Catalysis Under Transient Conditions - American Chemical Society

5 Nitric Oxide Reduction by Hydrogen over Rhodium

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Using Transient Response Techniques BRUCE J . SAVATSKY and ALEXIS T. B E L L University of California, Department of Chemical Engineering, Berkeley, C A 94720

The catalyzed reduction of nitric oxide over rhodium is of considerable interest in view of extensive research (1-7) showing that catalytic convertors containing rhodium are particularly effective for controlling the emission of nitric oxide from auto mobiles. While it has been established that carbon monoxide and hydrogen are the primary agents participating in NO reduction, little is known thus far concerning the mechanism and kinetics of the reduction process. Studies by Kobylinski and Taylor (1) with Rh/Al2O3 have shown that H2 is a more effective reducing agent than CO, as evidenced by the fact that the temperature required to achieve a given degree of conversion is lower using H2 rather than CO as the reducing agent. It was also demonstrated that even when H2 and CO are combined, H2 is still the preferred reducing agent. A more detailed investigation of NO reduction by H2 has been reported by Yao et al. (6). These authors noted that the global activation energy for NO reduction decreased from 14.7 to 8.8 kcal/ mole as the Rh loading increased from 0.34 to 12.4%. Nitrogen and N2O were observed as the major products with N2O being the dominant product under all conditions studied. It was also noted that the ratio of N2/N2O was not very much affected by either the reactant concentrations or the catalyst temperature. Although not observed directly, the authors proposed that a sur face complex containing two NO molecules and two H atoms could be an important reaction intermediate. More recently, Myers and Bell (8) have investigated the interaction of NO with H2 over Rh/SiO2 using temperature programmed reaction (TPR). Their results suggest that NO dissociation to form Ν and O atoms is the first step of the reaction sequence. Ammonia and H2O are believed to be formed by the stepwise hydrogenation of the atomic species and the small amount of N2 observed is formed by the recombination of Ν atoms. The present work was undertaken to investigate the applicability of transient response techniques [e.g., ref. (9-11)] for characterizing the mechanism and kinetics of NO reduction. Mass spectrometry was used to trace the dynamics of product formation and in situ infrared spectroscopy was used to 0097-6156/82/0178-0105$09.25/0 © 1982 American Chemical Society Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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observe the s t a t e of adsorbed Ν0· Two types of experiment were performed. The f i r s t i n v o l v e d the r e d u c t i o n of preadsorbed NO i n a constant flow of H2. The second type of experiment i n v o l v e d the s u b s t i t u t i o n of ^NO f o r ^NO, e i t h e r during s t e a d y - s t a t e r e d u c t i o n or j u s t p r i o r to the a d d i t i o n of H2 to a f l o w i n g stream of NO. I t w i l l be shown that the data obtained from these experiments can be i n t e r p r e t e d i n terms of a mechanism of NO deduced from evidence presented i n the recent l i t e r a t u r e . Experimental C a t a l y s t . A 4.4% Rh/Si02 c a t a l y s t was used f o r a l l of the work presented here. The c a t a l y s t was prepared by impregnation of Davison 70 s i l i c a g e l w i t h an aqueous s o l u t i o n of RhCl3. The f r e s h l y prepared m a t e r i a l was d r i e d and then reduced i n f l o w i n g H2 at 673K f o r 2 h r . The d i s p e r s i o n of the c a t a l y s t was d e t e r mined to be 25% by H2 chemisorption. Apparatus. F i g u r e 1 shows a schematic of the r e a c t o r . The r e a c t o r body was made of s t a i n l e s s s t e e l f l a n g e s . To e l i m i n a t e the c a t a l y t i c a c t i v i t y of the r e a c t o r , the i n t e r i o r w a l l s were coated w i t h aluminum and then o x i d i z e d to produce an alumina s u r f a c e . Calcium f l u o r i d e windows were mounted i n each h a l f of the body so that an i n f r a r e d beam can be passed through the r e a c t o r . These windows were sealed to the s t a i n l e s s s t e e l flanges by compression between two G r a p h o i l gaskets. The s e a l between the two halves of the r e a c t o r was made through a copper gasket, gold p l a t e d to e l i m i n a t e i t s c a t a l y t i c a c t i v i t y . The c a t a l y s t , a 15 mm diameter d i s k , weighing 51 mg, was held i n place between two aluminum r i n g s . The assembled r e a c t o r i s compact and has a dead volume of only 1.6 cm^. The r e a c t o r was heated by two 200W d i s k heaters (Thermal C i r c u i t s , I n c . ) , placed over the c i r c u l a r f a c e s , and the c a t a l y s t temperature was measured w i t h a 1/16" s t a i n l e s s - s t e e l sheathed thermocouple, fed through a port (not shown) i n the r e a c t o r body. The r e a c t o r was connected to the balance of the apparatus as shown i n F i g . 2. One of two premixed gas streams could be fed to the r e a c t o r . By s w i t c h i n g the i n d i c a t e d v a l v e , a step change i n e i t h e r r e a c t a n t or isotope c o n c e n t r a t i o n could be achieved. The e f f l u e n t from the r e a c t o r was sampled through a d i f f e r e n t i a l l y pumped i n l e t system i n t o an EAI 250B quadrupole mass spectrometer. The s i g n a l from the mass spectrometer was then fed to a micro processor-based data a c q u i s i t i o n system. This u n i t was designed to monitor as many as ten p r e s e l e c t e d mass peaks, as w e l l as the c a t a l y s t temperature and the gas f l o w r a t e . The time r e q u i r e d f o r the c o l l e c t i o n of one data set depended on the number of mass peaks tracked and the d w e l l time on each peak. The s h o r t e s t c y c l e time used i n the present work was 0.5 s, d u r i n g which the i n t e n s i t i e s of four mass peaks were read, together w i t h the c a t a l y s t tempera ture and the gas f l o w r a t e .

