Adsorption of Water, Methanol, and Formic Acid on Fe2NiP, a

Article pubs.acs.org/JPCC

Adsorption of Water, Methanol, and Formic Acid on Fe2NiP, a Meteoritic Mineral Analogue Danna Qasim,‡ Logan Vlasak, Aaron Pital, Thomas Beckman, Nsamba Mutanda, and Heather Abbott-Lyon* Department of Chemistry and Biochemistry, Kennesaw State University, Kennesaw, Georgia 30144, United States S Supporting Information *

ABSTRACT: The surface of an analogue to the meteoritic mineral schreibersite or (Fe,Ni)3P was investigated to provide insight into the interaction of the mineral surface with prebiotic molecules such as water, methanol, and formic acid. A protocol for creating synthetic metal-phosphide samples with a surface reflectivity suitable for reflection−absorption infrared spectroscopy (RAIRS) was developed and is outlined in this paper. Scanning electron microscopy coupled with energy dispersive spectroscopy revealed an average defect size less than 1 μm and evidence of subsurface phosphorus segregation. At surface temperatures between 120 and 140 K, RAIRS spectra indicate that water and formic acid interact molecularly with surface atoms, while methanol appears to dissociate into methoxy and protons upon adsorption. The observed infrared spectra provide insight into the adsorption geometries of these prebiotic molecules on synthetic schreibersite. This data suggests the importance of the schreibersite mineral surface in aqueous-phase schreibersite-mediated phosphorylation experiments that have been performed by others and strengthens the argument that schreibersite-induced chemistry could occur in astrochemical environments.

1. INTRODUCTION The heterogeneous catalytic properties of nickel phosphides, iron phosphides, and iron−nickel phosphides have been studied in a number of hydrotreating processes and in electrocatalysis.1−8 In hydrodesulfurization (HDS), nickel phosphides are highly active catalysts, in part due to the radius of phosphorus. The phosphorus atom is large enough to be positioned laterally with transition metals at the surface and can, therefore, participate in bonding with adsorbates.2 Iron and phosphorus have also exhibited a joint effort in enhancing the catalysis that is induced by nickel within an iron−nickel phosphide sample. Because the nickel to phosphorus charge transfer is small,2 and iron increases the electron density on nickel because of nickel’s higher electronegativity,8 nickel is able to act as a very electron-rich metal in these crystal structures. In addition, the presence of phosphorus has been shown to protect the nickel’s catalytic performance by way of the ensemble effect, an effect based on the number of surface atoms.2 Such examples demonstrate how iron−nickel phosphides have become useful catalysts in today’s industries. Naturally occurring iron−nickel phosphide is classified as the mineral schreibersite or (Fe,Ni)3P, which is typically found in iron and stony-iron meteorites.9 Significant quantities of schreibersite are believed to have been delivered to the early Earth during the Late Heavy Bombardment.10,11 Based on the promising applications of iron−nickel phosphides in heterogeneous catalysis, it is not surprising that schreibersite is currently under speculation for catalyzing chemical reactions on the early Earth that may have contributed to the origin of life. A number of recent studies on (Fe,Ni)3P show that schreibersite is able to © XXXX American Chemical Society

phosphorylate biomolecules and produce reactive phosphorus compounds in aqueous solutions.12−17 Despite this, there is little information on the mechanism of forming these phosphorylated products. Because the majority of schreibersite corrosion and phosphorylation reactions occur in the presence of the schreibersite mineral, we hypothesize that the mineral surface has a role in product formation by participating as both a source of phosphorus or reactive intermediates and a catalyst for the phosphorylation reaction. Molecules that are ideal candidates to interact with the schreibersite surface are species that are regarded as common molecules on the early Earth. Water (H2O), methanol (CH3OH), and formic acid (HCOOH) are prebiotically relevant and have been investigated on model systems that are comparable to schreibersite. In addition to their prebiotic relevance, these molecules also make excellent model systems because they have a vapor pressure high enough to be easily dosed into a vacuum system. Water, colloquially referred to as the elixir of life, has been detected in all stages of evolution in space: in jets emanating from stars,18 molecular clouds in the interstellar medium,19 and protoplanetary disks far from our Solar System,20 to nearby comets, asteroids, and Mars.21−23 The adsorption of H2O on iron and nickel terminated surfaces has been described in the literature,24−32 and the majority of the spectra from reference materials show that water forms clusters on iron and nickel surfaces at low temperatures (i.e., Received: February 9, 2017 Revised: June 1, 2017

A

DOI: 10.1021/acs.jpcc.7b01312 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Top view schematic of the experimental apparatus used for these experiments. The red lines represent infrared light traveling along a classical RAIRS pathway of approximately 80° to the surface normal.

