical calculation. The lowest-coordinated Au atoms on both surfaces are 7-coordinated, but interestingly and unexpectedly, the surface chemistry of NO differs very much on these two surfaces. The accompanying DFT theoretical calculation results reveal that the surface structure-dependent stability and decomposition reactivity of various types of (NO)2 dimer are responsible for the different adsorption and decomposition behaviors of NO on Au surfaces.
2. Experimental section and computational detail All experiments were performed in a Leybold stainless steel ultrahigh vacuum (UHV) chamber with a base pressure of 1.2 1010 mbar [36]. The UHV chamber was equipped with facilities for XPS, UPS, LEED, and differentially-pumped TDS measurements. The Au(9 9 7) and Au(1 1 0) single crystals purchased from MaTeck were mounted on the sample holder by two Ta wires spot-welded to the backside of the sample. The sample temperature could be controlled between 100 and 1273 K and was measured by a chromel–alumel thermocouple spot-welded to the backside of the sample. Prior to the experiments, the Au samples were cleaned by repeated cycles of Ar ion sputtering and annealing until LEED gave a sharp diffraction pattern, and no contaminants could be detected by XPS. NO (>99.9%, Nanjing ShangYuan Industry Factory) was used as received without any further purification, and the purity was further checked by quadrupole mass spectrometer (QMS) prior to experiments. The base pressure of the chamber during the course of NO exposure was controlled to be below 5 1010 torr; therefore, a line-of-sight stainless steel doser (diameter: 8 mm) positioned 2 mm in front of the Au surface was used for relatively
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large NO exposures. The exposures of NO reported herein were corrected with the enhancement effect of the doser (1000) [37]. All exposures were reported in Langmuir (1 L = 1.0 106 torr s) without corrections for the gauge sensitivity. During the TDS experiments, the Au sample was positioned 1 mm away from the collecting tube of a differentially-pumped QMS and heated to 650 K with a heating rate of 3.0 K/s. The signals with m/e = 30 (NO), 46 (NO2), 18 (H2O), 28 (CO and N2), and 44 (N2O and CO2) were monitored. XP spectra were recorded by a hemispherical energy analyzer (PHBIOS 100 MCD, SPECS GmbH) with a pass energy of 20 eV using Al Ka radiation (hm = 1486.6 eV). All the calculations were performed by density functional theory implemented in DMol3 package [38,39]. The double-numeric quality basis set with polarization functions (DNP), DFT semicore pseudopotential (DSPP), and PBE functional [40] were used for all the calculations. The Au(1 1 0)-(1 2) surface was simulated by seven layers gold atoms with the bottom three layers fixed, and the Au(9 9 7) surface was simulated by stepped four layers gold atoms with the bottom two stepped layers fixed. The supercells used for Au(1 1 0)-(1 2) and Au(9 9 7) were (8.394 Å 8.394 Å 26.9032 Å) and (11.871 Å 21.605 Å 24.535 Å), respectively, and the vacuum layer was larger than 15 Å. For the energy calculations, the real space cutoff was set as 4.5 Å, and the (5 5 1) and (4 2 1) k-points samplings were used for Au(1 1 0)-(1 2) and Au(9 9 7), respectively. The transition states of elementary steps were searched by synchronous transit method with conjugated gradient refinements [41]. The core level shift of N 1s and O 1s was calculated by comparing the relative total energy of substation of different nitrogen (oxygen) to oxygen (fluorine) in the same supercell. The isolated adsorbed NO molecule was used as the reference.
3. Results and discussion Fig. 1 shows LEED patterns and schematic structural illustrations of clean Au(9 9 7) and Au(1 1 0) surfaces. Clean Au(9 9 7) vicinal surface exhibits a LEED pattern with splitting spots forming a hexagonal symmetry, similar to the LEED patterns of Pt(9 9 7) and Cu(9 9 7) [42,43]. The (9 9 7) vicinal surface is created by cutting a Au(1 1 1) crystal at an angle of approximately 7° offset to obtain 0 direction separated by atomic height steps along the dense ½1 1 (1 1 1) terraces and is composed of close-packed (1 1 1) terraces and a mono-atomic step with {1 1 1} microfacet. The coordination numbers of atoms on the step and terrace of Au(9 9 7) surface are 7 and 9, respectively. Clean Au(1 1 0) exhibits a sharp (1 2) LEED pattern, a characteristic of reconstructed Au(1 1 0)-(1 2) surface that can be described by a missing-row model with paired rows in the second layer and buckled rows in the third layer. The Au(1 1 0)-(1 2) surface gives rise to on top of row atoms, side of row atoms, and trench atoms whose coordination numbers are 7, 9, and 11, respectively.
Fig. 1. LEED pattern (upper) and schematic structural illustration (bottom) of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces. Ep = 85 eV.
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Fig. 2. NO, N2, and N2O TDS spectra following the saturation exposure (100 L) of NO on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces at 105 K.
