Poster Topic 4. Emerging Sensing Materials and Technologies
Zirconium Phosphate-Based Porous Heterostructures: A New Class of Material2 for Ammonia Sensing 1 2
2
Thomas Simons ,José F. Blanco Villalba , José Jiménez-Jiménez , Enrique Rodríguez-Castellón , Ulrich Simon 1 RWTH Aachen University, 52074 Aachen, Landoltweg 1, Germany 2 University of Málaga, Faculty of Science, 29071 Málaga, Spain 1
[email protected]
1
Summary Metal phosphates such as zirconium phosphate are well known proton conductors based on the mobility of protons stemming from Brønsted-acid sites. Here we studied the electrical properties of a new porous zirconium phosphate based material by means of electrical impedance spectroscopy. We show that the proton conductivity is supported by solvate molecules like ammonia, which makes Porous Phosphate Heterostructures (PPH) suited for gas phase ammonia sensing.
Introduction In previous works it was shown that BrØnsted-acidic proton conducting zeolites could serve as ammonia sensing material [1] as well as for in situ monitoring of DeNOx-SCR (selective catalytic reduction of NOx with NH3) by applying impedance spectroscopy [2]. From these finding one may expect that other materials, which are proton conductors and which exhibit high porosity with accessible BrØnsted-acid sites can be suited for ammonia sensing as well. Therefore, we studied the ammonia sensing properties of a novel zirconium(IV) phosphate-based Porous Phosphate Heterostructure (PPH), where layers of zirconium phosphate are expanded by the formation of silica galleries in the interlayered space, as reported recently [3]. For comparison, we tested the non-porous zirconium(IV)hydrogenphosphate (ZrP) and thereby can demonstrate the enhanced ammonia sensing performance of the PPH.
Experimental Both materials were deposited as thick films (50 µm) on an interdigital electrode (IDE) structures (electrode spacing 125 µm) on an alumina substrate with an integrated backside heater. These sensor chips were placed in a steel measuring chamber equipped with a 4-channel gas control and mixing system (MKS). The complex electrical impedance at different temperatures and gas conditions was measured with an impedance measuring system (Solartron). In all measurements a gas flow of 100 sccm was applied. All samples were held at 400 °C in constant nitrogen flow for at least 1 h before measurements to avoid humidity effects.
Results In a first step both samples were heated up in a pure nitrogen atmosphere up to a temperature of 500 °C, in steps of 5 °C (100-300 °C), 10 °C (300-400 °C) and 20 °C (400-500 °C). To observe the proton conductivity the complex impedance in a frequency range from 10-1 to 106 Hz was measured at each step, after an equilibration time of 15 min. From these spectra, the resonance frequency and the corresponding admittance Y’ were determined. In an Arrhenius diagram, ln(Y’T) was plotted against the inverse temperature (see Fig. 1) to compare the activation energy of proton transport mechanisms of both materials.
Poster
Fig. 1:
Arrhenius diagrams of ZrP and PPH
Fig. 1 shows that both curves exhibit the same slope, indicating the same activation energy, which points to the same proton transport mechanism. Ex situ powder XRD measurements show that both samples are mostly anhydrous ZrP, formed during the initial heating step. To observe the ammonia sensing properties of the materials a second series of measurements were performed under selected temperatures between 100 and 250 °C. After an initial time under pure nitrogen the atmosphere was switched to 100 ppm of ammonia in nitrogen and the absolute value of the impedance |Z| at 10 kHz was recorded (in steps of 70 s) to monitor the effect of ammonia (see Fig. 2).
Fig. 2:
Ammonia sensing properties of PPH (left) and ZrP (right) at 150 °C
While ZrP shows only a very little impedance change upon ammonia application, the chemically related PPH shows a pronounced effect, i.e. an increase in proton conductivity. This difference could be explained by two reasons or a combination of both: (i) Different numbers of available Brønsted-sites for a solvate supported proton transport. In ZrP only the surface Brønsted-sites are accessible for the ammonia. In the expanded PPH also interlayer Brønsted-sites could contribute to the conduction. (ii) Terminal SiOH groups at the silica galleries in the PPH may serve as additional acidic groups to support an ammonia supported proton transport. Our ongoing work is devoted to the deciphering of the underlying mechanisms, e.g. by means of a combination of in situ impedance measurements and Diffuse Reflectance Infrared Fourier Transformed Spectroscopy. References [1] Rodríguez-Gonzáles L., Simon U. Meas. Sci. Technol. 21, 027003 (2010); doi:10.1088/0957-0233/21/2/027003 [2] T. Simons, U. Simon, Beilstein J Nanotech. 3, 667-673 (2012); doi:10.3762/bjnano.3.76 [3] R. Moreno-Tost, M. L. Oliveira, D. Eliche-Quesada, J. Jiménez-Jiménez, A. Jiménez-López, Enrique Rodríguez-Castellón, Chemosphere 72/4, 608-615 (2008); doi:10.1016/j.chemosphere.2008.02.065
