Ind. Eng. Chem. Prcd, Res. Dev. 1986, 25, 563-568
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Formaldehyde Oxidation on Nickel Oxide James Joseph Fostert and R. I. Masel' Deparfment of Chemlcal Engineering, Unlvwslty of Illlnois. Urbana, Illlnois 6 180 1
Aldehyde pollution will become an Increasing problem as the nation switches to alcohol-blended fuels. Catalytic oxidation is an attractive way to reduce aldehyde emissions, but there are no data available on the oxidation process. In the work reported here, the rate of formaldehyde Oxidation on a nickel oxide catalyst is studied at 493 K and pressures of about 1 atm. It is found that nickel oxide is quite effective in catalyzing the oxidation of formaldehyde to carbon dioxide and water. The oxidation rate shows a complicated dependence on the feed composition. The rate is enhanced by adding the product COPto the feed. Rate multiplicity is observed at oxygen partial pressures between 0.03 and 0.09 bar. The data have been fit by a complicated rate equation with six constants which was derived from a previously proposed mechanism for CO oxidation on nickel oxide.
Introduction This paper will consider formaldehyde (HCHO) oxidation on a nickel oxide catalyst. The oxidation of formaldehyde is an interesting problem. Lee and Geffers (1977) showed that an engine running on an alcohol-blended fuel produces much more aldehyde emissions than it would running on gasoline. Since the use of alcohol-blended fuels is on the increase in the United States, there could be a significant increase in aldehyde emissions. Catalytic oxidation is an attractive way to convert the aldehydes into something that is safe in the environment. Formaldehyde oxidation will be an important component in the overall oxidation process. Interest in formaldehyde oxidation has also come from work done on catalytic combustion. Formaldehyde is an important intermediate formed during the combustion process. No one has yet considered how the production and destruction of partially oxidized intermediates contribute to the overall catalytic combustion process. However, Hautman et al. (1981) have shown that in a gasphased combustor the production and destruction of carbon monoxide and formaldehyde play a key role in determining properties such as ignition delays and overall rates. Hautman's arguments should also apply to catalytic combustors. Thus it seem that oxidation of formaldehyde will have to be considered in the design of a catalytic combustion system. At present, though, there is no data available in the literature on gas-phase atmospheric pressure oxidation of formaldehyde on the kinds of catalysts likely to be used in catalytic combustion or emissions control. The closest paper is the work of McCabe and McCready (1984), who examine formaldehyde oxidation over platinum at below 1 Torr. There is also work of Bibin and Popov (1968), who examined ways of preventing formaldehyde from oxidizing after it is formed in the Reichhold-Skanska (methanol oxidation) process, and the work of Ai (1983), who studied the oxidation of formaldehyde to formic acid. Oxidation of formaldehyde in solution has been explored more extensively, the latest example of which is the paper of Jakse et al. (1985). However, no one has previously examined the oxidation of gaseous formaldehyde at 1 atm on a catalyst that would be expected to show high activity for the oxidation of formaldehyde to carbon dioxide and water.
* Correspondence should be addressed to this author.
Present address: Covington Research Center, Westvaco,
Covington, VA 24426.