below ∼200 K). In particular, water dimers were observed to chemisorb to both nickel and oxygen surface atoms using ultraviolet photoelectron spectroscopy (UPS) on a Ni(110) surface.29 Water can break apart into hydroxyls and protons at surface temperatures between 200−300 K. The dissociation of water and subsequent formation of a hydroxylated surface can happen at low surface temperatures, if the surface is covered with less than a monolayer of oxygen.26 On iron oxide surfaces, adsorption of water at temperatures below 200 K results in only weak physisorption.30,31 Hydroxyls have been observed on oxygen-depleted iron oxide surfaces, primarily at Fe2+ sites. On nickel oxide surfaces at low temperatures, it was observed that H2O dissociatively adsorbs at low coverage (10 langmuir dose) and molecularly chemisorbs at higher coverage (30 langmuir dose).32 CH3OH and HCOOH are organic molecules suggested to be common prebiotic species33,34 and contain an −OH group, which is the typical functional group that is phosphorylated in biochemical systems. Most of the experiments for CH3OH adsorption on iron- and nickel-containing substrates suggest that CH3OH dissociates into methoxy (CH3O−) at a wide range of surface temperatures,35−41,43 although there are few examples of molecular nondissociative adsorption.42 In some cases the dissociation of methanol was linked to defect sites on the surface.37 The O−H stretching frequency is not always observed. In the case of CH3OH adsorption on nickel oxide, it was proposed that the lack of an O−H vibrational frequency in the infrared region was due to the hydroxyl hydrogen interacting with a lattice oxygen.43 Formic acid adsorption studies have illustrated that HCOOH is able to both molecularly and dissociatively chemisorb on the surface of several iron- and nickel-containing surfaces.35,44−47 Dissociative adsorption of formic acid as formate anions CHO2− has been observed at surface temperatures above 160 K and is typically identified by the O−C−O symmetric and asymmetric stretches (∼1350 and 1550−1600 cm−1, respectively).35,44−47

To study the interaction between a chemically active surface and the surrounding molecules, reflection−absorption infrared spectroscopy (RAIRS) has been exploited in numerous surfacescience experiments. However, because RAIRS requires a surface that is both reflective in the infrared region and also has sufficient conductivity to allow an image dipole to be created, it has typically been performed on single-crystal metal surfaces or thin metal-oxide films grown on single-crystal metals. Transmission infrared spectroscopy has been performed on metal phosphides.4,8,48 These studies showed that compared to the nickel−nickel distance, the iron−iron distance in Fe2P is relatively large and allows for multiple CO molecules to adsorb to one site.8,48 Previous experiments have revealed that nickel is more susceptible to interacting with CO, giving rise to a sharp and intense peak at approximately 2079−2086 cm−1, than iron is, which results in a broad and weak peak at approximately 2000−2005 cm−1.8 Because of the strength of the interaction and the angle of adsorption, the infrared signal for CO adsorbed on Ni2P particles was much stronger than on Fe2P.8,48 Also, Prins and Bussell discuss that while monometallic metal phosphides have good activity for catalytic processes such as hydrodeoxygenation, bimetallic phosphides are likely to provide better reactivity and should be more thoroughly screened for catalytic activity.4 It should be noted that we recently published two RAIRS spectra for water adsorption on synthetic schreibersite or Fe2NiP illustrating the temperature dependence of water adsorption on the mineral surface.49 However, the work presented here is the first detailed investigation of adsorption on a metal phosphide surface using RAIRS. Here, we report the development of a synthetic metalphosphide, meteoritic mineral surrogate that is conducive to RAIRS. The RAIRS spectra of H2O, CH3OH, and HCOOH on Fe2NiP at low surface temperatures (120−140 K) have been measured. A step-by-step procedure on the formation of a flat and reflective metal-phosphide sample is outlined. Additionally, a record of the change in the surface morphology and B


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Figure 2. Elemental maps of (a) iron, (b) nickel, and (c) phosphorus superimposed on a secondary electron image of a region of the polished sample.

user facility at the Georgia Institute of Technology. An accelerating voltage of 10 kV was used. 2.2. Development of a Metal-Phosphide Mineral Surface Conducive to RAIRS. Although we have published two RAIRS spectra of water on schreibersite previously,49 this is the first detailed RAIRS study of a metal phosphide surface. The development and theory behind RAIRS spectroscopy on metal surfaces is well-established.50,51 Briefly, the primary factors governing RAIRS signal intensity are the index of refraction (n) and extinction coefficient (k) of the conductive surface and the dielectric response of the substrate at infrared frequencies. To the best of our knowledge, these values have not been measured for metal phosphides such as schreibersite. More generally, RAIRS signal intensity requires a surface that is both reflective in the infrared region and also has sufficient conductivity to allow an image dipole to be created. An image dipole is a shift in the electron density within a surface that results when an ion, a polar molecule, or an induced dipole resulting from a vibrational motion is brought into close proximity with a conductive surface. The dipole moment of the adsorbed species is mirrored by freely moving electrons within the conductive surface. Although they are not metals, iron and nickel phosphides have been characterized as very low band gap materials (i.e., 0.50 eV or less),5 showing low resistance and the flow of electrical current when measured by a standard multimeter. Iron−nickel phosphides have other similarities to metals including their density (c.f., 7.0−7.8 g·cm−3 for schreibersite and 7.87 g·cm−3 for iron) and metallic luster when polished. Therefore, we anticipated that metal phosphides like synthetic schreibersite might produce an image dipole and obey the same selection rules as metals.52,53 The experimental results presented below provide experimental confirmation for this hypothesis, and supporting theoretical calculations for this system would be highly desirable in the future. Unfortunately, iron−nickel phosphides are not available commercially, and there is currently no procedure for growing single-crystals of these substances. A procedure for the creation of solid samples smooth enough for RAIRS measurements is outlined here. Naturally occurring schreibersite has a slightly variable composition designated by the chemical formula (Fe,Ni)3P. Meteoritic schreibersite generally contains an empirical formula of Fe2NiP, which is the ratio used in the synthetic schreibersite studied in this work. Powders of iron, nickel, and red phosphorus (2:1:1) were heated under an argon atmosphere at 820 °C for 235 h by La Cruz.49 The sample was then pulverized, sieved, and pressed into an iron plate with a hydraulic press capable of applying 20 tons of pressure. To

