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Spatially Resolved Product Formation in the Reaction of Formic Acid with Calcium Carbonate (101h4): The Role of Step Density and Adsorbed Water-Assisted Ion Mobility Courtney R. Usher, Jonas Baltrusaitis, and Vicki H. Grassian* Department of Chemistry and the Central Microscopy Research Facility, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed September 29, 2006. In Final Form: February 7, 2007 The reaction of calcium carbonate (101h4) single-crystal surfaces with formic acid (HCOOH) vapor was investigated using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM images indicate the reaction produces rather well-defined crystallites, preferentially at step edges and at distinct angles to one another and mirroring the rhombohedral structure of the calcite surface, while exposing unreacted carbonate surface. The size and surface density of the crystallites depend upon substrate step density, exposure time, and relative humidity. XPS data confirmed the crystallite composition as the expected calcium formate product. The AFM images show erosion and pit formation of the calcite surface in the vicinity of the product crystallites, clearly providing the spatially resolved characterization of the source of Ca ions. AFM experiments exploring the effects of water vapor on the reacted surface show that the calcium formate crystallites are mobile under conditions of high relative humidity, combining to form larger crystallites and nanometer-sized crystals with an orthorhombohedral habit consistent with the R form, as confirmed by X-ray diffraction. The implications for the reactions described here are discussed.
Introduction The ubiquitous presence of calcium carbonate in the natural environment lends itself to a wide range of studies. Important aspects of the chemistry of this material in the environment include the following: (i) its utility in the sequestration of toxic heavy metals in the geosphere;1-5 (ii) its capacity to affect the chemical balance of the atmosphere;6-8 (iii) its significant role in CO2 exchange;9 (iv) energy storage;10 (v) its part in acid precipitation, e.g., by systems affected by acid mine drainage;11-14 and (vi) its dissolution behavior.15-18 Calcium carbonate is also a prevalent structural material for buildings and construction and is present in a wide array of different museum pieces. Due to the presence of formic acid in both outdoor and indoor environments, and its potential reactivity with calcium carbonate (1) Pingitore, N. E.; Lytle, F. W.; Davies, B. M.; Eastman, M. P.; Eller, P. G.; Larson, E. M. Geochim. Cosmochim. Acta 1992, 56, 1531. (2) Stipp, S. L.; Hochella, M. F., Jr.; Parks, G. A.; Leckie, J. O. Geochim. Cosmochim. Acta 1992, 56, 1941. (3) Garcia-Sanchez, A.; Alvarez-Ayuso, E. Miner. Eng. 2002, 15, 539. (4) Kelly, S. D.; Newville, M. G.; Cheng, L.; Kemner, K. M.; Sutton, S. R.; Fenton, P.; Sturchio, N. C.; Spoetle, C. EnViron. Sci. Technol. 2003, 37, 1284. (5) Chada, V. G. R.; Hausner, D. B.; Strongin, D. R.; Rouff, A. A.; Reeder, R. J. J. Colloid Interface Sci. 2005, 288, 350. (6) Usher, C. R.; Michel, A. E.; Grassian, V. H. Chem. ReV. 2003, 103, 4883. (7) Gustafsson, R. J.; Orlov, A.; Badger, C. L.; Griffiths, P. T.; Cox, R. A.; Lambert, R. M. Atmos. Chem. Phys. 2005, 5, 3415. (8) Laskin, A.; Iedema, M. J.; Ichkovich, A.; Graber, E. R.; Traniuk, I.; Rudich, Y. Atmospheric Chemistry. Faraday Discuss. 2005, 130, 453. (9) Robbins, L. L.; Fabry, V. In Carbon Dioxide Chemistry: EnVironmental Issues; Paul, J., Pradier, C., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1994; p 301. (10) Sundquist, E. T. In The Carbon Cycle and Atmospheric CO2: Variations Archean to Present; Sundquist, E. T., Broecker, W. S., Eds.; Geophysical Monograph 32; American Geophysical Union: Washington, D.C., 1985; p 5. (11) Davison, W.; House, W. A. Water Res. 1988, 22, 577. (12) Eckstein, Y.; Hau, J. A. J. Hydrol. (Amsterdam) 1992, 131, 369. (13) Wicks, C. M.; Groves, C. G. J. Hydrol. (Amsterdam) 1993, 146, 13. (14) Xu, C. Y.; Schwartz, F. W.; Traina, S. J. EnViron. Eng. Sci. 1997, 14, 141. (15) Brown, C. A.; Compton, R. G.; Narramore, C. A. J. Colloid Interface Sci. 1993, 160, 372. (16) Liang, Y.; Baer, D. R. Surf. Sci. 1997, 373, 275. (17) de Leeuw, N. H.; Parker, S. C. Phys. ReV. B 1999, 60, 13792. (18) Arvidson, R. S.; Ertan, I. E.; Amonette, J. E.; Luttge, A. Geochim. Cosmochim. Acta 2003, 67, 1623.