Although the lowest-coordinated Au atoms are 7-coordinated on both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces, unexpectedly and very interestingly, the chemisorption and surface reaction of NO differ greatly on these two surfaces. Fig. 2 compares NO, N2O, and N2 TDS spectra following the saturation exposure of NO on both surfaces at 105 K. It is noteworthy that the fragmentation contribution of N2O to the signals of NO and N2 in mass spectroscopy, respectively, amounting to 31% and 11% [44], was subtracted
in all presented NO and N2 TDS spectra in the paper. The as-measured TDS profiles corresponding to Fig. 2 are shown in Fig. S1. The desorption traces of fragments at m/z = 12 and 14 (Fig. S2) support our assignment of the desorption peak of m/z = 28 to either N2 formed as the decomposition product of NO(a) or CO(a) formed due to the chemisorption of residual CO in the UHV chamber. Following the saturating NO exposure at the same temperature, a single NO desorption peak (denoted as a peak) centering at 144 K was observed for the Au(9 9 7) surface; However, for the Au(1 1 0)-(1 2) surface, besides a similar a desorption peak, another stronger b desorption peak centering at 123 K with a shoulder at 116 K was also observed. More striking differences come from the desorption features of N2O and N2 formed as the products of NO decomposition. The N2O and N2 desorption peaks are very obvious for the Au(1 1 0)-(1 2) surface but neglectable for the Au(9 9 7) surface; moreover, the N2O and N2 desorption peaks for the Au(1 1 0)-(1 2) surface are clearly associated with the b-NO desorption peak. These results demonstrate an unexpected structure sensitivity of NO chemisorption and decomposition on Au surfaces because the lowest-coordinated Au atoms on Au(1 1 0)-(1 2) and Au(9 9 7) surfaces are same. It can be thus concluded that surface sites other than the 7-coordinated Au atoms on the Au(1 1 0)-(1 2) surface are most catalytically active in decomposing NO at the saturating NO(a) coverage. LEED observations show that the reconstructed Au(1 1 0)-(1 2) surface is not lifted by NO chemisorption under our experimental conditions. Fig. 3A, B, and C, respectively, show NO, N2O, and N2 TDS spectra following various NO exposures on Au(9 9 7) at 105 K, and Fig. 3D
Fig. 3. (A) NO, (B) N2O, and (C) N2 TDS spectra following various exposures of NO on Au(9 9 7) surface at 105 K: (a) 0 L; (b) 1 L; (c) 5 L; (d) 10 L; (e) 20 L; (f) 50 L; (g) 100 L. (D) Integrated NO, N2O, N2 TDS peak areas and N2O/NO, N2/NO peak area ratios as a function of NO exposure.
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Fig. 4. (A) NO, (B) N2O, and (C) N2 TDS spectra following various exposures of NO on Au(1 1 0)-(1 2) surface at 105 K: (a) 0 L; (b) 1 L; (c) 2 L; (d) 5 L; (e) 10 L; (f) 20 L; (g) 50 L; (h) 100 L. (D) Integrated NO, N2O, N2 TDS peak areas and N2O/NO, N2/NO peak area ratios as a function of NO exposure.
shows the integrated NO, N2O, NO desorption peak areas and the N2O/NO, N2/NO desorption peak area ratios. At the lowest NO exposure (1 L), a single a-NO desorption peak appears at 144 K; meanwhile, a N2O desorption peak and a very tiny N2 desorption peak appear at 133 K. Previous studies have clearly demonstrated that NO does not adsorb on Au(1 1 1) at the temperature as low as 95 K [17,18]. Thus, the a-NO desorption peak can be reasonably assigned to NO species chemisorbed on the (1 1 1) step of Au(9 9 7) surface whose Au atoms are 7-coordinated. This agrees with the general observation that low-coordinated Au atoms exhibit enhanced chemisorption ability. The TDS results also demonstrate that some chemisorbed a-NO species undergoes decomposition reactions upon heating to give rise to the desorption traces of N2O and N2. With the increase of NO exposure, the NO desorption peak grows and a weak shoulder develops, but its peak maxima do not shift; the N2O desorption peak grows and gradually develops into a main peak with two shoulders; the N2 desorption peak initially slightly grows, but then weakens. As shown in Fig. 3D, the chemisorption of NO on Au(9 9 7) at 105 K saturates after an exposure of 50 L NO. We herein employ the N2O/NO and N2/NO desorption peak area ratios to approximate the decomposition reactivity of chemisorbed NO species. It could be seen from Fig. 3D that both N2O/NO and N2/NO desorption peak area ratios decrease rapidly with the increase of NO exposure and thus the surface coverage of chemisorbed NO species on Au(9 9 7). Thus, NO molecules chemisorbed on Au(9 9 7) at a low coverage are more prone to decompose than those at high coverages.
