Catalytic Control of Air Pollution - American Chemical Society

Chapter 2

Methane Oxidation over Noble Metal Catalysts as Related to Controlling Natural Gas Vehicle Exhaust Emissions

Downloaded by UNIV OF SYDNEY on September 4, 2013 | http://pubs.acs.org Publication Date: June 10, 1992 | doi: 10.1021/bk-1992-0495.ch002

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Oh , P. J. Mitchell , and R. M. Siewert 2

Physical Chemistry Department and Thermosciences Department, General Motors Research Laboratories, Warren,MI48090 Natural gas has considerable potential as an alternative automotive fuel. Methane, the principal hydrocarbon species in natural-gas engine exhaust, has extremely low photochemical reactivity but is a powerful greenhouse gas. Therefore, exhaust emissions of unburned methane from natural-gas vehicles are of particular concern. This laboratory reactor study evaluates noble metal catalysts for their potential in the catalytic removal of methane from natural-gas vehicle exhaust. Temperature run-up experiments show that the methane oxidation activity decreases in the order Pd/Al O > Rh/Al O > Ρt/Al O . Also, for a l l the noble metal catalysts studied, methane conversion can be maximized by controlling the Ο concentration of the feedstream at a point somewhat rich (reducing) of stoichiometry. 2

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In r e c e n t y e a r s n a t u r a l gas has r e c e i v e d i n c r e a s e d a t t e n t i o n as an alternative fuel f o r motor vehicles because of i t s p o t e n t i a l t e c h n i c a l , economic and e n v i r o n m e n t a l advantages. N a t u r a l g a s , which c o n s i s t s p r i m a r i l y o f methane (85-95% o f t h e t o t a l HC), has e x c e l l e n t knock r e s i s t a n c e and i g n i t i o n capability over a wide range o f a i r fuel ratios (1 ). These properties permit n a t u r a l - g a s engines to operate a t h i g h compression r a t i o s and w i t h v e r y f u e l - l e a n m i x t u r e s , r e s u l t i n g i n s u b s t a n t i a l l y h i g h e r f u e l e f f i c i e n c i e s than a r e p o s s i b l e with g a s o l i n e engines. Also, natural gas i s l e s s e x p e n s i v e than g a s o l i n e on an energy basis and i s r e a d i l y a v a i l a b l e from abundant domestic s u p p l i e s ( 2, 3 ) . Environmental b e n e f i t s o f n a t u r a l gas i n c l u d e e x t r e m e l y low p h o t o c h e m i c a l reactivity ( 4 ) , reduced c o l d s t a r t CO e m i s s i o n s (/), and z e r o e v a p o r a t i v e e m i s s i o n s . Furthermore, the low c a r b o n c o n t e n t o f methane would l e a d t o r e d u c e d CO2 e m i s s i o n s (the most common "greenhouse" gas) from natural gas v e h i c l e s . However, methane i t s e l f i s a much more p o w e r f u l greenhouse gas than CO2 ( 3 , 5 ) and t h u s , though n o t c u r r e n t l y r e g u l a t e d , exhaust e m i s s i o n s of unburned methane from n a t u r a l - g a s v e h i c l e s a r e a n t i c i p a t e d t o be of p a r t i c u l a r c o n c e r n i n t h e f u t u r e .

0097-6156/92/0495-0012$06.00/0 © 1992 American Chemical Society In Catalytic Control of Air Pollution; Silver, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Methane Oxidation over Noble Metal Catalysts

