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Receiued for reuieiu March 27, 1974 Resubmitted August 17,1976 Accepted September 7,1976
Selective Oxidation of Hydrogen in Carbon Monoxide/Air Streams. Application to Environmental Monitoring J. R. Setter' and K.
F. Blurton
Energetics Science, Incorporated, Elmsford, New York 10523
In order to use an electrochemical analyzer for monitoring carbon monoxide concentrations (0-200 ppm) in air streams containing high hydrogen concentrations (0-2%), it was necessary to filter the hydrogen selectively prior to the gas analyzer. Selective oxidation of hydrogen was achieved by passing the gas mixture through a high temperature (982 K) "uncatalyzed" Vycor tube. Under these conditions approximately 92% of the hydrogen but only 28% of the carbon monoxide was oxidized and this separation efficiency was sufficiently high for successful operation and application of the carbon monoxide the analyzer.
Introduction
A continuous problem in air pollution monitoring is the quantitative analysis of trace constituents in an air stream containing a variety of gaseous substances which interfere with the analytical technique. One way of dealing with this situation is to prefilter the interferents selectively and this is most commonly achieved with selective gas sorbents. 22
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,No. 1, 1977
In the development of a portable analyzer for continuously monitoring carbon monoxide (0-200 ppm) in environments containing high hydrogen concentrations (20 OOO ppm), it was necessary to prefilter hydrogen selectively since the electrochemical analyzer used had an interferent signal due to Hz (100 ppm of H2 gave a signal equivalent to 1 ppm of CO (Blurton and Bay, 1974)). This separation was achieved by the selective catalytic oxidation of hydrogen in the gas stream. It
is the first time this technique has been reported to provide solutions to such air pollution monitoring problems. Although the selective oxidation of CO from Hz/CO/air streams has heen studied by Cohn (1965) and Shishu and Kowalczyk (19741, the selective oxidation of HZhas not heen reported. Investigations by Close and White (1975) and Matsushima et al. (1975) in ultrahigh vacuum suggested that the selective oxidation might he possible with palladium catalysts by operating at a sufficiently high temperature where there is a change in the kinetics of the CO oxidation reaction from a Langmuir-Hinshelwood to an Eley-Rideal mechanism. However, while this approach was partially successful, it was not sufficiently effective a t reducing the Hz instrument interference and a preferred approach was the "uncatalyzed" oxidation of hydrogen in the presence of CO. The results of this kinetic study are reported here together with the developmental considerations encountered when the "filter"-reactor was interfaced with the carbon monoxide analyzer. During the course of this investigation we found there were striking similarities between our results and the fundamental catalytic rate data for heterogeneous CO oxidation in "clean" systems. This observation provides a demonstration of the important connection between fundamental catalytic studies and the practical application of catalysis which are often described as being separated by a "pressure gap" (Schmidt, 1975).
Experimental Section The experimental apparatus consisted of an inlet metering system, a reactor, and an analytical system (Figure 1). The inlet metering system consisted of feed lines consisting of a 200 ppm (parts per million) CO/air mixture, a 2.4% Hz/air mixture and air, respectively. The CO and H2 concentrations were calculated on a volumetric basis. The amount of CO and Hz in the gas mixture was varied by adjusting the flows of these gases. In some experiments water vapor in the range 0% to 85% RH a t 22 "C was introduced into the feed line by passing the gas mixture over the surface of water contained in a bottle immediately prior to the reactor. The design and the temperature profile of the integral flow reactor used to study the uncatalyzed reactions was described by Stetter and Blurton (1976). The reactor consisted of an insulated Vycor tube (id. 4 mm) with a 1.5-cm heated reaction zone and a thermocouple external to the tube. All the temperatures reported are the maximum temperature a t the center of the tube and they were calculated from the value of the external thermocouple and temperature profile data. For the study of the catalyzed reactions, the heated zone along the Vycor tube was extended to 10 cm. At the flow rates studied, the pressure drop across the catalyst-charged reactor was in the range 3-10 in. of water. The system conditions investigated were reactor temperatures varying from 373 to 1000 K, continuous total sampling flows varying from 100 to 400 cm3/min, carbon monoxide concentrations in the range &2M) ppm, and hydrogen concentrations in the range 0-2.4% Hz. Septum-sealed syringe sampling ports were placed before and after the reactor and in the sample mixing bottle so that samples could he obtained for analysis. Hydrogen, O2 and CO were analyzed by gas chromatography using a 5A molecular sieve column for separation, UHP N2 carrier gas, and a thermal conductivity detector for Hz and 0 2 analysis, and an electrochemical detector for CO analysis (Stetter et al., 1976). In a later part of the study the effluent from the reactor was passed directly to the electrochemical carbon monoxide analyzer. The catalyst was silica gel containing 0.01 w t % P d and 25 mg was placed in the reactor. A dilute Pd concentration was
Figure 2. Transmission electron micrograph of Pd supported on silica gel (0.01%Pd); 300 OOOX magnification.