Bell and Hegedus; Catalysis Under Transient Conditions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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Transmission i n f r a r e d s p e c t r a of species adsorbed on the c a t a l y s t were taken w i t h a D i g i l a b FTS-10M F o u r i e r - t r a n s f o r m i n f r a r e d spectrometer, u s i n g a r e s o l u t i o n of 4 cm~l. To improve the s i g n a l - t o - n o i s e r a t i o , between 10 and 100 i n t e r f e r o g r a m s were co-added. Spectra of the c a t a l y s t taken f o l l o w i n g r e d u c t i o n i n H2 were s u b t r a c t e d from s p e c t r a taken i n the presence of NO to e l i m i n a t e the spectrum of the support. Because of the very short o p t i c a l path through the gas i n the r e a c t o r and the low NO p a r t i a l pressures used i n these s t u d i e s , the spectrum of gasphase NO was extremely weak and d i d not i n t e r f e r e w i t h the o b s e r v a t i o n of the spectrum of adsorbed s p e c i e s . Results Reaction of Preadsorbed NO. These experiments were c a r r i e d out u s i n g the f o l l o w i n g procedure. P r i o r to each experiment, the c a t a l y s t was reduced i n H2 f o r 5 min a t 423 K. The c a t a l y s t was then exposed t o a 0.28% NO/Ar mixture f o r 5 to 60 s. I t was noted that NO a d s o r p t i o n was accompanied by a p a r t i a l r e d u c t i o n of the adsorbing NO w i t h adsorbed H2 l e f t on the c a t a l y s t s u r f a c e from the p e r i o d of r e d u c t i o n . I n f r a r e d s p e c t r a taken a t the end of the NO a d s o r p t i o n p e r i o d showed an i n t e n s e band at 1660-1680 cm""l, the p o s i t i o n of the band s h i f t i n g upscale w i t h the d u r a t i o n of adsorpt i o n , and a much weaker band a t 1830 cm*~l. Based on previous s t u d i e s of NO a d s o r p t i o n on supported Rh c a t a l y s t s (12-14) and analogy w i t h the s p e c t r a of Rh n i t r o s y l s (15-17), the band a t 1660-1680 cm"^ can be assigned to a N0 ^~ s t r u c t u r e and the band at 1830 cm"^ t o a N0 s t r u c t u r e . The r e d u c t i o n of adsorbed NO was i n i t i a t e d by s u b s t i t u t i n g a 10% H^/Ar mixture f o r the NO/ Ar mixture. While the formation of products began immediately, no change i n the c a t a l y s t temperature was observed during the course of the r e a c t i o n . Figure 3 i l l u s t r a t e s a t y p i c a l s e r i e s of product responses. N i t r o g e n and N2O were formed immediately upon contact of the c a t a l y s t w i t h H2 and the maximum i n the s i g n a l s f o r these products was observed i n about 1.2 s. The hydrogen-containing products, NH3 and H2O, a l s o appeared as sharp peaks, but the maximum i n the s i g n a l s f o r these products occurred a t 2.5 s. Changes i n the experimental c o n d i t i o n s d i d not a l t e r the q u a l i t a t i v e f e a t u r e s of the responses. I n a l l cases the peaks f o r N2 and N2O occurred simultaneously and preceded the peaks f o r NH3 and H2O. I t was f u r t h e r observed that the r e a c t i o n temperature, d u r a t i o n of NO exposure, and the H2 p a r t i a l pressure during NO r e d u c t i o n a f f e c t e d the i n t e n s i t i e s of the product peaks and the time delay between the N2/N2O peaks and the NH3/H2O peaks. To a much l e s s e r degree, the r e a c t i o n c o n d i t i o n s a l s o a f f e c t e d the time a t which the N2 and N2O s i g n a l s reached a maximum. a

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Figure 3. The transient responses for N , N 0, H O and NH , during the reduc tion of preadsorbed NO. Before reaction, NO was adsorbed for 15 s: P = 1.0 X 10' atm, P = 2.8 χ 10 atm, and Τ = 423 Κ. 2

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F i g u r e 4 shows t h e e f f e c t s o f NO e x p o s u r e t i m e on t h e a b s o r b a n c e o f t h e band a t 1660-1680 c m ~ l , o b s e r v e d p r i o r t o r e a c t i o n , and t h e maximum i n t e n s i t i e s o f t h e NH3, N 2 , N2O, a n d H2O s i g n a l s , observed during r e a c t i o n . The a b s o r b a n c e o f t h e i n f r a r e d band p r o v i d e s a measure o f t h e q u a n t i t y o f NO a d s o r b e d on t h e Rh s u r f a c e . As may be s e e n , t h e c o v e r a g e by a d s o r b e d NO i n c r e a s e s r a p i d l y f o r NO e x p o s u r e s up t o 15 s and t h e n l e v e l s o f f a t a p l a t e a u . The p r o d u c t maxima show a s i m i l a r t r e n d , b u t f o r NO e x p o s u r e s g r e a t e r t h a n 20 s, t h e s i g n a l s f o r H2O, NH3, and N2 show a s l i g h t d e c l i n e . A s s u m i n g t h a t t h e maximum s i g n a l i n t e n s i t y f o r e a c h p r o d u c t i s p r o p o r t i o n a l t o t h e t o t a l amount o f p r o d u c t f o r m e d , t h e r e s u l t s p r e s e n t e d i n F i g . 4 i n d i c a t e t h a t f o r NO e x p o s u r e t i m e s g r e a t e r t h a n 20 s t h e amount o f a d s o r b e d NO u n d e r g o i n g r e d u c t i o n d e c r e a s e s s l i g h t l y w i t h i n c r e a s i n g NO c o v e r a g e . Such a t r e n d s u g g e s t s t h a t a t h i g h NO c o v e r a g e s , a f r a c t i o n o f t h e a d s o r b e d NO d e s o r b s a n d , h e n c e , i s n o t r e d u c e d . The t i m e o f e x p o s u r e t o NO h a d no e f f e c t on t h e t i m e r e q u i r e d f o r t h e N2 and N2O s i g n a l s t o r e a c h a maximum d u r i n g NO r e d u c t i o n . However, t h e NO e x p o s u r e t i m e d i d a f f e c t t h e t i m e a t w h i c h t h e NH3 and H2O s i g n a l s a t t a i n e d a maximum. As shown i n F i g . 5, t h e d e l a y b e t w e e n t h e maximum f o r t h e N2 peak a n d t h e maximum f o r t h e NH3 peak was t h e same a s t h e d e l a y b e t w e e n t h e N2 peak and t h e H2O peak. I n b o t h c a s e s , as t h e exposure time i n c r e a s e s , t h e d e l a y i n c r e a s e s f r o m 0 t o 2.5 s and t h e n l e v e l s o f f f o r NO e x p o s u r e s o f more t h a n 20 s. R e s u l t s s i m i l a r t o t h o s e shown i n F i g s . 4 and 5 were a l s o o b t a i n e d a t o t h e r temperatures. I n c r e a s i n g t h e temperature from 398 t o 473 Κ h a d two p r i n c i p a l e f f e c t s . The f i r s t was t o p r o d u c e n a r r o w e r b u t more i n t e n s e p r o d u c t p e a k s . The s e c o n d e f f e c t was t o r e d u c e t h e d e l a y between t h e N2 peak and e i t h e r t h e NH3 o r H2O p e a k . Thus, f o r e x a m p l e , f o r NO e x p o s u r e t i m e s o f g r e a t e r t h a n 20 s, t h e d e l a y d e c r e a s e d f r o m 7.5 s a t 398 Κ t o 0.5 s a t 473 K. The e f f e c t s o f t h e H2 p a r t i a l p r e s s u r e , u s e d t o r e d u c e t h e a d s o r b e d NO, on t h e maximum p r o d u c t s i g n a l i n t e n s i t i e s and t h e t i m e s a t w h i c h t h e p r o d u c t p e a k s a p p e a r e d was e x p l o r e d f o r a c o n s t a n t NO e x p o s u r e t i m e o f 35 s and a t e m p e r a t u r e o f 423 K. The r e s u l t s i n F i g . 6 show t h a t t h e maximum i n t e n s i t y o f e a c h p r o d u c t s i g n a l i n c r e a s e s s l i g h t l y a s t h e p a r t i a l p r e s s u r e o f H2 i s i n c r e a s e d . B o t h t h e t i m e a t w h i c h N2 and N2O s i g n a l s r e a c h a maximum and t h e d e l a y o f t h e NH3 and H2O s i g n a l s a r e a f f e c t e d b y t h e H2 p a r t i a l p r e s s u r e . F i g u r e 7 show t h a t t h e t i m e f o r maximum p r o d u c t i o n o f N2 and N2O d e c r e a s e s f r o m 3 t o 1.5 s a s t h e H2 p a r t i a l p r e s s u r e i n c r e a s e s f r o m 0.01 t o 0.1 atm a n d F i g . 8 shows t h a t t h e t i m e d e l a y f o r t h e maximum p r o d u c t i o n o f NH3 and H2O d e c r e a s e s f r o m 5.5 t o 3.0 s. I s o t o p i c T r a c i n g w i t h ^NO. To t r a c e o u t t h e d y n a m i c s o f n i t r o g e n i n c o r p o r a t i o n i n t o t h e p r o d u c t s u n d e r c o n d i t i o n s where t h e s u r f a c e c o v e r a g e by a d s o r b e d NO r e m a i n s c o n s t a n t , e x p e r i m e n t s


