Zinc Oxide and Copper Oxide surfaces as potential adsorbent layers for Greenhouse gases and Hydrogen: A Molecular Dynamics Analysis

doi:10.21203/rs.3.rs-3134640/v1

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Zinc Oxide and Copper Oxide surfaces as potential adsorbent layers for Greenhouse gases and Hydrogen: A Molecular Dynamics Analysis

https://doi.org/10.21203/rs.3.rs-3134640/v1

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Molecular dynamics (MD) is used to simulate the adsorption characteristics of Hydrogen and greenhouse gases like Methane and Carbon dioxide on Zinc oxide and Copper oxide surfaces at room temperature. The simulation based on the Reax-FF inter-atomic potential was used to quantify and illustrate the adsorption process of hydrogen on the chosen adsorbents at different incident energies. Our simulation results have shown that Zinc oxide is a more effective adsorbent for hydrogen than Copper oxide at room temperature. The adsorbent properties and adsorption mechanism remain relatively constant regardless of variations in the incident velocity of the adsorbate. The results of methane adsorption at 300 K indicate that Zinc oxide is a more effective adsorbent than Copper oxide. However, both adsorbents have equal efficiency in adsorbing carbon dioxide at the same temperature. Molecular dynamics simulation at different temperatures ranging from 50 K-450 K reveals that Copper oxide can adsorb methane molecules at low temperatures while Zinc oxide retains its adsorption properties regardless of temperature.

Adsorption

LAMMPS

Molecular Dynamics

Reax-FF

Hydrogen

Carbon dioxide

Methane

Zinc oxide

Copper oxide

Hydrogen(H) is one of the most promising potential sources of futuristic clean energy fuel in meeting the increased demand for energy consumption. However, due to its low energy density per unit volume and low ambient temperature density, improved storage technologies with the potential for higher energy density must be developed [1]. On the other hand, Methane (CH₄) and Carbon dioxide (CO₂) are the most hazardous air pollutants and primary contributors to the greenhouse effect and global warming [2, 3] requires strategic means of detecting and reducing their emissions into the atmosphere [4]. Apart from these environmental impacts, the adsorption of hydrogen and methane activates catalytic activity [5–7] on the surface of selected adsorbents and carbon dioxide molecule is used in solvent recovery [8] and post-combustion capture processes [9].

These gases are detected through a surface phenomenon called Adsorption [10]. Adsorption is the process by which atoms or molecules adhere to a solid surface. It is a fundamental process underlying gas sensing application. The ability of a material to selectively adsorb certain gases is crucial for developing effective gas sensors. Thus, the choice of adsorbent is a critical factor in determining the adsorption mechanism.

The whole adsorption process relies on the properties of the chosen adsorbent surface. The most commonly used and studied adsorption mechanisms are on nanostructured metal oxides [11–13], carbon-based materials [14], polymer resins, and activated metal oxides [15, 16].

N-type Intrinsic semiconducting Zinc oxide (ZnO) and P-type copper oxide (CuO) are the most widely used nontoxic and environment-friendly metal oxides. ZnO and CuO possess immense applications for industrial purposes in various fields [17–19]. Surface area, porosity, chemical stability, and renderability are some characteristics that make us choose these metal oxides as an adsorbent for our study. Numerous techniques exist for determining the adsorption of gases on solid adsorbents under specific conditions. Experimental results are available on sensing applications of both ZnO and CuO for different gases, including methane, Carbon dioxide, and Hydrogen, performed under several conditions [20–27]. There are also several publications on the use of ZnO as a potential hydrogen storage material [28, 29].

Molecular dynamics simulations (MD) [30] offer an alternative to experimental research for investigating the adsorption mechanism of different gases on various adsorbent surfaces. Pingan Liu et al. [31] studied the adsorption of ethanol molecules on the Aluminium surface and pointed out the role of each functional group in the adsorption process of ethanol. Focusing on the effects of temperature and pressure, Fajar Fatriansyah et al. [32] investigated hydrogen adsorption across various graphene frameworks. The author validated the accuracy of the molecular dynamics simulation by comparing its results with those obtained through experimental research. D Dhaneswara et al. [33] performed an MD simulation of hydrogen adsorption on silica and highlighted the capability of silica as a good hydrogen storage material. The impact of temperature and pressure on nitrogen adsorption in amorphous silica was also examined in the paper by J.F.Fatriansyah and co-authors. [34]. These studies demonstrate the efficiency of using MD simulations to provide comprehensive insights into atomic behaviour and adsorbed structures.

