Chapter 9 Carbon Monoxide Oxidation on Model Single -Crystal Catalysts
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Charles H. F. Peden Inorganic Materials Chemistry Division, Sandia National Laboratories, Albuquerque, N M 87185-5800
The activity of a variety of model single crystal catalysts [e.g., Rh(111), Rh(100), Ru(001), Ru(101), Pt(100), Pd(110), Ir(111), and Ir(110)] for CO oxidation has been studied. Kinetic measurements as a function of temperature and partial pressure of O and CO in a high-pressure reactor - U H V surface analysis apparatus over the temperature range 425 to 725 K, and pressure range of 0.1 to 600 torr have been made. From the measured rate parameters, the reaction mechanisms for a variety of experimental conditions are proposed. In addition, a number of ex-situ probes of the surface composition and structure are used, including Auger electron, X-ray photoelectron, and high-resolution electron energy loss spectroscopies, temperature programmed desorption, and low -energy electron diffraction. Recently, we have also used fourier transform reflection-absorption infrared spectroscopy as an in-situ probe of reaction intermediates. The behavior of the various metals is exemplified by Rh and Ru whose reactivity patterns will be compared and contrasted in this paper. For all of the metal surfaces studied to date, the reaction kinetics (specific rates and their dependence on the reactant partial pressures) that are measured on the model single crystal catalysts are in excellent agreement with results obtained previously for high surface area supported catalysts, demonstrating the structure insensitivity of this reaction. For Rh, Pt, Pd and Ir, we observe simple Arrhenius behavior over most of the temperature range studied with activation energies ranging between 22 and 33 kcal/mol, close to the desorption energy of CO from these surfaces. These results (and the partial pressure dependencies of the reaction) are consistent with the generally accepted model in which the formation of CO occurs by the Langmuir-Hinshelwood reaction between CO molecules and Ο atoms, both chemisorbed to the metal surface. In contrast, the reaction kinetics measured on Ru are not readily reconciled within such a model, and further suggest that the mechanism may involve the direct reaction between gas-phase or weakly bound (physisorbed) CO molecules and chemisorbed oxygen on Ru (Eley-Rideal mechanism). 2
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0097-6156/92/0482-0143$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.
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The development of coupled high-pressure (« 1 atm.) microcatalytic reactor/ultra-high vacuum (UHV) surface science apparatus has resulted in significant new insights into the mechanisms of a number of catalytic processes (for recent reviews, see references 1-3). Notably, the chemical and physical state of the surface prior and subsequent to reaction can be analyzed without exposing the sample to an ambiant other than the reactants. A number of exsitu electron spectroscopic probes of the surface composition and structure can be used, including Auger electron (AES), X-ray photoelectron (XPS), and highresolution electron energy loss (HREELS) spectroscopies, low-energy electron diffraction ( L E E D ) , and temperature programmed desorption (TPD). Recently, the utility of fourier transform infrared reflection-absorption spectroscopy (FT-IRAS) as an in-situ probe of reaction intermediates present at high-pressure reaction conditions in the microcatalytic reactor has been demonstrated (4,5). It is the purpose of this paper to describe the variety of useful information obtained in these type of coupled kinetic and spectroscopic studies. Specifically, the focus will be on the CO oxidation reaction on single crystal Rh and Ru in which we have used all of the above listed experimental probes to unravel the mechanistic details of this process for these two metals. The catalytic oxidation of CO by 0 over group VIII metals is important for the control of automotive exhaust emissions (6,7). As such, considerable attention has been focused on the kinetics and mechanisms of this reaction (8). Furthermore, the relative simplicity of the reaction on a metal surface makes it an ideal model system of a hetereogeneous catalytic process - a process involving molecular and dissociative (atomic) adsorption, surface reaction, and desorption of products. This additional motivation has led to numerous fundamental studies of the various elementary steps of the reaction; for example, the adsorption and desorption of CO and 0 , and the surface reaction between chemisorbed CO molecules and Ο atoms (= CO(ad) and O(ad), respectively) (9). In most cases, such studies have provided strong evidence that C 0 formation on supported and unsupported transition metal surfaces, under steady-state conditions, is a result of this surface reaction, a process defined as the Langmuir-Hinshelwood (L-H) mechanism (9). An alternate mechanistic proposal, the Eley-Rideal (E-R) mechanism, involves the reaction between a gas-phase or physisorbed CO molecule and O(ad) (9). This mechanism, although often invoked to explain kinetic and spectroscopic data, has yet to be unambiguously established for the CO oxidation reaction. The results to be reviewed in this paper concern a direct comparison between the activity for the CO oxidation reaction, and in-situ and post-reaction spectroscopic characterization of model single crystal Rh and Ru surfaces. In this way, we demonstrate that while the L - H mechanism seems most appropriate for explaining the Rh data, the E-R mechanism may well best account for the results obtained on Ru. In addition, we describe kinetic and spectroscopic data obtained on Rh under highly oxidizing conditions in which tne L-H reaction mechanism is altered due to the oxidation of the Rh surface. 2
