Method for the detection of NO in fluids

The invention relates to a method for the detection of nitrogen oxide (NO) in fluid media.

Several methods are known for the denitrification of diesel engine exhaust gases.

On the one hand, catalytic denitrification can be carried out using ammonia or hydrocarbons as reducing agents. On the other hand, adsorber catalysts can denitrify by adsorbing NO_x enable, which must be regenerated at regular, short intervals by the use of reducing agents.

In addition, nitrogen oxide emissions can be reduced by returning the exhaust gas to the combustion chamber. This can be intensified by the recirculated exhaust gas using suitable adsorbent materials with NO_x is enriched.

The material-assisted denitrification processes for the reducing agent dosing and the adsorption / regeneration times provide knowledge of the load-dependent and speed-dependent instantaneous NO_xEmissions ahead. This can be done either by using a NO sensor or by using map values ​​that have been stored in a memory.

NO_xMaps do not apply to the individual engine, but only to the series, so that production-related fluctuations in the NO_x- Raw gas content can occur with the same map points. In addition, the current catalytic converter status (temperature, NO_x- and reducing agent loading) may be different for the same map points. Therefore, a NO sensor-controlled control would be preferable. Using an NO sensor, the exhaust gas recirculation rate could be controlled so that a minimal NO_xEmission occurs.

A number of sensor principles for measuring the NO content in gases are described in the literature. However, only a few of them are suitable for the conditions in hot real exhaust gas.

In principle, the use of ceramic solid electrolytes offers a high potential for use, since these sensors are characterized by high selectivity, high temperature resistance and low construction costs. This is shown not least by the great success of the λ sensor based on ZrO₂ as an oxygen ion conductor. A transfer of this principle to NO requires an NO⁺- solid electrolytes.

In the <b>US 5,466,350</b> is an amperometric thin film solid electrolyte detector for NO, which is based on the passage of nitrosonium cations (NO⁺) based on the solid electrolyte. Four electrodes are required in a bipotentiostat arrangement: a first and second working electrode, a common reference electrode and a common counter electrode. In addition, a diffusion barrier is used in front of the working electrode, which acts as an anode, and ensures that the sensor operates under diffusion-controlled conditions. This is the only way to ensure that the current-voltage characteristic is proportional to the NO concentration in the gas.

As a solid electrolyte in the <b>US 5,466,350</b> NO-β-Al₂O₃ used that in two steps from Na-β-Al₂O₃ is manufactured. First, the NO-β-Al₂O₃ in an AgNO₃-Melt with Ag⁺ to Ag-β-Al₂O₃ exchanged. In a second step, Ag-β-Al₂O₃ with NO⁺ exchanged using NOCl ions, so that ultimately NO-β-Al₂O₃ is obtained.

This second step requires a medium that is simultaneously well soluble for both NO⁺ as well as for Ag⁺ and has good oxidation resistance at temperatures around 200 ° C.

The state of the art according to <b>US 5,466,350</b> has the following disadvantages:<ul id="ul0001" list-style="dash"><li>The bipotentiostat arrangement uses four electrodes, which is a significantly more complicated arrangement compared to a two-electrode arrangement in terms of design, manufacture and operation.</li><li>A diffusion barrier must be integrated so that the ion current in the solid electrolyte is proportional to the NO concentration in the gas phase in an amperometric measuring principle.</li><li>The proposed amperometric sensor principle with diffusion limitation is also inherently slow because it is diffusion-dependent. A dependency of the sensor signal on the surface quality (occupancy by soot particles) also occurs here. As a result, no constant dependence on sensor signal and NO content can be achieved in real vehicle exhaust gas.</li><li>The main disadvantage of the amperometric measurement principle is also the cross sensitivity to water vapor, which is present in the exhaust gas in high concentrations. Due to the voltage of at least 1.6 V to be applied for the NO oxidation (note: literature value 1.31 V refers to the calomel electrode), reactions with water vapor also take place at the first working electrode (anode), such as:<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>2H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext> → O</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msup><mrow><mtext> + 4H</mtext></mrow><mrow><mtext>+</mtext></mrow></msup><msup><mrow><mtext> + 4e</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><mtext> (1.229 V)</mtext></mrow></math><img file="EP0871031A2_D0001.tif" /></maths><maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext>2NO + H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>O → N</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>O</mtext></mrow><mrow><mtext>4</mtext></mrow></msub><msup><mrow><mtext> + 4H</mtext></mrow><mrow><mtext>+</mtext></mrow></msup><msup><mrow><mtext> + 4e</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><mtext> (1.035 V)</mtext></mrow></math><img file="EP0871031A2_D0002.tif" /></maths>The protons formed are also transported through the electrolyte and, like NO, reduced on the second working electrode, which acts as a cathode. The measured current thus consists of the reduction of NO⁺ and H⁺ together.</li></ul>

