Introduction. ch1.1. James A. Dumesic, George W. Huber and Michel Boudart. 1.1 Principles of Heterogeneous Catalysis

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1 ch. Introduction. Principles of Heterogeneous Catalysis James A. Dumesic, George W. Huber and Michel Boudart.. Introduction Heterogeneous catalysis is of vital importance to the world s economy, allowing us to convert raw materials into valuable chemicals and fuels in an economical, efficient, and environmentally benign manner. For example, heterogeneous catalysts have numerous industrial applications in the chemical, food, pharmaceutical, automobile and petrochemical industries [ 5], and it has been estimated that 90% of all chemical processes use heterogeneous catalysts [6]. Heterogeneous catalysis is also finding new applications in emerging areas such as fuel cells [7 9], green chemistry [0 2], nanotechnology [3], and biorefining/biotechnology [4 8]. Indeed, continued research into heterogeneous catalysis is required to allow us to address increasingly complex environmental and energy issues facing our industrialized society. Discussing the principles of heterogeneous catalysis is difficult, because catalysts are used for a wide range of applications, involving a rich range of surface chemistries. Moreover, the field of heterogeneous catalysis is highly interdisciplinary in nature, requiring the cooperation between chemists and physicists, between surface scientists and reaction engineers, between theorists and experimentalists, between spectroscopists and kineticists, and between materials scientists involved with catalyst synthesis and characterization. Furthermore, industrial catalysts are complex materials, with highly optimized chemical compositions, structures, morphologies, and pellet shapes; moreover, the physical and chemical characteristics of these materials may depend on hidden or unknown variables. Accordingly, principles of heterogeneous catalysis are typically formulated from studies of model catalysts in idealreactors with simplified reactants under mild pressure conditions e.g., bar), rather than from catalytic performance data obtained with commercial catalysts in complex reactors using mixed feed streams under industrial reaction conditions. The principles derived from these more simplified studies advance the science of heterogeneous catalysis, and they guide the researcher, inventor, and innovator of new catalysts and catalytic processes...2 Definitions of Catalysis and Turnover The definition of a catalyst has been discussed many times [9]. For example, a catalyst is a material that converts reactants into products, through a series of elementary steps, in which the catalyst participates while being regenerated to its original form at the end of each cycle during its lifetime. A catalyst changes the kinetics of the reaction, but does not change the thermodynamics. Another definition is that a catalyst is a substance that transforms reactants into products, through an uninterrupted and repeated cycle of elementary steps in which the catalyst participates while being regenerated to its original form at the end of each cycle during its lifetime [20]. The main advantage of using a heterogeneous catalyst is that, being a solid material, it is easy to separate from the gas and/or liquid reactants and products of the overall catalytic reaction. The heart of a heterogeneous catalyst involves the active sites or active centers) at the surface of the solid. The catalyst is typically a high-surface area material e.g., m 2 g ), and it is usually desirable to maximize the number of active sites per reactor volume. Identifying the reaction intermediates and hence the A list of abbreviations/acronyms used in the text is provided at the end of the chapter. Corresponding author. References see page 4

2 ch. 2. Principles of Heterogeneous Catalysis mechanism for a heterogeneous catalytic reaction is often difficult, because many of these intermediates are difficult to detect using conventional methods e.g., gas chromatography or mass spectrometry) because they do not desorb at significant rates from the surface of the catalyst especially for gas-phase reactions). Heterogeneous catalysts typically contain different types of surface sites, because crystalline solids exhibit crystalline anisotropy. Equilibrated single crystals expose different faces with different atomic structures so as to minimize total surface energy. It would be surprising, in fact, if different crystallographic planes exposing sites with different coordination environments possessed identical properties for chemisorption and catalytic reactions. Moreover, most catalytic solids are polycrystalline. Furthermore, in order to achieve high surface areas, most catalysts contain particles with sizes in the nanometer length scale. The surfaces of these nanoscopic particles contain sites associated with terraces, edges, kinks, and vacancies [2]. If the catalyst contains more than one component as is generally the case), the surface composition may be different from that of the bulk and differently so for each exposed crystallographic plane. Solids normally contain defects of electronic or atomic nature; in addition, they contain impurities which are either known or unknown in the bulk, but are mostly unknown at the surface. Finally, the surface atomic structure and composition may change with time-onstream as the catalytic reaction proceeds. In short, it is normal to expect that a catalytic surface exposes a variety of surface sites, in contrast to displaying a single type of active site. Indeed, it is so normal today to expect such complexity that it seems surprising that, in 925, when Taylor formulated