Distance-Dependent Radiation Chemistry: Oxidation versus

Article pubs.acs.org/JPCC

Distance-Dependent Radiation Chemistry: Oxidation versus Hydrogenation of CO in Electron-Irradiated H2O/CO/H2O Ices Nikolay G. Petrik,*,† Rhiannon J. Monckton,‡,§,∥ Sven P. K. Koehler,‡,§,∥ and Greg A. Kimmel*,† †

Physical Sciences Division, Pacific Northwest National Laboratory, MSIN K8-88, P.O. Box 999, Richland, Washington 99352, United States ‡ School of Chemistry and §Photon Science Institute, The University of Manchester, Manchester M13 9PL, U.K. ∥ UK Dalton Cumbrian Facility, The University of Manchester, Moor Row, Whitehaven CA24 3HA, U.K. S Supporting Information *

ABSTRACT: Electron-stimulated oxidation of CO in layered H2O/CO/ H2O ices was investigated with infrared reflection−absorption spectroscopy (IRAS) as a function of the distance of the CO layer from the water/vacuum interface. The results show that while both oxidation and reduction reactions occur within the irradiated water films, there are distinct regions where either oxidation or reduction reactions are dominant. At depths less than ∼15 ML from the vacuum interface, CO oxidation to CO2 dominates over the sequential hydrogenation of CO to methanol (CH3OH), consistent with previous observations. At its highest yield, CO2 accounts for ∼45% of all the reacted CO. Another oxidation product is identified as the formate anion (HCO2−). In contrast, for CO buried more than ∼35 ML below the water/ vacuum interface, the CO-to-methanol conversion efficiency is close to 100%. Production of CO2 and formate is not observed for the more deeply buried CO layers, where hydrogenation dominates. Experiments with CO dosed on preirradiated ASW samples suggest that OH radicals are primarily responsible for the oxidation reactions. Possible mechanisms of CO oxidation, involving primary and secondary processes of water radiolysis at low temperature, are discussed. The observed distance-dependent radiation chemistry results from the higher mobility of hydrogen atoms that are created by the interaction of the 100 eV electrons with the water films. These hydrogen atoms, which are primarily created at or near the water/vacuum interface, can desorb from or diffuse into the water films, while the less-mobile OH radicals remain in the near-surface zone, resulting in preferential oxidation reactions there. The diffusing hydrogen atoms are responsible for the hydrogenation reactions that are dominant for the more deeply buried CO layers.

I. INTRODUCTION Radiation- and photo-induced (“nonthermal”) processes in water and aqueous systems are very important in a broad range of applications, including nuclear technology,1−8 astrochemistry and planetary sciences,9−17 radiation and photocatalysis,18−21 and radiation biology and medicine.22−25 Various types of highenergy radiation (α, β, γ, etc.) interacting with condensed matter generate cascades of low-energy electrons, the most abundant of which have energies below 70 eV.26 Those electrons are responsible for most of the radiation-induced chemical transformations.26,27 For this reason, fundamental studies of radiation chemistry often use low-energy electrons. However, because of their large scattering cross sections and correspondingly short mean-free paths, experiments utilizing low-energy electrons are performed in vacuum with solid forms of wateramorphous solid water (ASW) or crystalline iceas model systems for condensed water. Molecular ices also have direct relevance to astrochemistry and planetary sciences.9−17 Molecular clouds, which include dust grains coated with molecular ices, are a birthplace for molecules in space. The dominant components of the ice mantles are H2O, CO, and CO2,10 and at the typical © 2014 American Chemical Society

temperatures of these clouds (10−100 K), the water is predominantly found as ASW.9,10 Comets and some moons of the gas giant planets are primarily composed of, or covered by, ices.12,15,17,28,29 All of these objects are exposed to various types of radiation including energetic (keV−MeV) ions, UVVUV photons, and cosmic rays. Radiolysis of ices is an essential component of molecular synthesis/decomposition in space.9−17 The ices in space are typically not pure but a mixture of water with various simple molecules like CO, CO2 H2CO, CH3OH, CH4, NH3, and others.9,10,29−31 A substantial number of papers have been published in the past few decades investigating the radiolysis and VUV photolysis of water−molecular ice mixtures for astrochemistry and planetary sciences, including H2O + CO systems.30,32−39 A broad variety of ice compositions, radiation sources, experimental conditions, and techniques were employed, which makes comparing the different results difficult. Previous investigations of H2O + CO ices using infrared (IR) spectroscopy and other methods found two major radiationReceived: September 27, 2014 Revised: October 31, 2014 Published: November 4, 2014 27483

