Kinetics and Mechanism of Carbon Monoxide Oxidation on Platinum

Downloaded by NORTH CAROLINA STATE UNIV on October 12, 2012 | http://pubs.acs.org Publication Date: January 23, 1992 | doi: 10.1021/bk-1992-0482.ch004

Chapter 4

Kinetics and Mechanism of Carbon Monoxide Oxidation on Platinum, Palladium, and Rhodium Foils 1

George W. Coulston and Gary L. Haller Department of Chemical Engineering, Yale University, New Haven, CT 06520 The kinetics of CO oxidation have been measured on Pt, Pd and Rh, as well as the heat of adsorption of CO, ΔΗ O, under reaction conditions. It is observed that a linear correlation exists between the activation energy for the surface reaction, E , and ΔΗ O (a Polanyi relation). It is C

SR

C

proposed that ΔΗ O is an indirect measure of the ability of chemisorbed CO to extract strongly bound Ο atoms from the surface. C

We have been interested in the dynamics of CO oxidation on Pt and have investigated this system in some detail via the analysis of the infrared emission from the desorbed product CO2 (1). Very recently we have extended our study of CO oxidation to Pd (2,3,4) and Rh (3,4) and improved our CO oxidation on Pt analysis as a result of increased spectral resolution (3,4). As a necessary preliminary set of experiments, we also investigated the kinetics of CO oxidation on Pd, Rh and Pt. As a result of signal to noise constraints, our primary objective was to determine conditions of CO/O2 ratio and surface temperature that would provide the maximum rate and therefore the optimum CO2 density for the dynamics experiments. From our previous work on Pt foil we knew that CO/O2 ratios of about one and a surface temperature just above that of the maximum rate were likely to be near optimum. It is well known that on all of the metals of group VIII in the second and third transition (5), the reaction 1Current address: Ε. I. du Pont de Nemours & Company, Experimental Station, Wilmington, DE 19880-0262 0097-6156/92/0482-0058$06.00/0 © 1992 American Chemical Society

In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

4. COULSTON & HALLER

Kinetics and Mechanism of CO Oxidation 59

rate passes through a maximum as a function of CO/O2 ratio at a fixed surface temperature (at a ratio of around one) and as a function of surface temperature (at a fixed C O / 0 ratio). The origin of the maximum is similar in both cases, i.e., at high CO/O2 ratios or low surface temperatures, the surface is covered with CO which inhibits the adsorption of O2 and thus the rate. As the surface temperature is raised, CO is desorbed and the rate Increases but passes through a maximum at some temperature where the rate of desorption of CO begins to compete with the rate at which CO finds an adsorbed oxygen with which to react. There is an analogous optimum when the ratio of CO/O2 is varied at constant temperature. Both the reactant ratio (at fixed surface temperature) and the optimum surface temperature (at fixed reactant ratio) will, of course, depend on the particular metal under consideration. At 373K and a stoichiometric reactant ratio, the three metals have rates in the order Pt>Rh>Pd (5). At this temperature all three metals are significantly CO inhibited, first order in oxygen, zero order in CO and the activation energy is essentially equal to the heat of adsorption of CO; the absolute pressure is a less important factor, but a recent study indicates that the rate does vary on Pd by a factor of about 0.74 at 445K when the total pressure is varied by eight orders of magnitude (6). As we will see below, for a stoichiometric ratio but with the surface temperature at 573K the order of rates is Pd>Rh>Pt, just the reverse of that at 373K, and if maximum rates at a stoichiometric reactant ratio are compared (different surface temperatures) the metals are ordered Pd>Pt>Rh. Since the binding energy of CO is not a strong function of the metal, we surmise that what is most affecting this behavior is the ability of the metals to interact with oxygen and, in turn, how this affects the competition with CO as a function of temperature. We will compare the activity of Pt, Pd and Rh around stoichiometric reactant ratios and optimum surface temperature and reactant fluxes in the range of 0.002 - 0.004 Torr total pressure. The reactants are directed at the foil surfaces with a free jet and the rates have been followed by mass spectrometry. While the combination of the free jet interception, foil surface and mass spectrometric detection do not allow us to calculate absolute turnoverfrequencieswith a high degree of accuracy, the relative rates are good to within a few percent so that nothing is compromised in the kinetic comparison.


