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Article

First Principles Study of O2 Dissociative Adsorption on Pt-Skin Pt3Cu(111) Surface

Key Laboratory of Advanced Functional Materials, School of Science, Kaili University, Kaili 556011, China
*
Author to whom correspondence should be addressed.
Submission received: 9 May 2024 / Revised: 6 June 2024 / Accepted: 9 June 2024 / Published: 14 June 2024
(This article belongs to the Section Computational Catalysis)

Abstract

:
The O2 dissociative adsorption serves as a pivotal criterion for assessing the efficacy of oxygen reduction catalysts. We conducted a systematic investigation into O2 dissociative adsorption on the Pt-skin Pt3Cu(111) surface by means of the density functional theory (DFT). The computational findings reveal that the O2 adsorption on Pt-skin Pt3Cu(111) surface exhibits comparatively lower stability when contrasted with that on the Pt(111) surface. For O2 dissociation, two paths have been identified. One progresses from the t-f-b state towards the generation of two oxygen atoms situated within nearest-neighbour hcp sites. The other commences from the t-b-t state, leading to the generation of two oxygen atoms occupying nearest-neighbour fcc sites. Moreover, the analysis of the energy barrier associated with O2 dissociation indicates that O2 on the Pt-skin Pt3Cu(111) surface is more difficult to dissociate than on the Pt(111) surface. This study can offer a valuable guide for the practical application of high-performance oxygen reduction catalysts.

1. Introduction

Binary alloys are known to exhibit superior catalytic properties and a wider application potential than pure metals [1,2,3]. This is because that alloying can alter the adsorption properties, electronic structure and distribution of active sites on the alloy surface by adjusting the composition of the alloy, thereby influencing the catalytic efficiency and product selectivity of the catalysts in key catalytic reaction steps [4,5,6]. A multitude of studies have highlighted the pivotal significance of dissociative adsorption of particular intermediates across diverse catalytic processes, which profoundly influence catalytic performance [7,8,9,10,11,12]. For instance, Duan et al. [12] examined the influence of transition metals in the subsurface layer of Pt alloys on the mechanism of the oxygen reduction reaction (ORR), revealing that O2 dissociative adsorption was one of the key steps in the ORR. The findings of Maatallah et al. [9] indicated that H2 dissociative adsorption on the catalyst surface has a significant impact on the catalytic efficiency of hydrogenation reactions. Mortensen et al. [10] investigated N2 dissociative adsorption on the Fe surface and further demonstrated that this process has a significant impact on the efficiency of ammonia synthesis. Therefore, an understanding of the dissociative adsorption process is beneficial for the development of efficient and selective binary catalysts.
Among the various catalytic reactions, the ORR is considered to be the most critical in the fuel cells [13,14], as it determines the efficiency and stability of fuel cells. Pt, the most important electro-catalyst for this reaction [15], has severely hindered the widespread practical utilisation of fuel cells due to its high overpotential at small currents [16] and high cost [17]. Consequently, the majority of research in this field has been directed towards the identification of ORR electro-catalysts with enhanced catalytic activity and stability at a reasonable price over the past five decades. Bimetallic catalysts have attracted considerable interest from the catalysis industry due to their superior reactivity and stability compared with monometallic catalysts [18,19]. In particular, numerous experiments have demonstrated that alloying Pt with Cu can reduce the amount of precious metal Pt used, as well as improving the ORR catalytic activity and stability [20,21,22,23]. For example, Matsui et al. [20] investigated the ORR performance of Pt-Cu fuel cell electrocatalysts. The results show that the ORR activity is affected by the structure formation of Pt-Cu alloys, and the electrocatalytic activity increases with the increase in the Cu/Pt molar ratio. It is noteworthy that the best ORR performance and durability were achieved at a Cu/Pt molar ratio of 1. Deng et al. [21] prepared an efficient ORR electrocatalyst consisting of monodisperse nanoscale Pt-Cu intermetallic compounds on hollow mesoporous carbon spheres (HMCS). The catalyst exhibits a beneficial electronic structure with a theoretical overpotential as low as 0.33 V and enhanced copper stability. Zysler et al. [22] presented a solvothermal method for the synthesis of a carbon-supported octahedral Pt-Cu alloy, which showed high efficiency in ORR. In particular, a specific activity of 1.02 mA/cm2 was achieved after 10,000 cycles (accelerated degradation test) in which 84% of the electrochemical surface area was maintained. Xiao et al. [23] uses a simple polyol method to prepare carbon-supported Pt-C catalysts, followed by electrochemical dealloying to form Pt-Cu/C catalysts with Pt-rich porous shells, which improves the catalytic activity and stability of ORR. Although the study of the O2 dissociative adsorption is a valuable guide for the practical application of high-performance oxygen reduction catalysts, relevant theoretical studies are rarely found in Pt-Cu alloys due to the complexity of the reaction process.
In this study, we conducted a systematic investigation into O2 dissociative adsorption on Pt-skin Pt3Cu(111) surface by means of density functional theory (DFT). The computational findings reveal that the O2 adsorption on the Pt-skin Pt3Cu(111) surface exhibits comparatively lower stability when contrasted with that on the Pt(111) surface. For O2 dissociation, two paths have been identified. One progresses from the t-f-b state towards the generation of two oxygen atoms situated within nearest-neighbour hcp sites. The other commences from the t-b-t state, leading to the generation of two oxygen atoms occupying nearest-neighbour fcc sites. Moreover, the analysis of the energy barrier associated with O2 dissociation indicates that O2 on Pt-skin Pt3Cu(111) surface is more difficult to dissociate than on the Pt(111) surface. The present paper is organized as follows: in Section 2, we present the computational details. Section 3 presents the calculated results and discussion, followed by conclusions in Section 4.

