Dosimeter-Type NOx Sensing Properties of KMnO4 and Its Electrical Conductivity during Temperature Programmed Desorption

An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT) for NOx storage catalysts (NSC) enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380 °C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristics are recovered periodically by heating to 650 °C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD). The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1) time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2) during the short-term thermal NOx release.

and K 3 MnO 4 , as well as manganese oxide MnO x existing in different oxidation states [22,[24][25][26][27][28][29][30]. The decomposition of KMnO 4 is reported by Boldyrev [25,26] to become noticeable in the temperature range from 205 to 280 °C. KMnO 4 -impregnation and firing decreased the surface area of the powder to 100 m 2 /g (obtained by BET method (named after the originators of the method: Brunauer, Emmett, and Teller) relying on the adsorption of gases to determine the specific surface area of powders). The SEM (scanning electron microscope) analysis of the fired KMnO 4 /La-Al 2 O 3 powder is given in Figure 1 as a backscatter electron (BSE) image. The powder consists of spherical La-Al 2 O 3 -rich particles in the range of some tens of µm, partly embedded in a potassium and manganese comprising matrix, which was confirmed by energy-dispersive X-ray spectroscopy (EDX) measurements. The KMnO 4 /La-Al 2 O 3 powder was mixed with an organic binder (KD2721, Zschimmer & Schwarz) in order to obtain a processable paste. The paste was deposited by spatula onto a 96% pure alumina substrate equipped with gold interdigitated electrodes (area 5 × 6 mm, finger width/distance 100 µm) and fired at 600 °C. The sample was pre-conditioned for several hours at temperatures up to 650 °C in NO x containing oxygen-rich atmospheres. The electrical properties of KMnO 4 /La-Al 2 O 3 were investigated in a test apparatus as sketched in Figure 2. Following installation in a quartz-tube furnace with inner diameter of 22 mm, the sample was heated to temperatures between 300 to 650 °C. The KMnO 4 /La-Al 2 O 3 layer was exposed to a 2 L/min lean gas flow (10% O 2 , 50% N 2 humidified with a water bubbler at room temperature, and 5% CO 2 diluted in N 2 balance) with a gas exchange time of the system in the range of 8 s. The impedance Z of the KMnO 4 /La-Al 2 O 3 sample was recorded by an impedance analyzer (Alpha High Performance Frequency Analyzer, Novocontrol).

Electrical Properties in the Unloaded State
The electrical properties of KMnO 4 /La-Al 2 O 3 in the unloaded state (after regeneration of the NO x sorption sites) were evaluated from 300 to 650 °C by impedance spectroscopy in the frequency range of 1 Hz to 1 MHz. Plotting the impedances in the complex plane as Nyquist plots (real part Z' and imaginary part Z") yields near semicircular spectra at higher frequencies. This allows the bulk impedance to be modeled by a resistance R in parallel to a constant phase element CPE (R||CPE). The corresponding impedance Z CPE expressed as a function of the model parameters n (ranging from 0 to 1) and Q as well as the angular frequency ω is given in Equation (1): (1) In Figure 3(a and b), examples for the corresponding Nyquist plots of the measured impedance data (dots) for 380 and 650 °C, together with the fitted R||CPE curves in the upper frequency range (solid curves) and the corresponding fitting parameters, are displayed. While the KMnO 4 /La-Al 2 O 3 sample has a resistance of 180 kΩ at 380 °C, it decreases to 3.4 kΩ upon heating to 650 °C.
The electrical conductivity σ of the KMnO 4 /La-Al 2 O 3 specimen was estimated from the fitted R-values of the impedance in the R||C dominated frequency range taking into account the electrode geometry. The electrode geometry is estimated from the capacitance of the uncoated structure, assuming a parallel-plate capacitor, as described in [31]. The resulting Arrhenius-like representation of σ as a function of inverse temperature 1/T in Figure 3(c) gives a thermal activation energy of the conductivity E A of 0.8 ± 0.1 eV. This thermally activated conductivity leads to an almost two decades increase from ~5·10 −7 S/cm at 380 °C to 3·10 −5 S/cm at 650 °C.
Information regarding the electrical conductivity of KMnO 4 -based materials above the decomposition temperature in the literature is limited. In thermoelectric tests, KMnO 4 and its decomposition products were identified as n-type semiconductors by Boldyrev and Kabanov, and in the literature cited therein [26,28]. Upon thermal decomposition, the conductivity of KMnO 4 was found to increase, and depending on morphology, the conductivity was reported to range from 10 −6 to 10 −8 S/cm at 170 to 210 °C [26,28].
The lower conductivity of the KMnO 4 /La-Al 2 O 3 based material under investigation compared to the reported conductivity of pure KMnO 4 is attributed to the less conductive La-Al 2 O 3 particles serving as support oxide in the applied sensitive coating. Recently published results on the K 2 CO 3 /La-Al 2 O 3 system indicate a significant contribution of La-Al 2 O 3 to the measured conductivity given its lower conductivity than that of pure K 2 CO 3 [32]. Figure 3. Thermal activated conductivity of KMnO 4 /La-Al 2 O 3 : (a) Nyquist plot of the impedance Z at 380 and 650 °C, (b) enlargement of 650 °C data, (c) Arrhenius-like representation of conductivity σ from 300 to 650 °C. Further time-continuous impedance measurements were conducted at 10 kHz (see Section 3.1) and the 10 kHz data points in (a) and (b) are highlighted.

