M olecular hydrogen (H 2 ) sorbents are appealing materials for storing hydrogen fuel onboard vehicles. The uptake and release of H 2 fuel in the sorbent materials can be fast and require less heat transfer. To use the H 2 sorbents at near ambient conditions, the binding energy of H 2 in these materials must be within certain range (e.g., 20Ϫ40 kJ/mol). 1,2 Theoretical studies predicted 3,4 that Kubaslike interactions between transition metal (TM) centers and coordinated H 2 could fall within this desirable energy range. Such predictions are consistent with recent experimental studies by using metalϪorganic frameworks (MOFs) with under-coordinated TM. 5Ϫ8 Attempts to anchor TM directly on carbon nanostructures, however, have not yet been successful. Recently, Hamaed et al. used organometallic precursor to successfully graft Ti onto the inner surface of mesoporous silica. 9 Though this work demonstrated the feasibility of individually dispersing Ti and the capability of binding multi-H 2 by dispersed Ti, mesoporous silica has a relatively small surface-to-volume ratio and may be too heavy for practical hydrogen storage. So far, no practical H 2 sorbent is available. Finding the right material for onboard storage is still a grand challenge. Concerning TM-based organometallic sorbents, several conditions are required at the same time: First, the substrate materials possess high surface-to-volume ratio and are lightweight. Second, the TM atoms are undercoordinated and well-exposed to accommodate multi-H 2 . Third, these unsaturated TM atoms, despite their high chemical reactivity, 10 do not form clusters. These require that the anchoring bonds between the TM atoms and the substrate are strong and the TM coverage is also optimized. Along the line of strengthening the anchoring bonds, several strategies have been suggested, such as functionalizing organic molecules, 11 employing defect sites in carbon materials, 12,13 and directly integrating metal atoms into the skeleton. 14,15 Alternatively, graphene oxide (GO) can be a potential substrate to covalently anchor TM atoms by simultaneously satisfying all these three conditions. GO has large surface-to-volume ratio and is intrinsically lightweight (condition 1). GO possesses ample O sites on the surfaces. Oxygen is the key in anchoring under-coordinated Ti (condition 2) and enhancing the TMϪ substrate binding (condition 3), as having been experimentally demonstrated on mesoporous silica. 9 Although GO has been routinely synthesized and extensively studied, 16Ϫ24 currently its precise atomic structures are still under intense investigation. In fact, the O content of GO can vary greatly, depending

The oxygen reduction reaction (ORR) is of high industrial importance. There is a large body of literature showing that metal-based catalytic nanoparticles (e.g. Co, Mn, Fe or hybrid Mn/Co-based nanoparticles) supported on graphene act as efficient catalysts for the ORR. A significant research effort is also directed to the so-called “metal-free” oxygen reduction reaction on heteroatom-doped graphene surfaces. While such studies of the ORR on nonmetallic heteroatom-doped graphene are advertised as “metal-free” there is typically no sufficient effort to characterize the doped materials to verify that they are indeed free of any trace metal. Here we argue that the claimed “metal-free” electrocatalysis of the oxygen reduction reaction on heteroatom-doped graphene is caused by metallic impurities present within the graphene materials.

Nanostructured forms of stoichiometric In 2 O 3 are proving to be efficacious catalysts for the gas-phase hydrogenation of CO 2. These conversions can be facilitated using either heat or light; however, until now, the limited optical absorption intensity evidenced by the pale-yellow color of In 2 O 3 has prevented the use of both together. To take advantage of the heat and light content of solar energy, it would be advantageous to make indium oxide black. Herein, we present a synthetic route to tune the color of In 2 O 3 to pitch black by controlling its degree of non-stoichiometry. Black indium oxide comprises amorphous non-stoichiometric domains of In 2 O 3-x on a core of crystalline stoichiometric In 2 O 3 , and has 100% selectivity towards the hydrogenation of CO 2 to CO with a turnover frequency of 2.44 s −1 .

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