Surface Chemistry of Trimethyl Phosphate on α-Fe2O3 - The


The chemistry of trimethyl phosphate (TMP) was examined on the (012) crystallographic face of hematite (α-Fe2O3) using temperature-programmed desorpt...
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Surface Chemistry of Trimethyl Phosphate on α-Fe2O3 Michael A. Henderson* Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P. O. Box 999, MS K8-87 Richland, Washington 99352, United States

bS Supporting Information ABSTRACT: The chemistry of trimethyl phosphate (TMP) was examined on the (012) crystallographic face of hematite (α-Fe2O3) using temperature-programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS), and Auger electron spectroscopy (AES). TMP adsorbed at Fe3+ sites on the clean α-Fe2O3(012) surface through lone pair electrons on the PdO oxygen atom. A small portion of adsorbed TMP desorbed without decomposition; however, the majority of adsorbed TMP decomposed on the clean surface in a two-step process. The first step, occurring at or below room temperature, involved displacement of one methoxy group of TMP to form a surface methoxy and adsorbed dimethyl phosphate (DMP). In the second step, adsorbed DMP decomposed above 500 K to a 1:1 ratio of gaseous methanol and formaldehyde leaving phosphate on the surface. The phosphate was stable on the α-Fe2O3(012) surface to 950 K. Identification of these steps was assisted by using the chemistry of methanol on the clean surface. Coadsorption of TMP and water led to a small degree of hydrolysis between these two molecules in the multilayer but no significant changes in the chemistry of TMP molecules adsorbed on the surface.

1. INTRODUCTION Adsorbed phosphate species are known to influence the surface chemistry of iron oxides and oxyhydroxides.116 Phosphates are common adsorbates in the geochemistry and biogeochemistry of many iron-bearing minerals.1,12,13,15,16 For example, adsorbed phosphates have been shown to play an important role in the nucleation, growth, and dissolution patterns of iron oxide nanoparticles by selectively blocking certain facets compared to others.1,10,11,13,1723 This is partly due to the ability of surface phosphates to bind to oxide surfaces in a variety of adsorption configurations with varying degrees of stability depending on the extent of surface coordination, the pH, and the surface structure. Organophosphorus compounds have been utilized by researchers to better understand how phosphates interact with iron oxides and a variety of other oxide surfaces.9,2438 Many organophosphorus compounds are of sufficient volatility to allow gaseous exposure to surfaces at RT without the influence of a solvent. For example, trimethyl phosphate (TMP: OdP(OCH3)3) has been extensively used to study phosphateoxide interactions.9,24,2729,31,32,34,38 This molecule binds strongly to oxide surfaces, in part or whole through the lone pairs on the PdO center, and experiences hydrolysis of its POCH3 linkages through the influence of adsorbed OH/H2O. As such, the surface chemistry of organophosphorus compounds on oxides involves both oxide surface structure and the influence of adsorbed OH and H2O coverages. The ultrahigh vacuum (UHV) condition is ideally suited for examining both of these factors. Previous UHV work on the α-Fe2O3(012) surface3942 has permitted this author and co-workers to draw direct correlations between surface structure and surface chemistry. In this study, the (1  1) and (2  1) surfaces of α-Fe2O3(012) were employed as model hematite surfaces to examine the surface chemistry of TMP on hematite. The (1  1) surface, in particular, is of interest because r 2011 American Chemical Society

it has a nonpolar, ordered surface structure consistent with bulk termination along the [012] direction, and it has a fully oxidized (Fe3+) state consistent with stoichiometric α-Fe2O3. In contrast, the (2  1) surface possesses both Fe2+ and Fe3+ sites as a consequence of vacuum reduction, although its surface structure is not well-understood.3941 The aim of this study is to better understand the surface chemistry of organophosphorus compounds on hematite surfaces and how coadsorbed water influences that chemistry.

2. EXPERIMENTAL METHODS The UHV system used in this study was equipped with instruments for conducting TPD, SSIMS, AES, HREELS, and low energy electron diffraction (LEED) measurements. The base pressure of the chamber was 2  1010 torr. The α-Fe2O3(012) crystal (also referred to as the (0112) surface in the “right handed” four-vector coordinate system39) was 7  7  1 mm3 in size. The surface was cleaned by sputtering with Ar+ and annealing in UHV at 950 K and was monitored by AES and SSIMS.3941 The crystal was oxidized in O2 at 750 K and cooled in O2 to generate the fully oxidized (1  1) surface structure. The ideal (1  1) surface possesses two Fe3+ sites per surface unit cell corresponding to a surface site density of 7.29  1014 Fe3+/cm2, which is designated as the 1 monolayer (ML) coverage. The (2  1) surface was produced by annealing in UHV at 950 K. TMP surface chemistry on α-Fe2O3(012) resulted in P deposition on the crystal which could not be removed by annealing in vacuum at 950 K (the heating limit in these studies). Therefore, the crystal Received: September 16, 2011 Revised: October 25, 2011 Published: October 27, 2011 23527

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The Journal of Physical Chemistry C was routinely sputtered following every TMP experiment to reestablish a clean surface for the next experiment. Water and methanol (Aldrich ACS reagent grades) were purified with liquid nitrogen “freezepumpthaw” cycles. TMP is liquid at RT with a vapor pressure of about 0.8 torr.43 TMP was dosed onto the crystal using a pinhole aperture doser, the end of which was situated ∼1 mm from the crystal face during dosing. The inner diameter of the doser was smaller than the dimensions of the crystal, which ensured that TMP was dosed only on the crystal surface. The TMP source (Aldrich) had a cited purity of >99%; however, vapor from the source had detectable amounts of acetone and methylene chloride as impurities based on quadrupole mass spectrometry (QMS) measurements. The contributions from these gases when dosing TMP were diminished by pumping on the source at RT for long periods of time; however, they were not removed completely. Both contaminants were detectable in TPD experiments of TMP on α-Fe2O3(012). Fortunately, both of these molecules were displaced from the surface by multilayer coverages of TMP at the dosing temperature (120 K) as evidenced by their desorption features occurring below 180 K (see Supporting Information). As long as the initial TMP coverage was in excess of 1 ML coverage, no adverse effects of these impurities have been seen in TPD. The exposure of TMP routinely used throughout this study corresponded to roughly two layers of TMP on the clean α-Fe2O3(012) surface. Methanol and water were not detected as contaminants emitted from the doser, either as impurities in the source or as a result of TMP decomposition on the walls of the gas-handling system. The caveat to this was that the gas-handling system had to be extensively degassed and guarded against water exposure. Exposure of TMP to a water-covered gas line resulted in production of gaseous methanol, presumably through hydrolysis of TMP. (In contrast, the main dosing gas line was not affected, in terms of subsequent TMP use, by exposure to methanol.) Passivation of the gas line was accomplished by repeated cycles of TMP exposure and evacuation. In order to preserve the TMP gas line, water was exposed to the crystal by backfilling the chamber through a separate (independent) gas line. As a result, water TPD spectra in the coadsorption experiments were meaningless, being dominated by desorption signals from peripheral surfaces (such as the crystal mount and heating leads).

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Figure 1. Mass 29, 31, and 110 TPD spectra from multilayer coverages of TMP adsorbed at 120 K on the clean (1  1) and (2  1) surfaces of α-Fe2O3(012).

Comparisons of the QMS cracking patterns of standards, obtained both from the literature44 and from the QMS used in this study, lead to assignment of the mass 31 signal to methanol desorption and the mass 29 signal to both methanol and formaldehyde. (The methodology for deconvoluting these signals is discussed in the Supporting Information) Both masses also contributed signals in the TMP desorption features identified by the mass 110 signal. For the (1  1) surface, coincident mass 29 and 31 signals were detected at 560 K (15 K higher in temperature for the (2  1) surface). Previous work for methanol on α-Fe2O3(012)42 suggests that the coincident mass 29 and 31 signals evolving above 500 K in Figure 1 can be assigned to disproportionation of surface methoxy groups resulting in equivalent amounts of CH3OH and H2CO according to reaction 1 2CH3 OðaÞ f CH3 OHðgÞ þ H2 COðgÞ

3. RESULTS AND DISCUSSION 3.1. TMP on the Clean Surface. Figure 1 shows mass 29, 31, and 110 TPD spectra from multilayer TMP exposures at 120 K on the (1  1) and (2  1) surfaces of α-Fe2O3(012). These three masses in TPD were found to adequately characterize the thermal chemistry of TMP on α-Fe2O3(012). The mass 110 signal (filled-in traces) is the major QMS cracking fragment of TMP44 and therefore was used to portray the TPD properties of molecular TMP. For both surfaces, the mass 110 peak at 190 K was assigned to multilayer TMP desorption. The only other TMP desorption signal detected occurred in broad and weak desorption features that extended from the multilayer desorption peak to ∼350 K for both surfaces, indicative of some molecular TMP desorption occurring from the first layer on both surfaces. The amount of TMP in these features was not quantified. However, the majority of first layer TMP on either surface decomposed rather than desorbed as evidenced by the amounts of decomposition products in TPD and P left on the surface in AES (see below).

ð1Þ

where “a” and “g” represent adsorbed and gaseous species, respectively. (The formal charge on the methoxy species will dictate the extent to which the surface is reduced by this reaction.42) The mechanism for this disproportionation reaction is not clear, particularly as to whether it involves a concerted reaction between two methoxy groups or whether the decomposition of one methoxy to formaldehyde feeds hydrogen (via the surface) to other methoxy groups to form methanol. It is clear from TPD that little or no water was formed from either TMP or CH3OH (see the Supporting Information). This suggests that protons from the methoxy disproportionation process were conserved without generating water from surface O2 sites. In addition to the high temperature mass 29 and 31 desorption features, weaker signals at these masses were detected at 350 and 400 K on the (1  1) and (2  1) surfaces, respectively. The mass 29-to-31 ratio in these features was different from that in the high temperature features and was consistent with recombinative 23528

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The Journal of Physical Chemistry C

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desorption of methanol42 according to reaction 2 CH3 OðaÞ þ OHðaÞ f CH3 OHðgÞ þ OðbrÞ

ð2Þ

where “O(br)” denotes a surface bridging oxygen anion site. The 350 and 400 K peak temperatures and shapes were similar to those seen from the TPD of methanol on the clean (1  1) and (2  1) surfaces, respectively.42 In contrast to dosed methanol, the source of hydrogen needed to render recombinative CH3OH desorption in these TPD states from dosed TMP is not apparent because the parent TMP molecule does not possess acid protons. As stated in the Experimental Section, water and methanol were not detected as contaminants in the TMP source, so “dosed” sources of protons can be excluded. M€akie et al.9 recently proposed that TMP decomposes at RT on α-Fe2O3 nanoparticles to generate adsorbed dimethoxyphosphate species (DMP: PO2(OCH3)2) and adsorbed methoxy groups, as shown in reaction 3: TMPðaÞ þ OðbrÞ f DMPðaÞ þ CH3 OðaÞ

ð3Þ

According to these authors, some adsorbed methoxy groups decomposed at RT to generate formate according to reaction 4: CH3 OðaÞ þ 2OðbrÞ f HCOOðaÞ þ 2OHðaÞ

ð4Þ

Combining reactions 2 and 4 essentially amounts to utilizing hydrogen from decomposition of some CH3O groups to generate CH3OH from other CH3O groups (reaction 5): 3CH3 OðaÞ f 2CH3 OHðgÞ þ HCOOðaÞ

ð5Þ

However, there are a few issues associated with using reactions 35 to explain the 350400 K CH3OH TPD features in Figure 1. First, the formal charges of species in these reactions are not understood, and the possibility of redox changes in the surface Fe cations is not considered. Second, the only evidence for CH bond cleavage (leading to adsorbed formate) at low temperature in this study was the presence of CH3OH at 300450 K. CH bond cleavage was not detected on the α-Fe2O3(012) surface from adsorbed methanol.42 Formate has been detected in the decomposition of methanol on hematite powders.45,46 Formate decomposes above 500 K on α-Fe2O3 to CO and/or CO2.46,47 The absence of a signal in the TMP TPD data corresponding to formate decomposition (to CO or CO2) may be due to interference from cracking of CH3OH and H2CO occurring above 500 K. The amount of formate that would be generated based on reaction 5 should be half the amount of methanol formed at 350400 K. Using CH3OH TPD on the clean surface as a standard (see the Supporting Informtion), the amounts of CH3OH evolving according to reaction 5 (between 300 and 480 K) on the (1  1) and (2  1) surfaces were estimated at ∼0.22 and 0.20 ML, respectively (Figure 1). These estimates place the potential formate coverage at only ∼0.1 ML, which could be difficult to detect in these TPD experiments. Third, it is unclear why some CH3O groups would experience CH bond cleavage at low temperature (e300 K) while other CH3O groups would be stable on the surface to high temperature (>500 K). The explanation of this latter issue may be found in the different reactivities of POCH3 groups and adsorbed methoxy species on the α-Fe2O3(012) surface (to be discussed). The amount of CH3O that contributed to the CH3OH + H2CO desorption features above 500 K can be estimated essentially by doubling the amount of CH3OH detected. Using the mass 31 TPD peak area from CH3OH on the clean (1  1) surface as a standard,42 the coverages of CH3O responsible for

Figure 2. AES from the clean α-Fe2O3(012) surface (solid circles) and the clean α-Fe2O3(012) surface after TPD of a multilayer coverage of TMP (empty circles).

the CH3OH + H2CO TPD peaks above 500 K were 0.64 and 0.63 ML for the (1  1) and (2  1) surfaces, respectively. If the mechanism for TMP decomposition on hematite proposed by M€akie and co-workers9 is adopted (see reactions 35), then the coverage of TMP (or more precisely, DMP) responsible for the high temperature CH3OH + H2CO desorption states should be half the amount of CH3O that led to these desorption states, or ∼0.32 ML. In this model, each TMP decomposed to produce one adsorbed DMP species (reaction 3), which then decomposed at high temperature to generate two methoxy groups (reaction 6) DMPðaÞ þ 2OðbrÞ f PO4ðaÞ þ 2CH3 OðaÞ

ð6Þ

which in turn disproportionated (reaction 1). This estimate is also consistent with the amount of CH3OH detected between 300 and 480 K (reaction 5, see above). AES analysis after TPD not only provided additional evidence for TMP thermal decomposition on hematite but also a means of estimating the extent of TMP decomposition. Figure 2 presents AES spectra obtained from the clean surface (filled circles) and following TPD (up to 700 K) of multilayer TMP exposures (empty circles). The AES spectrum of the clean surface is consistent with that of α-Fe2O3.11,26 The AES spectrum after TPD of a multilayer TMP exposure on the (1  1) α-Fe2O3(012) surface has a prominent P feature at 108 eV, consistent with phosphate deposition on α-Fe2O3.26 This spectrum was free of C indicating that all the methoxy groups associated with the deposited P evolved as gaseous products during TPD. The peak-to-peak height ratio of the P 108 eV feature to that of the Fe 703 eV feature was ∼0.28 (0.22 if the Fe 651 eV line is used). This ratio is comparable to the ∼0.25 ratio observed after thermal decomposition of DMMP on a polycrystalline α-Fe2O3 surface.26 Nooney et al.11 used AES to analyze the uptake of phosphate from solution by an α-Fe2O3 thin film. These authors found that the phosphate uptake reached first layer saturation rapidly followed by the slower growth of iron 23529

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The Journal of Physical Chemistry C phosphate clusters. They estimated that first layer phosphate saturation on α-Fe2O3 occurred at a coverage of ∼3  1014 molecules per cm2 with an AES P/Fe(651) ratio of ∼0.35. Comparison of the AES results of Nooney et al. with those in this study after TMP TPD suggests that the phosphate coverage resulting from TMP decomposition on α-Fe2O3(012) was roughly 63% of that obtained from the solution phase phosphate uptake measured by Nooney and co-workers. This phosphate coverage on the α-Fe2O3(012) surface would correspond to ∼0.26 ML of TMP based on the cation site density on the ideal (1  1) surface (7.29  1014 cm2). This analysis should be taken as a low-sensitivity estimate of TMP decomposition compared to estimates obtained by TPD because AES measurements can be dependent on the sample and spectrometer conditions. Also, the uptake of phosphate from solution on a polycrystalline film is likely to be greater than the decomposition level measured by TPD in this study simply based on molecular size constraints. The vibrational properties of TMP on the (1  1) surface of α-Fe2O3(012) were examined in Figure 3 using HREELS to provide additional insights into TMP decomposition. Trace a shows the raw spectrum from the clean surface. The primary phonons of α-Fe2O3 were at 380, 485, and 640 cm1, with multiple scattering phonon losses located above 800 cm1. (The latter were not overtones or combinations but simply electrons that experienced multiple scattering events.) The multiple phonon modes were predominately removed by Fourier deconvolution,48 leaving a small remnant feature at 1280 cm1 (trace b). The remnant is likely from multiple scattering events, with one event being nondipole related and thus not removed with Fourier deconvolution.48 Trace c corresponds to the HREELS spectrum from a multilayer TMP exposure on the clean (1  1) surface at 120 K. The prominent TMP-related losses were at 850, 1045, 1255, 1450, and 2980 cm1. Based on vibrational studies of condensed phase TMP, these losses can be assigned to ν(POC), ν(POC), ν(PdO), δ(CH3), and ν(CH3) modes, respectively.28,31,43,4955 The PdO stretching mode is typically a sensitive indicator of structure in organophosphorus compounds. The PdO group in an isolated TMP molecule has a ν(PdO) frequency above 1300 cm1. Configurations of TMP molecules that allow intermolecular or intramolecular CH 3 3 3 OdP interactions have been shown to red shift the ν(PdO) mode to below 1300 cm1.31,4955 For example, the Viswanathan group4952 employed theory and vibrational spectroscopy to study the conformations of TMP trapped in various matrices. They found that conformations of TMP that permitted intramolecular CH2H 3 3 3 OdP interactions resulted in a ν(PdO) frequency red-shifted to ∼1287 cm1. (The effects of conformation changes in TMP were also seen to a lesser extent in the ν(POC) modes (8001000 cm1).) These researchers observed an additional red shift in the ν(PdO) mode to below 1270 cm1 for TMP and H2O coisolated in N2 or Ar matrices that could be attributed to single or multiple HOH 3 3 3 OdP interactions. Similarly, Reva et al.53 observed conformational changes in matrix-isolated TMP using FTIR. These interactions have also been detected in the gas phase.28,43 The prominent ν(PdO) at 1255 cm1 in the HREELS spectrum of multilayer TMP (Figure 3c) is consistent with intermolecular or intramolecular interactions between PdO and CH3O groups. Desorption of the multilayer occurred by heating to 200 K (see Figure 1), and the HREELS spectrum from the resulting surface (Figure 3d) was similar to the multilayer spectrum with a few exceptions. One difference in the HREELS spectrum after multilayer TMP desorption was the weaker intensities of the 850

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Figure 3. Fourier deconvoluted HREELS spectra for TMP on the (1  1) α-Fe2O3(012) surface at 120 K: (a) clean surface, not deconvoluted; (b) clean surface, deconvoluted; (c) multilayer TMP; and (df) “c” heated to 200, 400, and 700 K, respectively. All spectra recorded at 120 K. Spectra (bf) are multiplied by 10 and displaced vertically for clarity.

