Noble-Metal-Supported GeS Monolayer as Promising Single-Atom

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Noble Metal Supported GeS Monolayer as Promising Single Atom Catalyst for CO Oxidation Sharmistha Karmakar, Chandra Chowdhury, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02442 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

Noble Metal Supported GeS Monolayer as Promising Single Atom Catalyst for CO Oxidation Sharmistha Karmakar, Chandra Chowdhury and Ayan Datta* Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur – 700032, Kolkata, WestBengal, India.

ABSTRACT: The selection of a suitable substrate material for single atom catalysis (SAC) is a key step in designing new catalyst to achieve enhanced performance and selectivity. In the present study, we have explored the feasibility of GeS monolayer to serve as substrate for noble metal atom (Pd, Pt, Au, Ag, Rh and Ir) SAC. Our exploratory study indicates that metal atoms namely, Pd, Pt, Rh and Ir, show considerable binding energies to the GeS monolayer with moderate to high diffusion barriers, which thereby, reduces their clustering tendency. Examination of their catalytic activity towards CO oxidation reveals that both the Ir-GeS and Rh-GeS systems possess appreciable binding energies towards CO and O2; an essential requirements for the initiation of catalytic cycle. CO oxidation on Ir-GeS SAC is studied in details for three distinct mechanisms namely, Eley–Rideal (ER) and bimolecular Langmuir–Hinshelwood (BLH) and trimolecular Eley–Rideal (TER) mechanism. Computation of activation barriers show that Ir–GeS SAC prefers the less common TER mechanism where two CO molecules and one O2 molecule react to form the OOC–Ir–COO intermediate which then dissociates into two CO2 molecules. Additionally, microkinetic analysis predicts a maximum CO oxidation rate of 5.34×103 s–1 following the TER mechanism for the Ir–GeS system. Thus, the present study suggests that Ir supported GeS can act as a potent SAC for low temperature CO oxidation.

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Introduction: CO emission from automobiles and industrial processes is considered as a fundamental problem for clean and sustainable energy solutions for mankind. Therefore, efficient oxidation of harmful CO gas to relatively benign CO2 is highly desirable for environmental protection as well as for preventing fuel cell poisoning.1,2 CO oxidation is also the most widely studied chemical transformation in the field of heterogeneous catalysis. Previous studies indicate that some noble metal surfaces like Pt, Pd, Rh and Au can effectively catalyze CO oxidation.3-5 However, high operational temperature, low durability and high cost limit their applications in large scale processes. Moving from bulk materials to supported nanoparticles is known to enhance the catalytic performance for many reactions including CO oxidation.6 Haruta and co-workers have shown that transition metal oxide supported Au clusters exhibit remarkable catalytic activity for low temperature CO oxidation compared to its bulk form.7-9 Even in their nanocluster form the overall efficiency per atom is still lower compared to the homogeneous analog because of fewer numbers of catalytically active surface sites. Further downsizing from nanoclusters to subnanoclusters, improves the efficiency, as now majority of the metal atoms reside on the surface and available for catalytic reactions.10 The surface metal atoms are coordinately unsaturated in most of the cases and possess partially occupied d-states which are responsible for their high catalytic activity. The performance of these nanoclusters strongly depends on their size and morphology and hence, the local coordination environment around each metal atom is an important factor for overall reactivity.11-13 Therefore, the maximum efficiency in heterogeneous catalysis can only be achieved when all metal atoms are catalytically active as that for single atom catalyst (SAC).14-16 In SAC, isolated metal atoms are dispersed finely on a substrate with large surface-to-volume ratio and these materials show excellent catalytic activity for a range of chemical reactions namely, hydrogen evolution reaction (HER), hydrogenation of organic molecules, oxidation process of small molecules and water-gas shift reaction.17-20 The stability and activity of SACs are strongly governed by the nature of supporting materials since, single metal atoms are highly mobile and possess large surface free energies; they tend to aggregate into metal clusters on the support. Therefore, strong metal-substrate interaction is highly desirable for efficient anchoring the metal atoms on surface sites which in turn improves the durability and performance of the catalyst. Several metal oxides e.g., Al2O3, FeOx, TiO2 and