Fig. 4A, B, and C, respectively, show NO, N2O, and N2 TDS spectra following various NO exposures on Au(1 1 0)-(1 2) at 105 K, and Fig. 4D shows the integrated NO, N2O, NO desorption peak areas and the N2O/NO, N2/NO desorption peak area ratios. At the lowest NO exposure (1 L), a single a-NO desorption peak appears at 128 K and can be assigned to NO species chemisorbed on the 7coordinated on top of row Au atoms; meanwhile, weak N2O and N2 desorption peaks also appear at 128 K. With the increase of NO exposure, the a-NO desorption peak grows and reaches its saturation after an exposure of 5 L NO. Both N2O and N2 desorption peaks grow and broaden with two peak maxima at 121 and 135 K. When the NO exposure exceeds 5 L, an additional NO desorption peak (b peak) emerges at 116 K. Obvious N2O and N2 desorption peaks were observed to accompany the b-NO desorption peak. With the further increase of NO exposure, the b-NO desorption peak grows and approaches the saturation after an exposure of 100 L NO; accordingly, the b-N2O and N2 desorption peaks also grow. As shown in Fig. 4D, the dependence of N2O/NO and N2/NO desorption peak area ratios on the NO exposure varies oppositely before and after the exposure of 5 L NO. For NO exposures below 5 L where only the a-NO desorption peak appears, the N2O/NO and N2/NO desorption peak area ratios decrease with the increase of NO exposure. Thus, the NO molecules chemisorbed on the 7-coordinated Au atoms of Au(1 1 0)-(1 2) and Au(9 9 7) surfaces exhibit a similar trend toward decomposition reaction in which the chemisorbed NO molecules at a low coverage are more prone to decompose than those at high coverages. However, for NO
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exposures above 5 L where the a-NO desorption peak saturates, but the b-NO desorption peak appears and grows, the N2O/NO and N2/NO desorption peak area ratios increase with the increase of NO exposure. This demonstrates that the b-NO molecules chemisorbed on the Au(1 1 0)-(1 2) surface are more prone to decompose than the a-NO molecules, and their decomposition probability increases with the increase in their surface coverage. No obvious oxygen desorption was observed in the TDS experiments although the formation of N2O and N2 is very clear on the Au(1 1 0)-(1 2) surface. This could be due to the facile reaction of the resulted O(a) adatoms on the Au surface with both CO in the residual gas of the UHV chamber [45] and NO to form NO2(a) on the surface [18–20]. The formation of NO2(a) on the Au(1 1 0)(1 2) after 100 L NO exposure could be inferred by comparing its NO TDS spectrum with that following 10 L NO2 exposure on Au(1 1 0)-(1 2) (Fig. S3), in which the NO2 desorption peaks were observed at 205 and 312 K. Other indications for the formation of NO2(a) include the upshift of the desorption temperatures of aNO, a-N2O, and CO desorption features with the growth of the bNO desorption peak. It has been demonstrated that NO2(a) on Au surfaces could stabilize the co-adsorbed species [46]. The adsorption of NO on Au(9 9 7) and Au(1 1 0)-(1 2) was also studied by XPS. No signal could be observed by XPS for NO adsorption on Au(9 9 7). This could be due to the low coverage of NO(a) on Au(9 9 7). NO(a) chemisorbed on the 7-coordinated (1 1 1) step atoms of Au(9 9 7) at most only amounts to 0.125 ML (monolayer). A clear O 1s XPS peak was observed at 531.9 eV after an exposure of 5 L NO on Au(1 1 0)-(1 2) (Fig. 5A), and a very weak N 1s XPS peak could be identified at 401.5 eV (Fig. 5B). These features could be reasonably associated with the a-NO desorption peak in the corresponding TDS spectra. With the increase of NO exposure, both O 1s and N 1s features grow, and the O 1s feature slightly shifts to 531.6 eV; meanwhile, new O 1s and N 1s features, respectively, appear at 534.1 and 405.3 eV. Fig. 6 shows the O 1s and N 1s XPS spectra after a 100 L NO exposure on Au(1 1 0)-(1 2) at 105 K followed by annealing at elevated temperatures. After the exposure at 105 K, two O 1s features at 531.6 and 534.1 eV and two N 1s features at 401.6 and 405.3 eV were observed. After the annealing at 125 K, the O 1s feature at 534.1 eV and the N 1s feature at 405.3 eV completely disappear, and the N 1s feature at 401.5 eV weakens and shifts to 402 eV, but the O 1s feature at 531.6 eV only slightly weakens. These observed changes in the XPS spectra correspond to the b-NO, b-N2O and b-N2 desorption peaks in the TDS spectra. The O 1s feature at 531.6 eV and the N 1s feature at 402 eV weaken after the annealing at 170 K and completely disap-