materials, the study of the interaction of formic acid with calcium carbonate is of interest from a number of perspectives. Many sources of atmospheric formic acid have been postulated. Several direct sources are known, including emissions from automobiles,19,20 combustion of coal and biomass, and natural release from vegetation.21 Production from gas-phase reactions such as the oxidation of olefins by ozone and the oxidation of aldehydes and non-methane hydrocarbons has also been considered, though these chemical pathways have not been conclusively deemed significant.22 Photochemical oxidation of biogenically released isoprenes to yield formic acid is considered to be an important source.23,24 Major loss mechanisms for formic acid include wet and dry deposition, with slow gas-phase removal by reaction with OH (0.0027 ppb/h).25 The potential interaction of a gasphase compound as prevalent as formic acid with an abundant mineral dust component deserves close study. Carbonates make up a significant component of mineral dust aerosol,6 and formic acid has been shown to react with calcium carbonate to yield a surface layer of calcium formate.26 The organic layer produced on atmospheric particulates containing a large fraction of calcium carbonate may have important consequences with regard to the fate of those particulates, which may, in turn, have consequences for the Earth’s climate. Organic acids in the atmosphere (e.g., formic, acetic) may also be candidates for cloud condensation nuclei in their aerosol forms.27 In the museum environment, wooden specimen cases often emit a variety of trace pollutant gases, including carboxylic acids such as formic acid, which is produced as an oxidation product (19) Kawamura, K.; Ng, L. L.; Kaplan, I. R. EnViron. Sci. Technol. 1985, 19, 1082. (20) Grosjean, D. EnViron. Sci. Technol. 1989, 23, 1506. (21) Keene, W. C.; Galloway, J. N., J. Geophys. Res. 1986, 91, 14466. (22) Calvert, J. G.; Stockwell, W. R. EnViron. Sci. Technol. 1983, 17, 428A. (23) Andreae, M. O.; Talbot, R. W.; Andreae, T. W.; Harriss, J. Geophys. Res. 1988, 93, 1616. (24) Jacob, D. J.; Wofsy, S. C. J. Geophys. Res. 1988, 93, 1477. (25) Khwaja, H. Atmos. EnViron. 1995, 29, 127. (26) Al-Hosney, H. A.; Carlos-Cuellar, S.; Baltrusaitis, J.; Grassian, V. H. Phys. Chem. Chem. Phys. 2005, 7, 3587. (27) Yu, S. Atmos. Res. 2000, 53, 185.
