Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For nitrogen or nitrogen containing compound
Reexamination Certificate
2002-02-01
2004-07-20
Olsen, Kaj K. (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For nitrogen or nitrogen containing compound
C204S424000, C204S409000
Reexamination Certificate
active
06764591
ABSTRACT:
TECHNICAL FIELD OF INVENTION
The present invention relates to total nitrogen oxide (NO
X
) measurement systems for use in harsh environments. The present invention relates to a nitrogen oxide (NO
X
) measurement system having a platinum and zeolite based catalyst filter and a sensor element having a potential which varies in response to a NO
X
component in a gas being measured.
BACKGROUND OF THE INVENTION
There is a continuing need for high temperature NO
x
sensors for combustion environments due to government regulations and negative effects on ecosystems and health. The two main types of sensors that have been tested for NO
x
are the solid electrolyte (potentiometric and amperometric) and semiconducting types. One of the main drawbacks of these sensors that has hindered their development is the lack of selectivity between the two main NO
x
components of interest, NO and NO
2
. In combustion environments NO is often the dominant NO
x
species with NO
2
being present to a lesser amount and it would be ideal to have a selective sensor for each. However, the majority of ceramic sensors cannot distinguish between the two species giving a signal response to both NO and NO
2
. Typically the signals are in opposite directions, although there are some sensors where the NO and NO
2
signal was shown to go in the same direction. Nevertheless because the sensors respond to both gases it would be difficult to determine the level of NO and NO
2
in a mixture. The majority of reports do not test mixtures of NO and NO
2
together, which is most likely because the signal due to NO+NO
2
would be less than that of NO or NO
2
tested separately because of cancellation effects. In certain studies of solid electrolyte sensors they have been made to be semi-selective to NO or NO
2
by polarizing the sensor electrodes.
Another approach has been to develop systems to detect total NO
x
, which would be a signal due to the sum of NO+NO
2
. One of the methods proposed to do this has been to build a two-chamber device out of yttria-stabilized zirconia (YSZ) and in the first chamber to electrochemically oxidize all the incoming NO
x
gas to NO
2
and then detect the NO
2
as a “total NO
x
” signal in the second chamber. This method and its variations have been extensively represented in the patent literature. A second method to detect total NO
x
has been to use a chemical or catalytic filter placed before the sensor to alter the incoming gas. For example, materials used as chemical filters such as Mo converters can convert all the NO
x
to NO under certain conditions and KMnO
4
was shown to partially oxidize NO to NO
2
but the disadvantage is that they are both consumed over time and have to be replaced.
Catalytic filters equilibrate the incoming NO
x
to a thermodynamically defined ratio depending on the oxygen content of the gas and the temperature. The advantage of the catalytic filter is that it is not consumed in the reaction. The use of a NO
x
equilibration catalytic filter before a sensor has the advantage of simplicity and longer life. A Pt-SiO
2
/WO
3
catalyst layer was used on an amperometric design to equilibrate NO
x
to NO
2
at 150° C. but the effect of higher temperatures was not investigated. Some other catalytic filter materials that have been tested for NO
x
equilibration at various temperatures for possible sensor use are Pt black catalyst, Pt on cordierite, Mn
3
O
4
, Co
3
O
4
and Pt on Al
2
O
3
.
The use of zeolites as a sensor filter for alcohols has been shown before. The zeolite's own properties can be used to transform the incoming gas or it can be used as a support for an additional catalyst. It has also been shown before that gases such as CO, which is also present in a combustion environment, can interfere with the signal for NO
x
. Thus to measure an accurate level of NO
x
the CO cross-sensitivity must be minimized. Our approach in this study was to develop a system that could detect the total NO
x
gas concentration in a background of O
2
and N
2
at high temperatures with minimal CO interference. We used a non-selective YSZ air reference sensor to detect the NO
x
and a NO
x
equilibration/CO oxidation filter placed before the sensing electrode composed of a Pt catalyst dispersed onto a zeolite Y support. The sensor and the filter were maintained at different temperatures to provide a driving force for the NO
x
equilibration reactions.
SUMMARY OF THE INVENTION
The present invention presents a novel measurement system for determining total NO
X
concentration, from a gas sample. Total NO
X
includes pure NO, pure NO
2
and mixtures thereof. The measurement system comprises a gas conduit having an upstream end and a downstream end. The gas conduit carries a gas comprising NO
X
. The gas introduced into a measurement system of the present invention typically has concentrations of NO, NO
2
and CO in the range of 0 to 1000 ppm. Further, the gas typically contains 2 to 3% oxygen (O
2
). Disposed within the gas conduit is a catalyst filter comprising platinum and a zeolite. The gas flowing through the gas conduit interacts with the catalyst filter at a particular temperature to form an equilibrium mixture of NO and NO
2
from the gas comprising NO
X
. The measurement system further comprises a sensor element having two electrodes on a solid electrolyte yttria-stabilized zirconia; a sensing potentiometric electrode disposed downstream of the catalytic filter device so as to contact the equilibrium mixture of NO and NO
2
and a reference potentiometric electrode. Typically, the reference potential electrode is referenced to air.
It is preferred that the catalyst filter contain between 1 to 5% by mass of platinum. Further, it preferred that zeolite Y be used as the zeolite.
It is preferred that the catalytic filter be placed in the gas conduit so as to maximize exposure of the catalyst filter to the gas stream, thereby better effectuating the equilibrium formation of NO and NO
2
. Further, it is preferred that the catalyst filter be temperature controlled. Heating of the catalytic filter may be accomplished by any conventional means. By adjusting the temperature of the catalytic filter, the equilibrium concentration of NO and NO
2
can be adjusted. It is most preferred that the temperature of the catalytic filter be maintained at a temperature below approximately 700° C., to avoid decomposition of the zeolite.
It is preferred that yttria-stabilized-zirconia be used as the solid electrolyte. It is also preferred that the reference potentiometric electrode is constructed of platinum. It is further preferred that the sensing potentiometric electrode is constructed from platinum, chromium oxide or cobalt oxide. It is preferred that the sensor element be temperature controlled. Temperature control of the sensor element may be accomplished by any conventional means. It is most preferred that the electrolyte, along with the sensing potentiometric electrode and the reference potentiometric electrode are maintained at a temperature above approximately 400° C. and below approximately 600° C. Finally, it is preferred that the temperature of the catalytic filter is maintained at a different temperature than the temperature of the sensor element to improve the magnitude of the signal. The greater the temperature difference between the catalytic filter and the sensor element, the larger the magnitude of the signal. It is most preferred to have at least a 100° C. temperature difference between the catalytic filter and the sensor element. Additionally, it is preferred that the temperatures of the catalytic filter and the sensor element be known in order to establish the calibration curve for the measurement system.
A method of determining the total NO
X
content in a gas of the present invention comprises: (a) exposing the gas comprising NO
X
to a catalytic filter comprising platinum and a zeolite for a sufficient time so as to form an equilibrium mixture of NO and NO
2
from the gas comprising NO
X
, (b) exposing the equilibrium mixture of NO and NO
2
to a sensor
Dutta Prabir K.
Szabo Nicholas F.
Olsen Kaj K.
Standley Law Group LLP
The Ohio State University
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