his principle of active sites or active centers, the report created so much attention and remains one of the most often cited in heterogeneous catalysis [22]. The relative importance of surface structure as influenced by crystalline anisotropy, surface defects, and surface composition underlines the difficulty of identifying the active sites, either simple or complex, that are responsible for turning over the catalytic cycle. The identification and counting of active sites in heterogeneous catalysis became the Holy Grail of heterogeneous catalysis in 925, and the situation remains the same today. The activity of a catalyst is defined by the number of revolutions of the catalytic cycle per unit time, given in units of turnover rate TOR) or turnover frequency TOF). In cases where the rate is not uniform within the catalytic reactor or within the catalyst pellets, it is useful to report the rate as a site time yield STY), defined as the overall rate of the catalytic reaction within the reactor normalized by the total number of active sites within the reactor, again in units of reciprocal time. Catalysis by solid materials has been observed quantitatively at temperatures as low as 78 K and as high as 500 K; at pressures between 0 9 and 0 3 bar; with reactants in the gas phase or in polar or non-polar solvents; with or without assistance of photons, radiation or electron transfer at electrodes; with pure metals as unreactive as gold and as reactive as sodium; with multicomponent and multiphase inorganic compounds and acidic organic polymers; and at STYs as low as 0 5 s one turnover per day) and as high as 0 9 s gas kinetic collision rate at 0 bar). TOFs of commonly used heterogeneous catalysts are commonly on the order of one per second. The life of the catalyst can be defined as the number of turnovers observed before the catalyst ceases to operate at an acceptable rate. Clearly, this number must be larger than unity, otherwise the substance used is not a catalyst but a reagent. Catalyst life can either be short, as in catalytic cracking of oil, or very long, corresponding to as many as 0 9 turnovers in ammonia synthesis...3 Steps in a Heterogeneous Catalytic Reaction During an overall catalytic reaction, the reactants and products undergo a series of steps over the catalyst, including:. Diffusion of the reactants through a boundary layer surrounding the catalyst particle. 2. Intraparticle diffusion of the reactants into the catalyst pores to the active sites. 3. Adsorption of the reactants onto active sites. 4. Surface reactions involving formation or conversion of various adsorbed intermediates, possibly including surface diffusion steps. 5. Desorption of products from catalyst sites. 6. Intraparticle diffusion of the products through the catalyst pores. 7. Diffusion of the products across the boundary layer surrounding the catalyst particle. Accordingly, different regimes of catalytic rate control can exist, including: i) film diffusion control Steps and 7); ii) pore diffusion control Steps 2 and 6); and iii) intrinsic reaction kinetics control Steps 3 to 5) of catalyst performance. In addition to mass transfer effects, heat transfer effects can also occur in heterogeneous catalysis for highly exothermic or endothermic reactions especially in combustion or steam reforming). Figure shows a general effect of temperature on the reaction rate for a heterogeneous catalyst. At low temperatures, diffusion through the film and pores is fast compared to rates of surface reactions, and the overall reaction rate is controlled by the intrinsic reaction

3 ch...4 Desired Characteristics of a Catalyst 3 ln rate) Slope = 3-5 kj/mol Film diffusion controlled regime Slope = E a /2R Pore diffusion controlled regime Increasing Temperature / Temperature Intrinsic regime Slope = E a /R Fig. General effects of temperature on catalytic activity. The intrinsic activation energy is equal to E a,andristhegasconstant. kinetics. As the temperature is increased, the rates of surface reactions typically increase more rapidly than the rates of diffusion, and the overall rate of the catalytic process becomes controlled by intraparticle diffusion. The apparent activation energy in this regime is equal to the intrinsic activation energy divided by two. As the temperature is increased further, mass transfer through the external boundary layer becomes the controlling step. The onset of diffusion limited regimes can be altered by changing the reactor design, the catalyst pore structure, the catalyst particle size, and the distribution of the active sites in the catalyst particles. Values of various dimensionless groups can be calculated to estimate the extents to which transport phenomena may control catalytic performance for specific operating conditions [23 3]; however, these calculations are most reliable for cases where the intrinsic reaction kinetics are known. In these cases, it is possible to make catalysts with structures designed to provide adequate rates of diffusion and yet offering high surfaces areas, leading to high rates of reaction per reactor volume, such as the design of specific pore size distributions e.g., bimodal distributions containing large pores leading to high accessibility of the active sites within the interior of the catalytic pellet, and small pores that branch from the larger pores leading to high surface areas), the formulation of unique pellet shapes that lead to high accessibility of the active sites but do not cause large pressure drops through the catalytic reactor), and the synthesis of catalyst pellets containing a spatial distribution of the active material within the catalyst pellet [32]. In some cases, transport effects can be used to