J. M. White and Charles T. Campbell1 university of Texas Austin. TX 78712
I
Surface Chemistry in Heterogeneous Catalysis An emerging discipline
The feasible operation of a great number of industrial chemical processes depends directly upon the efficient use of solid catalysts. Coal gasification, hydrocarbon cracking, reformation. hvdroeenations and oxidations. for examole. are all econo&Ea~l~iarge scale processes and'rely heaviiy upon heterogeneous catalytic reactions. A detailed understanding of these, and other; heterogeneous processes evolves from knowledge of the interface between the reactants and the catalyst, namely the surface. The recent development of powerful techniques for probing the atomic and molecular species in the topmost layers of a solid has spurred the interest of scientists in refining their understanding of the details of catalvst behavior. The result has been the emergence of an " exciting discipline: surface chemistry in heterogeneous catalvsis. Since the nrincinles which undergird this area are " similar to those which support all of chemistry, our understanding of solids, liquids, and gases can be used to advantage. --n--
In this paper the important role which surfaces play in catalysis is highlighted hy focusing on the ratalyzed oxidation of carbon monoxide, a widely studied model reaction that also has practical significance in automobile exhaust control. In addition to providing an ample demonstration of how surfaces exert their influences in heteroeeneous ohenomena. this overview also serves to illustrate how experimental problems in this field are being attacked. Surfaces: Fundamental Importance While most of our chemical education treats in detail the three states of matter, little, if any, attention is paid a unique and vital feature of systems involving more than one phase-namely the boundary region that divides these phases. On an atomic level, interfaces occur over small distances and are the seat of communication between ohases. In the solid metal-gaseous oxygen system, for example, it is not really the oxvren uressure u,hich determines the stabilitv of the solid m&l okide, hut rather the concentration of dxygen atoms chemisorhed on the metal surface. For this reason, other sources of chemisorbed oxygen such as gaseous NO and COz serve in the efficient formation of metal oxides. In short, surfaces are not to be denied, and they should he given a status equal to that of the hulk phases in many problems. Surfaces are of critical importance for a host of practical and fundamental problems including corrosion of materials, biological membrane action, tertiary oil recovery, photo-electrochemistry, bonding and lubrication. But in no area of chemistry and technology do surfaces play a more important role than in heterogeneous catalysis, and it is this subject upon which we focus. The Source and the Thrust of Catalytic Chemistry We will consider heterogeneous catalysts as solid materials which increase the rate of specific liquid or gas phase reactions without themselves being permanently altered. Researchers have found active catalysts for a wide range of reactions, many of which are used to produce enormous quantities of products by the chemical and petroleum industries. Because the rate enhancement is often several hundredfold, catalysts lead to tremendous economic henefits through either increased production or decreased energy consumption. The increase in reaction rate has classically been attributed to a stabiliza-
tion by the catalyst of the reaction transition state (in this case occurring on the surface) and the resultant decrease in overall activation harrier for the process. As we shall see, the actual operation of catalysts is often far more complex and intriguing than this simple model suggests. In surveying recent developments'in the surface chemistry of the catalyzed carbon monoxide oxidation reaction. we will take note of the tvoes of molecular level events (interactions) that take placcas a catalyst latches onto the reactants and guides their reassembly into proaucts. In such a description, an attempt is usually made to dissect the overall process into a set of separate elementary steps (mechanism), and it is the identification and characterization of these steps that lies at the heart of catalytic chemistry. An Illustrative and Simple Catalytic Reaction A mixture of CO and Oz gas at 450 K would remain unreacted for geological times in the absence of a catalyst. Yet the.addition of as little as a few hundred milligrams of a solid transition metal catalyst (MI such as Pt, Pd, Rh, or Ir to the reaction vessel enables the production of up to a mole of COz per minute (1).This reaction, written: -
is a verv.simnle . one and has. therefore. received attention in many laboratories, including our own; as a model catalytic system for detailed analysis. The dream is to gain a fundamental understanding of the reaction mechanism, to apply this insight to more com~licatedreaction systems, and from this knowledge to constr;ct a framework f k t h e development of improved catalysts. Broadly speaking, we can divide a catalytic process into three stages: (1)transport of reactants, in this case CO(g) and Ode) - to the surface. (2) strone interactions at the surface. and (3) transport of prdd&ts, ~ & g ) , away from the surfaee. I t is the second of these three stages that catches the attention of the surface chemist, who further subdivides it into physisorntion. chemisor~tion.decomoosition. reaction, diffusion, an2 desdrption. W; expect thehetails to depend upon the properties of the metal, the CO, the 02, and the C0z. The gaseous species are already well-known, hut the surface chemist is faced initially with the problem of producing a metal surface with known and reproducible properties. This problem requires the ene era ti on of an atomically clean surface, often a specitic face o f a single crystal, so that the poritiuns of all the surface metal atom>are wtll-kn~>rvn.'l'ypicnlly,ultrahigh vacuum instrumentation is used in the preparation and preservation of these clean surfaces. Overall Kinetics: A Complex Mechanism 7'he I't-catalyzed steudy-state rate of CO? production \ ~ S I I CO S pressure is s h ~ w nin Figure 1 for a lixed O? p r e s i ~ ~ r e and various temperatures. The ~omplexityof the reaction mechanism is evidenced by two observations: (1)The apparent activation energy goes abruptly from positive values to zero as the temperature is increased, and (2) the order with Supported in part by National Science Foundation Grant, CHE7107827.
