Solar Light Degradation of Trimethyl Phosphate and Triethyl

Article pubs.acs.org/JPCC

Solar Light Degradation of Trimethyl Phosphate and Triethyl Phosphate on Dry and Water-Precovered Hematite and Goethite Nanoparticles Peter Mak̈ ie,†,‡ Per Persson,‡ and Lars Ö sterlund*,†,§ †

FOI, CBRN Defence and Security, SE-901 82, Umeå, Sweden Department of Chemistry, Umeå University, SE-901 87, Umeå, Sweden § Department of Engineering Sciences, The Ångstrom Laboratory, Uppsala University, P.O. Box 534, SE-751 21, Uppsala, Sweden ‡

S Supporting Information *

ABSTRACT: We report on the solar-light-mediated degradation of trimethyl phosphate (TMP) and triethyl phosphate (TEP) on hematite and goethite nanoparticles in synthetic air. Adsorption and photoreactions of TMP and TEP were studied by in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) on dry and water-precovered nanoparticles in dark and under simulated solar light irradiation. Two-dimensional correlation analysis of infrared spectra was used to identify surface products as a function of reaction time. The optical properties of the hematite and goethite nanoparticles were investigated with optical spectrophotometry. The optical band gap was determined by analysis of the Tauc relationship around the band gap energy, Eg, yielding band gap energies of 2.14 and 2.28 eV for hematite and goethite nanoparticles, respectively. It is found that both TMP and TEP are readily photodegraded upon solar light irradiation (employing AM1.5 filters with 1735 W m−2), yielding surface orthophosphate as the final product. The first step in the dissociation of TMP and TEP is displacement of the methoxy and ethoxy groups, respectively, yielding adsorbed dimethyl phosphate (DMP) and methoxy, and diethyl phosphate (DEP) and ethoxy intermediates. Further photodegradation displaces additional methoxy and ethoxy groups with adsorbed orthophosphate as final reaction product. Methoxy and ethoxy fragments are simultaneously oxidized to carboxylates and carbonates. Photodegradation of TMP and TEP is promoted by OH radicals, which is evidenced by the higher photodegradation rate on water-precovered surfaces. The rate of TMP degradation is higher than that for TEP, contrary to what is expected from their corresponding bulk hydrolysis rates, but consistent with their surface reactivity in dark, where TMP is observed to dissociate at room temperature but not TEP (or only very slowly). The photodegradation rate is higher on the goethite nanoparticles than the hematite nanoparticles on both dry and water precovered surfaces. The TMP and TEP photodegradation rate constants are found to be 0.025 (0.058) and 0.008 (0.023) min−1, respectively, on water-precovered hematite (goethite) nanoparticles. of dissolved iron.4 In addition, hematite and other iron (hydr)oxides are also important in atmospheric chemistry and aerosol photochemistry, especially in connection with oxidation of atmospheric pollutants.5 Moreover, the levels of available phosphorus in soils are low due to adsorption and precipitation on iron (hydr)oxide minerals. Sunlight exposure may be an important factor in increasing the bioavailable phosphorus compounds in water and soil by photoreduction of iron, and transformation of organic to inorganic phosphorus on solid particles, with concomitant phosphorus release.6 In particular, anthropogenic sources of organophosphorus (OP) compounds originating from pesticides, industrial and drain wastes, military

1. INTRODUCTION Iron (hydr)oxides play a central role in many environmental processes. Many of these processes are stimulated by light. In order to fully understand these environmental processes at a molecular level, the photocatalytic properties of various forms of iron (hydr)oxides must be characterized. Solar-light-induced molecular transformations on earth are known to be supported by abiotic photochemical energy transduction at mineral surfaces in soils and sediments1 and also are proposed to be the reasons for the paucity of organic compounds on Mars, due to photocatalytic degradation reactions on iron oxides.2 Photoreductive dissolution of particulate and colloidal iron (hydr)oxide yielding dissolved Fe(II) is an important process in oxygenated aquatic environments,3 which in combination with biochemical processes displace the iron speciation far from thermodynamic equilibrium, maintaining high concentrations © 2012 American Chemical Society

Received: March 20, 2012 Revised: June 4, 2012 Published: June 12, 2012 14917

dx.doi.org/10.1021/jp3026732 | J. Phys. Chem. C 2012, 116, 14917−14929

The Journal of Physical Chemistry C

Article

operando diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) is used to follow adsorption and photoreactions of TMP and TEP with and without irradiation with simulated sunlight. The influence of optical properties, water coverage, and length of the aliphatic group on the organophosphate (methoxy and ethoxy) on the decomposition mechanism and degradation kinetics during solar light exposure is discussed. To the best of our knowledge, this is the first study of gas−solid photoreactions of organic molecules on iron (hydr)oxides.

