Article pubs.acs.org/JPCC
N2 Emission via Intermediate N2O in a Steady-State NO + CO + D2 Reaction on Stepped Pd(211) by Angle-Resolved Desorption Tatsuo Matsushima*,† and Anton Kokalj‡ †
Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan Department of Physical and Organic Chemistry, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
‡
ABSTRACT: The role of the byproduct N2O has long been unknown in the NO reduction on the best metal catalysts regardless of many mechanistic studies. This paper clarifies how N2O works as an intermediate that emits N2 in the main pathway of deNOx processes. The offnormal emission of N2 from the decomposition of intermediate N2O(a) has been separately studied in a steady-state NO + CO + D2 reaction on stepped Pd(211) as well as NO + CO and NO + D2 reactions by means of angle-resolved product desorption. In these reactions, the N2 emission commonly takes place through either N2O(a) → N2(g) + O(a) or N(a) + N(a) → N2(g). With increasing surface temperature, a channel change from the former to the latter occurs slowly in the NO + CO reaction, whereas it proceeds quickly in the NO + D2 and NO + CO + D2 reactions around a kinetic transition. With increasing D2 pressure in the NO + CO + D2 system, the N2 emission via N2O(a) decomposition increases or remains constant below the kinetic transition, whereas CO2 formation is largely reduced and water formation is steeply increased, indicating a sharp change in the channel of surface-oxygen removal via reaction with CO to that with deuterium. The mechanism of N2 swing desorption in the off-normal emission has been elaborated on stepped Pd(211) as well as Pd(110) by referring to the potential energy surfaces as calculated by the density functional theory (DFT).
1. INTRODUCTION The N2 emission of the catalyzed NO reduction on palladium and rhodium surfaces (the best deNOx metal catalysts) takes place through either N2O(a) intermediate decomposing to N2(g) + O(a) or nitrogen association as N(a) + N(a) → N2(g).1,2 Our previous paper reported a sharp change in the N2 emission channel from the former to the latter at ∼500 K in a steady-state NO + D2 reaction on stepped Pd(211) = [(S)3(111) × (001)].3 The present paper delivers the first angle-resolved desorption (ARD) analysis of a steady-state NO + CO + D2 reaction on the same surface. It reports a similar channel change as well as an enhanced N2O pathway with increasing D2 pressure and another change in the channel of surface-oxygen removal via reaction with CO to that with deuterium. For comparison, the paper also delivers new ARD measurements of both steady-state NO + CO and NO + D2 reactions. Furthermore, the off-normal N2 desorption mechanism has been elaborated on both Pd(110) and stepped Pd(211) using density functional theory (DFT) calculations. The N2 emission via intermediate N2O(a) has received much attention because of the potential to provide deNOx catalysts that will work at low temperatures;4−12 i.e., the formation of the harmful byproduct N2O is relatively enhanced in the course of NO reduction at low temperatures where the intermediate N2O(a) pathway that yields both N2 and N2O prevails.2,3,13,14 The decomposition of intermediate N2O(a) also plays a main role in N2 emission in the catalytic removal of aqueous nitrate (NO3−) and nitrite (NO2−) ions.15−17 Furthermore, this decomposition provides a stage suitable for examining the energy partitioning in repulsive product desorption because the off-normal N2 emission yields a remarkable anisotropy. This © 2015 American Chemical Society
examination confirms the mechanism of desorption dynamics sensitive to surface sites, since the product energies are connected to the transition state structure via the energy partitioning.14 For this peculiar emission, the swing-desorption model of energetic N2 has been proposed after the N−O bond rupture in the lying N−N−O on the basis of the potential energy surface (PES) of the nascent N2 motion;3,14 the repulsive forces operative from the counterproduct O(a) make the nascent N2 swing over the bonding metal atom to the opposite side, leading to its off-normal desorption.18 Calculations of the PES, however, were preliminary because only one transition state structure was assumed. In this paper, three different cases are examined to elaborate the swing desorption mechanism, predicting that the collimation angle (the maximum flux position) largely depends on the structure of the sites toward which the nascent N2 swings. Surface nitrogen and oxygen deposited from NOx must be removed to keep catalysts active. In the presence of gaseous CO as a reducer, deposited O(a) is converted to CO2, whereas N(a) is removed as either N2 or N2O on Pd, Pt, and Rh surfaces.7−10,19−25 N(a) removal has long been proposed to proceed via temperature-dependent contributions of N(a) + NO(a) → N2O(g), N(a) + NO(a) → N2(g) + O(a), and 2N(a) → N2(g) steps. These elementary steps have been derived from temperature-programmed desorption (TPD) of NO- and/or N-covered surfaces.13,19,20,26−33 There, the N2O Received: March 6, 2015 Revised: April 24, 2015 Published: April 27, 2015 11699
DOI: 10.1021/acs.jpcc.5b02210 J. Phys. Chem. C 2015, 119, 11699−11713
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The Journal of Physical Chemistry C
Figure 1. (a) Top and side views of stepped Pd(211), definition of crystal axes, and the definition of the desorption angle, θ. Adsorption structures of the N2O intermediate as predicted by DFT calculations: (b) the most stable adsorption form, (c) adsorption form on the terrace, and (d) transitionstate structure for NN−O dissociation (before the N2 swing motion).