With regard to the production of the NO⁺-conducting solid electrolytes according to the <b>US 5,466,350</b> there are the following disadvantages:<ul id="ul0002" list-style="dash"><li>The proposed manufacturing process for the NO⁺-conducting solid electrolytes require working in aggressive, not hydrolysis-resistant molten salts and is carried out in a very cumbersome manner in several steps. In particular, the cleaning of the solid electrolytes after ion exchange with water and alcohol, which is necessary when using a molten salt, is problematic and can lead to damage to the electrolyte (hydrolysis of NO to ENT)₃). The limitation to 190 ° C (beginning decomposition of the NO salt melt) and the concentration compensation in the melt and electrolyte as the only driving force makes the complete ion exchange in dense layers of β '' - Al₂O₃ impossible. Gas-tight thin layers of pure β '' - Al₂O₃ are however necessary for a technical implementation of the results for a fast sensor.</li><li>A fundamental disadvantage of the exchange reactions in the melt or in solution is that the driving force of the reaction is the formation of insoluble secondary products, such as AgCl, when using an Ag⁺Ion conductor in a NOCl-containing melt. It is known from preliminary investigations (RH Radzilowski, JT Kummer, Inorg. Chem. 8 (1996) 2531; S. Maraun, diploma thesis, University of Essen, 1996) that these insoluble residues can deposit on the solid electrolyte and thus the exchange through passivation prevent. This forces the use of starting materials that are as finely powdered as possible, which after the exchange must first be chemically processed and compacted into shaped bodies. As will be shown below, moldings of the desired shape can be used directly in the process according to the invention.</li><li>In US 5,466,350 Na-β-Al₂O₃ went out. Na-β-Al₂O₃ points towards Li-stabilized Na-β '' - Al₂O₃ changed layer sequence and the resulting lower ionic conductivity and greater sensitivity to hydrolysis. As a rule, it contains high (&gt; 1%) portions of NaAlO₂, which is responsible for the sensitivity to hydrolysis. A quantitatively complete exchange of Na⁺ against Ag⁺ is not possible. The remaining Na leads to a strong susceptibility to hydrolysis in real exhaust gas and thereby to a rapid destruction of the electrolyte in real exhaust gas.</li></ul>

In summary, there are the following disadvantages for the implementation of a sensor according to the <b>US 5,466,350</b>, especially with regard to for automotive applications:<ul id="ul0003" list-style="dash" compact="compact"><li>no purely potentiometric measuring principle (λ sensor), therefore slow equilibrium setting and complex sensor technology;</li><li>low hydrolysis resistance of the solid electrolyte leads to destruction when operating in real exhaust gas;</li><li>elaborate production process, no complete conversion to NO ion conductor possible.</li></ul>

The invention is therefore based on the object of providing a method for detecting NO in fluid media with which the disadvantages of US Pat. No. 5,466,350 mentioned can be overcome.

This object is achieved with the method according to claim 1. Advantageous designs and a method for producing the NO⁺conductive solid electrolytes are the subject of further claims.

The process according to the invention comprises the following process steps:<ul id="ul0004" list-style="dash" compact="compact"><li>Positioning a nitrosonium (NO⁺) -conducting solid electrolytes between two porous electrodes,</li><li>immediate exposure of one of the two electrodes to the NO-containing fluid medium, without the interposition of a diffusion barrier,</li><li>Measuring the voltage difference between the two electrodes, which is a measure of the NO concentration in the NO-containing fluid medium.</li></ul>

The method according to the invention overcomes the disadvantages of the prior art in that a potentiometric and not an amperometric measurement is carried out for NO detection. This means that only two electrodes are required. In principle, potentiometric sensors achieve a fast signal response, since no diffusion equilibrium has to be set. Furthermore, the measurement signal is independent of changes in the sensor surface, which typically occur during operation in real exhaust gas.

Furthermore, the NO⁺conductive solid electrolyte can be obtained directly from the gas phase instead of via ion exchange in the salt melt. This can be done in simple, gas-tight reactors and is therefore also easy to handle in large-scale use.

The method according to the invention can be used in particular in the exhaust gas of both lean and λ = 1-operated motor vehicles to determine the instantaneous nitrogen oxide concentration.