composition throughout sample development is presented. At 135 K in an anoxic environment, methanol is shown to form bonds with surface atoms and dissociate upon contact with the surface. This suggests that chemistry may occur on schreibersite grains within asteroids, comets, and even circumstellar envelopes (i.e., environments that are not rich in liquid). Proposed adsorption geometries of H2O, CH3OH, and HCOOH on Fe2NiP are also discussed. These results illustrate that even at surface temperatures much lower than the temperatures found in aqueous-phase experiments (c.f., 135 K versus 353 K),14 the synthetic schreibersite of the Fe2NiP surface is chemically active.

2. MATERIALS AND METHODS 2.1. Ultrahigh Vacuum Experimental Setup. A schematic of the ultrahigh vacuum (UHV) experimental apparatus, which reaches a base pressure of ∼3 × 10−10 Torr, is shown in Figure 1. Details about the chamber and sample manipulator can be found in the Supporting Information. Synthetic iron−nickel phosphide (Fe2NiP) and iron−nickel (FeNi) surfaces were cleaned between experiments by three cycles of argon ion bombardment (30 min with an argon pressure of ∼5.0 × 10−5 Torr) and flashing (set-point temperature of 550 K). Dissolved gas impurities in H2O (deionized), CH3OH (Sigma-Aldrich, 99.9% purity), and HCOOH (Sigma-Aldrich, ≥95% purity) were removed by three freeze−pump−thaw cycles with liquid nitrogen. Before each new type of molecule was dosed via a variable leak valve, the gas manifold was baked and then passivated by repeated exposure to a high pressure of the molecule of interest. Ice thicknesses were reported by dosage where 1.0 langmuir is equivalent to 1.0 × 10−6 Torr·s of exposure to an ambient gas. Assuming the molecular sticking coefficient is unity and the surface is close packed with about 1 × 1015 atoms·cm−2, when the surface temperature is low (120−140 K), a 1.0 langmuir dose would result in approximately one monolayer of adsorbed molecules on the surface. Reflection−absorption infrared spectroscopy (RAIRS) measurements were acquired by reflecting infrared light off the surface at a grazing incidence angle of approximately 80° from the surface normal (i.e., 10° from the plane of the surface). Each spectrum was acquired using at least 250 scans with a spectrometer resolution of 4 cm−1 and is referenced to a singlebeam spectrum of the clean synthetic Fe2NiP surface. Scanning electron microscopy images were acquired using a Hitachi SU8230 Cold Field Emission SEM/EDS (scanning electron microscopy coupled with energy dispersive spectroscopy) system at the Institute for Electronics and Nanotechnology C


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Figure 3. Secondary electron images of the synthetic schreibersite surface (a) before and (c) after flattening and polishing the surface with Buehler products. An elemental map of phosphorus (b) superimposed on the secondary electron image (a).

form a robust solid sample, the sample was sintered under an argon atmosphere at 950 °C for 1 h. Pressing under high pressure and sintering the Fe2NiP was necessary because iron− nickel phosphides are very hard and very brittle. The procedure for flattening and polishing the sample is described in detail in the Supporting Information. Characterization of the sample throughout the development procedure was conducted by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) in order to obtain maps and spatial distribution of elements on the surface and identify elements in the near-surface region. Figure 2 illustrates phosphorus segregation in the near-surface region across the sampled area. X-ray diffraction data of similar synthetic schreibersite samples suggests that the phosphorusdepleted areas are iron−nickel alloys.49 This segregation was most likely caused by heating of the sample to 950 °C during sintering to create a solid sample sturdy enough for polishing. Figure 3 shows images of the sample before (a) and after (c) flattening and polishing. Additionally, an elemental map of phosphorus on the unpolished sample is shown in Figure 3b. The lack of bright features in Figure 3c is an indication of the reduction of edges within the probed area because the probability that an electron will escape from an edge is higher than the probability that an electron will escape from a flat terrace (i.e., the edge effect).54 The lattice structure of iron− nickel alloy minerals like taenite and kamacite is isometric hexoctahedral (i.e., simple cubic), while the lattice structure of schreibersite is a tetragonal disphenoid.55,56 This difference in crystal structure may play a role in the apparent surface roughness of the schreibersite regions compared to the phosphorus-depleted regions as measured by SEM/EDS. Regardless, the Fe2NiP surface shown in Figure 3c still retains many defect sites and, therefore, is similar to the mineral’s naturally rough topography in meteoritic samples, while still being smooth enough for RAIRS experiments. Electron diffraction spectroscopy (EDS) was used to determine the composition of the sample and to verify whether any contamination was present. It should be noted that EDS probes the near-surface region, and in these experiments, the maximum probe depth was approximately 1 μm below the surface. Table 1 lists the atomic percentages for both unpolished and polished Fe2NiP samples obtained by EDS. The reduction of phosphorus found after polishing the surface plus the elemental map shown in Figure 2c suggest that phosphorus is concentrated in areas of the sample that are not flat. This concentration of phosphorus in rough regions is visually apparent when comparing panels a and b of Figure 3. The correlation of the phosphorus concentration to the