used so that the percentage of the CO reacting could he determined over a wide temperature range. It was prepared by stirring silica gel in an aqueous solution of PdCly2HzO in triply distilled water, subsequently evaporating the slurry to dryness, and then reducing at 578 K for 8 h in a stream of pure hydrogen. The P d crystallite diameter as determined by transmission electron microscopy was in the range of 200-750 A (Figure 2). Results The initial experiments were concerned with optimizing the reactor operating conditions in order to minimize the hydrogen concentration entering the CO analyzer. Figure 3 shows the % CO reacted with the Pd catalyst from a CO/air mixture. There was a sharp increase in the CO oxidation rate at 453 K (onset temperature), a maximum in the oxidation rate between 523 and 537 K (transition temperature), and a decrease in the reaction rate at the higher temperatures. The reaction order for CO oxidation was negative at temperatures equal to and less than 453 K and positive a t temperatures greater than and in the range of the transition temperature. In this temperature range approximately 95% reaction of hydrogen was observed with this P d catalyst from an Hz/air stream. These results suggested that it should he possible to obtain selective oxidation of the Hz from Hz/CO mixtures a t reactor temperatures greater than 550 K. Figure 4 shows the % CO and Hz reacted from air streams containing both CO and Hz. Ind. Eng. Chem.;Prod. Res. Dev., Vol. 16, No. 1, 1977 23
100
t
I
/;I l,,.---I
40
I
d
e7 3
373
,073
T€MPERII-URE
(IC’
F i g u r e 5. “Uncatalyzed” reaction of CO and H Z from CO/Hz/air mixtures. Inlet gas composition was 1.3% Ha, 100 ppm of CO, balance air: 0, Hz reacted; 8, CO reacted. Flow rate 200 cm3/min.
=
I
T
N
:
I
I00
I
I’
4 i’ 0
313
413
513
673
EMPERRTURE
173
813
913
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Figure 4. Catalyzed reaction of CO and Hz from CO/Hz/air mixtures. Inlet gas compositions was 1.3% Hz, 100 ppm of CO, balance air: 0, H? reacted; m , CO reacted. Flow rate 200 cm3/min.
Comparison of the data in Figures 3 and 4 shows that there was a marked increase in the fraction of CO reacting and a small decrease in the amount of Hz reacting in the combined CO/Hz/air streams. For example, 93% of the H2 and 46%of the CO reacted at 953 K and this separation efficiency was not sufficient for our application. Higher operating temperatures were not investigated because catalyst stability was questionable and the shape of the reactionhemperature curve (Figure 4) indicated that significantly higher separation efficiencies would not be achieved. In order to determine the reason for the different fractions of CO reacting in the absence and presence of H2, a sample of the effluent from the reactor a t 623 K was collected and analyzed by mass spectroscopy. No methanol, but approximately 5 ppm of formaldehyde was detected. Although only a small fraction of the CO apparently reacted, by this Fischer-Tropsch type reaction, this may account for the differences observed between Figures 3 and 4 due to the experimental difficulties in collecting and storing samples prior to mass spectrometric analysis. However, it is interesting to note that in this system a difference in the reaction mechanism of the two parallel oxidation reactions results in the observed catalytic selectivity and forms the basis of the separation process. In view of the results obtained by the catalytic oxidation of CO and Hz, the experiments were repeated with a Vycor reaction tube without the catalyst charge to investigate the role of homogeneous and wall reactions. Figure 5 shows that a t temperatures greater than 973 K only 28% of the CO reacted whereas 95% of the hydrogen reacted in a CO/HZ/air stream and this was a significant improvement in separation efficiency over that obtained with a catalyst (Figure 4). The amount of CO reacting was essentially independent of the CO and H2 concentrations entering the reactor within the experimental range of gas concentrations (Figure 6), an impor24
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
0 0
A0
20
60
CO
80
100
I20
140
I60
!BO
[ppm)
CONCENTRATION
F i g u r e 6. “Uncatalyzed” reaction of CO and Hz from CO/HZ/air mixtures. Reactor temperature 989 K; flow rate 200 cm3/min: 0 , HP reacted; 8, CO reacted. ACTVRL
CO
CCNCCNTRIT,DU
20
/
o v
eo
60
100
140
le0
CNeLYZER RERDING ( p p m )
Figure 7. CO reading on electrochemical analyzer as a function of CO concentration. Reactor temperature 982 K; flow rate 200 cm3/min: 0 , readings with CO/air mixtures; 8, readings with CO/Hz/air mixtures (0.5-2% Ha).
tant consideration in instrument design and performance. It should be noted that no CO reacted in this temperature range when there was no Hz in the mixture, again suggesting the possibility of hydrocarbon formation as being responsible for the reacted CO in these mixtures. Figure 7 shows the readings obtained when the stream exiting from the reactor furnaces was passed directly into the CO analyzer. In these experiments the CO and Hz concentration in air was varied from 0 to 200 ppm and 0.5% to 2%, respectively. When 2% Hz or more was passed directly into the CO analyzer with or without CO, (Le., with oxidation filter by-passed), the analyzer gave a reading equivalent to over 200 ppm of CO and the readings were not reproducible. With carbon monoxide air mixtures passing through the reactor and no H2 in the gas stream, typical CO analyzer responses were
Table I. CO Analyzer Readings Actual CO concn, ppm
% Hz in inlet stream
75 75 75 75 75 100 100 100 100
fz
Analyzer reading, ppm CO
0 0.4 1.5 1.6 1.95 0
75 74 79 77 81
100 96 93 95
1.2
1.3 1.8
2 . m’ ”
I
3,Om
,o,ooo
6.000
PPM
20,mo
I5,OOO
H,
I Z5,OOO
IN
Figure 8. Reaction of Hz from Hz/air mixtures: 0,1012 K; a, 1005 K; A ,994 K; flow rate 200 cm3/min.