0196-432118811225-0563$01.50/0
The objective of the work here will be to do that. We have chosen to work with a nickel oxide catalyst. The works of Prasad et al. (1984) and Krylov (1970) indicate that nickel oxide is quite effective in catalyzing the oxidation of CO and HP. It also shows good dehydrogenation properties. Thus, it seems to be a good candidate as a formaldehyde combustion catalyst. The purpose of the work here is to see if nickel oxide is a good catalyst for the total oxidation of formaldehyde and to examine the kinetics of the oxidation reaction. Experimental Section A schematic of the apparatus is given in Figure 1. Gaseous formaldehyde mixes with oxygen, helium, and carbon dioxide in the gas manifold and then passes into the reactor oven, through a preheater, and then to a modified Berty reactor that holds the catalyst. The formaldehyde is oxidized in the reactor, and the effluent from the reactor (including unreacted formaldehyde) was analyzed by using a gas chromatograph. The entire apparatus is made of glass, anodized aluminum, and gold. It is heated above 383 K to keep the formaldehyde from polymerizing. The inertness of the apparatus was tested by passing formaldehyde, oxygen, and helium through the apparatus without the catalyst and checking the conversion with the gas chromatograph. No combustion products were detected. [Axeford and Norrish (1948) showed that there is negligible gas-phase oxidation of formaldehyde under the conditions used here.] Two infrared cells are also shown in Figure 1, but they were not used in the experiments presented here. Gaseous formaldehyde is generated by heating a mixture of paraformaldehyde (Fisher Scientific purified grade) with Dow Corning 705 diffusion pump oil following the method of Morrison and Boyd (1976). The oil acts as a heattransfer fluid. Helium (99.996%)was obtained from AGA Gas Inc., oxygen (99.5%)from NCG Industrial Gases, and carbon dioxide (99.99%)from Air Products. The flowrates of these gases are measured with Gilmont No. 1 flowmeters calibrated at the operating temperatures. The catalytic reactor used in the work here is modeled after the reactor of Berty (1974). The reactor holds the catalyst in a fixed bed. Gas is circulated through the bed at high enough velocity so that the bed acts like a differential reactor. The gas is recycled within the reactor with an internal recycle ratio of 20 or more and a conversion per pass of less than 1% , so that the whole unit acts as an ideal CSTR. The system differs from a commercial Berty reactor only in that the reactor is turned upside down to 0 1986 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
---- COZ Methyl Formate .. . . . . . .
-.-
ROTAMETERS
/MAN'FoLD
Dimethoxymethane Total Conversion
I
i
MODIFIEDs BERTY REACTOR CUTLETTOTRAPS AND VACUUM
Day From Regeneration
Figure 2. Variation in the product distribution over the catalyst life cycle as determined by GC/MS.
I I I I I
L-
FORMALDEHYDE GENERATOR
Figure 1. Schematic of the apparatus.
allow the impeller motor to be placed above the reactor, and the materials of construction were changed from those in a standard Berty reactor to make them compatible with formaldehyde. Thus, the reactor flow patterns are almost the same as those in a standard Berty reactor. The reactor was tested by using the procedure of Carberry (1976). Dryright was placed in several places in the system including the feed line, catalyst bed, and exit. Helium flow was established. Then a pulse of water was injected into the reactor. It was found that the water concentration was uniform throughout the catalyst bed and the same in the catalyst bed and the exit. The inlet showed considerably higher water concentrations. These results told us that the bed was uniformly mixed and that the exit concentration was equal to the concentration in the catalyst bed and different from that in the inlet. A Carle 1000 gas chromatograph (GC) with a Porapak CST 50180 mesh column and a thermal conductivity detector was used to analyze the inlet and outlet composition from the reactor. The GC is interfaced to an Aim 65 microcomputer, which integrates the chromatographic peaks and measures their elution times. The product peaks were identified by connecting the GC to a large mass spectrometer and doing GC/MS. The catalyst used for the work reported here was made from a 1.75 in. X 5 in. X 0.005 in. spiraled piece of nickel foil. The foil was oxidized in a muffle furnace for 15 h at 873 K in air to produce a thick nickel oxide coating. The foil was then positioned in the reactor so that the reactant gases would flow parallel to the catalyst surface. The catalyst was activated by oxidizing the foil at 873 K in oxygen for 2 h and then reducing it in a 3:l formaldehyde to oxygen mixture at 453 K for about 30 min. Some runs were also done on a catalyst that was partially poisoned by leaving it in the 3:l formaldehyde-oxygen mixture for 3 days. The reaction was qualitatively similar on both catalysts. However, less formaldehyde was converted to carbon dioxide and water on the catalyst that had been reduced for 3 days. A series of experimental runs were begun by heating the catalyst 20-30 deg above the desired reaction temperature. Next, the formaldehyde,oxygen, and helium flowrates were adjusted to the reaction conditions. Finally, the catalyst was cooled to the desired temperature, and the investigations were started. The experiments consisted of varying the formaldehyde, oxygen, helium, and carbon dioxide flowrates and measuring the inlet and outlet composition.
