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Catalytic decomposition of N2O on PdxCuy alloy catalysts: A density functional theory study
Applied Surface Science 510 (2020) 145349
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Catalytic decomposition of N2O on PdxCuy alloy catalysts: A density functional theory study Kyeounghak Kima,1, Seungyeon Baekb,1, Jae Jeong Kimb, Jeong Woo Hana, a b
T
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Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea School of Chemical and Biological Engineering, Seoul National University, Gwanak 1, Gwanak-gu, Seoul 08826, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: N2O reduction reaction Density functional theory (DFT) PuxCuy alloy Metal catalyst
The density functional theory (DFT) calculations were performed to investigate the catalytic activities of Pd, Cu, and PdxCuy alloy catalysts for N2O reduction reaction (NRR). The activation and dissociation of N2O on PdxCuy catalysts were explored. The rate-determining step for the NRR was the dissociation of N2O into N2 and O. The electronic structure of PdxCuy alloys was determined by both ligand and strain effects, but was not closely related to the catalytic activity for NRR because two kinds of surface Pd and Cu atoms are differently involved in active sites for molecular adsorption and dissociation. Owing to the different role of Pd and Cu in NRR, the PdCu catalyst had both strong N2O adsorption strength and easier N2O dissociation than did pure and Pd3Cu catalysts, and therefore had the highest catalytic activity for NRR among the PdxCuy alloy catalysts. These fundamental findings were further applied to predict the thermal and electrochemical NRR.
1. Introduction Reduction of nitrous oxide (N2O) has been considered as one of the environmental problems owing to its substantial contributions to ozone depletion and the greenhouse effect. The N2O is mainly generated by the three-way catalyst in automobiles, by microbial action in soils or oceans, and by the process to manufacture nitric acid (HNO3) [1]. Despite such harmful effects, N2O has a lifetime of about 150 years [2], so these anthropogenic emissions have caused the total amount of N2O in the atmosphere to increase steadily. To reduce its emissions, several techniques have been suggested: (1) thermal decomposition, (2) catalytic decomposition, (3) electrochemical reduction, (4) ammonia oxidation during the process of nitric acid production, (5) selective catalytic reduction, and so on [3–6]. For thermal decomposition, however, the very high-temperature requirement of up to 800 °C led the development of enhanced catalyst to effectively reduce the emission of N2O by converting it into less harmful gases such as N2 and O2. Even though the electrochemical reduction is possible at low temperatures, the competition with H2O reduction lowers the activity for N2O reduction because electrochemical reactions occur in aqueous solutions [7]. It has been reported that kinetic factors such as adsorption and dissociation of N2O on catalyst surfaces are the key factors to determine the electrochemical activity of N2O reduction, and these factors must be affected strongly by
the characteristics of catalyst surfaces [8]. Therefore, N2O reduction activity is strongly dependent on the chemical composition of catalysts, regardless of the reduction technique. Since the catalytic activity of support itself is very low [9], many of previous kinetic studies have shown that the key elementary steps of N2O reduction reaction (NRR) are N2O adsorption and dissociation on metal surfaces [8–10]. Therefore, the appropriate selection of metal catalyst is the most important factor in the design of NRR catalyst. For this reason, the catalytic activity and reaction mechanism for NRR on various transition metal catalysts (Co, Ni, Cu, Rh, Pd, Ir, and Pt) have been widely investigated [9,11,12]. In addition to single metal catalysts, Chen et al. recently suggested core-shell structure of Fe@Rh/SBA15 as an enhanced catalyst with high N2O decomposition activity and low cost compared to alloy (RhFe/SBA-15) catalyst from experiments and DFT calculations [13]. Although core-shell catalyst significantly reduces the cost for catalysts, the price of Rh is still very expensive to apply for N2O reduction in practical application. On the other hand, Pd and Cu are relatively inexpensive but have high electrochemical activity for N2O reduction [7]. PdxCuy alloy catalyst showed considerably higher mass activity than pure Pd or Cu catalyst for the electrochemical activity of NRR; theoretical results proposed that the increased catalytic activity may be a result of an optimal electronic structure at an optimal surface alloy ratio of PdxCuy [14]. However, the mechanism of the NRR on Pd/Cu alloy catalyst was
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Corresponding author. E-mail address: [email protected] (J.W. Han). 1 K. Kim and S. Baek contributed equally to this work. https://doi.org/10.1016/j.apsusc.2020.145349 Received 30 September 2019; Received in revised form 19 December 2019; Accepted 9 January 2020 Available online 20 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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using the climbing image-nudge elastic band (CI-NEB) method with five intermediate images to determine the minimum energy pathway (MEP) [21].
not clearly investigated. DFT calculations have shown that it can be applied to explore reaction mechanism and provide fundamental understanding for not only NRR, but also N2O formation and dissociative adsorption at the atomic scale [15–17]. In this paper, therefore, we first focus on elucidating the fundamental surface properties such as the molecular adsorption/desorption and conversion of N2O on the PdxCuy surfaces as well as physical and chemical properties of the PdxCuy catalysts for the NRR. Furthermore, we applied these findings to predict the thermal reduction and electrochemical reduction of N2O on the PdxCuy surfaces.