improve the selectivity of a catalyst, such as in the case of shape-selective catalysis in zeolites [33 36]. In the following sections, we focus on various factors controlling the intrinsic reaction kinetics of catalysts, and we refer the reader to other articles for further discussion on transport effects in heterogeneous catalysis [23 3]...4 Desired Characteristics of a Catalyst The following list provides several of the key attributes of a good catalyst: The catalyst should exhibit good selectivity for production of the desired products and minimal production of undesirable byproducts. The catalyst should achieve adequate rates of reaction at the desired reaction conditions of the process remembering that achieving good selectivity is usually more important than achieving high catalytic activity). The catalyst should show stable performance at reaction conditions for long periods of time, or it should be possible to regenerate good catalyst performance by appropriate treatment of the deactivated catalyst after short periods. The catalyst should have good accessibility of reactants and products to the active sites such that high rates can be achieved per reactor volume. The first three key attributes of a good catalyst are influenced primarily by the interactions of the catalyst surface with the reactants, products, and intermediates of the catalytic process. In addition, other species may form on the catalyst surface e.g., hydrogen-deficient carbonaceous deposits denoted as coke) that are not directly part of the reaction scheme or mechanism) for the overall catalytic process. The principle of Sabatier states that a good heterogeneous catalyst is a material that exhibits an intermediate strength of interaction with the reactants, products, and intermediates of the catalytic process [37, 38]. Interactions of the catalyst surface with the various adsorbed species of the reaction mechanism that are too weak lead to high activation energies for surface reactions and thus low catalytic activity, whereas interactions of the catalyst with adsorbed species that are too strong lead to excessive blocking of surface sites by these adsorbed species, again leading to low catalytic activity. The principle of Sabatier is elegant in its simplicity and generality, but it is deceptively difficult to use in practice. In particular, this principle applies to a catalyst in its working state, and the nature of the catalyst surface can be expected to be dependent on the nature of the catalytic reaction conditions. For example, one may begin the catalytic reaction with the heterogeneous catalyst References see page 4

4 ch. 4. Principles of Heterogeneous Catalysis in a given oxidation state e.g., containing zero-valent metal particles following treatment of the catalyst in H 2 at elevated temperature); however, the nature of the surface can be changed dramatically upon interaction with strongly adsorbed species, such as the formation of carbonaceous deposits coke), and formation of oxides, carbides, nitrides, or sulfides upon interaction with O, C, N, or S species, respectively [39 44]. In this case, the interactions of these oxide, carbide, nitride, or sulfide surfaces with the adsorbed species enter into the reaction mechanism. Of even greater complexity is the fact that a variety of different types of sites are typically present on a catalyst surface e.g., sites having different coordination and/or chemical composition), and a majority of the observed catalytic activity may be caused by the contributions from a small fraction of the sites present on the catalyst surface. In this case, the adsorbed species interact with these special surface sites e.g., steps and defect sites on a metal nanoparticle, or sites present at the metal support interface of a supported metal catalyst). Another factor that complicates catalyst design is that the strengths of interaction of the surface with adsorbed species typically depend on the surface coverages by adsorbed species. For example, the interaction of a transition metal surface with adsorbed CO may be very strong at low surface coverages e.g., binding energy of nearly 200 kj mol ), suggesting that these surfaces would be completely covered and thus poisoned by adsorbed CO at moderate pressures and temperatures; however, these surfaces may carry out catalytic reactions in the presence of gaseous CO at these pressures and temperatures because the differential heat of CO adsorption decreases significantly e.g., to binding energies near 00 kj mol ) as the surface coverage by adsorbed CO increases [45, 46]. Accordingly, there is a relationship between activity and the interaction of the surface with adsorbed species at the surface coverage regime appropriate for the catalytic reaction conditions. The aforementioned complications caused by the presence of different types of sites on the surface, and the effects on the surface binding energies caused by changes in surface coverages, clearly make it difficult to interpret the performance of a heterogeneous catalyst in quantitative detail. Tools are certainly available to address these complications, such as kinetic Monte Carlo simulations combined with results from density functional theory DFT) calculations [47 50]. Yet, from a different point of view, the presence of different types of sites and the effects of surface coverage may well contribute to the robustness of the heterogeneous catalyst for operation over a wide range of reaction conditions. In general, the presence of different types of sites and the effects of surface coverage both contribute to surface non-uniformity different types of sites producing apriornon-uniformity, and effects of surface coverage causing