deposits, etc. are important to evaluate, due to their environmental (i.e., eutrophication) and health risks. The photocatalytic properties of semiconducting oxides, such as iron (hydr)oxides, are governed by photoinduced band gap excitations of valence electrons to the conduction band within the light penetration depth of the material.7 These excitations create reactive electron−hole pairs, which, after scattering toward the surface of the oxide, may react with adsorbed molecules through interfacial charge transfer reactions. Holes may react with hydroxyls, producing highly reactive hydroxyl radicals, or directly oxidize adsorbed organic molecules. Electrons can reduce molecular oxygen, thus producing superoxide radicals. These reactive species can degrade a multitude of organic molecules.8,9 The optical band gap of hematite is reported to be 2.1−2.2 eV, similar for both bulk and nanostructured materials.10−13 The optical properties of goethite are less studied, with indications that the band gap energy is slightly higher than for hematite (2.1−2.3 eV).14,15 The fact that visible light may be employed to promote photocatalytic reactions on iron (hydr)oxides has spurred interest in these materials for a wide range of areas of application employing solar light, ranging from solar fuel and electricity production to fine chemicals synthesis and environmental remediation technologies.16−19 However, iron oxides are known to have poor photoconversion efficiency, which is attributed to the existence of deep electron traps and short diffusion length of photogenerated holes, and have spurred interest in characterizing and modifying the electronic properties by manufacturing of nanostructured iron (hydr)oxide materials.5,20 Considering photoinduced OP transformations on transition metal oxides, most published work concern the pesticide and warfare agent simulant dimethyl methyl phosphonate (DMMP) on TiO2,21−24 and more sparsely other oxides.25,26 Photodegradation of trimethyl phosphate (TMP) has been studied on TiO2,27,28 and triethyl phosphate (TEP) on TiO2.27,28 Complete mineralization of the organophosphorus compounds was evidenced, with main reaction intermediates being dimethyl phosphate (DMP) and monomethyl phosphate (MMP), and diethyl phosphate (DEP) and monoethyl phosphate (MEP), for TMP and TEP, respectively. In addition, photodegradation of pesticides in suspension of TiO2 has been studied for water-cleaning applications.29 Other studies indicate that fragments of TMP, TEP, and DMMP can be formed during photolysis of pesticides.30 There are comparably more studies on the thermally induced reactions of OP on transition metal oxides. These include DMMP on iron oxides31,32 and TiO2.33−35 Recently, we reported on the adsorption and surface chemistry of TMP on hematite, maghemite, and goethite nanoparticles. Similarly, Henderson examined the surface chemistry of TMP on the hematite α-Fe2O3(012) surface.36 In these studies it was shown that TMP adsorbs at Fe3+ sites through lone pair electrons on the PO oxygen atom. A fraction of adsorbed TMP dissociated to yield surface methoxy already at room temperature and was further converted to orthophosphate at elevated temperatures or through a slow hydrolysis reaction with coadsorbed water. To our knowledge, there are, however, no published works on OP photoreactions on iron oxides and hydroxides. Here, we present a comparative study of the solar-lightinduced reactivity of two simple OP compounds, TMP and TEP, preadsorbed on well-characterized hematite and goethite nanoparticles, with and without preadsorbed water. In situ and

2. EXPERIMENTAL SECTION 2.1. Materials. The synthesis procedures of hematite and goethite nanoparticles have been described in detail elsewhere.37,38 The nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, BET isotherm, and BJH pore size distribution measurements.37 The hematite and goethite samples were annealed at 673 and 473 K, respectively, for 20 min in order to remove organic residues prior to adsorption experiments.37 A summary of the physical properties of the hematite and goethite nanoparticles is shown in Table 1. Table 1. Physical Properties of Hematite and Goethite Nanoparticles d (nm)

material hematite goethite

TEM ∼30 ∼11a ∼62b

XRD 30.74c 8.06d 31.83e

Fe:O ratio (XPS)

BET surface area (m2 g−1)

BJH average pore width (nm)

2:3.031 1:1.981

28.98 100.06

22.2 13.6

a

Average calculated as an average of height and width. bAverage calculated from the length of the long-axis. cAverage calculated from the ⟨012⟩, ⟨104⟩, ⟨110⟩, and ⟨116⟩ reflections. dAverage calculated from the ⟨110⟩ reflection. eAverage calculated from the ⟨002⟩ reflection.

2.2. In Situ Diffuse Reflectance Fourier Transform Infrared Spectroscopy. An operando diffuse reflectance Fourier transform spectroscopy (DRIFTS) setup was employed to measure adsorbed molecules as a function of gas dosing and simultaneous solar light irradiation, using a Bruker IFS-66v/S spectrometer equipped with a broad band MCT detector.39 Samples of dry nanoparticles were placed in a modified, temperature-controlled reaction cell (Praying Mantis, Harrick Scientific Inc.). Background spectra were collected in a feed of synthetic air (20% O2 in N2, 100 mL/min). The DRIFT spectra were collected at 4 cm−1 resolution, and each spectrum was averaged over 128 scans. All reported spectra are raw spectra without smoothing. The spectra have been intensity shifted to clarify time evolution of vibrational bands. Gas dosing was performed by feeding gas through a gas generator system connected to the reaction cell.40 Briefly, liquid OP was evaporated through a diffusion tube connected to a temperature-controlled reservoir held at constant T = 62 ± 0.2 °C. Gas phase OP was fed into a stream of synthetic air directed into the reaction cell, yielding gas injection rates of 6.08 and 2.56 μg min−1 for TMP and TEP, respectively (equivalent to 19 and 6 ppbv, respectively). After collection of background spectra, TMP and TEP were dosed for 22 and 36 14918



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Figure 1. (a) Absorptance, (b) reflectance, and (c) absorption coefficient for hematite and goethite films deposited on quartz substrates, as measured by UV−vis spectrophotometry. The film thicknesses used for calculating the absorption coefficient were obtained from white beam interferometry (values in parentheses in the legend).