2. Computational Details

Spin-polarized DFT calculations were carried out using the Vienna ab initio simulation package (VASP) [24,25,26] and the projector-augmented wave method [27,28]. The exchange-correlation functional was described within the generalized gradient approximation proposed by Perdew et al. [29]. A kinetic energy cutoff of 400 eV was used with a plane-wave basis set, and the electric dipole was neglected. Brillouin zone integration for the surface structures was carried out using 5 × 5 × 1 Monkhorst-Pack grids [30] for a 2 × 2 unit cell. The converge criteria for force and electronic self-consistency were 0.02 eV/Å and 10−5 eV, respectively. We did not consider the van der Waals interaction because the chemical adsorption and dissociation processes of O2 involve strongly bonded interactions like covalent bonds, and the system does not involve significant van der Waals interactions or dispersion forces.
Previous studies have shown that Pt3M (M = transition metals) alloys with LI2 structures can be used to study a variety of catalytic reactions, including oxygen reduction reactions [31,32]. Moreover, due to the influence of surface segregation energy, when the concentration of Pt in the alloy is slightly higher than 75%, the surface of Pt3M(111) tends to form the so-called ‘Pt-skin’, i.e., the outermost layer is pure Pt, while the second layer and the following layers still have the LI2 crystal structure [33,34]. In this study, we chose this Pt-skin Pt3Cu(111) surface to investigate the O2 dissociative adsorption process at the surface of the Pt-Cu alloy. In DFT calculation, the bulk lattice constant is an important indicator of the accuracy of the calculation. For the lattice constant of bulk Pt, we calculated a value of 3.97 Å, which is identical to the experimentally measured value (3.97 Å) [35] and also exactly equal to the previous DFT calculation (3.97 Å) [36]. For the lattice constant of bulk Pt3Cu, we calculated a value of 3.90 Å, which is also exactly equal to the previous DFT calculated value (3.90 Å) [31,36]. Therefore, despite the fact that the geometry differences for the Pt-skin Pt3Cu(111) and Pt(111) surfaces are so small (<0.1 Å), we can still conclude that our DFT calculations are reliable. In fact, it is a common phenomenon that the geometrical differences for the Pt3M (M = transition metals) and Pt are too small [31,37]. For example, Pašti et al. [37] investigated the bulk lattice constants of Pt and Pt3M using DFT calculations. The calculations showed that the bulk lattice constants of Pt, Pt3Pd, Pt3Rh, Pt3Fe, Pt3Co and Pt3Ni were 4.001, 3.991, 3.965, 3.941, 3.922 and 3.911 Å, respectively. Sankarasubramanian et al. [31] also investigated the bulk lattice constants of Pt with Pt3M by DFT calculations. The calculated results show that the bulk lattice constants of Pt, Pt3Fe, Pt3Co, Pt3Cu and Pt3Ni are 3.95, 3.91, 3.89, 3.90 and 3.87 Å, respectively. The bulk lattice constant of an alloy is believed to be primarily influenced by the atomic radii of the constituent elements [38,39]. The elements with larger atomic radii increase the lattice constant, while elements with smaller atomic radii decrease the lattice constant. Since the difference between the atomic radius of Pt (2.39 Å) and that of Cu (2.26 Å) is small [40]. Therefore, the difference in bulk lattice constants between Pt and the Pt3Cu is also very small. Figure 1 shows the slab model of Pt-skin Pt3Cu(111) surface, which consists of four atomic layers separated by a 15 Å thick vacuum layer. In the slab model, the atomic structure within the two lowest layers was fixed, whereas the remaining atoms underwent a complete relaxation.
The adsorption energy ( E a d s , O 2 ) of O2 molecule at different adsorption site is calculated as follows:
E a d s , O 2 = E O 2 s l a b E s l a b E O 2
In this formulation, E O 2 s l a b represents the energy of the slab with O2 molecule, E O 2 represents the energy of the isolated O2 molecule, and E s l a b represents the energy of the slab.
The climbing-image nudged elastic band (CI-NEB) method is a computational method employed to identify transition states in chemical reaction [41,42]. We also have used this method to study the transition states and minimum energy path (MEP) for O2 dissociation on the Pt-skin Pt3Cu(111) surface. The energy barrier ( E a ) or activation energy can be calculated as follows:
E a = E T S E I S
In this formulation, E I S and E T S represent the total energies of the initial state (IS) and the transition state (TS), respectively.