dx.doi.org/10.1021/jp509785d | J. Phys. Chem. C 2014, 118, 27483−27492

The Journal of Physical Chemistry C

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induced processes for CO buried in the ASWhydrogenation/ reduction (mainly to methanol) and oxidation (mainly to CO2).30,32−39 Typically, IR spectra revealed that the products of both hydrogenation and oxidation of CO appeared simultaneously within the irradiated films. The composition of the H2O + CO ice is an important parameter for the experiments. In many cases, H2O + CO gas mixtures with comparable concentrations of both components are dosed on a cold substrate. In these systems, there is a high probability of direct radiation impact on the CO. Moore and coworkers,30,32 Bennett et al.,33 Laffon et al.,39 and Watanabe et al.34 irradiated H2O + CO mixtures with H2O:CO ratios ranging from 0.5:1 to 5:1 at 10−20 K with 0.8 MeV protons, 5 keV electrons, soft X-rays ( 15 ML, more of the CO is reduced to methanol than is oxidized to CO2. The observation of both oxidation and reduction of CO is consistent with earlier investigations.29,30,32−34 For θCap = 10 ML, CO2 accounts for ∼45% of all the converted CO. Another product of radiation-induced oxidation of CO is identified as the formate anion (HCO2−). For pure ASW samples irradiated at low temperatures and then dosed with CO, CO2 is produced, suggesting that OH radicals are the main oxidants. The results presented here provide new insights into the distribution of oxidation and reduction reactions within irradiated water films and the processes leading to these different zones of reactivity within the films.

II. EXPERIMENTAL PROCEDURE The experiments were carried out in an ultrahigh-vacuum (UHV) system that has been described previously,40 using procedures very similar to our previous investigation of the electron-stimulated reactions in layered H2O/CO/H2O systems.40 Here we give a brief overview of the main components and procedures. The system is equipped with a molecular beamline for dosing water, CO, and other adsorbates on the sample, a closed-cycle helium cryostat for sample cooling, a low-energy electron gun (Kimball Physics, model ELG-2), a quadrupole mass spectrometer (Extrel, model EXM720), and a Fourier-transform infrared (FTIR) spectrometer (Bruker, Vertex 70) for infrared reflection absorption spectroscopy (IRAS) performed in external reflection mode. The IR beam is incident on the sample at an angle of 84° with respect to the surface normal. The IR beam path is purged with dry N2, and any residual contributions from gas-phase CO2 and water have been subtracted from the IR spectra. As discussed in the Introduction, the key aim of these experiments is to investigate the nonthermal reactions of water and CO versus the position of a thin CO layer within the water films. Thus, controlling and characterizing the distribution of CO within the water films is crucial for the success of the endeavor.46,47 Since electron-stimulated reactions that can occur at the water/Pt(111) interface41,42,45 are not the focus of the current experiments, a smooth, dense ASW layer of at least 30 ML was initially deposited on the Pt(111) at 100 K.48,49 Various control experiments (data not shown) showed such films are sufficiently thick that reactions occurring at or near the water/Pt(111) interface did not influence the CO− water reactions. The CO was deposited on the ASW spacer layer at T < 30 K and buried under ASW “cap” layers of varying coverages, θCap. The ASW cap layer was constructed by adsorbing 4 ML of ASW at T < 30 K, then setting the temperature to 100 K, and adding additional water to achieve the desired total cap layer coverage. While heating the sample to 100 K to add any additional water to the cap layer, approximately 1/3 of the adsorbed CO desorbs, but the remaining CO is trapped within the film and, in the absence of electron irradiation, will not desorb on the time scale of the experiments. The coverage of the trapped CO, θCO, was ∼2.5 × 1014 cm−2. This procedure was developed based on several control experiments that were conducted to determine the distribution of CO within the ASW films, which are described