3050 and 2790 cm−1 and in the fingerprint region between 1600 and 1000 cm−1. Prior to analysis the spectra were baseline corrected by fitting a straight line between the end points of both regions. The first spectrum in each series corresponds to the time point immediately before irradiation. The obtained spectral data sets display variation as a function of irradiation time (external perturbation), which is caused by photodegradation of TMP and TEP and evolution of surface reaction intermediates. To quantify these variations the synchronous and asynchronous correlation patterns are calculated in the fingerprint and methyl stretching regions separately and combined in correlation plots and/or in slice spectra. In the synchronous correlation plots, diagonal autopeaks represent major spectral variation as a function of irradiation time at two identical frequencies, and off-diagonal cross peaks show the correlated response due to irradiation at two different frequencies. The signs of the latter determine the phases of the synchronously correlated cross peaks: positive correlation of two peaks indicates either increment or decrement of both peak intensities, while negative correlation implies increment of one peak and decrement of the other. The asynchronous correlation plot contains only off-diagonal cross peaks displaying uncorrelated (out of phase) peak responses as a function of irradiation time. The signs of asynchronous correlation peaks indicate the sequences of two peak intensity

min, respectively, followed by 20 and 15 min of purging in dry synthetic air and finally solar light irradiation. The resulting surfaces prepared in this manner are partially dehydroxylated, as evidenced by simultaneous DRIFTS analysis. Trimethyl phosphate (liquid TMP, 98% GC purity, Merck), triethyl phosphate (liquid TEP, 98% GC purity, Sigma-Aldrich), and dimethyl phosphate (liquid DMP, 98% GC purity, Acros Organics) were used without further purification. Experiments involving preadsorbed water were undertaken by directing synthetic air through a temperature-controlled water bath (T = 14 ± 0.2 °C), providing a relative humidity of 17% in the reaction cell. This corresponds to an equilibrium water coverage of ca. four monolayers (ML), assuming that 1 ML = 1019 water molecules m−2, and a desorption energy equal to water sublimation energy (10 kcal mol −1). In the experiments, 20 min of water vapor exposure was followed by 20 min purging in synthetic air. To ensure reproducibility, the water exposure was simultaneously measured by in situ DRIFTS. A rough estimate assuming zero-order desorption showed that 0.5% of the water coverage desorbs during the purging period (20 min). Background DRIFT spectra were collected after the water exposure and purging period. DRIFT spectra were analyzed with 2D FTIR correlation spectroscopy using the 2D-Shige program,41 following the general procedures described by Noda and Ozaki.42 The analysis was performed in the methyl stretching region between 14919



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3. RESULTS AND DISCUSSION 3.1. Solar Light Absorption in Hematite and Goethite Nanoparticles. Figure 1 shows the absorptance, A(λ) = 1 − T(λ) − R(λ), the reflectance, R(λ), and the absorption coefficient, α, determined from eq 1, for hematite and goethite powders, respectively, deposited on quartz substrates. Due to film inhomogeneity, the absolute value of α is somewhat uncertain, with the main errors originating from film thickness inhomogeneity (Δd ≈ 40%) and variation in T due to optically transparent pin holes in the films. The latter is expected to significantly underestimate α above the optical band gap, Eg, since T → 0 in this region (see eq 1). This is also evident by comparisons with the higher α values reported for denser films prepared by physical vapor deposition methods.12 The spectral dependences of the optical absorption around the band edges in the oxides are, however, unaffected by this and lead only to minor errors in estimated band gap energies. It is clear from Figure 1a that the optical absorption edge lies in the wavelength region λ = 500−600 nm for both samples and is lower for goethite than for hematite. The energy at which the slope of A in Figure 1a changes abruptly marks the demarcation energy, Ed, which may be used to approximate the transition between the low-energy absorption Urbach tail and the absorption due to electronic interband transitions.45 This analysis yields an approximate value of the band gap energy Ed = 606 nm (2.0 eV) for hematite. A similar analysis for goethite is complicated due to the additional absorption peak at 640 nm and the weaker Urbach tail. A rough extrapolation indicates that Ed ≈ 560 nm (∼2.2 eV) for goethite. More elaborate studies show that determination of band gap energy values for hematite from optical spectrophotometry data depends sensitively on the assumptions made for the type of electronic interband transition.12,46 Band gap energies ranging from 1.9 to 3.1 eV have been reported for hematite depending on whether indirect and direct electronic transitions are assumed within a parabolic band model, viz.47

variations. The graphical presentation of the 2D correlation analysis is provided in the Supporting Information. 2.3. Optical Spectrophotometry. Optical spectrophotometry measurements in the wavelength region 200−900 nm were performed with a Perkin-Elmer Lambda 900 double-beam spectrophotometer equipped with a Spectralon coated integrating sphere. Total reflectance and transmittance, as well as diffuse reflectance and transmittance, were measured on thin films. For strongly absorbing films the absorption coefficient can be calculated from the special absorption (1 − T)/R according to Hong et al.43 ⎛ 1 − R corr(λ) ⎞ α = d −1 ln⎜ ⎟ ⎝ Tcorr(λ) ⎠

(1)

where d is the film thickness, and Rcorr and Tcorr are the corrected values of the total reflectance and transmittance, respectively.44 Films were made by vigorously mixing sample powders in water to obtain opaque slurries with well-dispersed particle aggregates, which were then deposited on quartz substrates. The films were subsequently dried at 423 K. The procedure was repeated three to five times, to deposit sufficiently thick films for accurate measurements below the absorption band edge. This resulted in inhomogeneous, porous films with optically visible pin holes, which affected the absolute determination of T, in particular above the optical absorption edge (in the UV region). The film thicknesses, as measured with white beam interferometry (Veeco Wkyo NT2000), was in the range 3−5 μm, with an estimated accuracy (from lines scans) no better than 40%. 2.4. Solar Light Irradiation. Simulated solar irradiation was performed with a Xe arc lamp source, equipped with a focusing lens assembly, operating at 200 W. The light was passed through a 75 mm water filter to remove excess infrared heat and a set of filters (AM1.5), as described in detail elsewhere.39 A fused silica fiber optics bundle was used to direct the light into the reaction cell through a CaF2 window. The total photon irradiance, incident at angle of 25° on the powder samples, was E = 173.5 mW cm−2 measured with a thermopile detector. The absorbed irradiance, Eabs, was calculated from the measured absorptance in films according to Eabs =

∫ Eλ(1 − Tcorr − Rcorr) dλ

αhν = B(hν − Eg ) p

Here α is the absorption coefficient, B is a constant related to the density of states at the valence and conduction bands, Eg is the band gap energy, and the exponent p describes the type of electronic transition in bulk semiconductors: p = 2, 1/2, 3, and 3 /2 for indirect allowed, direct allowed, indirect forbidden, and direct forbidden transitions, respectively.48 We adopt a less restrictive approach and rewrite eq 5 in the form

(2)

where Eλ is the measured spectral irradiance (in units of mW cm−2 nm−1) of the lamp source (Figure SI1, Supporting Information) and the integration was performed over the spectral bandwidth of the lamp source from 200 to 800 nm. In the DRIFTS setup, with light incident on a thick bed of nanopowders and T = 0 in the wavelength region of interest above the band gap energy, Eg, eq 2 is then approximated to Eabs =

∫200

λ = hc / Eg

Eλ(1 − R corr) dλ

p d[ln(αhν)] = d(hν) (hν − Eg )

∫200

λ = hc / Eg

Eλ(1 − R corr) dλ hc /λ

(6)

A plot of eq 6 as a function of hν yields a discontinuity at Eg. Hence, Eg can be determined without any assumptions about the nature of the optical transition, as previously demonstrated for nanostructured CdS−ZnO thin films.49 The Eg value obtained in this manner for hematite, Eg = 2.14 eV, is slightly higher than the demarcation energy, Ed, and is in good agreement with previous reports of Eg, which are in the range Eg = 2.1 − 2.2 eV.11−13 For goethite we determine Eg = 2.28 eV, which is in fair agreement with the rough estimate of Ed above, as well as previous studies.14,15 For comparisons, if we employ the Tauc relationship (eq 5), we obtain with p = 2 (indirect transition) Eg ≈ 2.0 eV for hematite, and with p = 1/2 (direct transition) Eg ≈ 2.3 eV for goethite, which is in good

(3)

The total absorbed photon flux per unit area, Fabs, above the band gap of the oxides was accordingly calculated from the equation Fabs =

(5)

(4) 14920



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Figure 2. In situ DRIFT spectra displaying 22 min of TMP dosing (D), followed by 20 min purging in synthetic air (P), and synthetic solar light irradiation (hν) on nanopowders of (a) hematite and (b) goethite. The band positions after 256 min irradiation are indicated in parentheses, and bands originating from dissociation products are shown in normal typeface. The spectra are shifted along the ordinate axis to clarify band evolvement during the irradiation.

degradation, which is primarily indicated by the depletion of the TMP ν(PO) bands at in the 1225−1255 cm−1 region and the TMP νa((P)−O−C) bands at ∼1050 cm−1. This is accompanied by a distinct increment of the bands ∼1080 cm−1, present as a shoulder prior to irradiation on both hematite and goethite. The 2D synchronous correlation analysis corroborates that this band is negatively correlated to bands originating from adsorbed TMP (Figure SI3, Supporting Information). The band at ∼1080 cm−1 has been attributed to the νs(P−O) mode in dimethyl phosphate (DMP),51−53 which is also supported by our reference spectrum of liquid DMP in bulk water (Supporting Information, Figure SI2b). The intensity of the TMP νa((P)−O−C) band decreases in-phase with the intensity of the phosphoryl ν(PO) band, as shown by 2D correlation analysis (Supporting Information, Figure SI3). It is clear from Figure 2 that the rate of TMP degradation, as measured by decreasing intensity of the phosphoryl ν(PO) band, is higher

agreement with the studies quoted above, both with respect to Eg and type of transition.12,50 3.2. Solar Light Decomposition of TMP on Hematite and Goethite Nanoparticles. 3.2.1. Dry Surfaces in Synthetic Air. Figure 2 shows DRIFT spectra obtained during TMP dosing (D), purging in synthetic air (P), and subsequent irradiation with synthetic solar light (hν), respectively, on hematite and goethite nanoparticles. The main vibrational bands during the dark period are attributed to adsorbed TMP (Table 2). In addition, weak absorption bands that are absent in liquid TMP (Supporting Information, Figure SI2a) appear in the spectra. This is due to a dissociation pathway, whereby a methoxy group is displaced from TMP to form surface methoxy, as previously shown.36,37 Compilation of bands and mode assignments originating from dissociation products are given in Table SI1 (Supporting Information). It is clear from Figure 2 that simulated solar light exposure enhances TMP 14921



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Table 2. Vibrational Frequencies and Mode Assignments of Adsorbed TMP and TEP on the Dry and Water-Precovered Hematite and Goethite Nanoparticlesa ν̅(cm−1) hematite TMP

TEP

a

goethite

dry

humid

dry

humid

liquid

assignment

2962 2930 2861 1227 1191 1048 865 2986 2939 2916 2877 1446 1398 1373 1214 1166 1032

2964 2915 2862 1233 1191 1053 862 2986 2939 2914 2879 1446 ∼1398 1373 1219 1167 1034

2962 2919 2860 1255 1190 1052 863 2984 2937 2914/2908 2878 1451 1395 1370 1240/1215 1167 1034

2963 − 2860 1245 1190 ∼1049 ∼861 2985 2938 2913 2877 1447 1396 1374 1232 1166 1035