NO x Sensing Properties
Similar to passive samplers, dosimeter-type gas sensors are operated in two alternating steps: Analyte molecules are progressively accumulated in the sensitive layer during a sorption period, followed by a regeneration procedure to release the formerly sorbed molecules. The focus of the next section is on the evaluation of the dosimeter-type NO x sensing characteristics of KMnO 4 /La-Al 2 O 3 during NO x sorption as well as the efficiency of thermal regeneration.

Experimental Setup and Data Evaluation
To study the effect of NO x in the low ppm range, the KMnO 4 /La-Al 2 O 3 sample was exposed to various NO and NO 2 concentrations, c NO,in and c NO 2 ,in for defined time intervals t NOx,in . NO x was admixed to the 2 L/min lean base gas flow (10% O 2 , 50% N 2 humidified with a water bubbler at room temperature, and 5% CO 2 diluted in N 2 balance). The outlet concentrations were determined by a chemiluminescence detector, as illustrated in Figure 2. In accordance to the reported catalytic activity of Mn-containing LNTs [23,[33][34][35][36], as well as to results on LNT-based NO x dosimeters [6,18], the NO x sorption studies were performed at a sorption temperature T sorption of 380 °C, with periodic heating to 650 °C for regeneration.
The sample impedance was recorded continuously during NO x exposure. Since the fitted n-parameters of Z CPE (Equation (1)) of KMnO 4 /La-Al 2 O 3 in the high frequency range were found to be

°C
close to 1 (≈0.95), Q can be approximated by the capacitance C and the R||CPE equivalent circuit model can be simplified to an R||C circuit. Thus, R is calculated from the absolute value of the impedance |Z| and the phase angle φ at a fixed frequency according to Equation (2). From the Nyquist plots in Figure 3, 10 kHz (marked in red) was selected as an appropriate measurement frequency to monitor the temperature dependent electrical properties in the R||C dominated range over time and was used, if not denoted otherwise. The absolute value of the relative resistance change due to NO x exposure ∆R rel is denoted as the sensor signal, and is defined by Equation (3), with R 0 being the base resistance in the NO x unloaded state: The analysis in terms of dosimeter-type sensing properties during NO x sorption at constant flow rates is illustrated in Figure 4. During the NO x loading stage, ∆R rel is expected to increase in the presence of NO x at T sorption due to progressive NO x accumulation, without recovery ( Figure 4(a)). In the case of a constant flow rate, the cumulated NO x exposure (or dose) A NOx,in is given by the time integral of c NOx,in as sketched in Figure 4(b), resulting in the unit ppm·s [6,16,37].