H

CO + H → HCO → H 2CO → H3CO(H 2COH) → H3COH

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in our previous paper and its Supporting Information.40 Since previous research has shown that the diffusion of atoms and small molecules in dense ASW is negligible below ∼120 K,50−53 the CO remains trapped within the first 4 ML when additional water is dosed at 100 K. The incident energy of the electrons, Ei, used to irradiate the films was 100 eV, and the instantaneous current densities were ∼1.5 × 1015 cm−2 s−1. The electron beam was smaller than the molecular beam spot size on the sample (∼1.5 mm and 7.5 mm, respectively). To produce a uniform electron fluence across the films, the electron beam was rastered over the surface.42 Low-energy electrons, such as the 100 eV electrons used here, efficiently sputter water films.43 Therefore, to maintain approximately constant ASW cap layer coverages during the experiments, additional water was dosed during the experiments to compensate for the electron-stimulated sputtering. After initially preparing the layered ASW films with the trapped CO, as described above, IRAS spectra were recorded prior to irradiating the films. After electron irradiation, additional water was dosed to account for the amount sputtered during the irradiation, and an IRAS spectrum for the irradiated film was obtained. The sequence of electron irradiation, water dosing, and IRAS was then repeated the desired number of times to obtain a series of IRAS spectra for increasing electron fluences while maintaining an approximately constant water coverage. Because of the small amount of CO initially buried with the ASW films, the electron-stimulated reaction products can be difficult to discern in the raw IRAS spectra. However, the reaction products are more readily observed in differential absorbance spectra obtained by subtracting the irradiated ASW films without buried CO from the corresponding spectra with the buried CO (for the same electron fluences). The difference spectra are particularly useful since electron irradiation leads to spectral changes in the absorbance features of the ASW, even in the absence of CO. These changes can make it difficult to observe the small signals associated with the CO−H2O reaction products. However, even using these differential absorbance spectra, there are some residuals left from the major water bands after subtraction.

Figure 1. IRAS spectra for electron-irradiated 12C16O/H216O films with θcap = 10 ML and Tirr = 100 K. (a) Absorbance spectra for the samples before and after irradiation (black and red lines) as well for a sample irradiated without any coadsorbed CO (green line). (b) Differential absorbance spectra versus irradiation fluence from 0 to 5.9 × 1015 e−/cm2 (black, red, blue, and green lines). The IR spectra of formic acid (HCOOH, pink line) and formaldehyde (H2CO, orange line) buried in H2O are shown for comparison. The spectra for formic acid and formaldehyde have been arbitrarily scaled. The IR spectrum of the formyl anion (HCOO−) in liquid water58 is also shown (light blue line).

III. RESULTS Figure 1a shows the IRAS spectra for a CO/ASW film prior to electron irradiation (black lines) and after irradiation at 100 K with 100 eV electrons and a fluence of 1.6 × 1015 e−/cm2 (red lines). The CO was deposited on a 30 ML ASW layer and was capped with a 10 ML ASW layer as described in the Experimental Procedure section. The spectra are dominated by the broad water bands associated with the OH stretch at ∼3350 cm−1 (ν1 + ν3), the scissors mode (ν2) at ∼1650 cm−1, hindered rotations (νR) at ∼900 cm−1, and combination bands at ∼2240 cm−1 (νR + ν2, 3νR).54−56 The low coverage of CO (∼2.5 × 1014 molecules/cm2) results in a relatively narrow CO peak at 2136 cm−1 (Figure 1), similar to earlier reports.30 The loss of the CO peak due to electron irradiation and the appearance of a CO2 peak (see expanded scale inset) along with changes in the OH stretch region are readily apparent in the IRAS spectra, while other changes can only be seen in differential absorbance spectra such as in Figure 1b. Figure 1a also shows the spectrum of an electron-irradiated neat water film deposited using the same procedure but without CO (i.e., 30 ML ASW plus a 10 ML ASW cap) (green lines).

The black, red, blue, and green lines in Figure 1b show a series of differential absorption spectra from irradiated ASW films with and without trapped CO for increasing electron fluences. For comparison, we also show experimental IR spectra for formic acid (HCOOH, pink line) and formaldehyde (H2CO, orange line) buried in the ASW layer using the same procedure that was used to trap the CO layers. As seen in the figure, new features at 1018, 1352, 1387, 1594, ∼1700, and 2341 cm−1 appear in the irradiated sample. Peaks at 1018, 2831 (not shown), and 2856 cm−1 (not shown) are due to methanol, and the peak at 2341 cm−1 corresponds to CO2. These two molecules were reported earlier as main products of irradiated H2O/CO ices.30,32−39 For the irradiated films, a new peak at 1594 cm−1 (see Figure 1b) is quite similar to the asymmetric stretch of CO in the formate anion HCO2− (Figure 1b, light blue line).57−59 The formate anion has a characteristic doublet due to the CO symmetric stretch and CH bend modes, which we also see in the spectra of irradiated CO at 1352 and 1387 cm−1. A similar triplet of 1589, 1384, and 1353 cm−1 peaks was also attributed to the formate anion in IR spectra of proton irradiated H2O + 27485



