Article pubs.acs.org/JPCC
Adsorption and Subsequent Reaction of a Water Molecule on Silicate and Silica Cluster Anions Masashi Arakawa,* Tsubasa Omoda,† and Akira Terasaki* Department of Chemistry, Faculty of Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
ABSTRACT: We present reactions of size-selected free silicate, MglSiOm−, and silica, SinOm−, cluster anions with a H2O molecule focusing on H2O adsorption. It was found that H2O adsorption to MglSiOm− with l = 2 and 3 (m = 4−6) is always followed by molecular oxygen release, whereas reactivity of the clusters with l = 1 (m = 3−5) was found to be much lower. On the contrary, in the reaction of SinOm− (n = 3−8, 2n − 1 ≤ m ≤ 2n + 2), a H2O adduct is observed as a major reaction product. Larger and oxygenrich clusters tend to exhibit higher reactivity; the rate constants of the adsorption reaction are 2 orders of magnitude larger than those of CO adsorption previously reported. DFT calculations revealed that H2O is dissociatively adsorbed on SinOm− to form two SiO3(OH) tetrahedra. The site selectivity of H2O adsorption is governed by the location of the singly occupied molecular orbital (SOMO) on SinOm−. The present findings give molecular-level insights into H2O adsorption on silica and silicate species in the interstellar environment.
1. INTRODUCTION Water is a ubiquitous molecule on the Earth and is involved in various chemical processes in nature. The origin of water in the inner solar system is a subject of continuing debates.1,2 Even in the early stage of planetary formation, that is, in molecular clouds and protoplanetary disks, H2O is one of the abundant molecules.3 In addition to water, silicates such as pyroxene (e.g., MgxFe1−xSiO3) and olivine (Mg2xFe2(1−x)SiO4) exist as main constituents of interstellar dusts.4,5 It has recently been reported that silica (SiO2) is ubiquitously present throughout the planetary formation process.6,7 Although consensus has not been achieved, it is one of the prevalent hypotheses that hydrous silicates are the water-delivery source to the Earth.8 In this context, adsorption of water on silicates and silica might be a key process to understand the origin of water on the Earth. From this perspective, adsorption of a water molecule9−11 and a hydrogen atom12 on surfaces of forsterite (Mg2SiO4), which is the magnesium-rich end-member of olivine solid solution series, has been investigated. A recent theoretical study reported that dissociative adsorption of H2O is the most energetically favorable process for (100), (010), and (110) surfaces.11 In molecular clouds and protoplanetary disks, silicates and silica exist as free radicals and ions as well as crystalline and amorphous forms because of intense cosmic radiation. Ion− molecule reactions of ions consisting of Si, O, Mg, and Fe with gas molecules, for example, H2O, H2, and CO, might contribute to chemistry in the interstellar environment such as molecular evolution and delivery of water to the Earth. In this regard, we have reported the formation of clusters with mineral compositions13 and reaction of silicon-oxide cluster anions, SinOm−, with a CO molecule14 because gas-phase clusters © XXXX American Chemical Society
provide good models of free radicals and ions. Agglomeration of silicon-oxide clusters has been investigated to understand the growth process of silicate and silica particles in space.15,16 Reactions of silicate and silica clusters with H2O, however, have not been reported; here we examine clusters consisting of Si, O, and Mg as silicate clusters and silicon-oxide clusters (SinOm− with m/n ≈ 2) as silica clusters. In the present study, we investigate the reaction of silicate and silica cluster anions, that is, MglSinOm− and SinOm−, respectively, with a H2O molecule to demonstrate adsorption and subsequent reaction of H2O on these radical anions.