We calculated the rate on the basis of the conversion of formaldehyde and oxygen r = CFOXF/T
(1)
where CFo is the feed formaldehyde, Xp is the conversion, and T is the residence time and had an independent check from the C02 production. During the experiments, the total reactant pressure was typically about 0.1 bar; helium is added to keep the total pressure about 1.0 bar (- 1atm). Typical residence times were around 1 s.
Results The experiments showed that the activity and selectivity of the nickel oxide catalyst varied with time. Figure 2 shows the measured product distribution as a function of the time from when the catalyst was last regenerated. A freshly regenerated catalyst oxidizes the formaldehyde to methyl formate and a small amount of carbon dioxide. The carbon dioxide concentration increases in the first 2 days, while the methyl formate concentration declines. The product distribution then stabilizes, with carbon dioxide as the predominant product. After 12 days, dimethoxymethane and methyl formate start to form again while the COz production drops away. No dimethoxymethane or methyl formate was detected in the feed, so it was clear that the catalyst was producing some undesired products. Hence, the experiments are stopped, and the catalyst is regenerated. The results reported here were obtained on days 2-12 when C02 and barely detectable amounts of methyl formate were formed. Later, it was found that the regeneration cycle could be eliminated by heating the reactor oven to 520 K in a 5% oxygen in helium mixture between series of experimental runs. Over 200 experimental runs were performed to identify the variations in rate with varying formaldehyde, oxygen, and helium concentration. In one of the series of runs, the oxygen concentration in the feed was fixed, and the fraction of the oxygen that is converted was measured as a function of the formaldehyde concentration in the feed. Figure 3 shows some typical data. The oxygen conversion increases linearly with increasing formaldehyde concentration and then levels off. To put the data in Figure 3 in perspective, at the conditions shown, a 30% conversion corresponds to a turnover number of about 3/s, so the catalyst is clearly quite active for formaldehyde oxidation. To proceed, one needs to learn why the rate levels off. Unfortunately, one cannot tell why the rate levels off from the data in Figure 3. Clearly, there could be something special about the rate equation (e.g., Langmuir-Hinshelwood kinetics) that causes the rate to level off. However, notice that at fixed feed oxygen concentration when the rate goes up, the oxygen concentration in the reactor goes
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 565 I
I
0.007
-
0.005
-
8
m
,$ 5 z
- 0.010
c
0
6
d
c
0,003
N
Formaldehyde Pressure In Feed, Bar
Figure 3. Plot of the conversion of oxygen as a function of the formaldehyde partial pressure in the feed for a fixed feed oxygen concentration of 0.10 bar and a fixed residence time of 2.4 s. No COz was added to the feed. The small error bars in the f i e , which were added only at the insistence of the editor, represent the variation in rate according to whether it is calculated from the change in the formaldehyde, oxygen, or carbon dioxide concentration.
s.c
w"
0.001
02
0.I Feed Oxygen Partial Pressure, Bar
Figure 5. Plot of the production rate of carbon dioxide as a function of the oxygen partial pressure in the feed for a fixed feed formaldehyde concentration of 0.07 bar and a fixed residence time of 0.71 s. No C02 was added to the feed.