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CH3OH ice.60 The formate anion was also discussed as a possible candidate for the small nonresolved shoulder around 1550−1600 cm−1 and a small peak at 1358 cm−1 in an electron irradiated, equimolar mixture of H2O + CO.33 There is no measurable peak of the trans- (cis-) HOCO (hydrocarboxyl) radical observed in the spectra of the irradiated samples around 1848 (1797) cm−1.61,62 HOCO is considered as an intermediate in the CO oxidation with OH radicals to CO2,63 but the published data on this product are controversial. HOCO was reported in the spectrum of electron-irradiated equimolar mixture of H2O + CO33 and among the reaction products of OH radicals with solid CO.62 On the other hand, the HOCO peak was not observed in IR spectra of proton irradiated H2O + CO ice (20:1)30 or in the layered H2O/CO/ H2O films irradiated with electrons,36 while a big CO2 peak was detected in both cases. It is possible that the HOCO radical can only be observed in the systems with relatively high concentrations of CO, which minimizes the probability for this radical to react with the products of water radiolysis. The species responsible for the broad feature near ∼1700 cm−1 in Figure 1b is unclear. Changing the isotopic composition of the CO (12C16O and 13C16O) and the water (H216O, H218O, and D216O) results in red-shifts for both the formate peak and the ∼1700 cm−1 peak relative to 12C16O in H216O (see Figure S1). The ∼1700 cm−1 feature can also be seen in D2O, but it appears relatively smaller and sharper. It is close to the 1715 cm−1 peak from the CO stretch of formic acid (see Figure 1b, pink line).34,64 However, formic acid should also have a strong peak near 1230 cm−1, which is not seen in the spectrum of irradiated CO. The frequency of the νS(CO) stretch mode of formaldehyde buried in ASW is at 1734 cm−1 (Figure 1b, orange line), but the formaldehyde peak is much narrower than the broad peak observed in the irradiated films. Formaldehyde also has the CH2 scissors peak at 1496 cm−1, which we do not see in the spectra of irradiated CO + H2O. (Formaldehyde is observed for CO buried deeper in the ASW layer, where the hydrogenation reactions dominate.40) The ν2 mode of bicarbonate (HCO3−) could also have a peak near ∼1700 cm−1; however, it should also be accompanied by an equally intense peak at ∼1350 cm−1, which is not observed.65−67 Because the ∼1700 cm−1 peak appears to be relatively intense, we also do not expect it to be associated with a highly reactive species, such as HCO2, that might be involved in the reactions leading to the formate anions (see discussion in section IV.b). Figure 2 shows the coverages of CO, CO2, and methanol (derived from the integrated IRAS signals) versus electron fluence for a film where a CO layer was dosed on a 30 ML ASW film, capped with a 10 ML ASW film, and then irradiated at 100 K. The amount of CO decreases exponentially as the fluence increases (Figure 2, black circles). In this experiment, CO2 is the major product. At their maximum yields, about 45% of the CO has been converted to CO2 and 18% to methanol. At higher electron fluences, the CO2 decreases indicating radiation-induced decomposition of CO2 in the ASW. The CO2 yield versus fluence can be nicely fit to two exponentials: θCO2(Φ) = K[exp(−k1Φ) − exp(−k2Φ)], describing the sequential reaction CO −k1→ CO2 −k2→ products, where K = θCO(0) × k1/(k2 − k1) and Φ is the electron fluence. No new products are observed in the IRAS spectra at higher doses, when CO2 decays.

Figure 2. CO (black circles), CO2 (red circles), and CH3OH coverages (green squares) versus electron fluence in irradiated CO/ ASW films for θCap = 10 ML and Tirr = 100 K. The purple triangles and blue diamonds show the (arbitrarily scaled) integrated absorbances of formate (HCO2−) and an unidentified species with a peak at 1700 cm−1, respectively.