Table 1. Composition of Synthetic Schreibersite Samples by SEM/EDSa percent composition (%)

a

element

unpolished

polished

Fe Ni P C O

53.6 15.9 18.2 9.7 2.6

58.4 18.3 10.2 9.7 3.4

±0.5% uncertainty in measurements.

morphology of the subsurface, in addition to showing dependency on the thermal history of the sample, further supports the argument that the mineral is more than just a source of phosphorus. To test the reproducibility of our procedure for creating synthetic schreibersite samples, the procedure described above and in the Supporting Information was used to produce a synthetic Fe3P sample from commercially available Fe3P powder (Alfa Aesar, 99.5% purity, 40 mesh). SEM reveals that the Fe3P surface has more defect sites than the Fe2NiP sample used in these experiments, while EDX analysis reveals a similar distribution of iron, phosphorus, and oxygen on the surface with a significantly higher percentage of carbon in the sample (see Figure S1).

3. RESULTS AND DISCUSSION 3.1. RAIRS of H2O on Fe2NiP. Figure 4 shows the RAIRS spectra after depositing increasing amounts of H2O onto a synthetic Fe2NiP surface. The experimentally observed RAIRS frequencies and their relative intensities are 3420−3413 cm−1 (strong), 878−812 cm−1 (medium), and 1649−1548 cm−1 (weak). The spectral features in Figure 4 have been compared to vibrational data from experiments with model systems similar to schreibersite, as outlined in Table 2.25−29,32 Some of these experiments incorporated high-resolution electron energy loss spectroscopy (HREELS) to probe the vibrational frequencies. HREELS and RAIRS are both sensitive to dipole interactions at the ice−mineral interface and should yield similar frequencies for the same sample under investigation, as observed in Figure 1 in Frederick et al.57 Based on these comparisons, the peak at 3420 cm−1 is assigned to the hydrogen-bonded O−H stretch of H2O because the free (i.e., nonhydrogen-bonded) O−H stretch has a stretching frequency of ∼3580 cm−1 for H2O adsorbed on oxidized iron surfaces.26 Because hydrogen bonding lowers the D


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be either direct, the result of the adsorbate molecules interacting with surface phosphorus atoms, or they could be indirect, the result of changes in the electronic structure of the solid as a result of phosphorus incorporation (i.e., synergistic effects). Because of the shift of the O−H stretching frequency of H2O bound to an FeNi versus an Fe2NiP substrate, water most likely interacts with phosphorus on the surface. Therefore, we propose that water is binding molecularly to Fe2NiP through the interaction of the oxygen atom in H2O with P atoms in the surface, leaving the hydrogens of H2O extending into the vacuum. This is further supported by RAIRS and TPD spectra of H2O on Fe2NiP presented in La Cruz et al.49 3.2. RAIRS of CH3OH on Fe2NiP. Figure 5 shows the vibrational spectra after a synthetic Fe2NiP surface was exposed

Figure 4. RAIRS spectra of H2O adsorbed to Fe2NiP with a dosage of 0.2−1.0 langmuir. Experiments were run with a sample temperature of 125 K. Each spectrum shown is referenced to a single-beam spectrum of the clean, synthetic Fe2NiP surface.

potential energy of the O−H potential well,58 the hydrogenbonded O−H stretch has a lower frequency than the free O−H stretch. The shoulder along the O−H peak in Figure 4 at ∼3305 cm−1 suggests small island formation of amorphous H2O ice on the surface rather than two-dimensional layer growth.51 The libration mode for H2O observed at 891−810 cm−1 also indicates that water ice is on the surface. A blue-shift in the libration is observed with increasing coverage as shown in Figure 4, as illustrated by the vertical line at 891 cm−1 going through the center of the high dosage peaks (i.e., 0.6−1.0 langmuir), but being left-of-center for the low dosage peaks (i.e., 0.2 and 0.4 langmuir). While the blue-shift could suggest that the lowest dosages of water correspond to submonolayer coverage on the surface, the higher dosages correspond to multilayers. Additionally, no blue-shift is observed in the O−H stretch region of the spectra as shown in Figure 4. (Note that blue-shifts are often attributed to dipole coupling, in which neighboring molecules collectively oscillate with a different vibrational frequency than the individual molecules.58−61) Therefore, the coverage of all dosages may correspond to multilayers. As seen in Table 2, the O−H stretching frequency of H2O in this experiment is close to the O−H stretching frequency of H2O found in other experiments on both iron and nickel surfaces. From this comparison, it is not conclusive as to what surface sites H2O preferentially binds to. However, the proposed adsorption geometry of water on the Fe2NiP surface is further clarified by control experiments of H2O adsorbed on an iron−nickel surface. The RAIRS data for H2O on FeNi can be found in the Supporting Information (see Figure S2). Differences in the RAIRS frequencies and line profiles between the Fe2NiP experiments and the FeNi experiments are proposed to be due to phosphorus. These differences could