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Figure 4. The maximum intensity of the reaction products and the absorbance of the IR band at 1680 cm' , as a function of NO exposure time during the reduction of preadsorbed N0 : P = 1.0 χ 10' atm, P = 2.8 X 10~ atm, and Τ = 423 Κ. 1

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Figure 5. The effect of NO exposure time on the time delay in the maximum production of H 0 and NH relative to N during the reduction of preadsorbed NO: P =ζ10χ 10 atm, P = 2.8 χ 10' atm, and Τ = 423 Κ. 2

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Figure 8. The effect of H partial pressure on the time for the maximum production of N and N 0, during the reduction of preadsorbed NO: P = 2.8 χ 10' atm, Τ = 423 Κ, and t ~ 30 s.

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were performed i n which N 0 was s u b s t i t u t e d f o r N 0 . Because of problems i n r e s o l v i n g products w i t h very n e a r l y i d e n t i c a l molecu l a r weights, only the responses f o r the isotopes of N2 were followed. 1 5

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Two experiments were performed. I n the f i r s t , the c a t a l y s t was i n i t i a l l y exposed t o a ^NO/Ar mixture f o r 60 s a f t e r which t h i s mixture was replaced by a ^NO/I^/Ar m i x t u r e . The responses for N2 and Ν Ν were recorded as f u n c t i o n s of time, but the responses f o r ^2 could not be r e s o l v e d from that f o r N 0 , s i n c e the two s p e c i e s have n e a r l y the same value of m/e. To e s t a b l i s h the N2 response, the experiment was performed a second time but now r e v e r s i n g the order i n which ^ N 0 and ^ N 0 were i n t r o d u c e d . The response f o r the N2 s i g n a l i n t h i s case i s i d e n t i c a l to that f o r * N i n the f i r s t case. F i g u r e 9 i l l u s t r a t e s the p a r t i a l pressure responses f o r N , N N , and N observed f o l l o w i n g i n t r o d u c t i o n of the I ^ - c o n t a i n i n g stream. The N2 response begins immediately but then passes through a maximum as the adsorbed ^ N 0 i s consumed. The l ^ N ^ N response e x h i b i t s an i n d u c t i o n p e r i o d of about 2.5 s, a f t e r which i t a l s o r i s e s r a p i d l y and then passes through a maximum. The N2 response begins a f t e r a 2 s delay and r i s e s m o n o t o n i c a l l y t o a maximum v a l u e i n about 9 s. The second experiment was performed t o determine the responses of N , N N , and ^2 N0/H /Ar mixture was r e p l a c e d by a ^NO/I^/Ar m i x t u r e . This experiment was i n i t i a t e d by exposing the c a t a l y s t to a ^ N 0 / A r mixture f o r 60 s and then r e p l a c i n g t h i s mixture by a ^NO/I^/Ar m i x t u r e . A f t e r 2 min of r e a c t i o n , the NO/H /Ar mixture was r e p l a c e d by a NO/H /Ar m i x t u r e , and the responses f o r N2 and N^^N were f o l l o w e d as f u n c t i o n s of time. For the same reason d i s c u s s e d e a r l i e r , the response of N could not be r e s o l v e d from that f o r N 0 . To de termine the ^2 response, the experiment j u s t d e s c r i b e d was repeated, but the sequence i n which ^NO and ^NO were i n t r o d u c e d was reversed. The N2 response obtained i n t h i s case was used to represent the response f o r the f i r s t case. F i g u r e 10 i l l u s t r a t e s the responses f o r N^N, and following the s u b s t i t u t i o n of N 0 f o r N 0 . The N s i g n a l i s un a f f e c t e d d u r i n g the f i r s t second a f t e r the i s o t o p i c s u b s t i t u t i o n but then d e c l i n e s m o n o t o n i c a l l y to z e r o . I t i s s i g n i f i c a n t t o observe that the shape of the N2 response f o r t > 2 s and the shapes of the N^N and N responses are v i r t u a l l y i d e n t i c a l to those observed i n F i g . 9 . 1 4

