Effects of Surface Coordination on the Temperature-Programmed

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J. Phys. Chem. C 2007, 111, 17072-17081

Effects of Surface Coordination on the Temperature-Programmed Desorption of Oxalate from Goethite Jean-Franç ois Boily,* Ja´ nos Szanyi, and Andrew R. Felmy Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: July 16, 2007; In Final Form: August 27, 2007

The temperature-programmed desorption (TPD) of electrostatically, hydrogen-, and metal-bonded oxalate complexes at the goethite surface was investigated with concerted Fourier transform infrared (FTIR) measurements (TPD-FTIR) to 660 K and mass spectrometer analyses of the evolved gases to 900 K. These reactions took place with the concomitant dehydroxylation reaction of goethite to hematite and decarbonation of bulk-occluded carbonate. The measurements revealed three important stages of desorption. Stage I (300440 K) corresponds to the desorption of electrostatically and/or unbound oxalate molecules in the goethite powder with a thermal decomposition reaction pathway characteristic of oxalic acid. Stage II (440-520 K) corresponds to the desorption of hydrogen-bonded surface complexes due to the loss of surface hydroxyls at 3660 and 3550 cm-1, leading to a partial desorption via oxalic acid thermal decomposition pathways and to a partial conversion to metal-bonded surface complexes. Finally, Stage III (520-660 K) corresponds to the thermal decomposition of the metal-bonded oxalate complex, proceeding through a two-electron reduction pathway.

1. Introduction Interactions between organic acids and mineral surfaces are ubiquitous to a wide range of natural and industrial settings. The adsorption of organic acids on mineral surfaces has for a large part been focused on aqueous systems1-5 in the field of environmental (geo)chemistry because of the importance of water-laden natural environments. Dry mineral surfaces have, however, been of greater interest to the catalysis literature6-9 and have provided detailed information on the gas-phase reactivity of mineral surface hydroxyls because of the wider range of experimental techniques (e.g., vacuum or gas-phase environments) that are applicable to these systems. Reactions at the wet and dry surfaces of goethite (R-FeOOH), a mineral of widespread importance, have been studied in both environmental and industrial contexts.10-16 Surface reactions in the absence of water have notably involved a number of adsorbates including deuterium oxide, hydrofluoric acid, nitrous oxide, carbon dioxide, phosphate, methanol, acetic acid, benzoic acid, pyridine, and oxalate.10-15 Oxalate has in particular been the focus of several studies because of its widespread occurrence and importance and because of the strength of the complexes it forms with metals.5,17-21 The speciation of oxalate on the goethite surface has been more explicitly resolved in the presence of water by Persson and Axe5 and consists of a fivemembered chelate metal-bonded (MB) complex and of a complex that is in outer-sphere coordination. The important inhomogeneous peak broadening in the asymmetric C-O stretch of the latter species moreover indicates interactions with surface hydroxyls of different acidities and therefore of a mixture of hydrogen-bonded (HB) complexes (no intermediate water molecule) and electrostatically bonded (EB) complexes (with intermediate water molecules). A schematic representation of these species is shown in Figure 1. The EB/HB complexes * Corresponding author. E-mail: [email protected]; tel.: 509-376-0615; fax: 509-376-3650.

Figure 1. Schematic representation of metal-bonded (MB), hydrodenbonded (HB), and electrostatically bonded (EB) oxalate complexes on the {110} surface of goethite. The largest hollow circles represent the area taken by surface oxygen atoms and water molecules at the goethite/ water interface.

dominate the surface speciation of minerals circumneutral to alkaline aqueous suspensions. They are, however, outcompeted by the MB complex in more acidic conditions where OH2/ carboxylate exchange is facilitated by the protonation of surface OH groups. The possible persistence of these complexes to the dry state can provide an opportunity to investigate these species with additional techniques. Combined Fourier transform infraredthermal-programmed desorption (FTIR-TPD) can provide, in particular, further insight into the surface speciation, the relative thermal stabilities, and the different mechanisms by which the EB/HB and MB decompose into gaseous components. In a previous study,22combined TPD-FTIR experiments provided detailed information on the thermal decomposition of nanoparticles of goethite containing occluded carbonate. The decomposition reaction had previously been shown to proceed as23,24

2R-Fe[O(1-x)(CO3)x]OH(s) f R-Fe2O3(s) + H2O(g) + 2xCO2(g) (1) where x is the fraction of carbonate in the goethite bulk of the goethite-carbonate solid solution R-Fe[O(1-x)(CO3)x]OH(s). The TPD data revealed a dehydration event at 485 K and a single dehydroxylation event at 560 K. The first event coincides with the release of CO2 arising from carbonate adsorbed on the