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Vehicle emission tests conducted a t G e n e r a l Motors R e s e a r c h L a b o r a t o r i e s w i t h two d i f f e r e n t g a s o l i n e e n g i n e s c o n v e r t e d t o o p e r a t e on n a t u r a l gas (6) showed that poor methane c o n v e r s i o n (2). Because o f t h e wide i g n i t i o n range of n a t u r a l g a s , however, a c t u a l n a t u r a l gas e n g i n e s c a n p o t e n t i a l l y be o p e r a t e d under a v a r i e t y o f a i r - f u e l r a t i o conditions. The c u r r e n t l y favored operating strategies f o r natural gas e n g i n e s include fuel-lean and stoichiometric combustion ( 7 ) . Since catalytic activity i s often a sensitive function of the s t o i c h i o m e t r y of the r e a c t i o n environment ( 12 ) , i t i s e n t i r e l y p o s s i b l e that methane conversion e f f i c i e n c y i n n a t u r a l - g a s engine exhaust may v a r y s u b s t a n t i a l l y with the a i r - f u e l r a t i o . With t h i s possibility i n mind, we conducted laboratory methane oxidation experiments w i t h single-component noble metal c a t a l y s t s o v e r wide ranges o f t e m p e r a t u r e s and feedstream stoichiometries. The r e s u l t s of such l a b o r a t o r y e x p e r i m e n t s a r e o f p r a c t i c a l i n t e r e s t because (1) methane i s t h e p r i n c i p a l hydrocarbon s p e c i e s i n n a t u r a l - g a s engine e x h a u s t , (2) i t s o x i d a t i o n characteristics have n o t been examined under c o n d i t i o n s likely t o be encountered i n natural-gas vehicle e x h a u s t , and (3) n o b l e metals a r e ranked among t h e most a c t i v e c a t a l y s t s f o r methane o x i d a t i o n ( 7 , S ) . Experimental Catalysts. A l l o f the c a t a l y s t s were p r e p a r e d by i n c i p i e n t wetness i m p r e g n a t i o n o f a γ-alumina s u p p o r t (3 mm d i a m e t e r s p h e r e s , 96 m^/g BET a r e a ) w i t h aqueous s o l u t i o n s of the metal salts. The m e t a l loadings, metal salts used i n the p r e p a r a t i o n , and CO/metal and H/metal r a t i o s d e t e r m i n e d from s t a t i c c h e m i s o r p t i o n measurements a r e l i s t e d i n T a b l e 1. The noble m e t a l l o a d i n g s o f t h e c a t a l y s t s were chosen t o p r o v i d e a similar number of a c t i v e m e t a l atoms i n a l l cases. A f t e r i m p r e g n a t i o n t h e c a t a l y s t s were d r i e d i n a i r o v e r n i g h t a t room t e m p e r a t u r e and then calcined i n f l o w i n g a i r a t 500°C f o r 4 h. Such p r o c e d u r e s r e s u l t e d i n t h e d e p o s i t i o n o f t h e n o b l e m e t a l s near t h e p e r i p h e r y o f t h e c a t a l y s t beads. The o b s e r v a t i o n o f a CO/Rh r a t i o i n e x c e s s o f u n i t y f o r Rh/Al2Û3 i n d i c a t e s t h a t a t l e a s t p a r t o f the Rh e x i s t s as i s o l a t e d atoms o r i o n s , f o r m i n g a d i c a r b o n y l s p e c i e s d u r i n g CO c h e m i s o r p t i o n (13,14).

In Catalytic Control of Air Pollution; Silver, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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CATALYTIC CONTROL OF AIR POLLUTION T a b l e 1.

Catalyst

Metal Loading (wt%)

Properties

Noble M e t a l Precursor

Chemisorption H/M CQ/M

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0.20

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H PtCl6

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0.70

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0.16

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PdCl

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0.32

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1.25

0.42

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Rh/Al 0 2


Catalyst

SOURCE:

3

Adapted

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from r e f . 35.

R e a c t o r System and Analytical Methods. The i n t e g r a l f l o w r e a c t o r system used i n t h i s s t u d y was s i m i l a r t o t h a t employed i n a p r e v i o u s study ( 7 5 ) . The reactor was a 2.5 cm o.d. s t a i n l e s s s t e e l tube housed i n an e l e c t r i c f u r n a c e . The f e e d gases were passed downward t h r o u g h l a y e r s of s i l i c o n c a r b i d e p a r t i c l e s and the c a t a l y s t p e l l e t s . The s i l i c o n c a r b i d e l a y e r l o c a t e d upstream of the c a t a l y s t bed s e r v e s as an i n e r t heat transfer medium and also helps establish f u l l y d e v e l o p e d f l o w i n the reactive section. Temperatures were measured with a chromel-alumel thermocouple positioned along the r e a c t o r centerline with i t s t i p located a few m i l l i m e t e r s below the top of the c a t a l y s t bed. A l l the e x p e r i m e n t s r e p o r t e d h e r e were done u s i n g 15 cm^ o f c a t a l y s t and a total feedstream f l o w r a t e o f 13 L/min (STP), y i e l d i n g a space velocity of 52 000 h" . The f e e d s t r e a m c o n t a i n e d 0.2 v o l % CH4, 0.1 v o l % CO i f p r e s e n t , and v a r i a b l e l e v e l s of 0 i n a He background. 1