3. Results and discussion 3.1. Vertical adsorption of N2O To investigate the mechanism of NRR, we first examined the molecular adsorption of N2O as the first elementary step. Two main adsorption configurations of vertical and parallel adsorptions have been reported as the most stable and activated adsorption configurations of N2O on metal surfaces, respectively [11,13,22]. Our calculated adsorption energies were also higher for vertical adsorption configuration (Fig. 2) than for parallel adsorption on all surfaces (Fig. 3). For vertical adsorption, Pd has higher adsorption affinity of −0.44 eV than Pd3Cu (−0.42 eV), PdCu (−0.40 eV), and Cu (−0.18 eV). Although the adsorption strength decreases with increase in the amount of Cu in Pd, the degree of this decrease on PdxCuy alloys is not significant compared to pure Pd. The small degree of change may be a consequence of the adsorption site of N2O (Fig. 2): on PdxCuy alloy surfaces, N2O adsorbs preferentially at the top of a Pd atom rather than a Cu atom. The N2O adsorbs vertically to one surface atom, so the adsorption strength is mainly determined by the interaction between adsorbate and the surface atom to which the NO2 is directly bound. This conclusion is confirmed by the local charge transfer between one surface atom and the adsorbed N2O (Fig. S1). Therefore, the adsorption energy of vertically adsorbed N2O on PdxCuy alloy catalysts might be similar to that on Pd. The small energy difference might be attributed to the electronic and geometric effects from Cu. We will discuss this possibility in detail in Section 3.4.
2. Computational details DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) [18]. Exchange-correlation effects were treated using the Perdew-Burke-Ernzerhof (PBE) functional based on the generalized gradient approximation (GGA). A plane-wave was expanded with a cutoff of 400 eV. A 4 × 4 × 1 Monkhorst-Pack k-point mesh was used [19], with a (4 × 4) surface unit cell. The conjugate gradient method was used for ionic relaxation. Methfessel–Paxton smearing was used to treat partial occupancies with a width of 0.2 eV. All structures were relaxed until the force on each atom was < 0.03 eV/ Å. DFT-D3 was employed to correct for van der Waals interactions [20]. For gas-phase molecules, we calculated the total energy of an isolated gas-phase molecule of N2O, N2, and O2 in a 20 × 20 × 20 Å supercell with a 12 × 12 × 12 Monkhorst-Pack k-point mesh, respectively, to avoid interaction between the periodic images. The total energy of gasphase atomic O was calculated by half of the total energy of O2. The lattice constants of bulk FCC structures were optimized as: Pd, 3.886 Å; Cu, 3.568 Å; Pd3Cu, 3.820 Å; PdCu, 3.760 Å. We used uniformly-distributed alloy models because evident secondary phase or islanding has not been observed on PdxCuy catalysts (Fig. 1). A three-layer slab model with fixed one bottom layer was used to describe the metal surface. A vacuum thickness of 15 Å was used to exclude the artifactitious interaction between periodic slabs. The adsorption energy (Eads) for each adsorbate was calculated as
Eads = EA − Esurf − EA (g ) ,
3.2. Geometry transformation of adsorbed N2O The N2O bends before dissociating into N2 and O, so both N and O atom adsorb parallel to the surface [11,13,17]. This transformation of adsorption geometry from vertical to parallel adsorption is considered as an activation step for the N2O molecule on the surfaces before the dissociation reaction. The energies required for the transformation are 0.12 eV on Pd, 0.15 eV on Pd3Cu, 0.15 eV on PdCu, and 0.06 eV on Cu
(1)
where EA is the total energy of the adsorbate-substrate system, Esurf is the total energy of the bare surface, and EA(g) is the total energy of an isolated gas-phase molecule. The reaction barriers were calculated
Fig. 1. Optimized structures of (a) Pd(1 1 1), (b) Cu(1 1 1), (c) Pd3Cu(1 1 1), and (d) PdCu(1 1 1) surfaces. Silver sphere: palladium; blue sphere: copper. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Fig. 2. Optimized structures of vertically adsorbed N2O on (a) Pd(1 1 1), (b) Cu(1 1 1), (c) Pd3Cu(1 1 1), and (d) PdCu(1 1 1) surfaces.
Fig. 3. Optimized structures of parallelly adsorbed N2O on (a) Pd(1 1 1), (b) Cu(1 1 1), (c) Pd3Cu(1 1 1), and (d) PdCu(1 1 1) surfaces.
Fig. 4. Relative energy profile and snapshots of initial state (IS), transition state (TS), and final state (FS) for the geometry transformation of adsorbed N2O on Pd (1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) surfaces. 3
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Table 1 Adsorption energies for vertically adsorbed N2O, parallelly adsorbed N2O, N2, and atomic O on Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) surfaces. The transformation energies of N2O from vertical to parallel adsorption configurations on Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) surfaces. Superscript “b” represents the energy barriers for geometry transformation. Adsorption energy (eV)
Pd Pd3Cu PdCu Cu
Table 2 (a) N2O dissociation energy, (b) optimized lattice parameter, and (c) d-band center of Pd, Pd3Cu, PdCu, and Cu catalysts. Superscript “b” represents the energy barriers for N2O dissociation.
