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HCl Adsorption and Ionization on Amorphous and Crystalline H2O Films below 50 K Patrick Ayotte,*,† Patrick Marchand,† John L. Daschbach,‡ R. Scott Smith,‡ and Bruce D. Kay*,‡ † ‡
Departement de Chimie, Universite de Sherbrooke, 2500 Boulevard Universite, Sherbrooke, Quebec, Canada J1K 2R1 Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland, Washington 99352, United States ABSTRACT: Molecular beams were used to grow amorphous and crystalline H2O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm1 are superimposed. The band centered near 2530 cm1 is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.
I. INTRODUCTION Ever since the formation of the ozone hole was linked to heterogeneous atmospheric chemistry processes occurring at the surface of the solid particles forming polar stratospheric clouds, physical chemists have become keenly interested in understanding the interaction of various halogen reservoir species with the surface of ice. Of the many elementary heterogeneous phenomena and chemical reactions that play a role in the destruction of ozone, by far the ones that have attracted the most attention are the adsorption of HCl and ClONO2 onto ice, and their bimolecular reactions to form molecular chlorine.13 From a fundamental perspective, this problem confronted our preconceptions about acidbase chemistry in the confined geometry of environmentally relevant aqueous solutions and mixed molecular solid interfaces.4 Given that the surface propensities of various ions can give rise to appreciable surface segregation, conventional wisdom might be challenged as we stride to develop our understanding of how strong and weak acids behave at aqueous interfaces.59 In the case of HCl, the excess free energies for segregation of chloride and hydronium to the surface of water are relatively small, estimated to be at most ∼8 kJ/mol for H3Oþ at STP and probably much less for Cl. On the other hand, thermodynamic considerations indicate that dilute hydrochloric acid solutions should become even more strongly acidic at low temperatures (e.g., because of the large negative enthalpy of the reaction HCl(aq) þ H2O(aq) f Cl(aq) þ H3Oþ(aq), ΔHr = 57 kJ/mol). r 2011 American Chemical Society
Furthermore, the recent observation of acid ionization in the HCl 3 (H2O)4 cluster solvated in helium nanodroplets demonstrated that short-ranged H-bonding interactions might be more important than long-range electrostatics to promote ionic dissociation as a mere four water molecules suffice to dissociate HCl, albeit in a very specific H-bonding architecture.10,11 Therefore, it appears like the balance of energetics for hydrochloric acid adsorption at the surface of condensed water at low temperatures (supercooled water, amorphous or crystalline ice) could be strongly dominated by thermodynamic considerations favoring extensive dissociation. Nevertheless, the ice surface has been described as presenting a poor solvation environment to adsorbates. Therefore, it may well display adsorption sites characterized by unfavorable H-bonding topologies for ionic dissociation separated by sizable kinetic barriers that could trap metastable molecularly adsorbed HCl at low temperatures on ice.12,13 The binding energy1224 for HCl at these sites has been estimated to lie in the 2549 kJ/mol range, compared to the 6470 kJ/mol adsorption energy onto sites were dissociative adsorption occurs, a value only slightly smaller than the heat of dissolution in aqueous solutions at infinite dilution, 75 kJ/mol. Special Issue: Victoria Buch Memorial Received: October 31, 2010 Revised: March 31, 2011 Published: April 20, 2011 6002
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The Journal of Physical Chemistry A Of the many experimental methods used throughout the past 25 years to study the adsorption of HCl onto ice (for a review, see ref 25), infrared spectroscopy has triggered the most intense efforts as it can probe the chemical nature of the adsorbate directly, in situ, and nondestructively. The most early works reported the observation of a Zundel continuum, which was interpreted as signaling the presence of aqueous protons, leading their authors to conclude HCl adsorbs dissociatively onto ice.2629 Using different sample preparation methods25,26,3032 or experimental techniques,33,34 other investigators reported observations of molecularly adsorbed HCl at the surface of ice films or ice nanoparticles. There thus remains some confusion regarding to what extent the different preparation methods used to synthesize ice samples, the various dosing methods used to expose the ice to HCl