High-Pressure Photodissociation of Water As a Tool for Hydrogen Synthesis and Fundamental Chemistry

High-pressure photodissociation of water as a tool for hydrogen synthesis and fundamental chemistry Matteo Ceppatelli, Roberto Bini1, and Vincenzo Schettino European Laboratory for Non-Linear Spectroscopy, Via N. Carrara 1, I-50019 Sesto Fiorentino, Florence, Italy; and Dipartimento di Chimica, Universita´di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Florence, Italy Edited by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, and approved May 14, 2009 (received for review February 18, 2009) High-pressure methods have been demonstrated to be efficient in hydrocarbons triggered by 2-photon (TP) excitations realized providing new routes for the synthesis of materials of technolog- with cw (continuous wave) low-power laser sources that, because ical interest. In several molecular compounds, the drastic pressure of the small cross-section of TP transitions, ensure catalytic conditions required for spontaneous transformations have been amounts of excited species (5, 6, 12, 15). lowered to the kilobar range by photoactivation of the reactions. Among simple molecular systems, water is of primary impor- At these pressures, the syntheses are accessible to large-volume tance because of its abundance on the Earth‘s surface and applications and are of interest to bioscience, space, and environ- because it is the most abundant polyatomic molecule in the mental chemistry. Here, we show that the short-lived hydroxyl visible universe (16). In addition, chemical reactions taking place radicals, produced in the photodissociation of water molecules by in liquid water are essential for many processes in environmental near-UV radiation at room temperature and pressures of a few science and biology. The photodissociation of water is the tenths of a gigapascal (GPa), can be successfully used to trigger principal step in many chemical reactions occurring in the chemical reactions in mixtures of water with carbon monoxide or Earth’s atmosphere, in planets, comets, or other space environ- nitrogen. The detection of molecular hydrogen among the reaction ments. Absorption of vacuum UV (VUV) photons can give rise products is of particular relevance. Besides the implications in to single and multiple ionization with the formation of neutral fundamental chemistry, the mild pressure and irradiation condi- and ionic fragments, either in the ground or in the excited states, tions, the efficiency of the process, and the nature of the reactant able to trigger many chemical reactions. and product molecules suggest applications in synthesis. Photoreactions in gas mixtures containing CO, CO2,CH4, NH3,N2, and H2O [components of the primitive Earth atmo- high-pressure chemistry ͉ photocatalysis sphere (17, 18)] have been studied to investigate the photochem- ical abiotic formation of bioorganic compounds (19). The high- energy photons (␭ Ͻ 185 nm) used in these experiments should he application to molecular systems of suitable external pres- simulate the solar irradiation in planetary or primitive Earth sures causes a density increase that determines a strengthening T environments. The choice of these drastic irradiation conditions of the intermolecular interactions leading first to changes in the is dictated by the energies required to excite, and eventually aggregation state of the material, and then to crystalline phase dissociate, these simple molecules. More than 10.5 eV (118 nm) transitions (1). Upon further compression, the possible overlap of is necessary to dissociate at ambient pressure the triple bonds of the electronic density of nearest-neighbor molecules can also trigger N2 and CO for the generation of the precursors of bioorganic a complete reconstruction of the chemical bonds, giving rise to new compounds (20). Slightly lower energies are required for pho- materials in some cases recoverable at ambient conditions (2). After togeneration of OH and H radicals from water molecules. For the pioneering work on the pressure-induced polymerization of instance, evidence for radical formation has been reported in cyanogen (3) and acetylene (4) crystals, several simple molecular 1-photon (184.9 nm) absorption experiments (21). Photolysis at crystals have been reported to react upon application of suitable 184.9 nm in low-pressure gaseous CO/H2O mixtures has been external pressure, giving rise to high-quality polymeric (5–7) and used to explain the presence of methane in the Martian atmo- amorphous (8–10) materials of potential technological interest. sphere (22). Mixed ices containing H2O and CO (23) or C2H2 These transformations require pressures from several to 100 GPa, (24) irradiated at 10 K with wavelengths Ͻ120 nm produce small as in the nitrogen case (7), but the reaction threshold pressure has amounts of CH3OH, H2CO, and CO2, in addition to some CO been successfully lowered in almost all of the unsaturated hydro- and CH4 in the case of C2H2. carbons studied so far by a