induced non-uniformity). At a selected set of reaction conditions, an optimal set of surface binding energies exists that satisfy the principle of Sabatier as discussed below). Accordingly, the performance of a heterogeneous catalyst with a non-uniform surface will be dominated by the subset of the sites having surface binding energies closest to the optimal values. At higher temperatures, other sites having stronger binding energies with adsorbed species will become the dominant contributors to the observed catalytic activity, whereas sites having weaker binding energies with adsorbed species will control catalytic activity at lower temperatures. Thus, while the effects of surface non-uniformity make it more difficult to predict the performance of a heterogeneous catalyst from a molecular-level understanding, these effects may serve to broaden the range of reaction conditions over which the catalyst can operate effectively. In this respect, our desire to design catalysts having very high selectivity is guided by the synthesis of uniform catalysts, where each site has the optimal properties for production of the desired reaction product. This strategy leads to the idea of highly selective, single-site catalysis as discussed by Thomas et al. [5]. In contrast, the design of catalysts that operate over a wide range of reaction conditions is guided by the synthesis of non-uniform catalysts, such that different subsets of sites control catalyst performance at different reaction conditions. The disadvantage of using non-uniform catalysts, however, is that different sites may display different selectivities for the production of various products, and control over catalytic selectivity may thus be limited [22]...5 Reaction Schemes and Adsorbed Species We now explore further the principle of Sabatier using a specific example: water-gas shift over a metal catalyst e.g., Cu). This reaction CO + H 2 O CO 2 + H 2 )is of importance for the production of H 2 from steamreforming of fossil fuels, and for controlling the CO : H 2 ratio in synthesis gas mixtures used in methanol and Fischer Tropsch synthesis processes. For this example, we consider the reaction scheme shown in Fig. 2, where * represents a surface site. The stoichiometric numbers, σ i, and σ i,2, indicate the number of times that step i occurs to give the overall reaction for reaction schemes and 2, respectively. In this sequence of steps, the water-gas shift reaction can take place via the formation of carboxyl species COOH) or through the formation of formate species HCOO) [52]. In the absence of a catalyst, the rate of watergas shift via this mechanism is negligible, because the

5 ch...5 Reaction Schemes and Adsorbed Species 5 s i, s i,2. CO + CO 2. H 2 O + H 2 O 3. H 2 O + OH +H 2 4. CO + OH COOH + 5. COOH + OH CO 2 + H 2 O 6. CO 2 CO H H CO + OH HCOO 0 9. HCOO CO 2 + H 0 overall CO + H 2 O CO 2 + H 2 Fig. 2 Assumed reaction mechanism for water-gas shift reaction over Cu. Adapted from Ref. [52]. reaction intermediates e.g., OH,H, COOH,HCOO ) are at very low concentrations in the gas phase. For example, the enthalpy change for step 3 in the gas phase is approximately 500 kj mol. However, adsorption of the reaction intermediates onto the catalyst surface allows these steps to take place with small enthalpy changes. In thecaseofcopper,thebindingenergiesofhandoh are approximately 250 and 280 kj mol on Cu), such that the enthalpy change for step 3 on the catalyst surface is now slightly exothermic. According to the principle of Sabatier, a good catalyst is a material that adsorbs reaction intermediates with intermediate strength. However, we now must distinguish between reactive intermediates and spectator species on the catalyst surface. In the above reaction scheme, we see that the watergas shift reaction can take place through adsorbed carboxyl species or formate species. Results from DFT calculations indicate that adsorbed formate species have lower energy compared to adsorbed carboxyl species on copper surfaces, suggesting that path 2 for water-gas shift σ i,2 ) would be favored versus path σ i, )based on thermodynamic arguments. However, the activation energy for step 4 is considerably lower than that for step 8, and the primary path for water-gas shift over copper involves the formation and subsequent reaction of adsorbed carboxyl species. Accordingly, the most stable species on the catalyst surface are not necessarily the most reactive species. This idea leads us to distinguish between a most abundant surface intermediate MASI) and a most abundant reactive intermediate MARI). In certain cases, the MASI and the MARI may be the same species, but in other cases such as in this case of water-gas shift on copper), the MASI is a spectator species that 0 0 does not participate in the overall reaction. In this latter case, the spectator species inhibits the overall reaction by blocking surface sites, and it serves no useful role in the overall reaction scheme. For purposes of elucidating catalytic reaction schemes it is essential to distinguish between reactive intermediates and spectator species. This distinction is of paramount importance in spectroscopic studies of adsorbed species on catalyst surfaces, where the detection of a specific adsorbed species using a spectroscopic method e.g., the detection by infra-red IR) studies of adsorbed ethylidyne species on platinum surfaces during ethylene hydrogenation [53]) does not guarantee that this species is a reactive intermediate. Instead, these spectroscopic studies must be conducted under dynamic conditions e.g., so-called operando measurements, where spectroscopic and reaction kinetics data are collected simultaneously) to determine that the time