WO1995009361A1

WO1995009361A1 - Nitrogen monoxide no and ammonia nh3 detector

Info

Publication number: WO1995009361A1
Application number: PCT/DE1994/001115
Authority: WO
Inventors: Matthias Peschke; Hans Meixner; Helmut Schmelz; Asbjörn RAMSTETTER; Monika Seidl; Bertrand Lemire; Maximilian Fleischer; Christian Dahlheim
Original assignee: Siemens Aktiengesellschaft
Priority date: 1993-09-28
Filing date: 1994-09-23
Publication date: 1995-04-06
Also published as: EP0721584A1; JPH09503062A; DE4333006A1; KR960705206A; DE4333006C2

Abstract

The Nox emission of a Diesel engine may be substantially reduced by applying the SCR process. In this process, NH3 is injected into a catalyst through which the exhaust fumes are made to flow and in which the injected NH3 reacts with NO or NO2, forming nitrogen and water. As the exhaust fumes should not contain NO nor excess NH3, appropriate detectors are required to control NH3 leaks or to monitor or regulate the NH3 dosage. For that purpose, a nitrogen monoxide and ammonia detector contains an AlVO4 or FeVO4 thin layer as gas-sensitive element. The sensitivity to NO or NH3 of a vanadate layer produced by a special sputtering process is higher by several orders of magnitude than the transverse sensitivity to oxygen and hydrogen. The detector is not sensitive to methane, carbon monoxide and carbon dioxide. No masking effects occur, i.e. the sensitivity to NO and NH3 of the detector is not affected by the presence of other gasses. The invention is suitable as an air quality sensor, as a NH3 leak monitor and as a sensor for regulating a DENOx catalyst.

Description

Detector for the detection of nitrogen monoxide NO and ammonia NH ₃

The nitrogen oxide and particle emissions (dust) of a diesel engine optimized for performance and consumption can only be reduced insignificantly by means of combustion technology. In order to also be able to comply with the exhaust gas values prescribed by law in the future, post-treatment of the diesel engine exhaust gases is therefore unavoidable.

A significant reduction in the NO _x emission of an engine with excess air can be achieved by using the so-called "selective" atalytic eduction method. In the SCR process, gaseous ammonia NH3, ammonia in aqueous solution or urea as a reducing agent is injected into the exhaust system, so that the chemical reactions in particular on a catalyst

4NO + 4NH3 + O2 → 4N2 + 6H2O 2N02 + NH3 + O2 → 3N2 + 6H2O

can expire. About 0.9 to is required to completely reduce 1 mol of NO _x in the diesel engine exhaust gas

1.1 moles of NH3. If less ammonia NH3 is injected, the catalyst no longer works with the highest efficiency.

Overdosing should also be avoided, as otherwise unused ammonia NH3 will get into the atmosphere. Sensors with which one could measure the NH3 slip or check or regulate the NH3 dosage would therefore be advantageous. There is a desire on the part of the automotive industry to control air conditioning systems and ventilation systems in such a way that the concentration of pollutants in the gas cell of a car always remains below a threshold which is harmless to human health. This requires, for example, a sensor for nitrogen oxides N0 _X , which reduces or interrupts the fresh air supply from a certain NO _x concentration and switches the ventilation system to air recirculation mode. Similar to an NH3 sensor, a detector that responds to nitrogen oxides could also be used to control a diesel catalytic converter.

A NO _x detector is known from / l /, the sensitive element of which consists of a mixture of the metal oxides Al2O3 and V2O5. However, the known detector does not respond to ammonia NH3. In addition, there are considerable difficulties in distinguishing the nitrogen oxides NO and NO2.

The aim of the invention is to create a detector with which both ammonia NH3 and nitrogen monoxide NO can be detected in a gas mixture. A detection of these gases should also be guaranteed if their concentration is in the ppm range. In addition, a method is to be specified with which highly sensitive vanadate layers can be produced. According to the invention, these objects are achieved by a detector according to patent claim 1 and a method according to patent claim 9.