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CeO2 and metal surfaces (Cu, Ag) have been demonstrated as promising substrate for SACs.17,18,20-22 Qiao et al. synthesized single atom catalyst based on Pt1/FeOx systems which shows excellent catalytic activity for low temperature CO oxidation.21 Apart from traditional metal oxide surfaces, other two-dimensional materials such as defective graphene and hexagonal boron nitride (h-BN), graphitic carbon nitride (g-C3N4), graphyne, silicene, molybdenum disulfide (MoS2), graphdiyne, C2N monolayer, have attracted considerable interest as substrate materials for SACs due to their large surface area and high thermal stability.23-27 Some of SACs based on 2D-materials have already been characterized experimentally and they exhibit interesting reactivity pattern for various kind of reactions.28 Therefore, the search for suitable support materials for SACs which triggers up both the performance and longevity of the catalyst is still going on. Among other two-dimensional (2D) layered materials, phosphorene offers suitable coordination sites for adatom adsorption and possesses moderate to high binding energy with various alkali metals, alkaline earth metals, transition metals and noble metals.29,30 High thermal stability and strong adatom binding strength make phosphorene a promising candidate for SAC application. However, high reactivity towards atmospheric oxygen limits its application in reactions involving molecular O2 such as, oxygen reduction reaction (ORR), epoxidation reaction and CO oxidation.31 The 2D analog of group IV monochalcogenides, germanium monosulfide (GeS) monolayer, displays similar structural and electronic characteristics as phosphorene. The bulk form of GeS is a van der Waals solid with an orthorhombic crystal structure and therefore, synthesis of GeS monolayer can be achieved by efficient physical or chemical exfoliation techniques from its bulk phase.32,33 The GeS monolayer possesses uniform anchoring sites for adatom adsorption and it is kinetically resistant to oxidation at ambient conditions.34 Recently, the GeS monolayer has been predicted to show potential application in the field of catalysis such as water splitting, ORR, Li-ion battery etc, while multilayered GeS has already been tested successfully for the mentioned reactions.35,36 Therefore, large surface-to-volume ratio, high thermal and chemical stability make GeS monolayer as a promising substrate for SAC application. In this manuscript, we have investigated the potential of GeS monolayer as a substrate in SAC application for noble metal atoms (Pd, Pt, Au, Ag, Rh and Ir) using density functional theory.


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Next, we have examined their catalytic activity for CO oxidation. Our results indicate that, PdGeS, Rh-GeS, Ir-GeS and Pt-GeS, possess large metal binding energies along with moderate to high diffusion barriers which reduce the possibility for aggregation of the catalysts. Additional calculations reveal that both the Ir-GeS and Rh-GeS systems can be used for CO oxidation reaction as both of them bind appreciably with the reactant molecules, an essential requirement for the initiation of catalytic cycle. Evaluation of reaction pathways and estimation of activation barriers for the Ir-GeS SAC show that the highest barrier of CO oxidation is ~0.11 eV, along the favorable tri-molecular Eley-Rideal (TER) mechanism..

Computational Details: All electronic structure calculations were carried out with the projector augmented wave method within the framework of density functional theory (DFT), as implemented in Vienna Ab Initio Simulation Package (VASP).37 The exchange-correlation energy was described with the generalized gradient approximation (GGA) as parameterized by the Perdew−Burke−Ernzerhof (PBE) functional.38 An energy cutoff of 400 eV was used for plane wave basis set. The van der Waals interactions were incorporated by the semiempirical correction scheme of Grimme (DFTD2) method.39 All structures were relaxed to reach the convergence threshold of 10-4 eV for energy. All calculations were performed with spin-polarization effect. The Brillouin zone was sampled by a Monkhorst−Pack special k-point mesh of 5 × 5 × 1 for structural relaxation and 7 × 7 × 1 for densities of states (DOS) calculations. A vacuum of about 20 Å was set between two adjacent GeS layers to avoid any artificial interactions induced by periodically repeated images. The charge transfer between the adsorbate and the GeS monolayer was modeled using Bader charge analysis software.40 The transition state (TS) structures for each step were located using the climbing-image nudged-elastic-band (CI-NEB) method41 and six intermediate images were inserted along the minimum energy path (MEP) in order to find the actual TS. Frequency calculation at the optimized transition state verifies the nature of vibrational mode of the TS along MEP. For all the pathways, the activation barriers (∆E‡) and the reaction energies (∆E) were evaluated as follows: ∆E‡ = ETS – EIS and ∆E = EFS – EIS; where IS, TS and FS correspond to initial state, transition state and final state, respectively.


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Results and Discussion: Adsorption of single metal atom on GeS monolayer: The ability of a metal atom to act as catalyst for a specific reaction strongly depends upon its interaction with the support material. The structural asymmetry in GeS monolayer offers six different binding positions namely, hollow sites (H1 and H2), bridge sites (B1 and B2) and top sites (T1 and T2). The various adsorption positions of M atom on GeS monolayer can be described as :(1) above the center of GeS hexagon coordinating with two S and one Ge atoms, H1 site; (2) above the center of GeS hexagon coordinating with two Ge and one S atoms, H2 site; (3) on the top of Ge atom, T1 site; (4) on the top of S atom, T2 site; (5) above the midpoint of GeS bond, B1 site and (6) above the midpoint of non-bonded Ge-S fragment, B2 site. The binding energy (Eb) per M atom can be written as: Eb= (EGeS + EM) - EGeS+M ; where, EGeS corresponds to the energy of pristine 3×3 GeS monolayer, EM is energy of metal atom and EGeS+M is energy of GeS monolayer containing one M atom. A positive value of Eb indicates favorable binding to the surface. Scheme 1 shows the various possible sites for metal atom adsorption over GeS monolayer. We have evaluated the binding energy of each metal atom for all possible sites in GeS monolayer (Table 1).

Scheme 1: Top view of different metal adsorption sites on GeS monolayer. The color codes of the atoms are: Green: Ge, Yellow: S, Blue: M.