Pd Pd3Cu PdCu Cu
N2O transformation energy (eV)
Vertical N2O
Parallel N2O
N2
O
−0.44 −0.42 −0.40 −0.18
−0.32 −0.27 −0.25 −0.12
−0.53 −0.50 −0.47 −0.15
−1.53 −1.35 −1.44 −1.66
0.12 0.15 0.15 0.06
(0.17)b (0.16)b (0.15)b (0.06)b
Dissociation energy (eV)
Lattice parameter (Å)
d-band center (eV)
−1.87 −1.72 −1.79 −1.83
3.89 3.82 3.76 3.57
−1.67 −1.64 −1.64 −2.29
b
(0.25) (0.16)b (0.09)b (0.08)b
3.4. Electronic structure of PdxCuy catalysts The position of the d-band center has been suggested as a key descriptor to predict the catalytic activity of metal catalysts for various redox reactions [24,25], so we also examined the d-band center on Pd, Cu, and PdxCuy catalysts. The d-band center of Pd (−1.67 eV) is higher than that of Cu (−2.29 eV); this result is consistent with other work [26]. However, even though PdxCuy alloys consist of Pd and Cu, the alloys have a higher d-band center position of −1.64 eV (Pd3Cu and PdCu) than those of pure Pd and Cu catalysts, not the value between them (Fig. 6(a)). To identify the reason for the upper position of the dband center of PdxCuy alloy catalysts, we examined the shift in the dband center of constituent elements in PdxCuy alloy catalysts. In PdxCuy alloys, Cu shows significant upshift of the d-band center, whereas the Pd shows a small upshift (Fig. 6(b)). The difference can be explained by the strain and ligand effects of constituent elements in alloy catalysts. We first examined the effective strain on Cu and Pd atoms in PdxCuy alloys (Fig. 6(c)). The strain effect was induced by the size mismatch between the constituent elements, so the effective strain on each constituent element was calculated using the difference in the lattice constants of pure and alloy catalysts. For example, the effective strain (%) of Cu atoms in PdCu was calculated as (PdCu lattice/Cu lattice − 1) × 100. As the amount of Pd increased, the lattice constant increased to 3.76 Å in PdCu and to 3.82 Å in Pd3Cu, because Pd has a larger lattice constant (3.89 Å) than Cu (3.57 Å). This difference indicates that Cu atoms in PdxCuy catalysts are subjected to tensile strain, whereas Pd atoms in the alloys are subjected to compressive strain. It has been known that the tensile strain results in the upshift of the d-band center of Cu, whereas the compressive strain downshifts the position of the dband center of Pd. This tendency actually reported for PdxCuy alloy nanoparticle models [27]. In our results, the larger shift of d-band center position of Cu than of Pd might occur because the effectively applied strain is larger on Cu than on Pd in PdxCuy catalysts. However, the upshift of the d-band center of Pd in Fig. 6(b), which feels compressive strain, cannot be reasonably explained by only the strain effect.
(Fig. 4 and Table 1). In addition, the calculated barriers for the geometry transformation of adsorbed N2O surfaces were: Pd, 0.17 eV; Pd3Cu, 0.16 eV; PdCu, 0.15 eV; Cu, 0.06 eV. The higher transformation energy (and thus higher transformation barrier) on Pd(1 1 1) comes from the higher adsorption energy of vertically adsorbed N2O in an initial adsorption configuration than Cu (1 1 1) (Fig. 2 and Table 1). In addition, in the transformed parallel configuration, the lower adsorption energy of atomic O on Pd than Cu (Table 1) that is one of the atoms involved in the parallel adsorption of N2O also assists the tendency. This implies that the presence of Cu facilitates the ease of N2O activation for its dissociation.
3.3. Dissociation of N2O After the geometry transformation of adsorbed N2O, N2O dissociates into N2 and O. This step was commonly considered as a key elementary step for both the electrochemical reduction reaction (N2O + 2H+ + 2e− → N2 + H2O or N2O + H+ + 2e− → N2 + OH−) [8] and the gas-phase thermal reduction reaction (2N2O → 2 N2 + O2) [11]. The calculated dissociation energy of N2O was lower on pure Pd and Cu surfaces than on PdxCuy alloy surfaces (Fig. 5 and Table 2) while they were highly exothermic for all catalysts. The calculated dissociation barriers for the surfaces were: Pd, 0.25 eV; Pd3Cu, 0.16 eV; PdCu, 0.09 eV; Cu, 0.08 eV. In all cases, both transformation and dissociation barriers of the adsorbed N2O are evidently decreased as the amount of Cu increased. This implies that Cu increases the catalytic activity for the N2O dissociation (Table 2). Although the PdCu shows a similar dissociation barrier (0.09 eV) to that of Cu (0.08 eV), the overall activity of PdCu would be higher than Cu catalysts when we consider the higher adsorption strength of N2O on PdCu surface than Cu [23].
Fig. 5. Relative energy profile with initial state (IS), transition state (TS), and final state (FS) of N2O decomposition on Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) catalysts. 4
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Fig. 6. (a) d-band center of Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1). (b) Degree of d-band center shift of each atom in PdxCuy catalysts. (c) The effective strain of Pd and Cu atoms in PdxCuy catalysts. (d) Amount of charge transfer of each atom in PdxCuy catalysts.