The integrated IRAS signals of formate (1594 cm−1) and the 1700 cm−1 feature versus electron fluence are also shown in Figure 2 (purple triangles and blue diamonds, respectively). Since we do not have coverage calibrations for these peaks, the intensities have been arbitrarily scaled. The formate grows with fluence in a fashion quite similar to the CO2, maximizing near Φ ∼ 1.5 × 1015 electrons/cm2, but then decays at a rate that is slower than for CO2. The ∼1700 cm−1 feature evolves differently: It maximizes at a lower fluence near Φ ∼ 0.7 × 1015 electrons/cm2 and then decays. In control experiments, we have irradiated CO2 buried in ASW under a 10 ML cap. In that case, the CO2 signal decays with a rate comparable to the CO2 decay in Figure 2 (see Figure S2a). The methanol yield from irradiated CO2/ASW films is approximately half its yield in CO/ASW films with the same θCap. On the other hand, both the formate peak at 1594 cm−1 and the ∼1700 cm−1 feature are detected in the irradiated CO2/ASW films (see Figures S1 and S2b). The formate yield from the CO2/ASW films is comparable to the formate yield from CO/ASW films. No other products are seen in the IR spectra. From the data in Figure 2 and Figure S1, one might assume that formate (and the 1700 cm−1 feature) is produced from CO2, which is itself produced when irradiating the CO/ ASW films, e.g., CO → CO2 → HCOO−. However, if this was the case, then we would expect a kinetic signature of such a reaction scheme. Specifically, the production of the formate would be delayed relative to CO2. Instead, the initial formate and CO2 kinetics are similar (see Figure 2). This indicates that in the CO + H2O system, most of the formate is produced directly from CO and not from CO2. From the data presented in Figure 2, the initial reaction yields, which we define as the number of reactions per incident electron, can be determined for a cap layer coverage of 10 ML. Similar experiments were performed for a range of cap layer coverages from 4 to 60 ML. Figure 3 shows the initial yields for the CO decomposition and the CO2 and CH3OH accumulation as a function of the H2O cap layer coverage. Because the number of reactions per incident electron decrease as θCap increases, the maximum electron fluence used for each coverage increased from 4 × 1014 electrons/cm2 for 4 ML to 1.2 × 1017 electrons/cm2 for 60 ML. The CO reaction yield decreases 27486



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Figure 3. CO (black circles), CO2 (red circles), and CH3OH (green squares) reactions per incident electron versus θCap. The IRAS signal for formate (arbitrarily scaled) is also shown (purple triangles). CO is lost, and the other species are produced during electron irradiation. Inset: the CO reactions per incident electron for larger range of cap layer coverages. The open circles show the results from ref 40.

dramatically from ∼2 to ∼0.0015 reactions per electron for 4 ML ≤ θCap ≤ 60 ML (Figure 3, black circles). Among the products, CO2 has the largest yield for θCap < 15 ML with its yield decreasing from ∼0.4 to ∼0.0013 reactions per electron for 4 ≤ θCap ≤ 30 ML (Figure 3, red circles). For the thinnest ASW cap layers, electron-stimulated desorption (ESD) of the CO is significant (see Figure S3), limiting the amount of CO2 that is produced. For CH3OH, the initial yield decreases from ∼0.05 to ∼0.0015 reaction per electron for 4 ≤ θCap ≤ 60 ML (green squares). For θCap > 15 ML, methanol is the most common reaction product, and for θCap > 30 ML nearly all the CO is converted to methanol. Since we do not have a coverage calibration for the formate peak, we have calculated the initial slope of the HCOO− integrated IR signal versus electron fluence, which is also shown in Figure 3 in purple triangles (arbitrary scale). The formate yield decreases by ∼300 times for 4 ≤ θCap ≤ 40 ML. We have previously investigated the electron-stimulated reactions for films with cap layer coverages as large as 400 ML.40 Those experiments showed that for θCap > 60 ML the CO reaction rate was proportional to 1/θCap while for θCap < 60 ML, the CO reaction rate increased significantly. The inset to Figure 3 shows the CO reactions per incident electron from the earlier results (open circles) along with the results presented here (solid circles). The results of both experiments are consistent and again show a change in behavior for the cap layer coverage of ∼60 ML. The temperature dependence of the electron-stimulated reactions in the buried CO layers depends on the position of the CO layer within the ASW films. For example, the electronstimulated reactions per incident electron for CO with θCap = 14 ML decreases by a factor of ∼3 when the irradiation temperature decreases from 100 to 20 K, as depicted in Figure 4a (black symbols). Accumulation of the main product, CO2, also slows down correspondingly (red symbols). The subsequent radiation-induced CO2 decomposition is suppressed even more significantly at 20 K. The slower loss of CO2 at 20 K is consistent with control experiments where the

Figure 4. CO coverage (black circles) and CO2 coverage (red circles) versus electron fluence in irradiated CO/ASW films. The filled and open symbols are for irradiations performed at 100 and 20 K, respectively. (a) θCap = 14 ML. (b) θCap = 45 ML.