constant for the formation or disappearance of the surface species is the same as the time constant for the overall catalytic reaction [54, 55]. The overall catalytic reaction is given by a linear combination of elementary steps, and the enthalpy change for the overall reaction, H,isgivenby: H = i σ i H i ) where H i are the enthalpy changes for elementary steps i. From the principle of Sabatier, it is now clear that the overall value of H should be composed of approximately equal contributions from each of the values of H i, giving rise to a relatively flat potential energy diagram of energy versus reaction coordinate in moving from reactants, through adsorbed intermediates, to products. Specifically, any value of H j that is very negative must be balanced by a value of H k that is very positive, such that the surface will become highly covered and poisoned) by the adsorbed species produced in step j, and the activation energy for step k will be high. Both of these effects lead to low catalytic activity. We note that the reaction mechanism can certainly contain steps with positive values of H i, because the intermediates produced in such a step can be consumed by following steps having negative values of H i. This situation is termed kinetic coupling, where the conversion of an unfavorable step is increased by its combination with a favorable step that consumes the unstable products of the first step. The highest value of H i for a surface reaction that can be tolerated can be estimated from transition state theory. The value of H i,max depends on the overall rate of the reaction TOF), the surface coverage by the surface species that reacts in this step θ A ), a frequency References see page 4

6 ch. 6. Principles of Heterogeneous Catalysis factor ν oftheorderof0 3 s ), and the temperature T, as given by: TOF = ν exp H ) i,max θ A 2) For a reaction operating at 500 K with a TOF of s, the maximum value of H i that can be tolerated for a species with high surface coverage θ A approaching unity) corresponds to 25 kj mol, which is still a rather high value. In practice, the highest value of H i that could be tolerated would be lower than this value of 25 kj mol, because the surface coverage by the reactive intermediate would typically be lower than unity and the above analysis assumes that the activation energy for the reverse of step i i.e., the exothermic direction for this step) is equal to zero. This situation where the overall enthalpy change is shared fairly equally between the various steps of the reaction scheme is a necessary condition for high catalytic activity, but it is not a sufficient condition, because we have not yet considered the transition states for the various elementary steps. The aforementioned reaction scheme for water-gas shift involving the formation of carboxyl species contains seven steps, thereby requiring the determination or estimation) of 3 rate constants to describe the reaction kinetics completely; that is, a forward and reverse rate constant for each step k for,i and k rev,i ) constrained by the relationship that these rate constants must give the proper equilibrium constant for the overall reaction, K eq,asgivenbelow: ) σi kfor,i = K eq 3) i k rev,i However, it is a rare case that all of these rate constants are kinetically significant. Thus, while we generally have the desire to know the values for as many rate constants as possible, we typically need to know only the values of a limited number of these rate constants to describe the performance of the catalyst for the reaction conditions of interest. Unfortunately, at the outset of research on a given catalyst process, we usually do not know which rate constants will be kinetically significant. Accordingly, an important objective of research into a given catalytic process is to identify which steps are kinetically significant, such that further research can focus on altering the nature of the catalyst and the reaction conditions to enhance the rates of these kinetically controlling steps. This situation is illustrated in Fig. 3 for the above case of water-gas shift involving carboxyl species, according to which the rate is controlled bysteps3and4,whereassteps,2,5,6,and7are quasi-equilibrated. The net rate of step 3 in Fig. 3, is twofold faster than the net rates of all other steps, because the stoichiometric number of step 3 is equal to 2 whereas all other stoichiometric numbers are equal to. Importantly, the net rate of each step divided by its stoichiometric number is equal to the net rate of the overall catalytic reaction. This equality is due to the principle of kinetic steady state, as stated by Bodenstein see Ref. [38]), according to which determining the rate of one single reaction typically the overall reaction) allows one to calculate the net rates of all the other individual reactions. The Bodenstein principle is an important foundation of our thinking about how catalytic cycles turn over. This principle also shows that the notion of a slow step in a catalytic cycle at the steady state is a misnomer, because all steps proceed at the same net rate...6 Conditions for Catalyst Optimality It can be shown that the net rate of the overall catalytic reaction is controlled by kinetic parameters which depend only on the properties of the transition states for the kinetically significant steps relative to the reactants and possibly the products) of the overall reaction [56]. The overall rate is also controlled by an additional kinetic parameter for each surface species that is abundant on CO + H 2 O + H 2 O + CO +OH COOH +OH CO 2 2H CO H 2 O OH + H COOH + CO 2 +H2 O CO 2 + H Fig. 3 Rates of forward and reverse steps in the water-gas shift reaction on Cu.