The advantage that can be achieved with the invention is, in particular, that the detector can still be operated without problems even at the temperatures of 500 to 600.degree Cross sensitivity to oxygen O2 and hydrogen H2 is. The detector does not respond to methane CH4, carbon monoxide CO and carbon dioxide CO2. There are also no masking effects, ie the sensitivity of the detector to NO and NH3 is affected by Presence of other gases not changed. A distinction can also be made between nitrogen oxides NO and NO2, provided that only one of the two gases is present in the sample gas.

The dependent claims relate to advantageous further developments of the invention explained below with reference to the drawings. This shows:

1 and 2 the schematic structure of the detector according to the invention

Fig. 3 shows the comb electrodes of the detector

4 process steps for the production of the comb electrodes FIG. 5 the AI2O3-V2O5 sandwich structure deposited on the comb electrodes FIG. 6 to 10 the sensitivity of the AIVOψ thin layer of the detector produced according to the invention to nitrogen monoxide NO, ammonia NH3 and others Gases

Figures 1 and 2 show a detector according to the invention, the substrate 1 consists of a very good electrical insulating material such as glass, beryllium oxide BeO, aluminum oxide Al2O3 or silicon (with Si3N4 SiO2 insulation). On the substrate 1, which is between 0.1 and 2 mm thick, there are two platinum electrodes 2, 2 'forming an interdigital structure, a vanadate layer (AIVO4 or

FeVθ4) as NH3 or. NO-sensitive element and a temperature sensor 4 are arranged. The passivation layer made of silicon oxide denoted by 5 shields the connecting lines 6, 6 'and 7, 7' respectively assigned to the two comb electrodes 2, 2 'and the temperature sensor 4 from the oxygen present in the measuring gas.

In order to be able to set the desired operating temperature of up to 600 ° C. and to be able to keep it constant regardless of external influences, the detector is actively heated with the aid of a resistance layer arranged on the back of the substrate 1. The resistance layer designated 8 in FIG. 2 consists for example of platinum Pt, gold Au or an electrically conductive ceramic and has a meandering structure. Also shown is the approximately 10 to 100 nm thick and made of titanium Ti, chromium Cr, nickel Ni or tungsten W metal layer 9, which improves the adhesion between the substrate 1 and the platinum electrodes 2, 2 '.

The dimensions of the comb electrodes 2 and 2 'depend on the specific resistance of the sensor layer 3 applied above them in the desired temperature range. For example, the comb structure 2, 2 ′ can have thicknesses of 0.1 to 10 μm, widths of 1 to 1000 μm and electrode spacings of 1 to 100 μm. For a 1 μm thick AIVO4 layer 3, the following dimensions lead to well-measurable specific resistances in the temperature range between 500 and 600 ° C.: electrode thickness D = 1.5 μm, length of the interdigital structure L = 1 mm, electrode spacing S = 50 μm.

FIG. 3 shows a true-to-scale illustration of an interdigital structure in a top view. In this exemplary embodiment, a resistance layer 10 made of platinum is used as the temperature sensor. To produce the comb electrodes 2, 2 ', a 1.5 μm thick platinum layer 11 is first deposited on the heated corundum substrate 1 in a sputtering system (see FIGS. 4a, b). The structuring of this

Layer 11 takes place in a positive photo step, in which the photoresist 12 is applied at the location of the electrodes to be produced and exposed through a mask 13 (see FIG. 4c, d, e). The developed photoresist 12 protects the platinum layer 11 during the subsequent etching step (see FIG. 4f). After removal of the photoresist 12 with acetone, the desired comb electrodes 2 and 2 '(see FIG. 4g) are obtained, on which the sensitive vanadate layer 3 is subsequently deposited (see FIG. 4h). The use of gold Au instead of platinum Pt as the electrode material has no influence on the gas sensitivity of the vanadate layer 3.