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The gas stream entering and leaving the r e a c t o r was a n a l y z e d using a V a r i a n 6000 gas chromatograph equipped with a thermal conductivity detector. A single column (0.32 cm d i a m e t e r by 1.5 m length) containing Molecular Sieve 5A was employed, and the c h r o m a t o g r a p h i c s e p a r a t i o n s were c a r r i e d out i s o t h e r m a l l y a t 60°C i n a He c a r r i e r gas. Individual s p e c i e s i n the r e a c t i o n m i x t u r e were identified their elution times and and quantified by comparing integrated commercially-supplied c a l i b r a t i o n areas with t h o s e of gases. activity of the catalysts was The methane oxidation c h a r a c t e r i z e d i n two ways: (1) temperature run-up e x p e r i m e n t s w i t h f i x e d f e e d c o m p o s i t i o n , and (2) v a r i a b l e c o m p o s i t i o n e x p e r i m e n t s a t a f i x e d temperature. S i n c e our primary i n t e r e s t here i s i n e v a l u a t i n g c a t a l y s t s ( r a t h e r than detailed kinetics) f o r p o t e n t i a l automotive a p p l i c a t i o n , we chose t o measure c a t a l y s t performance under c a r e f u l l y c o n t r o l l e d i n l e t c o n d i t i o n s to the catalyst bed. A l l the methane o x i d a t i o n a c t i v i t i e s r e p o r t e d h e r e a r e t h o s e f o r the f r e s h c a t a l y s t s . The r e a c t o r temperature was controlled by a thermocouple p l a c e d a t the o u t s i d e s u r f a c e of the r e a c t o r tube. However, temperature v a l u e s quoted i n t h i s s t u d y ( r e f e r r e d t o as c a t a l y s t temperature) a r e those a c t u a l l y measured j u s t below the top of the c a t a l y s t bed.


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Results O x i d a t i o n A c t i v i t y o f Noble Metal C a t a l y s t s i n CH4-CO-O2 M i x t u r e s . Although our p r i n c i p a l focus i s on the c a t a l y t i c o x i d a t i o n of methane, we f i r s t conducted laboratory reactor e v a l u a t i o n s o f the t h r e e n o b l e m e t a l s P t , Pd, and Rh i n f e e d s t r e a m s t h a t c o n t a i n e d CO i n a d d i t i o n t o methane and 02· We added CO t o t h e f e e d because CO, which i s present i n natural-gas vehicle exhaust i n significant q u a n t i t i e s ( 6 ) , has been shown t o s i g n i f i c a n t l y a f f e c t the o x i d a t i o n a c t i v i t y of noble metal catalysts f o r some h y d r o c a r b o n s (16-18). F u r t h e r m o r e , the p r e s e n c e o f CO i n the f e e d s t r e a m a l l o w s one t o examine the c o n v e r s i o n efficiencies of b o t h methane and CO simultaneously. F i g u r e 1 shows the s t e a d y - s t a t e c o n v e r s i o n s o f CH4 and CO as a f u n c t i o n o f temperature f o r the Pt/Al203» Pd/Al2Û3, and Rh/Al2Û3 catalysts. The e x p e r i m e n t s were conducted i n an o x i d i z i n g f e e d s t r e a m c o n t a i n i n g 0.2 v o l % CH4, 0.1 v o l % CO, and 1 v o l % O2. The CO c o n v e r s i o n over t h e Pt/Al2Û3 and Pd/Al203 c a t a l y s t s i s n e a r 100% a t t e m p e r a t u r e s as low as ~200°C; however, the Rh/Al2Û3 c a t a l