In this paper, we performed MD calculations to simulate the adsorption characteristics of Hydrogen, Methane, and Carbon dioxide gases on adsorbent materials like Zinc oxide and Copper oxide at room temperature. The paper primarily focuses on the difference in the total surface energy of the adsorbent surface and its surface morphology on adsorbing various gases, the percentage of adsorbed gas and corresponding adsorption energy under different conditions, followed by the adsorption mechanism of individual gases on the chosen surfaces.

We have adopted a computational simulation study using Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [35], that performs Molecular dynamics (MD) calculations to study the adsorption mechanism of different gases. The obtained model is visualized using OVITO [36] and VMD (Visual Molecular Dynamics) [37] software.

The hexagonal-shaped wurtzite structure of Zinc oxide (ZnO) [38]- an n-type material and monoclinic structured Copper Oxide (CuO)- a p-type material with 2700 and 5400 atoms, respectively which has a dimension of 15 x 15 x 3-unit cell is chosen as the adsorbent [39] as shown in Fig. 1. The lattice parameters for the chosen substrate structures are shown in Table 1. The topmost layer of both the adsorbent is kept mobile and is the thermally controlled layer of the adsorbent, while the bottom layers are kept fixed.

Table 1

Lattice parameters for the chosen structures- ZnO and CuO
	Unit cell parameters
Structure	a(Å)	b(Å)	c(Å)	α⁰	β⁰	γ⁰
Zinc Oxide (ZnO)	3.350	3.350	5.220	90	90	120
Copper Oxide (CuO)	2.939	2.939	5.161	88.19	88.19	87.40

The temperature control of the mobile group of adsorbents is performed using the Noose-hoover thermostat (NVT ensemble) [38] with a damping factor of 50. Periodic boundary conditions are mandated in the x, y, and z directions. The region above the adsorbent (ZnO and CuO), at the height of 30 Å, is filled with a layer of adsorbate -Hydrogen, Carbon dioxide, and Methane which is initially minimized using Avogadro [40].

In this study, the atomic interactions between the atoms are modelled using the Reax-FF inter atomic potential [41]. The Reax-FF parameters involved in each potential function are developed empirically by quantum mechanical calculations followed by iterative training and optimization of sets. Reax-FF is a un harmonic type potential that considers both bonded and non-bonded interaction in or between molecules or atoms in a system. In Reax-FF, the total interaction energy of the system, ${E}_{system}$ is expressed as:

$${E}_{system} = {E}_{bond}+ {E}_{over} +{E}_{val}+ {{E}_{tor}+{E}_{Coulomb}+E}_{Vd waal{\prime }s }$$

Where ${E}_{bond}$is the energy associated with the breakage or formation of a bond between two atoms, ${E}_{over}$ is the energy penalty term related to over coordination of atoms, ${E}_{val}$ and ${E}_{tor}$ are the energy related to three body valence angle strain and four body torsional angle strain respectively. These four terms related to bonded interaction are calculated from Bond order which in turn depends on the inter-atomic position of atoms. ${E}_{Vd waal{\prime }s}$ represents the long-range attraction and short-range repulsive interaction, ${E}_{Coulomb}$ represents the geometry dependent electrostatic and polarization interaction between atoms. ${E}_{Vd waal{\prime }s}$ and ${E}_{Coulomb}$ represents the non-bonded interaction terms in Reax-FF potential function.