10.1021/la062866+ CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
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from the aldehydes used in the treatment of the case wood.28-31 Formic acid is routinely found to pollute the interior of these specimen cases, and can cause serious damage to collections that include lead, zinc, vitreous enamels, and calcite (e.g., shells, fossils, statues). The corrosion of calcareous materials by formic acid has been termed “Byne’s disease” and is irreversible. Though the corrosion products can be removed, the surface is irretrievably affected by the reaction that causes pitting and scarring.32 The same damage can occur in outdoor environments as well, where calcite-containing building materials and statuary are exposed to the atmosphere. In this study, atomic force microscopy (AFM) is used to investigate the reaction of formic acid with single-crystal calcite surfaces. AFM can provide a more detailed understanding of the molecular aspects of the reaction, giving information about heterogeneity and spatial relationships of product formation on the calcite surface that cannot be differentiated very easily from spectroscopic techniques alone. AFM will provide insights into any islanding, clustering, and surface heterogeneity that may occur. AFM will also allow the reaction to be observed under ambient conditions, unlike scanning electron microscopy which can yield spatial information, but is typically performed under high vacuum, and thus may not likely be comparable to environmentally relevant conditions. Furthermore, the importance of a number of variables, including surface step density and adsorbed water, in this gas-surface reaction can be more easily studied using AFM. These variables are shown here to be important in the reaction of CaCO3 (101h4) with formic acid. Experimental Methods Most images shown were collected in intermittent-contact mode (tapping mode) using a Nanoscope IIIa MultiMode atomic force microscope, J-scanner head (Digital Instruments), equipped with an extender electronics module for phase imaging. Phase imaging allowed for the spatial variations in the chemical composition of the surfaces examined to be detected. One set of experiments (data shown in Figure 1) was performed using an MFP-3D SA atomic force microscope (Asylum Research). Rectangular silicon cantilevers (125 µm length) from MikroMasch were used in all experiments (NSC15/Al BS, backside of probe aluminum-coated, with resonant frequency between 265 and 400 kHz and typical force constant of 40 N/m). Scan rates for these images were 1-2 Hz, adjusted for best image quality, leading to image acquisition times up to 10 min. Image “flattening” was necessary due to the bowing of images that results from the use of the tube scanner with which the AFM instrument was equipped; flattening and image analyses were performed using the shareware program WSxM (http://www.nanotec.es), similar to the image-rendering program available with the AFM software. Bulk calcium carbonate (Icelandic spar) single-crystal pieces, obtained from Atomergic Chemicals Corp. and Ward’s Scientific, were cleaved with a razor edge to expose a fresh surface (about 1 cm2 in area) of calcium carbonate (101h4). These fresh samples were imaged as quickly as possible prior to reaction with formic acid (HCOOH). Formic acid (Alfa Aesar) was used as received and has a vapor pressure at 20 °C of 35 Torr. In most experiments, saturation pressure of HCOOH was used; reaction times, rather than pressures, were varied. For these reactions, calcite was exposed to HCOOH in a covered beaker containing both the calcite sample and a small amount of liquid HCOOH in a separate, open vial. In most of these experiments, relative humidity in the experiments was not controlled, (28) Packman, D. F. The Acidity of Wood. Holzforschung 1960, 14, 178. (29) Arni, P. C.; Cochrane, G. C.; Gray, J. D. J. Appl. Chem. 1965, 15, 305. (30) Arni, P. C.; Cochrane, G. C.; Gray, J. D. J. Appl. Chem. 1965, 15, 463. (31) Budd, M. K. Appl. Mater. Res. 1965, 4, 124. (32) Padfield, T.; Erhardt, D.; Hopwood, W. In Trouble in Store. Science and Technology in the SerVice of ConserVation, 1982; Brommelle, N. S., Thomson, G., Eds.; 1982, pp 24.
Usher et al. but dictated instead by ambient levels in the laboratory, which was typically 30%. In experiments investigating changes due to relative humidity, the saturation pressure of water vapor at 20 °C was used (21 Torr) with varying exposure times. Calcite samples were exposed to water vapor in the same fashion as they were to formic acid. X-ray photoelectron spectroscopic measurements were made using a Kratos Axis Ultra spectrometer. A monochromatic Al KR X-ray source was used. For survey scans covering the range of 1200 to -5 eV, a pass energy of 160 eV, a step size of 1 eV, a dwell time of 200 ms, and an X-ray spot size of ∼700 µm × 300 µm were used. For region scans (O 1s, Ca 2p, C 1s) with a typical width of 20-50 eV depending on the peak examined, the spot size remained unchanged, with pass energy of 20 eV, step size of 0.1 eV, and dwell time of 1000 ms. Binding energies are readable to 0.1 eV. All spectra were analyzed using the commercially available CasaXPS data processing software.33 All spectra were charge-calibrated with respect to the adventitious carbon C 1s peak at 285.0 eV. A Shirley-type background was used to subtract the inelastic background, and mixed Gaussian/Lorentzian (GL(30)) line shapes were used to curve-fit the selected element envelope. Analysis times ranged from approximately 3 to 8 min. X-ray diffraction (XRD) studies were performed using a Siemens/ Bruker D-5000 diffractometer equipped with a Cu KR source (λ ) 1.54 Å) and Kevex energy-sensitive detector. A sample of calcite which contained the calcium formate product, produced after lengthy exposure of the calcite single crystal to formic acid vapor, was analyzed in air at room temperature in the region 2θ ) 10-50° and scanned using a 0.02° step size.