y s t was o b s e r v e d t o be much l e s s active f o r CO o x i d a t i o n i n t h i s o x i d i z i n g environment, requiring ~350°C f o r complete CO conversion. In agreement w i t h e a r l i e r literature reports o f e x t r e m e l y low methane o x i d a t i o n r a t e s (10,11,19), h i g h temperatures i n e x c e s s o f 500°C a r e r e q u i r e d f o r 50% c o n v e r s i o n o f the methane over the n o b l e m e t a l s i n the o x i d i z i n g f e e d s t r e a m employed i n F i g u r e 1. The low r e a c t i v i t y of CH4 toward O2 i s a l s o reflected i n t h e absence o f c a t a l y s t l i g h t o f f d u r i n g t h e methane o x i d a t i o n . The methane o x i d a t i o n a c t i v i t y i n the p r e s e n c e of e x c e s s O2 decreases i n the o r d e r Pd/Al203 > Rh/Al2Û3 > Pt/Al2Û3, and t h i s activity ranking i s consistent w i t h the o b s e r v a t i o n s o f F i r t h and H o l l a n d (9) and of Yu Yao (10). I n the o x i d i z i n g f e e d s t r e a m of F i g u r e 1, c a r b o n d i o x i d e was t h e o n l y c a r b o n containing reaction product d e t e c t e d a t temperatures above 200°C (where t h e CO i n t h e f e e d was c o m p l e t e l y c o n v e r t e d ) , i n d i c a t i n g the complete o x i d a t i o n o f the methane i n the f e e d . Complete o x i d a t i o n o f methane was a l s o observed i n CO-free oxidizing f e e d s , as w i l l be d i s c u s s e d l a t e r i n t h i s paper. S i n c e a c t u a l n a t u r a l - g a s v e h i c l e s can p o t e n t i a l l y o p e r a t e over a wide range o f a i r - f u e l r a t i o s , i t i s o f p r a c t i c a l i n t e r e s t t o examine the performance o f the n o b l e m e t a l catalysts under n e t - r e d u c i n g conditions as w e l l . F i g u r e 2 shows s t e a d y - s t a t e CH4 and CO c o n v e r s i o n s as a f u n c t i o n o f temperature i n a reducing feedstream c o n t a i n i n g 0.2 v o l % CH4, 0.1 v o l % CO, and 0.33 v o l % O2. A l l t h r e e noble metals exhibit very similar low-temperature CO oxidation a c t i v i t y , r e a c h i n g n e a r l y 100% CO c o n v e r s i o n a t t e m p e r a t u r e s as low as 200°C. However, the CO c o n v e r s i o n e f f i c i e n c i e s over the n o b l e m e t a l c a t a l y s t s b e g i n t o f a l l below 100% near 450°C and then c o n t i n u e to d e c l i n e w i t h a f u r t h e r i n c r e a s e i n temperature. F o r each o f the n o b l e m e t a l c a t a l y s t s , the temperature r e q u i r e d f o r t h e o n s e t o f the CH4 o x i d a t i o n i n t h e r e d u c i n g f e e d s t r e a m of F i g u r e 2 i s s i m i l a r to t h a t o b s e r v e d i n F i g u r e 1 f o r the o x i d i z i n g f e e d s t r e a m . N o t i c e , however, t h a t once the r e a c t i o n began, the CH4 c o n v e r s i o n i n c r e a s e d much more s h a r p l y w i t h temperature (leveling o f f between 80 and 95% for Τ > 550°C) i n the r e d u c i n g feedstream than i t does i n the o x i d i z i n g feedstream. The o b s e r v a t i o n t h a t the d e c r e a s e i n CO c o n v e r s i o n i s g e n e r a l l y accompanied by an i n c r e a s e i n CH4 c o n v e r s i o n