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Table 1 clearly shows that the hollow sites, H1 and H2, are the most probable sites for adatom adsorption and they possess similar Eb values with maximum difference of 0.4 eV for Pt-GeS. Iratom shows the highest adsorption energy of 5.26 eV while the Ag atom has the lowest Eb value of 1.18 eV. The binding strength decreases in the following order as Ir > Pt > Rh > Pd > Au > Ag. The calculated adsorption energies for various single metal atoms are comparable with other 2D materials such as graphyne, graphdiyne and MoS2, and are much larger compared to pristine graphene and h-BN sheet.23,26,42 The most stable configurations for Ir, Pt and Rh atoms at H1 site (see Figure 1) show that the metal atoms lie very close to the GeS2 triangular surface and forms strong bond with the neighboring sulfur atoms with an average bond distance of 2.25Å , 2.29Å and 2.27Å, respectively. They prefer to form square-planer geometry in this site causing a small local distortion although the overall GeS monolayer remains unperturbed as can be seen from the side view of Ir-GeS and Rh-GeS in Figure 1(a) and 1(b). On the otherhand, the other metal atoms at H1 site (Pd, Au and Ag) lie vertically above the GeS2 triangular surface with an average dM-S value of 2.36Å, 3.16Å and 2.85Å respectively. The adatom (Ir, Pt, Rh, Pd, Au and Ag) adsorption at H2 site follow similar characteristics with the latter case (Pd, Au and Ag atoms at H1 site) and all of them lie vertically above the Ge2S triangular plane with a vertical distance of 1.22Å, 1.11Å, 1.26Å, 1.19Å, 1.36Å and 1.59Å, respectively. To gain deeper insight into the binding nature we have evaluated the spin-polarized partial density of states projected on different orbitals for Ir-GeS, Rh-GeS and Au-GeS systems as shown in Figure 1(d), 1(e) and 1(f), respectively. The PDOS plots for Ir-GeS and Rh-GeS show that the s, p and d states of the metal atoms are significantly broadened compared to the free metal atoms (see Figure S1 in Supp Info.) and the p and s states have downshifted to the Fermi level. The similarity of s and p states of adsorbed Ir and Rh with the overall DOS plot of these metal atoms indicates spd-type hybridization in these metal atoms. The spd type orbitals largely overlap with the neighboring sulfur 3p states which can be clearly seen in Figure 1(d) and 1(e). The hybridization between metal states with the sulfur 3p states creates strong interaction with the Ir/Rh atom and the GeS monolayer. Presence of sharp peaks near Fermi level for Ir-GeS and Rh-GeS indicate high chemical reactivity for these systems.23 On the otherhand, the Au-GeS system shows slightly different behavior. In this case although the s and p states of adsorbed metal atom (Au) get stabilized compared to the free Au atom, they do not mix with the d states of the metal atom. Here, the d states of Au atom are comparatively less broadened than the previous case and show


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an overlap region with the sulfur 3p states suggesting a moderate binding interaction. The PDOS plots of spin-up and spin-down channels for Ir-GeS, Rh-GeS and Au-Ges are asymmetric in nature which makes the systems magnetically active. The charge difference density analysis (see Figure 1(a) and 1(b)) shows that significant amount of charge is shared within the bonded atoms for Ir-GeS and Rh-GeS which is the characteristics of covalent bonding. Table 1: Metal Binding Energies (Eb in eV) calculated at H1, H2, B1, B2, T1 and T2 sites and clustering energy (Ecluster in eV) for all M-GeS systems.

Pd Pt Rh Ir Ag Au

H1 3.05 4.70 4.63 5.26 1.03 1.62

H2 3.29 4.30 4.65 5.05 1.18 1.83

B1 1.60 2.09 2.39 1.28 0.02 0.55

B2 2.36 3.03 3.45 3.44 0.47 1.06

T1 1.28 1.62 1.55 1.35 0.48 1.12

T2 1.44 1.88 1.92 1.55 0.37 0.86

Ecluster -0.78 -2.16 -1.81 -2.08 -0.50 -0.81

The durability of SACs strongly depends upon the mobility of metal atoms and their clustering tendency over GeS monolayer. Low diffusion barrier and large (positive) clustering energy enhances the aggregation tendency and thereby reduces its effectiveness as single metal catalyst.23,26 The aggregation tendency of the metal atoms strongly depends upon the metal dimer adsorption energy on GeS monolayer. Weak adsorption of metal dimer on GeS surface compared to the isolated metal atom lowers the risk of metal clustering. Thus, the clustering energy (Ecluster) can be evaluated as43 Ecluster= Eb(dimer) - Eb(mono) where, Eb(dimer) and Eb(mono) are the per atom binding energy of the dimer and the isolated metal atom in eV. A negative value of Ecluster indicates that the metal atoms are prone toward dispersion. Table 1 shows that all metal atoms possess negative clustering energy which favors the uniform distribution of metal atoms on GeS surface. Next, we have estimated the diffusion barrier for all metal atoms from its most stable hollow site (H1/H2) to the neighboring hollow site (H2/H1) via metastable B2 configuration. The diffusion barrier was estimated as the energy difference between the metastable configuration and the most stable site. The calculated energy barriers for diffusion were found to be 0.93 eV, 1.67 eV, 1.2 eV, 1.82 eV, 0.71 eV and 0.77 eV


