Reduction of N2O by CO via Mans–van Krevelen Mechanism over

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Reduction of N2O by CO via Mans−van Krevelen Mechanism over Phosphotungstic Acid Supported Single-Atom Catalysts: A Density Functional Theory Study Li-Long Zhang,† Xue-Mei Chen,† and Chun-Guang Liu*,†,‡ †

College of Chemical Engineering, Northeast Electric Power University, Jilin City, 132012, P. R. China State Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, Ministry of Science and Technology of China, School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, 15 Yu Cai Road, Guilin, 541004, P. R. China

‡

Inorg. Chem. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 03/29/19. For personal use only.

S Supporting Information *

ABSTRACT: In general, reduction of N2O by CO is first performed by N2O decomposition over a catalyst surface to release N2 and form an active oxygen species, and subsequently CO is oxidized by the active oxygen species to produce CO2. However, the strong adsorption behavior of CO on the catalyst surface usually inhibits adsorption and decomposition of N2O, which leads to a low activity or poisoning of catalysts. In the present paper, a Mans−van Krevelen (MvK) mechanism has been probed based on a series of phosphotungstic acid (PTA) supported single-atom catalysts (SACs), M1/PTA (M = Fe, Co, Mn, Rh, Ru, Ir, Os, Pt, and Pd). Although the calculated adsorption energy of CO is exceedingly higher than N2O for our studied systems, the adsorbed CO could react with the surface oxygen atom of the PTA support through the MvK mechanism to form an oxygen vacancy on the PTA surface. N2O acts as an oxygen donor to replenish the PTA support and release N2 in the whole reaction process. This proposed reaction mechanism avoids competitive adsorption and poisoning of the catalyst caused by CO. The calculated adsorption energy, oxygen vacancy formation energy, and the free energy profiles show that the catalytic activity of Pd1/PTA, Rh1/PTA, and Pt1/PTA SACs is quite high, especially for Pt1/PTA and Pd1/PTA systems. Meanwhile, molecular geometry and electronic structure analysis along the favorable reaction pathway indicates that the metal single atom not only plays the role of adsorbing CO and activating surface atoms of the PTA support but also works as an electron transfer media in the whole reaction process. We expect that the present calculated results could provide some clues for the search for appropriate catalyst for reduction of N2O to N2 by CO at low temperature. found to be a no energy barrier process. FeO+ subsequently is restored into Fe+ by reacting with CO, and release CO2 to complete the whole cycle. Recently, Maihom and Limtrakul et al.21 also made great progress in reaction of CO and N2O on coordinatively unsaturated metal−organic frameworks by using density functional theory (DFT) calculations. According to their DFT mechanism investigation, N2O first adsorbed and dissociated to provide an oxygen source because metal− organic frameworks could not react directly with CO. In another case, the same group reported that N2O can be reduced by CO over an iron-embedded graphene catalyst.24 The direct decomposition of N2O on the catalyst to generate an Fe−O intermediate, which is an active species for the CO oxidation, is also proposed to be the first step in this catalytic system. These reported reaction mechanisms can be summarized as two steps, N2O decomposition and CO oxidation:

1. INTRODUCTION Along with the broad applications of the nitrogenous chemicals in agriculture and industrial processes, atmosphere concentrations of nitrous oxide (N2O) have increased from 270 ppb before the industrial revolution to 328.9 ppb in 2016.1 N2O has become the third most significant anthropogenic greenhouse gas and the most abundant stratospheric-ozonedepleting substance.2−4 How to convert N2O to eco-friendly N2 is thus a major environmental issue. A number of catalytic systems including noble metals,5,6 perovskite-like mixed oxides,7−9 copper, cobalt, iron, and titanium-containing acidic zeolites,10−14 carbon nanotubes,15,16 metal-substituted hexaaluminates,17 metalloporphyrins,18−21 and metal alloys,22 etc., have been demonstrated to be efficient for the conversion of N2O into N2. Among them, catalytic reduction of N2O to N2 by CO is considered the most promising and attractive process because this catalytic process can eliminate two environmentally harmful industrial gases at the same time.21 To the best of our knowledge, the catalytic reduction of N2O by CO was first reported by Kappes and Staley23 in 1981. In their research, decomposition of a N2O molecule on Fe+ to form a FeO+ complex, accompanied by the release of N2, is © XXXX American Chemical Society

