Oxidation of Methanol over a Palladium Monolithic Automotive Catalyst

studied using a monolithic catalyst containing palladium. Experimental conditions simulated those in the catalytic converter of a methanol-fueled auto...
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Environ. Sci. Technol. 1996, 30, 1997-2003

Oxidation of Methanol over a Palladium Monolithic Automotive Catalyst KEVIN A. FRANKEL, TYSON M. MEW, PHOOI K. LIM, AND GEORGE W. ROBERTS* Department of Chemical Engineering, North Carolina State University, P.O. Box 7905, Raleigh, North Carolina 27695-7905

The oxidation of methanol and its partial oxidation products, carbon monoxide and formaldehyde, were studied using a monolithic catalyst containing palladium. Experimental conditions simulated those in the catalytic converter of a methanol-fueled automobile. With a properly activated catalyst and a fuel-lean feed, carbon dioxide was the only carbon-containing product, and the methanol conversion above the lightoff temperature of the catalyst appeared to be controlled by mass transfer from the bulk gas stream to the surface of the monolith. Under fuel-rich conditions, carbon monoxide and formaldehyde were formed along with carbon dioxide. The highest formaldehyde yield, about 4% of the methanol reacted, occurred at a relatively low feed temperature, about 350 K, essentially the light-off temperature of the catalyst. The formaldehyde yield decreased as the feed temperature increased and was essentially zero above about 600 K. When a thermally-deactivated catalyst was used, the formaldehyde yield increased significantly and formaldehyde was observed over a wider range of feed temperatures and under fuel-lean conditions. With the properly activated catalyst, multiple steady states were observed in the form of a reaction hysteresis with respect to feed temperature.

Introduction The automobile is one of the leading sources of air pollution and one of the leading consumers of energy. Over the past several decades, the combined national objectives of decreasing dependence on imported petroleum and improving environmental quality have created renewed interest in alternative automotive fuels such as methanol (CH3OH). Methanol has several real or potential advantages relative to conventional motor gasoline, e.g., high octane number, lower evaporative emissions due to its lower vapor pressure, the absence of polluting contaminants such as sulfur and nitrogen, higher power and thermal efficiency, and emissions that have relatively low photochemical reactivity (1-3). However, studies have shown that vehicles * Corresponding author e-mail address: [email protected]; fax: (919)515-3465.

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 1996 American Chemical Society

operating on methanol emit unburned CH3OH, formaldehyde (HCHO), carbon monoxide (CO), and oxides of nitrogen (NOx) (1, 4-7). Of these emissions, HCHO has generated the greatest public concern because it is a suspect carcinogen and is photochemically reactive in the atmosphere. The automobiles involved in these studies were equipped with catalytic converters, but the catalysts were designed for use with conventional gasoline rather than with alcohol fuels. A number of previous studies have been aimed at developing catalysts specifically for oxidation of the species that are found in the exhaust from a methanol-fueled internal combustion engine (8-12). There are some significant differences between this exhaust stream and one from a conventional, gasoline-fueled engine. For example, catalyst poisons such as sulfur that are present in gasoline are essentially absent in the methanol engine stream, although poisons contained in the engine oil, e.g., phosphorus, zinc, and calcium, still are present. Moreover, species such as CH3OH and HCHO are relatively easy to oxidize catalytically compared to most components of gasoline engine exhaust. McCabe et al. (10-12) concluded that either Pt or Pd catalysts gave the highest activity and lowest HCHO yields without CO in the feed. They also observed that the presence of CO decreased the activity of both catalysts, an effect that was confirmed by the study of Brewer et al. (13). Rhodium, an important component of present catalysts for gasoline engines, had low CH3OH oxidation activity and produced a relatively high yield of HCHO (10). Recently, Brewer et al. (13) contended that a monolithic catalyst containing only Pd on a γ-alumina substrate was the best overall catalyst for CH3OH oxidation because of its greater durability compared to base metal and bimetallic catalysts and because it costs less than a platinum-only catalyst. Newkirk et al. (14) tested a number of monolithic catalysts on M90 fuel (90% CH3OH/10% gasoline). A catalyst containing only Pd was among the best of those evaluated. McCabe and Mitchell (10) studied the effects of various feed components on the kinetics of CH3OH oxidation. They suggested that the effect of each species on the apparent catalyst activity can be explained by how strongly the species adsorbs on the catalyst. Species that adsorb strongly compared to CH3OH or that decompose to form strongly adsorbed products will inhibit the oxidation of CH3OH. Brewer et al. (13) carried out the most complete study of the kinetics of CH3OH oxidation on a palladium-coated monolith. They determined the reaction orders in CH3OH and O2 to be 0.55 and 0.74, respectively. A term to account for the inhibition of the reaction by CO was included in the rate equation, and the overall rate constant was described as a temperature-dependent weighted average of the rate constants for reduced and oxidized sites. A number of studies have been directed toward understanding the chemistry and kinetics of the oxidation of CH3OH. McCabe and McCready (15) studied CH3OH oxidation over Pt wires and suggested that the reaction follows the sequential path

