16344
J. Phys. Chem. 1995, 99, 16344-16350
Effect of NO Pressure on the Reaction of NO and CO over Rh(ll1) Haryani Permana and K. Y. Simon Ng Department of Chemical Engineering, Wayne State University, Detroit, Michigan 48202 Charles H. F. Peden Environmental Molecular Sciences Laboratory, Pacifc Northwest Laboratories,t P.O. Box 999, Richland, Washington 99352 Steven J. Schmieg and David N. Belton* Physical Chemistry Department, G. M. NAO R&D Center, P.O. Box 9055, Warren, Michigan 48009 Received: April 24, 1995; In Final Form: August 17, 1 9 9 9
We have examined the reaction of NO with CO over a Rh( 111) catalyst under conditions of both low (0.8 Torr) and high (8 Torr) NO pressures. Our results show that the same three products are formed (C02, N20, and N2) regardless of the NO pressure employed. However, the selectivity of the reaction and the E, for product formation are profoundly affected when the NO pressure is lowered by tenfold. For high NO pressures the selectivity is insensitive to either NO pressure (1 Torr < PNO< 40 Torr) or reaction temperature (T < 673 K). In sharp contrast to this high-pressure behavior, when the NO pressure is lowered to 0.8 Torr, then the selectivity is a strong function of reaction temperature, with N20 being the major product below 635 K and N2 the major product above 635 K. It is exactly this switch over in selectivity at elevated temperature that has always been seen over low-loaded Rh/A1203 catalysts but until this time not reported for singlecrystal catalysts. On the basis of our new results we can say that Rh(ll1) has the characteristic selectivity behavior of practical MA203 catalysts. These results are encouraging because they show that the mechanistic insight we gain over Rh( 1 11) is more broadly applicable to supported catalysts than was previously thought.
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1. Introduction One of the most important reactions occurring in automobile catalytic converters is the reaction between carbon monoxide (CO) and nitrogen oxides (NO,) over rhodium-containing catalyst particles.' In the future, this reaction must be run much more efficiently because government regulations require that the emission levels of NO,, CO, and hydrocarbons (HC's) must fall substantially.2 For example, future standards require that, compared to 1992 levels, NO, must be reduced by a factor of 5, CO reduced by a factor of 2, and HC reduced by a factor of The extremely low HC standards tend to require a slightly lean (excess 02) engine exhaust so that HC conversions in excess of 99% can be obtained in the converter. Unfortunately, lean exhaust is unfavorable for tight NO, control because 0 2 competes with NO to be the oxidant for the CO in the exhaust.' As a result of this competition, NO, emission levels are strongly dependent on the kinetics of the NO-CO reaction; therefore, it is important that we understand NO, reduction kinetics in much more detail than has been previously achieved. Part of this understanding comes from measuring NO-CO reaction rates and modeling the kinetics at the elementary step level using experimentally measured rates for the elementary reaction In previous papers we reported that reaction of NO and CO over Rh and Pt-Rh single crystals gives the same three products (COz, N20, and N2) as are observed for reaction over supported Rh and Pt-Rh We also reported that the selectivity
' Pacific Northwest Laboratory is a multiprogram National Laboratory operated for the US.Department of Energy by Batelle Memorial Institute under contract number DE-AC06-76RLO 1830. * Author to whom correspondence should be addressed. Abstract published in Advunce ACS Absrructs, October 1, 1995. @
0022-365419512099-16344$09.00/0
for N20 (ratio of N20 to N20 N2) was remarkably insensitive to reaction conditions of temperature, NO pressure (PNO> 1 Torr), and CO pressure. In those papers we noted that Rh3 and Pt-Rh4 single crystals have temperature and pressure dependent selectivities very similar to that of highly loaded (4.6 wt %) Rh/Si02 catalysts8 but quite different from that of low-loaded RhIAl2O3 catalyst^.^-'^ These low-loaded (0.1 wt %) Rh/&O3 catalysts switch over from making primarily N20 at low temperatures to making exclusively N2 at higher temperatures?-15 On the basis of our previous work we were unable to explain why high-loaded RWSi02 and single-crystal catalysts appeared so different from low-loaded Rh/Al2O3 catalyst^.^.^ In this paper we make an important extension to our previous work that allows us to resolve the apparent conflict in the behavior of these different Rh catalysts. This new insight was gained by reconfiguring our reactor to operate as a flow reactor and improving the sensitivity of our gas chromatograph. These two changes allow us to run the NO-CO reaction using NO pressures 2 . 