desorption strengths are moderate. The d-band center is a normalized value to represent the electronic structure of all atoms in alloy catalysts. For example, the catalytic activities of oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) mainly occur by oxygen dissociation and hydrogen formation, respectively, so the interaction between surface atoms and the simple component of the reaction, that is atomic oxygen or atomic hydrogen, can be the key descriptor to predict the catalytic activity of ORR and HER. Their most stable adsorption sites were the hollow sites in which all surface atoms participate in the adsorption of atomic oxygen or hydrogen. For this reason, the d-band center could be reasonably used for predicting the catalytic activity of these reactions. However, our results suggest that catalytic activity for NRR cannot be predicted reliably by using only a simple descriptor such as a d-band center. When the different kinds of active sites are identically involved in the catalytic reactions, the activities cannot be easily determined. In PdxCuy system, two kinds of surface Pd and Cu atoms differently provide the active sites for molecular adsorption and dissociation, respectively. For the molecular adsorption of N2O, an N atom in N2O vertically binds to only one surface atom. Thus, the interaction can be mostly determined by the characteristics of the surface atom; the N2O molecule adsorbs preferentially onto the Pd atoms of PdxCuy surfaces with the adsorption energy of −0.42 eV on Pd3Cu and −0.40 eV on PdCu (Figs. 2, 3 and S2). This implies that PdxCuy catalysts take an advantage of easier molecular adsorption of N2O as in Pd catalyst (−0.44 eV). Contrary to the adsorption of N2O and N2 onto the PdxCuy alloy, the atomic O adsorption as a reaction product of N2O dissociation is affected by both surface Pd and Cu atoms because it adsorbs at a hollow site of Pd-Cu (Fig. S3). For the N2O dissociation, on the other hand, the PdCu shows closer catalytic activity to Cu (dissociation barrier, 0.09 eV on PdCu and 0.08 eV on Cu) than Pd (0.25 eV). To sum, the adsorption strengths of N2O, N2, and O are determined
The electronic structure could also be changed by the ligand effect. As two metals with different Fermi levels formed alloy, the charges (mainly d-band level of electrons) would flow from the metal with a higher Fermi level to the lower one [28]. In this respect, Tang et al. previously showed a linear relationship of the amount of charges transferred between core and shell elements with the d-band center position [29]. Likewise, in this study, the transferred charges could make extra electrons or electron deficiency, thereby changing the electronic structure of each atom in PdxCuy alloys. This was confirmed by the shift of d-band center in Pd and Cu of PdxCuy alloys (Fig. 6). To verify whether this process occurs here, we conducted Bader charge analysis to calculate the amount of charge transfer between Pd and Cu in PdxCuy alloy catalysts. After the formation of alloy catalysts, the charge was transferred from Cu to Pd, where the gain of charge in Pd results in the upshift of the d-band center [27]. Despite the opposite tendency of tensile strain (upshift of d-band center) and charge depletion (downshift of d-band center) in Cu, the overall d-band position of Cu was upshifted in PdxCuy alloy catalysts; this result implies that the strain effect is more dominant for determining the d-band center position of Cu in PdxCuy alloy catalysts. On the other hand, the d-band center of Pd was a little upshifted in PdCu alloy catalyst even though it is subjected to compressive strain. Considering that Pd gained charges from Cu in PdxCuy alloy catalyst, the d-band center of Pd was more affected by the ligand effect than by the strain effect. As a result, the d-band center was overall upshifted on PdxCuy alloy catalysts, but was not located at the middle point between Pd and Cu.
3.5. Catalytic activity of NRR The d-band theory suggests that an optimal position of d-band center increases catalytic activity because both adsorption and 5
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predict the overall activity of NRR because two kinds of surface Pd and Cu atoms are differently involved in the active sites for molecular adsorption and dissociation. Our findings can be further applied to predict the catalytic activities of thermal and electrochemical reductions of N2O on the PdxCuy catalysts as the practical applications of our study. CRediT authorship contribution statement Kyeounghak Kim: Conceptualization, Methodology, Investigation, Writing - original draft, Visualization. Seungyeon Baek: Conceptualization, Investigation, Resources, Writing - original draft. Jae Jeong Kim: Supervision, Project administration, Funding acquisition. Jeong Woo Han: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Fig. 7. Schematic illustration for the relationship between catalytic activity and PdxCuy alloy components.
Declaration of Competing Interest
by the electronic structures of Pd, Pd, and Pd-Cu, respectively. Owing to this complexity, dissociation energy might not simply follow the trend of d-band center position. However, it can provide a merit for the higher catalytic activity of NRR; since PdxCuy catalysts have both Pd and Cu atoms, Pd atoms enhance the molecular adsorption ability of N2O while Cu atoms lower the dissociation barrier of N2O, thereby resulting in the enhanced catalytic activity for NRR (Fig. 7).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was financially supported by the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173020032120) and the Basic Science Research Program (NRF2018R1A2B2002875) through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT). This work was also supported by the Super Computing center/KISTI with supercomputing resource including technical support (KSC-2018-C3-0021). The earth image of sun rising over planet earth, which was used for table of content, was taken from the website of Jooinn under permission.
3.6. Thermal and electrochemical reduction reactions of N2O on PdxCuy catalysts Our fundamental findings can be applied to a range of the practical catalytic reactions. We first consider the thermal reduction of N2O. In this reaction, the O2-formation step is followed by the dissociation of N2O. Two atomic O atoms formed by the dissociations of N2O finally form an O2 molecule that further desorbs from surface as a product. The calculated formation energies of O2 on surfaces were: 1.86 eV on Pd; 1.53 eV on Pd3Cu; 1.90 on PdCu; 2.40 eV on Cu. Since the reaction energies for O2 formation are much higher than those for N2O dissociation or activation on the Pd, Cu, and PdxCuy surfaces, the O2-formation step is considered as a rate-determining step for the thermal reduction of N2O. Chen et al. previously confirmed that the O2 formation requires more energy than N2O dissociation on Rh-M surfaces (M = Co, Cu, and Ni) [11], and has a linear relationship between reaction energy and reaction barrier for O2 formation. This implies that Brønsted-Evans-Polanyi (BEP) relationship is also effective in the O2 formation on metal surfaces. Without the calculation of reaction barrier for O2 formation, therefore, we can reasonably predict that Pd3Cu would have the highest catalytic activity for the thermal reduction of N2O due to the easiest O2 formation. Meanwhile, for the electrochemical NRR, adsorption and dissociation of N2O have been known as the key steps to determine the catalytic activity rather than hydrogen evolution reaction from H+ in aqueous solutions or other steps such as formation of OH− or H2O as the reaction product [8,30]. The better electrochemical activity on PdxCuy alloy catalyst than on pure Pd or Cu was indeed observed in recent experiment [14]. Thus, our results might also be directly applied to describe the catalytic activity for electrochemical reduction of N2O.