The extraordinary properties of the detector according to the invention are based on the method for producing the gas-sensitive layer. In contrast to the calcination process known from /! /, The sensitive layer is applied in a special sputtering process and then annealed for several hours. The comb electrodes can be coated, for example, in the Leybold Z490 sputtering system. Metallic vanadium V and aluminum Al serve as starting materials; H. are atomized in a plasma consisting of 80% argon and 20% oxygen from appropriate targets and are deposited on the heated substrate. The sandwich structure 14 shown in FIG. 5 is built up by alternately atomizing the two targets. It has a thickness of approximately 1 μm and consists of 60 to 80 V2O5 and AI2O3 layers approximately 10 to 15 nm thick, the Al2θ3 content being 50% to a maximum of 70%. The sputtering parameters are given in the table below.

Residual gas pressure approx. 2 - 4 x 10 ^{~ 6} mbar

Sputtering gas pressure 4.2 x 10 - *** - "mbar

Sputter gas 20% O ₂ /80% Ar

DC potential AI target: 155 V

V target: 225 V.

Substrate temperature approx. 250 ° C

In order to produce a homogeneous mixed oxide, the sandwich structure 14 is annealed in air in a high-temperature furnace for about 5 to 15 hours. The furnace temperature has a decisive influence on the topography and the phase of the Al2O3 V2O5 layers. Layers which have been annealed at temperatures T between 550 ° C. T T <610 ° C. show an optimal sensitivity for ammonia NH3 and nitrogen monoxide NO consist of equal proportions of V2O5 and AI2O3. The aluminum vanadate AIVO4, which is responsible for the high gas sensitivity, results from the tempering. The maximum working temperature of the vanadate layer is about 600 ° C. Aluminum vanadate AIVO4 has a triclinic unit cell with ca = 0.6471 nm, b = 0.7742 n, c = 0.9084 nm, α = 96.848 A, ß = 105.825 A and χ = 101.399 A, the volume of which is V = 0 , 4219 nm ³ .

Layers with an Al2O3 content of more than 50% show a somewhat smaller measuring effect. However, they can also be used at higher temperatures of up to 680 ° C.

The following diagrams are intended to document the sensitivity or sensitivity of the AIVO4 thin films produced by the described method to different gases. The size o / o (tfo: conductivity of the sensitive layer in synthetic air (80% N2 20% O2)) is plotted as a function of the time t or the concentration of the respective gas.

Even the presence of very small amounts of nitrogen monoxide NO and ammonia NH3 in dry synthetic air leads to a significant increase in the conductivity of the aluminum vanadate AIVO4 (see FIGS. 6 and 7). The conductivity changes by about 75% when 10 ppm nitrogen monoxide NO is added to the air. The addition of 10 ppm ammonia NH3 increases the conductivity by more than a factor of 6.

As shown in FIG. 8, the specific resistance of the AIVO4 thin film increases in the presence of nitrogen dioxide NO2. Since the vanadate shows a completely different behavior compared to nitrogen monoxide NO (reduction in specific resistance, see FIG. 6), one can clearly distinguish the two nitrogen oxides from one another. In addition to nitrogen monoxide NO and ammonia NH3, the vanadate layer also responds to changes in the oxygen partial pressure and hydrogen H2 (see Fig. 9). The cross sensitivity to oxygen O2 and hydrogen H2 is, however, considerably less than the reaction to nitrogen monoxide NO and ammonia NH3. For example, 500 ppm hydrogen H2 in air results in almost the same change in conductivity as the addition of 10 ppm nitrogen monoxide NO. The gases carbon monoxide CO (up to 1500 ppm), methane CH4 (up to 5000 ppm) and carbon dioxide CO2 (up to 1%) up to the concentrations indicated in brackets are not detectable. In humid air (80 mbar H2O), a clear decrease in the NH3 sensitivity is observed; however, it still remains twice as large as the sensitivity to nitrogen monoxide NO (see the right part of FIG. 9).

