Identifying active functionalities on few-layered graphene catalysts for oxidative dehydrogenation of isobutane.

The general consensus in the studies of nanostructured carbon catalysts for oxidative dehydrogenation (ODH) of alkanes to olefins is that the oxygen functionalities generated during synthesis and reaction are responsible for the catalytic activity of these nanostructured carbons. Identification of the highly active oxygen functionalities would enable engineering of nanocarbons for ODH of alkanes. Few-layered graphenes were used as model catalysts in experiments to synthesize reduced graphene oxide samples with varying oxygen concentrations, to characterize oxygen functionalities, and to measure the activation energies for ODH of isobutane. Periodic density functional theory calculations were performed on graphene nanoribbon models with a variety of oxygen functionalities at the edges to calculate their thermal stability and to model reaction mechanisms for ODH of isobutane. Comparing measured and calculated thermal stability and activation energies leads to the conclusion that dicarbonyls at the zigzag edges and quinones at armchair edges are appropriately balanced for high activity, relative to other model functionalities considered herein. In the ODH of isobutane, both dehydrogenation and regeneration of catalytic sites are relevant at the dicarbonyls, whereas regeneration is facile compared with dehydrogenation at quinones. The catalytic mechanism involves weakly adsorbed isobutane reducing functional oxygen and leaving as isobutene, and O2 in the feed, weakly adsorbed on the hydrogenated functionality, reacting with that hydrogen and regenerating the catalytic sites.

In situ Raman spectroscopy of LiFePO4: size and morphology dependence during charge and self-discharge.

Previous studies of the size dependent properties of LiFePO4 have focused on the diffusion rate or phase transformation pathways by bulk analysis techniques such as x-ray diffraction (XRD), neutron diffraction and electrochemistry. In this work, in situ Raman spectroscopy was used to study the surface phase change during charge and self-discharge on a more localized scale for three morphologies of LiFePO4: (1) 25 ± 6 nm width nanorods, (2) 225 ± 6 nm width nanorods and (3) ∼2 µm porous microspheres. Both the large nanorod and microsphere geometries showed incomplete delithiation at the end of charge, which was most likely caused by anti-site defects along the 1D diffusion channels in the bulk of the larger particles. Based on the in situ Raman measurements, all of the morphologies studied exhibited self-discharge with time. Among them, the smallest FePO4 particles self-discharged (lithiated) the fastest. While nanostructuring LiFePO4 can offer advantages in terms of lowering anti-site defects within particles, it also creates new problems due to high surface energies that allow self-discharge. The in situ Raman spectroscopy also showed that carbon coating did not provide significant improvement to the stability of the lithiated particles.

Trends in the Catalytic Activity of Transition Metals for the Oxygen Reduction Reaction by Lithium.

Periodic density functional theory (DFT) calculations indicate that the intrinsic activity of Au, Ag, Pt, Pd, Ir, and Ru for the oxygen reduction reaction by Li (Li-ORR) forms a volcano-like trend with respect to the adsorption energy of oxygen, with Pt and Pd being the most active. The trend is based on two mechanisms: the reduction of molecular O2 on Au and Ag and of atomic O on the remaining metals. Step edges are found to be more active for catalyzing the Li-ORR than close-packed surfaces. Our findings identify important considerations in the design of catalyst-promoted air cathodes for nonaqueous Li-air batteries.