the decomposition of adsorbed N2O on Pd(211) (Figure 1a).44 The polar angle sign is defined in accordance with the work of Ikai and Tanaka.19 It is positive in the step-up direction and negative in the step-down direction; the off-normal N2 emission appears on the latter side (see Figure 1a). A palladium crystal (from Surface Preparation Laboratory, Netherlands) in a disk-shaped slice (with a diameter of 10 mm) was mounted on the top of a manipulator. The LEED pattern shows a sharp (1 × 1) form after cleaning by repeated cycles of Ar+ ion bombarding, heating in oxygen, reducing in CO, and subsequent annealing to 1100 K. Without further purification, commercial 15NO (isotope purity = 99%), 15N2O (99%), D2 (99%), and 13CO (99%) were separately back-filled. AI-mass signals with mass/charge ratios of 4 (D2), 20 (D2O), 21 (15ND3), 29 (13CO), 30 (15N2), 31 (15NO), 32 (O2), 45 (13CO2), and 46 (15N2O) can be simultaneously monitored. Hereafter, isotopes 15N and 13C are simply designated as N and C in the text. The AI or AR signal is determined to be the signal difference between a desired surface temperature and room temperature. 2.B. Computational. DFT calculations were performed within the generalized gradient approximation as implemented in the PWscf code contained in the Quantum ESPRESSO distribution.45 In our previous publications,46,47 the Perdew− Burke−Ernzerhof (PBE) functional was used.48 However, this functional significantly overestimates the N2 bonding to late transition metal surfaces; this shortcoming is of crucial importance when studying desorption of the product N2. For this reason, the revised PBE (revPBE) functional of Zhang and Yang,49 which reduces the overbinding of N2, is currently used. The Kohn−Sham orbitals were expanded in a plane wave basis set to a kinetic energy cutoff of 35 Ry (350 Ry for the chargedensity cutoff), whereas core electrons are described implicitly by ultrasoft pseudopotentials.50 Brillouin-zone (BZ) integrations were performed with the special-point technique,51 and Fermi-surface effects were treated using the smearing technique of Methfessel and Paxton52 using a smearing parameter of 0.03 Ry. The Pd(211) surface was modeled with a slab consisting of 12 (211) layers (this slab thickness corresponds to a Pd(111) slab four-layers thick), and the molecule was adsorbed on one side of the slab. The bottom three layers were constrained to
pathway had been treated as a side reaction rather than to yield N2, probably because no spectroscopic observations of N2O(a) are successful in steady-state NO reduction.27,34−38 In fact, the surface residence time of this intermediate is too short to be detected by surface-vibration spectroscopy in the course of catalyzed NO reduction above ca. 450 K because of the small adsorption heat and fast decomposition.2,3,14 Furthermore, before our ARD analysis, no direct evidence had been found of the decomposition of intermediate N2O(a) to N2 in the NO reduction.13,31−33,39,40 At high pressures, readsorption of the product N2O and its subsequent reduction to N2 have been considered.4−9,30 Thus, except for our ARD analysis,1−3 most of the kinetic analyses of the steady-state NO reduction reported so far have been unsuccessful in examining the N2O pathway to N 2 because the amount of N 2 O(a) was immeasurable.4−9,24,27,29,32,39 In our method, the N2O pathway to N2 can be separately examined because the resultant N2 is offnormally emitted, whereas in emission through the association of N(a) + N(a), the product is desorbed nearly along the surface normal.1−3,19,20,40−42
2. TECHNICAL 2.A. Experimental Section. The apparatus consists of three ultrahigh vacuum chambers, described previously.1 Briefly, a reaction chamber has reverse-view low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy optics, and a mass spectrometer for angle-integrated (AI) signals. An analyzer, which is separately evacuated, has a mass spectrometer for angle-resolved (AR) measurements in a pulsecounting mode. The chopper house between the reaction chamber and the analyzer is rapidly and separately evacuated with a cooled (∼25 K) copper plate and a tandem turbomolecular pump. The produced N2 flux is measured with an AR mass spectrometer without sensitivity corrections due to different velocities as a function of the desorption angle (θ = polar angle) because no serious shift is caused in constructing the angular distribution.43 These ARD measurements can be conducted without disturbing a steady-state reaction in the reaction chamber as confirmed by the AI-mass spectrometer.1 On the present surface, the desorption angle is scanned in the normally directed plane along the [1̅11] direction because of the maximized product N2 desorption in 11700