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for Pd, Pt, Rh, Ir, Ag and Au, respectively. Hence, the Au, Ag and Pd atoms possess relatively lower activation barriers for diffusion while Ir and Pt atoms have significantly large barriers and the barrier height for Rh is moderate. The kinetic stability of a single metal atom at its most stable site can be estimated using Arrhenius equation as follows: τ = (1/k) and k = A exp(-Ea/RT) where τ is the life time , k represents the 1st order rate constant, A is the pre-exponential factor (assumed as 1013 s-1), R is the gas constant and T is the temperature.44 The typical life time of metal atoms at its most stable site were estimated to be 5.6 m, 9×109 days, 124 days, 3×1012 days, 0.07 s and 0.7 s for Pd, Pt, Rh, Ir, Ag and Au, respectively, at 300K. Therefore, it can be concluded that sufficiently large diffusion barriers alongwith negative clustering energy for Rh, Ir and Pt make them immobile over GeS surface and reduce the probability of metal clustering at ambient conditions. On the otherhand, Pd-SAC, Ag-SAC and Au-SAC are kinetically unstable at room temperature and undergo facile diffusion from one site to another. Nevertheless, reducing the operational temperature of the catalyst to 240 K might make Pd-SAC suitable for SAC application. To check the thermal stability of the most promising SAC (Ir-GeS) we have performed ab initio molecular dynamics (AIMD) simulation of a single Ir atom over GeS monolayer at 300 K (see Supp Info). From the RMSD plot (Figure S2 in Supp Info) it is evident that the average positional fluctuation of each atom equilibrates within few picoseconds which is clearly suggestive of thermal stability of the system with no structural disintegration. Also the radial distribution function between Ir-Ge and Ir-S pairs maximize at 2.7 Å and 2.3 Å which are indeed very close to DFT calculated bond lengths at H1 site (Figure S3 in Supp Info). Therefore good agreement between the AIMD equilibrated structures with DFT optimized structure essentially suggests that the structures obtained from DFT can reasonably model the single atom catalyst.


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Figure 1: Optimized structures (top view and side view) and charge density plots for (a) Ir-GeS, (b) Rh-GeS and (c) Au-GeS. The spin-polarized PDOS plots of selected metal orbitals (4d, 5s and 5p for Rh and 5d, 6s and 6p for Au and Ir) and coordinated sulfur atoms’ orbitals (3p and 3d) for (d) Ir-GeS, (e) Rh-GeS and (f) Au-GeS. The color codes of the atoms are: Green: Ge, Yellow: S, Blue: Ir, Purple: Rh and Sky blue: Au.

CO and O2 adsorption over M-GeS: The efficiency of a catalyst strongly depends upon its ability to bind the reactive molecules effectively at the catalytic active centers. Thus, the adsorption CO and O2 molecules over M-GeS surfaces were first investigated. All possible initial configurations including the end-on and side-on configuration were examined to determine the energetically favorable configurations. Adsorption of CO and O2 molecules at both H1 and H2 sites for each metal atom were considered and they furnish similar binding trends. Therefore we have listed the adsorbate binding energy (Eb) and other electronic and structural parameter for CO and O2 adsorption at


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the most favorable configuration of M-GeS. The Eb value can be evaluated in a similar fashion to the metal adsorption energy and a positive value indicates exothermic binding. Table 2: Adsorption energies [Eb(CO) and Eb(O2) in eV] for CO and O2 molecules adsorption over M-GeS, bond lengths (dC-O and dO-O in Å) of the adsorbed molecules, extent of charge transfer (Q) between the adsorbate and the M-GeS sheet and bonding distances (dCO-M and dO2-M in Å) between the M atom in M-GeS and the adsorbed molecules for all M-GeS SACs.

Eb(CO) dC-O dCO-M Q(CO) Eb(O2) dO-O dO2-M Q(O2)

Pd 1.53 1.16 1.91 0.22 0.79 1.38 2.03, 2.07 0.68

Pt 1.44 1.17 1.85 0.24 0.17 1.26 2.60 0.16

Rh 1.69 1.17 1.86 0.31 1.73 1.35 1.95,2.09 0.58

Ir 2.06 1.17 1.86 0.37 1.49 1.41 1.96,2.04 0.76

Ag 0.73 1.15 2.05 0.13 0.92 1.33 2.13,2.59 0.49

Au 0.96 1.16 1.96 0.15 0.97 1.31 2.08 0.46

CO adsorption on M-GeS surfaces occurs predominantly through end-on configuration where the O-C moiety points towards the metal centre forming an M-C bond and the CO molecule lies perpendicular to the GeS surface with a tilt with respect to the sheet. Bonding interactions between CO and metal centre occur in two steps. First, it donates the lone pair electrons to the metal vacant d or s orbital and then the occupied metal d orbital interacts with the low lying π* orbital of the CO molecule causing a reverse charge transfer from the metal centre to CO molecule. The availability of suitably arranged metal orbitals is highly desirable for strong CO binding. All the metal atoms in M-GeS show appreciable binding strength with CO and the Eb values vary from 0.73 eV to 2.06 eV in the whole series. The Ir-GeS SAC furnishes the highest CO adsorption energy of 2.06 eV while the Ag-GeS SAC possesses the lowest Eb=0.73 eV. The calculated binding energies for the different M-GeS substrates follow the order as: Ir > Rh > Pd > Pt > Au > Ag. The above series for CO adsorption energy can be divided into two subsets; the first set comprises the Pd-GeS, Pt-GeS, Rh-GeS and Ir-GeS systems which show quite strong CO binding with Eb ≥ 1.5 eV. However, the other subset comprising Ag-GeS and Au-GeS have comparatively lower Eb value (< 1 eV). The CO adsorption energies can be nicely correlated with the amount of charge transfer (Q) from metal to the adsorbate and the M-C bond


