AmericanChemicalSociety.com
Reports: AC5 47684-AC5: Transmission Infrared Studies of Hydrogen Storage Materials
The overall goal of this project is to use infrared spectroscopy to gain a fundamental understanding of the reaction mechanisms associated with the use of boron-containing materials for hydrogen storage applications. Among the many formidable obstacles standing in the way of practical hydrogen fuel cell-powered vehicles is the challenge of storing enough hydrogen on board so that performance comparable to today’s gasoline-powered vehicles can be achieved. Complex chemical hydrides are seen as one of the most promising ways of storing hydrogen at high enough densities to meet a target of 9.0% hydrogen by system weight. The material used in the storage system must therefore have an even higher hydrogen weight percent, which generally means that only hydrides of elements of low atomic number are likely to meet the challenge.
Boron is second only to carbon in the rich variety of compounds formed with hydrogen, and boron hydrides and related compounds are being intensely studied as potential hydrogen storage materials. Because they are stable and relatively safe to handle, NaBH4, and especially LiBH4 with a hydrogen weight percentage of 18.1, are of particular interest for hydrogen storage applications. A recent report showed that Ca(BH4)2 can serve as a material for the reversible storage of hydrogen according to the following chemical equation,
3Ca(BH4)2 ↔ CaB6 + 2CaH2 + 10H2,
which implies 9.6% of the mass of calcium borohydride is available for reversible release. The reverse reaction involves the interaction of H2 gas with solid CaB6, implying that the key B-H bond forming reaction takes places at the surface of the hexaboride. It is therefore of fundamental interest to study the surface chemistry of calcium and other metal hexaborides.
Infrared spectroscopy is being used in two different forms, transmission through powdered samples and reflection from single crystal surfaces, to support the goals of this project. Transmission infrared spectroscopy studies of metal borohydrides have the potential to detect the formation of intermediates that might form in the decomposition process. The identification of such intermediates is a key part of establishing the mechanism of hydrogen release from hydrogen storage materials. A second technique, infrared reflection absorption spectroscopy (RAIRS), is being used to explore the interaction of small molecules, including H2, with hexaboride surfaces as a way to gain insights into the mechanism of the reverse reactions needed to reform the starting compounds.
Recent research has indicted that stable intermediates containing the B12H122- anion are formed from the decomposition of borohydrides. In the case of LiBH4, the release of hydrogen might then occur according to the following equation:
12LiBH4 ↔ Li2B12H12 + 10LiH + 13H2 ↔ 12LiH + 12B(s) + 18H2(g),
which could limit the total amount of hydrogen released. To ascertain if such an intermediate is formed from the corresponding borohydrides, IR spectra of K2B12H12 as a function of temperature were obtained.
To conduct transmission IR studies of various borohydrides, an apparatus that permits spectra of the solid compounds over a wide range of temperatures under both a low vacuum and under an ambient of gas was constructed. We have obtained spectra in the mid-IR range for LiBH4, NaBH4, Ca(BH4)2, and K2B12H12. Each compound displays intense peaks in the B-H stretch region, and for the BH4- compounds there is an intense peak in the BH4 deformation region as well. For example, NaBH4 has four prominent peaks at 1127 cm-1 due to the asymmetric BH4 deformation fundamental (ν4), the B-H asymmetric stretch fundamental (ν3) at 2291 cm-1, the overtone of ν4 at 2224 cm-1, and a peak at 2385 cm-1 due to a combination of the symmetric deformation mode (ν2) with the ν4 mode. In contrast, K2B12H12 shows a single intense B-H stretch at 2484 cm-1, and weaker peaks due to B-H bending modes at 1079 and 720 cm-1. These are the only peaks observed in the spectrum. The high symmetry of the icosahedral B12H122- anion accounts for the simplicity of its IR spectrum, with only three IR active fundamentals predicted. Since the B-H stretch of B12H122- occurs at a much higher value than the B-H stretch of BH4-, it should be possible to detect formation of the former if it forms from the decomposition of the latter. Spectra obtained of NaBH4 after annealing to high temperature shows the development of a peak at 2433 cm-1. While this is higher than the peaks in the B-H stretch region of BH4-, it is still below the value for the IR active B-H stretch of B12H122-. Thus, while the results suggest formation of a distinct intermediate that contains B-H bonds, the details are not entirely consistent with formation of an Na2B12H12 intermediate. Because of the somewhat ambiguous results obtained so far, a variety of approaches are being taken to optimize the quality of the spectra. Once the optimum conditions are found, the experiments will be repeated and the spectra further analyzed to arrive at conclusions that are fully consistent with all available evidence. When this point is reached, the work will be submitted for publication.
Boron forms hexaborides with many metals and these hexaborides all have the same cubic structure with remarkably similar lattice constants. The properties of the various MB6-borides are all quite similar, regardless of the metal. The lattice constants of CaB6 and LaB6 are particularly similar at 4.146 and 4.154 Å, respectively. As the large single crystals needed for meaningful surface science studies are readily available for LaB6 but not for CaB6, we focused our hexaboride surface chemistry studies on LaB6. Although we hypothesized that H2 would react with LaB6 surfaces to form B-H-bonds that would be readily detected through a strong B-H stretching vibration, we failed to detect such evidence for B-H bond formation with RAIRS. To determine if this indicated a general lack of reactivity of surface of LaB, we have used RAIRS to study the adsorption and thermal reactions of CO, O2, and H2O on the LaB6(100) and LaB6(111) surfaces. Papers describing the work with CO and O2 were recently published and a manuscript on the H2O studies is in preparation.
