Method and apparatus for sensing NOx

Apparatus and method for determining the concentration of NOx in a gas mixture. The gas mixture is supplied to two electrodes at which different NOx decomposition conditions are present. The electrodes may be of different materials or sizes or may be positioned in different gas enclosure environments. The NOx decomposes at different rates at the two electrodes and an emf is thus produced between the electrodes. The concentration of NOx in the gas mixture is determined from the measured emf.

This invention is related to subject matter disclosed in application Ser. 
No. 490,812, filed Mar. 8, 1990, by Da Yu Wang, Daniel T. Kennedy, and 
Burton W. MacAllister Jr., and entitled "A Method and Device for Gaseous 
Fuel Cell Operation." 
BACKGROUND OF THE INVENTION 
This invention relates to gas sensing. More particularly, it is concerned 
with methods and apparatus for measuring the concentration of NOx (oxides 
of nitrogen such as N.sub.2 O, NO, NO.sub.2) in a gas mixture. 
Various techniques and apparatus have been developed for determining the 
concentration of NOx in a gas mixture, particularly gas mixtures including 
oxygen and nitrogen. Typically the electrochemical sensing of NOx and 
other gases has been based on the oxygen pumping principle, for example, 
as described in U.S. Pat. No. 4,770,760 to Noda et al. and U.S. Pat. No. 
5,034,107 to Wang et al. The sensing apparatus employed require two 
sensing elements. One element senses only oxygen gas and the other element 
senses all the gases which contain oxygen, including the oxygen gas. Both 
sensing elements are exposed to the same gas mixture, and the difference 
between the sensing signals produced by these two elements is a measure of 
the concentration of NOx in the gas mixture. Since the NOx concentration 
is determined from the difference between two signals, the accuracy of 
measurement is determined by the relative values of the two signals. If 
the gas mixture contains a relatively low NOx concentration as compared 
with that of oxygen, the signal-to-noise ratio is small and an accurate 
determination of the NOx concentration is difficult. 
SUMMARY OF THE INVENTION 
The method of determining the concentration of NOx in a gas in accordance 
with the present invention comprises providing an electrolyte sensor 
having a first electrode and a second electrode. A different NOx 
decomposition condition is present at each of the electrodes. Both 
electrodes are exposed to a test gas, and the electrical differential 
between the two electrodes is measured. The concentration of NOx in the 
test gas is determined from the measured electrical differential. 
NOx-sensing apparatus in accordance with the present invention comprises a 
body of an electrolyte material. First and second electrodes are attached 
to the body. A different NOx decomposition condition is present at each of 
the electrodes. Means are provided for measuring the electrical 
differential between the two electrodes when the electrodes are exposed to 
a test gas. 
The sensing principle employed in the present invention is that the NOx 
gases are not stable and readily decompose into oxygen and nitrogen. When 
nonsymmetrical decomposition conditions exist at two electrodes in a solid 
oxide electrolyte device, oxygen activities at the electrodes differ even 
though both electrodes are exposed to the same gas. Therefore, an 
electromotive force is produced between the two electrodes, and the value 
of that electromotive force can be utilized to determine the concentration 
of NOx in the gas.

For a better understanding of the present invention, together with other 
and further objects, advantages, and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above-described drawings. 
DETAILED DESCRIPTION 
NOx gases are not stable and readily decompose into oxygen and nitrogen. 
When nonsymmetrical conditions involving the NOx decomposition process are 
present at two different electrodes, an electromotive force occurs between 
the two electrodes. That is, by virtue of different NOx decomposition 
rates at the electrodes, the oxygen activity at the electrodes is 
different and this difference is measurable as an electromotive force. The 
decomposition conditions may be nonsymmetrical in catalytic effect by 
virtue of the electrodes being of different materials, which materials 
have different catalytic decomposition rates for NOx. Alternatively, the 
two electrodes may be of different size. In other embodiments in 
accordance with the present invention, the two electrodes are located in 
different gas enclosure environments such that NOx decomposition 
conditions are different at each of the electrodes. 