0.006 0.004
6
3
\
0.004
6
\
m
m
0
c
B
c
0,002
0.003
0 0.02 0,04 006 0.08 0.10 0.12 0.14
'0
Reoctor Formaldehyde Pressure (Ears)
Figure 4. Effect of increasing the formaldehyde partial pressure in the reactor on the rate of formaldehyde oxidation on an activated and partially poisoned catalyst. During these experiments the oxygen partial pressure in the reactor was set at 0.0784 f 0.005 bar by adjusting the feed oxygen concentration appropriately. No COz was added to the feed.
down. The rate is dependent on the oxygen concentration in the reactor, not the oxygen concentration in the feed. Thus the reduction in the reactor oxygen concentration when additional formaldehyde is added to the feed could cause the rate to level off. As a result, if one only compared experiments at fixed-feed conditions, one could not tell whether the trends observed are due to purely kinetic phenomenon or are instead artifacts due to depletion of one or both of the reactants. In order to avoid these difficulties, some additional experiments were done where the oxygen flowrate into the reactor was adjusted so that the oxygen concentration in the reactor was constant; the formaldehyde concentration was varied. Figure 4 shows some of the data from these experiments. The graph indicates that on both an activated and a partially poisoned catalyst the reaction rate varies linearly with the formaldehyde concentration in the reactor. The rate is lower with the poisoned catalyst. Other experiments made with the partially poisoned catalyst indicate that the poisoned catalyst performs similarly to the activated catalyst, except that it shows a lower activity. The rest of the experiments reported here were done with an activated catalyst at 493 K. Figure 5 illustrates the changes in the reaction rate with variations in the oxygen concentration at a fixed formaldehyde feed concentration. The effects of varying oxygen concentration were much more complicated than the effects of varying formaldehyde concentration. Over the range of conditions shown, there are two steady states for each feed condition. Further, the steady states were only
0.002 0.03
0.05
007
0.09
Reoctor Oxygen Pressure (Ears)
Figure 6. Effect of increasing reactor oxygen partial pressure on the reaction rate measured on an activated catalyst at 493 K. During these experiments the formaldehyde concentration in the reactor was fixed at 0.078 f 0.007 bar by adjusting the feed concentration accordingly. No C02 was added to the feed.
marginally stable; a small perturbation in flowrate would induce the reactor to jump from one steady state to another. Multiple steady states were seen over a wide variation of feed conditions and on both activated and poisoned catalysts. Thus, it is clear that formaldehyde oxidation over a nickel oxide catalyst is a fairly complicated reaction. As above, there is some ambiguity in the data in Figure 5 in that the reactor concentrations of all of the species are varying at each point on the curve, and it is the reador concentration, not the feed concentration, that determines the rate. One way to avoid this difficulty is to do a series of runs where the feed conditions are adjusted so that the formaldehyde concentration in the reactor is held constant; the oxygen concentration is still allowed to vary. Figure 6 shows the results of these runs. Again, two stable steady states are seen for each set of feed conditions, indicating that the observation of multiple steady states is not affected by the depletion of formaldehyde. The results in Figure 6 are interesting. Notice that if the rate was dependent on only the formaldehyde and oxygen concentration and the reactor concentrations of both species are fixed, only one steady state should be seen. Thus, the observation of multiplicity at fixed oxygen and formaldehyde concentration indicates that the rate is not solely a function of the formaldehyde and oxygen concentration. The only other compounds with appreciable gas-phase concentrations in the reactor were the combustion products, water and carbon dioxide. Thus, it seemed likely that the addition of one or both of these
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
20
-
0.016 Oa021-----l
.> L!
a
c
9 b
Oxygen I Farmoldehyde
I/ :
Oo Ot05
0,IO
Feed C02 Pressure Bar
Figure 7. Plot of the difference between the oxygen and formaldehyde concentration at the entrance and exit of the reactor as a function of the partial pressure of COz added to the feed for a fixed feed oxygen partial pressure of 0.133 bar, a fixed feed formaldehyde partial pressure of 0.081 bar, and a fixed residence time of 1.2 8.
0-02 ao4 0~06 am 0.10 Reactor Formaldehyde Pressure (Bars)
0.12
Figure 9. Effect of increasing the formaldehyde pressure in the reactor on the rate of formaldehyde oxidation at a fixed feed oxygen pressure of 0.085 bar and a fixed feed COPpressure of 0.112 bar; r = 0.9 s. Note that COz and O2 are in large excess, so that their concentration is almost constant throughout the reactor. The rates here are based on conversion of formaldehyde.