Cumulative NO x Detection at 380 °C
The presence of NO x was found to decrease the resistivity of KMnO 4 /La-Al 2 O 3 , with the electrical response continuing to satisfy the R||C equivalent circuit (not shown). The temporal dependence of R on NO x at 380 °C was studied by exposing the sample to pulses of NO and NO 2 for periods of t NOx,in = 100 s with concentrations ranging up to 16 ppm. The pulse heights in terms of c NO,in and c NO 2 ,in , together with the resulting sensor response, are displayed in Figure 5(a). The sensor response ∆R rel (Equation (3)) increases stepwise in the presence of NO and NO 2 without any recovery at 0 ppm NO x . The slope of ∆R rel vs. t increases with c NOx,in . The characteristic line in Figure 5(b) is extracted from the measured data points and the course of the NO x concentration according to Figure 4. ∆R rel correlates almost linearly with the cumulated NO x exposure A NOx,in , independent of the NO x species, up to at least 40% signal change with a NO x sensitivity of 4.8%/1,000 ppm·s. The specimen thus provides comparable sensitivity to both NO and NO 2 . Small deviations of the NO 2 related data points from linearity at the initial stage of exposure may originate from NO 2 adsorption on the inner surface of the feed lines. The sensing characteristics and in particularly the sensitivity of NO x dosimeter with a comparable sensitive material were found to be dependent on the temperature as well as on the thickness of the sensitive layer [6,16].  (3)) during cyclic exposure to NO or NO 2 , (b) resulting linear ∆R rel vs. A NOx,in characteristic line. Figure 5 indicates the strong and progressive sorption of NO and NO 2 onto KMnO 4 /La-Al 2 O 3 at 380 °C with corresponding impact on its electrical conductivity. As in K-Mn-containing LNTs [21,23], NO x is expected accumulate on KMnO 4 /La-Al 2 O 3 by forming stable nitrates on the potassium sites generated upon KMnO 4 decomposition [22]. For LNTs it is well known that NO is first oxidized to  NO 2 on redox active sites provided by e.g., precious metals, followed by chemical NO 2 storage by reaction with the alkaline (earth-) carbonates, mainly BaCO 3 or K 2 CO 3 , to form nitrates [38][39][40]. The observed increase in the conductivity of fully formulated LNTs in NO x enables their application as total NO x sensors [6,11,41] or for in-situ diagnostics of automotive catalysts [41][42][43].
The requirement of an incorporated oxidant, for the purpose of NO sorption, was verified by electrical means. Pure BaCO 3 or K 2 CO 3 , on the other hand, accumulates only NO 2 , enabling conductometric NO 2 dosimetry, without NO cross-sensitivity [15,32]. Given the ability of KMnO 4 /La-Al 2 O 3 to detect either NO or NO 2 , the oxidizing properties of KMnO 4 /La-Al 2 O 3 are demonstrated to be sufficient to convert NO to NO 2 prior to nitrate formation. This is consistent with MnO x , as a product of KMnO 4 decomposition [25,27], being known as an effective oxidizing agent in NO x reduction catalysts [22,24,33,34,36,44]. The contribution of MnO x to the NO x sorption capacity at 380 °C cannot be excluded [45][46][47][48].
The linear correlation between ∆R rel and A NOx,in in the low loading state of the NO x dosimeter based on KMnO 4 /La-Al 2 O 3 in Figure 5 points on a sorption rate proportional to the NO x concentration. This linearity provides a dual-mode functionality: while the sensor response corresponds directly to the cumulated NO x exposure during the sorption period, the course of c NOx,in can be determined via the signal derivative as described in [6,14]. Furthermore, these results demonstrate that decomposed KMnO 4 can be utilized in NO x dosimeters and catalysts without any need for expensive precious metal additives due to its intrinsic oxidizing nature.

NO x Concentration Sensitivity at 650 °C
An important criterion for a useful sensor is the ability to refresh or regenerate the device following accumulation of the target gas analyte, which in this study is NO x . The decreased thermodynamic stability of the formed nitrates upon heating limits the catalytic activity of LNTs [38][39][40]49] and alters the cumulative NO x sensing characteristics of carbonates and LNT-based sensors [6,17,18,32]. According to Becerra et al. [22], nitrate and nitrite-like compounds formed on KMnO 4 -based materials decompose in the temperature range of ~550 to 670 °C. Hence, a thermal release of sorbed NO x , leading to a recovery of the sorption sites of KMnO 4 /La-Al 2 O 3 , seems feasible.
The effect of NO x on the resistivity of KMnO 4 /La-Al 2 O 3 was studied at 650 °C to investigate this temperature as being suitable for regeneration. The sample was exposed to the NO x concentration profile shown in Figure 6(a) with up to 8 ppm NO and 75 ppm NO 2 . ∆R rel is calculated from the impedance at 1 MHz due to the increased conductivity. Again, the conductivity of KMnO 4 /La-Al 2 O 3 increases in the presence of NO x . But, as shown in Figure 6(b), at 650 °C, the value for ∆R rel follows the course of c NOx,in (instead of ∫c NOx,in dt) being characteristic of a common concentration-detecting gas sensor response. Despite the corresponding concentration-related characteristic line in Figure 6(c), which gives a linear correlation, the low sensitivity of only 2.7%/100 ppm NO 2 limits the application of KMnO 4 /La-Al 2 O 3 as a NO x sensing material operated at 650 °C.
The reversibly sensor response at 650 °C in Figure 6 indicates that the equilibrium of the NO x sorption on the KMnO 4 -based material is shifted to the side of the reactants and the resulting fast desorption goes along with the loss of NO x accumulation capability. Hence, 650 °C seems an appropriate temperature to release formerly sorbed NO x and to recover the sorption capacity, as well as the electrical properties of KMnO 4 /La-Al 2 O 3 . The reversibility of the sensor response of KMnO 4 /La-Al 2 O 3 at 650 °C is consistent with results on an LNT-based NO x dosimeter [17].