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CATALYTIC CONTROL OF AIR POLLUTION

Figure 1. Conversions of (A) CO and (B) C H over the alumina-supported noble metal catalysts as a function of temperature in an oxidizing feedstream containing 0.2 vol% C H , 0.1 vol% CO, and 1 vol% 0 . (Reproduced with permission from ref. 35. Copyright 1991 Academic.) 4

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Catalyst Temperature (°C)

Figure 2. Conversions of (A) CO and (B) C H over the alumina-supported noble metal catalysts as a function of temperature in a reducing feedstream containing 0.2 vol% C H , 0.1 vol% CO, and 0.33 vol% 0 . (Reproduced with permission from ref. 35. Copyright 1991 Academic.) 4

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in Figure 2 s t r o n g l y suggests that the l o s s i n CO c o n v e r s i o n e f f i c i e n c y at elevated temperatures i s due p r i m a r i l y t o p a r t i a l o x i d a t i o n o f methane t o CO under t h e o x y g e n - d e f i c i e n t c o n d i t i o n s . I n f a c t , o u r e x p e r i m e n t s w i t h CH4-O2 m i x t u r e s ( i . e . , no CO i n t h e f e e d ) under r e d u c i n g conditions confirmed the formation o f CO as t h e principal carbon-containing partial o x i d a t i o n product o f methane oxidation. T h i s a s p e c t w i l l be d i s c u s s e d l a t e r i n t h e p a p e r . Comparison o f F i g u r e s IB and 2B shows t h a t a t t e m p e r a t u r e s above 550°C t h e CH4 c o n v e r s i o n over each of the noble metal c a t a l y s t s i s higher i n the r e d u c i n g environment than i t i s i n the o x i d i z i n g environment. T h i s suggests t h e p o s s i b i l i t y t h a t t h e CH4 c o n v e r s i o n may be s e n s i t i v e t o t h e s t o i c h i o m e t r y o f t h e gas-phase environment and thus t h e r e may e x i s t an optimum f e e d s t r e a m s t o i c h i o m e t r y a t which a maximum CH4 c o n v e r s i o n occurs. To e x p l o r e t h i s p o s s i b i l i t y , we measured steady-state CH4 and CO conversions at a catalyst temperature o f ~550°C u s i n g f e e d s t r e a m s c o n t a i n i n g 0.2 v o l % CH4, 0.1 v o l % CO, and v a r i a b l e l e v e l s o f O2. A wide range o f f e e d s t r e a m s t o i c h i o m e t r i e s were c o v e r e d by i n c r e a s i n g t h e 0 c o n c e n t r a t i o n s t e p b y - s t e p w h i l e h o l d i n g t h e temperature and t o t a l f l o w r a t e c o n s t a n t . The r e s u l t s of such e x p e r i m e n t s a r e shown i n F i g u r e 3, where t h e CH4 and CO c o n v e r s i o n s a r e p l o t t e d a g a i n s t the 0 c o n c e n t r a t i o n i n the feed. I t c a n be seen i n F i g u r e 3B t h a t f o r each o f t h e n o b l e m e t a l c a t a l y s t s , t h e CH4 c o n v e r i s o n goes through a maximum a t an 0 c o n c e n t r a t i o n somewhat l e s s than i t s s t o i c h i o m e t r i c v a l u e o f 0.45 v o l % ( i . e . , i n a n e t - r e d u c i n g f e e d ) and then d e c l i n e s s h a r p l y as t h e O2 c o n c e n t r a t i o n i s i n c r e a s e d further. The optimum i n l e t oxygen c o n c e n t r a t i o n f o r P t / A l 0 3 and Rh/Al2Û3 lies between 0.40 and 0.42 v o l % O2; however, a maximum CH4 c o n v e r s i o n over Pd/Al2Û3 was o b s e r v e d a t a lower i n l e t O2 c o n c e n t r a t i o n o f ~0.35 v o l % . The CO c o n v e r s i o n s measured d u r i n g t h e same e x p e r i m e n t s a r e p l o t t e d i n F i g u r e 3A. F o r all three noble metal catalysts, t h e CO c o n v e r s i o n increases m o n o t o n i c a l l y w i t h i n c r e a s i n g O2 c o n c e n t r a t i o n i n t h e f e e d , r e a c h i n g an a s y m p t o t i c l e v e l o f 100% a t an O2 c o n c e n t r a t i o n between 0.45 and 0.5 v o l % . Under reducing c o n d i t i o n s , t h e CO c o n v e r s i o n e f f i c i e n c y d e c r e a s e s i n t h e o r d e r P t / A l 2 0 3 > Pd/Al203 > Rh/Al2032

2

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Oxidation A c t i v i t y o f Noble M e t a l C a t a l y s t s i n CH4-O2 M i x t u r e s . A d d i t i o n a l methane o x i d a t i o n e x p e r i m e n t s were c a r r i e d o u t over t h e same a l u m i n a - s u p p o r t e d noble metal c a t a l y s t s w i t h o u t CO i n t h e f e e d . Comparison o f t h e r e s u l t s o f t h e s e e x p e r i m e n t s w i t h those p r e s e n t e d i n t h e p r e v i o u s s e c t i o n a l l o w s one t o examine how t h e CH4 c o n v e r s i o n c h a r a c t e r i s t i c s of the noble metal catalysts a r e a f f e c t e d by t h e p r e s e n c e o f CO i n t h e f e e d . A l s o , e x p e r i m e n t s under c o n d i t i o n s where CO i s absent i n t h e r e a c t a n t gas m i x t u r e p r o v i d e u s e f u l i n s i g h t i n t o the o r i g i n o f t h e CO o b s e r v e d d u r i n g our p r e v i o u s methane o x i d a t i o n experiments i n a reducing environment and t h e t e n d e n c i e s o f t h e v a r i o u s n o b l e m e t a l s t o p a r t i a l l y o x i d i z e methane t o CO. Temperature run-up e x p e r i m e n t s conducted i n an o x i d i z i n g CH4-O2 feedstream (0.2 v o l % CH4, 1 v o l % O2, no CO) r e v e a l e d methane conversion c h a r a c t e r i s t i c s s i m i l a r t o those d e p i c t e d i n F i g u r e IB. S i m i l a r l y , t h e methane c o n v e r s i o n v s . temperature d a t a o b t a i n e d w i t h a CO-free reducing feedstream c o n t a i n i n g 0.2 v o l % CH4 and 0.33 v o l % O2 e x h i b i t c l o s e similarities t o those shown i n F i g u r e 2B. The s i m i l a r i t i e s i n methane o x i d a t i o n f e a t u r e s under r e d u c i n g c o n d i t i o n s between t h e c a s e s with and w i t h o u t CO i n c l u d e (1) t h e onset o f



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Figure 3. Conversions of (A) CO and (B) C H over the alumina-supported noble metal catalysts at a catalyst temperature of 550 * C in feedstreams containing 0.2 vol% C H , 0.1 vol% CO, and variable levels of O (Reproduced with permission from ref. 35. Copyright 1991 Academic.) 4