To study the adsorption characteristics of hydrogen on Copper Oxide and Zinc Oxide, the simulation is performed for different incident energies ranging from 0.1 eV to 0.5 eV with a step size of 0.1 eV, maintaining the adsorbent temperature at 300 K. While methane adsorption and carbon dioxide adsorption on the same adsorbent are performed with the root mean square velocity (RMS) of the molecule attained at 300 K. For their comparative analysis, the same method is adopted for hydrogen adsorption as well. To enhance our simulation, we conducted an adsorption investigation of the adsorbates at different adsorbent temperatures ranging from 50 K to 450 K, with an increment of 50 K while maintaining a constant velocity equivalent to their RMS velocity. The molecular dynamics simulation is performed for 150 ps (deposition time) with a time step of 0.001 ps.

Under these conditions, it is feasible to observe and analyse the surface morphology, adsorption mechanism, and variations in the total energy of surface atoms. The adsorption process throughout our study is characterized using ‘adsorption energy’, ‘Surface energy plots’, and ‘percentage of adsorbed particles.’ Adsorption energy is the energy released or adsorbed when a molecule or particle is adsorbed onto a surface [42]. It is considered a measure of the strength of interaction between the adsorbate and the adsorbent. In MD simulations, adsorption energy, ${E}_{adsorption}$ is calculated as follows.

$${E}_{adsorption} = {E}_{adsorbate}+ {E}_{adsorbent} - {E}_{pair}$$

where ${E}_{adsorbate}$ is the potential energy of the adsorbate (H₂, CO₂ and CH₄), ${E}_{adsorbent }$ denotes the potential energy of the adsorbent (ZnO and CuO) and ${E}_{pair}$ is the total potential energy after the adsorption, which includes adsorbed particles and the adsorbent.

3.1 Adsorption of Hydrogen on Zinc oxide (ZnO).

3.1.1 Single-molecule adsorption mechanism of Hydrogen on ZnO:

Hydrogen adsorption was initiated from the height of 30 Å above the surface of ZnO. Hydrogen gets adsorbed on the ZnO surface within a time of 1400 fs. The adsorption distance is 3.4 Å, about which the adsorbed hydrogen undergoes motion throughout the surface of Zinc oxide. This indicates that hydrogen adsorbed as a molecule on the surface of Zinc oxide is through physisorption [42]. Depiction of the path taken by a hydrogen molecule over time as it moves towards the ZnO adsorbent can be observed in Fig. 2.

The adsorbed hydrogen is observed to be strongly bound to the ZnO surface. This is indicated by the minimum energy value of the curve in Fig. 3(a). The fluctuation of the curve seen about the minimum energy is because of the thermal vibration of the surface, which in turn results in the vibration of hydrogen on the ZnO surface.

3.1.2 Adsorption of Hydrogen on Zinc Oxide (ZnO) surface with variation in the incident energy of hydrogen:

The Zinc oxide surface terminated with zinc atoms is subjected to hydrogen clouds at varying energies at 300 K. This results in the Vibration of the first two alternating layers of Zn and O atoms. The incoming hydrogen gets attached to the oxygen atoms. This is because of the vibration of the Zn layer that causes its displacement, thereby exposing oxygen atoms lying beneath it. As a result, hydrogen gets adsorbed to the oxygen atoms in between two Zinc atoms, forming a diagonal arrangement on the surface of Zinc Oxide as in Fig. 4(e). Hydrogen is seen to adsorb as both molecules – Physisorption and atoms- chemisorption [43] on the surface of ZnO, Fig. 4(f). However, the majority of hydrogen is seen to adsorb as molecules, resulting in the formation of relatively weak bond- physisorption with the surface atom.

The percentage of hydrogen adsorbed and the calculated adsorption energy of hydrogen on the Zinc surface with an increase in velocity from 1900 m s^− 1 (RMS) to 9800 m s^− 1 (0.5 eV) is shown in Fig. 5(a). The figure shows no significant variation in the percentage of adsorbed hydrogen on the Zinc oxide surface with the incident velocity of hydrogen.

