Theoretical insights into catalytic CO2 hydrogenation over single-atom (Fe or Ni) incorporated nitrogen-doped graphene

Enhanced CO oxidation performance over hierarchical flower-like Co3O4 based nanosheets via optimizing oxygen activation and CO chemisorption

Enhancing low-temperature activity is a focus for carbon monoxide (CO) elimination by catalytic oxidation. In this work, the hierarchical flower-like silver (Ag) modified cobalt oxides (Co3O4) nanosheets were prepared by solvothermal method and applied into catalytic CO oxidation. The doped Ag species in the form of AgCoO2 induced the prolongated surface Co-O bond and weaker bond intensity. Consequently, the oxygen activation/migration ability and redox capacity of Ag0.02Co were enhanced with more oxygen vacancies. The chemisorbed CO was preferentially converted to CO2 but not carbonates. The inhibited carbonates accumulation could avoid the coverage of active sites. According to Density functional theory (DFT) calculations, the electron transfer from AgCoO2 to Co3O4 promote electron donation ability of Co3O4 layer, benefiting for oxygen activation. Moreover, the longer Co-C and C-O bond length suggest the weakened chemisorption strength and higher active of CO molecule. The Ag modified Co3O4 exhibited more satisfactory activity at lower temperature. Typically, it realized 100% CO conversion at 90 °C, and displayed 6.3-fold higher reaction rate than pristine Co3O4 at 40 °C. Moreover, the Ag0.02Co exhibited outstanding long-term stability and water resistance. In summary, the optimized oxygen activation, CO chemisorption and interfacial electron transfer synergistically boosted the CO oxidation activity on Ag modified Co3O4.

Single-Atom Catalysts for Hydrogen Activation

Selective hydrogenation is one of the most important reactions in fine chemical industry, and the activation of H₂ is the key step for hydrogenation. Catalysts play a critical role in selective hydrogenation, and some single-atom catalysts (SACs) are highly capable of activating H₂ in selective hydrogenation by virtue of the maximized atom utilization and the highly uniform active sites. Therefore, more research efforts are needed for the rational design of SACs with superior H₂ -activating capabilities. Herein, the research progress on H₂ activation in typical hydrogenation systems (such as alkyne hydrogenation, hydroformylation, hydrodechlorination, hydrodeoxygenation, nitroaromatics hydrogenation, and polycyclic aromatics hydrogenation) is reviewed, the mechanisms of SACs for H₂ activation are summarized, and the structural regulation strategies for SACs are proposed to promote H₂ activation and provide schemes for the design of high-selectivity hydrogenation catalysts from the atomic scale. At the end of this review, an outlook on the opportunities and challenges for SACs to be developed for selective hydrogenation is presented.

Metal-Decorated InN Monolayer Senses N2 against CO2

Poor selectivity is a common problem faced by gas sensors. In particular, the contribution of each gas cannot be reasonably distributed when a binary mixture gas is co-adsorbed. In this paper, taking CO₂ and N₂ as an example, density functional theory is used to reveal the mechanism of selective adsorption of a transition metal (Fe, Co, Ni, and Cu)-decorated InN monolayer. The results show that Ni decoration can improve the conductivity of the InN monolayer while at the same time demonstrating an unexpected affinity for binding N₂ instead of CO₂. Compared with the pristine InN monolayer, the adsorption energies of N₂ and CO₂ on the Ni-decorated InN are dramatically increased from -0.1 to -1.93 eV and from -0.2 to -0.66 eV, respectively. Interestingly, for the first time, the density of states demonstrates that the Ni-decorated InN monolayer achieves a single electrical response to N₂, eliminating the interference of CO₂. Furthermore, the d-band center theory explains the advantage of Ni decorated in gas adsorption over Fe, Co, and Cu atoms. We also highlight the necessity of thermodynamic calculations in evaluating practical applications. Our theoretical results provide new insights and opportunities for exploring N₂-sensitive materials with high selectivity.

Nitrogen atom coordination tuned transition metal catalysts for NO oxidation and reduction

Developing an efficient catalyst for NO oxidation and reduction at ambient temperature is a significant challenge. Recent studies have suggested that the N-coordinated transition metal (TM) single atom catalysts (SACs) have high catalytic activity and stability. Herein, we report the activation potential of a series of 3d TM atoms supported on N coordination-tuned graphene (GR) for NO oxidation and reduction. The results show that the N coordination pattern can greatly alter the catalytic reactivity of TM on the catalysts, and the TM atom on the catalysts with three-coordinated pyridinic nitrogen TM-N₃@GR exhibit the strongest chemical activity. Among the TM-N₃@GR catalysts, Ti-N₃@GR is the most promising candidate. The rate constants and equilibrium constants were calculated to evaluate the kinetic and thermodynamic feasibility of the catalytic reaction, respectively. Our results demonstrate that the reduction of NO to N₂ on Ti-N₃@GR can occur at ambient temperature with a large exotherm of 6.99 eV, and the oxidation of NO to NO₂ on Ti-N₃@GR can easily proceed when the temperature reaches 360 K with a large equilibrium constant. Our studies are of great significance for understanding the performance of N coordination-tuned catalysts and designing Ti-based catalysts for NO oxidation and reduction.