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r



20


methane o x i d a t i o n a t a comparable temperature f o r each of the n o b l e metals, (2) the same activity ranking (Pd/Al2Û3 > Rh/Al2Û3> Pt/Al2Û3) f o r low methane c o n v e r s i o n s , and (3) s i m i l a r a s y m p t o t i c methane c o n v e r s i o n l e v e l s of ~90% f o r temperatures above 550°C. The observation that the CH4 conversion c h a r a c t e r i s t i c s during a temperature run-up a r e e s s e n t i a l l y unaffected by CO i n the f e e d i s not s u r p r i s i n g i n v i e w of the l a r g e disparity i n their reactivity toward 02· That i s , much h i g h e r temperatures are required f o r methane o x i d a t i o n t h a n f o r CO o x i d a t i o n , so t h a t a l l t h e CO has been c o m p l e t e l y r e a c t e d by the time t h e CH4 c o n v e r s i o n becomes s i g n i f i c a n t (see F i g u r e s 1 and 2 ) . For a l l three noble metal catalysts, methane o x i d a t i o n under r e d u c i n g c o n d i t i o n s produced CO and H2 as the p r i n c i p a l p r o d u c t s of p a r t i a l oxidation. Our o b s e r v a t i o n o f H2 as a r e a c t i o n p r o d u c t d u r i n g methane o x i d a t i o n i s perhaps not s u r p r i s i n g i n view o f t h e report of Frennet ( 20 ) t h a t at elevated t e m p e r a t u r e s methane chemisorption on n o b l e m e t a l s i s accompanied by H2 evolution. Experiments conducted with feedstreams containing no CO were p a r t i c u l a r l y c o n v e n i e n t i n examining the p r o d u c t d i s t r i b u t i o n s and the t e n d e n c i e s of the v a r i o u s n o b l e m e t a l c a t a l y s t s t o form CO d u r i n g methane o x i d a t i o n . T a b l e 2 shows the amount of CH4 consumed and t h e T a b l e 2. P r o d u c t D i s t r i b u t i o n s f o r Methane O x i d a t i o n Under Reducing C o n d i t i o n s a

CH4 Catalysts Pt/Al 0 2

Pd/Al 0 2

Rh/Al 0 2

SOURCE: a

3

3

3

Consumed (ppm)

CO2

Produced (ppm)

CO

Produced (ppm)

H2

Produced (ppm)

1005

820

163

600

1000

800

250

715

1560

1025

560

2700

Reprinted with permissionfromref.

35.

Copyright

1991

Academic.

0 . 2 v o l % CH4 and 0.15 v o l % 0 i n the f e e d , c a t a l y s t temperature = ~520°C, space v e l o c i t y = 52 000 h"* 2

amounts o f CO, CO2» and H2 produced d u r i n g methane o x i d a t i o n under r e d u c i n g c o n d i t i o n s (0.2 v o l % CH4, 0.15 v o l % O2, and no CO i n t h e f e e d ) o v e r the t h r e e n o b l e m e t a l c a t a l y s t s a t a c a t a l y s t temperature of ~520°C. F o r each of the n o b l e m e t a l c a t a l y s t s , t h e r e i s a good c o r r e l a t i o n between the amount of CH4 reacted and the sum of t h e amounts o f CO and CO2 produced d u r i n g the methane o x i d a t i o n ( c a r b o n balance c l o s u r e to w i t h i n 5%), i n d i c a t i n g that CO and CO2 a r e t h e only carbon-containing reaction p r o d u c t s under our e x p e r i m e n t a l


