Theoretical Design of Catalysts for the Heterolytic Splitting of H₂

Abstract

Here, we briefly review recent advances in H2 storage technologies relying on mixed proton–hydride and destabilized hydride materials. We establish a general relationship across different materials: the higher the effective H content, the higher the temperatures needed to completely desorb H2. Nevertheless, several systems show promising thermodynamics for H2 desorption; however, the desorption kinetics still needs to be improved by the use of appropriate catalysts. Prompted by the importance of heterolytically splitting stable dihydrogen molecules for proton–hydride technologies, we attempt to theoretically design novel H2 transfer catalysts. We focus mainly on M4Nm4H8 catalysts (M = V, Ti, Zr, Hf, and Nm = Si, C, B, N), which should be able to preserve their functionality in the strongly reducing environment of a H2 storage system. We are able to determine the energy of H2 detachment from these molecules, as well as the associated energy barriers. In order to optimize the properties of the catalysts, we use isoelectronic atom‐by‐atom substitutions, vary the valence electron count, and borrow the concept of near‐surface alloys from extended solids and apply it to molecular systems. We are able to obtain control over the enthalpy and electronic barriers for H2 detachment. Molecules with the coordinatively unsaturated > TiSi < unit exhibit particularly favorable thermodynamics and show unusually small electronic barriers for H2 detachment (> 0.27 eV) and attachment (> 0.07 eV). These and homologous ZrSi frameworks may serve as novel H2 transfer catalysts for use with emerging lightweight hydrogen storage materials holding 5.0–10.4 wt % hydrogen, such as Li2NH, Li2Mg(NH)2, Mg2Si, and LiH/MgB2 (discharged forms). Catalytic properties are also anticipated for appropriate defects on the surfaces and crystal edges of solid Ti and Zr silicides, and for TiSi ad‐units chemisorbed on other support materials.

Atypical compounds of gases, which have been called ‘noble’

“…So far we have discussed compounds of Xe and Kr in their positive oxidation states, predominantly those of divalent Ng The calculated structures of: (A) Xe VI F 5 Cl in C 4v symmetry; (B) Xe IV F 3 Cl in C 2v symmetry; (C) Xe IV F 3 (C 2 F) in C 2v symmetry; (D) FXe II SiF 3 in C 3v symmetry; (E) HXe II OOXe II H in C 2 symmetry; (F) FXe II OOXe II F in C 2 symmetry; note, the O-O bond is shorter than for HXe II OOXe II H; (G) FXe II (NF)Xe II F in C s symmetry; (H) ClXe II FXe II Cl + in D 'h symmetry. 146 Most of these compounds (A-G) are predicted to be unstable thermodynamically with respect to the substrates (eqn (12)- (17), but they are genuine minima on the PES's. Calculated bond lengths are given in Å .…”

Metal (boro-) hydrides for high energy density storage and relevant emerging technologies

“…Another broadly explored attempt to facilitate hydrogen release is the formation of protonic-hydridic compounds [291] via a combination of the moieties containing partially negatively charged H atoms bound to boron with those attached to nitrogen, which are partially positive. Such approach often increases the theoretical gravimetric content of hydrogen and lowers the temperature of decomposition, as compared with similar borohydrides lacking nitrogen [292].…”

Unprecedented flexibility of the >TiSi< group for the addition of H2

“…This feature only in part reflects the obstacles that arise from thermodynamics; some of the mixed H + /H À (and 'destabilized') materials for hydrogen storage could, in principle, exhibit much lower values of the thermal decomposition temperature, if proper catalysts are used. 5 Previously, we have reported that the hypothetical Ti 4 C 4 H 6 molecule is capable of low-barrier (B0.3 eV) heterolytic splitting and attachment of H 2 . 6 Unfortunately, H 2 binds too strongly to Ti 4 C 4 H 6 (by B1.0 eV) to be detached at modest temperatures.…”

“…This finding has triggered a set of systematic investigations of the related M 4 Nm 4 H 6 molecules (M = Ti, Zr, Hf, V, Nm = C, Si, B, N). 5 We have found that the molecular species containing the coordinatively unsaturated 4TiQSio or 4ZrQSio units, such as the M 4 Si 4 H 6 molecules (M = Ti, Zr), exhibit favourable thermodynamics and extremely small barriers for the attachment of H 2 . At that time we considered exclusively isomers, which are denoted here as I, II and III (see Section 2 of the present work).…”

“…At present, the temperature values corresponding to the release of all hydrogen stored are still pretty high for all reversible protic-hydridic materials. 5 It turns out that not even a single material is known that desorbs significant amounts of H 2 ( Z 7 wt%) below 180 1C, and can be recharged at modest (p,T) conditions. This feature only in part reflects the obstacles that arise from thermodynamics; some of the mixed H + /H À (and 'destabilized') materials for hydrogen storage could, in principle, exhibit much lower values of the thermal decomposition temperature, if proper catalysts are used.…”