Results and Discussion Calcite (101h4) Reaction with HCOOH as a Function of Exposure Time. The reaction of calcite with formic acid vapor at saturation pressure was examined as a function of exposure time. Figure 1 shows six AFM height images that outline the dependence of the extent of reaction of CaCO3 with HCOOH on exposure time. Figure 1a shows a typical AFM height image of the freshly cleaved calcite exhibiting the step edges characteristic of this fractured surface. Calcite has perfect rhombohedral cleavage along the (101h4) plane.34 This surface is not static after fracture in air. Many AFM studies have shown the accumulation of patches on the surface, which are thought to be the result of chemisorption of water to dangling bonds formed during fracture, forming surface-bound bicarbonate and calcium hydroxide products.35 Because of this tendency of calcite to react with the ambient environment, the sample in Figure 1a was imaged quickly enough after cleaving that these patches had not yet begun to form in the region scanned. The image displayed is 5 µm × 5 µm, and the heights of the steps on the calcite surface are around 5 Å. Figure 1b shows a 5 µm × 5 µm height image of the calcite surface after 2 min of exposure to saturation pressure HCOOH vapor. In this image, the features that form follow along stepedge lines and are at specific angles to one another; the features are at least 30 nm in height. Figure 1c displays a 5 µm × 5 µm height image of calcite after exposure to saturation pressure of HCOOH vapor for 5 min. The surface coverage has increased, with features continuing to form along step edges. Orientation of the features is similar in this circumstance, aligning at the same angles to each other as in the shorter exposure experiment. In Figure 1d, a 5 µm × 5 µm height image of calcite after 10 min of exposure to saturation pressure HCOOH vapor, it is seen that the crystals on the calcite surface have lengthened and grown larger, reaching at least 54 nm in height. In Figure 1e, the calcite has been exposed to saturation pressure for 30 min. By this time (33) Fairley, N. CasaXPS, 2.3.12; 1999-2006. (34) Rachlin, A. L.; Henderson, G. S.; Goh, M. C. Am. Mineral. 1992, 77, 904. (35) Al-Abadleh, H. A.; Al-Hosney, H. A.; Grassian, V. H. J. Mol. Catal. A. 2005, 228, 47.
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Figure 1. AFM height images (each 5 µm × 5 µm) of a calcite single crystal having undergone increasing exposure to HCOOH vapor at saturation pressure: (a) freshly cleaved calcite prior to HCOOH exposure (0 min); (b) 2 min; (c) 5 min; (d) 10 min; (e) 30 min; (f) 60 min.
the calcite surface is covered in large projections which continue to form at specific angles to one another; here, the crystallites reach at least 200 nm in height. By 60 min of exposure (Figure 1f) the surface features have grown to more than 270 nm in height and completely cover the calcite. The features in images Figure 1b-f are considered to be crystals of calcium formate, based on infrared studies of HCOOH reaction with calcite powder.26 At low exposures there is significant phase segregation, with product and surface clearly discernible, whereas at longer exposures, the underlying calcite surface is no longer visible. The formation of the organic salt is comparable to the results by Foster et al., who observed, using AFM, the formation of magnesium acetate islands on MgO (100) reacted with acetic acid.36 (36) Foster, M.; Passno, D.; Rudberg, J. J. Vac. Sci. Technol., A 2004, 22, 1640.
