Stability and Catalytic Performance of Single‐Atom Supported on Ti2CO2 for Low‐Temperature CO Oxidation: A First‐Principles Study

Single–atom manganese and nitrogen co-doped graphene as low-cost catalysts for the efficient CO oxidation at room temperature

We propose K1–xMo3P2O14 as a promising support for single-atom catalysts. A novel single-atom catalyst, Pt/K1–xMo3P2O14, was then designed for CO oxidation and its catalytic activity was investigated by means of first-principles computations. The agglomeration of Pt atoms is effectively inhibited by K1–xMo3P2O14. Eley–Rideal (ER), Langmuir–Hinshelwood (LH), and oxygen vacancy mechanism were considered for CO oxidation, and the results indicate that Pt/K1–xMo3P2O14 exhibits ultrahigh activity with a low energy barrier of 0.49 eV for low-temperature CO oxidation via LH mechanism. The computations would shed new light on the design of novel single-atom catalysts and widen the applications of mixed-valent molybdenum monophosphates.

A density functional theory study of the CO oxidation on Pt1 supported on PtX2 (X = S, Se, Te)

The CO oxidation mechanism on graphene with divacancy (DG) embedded with transition metal from Sc to Zn has been studied by using first principles calculations. The results indicate that O2 molecule is preferentially adsorbed on Sc, Ti, V, Cr, Mn, and Fe-DG, which can avoid the CO poisoning problem that many catalysts facing and is beneficial to the CO oxidation progress. Further study indicates that Mn-DG shows the best catalytic properties for CO oxidation with consideration of both Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) oxidation mechanisms. Along the ER mechanism, the reaction energy barrier for the first step (CO free + O2 pre-adsorbed → OOCO) is 0.96 eV. Along the LH mechanism, the energy barrier for the rate limiting step (CO adsorbed + O2 adsorbed → OOCO) is only 0.41 eV, indicating that the CO oxidation on Mn-DG will occur along LH mechanism. The Hirshfeld charge distributions of O2 and CO molecules is tuned by the embedded Mn atom, and the charge transfer from the embedded Mn atom to the adsorbed molecules plays an important role for the CO oxidation. The result shows that the Mn-embedded divacancy graphene is a noble-metal free and efficient catalyst for CO oxidation at low temperature.

To remove the toxic carbon monoxide (CO) gas from the environment, recently, researchers have taken great interest in single‐atom catalysts (SACs). In this study, we investigated various reaction pathways and barrier energies for the CO oxidation process onto Al‐decorated C 20 fullerene (Al@C 20 ) employing density functional theory (DFT). The outcomes validate that Al decoration on C 20 is energetically stable while the corresponding electronic properties show that the Al atom acts as the reactive site for catalytic activity. The bond distances and larger negative adsorption energies (−0.65 and −2.53 eV) evince that CO and O 2 species adsorbed strongly to the Al atom. For the oxidation of CO, two popular mechanisms are examined: Eley Rideal (ER) and Langmuir Hinshelwood (LH). The assessment of energy barriers discloses that the CO oxidation reaction progresses via the LH mechanism. Additionally, the CO + O* → CO 2 reaction proceeds very quickly onto the Al@C 20 catalyst, with a very small energy barrier (0.13 eV), indicating the excellent catalytic reactivity of the studied catalyst. These results propose that the designed catalyst can be valuable in the progress of novel noble metal‐free catalysts for harmful CO elimination from the environment.

Fe-doped C3N monolayer as a promising SAC for CO oxidation with low temperature and high reactivity

The catalytic oxidation of toxic carbon monoxide (CO) into carbon dioxide (CO 2 ) is one of the most significant reactions in heterogeneous catalysis and industrial chemistry. Hence, finding efficient and cost‐effective catalysts for CO oxidation is very critical. In this study, the electronic and catalytic properties of a Co atom incorporated B 36 cluster (Co@B 36 ) are investigated by means of first‐principles calculations. The results show that the Co atom can seriously tune the properties of the B 36 cluster due to significant hybridization between the 3d states of the Co atom and the B‐2p states of the surrounding B atoms. This induces a significant positive charge on the Co atom, providing an active site for O 2 and CO molecules to attack. Furthermore, the well‐known mechanisms for CO oxidation, namely the Eley‐Rideal (ER), Langmuir‐Hinshelwood (LH), termolecular Eley‐Rideal (TER), and new Eley‐Rideal (NER), are taken into account in this work. According to our findings, the LH and TER mechanisms are the most energetically efficient routes for CO oxidation; however, the favored mechanism may be depending on the relative concentrations of CO and O 2 molecules in the reaction mixture. The obtained activation energies through the TER and LH mechanisms are also comparable with those reported on noble metals. The findings of this work might help in the design and manufacture of noble‐metal free single atom catalysts for the removal of harmful CO molecules from the environment.