3. Results and Discussion

3.1. The Adsorption of O2

Previous studies have investigated the O2 adsorption behaviour on the Pt(111) surface [4,43]. The results indicated the existence of three distinct adsorption sites for O2 on the surface of Pt(111). These were designated as t-h-b, t-f-b and t-b-t. Nevertheless, for each type of adsorption site, there are a multitude of adsorption configurations on the Pt-skin Pt3Cu(111) surface. We endeavoured to identify all possible adsorption configurations, and only those deemed stable (e.g., t-h-b1 or t-b-t1) were marked with symbols, listed in Table 1 and plotted in Figure 2. This reveals the following distinctive features. The first feature is that the O2 adsorption on the Pt-skin Pt3Cu(111) surface exhibits comparatively lower stability when contrasted with that on the Pt(111) surface. For instance, at the most stable adsorption site, the O2 adsorption energy on the Pt-skin Pt3Cu(111) surface is measured at −0.45 eV, whereas on the Pt(111) surface, it is −0.62 eV. This is mainly due to the changes in electronic structure, strain effects and changes in the arrangement of surface atoms caused by alloying, which together result in a weakening of the interaction between oxygen molecules and surface atoms, thus reducing the adsorption energy. This optimised adsorption energy is conducive to improving the activity and stability of the catalyst in the oxygen reduction reaction [6,44]. The second feature is that the O2 bond length on the Pt-skin Pt3Cu(111) surface exhibits a reduced degree of elongation in comparison to its counterpart on the Pt(111) surface. This may be due to the fact that the alloying of Pt with Cu induces an increase in the electron density of the Pt atoms, which enhances the filling of the π-anti-bonding orbitals of the O-O bond and shortens the O2 bond length [2,6]. The third feature is that in the case of the Pt-skin Pt3Cu(111) surface, the number of electrons acquired by O2 is found to be fewer than that observed in its counterpart on the Pt(111) surface. This may be due to the fact that on a pure Pt surface, the O2 molecules adsorb more strongly and therefore obtain more electrons and form stronger metal–oxygen bonds. Whereas on the Pt alloy surface, the adsorption energy is optimised and is usually lower, and the O2 molecules obtain fewer electrons and remain weakly adsorbed, thus being more favourable for the subsequent reaction steps.
It is noteworthy that these three parameters including the adsorption energies (Eads), the bond lengths of O2 (dO-O) and the number of electrons gained by O2 (Nchg) are relatively less varied on the surface of Pt-skin Pt3Cu(111) compared to the Pt(111) surface. This is a common phenomenon in Pt alloys [4], and similar results are often found in the noble metals Au [45] and Pd [6]. The d-orbital electrons of these noble metals are more stable and saturated and do not easily participate in chemical reactions. Therefore, alloys composed of noble metals (such as Pt, Au or Pd) are prone to form chemically inert surfaces, which will result in low adsorption energy of O2 on the surfaces of noble metals and their alloys, and the change of adsorption energy will be very small [46]. In addition, the combination of two factors, the weak interaction of the adsorption process and the high stability of oxygen molecules themselves, also leads to little change in the O2 bond lengths [2,6]. Similarly, under the combined effect of low adsorption energy and high electronegativity of oxygen molecules, it is difficult for electrons to be transferred from the alloy surface to the adsorbed oxygen molecules, which also results in little change in the value of Nchg [6,44].