5 ~lower (down to 0.4 Torr) than was previously possible in our system. In this paper we show that the gas phase NO pressure is extremely important in determining the selectivity for N20 over Rh single-crystal catalysts, provided NO pressure is below 1 Torr. These new results show that N20 formation changes abruptly from zero order in NO pressure above 1 Torr to positive first order below 1 Torr. In this lower pressure regime N2 formation remains zero order in NO pressure, which makes the selectivity of the reaction extremely sensitive to NO pressure. In our previous papers we assumed that the zero-order NO kinetics observed above 1 Torr of NO would also apply to reactions at 0.8 Torr or lower; however, our new results show that in fact they do not. Because of this, the selectivity of the reaction is very sensitive to NO pressure. Factoring in these new results, we conclude that the previously 0 1995 American Chemical Society
Reaction of NO and CO over Rh(ll1)
J. Phys. Chem., Vol. 99,No. 44,1995 16345
reported differences3 in catalyst formulations arose primarily from subtly different reaction conditions of NO pressure and reaction temperature. With this in mind, we conclude that our results over Rh(ll1) are much more broadly applicable to supported catalysts than we previously thought and thus offer a better insight into the behavior of more practical catalytic systems.
2. Experimental Section 2.1. Apparatus. The experiments were performed in a custom-built system that couples an ultrahigh vacuum (UHV) analysis chamber to a moderate-pressure (< 100 Torr) reactor. The reactor and analysis chamber are separated with a gate valve. The UHV analysis chamber is equipped with a wide array of analytical techniques. For this study we used Auger electron spectroscopy (AES),X-ray photoelectron spectroscopy ( X P S ) , and low-energy electron diffraction (LEED). The reactor is equipped with a gas chromatograph (GC) for reactant and product detection. The gases used in these experiments were 99.0% NO (Scott Specialty Gases) and 99.99% CO in an Al cylinder (Scott Specialty Gases). Each gas cylinder is connected to a 10 and a 100 cm3 (STF')/min flow controller, whose outlets join into a common gas manifold. The concentration of each reactant is controlled by setting the relative flow rates of each gas. The gases are then trapped with an isopentane (2methy1butane)hiquid nitrogen slurry bath at 113 K prior to entry into the reactor. This trap excludes any metal carbonyls present in the CO cylinder and NO2 and C02 impurities present in the NO cylinder; however, the NO still had impurity levels of 0.40% N20 and 0.25% Nz. These contaminants in the NO supply were subtracted from any Nz0 and N2 that was produced by the reaction. The gases were leaked through the reactor at low pressure, as measured by a baratron gauge. The total pressure of the reactor is controlled using a metering valve, at the outlet of the reactor, that is connected to a mechanical roughing pump. The reactor has a volume of 0.370 L and is pumped with a turbomolecular pump with an ultimate base pressure of Torr. The sample could be transferred from the reactor to the UHV analysis chamber within 5 min after reactor pump-down with a typical base pressure of Torr during spectroscopic analysis. 2.2. Sample Preparation. The Rh( 1 1 1) sample was obtained from a boule of Rh oriented along the [loo]direction. The crystal was cut so that both sides of the sample were oriented to f0.5"of the (111)plane, as shown by the Laue diffraction pattern. The initial cleaning procedure consisted of flowing 99.9999% HZ at 50 cm3 (STP)/min over the sample in a quartz tube furnace for 40 h at 1275 K. This procedure has been shown to be effective at removing any low Z impurities from the bulk, such as C, B, P, and Si.I6 The sample was then sanded and polished using diamond paste, with the final polish utilizing a 0.25p m grit. After it was polished, the crystal was etched in hot HF/HNO3 (3:l)for several minutes. The crystal was mounted on the transfer device using two etched 0.015in Ta wires spot-welded to the edges of the sample for resistive heating. A 0.003 in chromel-alumel thermocouple was also spot-welded to the edge of the sample to monitor the crystal temperature. After being mounted, the sample was rinsed again with HNO3 to remove the spot-welding residue, followed by rinsing with distilled H20 and methanol. The Rh(ll1) sample was rectangular in shape (8.69mm x 6.02mm) with an area of 0.523cm2 per side and a thickness of 0.91 mm. The number of active sites was calculated to be 1.67 x 1015 (both sides). The edge area accounted for approximately 19% of the total area of the sample.