Promoting reactivity of graphene based catalysts to achieve LH mechanism for CO oxidation

We report herein a new sulfur-functionalized MXene Ti2C (Ti2CS2)-supported osmium-metal single-atom catalyst (SAC) Os1/Ti2CS2 with high low-temperature catalytic activity for CO oxidation. Using periodic density functional theory calculations, the most stable SAC, Os1/Ti2CS2, has been screened from a series of group 8–11 transition metal SACs M1/Ti2CS2 (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au). The calculations show that it is favorable for O2 and CO to be coadsorbed on the Os1 single atom (SA) of Os1/Ti2CS2 and the adsorption energy of the first O2 molecule is slightly higher than that of CO. Moreover, the termolecular co-adsorption of O2 + 2CO on Os1 SA is also possible, which is favorable for CO oxidation on Os1 SA through a novel three-molecule reaction mechanism. Accordingly, four different catalytic mechanisms, the Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R), termolecular Langmuir-Hinshelwood-A (TLH-A) and termolecular Langmuir-Hinshelwood-B (TLH-B), are systematically studied for CO oxidation by O2 on Os1/Ti2CS2. The theoretical studies indicate that the TLH-B mechanism is the most feasible for CO oxidation with the reaction barrier energy of only 0.74 eV, which is far lower than for L-H, E-R and TLH-A with barrier energies of 1.06, 1.09 and 1.47 eV, respectively. The results provide fundamental understanding to the surface chemistry of MXene and designing new sulfur-functionalized two-dimensional MXene catalytic nanomaterials.

The mechanism of CO oxidation by O2 on a single Au atom supported on pristine, mono atom vacancy (m), di atom vacancy (di) and the Stone Wales defect (SW) on single walled carbon nanotube (SWCNT) surface is systematically investigated theoretically using density functional theory. We determine that single Au atoms can be trapped effectively by the defects on SWCNTs. The defects on SWCNTs can enhance both the binding strength and catalytic activity of the supported single Au atom. Fundamental aspects such as adsorption energy and charge transfer are elucidated to analyze the adsorption properties of CO and O2 and co-adsorption of CO and O2 molecules. It is found that CO binds stronger than O2 on Au supported SWCNT. We clearly demonstrate that the defected SWCNT surface promotes electron transfer from the supported single Au atom to O2 molecules. On the other hand, this effect is weaker for pristine SWCNTs. It is observed that the high density of spin-polarized states are localized in the region of the Fermi level due to the strong interactions between Au (5d orbital) and the adjacent carbon (2p orbital) atoms, which influence the catalytic performance. In addition, we elucidate both the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms of CO oxidation by O2. For the LH pathway, the barriers of the rate-limiting step are calculated to be 0.02 eV and 0.05 eV for Au/m-SWCNT and Au/di-SWCNT, respectively. To regenerate the active sites, an ER-like reaction occurs to form a second CO2 molecule. The ER pathway is observed on Au/m-SWCNT, Au/SW-SWCNT and Au/SWCNT in which the Au/m-SWCNT has a smaller barrier. The comparison with a previous study (Lu et al., J. Phys. Chem. C, 2009, 113, 20156-20160.) indicates that the curvature effect of SWCNTs is important for the catalytic property of the supported single Au. Overall, Au/m-SWCNT is identified as the most active catalyst for CO oxidation compared to pristine SWCNT, SW-SWCNT and di-SWCNT. Our findings give a clear description on the relationship between the defects in the support and the catalytic properties of Au and open a new avenue to develop carbon nanomaterial-based single atom catalysts for application in environmental and energy related fields.