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Discussion Two g e n e r a l f e a t u r e s of the r e a c t i o n mechanism can be deduced from the o b s e r v a t i o n s made d u r i n g the r e d u c t i o n of adsorbed NO by H 2 . F i r s t , the c l o s e c o i n c i d e n c e of the N2


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and N 0 peaks suggests that these products share a common r a t e l i m i t i n g s t e p , such as, f o r example, the d i s s o c i a t i o n of adsorbed NO. Second, the delay i n the appearance of the NH3 and H2O peaks r e l a t i v e to the N and N 0 peaks suggests that formation of the former p a i r of products i n v o l v e s adsorbed r a t h e r than gaseous hydrogen. This deduction i s f u r t h e r supported by the manner i n which changes i n NO exposure time, H p a r t i a l pressure, and r e a c t i o n temperature a f f e c t the magnitude of the delay. F i g u r e 11 i l l u s t r a t e s a p o s s i b l e mechanism f o r NO r e d u c t i o n , which i s c o n s i s t e n t w i t h the q u a l i t a t i v e f e a t u r e s of the present r e s u l t s and i n f o r m a t i o n a v a i l a b l e i n the l i t e r a t u r e . A d i s c u s s i o n of the j u s t i f i c a t i o n f o r i n c l u d i n g s p e c i f i c elementary steps w i l l be presented next. The a s s o c i a t i v e a d s o r p t i o n of NO i s supported by s e v e r a l i n f r a r e d s t u d i e s (12-14). Spectra of NO adsorbed on reduced Rh show an i n t e n s e band at 1660-1740 cm*~l and a weaker band at 1830 cm . These f e a t u r e s are a s s o c i a t e d w i t h NO adsorbed as N0° and N0 , r e s p e c t i v e l y . I f the s u r f a c e i s p a r t i a l l y o x i d i z e d , a t h i r d band i s observed at 1910 cm" , a s s o c i a t e d w i t h N0^ . During steady-state r e d u c t i o n of NO by H , only the N0_""form of adsorbed NO i s observed (10). The r e v e r s i b i l i t y of NO a d s o r b t i o n i n t o t h i s s t a t e has been examined by Savatsky and B e l l (14)· Their work shows that at temperatures above 293 K, e q u i l i b r i u m desorpt i o n occurs when a stream of argon i s passed over a R h / S i 0 c a t a l y s t b e a r i n g preadsorbed NO. Evidence f o r d i s s o c i a t i v e chemisorption comes from s e v e r a l sources (8,19-21). TPD s t u d i e s conducted w i t h Rh s i n g l e c r y s t a l s (19-21) suggest that a p o r t i o n of the NO adsorbed at ambient temperatures occurs d i s s o c i a t i v e l y . F u r t h e r d i s s o c i a t i o n i s presumed to occur at e l e v a t e d temperatures s i n c e N and N 0 are observed during TPD at temperatures s l i g h t l y above the t h r e s h o l d f o r NO d e s o r p t i o n . A s i m i l a r behavior has a l s o been observed i n TPD s t u d i e s conducted w i t h R h / S i 0 ( 8 ) . While the exact mechanism of d i s s o c i a t i o n i s not e s t a b l i s h e d by these i n v e s t i g a t i o n s , i t seems p l a u s i b l e to propose that d i s s o c i a t i o n proceeds as i n d i c a t e d by r e a c t i o n 2 i n F i g . 11. Studies by a number of authors (8*22-27) have shown that H2 adsorbs d i s s o c i a t i v e l y on Rh and that t h i s process i s r e v e r s i b l e at the temperatures used i n the present s t u d i e s . As noted e a r l i e r , the atomic hydrogen formed by t h i s means i s b e l i e v e d to be r e s p o n s i b l e f o r the formation of NH3 and H 0. Consequently, these products are assumed to be formed by a sequence of LangmuirHinshelwood steps. While there i s no independent evidence to support t h i s hypothesis f o r the s y n t h e s i s of NH3, recent r e s u l t s reported by T h i e l et a l . (27) i n d i c a t e that the formation of H 0 from H and adsorbed 0-atoms does proceed v i a a two step sequence such as that represented by r e a c t i o n s 6 and 7 i n F i g . 11. 2

2

2

2

a

1

+

2

2

2

2

2

2

2

2


120

C A T A L Y S I S

N0

+ S

—

Na

rr-

(2)

N0

(3)

H

2

+ 2S

(4)

N0

+ N

Q

a

(5)

2N

(6) (7) (8)

0a

0H

N H

(10)

NH

a 2

+

a

H

— •

a

a

+H

n a

•

——

•

0a

a

+ 2S + 2S

2

0H

a

+ S

H 0 + 2S 2

NH

-— ——

+

2

a

C O N D I T I O N S

Q

N0

—

——

T R A N S I E N T

^ 2H

—— • · N

a

+ H

a

Q

Q

H

+

+ H

a

N

(9)

Figure 11.

^-

NO + S

(1)

U N D E R

Q

+ S

NHo

+ S

NH

+ 2S

3

The reaction mechanism proposed for the interpretation of transient response experiments.


5.

S A V A T S K Y

A N D

Nitric

B E L L

Oxide

Reduction

111

The s p e c i f i c a t i o n of r e a c t i o n s 4 and 5 f o r the formation of N 0 and N i s based on the TPD s t u d i e s reported by Castner et a l . (19), Campbell and White (20), B a i r d et a l . (21) and Myers and B e l l ( 8 ) . The work of Myers and B e l l has shown s p e c i f i c a l l y that the formation of N 0 does not occur v i a a R i d e a l - E l e y process. I t appears more l i k e l y that t h i s product i s formed v i a a LangmuirHinshelwood process. To determine whether the mechanism i n F i g . 11 could e x p l a i n the experimental o b s e r v a t i o n reported here, a model of the r e d u c t i o n process was developed and examined. The k i n e t i c s of NO adsorption and r e d u c t i o n of the adsorbed NO by H can be described by eqns. 1 and 2. 2

2

2

2

dP dt

,

_Q_ V

=

£

(Pj-P*) +

p RTM c

(1-e) I v ^ r . ε j

m

r

dâf dt

=

(1)

Σ Vijrj j

(2)

D e f i n i t i o n s f o r the v a r i a b l e s and constants appearing i n eqns. 1 and 2 are given i n the nomenclature s e c t i o n at the end of t h i s paper. The f i r s t of these equations represents a mass balance around the r e a c t o r , assuming that i t operates i n a d i f f e r e n t i a l manner. The second equation i s a species balance w r i t t e n f o r the c a t a l y s t s u r f a c e . The r a t e of elementary r e a c t i o n j i s rep resented by r j , and v ^ j i s the s t o i c h i o m e t r i c c o e f f i c i e n t f o r com ponent i i n r e a c t i o n j . The r e l a t i o n s h i p of r j to the reactant p a r t i a l pressures and surface species coverages are given by expressions of the form - kjP*^