molecules, and the specific temperature/coverage regimes probed in these various studies could influence the chemical nature of HCl adsorbed at the surface of ice. Furthermore, the various experimental techniques used to probe HCl adsorbates (i.e., spectroscopic or mass spectrometric) may also display different sensitivities toward their molecular and ionically dissociated adsorption states at the ice surface. In addition, the paucity of appropriate methods to characterize the ice surface structure and morphology, and those used to discriminate between the amount of HCl deposited onto, from that absorbed within the ice film, may also have contributed to the confusion regarding the relative importance of these various parameters in determining the adsorption state of HCl at the surface of ice. Vibrational spectra of HCl adsorbed onto ice are notoriously difficult to interpret as H-bonded HCl molecules and hydronium ions can both display similar vibrational signatures in the condensed phase. Specifically, broad and strong bands observed in the 24802550 cm1 range upon adsorbing HCl onto ice can be assigned either to reactant (i.e., HCl(ads)) or product (i.e., H3Oþ(ads)) species of the ionic dissociation reaction, HCl(ads) þ H2O(ads) f Cl(ads) þ H3Oþ(ads). Hydrated hydronium ions display a strong band in this range of the infrared spectra of concentrated mixtures of strong acids and water that is assigned to their asymmetric OH stretching vibrations. For example, the infrared spectrum of the (amorphous and crystalline) HCl monohydrate is dominated by a strong and broad band centered near 2550 cm1.26,3539 Thus, HCl molecules adsorbed onto ice that would react and form an amorphous solid or supercooled hydrochloric acid adlayer with a near equimolar stochiometry could yield a spectral feature in this range as reported by Delzeit et al.26 Alternatively, HCl molecules strongly H-bonded to H2O could also display a vibrational frequency in this range. For example, the HCl 3 H2O and HCl 3 (H2O)2 binary clusters in the gas phase40 (argon matrices)41 display intramolecular HCl stretching frequencies of 2723 (2659) and 2464 (2390) cm1, respectively. It is thus certainly conceivable that adsorption sites may exist at the surface of ice which could offer H-bonding topologies that would result in molecularly adsorbed HCl12,13 displaying an intramolecular stretching frequency in the 2480 2550 cm1 range, red shifted from its gas phase frequency of 2886 cm1. The combined spectroscopic/modeling investigations of Devlin, Buch, and co-workers were very successful at identifying these H-bonding topologies.25,30 Methodological advances allowed Devlin to prepare three-dimensional (3D) arrays of ice nanoparticles displaying very high surface area that are ideally suited for surface spectroscopic studies. The surface of these ice nanoparticles is characterized by a very low dangling OH bond
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(d-OH) areal density that offers a poor solvating environment for certain adsorbates. Metastable, molecularly adsorbed HCl could thus be trapped at the surface of these nanoparticles at low temperatures (1060 K) and ionic dissociation reaction intermediates could thus be identified using their distinctive spectral signatures. Monte Carlo methods with empirical force fields were then used to identify likely adsorption sites upon which HCl could be trapped in metastable molecularly adsorbed states. The distinctive features of these adsorption sites were thus identified and used to build small cluster models amenable to high-level electronic structure calculations. This allowed harmonic frequencies to be evaluated providing detailed assignments for the experimentally observed spectral features and allowing in-depth understanding of the molecular-level configurations that accompany the discrete steps along the ionic dissociation pathway for HCl at the surface of ice. The configurations responsible for the HCl[1] band peaked near 2480 cm1 were interpreted as corresponding to singly H-bonded HCl molecules adsorbed at d-O sites while the HCl[2] band peaked at 1700 cm1 was proposed to correspond to doubly H-bonded HCl molecules that are also bound to d-O sites, but that simultaneously accept a single H-bond from an adjacent d-OH site. Configurations where d-O bound HCl accepts two H-bonds from neighboring d-OH sites were shown by previous quantum mechanics/molecular mechanics (QM/MM) work21 to dissociate promptly. Identifying these spectral features as resulting from molecularly adsorbed HCl required great care as they overlap strongly with the asymmetric OH stretching band of hydronium ions (i.e., for the HCl[1] band) and the water-bending band of ice (i.e., for the HCl[2] band). A more definitive identification by these authors required extensive isotope substitution studies as well as similarly detailed spectroscopic/computational studies of the adsorption of HBr onto ice nanoparticles. Heating of the HCl-covered ice nanoparticle arrays resulted in the disappearance of the molecularly adsorbed HCl bands concomitantly with the growth of bands assigned to hydronium ions.25,30 This was interpreted as resulting from the thermal activation of HCl ionic dissociation on ice and was supported by independent observations from Kang and co-workers33,34 of the decrease in the reactive ion scattering yields from molecular HCl (i.e., m/z = 169 assigned to CsHClþ), accompanied by the increase in the low energy (i.e., Csþ kinetic energy of 20 eV) sputtering yields from hydronium ions (i.e., m/z = 19 assigned to H3Oþ or corresponding signatures from other analogous deuterated isotopomers), observed with increasing temperature from 50 to 140 K. While the Devlin technique25,30 provided a novel preparation method for ice samples with interesting structures and morphologies, it did not allow precise, accurate, and uniform dosing of HCl onto the surface of the ice nanoparticles throughout the 3D array. Rather, HCl “clusters” were introduced in alternation with ice nanoparticles during the deposition of the 3D arrays. The sample was then heated to 60 K allowing HCl clusters to sublimate and HCl molecules to diffuse onto the surface of the ice nanocrystals throughout the 3D array. Therefore, it would be highly desirable to study HCl adsorption onto ice combining a more precise dosing methodology with a highly surface sensitive spectroscopic technique. Vibrational spectroscopy is a powerful tool allowing in situ, nondestructive investigations of adsorbates on surfaces however, precious insight into the chemical state of HCl molecules adsorbed onto ice could also be gleaned by probing the lack or presence of adsorbed chloride ions. These latter obviously 6003
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The Journal of Physical Chemistry A cannot be probed directly using infrared spectroscopy however, photoemission studies by Parent and co-workers4244 were able to probe the chemical nature of HCl molecules adsorbed onto the surface of ice from the perspective of their electronic structure. Indeed, they were able to identify a distinctive spectral signature for molecular HCl, namely the near-edge X-ray absorption fine structure (NEXAFS) associated with transitions * orbital in from the Cl2p orbital to the intramolecular σClH solid HCl.4244 This feature was observed upon condensing multilayer solid HCl at 20 K however, it was not observed when a single HCl adlayer was adsorbed onto ice at 20 or 90 K. Instead, a narrow peak at 200.1 eV photon energy was observed and assigned to transitions between adsorbed chloride ions 2p valence f 4s Rydberg orbitals. Such a characteristic feature was clearly identified for HCl exposures up to the saturation coverage for HCl at the surface of an ice film, but it was not observed when solid HCl condensed upon increasing the exposure beyond the first HCl adlayer. This provided very strong evidence signaling that HCl was largely dissociated at the surface of ice, even at temperatures as low as 20 K. Furthermore, this experimental observation of facile (or even spontaneous) ionization of this strong acid at the surface of ice is consistent with a large body of theoretical work based on electronic structure calculations addressing this issue.1224 Clearly, however, there still remains significant uncertainty surrounding the conditions under which HCl adsorbs molecularly onto ice. In this work, we report a detailed study of the adsorption of HCl onto amorphous and crystalline ice films at low temperature using a combination of vibrational spectroscopy and molecular beam methods. Molecular beam dosing allows very precise and reproducible quantities of adsorbates to be deposited at the surface of ice and to prepare composite samples with complex compositional profiles. Grazing-angle reflectionabsorption infrared spectroscopy (RAIRS) provides the sensitivity to detect submonolayer HCl coverages on a low surface area, dense amorphous or crystalline ice film. The experimental methodology used in this work is described in Section II and results are presented in Section III. Observations reported herein strongly suggest HCl adsorbs dissociatively onto ice and these finding are compared and contrasted with previous studies in Section IV. Finally, we summarize and conclude in Section V.