Figure 2 displays phase images of another calcite sample reacted with formic acid (10 min at saturation pressure). In Figure 2a (5 µm × 5 µm scan) it is seen that the calcite surface had exhibited a low density of wide and shallow steps (with approximately 14 edges in the 5 µm × 5 µm region shown), which led to the formation of rather sparsely dispersed formate crystals, seen to develop preferentially along step edges. Despite the preference for formation along step edges, where some of the crystals cross at a shallow angle, some cross obliquely, and some aligned directly along them, the direction of the step itself does not appear to determine the orientation of the crystals; rather the rhombohedral shape of the calcite (101h4) surface seems to dictate the directions of crystal growth. In one study of crystal growth on calcite surface, sodium nitrate crystals grown on cleaved calcite (101h4) also mimicked the underlying rhombohedral orientation.37 Other types of crystals grown on surfaces of calcite, from gas-phase and
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Figure 2. AFM phase images of calcium formate crystals that appear on single-crystal calcite surface after 10 min exposure to saturation pressure HCOOH vapor: (a) 5 µm × 5 µm image exhibiting widely spaced steps along which the formate crystals preferentially develop; (b) 2 µm × 2 µm close up of region of dense crystal formation; (c) expansion of circled region in b, 1 µm × 1 µm zoom displaying the erosion of the calcite surface around the step edges during production of calcium formate crystals. The crystals form in distinctly rhombohedral orientation, as shown by the overlaid rhomb in c. Also displayed is a line scan showing pit formation on the surface where erosion occurred due to the incorporation of surface material into the product crystals.
solution, were also seen to adopt shapes driven by the underlying substrate.38-41 The quadrilateral figure overlaid on the crystals in Figure 2c emphasizes the observed orientation in these experiments. The shape of the arrangement of crystals is similar to the shape of etch pits that form on calcite undergoing reaction with water (liquid or vapor). These etch pits are rhombohedral in shape and grow along the (481h) and (4h41) directions of calcite.42 The crystals on this surface are seen to grow along these directions as well, with the crystals in the (481h) direction seeming to be more numerous, at least initially, than in the (4h41) direction. The crystals obtain material directly from the calcite surface; the circle in the Figure 2b image (2 µm × 2 µm scan) shows where the step edge is eroded by the formation of the crystals; Figure 2c is an expansion of the region near the circle (a 1 µm × 1 µm scan), better showing the step-edge erosion around the crystals, indicating that the surface material is incorporated into the product. A line scan (Figure 2d) taken across one crystallite confirms the surface erosion in the crystallite vicinity. The erosion seems to occur in a direction not parallel to the crystals but along the (221h) direction. Another sample of calcite with a higher step density was reacted with HCOOH vapor at saturation pressure for 10 min. In a 5 µm (37) Finch, G. I.; Whitmore, E. J. Crystal Growth on Calcite Surfaces. Trans. Faraday Soc. 1938, 34, 640. (38) Kemnitz, E.; Hass, D.; Sto¨ber, R. Z. Chem. 1983, 23, 312. (39) Frincu, M. C.; Sharpe, R. E.; Swift, J. A. Cryst. Growth Des. 2003, 4, 223. (40) Lea, A. S.; Hurt, T. T.; El-Azab, A.; Amonette, J. E.; Baer, D. R. Surf. Sci. 2003, 524, 63. (41) Frincu, M. C.; Fleming, S. D.; Rohl, A. L.; Swift, J. A. J. Am. Chem. Soc. 2004, 126, 7915. (42) Morse, J. W.; Arvidson, R. S. Earth Sci. ReV. 2002, 58, 51.