(3)

and r

j

=

k

4

jWn

f o r R i d e a l - E l e y and Langmuir-Hinshelwood steps, r e s p e c t i v e l y . Equations 1 and 2 can be solved n u m e r i c a l l y using an a l g o r i t h m which handles s t i f f d i f f e r e n t i a l equations (28). Two sets of boundary c o n d i t i o n s are r e q u i r e d . For 0 t^o> corresponding to the p e r i o d of N O r e d u c t i o n , the values of P £ and P ^ are changed so that 2 Q

2

(7)

Table I l i s t s the values of the r a t e c o e f f i c i e n t s used to simulate the t r a n s i e n t response experiments shown i n F i g s . 3 through 8. These values were obtained i n the f o l l o w i n g manner (29). S t a r t i n g from a set of i n i t i a l guesses, the values of k were v a r i e d s y s t e m a t i c a l l y t o o b t a i n a f i t between the p r e d i c t e d product responses and those obtained from experiments i n which H was added suddenly t o a flow of NO. These experiments w h i l e not described here were i d e n t i c a l t o that presented i n F i g . 9, w i t h the exception that only 1%0 was used. Because of the l a r g e number of parameters i n the model, only a rough agreement could be achieved between experiment and theory even a f t e r 500 i t e r a t i o n s of the o p t i m i z a t i o n r o u t i n e (30). The parameter values obtained a t t h i s p o i n t were now used to c a l c u l a t e the responses expected during the r e d u c t i o n of adsorbed NO. These computations produced responses s i m i l a r to those observed e x p e r i m e n t a l l y ( i . e . , F i g . 3) but the appearance of the product peaks i n time d i d not c o i n c i d e w i t h those observed. To c o r r e c t f o r t h i s , the values of k^, ky, and kg were adjusted i n an e m p i r i c a l manner. The c r i t e r i a used at t h i s p o i n t were that the p r e d i c t e d depen dences of the delays between the appearance of the NH3 and H 0 peaks and the N and N 0 peaks on the d u r a t i o n of NO a d s o r p t i o n and the H p a r t i a l pressure during r e d u c t i o n agree as c l o s e l y as p o s s i b l e w i t h the dependences found e x p e r i m e n t a l l y and shown i n F i g s . 5 and 8. The r a t e parameters presented i n Table I were used together w i t h the parameter values l i s t e d i n Table I I t o p r e d i c t the product responses during the a d s o r p t i o n of NO on a hydrogen covered Rh surface and the subsequent r e d u c t i o n of the adsorbed 2

2

2

2

2


5.

S A V A T S K Y

A N D

B E L L

Nitric Oxide

Reduction

123

Table I . Rate C o e f f i c i e n t s Used to Simulate the T r a n s i e n t Response Experiments

Step //

1. -1.

k

i

4

1

2.8 χ 1 0 " s " atm" 1.6 s-1 1

1

2·

9.3 χ 1 0 "

s""

3.

2.8 χ 1 0 s " l atm" 2

-3.

1.9 x 1 0

4.

3.7 x Ι Ο

5.

9.9 x 10-1 - l

6.

6.8 s-1

7.

1.1 χ 1 0 l s-1

8.

6.0 s-1

9.

1.4 χ 1 0 s " l

10.

1.8 χ 1 0 s-1

_ 1

- 1

s"

1

1

1

s"" s

3

2


124

C A T A L Y S I S

Table I I

V

T R A N S I E N T

C O N D I T I O N S

Parameters and V a r i a b l e s Used to Simulate the T r a n s i e n t Pvesponse Experiments

Q =

9.55 cm

ε

=

0.96

r =

1.6

cm

3

(STP)/s

J

Pc "

0.8 gm/cm^

\

10"

=

U N D E R

5

mol Rh sites/gm c a t a l y s t


5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

125

Reduction

NO. The same parameters were then used to p r e d i c t the responses for the i s o t o p i c t r a c e r experiments reported i n F i g s . 9 and 10. The r e s u l t s of these c a l c u l a t i o n s are presented below. Figure 12 i s r e p r e s e n t a t i v e of the responses p r e d i c t e d f o r the product p a r t i a l pressures both during the p e r i o d of NO a d s o r p t i o n and the subsequent r e d u c t i o n of adsorbed NO. P r i o r to the a d d i t i o n of NO, the surface i s considered to be at e q u i l i brium w i t h the p a r t i a l pressure of H used to reduce the c a t a l y s t . At time zero the H flow i s terminated and NO i s added to the Ar c a r r i e r . Reaction products appear immediately due to the r e d u c t i o n of NO by adsorbed Η-atoms. A f t e r 30 s of NO exposure, the flow of NO i s terminated and the flow of H i s r e s t o r e d . The i n t r o d u c t i o n of H causes a sudden i n c r e a s e i n the product p a r t i a l pressures, which then pass through a maximum as the adsorbed NO i s consumed. I t i s noted that i n agreement w i t h experimental o b s e r v a t i o n ( i . e . , see F i g s . 3 and 4) the c a l c u l a t i o n s p r e d i c t that NH3 i s the dominant n i t r o g e n - c o n t a i n i n g product and that N and N2O are formed i n s u c c e s s i v e l y lower c o n c e n t r a t i o n s . The curves i n F i g . 12 show f u r t h e r that the production of N 0 and N a t t a i n maxima w i t h i n about 1 to 1.5 s a f t e r the i n t r o d u c t i o n of H but that the production of NH3 and H 0 occurs about 2 s l a t e r . Both of these f e a t u r e s are i n good agreement w i t h the behavior of the experimental data.. _ The Responses f o r θ^, θ^ο> ν shown i n F i g . 12. The value of 6g, which i s i n i t i a l l y 0.925, decreased during the p e r i o d of NO exposure at the same time that the value of 9^Q increases r a p i d l y from zero. I t should be noted that the p r e d i c t e d r a t e of accumulation of adsorbed NO i s q u a l i t a t i v e l y c o n s i s t e n t w i t h the dynamics of the band appearing at 1680 cm""l, a s s o c i a t e d w i t h No *~ shown i n F i g . 4. I t i s seen, though, that w h i l e the experimental r e s u l t s e x h i b i t a short i n d u c t i o n p e r i o d f o l l o w e d by a r a p i d r i s e i n the absorbance of the 1680 cm"" 1 band to a s a t u r a t i o n l e v e l , the p r e d i c t e d curve shows a smooth monotonie i n c r e a s e i n θ^ο· The vacancy coverage, θ , which i s i n i t i a l l y equal to 0.075, r a p i d l y decreases during the i n i t i a l p e r i o d of NO exposure but then very s l o w l y i n c r e a s e s . This behavior__can be a t t r i b u t e d to the f o l l o w i n g f a c t o r s . The f i r s t i s that θ i n e q u i l i b r i u m w i t h 0.10 atm of H i s l a r g e r than θ i n e q u i l i b r i u m w i t h 0.0028 atm of NO. C a l c u l a t i n g the e q u i l i b r i u m constants f o r H and NO ad s o r p t i o n and desorption of these gases, given i n Table I , one 2