DOI: 10.1021/acs.jpcc.5b02210 J. Phys. Chem. C 2015, 119, 11699−11713
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Figure 2. Temperature dependence of the N2 intensity in the AI mass spectrometer and AR-N2 signal at 31° off-normal toward the step-down direction in (a) a steady-state NO + CO reaction with PNO = 0.95 × 10−5 Pa and PCO = 1.3 × 10−5 Pa and (b) a steady-state NO + D2 reaction with PNO = 0.95 × 10−5 Pa and PD2 = 3.2 × 10−5 Pa. Signals observed in the direction of the increasing surface temperature are designated by filled symbols, and those in the downward direction are designated by open symbols.
peak at 475 K and follows a broad maximum at ∼550 K. The peak at 475 K is relatively much smaller in the AI-N2 signal, indicating a sharp angular distribution. The ratio of the AR signal to the AI signal is largely decreased above 500 K, indicating sharp changes in the angular distribution. The lesssharp temperature dependences at ∼475 K were previously reported, whereas the reactant pressure ratio was either much smaller or much higher than that in the present work.3 Here, a ratio of PD2/PNO = 3.4 is selected to show a sharp enhancement of this N2 emission at ∼475 K. In the NO + D2 reaction, the other products are D2O, ND3, and N2O. The AI-N2O signal is always small, peaks at ∼475 K, and decreases slowly at higher temperatures. The amount of D2O is always significant, whereas the ND3 signal becomes significant above ca. 480 K. As reference measurements, we have also examined the reaction of N2O + D2 → N2 + D2O. No steady-state N2O reduction is noticed under pressure conditions similar to those for the NO + D2 reaction, i.e., PN2O = 1.0 × 10−5 Pa, PD2 = 3.2 × 10−5 Pa, and TS = 350−700 K. 3.B. CO Pressure Dependence. The steady-state NO + CO reaction shows first-order dependence in CO at 475 K to near the saturation level (Figure 3a). The AR-N2 signal at −31° off-normal is also shown in the figure. It increases in a way parallel to the AI-N2 signal, indicating an angular distribution that is insensitive to the CO pressure. The kinetic transition appears when the CO pressure reaches ∼4 times the NO pressure. Above this pressure, the reaction is reduced, showing negative orders in CO. No sharp kinetic changes are found in contrast to those in the NO + D2 reaction.3 The angular distribution of desorbing N2 in the NO + CO reaction is insensitive to either the CO/NO pressure ratio or the surface temperature. The AR-N2 signal normalized to the unit surface area is plotted versus the desorption angle on the polar coordinates in the normally directed plane along the stepup and -down direction (Figure 3b−d). N2 desorption at TS = 475 K is sharply collimated around −30° off-normal in a similar way for both cases of PCO > PNO and PCO < PNO. N2 formation is enhanced at a higher PNO without changes in the angular
the bulk positions and the in-plane lattice spacing was fixed to the revPBE calculated equilibrium bulk Pd lattice parameter of 4.01 Å, while all other degrees of freedom were relaxed. Adsorption was modeled with a (3 × 1) supercell using the 3 × 3 × 1 uniformly shifted k-point mesh. The thickness of the vacuum region (the distance between the top of the admolecule and the adjacent slab) was set to ∼10 Å. The Pd(110) surface was modeled with a slab that consisted of six (110) layers (the bottom two layers were constrained), a (3 × 2) supercell, and the 3 × 3 × 1 uniformly shifted k-point mesh. The dissociation of N2O and the upright → flat transition of N2 were calculated using the climbing image nudged elastic band (CI-NEB) method.53,54 The transition states (TSs) were located as the configurations of the largest energy along the minimum energy paths (MEPs). The potential energy surface (PES) for the upright → flat transition of N2 was further described by a series of stepwise constrained relaxation calculations where only the two angles that determine the orientation of N2 were fixed (while other degrees of freedom were relaxed).