10.1021/jp075576q CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007

Effects of Surface Coordination goethite surface and occluded with nonstoichiometric water in the goethite bulk. A more important release of CO2 occurs at 125 K above the dehydroxylation event and arises from carbonate molecules of the goethite-carbonate solid solution. In this study, the same goethite nanoparticles were studied with the TPD-FTIR technique with oxalate anions adsorbed as both EB/HB and MB surface complexes. These experiments provide insight into (1) the relative thermal stabilities of EB/HB and MB complexes, (2) the different decomposition pathways of these complexes, and (3) clues to the relationship between surface hydroxyls and oxalate surface complexes. 2. Materials and Methods 2.1. Solutions. All solutions were made from doubly deionized water. The solutions used for the oxalate adsorption experiments were prepared in a total ionic strength of 0.1 mol‚dm-3 by addition of NaCl (Aldrich, dried at 220 °C for 2 h). A 20.0 mmol‚dm-3 HCl titrant solution in 80.0 mmol‚dm-3 NaCl was made from a stock solution of HCl (0.5 mol‚dm-3) that was standardized with tris-methyl aminomethane (Aldrich). A 20.0 mmol‚dm-3 NaOH titrant solution in 80.0 mmol‚dm-3 NaCl was made from a filtered 50% NaOH (Aldrich) stock solution and standardized potentiometrically against the 20.0 mol‚dm-3 HCl titrant. A 30 mmol‚dm-3 sodium-oxalate stock solution was made from Na2C2O4 (Aldrich) in 25 mmol‚dm-3 NaCl. A stock solution of 0.1 mol‚dm-3 NaCl was also used for the experiments. One solution of 30.0 mmol‚dm-3 Fe(III)oxalate at p[H+] 1.5 was made by adding Na2C2O4 salt to a solution of FeCl3 equilibrated at p[H+] 2.0 in deionized water. The resulting solution was titrated to p[H+] 1.5 and adjusted to a total ionic strength of 0.1 mol‚dm-3 by addition of NaCl. Finally another set of solutions, consisting of 0.5 mol‚dm-3 Fe(NO3)3 (Aldrich) and 2.0 mol‚dm-3 NaOH (Aldrich), were used for the goethite synthesis procedure. 2.2. Goethite Synthesis. Goethite was prepared as described in Boily et al.22 Briefly, a freshly precipitated ferryhydrite suspension was converted to goethite at 358 K at pH 12 for a period of 48 h. The resulting suspension was dialyzed for 2 weeks until the conductivity of the filtrate was about 5 times larger than distilled water. A stock suspension of 30 g‚dm-3 goethite was then equilibrated in 0.1 mol‚dm-3 NaCl and stored in a polyethylene bottle. The particles were confirmed to be composed solely of goethite by X-ray powder diffraction. The analysis of a 90-point B.E.T.25 adsorption/desorption N2(g) isotherm yielded a specific surface area of 97 m2/g, a micropore surface area of 2.1 m2/g, an average B.E.T. pore size of 13 nm, and B.H.J.26 pore sizes of 29 nm (adsorption) and 124 nm (desorption). 2.3. Adsorption of Oxalate. A batch adsorption experiment at 293 K was used to adsorb oxalate on the goethite surface at p[H+] 6.5. A propeller-stirred 30 g‚dm-3 goethite suspension in 0.1 mol‚dm-3 NaCl was degassed overnight at p[H+] 3 in a sealed Teflon reaction vessel exposed to a humid N2(g) atmosphere to evacuate dissolved and adsorbed CO2 species. Sodium oxalate was added to the suspension to achieve a total concentration of 1.7 µmol‚m-2 of goethite. The sample was then titrated to p[H+] 6.5 and transferred to a degassed polyethylene test tube, which was sealed and allowed to equilibrate for 24 h on an end-to-end rotator. The equilibrated suspensions were centrifuged, and the final oxalate concentration in the supernatant was analyzed by ion chromatrography (Dionex). 2.4. ATR-FTIR. Attenuated total reflectance (ATR) FTIR measurements were carried out on a single-bounce diamond ATR cell (DuraSamplIR) using a Bruker IFS 66/S FTIR

J. Phys. Chem. C, Vol. 111, No. 45, 2007 17073 spectrometer equipped with a N2(g) purge bench and operated with a DTGS detector at 4 cm-1 resolution. Each spectrum was an average of 500 scans and was collected at 293 K. The spectrum of the empty ATR cell was first taken after at least 20 min of evacuation period in the atmosphere of the purge bench. The samples were thereafter applied onto the sample holder without touching the cell and analyzed after an additional 20-min evacuation period. Different forms of samples were analyzed. The spectra of the oxalate anion in 0.1 mol‚dm-3 was obtained by subtraction of the spectrum of 0.1‚mol‚dm-3 NaCl solution from that of a 10 mmol‚dm-3 sodium-oxalate solution in 25 mmol‚dm-3 NaCl (total I ) 0.1 mol‚dm-3) at p[H+] 6.5. The solution was transferred with a pipet without touching the cell and was covered with a glass lens to minimize evaporation to the dry N2(g) atmosphere of the spectrometer. The lens was thereafter removed to evaporate the solution on the diamond cell and to collect the spectrum of the evaporated solution. The same procedure was employed to obtain the spectrum of the [FeC2O4]+ aqueous species, present in a concentration of 20.3 mmol‚dm-3 and in the presence of 4.0 in mmol‚dm-3 [Fe(C2O4)2]-, the 30 mmol‚dm-3 Fe-oxalate solution. Oxalate surface complexes on goethite were also analyzed in the wet and dry states using a goethite suspension equilibrated with 1.7 µmol‚m-2 oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl. The net spectrum of oxalate adsorbed on the wet goethite surface was obtained by first acquiring the spectrum of the supernatant, using the aforementioned method for analyzing aqueous solutions. The sample was then removed by capillary action with a lens tissue and washed several times with water, without touching the cell. The centrifuged wet goethite paste was thereafter applied with a spatula, covered with a glass lens, and analyzed with the spectrometer. The glass lens was then removed, and the paste was allowed to dry in the atmosphere of the purge bench for a period of 24 h. During this time, scans of the drying paste were taken every 30 min to monitor the drying procedure. The dryness of the solid was evaluated by the loss of the water stretching and bending modes. The solid was thereafter exposed to a solution of deuterium oxide (Cambridge Isotopes, Inc.) and analyzed again to dryness. This procedure, which was repeated twice, was necessary to investigate potential residual water modes. Finally, the spectrum of a goethite suspension equilibrated in 0.1 mol‚dm-3 NaCl at p[H+] 6.5 in the absence of oxalate was collected after evaporation. The net spectrum of adsorbed oxalate on the wet goethite surface was obtained by subtraction of the spectrum of the wet paste from those of the supernatant and of the dry goethite thin film in the absence of oxalate. This latter spectrum was necessary to remove the goethite bending overtone at 1785 cm-1 and the bending combination overtone at 1662 cm-1.27-30 The net spectrum of oxalate adsorbed on the dry goethite surface was also obtained by subtraction of these modes, which were more substantial in the absence of water. All spectral subtractions were carried out with the computer program Opus 5.0 (Bruker). 2.5. TPD-FTIR. TPD-FTIR measurements were carried out by exposing ground goethite powder (∼10 mg) pressed onto a fine tungsten mesh to a temperature gradient of 12 K/min from 300 to 880 K at 10-9 Torr. The sample consisted of a centrifuged goethite paste that was equilibrated at p[H+] 6.5 with 1.7 µmol‚m-2 oxalate in 0.1 mol‚dm-3 NaCl. The paste was dried overnight at 293 K and then ground into a powder. The setup consists of an 2 3/4” stainless steel cube IR cell equipped with KBr IR windows and that is connected to the gas handling manifold and the pumping station. The sample is

