Fe-doped carbon nanotubes: towards the molecular design of new catalysts for the oxygen reduction reaction

Using DFT computational methods, single-walled carbon nanotubes (CNT) are explored in different geometric configurations (armchair, chiral and zigzag) doped with Fe. Geometry, electronic structure and magnetic properties are investigated for all systems, in order to evaluate a potential application of these structures as electrocatalysts in efficient and low-cost fuel cells. In search for a better electrode material, we turn our attention on nature for help. Oxygen molecules are well-known to reveal a remarkable affinity to the heme group. Therefore, we model the adsorption/dissociative behavior of oxygen molecules on carbon nanotubes doped with Fe atoms. We analyze in detail the effect of the chiral nature of carbon nanotubes that governs their electric, magnetic and chemical behavior. Our results indicate that the dissociation phenomenon involving the armchair (5,5) Fe@CNT is more favored than other chiralities and other doped CNT systems, leading to the lowest activation barrier.

Theoretical investigation of the ORR on boron-silicon nanotubes (B-SiNTs) as acceptable catalysts in fuel cells

Here, the potential of boron doped silicon nanotubes (7, 0) as ORR catalysts is examined. Acceptable paths for the ORR on studied catalysts are examined through DFT. The optimum mechanism of the ORR on the surface of B2-SiNT (7, 0) is shown. The ORR on the surface of B2-SiNTs (7, 0) can continue through LH and ER mechanisms. The calculated beginning voltage for the ORR on B2-SiNTs (7, 0) is 0.37 V and it is smaller than the beginning voltage (0.45 V) for platinum-based catalysts. In the acidic solution the beginning voltage for the oxygen reduction process can be evaluated to be 0.97 V, which corresponds to 0.37 V as a minimum overvoltage for the ORR. The B2-SiNTs (7, 0) are suggested as an ORR catalyst in acidic environments.

DFT Study of the Oxygen Reduction Reaction Activity on Fe-N₄-Patched Carbon Nanotubes: The Influence of the Diameter and Length

The influences of diameter and length of the Fe−N₄-patched carbon nanotubes (Fe−N₄/CNTs) on oxygen reduction reaction (ORR) activity were investigated by density functional theory method using the BLYP/DZP basis set. The results indicate that the stability of the Fe−N₄ catalytic site in Fe−N₄/CNTs will be enhanced with a larger tube diameter, but reduced with shorter tube length. A tube with too small a diameter makes a Fe−N₄ site unstable in acid medium since Fe−N and C−N bonds must be significantly bent at smaller diameters due to hoop strain. The adsorption energy of the ORR intermediates, especially of the OH group, becomes weaker with the increase of the tube diameter. The OH adsorption energy of Fe−N₄/CNT with the largest tube diameter is close to that on Pt(111) surface, indicating that its catalytic property is similar to Pt. Electronic structure analysis shows that the OH adsorption energy is mainly controlled by the energy levels of Fe 3d orbital. The calculation results uncover that Fe−N₄/CNTs with larger tube diameters and shorter lengths will exhibit better ORR activity and stability.

Nitrogen-doped carbon nanotube as a potential metal-free catalyst for CO oxidation

We elucidate the possibility of nitrogen-doped carbon nanotube as a robust catalyst for CO oxidation.

The effect of nano confinement on the C-h activation and its corresponding structure-activity relationship

The C–H activation of methane, ethane, and t-butane on inner and outer surfaces of nitrogen-doped carbon nanotube (NCNTs) are investigated using density functional theory. It includes NCNTs with different diameters, different N and O concentrations, and different types (armchair and zigzag). A universal structure-reactivity relationship is proposed to characterize the C–H activation occurring both on the inner and outer surfaces of the nano channel. The C–O bond distance, spin density and charge carried by active oxygen are found to be highly related to the C–H activation barriers. Based on these theoretical results, some useful strategies are suggested to guide the rational design of more effective catalysts by nano channel confinement.

Catalytic performance of carbon nanotubes in H2O2 decomposition: experimental and quantum chemical study

The catalytic performance of multi-walled carbon nanotubes (MWCNTs) with different surface chemistry was studied in the decomposition reaction of H2O2 at various values of pH and temperature. A comparative analysis of experimental and quantum chemical calculation results is given. It has been shown that both the lowest calculated activation energy (∼18.9 kJ/mol) and the highest rate constant correspond to the N-containing CNT. The calculated chemisorption energy values correlate with the operation stability of MWCNTs. Based on the proposed quantum chemical model it was found that the catalytic activity of carbon materials in electron transfer reactions is controlled by their electron donor capability.

Simulating adsorption of organic pollutants on finite (8,0) single-walled carbon nanotubes in water

Understanding the mechanism and thermodynamics of the adsorption of chemicals on carbon nanotubes (CNTs) is important to risk assessment and pollution control of both CNTs and chemicals. We computed the adsorption of cyclohexane, benzene derivatives, and polycyclic aromatic hydrocarbons (PAHs) on (8,0) single-walled carbon nanotubes by the M05-2X of density functional theory. The computed adsorption energies (E(a)) in the aqueous phase are lower than those in the gaseous phase, indicating that the adsorption in the aqueous phase is more favorable. The contribution of π-π interactions and the enhancing effect of a -NO(2) substituent on the adsorption were quantified. For a hypothetical aromatic with the same hydrophobicity (logK(OW)) to cyclohexane, π-π interactions contribute ca. 24% of the total interactions as indicated by E(a). -NO(2) enhances the π-π interactions due to its electron withdrawing effects, and contributes 24% to the value of E(a). Simple linear regression showed the computed Gibbs free energy changes for the adsorption correlate significantly with the experimental values (r = 0.97, p < 0.01). The correlation together with the computed thermodynamic parameters may be employed to predict the adsorption affinity of other chemicals. The study may pave a new way for evaluating/predicting the adsorption affinity of organic compounds on SWNTs and probing the adsorption mechanisms.