3.2. The Electronic Structure of Pt-Skin Pt3Cu(111) Surface

In surface science, the d-band theory can be used to explain the adsorption behaviour on metal surfaces. According to this theory [47,48], the interaction between adsorbed molecules or atoms and the metal surface is primarily determined by the energy level distribution of the d-electrons on the metal surface, and the position of d-band centre is indicative of the position of d-electrons’ energy level. If the position of d-band centre is elevated, it can be inferred that the d-electron energy level on the metal surface is proximate to the Fermi energy level. This typically indicates that the metal surface is more adept at adsorbing the adsorbate. Conversely, if the d-band centre is situated at a considerable distance from the Fermi level, this suggests that the metal surface is less capable of adsorbing adsorbates. To further investigate the influence of alloying on the O2 adsorption behaviour, we calculated the d-band DOS for Pt on the top-most atomic layer of both Pt-skin Pt3Cu(111) and Pt(111) surfaces in the absence of adsorption. The results are shown in Figure 3. The d-band centre for Pt atoms on the alloy surface is displaced from the Fermi level into the lower energy region, compared with pure Pt. This indicates that alloying Pt with Cu reduces the interaction of O2 with its surface. This is consistent with our calculations; above that, the affinity of O2 for the Pt-skin Pt3Cu(111) surface is attenuated.
It is generally accepted that the geometric (strain) and electronic (ligand) effects are the two principal factors that can influence the d-band centre [49,50,51]. When a second metal is added to a pure metal, the original geometric and electronic properties of the metal are altered, as shown in Table 2. We can find that the addition of Cu with a smaller atomic radius to Pt results in a decrease in the Pt-Pt interatomic distance. This suggests that the compressive strain drives d-band centre for Pt on the alloy surface into the lower energy region, which in turn leads to a decrease in the adsorption strength. Therefore, the geometric effect may be one of the principal factors affecting the adsorption behaviour. Furthermore, a Bader charge analysis was conducted on Pt-skin Pt3Cu(111) and Pt(111) surfaces. The topmost Pt atoms on the alloy surface exhibited a more negative charge than those on the Pt(111) surface. This calculation suggests that electrons are transferred from Cu atoms in the second layer to Pt atoms in the topmost layer. However, the accumulation of excess electrons on the surface of the alloy leads to an augmentation of electrostatic repulsion [44], which will also indirectly contribute to the decrease in the adsorption strength of O2 on the alloy surface. This finding aligns with the calculation results in Table 1. Therefore, we can infer that the decrease in the bonding strength of O2 to the surface of Pt-skin Pt3Cu(111) is a result of both the geometric effect and electronic effect.