pear after the annealing at 325 K, corresponding to the a-NO and NO2 desorption peaks in the TDS spectra. XPS studies of NOx adsorption on Au surfaces are few, and only three relevant works have been found. We have recently studied the adsorption of NO2 on Au(9 9 7) and identified the N 1s and O 1s binding energy of NO2(a) on Au(9 9 7), respectively, at 401.3– 402 and 531 eV [35]. Vinod et al. studied the adsorption of NO and N2O on Au(3 1 0) at 80 K by XPS and TDS [25]. N2O(a) on Au(3 1 0) formed by the adsorption of N2O at 80 K exhibited two N 1s peaks at 401.5 and 405.5 eV that were, respectively, assigned to end-N (NNO) and middle-N (NNO) of chemisorbed N2O(a) species [25]. On Ag(1 1 1), the N2O(a) formed after NO adsorption at 90 K gave two N 1s features at 401.8 and 405.5 eV and an O 1s signals at 534.1 eV [47]. An exposure of NO (5 106 mbar, 30 min) on Au(3 1 0) at 80 K resulted in two N 1s features at 401.5 and 403.5 eV that were, respectively, assigned to N2O(a) formed by NO decomposition and NO(a) [25]. The assignment of the N 1s feature at 403.5 eV to chemisorbed NO(a) on Au(3 1 0) is questionable since this value is much higher than the N 1s binding energy of chemisorbed NO(a) on Ag(1 1 1) (401.8 eV) [47] and Cu(1 1 1) (401 eV) [48] and the N 1s binding energy of chemisorbed NO2(a) on Au(9 9 7) (401.3–402 eV) [35]. Bukhtiyariv et al. recently employed in situ XPS to study the decomposition of NO on Au(1 1 1), Au(3 1 0), and Au(5 3 3) surfaces at elevated NO pressure up to 7 Pa and temperatures between 300 and 500 K [26]. They identified the N 1s binding energy of chemisorbed N(a) on Au(3 1 0) and Au(5 3 3) to be 399.4 eV. They also observed a N 1s feature at 402.7 eV after NO exposure on Au(5 3 3) at 300 K that was assigned to N2O(a). It is noteworthy that, in the Vinod et al.’s work [25] and Bukhtiyariv et al.’s work [26], the employed NO pressures were high and might result in surface reactions other than NO decomposition and thus form N-contained surface species other than N(a), NO(a), and N2O(a). Comparing the TDS and XPS results of NO adsorption on Au(1 1 0)-(1 2), we assigned the O 1s feature at 531.9 eV and the N 1s feature at 401.5 eV to chemisorbed NO species, the O 1s feature at 534.1 eV and the N 1s features at 401.5 and 405.3 eV to chemisorbed N2O(a) species, and the O 1s feature at 531.6 eV and the N 1s feature at 402 eV to chemisorbed NO2(a) species. These XPS results clearly demonstrate that the bNO species is accompanied by N2O(a) during the exposure of NO on Au(1 1 0)-(1 2) at 105 K. N2O was reported to weakly chemisorb on clean Au(3 1 0) surface, but the interaction could be much enhanced with the coexistence of oxygen adatoms on the surface [25]. Thus, the b-NO species is much more facile to decompose into N2O(a) than the a-NO species on Au(1 1 0)-(1 2), whereas the
Fig. 5. (A) O 1s and (B) N 1s XPS spectra following indicated NO exposures on Au(1 1 0)-(1 2) surface at 105 K.
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Fig. 6. (A) O 1s and (B) N 1s XPS spectra after 100 L NO exposure on Au(1 1 0)-(1 2) surface at 105 K followed by annealing at indicated temperatures.
a-NO species binds the surface more strongly than the b-NO species. Above experimental results of adsorption and decomposition of NO on Au(1 1 0)-(1 2) and Au(9 9 7) can be summarized as follows: (1) Low-coordinated Au atoms exhibit an enhanced reactivity toward NO adsorption. NO does not chemisorb on Au(1 1 1) at 95 K [17,18], but chemisorb on the (1 1 1) step of Au(9 9 7) surface and on Au(1 1 0)-(1 2) that expose low-coordinated Au atoms at 105 K. (2) Chemisorbed NO species on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces can decompose to form N2O and N2 at low temperatures, and its decomposition reactivity depends on both the chemisorption site and the surface coverage. The a-NO species chemisorbed on the 7-coordinated Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces exhibits similar decomposition behaviors. However, on Au(1 1 0)-(1 2), the a-NO(a) species that binds the surface more strongly exhibits a much poorer decomposition reactivity than the b-NO species. This exemplifies an important concept in heterogeneous catalysis that the most strong chemisorption site on the catalysts surface sometimes is not most catalytically active. Moreover, the decomposition reactivity of a-NO species on both Au(9 9 7) and Au(1 1 0)-(1 2) decreases with the increase in its coverage, whereas the decomposition activity of b-NO on Au(1 1 0)-(1 2) increases with the increase in its coverage. The structure sensitivity of low-temperature NO decomposition catalyzed by Au surfaces can thus be concluded. These experimental results are very interesting, particularly concerning the distinctly different decomposition reactivities between the a- and b-NO species. The decomposition of NO on transitional metal surfaces has been extensively studied, and two mechanisms can lead to the N2O formation: one is the direct N– O bond breaking followed by a reaction of NO(a) with N(a) [49]; the other is the (NO)2 dimer mechanism in which N2O is formed by the N–O bond breaking of (NO)2 dimer [50,51]. The N2O formation mechanism by NO decomposition on Au surfaces remains controversial. Vinod et al. [25] proposed that the formation of N2O upon NO adsorption on Au(3 1 0) at 80 K was due to the reaction of adsorbed NO(a) with atomic N(a) formed by NO(a) decomposition. However, Chau et al. [17] proposed that N2O was formed via the (NO)2 dimer mechanism on Au field emitter tips under NO gas flow conditions at 300 K because of the observation of + the desorption of NO+, ðNOÞþ 2 , and N2O . The distinctly different decomposition reactivities between the a- and b-NO species on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces offer an opportunity to clarify the NO decomposition mechanism on Au surfaces. Thus, we