photoactivation of the high-pressure Despite these important results, studies employing lower- reactions (11). In some cases, an increasing selectivity (5) or the energy photons, where the solar radiation is peaked, are almost opening of new reaction paths (12) is also observed. These syntheses missing. Two-photon absorption experiments (281–286 nm) have are appealing because only physical methods are used, thus repre- shown the formation of OH and H radicals (25), suggesting the senting an interesting perspective for a green chemistry (13). possibility of using lower-energy photons to generate radicals The mechanisms governing the photoinduced reactivity of through multiphoton absorption processes. In this respect, the molecular systems derive from the structural and charge density high-pressure photodissociation of water is of particular rele- distribution changes after an electronic transition. In general, the vance because it could occur with much lower irradiation excited molecule is characterized by a reduced binding order that energies because of the red shift of the electronic transitions with determines molecular bonds stretching, a lowering of rotational pressure, whereas the higher density could increase the effi- and torsional barriers, an increase in the polarity, and even ciency of the OH and H radicals as reaction initiators. The dissociation and ionization. These species can be particularly irradiation of water under high pressure is an almost unexplored aggressive from a chemical point of view and, depending on their topic despite the fact that these conditions are encountered in lifetime and free mean path, can trigger and propagate a reaction (14). It is therefore evident that these reactions become more and more relevant with increasing pressure because the increas- Author contributions: M.C. and R.B. designed research; M.C. performed research; M.C. and ing density of the materials results in reduced intermolecular R.B. analyzed data; and R.B. and V.S. wrote the paper. distances that favor the interaction between excited and ground- The authors declare no conflict of interest. state molecules. The efficiency of these processes is also attested This article is a PNAS Direct Submission. to by several reactions occurring in pure condensed unsaturated 1To whom correspondence should be addressed. E-mail: [email protected]fi.it. 11454–11459 ͉ PNAS ͉ July 14, 2009 ͉ vol. 106 ͉ no. 28 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0901836106 Downloaded by guest on October 2, 2021 S (1) N2 stretching mode 0 S (2) S (3) S (0) 0 0 0 e ) d Intensity (a.u.) a.u. 2320 2340 2320 2340 ( Raman shift (cm-1) c Intensity Intensity b a b a c 400 600 800 1000 1600 1800 2 Raman shift (c m -1) Fig. 2. Raman spectra of the N2/H2O mixture at 0.1 GPa. (Line a) Spectrum measured before irradiation in the nitrogen-rich region (see Fig. 1(2). (Line b) Spectrum measured in the water-rich region after3hofirradiation with 500 mW of the 350-nm line focused to irradiate all of the sample (150 ␮min Ϫ1 diameter), the 2 strong bands just Ͼ1,000 cm are the R2 and R1 ruby fluorescence lines. (Line c) Spectrum measured in the nitrogen-rich region (same conditions as line b). (Line d) Spectrum measured in the nitrogen-rich region after 5 additional hours of irradiation with 200 mW of the 350-nm line focused to 40 ␮m to increase the power density by a factor 6. (Line e) Reference c c spectrum of a H2/Ar mixture (5% H2 molar fraction) at 1.4 GPa. ance nor new bands in the Raman spectra are detected after the irradiation, thus indicating the chemical stability of the clathrate Fig. 1. Photomicrographs illustrating the history of the N2/H2O mixture. (1) under these conditions. The clathrate is reported to be stable at The sample at 0.6 GPa after the loading. The Raman spectra clearly show that ambient temperature down to 0.14 Gpa, where it decomposes the b region is mainly fluid nitrogen, whereas the solid fraction a is the (29). The pressure is lowered down to 0.1 GPa observing the nitrogen clathrate hydrate (28). (2) At 0.1 GPa after the clathrate decompo- clathrate decomposition and the obtainment of a 2-fluid phase sition and before the irradiation cycles. The c region is essentially composed of system. The Raman spectra indicate that the smaller phase of the water. (3) After3hofirradiation with 500 mW of the 350-nm line focused to sample (labeled c in Fig. 1) is essentially composed of water, irradiate all of the sample (150 ␮m in diameter). (4) After 5 additional hours of irradiation with 200 mW focused in the nitrogen-rich region to a spot of 40 whereas the largest one is composed of nitrogen. A new homo- ␮m to increase the power density by a factor 6 with respect to the previous geneous irradiation (500 mW, 3 h) of all of the fluid sample gives irradiation cycle. rise to remarkable changes in the sample appearance: Some CHEMISTRY bubbles form in the nitrogen-rich region, whereas the extension of the water region reduces.

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