Efficiency of Thermal Regeneration
In the following test series, the efficiency of thermal regeneration, the reproducibility of the dosimeter-type NO x sensing characteristics, and the influence of NO x exposure time were studied. The same KMnO 4 /La-Al 2 O 3 sample was exposed to 8 ppm NO 2 or NO at 380 °C in periods of 250, 500, 750, 1,000, and 2,000 s. Between each NO x exposure period, the sample was regenerated at 650 °C for about 5 min in the lean gas flow. The sensor responses as a function of t NO 2 ,in and t NO,in are compared in Figure 7. The five NO 2 -borne curves of ∆R rel depicted in Figure 7(a) are almost identical in the corresponding overlapping time scales; please note that the data points corresponding to the longest NO 2 exposure of 2,000 s are partly masked by the other data curves. The corresponding NO curves up to 1,000 s in Figure 7(b) are overlapping as well. The sensor behaves linearly (following an initial incubation period) up to a resistance change of about 40%, with the slope of ∆R rel in Figure 7(a) being nearly constant up to about 1,000 s (8,000 ppm·s NO 2 ). The nonlinearity at the beginning of NO x exposure in Figures 5 and 7, i.e., the slight initial slope increase during the first 375 s in NO 2 (3,000 ppm·s NO 2 ), is assumed to be caused by NO x (in particular NO 2 ) being adsorbed on the feed gas lines resulting in a delayed sensor response. Further NO 2 exposure leads to a decrease in the slope and ∆R rel reaches a value of 70% after half an hour in 8 ppm NO 2 . By definition, ∆R rel cannot reach 100% as the conductivity increases, resulting in a flattening of the curve of ∆R rel with continuing NO x loading. It is expected that a greater sensitive layer thickness would increase the linear range to higher NO x levels, but at reduced sensitivity (slope d(∆R rel )/dA NOx,in ), as reported for LNT-based dosimeters [16]. The reproducibility of the sensor response in Figure 7 indicates that the sorption sites of the KMnO 4 /La-Al 2 O 3 -based dosimeter material can be recovered by releasing sorbed NO x thermally. Heating up to 650 °C restores the NO x sensing characteristics at 380 °C, independent of the former NO x exposure duration. The base resistance in the unloaded state R 0 was found to decrease slightly with time without impacting the NO x sensitivity. This might be attributed to small morphological changes during thermal aging, which, however, are too small to be seen by SEM. It is noteworthy to mention that the dosimeter principle avoids such long term signal drifts by definition, since the zero level of ∆R rel is reset after each regeneration step. This is one of the key advantages of conductometric dosimeters compared to classical semiconductor gas sensors.