2

2

2

2

2

2

2

2

a n d

θ

a r e

a

ν

ν

2

ν

2

2

Ν0

concludes that θ"^ = 0.075 and θ" = 0.2. It i s this difference i n the s t r e n g t h of a d s o r p t i o n which d r i v e s the decrease i n _θ . Figure 13 shows, though, that during the a d s o r p t i o n of NO, θ f a l l s below 0.02 and then g r a d u a l l y climbs back up. This i n crease i s due to formation of a v a r i e t y of surface intermediates formed during the i n i t i a l moments of NO a d s o r p t i o n , which are not r e l e a s e d i n s t a n t a n e o u s l y as r e a c t i o n products. The surface ν

ν

ν


oo

1

Ο H

Π Ο

w

H

&o

w

H

σ

d

C/5

κ! GO

>

> Η

G\

Ο


to

ι—^

--Ι

as ο ο*

ft

χ

Β*

ο

w

r r

Ο

w

>

GO

> < >

GO

H


128

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

coverage by these species reduces the vacancy f r a c t i o n . As time progresses the i n t e r m e d i a t e s a r e consumed, and the surface g r a d u a l l y exposes a g r e a t e r number of vacant s i t e s . Upon i n t r o d u c t i o n of H 2 , f o l l o w i n g NO a d s o r p t i o n , the magnin i t u d e of immediately i n c r e a s e s w h i l e that of θ^ο decreases. The f i r s t of these changes r e f l e c t s the d i f f e r e n c e between the r a t e s of H2 a d s o r p t i o n and consumption i n the r e d u c t i o n of NO. The decrease i n θ^ο occurs f o r two reasons: a) i n h i b i t i o n of the r e a d s o r p t i o n of desorbing NO as a consequence of H2 a d s o r p t i o n ; and b) the consumption of adsorbed NO by r e d u c t i o n . I t should be noted that the d i s s o c i a t i o n of NO, and hence the r a t e of NO r e d u c t i o n i s a c c e l e r a t e d by the c r e a t i o n of vacant s i t e s . The i n c r e a s e i n θ , seen i n F i g . 13, can be a s c r i b e d t o a consumption ν

ί

of chemisorbed NO and the f a c t that θ i s g r e a t e r than θ , as was d i s c u s s e d e a r l i e r . _ _ Figure 14 shows the computed responses f o r θ^, θο> and 9QH» These coverages i n c r e a s e s l o w l y d u r i n g NO a d s o r p t i o n but r i s e r a p i d l y and pass through a__maximum when H2 i s added t o the f l o w . The responses f o r Θ^Η Θ^Η s i m i l a r i n shape t o the response f o r 9 but are s i g n i f i c a n t l y s m a l l e r i n magnitue and, hence, have not been shown in__Fig. 14. I t i s s i g n i f i c a n t to note that the maxima i n 0Q and 6fj occur a t times which c o i n c i d e w i t h the time a t which the r a t e s of N * 2 ° ^ach a maximum. S i m i l a r l y , the maxima i n 9QH d Θ^Η P p e a r a t times n e a r l y i d e n t i c a l t o the times a t which the r a t e s o f 1^0 and NH3 formation reach a maximum. C a l c u l a t i o n s s i m i l a r t o those j u s t d i s c u s s e d were a l s o c a r r i e d out f o r NO exposure times between 5 and 30 s. F i g u r e 15 i l l u s t r a t e s the e f f e c t s of NO exposure time on the p r e d i c t e d maximum i n t e n s i t y of each product peak. The curves appearing i n t h i s f i g u r e may be compared w i t h the experimental r e s u l t s shown i n F i g . 4. I t i s noted that w h i l e the d i s t r i b u t i o n of products p r e d i c t e d f o r each NO exposure time i s q u a l i t a t i v e l y c o n s i s t e n t w i t h that observed e x p e r i m e n t a l l y , the shape of the product peak i n t e n s i t y curves i s not. The experimental data show a r a p i d i n i t i a l i n c r e a s e which i s f o l l o w e d by the attainment of a broad maximum. By c o n t r a s t , the p r e d i c t e d curves show a slow monotonie increase. The e f f e c t of NO exposure time on the time a t which the N2 and N2O s i g n a l s a t t a i n a maximum i s shown i n F i g . 16. I t i s seen that the model of NO r e d u c t i o n p r e d i c t s that N2 formation peaks about 0.5 s a f t e r the peak i n the N2O formation and that the peak times f o r both products d e c l i n e by about 0.5 s as the NO exposure time i s i n c r e a s e d from 5 to 30 s. These trends are i n good agreement w i t h the data. I t should be noted that s i n c e a product a n a l y s i s c o u l d be taken only once every 0.5 s, i t was not p o s s i b l e to determine product peak p o s i t i o n s w i t h an accuracy of b e t t e r than 0.5 s. Consequently, both the p r e d i c t e d d i f f e r e n c e between γ

A N D

γ

a r e

Q H

anc

N

r

2

a n

a

2


Η*

to

a

«·* δ*

Ci

Ο

w

r r

Ο

W

>

>

00

>

ο

ο >


t—^ U> h-*

"S.