× 5 µm region, the unreacted calcite had at least 140 step edges available for reaction; i.e., the step density is a factor of 10 times higher than in the previous example. Figure 3 displays the unreacted surfaces (5 µm × 5 µm) for both low and high step density cases (left images, a and b, respectively), along with corresponding reacted surfaces (1 µm × 1 µm, right images). As in the sample with low step density, calcium formate crystallites form at step edges in the (481h) and (4h41) directions, but the higher step density leads to a greater surface coverage of crystallites. For comparison, the crystallites in 1 µm × 1 µm images of each surface were counted and divided into three categories: particles in the (481h) direction, those in the (4h41) direction, and those that could not be definitively assigned, i.e., without obvious length in either direction. The sample with low step density had approximately 60 particles in the 1 µm × 1 µm image, with 40% of them along the (481h) direction, 25% in the (4h41) direction, and 35% without preferential direction. In this 1 µm × 1 µm image, all the particles have clearly formed along step edges. By contrast, the high step density sample had, in the same size sample field, approximately 335 total particles; about 65% in the (481h) direction, 20% in the (4h41) direction, and 15% assigned as having neither preferential direction. Crystallites have covered most of the surface, falling generally along step edge directions. In both cases, crystallite growth along the (481h) direction seems to be favored over the (4h41) direction, suggesting that there may be a lower kinetic barrier along that direction for ion mobility and crystallization. To confirm the chemical identity of the species formed on the calcite surface after reaction with formic acid, an X-ray photoelectron spectrum was obtained from the surface of calcite
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Figure 3. AFM height images of (a) low step density and (b) high step density (∼10 times greater than in a) calcite before (left images, 5 µm × 5 µm) and after reaction (right images, 1 µm × 1 µm) with formic acid vapor (10 min). Greater step density leads to greater surface coverage of calcium formate crystallites over identical exposure times.
exposed to 14 Torr HCOOH for 15 min. The spectrum is shown in Figure 4 along with spectra of unreacted calcite and calcium formate standard for comparison. Only C 1s data are shown. Unreacted calcite (bottom spectrum) gives notable peaks at 285.0 eV (so-called adventitious carbon43) and 289.9 ( 0.1 eV (CO32-, carbonate44). The major peak from calcium formate standard (upper spectrum) lies at 288.6 eV. Calcite that has been reacted with 14 Torr HCOOH shows a marked increase in the formate peak at 288.8 eV, surpassing carbonate (at 289.9 eV) in intensity. The simple balanced reaction of calcium carbonate with formic acid (eq 1)
CaCO3 + 2HCOOH f Ca(HCOO)2 + CO2 + H2O (1) shows that carbonate groups would indeed be consumed, replaced by the formate ion coordinated to surface calcium atoms. The XPS results corroborate FT-IR data26 that the product of the solid CaCO3/gas-phase HCOOH reaction is calcium formate. The product phase was also determined by X-ray diffraction, where a single crystal of calcium carbonate was reacted with formic acid vapor. The peak locations agreed with standard diffraction data for calcium formate in the orthorhombic (R) phase. The most intense peak was at 2θ ) 15.88°, corresponding to the (210) plane, and its intensity was enhanced relative to the intensity of the peak in the calcium formate standard diffraction data, whereas the other peaks in the pattern did not show a concomitant increase in intensity with the (210) peak. This (43) Mathez, E. A. Geochim. Cosmochim. Acta 1987, 51, 2339. (44) Stipp, S. L.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1991, 55, 1723.
indicates a slight preference for the (210) orientation on the calcite surface. Effects of Water Vapor on the CaCO3 (101h4)/HCOOH System. Figure 5 compares the surfaces of acid-reacted calcite before (a) and after (b) exposure to 10 min saturation pressure water vapor. Both are phase images with dimensions 5 µm × 5 µm. Prior to reaction with water vapor, the calcite surface shown in the 5 µm × 5 µm image was decorated with more than 500 individual crystallites, with an average length of approximately 190 nm. Half of these were along neither the (481h) nor the (4h41) direction; of these particles, 50% were located along step edges and 30% of the crystallites were along the (481h) direction, with the remaining particles, about 15%, in the (4h41) direction. On the calcite (101h4) surface, calcium atoms in the (481h) and (4h41) directions are approximately 6.4 Å apart, with carbonate groups lying between them (calculated from distances along the (22h1) and (010) directions in Lea et al.40). Using CrystalMaker (http://www.crystalmaker.com) and atomic coordinates of R-calcium formate from Watanabe and Matsui,45 the distance between calcium atoms nearest the (210) plane of calcium formate was determined to be approximately 6.2 Å. The product crystals that have grown enough to exhibit direction on the calcite surface are observed to orient themselves preferentially in the (481h) and (4h41) directions. The closeness in the distances of calcium atoms in the (210) orientation of calcium formate and on the (101h4) face of calcite may direct the formation of product crystallites along the observed dimensions. (45) Watanabe, T.; Matsui, M. Acta Crystallogr. 1978, B34, 2731.
