Qinghai Cai

Silicon-Doped Graphene: An Effective and Metal-Free Catalyst for NO Reduction to N2O?

Density functional theory (DFT) calculations were performed on the NO reduction on the silicon (Si)-doped graphene. The results showed that monomeric NO dissociation is subject to a high barrier and large endothermicity and thus is unlikely to occur. In contrast, it was found that NO can easily be converted into N2O through a dimer mechanism. In this process, a two-step mechanism was identified: (i) the coupling of two NO molecules into a (NO)2 dimer, followed by (ii) the dissociation of (NO)2 dimer into N2O + O(ad). In the energetically most favorable pathway, the trans-(NO)2 dimer was shown to be a necessary intermediate with a total energy barrier of 0.464 eV. The catalytic reactivity of Si-doped graphene to NO reduction was interpreted on the basis of the projected density of states and charge transfer.

Can Si-doped graphene activate or dissociate O2 molecule?

Recently, the adsorption and dissociation of oxygen molecule on a metal-free catalyst has attracted considerable attention due to the fundamental and industrial importance. In the present work, we have investigated the adsorption and dissociation of O(2) molecule on pristine and silicon-doped graphene, using density functional theory calculations. We found that O(2) is firstly adsorbed on Si-doped graphene by [2+1] or [2+2] cycloaddition, with adsorption energies of -1.439 and -0.856eV, respectively. Following this, the molecularly adsorbed O(2) can be dissociated in different pathways. In the most favorable reaction path, the dissociation barrier of adsorbed O(2) is significantly reduced from 3.180 to 0.206eV due to the doping of silicon into graphene. Our results may be useful to further develop effective metal-free catalysts for the oxygen reduction reactions (ORRs), thus greatly widening the potential applications of graphene.

Doping of calcium in C(60) fullerene for enhancing CO(2) capture and N(2)O transformation: a theoretical study.

Recently, capturing or transforming greenhouse gases, such as CO(2) and N(2)O, have attracted considerable interest from the perspective of environmental protection. In the present work, by studying CO(2) and N(2)O adsorption on pristine and calcium (Ca)-decorated fullerenes (C(60)) with density functional theory (DFT) methods, we have evaluated the potential application of this C(60)-based complex for the capture of CO(2) and transformation of N(2)O. The results indicate that the adsorptions of CO(2) and N(2)O molecules on the pristine C(60) are considerably weak accompanied by neglectable charge transfer. When C(60) is decorated with Ca atoms, however, it is found that CO(2) and N(2)O adsorptions on the C(60) are greatly enhanced. Up to five CO(2) molecules can be adsorbed on the CaC(60) system due to the electrostatic interaction. For N(2)O molecule, it is first molecularly adsorbed on the Ca atom with the adsorption energy of -0.534 eV, followed by the N(2) formation with a low barrier and high exothermicity. Moreover, when four Ca atoms are decorated on the surface of C(60), the maximum number of the adsorbed CO(2) molecules is 16. Our results might be useful not only to widen the potential applications of fullerene but also to provide an effective method to capture or transform greenhouse gases.

Catalyst-free achieving of controllable carbon doping of boron nitride nanosheets by CO molecules: a theoretical prediction

Controllable carbon (C) doping in a boron nitride (BN) nanostructure can render it exciting magnetic and conductive properties, which would be very valuable for its potential applications in optoelectronics and spintronics. Thus, searching for an efficient method to achieve C-doped BN nanostructure is of vital importance. Here, using density functional theory (DFT) calculations, we propose a mechanism to obtain C-doping of BN nanosheet by the interactions of two CO molecules with three kinds of defective BN nanosheets, including B or N vacancy and BN divacancy. The results show that the proposed mechanism in the present work has the following advantages: (i) the activation energies are only 0.30 and 0.37 eV for BN sheet with B and N vacancy, respectively, suggesting that this reaction can easily occur. For BN sheet divacancy configuration, because the released energy of CO-coadsorption (−5.49 eV) can completely offset the subsequent barrier (1.72 eV), C-doped BN nanosheet can also be achieved using BN nanosheet with divacancy as a reactant. (ii) No catalyst is needed, thus no extra step is needed to remove the catalyst. (3) The harmful CO molecule can be used as a reactant and transformed into CO2 or O2 molecule. (4) The selectivity of CO for vacancy defect sites is high. The present results provide an effective theoretical method to synthesize C-doped BN nanosheets, which would be useful for the development of BN nanosheet-based devices.

Theoretical insights into the effects of the diameter and helicity on the adsorption of formic acid on silicon carbide nanotube

The anchoring of small organic molecules onto the semiconductor surface has a great application for developing various molecular devices, such as novel solar cells, fuel cells, hybrid systems, sensors, and so on. In the present work, by carrying out detailed density-functional theory calculations, we have investigated the adsorption of the formic acid (HCOOH) molecule on planar and various curved silicon carbide (SiC) nanotubes. By considering both the molecular and dissociative adsorptions of HCOOH on these SiC nanomaterials, we found that the HCOOH molecule prefers to dissociate into HCOO and H group. Interestingly, different adsorption modes were found for HCOOH on SiC nanotubes, i.e. dissociative monodentate or bidentate adsorption, which depends on the tube diameter and helicity. For (n, 0) SiC nanotube, the monodentate adsorption mode is energetically favorable when n is less than 10. However, HCOOH prefers to be adsorbed on other (n, 0) SiC nanotubes in a bridged bidentate mode, which is similar to those of on (n, n) SiC nanotubes or planar SiC sheet. Moreover, upon HCOOH adsorption, these SiC nanomaterials remain to be of the semiconducting nature and their band gaps are decreased to different degrees. In addition, we also explored the effects of HCOOH coverage on its adsorption on SiC nanotube.