II. EXPERIMENTAL SECTION Ice films were deposited onto a Pt(111) single crystalline substrate, 1 cm in diameter and 1 mm thick, that was spot-welded to 2 mm diameter Ta wires clamped to gold-plated copper jigs mounted onto a closed-cycle helium cryostat. The temperature of the substrate could be controlled using a computer-based PID algorithm that drove resistive heating through the Ta wires and that measured the substrate temperature with a K-type thermocouple spot-welded to the back of the crystal. The substrate was cleaned by repeated sputter-anneal cycles: Neþ ion sputtering for 10 min at 10 μamp and 1.5 keV at 20 K followed by annealing in ultrahigh vacuum (UHV) for 5 min at 1300 K. The highly reproducible and quantitative control over dosing of gaseous HCl on ice required for this study was achieved through molecular beam techniques. Dosing of H2O and HCl onto the clean Pt(111) substrate was performed using a quadruply differentially pumped molecular beam. Neat gases (P = 2 Torr for H2O and 1 Torr for HCl) were expanded in high vacuum through the 1 mm diameter hole of a stainless steel
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nozzle creating a quasi-effusive molecular beam that was skimmed and collimated by three stages of differential pumping. The same nozzle was used for dosing H2O and HCl thereby ensuring complete overlap of the HCl beam with the ice film. The diameter of the beam exceeded slightly the dimensions of the Pt(111) crystal ensuring that the film thickness was homogeneous over the whole substrate surface probed by the infrared beam used for vibrational spectroscopic investigations. The temperature of the sample could then be raised at a constant rate using computer control allowing temperature programmed desorption (TPD) experiments to be performed. Desorption fluxes were analyzed in a line-of-sight geometry using a quadrupole mass spectrometer. TPD spectra for coverages up to a few monolayers (ML) were used to evaluate the relative molecular beam fluxes for H2O and HCl as well as to establish the cleanliness of the platinum substrate. Relative coverages for H2O ((20%) and HCl ((50%) were defined as their saturation coverages onto Pt(111) respectively.45 The ice film thickness and phase, as well as the chemical state of the HCl adsorbates, were established using grazing incidence (82° < θIR < 85°) RAIRS. The infrared beam emanating from a commercial Fourier transform IR (FTIR) spectrometer (Bruker Equinox 55 or Nicolet Nexus 670) was steered and focused onto the platinum substrate, collected and refocused onto a mercurycadmium-telluride (MCT) detector. The beam path between the FTIR bench, the UHV chamber, and the detector was purged from H2O and CO2 atmospheric contamination by flowing clean dry N2. Experiments using BaF2 focusing lenses display a low wavenumber cut off of ∼1200 cm1, whereas those where KBr optics were used to extend down to the MCT band gap of ∼650 cm1. Raw spectra are reported with absorbance defined as log[I(ω)/Io(ω)] where Io(ω) is the power spectrum reflected from the clean Pt(111) substrate and I(ω) is that from the film/ Pt(111) system recorded at the same temperature. Interferograms were averaged over 256 scans in order to acquire these power spectra. Difference spectra are reported with absorbance defined as log[I1(ω)/I2(ω)] with I1(ω) and I2(ω) defined as the power spectra reflected from two different film/Pt(111) systems at the same temperature. RAIRS spectra display complex features due to trivial optical effects arising from interference between the multiple reflections from the filmsubstrate and filmvacuum interfaces.46 These optical effects can complicate their interpretation in terms of vibrational spectra and mask the subtle Zundel continuum that is expected to be observed as a result of aqueous protons9 arising from the dissociative adsorption of such a strong acid at the surface of ice. Therefore, we modeled absorbance spectra for thin transparent films adsorbed onto the ice surface in order to distinguish small changes in reflectivity due to optical effects from absorption by the Zundel continuum using a simple classical optics model described previously.9
III. RESULTS Results from molecular beam reflection experiments (the socalled King and Wells technique) for HCl on Pt(111) and crystalline ice (CI) substrates at various temperatures are reported in Figure 1. The HCl molecular beam impinged at 45° incidence with a flux of ∼0.2 ML/sec at the substrate determined by the exposure required to obtain a saturation coverage for HCl on Pt(111) as reported previously.45 The reflected flux was measured as a function of time with a quadrupole mass spectrometer 6004
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Figure 1. Intensity of the HCl flux reflected by the substrate as obtained using the King and Wells method. The incoming HCl flux is supplied at 45° incidence by a quadruply differentially pumped quasi-effusive molecular beam and collected in line-of-sight by a quadrupole mass spectrometer. The flux reflected by the Pt(111) substrate at 300 K (black trace) is considered identical to the molecular beam flux. The fluxes initially reflected by a crystalline ice film at 60 K (red trace) and 80 K (blue trace) indicate unit adsorption probability for HCl onto ice. The reflected flux increases sharply after 5 s exposure of the ice film at 80 K allowing the saturation coverage for HCl on ice to be determined.
