Understanding the effects of metal particle size on the NO2 reduction from a DFT study

Applied Surface Science 489 (2019) 1019–1029

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Understanding the effects of metal particle size on the NO2 reduction from a DFT study B.M. Pascuccia, G.S. Oteroa, P.G. Belellia, M.M. Brandab, a b

T

⁎

IFISUR, CONICET, Av. Alem 1253, Bahía Blanca 8000, Argentina INFAP, CONICET, Av. Ejército de los Andes 950, San Luis 5700, Argentina

ARTICLE INFO

ABSTRACT

Keywords: Cu Ag Au Nanoparticles NO2 Adsorption Reduction DFT

The study of the effect of particle size and low coordination sites in metal nanoparticles (Cun, Agn and Aun with n = 19, 38, 55, 79 and 116) on the reduction of NO2 to NO + O, was carried out using density functional theory (DFT) calculations. All metal nanoparticles have shown to be more favorable for the NO2 adsorption than the (111) extended surfaces. The adsorption energy order of NO2 found for both configurations, O-down (most stable) and N-down, was Cu > Ag > Au. The dissociation energy values of NO2 on Cu and Ag nanoparticles decrease with the increment of the particle size, however, considering the activation barrier values, the most reactive substrates evaluated were Cu(111) surface, and the Cu19, Cu116. The activation barriers (Eact) for the larger nanoparticles, were obtained using a non-traditional relationship of Brönsted Evans Polanyi (BEP), from the Eact calculated on the smaller ones. The BEP relations highly depend on the reaction product configurations and also on the structure of the active site. Notwithstanding that the nanoparticles improve the NO2 adsorption and the dissociation energies are lower than those corresponding to the extended surfaces, the activation barriers are higher.

1. Introduction Reduction reactions involving NOx are of great interest in many fields of basic and applied research, including surface science [1], environmental catalysis [2,3] and many areas of biology and physiology [4]. The formation of NOx products is a consequence of incomplete combustion of fossil fuels, being these oxides the most dangerous and harmful pollutants in our atmosphere. When NOx are combined with moisture they produce nitric acid, which is one of the principal constituents of the acid rain and photochemical smog in the planet [5]. One way of avoiding the presence of NOx is to achieve its removal through the reduction to N2 and O2. Nitrogen oxides have been identified as toxic and irritant gases with significant risks for people with respiratory diseases. Exposure of NO2 at concentrations higher than 1.0 ppm induces anomalous changes in lung function. In repeated exposures at high concentrations of NO2 (1–5 ppm), an increase in respiratory rate and a reduced gas exchange capability in the lungs were evidenced [6]. Some recent works [7–11] proved that the use of metal nanoparticles greatly improves the catalytic activity compared to the extended surfaces. Moreover, several research groups have concluded that size plays a key role in the catalytic activity of nanoparticles [12–15]. In ⁎

particular, while gold surfaces show very low reactivity, small Au nanoparticles (1–6 nm) are highly reactive materials [16–20]. Catalysts based on nanoparticles improve the chemical reactions through the reduction of the activation energy, increasing their efficiency. Gold nanoparticles can, for example, catalyze the oxidation of CO at room or lower temperatures, which are significantly lower than those needed when supported metal catalysts are used. Selective oxidation reactions of hydrocarbons also occur at low temperature on supported gold catalysts [21,22]. The reactivity of metal nanoparticles is a fundamental issue of heterogeneous catalysis [23–25], however much remains to be studied. These systems have different physical and chemical properties with respect to the bulk and extended surfaces [26]. The main cause of these differences is that nanoparticles have low coordination sites at the edges and corners. The number of this type of sites varies according to the size and shape of the nanoparticle. Theoretical studies of O2 dissociation on Au nanoparticles [58,28] showed that, besides the influence of the low coordination sites, there is a critical size for which the reaction is favored. Thus for a given reaction in study, systematic studies with increasing size of nanoparticles are needed and then compared to the corresponding studies on metallic surfaces. NOx adsorption and its dissociation have been experimentally

Corresponding author. E-mail address: [email protected] (M.M. Branda).

https://doi.org/10.1016/j.apsusc.2019.05.318 Received 17 November 2018; Received in revised form 24 May 2019; Accepted 27 May 2019 Available online 29 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 489 (2019) 1019–1029