B
1
0.1
I
1
0
in (lo3) ( K - ~ ) Figure 1. Specific rates of COz, N20, and N2 formation for the NOCO reaction over Rh( 111) as a function of inverse temperature using (A) PNO= 0.8TOITand Pco = 4 TOITand (B)PNO= Pco = 8 TOIT.
A sharp (1 x 1) LEED pattern was obtained on the front and back of the Rh(ll1) sample by Ar+ sputtering (2keV, -12 pA) for -40 h with the sample at 875 K, followed by annealing at 1275 K for 20 min. This extensive sputtering treatment was necessary to obtain a sharp LEED pattern and has been successful in ordering Rh single-crystal surfaces. Next, the sample was placed in the reactor and treated in an 8 Torr of CO/8 Torr of 0 2 gas mixture at 525 K to remove residual carbon contamination in the near-surface region. The surface cleanliness was monitored with AES and X P S , and the surface order was checked periodically with LEED. 2.3. Gas Chromatography Measurements. Reactions were performed in a flow mode. All products and reactants were measured with a Varian 3400 gas chromatograph (GC)using simultaneous injection of sample (2 x 5 mL) into each of two columns operated at 313 K with a He carrier gas. One gas sample entered a molecular sieve column where N2, NO, and CO were separated before passing on to the detector for analysis. The other sample entered a delay column followed by a Hayesep DIP column that delayed entry and subsequent elution of components until the last molecular sieve component (CO) eluted. At this point a valve was switched to direct the Hayesep DIP effluent to the detector for the measurement of C02 and N20. Column effluents were monitored using a thermal conductivity detector (TCD). The TCD filament temperature was maintained at 573 K, and the detector temperature, at 423 K. Using this arrangement, we were able to detect Nz,NO, CO, COz, and NzO.
3. Results C02, N20, and N2 were produced from the NO-CO reaction between 523 and 673 K. Figure 1 shows turnover numbers (TONs) as molecules/(site s) for the three products obtained on Rh(1 1 1). TONs are given on a per Rh site basis by assuming that the number of exposed surface Rh sites is 1.67 x
Permana et al,
16346 J. Phys. Chem., Val. 99, No. 44,1995
TABLE 1: Apparent Activation Energies and Frequency Factors for COZ,NzO,and N1 Formation from the NO-CO Reaction over Rh(ll1) Using Two Different Reaction Conditions
co2
NO
Vb
34.2 5.0 x 1OI3
35.7 1.1 x 1014
32.1 1.6 x l o i 2
PCO= 4.0 Torr and PNO= 8.0 Torr Ea" 50 A) particles than we previously suspected. Specifically, when the reaction is run using the same NO pressures as were employed for those supported catalysts, Rh(ll1) shows the switch in selectivity that has been classically attributed as a characteristic of supported Rh catalysts. 4.2. Mechanistic Considerations. Previously, we developed a model to explain the kinetics in the higher pressure NO regimes (’1 Torr). The experiments reported here represent part of our ongoing research to test the validity of that model. In our model of the reaction there are six important elementary steps,
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+ S CO(ad) NO(g) + S NO(ad) NO(ad) + S - N(ad) + O(ad) CO(ad) + O(ad) - CO,(g) + 2s NO(ad) + N(ad) - N20(g) + 2s N(ad) + N(ad) - N,(g) + 2s CO(g)
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(3) (4)
(5)
Using these six steps and rate constants that are taken primarily from elementary step measurements under UHV conditions, we were able to quantitatively model reaction kinetics in the high (> 1 Torr) NO pressure regime.5 A recent improvement to our understanding came when we re-examined the NO dissociation reaction and quantitatively assessed the degree to which NO inhibits its own disso~iation.’~ As a result of “self-inhibition”, the NO dissociation rate slows down dramatically for NO coverages over about l/4 ML. At coverages near saturation NO, the NO dissociation rate is roughly 500x slower than that in the low coverage limit.I9 When we incorporate this measurement into our model, it predicts that NO dissociation is the ratelimiting step, that @NO is quite high, and that OCOis very low provided the reaction is run under these conditions: 528 K < T < 648 K, 1 TOIT< Pco < 40 T o r , and 1 TOIT< PNO< 40 Torr. The stability in NO and CO coverages has two effects. First, high surface NO coverages keep NO dissociation as the rate-limiting step in the reaction. Second, stable NO coverages (ONalso remains constant, and steps 5 and 6 have similar activation energies) make the N20 selectivity insensitive to reaction pressures and temperatures. As one can see, high and
al.