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lengths. Table 2 shows that the Q (CO) values follow similar order with the CO binding energy and it ranges from 0.13-0.37 |e|. Another important structural parameter is M-C-O angle; the CO molecule binds in a linear fashion for the first set with ∠M-C-O values ranging from 176º to 179º while the other two systems possess a bent geometry with ∠M-C-O ≈ 160º. The bond lengths for the adsorbed CO (1.15Å - 1.17Å) change slightly from its gas phase value (1.14 Å). A linear geometry for CO adsorption indicates strong back-donation from the metal center to the CO molecule which has also been validated from the amount of charge transfer. Therefore, strong back-donation, short M-C bond length and large amount of charge transfer facilitate CO binding to the metal centre. We have shown the most favorable CO adsorption configuration for Ir-GeS and Rh-GeS in Figure 2(a) and 3(a), respectively and the PDOS plots of selected species have been drawn to get deeper insight into the bonding interaction for these systems.


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Figure 2. The most plausible adsorption structures and their corresponding PDOS plots for CO and O2 adsorption on Ir-GeS: (a) the optimized structure for CO adsorption on Ir-GeS, (b) PDOS plots of gas-phase CO, adsorbed CO and Ir 5d state for CO adsorbed Ir-GeS system, (c) the optimized structure for O2 adsorption on Ir-GeS, (d) PDOS plots of gas-phase O2, adsorbed O2 and Ir 5d state for O2 adsorbed Ir-GeS system. The color codes of the atoms are: Green: Ge, Yellow: S, Blue: Ir, Ass: C and Red: O. Figure 2(b) and 3(b) show the PDOS plots projected on adsorbed CO, gas phase CO and metal d state for Ir-GeS and Rh-GeS systems, respectively. Figure 2(b) and 3(b) clearly depict that the 2π* peaks of the adsorbed CO have downshifted from its gas phase value of 6.0 eV to ~2.5 eV and strong interaction of 2π* peaks with the metal d-states near the Fermi level facilitates the back-donation of electrons from metal to the adsorbate activating the CO moiety. The 5σ peak of the adsorbed CO is now significantly broadened over a large energy (~0 to ~ -5 eV) region and merged with the 1π peak of adsorbed CO. Strong hybridization of 5σ, 2π* and 1π orbitals of the adsorbate with metal (Ir and Rh) d-states near the Fermi level enables efficient binding of CO molecule for both cases. These systems are magnetically active as the spin-up and spin-down channels are asymmetric. For O2 adsorption, all the substrates except Pt-GeS prefer side-on configuration where the O2 molecule lies almost parallel to the GeS sheet. The Pt-GeS adopts end-on configuration with very low adsorption energy of 0.17 eV. For rest of the surfaces, the binding is quite strong (0.8 eV-1.7 eV) and the adsorption energies decrease in the following order: Rh > Ir > Au > Ag > Pd. The bonding characteristics of O2 molecule are similar to that of CO. Here, the electron donation takes place from 1π orbital of O2 instead of 5σ and the anti-bonding 2π* orbital is partially filled. The average M-O bond lengths are ~2 Å for Ir-GeS, Rh-GeS and PdGeS while the same for AuGeS and Ag-GeS are 2.3 Å as they possess two unequal M-O bonds. On the other and, the bond length of the adsorbed O2 gets significantly elongated compared to its free state (1.23 Å). The change in O-O bond length (∆dO-O) has been calculated to be 0.15 Å, 0.03 Å, 0.12 Å, 0.18 Å, 0.10Å and 0.08 Å for Pd, Pt, Rh, Ir, Ag and Au, respectively. The O-O bond elongation in adsorbed state occurs because of large amount of charge transfer from the substrate to the O2 moiety and the transferred e- s occupy the anti-bonding 2π* state which reduces the bond order. The charge transfer is minimal for the weakly coordinating Pt-GeS system while for other


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systems, the O2 molecules gain significant number of electrons from the M-GeS sheet and the amount of charge transfer ranges from 0.46 |e| to 0.76 |e| being highest for the Ir-GeS system. The O2 binding energy with M-GeS (except Pt-GeS) is similar to other well characterized SAC materials and it is highly activated for further reaction.26 We have shown the most favorable adsorption structure of O2 molecule for Ir-GeS and Rh-GeS systems in Figure 2(c) and 3(c) respectively. The corresponding PDOS plots projected on adsorbed O2 state, gas phase O2 and metal d ortibals have been shown in Figure 2(d) and 3(d). As can be seen from Figure 2(d), for Ir-GeS the down spin channel of the anti-bonding 2π* orbital of the adsorbed O2 gets downshifted and broadened near Fermi level which indicates a charge transfer event from the substrate to the O2 2π* state. However, the 1π orbital of the adsorbed O2 split into two peaks near -6.3 eV showing observable downshifting. Therefore, strong overlap of the metal d-states with the adsorbed O2 states ensures efficient binding to the metal centre. The PDOS plots of Rh-GeS in Figure 3(d) also show similar behavior. Therefore, the Ir-GeS, Pd-GeS and Rh-GeS SACs show appreciable binding strength with CO and O2 which is the prerequisite for the initiation CO oxidation reaction. Therefore, in order to check their catalytic performance for CO oxidation, we have studied all possible reaction mechanisms of CO oxidation for Ir-GeS SAC.


