WO2000025119A1 - Gas sensor with a perovskite structure for determining hydrocarbon - Google Patents

Abstract

Disclosed is a resistive gas sensor that determines hydrocarbon in exhaust gases, comprising a semiconductor metal oxide perovskite structure of the general formula A1-x-z1 A'x B1-y-z2 B'y O3, wherein x is between 0 and 0.05; z1 is between 0.01 and 0.1; y is between 0 and 0.1 and z2 is between 0 and 0.005.

Description

description

Gas sensor for hydrocarbon determination with perovskite structure

The present invention relates to a resistive gas sensor for hydrocarbon determination in exhaust gases and a method for its use.

Resistive gas sensors are known. It is also known that sensors can be made of semiconducting metal oxide with a perovskite structure of the general formula Aι_ _x _ _zl A ' _x Bι_ _y _ _z2 B' _y 0 ₃ . Such a semiconductor sensor is described in EP 0 365 567 B1. The gas sensor described there should have a particularly high response speed for measuring the partial pressure of oxygen. A certain stoichiometric composition is proposed. However, the previously known sensors are not satisfactory in every respect, in particular with regard to their use in the field of exhaust gas monitoring in automobiles.

Exhaust gas monitoring is carried out here to minimize pollutant emissions. Exhaust gas monitoring data ensures that the combustion in the engine runs optimally on the one hand and that the pollutants that are nevertheless emitted can be optimally converted in a downstream catalytic converter. For this purpose, the exhaust gas itself is checked with the monitoring data, as is the quality of the catalyst. In automobiles, it is initially common to regulate the air and fuel supply to the engine in such a way that an ideal stoichiometric ratio is achieved which allows the fuel to be converted completely in the engine without excess air being introduced. The ratio of air to fuel is referred to as λ-value and is ideally 1. However, an auto- moves upwards and downwards from the ideal value 1 due to the finite control speed and the constant load changes λ.

In lean phases, the excess oxygen can be stored on or in the catalyst. If the fuel supply to the engine then rises so much that there is a rich mixture there, ie more fuel is supplied than can be reacted with the introduced air, the oxygen previously deposited on the catalytic converter can be used

Afterburning of the exhaust gas can be used. The exhaust gas composition behind the catalytic converter is therefore comparatively constant with a functioning catalytic converter.

The exhaust gas composition is usually measured using a so-called λ-

Probe measured, which responds to the oxygen partial pressure that suddenly changes by many orders of magnitude at point λ = 1. If such a λ probe is arranged behind a functional catalytic converter, it will output an average value that corresponds exactly to the signal at λ = 1. If, on the other hand, the catalytic converter is defective or aged, so that oxygen can no longer be deposited and / or used for exhaust gas afterburning, the typical fluctuations in the λ value that occur regularly in automobile operation will also have an effect behind the catalytic converter and with a λ probe arranged there be measurable. Is obtained from a comparison between the λ-value which is upstream of the catalyst, and that λ-value which can be determined by an appropriate sensor downstream of the catalyst, so it can advertising obtained an indication of the oxygen storage capacity of the catalyst, ^• the. In principle, this makes it possible to determine the quality of a catalytic converter. A disadvantage of the previously known determination of the catalyst quality is that there is only an indirect connection between the oxygen storage capacity and the actual catalyst conversion efficiency. The advantage of the fast measuring speed obtained in the known prior art also brings no significant benefit. The information obtained up to now from the comparison of the sensor values before and after the catalytic converter is therefore not as precise as is desirable due to the increasing demands on exhaust gas monitoring and online monitoring of all exhaust gas-relevant components or is required by legal regulations. It is therefore desirable to determine the quality and quality of a catalyst better than was previously possible.

The object of the present invention is to provide something new for commercial use.

The solution to this problem is claimed independently. Preferred embodiments are claimed dependent.

The invention is based on the knowledge that the hydrocarbon emissions in the exhaust gas behind the catalytic converter represent a much better measure of the quality of the catalytic converter than the lambda value detected with a λ sensor. The hydrocarbon emissions per distance covered are a direct measure of the catalytic effect. It was further recognized that a particularly suitable exhaust gas sensor for use behind an exhaust gas catalyst can be obtained from a semiconducting metal oxide with a perovskite structure, which has the general formula Aι_ _x _ _zl A ' _x Bι_ _y _ ₂₂ B' _y 0 ₃ and where x between 0 and 0.05; zl between 0.01 and 0.1; y is between 0 and 0.1 and z2 between 0 and 0.005. In contrast to previously known semiconducting metal oxide gas sensors with a perovskite structure, it is not important in the present case to obtain a particularly rapid response to oxygen, but rather to be able to measure the hydrocarbons particularly easily and precisely. It was recognized that this is possible by varying only the stoichiometry of metal oxide gas sensors known per se in a perovskite structure. The mechanism underlying the improved properties has not yet been fully understood. However, it is noted that with the composition of the invention in which zl is between 0.01 and 0.1, there is a net excess of oxygen in the resulting crystal. It is assumed that this excess of oxygen, especially when doping substances are present at the same time, is responsible for the improved measurability of the hydrocarbons. These are present if at least one of x or y is not equal to 0.