7 ch...6 Conditions for Catalyst Optimality 7 the catalyst surface. Specifically, the net rate of the overall reaction is determined by the kinetic parameters as well as by the fraction of the surface sites, θ,thatisavailable for the formation of the transition states; the value of θ is determined by the extent of site blocking by abundant surface species. To illustrate how to determine the optimal activity of a catalyst, we consider an example in which the reaction scheme contains a single rate-controlling step and a single abundant surface species. According to results obtained using DeDonder relations discussed in Section ) [56], we may write this reaction scheme in terms of a quasi-equilibrated step involving the transition state for the rate-controlling step, TS i,and a second equilibrated step involving the formation of the most abundant surface species, A,asgivenbelowand:. Reactants + 2 TS i 4) 2. A + A 5) The overall rate of the reaction, r net, as will be discussed in Section , is now given by: r net = ν o S exp σ R H ) o Fa i )θ 2 z/σ tot ) 6) θ = S o + exp 2 R H 2 o ) 7) a A where Fa i ) is a function of the activities a i )ofthereactants and/or products of the overall reaction. Neglecting entropy effects, as we change the nature of the catalyst for constant reaction conditions, the primary items in the above equations that change are H o and H2 o.note, we implicitly assume that the reaction mechanism does not change.) Accordingly, the overall rate of the reaction for different catalysts is given by: r net = C exp H o ) + C 2 exp H 2 o C = ν σ exp C 2 = exp o S R S o 2 R )) 2 8) ) ) Fa i ) z /σ tot 9) ) a A 0) We next consider that the surface properties of the catalyst are described in terms of some fundamental catalyst property, x. This property x could be a heat of adsorption of one of the reactants [57], the heat of formation of a bulk compound that can be correlated with a heat of adsorption [58], the position of the catalytic element along a horizontal series in the Periodic Table, an electronic property of the catalyst such as Pauling s d-band character of the metal [59], or the d-band center of the metal [60]. The optimal catalyst can thus characterized by the following relationship: dr net = 0 = + 2C exp C exp + C 2 exp H o ) o H C 2 exp + C 2 exp ) o d H H 2 o )) 2 H 2 o H o 2 ) d H o 2 )) 3 ) This relationship may be simplified to give: d H o 2C 2 exp H o ) 2 d H o 2 d H = )) 2 o = 2θ A + C 2 exp H o 2 2) Thus, for the optimal catalyst, the surface coverage by themostabundantsurfacespeciesisequalto: θ A = d H o 2 d Ho 2 = ω 2ω 2 3) where the values of ω i are defined as: ω = d Ho ω 2 = d Ho 2 4) 5) In the above derivation, we assume that ω and ω 2 have the same sign, such that variations in x change the enthalpy of the transition state and the MASI in the same direction. We also assume that d 2 H o )/2 and d 2 H o 2 )/2 are small or zero. This assumption is valid if we are searching for improved catalysts over asmallrangeofx, which typically occurs when testing catalysts. In fact, when we vary x over a large range, then the mechanism of the catalytic reaction would probably change. References see page 4

8 ch. 8. Principles of Heterogeneous Catalysis Intermediate energies Rate net is maximum as H tran 0 Case : w >> w 2 H tran H MASI Rate Intermediate energies Rate net is maximum at w = 2q MASI w 2 Case 2: w w 2 H MASI H tran Rate x Fundamental catalyst parameter) x Fundamental catalyst parameter) Rate net is maximum at w = 2q MASI w 2 Intermediate energies Case 3: w << w 2 H tran H MASI Rate x Fundamental catalyst parameter) Fig. 4 Reaction rates and energies of transition state and most abundant surface intermediate MASI) as functions of fundamental catalyst parameter x. First, we will consider Case shown in Fig. 4, where ω ω 2. In this case, a maximum rate does not exist and the parameter x should be adjusted to its lowest possible value i.e., the strongest bonding to the surface) which would decrease the enthalpy of the transition state as much as possible. In this case the optimal catalyst operates at high surface coverage. We now consider Case 2, where ω ω 2; that is, x changes the enthalpies of the transition state and the MASI by similar amounts. This situation is probably more physically realistic, and if the MASI is in fact a reactive intermediate i.e., if it is a MARI), then the family of catalysts described by the variation of x follows the Brønsted Evans Polanyi Semenov relationship, which relates the thermodynamics and kinetics of the system. Here, there is a clear maximum in the rate versus x, as shown in Fig. 4. The plot of rates versus x appears as a volcano-type curve which decreases symmetrically on both sides. In the case where ω is equal to ω 2,the surface coverage by the MASI on the optimal catalyst is equal to 0.5. For Case 3, ω ω 2, corresponding to the situation where x changes the enthalpy of the MASI more significantly than the transition state. The optimal catalyst in this case has a low surface coverage by the MASI. A maximum rate occurs in this case, as shown in Fig. 4, provided that ω > 0; however, the plot of rate versus x is not symmetric with respect to x. Wenotethatasx increases for these three cases, the number of vacant sites on the catalyst increases. Cases 2 and 3 clearly illustrate the principle of Sabatier, in which a maximum rate occurs at some moderate level of interaction of the catalyst surface with the intermediates and adsorbed species. While Case appears to contradict Sabatier s principle, the situation where ω ω 2 is highly unlikely. In particular, this situation corresponds to the case where the catalyst interacts more strongly with the transition state than with any of the reactive intermediates. However, if the activation energies for the elementary steps of the mechanism are positive, then the reactants and/or products of the elementary step involving the ratecontrolling transition state are more strongly adsorbed on the surface than is the transition state, leading to the situation described by Case 2 or 3. We may generalize the above expression for catalyst optimality to the case where the surface contains several abundant surfaces species, A,B,C,andD,leadingto the following expression: ω = 2ω Aθ A + ω B θ B + ω C θ C + ω D θ D ) 6) In this case, the nature of the optimal catalyst is controlled by the change in the binding energy of the transition state with respect to x ω ) compared to the changes in the bindings energies of species A, B, C, and