performed DFT calculation studies of adsorption and decomposition of NO on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces. The adsorption of NO on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces was firstly investigated. On both surfaces, the most preferable site for NO adsorption was found to be the most coordinationunsaturated (7-coordinated) Au atoms along the ridge. Fig. 7 shows the adsorption structures of NO(a) on the top and bridge sites of 7coordinated Au atoms, and Table 1 summarizes the corresponding adsorption energy and geometric structure. The adsorption energy Eads is defined as follows: Eads = (Esurf + nENO Etot)/n, where n is number of NO unit (for NO(a), n = 1; and for (NO)2 dimer, n = 2), Esurf is the energy of clean Au(1 1 0) or Au(9 9 7) surface, ENO is the energy of NO gas phase, and Etot is the energy of adsorbed system. The adsorption energies of NO(a) on the top and bridge sites of 7coordinated Au atoms vary between 0.50 and 0.60 eV. These values agree with previous reported adsorption energies of NO(a) on stepped Au surfaces such as Au(1 1 0)-(1 2), Au(2 1 1), Au(3 2 1), and Au(3 2 2) [23,24,29]. The similar adsorption energies of NO(a) on the top and bridge sites of 7-coordinated Au atoms imply that NO(a) on Au ridge can migrate easily. The adsorption energies of atop NO(a) on 9-coordinated Au atoms of Au(9 9 7) (the (1 1 1) terrace) and Au(1 1 0)-(1 2) (the side of row atoms) were calculated to be 0.37 and 0.33 eV, respectively. These values agree with previous reported adsorption energy of atop NO(a) on Au(1 1 1) [19,20,28]. These calculation results clearly demonstrate that low-coordinated Au atoms exhibit enhanced chemisorption ability toward NO. However, comparing with gas-phase NO (Table 1), the N–O bond distance of NO(a) on both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is only a little bit elongated, indicating that the adsorption of NO on the Au surfaces does not weaken the N–O bond much. As the result, the activation barriers for the direct decomposition of atop NO(a) on Au(9 9 7) and Au(1 1 0)-(1 2) into N(a) and O(a) were calculated to be 3.68 and 4.68 eV, respectively. Therefore, the formation of N2O observed experimentally after NO adsorption on Au(9 9 7) and Au(1 1 0)-(1 2) at 105 K under our experimental conditions is not likely resulted from the reaction of adsorbed NO(a) with atomic N(a) formed by NO(a) decomposition. DFT calculation results demonstrate that two NO(a) molecules adsorbed on neighboring ridge Au sites of Au(9 9 7) and Au(1 1 0)(1 2) surfaces facilely form adsorbed (NO)2 dimer without barrier for the flat potential surface along the ridge gold atoms. The formed (NO)2 dimer on the ridge Au atoms of Au(9 9 7) surface (denoted as (NO)2 shown in Fig. 8A) exhibits an adsorption energy of 0.75 eV (Table 1), and the formed (NO)2 dimer on ridge Au atoms of Au(1 1 0)-(1 2) surface (denoted as (NO)2-I shown in Fig. 9A)
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Fig. 7. Top views of NO(a) adsorbed on top site (A) and bridge site (B) of Au(1 1 0)-(1 2) ridge atoms and on top site (C) and bridge site (D) of Au(9 9 7) ridge atoms. The insets show the corresponding side views. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 Adsorption energy and structural parameters of various surface species, surface intermediates and transition states for NO adsorption and decomposition on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces shown in Figs. 10 and 11. The calculated structural parameters of free NO, (NO)2 and N2O are also included for the comparison. Species
Eads (eV)
dAu–N (Å) dAu–O (Å)
Gas-phase NO Gas-phase (NO)2 Gas-phase N2O Au(9 9 7)
Bridged-NO(a) Atop NO(a) TS0 in Fig. 10
0.57 0.59
(NO)2
0.75
(NO)2
0.54
(NO)2
0.38
dN–N (Å)
1.164 1.167 1.186
2.007 1.161
2.320(2.325) 2.159 2.093/2.089/2.496 2.157 2.216/2.213
1.181 1.170 1.975 1.180/1.179
1.956
1.245/1.246
1.430
1.296/1.316
1.241
1.167/1.170
2.101
1.184/1.187
1.810
1.310/1.295
1.332
1.766/1.269
1.185
(NO)2-I
0.71
2.316/2.317 2.579/2.573 – 2.310(2.324)/2.209 – 2.955/3.022 2.831/2/905 2.744/2.731 – 2.206/2.359 – 2.189(2.22)/2.386 2.29 2.134 2.076(2.099) 1.986 2.221(2.215)
(NO)2-II
0.59
2.575(2.667)/2.540(2.680)
1.174/1.175
1.978
(NO)2
0.50
1.247/1.247
1.424
(NO)2
0.36
2.300/2.306 2.535/2.531 – 2.272(2.261)/2.217 3.366/2.820 2.675/2.945 2.918/2.077 4.315/4.326 5.375/5.561 3.145/2.764 – 2.339/2.185(2.181)
1.425/1.306
1.239
1.167/1.175
1.939
1.211/1.224
1.637
1.167/1.165
2.026
1.284/1.681
1.199
TS1 in Fig. 10 TS2 in Fig. 10 TS3 in Fig. 10 TS4 in Fig. 10 Au(1 1 0)
dN–O (Å)
Bridged-NO(a) Atop NO(a) TS0 in Fig. 11
0.56 0.57
TS1 in Fig. 11 TS2 in Fig. 11 TS3 in Fig. 11 TS4 in Fig. 11
exhibits an adsorption energy of 0.71 eV (Table 1). Note that the adsorption energy of (NO)2 dimer is referred to each NO molecule. Thus, (NO)2 dimer adsorbed on the ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is more stable than corresponding NO(a). Comparing with free (NO)2 dimer molecule (Table 1), these chemisorbed structures of (NO)2 on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces do not activate (NO)2 much. Meanwhile, the N–O bond distance of (NO)2 dimer adsorbed on the ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is very close to that of corresponding NO(a). These results demonstrate that the (NO)2 dimer
1.183 1.171 2.128 1.178
1.977