The variation in total energy of the ZnO surface (First Zn and O layer) on the incidence of hydrogen at different incident energy with the simulation time is shown in Fig. 3(b). The Total energy of pure ZnO surface vibrating at 300 K is also shown as a reference for comparison. From the graph, it is observed that on the incidence of hydrogen, the total energy of the surface layer decreases when compared to the total energy of the surface before hydrogen adsorption Fig. 3(b). The adsorption of hydrogen molecules on the surface of zinc oxide involves the process of bond formation (attractive interaction) between them. This exothermic process releases energy, which causes a reduction in the surface energy of the zinc oxide. As additional hydrogen is adsorbed, the surface coverage of ZnO becomes higher, and the adsorption process becomes less energetically favourable. This is because the hydrogen atoms start to experience repulsive interactions with each other, which makes it more difficult for additional hydrogen molecules to get adsorbed on the surface. At this point, the surface energy of the zinc oxide starts to increase, Fig. 3(b), as the energy required to adsorb additional hydrogen molecules on the surface increases. At the end of the simulation, the surface energy of zinc oxide re-establishes its original value (energy before hydrogen adsorption). This occurs because the ZnO surface gets saturated with hydrogen molecules, leading to a decrease in the efficiency of energy transfer. This behaviour is observed consistently across all incident energies or velocities of hydrogen, neglecting some randomness in the attained minimum value.

3.2 Adsorption of Hydrogen on Copper oxide (CuO).

3.2.1 Single-molecule adsorption mechanism of Hydrogen on CuO:

Hydrogen gets adsorbed on the mobile surface of copper oxide within 600 fs, Fig. 6(a). Figure 6(a)-6(c) shows the path of hydrogen molecules toward the CuO surface. Immediately after the adsorption, hydrogen liberates from the surface as a free particle, Fig. 7 (a).

In CuO, hydrogen rarely adsorbs on the surface; this observation is strengthened by the sharp drop in the potential energy curve, immediately after which the potential energy of the hydrogen molecule increases, Fig. 7(a). The sudden increase in energy of the hydrogen is a result of the liberation of adsorbed hydrogen from the surface.

3.2.2 Adsorption of Hydrogen on Copper oxide (CuO) surface with variation in the Incident energy of hydrogen:

Hydrogen, located at the height of 30 Å, is allowed to incident at different energies on the Cu-terminating CuO surface. The temperature of the top layers of the surface is maintained at 300 K. This results in the vibration of the first two alternating layers of Cu and O atoms. Most of the incoming hydrogen is seen to rebound as it approaches the surface. Hence, the adsorbed number of hydrogen is significantly less compared to ZnO.

The Adsorption process of hydrogen on CuO is shown in Fig. 8(a)-8(d). The number of hydrogen adsorbed and the adsorption energy of hydrogen on the copper surface with an increase in velocity from 1900 m s^− 1 (RMS) to 9800 m s^− 1 (0.5 eV) is shown in Fig. 5(b). From the obtained data, there is a significant reduction in the hydrogen adsorption on the Cu surface, and there is no change in their percentage with an increase in velocity as in ZnO. The decreased percentage of hydrogen adsorbed compared to that on ZnO is attributed to the higher vibrational energy of the Cu layer at 300K. Hydrogen molecules do not adsorb on the copper oxide (CuO) surface because of the strong bond between copper and oxygen, stronger than the bond between hydrogen atoms in H₂. Consequently, the attractive forces between the CuO surface and H₂ molecules are relatively weak compared to those between H₂ molecules. This results in hydrogen molecules unable to form stable bonds with the CuO surface and create adsorption sites. The average total surface energy of CuO at 300 K is obtained as -115.66 kcal mol^− 1, greater than that of Zinc oxide, whose vibrational energy is -167.08 kcal mol^− 1 at the same temperature. Due to these reasons, a large portion of hydrogen particles recoils from the copper surface. Hence copper terminating surface of pure copper oxide rarely adsorbs hydrogen.

Even though the surface of copper is observed to contain hydrogen in Fig. 8, as time evolves, the hydrogen is seen to leave the surface through constant vibration. Total energy plots for various incident velocities of hydrogen are illustrated in Fig. 7(b). The total energy of pure CuO surface vibrating at 300 K is also shown as a reference for comparison. From the obtained plots, there is no significant change in the total surface energy of mobile atoms compared to that of copper surface without hydrogen. The surface energy shows nearly the same variation as pure CuO surface without hydrogen. This again proves that hydrogen adsorption is weak on copper oxide. Moreover, there is no difference in the energy curve of the copper surface upon incidence at different velocities. The curves show an almost similar trend. Thus, the adsorption process is not significantly affected by the incident velocity of hydrogen.