CH3OH f HCHO f CO f CO2

(A)

The last two parts of this sequence are consistent with the results of Sodhi et al. (16).

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FIGURE 1. Equipment diagram.

Reaction sequence A implies that high concentrations of HCHO will be produced if the rate of CH3OH oxidation is high relative to the rate of HCHO oxidation or decomposition. McCabe and co-workers have studied the production and destruction of HCHO. They determined that HCHO yields were greatest under either very fuel-lean or very fuel-rich conditions and were lowest under stoichiometric or slightly rich conditions (16). However, in a number of laboratory studies with carefully-conditioned Pd and Pt catalysts, the measured concentrations of HCHO have been very low (17), approximately 1-2% of the CH3OH converted over a range of air/fuel ratios from half to twice stoichiometric. Several studies (17, 18) have shown that the HCHO concentrations in the effluent from the catalyst increase as the catalyst ages. McCabe et al. (17) studied the effects of Pb poisoning and thermal deactivation on an unspecified noble metal catalyst. Lead deactivation was effected by aging a catalyst on a pulse flame combustor for a total of 1320 simulated mi using isooctane fuel doped with 4 g of Pb/gal. The light-off temperature, a measure of the intrinsic catalytic activity, and the peak HCHO concentration increased significantly for the Pb-deactivated catalyst compared to a normally-aged catalyst. The maximum HCHO yield over the Pb-deactivated catalyst was initially about 12% of the CH3OH converted. However, this yield decreased with time on-stream to a steady-state maximum level of about 4%, presumably due to restoration of catalyst activity resulting from loss of Pb from the catalyst surface. Thermal deactivation was achieved by aging a catalyst on a pulse flame combustor at conditions that included about 30 h at elevated temperatures (T ≈ 1200 K). Formaldehyde yields were higher for the thermally-deactivated catalyst compared to a normally-aged catalyst. However, the maximum HCHO yield was only about 3-4% of the CH3OH converted. Many previous studies involved catalyst systems, e.g., powders, beads, and wires, that are not physically representative of the monolithic catalysts that are used on today’s automobiles. Moreover, many studies also involved experimental conveniences that make the data difficult to

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correlate to actual automotive catalyst operation, e.g., low methanol conversions, measurement of the catalyst temperature rather than the temperature of the gas entering the catalyst, and the use of isothermal reactors. In addition, the concentrations of formaldehyde measured in most laboratory studies do not explain the higher formaldehyde emissions measured on actual automobiles. The overall objective of this research was to develop an improved understanding of the oxidation of methanol and its partial oxidation products under conditions comparable to those existing in automotive catalytic converters. Particular emphasis was placed on determining the conditions that lead to the production of formaldehyde.