adsorbed on the ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces via the N atoms does not promote the activation of the N–O bond. Besides the (NO)2 dimer species, other types of (NO)2 dimer were found to form on Au(9 9 7) and Au(1 1 0)-(1 2) surfaces. On the Au(9 9 7) surface, (NO)2 dimer with its O atom bonded with the ridge Au atoms and its N atom bonded with the terrace Au atoms (denoted as (NO)2 shown in Fig. 8B) exhibits an adsorption energy of 0.53 eV. It is noteworthy that adsorbed (NO)2 dimer compete with (NO)2 for the ridge Au atoms on Au(9 9 7). On the
Z. Wu et al. / Journal of Catalysis 304 (2013) 112–122
119
Fig. 8. Top views of (NO)2 dimer adsorbed on Au(9 9 7) surface. The insets show the corresponding side views. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. Top views of (NO)2 dimer adsorbed on Au(1 1 0)-(1 2) surface. The insets show the corresponding side views. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Au(1 1 0)-(1 2) surface, two additional types of adsorbed (NO)2 dimer were identified: (NO)2-II (Fig. 9B) with both N atoms bonded with the bridge site of edge Au atoms of the trench exhibiting an adsorption energy of 0.59 eV, and (NO)2 (Fig. 9C) with both N atoms and O atoms bonded with the top site of edge Au atoms of the trench exhibiting an adsorption energy of 0.50 eV. The (NO)2 dimer species on both Au(9 9 7) and Au(1 1 0)-(1 2) surfaces is thermodynamically much more stable than other types of (NO)2 dimer species; however, as demonstrated from their geometric parameters summarized in Table 1, it is much less activated than other types of less stable (NO)2 dimer species. The N–O bond distance of (NO)2 dimer on Au(9 9 7) is greatly elongated to 1.246 Å, and the N–N bond is greatly shortened to 1.430 Å. Similarly, the N–O bond distance of (NO)2 dimer on Au(1 1 0)-(1 2) is greatly elongated to 1.247 Å, and the N–N bond is greatly shortened to 1.424 Å. These theoretical calculation results also exemplify the concept that the most strongly chemisorbed surface species is not always most strongly activated. We then searched for the decomposition pathways of various types of (NO)2 dimer species on the Au(9 9 7) and Au(1 1 0)(1 2) surfaces. Fig. 10 presents the results on the Au(9 9 7) surface. The (NO)2 dimer firstly converts to an intermediate species bonded with the surface via the O atom (denoted as (NO)2 dimer) with an activation energy of 0.62 eV; then, the (NO)2 dimer facilely decomposes to O adatom and N2O with an activation energy of 0.17 eV. The (NO)2 dimer exhibits an adsorption energy of 0.38 eV, two N–O bond distances of 1.296 and 1.316 Å, and the N–N bond distance of 1.241 Å (Table 1). The decomposition of (NO)2 dimer was found to follow various pathways: one is that the (NO)2 dimer firstly converts to (NO)2 dimer with an activation energy of 1.24 eV followed by the decomposition pathway of (NO)2 dimer; another is that the (NO)2 dimer firstly converts to (NO)2 dimer with an activation energy of 1.34 eV followed by the decomposition pathway of (NO)2 dimer. These calculation results demonstrate that the (NO)2 dimer species with a small adsorption energy on the Au(9 9 7) surface exhibits a higher decomposition
reactivity to form N2O than the (NO)2 dimer species with a large adsorption energy. This is reasonable because comparing the (NO)2 dimer species, the (NO)2 dimer species exhibits a weakened N–O bond and a strengthened N–N bond (Table 1). On the Au(1 1 0)-(1 2) surface (Fig. 11), the (NO)2 dimer firstly converts to an intermediate species bonded with the surface via the O atom (denoted as (NO)2 dimer) with an activation energy of 0.49 eV; then, the (NO)2 dimer facilely decomposes to O adatom and N2O with an activation energy of 0.11 eV. The (NO)2 dimer exhibits an adsorption energy of 0.36 eV, two N–O bond distances of 1.425 and 1.306 Å, and the N–N bond distance of 1.239 Å (Table 1). The decomposition of the most stable (NO)2-I dimer adsorbed on the ridge Au atoms could follow various pathways: one is that the (NO)2-I dimer firstly converts to (NO)2 dimer with an activation energy of 0.70 eV followed by the decomposition pathway of (NO)2 dimer; another is that the (NO)2-I dimer firstly converts to (NO)2 dimer with an activation energy of 0.95 eV followed by the decomposition pathway of (NO)2 dimer. Similar to the case of Au(9 9 7) surface, less stable (NO)2 dimer species adsorbed on the Au(1 1 0)-(1 2) surface exhibits a higher decomposition reactivity to form N2O. Above theoretical calculation results can be summarized as follows: (1) Low-coordinated Au atoms exhibit an enhanced ability to adsorb NO. NO(a) adsorbed on the 7-coordinated Au atoms is less stable than the (NO)2 dimer species adsorbed on the same sites, but exhibits a similar stability to other types of (NO)2 dimer species. (2) (NO)2 dimer adsorbed on the 7-coordinated Au atoms is more stable than NO(a) adsorbed on the same sites, but the activation energy of (NO)2 dimer decomposition is much lower than that of NO(a) decomposition. Thus, the active surface species for the decomposition of NO on Au surfaces is (NO)2 dimer. (3) The reactivity of (NO)2 dimer species on Au surfaces to decompose into O adatom and N2O sensitively depends on its structure, and multireaction pathways coexist for the (NO)2 dimer decomposition reaction. Particularly, the (NO)2 dimer species with a smaller adsorption energy exhibits a higher decomposition reactivity to form