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4. Conclusion The mechanism of N2O decomposition on Pd, Cu, and their alloys was investigated by DFT calculations. We found that Pd strongly interacts with N2O, which makes the easier molecular adsorption of N2O onto the metal surface. On the other hand, Cu has the lower binding strength of N2O, but easily dissociates it into N2 and O. Therefore, PdxCuy alloy catalyst can take both advantages from Pd and Cu, thereby showing the enhanced catalytic activity for NRR. Interestingly, in this PdxCuy system, it was found that d-band center could not be used to 6
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Catalytic decomposition of N2O on PdxCuy alloy catalysts: A density functional theory study Kyeounghak Kima,1, Seungyeon Baekb,1, Jae Jeong Kimb, Jeong Woo Hana, a b
T
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Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea School of Chemical and Biological Engineering, Seoul National University, Gwanak 1, Gwanak-gu, Seoul 08826, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: N2O reduction reaction Density functional theory (DFT) PuxCuy alloy Metal catalyst
The density functional theory (DFT) calculations were performed to investigate the catalytic activities of Pd, Cu, and PdxCuy alloy catalysts for N2O reduction reaction (NRR). The activation and dissociation of N2O on PdxCuy catalysts were explored. The rate-determining step for the NRR was the dissociation of N2O into N2 and O. The electronic structure of PdxCuy alloys was determined by both ligand and strain effects, but was not closely related to the catalytic activity for NRR because two kinds of surface Pd and Cu atoms are differently involved in active sites for molecular adsorption and dissociation. Owing to the different role of Pd and Cu in NRR, the PdCu catalyst had both strong N2O adsorption strength and easier N2O dissociation than did pure and Pd3Cu catalysts, and therefore had the highest catalytic activity for NRR among the PdxCuy alloy catalysts. These fundamental findings were further applied to predict the thermal and electrochemical NRR.
1. Introduction Reduction of nitrous oxide (N2O) has been considered as one of the environmental problems owing to its substantial contributions to ozone depletion and the greenhouse effect. The N2O is mainly generated by the three-way catalyst in automobiles, by microbial action in soils or oceans, and by the process to manufacture nitric acid (HNO3) [1]. Despite such harmful effects, N2O has a lifetime of about 150 years [2], so these anthropogenic emissions have caused the total amount of N2O in the atmosphere to increase steadily. To reduce its emissions, several techniques have been suggested: (1) thermal decomposition, (2) catalytic decomposition, (3) electrochemical reduction, (4) ammonia oxidation during the process of nitric acid production, (5) selective catalytic reduction, and so on [3–6]. For thermal decomposition, however, the very high-temperature requirement of up to 800 °C led the development of enhanced catalyst to effectively reduce the emission of N2O by converting it into less harmful gases such as N2 and O2. Even though the electrochemical reduction is possible at low temperatures, the competition with H2O reduction lowers the activity for N2O reduction because electrochemical reactions occur in aqueous solutions [7]. It has been reported that kinetic factors such as adsorption and dissociation of N2O on catalyst surfaces are the key factors to determine the electrochemical activity of N2O reduction, and these factors must be affected strongly by
the characteristics of catalyst surfaces [8]. Therefore, N2O reduction activity is strongly dependent on the chemical composition of catalysts, regardless of the reduction technique. Since the catalytic activity of support itself is very low [9], many of previous kinetic studies have shown that the key elementary steps of N2O reduction reaction (NRR) are N2O adsorption and dissociation on metal surfaces [8–10]. Therefore, the appropriate selection of metal catalyst is the most important factor in the design of NRR catalyst. For this reason, the catalytic activity and reaction mechanism for NRR on various transition metal catalysts (Co, Ni, Cu, Rh, Pd, Ir, and Pt) have been widely investigated [9,11,12]. In addition to single metal catalysts, Chen et al. recently suggested core-shell structure of Fe@Rh/SBA15 as an enhanced catalyst with high N2O decomposition activity and low cost compared to alloy (RhFe/SBA-15) catalyst from experiments and DFT calculations [13]. Although core-shell catalyst significantly reduces the cost for catalysts, the price of Rh is still very expensive to apply for N2O reduction in practical application. On the other hand, Pd and Cu are relatively inexpensive but have high electrochemical activity for N2O reduction [7]. PdxCuy alloy catalyst showed considerably higher mass activity than pure Pd or Cu catalyst for the electrochemical activity of NRR; theoretical results proposed that the increased catalytic activity may be a result of an optimal electronic structure at an optimal surface alloy ratio of PdxCuy [14]. However, the mechanism of the NRR on Pd/Cu alloy catalyst was
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Corresponding author. E-mail address: [email protected] (J.W. Han). 1 K. Kim and S. Baek contributed equally to this work. https://doi.org/10.1016/j.apsusc.2020.145349 Received 30 September 2019; Received in revised form 19 December 2019; Accepted 9 January 2020 Available online 20 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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using the climbing image-nudge elastic band (CI-NEB) method with five intermediate images to determine the minimum energy pathway (MEP) [21].
not clearly investigated. DFT calculations have shown that it can be applied to explore reaction mechanism and provide fundamental understanding for not only NRR, but also N2O formation and dissociative adsorption at the atomic scale [15–17]. In this paper, therefore, we first focus on elucidating the fundamental surface properties such as the molecular adsorption/desorption and conversion of N2O on the PdxCuy surfaces as well as physical and chemical properties of the PdxCuy catalysts for the NRR. Furthermore, we applied these findings to predict the thermal reduction and electrochemical reduction of N2O on the PdxCuy surfaces.