3.3 Adsorption of Methane (CH₄) on Zinc Oxide (ZnO) at its RMS velocity.

3.3.1 Single-molecule adsorption mechanism of Methane on ZnO:

The trajectory of the methane molecule traversing toward the ZnO surface is shown in Fig. 9. To study the adsorption mechanism at 300 K, the variation in potential energy of the adsorbate (Methane) with respect to time and distance, Fig. 10(a) is analysed.

According to the studies, the hydrogen atom of methane leads the adsorption process toward the Zinc oxide surface. This is attributed to the fact that the carbon atom of methane with zero lone pair is tetrahedrally bonded to four hydrogen atom that entirely covers it. As a result, the carbon atom lies in the centre. The carbon in methane cannot be exposed to the surface unless there is a breakdown in the bonds.

Methane molecule traversing towards zinc oxide gets adsorbed on the surface within 700fs, Fig. 10(a). Later the methane molecule constantly vibrates about the hydrogen atoms under the thermal effect, maintaining an average distance between the Zinc surface and hydrogen atom as 3.1 Å. Methane undergoes physisorption on the surface of zinc oxide at 300 K [42].

3.3.2 Adsorption of Methane on Zinc oxide (ZnO) surface at its RMS velocity:

Methane molecules on the zinc surface get adsorbed on ZnO following the mechanism stated in single-molecule adsorption. The percentile of adsorption and corresponding adsorption energy is represented in Table 2. Methane adsorbed on the surface at the final simulation time is shown in, Fig. 11. Incident methane molecules undergo constant vibration on Zn-surface due to the thermal energy of the Zn-layer at 300 K. Based on the calculated adsorption energy, we can conclude that the interaction between a methane molecule and a zinc-terminated surface is very weak, resulting in low adsorption energy when compared to the adsorption energy of hydrogen on zinc oxide.

Figure 10(b) illustrates how the total energy of the Zinc terminated surface of ZnO changes over time when methane molecules are introduced. On the introduction of methane molecules, the total energy of surface atoms decreases to a minimum value, as seen in Fig. 10(b). The surface energy of Zinc oxide gets transferred to the incoming methane molecules.

Consequently, the methane molecules adsorbed on the surface exhibit persistent vibrations around the lowest energy attained. This phenomenon leads to a reduction in the surface energy of the Zinc layer over time.

3.4 Adsorption of Methane on Copper oxide (CuO) surface at its RMS velocity.

3.4.1 Single-Molecule adsorption of Methane:

The trajectory of the methane molecule traversing towards the adsorbent- CuO is shown in Fig. 12. The adsorption mechanism of methane on the CuO surface is the same as that on the ZnO surface.

In the case of copper oxide, Fig. 13(a) adsorption process completes in a time of 400 fs, soon after it gets desorbed from the surface, liberating as a free particle, stating the fact that methane does not adhere to the copper oxide surface at 300 K. The potential energy curve plotted as a function of time and distance for both surfaces, Fig. 13(a), supports the above-stated observation of methane adsorption.

3.4.2 Adsorption of Methane on Copper oxide:

Observations indicate that before adsorption on the Cu surface, a significant number of methane molecules dissociate into individual atoms, carbon, and hydrogen. These atoms cover the adsorbent surface more prominently than methane molecules, Fig. 14. Only a very smaller proportion of methane molecules get adsorbed on the surface without dissociation. This characteristic of methane adsorption is reported in the case of many single crystal metals [6] and a few metal oxides [44] and forms the basis for catalysis.

Hence, compared to the zinc-terminating ZnO surface, the copper-terminating surface of CuO is a weak adsorbent for methane at 300 K. The number of molecules adsorbed, and the corresponding adsorption energy is shown in Table 2.