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Figure 3. The most plausible adsorption structures and their corresponding PDOS plots for CO and O2 adsorption on Rh-GeS: (a) the optimized structure for CO adsorption on Rh-GeS, (b) PDOS plots of gas-phase CO, adsorbed CO and Rh 4d state for CO adsorbed Rh-GeS system, (c) the optimized structure for O2 adsorption on Rh-GeS, (d) PDOS plots of gas-phase O2, adsorbed O2 and Rh 4d state for O2 adsorbed Rh- GeS system. The color codes of the atoms are: Green: Ge, Yellow: S, Purple: Rh, Ass: C and Red: O.

CO oxidation over Ir-GeS: Generally, CO oxidation over various SACs take place through two well established mechanisms namely, the Eley-Rideal (ER) and the bimolecular Langmuir-Hinshelwood (BLH) mechanism.42 In the ER mechanism, one kind of reactants (mostly O2 for CO oxidation) gets adsorbed on the


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surface while the other one (CO) interacts directly from the gas phase with the adsorbed species. For the BLH pathway, both the reactant molecules (CO and O2) co-adsorb on the neighboring sites before undergoing reactions. Another less common mechanism is tri-molecular Eley-Rideal (TER)45 one where initially two reactant molecules (2CO) get co-adsorbed on the catalytic center and the third molecule (O2) reacts from the gas phase with these activated species. The feasibility of various reaction pathways described above strongly depend upon the binding strength of the catalyst towards different reactant molecules and intermediates. We have systematically investigated all three reaction mechanisms (ER, BLH and TER) for CO oxidation over Ir-GeS system. The Ir-GeS system shows stronger binding towards CO moiety compared to the O2 , hence, the catalytic centers will be predominantly occupied by the CO molecule if 1:1 mixture of CO: O2 is used, limiting the possibility of ER mechanism. The CO oxidation following the BLH pathway starts with the co-adsorption of CO and O2 molecules over catalytic active center. The coadsorption energy of CO and O2 molecules over Ir-GeS is found to be 3.2 eV which is significantly larger compared to their individual adsorption energies (2.06 eV and 1.49 eV for CO and O2, respectively). Figure 4 represents the minimum energy pathways and intermediate structures for CO oxidation following BLH pathway. IS1-BLH represents the most plausible coadsorption configuration of CO and O2 where the Ir atom is attached with the C atom of CO and one of the O atoms (Oa) of O2 with a bonding distance of 1.86Å and 2.04Å respectively. The OaOb bond length in IS1-BLH is elongated to 1.31Å while the C-O length changes slightly to 1.17Å. The free end of the activated O2 (labeled as Ob) approaches the C atom of CO to generate a peroxide-like (OOCO) intermediate (MS1-BLH) via transition state, TS1-BLH. As a new C-O (C-Ob) bond is formed in the MS1-BLH, the Oa-Ob bond length is further elongated to 1.53Å. The activation barrier and the reaction energy for the IS1-BLH → MS1-BLH conversion are 0.45 eV and -0.24 eV, respectively. In the MS1-BLH configuration, the Oa-Ob bond gets weakened while strengthening of the C-Ob bond occurs. As a result, the MS1-BLH intermediate spontaneously dissociates into FS1-BLH i.e. physisorbed CO2 and an atomic O adsorbed on Ir center, by a barrierless process. As this step involves the formation of C═O bonds, it is associated with large exothermicity of -1.4 eV with respect to the MS1-BLH. The physisorbed CO2 molecule lays 2.93Å apart from the adsorbed O atom with a small binding energy of 0.3 eV.


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Therefore the newly formed CO2 molecule can be easily desorbed leaving behind the atomic Ocovered Ir-GeS for further reaction.

Figure 4. The reaction pathways for CO oxidation initiated by BLH mechanism on Ir-GeS surface. All energies are in eV. After spontaneous release of CO2 fragment, the atomic O resides on the top site of Ir-GeS with an Ir-O bond of 1.77Å and adsorption energy of 4.5 eV. Bader charge analysis shows that the adsorbed O atom acquires negative charge of 0.85|e|, implying the formation of O- species which then oxidizes another CO molecule by ER mechanism as shown in Figure 4. The interaction of atomic O in O-Ir-GeS with gaseous CO yields IS2-ER configuration where the CO fragment resides at 3.3Å and 3.05Å away from the preadsorbed O and Ir atom, respectively. The CO molecule shows moderate adsorption energy of 0.51 eV in this configuration. The oxidation process of IS2-ER→FS2-ER can occur via two pathways i.e. path A and path B, generating a physisorbed CO2 molecule at 3.4 Å away from the Ir center in Ir-GeS. In path A, the C atom of the CO approaches the adsorbed O and takes it away from the Ir atom by developing a new C-O