Equipment Figure 1 is a schematic diagram of the experimental apparatus. The liquid feed, consisting of either neat CH3OH or a mixture of CH3OH with HCHO and/or H2O, was stored in a buret (101). The flow rate was determined by measuring the time required for the liquid level to drop from one position in the buret to another. The liquid was delivered by a peristaltic pump (103) into the evaporator (104). All lines downstream of the evaporator were heated to prevent condensation. Components of the feed gas (CO2, O2, N2, CO) were drawn from cylinders (102). The flow rates of these gases were controlled and measured by mass flow controllers (106). The CO gas first passed through an activated-carbon carbonyl trap (105) to ensure that no iron or nickel would deposit on the catalyst. The mixed feed stream then entered the reactor (201), which was an Inconel 660 tube with an outside diameter of 2.5 cm and a wall thickness of 0.13 cm housed in a clam-shell furnace. The furnace had three heating zones. The first zone, 20 cm in length, served as a preheater to ensure a well-defined inlet temperature. This section contained three blank (uncatalyzed) ceramic monoliths, each 2.5 cm long and 1.9 cm in diameter, wrapped with a ceramic blanket as described below. The monoliths were designed to reduce entrance effects and center the inlet thermocouple. The temperature of the reactor feed stream was measured by a K-type, chromel-alumel ther-

mocouple in a stainless-steel sheath. The thermocouple was placed about 1.2 cm from the inlet face of the catalyst and was partially shielded from radiant heat by the blank monoliths. The intent of this placement and shielding of the inlet thermocouple was to measure the temperature of the inlet gas steam as closely as possible, with minimum influence of radiation from the hotter inlet face of the catalyst. The latter was found to contribute no more than 2 K to the measured inlet temperature. The second furnace zone was 30 cm in length and contained the catalyst. The catalyst was a ceramic (cordierite) monolith (62 cells per cm2) coated with palladium at a loading of 79 g/m3 on a γ-alumina substrate supplied by Allied Signal Environmental Catalyst Company. The catalyst sample used in the reactor was 0.95 cm in diameter and 2.5 cm in length. The catalyst was initially conditioned by passing a feed mixture containing 0.9 mol % methanol in air over the fresh catalyst at an inlet temperature of 573 K for 24 h. Early runs were repeated to ensure that no observable deactivation of the catalyst occurred during the study. The monolith was wrapped with a ceramic blanket to insulate the catalyst from the reactor wall and to prevent gas bypass around the outside of the catalyst. The third furnace zone was 10 cm in length and was heated to ensure that none of the vapor condensed after leaving the catalytic reactor. A second thermocouple was positioned 1.2 cm from the outlet face of the catalyst and was used to measure the outlet temperature. This thermocouple was also centered and protected from radiant heat by a blank monolith. After leaving the furnace, most of the product stream passed through a wet test meter (301), which determined the molar flow rate. A small fraction of the product stream was diverted to a Perkin Elmer Series 910 gas chromatograph/mass spectrometer [GC/MS (302)] equipped with a spectral library to determine the components and composition of the product stream. A 460 cm long packed column (80/100 mesh Haysep-R) was used to separate the stream components. The inlet stream could be directed to the wet test meter, and the GC/MS through a bypass line controlled by a three-way valve located prior to the reactor. With the information acquired from the wet test meter and the GC/MS, atomic and mass balances were made to test the quality of the data. The acceptable errors in the carbon, nitrogen, and oxygen atomic balances were (4.0%. The acceptable error in the hydrogen balance was set at (8.0% because the concentration of H2 could not be measured directly by the GC/MS. Therefore, any H2 present would contribute to the error in the hydrogen balance. Any data point that did not meet all four of these criteria was discarded. Energy balances were performed by comparing the experimentally measured change in bulk gas temperature across the catalyst, i.e., the measured outlet temperature minus the measured inlet temperature, to the theoretical adiabatic temperature rise. Acceptable error was (25%. Data points were discarded if this criterion was not met.