stable NO coverages are an important component in our model of how the NO-CO reaction runs over Rh catalysts at high pressures. The experiments reported here were specifically designed to get out of the zero order, temperature insensitive kinetic regime. This was not possible in our previous experiments due to limitations of our batch reactor and our product detection capabilities. By reconfiguring our reactor as a flow reactor and modifying our GC to have greater sensitivity, we can now access a kinetic regime where the reaction kinetics are much more sensitive to reaction conditions. As the data in Figure 2 show, we can break out of this regime even with relatively high NO pressures of 8 Torr of NO by raising the reaction temperature above 673 K. Presumably, once above 673 K (using 8 Torr of NO) the surface NO coverage will begin to fall, causing the selectivity of the reaction to be temperature dependent. However, as we stated earlier, at these high temperatures the reaction is so fast that the heat released by the oxidation of CO makes our sample temperature unstable and consumes the reactants much too fast to keep the conversion low. In our apparatus a more accessible pathway to this different kinetic regime is to lower the NO pressures. Since, in our model, the NO adsorption-desorption equilibrium controls the reaction rates by controling the NO dissociation rate, lowering the NO pressure sufficiently should perturb this equilibrium toward desorption. In Figures 1-4 we compare reaction rates and reaction orders under these two kinetic regimes, which differ primarily in the gas phase NO pressure. All these data show a sharp transition in the kinetic behavior for NO pressures less than 1 Torr. For Rh( 111) we find that the reaction orders in both CO (Figure 3) and NO (Figure 4) change from the zero-order kinetics previously reported3 to positive order in NO and negative order in CO when the NO pressure is less than 1 Torr. Fortuitously, the transition occurs just outside the range of pressures that were accessible to us in our previous experiments and serves as a good reminder that extrapolation of kinetics to conditions outside those for which measurements are available is not reliable unless a valid kinetic model for the reaction exists. As for the temperature dependence, Figure 1 shows that there are two differences in the Arrhenius behavior for the two cases we examined (4 Torr of CO/O.8 Torr of NO versus 8 Torr of COB Torr of NO). First, at lower NO pressure (Figure 1A) COZ is formed with an Ea 10 kcal/mol lower (see Table 1) than that when the higher NO and CO pressures are reacted. This difference is clearly outside our range of experimental error (roughly 3 kcdmol). We cannot assign this difference in E, to a single elementary step because for NO-CO the rates of the individual steps can be very similar, strongly coverage dependent, and thus very interdependent.’* In the future we hope to sort out these E, differences using a detailed kinetic model, but for now we note them primarily to point out that lowering the NO pressure from 8 to 0.8 Torr places us in a very different kinetic regime. The second and most important difference in the Arrhenius behavior for the two reaction conditions (4/0.8 versus 8/8) is that for the 4/03 case the N20 and N2 formation rates show a break in the Arrhenius plot (Figure lA), but no break is observed in the 8/8 case (Figure 1B). Of course this break in the Arrhenius plots means that the selectivity for N20 begins to decrease sharply above 600 K when we are reacting 0.8 Torr of NO (Figure 2 ) . A similar drop in the N20 selectivity occurs about 75 K higher for the 8/8 case. Any debate about the NOCO mechanism always centers around how to explain the temperature dependent selectivity of Figures 1 and 2. One idea is that both N2 and N20 come from a common N20 intermediate
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Reaction of NO and CO over Rh( 111)
J. Phys. Chem., Vol. 99, No. 44, 1995 16349