The invention is explained in more detail with reference to FIG. Show it:<dl id="dl0001" compact="compact"><dt>Fig. 1</dt><dd>Voltage of the NO sensor (NO-β-Al₂O₃) under currentless conditions for different NO concentrations (1%, 0.2%, ...... 60 ppm) over time.</dd><dt>Fig. 2</dt><dd>Dependence of the voltage of the NO sensor on the NO concentration.</dd><dt>Fig. 3</dt><dd>several process variants for producing a NO-β '' - Al₂O₃- solid electrolytes.</dd><dt>Fig. 4</dt><dd>Experimental setup for the production of a NO-conductive solid electrolyte by means of gas phase exchange.</dd><dt>Fig. 5</dt><dd>Time course of voltage and current when installing NO⁺ in the solid electrolytes during the gas phase exchange.</dd></dl>

Sensor structure and principle of NO detection

A gas-tight (&gt; 98% of the theoretical density) thin disk made of Li-stabilized Ag-β '' - Al is advantageously used for the sensor structure₂O₃ with a thickness of, for example, 200 µm. This disc is applied to a metallized ceramic base using a connection layer (eg glass solder). The connection takes place in a soldering process at a maximum of 1000 ° C depending on the glass solder selected. The connection to the base must be gas-tight and non-conductive. Pastes, CVD or PVD layers can be used as electrodes. The layers should have a small thickness and allow rapid gas diffusion to the electrolyte surface. The electrode material can in particular be graphite, noble metal or electronically conductive ceramic. The contact can be made via precious metal contacts.

When using the Ag⁺ against Na⁺ Ion exchange in an Ag melt, the silver deposited on the cathode or remaining on the anode takes over the function of the electrode.

Test gas (exhaust gas) and air are through a gas-tight layer (molded body) of a NO⁺-conductive solid electrolyte separately (Fig. 4). The redox reaction NO &lt;-&gt; NO occurs on the porous electrolytes (preferably platinum) on both sides⁺ instead of and enforces a potential between the electrodes determined by the concentration gradient. The potential is directly dependent on the difference in concentration on both sides, i.e. if air is used as a reference, it is proportional to the NO concentration in the exhaust gas. In contrast to the NO sensor after the<b>US 5,466,350</b>, which must be operated amperometrically, the functional principle described here is purely potentiometric and corresponds to that of the λ probe with its known advantages.

Using the example of NO-β-Al₂O₃ is shown in Fig. 1 that 60 ppm NO in the exhaust gas provide a well-measurable voltage. Furthermore, FIG. 2 shows that the greatest voltage changes occur with the concentration in the concentration range of 0-2000 ppm NO relevant for use as an exhaust gas sensor.

If the gases differ on both sides of the electrolyte with regard to their NO concentration, this results in a potential difference between the two electrodes according to Nernst's law:<maths id="math0003" num=""><math display="block"><mrow><mtext>U = (R x T) / F x In (pNO, sample gas / pNO, reference)</mtext></mrow></math><img file="EP0871031A2_D0003.tif" /></maths> With<ul id="ul0005" list-style="none" compact="compact"><li>R = gas constant</li><li>T = temperature in Kelvin</li><li>F = Faraday constant.</li></ul>

This potential difference is made up of the two standard potentials, which are determined at the anode by the concentration difference between NO in the sample gas and NO⁺ in the solid electrolyte and on the cathode between the concentration of NO in the reference and NO⁺ in the solid electrolyte. In contrast to the amperometrically operated sensor, the NO at the anode does not need to be NO for the measuring effect of a potentiometric sensor⁺ oxidized and reduced to NO at the cathode.

By selecting the NO partial pressure on the reference gas side, both the measuring range and the sensitivity of the sensor can be adapted to the requirements. If the NO partial pressure in the reference gas is higher than in the sample gas, the signal curve is very steep, with a smaller partial pressure it is flatter.

Since the voltage also depends on the temperature, electronic compensation must be carried out via a thermocouple if the temperature fluctuates.

A diffusion barrier is not necessary, since there is no conversion of NO at the electrodes and therefore diffusion to the electrode is irrelevant. Therefore, the sensor is also insensitive to fluctuating gas speeds.

Production of the NO

<b>+</b>

-conductive solid electrolyte

3 shows various advantageous process variants for producing an NO-β '' - Al₂O₃Solid electrolytes with Na-β '' - Al₂O₃ as an exemplary starting material.<ul id="ul0006" list-style="none" compact="compact"><li><u>1. Step:</u> Ion exchange Ag⁺ against Na⁺ in an AgNO₃-Melt at around 300^O C or by anode coating with silver paste with applied voltage and approx. 1000^OC.</li><li><u>2nd Step:</u> Ion exchange NO⁺ against Ag⁺ in the gas phase with voltage applied and approx. 300 ^OC.</li></ul>

The use of Li-stabilized β '' - Al proves to be particularly advantageous₂O₃ with a NaAlO₂-Content &lt;0.5% by weight. This makes the conductivity and hydrolysis resistance to Na-β-Al₂O₃ significantly improved (JL Sudworth, AR Tilley, The Sodium Sulfur Battery, Chapmann and Hall 1985, p. 20-56).