3.3. The Dissociation of O2

For the O2/Pt3Cu(111) system, the two adsorption configurations that exhibited the highest adsorption capacity, t-b-t1 and t-f-b1, were selected as initial states for the study of the O2 dissociation path. For the t-f-b1 state, three potential final states were examined, including the dissociated two oxygen atoms occupying the nearest-neighbour fcc and hcp sites (fcc × hcp), the nearest-neighbour hcp sites (2 × hcp), and the nearest-neighbour fcc sites (2 × fcc), respectively. Computational results indicated that the path to the 2 × hcp final state was the most reasonable elementary step for dissociation. Figure 4a illustrates the dissociation path with the t-f-b1 configuration as the initial state and the 2 × hcp configuration as the final state. It can be observed that during the dissociation process, the elongation of the O-O bond occurs along the O2 axis, culminating in the formation of the transition state (TS1), wherein one O atom relocates to the bridge site, while the other O atom migrates to the near-top position. This transition occurs concomitantly with the energy barrier of 0.61 eV. As the O-O bond is elongated further, it is broken because the near-top O atom continues to climb to the top site, while the O atom occupying the bridge site is displaced to the neighbouring hcp site, forming an intermediate state called “top-hcp”. However, the O atom occupying the top site is less stable than its counterpart situated within the hcp site, and it will continue to move to the other neighbouring hcp site. Following the overcoming of the energy barrier (0.23 eV) associated with the transition state (TS2), the O atom finally reaches the hcp site and completes the dissociation process. Figure 4b illustrates the dissociation path with the t-b-t1 configuration as the initial state and the 2 × fcc configuration as the final state. It can be observed that the O–O bond undergoes elongation as it rotates towards the adjacent bridge site, culminating in the formation of a transition state (TS1). This transition occurs concomitantly with the energy barrier of 0.65 eV. Subsequently, the O-O bond is broken, as the O atom occupying bridge site slides to neighbouring fcc site, while the near-top O atom continues to climb to the top site. The O atom occupying fcc site is in a stable state, whereas the O atom occupying the top site is in an unsteady state, following the overcoming of the energy barrier (0.09 eV) associated with the transition state (TS2), the O atom finally reaches fcc site and completes the dissociation process.
For the O2/Pt(111) system, numerous prior investigations have indicated that it is difficult for t-b-t state to dissociate directly; rather, it should convert to the t-f(h)-b states through the action of diffusion [4,43]. This leads to two dissociation paths: one pathway starts from the t-f-b state and ends with the 2 × hcp state, the other path starts from the t-h-b state and ends with the 2 × fcc state. The energy barriers associated with these two dissociation paths are 0.53 and 0.36 eV, respectively, which are lower than the corresponding energy barriers for the O2/Pt3Cu(111) system, in agreement with experimental observations [52,53].

4. Conclusions

We conducted a systematic investigation into O2 dissociative adsorption on the Pt-skin Pt3Cu(111) surface by means of density functional theory. The computational findings reveal that the O2 adsorption on the Pt-skin Pt3Cu(111) surface exhibits comparatively lower stability when contrasted with that on the Pt(111) surface. Bader analysis shows that in the case of the Pt-skin Pt3Cu(111) surface, the number of electrons acquired by O2 is found to be fewer than that observed in its counterpart on the Pt(111) surface. This suggests that the addition of Cu induces alterations in the surface electronic configuration, thereby impeding the favourable transfer of electrons to O2. By analysing the factors affecting the centre of the d-band of the metal catalyst, we can infer that the decrease in the bonding strength of O2 to the surface of Pt-skin Pt3Cu(111) is a result of both a geometrical effect and electronic effect. For O2 dissociation, two paths have been identified. One progresses from the t-f-b state towards the generation of two oxygen atoms situated within nearest-neighbour hcp sites (2 × hcp). The other commences from the t-b-t state, leading to the generation of two oxygen atoms occupying nearest-neighbour fcc sites (2 × fcc). Moreover, the analysis of the energy barrier associated with O2 dissociation indicates that O2 on the Pt-skin Pt3Cu(111) surface is more difficult to dissociate than on the Pt(111) surface. This study can offer a valuable guide for the practical application of high-performance oxygen reduction catalysts.

Author Contributions

Conceptualization, Y.Y. and G.W.; methodology, Y.Y.; software, Y.W.; validation, M.F.; formal analysis, Y.W.; investigation, M.F.; resources, X.F.; data curation, H.G.; writing—original draft preparation, Y.Y.; writing—review and editing, G.W.; visualization, H.G.; supervision, G.W.; project administration, M.Z.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Research for Doctoral Special Project of Kaili University, grant number: BS20240210. And the APC was funded by the Natural Science Research for Doctoral Special Project of Kaili University.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Acknowledgments