2

Experimental A detailed description of the complete apparatus, the foil cleaning procedure, and Auger analysis can be found in ref. 3 and 4, but a brief description of the kinetic measurements is given here. A quadrupole residual gas analyzer that was equipped with a Faraday cup detector was used to measure the overall rate of the CO oxidation. The CO and O2 were delivered to the surface through a free jet nozzle source, the axis of the nozzle intersecting the surface normal at a 30° angle and the 240 μιη orifice lying 1 cm away from the surface. Typically, mass flow controllers were set to deliver 10 ccm = 0.16 cm s each of CO 3

1


SURFACE SCIENCE OF CATALYSIS

60

and 0 (in the case of Pt and Rh; a factor of two lower In the case of Pd); this corresponds to a total flux at the surface of 2.2xl0 c m s e c and a density of 3.3xl0 c m . Prior to each experiment, a measurement was made of the quadrupole's sensitivity to 5xl0" Torr of argon. The rate of reaction was assumed to be proportional to the CC>2 ion current measured by the mass spectrometer. Changes in the C>2 and C O ion currents were not used because these signals did not provide the same degree of precision as the CC>2 signal. Of course, it was necessary to assume that the pumping speed for CO2 provided by the cryogenic pumps remained constant throughout the measurement. In the case of CO2, the dominant pumping mechanism would have been cryocondensation (as opposed to cyrosorption into the activated charcoal absorbent located in the bodies of the two cryopumps or cryotrapping by other pumped gases) because it was the most condensable gas in the system. Thus, the assumption that the pumping speed for C 0 is independent of the CO2 pressure in the chamber is very reasonable as long as the total pressure is well below Ι Ο where the thermal load on the pumps would be too large. Typical background pressure for the kinetic experiments was 4 x 1 0 ° Torr. 2

18

1

13

2

3

6

+

+

+


+

2

3

Results The conversion of CO to CO2 was measured at three CO:02 ratios, 2:1, 1:1 and 1:2, on Pt, Pd and Rh foils. As can be seen in Figures 1, 2 and 3, the surface temperature range was about 400 -HOOK for Pt, 300 900K for Pd and 300 - 750K for Rh, respectively. Below T , the surface temperature of the maximum rate, the kinetic form of the rate is m

r = (kiPo2)/(KP o) C

(D

where ki is the rate constant for molecular O2 adsorption and Κ is the equilibrium constant for CO adsorption. Since the adsorption of molecular oxygen is essentially non-activated, the apparent activation energy in this regime is approximately equal to the heat of adsorption of CO at the appropriate coverages and reaction conditions. The apparent activation energies in kcal/mol obtained from an Arrhenius plot below T and above T are collected in Table I. m

m

Discussion At surface temperatures above T , the reaction is proportional m

to CO coverage and some function of oxygen atom coverage, fl6o), i.e. r = k «e )eco 2

0


(2)

Kinetics and Mechanism of CO Oxidation61



400

_j

600

.

,

400

,

ι

600

800

,

,

1000

. — ι — . — . — . — ι —

800

1000

Surface Temperature/K

Figure 1. Carbon monoxide conversion on Pt. The CO:C>2 ratios are provided in the legend, the total nozzle flow rate was (lOccm CO):(5ccm 0 ) , (lOccm CO):(10ccm 0 ) , (5ccm CO):(10ccm 0 ), respectively, for 2:1, 1:1 and 1:2 C O / 0 ratios. 2

2

2

2



62


Figure 2. Carbon monoxide conversions on Pd. The CO:02 ratios are provided in the legend, the total nozzle flow rate was constant at 10 ccm.