Finally, a series of experiments was performed where excess oxygen and carbon dioxide were fed into the reactor and the formaldehyde concentration was varied. The conditions were such that the carbon dioxide and oxygen levels were essentially constant over the catalytic surface in all of the runs. The results shown in Figure 9 indicate that at low formaldehyde concentrations the rate varied linearly with the formaldehyde concentration. However, at formaldehyde concentrations above 0.06 bar the rate flattened out. A comparison of Figures 4 and 9 indicates that at comparable conditions the rate of reaction is much higher when excess CO, is added to the reactor feed. Other experiments at lower oxygen concentrations showed similar ! OO 0,02 0.04 0.m 0.08 results. Thus, the differences in rate between Figures 4 Reoctor CO, Pressure (Bars) and 9 can be attributed to an effect of the carbon dioxide added to the reactor feed. Figure 8. Variation in the rate found in an experiment where the COz concentration in the reactor was varied by adding COZ to the feed. The reactor oxygen was held constant at 0.11 f 0.010 bar while the reactor formaldehyde concentration was held constant at 0.0774 f 0.0013 bar. T was fixed at 0.8 s.
products would have an effect on the reaction rate. To test this possibility, water and carbon dioxide were added to the reactor feed. Water was found to have no effect on the reaction; however, the reaction rate was greatly increased when carbon dioxide was added. The effect of variations in the carbon dioxide concentration on the reaction rate at fixed feed oxygen and formaldehyde concentration is illustrated in Figure 7 . When C 0 2 was added to the feed, at fixed feed oxygen and formaldehyde concentration, it was found that the concentrations of formaldehyde and oxygen at the exit of the reactor dropped. Clearly, the reaction rate had increased when C 0 2 was added to the feed. This suggests that the multiplicity seen in Figures 5 and 6 occurs because the reaction is promoted by C02. It would be nice to interpret the data in Figure 7 in a quantitative way; however, that is difficult, since COSadditions change all of the concentrations in the reactor. As a result, another set of runs was done where the feed oxygen and formaldehyde concentrations were adjusted until the formaldehyde and oxygen concentrations in the reactor were constant. A rate was then calculated from the difference in the formaldehyde, oxygen, and C 0 2 concentrations at the entrance and exit of the reactor. The result is the plot in Figure 8, which illustrates the effects at COz additions on the rate at a fixed reactor formaldehyde and oxygen concentration. Again, it appears that CO, additions increase the rate; the curve was shifted from that in Figure 7 because the COz concentration in the reactor is always higher than the COz concentration in the feed.
Discussion The results in the last section show several unusual features. Apparently, formaldehyde oxidation on nickel oxide is autocatalytic; multiple isothermal steady states are seen. It is unusual to find a catalytic reaction exhibiting autocatalytic behavior and multiple steady states. It is useful to compare the results here to previous results for carbon monoxide oxidation over a nickel oxide catalyst. Conner and Bennett (1976) did an extensive study of CO oxidation on nickel oxide and found that the reaction is autocatalytic. They proposed the following mechanism to explain their data 02 + 2a 20(a) (2) co + O ( a ) CO,(LY) (3)
--
where a and /3 are two different sites on the surface. Autocatalytic behavior occurs because the COz produced during CO oxidation can react with the nickel oxide substrate to form a carbonate site. The presence of the carbonate site increases the rate of the reaction. Later work by Ueno et al. (1977) verified many of the qualitative features of Conner and Bennett’s mechanism, including the presence of the carbonate site. Conner and Bennett’s mechanism explains many of the qualitative features of our data. It reproduces the observation of a rate enhancement by the product COz,and it predicts the same kind of saturation phenomenon seen in Figures 3-9. Hence, it would he interesting to consider
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
Table I. Selection of the Rate Equations That Were Fit to the Data no. equation
A1
rms error k8CH@
klPCO~PCH-#O;12
587
0.070
A4
0.073
A5
0.074
A6
0.083
A7
0.152
A8
0.369
A9
0.621
whether Conner and Bennett's mechanism could explain our data. In order to test this possibility, rate equations for several variations of Conner and Bennett's mechanism were derived, and attempts were made to fit these equations to our data. Table I summarizes the results of these efforts. Generally, the correlations were quite good. For example, eq A1 and A2 both fit the data with a root mean square (rms) error of less than 7 % , where the rms error, ERMS, was calculated from measured rate - 1)11111 predicted rate
(8)