Results and Discussion This section is divided into four parts. First, the steadystate CH3OH conversion and product distribution will be described for feeds with CH3OH as the only fuel component. Next, the results of experiments with more than one fuel component, i.e., combinations of CH3OH, HCHO, and CO, will be presented, followed by the oxidation of CH3OH on

TABLE 1

Components of Feed Gas for the Different Series of Experiments series no.

feed components

symbol shape

1 2 3 4 6

N2, O2, CH3OH (base case) N2, O2, CH3OH, CO2 N2, O2, CH3OH, H2O N2, O2, CH3OH, H2O, CO2 N2, O2, CH3OH, H2O, CO2, HCHO, CO

b 9 ( 2 1

a deactivated catalyst. Hysteresis will be described in the last section. Table 1 shows the components of the feed for each series of runs. The concentrations of the different feed components were as follows: CH3OH ) 0.9 ( 0.05 mol %, HCHO ) 195 ( 25 ppm, CO ) 0.78 ( 0.04 mol %, CO2 ) 9.5 ( 0.60 mol %, H2O ) 8.1 ( 0.90 mol %, and O2 ) 1.0 -2.0 mol %, depending on the desired oxygen-to-fuel ratio. The balance was N2. All runs were done at a space velocity of about 100 000 h-1 (298 K and 1 atm) and at an inlet pressure of 3.5 psig. The space velocity was calculated by dividing the inlet gas flow rate, corrected to 298 K and 1 atm, by the catalyst volume, which was calculated from the measured length and diameter of the actual sample. A CH3OH concentration of 0.9 mol % is representative of the feed to the catalytic converter during cold start (17), and the CO2 and H2O concentrations are typical of a wide range of operating conditions. The 100 000 h-1 space velocity is typical of highway driving. The feed to the reactor always contained CH3OH, O2, and N2. In some experiments, CO2, H2O, or both were added. It should be noted that the concentrations of CO2 and H2O used in this study were representative of engine exhaust streams, while the concentrations used in previous studies were much smaller, thus justifying reexamination of these variables. Experiments were conducted at inlet temperatures ranging from 343 to 703 K and at various stoichiometric ratios ranging from 0.65 to 1.44. The stoichiometric ratio, φ, is defined by

φ)

moles of O2 in feed stoichiometric moles of O2 based on fuel in feed (1)

When φ > 1, the system is operating under fuel-lean conditions, i.e., the concentration of O2 in the feed exceeds that required to oxidize all of the fuel components to CO2 and H2O. Experiments with Methanol as the Only Fuel Component. Figure 2 shows the steady-state conversion of CH3OH under fuel-lean conditions for several different feed gas compositions, including those containing CO2 and/or H2O (series 2, 3, and 4). In almost every case, the CH3OH conversion was greater than 90%, even at very low inlet temperatures (Tin ≈ 340 K). Addition of CO2 and/or H2O to the feed stream had no significant effect on the CH3OH conversion, consistent with the results of McCabe and Mitchell (10). Brewer et al. (13) found that H2O had an inhibiting effect of the kinetics of CH3OH oxidation, using a catalyst very similar to the one employed in this research. It is possible that mass-transfer limitations, as discussed below, obscured the kinetic effect of H2O in this study. The only carbon-containing product observed from the oxidation of CH3OH under fuel-lean conditions was CO2.

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FIGURE 2. Methanol conversion under fuel-lean conditions. The data points show the conversion with methanol as the only fuel component. The dashed line shows the effect of adding 0.74% CO to the feed. The shapes of the data points indicate the feed composition, as shown in Table 1.

FIGURE 4. Oxygen conversion under fuel-rich conditions with methanol as the only fuel component. The shapes of the data points indicate the feed composition, as shown in Table 1. The stoichiometric ratios are as follows: series 1, 0.72; series 2, 0.72; series 3, 0.70; series 4, 0.65; series 6, 0.68.

compositions are below 345 K, and the light-off temperatures for the series 1 and 4 compositions are approximately 355 and 370 K, respectively. The lack of a consistent trend in these data suggests that the addition of CO2 and/or H2O to the feed gas does not have a significant influence on light-off temperature. At low inlet temperatures (Tin < 440 K), the CH3OH conversion was lower under fuel-rich conditions than under fuel-lean conditions. At high inlet temperatures (Tin > 515 K), CH3OH conversions approach similar levels under both fuel-rich and fuel-lean conditions.