Electrical Conductivity during Temperature Programmed Desorption (eTPD)
As in temperature programmed desorption (TPD) studies, the course of the NO x concentration due to NO x desorption during thermal regeneration gives quantitative information about the amount of stored NO x in KMnO 4 /La-Al 2 O 3 . Combining TPD with simultaneous electrical characterization (eTPD), an electrical readout of the cumulative sorbed NO x during the short thermal regeneration periods results. To obtain further insight into the relation between the NO x loading state and the electrical behavior during NO x sorption and release, eTPD is applied to the KMnO 4 /La-Al 2 O 3 formulation.

eTPD Setup and Data Evaluation
eTPD on KMnO 4 /La-Al 2 O 3 is performed with the experimental arrangement shown in Figure 2. The eTPD related data and their evaluation are summarized in Figure 8. To recover the NO x sensing characteristics of KMnO 4 /La-Al 2 O 3 in between the NO x sorption intervals in the lean gas flow in Figure 7, the sample was heated from 380 °C (T sorption ) to 650 °C (T desorption ) with a heating rate of 74 °C/min from 425 to 635 °C, while monitoring the impedance at 10 kHz. The temperature increase started at t heat , 50 s after the end of the preceding NO x dosing interval. The resulting NO x desorption curve (Figure 8(a)) is displayed as a NO x concentration c released . At t start = t heat + 50 s an increase of NO x is observed in the outlet until t end . As illustrated in Figure 8(b), at a constant flow rate, the time integral of c released , evaluated in the time interval of t end -t start = 300 s, reflects the released NO x amount A released . A released is expected to be proportional to the quantity of sorbed NO x , if the sorption sites are fully recovered by heating. The conductance G of KMnO 4 /La-Al 2 O 3 , with G = 1/R, was found to be affected by the temperature and the NO x loading level, both changing during thermal regeneration. As sketched in Figure 8(a), NO x release results in a convergence of G to G 0 = 1/R 0 , G 0 being the temperature dependent conductance in the unloaded state. The time integral of the conductance upon heating, relative to those of G 0 , is evaluated as the cumulative electrical response F G . F G is calculated according to Equation (4) and is shown in Figure 8(b) as the area between the curves corresponding to the two loading states. In the ideal case, F G would be a measurand for A released (Figure 8(c)) and would depend linearly on the cumulative sorbed NO x amount. (4)