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Figure 4. X-ray photoelectron spectra obtained for unreacted calcite, calcite reacted with HCOOH vapor, and a calcium formate standard. The spectrum for the CaCO3 + HCOOH experiment shows the peak fit contributions (dashed lines) of carbonate and calcium formate to the total C 1s signal.
After reaction with water vapor, the total number of crystallites in the 5 µm × 5 µm space has decreased by more than half to merely 200. Nearly 60% of the crystallites are oriented along the (481h) direction, with approximately 35% in the (4h41), with 5% remaining without orientation. The image in Figure 5b clearly shows not only the decrease in the number of crystallites but also the dramatic change in crystallite size after exposure to water, resulting from the coalescence of the initial calcium formate
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particles into larger crystals. Some crystallites grow as long as 1 µm or greater in one dimension and from 45 nm to over 150 nm in height (the average length of all the crystallites in the 5 µm × 5 µm region is about 300 nm). Some of these crystallites have grown large enough to distinctly show the orthorhombic dipyramidal shape of calcium formate crystals in the R phase.46,47 As described earlier, this is the phase that forms during this reaction, as confirmed by X-ray diffraction. For example, the crystal in the circled portion of the image in Figure 5b is enlarged for comparison to an idealized orthorhombic dipyramidal crystal. The orthorhombic habit lends itself to the long, thin shape of the crystals.48 Unmistakable in Figure 5b is the extent of erosion of the calcite surface during reaction with HCOOH, with CaCO3 surface exposed as the crystals combined, and patches of material surrounding the crystals of different composition than in regions between as evidenced by the contrast in the phase image. These patches are very thin, essentially two-dimensional, and are likely the product of the reaction of the fresh calcite surface with water vapor. The enlargement of these patches in an extremely humidified environment, without additional formic acid exposure, suggests that ions on the surface can be quite mobile under conditions of high relative humidity. At relative humidity of 55% one molecular monolayer of water covers the calcite surface; at RH > 55%, multilayer adsorption of water occurs.35 Above RH ) 90%, the water layer becomes more bulklike in property.49 This water layer can provide a suitable environment for the migration of ions. These images were obtained after the sample had been removed from saturation pressure of water vapor and were taken under laboratory conditions of much lower relative humidity; this would decrease the amount of available water on the surface, causing the calcium formate to collect into distinct crystals much larger than they had initially been. Finally, Figure 6 shows AFM height images (a, 5 µm × 5 µm; b, 2.2 µm × 2.2 µm) of calcite after simultaneous exposure to formic acid and water vapor (both at respective saturation
Figure 5. AFM phase images (both 5 µm × 5 µm) comparing (a) calcite after reaction with saturation pressure HCOOH vapor (10 min) and (b) that surface after exposure to saturation pressure H2O vapor (10 min).
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water and deliquesce,51 it may be that, in a significant layer of water at the surface of calcite, calcium formate takes on an amorphous phase and upon drying can crystallize into the R-phase as observed in Figure 5a,b.