I
E 0 P
3’5
31’6
3 ‘7
3 ‘8
31’9
3i0
-ivcaP-u=,E
Figure 9. Temperature dependence of reaction of Hz/air mixtures. Inlet HJ concentration is 1.01%;flow rate 200 cmJ/min.
reproducible to 0.5 ppm and there was excellent agreement between actual and measured CO concentrations (Figure 7). With H2 in the gas stream, the CO analyzer signal was reproducible to f3% and typical performance is also indicated in Figure 7. The maximum deviation between the instrument reading and actual CO concentration was 16 ppm and this occurred only a t the 200 ppm of CO concentrations. Although the specic effects of water vapor on the oxidation of Hz and CO in the reactor were not determined, the same readings were obtained on the analyzer with relative humidities in the range of 0-85%, and this was the highest R H tested. Table I shows the analyzer reading as a function of H2 concentration a t 75 and 100 ppm of CO. This again demonstrates the good agreement between the analyzer readings and the actual CO concentrations within the experimental precision of the reactor-instrument system.
A detailed study of the mechanism of the reactions occurring in this system was not carried out. However, in order to obtain an indication of the mechanism, we determined the amount of hydrogen reacting at three temperatures as a function of initial hydrogen concentration. The amount of Hz reacting at three temperatures is a linear function of the initial hydrogen concentration (Figure 8 ) . From the least-squares lines in Figure 8, the plot of the square root of the reactor temperature vs. the ppm of H2 reacted yielded a straight line (Figure 9). These empirical results are typical of the high temperature diffusion limit for a catalytic process. If reaction occurs at the tube walls and each reactive molecule (H2 or CO) which reaches the wall reacts, then, on the basis of molecular diffusion, a maximum separation of approximately 3.75/1 would be achieved in an ideal system of this type. This is remarkably similar to the 95% H2 vs. 28% CO reacted actually observed (Figure 6). Here it is postulated that a difference in a molecular property produces a difference in observed reaction rates and provides a separation of H2/ CO/air mixtures. A further observation made in this work is that there was remarkable agreement with the data reported here for CO oxidation (Figure 3) and the measurements in “clean” systems (Close and White, 1975; Matsushima and White, 1975 and Matsushima et al., 1975). This recent data with polycrystalline Pa) reactor with an O2 to CO Pd foils in a low pressure ( ratio of 0.2 showed a sharp onset temperature at 450 K, a transition temperature of 475 K, negative reaction order at temperatures less than the transition temperature, and a positive reaction order a t temperatures greater than the transition temperature. Similar results were observed with Pd single crystals (Ertl and Rau, 1969). There is, therefore, remarkable agreement between the results obtained in clean systems with those obtained with supported Pd catalysts in the presence of a diluent, co-reactive species, higher pressures, and extremely large 0 2 to CO ratios. Our high-pressure data, therefore, are consistent with the Langmuir-Hinshelwood and Eley-Rideal mechanistic interpretations and demonstrate for this reaction the close connection between catalytic studies in pure and practical systems. In conclusion, two examples of catalytic selectivity based on distinct kinetic (mechanistic) considerations have been described which have resulted in a selective separation process. As a result of this work, the portable battery-operated reactor previously described (Stetter and Blurton, 1976) was interfaced with the CO analyzer for use in high hydrogen environments. While perfect separation was not achieved, this process was capable of reducing hydrogen instrument interference over a wide range of concentrations to an acceptable level.
Literature Cited Blurton. K. R., 6ay.H. W., Am. Lab., 6 (7), 50 (1974). Cohn, J. G. E., US. Patent 3 216 783 (1965). Close, J. S., White, J. M.. J. Catal., 36, 185 (1975). Ertl, G., Rau, P., Surf. Sci., 15, 443 (1969). Matsushima, T., White, J. M., J. Catai., 39, 265 (1975). Matsushima, T., Almy, D.B.. Foyt, D. C., Close, J. S.,White, J. M., J. Catai., 39, 277 (1975). Schmidt, L. D., J. Vac. Sci. Techno/., 12, 341 (1975). Shishu, R. C., Kowalczyk, L. S.,Platinum Met. Rev., 18 (2), 58 (1974) Stetter, J. R., Blurton, K. F., Rev. Sci. instr., 47, 691 (1976). Stetter, J. R.. Rutt, D. R., Blurton, K. F., A n d . Chem., 48, 924 (1976).
Received for reuieu: July 28, 1976 Accepted November 29,1976
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 1, 1977
25