3. RESULTS 3.A. Temperature Dependence. Remarkable differences are found in the temperature dependence of the N2 formation in steady-state NO reduction with the reducing reagent between CO and D2 (Figure 2). The differences are wellreproduced in the subsequent decrease of the surface temperature (TS). N2 formation is commonly noticeable above ∼420 K. In the NO + CO reaction, the AI-N2 signal shows a broad peak at ∼520 K, and the AR-N2 at −31° offnormal (the collimation angle) is maximized in a similar way, except for the peak position at ∼500 K. This similarity indicates slow changes in the angular distribution of desorbing N2 with increasing surface temperature.3 The AI signal due to the counterproduct CO2 is approximately twice the AI-N2 signal. The AI signal of N2O is Pd(100) > Pd(110)21−23 or Rh(111) > Rh(100) > Rh(110).24 In the former model that omits the intermediate N2O(a) decomposition, these orders have been explained as “the more open surfaces run the NO dissociation
Figure 7. D2 pressure dependence of the amounts of adsorbed CO(a) and N(a) + NO(a) on Pd(211) in the course of the steady-state NO + CO + D2 reaction at TS = 460 K. PNO = 1.0 × 10−5 Pa, and PCO = 1.3 × 10−5 Pa. The amounts observed in the direction of the increasing D2 pressure are designated by closed symbols, and those in the decreasing D2 pressure are indicated by open symbols.
peak area due to the AI−D2 signal is always negative. However, the amount of deuterium-containing species (D(a) + OD(a) + OD2(a) + DN(a) + D2N(a)) is not determined because of the memory effect of the D2O signal.
4. DISCUSSION 4.A. Intermediate N2O(a) That Emits N2. As the NO + D2 reaction is predominant even in the presence of CO, our mechanism of this reaction on Pd(211) is briefly summarized.3 (i) The overall reaction proceeds below approximately TS = 500 K via NO(a) dissociation and the subsequent NO(a) + N(a) → N2O(a) step followed by either decomposition that emits N2(g) or desorption of N2O(g). The formation of N2O(a) via a dimer of NO ((NO)2(a) → N2O(a) + O(a)58,59 or (NO)2(a) + N(a) → N2O(a) + NO(a)60) is negligible on this surface.19,20 (ii) At higher temperatures, the nitrogen recombination, 2N(a) → N2(g), is relatively enhanced because of the high activation energy and decreased amounts of NO(a). (iii) The removal of deposited O(a) by D(a) is very fast, yielding either a ΘO ≫ ΘD or a ΘO ≪ ΘD condition, where ΘO and ΘD stand for the coverage of O(a) and D(a), respectively. The removal of N(a) as ammonia becomes significant above the critical D2 pressure (i.e., ΘO ≪ ΘD), making clear the channel change of the N2 emission. On the other hand, in the NO + CO reaction on the same surface, no sharp channel change of the N2 emission is found in dependence on either the surface temperature (Figure 2) or the CO pressure (Figure 3a) because of the lack of 2D(a) + O(a) → D2O and 3D(a) + N(a) → ND3 processes. Nevertheless, N2 emission still proceeds through either N2O(a) → N2(g) + O(a) or N(a) + N(a) → N2(g), as confirmed from the angular distributions of desorbing N2. These pathways to N2 are also operative in the NO + CO + D2 reaction. Under a high vacuum, as was used in the present work, the readsorption of the emitted product N2O and its subsequent decomposition can be neglected, even if its sticking probability is close to unity, because the partial pressure of emitted N2O is much less than that of NO (∼1/100 in the present work). Furthermore, we have confirmed with our apparatus that no steady-state N2O reduction is noticed on Pd(211) under 11705