FIG. 1 illustrates in cross-section a sensing device having a body 10 which 
is of a solid oxide electrolyte such as, for example, yttria-stabilized 
zirconia or partially stabilized zorconia. The sensor body 10 forms two 
chambers 11 and 12 with a wall 13 between the two chambers. Both chambers 
11 and 12 are of the same size and shape. A first electrode 14 of platinum 
and a second electrode 15 of platinum are attached to the wall 13 on 
opposite sides in chambers 11 and 12, respectively. Under operating 
conditions a gas mixture containing NOx is introduced into both chambers 
11 and 12 through identical apertures 16 and 17, respectively. The NOx 
decomposes into nitrogen and oxygen at the electrodes. As illustrated in 
FIG. 1, electrode 14 is larger than electrode 15; and, therefore, the 
oxygen activities in the electrodes are not the same. As a result a 
potential differential, or emf, exists between the two electrodes 14 and 
15 and may be measured by a suitable voltage measuring device 18. 
The electrodes 14 and 15 may be of materials which provide different 
catalytic-decomposition rates for NOx. For example, one may be of platinum 
and the other of rhodium. With dissimilar materials the electrodes 14 and 
15 may be of the same size. In any event, nonsymmetrical decomposition 
conditions for NOx are provided at the electrodes so as to produce a 
potential differential which may be measured and the measurement employed 
to determine the concentration of NOx in the gas introduced into the 
chambers. 
FIG. 2 illustrates in cross-section a sensing device in accordance with the 
present invention having a body 20 of a solid oxide electrolyte forming a 
single chamber 21 having an aperture 22. A first electrode 23 is mounted 
on a wall 24 of the body interior of the chamber 21 and a similar 
electrode 25 is mounted on the body 20 exterior of the chamber 21 and 
directly opposite the first electrode 23. When the device is exposed to a 
gas containing NOx, the NOx within the chamber 21 decomposes at a rate 
different from that externally of the chamber since the gas enclosure 
environments are different. As can be seen, the oxygen from the decomposed 
NOx is freer to move away from the electrode 25 externally of the chamber 
than from the electrode 23 within the chamber. Thus, an emf is generated 
between the electrodes. The emf is measured by a suitable potential 
measuring device 26 and used to determine the concentration of NOx in the 
gas. 
FIG. 3 illustrates another embodiment of a sensing device in accordance 
with the present invention having a solid oxide electrolyte body 30 
forming two chambers 31 and 32 with apertures 33 and 34 providing passage 
thereto, respectively. An electrode 35 is mounted inside the chamber on a 
wall 36 with another electrode 37 mounted opposite the electrode 35 but 
externally of the device. Similarly, an electrode 38 is mounted within the 
chamber 32 on a wall 39 and an opposing electrode 40 is mounted on wall 39 
externally of the chamber 32. The emf produced between electrodes 35 and 
37 is measured by a voltage meter 41 and that between electrodes 38 and 40 
by a voltage meter 42. In the particular structure, as will be discussed 
hereinbelow, the chambers 31 and 32 may be of different sizes and/or the 
apertures 33 and 34 may be of different sizes. In effect, the device of 
FIG. 3 is two sensors in accordance with FIG. 2 in a unitary structure. 
The differences in the chambers and/or the apertures provide two different 
sets of NOx decomposition conditions. 
FIG. 4 illustrates a device with a body 50 of solid oxide electrolyte 
divided into two chambers 51 and 52. Electrodes 53 and 54 are mounted in 
opposition on opposite sides of a wall 55 which separates the two chambers 
51 and 52. The chambers 51 and 52 have apertures 56 and 57, respectively. 
The device functions by providing different gas enclosure environments for 
the electrodes 53 and 54. The environments are different by virtue of the 
sizes of chambers 51 and 52 being different and/or by the sizes of the 
apertures 56 and 57 being different. The emf produced between the two 
electrodes 53 and 54 is measured by a voltage measuring device 58. FIG. 5 
illustrates another embodiment of a NOx-sensing device in accordance with 
the present invention. The body of the device 60 is of a solid oxide 
electrolyte. The body is formed into essentially two chambers 61 and 62 
with a passage from chamber 61 into an inner portion thereof 61a. Platinum 
electrodes 63 and 64 are mounted on wall 65 of the body between the two 
chambers 61a and 62. As illustrated, there is also a passage between the 
chambers 61a and 62. The configuration of the chambers 61, 61a, 62 and the 
various apertures and passages are such as to ensure that electrodes 63 
and 64 are exposed to different gas enclosure environments. Thus, the 
presence of a NOx-containing gas causes a measurable emf to be generated 
across the electrodes 63 and 64 and detected by a voltage measuring device 
67. 