2960 2914 2857 1278/1268 1187 1037 847 2985 2936 2910 2873 1445 1394 1370 1271/1261 1166 ∼1030

νas(CH3) νs(CH3) νs(CH3) νs(PO) ρr(P−O−CH3) νas((P)−O−C) νs((P)−O−C) νas(CH3) νas (CH3) νs(CH3) νs(CH2) δas(CH3) ρw(O−CH2) δas(O−CH2) νs(PO) ρr(P−O−CH3) νas((P)−O−C)

The band positions are evaluated immediately before irradiation. Assignment based on data from refs 26, 56, 75, 76.

complete dissociation of DMP. The relative amount of orthophosphate formation is, however, different on hematite and goethite. On hematite a weak shoulder due to orthophosphate appears at ∼1160 cm−1, which is not detectable during adsorption in the dark. In contrast, on goethite the orthophosphate related bands at 1170 and 1145 cm−1, which appear already in the dark evolve synchronously on goethite (Supporting Information, Figure SI3b). The band at 1145 cm−1 evolves more rapidly than that at 1170 cm−1 and are attributed to two different coordinated orthophosphate species, as previously shown in adsorption studies of orthophosphates on goethite in bulk water.60 3.2.2. Water-Precovered Surfaces in Synthetic Air. Figure 3 shows DRIFT spectra of TMP adsorbed on water precovered hematite and goethite nanoparticles, obtained under otherwise identical conditions as those presented in Figure 2. The negative bands apparent at ∼1630 cm−1 are due to the water δ(HOH) mode and indicate removal of surface water caused by evaporation during (dry) synthetic air exposure (an effect that is reduced by keeping the samples in synthetic air for 20 min prior to gas dosing) and rearrangement of water upon TMP adsorption. Similar to dry surfaces, the TMP photodegradation rate is higher on goethite than hematite, as probed by depletion of the TMP ν(PO) bands in the 1250−1220 cm−1 region. After 256 min irradiation, the TMP ν(PO) band is hardly discernible on goethite. Furthermore, the degradation rate is higher on water-covered surfaces compared to dry surfaces, as measured by the disappearance of the ν(PO) bands (see below). However, comparing the ratio of the DMP and TMP ν(P−O) bands at ∼1080 and 1050 cm−1, respectively, it is seen that this ratio is higher on dry goethite compared to the watercovered goethite sample. The higher rate of TMP removal on water-precovered goethite inferred from analysis of the ν(P O) bands implies that the rate of DMP decomposition to orthophosphate is also enhanced by preadsorbed water. Indeed, comparing Figures 2 and 3 it is evident that the concentration

on goethite compared to hematite. This is also seen by the more rapid evolution of the DMP bands on goethite. Photodegradation of TMP is accompanied by surface methoxy formation, as evidenced by the appearance of the characteristic νs(CH) bands in the 2820−2805 cm−1 regions.33,37,54−56 On goethite, the DMP bands appear as doublets at 1072 and 1079 cm−1, before irradiation, where the bands at lower frequency dominates. In analogy with TMP adsorption, this splitting has been interpreted as being due to two different types of adsorption sites on goethite, one due to Fe3+ coordination and the other due to hydrogen-bonding.37 It is apparent from Figure 2b that two overlapping absorption bands at 2807 and 2818 cm−1 also exist on goethite in the methoxy νs(CH) region, which indicate formation of chemically distinct methoxy and/or methanol species. During irradiation the surface methoxy group bands decreases, which is accompanied by evolution of new absorption peaks at 1580−1560 and ∼1350 cm−1. These bands are characteristic of the νa(OCO) and νs(OCO) modes, respectively, due to adsorbed formate and carbonate species,23,56−59 which are known to form upon surface methoxy oxidation on oxide surfaces.23,33 The relative intensities of the bands due to these oxidation products are significantly higher on hematite than on goethite. The presence of formate is also corroborated by the shoulder at ∼2875 cm−1 apparent on hematite, which may be assigned to a ν(CH) formate band,23,57 which is not visible on goethite. In the synchronous and asynchronous correlation plots (Supporting Information, Figure SI4) the methoxy bands at 2820−2805 cm−1 correlate negatively with the formate bands, indicating that the methoxy appears before formate and that methoxy decreases as formate increases. Collectively, these results show that the photoinduced TMP reactions follow the same pathways as those in the dark, with the same major intermediates. After prolonged irradiation new bands in the 1170−1140 and 1000−800 cm−1 regions evolve, which can be assigned to surface coordinated orthophosphate species.56,60,61 This implies 14922



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Figure 3. In situ DRIFT spectra displaying 22 min of TMP dosing (D), followed by 20 min purging in synthetic air (P), and synthetic solar light irradiation (hν) on nanopowders of (a) hematite and (b) goethite covered with preadsorbed water. The band positions after 256 min of irradiation are indicated in parentheses and bands originating from dissociation products are shown in normal typeface. The spectra are shifted along the ordinate axis to clarify band evolvement during the irradiation.