[Cat] + N2O(g) → [Cat]O + N2(g)

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Received: January 30, 2019

A

DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry [Cat]O + CO(g) → [Cat] + CO2 (g)

Mn, Rh, Ru, Ir, Os, Pt, and Pd. Our DFT calculations show that Rh1/PTA, Pd1/PTA, and Pt1/PTA SACs are the most active for N2O decomposition. Therefore, the possible mechanisms for reduction of N2O by CO over Rh1/PTA, Pd1/PTA, and Pt1/PTA SACs have been examined and proposed. This work shows the feasibility for utilizing Pt1/PTA and Pd1/PTA SACs for reducing environment of harmful gases, especially for development of catalyst for eliminating two environmentally harmful gases in a catalytic process at a low temperature.

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Although these reported catalysts have been demonstrated to be very active for such reactions, it cannot meet the requirement of the real-world catalytic converter since the adsorption ability of CO on most catalysts is stronger than that of N2O, which might lead to the poisoning of the catalyst at operating temperature. Under high temperature, a reversal of the adsorption ability for both gases occurs, resulting in the reduction of N2O to N2 by CO on the catalyst surface. Therefore, the reaction of CO and N2O is always studied at high temperature experimentally. To date, finding an appropriate catalyst is of vital importance for N2O oxidation by CO at low temperature. Single-atom catalysts (SACs) have been demonstrated to be effective in many applications such as fine chemicals production, fuel processing, and environment remediation, etc.25−44 The physicochemical properties of those single atoms could significantly differ from the corresponding cluster, subnanoparticles, and nanoparticles because of the unique coordination environment surrounding single atoms in a suitable support materials (most of them are metal oxides).30,31 It has been well-demonstrated that SACs displayed high activity and selectivity in some environmental, economic, and social important reactions. Meanwhile, the maximum atom efficiency of SACs (100% metal dispersity) offers a new idea to develop cost-effective catalysts using noble metals such as Pt, Au, Pd, Ir, Rh, Os, and Re, etc. The surface chemistry of polyoxometalates (POMs) has attracted considerable attentions because of their potential applications in the field of corrosion protection,45 electrochemistry,46 and oxidation catalysis,47−52 etc. Most of these applications are closely relative to the unique surface structure of POMs. Differing from the metal oxide with a periodic and network structure, POMs possess the discrete metal−oxo anionic structure, and are always considered as molecular analogues of metal oxides.47,53−63 This structural feature results in an interesting island effect on the surface of POM solid. And thus, POMs become the ideal SAC support materials because the “separated island” in the interface can effectively prevent the diffusion of single atoms and avoid agglomeration and the formation of metal particles. The POM-supported SAC has been successfully synthesized by Yan’s group first.47 A combination of experimental and theoretical study showed a Pt single atom can be effectively dispersed on the Keggin-type POM surface with a high metal loading (close to 1 wt %). This reported Pt1/POM SAC has been demonstrated to be active for catalyzing hydrogenation of nitrobenzene and cyclohexanone. The surface oxygen activity of POM supports in SACs has been explored in a recent study reported by the same group.53 A systematic experimental study shows that the surface oxygen species of an Rh1/PTA SAC (PTA = [PW12O40]3−) can oxidize CO to CO2 at a higher temperature between 150 and 400 °C, and the turn over frequencies are found to be between 0.2 and 1.7 s−1 from 165 to 195 °C. The mechanistic studies show that the surface oxygen of PTA reacts with adsorbed CO on the Rh single atom to form a surface oxygen vacancy, which is replenished by O2. And thus, the complete catalytic cycle is the typical Mans−van Krevelen (MvK) mechanism. In the present paper, we employed DFT calculations to probe a full reaction mechanism for reduction of N2O by CO based on a series of M1/PTA SACs, where the metal is Fe, Co,