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Fig. 10. Calculated decomposition reaction pathways of NO(a) and (NO)2 on Au(9 9 7) surface. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. The structural parameters of surface species and transition states are summarized in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
N2O. The (NO)2 dimer bonded with the Au(9 9 7) and Au(1 1 0)(1 2) surfaces via its O atom served as the intermediate species shown in Figs. 10 and 11 exhibits the smallest activation barrier for the decomposition reaction but meanwhile the lowest adsorption energy. These results agree with previous theoretical calculation results of (NO)2 dimer decomposition on Au(1 1 1) [28] and Ag(1 1 1) [51] in which (NO)2 dimer bonded with the surface via the O atom exhibits the lowest barrier for the decomposition reaction. However, there is a major difference regarding the stability of various (NO)2 dimer species on Ag(1 1 1) and Au surfaces. The (NO)2 dimer species bonded with Ag(1 1 1) via the O atom was calculated to be slightly more stable than the (NO)2 dimer species bonded via the N atom [51], while the (NO)2 dimer species bonded the surface via the O atom on Au(1 1 1) was calculated to be much less stable than the (NO)2 dimer bonded via the N atom [28], agreeing with our theoretical calculation results of Au(9 9 7) and Au(1 1 0)(1 2) surfaces. These theoretical calculation results suggest that the formation of chemisorbed (NO)2 dimer species and their decomposition should dominate the surface chemistry of NO on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces under our experimental conditions. Adsorbed (NO)2 dimer species will decompose into NO(g) as soon as it desorb from the surface and thus is indistinguishable from NO(a) in the TDS spectrum. No XPS result has been reported for the chemisorbed (NO)2 dimer species on metal surfaces. We have thus calculated the N 1s and O 1s binding energy shifts between chemisorbed NO and (NO)2 on both Au(1 1 0)-(1 2) and
Au(9 9 7) surfaces. The results show that the N 1s binding energy of chemisorbed (NO)2 is 0.2 eV higher than that of chemisorbed NO, while its O 1s binding energy is 0.3 eV lower than that of chemisorbed NO. Thus the experimentally observed O 1s feature at 531.9 eV and the N 1s feature at 401.5 eV could also be assigned to chemisorbed (NO)2 on Au surfaces. Therefore, although the presence of NO(a) cannot be excluded, on the basis of theoretical calculation results, we propose that the a-NO desorption peak in the TDS spectra largely arises from most stable (NO)2 dimer species chemisorbed on the 7-coordinated ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces (Figs. 7A, B and 8A), while the bNO desorption peak in the TDS spectra largely arises from less stable (NO)2-II and (NO)2 dimer species chemisorbed on the trench Au atoms of Au(1 1 0)-(1 2) surface (Fig. 8B and C). On the Au(9 9 7) surface, less stable (NO)2 dimer (Fig. 7C) needs to compete with more stable (NO)2 for the ridge Au atoms; thus, its formation is suppressed but still indicated by the weak low-temperature shoulder peak shown in Fig. 3A. The experimentally observed different decomposition reactivities of a- and b-NO species can be well explained by the calculated stability-dependent decomposition reactivity of various types of (NO)2 dimer species. The most stable (NO)2 dimer species chemisorbed on the 7-coordinated ridge Au atoms of Au(9 9 7) and Au(1 1 0)-(1 2) surfaces via the N atoms exhibits a high activation barrier (1.0 eV), and thus, only a small portion of a-NO species undergoes the decomposition reaction when heated (Fig. 3). The less stable (NO)2-II and (NO)2 dimer species chemisorbed on the trench Au atoms of Au(1 1 0)-(1 2) sur-
Z. Wu et al. / Journal of Catalysis 304 (2013) 112–122
121
Fig. 11. Calculated decomposition reaction pathways of NO(a) and (NO)2 on Au(1 1 0)-(1 2) surface. Green, yellow, blue, and red balls represent ridge Au atom, other Au atom, N atoms, and O atom, respectively. The structural parameters of surface species and transition states are summarized in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
face exhibit a low activation barrier (0.5 eV), and thus, the b-NO species can facilely undergo the decomposition reaction to form N2O(a) on the surface during the adsorption process at 105 K (Figs. 4 and 5). The catalytic performances of various supported Au nanocatalysts were examined in NO reduction by CO, H2, and hydrocarbons [14]. Supported Au nanocatalysts were active at low reaction temperatures, their activity sensitively depends on the catalyst structure, and the reduction product was dominantly N2O at low reaction temperatures but N2 at high reaction temperatures. Our comprehensive experimental and theoretical calculation results provide some deep insights into the fundamental understanding. Firstly, various types of chemisorbed (NO)2 dimer species form and coexist with NO(a) on Au surfaces. Secondly, chemisorbed (NO)2 dimer species on Au surfaces are more facile to decompose than NO(a); thus, chemisorbed (NO)2 dimer species, instead of NO(a), are the likely active surface species for low-temperature NO reduction catalyzed by supported Au nanocatalysts. Thirdly, N2O(a) and O(a) are the primary decomposition products of chemisorbed (NO)2 dimer species on Au surfaces. This agrees with previous report that the reduction product of NO reduction catalyzed by supported Au nanocatalysts was dominantly N2O at low reaction temperatures [14]. Fourthly, the decomposition reac-