3. Results and discussion 3.1. Vertical adsorption of N2O To investigate the mechanism of NRR, we first examined the molecular adsorption of N2O as the first elementary step. Two main adsorption configurations of vertical and parallel adsorptions have been reported as the most stable and activated adsorption configurations of N2O on metal surfaces, respectively [11,13,22]. Our calculated adsorption energies were also higher for vertical adsorption configuration (Fig. 2) than for parallel adsorption on all surfaces (Fig. 3). For vertical adsorption, Pd has higher adsorption affinity of −0.44 eV than Pd3Cu (−0.42 eV), PdCu (−0.40 eV), and Cu (−0.18 eV). Although the adsorption strength decreases with increase in the amount of Cu in Pd, the degree of this decrease on PdxCuy alloys is not significant compared to pure Pd. The small degree of change may be a consequence of the adsorption site of N2O (Fig. 2): on PdxCuy alloy surfaces, N2O adsorbs preferentially at the top of a Pd atom rather than a Cu atom. The N2O adsorbs vertically to one surface atom, so the adsorption strength is mainly determined by the interaction between adsorbate and the surface atom to which the NO2 is directly bound. This conclusion is confirmed by the local charge transfer between one surface atom and the adsorbed N2O (Fig. S1). Therefore, the adsorption energy of vertically adsorbed N2O on PdxCuy alloy catalysts might be similar to that on Pd. The small energy difference might be attributed to the electronic and geometric effects from Cu. We will discuss this possibility in detail in Section 3.4.
2. Computational details DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) [18]. Exchange-correlation effects were treated using the Perdew-Burke-Ernzerhof (PBE) functional based on the generalized gradient approximation (GGA). A plane-wave was expanded with a cutoff of 400 eV. A 4 × 4 × 1 Monkhorst-Pack k-point mesh was used [19], with a (4 × 4) surface unit cell. The conjugate gradient method was used for ionic relaxation. Methfessel–Paxton smearing was used to treat partial occupancies with a width of 0.2 eV. All structures were relaxed until the force on each atom was < 0.03 eV/ Å. DFT-D3 was employed to correct for van der Waals interactions [20]. For gas-phase molecules, we calculated the total energy of an isolated gas-phase molecule of N2O, N2, and O2 in a 20 × 20 × 20 Å supercell with a 12 × 12 × 12 Monkhorst-Pack k-point mesh, respectively, to avoid interaction between the periodic images. The total energy of gasphase atomic O was calculated by half of the total energy of O2. The lattice constants of bulk FCC structures were optimized as: Pd, 3.886 Å; Cu, 3.568 Å; Pd3Cu, 3.820 Å; PdCu, 3.760 Å. We used uniformly-distributed alloy models because evident secondary phase or islanding has not been observed on PdxCuy catalysts (Fig. 1). A three-layer slab model with fixed one bottom layer was used to describe the metal surface. A vacuum thickness of 15 Å was used to exclude the artifactitious interaction between periodic slabs. The adsorption energy (Eads) for each adsorbate was calculated as
Eads = EA − Esurf − EA (g ) ,
3.2. Geometry transformation of adsorbed N2O The N2O bends before dissociating into N2 and O, so both N and O atom adsorb parallel to the surface [11,13,17]. This transformation of adsorption geometry from vertical to parallel adsorption is considered as an activation step for the N2O molecule on the surfaces before the dissociation reaction. The energies required for the transformation are 0.12 eV on Pd, 0.15 eV on Pd3Cu, 0.15 eV on PdCu, and 0.06 eV on Cu
(1)
where EA is the total energy of the adsorbate-substrate system, Esurf is the total energy of the bare surface, and EA(g) is the total energy of an isolated gas-phase molecule. The reaction barriers were calculated
Fig. 1. Optimized structures of (a) Pd(1 1 1), (b) Cu(1 1 1), (c) Pd3Cu(1 1 1), and (d) PdCu(1 1 1) surfaces. Silver sphere: palladium; blue sphere: copper. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 2
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Fig. 2. Optimized structures of vertically adsorbed N2O on (a) Pd(1 1 1), (b) Cu(1 1 1), (c) Pd3Cu(1 1 1), and (d) PdCu(1 1 1) surfaces.
Fig. 3. Optimized structures of parallelly adsorbed N2O on (a) Pd(1 1 1), (b) Cu(1 1 1), (c) Pd3Cu(1 1 1), and (d) PdCu(1 1 1) surfaces.
Fig. 4. Relative energy profile and snapshots of initial state (IS), transition state (TS), and final state (FS) for the geometry transformation of adsorbed N2O on Pd (1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) surfaces. 3
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Table 1 Adsorption energies for vertically adsorbed N2O, parallelly adsorbed N2O, N2, and atomic O on Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) surfaces. The transformation energies of N2O from vertical to parallel adsorption configurations on Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) surfaces. Superscript “b” represents the energy barriers for geometry transformation. Adsorption energy (eV)
Pd Pd3Cu PdCu Cu
Table 2 (a) N2O dissociation energy, (b) optimized lattice parameter, and (c) d-band center of Pd, Pd3Cu, PdCu, and Cu catalysts. Superscript “b” represents the energy barriers for N2O dissociation.