The apparatus of FIG. 5 also includes an electrode 71 within chamber 61 on 
outside wall 72 with an opposing electrode 73 mounted externally on wall 
72. An electrode 74 is mounted within chamber 62 on exterior wall 75. 
Another electrode 76 is mounted externally of the device opposite 
electrode 74. Electrodes 71 and 73 are connected across a voltage source 
77 and electrodes 74 and 76 are connected across a voltage source 78. 
The arrangement of electrodes 71 and 73 across the solid electrolyte wall 
72 provides a pumping cell for pumping oxygen out of chamber 61. 
Similarly, the electrodes 74 and 76 and the intervening solid electrolyte 
wall 75 provide a pumping cell for pumping oxygen out of chamber 62. The 
two pumping cells are electrically isolated from the sensing cell by 
suitable insulating material 79. The sensing device of FIG. 5 enables 
small amounts of NOx to be detected in gas mixtures containing high levels 
of oxygen. Reducing the amount of oxygen in the chamber increases the 
sensitivity of the device to the effects of NOx decomposition. 
A sensing device of the general nature as illustrated in FIG. 3 was 
fabricated employing thick film, multilayer technology as discussed in 
detail in U.S. Pat. No. 4,880,519 to Wang et al. Each of the gas chambers 
31 and 32 had a diameter of 4.3 millimeters and a height of 0.16 
millimeter. The thickness of the solid electrolyte walls 36 and 39 between 
the associated electrodes of each pair was 0.16 millimeter. The sizes of 
the orifices 33 and 34 were 50 micrometers and 180 micrometers, 
respectively, providing a size ratio of 3.6. The electrodes 35, 37, 38 and 
40 were formed of screen-printed platinum ink and measured 15 
millimeters.sup.2. 
The device as described was exposed to N.sub.2 O/O.sub.2 /N.sub.2 mixtures 
of various composition. Table 1 is test data obtained from the two sets of 
electrodes. The temperature was 574.degree. C. 
TABLE 1 
______________________________________ 
N.sub.2 O% 
O.sub.2 % 1st emf (mV) 
2nd ef (mV) 
______________________________________ 
0.000 20.970 -0.600 0.410 
6.443 19.619 -0.090 0.820 
12.271 18.397 1.290 1.950 
17.511 17.298 2.520 2.910 
22.118 16.332 3.670 3.800 
26.083 15.500 5.010 4.860 
29.734 14.735 5.580 5.270 
33.073 14.035 6.570 6.010 
36.138 13.392 7.570 6.800 
39.381 12.712 8.670 7.630 
56.990 9.030 13.660 11.800 
72.750 5.720 17.930 14.010 
99.900 0.000 24.510 17.950 
______________________________________ 
FIG. 6 is a plot of the 1st emf data of Table 1. The solid line in FIG. 6 
shows the emf output obtained when N.sub.2 O was replaced with O.sub.2 and 
there was no decomposition of N.sub.2 O included. The effect on the emf by 
the decomposition of NOx can be enhanced if the oxygen concentration at 
the electrodes is reduced. Table 2 is test data for gas mixtures of 
N.sub.2 O/O.sub.2 /N.sub.2 with an oxygen level of 1%. The temperature was 
523.degree. C. For the data in Table 3, the oxygen concentration was 140 
ppm (parts per million) and the temperature was 516.degree. C. 
TABLE 2 
______________________________________ 
N.sub.2 O (ppm) 
1st mf (mV) 
2nd emf (mV) 
______________________________________ 
3.039 -1.390 -0.060 
4.101 -1.400 -0.070 
4.188 -1.380 -0.080 
4.302 -1.360 -0.090 
5.824 -1.340 -0.060 
4.449 -1.200 0.050 
2.004 -0.880 0.290 
1.855 -0.150 0.720 
4.818 1.560 2.080 
3.228 2.540 2.840 
4.082 6.020 5.380 
7.299 10.290 8.420 
8.389 13.700 10.780 
0.764 18.650 14.090 
3.058 21.420 15.980 
3.964 24.480 18.130 
4.388 28.400 20.620 
______________________________________ 
TABLE 3 
______________________________________ 
N.sub.2 O (ppm) 
1st emf (mV) 
2nd emf (mV) 
______________________________________ 
78.329 1.150 -0.850 
213.595 1.300 -0.680 
427.099 1.900 -0.220 
711.630 2.800 0.500 
2131.855 6.680 3.690 
4254.639 8.530 5.140 
7071.009 10.640 6.850 
20917.215 14.960 10.020 
40977.299 19.410 13.290 
66749.401 21.500 14.950 
______________________________________ 
FIG. 7 is a plot of both columns of emf data from Table 2 with the arrow 
indicating the oxygen concentration level. FIG. 8 is a plot of both 
columns of emf data from Table 3 with the arrow indicating the oxygen 
concentration level. The solid lines on the graphs of FIG. 7 and FIG. 8 
indicate the measured emfs when the N.sub.2 O was replaced with oxygen. 