10 shows the sensitivity of the AIVO4 thin layer in moist air (80 mbar H2O) at 500 ° C. and an NO content of 10 ppm. Another gas in the specified concentration was added to the moist air within the time intervals marked by a horizontal line. Between the 80th minute and the 110th minute, for example, the air also contained 1500 ppm carbon monoxide CO in addition to the 10 ppm nitrogen monoxide NO. As the measurement results show, the NO sensitivity of the AIVO4 layer is not influenced by the presence of carbon monoxide CO, methane CH4 and carbon dioxide CO2. The admixture of hydrogen H2 does not mask the NO sensitivity, but a clear cross-sensitivity can be determined. A similar effect is observed with oxygen O2 when its concentration decreases from 20% to 2%.

The detector according to the invention can be used, for example, as an air quality sensor in a motor vehicle. Its cross-sensitivity to oxygen O2 and hydrogen H2 is not a disadvantage here, since car exhaust gases do not contain quantities of oxygen and the oxygen concentration of the exhaust gases diluted in air remains almost constant.

/! / Sensors and Actuators 19 (1989) 259-265

Claims

Claims:

1. Detector for detecting nitrogen monoxide and ammonia with a sensor layer (3) arranged on an insulating base body (1) and a pair of electrodes (2, 2 ') contacting the sensor layer (3), characterized in that the sensor layer (3) consists of a vanadate eVθ4 or an admixture of a metal oxide Me2θ3 containing vanadate eVθ4.

2. Detector according to claim 1, with a sensor layer produced by the following method:

- Cover the pair of electrodes (2, 2 ') and the surface of the base body (1) between them with several

Metal oxide layers, so that a layer sequence

^Me 2 ⁰ 3 ^{~ v} 2 ° 5 ^{~ Me} 2 ⁰ 3 ^"v 2 ° 5 ^{~ etc} -

arises and

- Annealing the metal oxide layers.

3. Detector according to claim 2, so that the proportion of the metal oxide e2θ3 is 50% to a maximum of 70%.

4. Detector according to one of claims 1 to 3, that the sensor layer (3) has a thickness d <10 μm.

5. Detector according to one of claims 1 to 4, characterized in that the trivalent metal Me is aluminum or iron.

6. Detector according to one of claims 1 to 5, so that the pair of electrodes (2, 2 *) is designed as an interdigital structure.

7. Detector according to any one of claims 1 to 6, that a temperature sensor (4, 10) and / or a heating element (8) are arranged on the base body (1).

8. Method for producing a detector for the detection of nitrogen monoxide and ammonia in the - one consisting of an electrically conductive material

Layer (11) is deposited on an insulating base body (1),

at least one pair of electrodes (2, 2 ') which is not conductively connected to one another is produced by structuring the layer (11),

- Several metal oxide layers are deposited on the pair of electrodes (2, 2 ') and the intermediate surface of the base body (1) such that a layer sequence

e2θ - V2O5 - e2θ3 - V2O5 - ETC.

arises, where Me denotes a trivalent metal and

- The metal oxide layers are annealed for several hours, the temperature being selected so that vanadate MeV0 ₄ is formed.

9. The method according to claim 8, characterized in that the metal oxide layers are generated by reactive sputtering in an argon-oxygen atmosphere or by reactive electron beam evaporation.

10. The method according to claim 8 or 9, characterized in that more than 50 Me 0 ₃ - or V ₂ 0 ₅ layers are deposited.

11. The method according to any one of claims 8 to 10, so that only layers with a thickness d <20 nm are generated in each case.

12. The method according to any one of claims 8 to 11, that the metal oxide content is in the range between 50% and 70%.

13. The method according to any one of claims 8 to 12, d a d u r c h g e k e n n z e i c h n e t that a layer sequence

AI2O3 - V ₂ 0 ₅ - I2O3 - V2O5 - etc.

generated and annealed at a temperature between 550 ° C and 640 ° C ..

14. Use of a detector according to one or more of claims 1 to 7 as an air quality sensor or NH3 slip monitor.