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[1,30–32] and theoretically [33–36] studied over extended surfaces. Brown et al. [1] examined the structure of NO adsorbed on some metal surfaces and also they discussed the NO dissociation process. From theoretical results, the bonding of NO on the three-fold (fcc) site of the Cu(111) surface was reported [34]. This interaction is essentially ionic, with a small but noticeable π covalent contribution. On the other hand, Gajdoš et al. [37] found that NO preferentially adsorbs on hcp-hollow sites of Cu and Ag surfaces, while weak adsorptions were observed on Au bridge sites. Similar observations were made by Dumas et al. [38] using the Infrared Reflection-Absorption Spectroscopy (IRAS) technique. At low temperatures, the monomeric NO adsorbs on threefold hollow site of Cu(111) surface. The interaction of NO with Au(111) surface was theoretically studied using slab models with a different number of layers, to evaluate the effects of substrate thickness [39]. The authors suggested that the formation of NO2 from NO on regular sites of Au(111) would be limited by the lifetime of NO species weakly bound to the surface. Electronic properties observed for the NO adsorption and its oxidation on a regular surface of Au(111) and on a thin film of Au are qualitatively the same. NO2 species was less investigated. It was observed that, at elevated temperatures (300–500 K), NO2 is a very good oxygen source for the oxidation of thick films of Ag, Zn and Cu supported on Ru(001), with a dissociation probability close to one [31]. An XPS/UPS study showed that the NO2 adsorption on Ag(111) at 300 K is also dissociative, resulting in NO and O adsorbed products [40,41]. They observed a large excess of O(ads), due to the partial desorption of NO(ads). At low temperatures (90 K), the adsorption is dissociative again, obtaining the same products as at 300 K with a covering of 0.5 ML. However, NO desorption was not found. Although the adsorption and reduction of NOx on some metal surfaces have been studied in the past, recently the spectrum has been opened to the study of promising small particles. In order to gain a better understanding of the effect of low coordination sites present in metal nanoparticles, the catalytic reduction of NO2 to NO + O on octahedral nanoparticles of Cu, Ag, and Au and on their respective extended (111) surfaces, was studied using density functional theory (DFT) calculations.

Five metal nanoparticles with different shapes and sizes, formed by 19, 38, 55, 79, and 116 atoms, which exhibit low coordination sites, were selected in order to analyze the size effects on the surface reactivity. The nanoparticles were named as Men, where Me was Cu, Ag, and Au, with n = 19, 38, 55, 79, and 116 atoms. All the sites studied for the adsorption are displayed in Fig. 1. The geometry optimization was done considering the total relaxation of NO2 (or NO + O) and all the nanoparticle atoms, excepting the central ones. In almost all the cases, this procedure did not lead to noticeable changes; only the Au79 and Au116 nanoparticles exhibited an appreciable rearrangement of the atoms, converging to a rounded shape. Climbing-image Nudged Elastic Band method (CI-NEB) [52,53] combined with dimer [54] method were used to obtain the transition states (TS) for the NO2 reduction reaction. The minimum energy configurations and TS were confirmed by performing the frequency calculations. The initial and final states (IS and FS) were verified with all real values of vibrational frequencies. On the other hand, the TS was confirmed by obtaining a single imaginary frequency corresponding to the reaction path. Adsorption energies of NO2 were calculated for every possible site present on the Cu, Ag, and Au nanoparticles and (111) surfaces, applying the following equation: Eads = E(NO2/substrate) − E(NO2) − E (substrate), where E(NO2/substrate) is the total energy of NO2 adsorbed on a metal substrate, E(NO2) is the total energy of the free molecule, E (substrate) is the total energy of the (111) surface slabs or Men nanoparticles. Then, starting from the more favorable adsorption sites, the dissociation energies (Ediss) and the respective activation barriers (Eact) were calculated. The first reduction energies were calculated as Ediss = E(NO+O/substrate) − E(NO2/substrate), where E(NO+O/substrate) is the optimized total energy of NO and O co-adsorbed on the substrate and E(NO2/substrate) is the total energy of the NO2 adsorbed on the same substrate. Finally, the activation energies were expressed as Eact = ETS − EIS, where EIS and ETS are the total energies of the initial (NO2 adsorbed on a metal substrate) and transition states, respectively. 3. Results 3.1. NO2 adsorption

2. Computational details

Being in mind our work related to the NO2 adsorption on the extended Me(111) surfaces and on Me19 nanoparticles (with Me = Cu, Ag and Au) [55], in this work we carried out the study of all potential adsorption sites on metal nanoparticles formed by 38 atoms. On Me38, the Eads obtained for the most favorable sites are displayed in Table 1. The O-down configuration is also the most stable one on the three metals studied (see Fig. 2a), being about of 0.3 eV greater than the Ndown configuration. The most stable adsorption site is the same on all Me38 (O-down T1-T1edge). When NO2 is adsorbed on nanoparticles with N-down configuration, the interaction is through two metallic atoms of the edge (bridge sites, not shown). The stabilities of the adsorbed NO2 always follow the order Cu > Ag > Au. Although the obtained sequence is the same as on the Me(111) surfaces and Me19, the Eads are higher on Me38 than on the corresponding extended surfaces and even higher than on Au19 [55]. Considering the NO2 selectivity for specific interaction on Me19 and Me38, the O-down configuration, only this geometry was analyzed for the bigger nanoparticles with 55, 79, and 116 atoms. The optimized geometries of the NO2 adsorbed on these nanoparticles are displayed in Fig. 2 b, c, and d, respectively, and their adsorption energies are shown in Table 2. NO2 adsorption on nanoparticles is much stronger than on the (111) surfaces (more than 0.6 eV) [55]. In addition, it was found that the adsorption strength increases from Me19 to Me38, then decreases for Me55 and remains almost constant for larger nanoparticles, being Me38 the most reactive nanoparticle. For all nanoparticles, similar stretches of the OeN bonds were obtained with respect to the Me(111) surfaces.