Table 2

Percentage of adsorbed molecules (CO2 and CH4) at their RMS velocity on ZnO and CuO and their adsorption energy
Adsorbent	Methane		Carbon Dioxide
	Percentage Adsorbed (%)	Adsorption Energy (10⁴kcal /mol)	Percentage Adsorbed (%)	Adsorption Energy (10⁴kcal /mol)
Zinc oxide	73	-0.32	85	-0.042
Copper Oxide	5	-5.79	87	-0.047

The variation in the total energy of the surface on the incidence of methane molecules with the simulation time is shown in Fig. 13(b). Even though methane doesn’t adsorb on the copper surface as on zinc, the observed decrease in the surface energy is attributed to the interaction between adsorbed dissociated carbon and hydrogen atoms as well as the vibration of methane.

3.4.3 Effect of temperature on methane adsorption on Copper oxide and Zinc oxide:

Further, a low-temperature study of methane adsorption is performed to substantiate methane adsorption on CuO. The temperature of the mobile layer of adsorbent is varied from 50 K- 450 K with a step size of 50 K. The adsorption energy, percentage of adsorbed particles, and surface energy plots are compared for methane adsorption on CuO and ZnO.

Figure 15(a)-15(b) represents the percentage of particle adsorbed. The adsorbed rate of molecules is seen to decrease with temperature on both the adsorbents. Zinc oxide is seen to adsorb a very good percentage of molecules at low temperatures and moderate temperatures, Fig. 15(a)-15(b), but copper surface adsorbs more methane at lower temperatures compared to higher ones. At low temperatures, weak attractive interaction exists between methane molecules and copper surface. These interactions are more effective when the surface possesses low thermal energy. Hence low temperature provides a greater propensity to adhere to the surface.

Variation in total energy curves of adsorbent, Fig. 16(a)-16(b) strengthens our observations and calculations performed. At low temperatures, methane adsorption on ZnO decreases the surface energy of the Zn layer. The reason is because of energy transfer, as stated in the above cases. This indicates that methane adsorption takes place in a reasonable manner for all temperatures. Interaction between methane molecules and the surface can overcome the surface’s thermal energy. The variation in the percentage of methane adsorbed more or less remains the same with temperature. As a result, the variation in the total energy curve looks alike for all temperatures, expect some random fluctuations.

On CuO, methane adsorption dominates at low temperatures with more percentage of adsorbed molecules, Fig. 17(c). At higher temperatures, methane does not even adsorb on the copper surface, Fig. 17(d) because of very high thermal energy and increased repulsive interaction between the approaching methane molecule and the mobile surface. The total energy of the surface decreases on methane adsorption due to energy transfer between methane and copper surface. At the same time, since no molecules are adsorbed at high temperatures, the energy curve resembles the energy curve of the surface before adsorption at their corresponding temperatures, Fig. 18.

Methane molecules form a cluster at 450 K rather than adsorbing on the copper surface.

3.5 Adsorption of Carbon dioxide (CO₂) at its RMS velocity on Zinc Oxide (ZnO).

3.5.1 Single-molecule adsorption mechanism of carbon dioxide on ZnO:

The oxygen atom of Carbon dioxide leads the adsorption process of carbon dioxide molecules as it forms the most electronegative part of the molecule. The polar adsorbent surface (Zn terminated) shows a higher affinity towards oxygen than carbon. This favours oxygen of CO₂ being readily adsorbed. Due to the influence of temperature and the pursuit of attaining a stable configuration, molecules on the surfaces of both adsorbents undergo rapid vibration followed by a rotation. This is indicated by the fluctuating behaviour of the potential energy curve in Fig. 19(a). As inferred from the graph, the minimum adsorbed distance of the molecule on the surface is about 2.4 Å within a simulation time of 1300 fs. Hence it is concluded that carbon dioxide adsorption on the ZnO surface is through chemisorption [42].

3.5.2 Adsorption characteristics of Carbon dioxide on Zinc Oxide (ZnO):

According to the single molecule adsorption mechanism stated above, the initial stage of carbon dioxide adsorption involves the attachment of oxygen atoms. After adsorption, the molecules undergo rotation and eventually form a bent structure. Hence CO₂ molecules are found in their bent form on the surface of ZnO, as shown in Fig. 21(a). Both the oxygen atoms of the molecules are seen to adhere to the zinc atoms, resulting in their bent structure, Fig. 21(a). The number of molecules adsorbed, and adsorption energy is shown in Table 2.