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held on a tungsten mesh that is attached to a copper sample holder assembly. Temperature was monitored with a K-type thermocouple that was spot-welded to the top center of the tungsten mesh. The IR spectra were collected in transmission mode at 4 cm-1 resolution using a Nicolet Magna 750 spectrometer equipped with a MCT detector. Each spectrum was an average of 128 scans. Prior to spectral acquisition, a background spectrum was collected with the sample out of the IR beam. The IR cell was also connected to a UTI 100 mass spectrometer through a gate valve for analysis of the effluent gases. The FTIR spectra were manipulated using the computer program Matlab (The Mathworks, Inc.). Two-dimensional (2D) correlation maps were also generated from the FTIR data in the 300-660 K range using the method of Noda.30 The TPD traces were fitted with a Gaus-Lorentz function

TPD(T) ) e

-2.30258

{∑ 1

x2

}

e-x4 (T- x1) 2

n

2

1 + x32(T - x1)2

(2)

where A(λ) is the temperature-dependent TPD vector resulting from a linear combination of n peaks, T is temperature (K) and the four x1, x2, x3, and x4 variables are the adjustable parameters for each individual peak. The adjustable parameters were optimized by minimizing the sum-of-squares of the deviations of this model using a Levenberg-Marquardt iteration technique and calculated in the computational language of Matlab (The Mathworks, Inc.). 2.5. XPS. Four XPS measurements were carried out on two evaporated goethite suspensions that were equilibrated with and without 1.7 µmol/m2 oxalate at pH 6.5 in 0.1 mol‚dm-3 NaCl. The centrifuged samples were then evaporated on silicon plates in the dry N2(g) atmosphere of the FTIR spectrometer used for the ATR-FTIR measurements. Two of the four samples with and without adsorbed oxalate were heated in a vacuum oven at a rate of 12 K/min from 300 to 660 K, while two others were kept at 300 K. All samples were stored in a chamber purged with dry N2(g) overnight prior to XPS analysis. The measurements were performed using a Physical Electronics Quantum 2000 Scanning ESCA Microprobe. This system uses a focused monochromatic Al KR X-ray (1486.7 eV) source and a spherical section analyzer. The instrument has a 16element multichannel detector. The X-ray beam used was a 100 W, 100 µm diameter beam that was rastered over a 1.4 mm by 0.2 mm rectangle on the sample. The X-ray beam is incident normal to the sample, and the photoelectron detector was at 45° off-normal. Wide scan data was collected using a pass energy of 117.4 eV. For the Ag3d5/2 line, these conditions produce a fwhm of better than 1.6 eV. Narrow scan or highenergy resolution data is collected using a pass energy of 46.95. For the Ag3d5/2 line, these conditions produced a fwhm of better than 0.98 eV. The binding energy (BE) scale is calibrated using the Cu2p3/2 feature at 932.62 ( 0.05 eV and Au 4f at 83.96 ( 0.05 eV for known standards. The samples experienced variable degrees of charging. Low-energy electrons at ∼1 eV, 20 µA and low-energy Ar+ ions were used to minimize this charging. The reported spectra were not background-corrected but were scaled for the C 1s peak at 285.0 eV. All fitting procedures were carried out with the program CasaXPS (v. 2.3.12). 3. Results and Discussion 3.1. ATR-FTIR Spectra. The spectrum of the oxalate anion at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl is shown in Figure 2i. This species has an important symmetric stretching vibration, ν(C-

Figure 2. ATR-FTIR spectra of goethite and oxalate at 293 K. (i) 10 mM aqueous oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl; (ii) adsorbed oxalate in goethite wet paste at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl; (iii) goethite dried at p[H+] 6.5 with 1.7 µmol‚m-2 oxalate in 0.1 mol‚dm-3 NaCl; (iv) goethite dried at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl; (v) net spectrum of oxalate adsorbed on the dry goethite surface (iii-iv); (vi) evaporated solution of 10 mM aqueous oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl; (vii) evaporated solution of 30 mM Fe(III)-oxalate at p[H+] 1.5.

