An empirical study of the gas–solid reaction of carbon dioxide (CO2) with alanates is presented. This investigation was triggered by reports of hazards related to the reaction of lithium aluminum hydride with carbon dioxide, together with the recent emergence of alanates as potential hydrogen storage materials. Furthermore, the reduction of CO2 by hydrides is an alternative to the conventional CO2 reduction employing hydrogen gas in combination with a catalyst. Experimentally this work was carried out by studying the decomposition of alanate samples in a carbon dioxide atmosphere with thermogravimetric and ex situ IR spectroscopic techniques. It is shown that alanates react with CO2 at atmospheric pressure in two distinct temperature regions, yielding methane, hydrogen gas, and metal oxides as the major products. The experimental findings allowed us to postulate a mechanism for the complex metal hydride reduction of CO2, involving alane as a highly reactive intermediate. It is suggested that nucleophilic attack of alane hydride ions onto the carbonyl carbon of CO2 leads to sequential formation of aluminum formate and methoxy species which get converted into methane and metal oxides as the final products.

Configuration transitions of individual molecules and atoms on surfaces are traditionally described using an Arrhenius equation with energy barrier and pre-exponential factor (attempt rate) parameters. Characteristic parameters can vary even for identical systems, and pre-exponential factors sometimes differ by orders of magnitude. Using low-temperature scanning tunnelling microscopy (STM) to measure an individual dibutyl sulfide molecule on Au(111), we show that the differences arise when the relative position of tip apex and molecule changes by a fraction of the molecule size. Altering the tip position on that scale modifies the transition's barrier and attempt rate in a highly correlated fashion, which results in a single-molecular enthalpy-entropy compensation. Conversely, appropriately positioning the STM tip allows selecting the operating point on the compensation line and modifying the transition rates. The results highlight the need to consider entropy in transition rates of single molecules, even at low temperatures.

The reversibility of hydrogen sorption in complex hydrides has only been shown unambiguously for NaAlH4 doped with transition metal compounds. Despite a multitude of investigations of the effect of the added catalyst on the hydrogen sorption kinetics of NaAlH4, the mechanism of catalysis remains elusive so far. Following the decomposition of TiCl3-doped NaAlH4 by in-situ X-ray photoelectron spectroscopy (XPS), we link the chemical state of the dopant with those of the hydride and decomposition products. Titanium and aluminium change their oxidation states during cycling. The change of the formal oxidation state of Al from III to zero is partly due to the chemical reaction from NaAlH4 to Al. Furthermore, aluminium oxide is formed (Al2O3), which coexists with titanium oxide (Ti2O3). The interplay of metallic and oxidized Ti with the oxide skin might explain the effectiveness of Ti and similar dopants (Ce, Zr…).