FIGURE 3. Methanol conversion under fuel-rich conditions. The open data points show conversion with methanol as the only fuel component. The dashed line and solid symbols show the effect of adding 0.74% CO to the feed. The shapes of the data points indicate the feed composition, as shown in Table 1. The stoichiometric ratios are as follows: series 1, 0.72; series 2, 0.72; series 3, 0.70; series 4, 0.65; series 6, 0.68.

Under fuel-lean conditions, the CH3OH conversion above an inlet temperature of about 370 K was a weak function of inlet temperature, suggesting that the reaction was at least partially mass-transfer limited. A theoretical calculation of the conversion of CH3OH at the conditions of Figure 2 was carried out, assuming that the reaction was completely controlled by mass transfer of methanol from the bulk gas stream to the surface of the catalyst. Using the heat-transfer correlation for fully-developed flow in square channels given by Shah (19) and assuming that the j-factors for heat and mass transfer are equal and that the Schmidt and Prandtl numbers are equal (20), a CH3OH conversion of 99.7% was calculated. The agreement between this conversion and the experimentally-measured conversions at the highest inlet temperatures (g98.8% conversion for Tin > 625 K) supports the hypothesis that the oxidation of CH3OH is influenced substantially by mass transfer at the conditions of Figure 2. Figure 3 shows the data for experiments with several fuel-rich feed gas mixtures with similar stoichiometric ratios (φ ≈ 0.7). The light-off temperature is defined as the inlet temperature at which the conversion of CH3OH equals 50%. The light-off temperatures for the series 2 and 3 feed

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The O2 conversion for the experiments of Figure 3 is displayed in Figure 4. This conversion was greater than 90% for inlet temperatures above 370 K. The O2 conversion was a weak function of inlet temperature between about 350 and 500 K and was essentially independent of inlet temperature above 500 K. This behavior, supported by a theoretical calculation of the O2 conversion assuming complete mass-transfer control, suggests that the reaction was influenced by O2 mass transfer between the light-off temperature and about 500 K and was essentially controlled by O2 mass transfer above about 500 K. Figure 5 shows how the normalized product distributions for two fuel-rich systems (φ = 0.72) varied with inlet temperature. The yields in this figure are defined as

yield )

amount produced (mol) methanol reacted (mol)

(2)

For both feed compositions, the portion of CH3OH that oxidized at low inlet temperatures produced primarily CO2, with only a small formation of HCHO (yield ≈ 2%). This result is qualitatively and quantitatively similar to those of McCabe (17). As the inlet temperature and the CH3OH conversion increased, the HCHO and CO2 yields decreased; CO, which was not formed at very low inlet temperatures, became an important product. Referring to reaction sequence A, this suggests that the oxidation of CO is rapid at low inlet temperatures compared to the oxidation of CH3OH. The low inlet temperature data also suggest that HCHO consumption, either by oxidation or by decomposition, is rapid compared to CH3OH oxidation but not quite as rapid as CO oxidation since detectable quantities of HCHO are observed in the effluent. As the inlet temperature was increased, the CH3OH oxidation rate and the HCHO oxidation/decomposition rate increased significantly rela-

FIGURE 6. Formaldehyde yield with a normally-conditioned catalyst under fuel-rich conditions. FIGURE 5. Product distribution under fuel-rich conditions (O ≈ 0.72) with H2O (8.3%) or CO2 (10.1%) in the feed. Diamonds and dashed lines (series 3), H2O in feed; squares and solid lines (series 2), CO2 in feed. Solid symbols, CO2 yield; open symbols, CO2 yield + CO yield.