2. METHODS The experimental setup has been described in detail elsewhere.17,18 In brief, MglSinOm− and SinOm− were generated by a magnetron-sputter cluster-ion source. For the generation of MglSinOm−, two plates were employed as sputter targets: a magnesium plate (Kojundo Chem. Lab., 99.99%) with two 8 mm holes and a silicon plate (Kojundo Chem. Lab., 99.999%). Both plates were sputtered simultaneously in the presence of oxygen by placing the holed magnesium plate over the silicon plate. For the generation of SinOm−, only the silicon plate was sputtered after removing the magnesium plate. The sputtered atoms and ions were aggregated with oxygen in a buffer helium gas (99.99995%) cooled by liquid nitrogen to form clusters. After thermalized by collisions with a helium gas at liquidSpecial Issue: ISSPIC XVIII: International Symposium on Small Particles and Inorganic Clusters 2016 Received: November 20, 2016 Revised: January 11, 2017 Published: January 13, 2017 A
DOI: 10.1021/acs.jpcc.6b11689 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C nitrogen temperature, the cluster anions were mass-selected by a quadrupole mass filter (MAX-4000, Extrel CMS). The sizeselected reactant anions were guided by radio frequency octopole ion guides and quadrupole deflectors and were introduced into a linear 40 cm long reaction cell, where a H2O vapor at 298 K was introduced continuously by the vapor pressure of itself through a variable leak valve. The reactant anions passed through the reaction cell in 100−250 μs without being trapped. Ions produced upon reaction were identified by a second quadrupole mass analyzer (MAX-4000, Extrel CMS) and a channel electron multiplier to measure the yield of each reaction product. The partial pressure of H2O, PH2O, in the reaction cell was adjusted to be 9 × 10−3 Pa; the pressure was monitored outside the reaction cell by a residual gas analyzer (RGA100, Stanford Research Systems) and was converted to the pressure inside the reaction cell by calibration referring to a cross section of a known reaction.19 The pressure corresponds to the collision rate of 4 × 103 s−1, where the collisional cross section of H2O is estimated by the average dipole orientation theory, σADO = πe{2/(4πε0)2E}1/2{α1/2 + cμD(2/πkT)1/2}.20 Here α is the polarizability of H2O (1.48 Å3), e is the elementary charge, ε0 is the vacuum dielectric constant, c is the locking constant of H2O (0.25),20 μD is the permanent dipole moment of H2O (1.84 D), k is Boltzmann’s constant, T is the temperature, and E is the collision energy in the center-of-mass frame. The collision energy of the clusters with H2O was 7 eV in the laboratory frame (i.e., 0.8 eV in the center-of-mass frame for Mg2SiO4−). DFT calculations were performed by using the Gaussian 09 package21 employing the B3LYP functional22,23 with the augcc-pVDZ basis set24,25 to search for optimized geometrical structures of reactants and reaction products observed in the experiments. The relative energies were corrected for zeropoint-energy contributions. The charge distributions were obtained through natural bond orbital (NBO) analysis.26
Figure 1. Mass spectra of ions produced in the reaction of (a) MgSiO3−, (b) MgSiO4−, (c) MgSiO5−, (d) Mg2SiO4−, (e) Mg2SiO5−, (f) Mg2SiO6−, (g) Mg3SiO4−, (h) Mg3SiO5−, and (i) Mg3SiO6− with a H2O molecule. MgSiO3− and Mg2SiO4− are the clusters with a composition of enstatite and forsterite, respectively. The abscissa shows a mass shift, Δm, from the reactant, MglSiOm−. The intensity was normalized so that the summation of the peak intensities of all the reactant and product ions is to be unity.
3. RESULTS AND DISCUSSION 3.1. Reaction of MglSiOm− with H2O. We examined silicate clusters, MglSiOm−, in the composition range of m = 3− 5 for l = 1 and m = 4−6 for l = 2 and 3. These clusters represent compositions in the vicinity of enstatite (MgSiO3) and forsterite (Mg2SiO4), which are the magnesium-rich endmembers of orthopyroxene and olivine solid solution series, respectively. Figure 1 shows mass spectra of product ions upon reaction of MglSiOm− with a H2O molecule. The abscissa shows a mass shift, Δm, from the reactant, MglSiOm−. Several peaks of reaction products are observed for l = 2 and 3, whereas reaction products are hardly identified for clusters with l = 1. Three kinds of major products are observed in the mass spectra of l = 2 and 3: Δm = +18, −14, and −46. The peak at Δm = +18 observed for Mg2SiO6− indicates that a H2O adduct is produced. The peak at Δm = −14 is assigned to MglSiOm−1H2−; an O2 molecule is dissociated after adsorption of H2O. The peak at Δm = −46 is assignable to MglSiOm−3H2− or to Mgl−2SiOmH2−; mass spectrometry alone cannot distinguish the product. The production of MglSiOm−3H2− or Mgl−2SiOmH2− indicates dissociation of two O2 molecules or Mg2O, respectively, after H2O adsorption. The reactant clusters are classified into three types according to the reaction channels: only a H2O adduct (Δm = +18) is observed (type 1: Mg2SiO6−); dissociation of O2 occurs after H2O adsorption (Δm = −14) (type 2: Mg2SiO4−, Mg3SiO4−,