tivity of chemisorbed (NO)2 dimer species sensitively depends on its chemisorption configuration and thus the local structure of Au surface. This explains the structure sensitivity of NO reduction catalyzed by supported Au nanocatalysts [14]. A noteworthy observation is that the decomposition reactivities of various types of chemisorbed (NO)2 dimer species to N2O vary in a trend contrast to their adsorption energies on Au surfaces. Comparing with other supported metal nanocatalysts, a unique advantage of supported Au nanocatalysts is its exceptional high activity at low reaction temperatures. The observation that the decomposition reactivities of various types of chemisorbed (NO)2 dimer species to N2O vary in a trend contrast to their adsorption energies on Au surfaces sheds light on the nature of the exceptional high activity of supported Au nanocatalysts at low reaction temperatures. During the catalytic reaction, the coverage of a surface species on the catalyst surface is determined by the gas-phase pressure (liquid-phase concentration), the reaction temperature, and its adsorption energy on the catalyst surface. At high reaction temperatures, the surface species with the largest adsorption energy dominates on the catalyst surface, and its reactivity determines the observed catalytic activity; with the decrease in reaction temperature, its surface coverage increases, meanwhile, other types of surface species with smaller adsorption energies that do not compete
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the surface sites with the most stable surface species will also appear on the catalyst surface, and their coverages also increase; thus, the reactivities of all types of surface species present on the catalyst surface contribute to the observed catalytic activity. For a catalyst surface on which the surface species with small adsorption energies exhibit high reactivities, or better reactivities than the surface species with the largest adsorption energy, it will exhibit a high low-temperature catalytic activity or a better catalytic activity at low temperatures than at high temperatures. NO decomposition into O adatom and N2O on Au surfaces reported herein is a nice example. The activation energy of (NO)2 dimer species with the largest adsorption energy is much higher than that of other types of (NO)2 dimer species with small adsorption energies; thus, the Au surface is more active in catalyzing the NO decomposition into O adatom and N2O at low temperatures where (NO)2 dimer species with small adsorption energies are present on the surface than at high temperatures where only the (NO)2 dimer species with the largest adsorption energy is present on the surface. CO oxidation catalyzed by supported Au nanocatalysts is another example in which CO oxidation involves different types of surface species and proceeds with different reaction mechanisms at different regions of reaction temperatures [52–56]. We believe that above concept is also applicable for other heterogeneous catalytic reactions catalyzed by supported Au nanocatalysts with an exceptional high activity at low reaction temperatures.
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4. Conclusions
[26]
By a combined experimental and theoretical calculation study of NO adsorption and decomposition on the Au(9 9 7) and Au(1 1 0)-(1 2) surfaces, we have successfully understood the structure sensitivity of low-temperature NO decomposition on Au surfaces at the molecular level. (NO)2 dimer species, instead of NO(a), is the active surface species for the NO decomposition into O adatom and N2O. Depending on the Au surface structure, various types of (NO)2 dimer species form. The (NO)2 dimer species bonded with the lowest-coordinated Au atoms of the surface via the N atom is thermodynamically more stable than other types of (NO)2 dimer species but also exhibits a much larger activation energy for the decomposition reaction than other types of (NO)2 dimer species. Therefore, the Au surface exhibits a high activity in catalyzing the NO decomposition reaction at low temperatures where less stable, but highly reactive (NO)2 dimer species are present on the surface and facilely undergo the decomposition reaction. These conclusions learned from the well-defined model catalyst study provide deep insights into the fundamental understanding of structure sensitivity and exceptional high low-temperature catalytic activity of supported Au nanocatalysts.
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Acknowledgments
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This work was financially supported by National Basic Research Program of China (2013CB933104, 2010CB923301), National Natural Science Foundation of China (20973161, 21103156), MOE Fundamental Research Funds for the Central Universities.
[48]
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2013.04.013.
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