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bond as shown in TS2A-ER and finally forms a weakly adsorbed CO2. The activation barrier and reaction energy for the following step (IS2-ER→FS2-ER) via TS2A-ER are 0.89 eV and -1.51 eV, respectively. On the otherhand for path B, the IS2-ER initially yields intermediate MS1-ER by passing through TS2B-ER with an energy barrier of 0.39 eV. The MS1-ER intermediate comprises a bent CO2 configuration attached to the Ir atom with Ir-C and Ir-O bond lengths of 2.02 Å and 2.1 Å, respectively and the two C-O distances of the CO2 fragment are 1.21Å and 1.31Å. Formation of MS1-ER starting from IS2-ER is highly exothermic with ∆E= -1.83 eV. Ultimately, the MS1-ER gets converted to FS2-ER by crossing a barrier of 0.36 eV via TS3B-ER state and the corresponding reaction energy is +0.32 eV. The CO2 molecule is loosely bound to the Ir atom in the FS2-ER configuration with adsorption energy of 0.02 eV, therefore it will be desorbed easily from the Ir-GeS sheet making it free for another cycle. Although, the MS1-ER → FS2-ER conversion is endothermic in nature, formation of CO2 is still possible because the MS1-ER intermediate cannot go back to the IS2-ER state due to large reverse activation barrier (∆E‡=2.22 eV for MS1-ER → IS2-ER) and release of gaseous CO2 molecule makes the overall process entropically favorable. Hence, it can be concluded that the IS2-ER configuration will follow the path B as the formation of MS1-ER intermediate is kinetically and thermodynamically more favorable compared to the other path which eventually drives the reaction into forward direction. Detailed microkinetic analysis predicts a maximum CO2 formation rate of 4.7×10-3 s–1 at 298 K for BLH pathway and formation of the MS1-ER intermediate is the rate determining step according to Campbell’s degree of rate control (DRC) analysis for this pathway (see Supp Info). The tri-molecular ER (TER) pathway has been recently shown as another viable mechanism for CO oxidation over Pd and Au embedded defective h-BN and Pt-supported N-doped graphene sheet.45-47 In this mechanism, initially two CO molecules co-adsorb on the Ir-GeS sheet and then a free O2 molecule interacts physically with these adsorbed species and gets activated to form CO2 molecules in subsequent steps via OOC-Ir-COO intermediate. The co-adsorption energy of two CO molecules over Ir-GeS is 3.68 eV. IS1-TER and IS2-TER represent two possible initial configurations with comparable energies for TER mechanism with IS2-TER being more stable by 0.02 eV compared to IS1-TER. Figure 5 demonstrates the reaction profiles for CO oxidation and intermediate structures via TER mechanism. In the IS1-TER configuration, the O2 molecule lies in a perpendicular plane to the OC-Ir-CO moiety with an average distance of 3.05Å between


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the C atoms and O2 molecules. Whereas for IS2-TER configuration, the O2 moiety resides on the same plane with OC-Ir-CO and the corresponding C1-Oa and C2-Ob distances are 3.07Å and 3.14Å, respectively. As the physisorbed O2 molecule moves closer to the C atoms, two new C-O bonds start to develop and the Oa-Ob bond gets elongated accompanying the formation of OOCIr-COO (MS1-TER) intermediate with a small reaction barrier. The ∆E‡ values for the IS1TER→ MS1-TER and IS2-TER→ MS1-TER transformations are 0.18 eV and 0.11 eV, respectively and in the TS2-TER structure where the two O atoms (Oa and Ob) approach the C atoms simultaneously possess the lowest barrier. The MS1-TER intermediate comprises a pentagonal ring structure with an Oa-Ob distance of 1.49Å, as shown in Figure 5, and its formation is highly favorable thermodynamically (∆E = -1.57 eV). In the next step, the MS1TER intermediate dissociates into two physisorbed CO2 molecules by cleaving the Oa-Ob bond via TS3-TER with a small activation barrier of 0.2 eV and the corresponding reaction energy is 1.24 eV. Additionally, the newly formed CO2 molecules are loosely bound to the Ir-GeS sheet with adsorption energy of 0.12 eV and hence, they will desorb rather rapidly from the sheet resulting in the Ir-GeS SAC refurbished. Detailed microkinetic calculations following Campbell’s degree of rate control analysis predicts formation of the OOC-Ir-COO intermediate as the rate determining step for TER mechanism with a maximum CO2 formation rate of 5.34×103 s–1 at 298 K (see Supp Info).