that tends to desorb as N20 at low temperatures and readsorb and dissociate to form N2 at higher temperatures. Support for this mechanism was given in two papers that combined experiments and These papers nicely showed that the “common-intermediate” model has a solid basis for consideration, and we do not offer any direct evidence in this paper to contradict that model. However, at present the idea we favor is that N2 comes from an N N reaction and N20 comes from an NO N reaction. We suggest that at higher temperatures andor lower NO pressures the NO N N20 reaction loses out to a competing reaction, NO desorption. As NO desorption drives surface NO coverages down, N20 formation rates level off at high temperature. We estimate that a break in the Arrhenius plot would occur around 623 K if Ea for the NO N elementary step is approximately 33 kcaYmol and @NO is falling linearly versus UT. We made this estimation by doing a simple calculation that assumed that (1) the N20 formation is first order in ONOand ON(eq 5) and (2) ONis not decreasing with increasing temperature. The conclusion that ON,falls linearly with 1/T is based on preliminary reflectance IR measurements that show that surface NO coverages are declining as 1/T decreases. In the same temperature range in which N20 formation levels off (’623 K), the C02 formation rate is increasing linearly versus UT. The fact that there is no break in the C02 formation means that the NO dissociation rate is not leveling off as does the Nz0 formation rate (C02 formation rates are a direct measure of the NO dissociation rate because the CO 0 elementary step is very fast). This steadily increasing NO dissociation rate in tum means that the supply of N atoms must also be constantly increasing as the temperature is raised (UT falls). When the drop in N20 formation loses out to NO desorption, this eliminates what at lower temperature was the dominant N atom removal process (N20 formation). As a consequence of losing the major N removal channel while the N delivery rate is continuously increasing with temperature, the surface N coverages must rise sharply. Increasing N coverages in tum rapidly accelerate the N N reaction rate, which then takes over as the dominant N-atom removal process. In a recent papeI6 we showed that the N N reaction rate is extremely sensitive to surface N coverage with the rate for N atom recombination varying by 4 orders of magnitude between 0.1 and 0.5 ML. The rise in N coverage thus increases NZ formation. Above we offered the two competing explanations of the temperature dependent data. Temperature dependent kinetics are complicated because they result from overcoming Ea barriers as well as changing surface coverages. Frequently, pressure dependent data are more easily interpreted, since primarily surface coverages are changing and the effects of temperature on the Ea of individual steps are less important (one can imagine that if, as is commonly seen, the Ea for a process is coverage dependent, then changing coverages is changing the barrier). Figure 4 shows that below 1 Torr the N20 formation rate is first order in NO pressure, whereas N2 formation is zero order. In other words, NzO formation is extremely sensitive to gas phase NO pressure below 1 Torr of NO, but NZ formation is not affected down to an NO pressure of 0.4 Torr. In our view, these pressure dependent data are most consistent with our model, which has N2 and N20 formed via different pathways. We feel that the data indicate that surface NO coverages are falling, shutting down the NO N reaction, while surface N coverages remain constant, giving zero-order N2 kinetics. However, in all faimess we can also rationalize these data in terms of the N20 intermediate if we concluded that falling NO coverages offer more vacant sites for N20 dissociation. At this
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point in time, we cannot distinguish which of the two reaction mechanisms is correct. In order to make further progress on this problem, we need better rate constants for the elementary steps that make up the reaction mechanism. In our view, one can prove a reaction mechanism by independently measuring the rates of the elementary steps and applying rate constants from those measurements to a mathematical model to explain the overall reaction. Recently, we made some progress on this front when we re-examined the N N N26 and the NO dissociation reaction^.'^ However, at this time there are no measurements of the N20 formation and dissociation rates in temperature and coverage regimes applicable to the NO-CO reaction. We hope to make further progress in the near future by measuring the rates of elementary steps and measurements of steady-state NO and CO coverages with IR. But, regardless of the mechanistic details, we can with confidence state that the single most important process that controls the NO-CO activity and selectivity of Rh catalysts is the NO adsorptiondesorption equilibrium. Moving this equilibrium away from surface NO and toward gas phase NO causes the selectivity of the reaction to change from primarily N20 formation to exclusively N2 formation. We now know that differences in Ea of individual elementary steps are not what cause N2 to become the major N-containing product.