2. MODELS AND COMPUTATIONAL METHODS In the present paper, the solid structure of M1/PTA SACs was modeled by anchoring M3+ cation on the PTA surface with a one-toone manner because the PTA support possesses a nonperiodic structure and is the molecular analogue of metal oxide. It is wellknown that PTA anion with a Keggin structure can be expressed as [(W12O36)(PO43−)],64−67 where the PO43− subunit is trapped into the neutral W12O36 cage, which is a so-called clathrate structure. The welldefined neutral metal−oxo surfaces of the W12O36 cage are ideal catalyst supports. Introduction of M3+ cation on the surface of the W12O36 cage would generate a strong electrostatic interaction between M3+ ion and PO43− subunit, and thus effectively enhance metal− support interactions and thermodynamic stability. The surface oxygen atoms of the PTA support can be divided into three sets according to whether they are in terminal (Ot), bridging two metal atoms (Ob), and at the corners of the Keggin structure (Oc). According to previous experiments47 and our theoretical studies,63 the most stable anchoring site on the surface of [PW12O40]3− for coordination of various isolated single atoms is the 4-fold hollow site, which is formed by two Ob and Oc atoms (see Figure 1). In the present paper, the single metal atom studied here was anchored at the 4-fold hollow site of the PTA surface.

Figure 1. Polyhedral representation for M1/PTA SACs (a), highlighting the 4-fold hollow site of the PTA surface (b). All DFT calculations were carried out using Gaussian 09 Revision D.01 software68 with the M06L functional.69 The standard 6-31G(d) basis set was used for the main group elements; the pseudopotential basis set LANL2DZ70−72 was selected for all metal atoms. The vibrational frequencies of each structure were carried out at the same level in the present work. The M06L functional was derived from the Meta-GGA exchange M05 functional. It has been shown that the M06L functional displays good performance in calculations of molecular geometries, noncovalent interaction, thermochemistry, thermochemical kinetics, vibrational frequencies of the main-group, organometallic, and inorganometallic compounds. Meanwhile, the M06L functional is useful for these large-sized systems because of the computational advantages of local functionals.69 The accuracy of the M06L functional in prediction of geometry, vibrational frequency, reaction mechanism, etc., of POMs was also tested in our previous work.63,73 To the best of our knowledge, the structures obtained via the M06l functional are prone to virtual frequency and nonconvergence. In order to solve these crucial issues and make more accurate results, we have increased the quality of the integration grid B

DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Representative adsorption structures of catalyst that absorbed one CO molecule (a), two CO molecules (b), one CO2 molecule (c), one N-end N2O molecule (d), one O-end N2O molecule (e), and one N2 molecule (f) in their most favorable configuration.

Figure 3. Adsorption energy of one CO (black), two CO (red), one CO2 (blue), one N-end N2O (green), one O-end N2O (magenta), one N2 (cyan) on various M1/POM systems (a). The calculated formation energies for the oxygen vacancy on the PTA surface via removal of Ob or Oc atoms (b). (using the “ultrafine” level of grid precision) with the Gaussian 16. All of the structures discussed in the present work are minima or transition states on the corresponding potential energy surfaces, as confirmed by the correct number of imaginary frequencies. The single point calculations based on optimized geometries were carried out using the same functional with the polarization and diffusion basis sets 6-31+G(d) for main-group atoms and the relativistic energy-consistent pseudopotential basis set SDD for transition metal atoms to further refine the electronic energy. To ensure high quality results, we also increase the quality of the integration grids. In contrast to the previous results,73 the calculation parameters we set are the same as we did before. The natural bond orbital (NBO)74 method at the M06L/6-31G(d) level (LANL2DZ basis sets on the metal atom) was used to get charge distribution and Wiberg bond indices (WBI) of atoms. The adsorption energy Ead of the adsorbate is defined as the following equation

reaction was calculated by the energy difference between the transition state and initial state. An important character of the transition metal center is that it has the capability of adopting different spin states as a function of the ligand environment. In the present paper, all M1/PTA SACs and their adsorption complexes with various spin states studied here have been optimized at M06L/6-31G(d) levels (LANL2DZ basis sets on metal atom) in various spin states. The calculated relative energies of the metal−dinitrogen POM complexes in different spin states are summarized in Table S1. All the discussions in the following are based on the most stable spin states.