Calculations were performed from the density functional theory (DFT) that solves the Kohn-Sham equations for the valence electrons density [42–45]. The interaction between core states and valence electrons was treated through the Projector Augmented Wave (PAW) method proposed by Bloch [46], and the valence electronic states were expanded in a basis of plane waves with a cutoff of 415 eV for the kinetic energy. Moreover, the total energy threshold defining self-consistency of the electron density was set to 10−4 eV and a total force difference less than 10−2 eV for consecutive geometries was set for the structural optimization. The exchange-correlation effects were treated with the generalized gradient-corrected PW91 [47,48]. The Methfessel-Paxton smearing of width σ = 0.05 eV was applied and the reported total energies were then extrapolated to σ → 0 eV. Numerical integration in reciprocal space was performed using a Monkhorst-Pack grid of 5 × 5 × 1 for the extended surface, while Γ point was used for the nanoparticles. All of the calculations in the present work were developed at the spin-polarized level with the Vienna Ab-Initio Simulation Package (VASP) code [49–51]. Taking into account that dispersion effects on these systems were negligible, the calculations were done without the van der Waals correction. Extended Me(111) surfaces, where Me corresponds to Cu, Ag and Au metals, were represented with a periodic slab model of 5 layers and a 3 × 3 unit cell (molecular coverage θ = 0.11 ML). All the possible adsorption sites examined are displayed in Fig. 1. Two uppermost layers were allowed to relax completely together with the adsorbates and the other three layers of metal atoms were maintained fixed. 1020

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Fig. 1. a) Top view of Me(111) extended surface. b) Me19, c) Me38, d) Me55, e) Me79 and f) Me116 nanoparticles. Adsorption sites studied in all the substrates: T, B and H mean top, bridge and hollow sites, respectively.

greater the electronic transfer of the metal to the adsorbate, the greater the stability when it is adsorbed. Comparing the Bader charges of NO2 adsorbed on Me38 nanoparticles and with those on Me(111) surfaces (not shown), it is possible to note that the NO2 receives similar electronic charges from Cu and Ag substrates, while it takes more from Au38 with respect to Au(111). Besides, the interacting metal atoms on nanoparticles release more charge to NO2 than on the respective surfaces, pointing out the more localized interaction. This behavior could be the reason for the better stability of NO2 adsorbed on nanoparticles instead of on surfaces. The low coordination sites present in metal nanoparticles clearly improve the NO2 adsorption.

Table 1 Eads of NO2 on Me38 nanoparticles (in eV). The subscript “edge” means that the molecule is placed on top T1 in the parallel plane to the edge. T1 (100) and T1 (111) indicate that the molecule is placed on top T1 on the planes (100) and (111) respectively. NO2 O-down O-down O-down N-down

T1-T1edge T1-T1 (100) T1-T1 (111) T1-T1

Cu38

Ag38

Au38

−2.47 −2.35 −2.34 −2.16

−1.92 −1.82 −1.82 −1.63

−1.70 – −1.68 −1.44

Strongest adsorption energy values are written in bold.

The average values of both O-Me distances between the O atoms of NO2 molecule and the nearest metal atom of nanoparticles show no significant differences with the increasing size of nanoparticles, but they are shorter than those observed on Me(111) surfaces for the same adsorption geometry. This behavior evidences a stronger interaction of NO2 with nanoparticles mentioned above. For Cu nanoparticles, the average distance of OeCu is 1.97 Å, which is 0.1 Å longer than the corresponding distance in the copper oxide [56]. In cases of Ag and Au nanoparticles, the average distances are even longer than the metal oxides. These results indicate that the NO2 interaction is stronger with Cu nanoparticles than with Ag or Au ones. We analyze the charge transfers in Me38 because they are the most reactive nanoparticles for the NO2 adsorption. Bader charge analysis [57] (Table 3) indicates the oxidation of the two Cu atoms which interact with the oxygen atoms. These changes in the atomic charge go from almost zero (metallic character) for the clean nanoparticle to +0.18e when the NO2 is adsorbed. Similar behavior is observed for nanoparticle atoms of Ag and Au, with an average final charge of +0.14e and +0.11e, respectively. NO2 takes ~−0.6e, −0.5e and −0.4e from the two interacting metal atoms in Cu, Ag and Au nanoparticles, respectively. Both the nitrogen and the oxygen atoms are taking electronic charge from the metal in the following order: Cu > Ag > Au. This behavior agrees with the stability order of NO2 adsorption on the three metals and with the NeO bonds stretching; the

