Catalytic Activation of Carbon Dioxide - American Chemical Society

Chapter

10

Effect of Potassium on the Hydrogenation of Carbon Monoxide and Carbon Dioxide Over Supported Rh Catalysts

Downloaded by UNIV OF ARIZONA on November 30, 2012 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch010

S. D. Worley and C. H. Dai Department of Chemistry, Auburn University, Auburn, AL 36849

The reactions of hydrogen with carbon monoxide and carbon dioxide over Rh/Al O and Rh/TiO films, some of which contained potassium as an additive, have been investigated. For the CO hydrogenation reaction the presence of potassium caused the dissociation or desorption of the gem dicarbonyl, linear CO, and carbonyl hydride species, while it led to an enhancement of the bridged carbonyl species. For Rh/TiO films the hydrogenation of CO produced acetone and acetaldehyde as oxygenated products; the bridged carbonyl species was the likely precursor of these products. For the CO hydrogenation reaction the presence of potassium caused the dissociation or desorption of all CO species, and oxygenated products were not produced. Potassium significantly poisoned both reactions toward the production of methane. 2

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Considerable e f f o r t has been expended here toward the investigation of the CO and CO2 hydrogénation reactions over supported Rh catalysts (1-4). Although for Rh/X (X = A 1 0 and T i 0 ) at low pressure (50-100 Torr) and temperature (423-473 K) the predominant product for both of these reactions i s methane (1-4), there are means of a l t e r i n g the product distribution toward higher hydrocarbons and/or oxygenated products such as methanol. In general i t i s believed that methane and higher hydrocarbons are produced from dissociated CO or CO2 on supported t r a n s i t i o n metals, while undissociated CO i s thought to be a precursor to oxygenated products (_5). Basic support materials such as ZnO, MgO, Ι ^ Ο β , and Zr02 (6,7) as well as higher pressure (over 1 atm) (8,9) seem to s h i f t the product s e l e c t i v i t y toward oxygenates; however, AI2O3 and T1O2 as support materials can also lead to oxygenated products under special conditions. For example, Goodwin and coworkers (_5) have shown recently that added potassium causes the s e l e c t i v i t y for hydrogénation of CO over 3% Rh/Ti02 to s h i f t toward oxygenated products with acetaldehyde and acetone being 2

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0097-6156/88/0363-0133S06.00/0 © 1988 American Chemical Society

In Catalytic Activation of Carbon Dioxide; Ayers, W.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

134

CATALYTIC ACTIVATION OF CARBON DIOXIDE

present i n s i g n i f i c a n t q u a n t i t i e s . There has been c o n s i d e r a b l e recent interest i n t h e e f f e c t s o f a l k a l i m e t a l s on c a t a l y t i c r e a c t i o n s o v e r s i n g l e c r y s t a l s (10) and o v e r s u p p o r t e d t r a n s i t i o n metals (5,11). T h i s paper w i l l report the r e s u l t s o f recent work i n these l a b o r a t o r i e s c o n c e r n i n g t h e e f f e c t s o f p o t a s s i u m on t h e CO and C0£ hydrogénation r e a c t i o n s over s u p p o r t e d Rh catalytic films.


Experimental The Rh/Al203 and Rh/Ti02 c a t a l y s t s used i n t h i s study were p r e p a r e d i n a manner s i m i l a r t o those s t u d i e d p r e v i o u s l y here (1-4,12). Briefly, acetone/water solutions c o n t a i n i n g a p p r o p r i a t e amounts o f RhCl3*3H20, KC1, and alumina (Degussa Aluminum Oxide C, 100 m g ~ ) o r t i t a n i a (Degussa T i t a n i u m D i o x i d e P25, 50 m g " ) were sprayed u s i n g a s p e c i a l l y d e s i g n e d a t o m i z e r onto a h e a t e d 20 mm CaF2 i n f r a r e d window. Evaporation o f the solvents l e f t a uniform thin f i l m ( t y p i c a l l y 4.3 mg cm" ) o f t h e mixed solid m a t e r i a l s adhered t o t h e window. The window c o n t a i n i n g t h e f i l m was mounted inside an i n f r a r e d cell reactor (2-4) which was evacuated o v e r n i g h t . The sample f i l m was then evacuated a t 470 Κ f o r 1 h r , reduced a t 480 Κ by 85 T o r r doses o f hydrogen f o r 5, 5, 10, and 20 min p e r i o d s (each p e r i o d f o l l o w e d by e v a c u a t i o n t o £ a . 10~5 T o r r ) , and then evacuated f o r an a d d i t i o n a l hour a t 480 Κ t o a base p r e s s u r e o f 10"^ T o r r . F o r a t y p i c a l experiment the c e l l was then exposed t o a C0:H2 o r C02:H2 m i x t u r e (1:4) at C£. 82.5 T o r r t o t a l p r e s s u r e and h e a t e d rapidly t o some prescribed temperature. Methane gas and s u r f a c e i n t e r m e d i a t e formations during the r e a c t i o n s were monitored by infrared s p e c t r o s c o p y ( P e r k i n Elmer 983 w i t h d a t a system) ( 1 - 4 ) ; p r o d u c t d i s t r i b u t i o n s a t t h e end o f t h e experiment were measured by gas chromatograpy ( C a r l e 400). P r e s s u r e measurements were made w i t h an MKS B a r a t r o n c a p a c i t a n c e manometer (±0.01 T o r r ) . 2

