High-pressure photodissociation of water as a tool for hydrogen synthesis and fundamental chemistry

Abstract

High-pressure methods have been demonstrated to be efficient in providing new routes for the synthesis of materials of technological interest. In several molecular compounds, the drastic pressure conditions required for spontaneous transformations have been lowered to the kilobar range by photoactivation of the reactions. At these pressures, the syntheses are accessible to large-volume applications and are of interest to bioscience, space, and environmental chemistry. Here, we show that the short-lived hydroxyl radicals, produced in the photodissociation of water molecules by near-UV radiation at room temperature and pressures of a few tenths of a gigapascal (GPa), can be successfully used to trigger chemical reactions in mixtures of water with carbon monoxide or nitrogen. The detection of molecular hydrogen among the reaction products is of particular relevance. Besides the implications in fundamental chemistry, the mild pressure and irradiation conditions, the efficiency of the process, and the nature of the reactant and product molecules suggest applications in synthesis.high-pressure chemistry ͉ photocatalysis T he application to molecular systems of suitable external pressures causes a density increase that determines a strengthening of the intermolecular interactions leading first to changes in the aggregation state of the material, and then to crystalline phase transitions (1). Upon further compression, the possible overlap of the electronic density of nearest-neighbor molecules can also trigger a complete reconstruction of the chemical bonds, giving rise to new materials in some cases recoverable at ambient conditions (2). After the pioneering work on the pressure-induced polymerization of cyanogen (3) and acetylene (4) crystals, several simple molecular crystals have been reported to react upon application of suitable external pressure, giving rise to high-quality polymeric (5-7) and amorphous (8-10) materials of potential technological interest. These transformations require pressures from several to 100 GPa, as in the nitrogen case (7), but the reaction threshold pressure has been successfully lowered in almost all of the unsaturated hydrocarbons studied so far by a photoactivation of the high-pressure reactions (11). In some cases, an increasing selectivity (5) or the opening of new reaction paths (12) is also observed. These syntheses are appealing because only physical methods are used, thus representing an interesting perspective for a green chemistry (13).The mechanisms governing the photoinduced reactivity of molecular systems derive from the structural and charge density distribution changes after an electronic transition. In general, the excited molecule is characterized by a reduced binding order that determines molecular bonds stretching, a lowering of rotational and torsional barriers, an increase in the polarity, and even dissociation and ionization. These species can be particularly aggressive from a chemical point of view and, depending on their lifetime and free mean path, can trigger a...

High-pressure studies with x-rays using diamond anvil cells

“…The resonance is likely related to the O-O distance in x-ray induced reactions. Irradiation chemistry at HP is widely used for studying the optimal pathways of hydrogen synthesis [627], gas-loading [628,629], and synthesizing new compounds [630][631][632].…”

Backbone NxH compounds at high pressures

“…Indeed, recent theoretical work using ab initio evolutionary structure search predicted a new polymeric (NH) 4 hydronitrogen solid, which becomes more stable than ammonium azide and transtetrazene at 36 and 75 GPa, respectively [11]. However, using only thermodynamic stimuli is often insufficient to create new materials because of large kinetic barriers between different bonding schemes, which may require high temperature [3] or radiation [12][13][14] to create the necessary reaction conditions. Previous experimental works on high-pressure behavior of N 2 -H 2 mixtures found anomalous behavior of the N 2 and H 2 Raman vibron modes compared to pure materials [15,16].…”

Probing high-pressure reactions in heterogeneous materials by Raman spectroscopy

“…Recently, the possibility of inducing chemical reactions between the water and the guest molecules composing these systems has been proposed [23][24][25]. From a chemical point of view, clathrate hydrates are indeed an excellent solid state reaction environment, as every guest molecule is completely surrounded by the caging water molecules.…”

“…These hydrates, as well as those of several hydrocarbons [24,25], have been found to react under UV irradiation giving rise to heterogeneous reaction products, in which the presence of bubbles can be directly observed by using an optical microscope. Raman spectroscopy has been used to probe the presence of fluid hydrogen inside the bubbles, by the observation of the characteristic rotational bands [23].…”