3.2. NO2 dissociation In order to obtain the NO2 dissociation energies, all structure configurations considering every possible co-adsorption sites of NO + O species were studied. These calculations were carried out on nanoparticles formed by 38 and 55 atoms, and the results are depicted in Fig. 3a and b. On Me(111) and Me19 substrates, the NO2 dissociation was previously studied [55], while on Me38 and Me55 the results are depicted in Fig. 3a and b. Then, the co-adsorption study on Me79 and Me116 was restricted to the lowest energy configurations on the most favorable sites, previously found on smaller nanoparticles (Fig. 3c and d). It is important to mention that the dissociation energies were calculated considering only the O-down configuration (most stable one) of NO2 adsorbed on each substrate. On nanoparticles, NO species are adsorbed preferentially on bridge sites, whereas O atom on three-fold (H1 or H2) or four-fold coordinated hollow (H3) sites (see Fig. 3). After the NO + O co-adsorption, all nanoparticles evidenced some degree of deformation, mainly the smallest Au nanoparticles. The same behavior was reported by Roldán et al. [58] in his DFT study of O2 dissociation on Au nanoparticles. The dissociation order of NO2 is somewhat different with respect to the obtained for its adsorption, being in these cases Cu > Au > Ag, excepting for Me116 where the order is inverted on the heavier metals (Ag > Au). For sizes greater than Me38 the effectiveness of the nanoparticles for the 1021

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Fig. 2. NO2 adsorbed with O-down configuration on different sites: a) T1-T1 edge of Cu38; b) T1-T2 of Cu55; c) T1-T3 of Ag79 and d) T1-T1′ of Au116 nanoparticles.

nanoparticles show the highest reactivities with the biggest exothermic reaction energies (see Table 4). The charge analysis was performed for the co-adsorption of the NO and O species on the nanoparticles Me38 and Me116, being the most favorable ones for Au in the first size and for Cu and Ag in the second size (see Tables 5 and 6). On both series of nanoparticles, NO takes more electron charge from Cu > Ag > Au as it was also observed for NO2. Comparing a metal in the two nanoparticle sizes, we observed that NO gains slightly higher electronic charge when is adsorbed on Me116 than on Me38 and the magnitude of the increments are similar in the three metals. In all cases, NO receives electronic charge from the substrates, but it is not equally distributed between the N and O atoms with respect to the free NO. The NO interaction is the strongest on Cu116 (N gains ~0.6e and O loses ~0.1e), is intermediate on Ag116 (N gains ~0.5e and the O atom maintains its charge), and N takes ~0.4e and O practically does not change (0.07e) when the dissociation occurs on Au116. The polarization of the NO bond follows the order: Cu < Ag < Au (see Table 6). Thus, there is a concordance between the electronic transfer from the metal to the adsorbate and the better reactivity for the NO2 dissociation on Cu116 and Ag116 nanoparticles. On Au38, the charge released to NO is the smallest, N gains ~0.3e and O releases 0.06e, for this reason is the most stable situation for this noble metal (with low oxidation capacity). For both sizes, the charge gained by NO and O species is largely provided by the closest metal atoms, indicating localized interactions. The dissociated O atom on Cu and Ag nanoparticles is taking a charge close to one electron, accounting for a strong ionic character. Prior to co-adsorption, not all atoms are neutral in nanoparticles,

Table 2 Eads of NO2 on Me55, Me79 and Me116 nanoparticles (in eV). Nanoparticle

NO2 position

Cu

Ag

Au

Me55 Me79 Me116

O-down T1-T2 O-down T1-T3 O-down T1-T1′

−2.10 −2.14 −2.33

−1.48 −1.61 −1.64

−1.10 −1.17 −1.24

Table 3 Net charges of the interacting atoms for the O-down adsorption on Me38 nanoparticles, where Mex is the atom that interacts with Ox (x = a, b). Charges are in atomic units (e). Me38

a

q(Me ) q(Meb) q(N) q(Oa) q(Ob)

Free NO2

Cu

Ag

Au

0.00 −0.02 – – –

−0.02 −0.02 – – –

−0.02 −0.02 – – –

– – +0.67 −0.32 −0.32

NO2/Me38 Cu

Ag

Au

+0.18 +0.18 +0.56 −0.57 −0.57

+0.13 +0.14 +0.57 −0.57 −0.54

+0.11 +0.10 +0.58 −0.48 −0.47

NO2 dissociation begins to be greater than on (111) surfaces. For this reaction, the relationship between the dissociation energy and the size of nanoparticles has the same trend for Cu and Ag; a decrease of Ediss was found when increasing the particle size, except for Me79. In the case of Au nanoparticles, the most reactive one turned out to have 38 atoms, while the remaining nanoparticles presented similar behaviors. From an energetic point of view, Cu55 and Cu116 1022

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Fig. 3. NO + O co-adsorption geometries of: a) B1 + H3 on Ag38; b) B1 + H3 on Ag55; c) B3 + H2 on Au79 and d) B1 + H3 on Cu116, respectively. Table 4 Dissociation energies (Ediss, in eV) of the reaction: NO2 → NO + O on Me19, Me38, Me55, Me79 and Me116 nanoparticles; d(NeO): final distance between the dissociated fragments (in Å). In parenthesis, o.f. means that the adsorption sites are in opposite face. NO + O/