tive to the CO oxidation rate, and the HCHO oxidation/ decomposition rate increased relative to the rate of oxidation of CH3OH. This suggests that the activation energy for HCHO oxidation/decomposition is greater than the activation energy for CH3OH oxidation, which in turn is greater than the activation energy for CO oxidation. In the studies of HCHO oxidation over a Pt catalyst, Sodhi et al. (16) found that the activation energy for HCHO disappearance was greater than that for CO oxidation, consistent with the above interpretation of the results of Figure 5. Figure 5 shows data for two different runs, one with 7.7% H2O and no CO2 in the feed gas and one with 10.1% CO2 and no H2O in the feed gas. At the higher inlet temperatures, the CO2 yields are much higher for the H2Ocontaining feed gas. In fact, the quantity of CO2 formed with the H2O-containing feed gas is much greater than the amount that is calculated from the inlet concentrations of O2 and CH3OH and the measured yield of CO, assuming that all of the oxygen in both CO and CO2 comes from either O2 or CH3OH in the feed. This discrepancy, as well as the difference in CO and CO2 yields for the two different feed gases, is attributed to the water-gas shift reaction:

CO + H2O f CO2 + H2

(B)

This hypothesis is supported by the fact that significantly less water was produced per mole of methanol converted in the series 3 experiments (H2O in feed) than in the series 2 experiments (CO2 in feed). Figure 6 shows the HCHO yield versus inlet temperature for several feed compositions and for stoichiometric ratios between 0.65 and 0.82. With a normally-conditioned catalyst, HCHO was observed in the effluent only under fuel-rich conditions. Under stoichiometric or fuel-lean conditions, the effluent concentration of HCHO was always below the limit of detection, about 40 ppm. For each feed gas, the highest yield occurred at the lowest inlet temperature. The yield decreased as the gas inlet temperature was increased. This result is not completely consistent with studies by McCabe et al. (10, 11, 17), where the maximum yield of HCHO was observed at intermediate catalyst bed temperatures. This difference is probably not the result of differences between the catalysts studied. It may be due

to the adiabatic operation of the present catalyst, compared to the quasi-isothermal operation of McCabe’s reactors, or to the fact that different temperatures were measured, gas inlet in the present research and catalyst bed in the previous work. Experiments with Multiple Fuel Components. A set of experiments was carried out with a feed containing CH3OH (0.90 ( 0.05 mol %) and HCHO (195 ( 25 ppm). The feed also contained O2, N2, CO2, and H2O. There was no observable difference in CH3OH conversion or product distribution between these experiments and the previouslydescribed experiments with CH3OH as the only fuel component, provided that comparison was made at similar stoichiometric ratios and inlet temperatures. A second set of experiments was carried out with CH3OH (0.90 ( 0.05 mol %), HCHO (195 ( 25 ppm), and CO (0.78 ( 0.04 mol %) in a feed gas containing 7.4% H2O and 9.2% CO2. Figure 2 shows that the light-off temperature was about 435 K for a feed mixture with a φ of 1.22. With no CO in the feed, the light-off temperature was less than 345 K. Figure 3 shows a similar light-off temperature for a φ of 0.68. This data demonstrates that the presence of CO in the feed increased the light-off temperature significantly. This kinetic inhibition by CO is consistent with previous results (10-13). Above the light-off temperature, the CH3OH conversion was similar to that observed in the methanol-only runs, consistent with the hypothesis of masstransfer control at high inlet temperatures. Figure 7 shows the net conversion of the CO in the feed versus inlet temperature for the experiments discussed in the preceding paragraph and for a third set of experiments at φ ) 0.84 with the same feed gas. Under fuel-lean conditions, essentially all of the CO was oxidized once lightoff had occurred. Under fuel-rich conditions, the net conversion of CO went through a maximum at about 525 K. A possible explanation for this maximum derives from the competition for oxygen between the following reactions: k1

CH3OH + O2 98 CO + 2H2O

(C)

k3

CO + 1/2O2 98 CO2

(D)