The variation in total energy of the Zinc surface on the incidence of carbon dioxide molecules with the simulation time is shown in Fig. 19(b). The inbound carbon dioxide molecules receive a vibrational energy boost from the zinc oxide. The molecules adsorbed on the surface consequently show vibrations about the minimum energy. Over time, this mechanism causes the surface energy of the zinc layer to decrease. Finally, at the end of the simulation, the curve is shown to attain a new minimum of about − 171 kcal mol^− 1, Fig. 19 (b).

3.6 Adsorption of Carbon dioxide on Copper oxide surface at its RMS velocity.

3.6.1 Single-molecule adsorption mechanism of CO₂ on CuO:

Adsorption of CO₂ on CuO surface follows the exact mechanism as on ZnO. It is seen that adsorption takes place more rapidly on the copper surface within 700 fs when compared to the Zinc oxide surface. The minimum adsorbed distance of the molecule on the surface is about 2.3Å as inferred from the graph, Fig. 20(a). Hence it is clear that carbon dioxide adsorption on CuO is also through chemisorption [42].

3.6.2 Adsorption characteristics of Carbon dioxide on copper oxide (CuO):

Compared to Methane and Hydrogen, carbon dioxide shows potential adsorption characteristics on CuO. Following the same simulation condition, the incident carbon dioxide molecule adsorbs on the copper surface of CuO, Fig. 21(b). The molecule adsorbs in a linear form, preserving its linear structure. The carbon dioxide molecule interacts with the Cu atoms on the surface of the copper oxide, forming coordinate bonds between the oxygen atoms in CO₂ and the Cu atoms. The variation in the total energy of the copper surface on the incidence of carbon dioxide molecules with the simulation time is shown in Fig. 20(b). Adsorption of CO₂ causes a reduction in the surface energy, as the energy is transferred from the surface to the gas molecules that come in contact with it, resulting in their vibration. The table, Table 2, shows that both ZnO and CuO are equally good at adsorbing carbon dioxide molecules. The only difference lies in the adsorption mechanism of the molecule on the adsorbent.

The zinc-terminating surface of Zinc oxide at room temperature can be used as an adsorbent material for adsorbing hydrogen compared to copper-terminating copper oxide at the same temperature. Results of our MD simulation confirm the above statement with the help of computed adsorption energies and percentile count of adsorbed particles. In contrast to single-molecule hydrogen adsorption, which only exhibits physisorption, multi-molecule hydrogen adsorption is seen as both atoms and molecules. As a result, the hydrogen molecule cloud displays physisorption and chemisorption features. Further, it has been observed that the adsorption mechanism of hydrogen is not influenced by its velocity upon incidence on the surface, indicating that it is independent of the incident velocity. Carbon dioxide and methane adsorption were also performed on the same adsorbents at 300 K. The adsorbents are equally good at adsorbing carbon dioxide molecules by following entirely different adsorption mechanisms, as explained. Both adsorption mechanisms involve a chemical process of bond formation and, thus, chemisorption. Zinc oxide is found to adsorb methane molecules at low temperatures and moderate temperatures. In contrast, the copper shows physisorption of methane only at low temperatures and dissociative methane adsorption at room temperature. However, the copper surface does not even adsorb methane molecules at high temperatures. The adsorption capacity of both copper oxide and zinc oxide tends to decrease with an increase in temperature due to increased thermal vibration of the surface. Zinc oxide can be considered a better adsorbent for methane at moderate temperatures.

The simulation that was conducted has the potential to pave the way for the creation of cost-effective metal oxide-based hydrogen storage devices and improved sensors for the detection of pollutants like methane and carbon dioxide. It also provides insight into the catalytic activities of certain gases in the presence of metal oxides and other applications.

Acknowledgment

We acknowledge the support rendered by Adri van Duin and Swarit Dwivedi, Penn State University, Pennsylvania, United States, for providing Reax-FF parameter files.

Funding: No funding received for this work.
Authors’ contributions:

All authors have equally contributed to the conception, experimental design, carrying out simulations and manuscript composition.