Cun Site

Me19 Me38 Me55 Me79 Me116 a

NO B1 + O H1 NO B3 + O H3 NO B1 + O H3 (o.f.) NO B1 + O H3 NO B1 + O H3

Agn Ediss

d(NeO) a

0.24 0.07 −0.24 −0.01 −0.57

3.58 5.99 3.72 3.72 3.77

Aun

Site

Ediss

NO T1sup + O H1 NO B1 + O H3 NO B1 + O H3 (o.f.) NO B1 + O H3 NO B1 + O H3

d(NeO) a

1.75 1.31 1.21 1.48 0.98

4.06 4.40 4.27 4.37 4.24

Site NO B1 + O B1 (o.f.) NO B3 + O H1 (o.f.) NO B1 + O B1 (o.f.) NO B3 + O H2 NO B1 + O H3

Ediss

d(NeO) a

1.51 0.67 1.11 1.23 1.13

3.95 4.84 3.94 5.03 4.11

Ref. [55].

but they are neutral on average. Afterward, the Me atoms that interact with adsorbates release electronic charge and the nanoparticle is oxidized. Larger nanoparticles have a greater capacity to oxidize, for this reason the bigger size is more favorable to the NO2 dissociation on Cu116 and to a lesser extent on Ag116, while the smaller size of both analyzed is better for NO2 dissociation on the noblest metal (Au38).

barriers on Me(111) surfaces and on Me19 and Me38 nanoparticles were performed. In only two cases, Cu(111) and Ag(111) surfaces, the final geometries of the co-adsorbed NO and O were not the most stable configurations. In these cases, the NO O are on top and hollow fcc sites, respectively, due to the shorter NeO distances. We assume that the lowest energy pathways for NO2 dissociation will occur first to the nearest adsorption sites and then NO migrates to the final and more stable site. Dissociation (Ediss) and activation energies (Eact) are summarized in Table 7. The NO2 dissociation energy profiles for Me(111) and Me38 nanoparticles and their corresponding geometries are shown in Figs. 4–6.

3.3. Activation barriers Considering the lowest energy configurations of adsorbed NO2 and co-adsorbed NO + O on each substrate, calculations of activation 1023

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Table 5 Net charges of the interacting atoms for the NO and O co-adsorption on Me38 nanoparticles. Mea, b are the two metal atoms that interact with N; OD is the dissociated oxygen atom, which is adsorbed on a hollow site, and Me* are the metallic atoms in this site. Charge values are in atomic units (e). B3 sites for Cu38 and Au38, and B1 site for Ag38. H3 sites for Cu38 and Ag38, and H1 site for Au38. Me38

a

q(Me ) q(Meb) q(N) q(O) q(OD) q(Me*)

Free NO

Cu

Ag

Au

−0.03 +0.01 – – – −0.01

−0.05 +0.04 – – – 0.00

−0.08 0.00 – – – 0.00

– – +0.50 −0.47 – –

NO B + O H/Me38 Cu

Ag

Au

+0.20 +0.20 −0.08 −0.39 −0.99 +0.22

+0.13 +0.35 +0.08 −0.44 −0.91 +0.19

+0.10 +0.10 +0.18 −0.41 −0.76 +0.20

Table 6 Net charges of the interacting atoms for the NO and O co-adsorption on Me116 nanoparticles. Mea, b are the two metal atoms that interact with N; OD is the dissociated oxygen atom, which is adsorbed on a tetracoordinated hollow site. OD is linked with Meb and other three Me atoms which mean charge is Me*. Charge values are in atomic units (e). Me116

a

q(Me ) q(Meb) q(N) q(O) q(OD) q(Me*)

Free NO

Cu

Ag

Au

−0.07 −0.03 – – – −0.01

−0.06 −0.04 – – – −0.01

−0.05 −0.02 – – – 0.00

– – +0.50 −0.47 – –

NO B1 + O H3/Me116 Cu

Ag

Au

+0.17 +0.50 −0.10 −0.42 −1.00 +0.21

+0.13 +0.41 +0.04 −0.46 −0.90 +0.17

+0.09 +0.40 +0.13 −0.40 −0.74 +0.12

Fig. 4. Reaction energy profiles for NO2 reduction on: a) Cu(111) and b) Cu38. The optimized geometries of Initial (NO2 O-down), Transition and Final states are depicted.

Table 7 Reaction and activation energies of NO2 dissociation for the most favorable pathways on the (111) surfaces and on the nanoparticles of 19, 38, 55, 79 and 116 atoms, o.f. between parentheses means opposite face. The column “Eact (eV) Initial Descriptor (Fig. 8a)” reports the interpolated values, calculated with the IS predictor (see Eq. (7)). The last column reports the reverse activation energy values (E− act). Configuration of the FS

Substrate

Ediss (eV)

Eact (eV)

Eact (eV) initial descriptor (Fig. 8a)

E− act (eV)