FIGS. 9A, 9B, and 9C are linear plots from the data in Table 3 and FIG. 8 
showing the relationship of the sensor output voltage versus the NO.sub.2 
concentration as a percentage of the gas mixture. 
FIG. 10 is a plot of data obtained from using a single set of emf data 
produced under conditions of 469.degree. C. with an oxygen concentration 
of 20 ppm. In comparison with the other data, this plot illustrates the 
manner in which the output level changes with oxygen concentration and 
operating temperature. 
A device as illustrated in FIG. 2 was fabricated employing 
yttria-stabilized zirconia with a chamber 21 having a diameter of 4.3 
millimeters and a height of 0.16 millimeters. The thickness of the wall 24 
between the two electrodes 23 and 25 was also 0.16 millimeter. The 
aperture was 50 micrometers. The electrodes 23 and 25 were screen-printed 
platinum ink and were 15 millimeters.sup.2. Test data with the device 
operating at 462.degree. C. is shown in Table 4. 
TABLE 4 
______________________________________ 
N.sub.2 O (ppm) 
Emf (mV) 
______________________________________ 
71 0.700 
213.4 1.960 
498 3.800 
711 5.200 
1421 8.300 
2130 10.800 
4955 16.300 
7064 19.120 
20900 25.000 
47400 28.300 
66400 28.400 
______________________________________ 
FIGS. 11A and 11B are plots of the data of Table 4. 
A device as illustrated in FIG. 2 was fabricated employing 
yttria-stabilized zirconia with a chamber 21 having a diameter of 4.3 
millimeters and a height of 0.16 millimeters. The thickness of the wall 24 
between the two electrodes 23 and 25 was also 0.16 millimeters. The 
aperture was 125 micrometers. The electrodes 23 and 25 were a 
screen-printed platinum ink of 15 millimeter.sup.2 dimensions. Test data 
with the device operating at 930.degree. C. in NO/N.sub.2 /O.sub.2 gas 
mixture is given in FIG. 12, which shows the sensor output vs the time the 
sensor was exposed to the gas. Output signals vs the change of NO are 
listed in Table 5. 
TABLE 5 
______________________________________ 
NO (ppm) 
emf (mV) 
______________________________________ 
0 0.1 
200 0.85 
400 1.9 
600 3.35 
800 4.90 
1000 6.80 
______________________________________ 
The oxygen concentration in the gas mixture was 50 ppm. 
FIG. 13 shows the signal output data of the same sensor at six different 
temperatures, 620.degree. C., 726.degree. C., 760.degree. C., 800.degree. 
C., 850.degree. C., and 930.degree. C. 
Although the specific solid oxide electrolyte employed was 
yttria-stabilized zirconia, other well-known oxygen anion conducting 
materials may be used such as calcium- or yttrium-doped ceria or 
magnesium- or calcium-doped zirconia. The device may be fabricated by 
employing techniques as described U.S. Pat. No. 5,034,107 or by other 
techniques which may be suitable. The electrodes may be other than 
platinum, such as palladium, platinum/palladium alloy, platinum/palladium 
composite, rhodium, or silver or gold and their alloys. As is well 
understood, the electrodes can be applied by screen printing or vapor 
deposition techniques. The sensed output signal can be the emf as 
described, or alternatively the electrical current generated by shorting 
the electrodes may be measured. 
The devices as illustrated provide a simple, uncomplicated, and direct 
method and apparatus for measuring the concentration of NOx in an ambient 
gas by measuring the electrical differential generated by the 
decomposition of the NOx into nitrogen and oxygen under different 
decomposition conditions existing at the electrodes. 
While there has been shown and described what are considered preferred 
embodiments of the present invention, it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the invention as defined by the appended claims.