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fast as that with O(a), and the ammonia formation then competes with both N2O formation and N atom recombination. The deposited O(a) immediately reacts with two deuterium atoms to form D2O(g). This reaction is faster on palladium surfaces involving subsurface (or bulk) deuterium.81 An increased supply of subsurface D that takes place above the kinetic transition will lead to a higher reaction rate after removal of D(a) to some extent, because parts of the subsurface deuterium may contribute to oxygen removal after its backdiffusion. This explains the temporally enhanced reactivity after the terminated D2 supply, as shown in Figures 5 and 6. 4.D. Swing Desorption on Pd(110). The channel shift in the removal of O(a) or N(a) does not change the N2 distribution in the off-normal emission regardless of the reduced intensity or the changed surface composition. Thus, off-normal desorption is suggested to be controlled within small areas. For this peculiar distribution, the swing desorption of energetic N2 has first been proposed on nonreconstructed Pd(110) as described in the Introduction.18 Therefore, we first discuss the desorption scenario on this surface. Ikai and Tanaka have already confirmed the occurrence of off-normal emission of N2 on nonreconstructed (1 × 1) parts in their earlier AR-TPD report of NO-covered Pd(110) by means of LEED.82 The reconstructed part with a missing row form is stabilized by adsorbed oxygen,83 but it is inactive for this N2 emission. This missing-row form on Pd(110) is metastable after the removal of oxygen and is converted into the nonreconstructed (1 × 1) form above 355 K.83,84 The desorption model on inclined facets of reconstructed surfaces was again suggested by Hodgson for N 2 O decomposition on Pd(110).42,72 Later, reconstruction by N2O adsorption has been excluded by scanning tunneling microscopy (STM) observations.85,86 Furthermore, the off-normal N2 desorption is well-reproduced in the steady-state N2O + D2 (or CO) reaction on Pd(110) above 400 K both below and above the kinetic transition, being consistent with occurrence on the (1 × 1) part.87 On Pd(110)(1 × 1), the active N2O(a) is in a lying form, oriented along the [001] direction. It is bonded to the surface via terminal N and O atoms in bent geometry because of the charge back-donation from the metal.46,47 The swing motion of the nascent N2(a) is started by the N−O bond rupture in the lying N−N−O. Immediately after this rupture, highly repulsive forces are induced between the resultant N2 and the counterproduct O(a).18 At this moment, the bonding of the terminal nitrogen to the metal atom is not yet broken. Thus, the nascent N2 is swung over the bonding metal atom (passing the most stable standing form) to an inclined position on the opposite side. The sequential break of the terminal N−metal bond was predicted to let the N2 fragment leave the surface.14 To examine this final bond breaking, the potential energy surface of the swing motion has been constructed by means of DFT calculations. Few plausible scenarios for the N2O(a) dissociation (N−O bond cleavage) and concomitant desorption of the resulting N2 were examined on Pd(110). On this surface, the most stable N2O form is adsorbed on the top site, whereas N2O adsorbed at the bridge site is less stable.46,47,88 DFT calculations, however, reveal that the energy of the transition state for the dissociation of top-bonded N2O is larger than that for bridgebonded N2O. This implies that top-bonded N2O is more likely to first diffuse to a nearby bridge site and to dissociate there, because the respective cumulative energy barrier (ΔEtop→bridge +
faster (at high NO coverage) and are, thus, more selective for N2 because of higher N atom coverage”.23,24 According to our mechanism, on the other hand, selectivity at low temperatures is merely controlled by branching intermediate N2O(a) to decomposition and desorption. The selectivity to N2O should be smaller when the intermediate N2O(a) easily dissociates, i.e., the dissociation tendency should be in the inverse order of the selectivity to N2O. In fact, N2O(a) is desorbed without dissociation on Pt(111),67 Ir(111),68 Rh(111),69 Ni(111),70 and probably on Pd(111)71 as well. The small fraction of dissociation may proceed on structural defects.69 On the other hand, on open surfaces such as Pd(110),1,72 Ir(110),1 and