S*

%

χ

B" Ο

S

r r

W W

Ο

>

00

> < >

GO

H


132

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

tfj and t N o d the change i n these times w i t h the d u r a t i o n of NO a d s o r p t i o n l i e w i t h i n the l i m i t s of u n c e r t a i n t y f o r the experimental data. The p r e d i c t e d e f f e c t s of NO a d s o r p t i o n time on the time delay between the maxima i n NH3 and N production and the maxima i n H 0 and N production are shown i n F i g . 17. I t i s seen t h a t the p r e d i c t i o n s bound the experimental observations and show the the proper trend w i t h i n c r e a s i n g NO exposure. C a l c u l a t i o n s were a l s o performed to assess the a b i l i t y of the model to p r o p e r l y p r e d i c t the e f f e c t s of H p a r t i a l pressure on the product peak i n t e n s i t i e s and the time at which each peak occurs. Figure 18 shows the p r e d i c t e d e f f e c t of H partial pressure on the product peak i n t e n s i t i e s . In c o n t r a s t to the a n

9

2

2

2

2

2

2

trend observed e x p e r i m e n t a l l y and i l l u s t r a t e d i n F i g . 6, the model p r e d i c t s that i n t e n s i t i e s d e c l i n e w i t h i n c r e a s i n g p a r i t a l pressure. N e v e r t h e l e s s , the model does show that the changes are s m a l l , and i t provides a q u a l i t a t i v e l y c o r r e c t p i c t u r e of the product d i s t r i b u t i o n . F i g u r e 19 shows the p r e d i c t e d and e x p e r i m e n t a l l y observed time at which the formation of N reaches a maximum. As may be seen, the model provides a reasonably good d e s c r i p t i o n of the e x p e r i m e n t a l l y observed t r e n d . The e f f e c t of H2 p a r t i a l pressure on the time delay between the maximum i n the production of H 0 or NH3, and the maximum p r o d u c t i o n of N i s i l l u s t r a t e d i n F i g . 20. Here too, the c a l c u l a t e d curves are i n reasonable agreement w i t h the experimental data and i t i s noted that the model p r e d i c t s the same time delay f o r both NH3 and H 0 under most circumstances. To f u r t h e r t e s t the model, c a l c u l a t i o n s were performed to simulate the i s o t o p i c t r a c e r experiments presented i n F i g s . 9 and 10. I t should be noted that w h i l e the t r a c e r experiments were performed at 438K, the r a t e c o e f f i c i e n t s used i n the model were chosen to f i t the experiments i n which chemisorbed NO was reduced at 423 K. Figures 21 and 22 i l l u s t r a t e the n i t r o g e n p a r t i a l pressure and surface coverage responses p r e d i c t e d f o r an e x p e r i ment i n which 15^0 i s s u b s t i t u t e d f o r ^NO at the same time that H i s added to the NO f l o w . S i m i l a r p l o t s are shown i n F i g s . 23 and 24 f o r an experiment i n which ^NO i s s u b s t i t u t e d f o r ^NO during s t e a d y - s t a t e r e d u c t i o n . Comparison of F i g s . 21 and 23 w i t h F i g s . 9 and 10 shows that the p r e d i c t e d p a r t i a l pressure responses f o r the N isotopes are i n f a i r agreement w i t h those observed e x p e r i m e n t a l l y . The p r i n c i p a l d i f f e r e n c e s are that the experimental ^ N ^ N response i s s i g n i f i c a n t l y more asymmetric than the corresponding theor e t i c a l response and that a l l of the experimental t r a n s i e n t s occur over a s h o r t e r time frame than would be deduced from the t h e o r e t i c a l r e s u l t s . The s t r o n g asymmetry of the e x p e r i m e n t a l l y observed 2

2

2

2

2

2

14 15 N

N

response may

be due to the f a c t that ^ N

and 15

c o n t a i n i n g adspecies

are not randomly mixed but, r a t h e r , e x i s t


N

S A V A T S K Y

A N D

Nitric

B E L L

Oxide

Reduction

Figure 18. The predicted effects of P °n the intensities of H O NH , N , and N 0 at maximum production, during the reduction of preadsorbed NO: P o == 2.8 χ 10 atm; Τ = 423 Κ; and t = 30 s. H2

2

f

S

2

2

N

s

N0


*S

6-

M

\

\

\ \

E

"c7 5

(t NH, •t ) predicted

—

\

• Exptl.

ι

σ

± 4

ro X

Ε

-F 2 1

I

χ Ο

O.OI

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Η.

(atm)

Figure 20. Comparison of the predicted and the observed effects of P on the time delay in the maximum production of H 0 and NH relative to N , during the reduction of preadsorbed NO: P = 2.8 χ 10' atm, Τ — 423 Κ, and t = 30 s. H

2

N0

3

3

2

N0


5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

135

Reduction

Figure 21. Predictions of the partial pressure responses for N , N , and NN following substitution of a feedstream containing NO/Ar by a stream containing NO/H /Ar: P = 8.0 χ W atm, P = 8.0 χ 10 atm, and Τ = 423 Κ. 14

2

15

2

15

14

2

H

2

N0

3


14

15

136

C A T A L Y S I S

P° (atm) P,° A Β

0.080

NO

U N D E R

T R A N S I E N T

C O N D I T I O N S

(atm) P ° (atm) NO 0.992 0.008 0.912

(atm) P,°

A

5

0.008

r

1.0

5.0

0.8-

4.0

0.6-

3

0.4

2.0

0.2

1.0

0

δ

20 t (s) Figure 22. Predictions of the surface coverage responses for ïï , ÏÏ , $ , and ïï following substitution of a feedstream containing NO/Ar by a stream contain1ItN0

15N0

llfN

15

1!tN

:

7 4 λΤS~\

ι τι

/

A

r>

ο η

κ y

m-2

~*

r>

ο η

^ s

ιn-3

ι τ


Ο

ν

5.

S A V A T S K Y

A N D

B E L L

Nitric

Oxide

PR (atm) P°

Reduction

A Β

r

N0

0.912 0.912

0.008 0.008

••-A—·+. 120s

2.0

ε

0.080 0.080

(atm) P ° ( a t m )

(atm) P ° l 5

137

1.5

1.0

0.5

0 20 Figure 23. Predictions of the partial pressure responses for N , N , and NN following substitution of a feedstream containing NO/H /Ar by a feedstream con taining NO/H /Ar: P = 8.0 χ 10' atm, P = 8.0 χ 10~ atm, and Τ = 423 Κ. 14

14

15

2

H2

2

N0

2

15

2

2

3


14

15

138

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

Τ

PSH A Β

(atm) P £ ( a t m ) (atm) P° NO N0 0.912 0.008 0.912 0.008

(atm) P.° 2

0.080 0.080

r

,5

h—A—Η

Β

t (s) Figure 24. Predictions of the responses for Θ , Ô , Θ , and J following substitution of a feedstream containing NO/H /Ar by a feedstream containing NO/H /Ar: P = 8.0 χ 10' atm, P = 8.0 χ 10 atm, and Τ = 423 Κ. 15Ν0

14

15

2

H2

2

1]tN0

15Ν

1JfN

2

N0

3


5.