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Figure 5. The potential energy surfaces for CO oxidation on Ir-GeS following TER mechanism. All energies are in eV. Although, the Ir-GeS sheet shows stronger interaction with CO compared to O2, under oxygen rich environment the catalytic centers can be pre-occupied by O2 molecules resulting ER mechanism as a viable pathway for the initiation of CO oxidation. Therefore, we have investigated the possible occurrence of ER mechanism as the first step in CO oxidation. In the ER mechanism, a gaseous CO interacts physically with the pre-adsorbed O2 molecule as shown in Figure 6 to form a carbonate-like (CO3) intermediate or a CO2 molecule along with adsorbed atomic oxygen. IS1-ER represents the initial state for CO oxidation via ER pathway where the CO molecule resides 2.78Å apart from the Ob atom with a C-O bond length of 1.14Å and the corresponding CO adsorption energy is 0.11 eV. The O2 molecule adopts a side-on configuration in IS1-ER with Oa-Ob bond length of 1.41Å and hence, the adsorbed O2 is sufficiently activated for CO assisted O-O bond cleavage. As the CO molecule approaches one of the O-atom (Ob), a new C-O (C-Ob) bond starts to develop while the Oa-Ob bond is elongated from 1.4Å to 1.62Å in the transition state (TS1-ER). Finally, the Oa-Ob bond gets cleaved completely producing a physisorbed CO2 molecule and an atomic O adsorbed on Ir via TS1-ER with a moderate reaction barrier of 0.65 eV. This step (IS1-ER→FS1-ER) is highly exothermic with ∆E= -3.3 eV and the distance between the newly formed CO2 molecule and the remaining O atom (Oa) is 2.95Å. The CO2 molecule in FS1-ER shows weak binding (Eb = 0.33 eV) with the remaining O atom and hence, it could be released easily making O covered Ir-GeS free for another cycle of CO oxidation. In subsequent steps, the O adsorbed Ir-GeS oxidizes another CO molecule via ER mechanism in a similar fashion as mentioned before with activation barrier of 0.39 eV. Therefore the first step i.e. IS1-ER→FS1-ER, corresponds to the rate determining step as obtained from microkinetic calculations and degree of rate control analysis with an infinitesimally small CO2 formation rate of 1.8×10-15 s–1 at 298K (see Supp Info).


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Figure 6. The reaction profiles for CO oxidation on Ir-GeS via ER mechanism. All energies are in eV.

Detailed mechanistic study and kinetic calculations show that the activation barriers of the rate determining step (RDS) for ER, BLH and TER pathways are 0.65 eV, 0.39 eV and 0.11 eV, respectively, indicating TER mechanism as the most favorable one. Ir-GeS SAC not only possesses greater CO binding affinity compared to O2, the co-adsorption of 2CO molecules is also more favorable than the co-adsorption of CO and O2. Therefore, if 1:1 mixture of CO and O2 is used, TER mechanism is preferred over BLH for CO oxidation. Additionally, microkinetic analysis for all reaction pathways provides a quantitative estimate for the CO oxidation rate. The maximum rate for CO2 formation following the TER, BLH and ER mechanism are 5.34×103 s–1, 4.7×10-3 s–1 and 1.8×10-15 s–1 respectively, at 298K for 𝑝 𝐶𝑂 = 0.01 bar and 𝑝(𝑂! )=0.21 bar;45,48 which further validates the viability of TER pathway for CO oxidation on Ir–GeS SAC (see Supp Info). Therefore, depending upon initial conditions CO oxidation may follow TER or BLH mechanism. Whatever be the underlying mechanism of CO oxidation, the catalytic performance of Ir-GeS system for CO oxidation is comparable to other well studied SAC


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materials such as Au and Pd embedded defected h-BN and graphene, Pt supported MoS2 etc.42,45,46

Conclusion: Using first principle calculations, we have explored the potential of GeS monolayer as SAC substrate material for noble metal atoms (Pd, Pt, Au, Ag, Rh and Ir). Our studies indicate that Pd, Pt, Rh and Ir possess appreciable binding energies to the GeS monolayer alongwith moderate to high diffusion barriers and these properties decrease the adatom aggregation tendency thereby enhancing the durability of the catalyst. Next, we have examined their catalytic activity by using CO oxidation as a test reaction. For this purpose, adsorption of CO and O2 molecules on M–GeS (M=Pd, Pt, Au, Ag, Rh and Ir) systems are studied and detailed calculations reveal that both the Ir–GeS and Rh–GeS systems can be used for CO oxidation reaction as both of them bind appreciably with the reactant molecules which are the essential requirements for the initiation of catalytic cycle. Subsequently, we have investigated the detailed mechanistic pathways of CO oxidation for Ir–GeS SAC. CO oxidation on Ir–GeS can occur through three distinct mechanisms namely, Eley–Rideal (ER) and bimolecular Langmuir– Hinshelwood (BLH) and trimolecular Eley–Rideal (TER) mechanism. Estimation of activation barriers reveal that Ir–GeS SAC prefers the less common TER mechanism where the O2 molecule is activated by two preadsorbed CO molecules with an activation barrier of 0.11 eV to form the OOC–Ir–COO intermediate, which then dissociates into two CO2 molecules through the rate determining step with a barrier of 0.2 eV. Furthermore, microkinetic analysis predicts a maximum CO oxidation rate of 5.34×103 s–1 following the TER mechanism for Ir–GeS. Thus, the present study suggests that Ir supported GeS is very promising SAC for low temperature CO oxidation.

ASSOCIATED CONTENT Supporting Information: PDOS plots for free metal atoms and microkinetic analysis for all pathways of CO oxidation details of AIMD simulation. This material is available free of charge via the Internet.


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AUTHOR INFORMATION Corresponding Author: [email protected]; Phone No. +91-0332473-4971. ACKNOWLEDGMENT SK thanks CSIR India and IACS for SRF. AD thanks DST and BRNS for partial funding.

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Noble-Metal-Supported GeS Monolayer as Promising Single-Atom

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