Conflict of interest: The authors declare that there is no existing competing interest.
Ethical approval: Not Applicable
Consent to participate: Not Applicable
Availability of data and Code:

Data and code associated with the current submission will be made available with a DOI to an electronic repository

Supplementary information. Not Applicable

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No competing interests reported.

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Zinc Oxide and Copper Oxide surfaces as potential adsorbent layers for Greenhouse gases and Hydrogen: A Molecular Dynamics Analysis

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials and Methods.

3 Results and discussion

3.1 Adsorption of Hydrogen on Zinc oxide (ZnO).

3.1.1 Single-molecule adsorption mechanism of Hydrogen on ZnO:

3.2 Adsorption of Hydrogen on Copper oxide (CuO).

3.2.1 Single-molecule adsorption mechanism of Hydrogen on CuO:

3.3 Adsorption of Methane (CH₄) on Zinc Oxide (ZnO) at its RMS velocity.

3.3.1 Single-molecule adsorption mechanism of Methane on ZnO:

3.3.2 Adsorption of Methane on Zinc oxide (ZnO) surface at its RMS velocity:

3.4 Adsorption of Methane on Copper oxide (CuO) surface at its RMS velocity.

3.4.1 Single-Molecule adsorption of Methane:

3.4.2 Adsorption of Methane on Copper oxide:

3.4.3 Effect of temperature on methane adsorption on Copper oxide and Zinc oxide:

3.5 Adsorption of Carbon dioxide (CO₂) at its RMS velocity on Zinc Oxide (ZnO).

3.5.1 Single-molecule adsorption mechanism of carbon dioxide on ZnO:

3.5.2 Adsorption characteristics of Carbon dioxide on Zinc Oxide (ZnO):

3.6 Adsorption of Carbon dioxide on Copper oxide surface at its RMS velocity.

3.6.1 Single-molecule adsorption mechanism of CO₂ on CuO:

3.6.2 Adsorption characteristics of Carbon dioxide on copper oxide (CuO):

4 Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

Zinc Oxide and Copper Oxide surfaces as potential adsorbent layers for Greenhouse gases and Hydrogen: A Molecular Dynamics Analysis

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials and Methods.

3 Results and discussion

3.1 Adsorption of Hydrogen on Zinc oxide (ZnO).

3.1.1 Single-molecule adsorption mechanism of Hydrogen on ZnO:

3.2 Adsorption of Hydrogen on Copper oxide (CuO).

3.2.1 Single-molecule adsorption mechanism of Hydrogen on CuO:

3.3 Adsorption of Methane (CH4) on Zinc Oxide (ZnO) at its RMS velocity.

3.3.1 Single-molecule adsorption mechanism of Methane on ZnO:

3.3.2 Adsorption of Methane on Zinc oxide (ZnO) surface at its RMS velocity:

3.4 Adsorption of Methane on Copper oxide (CuO) surface at its RMS velocity.

3.4.1 Single-Molecule adsorption of Methane:

3.4.2 Adsorption of Methane on Copper oxide:

3.4.3 Effect of temperature on methane adsorption on Copper oxide and Zinc oxide:

3.5 Adsorption of Carbon dioxide (CO2) at its RMS velocity on Zinc Oxide (ZnO).

3.5.1 Single-molecule adsorption mechanism of carbon dioxide on ZnO:

3.5.2 Adsorption characteristics of Carbon dioxide on Zinc Oxide (ZnO):

3.6 Adsorption of Carbon dioxide on Copper oxide surface at its RMS velocity.

3.6.1 Single-molecule adsorption mechanism of CO2 on CuO:

3.6.2 Adsorption characteristics of Carbon dioxide on copper oxide (CuO):

4 Conclusion

Declarations

References

Additional Declarations

Status:

Version 1

3.3 Adsorption of Methane (CH₄) on Zinc Oxide (ZnO) at its RMS velocity.

3.5 Adsorption of Carbon dioxide (CO₂) at its RMS velocity on Zinc Oxide (ZnO).

3.6.1 Single-molecule adsorption mechanism of CO₂ on CuO: