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Interaction of Dimethylmethylphosphonate with Zeolite Y: Impedance-Based Sensor for Detecting Nerve Agent Simulants Xiaogan Li† and Prabir K. Dutta*,‡ School of Electronic Science and Technology, Dalian UniVersity of Technology, Dalian, Liaoning 116024, and Department of Chemistry, The Ohio State UniVersity, 120 W 18th AVenue, Columbus, Ohio 43210 ReceiVed: January 5, 2010; ReVised Manuscript ReceiVed: March 26, 2010
Dimethylmethylphosphonate (DMMP) is a simulant for the highly toxic organophosphate nerve agent Sarin (GB). The influence of DMMP on the ionic conductivity of zeolite Y is investigated by impedance spectroscopy. In the presence of 20-100 ppm of DMMP, the ionic conductivity of the sodium exchanged form of the zeolite showed an increase. The interaction between DMMP and the zeolite was elucidated by examining different cation-exchanged zeolites as well as external surface modification with ceria to deactivate acidic groups. A mechanism involving the binding of the sodium cation with the phosphonate group of DMMP that results in facilitated inter cage motion of the cation is proposed. The change in impedance measured at a single frequency of 3000 Hz allows for the use of Na+-exchanged zeolite Y as a sensor for detecting ppm level of DMMP. Sensing data were obtained over a temperature range of 300-350 °C, with best results at 320 °C. This device exhibited minimal response to CO, NH3, methane, and propane, possible interferents in the ambient air. Introduction Zeolites are crystalline aluminosilicates with pores and channels of molecular dimensions.1 Zeolites can be synthesized with different chemical compositions and distinct framework topologies. In addition, their ion-exchange properties, as well as adsorption and reactions of molecules within its cages have led to numerous applications in catalysis and separations.1 Use of the novel properties of zeolites for sensor applications is of more recent vintage. Zeolites have been adopted as catalytic filters in chemical gas sensors for improvement of the sensor selectivity.2 For example, platinum-doped zeolite Y has been used both as an electrode and a catalytic filter for a highly selective total NOx sensor.3 Of particular interest to the present study is the role of zeolite as an ionic conductor. The motion of extra-framework cations in response to an electric field is the basis of conductivity, and extensive studies have focused on this topic.4 In addition, the effect of different polar and nonpolar molecules on the ionic conductivity of zeolites has been investigated.5 The results from these studies have led to exploiting the change in ionic conductivity of zeolites in the presence of molecules to design sensors.6 At temperatures less than 110 °C, a change in the conductivity of hydrated zeolites is induced by alcohols and other substances.7 At temperatures of 400 °C, it was noted that Pt-exchanged zeolite Y showed increased impedance upon exposure to butane, and was proposed to arise from blocking effects of the butane on the cation transport in the channels of the zeolites.8 A large body of studies exist on the fundamental mechanism of the effect of ammonia on the conductivity of the different types of zeolites at high temperatures.9 This has led to successful development of ammonia sensors for automobile exhaust emission control. There are also reports on utilization of zeolites as ionic electrolytes for hydrocarbon sensors.10
Developing detection strategies for chemical warfare agents (CWAs) is an active area of research.11 Commercially available detection includes detection paper, gas detection tubes (e.g., Draeger system), a flame photometric detector (e.g., Proengin, Inc.), ion-mobility spectrometers (e.g., M90), surface acoustic wave arrays (e.g., M90), and gas chromatography-mass spectrometry devices (e.g., Hapsite). Recent studies using these commercial devices have concluded that none of these meet all the CWA detection requirements.12 Thus, new sensing principles and devices are being explored, including enzymatic, chemical, and supramolecular sensors.13 Electrochemical, in particular, chemoresistive-type sensors for the detection of CWAs are being explored.14 Thin-films of metallophthalocyanines (MPc’s) semiconductor also show promise for CWA detection.15 Although the benefits of electrochemical sensors are obvious (simple working principle, good sensitivity, easy miniaturization and low cost), the problems often are inadequate sensitivity and selectivity and slow response and recovery. There is a report on the detection of dimethylmethylphosphonate (DMMP) by observing the weight change upon adsorption of DMMP in the zeolite ZSM-5 using a quartz crystal microbalance.16 In this work, we focus on the influence of DMMP on the ionic conductivity of zeolite Y. DMMP is used as a simulant for the class of compounds containing phosphonate esters, such as pesticides and CWAs. The present study is a detailed examination of DMMP-zeolite interaction, the change in conductivity, and the mechanism involved. The potential for zeolites as sensors for DMMP is demonstrated. Previous work on DMMP-zeolite interactions has primarily focused on how acidic functionalities in zeolites can bring about DMMP destruction, with focus on remediation.17 Experimental Section
* Corresponding author. E-mail:
[email protected]. † Dalian University of Technology. ‡ The Ohio State University.
Materials Preparation. The sodium zeolite Y (NaY) with Si/Al ratio of ∼2.5 was purchased from Zeolyst. Zeolites with
10.1021/jp100088w 2010 American Chemical Society Published on Web 04/14/2010
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SCHEME 1: Procedures for Fabrication of the Zeolite Devices for Impedance Spectrocopy
different cations (MY) were obtained by ion exchange. Two grams of NaY was added to 200 mL of a 0.1 M aqueous solution of MCl (M ) NH4+, Li+, Na+, K+, Cs+, tetramethylammonium ion (TMA+)), the resulting suspension was stirred at room temperature overnight and filtered, and the procedure was repeated a second time. For preparation of LaNaY, LaCl3 aqueous solution (0.001 M) was used for ion-exchange to obtain a partially La3+-exchanged NaY sample. The external surface of the NaY particles was modified by treatment with tetrapropylammonium (TPA) and CeO2 species, respectively. For TPA coating, 1 g of NaY was stirred with a 0.01 M tetrapropylammonium hydroxide (TPAOH, Aldrich) solution, filtered, and washed. To obtain the CeO2 coating on the zeolite, hydrated Ce(NO3)4 (Aldrich) was dissolved in ethanol and then added to NaY. The mixture was stirred overnight and then heated to 80 °C to evaporate the solvent. Subsequently, the white powders were further calcined at 550 °C for 5 h to decompose the nitrate and form a ceria coating.18 Characterization. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku Geigerflex diffractometer using Nifiltered Cu KR radiation at 40 kV and 25 mA at 2θ between 20 and 80° at a scanning speed of 12°/min. The surface morphology of the synthesized powders and the zeolite films was examined using scanning electron microscopy (SEM: Phillips XL 30 ESEM FEG, FEI, USA) on gold-coated specimens. Diffuse reflectance infrared spectra (DRIFTS) were collected using a Perkin-Elmer infrared spectrometer equipped with a Pike Technologies diffuse reflectance chamber. All spectra were converted using the Kubelka-Munk transformation with KBr as the reference. Raman spectroscopy was performed using a Renishaw-Smiths Detection Combined Raman-IR Microprobe equipped with an argon ion laser. Zeolite Pellet Fabrication and DMMP Vapor Test. Scheme 1 demonstrates the procedure for fabricating zeolite pellets. The MY powders were compacted under an uniaxial force of 5000 tons to form a pellet with a diameter of 13 mm and a thickness of 1-4 mm. The pellet was calcined at 700 °C for 2 h, and gold paste was painted on the pellet surface according to two different electrode configurations: capacitive-type and paralleltype. These pellets were then heated to 600 °C for 2 h to burn out the binders and increase the adhesion between the gold and the surface of the pellet. Two gold wires served as the leads to the terminals of the impedance spectroscopy apparatus. The overall view of the testing setup is shown in Figure SS1 of the Supporting Information. The pellet was placed in a quartz tube in a programmable high temperature furnace. The gas vapors were introduced by bubbling air through liquid DMMP that was kept at ∼0.5 °C. The gas flow rate was controlled by precalibrated digital mass flow controllers (MFCs, Sierra) and the concentration was estimated based on the vapor pressure at
0.5 °C. The partial pressure of DMMP in dry atmosphere is around 10 Pa at 0.5 °C.19 By adjusting the gas flow rate bubbling through the liquid DMMP, different concentrations of DMMP was delivered to the testing chamber. For example, if the carrying gas flow rate is 80 sccm and the total flow rate is 100 sccm, then CDMMP can be estimated as
CDMMP )
10 80 × ) 80 ppm 5 100 10
Impedance spectroscopy (Solartron 1260) was employed to generate the cycling excitation voltage with a magnitude of 300 mV. A frequency range from 1 Hz to 107Hz was conducted during each scan. The impedance data was analyzed by commercial software Zplot and Zview (Scribner, Inc., USA). From the resistance of the samples obtained via data fitting, the conductivity and activation energy of ion transport in zeolites were calculated as follows. The conductivity (σ) obeys the following equations:
Rb ) F
L A
(1)
and
σ)
1 F
(2)
Here, Rb is the resistance (Ω), F is the resistivity (Ω · cm), L is the length of the pellet (cm); and A is the surface area of the pellet (cm2). Consequently, the activation energy (Ea), for cation transport in zeolites was determined by the Arrhenius equation:
( )
σT ) σ0 exp -
Ea RT
(3)
where σ0 is the pre-exponential conductivity, Ea is the activation energy (J · mol-1), and R is the gas constant ()8.314 J · mol-1 · K-1) Results Zeolite Y Characteristics. The zeolite Y (Si/Al ) 2.5) framework is made up of 1.3 nm supercages connected through 0.74 nm windows. These supercages are the site for interactions examined in this paper. Figure 1a is a schematic of a supercage (micrometer-sized crystals examined in this study have 109 such
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Li and Dutta the underlying zeolite layer. Figure 2c is an SEM of the crosssection of the pellet focused on the zeolite, and demonstrates that even at the pressures used in compacting the zeolite to form the pellet, there is resident intercrystalline porosity. The XRD in Figure 2d indicates that the pressure and heat treatment used to form the pellet did not disrupt the zeolite structure. Impedance Spectroscopy of DMMP-Zeolite Y. The impedance spectrum of a NaY pellet in flowing air (50 sccm/min) at 300 °C in the range of 1-107 Hz is shown in Figure 3a. The data is typical for a solid-state electrolyte between two metal electrodes, and has been reported previously for zeolite materials.4 The depressed semicircle arises from bulk conductivity, whereas the tail increasing with low frequency is due to polarization at the electrode-electrolyte interface, arising from the effect of blocking electrodes. This semicircle can be simulated by an equivalent electric circuit consisting of a resistor (R) in parallel with a capacitive component (CPE: constant phase element) representing the electrochemical cell (Chart 1).20 The impedance Z is expressed as
Z ) Z′ + iZ′′ ) R/(1 + ZCPE) ) R/(1 + iωτ)n
(4)
where Z′ is the real part and Z′′ is the imaginary part of the impedance, ZCPE represents the impedance contributed from the capacitive component of the equivalent circuit, ω is the angular frequency, τ () RC) is the relaxation time, and n is a fractional index whose magnitude is related to the suppression of the semicircle above the real axis. The peak frequency of the semicircle (ωp) also satisfies the following equation:
ωp · R · CPE ) 1
Figure 1. (a) Zeolite Y supercage along with cation positions and (b) molecular structure of DMMP.
interconnected cages) along with symbols for extraframework cation sites. Figure 1b shows the structure of DMMP, the analyte of interest; with a length of 0.57 nm, it should be able to penetrate into the supercage. Figure 2 shows the characteristic features of a typical zeolite pellet used in these studies. Figure 2a is a cross section of the pellet, showing the gold electrode and the zeolite. Figure 2b is the SEM top view of the Au electrode covering the pellet, indicating a porous layer through which molecules can reach
(5)
The best fit to the impedance curve of the NaY pellet in air was with the parameters shown in Table 1. Upon exposure of the NaY pellet to ∼100 ppm of DMMP at 200-400 °C, the semicircle changes shape, indicating a lower impedance, and the spectrum reverts back to the original upon reintroduction of air. A plot at 300 °C is shown in Figure 3a. The parameters for the best fit to these data are also included in Table 1. Upon DMMP introduction, the conductivity increases by ∼5%. The change in impedance of the NaY pellet with DMMP at 320 °C was monitored at a single frequency (3000 Hz), and the data is shown in Figure 3b. It is clear that there is a decrease in impedance upon introduction of ∼100 ppm DMMP (indicated by down arrow) with the signal stabilizing in ∼20 min and the process is reversed with a time constant of ∼25 min upon removing DMMP (indicated by the up arrow) in the air stream (these response times were optimal 320 °C). The impedance spectra were recorded with and without DMMP (∼100 ppm) over a range of temperatures from 200 to 400 °C. These curves were fit with eq 4 and using the conductivity, the Arrhenius plot obtained is shown in Figure 4a. The activation energy (Eact) changes from 78.3 kJ/mol in air to 76.2 kJ/mol in the presence of DMMP. Figure 4b shows a plot of the log of admittance (conductivity) as a function of frequency for the NaY pellet exposed to ∼100 ppm DMMP. There is a plateau region and then a sharp rise in conductivity around ∼7500 Hz. Surface versus Internal Cation Motion. The changes in impedance upon adsorption of DMMP can occur by interaction with cations at the surface or within the zeolite. Several surface modifications were examined. NaY was ion-exchanged with TPA+ ions. These TPA+ ions will only exchange Na+ ions on
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Figure 2. SEM micrographs of (a) the NaY/Au interface, (b) the top surface of the gold electrodes, (c) the cross section of the NaY pellet, and (d) XRD of the NaY pellet after heat treatment at 700 °C.
CHART 1: Equivalent Electrical Circuit Used for Fitting the Impedance Spectra with the Parameters Shown in Table 1
Figure 3. (a) Impedance spectra of the NaY pellet in DMMP and in air at 300 °C and (b) impedance change of the NaY pellet to ∼100 ppm DMMP at 320 °C with a single fixed frequency of 3000 Hz (arrows indicate turning DMMP on and off).
the surface and will not penetrate into the zeolite. Figure 5a shows the change in impedance with ∼100 ppm DMMP at 320 °C and is comparable to NaY (Figure 3b).
A second modification of the surface was based on CeO2 deposition on the zeolite surface.18 Cerium loading of 30 wt % is reported to be necessary for covering the external surface of the zeolite for acidic group deactivation, and was the basis for our loading choice.18 The choice of calcination temperature of 550 °C for uniform ceria deposition on the zeolite surface was also based on literature, since it has been shown that sintering at a higher temperature of 700 °C leads to aggregation of the ceria.18 The formation of ceria is confirmed by the power diffraction pattern (peaks at 2θ ) 28.5, 33.1, 47.2), as well as the IR/Raman spectrum (IR bands mainly from adsorbed carbonates at 1282, 1403, and 1392 cm-1, Raman band at 460 cm-1) shown in Figure SS2 (Supporting Information). Figure 5b shows the impedance change with 30% CeO2, and the data are comparable to that of NaY. In order to confirm that the mobility of the extraframework Na+ plays a role in the impedance changes observed upon DMMP adsorption, two experiments were carried out with the goal of altering the intrazeolitic free space. A sample of zeolite was ion-exchanged with La3+ (one La per five supercages) as to result in replacement of a fraction of the Na+ in the supercages. The impedance data obtained with ∼100 ppm DMMP for LaNaY at 320 °C is shown in Figure 6. NaY was also extensively ion-exchanged with TMA+, and the impedance of the zeolite pellet was measured with ∼100 ppm DMMP at
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TABLE 1: Fitted Parameters of the Impedance Spectra of the Zeolites in DMMP (∼100 ppm) and in Air at 300 °C bulk 300 °C in air in DMMP back in air
σ (×10
-6
-1
S · cm )
1.24 1.30 1.25
CPE (10
-11
interface (Wo) F)
9.21 9.49 9.30
320 °C. This data is also shown in Figure 6. It is clear that the most significant effect of the exchange of Na+ with TMA is to increase the response time to ∼50 min (defined on Figure 6), whereas with the La3+ exchanged sample, the response time is ∼20 min, longer than the ∼10 min with the NaY sample. Use of Impedance Changes for Developing a DMMP Sensor. The change in impedance of the NaY pellet upon exposure to DMMP can be used as the basis for a sensing device. In order to explore this further, we examined the response of the NaY pellet to ∼20-100 ppm DMMP, and this data is shown in Figure 7a. From this data, we defined a sensitivity parameter, S ) (Za - Zb)/Zb, where Za is the impedance in the presence of the analyte and Zb is the background response in air. A plot of S versus concentration is shown in Figure 7b and can be considered a calibration curve. In order to establish other optimization parameters for a possible device, the thickness of the pellet as well as the placement of electrodes on the sensitivity was examined, and this data is
5
n
ωp
Ri (×10 Ω)
CPEi (F)
ni
0.99 0.99 0.99
7143 7267 7121
3.72 3.05 3.30
7.01 7.16 6.91
0.50 0.49 0.49
shown in Figure 8. Thinner pellets resulted in higher sensitivities, although the placement of electrodes had a less significant effect. Figure 9 shows the effect of interferences from typical ambient constituents CO, NH3, and hydrocarbons (propane and methane) with a NaY pellet at 3000 Hz at 320 °C. Even with significantly higher concentrations of these gases, the interference effect on ∼100 ppm DMMP was minimal. Another optimization step involved examination of other possible extraframework cations, H+ (obtained by heating a NH4+exchanged zeolite Y), Li+, Na+, K+, and Cs+. These results are shown in Figure 10, expressed as the sensitivity, S, to ∼100 ppm DMMP. The Na+-exchanged sample showed the best performance, and with K+ and Cs+, there was an increase in impedance and very poor recovery. The inset data is for CsY, which shows that, upon introduction of DMMP, there is an increase in resistance, and the signal did not recover upon removing DMMP from the stream (shown by arrows).
Figure 5. Response of the surface-modified NaY sensors with (a) TPA and (b) 30% CeO2 to ∼100 ppm DMMP at 320 °C at a frequency of 3000 Hz.
Figure 4. (a) Arrhenius plots of the NaY pellet in air and ∼100 ppm DMMP, respectively. (b) Frequency-dependent conductivity spectrum of the NaY pellet at 320 °C.
Figure 6. Response of the (a) Na+ (NaY), (b) TMA+(TMAY) and (c) La3+(LaNaY) exchanged NaY sensors to ∼100 ppm DMMP at 320 °C at a frequency of 3000 Hz.
Interaction of DMMP with Zeolite Y
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Figure 9. Interference of the NaY based device (T ) 320 °C and frequency ) 3000 Hz) to other gases.
Figure 10. Cation influence on the sensitivity of Y-type zeolite pellets. (T ) 320 °C and frequency ) 3000 Hz). Figure 7. (a) Response curve of NaY pellet to different concentrations of DMMP from ∼20 to ∼100 ppm and (b) the correlation between the sensitivity and DMMP vapor concentrations at 320 °C at a frequency of 3000 Hz.
Figure 8. Influence of the pellet thickness and electrode configurations on the sensitivity of the NaY pellets to different concentrations of DMMP at 320 °C (frequency ) 3000 Hz).
Discussion Several decades of research on the ionic conductivity of zeolites have concluded that extraframework cation motions contribute to frequency-dependent conductivity.4 On the basis of the frequency-dependent conductivity, two relaxation pro-
cesses involving extraframework cation motion have been noted.4a,b At high temperature, a low frequency process arises from long-range Na+ movement from an SII site of one cage to the neighboring supercage (Figure 1a). The second motion relates to a low temperature, high frequency process and has been attributed to movement of Na+ within a supercage, e.g., from site SII via a thermally activated hopping process across SIII sites to the neighboring SII site (Figure 1a). Both these processes appear in the Argand diagram in Figure 3a under the semicircle. Previous studies of modulus plots versus frequency for zeolite Y have shown the presence of two peaks corresponding to these two processes.4d These studies have also noted that, with increase in temperature, the high frequency process moves out of the frequency window at which typical impedance spectroscopy measurements are made (1 Hz to 1 MHz). This suggests that, in the temperature range of interest (∼300-350 °C) in the present study, the motion is primarily of cations hopping between the cages. This direct current (DC)-type conductivity leads to accumulation of the charge at the electrode-electrolyte interface that leads to the low frequency tail in Figure 3a and is commonly referred to as the Warburg impedance. The activation energy for NaY (Figure 4a) for the low frequency hopping process measured in this study is 78.3 kJ/ mol; previous studies have reported energies of 804d and 71 kJ/mol.4n Upon introduction of DMMP, we observe a decrease in activation energy to 76.2 kJ/mol, indicating that the presence of DMMP facilitates cation transport. The measurement of the activation energy with DMMP assumes that the concentration of DMMP within the zeolite remains unchanged over the
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SCHEME 2: Proposed Model for the Interaction of DMMP with Cations Inside the Supercage: Cation Mobility Was Facilitated by the Re-orientation of the DMMP Molecules
temperature range. Since the experiments were carried out at temperatures greater than 250 °C, and the samples prior to experiments were maintained at 320 °C, the intrazeolitic water is mostly lost, and any effects from H2O on the signal are not considered. There has been considerable interest in the decomposition of DMMP on solid oxide surfaces as a possible method for remediation of CWAs.17,21,22 Since we observe recovery of the impedance signal upon removal of DMMP with multiple cycles, the interaction of DMMP with the NaY does not appear to result in significant decomposition. With oxide surfaces containing Bronsted/Lewis acid sites (including surface hydroxyls), the DMMP typically binds to the acid site via the PdO group, and decomposition begins at temperatures higher than 50 °C. Oxides with redox active metals (iron oxide) also bring about decomposition of DMMP. However, no significant decomposition was observed when DMMP was adsorbed on silica without acid sites.22 With zeolite Y, in the presence of water, hydrolysis of O-ethyl-5-2-(diisopropylamino)-ethyl methyl phosphonothioate (VX) was noted, as well as migration of these molecules into the supercages.17a Another phosphonothiate, O,S-diethyl phenylphosphonothioate (DEPPT) did not react on NaY, but did migrate into the zeolite cages. DMMP decomposition on nanocrystalline NaY (∼30 nm) has been noted at 200 °C, with decomposition products being CO2, formaldeyhyde, and dimethyl ether.17b Mechanistically, these reaction products are expected from acid-catalyzed reactions, and it was proposed that silanol groups on the external zeolite surface cause the DMMP decomposition. With the micrometer-sized crystals used in the present study (Figure 2C), the surface-induced decomposition should play a smaller role. Also, surface impregnation of ceria on zeolites should lead to deactivation of external acid sites without reduction of the effective pore radius. Since the CeO2-covered zeolite gives a comparable response to DMMP as with the parent NaY sample (Figure 5), the surface acidity in NaY does not promote decomposition of DMMP. Also, the comparable data of TPANaY and NaY shown in Figure 5 suggest that intrazeolite motion is responsible for the impedance changes. We propose a model involving specific interaction of DMMP with the extraframework cations as being responsible for altering the impedance characteristics. Our focus is on the low-frequency process, which involves long-range transport of Na+ from one supercage to another. As shown in Scheme 2, the model suggests that DMMP binds to the Na+ ions through the phosphonate groups, and motion of DMMP in the supercage promotes the hopping of Na+ ions, without complete dissociation from the lattice. DMMP binding weakens the electrostatic interaction and promotes hopping. The jump rate of the hopping process was estimated at 7500 Hz (Figure 4). The reorientation of the DMMP and therefore its mobility influences the response time. This model is consistent with the data shown in Figure 6 with TMA
and La3+-exchanged zeolite. The large diameter (6 Å) of TMA takes up significant volume of the supercages, and the crowding effect imposes steric constraints on the motion of the DMMP, thus increasing the response/recovery time. With partial La3+ exchange, the response and recovery times are a factor of 2 greater than those of the TMA sample (Figure 6b). The working hypothesis with La3+ was that it will replace 3 Na+, leading to partial emptying of the supercages, making room for DMMP orientation. On the basis of xenon NMR studies of LaNaY, two conflicting views have been expressed: that there is no change in intrazeolite space and that there is increased vacant space in the supercage, although there is agreement that La3+ does migrate into the sodalite cages and the hexagonal prisms upon dehydration.23,24 We observe an increase in response times on changing from NaY to LaNaY, although significantly less as compared to TMANaY, and propose that crowding within the cages does influence the DMMP motion. Possibly the most extensively studied zeolite-based system using impedance spectroscopy is interaction of ammonia with various zeolites, which provides an appropriate comparison with the present study.9 Typically, NH3 will interact with the protonexchanged form of the zeolite and change the low-frequency, intersite proton conductivity. Enhanced mobility of the proton in zeolites in the presence of NH3 is proposed to be due to reaction with the proton to form NH4+, which, upon reorientation can transfer the proton to another NH3 molecule or onto a new lattice site.5d The zeolite system can also be readily adapted as a device for sensing vapors of DMMP in the parts per million range. Optimization for device fabrication includes controlling thickness of the pellet, and Figure 8 suggests that thinner pellets (