2. OH ET AL.


21


conditions. (The p r e s e n c e of elemental carbon on t h e s u r f a c e i s r u l e d o u t because O2 exposure of the c a t a l y s t f o l l o w i n g s t e a d y - s t a t e r e a c t i o n a t low O2/CH4 r a t i o s d i d n o t produce s i g n i f i c a n t amounts o f CO o r CO2O F o r a l l t h e runs c o n d u c t e d under r e d u c i n g c o n d i t i o n s , i n c l u d i n g those presented i n Table 2, t h e mass b a l a n c e f o r oxygen a g r e e d w i t h i n 10% when CO, CO2, H2, and H2O were assumed t o be t h e only r e a c t i o n products. C o n s i d e r i n g the u n c e r t a i n t y a s s o c i a t e d with the H2 a n a l y s i s ( t y p i c a l p r e c i s i o n = ± 5 % o f t h e r e p o r t e d v a l u e ; our GC column was n o t o p t i m i z e d f o r H2 a n a l y s i s ) , i t appears t h a t no p a r t i a l oxidation products other than CO and H2 a r e formed i n s i g n i f i c a n t q u a n t i t i e s under our r e a c t i o n c o n d i t i o n s . Although trace amounts o f m e t h a n o l , formaldehyde, and f o r m i c a c i d have a l s o been r e p o r t e d as t h e r e a c t i o n p r o d u c t s i n some c a s e s ( 2 7 ) , we d i d n o t pursue t h i s p o i n t i n d e t a i l . The p r o d u c t s e l e c t i v i t y t o CO (i.e., t h e amount o f CO produced d i v i d e d by t h e amount o f CH4 consumed) f o r each n o b l e m e t a l c a n be c a l c u l a t e d from t h e d a t a o f T a b l e 2: 0.16 f o r Ρ ί / Α ^ Ο β , 0.25 f o r Pd/Al203, and 0.36 f o r Ι Ο ι / Α ^ Ο β . These c a l c u l a t e d s e l e c t i v i t i e s i n d i c a t e t h a t t h e tendency t o form t h e p a r t i a l o x i d a t i o n p r o d u c t CO decreases i n the order Rh/Al203 > Pd/Al203 > Pt/Al203» i n agreement with the ranking of the noble metal catalysts f o r CO c o n v e r s i o n o b s e r v e d under t h e r e d u c i n g f e e d s t r e a m c o n d i t i o n s o f F i g u r e 3. I t i s i n t e r e s t i n g t o n o t e t h a t t h e H2/CO r a t i o d i f f e r s s i g n i f i c a n t l y among the n o b l e m e t a l s : ~4 f o r Pt/Al203» ~3 f o r Pd/Al203» and ~5 f o r RI1/AI2O3. These compare w i t h a H2/CO r a t i o of 2 expected f o r the d i s s o c i a t i v e a d s o r p t i o n o f methane l e a d i n g t o t h e c l e a v a g e o f t h e C-H bond, a key s t e p i n methane o x i d a t i o n over noble metals (7,10,11). T h i s i m p l i e s t h a t f o r a l l t h r e e n o b l e m e t a l s , t h e CO g e n e r a t e d d u r i n g the methane o x i d a t i o n r e a c t s more readily w i t h oxygen than t h e H2. In f a c t , t h e h i g h H2/CO r a t i o o b s e r v e d f o r t h e RI1/AI2O3 c a t a l y s t i s c o n s i s t e n t w i t h t h e r e s u l t s o f s e p a r a t e r e a c t o r e x p e r i m e n t s w i t h C0H2-O2 m i x t u r e s ( 2 2 ) , which show that Rh/Al203 i s particularly e f f e c t i v e i n p r e f e r e n t i a l l y o x i d i z i n g CO i n t h e p r e s e n c e o f a l a r g e e x c e s s o f H2. Mechanistic Discussion Under t h e o x i d i z i n g f e e d s t r e a m c o n d i t i o n s considered here, the noble metal surface i s predominantly covered with oxygen and thus t h e c r i t i c a l s t e p i n t h e methane o x i d a t i o n i s t h e d i s s o c i a t i v e a d s o r p t i o n (and subsequent r e a c t i o n ) o f CH4 onto t h e oxygen-covered s u r f a c e (10,11,23-25). The s u p p o r t i n g e v i d e n c e f o r this conclusion i s given below. First, most kinetic s t u d i e s conducted over noble metal c a t a l y s t s i n the presence of excess O2 show t h a t t h e o b s e r v e d r a t e law i s f i r s t o r d e r i n methane and z e r o o r d e r i n oxygen (11,19,24). Second, oxygen a d s o r p t i o n on n o b l e m e t a l s i s f a s t under t h e r e a c t i o n c o n d i t i o n s considered here ( 2 6 ) , whereas methane adsorption i s a slow, a c t i v a t e d process (20). As a r e s u l t of the s u b s t a n t i a l difference i n adsorption strengths (O2 > CH4) , O2 i n h i b i t s t h e CH4 o x i d a t i o n under o x i d i z i n g c o n d i t i o n s by e x c l u d i n g t h e more weakly adsorbed s p e c i e s , CH4, from t h e a c t i v e s i t e s , as o b s e r v e d i n F i g u r e 3B. (The p r e s e n c e o f CO i n t h e f e e d would p r o b a b l y n o t s i g n i f i c a n t l y change the r e l a t i v e s u r f a c e c o n c e n t r a t i o n o f CH4 and O2 under r e a c t i o n c o n d i t i o n s , because CO tends to desorb r a p i d l y from t h e s u r f a c e a t the h i g h temperatures r e q u i r e d f o r t h e methane r e a c t i o n . )