' National Science Foundation Trainee. Volume 57, Number 7, July 1980 1 471
Figure 2. (81 Schematic of LEED pattern faadsorption of CO an RulOOl). (b) Simple model d (V 3 X 4 3 ) CO adswpl'ono h m y e m l m n t with ganratry of LEED panern: 1i.1 = ~ 3 1 5 1an0 16.1 = \ 3lb. lRet 8 ) Figm t . L o g a r h i c p l o t o f ~ r a t e d W ~ p o d u c t l o n , ~ , ~ W ~ ~ g ~ ~ ~
on polycrystalline Pt for a series of reaction tempratures. The 0 2 pressure is Pa. The slope here shows the reaction Mder with respect to constant at P, (Ref.2). respect to CO pressure (the slope) ranges for +1to 0 to -0.5 as the temwrature and pressure change. As usual for complex situations;scientists have been forced to dissect this reaGion intoseveral piecesin order to beein to understand the broad picture. Since the reaction is expected to occur between species adsorbed on the metal surface, the first mandate is a thorough understanding of the adsorption and desorption phenomena of the individual reactants and products. Adsorption Studles Surface physics has provided many fine techniques for analysis of the adsorption and desorption properties of a molecule. With these, a fairly clear picture of the interactions of CO and 0 2 gas with several transition metals has been achieved. Of primary interest is the nature of the complex formed upon adsorption. This information is provided by surface-sensitive soectrosconies which probe levels . the inerev ~ , (vibrational and electronic) of the adsorption complex. These include infrared-reflectance (IR) soectroscoDv. the electron spectroscopies-Auger (AES), pGotoelectrbi' (PES), and energy loss (EELS)-and work function measurements. Other methods, such as electron stimulated desorption (ESD), secondary ion mass spectrometry (SIMS), and thermal desorption spectroscopy (TDS), probe the surface composition directly. The ahove methods can sometimes provide quantitative measures of certain atomic or molecular moieties on the surface, and from these thermodynamic and kinetic information can be derived. Yet another set of techniques probe directly the location of atoms on the surface. ~hes'incl"de low energy electron diffraction (LEED), electron stimulated desorption ion angular distrihutions (ESDIAD), and angular resolved photoelectron spectroscopy (ARPES). All these structural techniques have detection, signal-to-noise and signal-tobackground limitations. Since they must be surface sensitive and since the surface of a 1mm thick metal foil comprises only ahout lo+% of the samples, these problems could he prohibitive were it not for the clever ahilitv of the techniaues. . . bv. their very nature, to probe only the surface region. Carbon monoxide bonds readilv to most transition metals at room temperature (3). One or more types of CO bonding occur, the most frequent types involving linear M-C=O and bridged ~~
~~
~
~
M
>c=o M bonds, as in metal carhonyls. These structures are inferred from the carhon-oxygen stretching frequency (1800-2000 472 / Journal of Chemicai Education
cm-I), as observed by IR or EELS (45) and from the M-C stretches (300-500 cm-1) as seen in EELS (4). The CO axis a rthe surface and the carbon end is is usuallv ~ e r ~ e n d i c u lto toward ;he surface. This geometry is confirmed by ESDIAD (6)and ARPES (7). On well-ordered sinele crvstal faces such as Ru (001), CO will often adsorb to forman ordered periodic structure, the geometry of which can he determined using LEED, as shown in Fig. 2. Like carbon monoxide, oxygen also adsorbs readilv on moat transition metals. At room t&perature it dissociat& to form adsorbed oxygen atoms, denoted O la), on the surface, as evidenced hy isotopic mixing expertments ( 9 ) .For some metals, this oxygen can then penetrate the outermost metal layers and difiuse toward the hulk. This sulnurtace oxvcen diffefi in its - - ~-~ ~~electronic structure from truly chemisorbei O(a), and this difference amears -. in the electron soectroscooic results (10.11). An understandine of the adsorbates. CO(a) and Oca). . .. is certainly not complete until their thermddynamic and kinetic properties are known. Since the concentration (or fractional coverage, 0 ) of the adsorbed species can be measured by several of the methods mentioned above, the rate of accumulation of CO(a) or O(a) starting from a clean surface can he measured as a function of temperature and pressure. For CO this rate, R a p , is given approximately by R,dm
= fcCDSoC0(1 - €IEo)
where fCwis the frequency of collisions of CO(g) with the metal surface (proportional to the pressure of CO) and S,CO is the sticking probability of CO od a perfectly clean suif;?ce ( 0 = 0). which is near unity for manv metals such as P d and Rh (12.13).The factor (1 -09 determines the probability that an i m.~ i. n ~. i CO n g molecule will strike an emntv . . metal site. After adsorption, the kinetics of desorption can be probed using thermal desorption spectroscopy. The sample is heated rapidly, while the evolution of CO(g) is monitored with a mass spectrometer. For metals that are active in the oxidation reaction, a pulse of COW is seen a t around 500 K. From the lineshape this pulse,