Evaluation of the Released Amount
In Figure 7, the resistance responses of KMnO 4 /La-Al 2 O 3 during an NO 2 sorption series at 380 °C with various exposure periods t NO 2 ,in are reported. The subsequent regeneration by heating to 650 °C to release the formerly sorbed NO x can be analyzed in terms of TPD. The corresponding outlet NO x concentrations c released in the lean 2 L/min gas flow, with a resolution of 0.1 ppm given by the CLD, for t NO 2 ,in up to 1,000 s are compared in Figure 9. After 1,000 s in 8 ppm NO 2 (red curve, as indicated in Figure 9), KMnO 4 /La-Al 2 O 3 starts to release NO x at about 400 °C. c released increases with temperature, and at about 550 °C, a maximum is reached at about 1.3 ppm. Shorter NO 2 exposure periods, representing a lower amount of NO x loading, yield lower peak heights of c released . At 650 °C, c released reaches zero for all curves, indicating the end of NO x release. Additionally, both peak maximum and desorption onset are shifted to lower temperatures with increasing t NO 2 ,in . The latter points to a lower stability of the sorbed NO x with increased NO x loading. Concerning the low values of c released , it should be considered that the evolved NO x is diluted in the 2 L/min lean gas flow and that the sensitive KMnO 4 coating amounts only to an area of about 30 mm 2 (5 × 6 mm).
The reproducibility of the dosimeter-type NO x sensing characteristics at 380 °C ( Figure 7) and the missing NO x accumulation at 650 °C ( Figure 6) reveal that the NO x sorption capacity of KMnO 4 /La-Al 2 O 3 can be recovered by heating to 650 °C. Therefore, the quantity of sorbed NO 2 on KMnO 4 /La-Al 2 O 3 can be estimated from the subsequently thermally released NO x amount A released , being the area under the desorption peak as shown in Figure 8(b). Figure 9(b) reveals that A released and hence the amount of NO x sorbed in KMnO 4 /La-Al 2 O 3 increases almost linearly (with only a small offset) with NO 2 exposure, reflected by t NO 2 ,in . Consequently, NO 2 is sorbed on KMnO 4 /La-Al 2 O 3 with a time constant sorption rate during the 8 ppm NO 2 exposure periods. After 1,000 s in 8 ppm NO 2 , resulting in a cumulated NO 2 exposure of 8,000 ppm·s, about 150 ppm·s NO x are released. This indicates that only about 1.9% of the NO 2 in the passing gas flow is sorbed in the KMnO 4 based sensitive layer. The gas velocity of 5.3 m/min together the sensitive area length of 6 mm amounts to a residence time of about 70 ms, being comparable to those in catalysts [37]. However, the huge gas volume above the sensitive layer in the 22 mm diameter quartz tube inhibits full NO x storage in this NO x dosimeter setup. The small offset of A released in Figure 9(b) amounts to about A offset ≈ 15 ppm·s. If one divides this value by the integration time of 300 s, one obtains an average concentration of 0.05 ppm NO x . A closer look at Figure 9(a) reveals that this is (roughly) the offset of the NO concentration measurement by the CLD with a resolution of 0.1 ppm. As a conclusion, the offset of A released can be attributed to an integration error. The analysis of the corresponding data after NO exposure yield the same qualitative results (data not shown here) but with a smaller offset. Hence, a further explanation might be the partial overlap of c released with the preceding decay of c NO 2 ,in due to NO 2 adsorption in the feed lines. Figure 9. NO x release during heating to 650 °C after 8 ppm NO 2 exposure for 250 s, 500 s, 750 s, and 1,000 s: (a) outlet NO x concentration c released , (b) area A released below the curve as depicted in Figure 8(b) as a function of NO 2 loading time t NO 2 ,in .
The observed sensor response ∆R rel of KMnO 4 /La-Al 2 O 3 during NO x sorption at 380 °C ( Figure 7) obviously corresponds to the amount of loaded NO x . In Figure 10, ∆R rel , caused by 8 ppm NO or NO 2 for up to 1,000 s, is related to A released , which is obtained from the subsequent regeneration shown in Figure 9. For NO exposure, as well as for NO 2 exposure, ∆R rel increases linearly with A released . Hence, in the investigated range, ∆R rel serves as a linear measure for the NO and NO 2 loading levels of KMnO 4 /La-Al 2 O 3 , and due to the constant NO x sorption rate (Figure 9(b)), also for the cumulated NO x exposure (NO x dose). Thereby, the conductivity of KMnO 4 /La-Al 2 O 3 is slightly more sensitive to NO compared to NO 2 . From a catalytic point of view, it would be expected that NO 2 in the gas flow influences the material's properties more than NO, since NO 2 can be sorbed directly as nitrate, whereas NO needs to be oxidized first [23,36]. However, the manganese oxide components of the decomposed KMnO 4 might become reduced upon oxidizing NO, thereby affecting the conductivity of KMnO 4 /La-Al 2 O 3 and hence the NO sensitivity. The delay in the sensor response resulting in an x-axis intercept in Figure 10 is expected to be caused by the already discussed inaccuracy of the determination of A released by integration of small values of evolved NO x (A Offset ≈ 15 ppm·s in Figure 9(b)). In addition, NO x A offset (in particular NO 2 ) adsorption to the feed lines lowers the sensor response but increases the analyzed value for the desorbed amount. Figure 10. Correlation between the sensor response ∆R rel during NO and NO 2 sorption and the NO x amount A released obtained from subsequent TPD.
Combining the classical TPD method, with the dosimeter-type electrical response of KMnO 4 /La-Al 2 O 3 , demonstrates the possibility of sensing NO x exposure and of electrically monitoring the NO x loading level of the NO x sorbent in-situ, both with linear correlation at low loading.