Figure 5. RAIRS spectra of CH3OH adsorbed to Fe2NiP with a dosage of 0.4−8.0 langmuir. Experiments were run with a sample temperature of 120 K. Each spectrum shown is referenced to a single-beam spectrum of the clean, synthetic Fe2NiP surface.

to varying dosages of CH3OH. Similar to the analysis of the H2O experiments, the adsorption of CH3OH on Fe2NiP was characterized by comparison of the vibrational bands in this experiment to band assignments in spectra from reference materials. This comparison is shown in Table 3. Because the adsorption frequencies are very similar for previous experiments of CH3OH adsorption on both nickelcontaining and iron-containing substrates (refer to Table 3), it is difficult to identify the preferred binding sites of CH3OH on Fe2NiP. At a dosage of 0.4 langmuir (Figure 5), the only feature observable is the C−O stretch at ∼1045 cm−1. For low dosages (i.e., 0.4−1.0 langmuir), the peak is broad, which is more clearly shown in the Figure S4. King and co-workers observed a similar feature for the C−O stretch of CH3OH adsorption on NiOx/ Ni(111) and stated that the feature corresponded to two different adsorption sites for methanol.37 However, the signalto-noise ratios are low for our spectra; therefore, the C−O feature cannot be definitively assigned to two different

Table 2. Vibrational Frequencies (cm−1) of Water (H2O) Adsorbed to Fe2NiP Compared to Vibrational Modes for the Adsorption of H2O on Other Iron- and Nickel-Containing Surfaces mode

Fe2NiP (this work)

Fe(100)25,26,28

Ni(110)27,29

NiO/Ni(110)32

H−O−H libration H−O−H bend O−H stretch

891−810 1649−1548 3420

820−800, 930, 690 1630−1620, 1600 3420

760−530 1610−1570 3300

− 1640−1550 3560, 3670

E


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Table 3. Vibrational Frequencies (cm−1) of Methanol (CH3OH) Adsorbed to Fe2NiP Compared to Vibrational Modes for CH3OH Adsorption on Other Iron- and Nickel-Containing Surfaces mode

Fe2NiP (this work)

Fe(110)40 Fe(100)39,41

α-Fe2O335

Ni(110)62 Ni(111)36

NiO(100)43 NiO/Ni(110)37

C−O−H bend C−O stretch CH3 bend CH3 bend C−H sym. stretch C−H asym. stretch O−H stretch

847−679 1048−1039 1135 1462 2836 2957 3337−3230

710−750, 820 1025−1061 1480 1450−1470 2850−2875 2938−2950 3230−3300

− − − 1460, 1450 2815 2920 3460

− 1020−1035 1130 1440−1455 2790−2810 2910−2955 3210

− 1040−1080 1150−1155 1450 2790−2830 2900−2940 −

methanol adsorption sites. Thus, all dosages appear to correspond to multilayers. The band at 3337−3230 cm−1 is assigned to the O−H stretch of CH3OH in the condensed phase (i.e., in multilayers) and is first visible at a dosage of 0.8 langmuir, as illustrated in Figure 5.63 The fact that the C−O stretch is observed at lower dosage than the O−H stretch may indicate that the O−H bond breaks upon contact with the surface, decomposing into methoxy groups (CH3) and protons. Alternatively, it may be that the O−H bond has a lower signal intensity because of its angle with respect to the surface plane. While CH3OH dissociation at low temperature (i.e., 90 K) has been observed on nickel surfaces,43,64 it does not occur on all metal surfaces.42 The CH3 deformation band (1462 cm−1), symmetric stretch (2836 cm−1), and asymmetric stretch (2957 cm−1) do not appear until 4.0 langmuir. Although these vibrational modes of the methyl groups might also be expected to appear at 0.8 langmuir, it has been observed in another RAIRS experiment that these modes do not appear until higher dosages.63 Most likely, this is due to the angle of the dipole moment of vibration with respect to the plane of the surface (i.e., the axes of the C− H bonds are tilted). At 4.0 langmuir, all the peaks intrinsic to CH3OH adsorption on a metal-containing surface are weak but observable, as shown in Figure 5. For larger doses of methanol (up to 50.0 langmuir as shown in Figure S3), the intensity of the peaks grows; however, the peak position remains constant. Because the vibrational features do not shift between 0.8 and 50.0 langmuir, we assign these dosages to multilayers. We propose that CH3OH dissociation on the Fe2NiP surface results in the oxygen of methoxy being bound to the surface with the methyl group extending into the vacuum, indicating that the Fe2NiP surface is chemically active to CH3OH at 130 K. The adsorption of CH3OH on an FeNi sample was additionally performed to investigate the possible interaction of CH3OH with lattice phosphorus atoms, and the RAIRS data is presented in Figure S5. There are no significant differences in the peak positions and intensities, suggesting that CH3OH interacts primarily with the metal atoms in Fe2NiP rather than with surface phosphorus atoms at 120−138 K. In both the CH3OH on Fe2NiP (Figure 5) and CH3OH on FeNi (Figure S5) a small feature at approximately 1230 cm−1 appears at dosages corresponding to multilayer coverage. A similar peak has been attributed to a C−C stretch from the decomposition of CH3OH on a platinum surface.65 Because this feature is not observed until multilayer coverage, this weak peak does not seem to indicate an activation of surface phosphorus atoms. 3.3. RAIRS of Formic Acid on Fe2NiP. Figure 6 displays the vibrational frequencies corresponding to the adsorption of HCOOH on the surface of the Fe2NiP synthetic mineral. Most of the vibrational modes observed in Figure 6 have been observed previously in studies of HCOOH or formic acid ices