O)sym, at 1307 cm-1 and an asymmetric stretch, ν(C-O)asym, at 1570 cm-1. These modes are retained on the spectrum of oxalate adsorbed on the wet goethite surface (Figure 1ii) and correspond to the EB/HB complex of Persson and Axe.5 The bands are considerably broader than those in aqueous solutions because interactions with surface hydroxyls of different acidities widen the distribution of C-O stretching energies. The 1570 cm-1 band, in particular, has experienced a strong inhomogeneous peak broadening toward stronger stretching frequencies indicating the formation of a range of weakly bound (e.g., electrostatic) carboxyl groups with a variety of surface hydroxyls. Four additional modes are also present at 1255, 1427, 1690, and 1715 cm-1, consistent with the results of Persson and Axe.5 These modes, which are close to those of the aqueous Fe(C2O4)+ complex,5 arise from a MB five-membered chelate complex with surface Fe(III) cations. The band positions and intensities of the 1690 and 1715 cm-1 are also affected by pH and can result from the formation of MB complexes of a range of binding strengths arising from (1) a diversity of surface chelation sites and (2) the dissolution and readsorption of Fe-oxalate complexes (J. Loring, personal communication). The absence of stretches at 1728 cm-1 (HC2O41-) and 1735 cm-1 (H2C2O40) and of ν(C-OH) bends at 1242 cm-1 (HC2O41-) and 1233 cm-1 (H2C2O40)5 also confirm the absence of protonated oxalate surface species at the goethite/water interface. Finally, it is noteworthy to mention that the band positions of the oxalate surface species of goethite are also close to those obtained by Hug and Bahnemann1 on anatase and lepidocrocite and thereby reveal a similarity in the surface speciation of oxalate at the mineral/water interface. The raw ATR-FTIR spectrum of oxalate adsorbed on the dried solid is shown in Figure 2iii. In addition to C-O stretches of oxalate, this spectrum contains modes from the overtone of the in-plane deformation δ(OH) ) 895 cm-1 at δ′(OH) ) 1789 cm-1 and of a combination mode of the in- and out-of-plane (γ(OH) ) 792 cm-1) deformations at δ(OH) + γ(OH) ) 1662 cm-1 (Figure 2iv). Subtraction of these modes yields the net

Effects of Surface Coordination

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Figure 3. FTIR spectra of goethite dried with 1.7 µmol‚m-2 oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl in the 1900-1000 cm-1 range obtained from the TPD-FTIR experiments from 300 to 660 K at an interval of 10 K. (a) Stacked raw spectra of (i) the ATR-FTIR spectrum of the evaporated sample for comparison and (ii) TPD-FTIR results; (b) overlapped raw TPD-FTIR spectra of (a) shown in i and those in the absence of oxalate shown in ii from Boily et al. (2006) showing the net effects of goethite dehydroxylation on this range of wavenumbers. The dashed lines highlight the range of positions of δ′(OH) and δ(OH) + γ(OH) in the 300-660 K range; (c) spectra of a corrected for the goethite bands of b(ii) revealing the C-O stretches for each temperature in the 1550-1900 cm-1 region. The circled region of the 300-440 K spectra highlight a preferential loss in the high-energy portion of the ν(C-O)asym stretch of the oxalate denoting a preferential loss of the most weakly bound complexes. (d) Spectra of a in the 1150-1350 cm-1 region.

spectrum of adsorbed oxalate of Figure 2v. This spectrum reveals the persistence of the EB/HB (1307 and 1582 cm-1) and MB (∼1255-60 (hidden in the tail of the pronounced 1307 cm-1 band), 1427, 1690, and 1715 cm-1) to the dry state. The absence of ν(CdO)and ν(C-OH) also indicates that the drying process did not protonate the oxalate anions, unlike the case of sulfate on an iron (hydr)oxide.32 Spectral deconvolution of the 1750-1500 cm-1 region (not shown) clearly resolved three expected Gauss-Lorentz components centered at 1582 (EB/ HB), 1690 (EB/HB), and 1715 (MB) but also one additional component at 1643 cm-1. In order to test whether this band arose from residual water bending modes, a dry goethite sample equilibrated with oxalate was exposed to two cycles of hydration/dehydration with deuterium oxide (not shown). The 1643 cm-1 band was, however, not affected by this procedure. The possibility that this mode arose from the drying of an unbound oxalate species was also investigated by comparison with the spectrum of an evaporated 10 mmol‚dm-3 sodium-oxalate solution at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl (Figure 2vi). Although this spectrum has a band close to 1643 cm-1, all other features, especially a marked component of the C-Osym stretch at 1336 cm-1, did not support this possibility. The possible formation of a dry Fe(III)-oxalate salt in the dry state was also investigated by evaporating a Fe(III)-oxalate solution dominated by oxalate-bound iron (Figure 2vii). The resulting spectrum indicated the formation of a mixed Fe(III)-oxalatehydroxide solid but of little similarity to the evaporated goethite-oxalate suspensions. The temperature dependence of the 1643 cm-1 band, to be discussed in the following section, will provide additional clues that these ATR measurements could not provide. 3.2. TPD-FTIR Data. 3.2.1. FTIR and XPS Spectra. The results of the TPD-FTIR experiments are shown in Figure 3. The low-temperature TPD-FTIR spectra (Figure 3a(ii)) compared well with the ATR-FTIR spectrum of the evaporated thin film (Figure 3a(i)). Increasing the temperature to 660 K eliminated most of the bands because of simultaneous goethite dehydroxylation and oxalate decomposition reactions. In a recent TPD-FTIR study,22 the same goethite preparation in the absence

