Journal Report

Du Qian

2012-9-12

Structural Selectivity of CO Oxidation on Fe/N/C CatalystsP. Zhang, X. F. Chen, J. S. Lian, and Q. Jiang

J. Phys. Chem. C 2012, 116, 17572−17579

We have performed extensive density functional theory calculations for the elementary steps in CO oxidation onFe/N/C active sites, including Fe-N4 and Fe-N3 porphyrin-like carbon nanotube (T-FeN4 and T-FeN3), Fe-N4 porphyrin-like graphene (G-FeN4), and Fe-N2 nanoribbon (R-FeN2).

Ead values of O2 are larger than that of CO on T-FeN3 and R-FeN2, suggesting that Fe in TFeN3 and R-FeN2 will be dominantly covered by the adsorbed O2 if the CO/O2 mixture is injected as the reaction gas. However, CO adsorption is more favorable than O2 on T-FeN4 and G-FeN4

TFeN3: Not only O2-2π*, but also O2-1π, O2-5σ, and O2-4σ*, interact with Fe-3d

T-FeN4 and G-FeN4 :The dominating contributions for O2 adsorption are the interactions between O2-2π* and Fe-3d orbitals.

The CO interact with Fe chiefly through hybridization between CO-5σ and Fe-3d, also CO-2π* and Fe-3d.Since the Ead value of CO on Fe/N/C is not only related to the electron donation from CO-5σ to metals but also to electron back- donation from metals to CO-2π*

T-FeN4 and G-FeN4

（ a） CO + O2 → OOCO → CO2 + O（ RDS） (b) CO + O → CO2

The electron transfer within the tube walls is easier on the T-FeN4 than on the G-FeN4 due to the larger curvature of the former.

T-FeN3

First, 3 CO molecules can adsorb on Fe in T-FeN3, 2 CO2 were produced.

Both of CO and O2 can be readily coadsorbed on the embedded-Fe atomO2 first dissociates rather than reacts with CO to form O−O−CO(a). CO oxidation(b)

R-FeN2

O2 first is adsorbed on Fe instead of reacting with twoCO to form COOOOC complex

3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4

Nanoparticles as Efficient Electrocatalysts for the Oxygen

Reduction

Zhong-Shuai Wu, Shubin Yang, Yi Sun, Khaled Parvez, Xinliang Feng and Klaus Mullen

J. Am. Chem. Soc. 2012, 134, 9082−9085

Catalysts for the oxygen reduction reaction (ORR) are keycomponents of fuel cells. Metals (Fe, Co, etc.) or metal oxides (Fe2O3, Fe3O4, Co3O4, IrO2, etc.) as well as nitrogencoordinated metal on carbon4 and metal-free doped carbon materials have been actively pursued. In this communication, we demonstrate Fe3O4/N-GAs, a novel class of monolithic Fe3O4 NPs supported on 3D N-doped graphene aerogels (N-GAs).

Fabrication process

1. Graphene oxide (GO） was dispersed in water by sonication, reaching a concentration up to 1.5 mg mL−1.2. Iron acetate (1−40 mg) and polypyrrole (PPy) (20 mg) were slowly added to 6 mL of the GO dispersion to form a stable aqueous suspension.3 .Ternary components were hydrothermally assembled at 180 °C for 12 h to form a graphene-based 3D hydrogel.4. The as-prepared hydrogel was directly dehydrated via a freeze-drying process to maintain the 3D monolithic architecture and then heated at 600 °C for 3 hunder nitrogen.

Figure1

Result and discussion

（ a） The XRD no apparent diffraction peak

could be identified at 20−30°, indicating that Fe3O4

NPs were efficiently deposited on the graphene

surface, suppressing the stacking of graphene

layers.

(b−d) Typical SEM images of Fe3O4/N-GAs.

(e) TEM and (f) HRTEM images of Fe3O4/N-GAs

(with sizes of 20−80 nm) revealing an Fe3O4 NP

wrapped by graphene layers.

Figure2

(a) Typical STEM image. (b) STEM image taken from the square region marked in (a)

(c-f） Corresponding elemental mapping images of (c) Fe, (d) C, (e) N, and (f) O.

(g, h) High-resolution XPSspectra of Fe3O4/N-Gas (g) Fe 2p; (h) N 1s

It is notable that the nitrogen content is much higher in the region of Fe3O4 NPs than in graphene layers (Figure 3e), indicating that Fe−N−C active sites have been established at the Fe3O4 NP interface.

The high-resolution N 1s scan, (Figure 3h) indicated the presence of two forms of nitrogen ,namely, pyrrolic N (401.0±0.2 eV) and pyridinic N (398.1±0.2eV)

Figure3

Figure4

(a) RRDE test of the ORR on Fe3O4/N-GAs, Fe3O4/N-GSs( graphene sheets), Fe3O4/N-CB (carbon black), both Fe3O4/N-GSsand Fe3O4/N-CB exhibited much higher ring currents than Fe3O4/N-Gas

(b) H2O2 yield

(c) electron transfer number of Fe3O4/N-GAs, Fe3O4/N-GSs, and Fe3O4/N-CB, n=3.72−3.95 for Fe3O4/N-GAs electrode over the whole potential range, emphasizing that the Fe3O4/N-GAs ORR proceeds mainly viaa four-electron mechanism.

(d) Peroxide percentage andelectron transfer number as functions of Fe3O4 loading