S A V A T S K Y

A N D

Nitric

B E L L

Oxide

139

Reduction

i n s m a l l patches. Such non-uniform mixing would impede the i n i t i a l formation of the i s o t o p i c a l l y mixed N . The s h o r t e r d u r a t i o n of the e x p e r i m e n t a l l y observed resonses i s not s u r p r i s i n g i n view of the f a c t that the experiments were performed at 438 Κ whereas the c a l c u l a t i o n s were done f o r a temperature of 423 K. 2

Conclusions Transient response experiments have revealed that the forma t i o n of N and N 0 during NO r e d u c t i o n by H over Rh proceeds without the i n t e r v e n t i o n of H . By c o n t r a s t , the formation of NH3 and H 0 i n v o l v e s the r e a c t i o n s of d i s s o c i a t i v e l y chemisorbed H w i t h Ν and 0 atoms, r e s p e c t i v e l y . The r e s u l t s obtained from experiments i n v o l v i n g the r e d u c t i o n of adsorbed NO and i s o t o p i c s u b s t i t u t i o n of ^NO f o r ^NO can be i n t e r p r e t e d on the b a s i s of the r e a c t i o n mechanism presented i n F i g . 11. Key elements of t h i s mechanism are that NO i s adsorbed r e v e r s i b l y i n t o a molecu l a r s t a t e , that r e d u c t i o n i s i n i t i a t e d by the d i s s o c i a t i o n of m o l e c u l a r l y adsorbed NO, and that a l l products are formed v i a Langmuir-Hinshelwood process. 2

2

2

2

2

2

Acknowledgment This work was supported by a grant from the NSF (CPE7826352). Nomenclature kj ^

-

P?

-

)?l

-

p

i

Rate c o e f f i c i e n t f o r r e a c t i o n j . Moles of Rh surface s i t e s per gram of c a t a l y s t [mol/gm]. P a r t i a l pressure of component i a t the r e a c t o r i n l e t [ atm ]. P a r t i a l pressure of component i a t the r e a c t o r o u t l e t [atm]. Average p a r t i a l pressure of component i i n the r e a c t o r , P

Q rj R t Τ V _ε 0 v-^j P r

m

c

Ξ

(

P

+

P

)

/

2

[ a t m ]

i i i - T o t a l v o l u m e t r i c flow r a t e [cm^/s]. - Rate of r e a c t i o n j [ s " ] . - Gas constant [atm · cm^/K · m o l ] . - Time [ s ] . - Temperature [ K ] . - Reactor volume [cm^]. - Reactor v o i d f r a c t i o n . - Surface coverage. - S t o i c h i o m e t r i c c o e f f i c i e n t f o r component i i n r e a c t i o n j . - C a t a l y s t d e n s i t y [gm/cm ]. 1

J


140

C A T A L Y S I S

U N D E R

T R A N S I E N T

C O N D I T I O N S

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

Kobylinski, T. P. and Taylor, B. W., J. Catal. 33, 376 (1974). Klimisch, R. J and Larson, J. G., Eds., "The Catalytic Chemistry of Nitrogen Oxides", Plenum Press, New York, 1975. Shelef, Μ., Cat. Rev.-Sci. Eng. 11, 1 (1975). Schlatter, J. C. and Taylor, K. C., J. Catal. 49, 42 (1977). Hegedus, L. L., Summers, J. C., Schlatter, J. C., and Baron, K., J. Catal. 56, 321 (1979). Yao, H. C., Yu Yao, Y. F., and Otto, Κ., J. Catal. 56, 21 (1979). Summers, J. C. and Baron, Κ., J. Catal. 57, 380 (1979). Myers, E. C. and Bell, A. T., subbmitted to J. Catal. Kobayashi, H. and Kobayashi, Μ., Catal. Rv.-Sci. Eng., 10, 139 (1974). Bennett, C. O., Catal. Rev.-Sci. Eng. 13, 121 (1976). Tamaru, K., "Dynamic Heterogeneous Catalysis", Academic Press, New York, 1978. Arai, H. and Tominaga, H., J. Catal. 43, 131 (1976). Solymosi, F. and Sarkany, J., Appl. Surface Sci. 3, 68 (1979). Savatsky, B. J. and Bell, A. T., submitted to J. Catal. Connely, N. G., Inorg. Chim. Acta Rev., 47 (1972). Nappier, Jr., T. E., Meek, D. W., Kirchner, R. H. and Ibers, J. Α., J. Am. Chem. Soc. 95, 4194 (1973). Goldberg, S. Z., Kubiak, C., Meyer, C. D. and Eisenberg, R., Inorg. Chem. 14, 1650 (1975). Hecker, W. C. and Bell, A. T., unpublished results. Castner, D. G., Sexton, B. A. and Somorjai, G. Α., Surface Sci. 71, 519 (1978). Campbell, C. T. and White, J. Μ., Appl. Surface Sci. 1, 347 (1978). Baird, R. J., Ku, R. C. and Wynblatt, P., Surface Sci. 97, 346 (1980). Mimeault, V. J. and Hansen, R. S., J. Phys. Chem. 45, 2240 (1966). Zakumbaeva, G. D. and Omaskev, Kh. G., Kin. Katal. 18, 450 (1977). Edwards, S. M., Gasser, R. P. Η., Green, D. P., Hawkins, D. S. and Stevens, A. J., Surface Sci. 72, 213 (1978). Kawasaki, Κ., Shibata, Μ., Miki, Η., and Kioka, T., Surface Sci. 81, 370 (1979). Yates, Jr., J. T., Thiel, P. A. and Weinberg, W. Η., Surface Sci. 84, 427 (1979). Thiel, P. A. Yates, Jr., J. T., and Weinberg, W Η., Surface Sci. 90, 121 (1979).


5.

S A V A T S K Y

A N D B E L L

Nitric Oxide

Reduction

28.

141

Hindmarsh, Α., "Gear: Ordinary Differential Equation Solver", UDID-3001, Rev. 1, August 20, 1972, Computer Center Library, University of California, Berkeley, CA. 29. Savatsky, B. J., Ph.D. Thesis, Department of Chemical Engineering, University of California, Berkeley, CA, 1981. 30. Nelder, J. A. and Mead, R., Computer J. 7, 308 (1964).

Received August 6, 1981.


Catalysis Under Transient Conditions - American Chemical Society

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