spectrum of liquid TEP is shown in Figure SI2a (Supporting Information). Similar to TMP, photodegradation of TEP is faster on goethite compared to hematite, as measured by the depletion of the ν(PO) bands at 1240−1215 cm−1. However, photodegradation of TEP is considerably slower than that of TMP. The decay of the TEP ν(PO) band is accompanied by distinct evolution of bands at ∼1075 cm−1 assigned to the νs(CO) vibrational mode in diethyl phosphate (DEP),63,64 analogous to the DMP formation discussed above. The 2D synchronous correlation shows that the DEP band is negatively correlated with TEP (Figure SI5, Supporting Information). We note in passing that in the case of TEP it is important to understand and eliminate spectral contributions originating from bulk hydrolysis products. Similar problems do not arise for TMP, which is more stable toward hydrolysis.65 Thus, the weak ∼1105 cm−1 band can be assigned to a vs(PO2−) band in DEP,63,64 and the weak 1215 cm−1 band on goethite signal

of orthophosphate species on water-precovered goethite is significantly higher than on dry goethite. Comparing the dry and water-precovered hematite samples, it is seen that the intensities of the νa(OCO) bands at ∼1580 and 1560 cm −1 are reduced, which means that the concentration of coordinated formate and carbonate species is reduced on the water-precovered samples, in good agreement with previous reports.62 This interpretation is supported by the decreased intensity of the formate ν(CH) shoulder at 2875 cm−1 in Figure 3. 3.3. Solar Light Decomposition of TEP on Hematite and Goethite Nanoparticles. 3.3.1. Dry Surfaces in Synthetic Air. Figure 4 shows DRIFT spectra obtained during TEP dosing (D), purging in synthetic air (P), and subsequent solar light irradiation (hν) on hematite and goethite nanoparticles. The main vibrational bands during the dark period originate from molecularly adsorbed TEP (Table 2). An ATR 14923



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Figure 4. In situ DRIFT spectra displaying 36 min of TEP dosing (D), followed by 15 min purging in synthetic air (P) and synthetic solar light irradiation (hν) on nanopowders of (a) hematite and (b) goethite. The band positions after 230 min of irradiation are indicated in parentheses and bands originating from dissociation products are shown in normal typeface. The spectra are shifted along the ordinate axis to clarify band evolvement during the irradiation.

small contributions from the νa(PO3−) vibration in DEP. For the sodium salt in water solution this band is located around 1233 cm−1.63 Surface-coordinated ethoxy fragments appear at ∼2850 cm−1, attributed to the νs(CH2) ethoxy band.56,66,67 Further photooxidation of surface ethoxy groups may yield a variety of oxidation products: carboxylic acids, aldehydes, and carbonates. However, it is challenging to discriminate between these species, in particular due to the similar frequencies of the various ν(OCO) bands. This is further complicated by the several TEP bending vibrational bands in the 1480−1370 cm−1 region. By comparisons with literature data and the results in the previous section, the following tentative assignment can be made. The distinct bands at ∼1580 and ∼1555 cm−1 are assigned to the νa(OCO) mode in adsorbed carboxylate and carbonate species, respectively.23,56−59 The corresponding symmetric νs(OCO) at ∼1350 cm−1 are also seen in the DRIFT spectra. The peak at ∼1425 cm−1 is attributed to the

δ(CH) mode in surface acetates.50,68 Similar to TMP, the ∼1135 cm−1 band that evolves on goethite is attributed to surface orthophosphate specie [νa(P−O)].56,60,61 The presence of orthophosphate is also supported by the evolution of the bands in the 980−900 cm−1 regime. The distinct orthophosphate bands which occur around 1170−1160 cm−1 on TMP degradation are obscured by the TEP ρr(P−O−CH3) rocking band present at ∼1166 cm−1 and the fact that the concentration of orthophosphate is much lower in the case of TEP photodegradation. 3.3.2. Water-Precovered Surfaces in Synthetic Air. Figure 5 displays DRIFT spectra obtained in the same manner as in the previous section, but with addition of preadsorbed water. As with TMP, the negative band at ∼1630 cm−1 is due to water evaporation and displacement of water by adsorbed TEP. It is apparent that photodegradation of TEP is more facile on humid compared to dry nanoparticles, as measured by the decay of the TEP ν(PO) bands at 1230−1220 cm−1 in combination with 14924



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Figure 5. In situ DRIFT spectra displaying 36 min of TEP dosing (D), followed by 15 min purging in synthetic air (P) and synthetic solar light irradiation (hν) on nanopowders of (a) hematite and (b) goethite covered with preadsorbed water. The band positions after 230 min of irradiation are indicated in parentheses and bands originating from dissociation products are shown in normal typeface. The spectra are shifted along the ordinate axis to clarify band evolvement during the irradiation.

from Figures 2−5 in the previous section. We have previously calibrated the DRIFT absorbance for TMP on nanoporous iron oxides,37 and shown that 1 au [log(1/R) absorbance units] due to the ν(PO) mode in TMP corresponds to N1au = 2.36 × 1021 molecules m−2 on dry iron oxide surfaces. We use this calibration also for TEP, since the ν(PO) mode is expected to have similar cross sections and the same stoichiometry for TMP and TEP. We also employ this procedure for waterprecovered samples, since the same TMP dose was employed in both types of experiments. (Note the infrared reflectivity changes after water adsorption. Hence, the calibration on dry surfaces cannot be used for the DRIFT absorbance on these samples, cf. Figures 2 and 3). In Figure 6 it is seen that the rate of TMP degradation in the dark (P) is higher on goethite than hematite. Assuming first-order reaction kinetics, the dark TMP dissociation rate is estimated to be kd = 0.001 and 0.004 min−1, respectively, on dry hematite and goethite, and kd = 0.008 and 0.017 min−1, respectively, on water-precovered hematite and

a higher rate of DEP formation, as probed by the increasing intensity of the DEP νs(CO) and vs(PO2−) modes at ∼1070 and 1105 cm−1, respectively. A considerably higher rate of TEP to DEP conversion takes place on humid goethite samples compared to dry samples, similar to the findings for TMP. Again identification of orthophosphate formation is complicated by the presence of the TEP ρr(P−O−CH3) rocking band at ∼1166 cm−1. However, 2D correlation unambiguously show that the ν(P−O) bands in the 1170− 1130 cm−1 region correlate synchronously with those in the 980−880 cm−1 regions, which supports the assignment of the growing peaks in the 1170−1130 cm−1 region to orthophosphate species. 3.4. Photodegradation Kinetics and Quantum Yield. Figures 6 and 7 show line profiles of the concentrations of TMP and TEP, respectively, as measured by the ν(PO) band, as a function of gas dosing (D), purging in synthetic air (P), and during simulated solar light irradiation (hν), deduced 14925