Conclusions
Figure 6. AFM height images of calcite simultaneously exposed to saturation pressure formic acid and water vapor: (a) 5 µm × 5 µm image showing two distinct types of features on the calcite surface, whose stepped nature is still somewhat visible; (b) 2.2 µm × 2.2 µm image of the surface in a, highlighting the distinct differences between the species formed during reaction.
pressures). The surface is irregularly covered with two clearly different species, one of larger height than the other. These two species are thought to be calcium formate (the taller features) and the surface-bound bicarbonate/hydroxide products, though no chemical identification of either feature has been determined. de Leeuw et al. calculated values of adsorption energies for water and formic acid onto calcite, which were determined to be -92.2 and -84.2 kJ mol-1, respectively, the greater adsorption energy of water likely due to the network of hydrogen bonding of water to surface oxygen and the steric hindrance introduced by the larger HCOOH molecules.50 The values are not dramatically different, however, and under simultaneous exposure to formic acid and water vapor it is assumed that there will be competition for surface sites. de Leeuw et al. suggest that HCOOH would not displace water molecules from the calcite surface.50 The lack of crystallinity in the calcium formate product may be due to the presence of water on the surface; as described above, surfaces at or above 55% RH will have water layers associated with them, and calcium formate becomes quite mobile in the presence of water. This surface may yet be quite wet, unlike that imaged in Figure 5a,b. While calcium formate is not observed to take up (46) Mentzen, B. F.; Comel, C. J. Solid State Chem. 1974, 9, 214. (47) Matsui, M.; Watanabe, T.; Kamijo, N.; Lapp, R. L.; Jacobson, R. A. T. Acta Crystallogr. 1980, B36, 1081. (48) Stipp, S. L. S. Mol. Simul. 2002, 28, 497. (49) Kendall, T. A.; Martin, S. T. Geochim. Cosmochim. Acta 2005, 69, 3257. (50) de Leeuw, N. H.; Parker, S. C.; Rao, K. H. Langmuir 1998, 14, 5900.
In this study, atomic force microscopy was used to observe, as a function of exposure time, the spatial changes that occur on the calcite surface after undergoing chemical reaction with formic acid vapor, as well as the effects of water vapor on the reacted surface. Upon exposure to formic acid vapor, the calcite surface becomes decorated with distinct crystallites that form primarily along step edges, aligned in a rhombohedral fashion that mirrors the calcite substrate; a larger fraction of the crystals grow in size in the (481h) direction than in the (4h41) direction, indicating a lower barrier along that direction. As exposure time increases, the density of these crystallites increases, as well as their size. Surface step density also increases the surface coverage of crystallites. The AFM images not only show the individual shape and orientation of the crystals on the calcite surface but also reveal significant surface erosion in their vicinity, strongly indicating incorporation of the surface material into the reaction product. X-ray photoelectron spectroscopy and X-ray diffraction were used to confirm the chemical nature of this reaction product as calcium formate. After introduction of water vapor, the crystallites are seen to coalesce into larger crystals with distinctly definable crystal habit, orthorhombic dipyramidal, which corresponds to the R-phase of calcium formate as corroborated by X-ray diffraction. This congregation of smaller crystals into larger crystals during exposure to water vapor indicates considerable mobility of the calcium formate on the calcite surface, and opens new areas of the calcite surface to further reaction. Simultaneous exposure of the calcite to formic acid and water vapor yields two products, one a presumably amorphous calcium formate phase and the other a calcium bicarbonate/hydroxide phase. This result suggests further phase segregation occurring during reaction. The segregation of the calcium formate product on the calcite surface, and the mobility of the Ca formate in the presence of water vapor, may have profound effects on the role of a particulate containing a high fraction of CaCO3 after reaction with formic acid as it cycles through arid and humidified environments during transport. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant Nos. CHE-0503854 and CHE-0443316. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors also gratefully acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research through Grant No. 42820-AC5. The authors would like to thank Jean Ross and Dr. Kenneth Moore of the Central Microscopy Research Facility for helpful discussions and Dr. Dale Swenson of the Department of Chemistry X-ray facility for assistance in the XRD experiments. LA062866+ (51) King, M. D.; Fisher, F. N.; Thompson, K. C.; Ward, A. D. Reactions on Atmospheric Mineral Aerosol; Central Laser Facility, CCLRC Rutherford Appleton Laboratory: Chilton, U.K., 2004/2005; p 135.