There is about a 15% overall variation (7% rms) in the measured rate over the regeneration cycle, so that a 7% rms error is comparable to the errors in the data. We have done some testing and found that the data taken on a freshly regenerated catalyst were better correlated by eq A1 than by eq A2,while the data taken on an aged catalyst were better correlated by eq A2 than Al. Both give errors comparable to the variations in rate over the regeneration cycle. Of course, this does not show that Conner and Bennett's mechanism is correct for formaldehyde oxidation; rather it only shows that Conner and Bennett's mechanism is consistent with the data. It is not obvious that consistency is enough. Notice that both eq A1 and A2 have six adjustable parameters. When
one has that many adjustable parameters, one always has to consider whether it is appropriate to conclude that the good agreement between the rate equations and the data is meaningful, even if there are 285 data points. However, we have not been able to find a Langmuir-Hinshelwood type rate equation with less than six adjustable constants that reproduces the qualitative trends reported here. The observed rate enhancement by COz could only occur if the product COz participates in the reaction. Yet, since the oxidation process starts spontaneously in the absence of COz, there must be a reaction pathway that does not involve C02. This implies that there are two reaction pathways: one involving C02,the other involving just the reactants. The final rate equation must include parameters for both pathways. Another complication is that the rate increases with increasing C02, 02,and HzCO concentrations, when the C02, 02,and H2C0 concentrations are low. However, at higher concentrations increasing the C 0 2 , 02,or H2C0 concentration has little effect on the rate. Such behavior implies that at high concentrations the active sites are completely saturated with reactants. Since the saturation is observed at both high and low concentrations of C02, it is clear that the saturation process affects both reaction pathways discussed above. We have not been able to find a LangmukHinshelwood type rate equation with less than six constants that would account for both of the reaction
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
I
I
1
1
Ql /
Conversion
Figure 10. Variation in the reaction rate, r, and the reactant replenishment rate, R, as a function of conversion for an example typical of the cases used to generate the data in Figure 4. r was calculated from eq A l , while R was calculated from eq 9.
pathways and the saturation phenomenon which are observed. We did try approximately 100 other rate equations with six to eight constants, but none of them fit with an rms error of less than 15%. Equations A1 and A2 can be derived by assuming that (1)formaldehyde dissociatively adsorbs onto the catalyst and (2) that there are two distinct active sites for the reaction of a suboxide site and a carbonate site. The work of Ueno (1977) shows that both sites exist, and we have verified the presence of the carbonate site and carbon monoxide from the dissociation of formaldehyde under our reaction conditions with IR. The numerical fit to the data shows that the reaction on the suboxide sites is important at low conversions. However, as soon as a significant amount of carbon dioxide is formed, the reaction on the COz sites predominates. The experiments reported here were performed under conditions where the reaction on the carbonate site produced a significant fraction of the total carbon dioxide produced in the reactor. Equations A1 and A2 also predict that one could observe isothermal multiplicity. Notice that the form of both equations is such that the reaction is autocatalytic. Lin (19791, Gray and Scott (1983), and Scott (1983) have studied the properties of autocatalytic reactions. They found that isothermal multiplicity is possible if the rate equation shows an “S” type curve on a rate/conversion graph. Figure 10 is a plot of the rate as a function of conversion calculated for conditions typical of those used to generate the data in Figure 6. The rate of oxygen consumption by chemical reaction, r, shown in the figure is calculated from eq Al. Also illustrated in the plot is the net rate, R, at which oxygen is replenished, where R is defined by R = Co0X/7 (9) and Coo is the initial oxygen concentration. Figure 10 shows that the rate increases with oxygen conversion for the range illustrated because the carbon dioxide produced by the reaction increases the rate more than the oxygen consumption reduces it. The net rate of oxygen replenishment, R, also increases because the rate at which oxygen is removed from the reactor (by fluid flow) decreases with increasing conversion. The two curves intersect at three points. Therefore, there is a possibility of three steady states. However, the analysis of Matsuura and Kato (1967) shows that the middle point is unstable; small variations in concentration will cause the reactor to jump to one of the other two steady states. Hence, there should be two stable steady states. The experiments here have shown only two steady states.