3. RESULTS AND DISCUSSION 3.1. Screening Eligible Catalyst. According to previous investigations, the ability to efficiently capture the adsorbates around the active site is an important criterion for an eligible catalyst. As mentioned above, reduction of N2O by CO would result in releasing of N2 and CO2. In order to elucidate the catalytic performance of these catalysts studied here, understanding the CO, CO2, N2O, and N2 adsorption over M1/PTA SACs surfaces should be the primary step. We have examined a

Ead(A) = EA‐M − EM − EA where EA‑M, EM, and EA represent the total energies of the catalyst after adsorption, the clean catalyst, and the isolated adsorbed atom/ molecule, respectively. The energy barrier Ea of each elementary C

DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Calculated free energy profile (kcal mol−1 and bond length in Å) for reduction of N2O by CO on Pt1/PTA SAC via the MvK mechanism

tends to react with the surface oxygen of the PTA support to generate an oxygen vacancy, which is the potential adsorption site for the N2O molecule. Due to the very strong adsorption behavior for two CO molecules relevant to one CO molecule over the series of M1/PTA SACs studied here, the adsorption configuration with two CO molecules has been assigned as the starting reactant for the MvK mechanism, which implies one of the adsorbed two CO molecules would react with the surface oxygen species of the PTA support to form a CO2 molecule and a deoxygenated PTA with an oxygen vacancy. As shown in Figure 2b, the surface oxygen species, Ob and Oc atoms, around the anchoring single metal atom are the potential reacting atom for CO oxidation, which are not equal chemically. In order to identify the difference of Ob and Oc atoms, the formation energy (Efvac) for the oxygen vacancy on the PTA surface via removal of Ob or Oc atoms has been calculated based on the reaction M1(CO)2/PTA → M1(CO)1/ PTA′ + CO2, where M1(CO)2/PTA is the di-carbonyl complex with the intact PTA support, and M1(CO)1/PTA′ is the mono-carbonyl complex with the deoxygenated PTA support (PTA′) (Efvac = EM1(CO)1/PTA′ + ECO2 − EM1(CO)2/PTA). In light of this formula, SAC with a negative Efvac value implies thermodynamically allowed for formation of an oxygen vacancy. The calculated Efvac values have been compared in Figure 3. For the Ob atom, the calculated Efvac values decrease in the following order: Os (23.59) > Ir (6.15) > Ru (5.48) > Rh (−21.73) > Pt (−28.71) > Pd (−45.43). Only the Rh, Pt, and Pd systems have the negative Efvac values, indicating formation of an oxygen vacancy on the PTA surface via removal of an Ob atom is thermodynamically allowed. For the Oc atom, the calculated Efvac values decrease in the following

number of adsorption configurations for each adsorbate (CO and N2O) and product (CO2 and N2). Only the geometric structure of the most stable configuration has been listed in Figure 2. And the calculated adsorption energy for these gases has been compared in Figure 3. As expected, the calculated adsorption energy for CO is significant when compared to other gases studied here. Moreover, this strong adsorption behavior of the CO molecule relevant to other gases studied here is not sensitive to the metal effects according to our DFT calculations. This result indicates that the presence of CO would significantly inhibit the adsorption of N2O, CO2, and N2. The calculated Ead value for CO decreases in the following order: Mn (−26.50) > Fe (−35.88) > Co (−39.82) > Pd (−50.41) > Ru (−68.05) > Pt (−71.78) > Rh (−75.31) > Ir (−107.96) > Os (−108.66), indicating M1/PTA SACs (M = Fe, Co, Mn) display poor adsorption behavior for CO when compared with others. And thus, the three SACs may not be appropriate as the catalyst for reduction of N2O by CO studied here. In addition, the very strong adsorption of two CO molecules on the same single metal atom (M = Ru, Rh, Os, Ir, Pt, and Pd) suggests that co-adsorption of CO2, N2O, and N2 with CO would not be considered in this work. We will revolve around the configuration of adsorbing two CO molecules on M1/PTA (Ru, Rh, Os, Ir, Pt, and Pd; POM = [PW12O40]3−) in the following discussions. This feature also has been confirmed by an experimental study based on an Rh1/PTA SAC.62 Due to the very strong adsorption behavior of CO, the N2O cannot be adsorbed and dissociated over M1/PTA SACs studied here. Thus, we considered the MvK mechanism for reduction of N2O to N2 in the presence of CO. According to the MvK mechanism, adsorbed one or two CO molecules D

DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

meanwhile, the oxygen vacancy of the Pt1/PTA is completely replenished. Furthermore, it can be found that desorption of N2 would be rather easy because the calculated desorption energy is −4.78 kcal mol−1. According to our DFT-derived energy profiles, a completed cycle for reduction of N2O by CO catalyzed by the Pt1/PTA SAC has been proposed in Figure 4. As a whole, the ratedetermining step in this catalytic cycle is the decomposition of N2O to N2 (16.24 kcal mol−1). Like most studies derived from DFT calculations, the entropy effects have not been considered in determination of the rate-determining step in this work. The Rh1/PTA and Pd1/PTA SACs also have been systemically examined along the same reaction pathway for reduction of N2O by CO. The calculated free energy profile is shown in Figures S1 and S2. For Rh1/PTA SAC, the ratedetermining step was calculated to be CO oxidation by the surface oxygen atom of the PTA support with a barrier of 25.12 kcal mol−1. For Pd1/PTA SAC, the calculated rate-determining barrier is the N2O decomposition with a barrier of 25.05 kcal mol−1. The calculated rate-determining barriers of Rh1/PTA and Pd1/PTA SACs are both higher than that of Pt1/PTA SAC. In order to clearly elucidate the capacity for reduction of N2O by CO for Rh1/PTA, Pd1/PTA, and Pt1/PTA SACs, the calculated free energy profiles for the favorable pathways in the three systems have been compared in Figure 5. It can be found that the whole reaction pathway can be divided into two steps, CO oxidation and N2O decomposition.

order: Os (72.64) > Ir (56.68) > Ru (32.3) > Rh (18.61) > Pt (−24.40) > Pd (−38.78), indicating Pt and Pd systems are thermodynamically allowed for the formation of an oxygen vacancy on the PTA surface via removal of an Oc atom. Accordingly, we will focus on the Rh1/PTA, Pt1/PTA, and Pd1/PTA systems in the following discussions. 3.2. Reduction of N2O by CO via MvK Mechanism. The calculated free energy profile for reduction of N2O by CO via the MvK mechanism catalyzed by Pt1/PTA SAC is shown in Figure 4. As mentioned above, the configuration with adsorption of two CO molecules on the single Pt site is assigned as the initial state, which is denoted as CO* in this work. Along with the MvK pathway, one of the two CO molecules in CO* could react with the surface oxygen species (Ob and Oc atoms) of the PTA support to form an oxygen vacancy and a CO2 molecule. Our previous study has shown that the Ob and Oc atoms on the PTA surface are not equal chemically for CO oxidation.73 And thus, the formation of an oxygen vacancy via removal of Ob and Oc atoms should be both considered. As shown in Figure 4, the absorbed CO is supposed to react with Ob and Oc to form a CO2 molecule via transition states, TS1b and TS1c, respectively. The calculated energy barrier for TS1b and TS1c is 7.40 and 18.85 kcal mol−1, respectively. And the calculated reaction energy for the formation of CO2 is 20.24 kcal mol−1 and 20.72 kcal mol−1 via removal of Ob and Oc atoms, respectively. All of these results indicate that the surface Oc atom of the PTA support has a high reactivity for CO oxidation because of a low barrier and comparable reaction energy when compared with that of the Ob atom. In order to analyze the reason for the difference in barrier of the two transition states, the geometric parameters of TS1c and TS1b have been compared in detail (see Figure 4): (i) The Pt−C1 bond is slightly elongated from 1.93 Å in CO* to 1.98 Å in TS1c, and the optimized Pt−C1 bond increases from 1.93 Å in CO* to 1.95 Å in TS1b. (ii) The optimized Pt−Oc bond of 2.72 Å in TS1c is distinctly longer than Pt−Ob of 2.13 Å in TS1b. The change of the Pt−Oc bond in TS1c (ΔrPt‑Oc) relevant to that in the initial configuration CO* is significant when compared with that of the Pt−Ob bond in TS1b (ΔrPt‑Oc = 0.34 Å vs ΔrPt‑Ob = 0.07 Å). (iii) The C1−Oc bond in TS1c is also longer than the C1−Ob bond in TS1b (1.89 vs 1.72 Å). These results all show that the strain of the three-memberedring structure (Pt−C1−Oc) in TS1c is smaller than that of the TS1b because of a relaxed structure, and thus a low barrier for TS1c. We can draw a conclusion that the formation of an oxygen vacancy via removal of a surface Oc atom is more favorable than that of an Ob atom for CO oxidation with the MvK mechanism. And thus, in the following sections, only the surface Oc atom is considered for N2O decomposition. The formed CO2 molecule desorbs without energy cost because the calculated process from CO2*@Pt-Ov,c to Pt-Ov,c is slightly exothermic, as illustrated in Figure 4. Subsequently, CO is adsorbed on the Pt metal site and oxygen vacancy Oc is filled with N2O. Then it can be found that the calculated barrier of the N2O decomposition is 16.24 kcal mol−1 in Figure 4. In the absorbed structure [CO* + N2O*]c, the optimized Pt−O4 and O4−N1 bond distances are 3.14 and 1.21 Å, while they are 2.97 and 1.34 Å in the TS2c, respectively. It suggests that the O4−N1 bond would break and the Pt−O4 bond would form in the coming. In addition, the O4−N1−N2 angle changes from 179° in the adsorbed state [CO* + N2O*]c to 142° in the TS2c. After across TS2c, the N2 is formed in N2*+Ptv;