and Mg3SiO5−); and a peak is observed at Δm = −46 in addition to Δm = −14 (type 3: Mg2SiO5−and Mg3SiO6−). We found that the type of the clusters is correlated with the location of the singly occupied molecular orbital (SOMO) by referring to optimized geometries of MglSiOm− and their isosurfaces of SOMOs shown in Figure 2. In the type-1 cluster, the SOMO is localized in the vicinity of two dangling O atoms on a Mg atom, whose local charges are −0.40e. The SOMOs of the type-2 clusters are localized in the vicinity of one of the Mg atoms. In the type-3 clusters, the SOMOs are localized in the vicinity of a dangling O atom on a Mg atom, where the local charge is −0.80e. It was thus found that the location of SOMO and the charge distribution in the silicate cluster play a role in determining the reaction channel. The reaction experiment on silicate clusters revealed that simple H2O adsorption is not a dominant reaction channel, except for Mg2SiO6−. H2O adducts are hardly identified for clusters with l = 1, and those with l = 2 and 3 exhibited B
DOI: 10.1021/acs.jpcc.6b11689 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
dissociation of O2 or Mg2O immediately after adsorption of H2O. 3.2. Reaction of SinOm− with H2O. We next focus on Mgfree silica clusters, SinOm− (n = 3−8, 2n − 1 ≤ m ≤ 2n + 2), expecting H2O adsorption. Figure 3a−d and e−h shows mass spectra of product ions upon reaction of Si5Om− and Si7Om− with a H2O molecule, respectively. The abscissa shows a mass shift, Δm, from the reactant. The peak at Δm = +18, which is observed for all of the mass spectra shown in Figure 3 except for Si5O9− (Figure 3a), indicates that one H2O molecule is adsorbed on SinOm− to form SinOm+1H2−. In the reaction of Si5O9−, Si4O9H2− is observed instead of Si5O10H2−, suggesting that SiO is dissociated after H2O adsorption. This was the only dissociation product observed in the experiments for n = 3−8. We assumed pseudo-first-order reaction kinetics for evaluation of reaction-rate constants because the density of H2O can be treated as a constant during the reaction. Because all of the products observed for SinOm− were H2O adducts, rate constants of H2O adsorption, k, were obtained by evaluating the yield of reaction products as follows k = −[ln{Irct /(Irct + Ipro)}]/(n H2Ot )
(1)
where nH2O is the number density of H2O in the reaction cell, t is the time for the clusters to pass through the reaction cell, and Irct and Ipro are the intensities of the reactant and the reaction products, respectively. The rate constants thus obtained are shown in Figure 4 for all of the clusters examined for SinOm−. Compositions of hatched pixels were not examined because we focused on clusters with compositions in the vicinity of silica, SiO2, and, in addition, because those compositions were hardly produced in the present cluster-source condition. No H2O adduct was observed for Si3O5−, Si3O8−, Si4O7−, and Si4O10−,
Figure 2. Optimized geometries of Mg2SiO4− (A), Mg2SiO5− (B), Mg2SiO6− (C), Mg3SiO4− (D), Mg3SiO5− (E), and Mg3SiO6− (F). Isosurface of the singly occupied molecular orbital (SOMO) is shown below each geometry. Local charges in the unit of elementary charge, e, are given in blue. Isovalues are 0.032. Cartesian coordinates of all the atoms are available in Table S1 of the Supporting Information.
Figure 3. Mass spectra of ions produced in the reaction of (a) Si5O9−, (b) Si5O10−, (c) Si5O11−, (d) Si5O12−, (e) Si7O13−, (f) Si7O14−, (g) Si7O15−, and (h) Si7O16− with a H2O molecule. The abscissa shows a mass shift, Δm, from the reactant, SinOm−. The inset in panel e shows the enlarged mass spectrum in the vicinity of Δm = +18. The intensity was normalized so that the summation of the peak intensities of all the reactant and product ions is to be unity. C
DOI: 10.1021/acs.jpcc.6b11689 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 5. Optimized geometries of H2O adducts of SinOm− for n = 4 and 5 in the composition range of 2n − 1 ≤ m ≤ 2n + 1. The spin states of all of the geometries were doublet. Binding energies, D0, of H2O to SinOm− are given in parentheses. Adsorbed H and OH are highlighted with dashed blue circles. Cartesian coordinates of all of the atoms are available in Table S3 of the Supporting Information.
are formed through adsorption of OH on a 3-fold Si site and H on a dangling O atom. On the contrary, Si4O7− is not able to form two SiO3(OH) tetrahedra by adsorbing a H2O molecule. Therefore, the two SiO3(OH) tetrahedra should be a key structure for H2O adsorption. The dissociation barrier of an O−H bond could affect the reactivity if the barrier is high. Although the dissociation barrier on SinOm− has not been reported, it is reported for Mn4O4+ that the barrier of water splitting is low: 0.15 eV for a ring-shaped isomer and