If the activation energy of reaction C is greater than that of reaction B, as previously hypothesized in connection with Figure 5, the following behavior would result: (1) At the lowest inlet temperatures, the oxidations of CH3OH to CO and of CO to CO2 are relatively slow. The

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FIGURE 7. Effect of stoichiometric ratio on net carbon monoxide conversion. Feed contained CH3OH, CO, HCHO, CO2, and H2O (series 6). Open symbols, O ) 0.84; solid symbols, O ) 0.68.

conversion of CH3OH is moderate, the net conversion of CO is small, and O2 is not completely consumed. As the inlet temperature is increased, all three conversions increase. (2) At intermediate inlet temperatures above light-off, the conversion of O2 exceeds 90% and is partially determined by the rate of mass transfer. Reactions C and D must compete for an insufficient supply of O2, since φ < 1. As the inlet temperature is increased, k1/k3 increases and reaction C becomes progressively faster than reaction D. The O2 that reaches the catalyst surface increasingly goes into reaction C, increasing CO production. The amount of O2 available for reaction D decreases with increasing inlet temperature. The overall result is that the net conversion of CO declines. The maximum in CO conversion is a consequence of the transition from region 1 to region 2. (3) At the highest inlet temperatures, O2 is preferentially consumed by reaction C. Reaction D proceeds only to the extent permitted by the excess O2, i.e., the amount remaining after reaction C has gone to completion. In order to test this last element of the hypothesis, a theoretical CO conversion was calculated for the runs with φ ) 0.68 and 0.84 by assuming that O2 reacts first with CH3OH, such that reaction C goes to completion. The remaining O2 then reacts with CO, leaving some CO unconverted due to the stoichiometric deficiency of O2. The water-gas shift reaction (B) was neglected in this calculation since the feed gas contained both CO2 and H2O. The results of these calculations are shown as horizontal lines at the right-hand side of Figure 7. The experimental data at the highest inlet temperatures agree reasonably well with the CO conversion predicted by this model, especially at φ ) 0.84. At a given inlet temperature, the catalyst surface is hotter for φ ) 0.84 than for φ ) 0.68. If the experiments with φ ) 0.68 had been run at higher inlet temperatures, the experimental data might have approached the model more closely. Runs with Deactivated Catalyst. Three runs were made with a thermally-deactivated catalyst. It has been reported (4, 5, 16, 17) that HCHO emissions increased with mileage in fleet studies. Thermal deactivation is one possible cause of such an “aging” effect. If an engine misfires, abnormally high concentrations of fuel and air pass over the catalyst, leading to high catalyst temperatures. This phenomenon was simulated in the catalyst conditioning process by passing a feed stream containing approximately 4 mol % CH3OH at a φ of 3.5 over the catalyst at an inlet temperature

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FIGURE 8. Methanol conversion with a thermally-deactivated catalyst under fuel-rich conditions.

FIGURE 9. Formaldehyde yield with a thermally-deactivated catalyst. Open symbols, fuel-rich; solid symbols, fuel-lean. Formaldehyde yield with a normally-conditioned catalyst is not measurable under fuellean conditions.

of 575 K for 4 h. The adiabatic temperature rise for these conditions is about 700 K, which produces a theoretical effluent gas temperature and catalyst surface temperature of around 1275 K. According to McCabe (17), thermal deactivation may occur when the temperature of the catalyst surface exceeds about 1175 K. Two of the three runs were done under fuel-rich conditions (φ ) 0.84 ( 0.04). Figure 8 shows the conversion of CH3OH for these runs compared to that of a normallyconditioned catalyst. Methanol conversion was much lower with the deactivated catalyst over the whole range of inlet temperature. The light-off temperature of the deactivated catalyst was between 415 and 455 K for the three runs, approximately 70-100 K higher than with the normallyconditioned catalyst at comparable gas compositions. This suggests that the oxidation of CH3OH over the thermally deactivated catalyst was kinetically controlled over a much longer length of catalyst at a given inlet temperature as compared to the normally-conditioned catalyst. Similar results were obtained under fuel-lean conditions with the deactivated catalyst. Formaldehyde was produced in all three runs with the deactivated catalyst, under both fuel-lean and fuel-rich conditions and at most inlet temperatures, as shown in Figure 9. At all inlet temperatures, the HCHO yields with the deactivated catalyst were significantly higher than those with the normally-conditioned catalyst. The HCHO yield from the deactivated catalyst decreased with inlet tem-