4.


COULSTON & HALLER

I '

1

I

I

•

I

300

I

I

•

I

1

1

I

I

I

I

I

400 500 600 Surface Temperature/K

700

Figure 3. Carbon monoxide conversion on Rh. The CO:C>2 ratios are provided in the legend, the total nozzle flow rate was (lOccm CO):(5ccm 0 ), (lOccm CO):(10ccm O2), (5ccm CO):(10ccm 0 ), respectively, for 2:1, 1:1 and 1:2 C O / 0 ratios. 2

2

2


64


Table I. Apparent activation energies (kcal/mol) determined from the rates of reaction with various CO:Q ratios on Pt, Pd and Rh 2


Metal

CO:0

2

Ε (TT ) m

ESR

m

Pd

1:2 1:1 2:1

32.7 28.1 22.3

-5.0 -4.5 - 4.4

27.7 23.6 17.9

a

1:2 1:1 2:1

24.0 23.1 14.0

-8.6 -8.3 - 10.1

15.4 14.8 3.9

Rh

1:2 1:1 2:1

13.8 13.1 19.5

-6.4 -6.6 -6.9

7.4 6.5 12.6

Pt

a

The activation energy on Pt for T>T was corrected for changes in the oxygen coverage with surface temperature. m

The carbon monoxide will be essentially in equilibrium with the surface. The oxygen coverage may be assumed to be high and the CO coverage very low, as will be argued below. Thus, Αθ ) is likely to be only slightly temperature dependent (3) (and not much effected by the oxygen coverage). In addition, we assume that the A H determined from E(TT as well as at T

Eapp = E(T>T ) = E m

S R

- |AH

c o

I

(3)

The activation energies and rate data may be used to estimate the coverages of the reactants. The activation energies in Table 1, Ε (TTm on each surface by equating the rate of oxygen adsorption to twice the rate of C O 2 production and, since oxygen was expected to adsorb through a 0 Ο m

c o

m




Table Π. Estimated carbon monoxide and oxygen coverages at T on Pd, Pt and Rh at various CO:C>2 ratios m

6 o

6o

a

C

CO:0

Metal


Pd

atT

2

4x10-3 5x10-3

0.3

2x10-2 8x10-3

0.1 0.15

2x10-2

0.1

1:2

1x10-2

0.4

1:1

1x10-2 1x10-2

0.1

1:2 1:1 2:1

2:1 a

Assuming 8 o

m a x

C

m

0.4

2:1

Rh

atT

m

3x10-3

1:2 1:1

Pt

b

b

= 10. Assuming 0 o

0.2

0.1 m a x

= 0.5.

precursor state, (7,8) the rate of oxygen adsorption was assumed to be first order in the concentration of vacant sites. Allowing for the possibility of oxygen desorption and recognizing that the CO coverage was already very low at T . we estimated the oxygen coverage using the relation m

1.ÎXÇO|(!JÇQ) θ

θιη

_

\2S°HPQ ι +

2

2F S° 02

( 4 )

where Xco is the CO fractional conversion, F02 is the flux of oxygen to the surface, and Pc ο and Po are the pressures of CO and 0 , respectively, in the nozzle. The sticking coefficient S ° was taken as 0.4 for Pd (9), 0.1 for Pt (10), and 0.1 for Rh (1 J). The rate constant kd in equation (4) was small enough to be neglected for the cases of Rh and Pd. However, taking the heat of adsorption of oxygen on Pt to be 55 kcal/mol and independent of coverage and assuming a normal first order desorption preexponential of 1 0 s indicated that oxygen desorption may have become appreciable at temperatures above 950K on the Pt foil. The coverage of oxygen on these surfaces for T

Kinetics and Mechanism of Carbon Monoxide Oxidation on Platinum

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