Figure 6. RAIRS spectra of HCOOH adsorbed to Fe2NiP with dosages of 0.4−8.0 langmuir. Experiments were run with a sample temperature of 135 K. Each spectrum shown is referenced to a singlebeam spectrum of the clean, synthetic Fe2NiP surface.

and agree well with the formation of crystalline, multilayer ice. In particular, it has been reported that the features at 1222 and 1263 cm−1, 1622 and 1714 cm−1, and 2586 and 2744 cm−1 are indicative of splitting due to crystalline ice formation,66 although the type of crystalline phase (e.g., hexagonal, cubic, etc.) is not specified. Additionally, the feature at 3020 cm−1 is indicative of crystalline formic acid ice formation. Notably absent are absorptions at approximately 1350 and 1550 cm−1, which would indicate the deprotonation of formic acid to formate (HCOO−) on the surface of schreibersite. These features have been observed after adsorption of HCOOH on both nickel and hematite surfaces at temperatures higher than those investigated here (i.e., ≥160 K in previous work versus our measurements at 135 K).35,44−47,67 However, we were not able to identify these absorption bands in the experiments on Fe2NiP. Therefore, it is proposed that HCOOH does not dissociate upon contact with the surface but rather molecularly adsorbs. Future experiments of temperature-dependent RAIRS are planned to determine the specific temperature at which formic acid will dissociate on synthetic schreibersite. A comparison between the features identified in Figure 6 and relevant literature values is presented in Table 4. Table 4 illustrates that the vibrational modes for HCOOH bound to Fe2NiP are in the best agreement with the frequencies observed for formic acid ices deposited at 135 K.66 As outlined by Cyriac and Pradeep,66 the splitting of certain vibrational modes (e.g., the C−O bend, CO stretch, and O−H stretch) is characteristic for crystalline formic acid ice, and this is not F


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Table 4. Vibrational Frequencies (cm−1) of Formic Acid (HCOOH) Adsorbed to Fe2NiP Compared to Vibrational Modes for HCOOH Ice and the Adsorption of HCOOH on Other Surfaces mode O−C−O bend O−H libration C−H bend C−O stretch O−C−O− sym. stretch C−O−H bend O−C−O− asym. stretch CO sym. stretch O−H bend overtone C−O−H bend + O−C−O bend C−H bend + H−C−O bend O−H stretch C−H stretch O−H stretch a

Fe2NiP (this work) 727 974 1082 1222, − 1380, − 1622, 1892 2139 2465 2586, 2914 3020

1263 1393 1714

2744

HCOOH(s)66

a-Fe2O335

Ni(110)44

NiO(100)/Mo(100)47,a

NiO(111)/Ni(111)45,46,67

711 927 1072 1217 − 1373, 1389 − 1614, 1702 1890 − − 2581, 2755 2953 3047

− − − − 1348 1376 1555 1630, 1720 − − − − 2880 3380, 3460

770 − 1000 − 1352 1384 1600 − − − − − 2840 −

− − − 1020/996 − 1397/1387 − 1672/1709 − − − − 2985/2975 3466/2500−3500

778 − − 1245 1360 − 1570 1710 − − − − 2947−2860 −

Monomer/dimer at 90 K.