There is also an exchange rate of&gt; 99% for the ion exchange Na⁺ against Ag⁺ in the AgNO₃-Melt reached. This significantly improves the sensitivity of the sensor to hydrolysis, since only traces of sodium or NaAlO are left in the replaced electrolyte₂ are included.

To avoid the Ag⁺ against Na⁺ After the sensor assembly, the ion exchange in the melt can also be coated with a silver paste on the anode. When the melting temperature of Ag (961 ° C) is reached and a sufficiently high voltage is applied between the anode and cathode, a current flow is measured which is based on the ion exchange Ag⁺ against Na⁺ is due. This exchange should take place on the anode and cathode side under inert gas.

Before the ion exchange, the electrolyte can be freed of any cover layers that inhibit ion exchange (NaOH and Na₂CO₃) by heating the electrolyte to over 850 ° C before applying the voltage.

For the ion exchange Ag⁺ against NO⁺ in the gas phase, the sensor assembly is heated to 300 ° C and an NO-containing carrier gas is applied to the anode. When a sufficiently high voltage is applied between the anode and cathode, a current flow is measured which is based on the ion exchange NO⁺ against Ag⁺ is due. Damage to the electrolyte due to cleaning in H₂O does not take place since no molten salt has to be removed. The cathodically deposited metallic silver can remain as an electrode on the electrolyte surface.

Exemplary embodiment: gas phase exchange of the solid electrolyte

A complete Ag-β '' - Al₂O₃Solid electrolyte 2 is contacted on both sides with electrodes 1,3 and installed in a gas-tight chamber in such a way that it is only exposed to NO-containing carrier gas on one side, as shown in FIG. 4. The membrane is heated to at least 300 ° C via the gas flow in order to achieve sufficient ion conductivity of the electrolyte. The NO-facing electrode is switched as an anode, whereas the cathode is surrounded by air.

5 shows the time course of current and voltage during the ion exchange. A voltage is applied which is slowly increased when the redox potential of NO / NO is reached⁺ a current flows which increases with the voltage. Due to polarization, the current becomes somewhat smaller at constant voltage.

The current flow quickly drops at a constant voltage after switching off NO and is available again after switching on NO. The current flow is thus clearly attributable to the following processes:<ul id="ul0007" list-style="none" compact="compact"><li>1. Oxidation from NO to NO⁺ at the anode.</li><li>2nd Transport of NO⁺ through the electrolyte (ion exchange).</li><li>3rd Reduction of NO⁺ to NO at the cathode.</li></ul>

Since the electrolyte is completely filled with Ag⁺ was loaded, an ion exchange of NO⁺ against Ag⁺ have taken place. By calculating the amount of substance required for any exchange of M⁺ by NO⁺ the duration of the electrolysis can be determined directly, i.e. the duration of the electrolysis directly determines the degree of exchange (principle of the pump cell). Since elemental metal or corresponding oxides / hydroxides are deposited on the working electrode during electrolysis, the installation of NO⁺ In addition to the current / time measurement, gravimetric monitoring is also carried out, which makes further analysis unnecessary.

Gases and solid electrolytes to be used advantageously for gas phase exchange

Pure NO and oxygen-free NO-containing carrier gases or gas mixtures are suitable as the gas phase.

As precursors for NO⁺-Exchanged solid electrolytes are suitable for water-free, cation-conducting solids. These include:<ul id="ul0008" list-style="dash" compact="compact"><li>Me-β-Al₂O₃, with Me = Li⁺, N / A⁺, K⁺, Rb⁺, NH₄⁺, Ag⁺</li><li>Me-β '' - Al₂O₃, with Me = Li⁺, N / A⁺, K⁺, Rb⁺, NH₄⁺, Ag⁺</li><li>β-ferrites (Fe₂O₃)</li><li>α-Corundum Sodalite</li><li>natural and synthetic layered silicates</li><li>Oxide framework structures in pyrochlore or defect pyrochlore structure of the general form AB0₃, with A = monovalent cations and B = Sb, Nb, W, Ta, as well as combinations of these elements with B = NbTa</li><li>Zeolites, zeolite-related framework structures such as AIPOs or SAPOs as well as mesoporous metal oxides</li><li>Hydrated V₂O₅</li><li>Vanadates and phosphates</li><li>Scheelite e.g. approx_1-xAg_xWHERE₄</li><li>Argyrodite Ag_xMX₆ (x = 7-9, M = metal; X = S, Se, Te) e.g. Ag₇TaS₆, Ag₉GaSe₆</li></ul>