Single transition metal atom embedded into a MoS2 nanosheet as a promising catalyst for electrochemical ammonia synthesis

The electrochemical reduction of N2 to NH3 (NRR) under ambient conditions is significant for sustainable agriculture. Here, by means of density functional theory (DFT) computations, the potential of a series of single transition metal (TM) atoms embedded into a MoS2 monolayer with an S-vacancy (TM/MoS2) as electrocatalysts for NRR was systematically investigated. Our DFT results revealed that among all these considered candidate catalysts, the single Mo atom embedded into the MoS2 nanosheet was found to be the most active catalyst for NRR with an onset potential of -0.53 V, in which the hydrogenation of the adsorbed N2* to N2H* is the potential-determining step. The high stabilization of the N2H* species is responsible for the superior performance of the embedded Mo atom for the NRR, which is well consistent with its d-band center. Our findings may facilitate the further design of single-atom electrocatalysts with high efficiency for NH3 synthesis at room temperature.

An organic polymer-grafted ionic liquid as a catalyst for the cycloaddition of CO2 to epoxides

A facile method for synthesizing benzyl chloride polymer (BCP) has been developed and BCP immobilized N-methylimidazolium chloride (BCP-IMCl) was prepared by grafting N-methylimidazole ionic liquid on the BCP surface. The compositions and structures of BCP and BCP-IMCl were determined by gel permeation chromatography (GCP), FT-IR, NMR, and elemental analysis. BCP-IMBr, produced via the substitution of Cl− by Br−, was demonstrated to be an efficient catalyst for a solvent-free cycloaddition of CO2 to propylene epoxide (PO) to form propylene carbonate (PC), exhibiting a TON of 88.6 mol/molIL and a 98.7% selectivity towards PC formation. The catalyst can be easily recovered and effectively reused without a significant loss in its activity and selectivity, which would lead to its potential application foreground for the environmentally friendly synthesis of PC.

Two-dimensional iron–tetracyanoquinodimethane (Fe–TCNQ) monolayer: an efficient electrocatalyst for the oxygen reduction reaction

Searching for efficient, cheap, and stable non-Pt electrocatalysts for the oxygen reduction reaction (ORR) has been a major challenge for the development of fuel cells. Herein, we systematically investigated the potential of the experimentally synthesized two-dimensional (2D) metal–tetracyanoquinodimethane (M–TCNQ, where M denotes Mn, Fe, and Co) monolayers as novel ORR catalysts by means of density functional theory (DFT) computations. Our results revealed that O2 molecules can be chemisorbed and efficiently activated on the M–TCNQ monolayers, and the subsequent oxygen reduction can readily proceed via a 4e− pathway. Among the monolayers, the Fe–TCNQ monolayer exhibits the highest catalytic activity with onset potentials of 0.63 and −0.20 V in acidic and alkaline media, respectively. Remarkably, its electrocatalytic performance could be further enhanced by the attachment of axial halogen ligands. Therefore, the Fe–TCNQ monolayer might serve as a promising alternative to Pt-based catalysts for the ORR in fuel cells.

Can trans-polyacetylene be formed on single-walled carbon-doped boron nitride nanotubes?

Recently, the grafting of polymer chains onto nanotubes has attracted increasing attention as it can potentially be used to enhance the solubility of nanotubes and in the development of novel nanotube-based devices. In this article, based on density functional theory (DFT) calculations, we report the formation of trans-polyacetylene on single-walled carbon-doped boron nitride nanotubes (BNNTs) through their adsorption of a series of C2H2 molecules. The results show that, rather than through [2 + 2] cycloaddition, an individualmolecule would preferentially attach to a carbon-doped BNNT via “carbon attack” (i.e., a carbon in the C2H2 attacks a site on the BNNT). The adsorption energy gradually decreases with increasing tube diameter. The free radical of the carbon-doped BNNT is almost completely transferred to the carbon atom at the end of the adsorbed C2H2 molecule. When another C2H2 molecule approaches the carbon-doped BNNT, it is most energetically favorable for this C2H2 molecule to be adsorbed at the end of the previously adsorbed C2H2 molecule, and so on with extra C2H2 molecules, leading to the formation of polyacetylene on the nanotube. The spin of the whole system is always localized at the tip of the polyacetylene formed, which initiates the adsorption of the incoming species. The present results imply that carbon-doped BNNT is an effective “metal-free” initiator for the formation of polyacetylene.

DFT-based study on the mechanisms of the oxygen reduction reaction on Co(acetylacetonate)2 supported by N-doped graphene nanoribbon

Development of low-cost and highly efficient electrocatalysts for oxygen reduction reaction (ORR) is still a great challenge for the large-scale application of fuel cells and metal–air batteries. In this work, by means of density functional theory (DFT) computations, we have systemically explored the anchoring of Co(acac)2 (acac = acetylacetonate) on N-doped graphene nanoribbon and its potential as the ORR electrocatalyst. Our DFT computations revealed that N-doped graphene nanoribbon can be used as the anchoring material of the Co(acac)2 complex due to the formation of a Co–O4–N moiety, thus ensuring its high stability. Especially, an O2 molecule can be moderately activated on the surface of the anchored Co(acac)2 complex, and the subsequent ORR steps prefer to proceed though a more efficient 4e pathway with a small overpotential (0.67 V). Therefore, the hybridization of Co(acac)2 with N-doped graphene can give rise to outstanding catalytic performance for ORR in fuel cells.