Rh(110),1 N2O(a) is largely dissociated below 100 K in subsequent heating, thus emitting N2. It also dissociates, to a small extent, below 100 K on Rh(100)1 and Pd(211).44 No dissociation takes place on Pd(100).73 On flat surfaces as well as stepped surfaces, N2O is commonly formed as N(a) + NO(a) → N2O(a) in TPD procedures of NO-exposed surfaces when N(a) is deposited.19,20,33,74 4.C. Enhanced N2O Pathway. Surface species mostly consist of NO(a) and N(a) on Pd(211) at TS = 450−490 K in the presence of a nearly equimolar mixture of NO and CO, although both molecules adsorb efficiently on the clean surface.75,76 On this surface, NO is partly dissociated at these temperatures. The deposited O(a) and CO(a) or N(a) and NO(a) can be removed as CO2 or N2O and N2, and the resultant vacant surface sites are then populated by either CO or NO. Nevertheless, the surface is eventually covered by NO(a) + N(a), suggesting repulsive interactions are operative among CO(a), O(a), NO(a), and N(a). The activation energy of the reaction of CO(a) + O(a) → CO2(g) or N(a) + NO(a) → N2O(a) is significant, and actually, these adsorbed species are copresent below 420 K.19,20 On the other hand, the reaction of 2H(a) + O(a) → H2O(a) on Pd already proceeds at ∼220 K.77,78 The high reactivity of D(a) is still kept, even in the presence of O(a), N(a), CO(a), and NO(a), because of the fast diffusion and a small activation barrier for the reaction with O(a), around 29 kJ/mol.79 Thus, this reaction can control the coverage of D(a) and O(a); i.e., only either ΘO ≫ ΘD or ΘO ≪ ΘD is possible in the steadystate NO + CO + D2 reaction under a high vacuum. This fast reaction provides unoccupied sites suitable for the adsorption and dissociation of NO as well as for the decomposition of N2O(a). Eventually, with increasing D2 pressure, the amount of N(a) + NO(a) is increased, and the pathway of N(a) + NO(a) → N2O(a) → N2(g) + O(a) is enhanced. The amount of surface oxygen deposited from NO(a) dissociation must be significant below the critical kinetic point. At the kinetic transition point, the amounts of surface deuterium and oxygen sharply change in the opposite directions. Thus, D(a) accumulates above the kinetic transition point, and then both the N2O(a) formation itself and its decomposition are reduced. On the resultant D(a)-rich surface, ND3 formation starts at higher temperatures, due to the somewhat higher activation energy of the 3D(a) + N(a) reaction. ND(a) and ND2(a) may be present above the kinetic transition point,80 but these species are minor below the kinetic transition point due to depleted D(a). These sequential changes of surface species are due to the order of the activation energy of the four processes, as 2D(a) + O(a) → D2O < N(a) + NO(a) → N2O(a) ≤ 3D(a) + N(a) → ND3 < 2N(a) → N2. The reaction of D(a) with N(a) is not as 11706
DOI: 10.1021/acs.jpcc.5b02210 J. Phys. Chem. C 2015, 119, 11699−11713
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Figure 8. Minimum energy path (MEP) for the upright-N2 → flat-N2 transition on Pd(110) for top- and bridge-bonded N2; ΔE is defined as ΔE = Esite − E0site, where E0site is the potential energy of the equilibrium upright-N2 adsorbed at site, Esite is the energy of a given configuration at that site, and site is either top or bridge. Magnitudes for the adsorption energy of top- and bridge-bonded N2 are also indicated (notice that for the bridge site the transition activation energy is larger than the desorption energy and vice versa for the top site). Each point on the MEP corresponds to a corresponding structure on the pertinent filmstrip (upper (lower) filmstrip corresponds to bridge (top) site). The inset shows the definition of the N−N−metal bend angle t1, the tilt angle t2 between the N−metal bond and the surface normal, and the angle tc between the surface normal and the axis linking the N2 mass center with the adsorption site. For all but the first point on the MEP, the (t1, t2) angles are also stated.