In addition to schreibersite-mediated chemistry in the aqueous solution, such chemistry could occur in regions beyond Earth. This laboratory investigation, along with laboratory studies by La Cruz et al.49 and Bryant and Kee,16 shows that the mineral induces chemistry under anoxic conditions (i.e., low-oxygen conditions similar to that of extraterrestrial space). Bryant et al. have performed UVirradiation experiments on iron phosphide (Fe3P) and seen activity for schreibersite,16 and UV photochemistry is an important process in complex organic molecule formation in the interstellar medium.72 Sample return from the Stardust mission showed that schreibersite is a component of the comet Wild 2.73 Astronomical observations of circumstellar envelopes have detected phosphorus species in the gas phase,74−79 and it is likely that the condensed phosphorus in these envelopes is commonly locked inside schreibersite grains.80 It is evident from the compilation of these astronomical studies that schreibersite could be distributed in various astrochemical environments, and the possibility of schreibersite-mediated chemistry occurring in such environments is worth considering.

observed in the H2O and CH3OH experiments because H2O and CH3OH were dosed at a surface temperature below their individual amorphous-to-crystalline phase transition temperatures.68−70 No frequencies corresponding to the interaction of HCOOH and phosphorus atoms on the surface of Fe2NiP were observed, as shown in Figure 6. Control experiments of HCOOH on an iron−nickel sample are shown in Figure S6. While not all of the peaks observed in Figure 6 were also visible in Figure S6, all the spectral features observed in Figure 6 can be assigned to formic acid based on comparison to other experiments of formic acid ices and formic acid adsorbed on iron- and nickel-containing surfaces. Therefore, the spectra presented here do not indicate a new interaction between formic acid and phosphorus atoms in schreibersite at the temperature of our experiments (∼135 K). 3.4. Implications for Schreibersite-Mediated Chemistry in the Aqueous Phase and in Astrochemical Environments. SEM/EDS and RAIRS data suggest that the mineral surface plays an active role in the aqueous-phase phosphorylation experiments performed by Gull et al.,14 doing more than simply corroding the mineral to release a reactive form of phosphorus. As stated previously, SEM/EDS images show that the concentration of phosphorus within the mineral will not be uniform, even in the ideal case of a synthetically created mineral. Given that areas with increased defects sites (e.g., step edges, kinks, dislocations, adatoms, etc.) tend to have higher initial concentrations of phosphorus compared to relatively flat regions and that defect sites are known to be more reactive than terrace sites on surfaces,71 the surface of schreibersite seems likely to be key to understanding phosphorylation by the mineral. Additionally, the study by La Cruz et al.49 demonstrated that the temperature of the mineral surface directly influences the reaction of water with phosphorus at the surface. The experiments shown here extend that first study by showing that the mineral surface interacts with H2O and CH3OH at surface temperatures significantly lower than temperatures used in the aqueous-phase experiments with schreibersite. This low-temperature reactivity demonstrates that the electronic environment at the mineral surface also influences the chemistry at the surface, which further suggests that the mineral surface could have a notable part in the phosphorylation reactions.

4. CONCLUSIONS A custom-built UHV apparatus and a synthetic schreibersite sample were optimized in order to perform RAIRS experiments on the surface of this metal phosphide. Before the ice−mineral interface was studied, the surface of the mineral analogue was characterized by SEM/EDS to record the initial surface morphology. These studies showed the atomic composition near the polished surface and size of the defect sites at the surface. An SEM image of the unpolished surface and a phosphorus elemental map overlaid on the same SEM image demonstrate that the concentration of phosphorus in this Fe2NiP sample is dependent on the surface morphology and on the thermal history of the sample, respectively. An extensive RAIRS study was performed for the adsorption of H2O, CH3OH, and HCOOH on Fe2NiP and FeNi substrates. Experiments with H2O on Fe2NiP showed that H2O most likely binds with lattice phosphorus at low temperatures (i.e., 120− 140 K). At low CH3OH dosages, methoxy and protons are formed on the surface and CH3OH does not appear to interact with lattice phosphorus. In contrast to CH3OH, there is no apparent evidence that formic acid dissociates upon contact G