zl can be between 0.015 and 0.03, a preferred value being 0.02. This ensures a sufficiently large excess of oxygen in the crystal. At the same time, y can approximately correspond to zl. This ensures that the amount of doping atoms present in the crystal, especially donors, approximately corresponds to that of the excess oxygen atoms. Here too, the reason why the gas sensors constructed in this way are particularly well suited for the intended purpose is not yet fully understood. The improved properties may be due in particular to the fact that the conductivity caused by oxygen migration in the crystal and by hole or electron movement is approximately the same, which may reduce cross-sensitivity. Component A 'can be selected from one or more lanthanides and in particular one of lanthanum, cerium, praseodymium or neodymium.

Component A itself is preferably selected from calcium, strontium, barium or lead. Component B will typically be titanium, so that one contains a calcium titanate, strontium titanate, barium or lead titanate, in which lanthanides are initially provided on one of the alkaline earth crystal lattice sites. The titanium lattice sites can be partially occupied by tantalum, niobium, iron, chromium and / or vanadium, depending on the choice of z2 and B ^' .

A particularly preferred material consists of Sro, ₉₈ Ti _0/98 Ta _o , o ₂ 0 ₃ . It is therefore a strontium titanate in which excess oxygen and additional tantalum have been introduced.

The gas sensor composed according to the invention can be arranged in an automobile behind the catalytic converter. When its electrical conductivity is determined, practically no cross-sensitivity to nitrogen oxide, NO, carbon monoxide and hydrogen is observed. The gas sensor is therefore practically insensitive to the most important secondary components in the exhaust gas and, apart from hydrocarbons, only responds to oxygen in the exhaust gas.

The invention is described below only by way of example using a preferred embodiment.

For the formation of a gas sensor with a metal oxide having the composition Sr ₀ 98 Ti ₀ 98 Ta _0, o20 ₃ are in the required amounts of strontium carbonate, SRC0 _3, and titanium dioxide, Ti0 _2, ground together in water for more than twenty hours in a ball mill, and thereby intimately mixed. Then under vigorous stirring, a solution of tantalum pentachloride, TaCl ₅ in ethanol was added dropwise together with a buffering ammonium carbonate solution (NH ₄ ) ₂ CO ₃ . This buffering ammonium carbonate solution later burns without residue. The solution is freed from excess water by suction, dried, pulverized and calcined at 1100 ° C. for the reaction. The powder obtained is ground again for twenty hours in order to comminute agglomerates. The resulting particles are smaller than 10 microns.

The powder obtained is processed into a screen-printable paste and / or a sprayable material and screen-printed onto a suitable carrier in a thickness of between 1-100 micrometers, a sensitive layer of twenty micrometers being preferred.

The gas sensor obtained is installed behind a catalytic converter in the exhaust gas path of an automobile and its electrical conductivity is recorded. It shows no cross-sensitivity to NO, CO and H ₂ , a clear oxygen characteristic and a clear and clear sensitivity to hydrocarbons.

The signal derived from the electrical conductivity of the gas sensor is a measure of the concentration of hydrocarbons in exhaust gases. Since the concentration or absolute amount of hydrocarbons per distance covered is a direct measure of the catalyst action, a simple and precise check of the catalyst is thus made possible. If the hydrocarbons covered for each distance covered increase continuously over a sufficiently long period of time, this shows that the catalytic converter is faulty. The new gas sensor enables online monitoring of the exhaust gas quality.

Claims

claims 1. Resistive gas sensor for hydrocarbon determination in exhaust gases with a semiconducting metal oxide in a perovskite structure of the general formula Aι-χ- _z i A ' _x Bι- _y _ _z B _y 0 ₃ , characterized in that x between 0 and 0.05; zl between 0.01 and 0.1; y is between 0 and 0.1 and z2 between 0 and 0.005. 2. Resistive gas sensor according to one of the preceding claims, characterized in that zl is between 0.015 and 0.03, in particular 0.02. 3. Resistive gas sensor according to one of the preceding claims, wherein A ^{1 is} selected from one or more elements of the lanthanides, in particular from lanthanum, cerium, praseodymium, neodymium. 4. Resistive gas sensor according to one of the preceding claims, wherein A is selected from one or more, preferably one of calcium, strontium, barium and lead. 5. Resistive gas sensor according to one of the preceding claims, wherein B is titanium. 6. Resistive gas sensor according to one of the preceding claims, wherein B 'is selected from one or more Tantalum, niobium, iron, chromium and vanadium. 7. Resistive gas sensor according to one of the preceding claims with the composition Sr ₀ , ₉₈ Ti ₀ , ₉₈ Ta _0f02 0 _3rd 8. The method for using a gas sensor according to one of the preceding claims, characterized in that the gas sensor is arranged behind a catalyst in the exhaust gas stream of an internal combustion engine, a value characterizing its electrical conductivity is determined and an exhaust gas hydrocarbon concentration is determined therefrom.