Figure 5. Calculated free energy profile (kcal mol−1) of reduction of N2O by CO on Rh1/PTA, Pt1/PTA, and Pd1/PTA in the favorable pathway via the MvK mechanism.

For the first step, the calculated barrier is 6.30, 25.12, and 7.40 kcal mol−1 for Pd1/PTA, Rh1/PTA, and Pt1/PTA, respectively. This result shows that it is rather liable to form oxygen vacancies on the PTA surface for the three systems, especially for Pd1/PTA and Pt1/PTA SACs. Meanwhile, the calculated relative energy of intermediates, CO2*@M-Ov and M-Ov, which correspond to the formation of an oxygen vacancy, shows a strong thermodynamic driving force in Pd1/ PTA SAC. Thus, formation of an oxygen vacancy on the PTA surface over the Pd1/PTA SAC is both kinetically and thermodynamically favorable. For the second step, the calculated barrier for N2O decomposition is 25.05, 22.99, E

DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and 16.25 kcal mol−1 for Pd1/PTA, Rh1/PTA, and Pt1/PTA, respectively. These results indicate that it is very easy to carry out, especially for Pt1/PTA SAC, which has the lowest barrier among the three systems. In summary, the reaction curve of the Rh1/PTA system locates in the upper and has the highest activation energy barrier for the CO oxidation, which also intuitively shows that it is the most unfavorable among the three systems both kinetically and thermodynamically. The Pt1/PTA system has the lowest barrier among the three systems, as supported by the calculated rate-determining barrier in the whole pathway is only 16.25 kcal mol−1 according to our DFT calculation. Thus, the Pt1/PTA system has a high activity for reduction of N2O by CO via the MvK mechanism. Although the Pd1/PTA system has a high energy barrier for the second step, the thermodynamic advantage of the Pd1/PTA system also indicates a high activity for reduction of N2O to N2 by CO. 3.3. Electronic Structure of SACs for the Whole Catalytic Cycle. It is well-known that a metal single atom in SACs plays a crucial role in the whole catalytic process. We first consider the anchoring effects of a single metal atom on the surface oxygen species of the PTA support. The key geometric parameters and NBO charges of the PTA support and M1/PTA (M = Rh, Pd, and Pt) are compared in Table 1.

The charge variation of metal atoms along the favorable reaction pathway is summarized in Figure 6 so as to further

Figure 6. Charge (q, |e|) variation of the Rh, Pd, and Pt atoms during CO oxidation along their favorable reaction pathways.

comprehend the role of anchoring Rh, Pt, and Pd atoms. For the whole cycle of the three systems, it can be found that a remarkable charge fluctuation occurs for the three metal atoms and the changing trend is rather similar. For the transition state corresponding to the CO oxidation, from MCO* to MTS1, the calculated NBO charges of the three metal single atoms are increased because the charge transfers from metal to CO. This will further weaken the CO bonding and enhance its activity. For the transition state about the N2O decomposition step, from M[CO* + N2O*] to MTS2, the calculated NBO charge increases, indicating N2O is activated because of the electron transfer from metal to the antibonding orbital of N2O. In all the three systems, the metal single atom receives the electrons in the intermediate M[CO* + N2O*], which directly reflects the electron acceptor properties of metal atoms as coadsorption of CO and N2O. The metal atom accepts and stabilizes excess electrons from adsorbed N2O and CO and promotes the adsorbed behavior of SACs studied here for both gases, which leads to the series of intermediates in the three reaction pathways are the thermodynamically stable species. In the desorption process of CO2 and N2, the metal charge changes rather little. As a result, the CO2 and N2 will be exceedingly easy to desorb because of their weak interaction with metals.