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Experimental The experiments reported here involved the use of several small high-pressure (< 1 atm.) reactors directly coupled to ultra-high vacuum (UHV) surfacescience apparatus. Detailed descriptions of the procedures are contained in the original literature (1016). Briefly, the single crystals are mounted on the sealed end of a reentrant tube connected to a retractable bellows allowing for
In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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PEDEN
Carbon Monoxide Oxidation on Model Singe-Crystal Catalysts
sample transfer in vacuum from the reactors to the U H V analysis chambers. The crystals were heated resistively and temperature monitored by thermocouples spot welded to the sample edges. Cooling was accomplished by partially filling the reentrant tube with liquid nitrogen. The surface analysis apparatus used here typically contain a cylindrical mirror analyzer (CMA) for AES and a quadrupole mass spectrometer for TPD. In addition, one of the systems had capabilities for HREELS, L E E D and XPS (14). Finally, a separate apparatus was used for in-situ FT-IRAS studies of the CO oxidation reaction over Ru(OOl) which also contained U H V surface analysis equipment for AES, TPD and L E E D (5). High-pressure reactions were performed by heating the crystals for several minutes after charging the reactor with the premixed gases (CO and 0 ). CO and 0 were both ultra-high purity grades and the C O was further purified prior to mixing by passing it through a liquid-nitrogen cooled trap to prevent the introduction of volatile metal carbonyls into the reactor. One of the systems used contained a gas chromatograph for the quantitative determination of product ( C 0 ) formation (10). Additional more qualitative probes of reaction rates included the appearance of gas-phase C 0 in the FT-IRAS spectra (15), and a mass spectrometric measurement of the product gas mixtures leaked into the U H V chamber (16).
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Results and Discussion Steady-State Kinetics for Stoichiometric Reaction Conditions. As will be seen below, the CO oxidation reaction mechanism is very sensitive to the state of the surface. We begin the discussion with a measurement of the kinetics for reaction at or near stoichiometric conditions (Le., reactant partial pressure ratios, C 0 / 0 = 2). Figure 1 is an Arrhenius plot comparing C 0 formation rates obtained on several single crystal catalysts (10,11,13). These results can be directly compared to previously published rates obtained on high-surface area supported catalysts containing these same metals as shown in Figure 2 (17-23). Note that rates obtained on different single crystal faces of Ru, Ir and Rh (Figure 1) give identical results showing that at least for these metals there is no intrinsic sensitivity of the reaction to the surface structure. This conclusion is reinforced by the very good agreement of the results on the supported and unsupported catalysts (Figure 2). Furthermore, it demonstrates that the single crystal metal surfaces are, in fact, very good models for the practical materials. The implication here is that the post-reaction analysis we carry out will be relevant to understanding the operation of the actual catalysts. This agreement extends also to the measured pressure dependence of the reaction for these systems (e.g., see reference 17a). From these kinetics, models of the mechanism of the CO oxidation reaction have been developed. 2
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Reaction Mechanism for stoichiometric conditions on Rh (and Pt, Pd and Ir). The dependence of the C 0 formation rate on the partial pressures of both CO and 0 for reaction over a R h ( l l l ) single crystal catalyst and a supported R h / A l 0 3 catalyst were both found to be first order in 0 pressure and negative first order in CO pressure (11,17a). This behavior was accounted for by a kinetic model developed by Oh, et al. (17a) which correctly predicts this observed behavior assuming a Langmuir-Hinshelwood (L-H) reaction mechanism between chemisorbed CO molecules and dissociatively adsorbed Ο atoms. It should be noted that these model calculations use only parameters, such as CO and 0 adsorption energies, obtained from experimental U H V determinations. For the reaction conditions we used, the model predicts that 2
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In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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