loss of buried CO2, irradiated at 100 and 20 K, was measured directly (see Figure S4). The modest temperature dependence of the reactions for θCap = 14 ML (Figure 4a) is in sharp contrast to the temperature dependence for θCap = 45 ML, where the CO decay probability decreases by more than 2 orders of magnitude in the same temperature range (Figure 4b). Here, CO hydrogenation is the dominant process, and methanol is the main product. The results in Figure 4b are consistent with the dramatic temperature dependence of hydrogenation reactions reported earlier for CO deeply buried in ASW.40 For thin ASW cap layers, the other productsformate and the species responsible for the peak at ∼1700 cm−1are also observed in the samples irradiated at 20 K (Figure S5). For formate, the initial reaction yield is significantly lower at 20 K, while the other species is similar at both temperatures. As a result, the peak at ∼1700 cm−1 appears before the formate at 20 K, while the two species appear simultaneously at 100 K. The distinct kinetics for the two peaks shows that they are associated with different products. To explore the mechanism(s) by which CO2 is produced, we have performed a series of experiments where an ASW film was irradiated with 100 eV electrons at 20 K and then annealed to various temperatures or irradiated at 100 K. Next, CO was adsorbed onto the preirradiated ASW films and capped with a 15 ML ASW film. The amount of CO2 produced was then monitored via IRAS (Figure 5a) or by measuring the amount of CO2 that desorbed concomitantly with the water during temperature-programmed desorption (Figure 5b). For an ASW film preirradiated at 20 K without any further annealing, a substantial amount of CO2 is produced (Figure 5a, red line), 27487



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induced oxidation of CO in low-temperature ices occurs via a barrierless reaction between thermalized CO and OH radicals: CO + OH → CO2 + H.33,34,39,62,68−71 As we will discuss later, these results are also in agreement with earlier experiments on the mobility and reactions of OH radicals in ASW films in the temperature range ∼60−100 K.45,72

IV. DISCUSSION a. Distance-Dependent Radiation Chemistry. As discussed above, previous studies30,32−39 have reported the simultaneous radiation-induced hydrogenation (mainly to methanol) and oxidation (mainly to CO2) of CO in CO/ H2O films. On the other hand, only hydrogenation products (HCO, CH2O, and CH3OH) were observed in our previous study when CO is positioned deep enough (60−400 ML) from an ASW/vacuum interface that was exposed to 100 eV electrons.40 Here, we have explored smaller cap layer coverages (4 ML ≤ θCap ≤ 60 ML) for the buried CO layer. For these smaller cap layer coverages, oxidation products such as CO2 and formate are observed, and they become the dominant products for θCap < 15 ML (Figure 3). The fraction of methanol among the products decreases with decreasing θCap and becomes negligible for θCap < 10 ML (Figure 3). These results show that placing CO at different positions within the ASW films allows one to map zones with very different radiation chemistrypreferential oxidation in the near-surface region and preferential hydrogenation further into the ASW films. Figure 6 illustrates these zones within the ASW film exposed to energetic electrons from the vacuum side. The various reactions occurring within irradiated ASW films suggest a plausible explanation for the different reactions zones depicted in Figure 6. In particular, H atom desorption from,73−75 and diffusion into,40 electron-irradiated ASW films play a significant role in creating an oxidizing region near the

Figure 5. (a) IRAS spectra for 13CO dosed at 20K on H216O film preirradiated with electrons at 20 K (middle red trace) and 100 K (upper blue trace), as well as without irradiation (lower black trace) and then capped with θcap = 15 ML at 20 K Inset: expanded 13CO peak showing the loss of CO for the film irradiated at 20 K. (b) Amount of CO2 produced versus annealing temperature for ASW films preirradiated at 20 K prior to the CO dose. For this experiment, the CO2 yield was measured by monitoring the CO2 desorption during temperature-programmed desorption (TPD) of the ASW films.

with a corresponding decrease in the CO signal (Figure 5a, inset). In contrast, very little CO2 is produced for a film preirradiated at 100 K (Figure 5a, blue line). For the ASW films preirradiated at 20 K and then annealed prior to the CO dose, the CO2 production decreases to zero in the annealing temperature range between 50 and 100 K (Figure 5b). However, annealing the preirradiated films af ter the CO is dosed and capped with water results in very little change to the CO and CO2 peaks (data not shown). The results in Figure 5 provide important information regarding the mechanism of radiation-induced oxidation of CO buried in an ASW matrix. In these experiments, the CO reacts with products of water radiolysis that remain on the ASW surface after the end of irradiation. Therefore, the data indicate that CO oxidation can proceed by an indirect process; i.e., it does not necessarily involve any direct electron-induced impact on the carbon monoxide. The water radiolysis products that oxidize the CO are thermalized and stable at 20 K. Furthermore, the reaction of CO → CO2 is efficient at 20 K, indicating it is practically barrierless. These observations are consistent with earlier studies indicating that the radiation-