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Figure 6. Number of adsorbed TMP molecules per unit area during (a) 22 min of dosing followed by 20 min purging in synthetic air and subsequently (b) irradiation of dry and water-precovered (“W”) nanopowder samples. The most intense ν(PO) TMP bands at 1255−1220 cm−1 were utilized for quantification. The error bars show 8% uncertainty of the measured absorbance (indicated for a few selected irradiation times).

Figure 7. Number of adsorbed TEP molecules per unit area during (a) 36 min of dosing followed by 15 min purging in synthetic air and subsequently (b) irradiation of dry and water-precovered (“W”) nanopowder samples. The most intense ν(PO) TEP bands at 1240−1215 cm−1 were utilized for quantification. The error bars show 8% uncertainty of the measured absorbance (indicated for a few selected irradiation times).

goethite samples [this is probably a lower estimate, since the DRIFT signal increases somewhat during the purging period (P) when no dissociation occurs, as for TEP; see Figure 7]. The solar-light-enhanced dissociation of TMP is shown in the right panel (Figure 6b), corresponding to the “hν” period in Figures 2 and 3, and reveals an exponential decay of TMP as a function of irradiation time. It is clear that the photodegradation rate is faster on water-precovered samples than dry samples, and that it is fastest on goethite. From Figure 7 it evident that TEP qualitatively follows the same trend as TMP, with the important exception that TEP does not dissociate in the dark (P). The fact that TMP and TEP photodegradation are promoted by preadsorbed water suggests a facile pathway involving OH• radicals, viz.

which is corroborated by previous reports that photoreduction of O2 is endothermic on hematite.69 Thus, we distinguish a fast photodegradation mechanism by OH• yielding adsorbed DMP and surface methoxy according to TMPad + OH• → DMPad + MeOad

or in the case of TEP TEPad + OH• → DEPad + EtOad

Further photo-oxidation displaces additional methoxy and ethoxy fragments and yields eventually surface orthophosphate, while the displaced fragments are further decomposed to formate and carbonates or desorb, which may be evidenced by the lower concentration of the latter species on waterprecovered surfaces [methanol and formaldehyde desorption have been reported for TMP on α-Fe2O3(012) surface36]. The distribution of intermediates and reaction sequences suggest that the OH radical reactions proceed by interactions with the methoxy and ethoxy carbon atoms on TMP and TEP, respectively, in agreement with previous studies.27 Assuming first-order reaction kinetics, the solar light degradation kinetics of TMP and TEP on hematite and goethite nanoparticles is described by −d[OP]/dt = kd[OP], where [OP] is the surface concentration of TMP or TEP and is

hVB+ + OH− → OH•

This reaction is thus more efficient than direct hole oxidation hVB+ + OP → OPox

where OP is either TMP or TEP. Similarly, we conclude from the above that OH radical reaction is faster than reactions mediated by O2-generated radicals on dry surfaces eCB− + O2 → O2− 14926



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Figure 8. First-order rate plots of TMP degradation in dark prior to irradiation (a), TMP photodegradation under simulated solar light irradiation (b), and TEP photodegradation under simulated solar light irradiation (c), on dry and water-precovered (“W”) hematite and goethite samples, respectively. Linear least-squares fits to the experimental data are shown by solid lines, from which the rate constants kd are determined. The ν(P O) vibrational band in TMP and TEP were used in the analysis. The error bars show 8% uncertainty of the measured absorbance.

Table 3. First-Order Rate Constants kd (in units of molecules m−2 min−1) and Lower Bounds of Quantum Yields, Φ, Determined from Eq 7 during the First 20 min of Irradiation Obtained from Figure 8a Φ × 10−5

kd × 1018 TMP dark TMP irradiation TEP irradiation

Hem

HemW

Goeth

GoethW

Hem

HemW

Goeth

GoethW

0.12 (0.001) 1.09 (0.009) 0.3 (0.005)

0.94 (0.008) 2.95 (0.025) 0.5 (0.008)

0.48 (0.004) 3.38 (0.031) 0.7 (0.010)

1.79 (0.017) 6.23 (0.058) 1.7 (0.023)

0.9 0.2

2.5 0.4

3.3 0.7

6.1 1.6

The values in parentheses are the rate constants in units of min−1. The rate constants for TMP during irradiation were obtained by subtracting the dissociation rate occurring during the purging period (“TMP dark”) (see Figure 6a).

a

proportional to the measured DRIFTS absorbance, A. Figure 8 shows plots of ln(A/A0) as a function of irradiation time on dry and water-preadsorbed hematite and goethite for TMP and TEP, respectively, which yield kd from the slope of the curves. It is seen that photodegradation is well-described by first-order reaction kinetics during the first 20 min of irradiation. In this model, we ignore competition for accessible OH radicals between TEP/TMP and organic fragments, which is reasonable during the initial irradiation period, where the surface concentration is dominated by TMP/TEP. After extended irradiation this assumption is likely not to be valid. Indeed, we observe deviation from first-order kinetics at t > 20 min. The rate constants obtained from the first-order reaction model analysis are shown in Table 3.