NO NO NO NO

Cu(111) Cu19 Cu38 Cu55

0.31a 0.24a 0.14 −0.24

0.61 1.53 1.58 –

– – – 1.70

0.30 1.29 1.44 1.94

Cu79 Cu116 Ag(111) Ag19 Ag38 Ag55

−0.01 −0.57 1.51a 1.75a 1.31 1.21

– – 1.84 2.35 1.81 –

1.66 1.55 – – – 2.11

1.67 2.12 0.33 0.60 0.50 0.90

Ag79 Ag116 Au(111) Au19

1.48 0.98 1.48a 1.51a

– – 1.74 2.58

2.02 2.00 – –

0.54 1.02 0.26 1.07

Au38

0.67

2.08

–

1.41

Au55

1.11

–

–

Au79 Au116

1.23 1.13

– –

2.32 2.27

top + O fcc B1 + O H1 B3 + O H3 B1 + O H3 (o.f.) NO B1 + O H3 NO B1 + O H3 NO top + O fcc NO T1 sup + O H1 NO B1 + O H3 NO B1 + O H3 (o.f.) NO B1 + O H3 NO B1 + O H3 NO top + O fcc NO B1 + O B1 (o.f.) NO B3 + O H1 (o.f.) NO B1 + O B1 (o.f.) NO B3 + O H2 NO B1 + O H3 a

1.09 1.14

Fig. 5. Reaction energy profile for NO2 reduction on a) Ag(111) and b) Ag38. The optimized geometries of Initial (NO2 O-down), Transition and Final states are depicted.

Ref. [55].

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dissociation (see Table 7 and Fig. 5a). In contrast, a competition between the dissociation and desorption of NO2 on Ag38 nanoparticle is observed, finding a small difference between these two energies. In case that NO2 dissociates on Ag38, the reverse reaction it is also very likely to occur, i.e., the formation of NO2 will occur from the NO and O fragments, because the inverse barrier is only 0.50 eV (see Fig. 5b). For the three analyzed substrates of Au (the (111) surface, Au19, and Au38) the activation energies are also very high (see Table 7 and Fig. 6). There is a clear preference for NO2 desorption versus its dissociation, however, it is observed that the difference between the activation energy and desorption energy decreases as the size of the nanoparticle increases (1.08 and 0.38 eV, for Au19 and Au38, respectively). Besides, the reverse of activation energies also increases with the increasing of the nanoparticles size. Despite that the dissociation energies improve with the nanoparticles, the activation barriers are too big. In all the cases, the geometries of the TS resemble the final geometries. 3.4. BEP relationships In order to find all the activation energy values for the most favorable reactions studied in this work, the Brönsted Evans Polanyi (BEP) method was used. The BEP relation [59,60] is an empirical rule stating that there is a linear relationship between the activation energy and the reaction energy for an elementary reaction. It is applied in many areas of chemistry and biochemistry, including heterogeneous catalysis, where it is often used to rationalize the observed variations in catalytic activity from one catalyst to the next or to estimate activation barriers [61]. The aim of this part is to find a linear relationship between the activation and reaction energies, from the activation values previously found with the NEB and dimer methods. In the traditional linear-BEP, this relation is defined as Eact = αEreac + β. In this way, a relationship has been sought between the reaction and activation energies obtained on Me(111) surfaces and on Me19 and Me38 nanoparticles to be extended to the biggest nanoparticles Me55, Me79, and Me116. In this work, we define the reaction energy (Ereac) as dissociation energy (Ediss). For all the reaction paths of the NO2 dissociation on all substrates studied, Fig. 7a shows the values of activation energies as a function of the reaction energies. The great dispersion of the data around the adjustment line is evident, from which it can be inferred that there is a poor linear relationship when all the reactions studied are considered together. Therefore, we proceeded to separate the reactions in such a way that they have a geometrically similar FS. Fig. 7b and c show the linear relationships with the traditional BEP for those reactions with the same FS configuration: NO bridge + O hollow and NO top + O hollow configurations, respectively. The relationship with the first geometry as FS did not give good results since the dispersion was very large. In contrast, a better linear relation with low dispersion was observed for the dissociation reactions of NO2 with FS geometry NO top + O hollow. However, this traditional BEP relationship is not useful for the sizes of nanoparticles analyzed, because most of the stable sites have a final geometry where the NO is adsorbed on a bridge site, instead of on top site. Searching for some linear relationship that allows predicting the activation energies, certain authors have found linear relationships based on different descriptors of the reaction energy. For example, Lee et al. [62] in a DFT study on RheNi binary metallic surfaces, considered descriptors based on the relative energy of the initial or final states for dissociation reactions of CeH, OeH, CeC, CeO and, C-OH bonds. Fajín et al. [63] found linear relationships for the dissociation reaction of methanol on different metal surfaces based on descriptors such as the co-adsorption energy of the CH3O and H species. Other authors, such as Wang et al. [64] studying transition metal surfaces, found the linear relationships with descriptors based on the dissociation energies of the CH4, H2O, NH3, and H2 molecules. Performing a wide revision of the

Fig. 6. Reaction energy profile for NO2 reduction on a) Au(111) and b) Au38. The optimized geometries of Initial (NO2 O-down), Transition and Final states are depicted. In parenthesis (o.f.) means opposite face.