E*bridge) is smaller than the dissociation barrier at the top site (E*top). ΔEtop→bridge is the energy difference between N2O bonded at bridge and top site, ΔEtop→bridge = EN2O(bridge)/Pd(110) − EN2O(top)/Pd(110), whereas E*bridge is the activation energy for dissociation at the bridge site. The reason is that during the dissociation at the top site the O fragment initially bonds to the top site and, only after the N−O bond is already broken, it diffuses to the stable bridge site, whereas during dissociation at the bridge site the O bonds directly to the stable bridge site. McCalman et al. have considered another scenario,88 where top-bonded N2O rotates slightly (helicopter-like) during dissociation such that O approaches the bridge site as the N−O bond is broken. According to our calculations, this scenario is plausible and displays a dissociation barrier similar to the cumulative barrier for dissociation at the bridge site. The cleavage of the N−O bond is only the first part of the process that leads to the inclined desorption of N2. Immediately after the N−O bond is broken, the respective fragments (O and N2) individually bind to the surface and then experience large mutual repulsive forces. Due to these forces, the N2 swings over the adsorption site to the opposite side. According to the above-described dissociation mechanisms, two different swinging processes should be examined: the first involves the swinging of N2 over the top site, and the second involves swinging over the bridge site (for both, swinging along the ±[001] direction is considered). It should be noted that, according to DFT calculations, two qualitatively different adsorption forms of N2 exist: upright and horizontal. For this
reason, the potential energy surface (PES) for swinging of N2 was considered using two different techniques. The first involves calculating the minimum energy path (MEP) for transition from the upright to the horizontal form of N2(a) (Figure 8), and the second comprises a series of calculations where the two pertinent angles (t1, t2) are scanned stepwise (Figure 9), i.e., at each “(t1, t2)-point” a constrained relaxation was performed, where t1 is the N−N−metal bend angle and t2 is the tilt angle of the N−metal bond against the surface normal (see also the graphical definition of the two angles in Figures 8 and 9). The (t1, t2) angles are also stated for each point calculated on the MEP in Figure 8. The PES for the swinging motion of N2 is considerably more repulsive for the bridge site (Figures 8 and 9), whereas the adsorption bonding of N2 is considerably stronger at the top site (see Figure 8). Figure 8, therefore, reveals that the PES for the swinging motion is repulsive enough to yield the desorption of N2 only at the bridge site where the barrier for the upright → horizontal transition is 0.21 eV larger than the adsorption energy magnitude of N2. In contrast, at the top site, the barrier for the upright → horizontal transition of N2 is 0.1 eV smaller than the N2−metal bond strength, which implies that N2 undergoes upright → horizontal transition rather than desorption. With reference to the MEP of Figure 8, it seems plausible that, at the bridge site, the N2−metal bond is broken somewhere between the intersection of the MEP curve with the |Eads| line (lower bound) and the transition-state (upper bound), i.e., after the third and before the sixth point on the MEP. The respective t1 11707
DOI: 10.1021/acs.jpcc.5b02210 J. Phys. Chem. C 2015, 119, 11699−11713
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Figure 9. 2D and 3D representations of the (t1, t2) potential energy surface (PES) for the swinging of N2 on top and bridge sites on Pd(110); the PES for the bridge site is shown in the upper row, and the top-site PES is shown in the bottom row. ΔE is defined analogously as in Figure 8.
Figure 10. MEP for the upright-N2 → flat-N2 transition along the [11̅1̅] direction on Pd(211). Each point on the MEP corresponds to a corresponding structure shown on the filmstrip. Note that the flat-N2 (last structure) does not correspond to a local minimum on the PES but was constrained to the bridge position (actually the axis of N2 was constrained in the [11̅1̅] direction for the whole MEP); if this constraint is lifted, then the N2 relaxes by displacing the N atom at the step-edge bridge site to a step-edge top site. The inset shows the definition of the N−N−metal bend angle t1 and the tilt angle t2 between the N−metal bond and the local (111) surface normal. For all but the first point on the MEP, the (t1, t2) angles are also stated. ΔE is defined analogously as in Figure 8. The horizontal blue dashed line indicates the desorption energy (Edes) of the upright-N2.