If, on the other hand, we consider that OP is distributed homogeneously in the porous oxide film, we may also obtain apparent first-order kinetics, even though intrinsic reaction kinetics is zero-order due to an exponentially decreasing intensity of the solar light intensity as a function of depth in the oxide.70 This can be considered an opposite limiting case, since (i) the OP is not expected to be evenly distributed in the film (length scale of the IR probe depth ∼1 mm) and (ii) the calibrated amount of OP prior to illumination is close to 1 ML after 22 and 36 min dosing times for TMP and TEP, respectively, assuming homogeneous distribution of OP, as we have previously shown.37 However, pursuing this analysis we find that the zero-order reaction rates are generally slightly lower (2−3 times) than those tabulated in Table 3. 14927



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compared to micrometer-sized and less anisotropic particles.5,19,71−73 There are conflicting results concerning whether hematite14,74 or goethite71 shows the highest photoactivity, in the few cases where such comparisons were made. A possible reason for these discrepancies may be found in the intrinsic materials properties. The highest reaction rates are reported for nanosized wires or rod-shape particles, such as our goethite nanoparticles. This may indicate that facet-dependent, anisotropic charge transport and quantum-size effects are important for the iron(hydro)oxides.5,72

As discussed above, TMP also dissociates in the dark (see Figure 6a). This means that kd determined from Figure 8 contains two contributions. Subtracting the dissociation rate in the dark, we find that solar-light-induced dissociation of TMP is ca. 3 times faster on goethite compared to hematite on dry surfaces, while it is ca. 2 times faster on goethite on waterpreadsorbed surfaces. The quantum yield for the photodegradation of TMP and TEP was determined from the measured decomposition rate (molecules m−2 s−1) and absorbed photon flux (photons m−2 s−1) from eq 4, i.e.

N A k Φ = 1au 0 d Fabs

4. CONCLUSIONS The photoinduced degradation of two simple organophosphates, TMP and TEP, has been studied in synthetic air on hematite and goethite nanoparticles. At room temperature TMP reacts weakly in the dark, whereby a methoxy group is displaced, yielding surface-bonded DMP, methoxy, and small amounts of formate and carbonates species. Under similar conditions, only minute amounts of dissociation products of adsorbed TEP are observed under dark conditions, and it may be considered nonreactive compared to TMP. Both TMP and TEP are, however, readily photodegraded and yield orthophosphate as the final surface product, which shows that also weakly reducing organic molecules can be photo-oxidized on iron (hydr)oxides. The goethite nanorods show higher reactivity than the more uniformly shaped hematite nanoparticles, indicating that the anisotropic morphology of iron (hydr)oxide nanoparticles with one dimension Eg, the integration in eq 4 is performed up to wavelengths corresponding to Eg determined in the previous section. This gives Fabs = 2.0 × 1021 m−2 s−1 (hν > 2.14 eV) for hematite and Fabs = 1.7 × 1021 m−2 s−1 (hν > 2.28 eV) for goethite. The quantum yields Φ calculated from eq 7 are shown in Table 3. Employing eq 4 gives a lower bound of Φ, since only photons that absorb within the penetration depth of light and create electrons and holes that reach surfaces with adsorbed OP can contribute to the observed photoreactions. Here, only the outermost surface region of the nanopowders is exposed to OP after 20 min adsorption time. In fact, photogenerated holes in hematite are reported to have a very short diffusion length of 2−4 nm.20 Since the penetration depth of light is on the order 1 μm in porous samples, Φ may be underestimated by 2 orders of magnitude. This situation is very different from studies of colloidal particles dispersed in aqueous suspensions, where a higher proportion of the incident light can be utilized because of the shorter diffusion path required for e−h pairs to reach the solid−liquid interface in well-dispersed nanoparticle suspensions. Since Fabs is similar for hematite and goethite (within 17%), it is evident that the relative values of Φ for dry and water-precovered hematite and goethite on samples follow the trends for kd. Previous reports on the photodegradation of organics in aqueous environments have proposed that only organics that are strongly reducing, complexing agents and react already in dark may be photo-oxidized by hematite.69 Our results show that also organic molecules that are not strongly reducing (TMP and TEP) and even nonreactive in dark (TEP) are readily photo-oxidized on both hematite and goethite. However, also in our studies we observe a trend of higher Φ for the more reactive specie TMP (which also dissociates in the dark), which may support the argument that the photoreactivity scales with an activation barrier correlated to the moleculesurface binding energy.69 This hypothesis should be verified by further studies. We are not aware of any other gas−solid studies of the photocatalytic degradation of organophosphorus compounds on iron (hydr)oxide surfaces. Instead, we make comparisons with published work on liquid−solid photodegradation of organics on iron (hydr)oxide. Most of these studies treat the hematite phase. From these studies it can be concluded that the photocatalytic properties of iron (hydr)oxides depend sensitively on the morphology, size, and phase.5,14,19,69,71−73 Many studies show that nanosized and oriented nanoparticles (wire, or rod structures) have different (often higher) reactivity

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ASSOCIATED CONTENT

S Supporting Information *

Plots of irradiance, reference ATR-FTIR spectra of TMP, TEP, and DMP, and 2D correlation an analysis. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Tel.: +46 18 471 6383. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank G. Westin for sample preparation and fruitful discussions and A. Mattsson for film thickness measurements. This work was funded by the Centre for Environmental Research, CMF (grant no. 0632299).

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REFERENCES

(1) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions, Occurrence and Uses; VCH: New York, 1996. 14928



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