Unraveling the Catalytic Performance of the Nonprecious Metal Single-Atom-Embedded Graphitic s-Triazine-Based C3N4 for CO2 Hydrogenation

Graphitic carbon nitride (g-C₃N₄) is regarded as a promising potent photoelectrocatalyst for CO₂ reduction. Here, extensive first-principles calculations and ab initio molecular dynamics (AIMD) simulations are performed to systematically explore the structural and electronic properties of nonprecious metal single-atom-embedded graphitic s-triazine-based C₃N₄ (M@gt-C₃N₄, M = Mn, Fe, Co, Ni, Cu, and Mo) monolayer materials and their catalytic performances as the single-atom catalysts (SACs) for CO₂ hydrogenation to HCOOH, CO, and CH₃OH. It is found that the atomically dispersed non-noble metal Mn, Fe, Co, and Mo sites anchored on gt-C₃N₄ can efficiently activate both H₂ and CO₂, and their coadsorbed state serves as a precursor to the hydrogenation of CO₂ to different C1 products. Among these SACs (M@gt-C₃N₄, M = Mn, Fe, Co, and Mo), Co@gt-C₃N₄ was predicted to have the best catalytic performance for CO₂ hydrogenation to C1 products, although their mechanistic details are somewhat different. The predicted energy barriers of the rate-determining steps for the conversion of CO₂ into HCOOH, CO, and CH₃OH on Co@gt-C₃N₄ are 0.58, 0.67, and 1.19 eV, respectively. The desorption of products is generally energy-demanding, but it can be facilitated remarkably by the subsequent adsorption of H₂, which regenerates M@gt-C₃N₄ for the next catalytic cycle. The present study demonstrates that the catalytic performance of gt-C₃N₄ can be well regulated by embedding the non-noble metal single atom, and the porous gt-C₃N₄ is nicely suited for the construction of high-performance single-atom catalysts.

CO2 capture and a route to transform it in formic acid: a theoretical approach

New organic frameworks (COFs) employing two coronene molecules forced to adopt a parallel conformation thus forming a molecular reactor are proposed. These COFs exhibit different distances between the coronene units, thus creating diverse electronic environments. The simulation of the trapping of CO₂ and H₂ molecules in the reactor hollow having distinct anchor fragments yields in the two cases formic acid. The analysis of the reaction profile allowed us to propose a thermodynamically favored process. The nature of the frontier molecular orbitals in the involved processes is also discussed. Reaction profile of CO₂ and H₂ process to yield formic acid.

Rational Design of Main Group Metal-Embedded Nitrogen-Doped Carbon Materials as Frustrated Lewis Pair Catalysts for CO2 Hydrogenation to Formic Acid

Developing efficient and inexpensive main group catalysts for CO₂ conversion and utilization has attracted increasing attention, as the conversion process would be both economical and environmentally benign. Here, based on the main group element Al, we designed several heterogeneous frustrated Lewis acid/base pair (FLP) catalysts and performed extensive first-principles calculations for the hydrogenation of CO₂. These catalysts, including Al@N-Gr-1, Al@N-Gr-2, and Al@C₂N, are composed of a single Al atom and two-dimensional (2D) N-doped carbon-based materials to form frustrated Al/C or Al/N Lewis acid/base pairs, which are all predicted to have high reactivity to absorb and activate hydrogen (H₂). Compared with Al@N-Gr-1, both Al@N-Gr-2 and Al@C₂N, especially Al@N-Gr-2, containing Al/N Lewis pairs exhibit better catalytic activity for CO₂ hydrogenation with lower activation energies. CO₂ hydrogenation on the three catalysts prefers to go through a three-step mechanism, i.e., the heterolytic dissociation of H₂, followed by the transfer of the hydride near Al to CO₂, and finally the activation of a second H₂ molecule. Other IIIA group element (B and Ga)-embedded N-Gr-2 materials (B@N-Gr-2 and Ga@N-Gr-2) were also explored and compared. Both Al@N-Gr-2 and Ga@N-Gr-2 show higher catalytic activity for CO₂ hydrogenation to HCOOH than B@N-Gr-2. However, the CO₂ hydrogenation path on Ga@N-Gr-2 tends to follow a two-step mechanism, including H₂ dissociation and subsequent hydrogen transfer. The present study provides a potential solution for CO₂ hydrogenation by designing novel and effective FLP catalysts based on main group elements.