The dissociation of NO2 on the Cu(111) surface is more favorable than its desorption (see Fig. 4a), since the desorption energy (1.51 eV) is more than twice greater than the activation energy (0.61 eV). However, the inverse reaction is also possible energetically, that is the formation of NO2 from the NO and O fragments, due the barrier is very low (0.30 eV). Here it is important to emphasize that the NO desorption implies an energy of 0.68 eV, which is more than twice the energy required for the reverse reaction. In this way, it would be expected that during the reaction, a small part of NO will desorb and most of it will oxidize and form NO2. Although the activation barriers are larger on the Cu19 and Cu38 nanoparticles than on the extended surface (see Table 7 and Fig. 4b), NO2 dissociation is more favorable than its desorption. The energies required for NO2 desorption on Cu19 and Cu38 are very high, of ~2.4 eV, while the activation barriers are of ~1.5 eV. Besides, on the Cu19, the reverse activation energy (NO2 formation), and the NO desorption energy are similar (1.29 eV and −1.23 eV, respectively). Although both values are high, these situations may compete during the reaction. In contrast, desorption of NO from Cu38 is more favorable than the formation of NO2 (1.25 eV and 1.44 eV, respectively), but in both cases high energies are required. From the previous analysis, it can be seen that while the adsorption and dissociation energies of NO2 on the Cu nanoparticles are energetically more favorable than on the Cu(111) surface, this improvement does not translate into activation energies. On the Cu(111) surface, the activation barrier is much lower than on nanoparticles, probably due to the smaller geometric change that undergoes in the TS, towards the FS. In nanoparticles the geometries of TS resemble the FS, a fragment of the NO2 molecule (NO) must migrate towards the final site, which is usually on another side of the nanoparticle. In almost all the cases studied, the NO always migrates long distances to its final adsorption position, while the dissociated O atom quickly searches for the nearest hollow site. In case of silver substrate, NO2 desorption on the Ag(111) surface and on the Ag19 nanoparticle is clearly more favorable than its 1025

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Fig. 8. Non-traditional BEP relationships with: a) Initial and b) Final descriptors for reactions with final geometries NO bridge + O hollow.

E TS =

(4)

EFS +

where:

BEP correlations, Panov et al. found that the energy of the individual bonds which are cleaved or formed during the reaction can be used as descriptors for plotting BEP correlations [65]. More recently, Reimers et al. [66] studied the sequential hydrogenation reactions of the CO molecule for methanol synthesis on surfaces of Zn, Ce and Ga oxides by DFT calculations. They found that traditional BEP relationships (Eact vs. Ereac) do not allow a good correlation to predict activation energy barriers, in contrast, BEP relationships referred to the initial and/or final states allowed to find them satisfactorily. If these descriptors are used, the linear relationship with respect to the initial state (IS) is, that is with respect to the energies of NO2 in gas phase (ENO2) and the bare substrate surface (EMe): (1)

EIS +

where:

E TS = ETS EIS = EIS

ENO2 ENO2

EMe EMe

ENO

1 E 2 O2

EMe

(5)

EFS = EFS

ENO

1 E 2 O2

EMe

(6)

EIS, ETS, and EFS are the energies of the initial, transition and final states, respectively. The ∆EIS is the adsorption energies of NO2 (Eads). Fig. S1 shows the corresponding graphs for these descriptors: the first one referring to the initial configuration (Eq. 1), and the second one referring to the final configuration (Eq. 4). There is also great data dispersion with respect to the adjustment line, although an improvement with respect to the traditional BEP is achieved especially for the final descriptor (Fig. S1b). In these figures, all cases are considered independently of the final geometry of the dissociated NO and O species. In search of better linear relationships, we selected the cases with the same geometric configuration of the final state, as explained above. The graphs of non-traditional BEP relationships for the final states NO bridge + O hollow and for NO top + O hollow are shown in Fig. 8 and Fig. S2, respectively. The Initial descriptor achieves a significant improvement in the finding of a linear relationship between ΔEIS and ΔETS when the NO2 dissociation reactions have the NO bridge + O hollow geometries as FS (see Fig. 8a), fulfilling the desired requirements that could not be previously achieved through the traditional BEP. When the reactions have the NO top + O hollow geometries as FS, the Final descriptor achieves the best linear correspondence between ΔEFS and ΔETS (see Fig. S2b). As seen above, these reactions also keep a linear correlation with the traditional BEP, but this relationship is not useful for the most stable configurations of the final geometry with the NO on bridge site, which is the preferred site on nanoparticles (see Table 4). For this reason, we use the linear correction obtained considering the Initial descriptor with the FS geometry NO bridge + O hollow. Next, we proceeded to obtain the Eact for the nanoparticles of the

Fig. 7. Activation energy (Eact) as a function of the reaction energy (Ereac) for: a) all the reactions studied, b) and c) separating by geometries of the final states. The line corresponds to the respective linear adjustment and the equation that describes it.