9 ch...6 Conditions for Catalyst Optimality 9 Dω A,ω B,ω C,ω D ) weighed by their respective surface coverages at the steady state. A bridge between the thermodynamics and kinetics of a reaction is provided by the Brønsted Evans Polanyi Semenov relationship, which states that there is a linear relationship between the activation energy E act of an elementary step and the heat of reaction if entropy effects are neglected [38]: E act = E 0 + α H 7) where H is the enthalpy of reaction, α is the transfer coefficient that varies between zero and one, and E 0 is a constant. In other words, if we neglect entropic effects, the activation energy of an elementary step in the exothermic direction is lower when the heat of reaction becomes more favorable i.e., H becomes more negative). DFT calculations have recently shown that Brønsted Evans Polanyi Semenov relationships are generally upheld in chemical reactions on catalyst surfaces [6 64]. We now consider the following catalytic reaction: A B + C 8) If we use the gas phase A species as the zero energy level as shown in Fig. 5) we can define the activation energy as: E act = E 0 + αh Bgas + B.E. B + H Cgas + B.E. C H Agas B.E. A ) = E 0 + αb.e. B + B.E. C B.E. A ) + α H gas 9) where the B.E. terms are the binding energies of the various species on the surface. A gas B gas + C gas 0 B.E. A A gas db.e. A db.e. B db.e. C Assume: = = a = 0 A* A* E act H a = B gas + C gas B* + C* B* + C* Fig. 5 Schematic potential energy diagram of energy versus reaction coordinate, showing the relationship between the energy of transition state, H =, and changes in the energies of adsorbed species. We now define the standard enthalpy for the formation of the transition state from gaseous species A as: H o = E act + B.E. A = E 0 + α H gas ) + αb.e. B + B.E. C ) + α)b.e. A 20) and we differentiate the enthalpy of the transition state with respect to x, leading to the following result: d H o db.e.b = α + db.e. ) C + α) db.e. A 2) This relationship shows how the change of the transition state enthalpy with respect to xω ) is related to the changes in the binding energies B.E.oftheadsorbed species ω 2 ).Ifα is equal to zero, then the change of the transition state enthalpy depends only on the change of the binding energy of A, and ω = ω 2.Inthiscase,the transition state is an early transition state that chemically looks similar to A [65]. If α is equal to, then the transition state is a late transition state that resembles the products of the reaction, and the change of the transition state enthalpy is related to changes of the binding energies of B and C. If db.e. A )/ = db.e. B )/ = db.e. C )/, then the late transition state gives rise to the situation where ω = 2ω 2. The above examples show how it is possible to maximize the activity of the catalyst. However, it is often more important to optimize the selectivity of the catalyst. Similar types of analyses can be carried out for these cases. In general, these situations are classified as being series selectivity challenges, such as A B C, where B is the desired product, and/or parallel selectivity challenges, such as A BcoupledwithA C. In these cases, if we want to optimize the selectivity of the catalyst, we search for some catalyst property, x, that decreases the enthalpy of the transition state for the desired reaction more than it decreases the enthalpies of the transition states for the undesired reactions. An undesired reaction that leads to progressive blocking of surface sites leads to deactivation of the catalyst with respect to time-on-stream. More generally, various mechanisms exist by which a catalyst can undergo deactivation, such as: i) poisoning; ii) thermal degradation sintering); iii) leaching of the active site; and iv) attrition [66]. The first three of these mechanisms are chemical in nature, whereas the last mechanism is physical e.g., the catalyst pellet breaks apart). Some catalysts do not show any measurable deactivation over periods of years, such as in ammonia synthesis. However, other catalysts lose an important fraction of their activity References see page 4

10 ch. 0. Principles of Heterogeneous Catalysis after less than a minute of contact with feed, as in catalytic cracking. In the latter case, if deactivation is caused by coking, the catalyst must be regenerated by continuous regeneration in an oxidizing atmosphere...7 Catalyst Design Given that the performance of a catalyst is controlled by a limited number of kinetic parameters, it is unclear why it is so difficult to design a catalyst from molecularlevel concepts. As noted above, during the early stages of research into a catalytic process, first we do not know which steps in the reaction mechanism are kinetically significant, and which species are most abundant on the catalyst surface under reaction conditions. Second, we do not often know the structure of the active site and its dependence on the nature of the reaction conditions. Third, we do not usually know how the activity and selectivity for the catalytic reaction depend on the structure of the active sites. Fourth, we do not typically know during these early stages the rates of various modes of catalyst deactivation e.g., sintering, phase changes, deposition of carbonaceous deposits on the surface, etc.), and we do not know whether the catalyst can be regenerated following deactivation. Finally, we must ensure that the texture of the catalyst and the geometry of the reactor are designed in such a way that mass transport of reactants and products to and from the active sites is sufficiently rapid that high rates of reaction per unit volume of reactor can be achieved. Because of these difficulties, the field of heterogeneous catalysis is highly interdisciplinary