temperature on the descending-temperature portion of the curve is substantially lower, about 375 K. The practical significance of the hysteresis phenomenon is that a higher inlet temperature is required to eliminate CH3OH and CO from the engine exhaust when the catalyst is cold, e.g., when the engine has just been started, than when the catalyst has been operating for a period of time. The phenomenon of hysteresis may be caused by axial heat conduction, by interactions between heat and mass transport to the catalyst surface, or from the particular kinetic expression that is associated with CO oxidation (23). The present data per se do not permit a definite conclusion as to the source of the hysteresis shown in Figure 10.

Acknowledgments FIGURE 10. Hysteresis of the methanol conversion. Circles, increasing inlet temperature; squares, decreasing inlet temperature. Solid symbols, steady state; open symbols, unsteady state.

perature, as it did with the normally-conditioned catalyst. Under fuel-rich conditions, the highest yield was about 7%, at an inlet temperature corresponding to light-off. Under fuel-lean conditions, the highest yield was about 20%, again at the light-off temperature. As stated earlier, HCHO was never observed in the effluent under fuel-lean conditions with a normally-conditioned catalyst. As noted previously, McCabe et al. (17) found that the maximum HCHO yield for a thermally-deactivated Pt/Rh three-way catalyst did not exceed about 4%. The higher yields observed in this study may be due to some combination of more severe aging conditions and a different catalyst composition. The deactivation of the catalyst upon exposure to surface temperatures in the region of 1275 K may be related in some way to the decomposition of PdO to Pd, which occurs between about 1050 and 1100 K depending on the nature of the catalyst support (21, 22). At a given temperature, PdO is more active than Pd metal for methane oxidation (22), but is less active for methanol oxidation (13) and formaldehyde oxidation (12). Therefore, the relationship between PdO decomposition and thermal deactivation, if such a relationship does exist, is likely to be complex. Hysteresis. Figure 10 shows the results of an experiment with a feed consisting of 1.72 mol % O2, 0.95 mol % CO, 0.82 mol % CH3OH, and the balance N2 (φ ) 1.02). The space velocity was approximately 100 000 h-1. The conversions of CH3OH, CO, and O2 were measured as a function of inlet temperature beginning at a temperature of about 340 K. The inlet temperature was gradually increased until it had reached about 530 K, at which point it was gradually decreased back to about 375 K. At certain points, the inlet temperature was held constant, the catalyst was allowed to come to steady state, and data were taken. As a check, some unsteady-state data were also taken while the inlet temperature was being slowly changed. The steady-state data are differentiated from the unsteady-state data, and the ascending-temperature data are differentiated from the descending-temperature data in Figure 10. Plots of CO concentration, outlet temperature, and O2 concentration versus inlet temperature are very similar to that shown for CH3OH conversion and are not presented here for the sake of brevity. Figure 10 shows a pronounced hysteresis. The light-off temperature for methanol on the ascending-temperature portion of the curve is about 435 K, consistent with the behavior shown in Figure 2. However, the extinction

This research was supported by a grant from the Environmental Protection Agency. The authors are grateful to Mr. Ronald G. Silver of Allied Signal Environmental Catalyst Company for supplying the catalyst used in this research.

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Received for review October 12, 1995. Revised manuscript received February 1, 1996. Accepted February 4, 1996.X ES950760N X

Abstract published in Advance ACS Abstracts, April 1, 1996.

VOL. 30, NO. 6, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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