Michael Trenary , University of Illinois (Chicago)
The overall goal of this project is to use infrared spectroscopy to study 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 percentage, which generally limits the choices to hydrides of elements of low atomic number.
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 for hydrogen storage applications. We have focused on the alkali metal borohydrides (MBH4) and ammonia borane (H3N:BH3, or AB) and the associated reaction chemistry associated with their reversible dehydrogenation. Because they are stable and relatively safe to handle, AB and LiBH4 with hydrogen weight percentages of 19.5 and 18.1, respectively, are of particular interest. 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.
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 metal hexaborides.
We used infrared spectroscopy 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 borohydrides and AB have the potential to detect 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 these materials. A second technique, infrared reflection absorption spectroscopy (RAIRS), was 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. 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).
This mechanism could limit the total amount of hydrogen released. To ascertain if such an intermediate is formed from the corresponding borohydrides, IR spectra of a K2B12H12 reference were obtained.
To conduct transmission IR studies of various borohydrides, an apparatus to acquire spectra of solids over a wide range of temperatures was constructed and used to obtain IR spectra for LiBH4, NaBH4, K2B12H12, and H3N:BH3. Each compound displays intense peaks in the B-H stretch region and for the borohydrides the one IR active B-H stretch fundamental occurs at ~ 2290 cm-1. The high symmetry of the icosahedral anion in K2B12H12 leads to a single intense B-H stretch at 2484 cm‑1. As this is much higher than the B-H stretch of BH4-, it should be possible to detect formation of B12H122- if it forms from the decomposition of BH4-. Spectra of NaBH4 after annealing to high temperature show a peak at 2433 cm-1. While this is higher than the B-H stretch 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.
In the course of our studies, we have come across various unanticipated challenges to using transmission IR spectroscopy to obtain spectra after annealing to the temperatures where dehydrogenation occurs. First, to obtain high transparency, the compound of interest must be diluted in an IR transparent material, such as KBr or CaF2, and the mixed powders pressed under high pressure to form a transparent sample. If the sample is not completely transparent, scattering of the IR radiation occurs, which leads to broad indistinct peaks. After annealing, the transparency can decrease, leading to artefacts in the absorbance spectra. Second, the compounds of interest can react with the host material. For example, ammonia borane is found to react with KBr to form KBH4. Third, any residual water in the sample will react with these compounds to form boron-oxygen bonds, which gives rise to intense IR peaks in the mid IR region. Fourth, the dehydrogenation temperatures can exceed the melting point of KBr, and although CaF2 can be used instead, the samples are less transparent resulting in broader peaks, making the spectra more difficult to interpret. We plan to document these experimental shortcomings of the method in a forthcoming 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 for the surfaces of LaB6, 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 have already been published and a manuscript on the H2O studies was recently accepted for publication.