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5. Conclusions We have examined the reaction of NO with CO over a Rh(1 11) catalyst under conditions of both low (0.8 Torr) and high (8 Torr) NO pressures. Our results show that the same three products are formed (COz, N20, and N2) regardless of the NO pressure employed. However, the selectivity of the reaction and the Ea for product formation are profoundly affected when the NO pressure is lowered by tenfold. These new results are very helpful in consolidating previously reported NO-CO kinetics over Rh catalysts. According to previous literature, the selectivity of low-loaded Rh/Al2O3 catalysts is markedly different from that of high-loaded Rh/SiOz and Rh single-crystal catalysts. In this paper we show that these differences in selectivity are most likely due to subtle differences in NO pressures and reaction temperatures employed in the different experiments. Our current results show that the selectivity for N20 is temperature dependent only for NO pressures less than 1 Torr and for reaction temperatures above about 600 K. The results are encouraging because we now have direct evidence that Rh single crystals possess the same selectivity properties that were previously reported only over supported catalysts. In our view, these results suggest that the mechanistic insight we gain over Rh(ll1) is more broadly applicable to supported catalysts than was previously thought. Since single crystals are the most tractable catalysts for measurements of the rates of elementary steps, we believe that experiments over Rh single crystals can provide a detailed NO-CO mechanism that is applicable to more practical catalysts.
References and Notes (1) Taylor, K. C. Rev. Sci. Eng. 1993,35,457.
(2) Calvert, J. G.; Heywood, J. B.; Sawyer, R. F.; Seinfeld, J. H. Science 1993,261, 31. (3) Belton, D. N.; Schmieg, S . J. J. Catal. 1993,144, 9. (4)Ng, K. Y . S . ; Belton, D. N.; Schmieg, S . J.; Fisher, G. B. J. Catal. 1994,146, 394. ( 5 ) Belton, D. N.; Schmieg, S. J.; Fisher, G . B.; Ng, K. Y. S . 40th National Symposium of the American Vacuum Society, November, 1993, Abstract number SS1-TuM1. ( 6 ) Belton, D. N.; DiMaggio, C. L.; Ng, K. Y.S . J. Catal. 1993,144, 273.
16350 J. Phys. Chem.. Vol. 99, No. 44, 1995 (7) Peden, C. H. F.; Belton, D. N.; Schmieg, S. J. J. Caral., accepted. (8) Hecker, W. C.; Bell, A. T. J. Caral. 1983, 84, 200. (9) Cho, B. K.; Shanks, B. H.; Bailey, J. E. J. Catal. 1989, 115, 486. (10) Cho, B. K. J. Catal. 1991, 131, 74. (11) Cho, B. K. J. Catal. 1992, 138, 255. (12) Oh, S. H.; Eickel, C. C. J. Cutal. 1991, 128, 526. (13) McCabe. R. W.: Wone. C. J. Catal. 1990, 121, 422. (14) Oh, S. H. J. Catul. 1990, 124, 477. (15) Cho, B. K. J. Caral. 1994, 148, 697. (16) Fisher, G. B.; Schmieg, S. J. J. Vac. Sci. Technol. A 1983, 1, 1064.
Permana et al. (17) Oh, S. H.; Fisher, G. B.; Carpenter, J. E.; Goodman, D. W. J. Caral. 1986, 100. 360. (18) Peden, C. H. F.; Goodman, D. W.; Blair, D. S.; Fisher, G. B.; Berlowitz, P. J.; Oh, S. H. J. Phys. Chem. 1988, 92, 1563. (19) Belton, D. N.; Ng, K. Y . S.; DiMaggio, C. L.; Schmieg, S. J. J . Catal., accepted. (20) Schmieg, S. J.; Belton, D. N. Appl. Catal. B 1995, 62, 127
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