Mechanistic insight into the selective catalytic oxidation for NO and CO on co-doping graphene sheet: A theoretical study

Metal-supported cerium dioxide catalysts are widely used in industrial processes. This study investigates copper-cerium dioxide (Cu-CeO2) as a promising catalyst for CO oxidation, noted for its cost-effectiveness and low activation temperature. However, the reaction mechanism remains unclear. We examine the effects of copper ion doping into CeO2 nanoparticles at 0, 5, 15 at%, synthesized via precipitation. X-ray diffraction (XRD) analysis confirmed sample purity, revealing partial amorphization at a higher doping level. Lower Cu concentration in CeO2 facilitated CO oxidation at a lower temperature. X-ray absorption spectroscopy (XAS) investigated the electronic and atomic structures, showing that CeO2 with a lower Cu doping resulted in a higher lattice oxygen content and moderate Cu ion reduction. Increased Cu doping decreased Ce-O coordination numbers, while Cu-O radial distance was larger at lower doping. In situ XAS identified moderate reduction of Ce and Cu, elucidating CO oxidation via the Langmuir-Hinshelwood (LH) mechanism at lower Cu concentration. The superior low-temperature CO oxidation performance is attributed to moderate Ce and Cu ion reduction coupled with increased oxygen adsorption on the catalyst surface. These findings provide insights into the relationship between the dopant concentration, electronic structure, and catalytic activity for optimizing ceria-based catalysts for efficient CO oxidation.

Recently, single atom catalysts (SACs) with isolated metal atom as the active site have received extensive attention for their excellent catalytic performance. However, limited by the strong aggregation tendency of monometallic atoms, the construction of SACs remains a formidable challenge. Herein, we developed a facile ternary copolymerization‐pyrolysis approach to synthesize a single cobalt atom catalyst (Co 1 @NC) by employing amino‐functionalized cobalt phthalocyanine (CoPc(NH 2 ) 4 ) as the metal precursor. Specially, CoPc(NH 2 ) 4 was copolymerized with melamine and 1,4‐phthalaldehyde to yield a terpolymer, thereby allowing CoPc to be more uniformly and stably distributed in the polymer network. Subsequently, the obtained terpolymer was pyrolyzed to afford Co 1 @NC. Aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (AC HAADF‐STEM) and X‐ray absorption spectroscopy (XAS) directly confirmed that the catalyst Co 1 @NC was a single cobalt atom catalyst. Furthermore, the large specific surface area ( S BET = 418.8 m 2 /g) and high cobalt content (2.71 wt%) of Co 1 @NC provided more cobalt active sites and therefore displayed excellent catalytic activity in the hydrogenation of nitrobenzene. In addition, the catalyst showed remarkable cyclic stability in five cycles and remained as the single atom catalyst.

The oxidation mechanism of CO on W-embedded graphene was investigated by M06-2X density functional theory. Two models of tungsten atom embedded in single and double vacancy (W-SV and W-DV) graphene sheets were considered. It was found that over W-SV-graphene and W-DV-graphene, the oxidation of CO prefers to Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanism, respectively. The two surfaces exhibit different catalytic activity during different reaction stages. The present results imply that W-embedded graphene is a promising catalyst for CO oxidation, which provides a useful reference for the design of a high-efficiency catalyst in detecting and removing of toxic gases.

We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first-principle density-functional theory (DFT) calculations. Calculations on selected model show that the low-coordinated Au atoms of the Au(29) nanoparticle carry slightly negative charges, which enhance the O(2) binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH). The overall LH reaction O(2(ads)) + CO((gas)) --> O(2(ads)) + CO((ads)) --> OOCO((ads)) --> O((ads)) + CO(2(gas)) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states (TS(LH1) and TS(LH2)) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au(29) + O(2(gas)) + CO((gas)) --> Au(29)-O(2(ads)) + CO((gas)) --> Au(29)-CO(3(ads)) --> Au(29)-O((ads)) + CO(2(gas)) requires an overall activation barrier of 0.19 eV, and the formation of Au(29)-CO(3(ads)) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au(29) nanoparticle is likely to occur at or even below room temperature. To gain insights into high-catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.