1
/
I
I
IO
20
1
30
Conversion
Figure 11. Same as Figure 7 except an excess of C 0 2was added to the feed.
Figure 11is a rate/conversion graph, similar to Figure 10, plotted for conditions typical of those used to generate the data in Figure 9. The experiments in Figure 9 were done with a large excess of carbon dioxide and oxygen in the reactor. The O(@)sites for C02 adsorption are saturated, so the COz produced by the reaction does not produce an increase in the rate. Figure 11illustrates that the formaldehyde replenishment curve and the consumption of HCHO curve intersect at just one point, and hence only a single steady state is seen. Again, we want to emphasize that the fact that the rate data is consistent with Conner and Bennett’s mechanism does not necessarily imply that Conner and Bennett’s mechanism is correct for this example.
Conclusions To summarize, it was found that nickel oxide is an active catalyst for the catalytic combustion of formaldehyde. However, the combustion process is complicated. The reaction is autocatalytic; isothermal multiplicity is seen. The data have been fit by a rate equation derived from Conner and Bennett’s (1976) mechanism for CO oxidation on nickel oxide. However, no direct evidence for this mechanism is presented.
Acknowledgment We acknowledge Fred Gibson for his able assistance with the experiments. This work was supported by the Army Research Office under Grant DAGG-29-80C-0111. Registry No. HCHO, 50-00-0; nickel oxide, 11099-02-8.
Literature Cited Ai, Mamoru J . Catal. 1983,83, 141. Axeford, D. W. E.; Norrish, R . G. W. Roc. R . Sac. London, A 1948, 192. 518. Berty, J. M. Chem. Eng. Prog. 1974, 70, 78. Bibln, V. N.; Popov, B. I. Kinet. Katal. 1988,9 , 618. Carberry, J. J. Chemical and Cata/yfic Reaction Engineering; McGraw-Hili: New York, 1976; p 403. Conner, W. C.; Bennett, C. 0.J . Cattli. 1976,4 1 , 20. Gray, P.; Scott, S. K. Chem. Eng. Soi. 1983,38, 29. Hautman, D. J.; Dryer, F. L.; Schug, K. P.; Glassman, I. Combusf. Sci. Techno/. 1981,25, 219. Jakse, F. P.; Friedman, R. M.: Delk, F. S.; Bulock, J. W. Appl. Cafal. 1985, 14, 303. Krylov, Oleg V. Cata/ysis by Nonmetals; Academlc: New York, 1970. Lee, W.; Geffers, W. AIChESymp. Ser. 1977, 73(165), 328. Lin, K. F. Can. J . Chem. Eng. 1979,57, 476. Matsuura, T.; Kato, M. Chem. Eng. Sci. 1967,22, 171. McCabe, R. W.; McCready, D. F. Chem. Phys. Left. 1984, 1 1 1 , 89. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 3rd ed.;Allyn Bacon: Boston, 1976; p 620. Prasad, R.; Kennedy, L. A.; Ruckensteln, E. Cafal. Rev. 1984. 26, 1. Scott, S.K. Chem. Eng. Sci. 1983,38, 1701. Ueno, A.; Hochmuth, J. K.; Bennett, C. 0. J . Cafal. 1977,49, 225.
Received for review June 14, 1985 Revised manuscript received January 9, 1986 Accepted June 19, 1986