Electrical Information upon Thermal Regeneration
The conductance upon releasing NO x provides information about the amount of previously sorbed NO x . This may also be useful for NO x dosimetry. Figure 11a depicts the courses of the conductance G during thermal regeneration after exposure to 8 ppm NO 2 , for the different loading states indicated by its specific NO x exposure period t NO 2 ,in . The course of the temperature is shown for comparison (black dots). G 0 reflects the conductance in the NO x unloaded state corresponding to t NO 2 ,in = 0.
Being thermally activated, G 0 increases by nearly two orders of magnitude, which agrees with Figure 3. At 380 °C, the conductance in the partly NO x loaded state G is higher than G 0 . The difference between G and G 0 corresponds to the cumulative NO x response, ∆R rel (Figure 7). With progressive temperature, log G increases like log G 0 . The difference between log G and log G 0 increases with t NO 2 ,in , indicating a correlation with the NO x loading level. Between about 480 and 530 °C, the curves of G start to converge to those of G 0 . Finally, above about 620 °C (230 s) all curves of G coincide with G 0 indicating that the unloaded state is recovered. A more detailed analysis reveals that the inflection point in the course of log G corresponds to the onset of NO x release shown in Figure 9(a) as c released . The temperature of the minimum in the slope of log G coincides with the temperature of the maximum of c released . Both are being shifted to lower temperatures, the higher the former loading level was. Hence, the convergence of the curves of log G to the reference in the unloaded state can be attributed to thermal NO x release from KMnO 4 /La-Al 2 O 3 , which decreases the temperature-dependent conductivity to the unloaded value.  Figure 11. eTPD results after 8 ppm NO x for up to 1,000 s: (a) conductance log G and temperature T during TPD, (b) cumulated electrical response F G (calculated acc. to Equation (4)) vs. the NO x loading time t NO 2 ,in , (c) F G as a function of the amount of released NO x A released for NO and NO 2 loading, as determined in Figure 9.
The comparison of the curves of the conductance G during regeneration (Figure 11(a)) suggests that the deviation of the course of log G from log G 0 might reflect the amount of previously sorbed NO x . In fact, Figure 11(b) reveals a linear correlation between F G calculated according to Equation (4) and the preceding sorption interval t NO 2 ,in . Accounting for the time constant NO and NO 2 sorption rate in the low loading state (exemplarily shown for NO 2 in Figure 9(b)), F G is also a linear function of A released as shown in Figure 11(c) for NO and NO 2 exposure, respectively. Therefore, besides of ∆R rel , the cumulated electrical response F G of KMnO 4 /La-Al 2 O 3 during regeneration may also be a suitable sensor signal for the cumulated NO x exposure and the in-situ loading level. Again, the resulting sensitivity to NO is slightly higher than those to NO 2 . Furthermore, the sensor response exhibits an offset, in particular, for NO 2 . Besides of the small integration error A offset when determining the area under the low level concentration curve during desorption in Figure 9a, these offsets likely originate from NO 2 adsorption in the feed lines. Considering the electrode geometry, the conductivity σ can be calculated from the conductance and the data from Figure 11(a) can be plotted in an Arrhenius-like representation in the area of a constant heating rate as depicted in Figure 12. The data points of the electrical characterization of the KMnO 4 /La-Al 2 O 3 sample after equilibration at various temperatures shown in Figure 3(c) are added to Figure 12 as black dots. Concerning the thermally activated conductivity in the unloaded state, the direct comparison reveals that the eTPD data agree well with those obtained in the equilibrated state. This confirms the recovery of the sorption sites by heating to 650 °C. The NO x saturated state of the carbonates was found to give a more pronounced transition in the curve of the equilibrated temperature-dependent conductivity upon thermal decomposition [32].
The investigation of NO x sorption on KMnO 4 /La-Al 2 O 3 demonstrates that eTPD enables one to correlate the analyte-induced electrical response quantitatively with the actual analyte loading state. This is achieved by observing the electrical properties and the gas desorption characteristics simultaneously. The eTPD method might enhance the understanding of the analyte sorption related electrical properties of functional materials applied for gas sensing or catalysis. Approaches on interpreting the conductivity during thermally releasing gas species are reported in the literature as well, but-to our knowledge-only without a simultaneous quantitative analysis of the desorption peak and hence with a missing correlation with the actual loading state under identical conditions. Colin et al. [50] and Fortin et al. [51] modeled the electrical influence of chemisorbed gases on semiconductors upon heating and verified it for the system oxygen-CdSe. The slope of the conductivity is reported to give information on desorption or binding energies of the species [51][52][53]. Rossé et al. [53] explains in detail the course of the resistivity in the Arrhenius-like representation during TPD being dependent on the heating rate and the amount of chemisorbed species. Additionally, the recovery of the initial loading state is described as a convergence to the unloaded reference. This description agrees fully with the interpretation of the results on NO x loaded KMnO 4 /La-Al 2 O 3 in Figure 11. However, applying eTPD, these electrical results were additionally verified by the analysis of the simultaneous desorption peak (Figure 9(a)). Yamazoe et al. [54] and Rodríguez-Gonzáles et al. [55] compared the conductivity with the evolution of desorbed gases as well, but these tests were performed on multiple samples in different setups. The Simon group [55][56][57] investigated the temperature dependent