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(2) Liu, P.; Rodriguez, J. A.; Asakura, T.; Gomes, J.; Nakamura, K. Desulfurization Reactions on Ni2P(001) and Mo2C(001) Surfaces: Complex Role of P and C Sites. J. Phys. Chem. B 2005, 109, 4575− 4583. (3) Oyama, S. T.; Gott, T.; Zhao, H.; Lee, Y.-K. Transition Metal Phosphide Hydroprocessing Catalysts: A Review. Catal. Today 2009, 143, 94−107. (4) Prins, R.; Bussell, M. E. Metal Phosphides: Preparation, Characterization and Catalytic Reactivity. Catal. Lett. 2012, 142, 1413−1436. (5) Sharon, M.; Tamizhmani, G. Transition Metal Phosphide Semiconductors for Their Possible Use in Photoelectrochemical Cells and Solar Chargeable Battery (Saur Viddyut Kosh V). J. Mater. Sci. 1986, 21, 2193−2201. (6) Callejas, J. F.; McEnaney, J. M.; Read, C. G.; Crompton, J. C.; Biacchi, A. J.; Popczun, E. J.; Gordon, T. R.; Lewis, N. S.; Schaak, R. E. Electrocatalytic and Photocatalytic Hydrogen Production from Acidic and Neutral-pH Aqueous Solutions Using Iron Phosphide Nanoparticles. ACS Nano 2014, 8, 11101−11107. (7) Nozaki, F.; Tokumi, M. Hydrogenation Activity of Metal Phosphides and Promoting Effect of Oxygen. J. Catal. 1983, 79, 207− 210. (8) Zhao, H.; Oyama, S. T.; Freund, H.-J.; Włodarczyk, R.; Sierka, M. Nature of Active Sites in Ni2P Hydrotreating Catalysts as Probed by Iron Substitution. Appl. Catal., B 2015, 164, 204−216. (9) Buchwald, V. The Mineralogy of Iron Meteorites. Philos. Trans. R. Soc., A 1977, 286, 453−491. (10) Pasek, M. A.; Harnmeijer, J. P.; Buick, R.; Gull, M.; Atlas, Z. Evidence for Reactive Reduced Phosphorus Species in the Early Archean Ocean. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 10089− 10094. (11) Pasek, M.; Lauretta, D. Extraterrestrial Flux of Potentially Prebiotic C, N, and P to the Early Earth. Origins Life Evol. Biospheres 2008, 38, 5−21. (12) Pasek, M. A.; Dworkin, J. P.; Lauretta, D. S. A Radical Pathway for Organic Phosphorylation during Schreibersite Corrosion with Implications for the Origin of Life. Geochim. Cosmochim. Acta 2007, 71, 1721−1736. (13) Pasek, M. A.; Lauretta, D. S. Aqueous Corrosion of Phosphide Minerals from Iron Meteorites: A Highly Reactive Source of Prebiotic Phosphorus on the Surface of the Early Earth. Astrobiology 2005, 5, 515−535. (14) Gull, M.; Mojica, M. A.; Fernández, F. M.; Gaul, D. A.; Orlando, T. M.; Liotta, C. L.; Pasek, M. A. Nucleoside Phosphorylation by the Mineral Schreibersite. Sci. Rep. 2015, 5, 17198. (15) Bryant, D. E.; Greenfield, D.; Walshaw, R. D.; Johnson, B. R.; Herschy, B.; Smith, C.; Pasek, M. A.; Telford, R.; Scowen, I.; Munshi, T.; et al. Hydrothermal Modification of the Sikhote-Alin Iron Meteorite under Low pH Geothermal Environments. A Plausibly Prebiotic Route to Activated Phosphorus on the Early Earth. Geochim. Cosmochim. Acta 2013, 109, 90−112. (16) Bryant, D. E.; Kee, T. P. Direct Evidence for the Availability of Reactive, Water Soluble Phosphorus on the Early Earth. H-Phosphinic acid from the Nantan meteorite. Chem. Commun. 2006, 22, 2344− 2346. (17) Pasek, M. A.; Kee, T. P.; Bryant, D. E.; Pavlov, A. A.; Lunine, J. I. Production of Potentially Prebiotic Condensed Phosphates by Phosphorus Redox Chemistry. Angew. Chem., Int. Ed. 2008, 47, 7918− 7920. (18) Vlemmings, W. H.; Diamond, P. J.; Imai, H. A Magnetically Collimated Jet from an Evolved Star. Nature 2006, 440, 58−60. (19) Van Dishoeck, E. F.; Herbst, E.; Neufeld, D. A. Interstellar Water Chemistry: From Laboratory to Observations. Chem. Rev. 2013, 113, 9043−9085. (20) Eisner, J. A. Water Vapour and Hydrogen in the TerrestrialPlanet-Forming Region of a Protoplanetary Disk. Nature 2007, 447, 562−564. (21) Mumma, M. J.; DiSanti, M. A.; Russo, N. D.; Fomenkova, M.; Magee-Sauer, K.; Kaminski, C. D.; Xie, D. X. Detection of Abundant

with the surface, and HCOOH does not appear to be interacting with phosphorus that is in the sample. The RAIRS data in this study show that the mineral surface is not inert and is, in fact, active at temperatures below aqueousphase phosphorylation experiments that were previously performed in other studies. The presence of methanol dissociation products on the surface and the interaction of water with lattice phosphorus observed in previous experiments shows that the electronic environment of the surface has an impact on the surrounding chemistry. In addition to schreibersite being observed in iron and stonyiron meteorites, it has been observed in cometary grains, and there are hints of it in observations of circumstellar envelopes. This phosphorus-containing mineral may be important in a variety of astrochemical environments, and our experiments indicate that it may be activated even under anoxic and relatively low-temperature (i.e., 120−140 K) conditions in extraterrestrial settings.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01312. Details about the experimental apparatus used in this work, the procedure for creating a synthetic iron−nickel phosphide sample conducive to RAIRS, and RAIRS data for a synthetic iron−nickel sample (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +1-470-578-6140. ORCID

Aaron Pital: 0000-0002-2965-4206 Thomas Beckman: 0000-0002-5511-3294 Heather Abbott-Lyon: 0000-0001-6844-2487 Present Address ‡

D.Q.: Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by NSF and the NASA Astrobiology Program, under the NSF Center for Chemical Evolution, CHE-1504217. This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). The development of the experimental apparatus was supported in part by a Cottrell College Science Award from the Research Corporation for the Advancement of Science. The authors thank Christopher Bennett for his insights on the RAIRS technique and Nikita La Cruz and Matthew Pasek for providing Fe2NiP powder and for useful discussions leading to this publication.

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REFERENCES

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Adsorption of Water, Methanol, and Formic Acid on Fe2NiP, a

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