E TS =

E TS = ETS

(2) (3)

With respect to the final state (FS), that is, with respect to the energies of NO in gas phase (ENO), O2 in gas phase (EO2) and the bare substrate surface (EMe): 1026

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larger sizes (55, 79 and 116 atoms) using the relationship corresponding to the equation obtained from Fig. 8a:

E TS = 1.67 EI + 3.09 eV

were selected in order to analyze the size effects on the surface reactivity. O-down configuration for the NO2 adsorption was the most stable one on the three metals studied. All the metal nanoparticles showed to be more reactive for the NO2 adsorption than the extended surfaces, being Me38 the nanoparticle most reactive. The adsorption energy order found for both configurations (O-down and N-down) was Cu > Ag > Au. NO species are adsorbed on nanoparticles preferentially on bridge sites, whereas O atom on three or four-fold coordinated hollow sites. From the Brönsted Evans Polanyi (BEP) relationship, we obtained the activation energy values for the NO2 reduction on the biggest nanoparticles, using the energy barriers of the directly calculated smaller ones, and then, interpolated for the larger nanoparticles. This relationship is highly dependent on the configuration of the reaction products and also on the structure of the active site. When the TS configuration is similar to the FS one, we found a linear relationship from the final descriptor and also from the traditional BEP. This was the case for the reactions with NO top + O hollow as final states. However, when the configuration of the TS is significantly different from the FS, the TS becomes independent of the FS; in these cases, the final descriptor and the traditional BEP are not valid relationships for obtaining the activation barrier. Notwithstanding the TS structure is far away from both the FS and IS structures, the Initial descriptor achieves a significant improvement finding a linear relationship between IS energies and TS energies for the NO2 dissociation reactions with NO bridge + O hollow as FS. Although there has not found a general predictive approach to the BEP relationships, we found in this work a very helpful prediction of the activation energies using alternative BEP descriptors and taking into account the FS configuration. While we cannot extend these results to other reactions, we take it into account for this same reaction in other substrates. In addition, we propose the extension of this behavior to other families of reactions with the same FS, that is, they could present non-traditional BEP linear relations with a specific descriptor. By examining all the activation barriers obtained by interpolation, it can be noticed that the activation energy slowly decreases as the size of the nanoparticle increases. In almost all the nanoparticles studied, the dissociation energies are lower than those corresponding to the extended surfaces, but the activation barriers are higher.

(7)

By entering the known value of the abscissas, ΔEIS, and by using Eq. (7), the ∆ETS values were calculated. Finally, the corresponding Eact were obtained by the difference with the ΔEIS. These values are reported in Table 7, in which all the values of the calculated Eact and Ediss were also collected. The activation energy for the reaction on Au55 could not be obtained since the final state presents a geometry with both adsorbates (NO and O) on bridge sites, and this configuration is not described by any of the relationships found. In agreement with the postulation of Norskov et al. [67], we have found that in case of the TS configuration is similar to the FS one, there is a linear relationship from the final descriptor and also from the traditional BEP. This has been observed when the reaction occurs on the three extended surfaces (see Figs. 4a, 5a, and 6a) and on Ag19 (not shown) with NO top and O hollow as FS. Conversely, if the TS is very different from the FS configuration, as in the cases with NO bridge and O hollow, the final descriptor and the traditional BEP are not valid descriptors for obtaining the activation barriers. The TS geometries on Me38 nanoparticles are very different from the FS configurations, with NO on bridge and O on hollow sites; in these cases, the alternative BEP which relates the energy of the IS with the energy of the TS, has demonstrated a significant improvement in the finding of a linear relationship with respect to the traditional BEP. It is possible to infer from Eqs. (1) and (4) that an important energy difference between ETS and EIS or between ETS and EFS, respectively, would result to a high Beta value and also a high Alpha value. On the other hand, the most outstanding feature of nanoparticles is to possess multiple edges and kinks that separate different surfaces; this causes the configuration of the FS of the dissociation reaction usually involves the co-adsorption of the fragments in different faces, thus moving away from the TS geometry. Something similar can happen between the IS and TS configurations. These could be the reasons which lead to an Alpha value greater than 1 in Eq. (7). We have observed this same phenomenon in a previous study of the NH3 dissociation on nanoparticles of Fe [68]. In particular, other published works have also obtained alpha values greater than one [69,70]. Despite the activation energies on copper nanoparticles do not decrease substantially with the increasing size, the condition for the favorable NO2 dissociation would be improved due to the reverse activation energy increase (E− act = Eact − Ediss, see Table 7). This means that the products would hardly form NO2 again (as it was also predicted for Cu(111) surface). The Cu116 nanoparticle seems to have the best energy requirements to NO2 desorption. In the case of Ag nanoparticles, NO2 always prefers to desorb than dissociate, except in Ag38 where desorption and dissociation may compete during the reaction. Gold nanoparticles are not good for NO2 dissociation, being Au38 also the most reactive one. Examining all the activation barriers obtained by interpolation, it can be noticed that the activation energies are lower for the Me38 size (except for Cu38); then an increase and afterward a decrement again is observed with the increasing size of the nanoparticle (since the NO2 adsorption energy decreases again). From all sizes of nanoparticles studied, Ag38 and Au38 seem to be a critical size for which the reaction is slightly favored. It is also appreciated that the dissociation energies in almost all the nanoparticles studied is lower than those corresponding to the extended surfaces, but the activation barriers are higher.

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4. Conclusions The reduction of NO2 to NO + O on different sizes of Cu, Ag, and Au nanoparticles and on their respective extended (111) surfaces was studied by means of the density functional theory (DFT) calculations. The metal nanoparticles constructed by 19, 38, 55, 79, and 116 atoms, 1027

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