Figure 6. Schematic illustration of the oxidation and hydrogenation “zones” within an ASW film that is irradiated with 100 eV electrons. Ionizations and electronic excitations of H2O by the incident electrons generate oxidizing and reducing species in the region near the ASW/ vacuum interface. A significant part of the primary products react, recombine, or desorb into the vacuum. However, some higher mobility products, such as hydrogen atoms or hydronium ions, can diffuse into the film, creating a zone of preferential hydrogenation for a deeply buried CO. Conversely, the region near the vacuum interface is enriched with oxidizing products, such as OH radicals, that are less mobile and less likely to desorb. CO layers placed in this region are preferentially oxidized. Thus, by changing the location of the CO layer, one can tune the chemical environment that the CO encounters within the irradiated ASW films. 27488



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surface of the film and a reducing region further in. When an energetic electron penetrates into the ASW film, it initiates a cascade of events. For 100 eV electrons, ionizations and excitations of water molecules are most likely within the first few water layers, and the majority of these events occur within the first ∼20 ML.41,42,76,77 Electronically excited water molecules, formed by direct excitation or via electron−ion recombination, can dissociate leading to H atom desorption, while leaving behind an OH.73,75 H atoms can also diffuse into the ASW films,40 again leaving behind an OH. As a result, the surface and near surface region of electron-irradiated ASW films becomes enriched with hydroxyls and other oxidizing species such as H2O2 and HO2.60,61 H241,42,78,79 and O73,75 also desorb during electron irradiation, but their yields are smaller, while H2O desorption does not change the balance of oxidizing and reducing agents. Since the electron-stimulated desorption of O2 proceeds through a sequence of reactions involving OH, H2O2, and HO2 intermediates,45,80 it also provides evidence of an oxidizing environment near the water/vacuum interface. In addition to contributing to the creation of the oxidation zone, H atom diffusion into the ASW films creates the reducing zone within the film. As result, CO (or other molecules) deposited at different places within the ASW film can be oxidized or hydrogenated, as seen experimentally. b. Insights into Mechanism of Radiation-Induced Oxidation in CO/H2O Systems. The major mechanism of radiation-induced oxidation in CO/H2O systems is associated with the reaction of OH radicals with CO:33,34,39,62,68−71,81 CO + OH → CO2 + H

the shape and intensity of the H2O bending mode band change under electron irradiation, it is difficult to observe these features unless one looks at the difference between IR spectra for irradiated films with and without buried CO. Second, most of our irradiations are performed at 100 K, which is the optimal temperature to maximize both of these peaks. Most of the previous studies were performed at significantly lower temperatures (e.g., 10−20 K), where these peaks are much smaller. Other parameters, such as the radiation type, fluence, etc., may also make these peaks harder to observe. In our experiments, the reactions leading to the production of formate are uncertain. However, a potential reaction pathway starts with the reaction of CO and OH. This reaction involves the formation of an energized HOCO* radical (in either cis- or trans-forms) that can decompose into reactants, proceed to products, or become thermally stabilized through collisions with a third species:63,68

The HOCO radical is extremely reactive, producing CO2 when reacting with OH, HO2, O2, or O, or producing CO when reacting with H radicals.63 trans-HOCO can also isomerize to the formyloxyl radical, HCO2, which is metastable with respect to dissociation into CO2 + H:83−87 trans‐HOCO → HCO2 → CO2 + H

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The formyloxyl radical, which has been studied in the gas phase via photoelectron detachment from the formate anion (HCO2−), is also very reactive. It has a large electron affinity (3.5 eV) for converting to formate.83 Thus, if the formyloxyl radical forms in the CO + OH reaction, it could be efficiently converted to the formate anion by capturing an electron, which are readily available in the zone of primary ionizations and excitations (see Figure 6):

(2)