positioned ∼1 cm away and in line-of-sight from the sample. After a prompt onset, the molecular beam flux reflected from clean Pt(111) at 300 K (black trace) displays a slow rise, probably due to adsorption and secondary reactions on the inner surfaces of the stainless steel vacuum chamber and mass spectrometer ionizer. The Pt(111) substrate was then cleaned thoroughly, its temperature was set to 145 K, and a 50 ML coverage crystalline ice film was grown upon it at 45° incidence. The substrate was then finally cooled back down to 60 K to measure the HCl beam flux reflected from the resulting crystalline ice substrate as a function of time displayed as the red trace in Figure 1, which shows almost no HCl scattered from the substrate. The sticking coefficients for HCl incident at 45° incidence upon ice, and upon HCl multilayers, are thus indistinguishable from unity within experimental error ((2%). Similar measurements (data not shown) found the HCl sticking coefficient to be unity for temperatures in the range of 20 to 60 K. Analogous measurements were made on dense films of ASW (data not shown) and the results were essentially identical to those obtained for a crystalline ice substrate. The blue trace in Figure 1 reports the behavior of the HCl flux reflected from a 50 ML coverage crystalline ice film, prepared as described above, but maintained at 80 K. The initial sticking coefficient for HCl is indistinguishable from unity within experimental error up to 4 s but the reflected HCl flux rises sharply thereafter until it eventually coincides with the flux reflected from Pt(111) at 300 K. This observation is consistent with previous reports29,4749 indicating that HCl adsorbs with high probability (S ≈ 1) onto crystalline ice up to a saturation coverage of ∼1 ML even for temperatures above the HCl condensation temperature (i.e., 72 K at this molecular beam flux). Molecular beam reflection experiments were performed on crystalline ice at various temperatures up to 140 K and the observations reported by Hodgson and co-workers48,50 were largely reproduced. Survey RAIRS spectra obtained during incremental dosing of 0.2 ML aliquots of gaseous HCl, up to 2.0 ML total cumulative HCl coverage, onto a 50 ML coverage crystalline ice film at 20 K
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Figure 2. Survey RAIRS spectra collected while dosing incremental 0.2 ML HCl doses onto a 50 ML coverage crystalline ice film at 20 K up to a cumulative HCl coverage of 2.0 ML. The left inset shows details of the dangling OH (d-OH; arrow indicates decreasing intensity with increasing HCl coverage) spectral range (36503750 cm1) while the right inset presents details of the HCl stretching spectral window (23002850 cm1). The growth of the HCl-solid feature is indicated by the vertical arrow. The peak position for the asymmetric OH stretching band of hydrated hydronium ions (H3Oþ at 2550 cm1) and for the intramolecular stretching band of molecularly adsorbed HCl (HCl[1] at 2480 cm1) are indicated by the vertical arrows. The raw RAIRS spectra are overlaid by superimposing them at 3000 cm1.
are displayed in Figure 2. We purposefully show raw absorbance spectra as baseline subtraction and other spectral manipulations may mask such subtle spectral features as the weak Zundel continuum. RAIRS spectra of thin ice films are dominated by the intense band located in the 31003500 cm1 range, which is assigned to the coupled intramolecular OH stretching vibrations of water molecules in ice. They also display a sloping baseline characterized by an increasingly negative absorbance with increasing wavenumbers, which is due to optical interference effects as described previously.9 The inset to the left in Figure 2 shows (baseline corrected) details of the dangling OH (i.e., d-OH) features peaking at ∼3690 cm1 while the inset to the right in Figure 2 shows (baseline corrected) details of the 23002850 cm1 range. This spectral window includes a band centered at 2770 cm1 assigned to the coupled intramolecular HCl stretching vibrations in amorphous solid HCl (labeled HClsolid) and a broad band centered near 2530 cm1. This spectral feature is similar to bands assigned to either intramolecular asymmetric OH stretching vibrations of hydrated hydronium ions (peak position near 2550 cm1 labeled by H3Oþ in the inset to Figure 2)26,35 or to strongly H-bonded HCl molecules adsorbed onto ice and forming a single H-bond with a water molecule’s dangling oxygen lone pairs (i.e., d-O) at the surface of ice (peak position near 2480 cm1 labeled by HCl[1] in the inset to Figure 2).25,3032 Upon applying the first two 0.2 ML HCl doses (red and green spectra), several changes are observed in the RAIRS spectra. First, the baseline is observed to vary wildly toward the lower wavenumbers range of the spectra (which is truncated below 1200 cm1 by the low wavenumbers cutoff of the BaF2 transmission window). Second, the d-OH feature (inset to the left) disappears almost completely below the detection limit imposed by the rms noise of the spectrometer (i.e.,