of oxalate was shown to undergo a dehydration event at 485 K, followed by a dehydroxylation event centered at 560 K. The latter event is associated with important losses in the bulk ν(OH) stretching region (3800-2700 cm-1), the in-plane and out-ofplane deformations, as well as the δ′(OH) overtone and the δ(OH) + γ(OH) combination mode (Figure 3b(ii)). Subtracting these two latter bands of Figure 3b(ii) from the spectra of Figure 3b(i) for each corresponding temperature in the 300-660 K range reveals the net bands of adsorbed oxalate molecules in the dry state (Figure 3c and d), just as was done for the ATRFTIR spectra. The resulting trends in the corrected spectra of Figures 3c and d can be separated in three distinct Stages at (I) 300-440 K, (II) 440-520 K, and (III) 520-660 K. These stages can also be clearly recognized by the temperature dependence of the intensities of the bands of the EB/HB and MB complexes, in Figure 3. The 300-440 K range (Stage I) is characterized by slight decreases in absorbance in the MB and EB/HB bands, which are better visualized in difference spectra (not shown). Particularly, the ν(C-O)sym band undergoes a red shift of 3 cm-1 while the higher stretching energy portion of ν(C-O)asym undergoes a concomitant loss in absorption around 1597 cm-1, that is, in the higher stretching energy region of the inhomogeneously broadened 1570 cm-1 band. These changes are taken as evidence of the loss of the most weakly bound oxalate complexes (EB/HB) because they affect only the strongest ν(C-O)sym and ν(C-O)asym, that is, those that belong to carbonyls that form the weakest hydrogen bonds with surface hydroxyls. Increasing the temperature from 440 to 520 K (Stage II) induced more dramatic changes in the C-O stretches of adsorbed oxalate. The ν(C-O)sym and ν(C-O)asym modes of the EB/HB oxalate complex underwent a concerted decrease in intensity with an inflection point at 460-470 K (Figure 4). The 1690 and 1715 cm-1 bands of the MB complex underwent an opposite, almost mirror-like, trend to those of the EB/HB complex, reaching maximum values at 500-520 K, again with an inflection point at 460-470 K. A similar trend is also seen in the 1255 cm-1 band of the MB, although it is not as clearly resolved because of contributions of the tail of the strong 1307


Boily et al.

Figure 4. Temperature dependence of the absorbance TPD-FTIR spectra of Figure 3c and d at wavenumbers characteristic of EB/HB and MB complexes.

Figure 5. FTIR spectra of goethite dried with 1.7 µmol‚m-2 oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl in the 3700-3500 cm-1 range. ATR-FTIR spectra of evaporated goethite thin films (i) in the absence of oxalate and (ii) in the presence of oxalate. TPD-FTIR spectra are shown in iii.

cm-1 band. A band at 1275 cm-1 gains importance in the 440490 K range and is proposed to be related to the loss of the 1307 cm-1 band through an isosbestic point at 1277 cm-1. As a whole, the loss of EB/HB bands and the increase of MB bands are taken as evidence of a thermally driven surface dehydration reaction of the type

≡FeOH20.5+‚‚‚O4C22- f ≡FeO4C2-1.5 + H2O

(3)

promoting the conversion of EB/HB to MB complexes in Stage II. This mechanism is moreover supported by the loss of surface OH groups in the same temperature range, seen through the discrete OH stretching vibrations in the 3550-3700 cm-1 range (Figure 5), including bands at 3660, 3648, and 3550 cm-1, which have been reported previously for the goethite surface.10-16 Although these bands are not considerably affected by the presence of oxalate, a result that can be explained by the relatively low oxalate surface loading (1.02 site/nm2) compared to the crystallographic site density (15.3 sites/nm2), they are affected by increasing temperatures. Figure 5 shows that increasing temperature induces a preferential desorption of the

Figure 6. Temperature dependence of the absorbance TPD-FTIR spectra of Figure 5.

3660 and 3550 cm-1 bands, relative to regions where no surface OH stretch is present (e.g., 3500 and 3660 cm-1) (Figure 6). The loss of the 3660 and 3550 cm-1 bands is particularly important in Stage II, precisely in the temperature range where EB/HB complex is desorbed. We therefore propose that the EB/ HB complexes are bonded to the corresponding surface hydroxyls and that the desorption reactions are triggered from the thermal instability of these groups. Interestingly, the 3648 cm-1 band follows a similar temperature dependence to those of the bulk OH stretches and must therefore arise from a less thermally labile surface hydroxyl, as will be discussed in a forthcoming communication. At temperatures above 520 K (Stage III), and up to the highest temperature studied by FTIR at 660 K, all bands underwent a concerted decrease in intensity. This important loss of vibrations denotes the removal of carbonyl-bearing compounds as well as hydroxides from the goethite bulk and surface, yielding a (hydro)hematite phase containing traces of carbonate.22 The loss of structural OH and nonstoichiometric water from the goethite bulk is in fact confirmed in the O 1s spectrum of the solid exposed to 660 K in vacuo, compared to those at 300 K (Figure 7). The changes undergone in the Fe 2p spectra are, however, more complex because the Fe multiplet structure of goethite and hematite are different.33-35 Second, the possible presence of reduced forms of Fe,36 either due to the partial reduction of hematite in the vacuum of the TPD-FTIR cell and/or due to the reduction by oxalate, as will be discussed in the following sections, adds to the complexity of the interpretations. The Fe 2p spectrum also shows that the low-temperature drying process in the presence of oxalate has no affect on the oxidation state and binding attribute of surface Fe(III) atoms. The 1643 cm-1 band, in contrast to those of the EB/HB and MB complexes, only underwent a slight monotonic decrease in with temperature throughout Stages I and II. It, however, underwent a similar temperature dependence as the MB bands throughout Stage III, with a sharp decrease in intensity up to 580 K. We therefore tentatively assign this band to a different MB species that exists only in the dry state. Although the loss of the oxalate hydration shell in the dry state could potentially affect a portion of the MB spectrum, elucidation of this band would require additional concerted experimental and theoretical efforts. The FTIR spectra have therefore provided evidence of a diversity of oxalate complexes adsorbed on the dry goethite surface. Additional information can also be extracted from a



Figure 8. Asynchronous 2D map of the goethite-corrected FTIR spectra from 300 to 660 K in the 1550-1750 cm-1 range. The enhanced spectral resolution from this map reveals several distinct C-O stretches arising from different EB/HB and MB surface complexes.

