Disordered Mesoporous Zirconium (Hydr)oxides for Decomposition of

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Disordered Mesoporous Zirconium (Hydr)Oxides for Decomposition of Dimethyl Chloro-Phosphate Jonathan Colón-Ortiz, John Landers, Wesley O. Gordon, Alex Balboa, Christopher J. Karwacki, and Alexander V. Neimark ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00843 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Applied Materials & Interfaces

Disordered Mesoporous Zirconium (Hydr)Oxides for Decomposition of Dimethyl Chloro-Phosphate

Jonathan Colón-Ortiz1, John M. Landers2, Wesley O. Gordon3, Alex Balboa3, Christopher J. Karwacki3, Alexander V. Neimark1,* 1Chemical 2National

and Biochemical Engineering Department, Rutgers University, Piscataway, NJ 08854

Research Council (NRC) Fellowship

3Edgewood

Chemical Biological Center, Aberdeen Proving Ground, MD, 21010

KEYWORDS: metal-oxides, mesopore formation, nanoporous structures, chemical warfare agent decomposition, surface modification, nerve agent simulants. ABSTRACT: A facile method for the formation of mesoporosity within non-porous zirconium hydr(oxides) (ZrO2/Zr(OH)4) is presented and investigates their detoxifying capabilities against dimethyl chlorophosphate (DMCP). Nanoaggregates of ZrO2/Zr(OH)4 appear to be deposited on larger thin flakes of the same material. H2O2 is used to induce surface oxygen vacancies of synthesized ZrO2/Zr(OH)4 and as a consequence mesopores with an average diameter of 3.1 nm were formed. Surface area of H2O2 treated ZrO2/Zr(OH)4 was increased by an order of magnitude and shows enhanced reactivity towards DMCP. DRIFTS spectroscopy is employed to assess the reactivity differences between the H2O2 treated and untreated ZrO2/Zr(OH)4. Peaks at 1175 and 1144 cm-1 indicate the presence of asymmetric stretching of O–P–O moiety within dimethyl phosphonate (DMHP), a decomposition product from DMCP, and a zirconium-bound methoxy group, respectively. It is suggested that the decomposition of DMCP proceeds through the consumption of bridged hydroxyl groups (b-OH) for both the untreated and H2O2-treated sample, as well as an additional hydrolytic decomposition pathway for the H2O2-treated sample.

oxides. Some examples of the physical changes, discussed herein, are the increased surface area to volume ratio and the transformation of crystalline phases into amorphous phases. A significant increase of the surface area comes from the formation of mesopores that enhances the catalytic efficiency. Given that CWAs are lethal, CWA simulants are used instead in experimental studies. Within nerve agent simulants, dimethyl methyl phosphonate (DMMP) is one of the most prevalent choices in literature to mimic sarin (GB) interaction on surfaces due to its low toxicity.26-30 One of the drawbacks of comparing DMMP reactivity against GB reactivity is that although these compounds have similar molecular size they differ significantly in their chemistry. DMMP lacks the highly reactive halogen atom that is present in GB, which is responsible for GB binding within acetylcholine esterase (AChE) enzymes causing inhibition of motor functions within the brain. By comparison, only a few studies have chosen the simulant dimethyl chlorine

INTRODUCTION Metal oxides in different forms find multiple applications including semi-conductors1-3, sensors4-6, water-purification7-8, and photocatalysts.9-11 Zirconium (hydr)oxides (ZrO2/Zr(OH)4) have been found to be promising in decomposing chemical warfare agents (CWA)12-13 and simulants.14-16 Moreover, Zr based metal organic frameworks (MOF), such as NU-100017-19 and UiO-66/UiO-6720-23 show exceptional results with respect to decomposing a variety of CWA and simulants in buffer solutions, where decomposition is proposed to occur at the Zr node. With the reactive nature of Zr(OH)4 as a baseline, we aim to enhance the reactivity by modifying the surface chemistry of ZrO2/Zr(OH)4 with the introduction of defects. Warm hydrogen peroxide solutions have been used to induce surface defects, such as oxygen vacancies, within metal oxides in order to increase their reactivity.24-25 One consequence of using hydrogen peroxide on metal oxides is that, in addition to surface chemistry alterations, there are also physical changes within the

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phosphonate (DMCP), even though it is considered a more accurate simulant for assessing reactivity.31-35 In this work, DMCP is employed to demonstrate the reactivity of ZrO2/Zr(OH)4 mesoporous catalysts that are synthesized through hydrogen peroxide treatment of non-porous ZrO2 particles.

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(version 5.981) was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were fitted using Gauss–Lorentz curves with an L/G ratio of 0.3, in order to determine the binding energy of the various components of each element core level. The instrumental resolution of the spectrometer is 0.5 eV. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The in situ IR spectra were recorded with a Nicolet 6700 (Thermo Electron Corporation, Madison, WI) spectrometer equipped with a MCT-A (HgCdTe) detector cooled with liquid N2, and with a diffuse reflection accessory (DiffusIR, Pike Technologies, Madison, USA) and a diffuse reflectance cell (DiffusIR environmental chamber, Pike Technologies, Madison, USA). For the DRIFT analysis, the resolution was 2 cm-1, and 128 transients were acquired per spectrum to provide a good balance between signal-to-noise and decent time resolution. The optical path was kept free of ambient CO2 and water contributions by flowing N2 through the spectrometer and diffuse reflection accessory. Before introducing the reactant DMCP to the catalyst, the surface materials were dried over 4 days in a N2 (g) purge and then placed into the DRIFTS cell. The cell was then evacuated under vacuum for approximately 120 minutes at room temperature. After evacuation, a He flow of 1.3 mL/min was introduced to the system to equilibrate the system before reactant gas introduction. After ~60 minutes, DMCP vapor was introduced into the system. The in-situ DRIFTS setup includes a glass saturator cell that holds the DMCP liquid, and once the He flows through the saturator, the vapor is carried into the system. A more detailed description of the DRIFTS setup is explained in the Supplemental Section. Once the DMCP is introduced into the system, an average of 128 scans per IR spectrum is taken every minute for 60 minutes. For all of the IR subtracted spectra shown below, the IR spectrum of the material before DMCP exposure was subtracted from the IR spectra after exposure.

EXPERIMENTAL SECTION Metal (hydr)oxides synthesis. All reagents for the synthesis of ZrO2/Zr(OH)4 were purchased from Sigma-Aldrich with a purity above 99% and were used as received. ZrO2/Zr(OH)4 were synthesized by combining 250 mL of 0.25M (aq) solutions of ZrO(NO3)2·xH2O with 1M (aq) NaOH at room temperature under stirring conditions. The resulting metal-hydroxide precipitates (approximately 8.5 g recovered) were washed three times with approximately 15 mL of deionized water while being filtered with a vacuum-filtration unit consisting of a polyethersulfone (PES) membrane with an average pore size of 0.2 µm. After filtering, the ZrO2/Zr(OH)4 were dried at 100 °C for 2 hours in a Fisher Isotemp 500 series oven. Formation of mesoporosity within ZrO2/Zr(OH)4. This method has been previously used for the formation of oxygen vacancies within metal-(hydr)oxides as presented elsewhere.24, 36 Half of the recovered metal-(hydr)oxides (approximately 4.0 g) were transferred into 200 mL of a 1M solution of H2O2 at 70 °C for 2 hours under stirring conditions. Subsequently, they were washed, filtered, and dried for 2 hours at 100 °C using the same protocol described in Section 2.1.1. The other half of the recovered metal-(hydr)oxides was left untreated and used for comparison. Surface area and porosity. Surface area and total pore volume of samples were obtained from an Autosorb-1 N2 gas adsorption instrument (Quantachrome Instruments). Adsorption/desorption isotherms were obtained at 77 K and 79 points were recorded including 11-points for BET surface area analysis using Quantachrome’s Autosorb software version 1.55. Pore size distributions were calculated using a cylindrical pore non-local density functional theory (NLDFT) model, based on the adsorption branch with N2 at 77 K adsorption isotherm kernel. Outgassing was performed at 300 °C for three hours. X-Ray Diffraction (XRD). X-ray diffractograms were obtained using a Phillips XPert diffractometer (Bragg-Brentano geometry) with a CuKα anode (1.5405 Å). The instrument was operated at 45kV and 40mA with a 0.02 °/step acquisition rate with a dwell time of 2 seconds/step from 2θ angles over the range of 10 ° to 60 ° using a 0.3 mm fixed receiving slit. The software used to collect x-ray diffractograms was X’Pert Data Collector version 2.0e. Transmission Electron Microscopy (TEM). Transmission electron micrographs were obtained by a JEOL 1200EX electron microscope with AMT-XR41 digital camera with an accelerating voltage of 80 kV and 2-seconds sample exposure time. Samples were supported on lacey carbon type-A 300 mesh copper grids. X-ray photoelectron spectroscopy (XPS). XPS analysis was carried out a Thermo Scientific K-Alpha XPS spectrometer with non-monochromatic Al K α radiation (1486.7 eV) with a 128 channel detection system using a 400 μm diameter analysis area. The Thermo Scientific Avantage software package

RESULTS AND DISCUSSION Mesopore Formation Within ZrO2/Zr(OH)4. Nitrogen adsorption experiments, Figure 1a, indicate that there is formation of mesopores during H2O2 treatment on the ZrO2/Zr(OH)4 samples as there is increased adsorption of nitrogen and the presence of a hysteresis loop on H2O2 treated ZrO2/Zr(OH)4. A non-local density functional theory (NLDFT) model was used to deconvolute the pore size distributions of untreated and H2O2 treated ZrO2/Zr(OH)4 based on the adsorption branch, Figure 1b. The NLDFT model estimates that the average pore size for H2O2 treated ZrO2/Zr(OH)4 is around 3.1 nm, while the untreated ZrO2/Zr(OH)4 remain essentially non-porous. In addition, the untreated ZrO2/Zr(OH)4 exhibit a surface area of 12.4 m2/g while the H2O2 treated ZrO2/Zr(OH)4 have an order of magnitude increase in surface area of 101.8 m2/g. Similar results were obtained for cerium (hydr)oxides prepared in the same manner, but not for zinc (hydr)oxides (see Figure S1 and Table S1). H2O2 has minimal effect on the formation of mesopores within zinc (hydr)oxides. This may be due to its single valence state that may not allow for a density of oxygen vacancies high enough to disrupt its crystalline structure.


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Figure 1. (a) Nitrogen adsorption isotherms for ZrO2/Zr(OH)4 (untreated – red, H2O2 treated – blue). Solid squares are adsorption points and open squares are desorption points. (b) Pore size distributions based on NLDFT model (untreated – red, H2O2 treated – blue) between both samples. The untreated sample, which is Impact of H2O2 on Surface Chemistry of ZrO2/Zr(OH)4. predominantly Zr(OH)4, is composed of approximately of 25.4 Oxidation states of ZrO2/Zr(OH)4. The core energy level spectra at.% ZrO2 and the H2O2-treated sample, which is predominantly of O 1s and Zr 3d are collected in Figure 2. For the untreated ZrO2, is composed of approximately of 22.2 at.% Zr(OH)4 ZrO2/Zr(OH)4, the position and shape of the peaks related to Zr based on the Zr–O and O–H components in the O1s peaks at 3d are representative for the oxidation state +4 37, Figure 2a, in 529.1 and 532.7 eV, respectively in Figure 2(b,d). which both the Zr 3d3/2 and Zr 3d5/2 are located at 181.2 and 183.8 eV, respectively.38 Of the four components on the O 1s Impact of H2O2 in the Morphology of ZrO2/ Zr(OH)4. The spectrum, Figure 2b, the first two peaks located at binding morphological structures of the ZrO2/Zr(OH)4 were studied by energies of 529.1 and 530.8 eV are assigned to the lattice Zr–O Transmission Electron Microscopy (TEM) and X-ray diffraction bonds and surface chemisorbed (XRD). The TEM images, Figure 3, show that nanoaggregates oxygen, respectively.37, 39-40 These surface chemisorbed species within 25-200 nm are densely supported on micron-sized thin 2arise from the presence of O , O2 , and O , which have been flakes for both untreated and H2O2-treated samples. The H2O2 associated with the formation of surface oxygen vacancies.40 It treatment appears to have an insignificant effect on the particle has been shown that the presence of oxygen vacancies within the surface of metal-oxides has proven to enhance the (b) (a) decomposition of CWA.24, 36, 40 The reactive oxygen vacancies present within the ZrO2/Zr(OH)4 as seen from our XPS studies in Figure 2(b,d) may have an important role in the decomposition of DMCP. However, isolating the effect of oxygen vacancies against the decomposition of DMCP were outside the scope of this study. The lower intensity peaks at 532.7 and 536.1 eV can be linked to the presence of the hydroxyl groups bonded to zirconium and the presence of (c) (d) humidity or surface bound water, respectively.41 Although there is evidence of mixed valence states in H2O2-treated metal oxides24 in our case there was no apparent change in the oxidation state of zirconium during the H2O2 treatment, Figure 2c. In addition, it can be seen in Figure 2d that the surface bound water molecules mostly disappear and the surface hydroxyls are significantly reduced when compared to the untreated ZrO2/Zr(OH)4. This reduction in O-H and H2O species from the Figure 2. (a) Zr 3d core level spectrum of untreated ZrO2/Zr(OH)4 sample after H2O2 treatment has been observed theoretically42 (b) O 1s core level spectrum of untreated ZrO2/Zr(OH)4 (c) Zr 3d as H2O2 molecules interact with O-H and H2O species adsorbed core level spectrum of H2O2 treated ZrO2/Zr(OH)4 (d) O 1s core on the surface of the sample by being cleaved and forming level spectrum of H2O2 treated ZrO2/Zr(OH)4. highly reactive H–O radicals that later react with other adsorbed species producing water molecules that leave the oxide surface. size distribution of the ZrO2/Zr(OH)4, although there is more However, treated and untreated samples contain hydroxyl abundance of large thin flakes on the treated samples. groups. Therefore, we refer to treated and untreated samples as (hydr)oxides as both, oxide and hydroxide, phases coexist


ACS Applied Materials & Interfaces 35

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Figure 3. (a) Aggregate size distributions of untreated ZrO2/Zr(OH)4, inset – TEM micrograph of untreated ZrO2/Zr(OH)4. (b). Aggregate size distributions of H2O2-treated ZrO2/Zr(OH)4, inset – TEM micrograph of H2O2 treated ZrO2/Zr(OH)4.

good agreement with the desorption value of 52.4±0.6 kJ/mol for DMCP determined by Wilmsmeyer et al. Furthermore, the calculated charge on the sp2 oxygen of GB was determined to be nearest in value to the charge on DMCP (-0.681 and -0.664 (a.u.)), respectively. Intuitively, this appears to be a coherent choice with the structural differences between GB and DMCP, which involves a substitution of the fluorine and methyl on GB with the chlorine and methoxy group on DMCP, respectively. DRIFTS measurements were performed to assess the reactivity differences between the H2O2 treated and untreated ZrO2/Zr(OH)4 samples. Time-resolved difference spectra are reported herein, with the initial (black) and final (brick red) in order to identify spectral changes with respect to the native material. Peak assignments were made based on DFT calculations of DMCP and corroborated with literature assignments for 1) DMCP in the gas phase,33 2) on previously reported solid substrates,34 3) DMCP adsorbed onto silica33 and 4) DMMP on Zr(OH)4.26 Characteristic spectral features are observed between the H2O2 treated and untreated ZrO2/Zr(OH)4. The decomposition of organophosphorus agents on metal (hydr)-oxides have been reported through a two-step mechanism of first bonding of the phosphorus moiety followed by hydrolysis to form dimethyl phosphonate (DMHP) followed by methanolysis to form methyl methylphosphonate and methanol.14 However, a close inspection of our spectral data indicates the picture to be more complex. The structure of ZrO2/Zr(OH)4 is known to consist of a complex array of reactive moieties ranging from various absorbed hydroxyl species (monodentate, bidentate) peroxide species, water and Lewis acid sites such as oxygen vacancies as shown in Scheme 1. It is generally agreed upon that the primary adsorption pathway of organophosphorous agents will occur through the P=O moiety on metal oxides.44 Within our investigation, it was revealed that several reaction pathways are likely to occur. For instance, upon DMCP adsorption to the H2O2-treated sample, there is an observed increase in the P=O at 1256 cm-1 stretching

In Figure 4 it can be seen that parental samples of ZrO2/Zr(OH)4 prior to H2O2 treatment exhibit the crystalline features that are found within cubic-lattice ZrO2, less intense but sharp peaks of Zr(OH)4 and impurities that come from the precursors used to synthesize the ZrO2/Zr(OH)4, in this case NaNO3. However, after the treatment, the cubic-lattice features disappear and the treated sample appears to be rather amorphous, suggesting that the H2O2 treatment vigorously transforms the crystallinity of the material. 2800

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Figure 4. X-ray diffractogram of untreated ZrO2/Zr(OH)4 (red) and H2O2 treated ZrO2/Zr(OH)4 (blue). Symbols above peaks represent phases of: (*) ZrO2, (†) Zr(OH)4, (‡) NaNO3.

Reactivity of H2O2 Treated ZrO2/Zr(OH)4 Against CWA Surrogates. Based on a previous study that investigated the interaction energy of 5 GB simulants with a silica surface, Wilmsmeyer et al.32 found a linear correlation between the density functional theory (DFT) calculated charge of the sp2 oxygen on each simulant with the experimentally determined desorption energy of said simulant from temperature programmed desorption (TPD) measurements. A further study determined the desorption value of GB to be 50±2 kJ/mol,43 in


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ACS Applied Materials & Interfaces that occurs for the first 30 min, which is the amount of time for the DMCP vapor to diffuse through the packed bed. After 30 min, there is a linear increase in both peaks, which suggests first order kinetics, the height increases for both peaks appear

mode with a decrease in a mode at 1288 cm-1 that follows, as shown in Figure S2. DFT calculations (see Figure S4) demonstrate that the P=O mode of DMCP is red shifted by nearly 25 wavenumbers for the DMHP product, in observance with our experiments. Examining the results of DMHP adsorption we see that the P=O mode at 1256 cm-1 of DMCP is no longer present, but a greater decrease in the 1288 cm-1 mode is observed, as shown in Figure S3. Scheme 1. Different reactive sites and hydroxyl groups present on

the ZrO2/Zr(OH)4 structure.

This broad peak is likely a convolution of several surface modes that obscures the DMHP P=O mode. Nevertheless these results would imply the conversion of DMCP to DMHP on the H2O2treated sample, and the rapid loss of the P=O mode in DMHP upon adsorption. This is in contrast to the control sample where the P=O mode related to DMHP is not observed. This indicates that the H2O2-treated sample allows for an additional reaction pathway that is absent in the control as shown in Scheme 2. Several reaction pathways are plausible based on how DMCP approaches the surface, either through P=O approach (A) or Cl approach (B). For both the control and H2O2-treated sample, DMCP likely undergoes Pathway 1a, whereby only bidentate (bridged) hydroxyls are consumed (see Figure 7 for the high wavenumber region) to form methyl chlorophosphate (MCP). This results in the hydrolytic decomposition through the loss of a methoxy group (CH3O) which can be observed in the low wavenumber region (see Figure S5). Pathway 2b demonstrates a hydrolytic mechanism of DMCP with the reaction of tightly bound water along with cleavage of the chlorine group to form DMHP, respectively. While Pathways 1b and 2a may be plausible, we do not see any spectral evidence through the formation of DMHP in the former. Additionally, while we cannot rule out the cleavage of the chlorine atom, detecting the chlorine surface mode that occurs below 1000 cm-1 is below the detection limit. DMCP binding peaks can be inferred from the low wavenumber region from 1400-900 cm-1. Several regions are observed that correspond to binding of the DMCP molecule and possible products. For the H2O2 treated sample (Figure 5), we observe peaks at 1175 cm-1 and 1144 cm-1 with the former representing the υa(O-P-O) mode and the latter the dissociative methoxy bound to a zirconium atom and possibly an additional υ(P-Ox) mode.26 Plotting the peak heights of the two largest modes in this region (1175 cm-1 and 1144 cm-1) as a function of time (inset of Figure 5), it can be observed that there is a lag phase

Scheme 2. a) Plausible reaction pathways involving b–OH groups affecting both the control and defective sample. b) Plausible reaction pathways involving water which only affects the defective sample. Red indicates reactive surface bound species. c) The two pathways concluded here

to be the same as in the untreated sample, Figure 6, but with significantly less intensity. Meanwhile, a third peak associated with the υs(O-P-O) mode can also be observed at lower wavelengths of 1080 cm-1 which is known to occur approximately 100 cm-1 lower than the higher energetic asymmetric peak.45 All three peaks represent possible products and are corroborated by the fact that they do not appear in the DFT calculated spectra nor in the gas phase of DMCP (Figure S6 & S7). The peak centered at 1030 cm-1 is associated with the υ(C-O) mode of DMCP. Peaks related to the P=O stretching liquid/multi-layer mode of DMCP can be observed at 1256 cm-1 and is supported by our DFT calculations, see Figure S7.



Δ Absorbance (a.u.)

Interfacial carbonate complexes were also found on the surface residing in the region of 1700-1300 cm-1 which are eventually displaced with exposure to DMCP, see Figure S8. The untreated ZrO2/Zr(OH)4 sample demonstrates two orders of magnitude decrease in the observed signal compared to the H2O2-treated sample, which may be a qualitative indicator for the lower adsorption of DMCP. It should be noted that for the H2O2-treated sample, the dominant peak is the asymmetric (OP-O) mode while for the untreated sample it is the lower energetic symmetric stretch. Peaks could not be observed below 900 cm-1 for both materials, as metal oxides strongly absorb in this region.

1364

Figure 6. Low wavenumber region of DMCP on untreated ZrO2/Zr(OH)4 between 1400-900 wavenumbers. Inset: Peak height as a function of time. O = 1115, X = 1095 cm-1. Modes in red represent those attributed to decomposition product. Time intervals between spectra are: 0, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 min


only one observed change at higher wavenumbers for the untreated sample, namely the decrease that occurs at 3700 cm1, which is related to the bidentate (bridged) hydroxyl groups (b–OH). It should be noted that the apparent differences between XPS and DRIFTS data regarding the presence of OH and water molecules adsorbed at the sample’s surfaces arises because the evacuation pressure was 3 orders of magnitude higher for XPS (~10-8 torr) than for DRIFTS (~10-5 torr) which leads to reduced amounts of weakly adsorbed species.

Figure 5. Low wavenumber region of DMCP on H2O2-treated ZrO2/Zr(OH)4 between 1400-900 wavenumbers. Inset: Peak height as a function of time. O = 1177, X = 1144 cm-1. A lag phase occurs for the first 30 minutes, after which a linear increase in products could be observed. Modes in red represent those attributed to decomposition product. Time intervals between spectra are: 0, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 min

2996 υa(CH3P) 2956 υa(CH3O)


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The contrast between the two samples is more apparent at higher wave numbers in the region between 4000-2700 cm-1 (Figure 7 and 8). Upon exposure to DMCP, there is a decrease for the H2O2-treated sample in the amount of free bi-dentate hydroxyl groups on the surface, occurring at 3700 cm-1 and 3628 cm-1, respectively, indicating the decomposition of DMCP upon contact with the surface. The inset of Figure 7 plots the decrease of bridged hydroxyls (b–OH) and tightly bound water modes at 3700 cm-1 and 3628 cm-1, respectively, the rate of which suggest first order kinetics. Meanwhile, there is also a reduction in the region that is attributed to the hydrogen bonding between the hydroxyls and DMCP in the range of 3550-3050 cm-1. The decrease of peaks at higher wavenumber regions coupled with the reaction products seen in lower wavenumber regions not only indicate the loss of water through desorption but also the loss of hydroxyl groups implying that DMCP decomposes forming the υa(CH3P) mode at 2996 cm-1, υa(CH3O) mode at 2956 cm-1, which are only seen on the H2O2-treated sample, see Figure S15. Based on the interaction with the non-reactive silica surface,33 adsorbed DMCP is expected to exhaust the surface hydroxyl group resulting in the decline of a sharp peak located at 3748 cm-1, which relates to the monodentate (terminal) hydroxyl groups (m–OH). Contrary to the H2O2 treated sample, there is

Figure 7. High wavenumber region of DMCP on H2O2-treated ZrO2/Zr(OH)4. O = 3700, X = 3628 cm-1. Modes in red represent those attributed to decomposition product. Time intervals between spectra are: 0, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 min

In order to assess if the peak increases are due to decomposition of DMCP or due to reversible adsorption of weakly bound multi-layer/liquid DMCP, a post-purge was performed with He for 2 hr (Figure S9-S11). It is likely that vibrational bands due to weakly bound species would decrease due to desorption. Meanwhile, the peak for the multi-layer/liquid DMCP mode at 1256 cm-1 appears to be unchanged (Figure S9). Similar trends can be observed for the pristine sample (Figure S10), albeit at a reduced signal. From the two samples, it can be concluded that the H2O2-treated sample provides additional reaction pathways,


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intervals between spectra are: 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28 min

where the pathway resulting in the asymmetric (O–P–O) mode is the primary adsorbed species.


At the same time, the Zr-bound methoxy mode at 1144 cm-1 experiences an initial increase before plateauing near 15 min of heating, followed by a slight decrease upon cooling. Similar non-monotonic trends could be observed for both the symmetric (O–P–O) mode at 1080 cm-1 and the (C–O) mode at 1030 cm-1. Meanwhile, the control sample shows a non-monotonic trend for the entire region (1400-900 cm-1). The trend associated with the pristine sample results in negative peaks after 10 min for the region, indicating the complete desorption of species possessing these vibrational modes. Post-heating experiments were also carried out with the hydrolysis product DMHP, see Figure S16. The presence of an isosbestic point, identified by the simultaneous decrease in the mode occurring at 1177 cm-1 and increase in the mode at 1144 cm-1, respectively, confirms the decomposition of the adsorbed DMHP. It should be noted that the spectral difference in the asymmetric (O–P–O) mode for DMHP changes much more rapidly compared to DMCP, indicating that DMHP is more reactive than DMCP.

Figure 8. High wavenumber region of DMCP on the untreated ZrO2/Zr(OH)4. Time intervals between spectra are: 0, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 min

To confirm the decomposition product, in-situ DRIFTS experiments were performed with the anticipated hydrolysis product dimethyl phosphonate (DMHP) (see Scheme 2, Figure S12-14). It can be seen that the adsorption of DMHP results in a large mode related to the asymmetric (O–P–O) binding mode, confirming the decomposition of DMCP. Notably less prominent is the mode near 1144 cm-1 which may be attributed to additional decomposition products. See Figure S15 for full spectra of DMHP and DMCP against H2O2-treated ZrO2/Zr(OH)4. Following the He purge, each sample was heated to 220 ˚C over the course of 20 min then allowed to cool for an additional 10 min in order to further confirm the reactive differences of the two catalysts. Figure 9 shows that the peaks located at 1251 cm-1 and 1177 cm1 experience monotonic declines, representing weakly bound DMCP and the decomposed asymmetric (O–P–O) mode along with the methoxy mode on DMCP, respectively, and continues to decline upon cooling the H2O2 treated sample.

CONCLUSIONS A facile method of H2O2 treatment is suggested for the formation of mesoporosity and reactive surface moieties within non-porous ZrO2/Zr(OH)4 which are found to enhance decomposition of DMCP. Detailed characterization of the H2O2-treated mesoporous ZrO2/Zr(OH)4 has provided comprehensive insights about its structural, morphological, optical and surface chemistry features. H2O2-treated ZrO2/Zr(OH)4 shows higher, up to an order of magnitude, decomposition of CWA simulant DMPC compared to the untreated ZrO2/Zr(OH)4 due to the increased surface area via the presence of mesopores and an additional reactive pathway. Our studies reveal that the detoxification of DMCP occurs by interacting with b-OH groups on sample surface that subsequently react to form methyl chlorophosphate (MCP) and DMHP. Formation of DMHP was only observed with the H2O2treated sample. The presence of the asymmetric stretch of the O–P–O moiety within DMHP suggests that DMCP is hydrolyzed to DMHP and later undergoes a subsequent methanolysis as Zr-bound methoxy groups were also found in the bulk of the catalyst. The H2O2 treatment of ZrO2/Zr(OH)4 has proven to easily and economically improve its ability to decompose nerve agents surrogates and shows the potential for CWA decomposition.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supporting Information. Physical characterization of ZnO, CeO2. Additional figures of DRIFTS depicting chemical interactions between DMCP and DMHP against H2O2-treated and untreated samples (PDF)

Figure 9. Low wavenumber region of the post heating of DMCP on H2O2-treated ZrO2/Zr(OH)4. O = 1144, X = 1177 cm-1. Modes in red represent those attributed to decomposition product. Time

AUTHOR INFORMATION



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Corresponding Author * Email: [email protected]

Author Contributions J.C.O. did all physico-chemical characterizations and synthesis of samples studied. W.O.G., and A.B. did all DRIFTS experiments while A.B. did the DFT simulations of DMCP and DMHP. J.L did the analysis of the DRIFTS and DFT results with W.O.G. and A.B. providing input. J.C.O. and J.L. co-wrote the manuscript. C.J.K. and A.V.N. supervised all work and provided corrections and modifications to manuscript. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest. Approved for public release; distribution unlimited.

ACKNOWLEDGMENTS Authors J.C.O and A.V.N received funding from by the Defense Threat Reduction Agency (DTRA) HDTRA1-14-1-0015. Authors J.L., A.B., W.O.G. and C.K. received funding from the Defense Threat Reduction Agency (DTRA)/Joint Science and Technology Office (JSTO) under project CB3587. J.C.O. would like to thank Dr. Thomas Emge for helpful discussions. The research was conducted while J. L. held a National Research Council (NRC) Associateship award at ECBC. J.L. would like to thank Dr. Monica McEntee for helpful discussions. J.L., W.O.G. and A.B. gratefully thank Dr. Michael Ellzy for the DMHP and for discussion.

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Disordered Mesoporous Zirconium (Hydr)oxides for Decomposition of

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