Table 1. Key Geometric Parameters (Bond Length in Å) and NBO Charges (q, |e|) for Clean PTA and M1/PTA SACs (M = Rh, Pd, and Pt) parameters

PTA

Rh1/PTA

Pt1/PTA

Pd1/PTA

W−Ob W−Oc ∠W−Ob−W ∠W−Oc−W q (Ob) q (Oc)

1.924 1.932 152.20 126.59 −0.712 −0.703

2.117 2.058 146.72 119.90 −0.666 −0.758

2.150 2.094 140.77 122.44 −0.713 −0.763

2.111 2.084 144.56 121.70 −0.682 −0.742

It can be found that anchoring of a single metal atom, Rh, Pd, and Pt, on the 4-fold hollow site of the PTA surface all weakens W−Ob and W−Oc bonds, as supported by the optimized W−Ob distance increases from 1.927 Å in PTA to 2.117, 2.150, and 2.111 Å for Rh1/PTA, Pd1/PTA, and Pt1/ PTA, respectively, and the optimized W−Oc distance increases from 1.932 Å in PTA to 2.058, 2.094, and 2.084 Å for Rh1/ PTA, Pd1/PTA, and Pt1/PTA, respectively. Meanwhile, anchoring a single metal atom on the PTA surface decreases angle degree of ∠W−Ob−W and ∠W−Oc−W, indicating anchoring effects push Ob and Oc atoms away from the PTA support surface. The calculated NBO charge shows that anchoring effects result in an electron transfer to the Oc atom in Rh1/PTA, Pd1/ PTA, and Pt1/PTA SACs because the Oc atom carries more negative charge when compared with that of the clean PTA support. By contrast, a contradictory trend has been found for the Ob atom in Rh1/PTA and Pt1/PTA SACs, where the calculated NBO charge indicates an electron transfer from the Ob atom. There is not a significant change in charge for the Ob atom in Pd1/PTA SAC. Meanwhile, a molecular orbital analysis shows that the Pt center in the Pt1(CO)2/PTA complex possesses a σxy2πx2−y22dxz2dyz2dz21 configuration (see Figure S3), indicating that the Pt metal center plays the role of electron acceptor when adsorption of two CO molecules on it.

4. CONCLUSIONS Catalytic reduction of N2O to N2 by CO is considered the most promising and attractive process because this process can eliminate two environmentally harmful industrial gases at the same time. Decomposition of N2O on the catalyst surface is usually taken as the initial step for this catalytic process. However, the strong adsorption behavior on the catalyst surface for CO seriously inhibits adsorption and decomposition of N2O and even leads to poisoning of the catalyst. In order to avoid competitive adsorption and poisoning of the catalyst caused by CO, the MvK mechanism for this reaction has been proposed according to our DFT calculations. The strongly adsorbed CO molecule on the single metal atom was oxidized by the surface oxygen atom of the PTA support to F

DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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form an oxygen vacancy, and N2O acts as an oxygen donor to replenish the oxygen vacancy and release N2 based on a series of PTA supported SACs, M1/PTA (M = Fe, Co, Mn, Rh, Ru, Ir, Os, Pt, and Pd). The results indicate the following: (i) M1/ PTA SACs (M = Rh, Pd, and Pt) possess good adsorption behavior for CO and thermodynamically allowed process for the formation of an oxygen vacancy on the PTA surface. (ii) The Rh1/PTA system has the highest barrier and relevant unstable intermediates, and thus is the most unfavorable both kinetically and thermodynamically. (iii) The Pt1/PTA system has a low energy barrier, and the Pd1/PTA system has a good thermodynamic advantage. Thus, both systems have the high activity for reduction of N2O by CO via the MvK mechanism. (iv) The metal single atom acts as an electron acceptor for binding CO, and works as an electron transfer media in the whole reaction process according to electronic structure analysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00290. The calculated relative energies of the M1/POM SACs and their CO adsorption complexes in different spin states. xyz coordinates for most relevant structures reported in this paper (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86 0432 64606919. Fax: 86 0432 64606919. E-mail: [email protected] or [email protected]. ORCID

Chun-Guang Liu: 0000-0002-1220-5236 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21373043).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00290 Inorg. Chem. XXXX, XXX, XXX−XXX