and 1045 cm1 losses (relative to the other modes). However, the most notable difference was that the ν(PdO) feature experienced an additional red shift to 1185 cm1 for adsorbed TMP. Previous studies of TMP adsorbed to surface cation sites on oxides have shown red shifts in the ν(PdO) mode that were attributed to TMP coordinated via lone pairs on the PdO oxygen. For example, Li et al.31 examined the coordination of TMP to MgO using FTIR. They observed a red shift in the ν(PdO) mode from the liquid phase value of 1275 cm1 to ∼1220 cm1 which indicated coordination of TMP through lone pairs on the PdO oxygen atom to Mg2+ surface cations. However, the ν(POC) mode (at ∼1042 cm1) and most of the other TMP modes for the adsorbed molecule did not experience significant shifts relative to the liquid phase modes. Kanan and Tripp28,29 observed a red shift in the ν(PdO) mode of TMP to 1268 cm1 resulting from coordination of TMP to surface OH groups on silica. They also observed a coincident red shift in the sharp ν(OH) mode of surface OH groups on silica from 3747 cm1 to a broad feature at 3262 cm1 after TMP desorption. Similarly, Kanan et al.27 found that TMP molecules coordinated to surface OH groups on WO3 had a ν(PdO) mode at 1275 cm1 whereas those coordinated to surface W6+ cations had a ν(PdO) mode at 1223 cm1. More recently, M€akie and co-workers9 examined the interaction of TMP with nanoparticles of hematite, maghemite (γ-Fe2O3), and goethite (α-FeOOH) using diffuse reflection FTIR. Based on red shifts in the ν(PdO) mode, they proposed that TMP coordinated primarily to Fe3+ sites on hematite and maghemite, but to surface OH groups on goethite. Heating to 400 K caused more significant changes in the HREELS spectrum of TMP on α-Fe2O3(012) (Figure 3e). At this temperature, most (if not all) of the adsorbed TMP molecules had clearly decomposed to some extent as evidenced by disappearance 23530

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The Journal of Physical Chemistry C of the 850 cm1 mode. Reva et al.53 found that the intense mode in the 820870 cm1 region which was found to be sensitive to the TMP conformation could not be described simply as a single, uncoupled POC stretch but was instead described as possessing both ν(POC) and ν(POC) character in the TMP molecule. Disappearance of this feature signaled decomposition of TMP. Additional changes in the HREELS spectrum of TMP seen by heating to 400 K included diminishment of the CH3 modes at 1450 and 2980 cm1 (consistent with desorption of some CH3OH) and weakening of the modes in the spectral range between 900 and 1400 cm1. The appearance of a weak peak at 3580 cm1 suggests some OH groups formed as a result of CH bond cleavage, supporting the mechanism depicted in reactions 35. All CH3-related modes were absent after heating to 700 K (Figure 3f). The only HREELS features remaining were located at 955 and 1260 cm1, with a poorly defined feature at ∼1100 cm1. Given that at 700 K the surface only contained P based on AES (Figure 2), the losses between 900 and 1300 cm1 can be assigned to surface phosphate. The strong feature at 1260 cm1 was either suggestive of at least one PdO bond existing in the adsorbed phosphate species or the reappearance of the phonon remnant from the “clean” surface (see Figure 3b). Elzinga and Sparks3 used FTIR to characterize phosphate on α-Fe2O3. As these authors pointed out, the number of PO stretching modes observed depended on the adsorption symmetry of the phosphate species. By varying the pH, these authors were able to vary the adsorption symmetry of phosphate between three different species with symmetries from C1 to C3v. Similar observations were made by TejedorTejedor and Anderson for phosphate adsorption on goethite.12 Comparisons with these studies are tenuous because of the availability of water in those studies to make POH groups but not in these UHV studies. Nevertheless, it would appear based on the number of features in Figure 3f between 900 and 1300 cm1 that either multiple forms of adsorbed phosphate were present on the surface or that the one form present had a low adsorption symmetry. The reactions associated with TMP decomposition on the (1  1) surface of α-Fe2O3(012) were probed with temperatureprogrammed SSIMS (TPSSIMS) in Figure 4. SSIMS is useful for monitoring changes in surface species as a function of surface treatment (e.g., heating). In these data, various secondary ion signals resulting from Ar+ sputtering were monitored in the static domain (i.e., 500 eV primary Ar+ and