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R e s u l t s and D i s c u s s i o n CO

Hydrogénation

The i n t e r a c t i o n o f CO w i t h s u p p o r t e d Rh c a t a l y s t s has been w e l l c h a r a c t e r i z e d by i n f r a r e d s p e c t r o s c o p y (13). The p r i m a r y s u r f a c e s p e c i e s o b t a i n e d a r e shown below. The "gem d i c a r b o n y l " s p e c i e s ( I ) e x h i b i t s two sharp i n f r a r e d bands a t ca_. 2030 and 2100 cm"Ι

OC

CO

Ο Ç

Rh

Rh

I

II

0

II Rn

Rh

III

which do n o t g e n e r a l l y s h i f t i n wavenumber w i t h changing s u r f a c e coverage. T h i s i n d i c a t e s t h a t d i p o l a r c o u p l i n g o f nearby adsorbed CO m o l e c u l e s i s minimal f o r supported Rh c o n t a i n i n g p r e d o m i n a n t l y this species. F o r Rh/X (X = AI2O3, T i 0 , or S i 0 ) catalysts 2

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10.

WORLEY AND DAI

Effect of Potassium on

Hydrogénation

135

containing less than 1% by weight Rh, species I i s the only species detected by infrared. The facts that the infrared bands for species I do not s h i f t with coverage and that no other species are detected for catalysts containing low Rh loading have led some workers to suggest that species I refers to Rh in a highly dispersed state, possibly even isolated Rh atoms (12,13). Work in several laboratories (12-14) has established that Rh in species I exists in the +1 oxidation state; this probably also contributes to i t s tendency to remain highly dispersed. Recent work has shown that the dispersion of Rh ions in species I may be a c t u a l l y caused by the presence of adsorbed CO (15,16). Figures 1 and 2 i l l u s t r a t e the effect of potassium on species I for a 0.5% Rh/Al203 catalyst f i l m . In this experiment the two catalysts were treated identically. They were held successively at temperatures of 300, 320, 380, 430 and 460 Κ for 30 min at each temperature in the presence of 1 χ ΙΟ"-* Torr of CO. The infrared band i n t e n s i t i e s were very similar through 380 K, probably indicating comparable CO coverages for the two samples. However, after 30 min at 430 Κ species I disappeared for the sample containing potassium (K:Rh = 2:1), but this phenomenon did not occur u n t i l after 30 min heating at 460 Κ for the sample which did not contain potassium. Clearly potassium did not s i g n i f i c a n t l y block species I s i t e s ; rather i t functioned to aid either CO bond dissociation or CO desorption from the surface, most probably through an electronic effect. The potassium may well be located on the support in close proximity to Rh sites. Some workers have observed K/CO interactions on single crystals ( 17). Such interactions can give r i s e to low frequency CO infrared bands (1400-1800 c m ) . A comparison of Figs. 1 and 2 indicates that such interactions do not occur appreciably for 0.5% RI1/AI2O3 catalysts which suggests that the K/CO interaction which causes enhanced d i s s o c i a t i o n or desorption of CO on or from Rh in species I may be one of long range. +

-1

When Rh/X catalysts containing higher Rh loadings (>1%) are employed, in addition to species I generally two other CO species can be detected by infrared. Species I I , the " l i n e a r " species, contains one CO molecule adsorbed on a Rh atom, while species I I I , the "bridged carbonyl" species, contains CO bridged across two Rh atoms (13). The infrared bands for these two species s h i f t to higher wavenumber as the CO surface coverage i s increased leading most workers to suggest that these two species correspond to clusters of Rh atoms rather than highly dispersed ions as for species I. Figure 3 shows a comparison of the behavior of the CO hydrogénation reaction over 2.2% Rh/Ti02 catalyst films with and without the presence of potassium. In spectrum 3a for a sample containing no potassium the four infrared bands normally observed are in evidence; the 2100 and 2030 cm"* bands correspond to species I, while the 2072 cm" band and the broad band near 1900 cm" can be assigned to species II and I I I , respectively. Upon heating at 440 Κ for 5.5 hr (spectrum 3b), species I and II disappeared, a new band was detected at 2047 cm" , the species III band declined in intensity, and bands corresponding to gas phase methane (3015, 1304 cm" ) and carbon dioxide (2349 cm" ) were produced. The band at 2047 cm" corresponds to a carbonyl hydride species rather than species I I . The carbonyl hydride 1

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CATALYTIC ACTIVATION OF CARBON DIOXIDE


136

Figure 1. Infrared spectra for the interaction of CO with a 0.5% Rh/Al203 f i l m (4.0 mg cm" ) at a background pressure of 1 χ 10~3 Torr as a function of temperature. 2


WORLEY AND DAI

Effect of Potassium on Hydrogénation

0.5% R h - K / A ï 2 0 3 K/Rh=2 CO: 1 Χ 10""3


0.1

ui ο ζ < m ce Ο

Catalytic Activation of Carbon Dioxide - American Chemical Society

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