Figure 7. XPS data of goethite dried in the presence and absence of 1.7 µmol‚m-2 oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl. (a) O 1s spectra and (b) Fe 2p spectra. The samples consist of goethite dried at 300 K and reacted in vacuo in a 12 K/min gradient to 660 K in the presence and absence of oxalate. In b the samples in the presence and absence of oxalate at 300 K are identical (both are overlapped in the figure), showing that oxalate does not reduce the surface in these conditions, while those at 660 K are considerably different. The sample in the absence of oxalate is even more reduced than that in the presence of oxalate, as can be seen in the reduction of the ∼718 eV satellite and the appearance of features below 709.6 eV. This difference may be caused by the stabilization of the redox state of surface ferric atoms by MB oxalate complexes.

2D correlation30 map of these data, as shown in Figure 8. The asynchronous 2D map provides an enhanced spectral resolution of bands that could not be identified in the FTIR spectra of Figure 3. We notably find several correlation squares in the high-energy C-O stretches (1500-1750 cm-1), revealing the existence of various EB/HB and MB complexes that arise from the coordination of oxalate with surface hydroxyls of different acidities and onto chelation centers of various strengths. 3.2.2. TPD Data. The liberation of compounds for the goethite bulk and surface under thermal decomposition is explicitly revealed in the TPD data of Figure 9. The detected mass spectrometer mass-to-charge (m/e) values correspond to H2O (m/e ) 18), CO2 (m/e ) 44), CO (m/e ) 28), HCOOH (m/e ) 46), HCOO- (m/e ) 45), and HCO+ (m/e ) 29). The two latter masses are mass spectrometer cracking fragments of HCOOH, and because they possess identical TPD profiles they will

henceforth be referred to as HCOOH*, namely, the sum of HCOOH, HCOO-, and HCO+. The TPD data can also be separated in the same three aforementioned Stages denoted in the FTIR spectra. Stage I encompasses a narrow event in the traces of CO and H2O at 430 K. The discreteness of this event also denotes a reaction involving oxalate molecules that are weakly bound to the goethite surface. This stage corresponds to the onset of CO2 and HCOOH* release, peaking in Stage II at 460 K. This peak is also correlated to the inflection point of the absorbance of the EB/HB complex (Figure 4) and therefore arises from the decomposition of a fraction of EB/HB complexes that did not convert to MB. Stage II ends with a local minimum in the H2O, CO, and CO2 traces at 520 K and is associated with maximum (minimum) absorbances in the FTIR bands of the EB/HB (MB) complexes. This provides evidence of the completion of the EB/ HB decomposition and of the EB/HB f MB conversion reactions (eq 3). Finally, Stage III corresponds to an accrued release of CO2 due to the release of both bulk-occluded carbonate22,37 and oxalate surface complexes. Comparison with the TPD data in the absence of oxalate22 reveals the specific contributions of oxalate to the TPD data of Figure 9. First, the important H2O trace results for the most part from the goethite dehydration and dehydroxylation reactions. Specific contributions from the decomposition of oxalate cannot, however, be explicitly deconvoluted, aside from the discrete 430 K event. The CO2 trace is, in contrast, 2.1 times larger than that in the absence of oxalate, as shown in Figure 7b(i). The net trace arising solely from the thermal decomposition of oxalate was estimated by subtraction of the goethitederived CO2 contributions from the data of Boily et al.22 (Figure 9b). The CO trace also required correction because an important portion of CO arises from the mass-spectrometer cracking of CO2. This is mostly apparent above 520 K where both CO and CO2 traces are identical in shape with a constant CO/CO2 intensity ratio of 0.27 (Figure 9b(ii)). Both traces are, moreover, identical in shape through the 300-900 K range in the absence of oxalate.22 Correcting the CO trace for this ratio yields the net CO trace of Figure 9c (ii) arising from the thermal decomposition of oxalate. Finally, the HCOOH* required no adjustments and arises solely from the decomposition of oxalate.


Boily et al. TABLE 1: Gauss-Lorentz Peak Areas of Components of the TPD Traces of CO, CO2, HCOOH*, and H2O Ta

Ab

CO

419 (I) 430 (I) 458 (II) 481 (II) >520(III)

2.04 3.41 2.03 1.19 0.00

CO2

413 (I) 430 (I) 457 (II) 500 (II) >520(III)

1.47 0.03 5.64 2.53 37.5

HCOOH*

419 (I) 430 (I) 458 (II) 491 (II) >520(III)

0.87 0.02 3.74 1.63 3.30

430 (I)

2.65

H2O a

b

Peak temperature (K). Individual peak area (au‚K).

TABLE 2: Stoichiometric Ratios of CO, CO2, HCOOH*, and H2O in Stages I, II, and III, Corrected for Bulk-Occluded CO2 stage

a

Ta

CO/CO2/HCOOH*

% gasb

I I

413-419 430

2.8:2.0:1.2 1.0:0.0:0.0

12

II II

457-458 481-500

1.0:2.8:1.8 1.0:2.1:1.4

26

III

>520

0.0:11.4:1.00

62

b

Peak temperature (K). Percentage of C-bearing gas released in the 300-880 K range, corrected for the goethite-occluded CO2 and mass-spectrometer-derived CO.