The data presented here support this reaction. For example, CO2 was produced at 20 K when CO was dosed onto preirradiated ASW films (Figure 5a). The low temperature at which the reaction is observed suggests that the CO2 is produced in a barrierless reaction between CO and one of the products of that remains on the ASW surface after electron irradiation. Previous studies of the O2 production in electronirradiated ASW films indicated that OH, H2O2, and HO2 are involved in a series of reactions leading to O2.44,45 Those studies showed that the concentration of OH was the largest for films irradiated at 20 K and decreased at higher temperatures due to increases in thermally activated reactions involving OH (e.g., OH + OH → H2O2 and OH + H2O2 → HO2 + H2O). In contrast, the concentrations of H2O2 and HO2 both are expected to increase at higher annealing or irradiation temperatures. Thus, the observed decrease in CO2 production versus annealing temperature for preirradiated ASW films (Figure 5b) closely matches the expected decrease in the OH concentration at the surface of the ASW films versus temperature45 and supports reaction 2 as a pathway for CO2 production. Because the reaction CO + O → CO2 has a significant activation barrier,70,82 this reaction is unlikely to be important for the CO2 produced by CO adsorption on preirradiated ASW films (Figure 5). However, since electron irradiation of water films produces energetic oxygen atoms,73,75 this reaction could potentially play a role in experiments with irradiated coadsorbed CO and ASW. For θCap < 35 ML, formate (HCO2−) and the unidentified peak near 1700 cm−1 are observed in the irradiated films. There is not much information in the literature on irradiated CO + H2O ices regarding these peaks, and there may be several reasons for this. First, these peaks are relatively broad, and they overlap with the intense band of the H2O bending mode. Since

CO + OH → HCO2 + e− → HCO2−

(5)

The formyloxyl radical can also be created from CO2 reacting with H atoms,84 which could explain the formate peak in IR spectra of irradiated CO2 on ASW (Figure S1): CO2 + H → HCO2 + e− → HCO2−

(6)

While it is plausible that the reactions leading to the observed formate anions involve HCO2, it is unlikely that the HCO2 is responsible for the 1700 cm−1 peak (see Figure 1b). In particular, HCO2 should be sufficiently reactive that its concentration within the irradiated films will be quite small. Obviously, more work needs to be done to assign this peak and understand the mechanism of its formation. For CO layers within ∼20 ML of the ASW/vacuum interface, the oxidation of CO to CO2 is weakly temperature dependent (Figure 4a), while for more deeply buried CO layers the hydrogenation of CO to methanol depends more strongly on the temperature (Figure 4b). The weak temperature dependence for CO oxidation probably reflects several factors. First, the ionizations and excitations that lead to the production of OH do not depend strongly on temperature,45 and since the CO + OH reaction can proceed even at low temperatures (Figure 5), CO oxidation is relatively efficient even at 20 K. Second, nonthermal effects within the penetration range of the incident electronssuch as electron-stimulated diffusion of various reaction products44may sustain the reactions at low 27489



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temperatures where otherwise the low diffusion rates of the reactants would limit the reaction rate. In contrast, for more deeply buried CO layers where the hydrogenation of CO to methanol is the dominant reaction, the reactants are thermalized and the H + CO reaction has a barrier88 that introduces a stronger temperature dependence to the reaction.40 It is also likely that the slow diffusion of H atoms into the bulk at 20 K limits the reaction rate.88

ASSOCIATED CONTENT

S Supporting Information *

Figure S1: the IR spectra for irradiated water films with various combinations of isotopes for both the CO and water; Figure S2: a comparison of the electron-stimulated reactions in CO/ ASW and CO2/ASW films, as measured with IRAS; Figure S3: the amount of CO2 and methanol produced by electron irradiation as a function of the water cap layer coverage; Figure S4: the amount of CO2 versus electron fluence in CO2/ASW films irradiated at 20 and 100 K; Figure S5: the amounts of CO, DCO2−, and the unidentified species at ∼1700 cm−1 (∼1681 cm−1 in CO/D2O films) versus electron fluence in CO/D2O films irradiated at 20 and 100 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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V. SUMMARY We have investigated the electron-stimulated reactions of CO layers buried in ASW films with 4 ML ≤ θCap ≤ 60 ML. Depending on θCap, both oxidation and reduction of the CO can be observed with infrared spectroscopy. For θCap < 15 ML, CO2 is the main oxidation product, accompanied by formate anion (HCO2−). For θcap ∼ 10 ML where the CO2 yield is maximized, it accounts for approximately 45% of all converted CO. Hydrogenation (to methanol) becomes the most important reaction channel for θCap > 15 ML, and above ∼35 ML the CO−methanol conversion efficiency is close to 100%. The observed distance-dependent radiation chemistry is associated with mobile hydrogen atoms diffusing deeper into the film and leaving behind a zone of preferential oxidation closer to the surface, while the less-mobile OH radicals remain in closer to the surface initiating the oxidation reactions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (N.G.P). *E-mail [email protected] (G.A.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. The work was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830. R.J.M. thanks the Dalton Cumbrian Facility program in part funded by the Nuclear Decommissioning Authority for financial support for her research visit to PNNL. 27490



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Distance-Dependent Radiation Chemistry: Oxidation versus

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