Pd Pd3Cu PdCu Cu
N2O transformation energy (eV)
Vertical N2O
Parallel N2O
N2
O
−0.44 −0.42 −0.40 −0.18
−0.32 −0.27 −0.25 −0.12
−0.53 −0.50 −0.47 −0.15
−1.53 −1.35 −1.44 −1.66
0.12 0.15 0.15 0.06
(0.17)b (0.16)b (0.15)b (0.06)b
Dissociation energy (eV)
Lattice parameter (Å)
d-band center (eV)
−1.87 −1.72 −1.79 −1.83
3.89 3.82 3.76 3.57
−1.67 −1.64 −1.64 −2.29
b
(0.25) (0.16)b (0.09)b (0.08)b
3.4. Electronic structure of PdxCuy catalysts The position of the d-band center has been suggested as a key descriptor to predict the catalytic activity of metal catalysts for various redox reactions [24,25], so we also examined the d-band center on Pd, Cu, and PdxCuy catalysts. The d-band center of Pd (−1.67 eV) is higher than that of Cu (−2.29 eV); this result is consistent with other work [26]. However, even though PdxCuy alloys consist of Pd and Cu, the alloys have a higher d-band center position of −1.64 eV (Pd3Cu and PdCu) than those of pure Pd and Cu catalysts, not the value between them (Fig. 6(a)). To identify the reason for the upper position of the dband center of PdxCuy alloy catalysts, we examined the shift in the dband center of constituent elements in PdxCuy alloy catalysts. In PdxCuy alloys, Cu shows significant upshift of the d-band center, whereas the Pd shows a small upshift (Fig. 6(b)). The difference can be explained by the strain and ligand effects of constituent elements in alloy catalysts. We first examined the effective strain on Cu and Pd atoms in PdxCuy alloys (Fig. 6(c)). The strain effect was induced by the size mismatch between the constituent elements, so the effective strain on each constituent element was calculated using the difference in the lattice constants of pure and alloy catalysts. For example, the effective strain (%) of Cu atoms in PdCu was calculated as (PdCu lattice/Cu lattice − 1) × 100. As the amount of Pd increased, the lattice constant increased to 3.76 Å in PdCu and to 3.82 Å in Pd3Cu, because Pd has a larger lattice constant (3.89 Å) than Cu (3.57 Å). This difference indicates that Cu atoms in PdxCuy catalysts are subjected to tensile strain, whereas Pd atoms in the alloys are subjected to compressive strain. It has been known that the tensile strain results in the upshift of the d-band center of Cu, whereas the compressive strain downshifts the position of the dband center of Pd. This tendency actually reported for PdxCuy alloy nanoparticle models [27]. In our results, the larger shift of d-band center position of Cu than of Pd might occur because the effectively applied strain is larger on Cu than on Pd in PdxCuy catalysts. However, the upshift of the d-band center of Pd in Fig. 6(b), which feels compressive strain, cannot be reasonably explained by only the strain effect.
(Fig. 4 and Table 1). In addition, the calculated barriers for the geometry transformation of adsorbed N2O surfaces were: Pd, 0.17 eV; Pd3Cu, 0.16 eV; PdCu, 0.15 eV; Cu, 0.06 eV. The higher transformation energy (and thus higher transformation barrier) on Pd(1 1 1) comes from the higher adsorption energy of vertically adsorbed N2O in an initial adsorption configuration than Cu (1 1 1) (Fig. 2 and Table 1). In addition, in the transformed parallel configuration, the lower adsorption energy of atomic O on Pd than Cu (Table 1) that is one of the atoms involved in the parallel adsorption of N2O also assists the tendency. This implies that the presence of Cu facilitates the ease of N2O activation for its dissociation.
3.3. Dissociation of N2O After the geometry transformation of adsorbed N2O, N2O dissociates into N2 and O. This step was commonly considered as a key elementary step for both the electrochemical reduction reaction (N2O + 2H+ + 2e− → N2 + H2O or N2O + H+ + 2e− → N2 + OH−) [8] and the gas-phase thermal reduction reaction (2N2O → 2 N2 + O2) [11]. The calculated dissociation energy of N2O was lower on pure Pd and Cu surfaces than on PdxCuy alloy surfaces (Fig. 5 and Table 2) while they were highly exothermic for all catalysts. The calculated dissociation barriers for the surfaces were: Pd, 0.25 eV; Pd3Cu, 0.16 eV; PdCu, 0.09 eV; Cu, 0.08 eV. In all cases, both transformation and dissociation barriers of the adsorbed N2O are evidently decreased as the amount of Cu increased. This implies that Cu increases the catalytic activity for the N2O dissociation (Table 2). Although the PdCu shows a similar dissociation barrier (0.09 eV) to that of Cu (0.08 eV), the overall activity of PdCu would be higher than Cu catalysts when we consider the higher adsorption strength of N2O on PdCu surface than Cu [23].
Fig. 5. Relative energy profile with initial state (IS), transition state (TS), and final state (FS) of N2O decomposition on Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1) catalysts. 4
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Fig. 6. (a) d-band center of Pd(1 1 1), Cu(1 1 1), and PdxCuy(1 1 1). (b) Degree of d-band center shift of each atom in PdxCuy catalysts. (c) The effective strain of Pd and Cu atoms in PdxCuy catalysts. (d) Amount of charge transfer of each atom in PdxCuy catalysts.