22


As the O2 c o n c e n t r a t i o n i n the feed i s d e c r e a s e d t o l e s s t h a n the s t o i c h i o m e t r i c v a l u e ( w h i l e h o l d i n g the c o n c e n t r a t i o n of the r e d u c i n g agents c o n s t a n t ) , however, CH4 can compete s u c c e s s f u l l y w i t h O2 f o r the a c t i v e s i t e s d e s p i t e i t s weaker adsorption strength. In t h i s case, the adsorption rates for both reactants would become comparable, yielding a high reaction rate and thus high CH4 conversion. With a further decrease i n i n l e t O2 c o n c e n t r a t i o n , insufficient amounts of O2 adsorb on the surface and the CH4 conversion becomes low because of the resulting k i n e t i c and/or s t o i c h i o m e t r i c l i m i t a t i o n s encountered d u r i n g the methane o x i d a t i o n . T h i s e x p l a i n s why the CH4 c o n v e r s i o n goes t h r o u g h a maximum as the i n l e t O2 c o n c e n t r a t i o n i s v a r i e d , as illustrated i n F i g u r e 3B. A s i m i l a r dependence of methane c o n v e r s i o n on exhaust s t o i c h i o m e t r y , including a maximum CH4 c o n v e r s i o n under somewhat richer than s t o i c h i o m e t r i c c o n d i t i o n s , was observed o v e r commercial n o b l e m e t a l catalysts i n a r e c e n t engine-dynamometer study w i t h methane f u e l (27).

The d e t a i l e d mechanism of methane oxidation on n o b l e m e t a l s i s not y e t w e l l u n d e r s t o o d . Methane c h e m i s o r p t i o n and methane-deuterium exchange e x p e r i m e n t s (20,28) have shown that the c h e m i s o r p t i o n o f methane on n o b l e m e t a l s involves d i s s o c i a t i o n t o adsorbed m e t h y l or methylene r a d i c a l s , as a r e s u l t of removal o f hydrogen atoms from the c a r b o n atom. The subsequent interaction of m e t h y l or methylene r a d i c a l s w i t h a d s o r b e d oxygen has been proposed to lead to e i t h e r d i r e c t o x i d a t i o n to CO2 and H2O or the f o r m a t i o n o f chemisorbed formaldehyde v i a methoxide, methyl p e r o x i d e , or methylene o x i d e intermediates (7,23,29,30). Recent s t u d i e s o f formaldehyde o x i d a t i o n on Pt by McCabe and McCready (31), and by L a p i n s k i et a l . ( 3 2 ) provided kinetic and spectroscopic evidence that the oxidation r e a c t i o n i n v o l v e s the dissociation of the adsorbed formaldehyde t o a d s o r b e d CO and adsorbed H atoms. These adsorbed CO and H atoms have been o b s e r v e d t o e i t h e r d e s o r b as CO and H2 o r r e a c t w i t h a d s o r b e d oxygen t o produce CO2 and H2O, depending on the s t o i c h i o m e t r y o f the r e a c t a n t gas m i x t u r e (31,32). Based on the above d i s c u s s i o n , the p r o p o s e d r e a c t i o n mechanism f o r methane o x i d a t i o n can be r e p r e s e n t e d by the f o l l o w i n g p a r a l l e l - c o n s e c u t i v e p r o c e s s . CH (g)

HCHO(g)

4

îi -H CH. (a)

C

>

V

(

a

)

+ 0 or CH -(a)

>

CO(g)

ÎI decomp. HCHO(a)

H (g) 2

îi îi +0 > C0(a) + 2H(a)

C

>

2

\

° 2 ^ + H 0(g) 2

direct

oxidation_

,'

1

The mechanism p r o p o s e d above i s consistent w i t h our o b s e r v a t i o n o f CO, CO2, H2, and H2O as the principal reaction p r o d u c t s o f the methane o x i d a t i o n under r e d u c i n g c o n d i t i o n s . A l t h o u g h not d i r e c t l y measured, material balance considerations indicate that no s i g n i f i c a n t amount o f gaseous formaldehyde was produced d u r i n g the methane o x i d a t i o n o v e r the n o b l e m e t a l c a t a l y s t s . This suggests that under our r e a c t i o n c o n d i t i o n s an adsorbed formaldehyde i n t e r m e d i a t e , once formed, may rapidly decompose t o CO(a) and Η(a) r a t h e r t h a n d e s o r b i n t o the gas phase as formaldehyde m o l e c u l e s .


2. OH ET A L


23

G i v e n the p r e s e n c e o f the r e a c t i o n products, CO, C02> #2» * H2O, i n the r e a c t i o n m i x t u r e , i t i s possible that the p r o d u c t d i s t r i b u t i o n f o r t h e methane o x i d a t i o n may be a f f e c t e d by the w a t e r gas s h i f t e q u i l i b r i u m r e a c t i o n a n (


CO2 + H

Catalytic Control of Air Pollution - American Chemical Society

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