The authors would like to acknowledge the support of the Natural Science Research for Doctoral Special Project of Kaili University (BS20240210) and the National Natural Science Foundation of China (51661013 and 12064019).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Side view and (b) top view for the slab model of the Pt-skin Pt3Cu(111) surface. Brick-red and grey spheres denote Cu and Pt atoms, respectively.
Figure 1. (a) Side view and (b) top view for the slab model of the Pt-skin Pt3Cu(111) surface. Brick-red and grey spheres denote Cu and Pt atoms, respectively.
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Figure 2. Top view of the stable adsorption configurations of O2 on Pt-skin Pt3Cu(111) surface. Red, grey and brick-red spheres represent O, Pt and Cu atoms, respectively. To enhance clarity, larger spheres are employed to depict the atoms situated in the topmost layer, and only the top two atomic layers are shown.
Figure 2. Top view of the stable adsorption configurations of O2 on Pt-skin Pt3Cu(111) surface. Red, grey and brick-red spheres represent O, Pt and Cu atoms, respectively. To enhance clarity, larger spheres are employed to depict the atoms situated in the topmost layer, and only the top two atomic layers are shown.
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Figure 3. The d-band DOS for Pt on the top-most atomic layer of both Pt-skin Pt3Cu(111) and Pt(111) surfaces in the absence of adsorption.
Figure 3. The d-band DOS for Pt on the top-most atomic layer of both Pt-skin Pt3Cu(111) and Pt(111) surfaces in the absence of adsorption.
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Figure 4. Two O2 dissociation paths on the Pt-skin Pt3Cu(111) surface. (a) The path starts from the t-f-b1 state and ends with the 2 × hcp (two O occupying the nearest-neighbour hcp sites), (b) the path starts from the t-b-t1 state and ends with the 2 × fcc (two O occupying the nearest-neighbour fcc sites).
Figure 4. Two O2 dissociation paths on the Pt-skin Pt3Cu(111) surface. (a) The path starts from the t-f-b1 state and ends with the 2 × hcp (two O occupying the nearest-neighbour hcp sites), (b) the path starts from the t-b-t1 state and ends with the 2 × fcc (two O occupying the nearest-neighbour fcc sites).
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Table 1. The O2 adsorption energies (Eads in eV), O2 bond lengths (dO–O in Å) and the quantity of electrons gained by O2 (Nchg) from the Pt-skin Pt3Cu(111) and Pt(111) surfaces.
Table 1. The O2 adsorption energies (Eads in eV), O2 bond lengths (dO–O in Å) and the quantity of electrons gained by O2 (Nchg) from the Pt-skin Pt3Cu(111) and Pt(111) surfaces.
Pt-skin Pt3Cu(111) Pt (111)
SiteEadsdO–ONchgSiteEadsdO–ONchg
t-f-b1−0.451.390.55t-f-b−0.621.400.56
t-f-b2−0.371.380.53t-h-b−0.431.390.54
t-h-b−0.151.370.52t-b-t−0.611.360.47
t-b-t1−0.301.350.45
t-b-t2−0.101.340.43
Table 2. The geometric and electronic characteristics of Pt-skin Pt3Cu(111) and Pt(111) surfaces computed utilizing DFT method.
Table 2. The geometric and electronic characteristics of Pt-skin Pt3Cu(111) and Pt(111) surfaces computed utilizing DFT method.
SurfacePt–Pt Distance (Å)Bader Charge (e)d-Band Centre (eV)
Pt-skin Pt3Cu(111)2.74−0.077−2.11
Pt(111)2.81−0.051−1.98
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Yu, Y.; Gu, H.; Fu, M.; Wang, Y.; Fan, X.; Zhang, M.; Wu, G. First Principles Study of O2 Dissociative Adsorption on Pt-Skin Pt3Cu(111) Surface. Catalysts 2024, 14, 382. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060382

AMA Style

Yu Y, Gu H, Fu M, Wang Y, Fan X, Zhang M, Wu G. First Principles Study of O2 Dissociative Adsorption on Pt-Skin Pt3Cu(111) Surface. Catalysts. 2024; 14(6):382. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060382

Chicago/Turabian Style

Yu, Yanlin, Huaizhang Gu, Mingan Fu, Ying Wang, Xin Fan, Mingqu Zhang, and Guojiang Wu. 2024. "First Principles Study of O2 Dissociative Adsorption on Pt-Skin Pt3Cu(111) Surface" Catalysts 14, no. 6: 382. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060382

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