°C
NH 3 loading level of zeolites being active for the selective catalytic reduction (SCR) of NO x . The conductivity upon heating reveals information on the conduction mechanism, the NH 3 desorption temperature as well as the SCR active temperature region allowing for in-situ reaction monitoring.
Kubinski et al. [58] demonstrated that the average resistance during the thermally-induced NH 3 release from an SCR zeolite catalyst correlates with the former NH 3 exposure. This enables in-situ monitoring of the amount of sorbed NH 3 with a higher sensitivity compared to those in the NH 3 sorption mode [58]. The equivalency of the two conductivity-related sensor responses ∆R rel and F G of KMnO 4 /La-Al 2 O 3 as a measure for the cumulated NO x exposure is demonstrated in Figure 13 as a monotone and almost linear correlation, independent of the type of exposed NO x species. Both values can be applied as NO x dosimeter-type sensor responses and correlate linearly with the quantity of sorbed NO x , enabling in-situ monitoring of the loading state, although they are analyzed upon NO x sorption at 380 °C (∆R rel ) and upon NO x release by heating up to 650 °C (F G ), respectively. Hence, dependent on the application and the information of interest, two different sensing modes are feasible with the proposed impedimetric NO x dosimeter based on KMnO 4 . In both cases, NO x is accumulated in the sensitive layer at sorption temperature and thermally released during periodic regeneration intervals. However, in the first method, the change in the conductance during NO x accumulation in the low loading state is monitored as a continuous and linear measure for the cumulated NO x exposure as well as for the amount of sorbed NO x . Concentration information can be obtained time-continuously from the signal derivative. On the contrary, in the second method, the integrated difference between the conductivity during NO x release upon heating and the conductivity in the unloaded state serves as the measurand. Unfortunately, no time-continuous information on the NO x concentration can be obtained. Figure 13. Correlation between electrical responses ∆R rel (during sorption) and F G (upon regeneration) affected by NO x exposure.
Simultaneous NO x detection in the sorption and release mode may be realized on one single sensor platform, with multiple independently heated sensitive layers, as described in [17]. A combination of both sensing modes to extract further information will be the focus of further research. The redundant sensing information is expected to enable a plausibility consideration of the time resolved sensor signal during NO x sorption and of the regeneration success. Additionally, the linear measurement range for the NO x exposure is expected to be enhanced in the regeneration mode. Another important issue of gas sensors is the sensitivity to other gases as well as poisoning of sensitive layers, e.g., by SO 2 [59,60]. Since interfering gases might affect NO x sorption and release as well as the temperature-dependent conductivity differently, a combination of the electrical responses upon NO x sorption and release might be particularly useful.

Conclusions
This initial study demonstrates the suitability of decomposed KMnO 4 deposited on La-stabilized alumina as dosimeter-type sensitive material with two different operation methods and as a NO x sorbent in catalysts with electrical in-situ characterization potential. The impedimetric sensor response to low levels of NO and NO 2 was found to be irreversible under isothermal conditions at 380 °C. These dosimeter-type sensing characteristics are reproducible if sorbed NO x is released by heating up to 650 °C to recover the sorption capacity. The resistance change of KMnO 4 /La-Al 2 O 3 in the low loaded state correlates linearly with the cumulated NO x exposure, enabling low level NO x detection due to the NO x oxidizing and sorbing capability of the KMnO 4 -based material. The sensor responds slightly more sensitively to NO than to NO 2 .
By combining the electrical response with thermal programmed desorption (eTPD), the change in the electrical properties of KMnO 4 /La-Al 2 O 3 can be related to the thermally released quantity of NO x . This novel method enables the quantitative correlation between the electrical response and the NO x loading in the sensor (or catalyst) material. The amount of NO x sorbed on KMnO 4 /La-Al 2 O 3 , estimated from the released amount, increases linearly with the cumulated NO x exposure (or dose), resulting in a time constant NO x sorption rate. The resistance change during NO x sorption correlates linearly with the amount of sorbed NO x and hence with the NO x exposure. Therefore, information on the NO x concentration can be obtained time-continuously from the signal derivative. Additionally, the thermally activated conductivity of KMnO 4 is affected by the NO x release upon heating. The deviation from the course of the temperature-dependent conductivity in the unloaded state is another linear measure for the previously stored amount of NO x . As a result, NO x exposure and NO x loading dependent electrical response can be analyzed either during NO x sorption or release enabling dosimeter-like NO x sensing or electrical in-situ monitoring.