Figure 9. (a) TPD data of goethite dried with 1.7 µmol‚m-2 oxalate at p[H+] 6.5 in 0.1 mol‚dm-3 NaCl for H2O (left ordinate axis) and CO, CO2, and HCOOH (right ordinate axis). The dashed lines denote the thresholds between Stages I, II, and III. (b) CO2 derived from the goethite bulk in the presence and absence of oxalate (i) and (ii) the CO2 trace scaled by a factor of 0.27 showing the fraction of CO generated by CO2 cracking in the mass spectrometer. (c) Gauss-Lorentz fitting results of Stages I and II in (i) the net CO trace (corrected for CO2 cracking), (ii) HCOOH*, and (iii) CO2.

All net CO, CO2, and HCOOH* were fitted by a GaussLorentz function (Figure 9c, Table 1). All three traces were correlated by events centered at 413-419, 430, 457-458, and 481-500 K of Stages I and II. Stage III is largely characterized

by the release of CO2, with a residual trace of HCOOH*. The overall stoichiometric ratios of these different events were estimated by the peak area and are reported in Table 2. These values will be used for the interpretations of the desorption mechanism in the following section. 3.3. Thermal Decomposition Reactions. The thermal decomposition of various oxalate compounds has been studied in both catalytic and geochemical contexts. The gas-phase thermal decomposition of oxalic acid has notably been described in great detail experimentally and theoretically.38-46 Other work assessed the roles of different solvents on the thermal decomposition products of oxalate47-54 including water55 as well as the effects of metal ion complexation.56 Metal-oxalate salts have also been studied and described extensively.57-62 The decomposition mechanisms of mineral-surface bound oxalate species, and especially the impact of surface coordination on the decomposition pathways, however, remain unknown. Decomposition pathways for EB/HB and MB oxalate species are expected to be different because of their different initial bonding environments. In the former complex, close interactions with desorbing OH and OH2 moieties could promote proton transfer to desorbing carbonyl groups and lead to decomposition pathways characteristic of oxalic acid.38-41 Inner-sphere metalcarbonyl interactions could, however, lead to decomposition pathways that are more characteristic of oxalate salts.63,64 The dominant gas-phase thermal decomposition pathway of oxalic acid in the 400-430 K range typically produces equimolar concentrations of carbon dioxide and formic acid:38-41

H2C2O40 H CO2 + HCOOH0

(4)



This reaction proceeds through two different dihydroxycarbene (:C(OH)2) radical intermediates. The first activated complex, O2C‚‚‚:C(OH)2, dissociates to form the :C(OH)2 radical. This species forms another activated complex with oxalic acid (O2CC(OH)2‚‚‚(OH2)C:)) or water (OH2‚‚‚(OH2)C:)) and produces HCOOH by a reciprocated hydrogen transfer.46 Another decomposition pathway, which occurs at higher temperatures,46 also produces CO

H2C2O40 H CO2 + CO + H2O

(5)

and involves an activated complex in which the hydrogen of one C-OH group is transferred to its counterpart C-OH group, forming the precursor of the C-OH2 moiety to H2O. The complexes then dissociate as H2O, CO, and CO2. Decomposition pathways of metal-oxalate salts are substantially different from those of oxalic acid vapors. The thermal decomposition of divalent metal-oxalate solids (Me2+), which were studied more extensively than their trivalent counterparts, produce either only carbon dioxide63

MeIIC2O4 H Me + 2CO2

(6)

leaving a metallic residue, or equimolar concentrations of carbon dioxide and carbon dioxide

MeIIC2O4 H MeIIO + CO + CO2

(7)

leaving an oxide residue, which may also disproportionate. There is also a strong correlation between decomposition temperature and metal-oxalate oxygen bond strength.57,58,62,63 The rupture of this bond is deemed to be the determining process in the decomposition of metal-oxalate solids,62 followed by the rupture of the weaker C-C bond.65 Metal cations of higher oxidation states (e.g., Fe3+), which are more relevant to the goethite surface, typically possess lower decomposition temperatures because of their larger reduction potentials. The decomposition ferric oxalate (Fe2(C2O4)3) solid in vacuum in fact occurs in the 410-450 K range, compared to 635 K in ferrous oxalate solids and roughly proceeds as63

Fe2(C2O4)3 f 2FeC2O4 + 2CO2

(8)

liberating CO2 and leaving a ferrous oxalate solid.59 Another decomposition pathway that is common in metaloxalate salts includes carbonyl-carbonate intermediates, first liberating carbon monoxide and ultimately carbon dioxide:64

MeIIC2O4 H MeIIOCO2 + CO H MeIIO + CO + CO2 (9) This pathway has, however, only been reported in alkali metalbearing iron-oxalate salts.60 In fact, the FTIR spectra of the goethite-oxalate solutions did not provide any evidence of the formation of carbonates on the goethite surface, judging from the spectra of carbonate surface complexes on iron (hydr)oxide surfaces published previously.11,22,66-69 Taking the combined FTIR and TPD data into consideration, and the decomposition pathways established previously, thermal decomposition reactions will be proposed in the following section. 3.4.1. Stage I. Although the FTIR data in the 300-440 K range revealed slight changes in FTIR intensity, they are associated with 12% (Table 2) of the released CO, CO2, and HCOOH in the whole TPD experiment. Because the more important changes took place in the high-energy portions of the symmetric and asymmetric C-O stretches of C2O42- (i.e.,

weaker interactions with O-H groups) and because the data in Stages II and III clearly revealed the decomposition of the EB/ HB and MB complexes, we propose that this event is associated with the most weakly bound oxalate species. The narrow range of temperature associated with the 430 K event can be used to support the fact that it involves oxalate molecules that are only weakly coordinated. The 413-419 K event is characterized by the release of roughly three CO molecules per two CO2 and one HCOOH*. Because HCOOH is a characteristic decomposition product of oxalic acid, we propose that the EB/HB complex and other adventitious weakly bound oxalate anions desorb as oxalic acid, stripping protons from the dehydroxylating surface, as discussed in the previous section (Figure 4). The EB/HB surface complex, here denoted as ≡FeaObHc ‚‚‚(OOC)22, can thereby undergo the following reaction:

≡FeaObHc ‚‚‚(OOC)22- H ≡FeaObHc-2 + H2C2O40 (10) The resulting oxalic acid is consequently proposed to decompose thermally at the goethite surface and/or in the gas phase via reactions 4 and 5, accounting for the release of two CO2, one HCOOH, and one CO. Accepting this decomposition scheme, however, requires an additional decomposition pathway to account for the remaining two CO molecules required to conform to the above-mentioned stoichiometry. The 430 K event provides clues to the possibility of a mechanism releasing only CO molecules. The sharpness of the H2O TPD data at 430 K enabled the determination of water released at this event and reveals a CO/H2O ratio of 3.41:2.65 (Table 2), with only negligible quantities of CO2 and HCOOH*. We therefore interpret this rough stoichiometric release to arise from the dehydration of weakly bound formic acid:

HCOOH H CO + H2O

(11)

Because the FTIR spectral changes of Stage I are difficult to monitor precisely, the formation of surface-bound formate remains speculative. We tentatively propose that the EB/HB complex first dissociates via the rupture of the central C-C bond, leaving two reactive ≡Fe-OH2‚‚‚O-C ¨ -O radicals, analogous to the dihydroxycarbene complex, which may then convert to HCOOH by reciprocated hydrogen transfer with an activated complex with H2O, as described previously. 3.4.2. Stage II. The FTIR data of the 440-520 K range revealed a considerable change in the surface speciation of oxalate and involved the conversion of the EB/HB to the MB complexes, as described in eq 10. The TPD data also revealed that a portion of the EB/HB complex was desorbed as oxalic acid and ultimately decomposed in the gas phase, representing 26% of the released carbon of oxalate. This is notably manifested by the 457-458 and 481-500 K events in CO, CO2, and HCOOH*. The overall release of roughly five CO2 molecules per two CO and three HCOOH* in this stage can be accounted for by both reactions 4 and 5: reaction 4 accounts for three CO2 molecules per three HCOOH* and reaction 5 for the remaining two CO2 for the two CO. The completion of these two reactions at 520 K leaves the dehydroxylating goethite surface to be dominated solely by the MB oxalate complex at the onset of Stage III. 3.4.3. Stage III. The five-membered MB chelate oxalate complex decomposes at temperatures above 520 K. It is consequently more thermally stable than the Fe2(C2O4)3 salt, which decomposes in the 410-450 K range.59 This can be explained by the different coordination environment of oxalate.

17080 J. Phys. Chem. C, Vol. 111, No. 45, 2007 However, because both products decompose by liberating only CO2 the mechanism for the MB complex can be similar to the one for ferric oxalate salts. The production of CO2 from the five-membered chelate complex requires the rupture of the central C-C bond, leaving an intermediate ≡FeIII(-O-C ¨ -O)2. An electron transfer to FeIII and the concomitant rupture of the FeIII-oxygen bond leads to the liberation of CO2:

≡FeIII(-O-C ¨ -O)2 f ≡FeII-O-C ¨ -O + CO2 (12) Although the XPS measurements provide evidence of important changes in the Fe 2p spectrum at 660 K treated in vacuo in the presence of oxalate, the one in the absence of oxlalate is in fact more substantial. They revealed a loss of the characteristic Fe(III) peaks and satellite features33-35 and the appearance of attributes of divalent surface Fe(II) atoms.36 The presence of the MB oxalate surface complexes should have thus stabilized surface Fe(III) atoms with respect to thermally driven reduction reactions in vacuo. However, given the compelling evidence35 of reduction reactions in ferric oxalate salts and the absence of XPS evidence of Fe atoms of oxidation states lower than 2+, we propose that the reaction product of eq 12 is a reactive intermediate that may react further with a vicinal FeIII site for a second electron-transfer reaction and the subsequent release of a second CO2 molecule. This reaction reaches completion at 800 K, as seen in the CO2 TPD data (Figure 9) after which the major source of CO2 is from goethite-occluded carbonate.22 Conclusions The thermal decomposition of oxalate sorbed in the dry state on the goethite surface proceeds in three determining stages. A first stage in the 300-440 K range consists of the desorption of weakly bound and residual supernatant oxalate, representing 12% of the oxalate content in the samples. This fraction of oxalate follows the decomposition pathways of oxalic acid vapor. A second stage in the 490-520 K range is characterized by the desorption of the EB/HB oxalate complex arising from the thermal decomposition of surface hydroxyls at 3660 and 3550 cm-1 and resulting in (1) additional MB complexes and (2) the gas-phase decomposition of 26% of the oxalate via the decomposition pathways of oxalic acid vapor. The remaining 62% of the oxalate desorbs as a five-membered MB chelate complex through the liberation of CO2. The resulting solid substrate surface contains FeII resulting from electron-transfer reactions from the metal-bound oxalate molecule. Acknowledgment. We thank M. Engelhard for the XPS analyses and E.S. Ilton for discussions on the XPS spectra. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Geosciences) Research Program. A portion of the research was performed in the Environmental Molecular Sciences Laboratory, 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. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under contract DE-AC060-76RLO 1830. References and Notes (1) Hug, S. J.; Bahnemann, D. J. Elec. Corr. Phenon. 2006, 150, 213. (2) Boily, J.-F., Persson, P.; Sjo¨berg, S. Geochim. Cosmochim. Acta. 2000, 64, 3453. (3) Yoon, T. H., Johnson, S. B., Musgrave, C. B., Brown, G. E. Geochim. Cosmochim. Acta. 2004, 68, 4505.

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