and t2 angles are approximately 20° for the lower and 40° for the upper bound. If instead of (t1, t2) angles the angle between the surface normal and the axis linking the adsorption site with the center of mass of N2 (angle tc, defined graphically in the inset of Figure 8) is considered, then respective tc values are in a range from approximately 30° (lower bound) to 50° (upper
bound). This range is consistent with the N2 collimation angles of 40−51° that are actually observed on Pd(110).1 For a more definitive discussion of the N2 desorption event, insight from DFT-based molecular dynamics (MD) simulations will be needed. 11708
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Article
The Journal of Physical Chemistry C
toward the adjacent bridge site and across the trough, which is 21/2dnn away (dnn is the nearest neighbor Pd−Pd distance); whereas on the Pd(211) it swings toward the bridge site at the step edge (see Figure 10), which is (31/2/2)dnn away and, thus, closer by ∼40%, as compared to that on Pd(110). As a consequence, the swinging MEP motion of N2 is more “stretched” on Pd(110) (smaller t1, larger t2) and more “twisted” on Pd(211) (larger t1, smaller t2) (compare the filmstrips in Figures 8 and 10). However, differences in the tc angles are smaller; for example, the tc angles for the TS structures are 50° and 37° on Pd(110) and Pd(211), respectively. However, more importantly than these differences in angles is the observation that the swinging PES is somewhat less repulsive on Pd(211) than that for the bridge site on Pd(110). Also, the bonding of N2 onto the (111) terrace is weaker than that on the bridge site of Pd(110). The energy difference between the N2 desorption energy and the barrier for the upright → horizontal transition (ΔEdes‑vs‑tr = Edes − E*⊥→∥) is, therefore, similar on Pd(110) and Pd(211). This energy difference is rather crucial because N2 will desorb only when the ΔEdes‑vs‑tr is negative. Moreover, it can be argued that, the more negative the ΔEdes‑vs‑tr is, the more repulsive the N2 tilting PES relatively is as compared to the N2 desorption barrier and the smaller (closer to the surface normal) the desorption angle should be. This similarity in ΔEdes‑vs‑tr on the two surfaces may explain why the desorption collimation angle is similar on the two surfaces, ∼50°, if measured with respect to the local (111) surface normal on Pd(211). In the literature, Ikai and Tanaka82,91 first proposed the desorption-mediated reaction of NO on Pd(110) as a mechanism of the off-normal N2 emission. In this mechanism, desorbing NO collides with N(a), inducing the off-normal desorption of the product. Using an isotope tracer, they skillfully confirmed that, on Pd(110) and stepped Pd(211), this off-normal N2 comes from the reaction of NO(a) with N(a); i.e., the heating of a surface with 15N(a) and 14NO(a) results in the emission of 14N15N off-normal and 15N2 along the surface normal. They argued that the orientation of the reaction coordinate of the above-described desorption-mediated N(a) + NO(a) → N2(g) + O(a) process is responsible for the offnormal N2 emission.19,20 However, they have never given any explanation of their reaction coordinate.40 Matsushima et al., on the other hand, reproduced the inclined N2 emission in the absence of NO(a) in the TPD procedures of several N2Ocovered surfaces, including Pd(110) as well as steady-state NO or N 2 O reduction on Pd(110) and Rh(110). 1,2 The contribution of the off-normal N2 emission pathway in NO reduction depends on the reaction conditions, such as the surface temperature and reactant gas composition, as seen on Pd(211) in this paper as well. The lack of difference in the desorption dynamics (spatial and/or velocity distributions) of high-energy product N2 between steady-state NO + CO (or H2) below ∼550 K and thermal decomposition or reduction of N2O indicates off-normal product emission from a common transition state. The state of desorbing NO has no effect on offnormal emission in NO reduction, although the participation of NO(a) is required for intermediate N2O(a) formation. Thus, the swing desorption mechanism is the first physically sound model that is able to predict the collimation angle of the off-normal N2 emission. It well predicts a bidirectional inclined desorption of N2 on fcc (110) surfaces in the plane along the [001] direction. On Pd(211), it naturally predicts a unidirectional inclined desorption of N2 because N2O can dissociate
The amount of energy released during the dissociation of N2O is considerably larger than the energy needed for desorption of N2. According to the DFT calculations, the maximum amount of released energy is ∼1.4 eV on Pd(110), calculated as ΔE = EN2(a)+O(a) − EN2O(a,TS); if the energy needed for the desorption of N2 is taken into account, the value is smaller by the corresponding amount. Released energy is initially predominantly distributed among the O and N2 fragments. Respective energy partitioning between the two fragments can be roughly estimated by integrating the repulsive forces as obtained from the MEP calculation of N2O dissociation, which yields 0.6 eV for O, 0.8 eV for N2, and