desorption strengths are moderate. The d-band center is a normalized value to represent the electronic structure of all atoms in alloy catalysts. For example, the catalytic activities of oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) mainly occur by oxygen dissociation and hydrogen formation, respectively, so the interaction between surface atoms and the simple component of the reaction, that is atomic oxygen or atomic hydrogen, can be the key descriptor to predict the catalytic activity of ORR and HER. Their most stable adsorption sites were the hollow sites in which all surface atoms participate in the adsorption of atomic oxygen or hydrogen. For this reason, the d-band center could be reasonably used for predicting the catalytic activity of these reactions. However, our results suggest that catalytic activity for NRR cannot be predicted reliably by using only a simple descriptor such as a d-band center. When the different kinds of active sites are identically involved in the catalytic reactions, the activities cannot be easily determined. In PdxCuy system, two kinds of surface Pd and Cu atoms differently provide the active sites for molecular adsorption and dissociation, respectively. For the molecular adsorption of N2O, an N atom in N2O vertically binds to only one surface atom. Thus, the interaction can be mostly determined by the characteristics of the surface atom; the N2O molecule adsorbs preferentially onto the Pd atoms of PdxCuy surfaces with the adsorption energy of −0.42 eV on Pd3Cu and −0.40 eV on PdCu (Figs. 2, 3 and S2). This implies that PdxCuy catalysts take an advantage of easier molecular adsorption of N2O as in Pd catalyst (−0.44 eV). Contrary to the adsorption of N2O and N2 onto the PdxCuy alloy, the atomic O adsorption as a reaction product of N2O dissociation is affected by both surface Pd and Cu atoms because it adsorbs at a hollow site of Pd-Cu (Fig. S3). For the N2O dissociation, on the other hand, the PdCu shows closer catalytic activity to Cu (dissociation barrier, 0.09 eV on PdCu and 0.08 eV on Cu) than Pd (0.25 eV). To sum, the adsorption strengths of N2O, N2, and O are determined
The electronic structure could also be changed by the ligand effect. As two metals with different Fermi levels formed alloy, the charges (mainly d-band level of electrons) would flow from the metal with a higher Fermi level to the lower one [28]. In this respect, Tang et al. previously showed a linear relationship of the amount of charges transferred between core and shell elements with the d-band center position [29]. Likewise, in this study, the transferred charges could make extra electrons or electron deficiency, thereby changing the electronic structure of each atom in PdxCuy alloys. This was confirmed by the shift of d-band center in Pd and Cu of PdxCuy alloys (Fig. 6). To verify whether this process occurs here, we conducted Bader charge analysis to calculate the amount of charge transfer between Pd and Cu in PdxCuy alloy catalysts. After the formation of alloy catalysts, the charge was transferred from Cu to Pd, where the gain of charge in Pd results in the upshift of the d-band center [27]. Despite the opposite tendency of tensile strain (upshift of d-band center) and charge depletion (downshift of d-band center) in Cu, the overall d-band position of Cu was upshifted in PdxCuy alloy catalysts; this result implies that the strain effect is more dominant for determining the d-band center position of Cu in PdxCuy alloy catalysts. On the other hand, the d-band center of Pd was a little upshifted in PdCu alloy catalyst even though it is subjected to compressive strain. Considering that Pd gained charges from Cu in PdxCuy alloy catalyst, the d-band center of Pd was more affected by the ligand effect than by the strain effect. As a result, the d-band center was overall upshifted on PdxCuy alloy catalysts, but was not located at the middle point between Pd and Cu.
3.5. Catalytic activity of NRR The d-band theory suggests that an optimal position of d-band center increases catalytic activity because both adsorption and 5
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predict the overall activity of NRR because two kinds of surface Pd and Cu atoms are differently involved in the active sites for molecular adsorption and dissociation. Our findings can be further applied to predict the catalytic activities of thermal and electrochemical reductions of N2O on the PdxCuy catalysts as the practical applications of our study. CRediT authorship contribution statement Kyeounghak Kim: Conceptualization, Methodology, Investigation, Writing - original draft, Visualization. Seungyeon Baek: Conceptualization, Investigation, Resources, Writing - original draft. Jae Jeong Kim: Supervision, Project administration, Funding acquisition. Jeong Woo Han: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Fig. 7. Schematic illustration for the relationship between catalytic activity and PdxCuy alloy components.
Declaration of Competing Interest
by the electronic structures of Pd, Pd, and Pd-Cu, respectively. Owing to this complexity, dissociation energy might not simply follow the trend of d-band center position. However, it can provide a merit for the higher catalytic activity of NRR; since PdxCuy catalysts have both Pd and Cu atoms, Pd atoms enhance the molecular adsorption ability of N2O while Cu atoms lower the dissociation barrier of N2O, thereby resulting in the enhanced catalytic activity for NRR (Fig. 7).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was financially supported by the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173020032120) and the Basic Science Research Program (NRF2018R1A2B2002875) through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT). This work was also supported by the Super Computing center/KISTI with supercomputing resource including technical support (KSC-2018-C3-0021). The earth image of sun rising over planet earth, which was used for table of content, was taken from the website of Jooinn under permission.
3.6. Thermal and electrochemical reduction reactions of N2O on PdxCuy catalysts Our fundamental findings can be applied to a range of the practical catalytic reactions. We first consider the thermal reduction of N2O. In this reaction, the O2-formation step is followed by the dissociation of N2O. Two atomic O atoms formed by the dissociations of N2O finally form an O2 molecule that further desorbs from surface as a product. The calculated formation energies of O2 on surfaces were: 1.86 eV on Pd; 1.53 eV on Pd3Cu; 1.90 on PdCu; 2.40 eV on Cu. Since the reaction energies for O2 formation are much higher than those for N2O dissociation or activation on the Pd, Cu, and PdxCuy surfaces, the O2-formation step is considered as a rate-determining step for the thermal reduction of N2O. Chen et al. previously confirmed that the O2 formation requires more energy than N2O dissociation on Rh-M surfaces (M = Co, Cu, and Ni) [11], and has a linear relationship between reaction energy and reaction barrier for O2 formation. This implies that Brønsted-Evans-Polanyi (BEP) relationship is also effective in the O2 formation on metal surfaces. Without the calculation of reaction barrier for O2 formation, therefore, we can reasonably predict that Pd3Cu would have the highest catalytic activity for the thermal reduction of N2O due to the easiest O2 formation. Meanwhile, for the electrochemical NRR, adsorption and dissociation of N2O have been known as the key steps to determine the catalytic activity rather than hydrogen evolution reaction from H+ in aqueous solutions or other steps such as formation of OH− or H2O as the reaction product [8,30]. The better electrochemical activity on PdxCuy alloy catalyst than on pure Pd or Cu was indeed observed in recent experiment [14]. Thus, our results might also be directly applied to describe the catalytic activity for electrochemical reduction of N2O.
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4. Conclusion The mechanism of N2O decomposition on Pd, Cu, and their alloys was investigated by DFT calculations. We found that Pd strongly interacts with N2O, which makes the easier molecular adsorption of N2O onto the metal surface. On the other hand, Cu has the lower binding strength of N2O, but easily dissociates it into N2 and O. Therefore, PdxCuy alloy catalyst can take both advantages from Pd and Cu, thereby showing the enhanced catalytic activity for NRR. Interestingly, in this PdxCuy system, it was found that d-band center could not be used to 6
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