in nature, and involves close collaboration between experts in such areas as catalyst synthesis, catalyst characterization, surface spectroscopy, chemical kinetics, chemical reaction engineering and, most recently, in theoretical calculations of catalyst structure and performance using density function theory. These broad studies can be grouped into three levels, as shown in Fig. 6. All studies of heterogeneous catalysis begin at the Materials Level. High-surface area catalytic materials must be synthesized with specific structures and textures, the latter referring to such features as the sizes of the various phase domains and the details of the pore structure. Clearly, the synthesis of catalytic materials must be guided by detailed characterization studies to determine the structures, compositions, and textures of the materials that have been prepared. These characterization studies should be conducted after the catalyst has been subjected to various treatment steps such as those treatments employed during activation of the catalytic material), and it is most desirable to carry out characterization studies of the catalyst under the actual reaction conditions of the catalytic process. Indeed, the properties of a heterogeneous catalyst are inherently dynamic in nature, and these properties often change dramatically with changes in the reaction conditions e.g., phase changes, surface reconstructions, changes in surface versus bulk composition, etc.) [67]. The central level of research and development of heterogeneous catalysts involves the quantification of catalyst performance this is known as the Catalyst Performance Level). These studies can be carried out in a preliminary fashion over a wide range of catalytic materials e.g., high-throughput studies) to identify promising catalysts Characterization studies: catalyst structure, composition, & texture, ideally under reaction conditions) Materials level Materials synthesis: catalytic materials with specific structures & textures Exploratory studies: promising leads for new catalytic materials & new catalytic reactions Catalyst performance level Reaction kinetics studies: activity, selectivity & stability for various reaction conditions Surface studies: surface composition and nature of surface sites, ideally under reaction conditions) Elucidation level Experimental studies: interactions of probe molecules with well-defined sites Theoretical studies: stability & reactivity of species on well-defined sites Fig. 6 Levels of study in heterogeneous catalysis research.

11 ch...8 Catalyst Development and reaction conditions for further studies. The performance of the catalyst is then documented in greater detail by determining catalytic activity, selectivity, and stability with respect to time-on-stream for various reaction conditions.thesemeasurementsmustbemadeatvarious conversions when multiple reaction pathways exist, because catalytic selectivities in these cases are different, depending on whether the desired products are formed in primary versus secondary reactions, or in series versus parallel pathways. We note here that various definitions of catalytic activity are used, depending on the nature of the study. For practical studies, catalytic activities can be reported as rates per gram of catalyst or per unit surface area. However, for more detailed studies or for research purposes, it is often desirable to report catalytic activities as rates per surface site i.e., TOFs), with the number of surface sites measured most often by selective adsorption measurements e.g., adsorption of H 2 or CO to titrate metal sites, adsorption of ammonia or pyridine to titrate acid sites). In some cases it is possible to report catalytic activity as rate per active site also called TOF), when it is possible to distinguish active sites from the larger number of surface sites using special probe molecules e.g., dissociative adsorption of N 2 to titrate sites for ammonia synthesis [68]; selective poisoning by adsorbates that compete with the reactants of the catalytic reaction [69]); or by transient isotopic tracing [70]. For the purposes of catalyst development, it is probably sufficient to work at the Materials Level and the Catalyst Performance Level. However, research into heterogeneous catalysis is dominated by studies conducted at a third level the Elucidation Level where the aim is to identify the fundamental building blocks of knowledge which can be assembled to build a molecular-level understanding of catalyst performance in order to guide further investigations to improve catalyst performance. At the Elucidation Level the studies are designed to determine the surface composition and nature of the surface sites on the catalyst [7 74]. Clearly, these investigations must be conducted with the catalyst under controlled conditions e.g., under ultra-high vacuum, after treatment with H 2, after calcination, etc.) and, where possible, such measurements should be made with the catalyst under reaction conditions. Moreover, the studies may be carried out on real catalytic materials and on more well-defined surfaces e.g., single crystals, or model samples formed by depositing known amounts of materials onto welldefined supports) [73, 75, 76]. Most measurements at the Elucidation Level involve studies of the interactions of specific probe molecules with the catalyst surface. These probe molecules may be the reactants, intermediates, or products of the catalytic reaction, or they may be more simple species chosen to monitor a specific functionality of the surface. Alternatively, a molecule may be used as a probe because it has an advantageous feature for spectroscopic identification e.g., CO for infrared studies, a 3 C-containing molecule for NMR studies). These studies of the interaction of probe molecules with surfaces are designed to determine the surface concentrations of different types of surface site, to determine the nature of the adsorbed species formed on the surface sites, and to determine the reactivities of the surface sites by monitoring the adsorbed species on the surface versus time, versus temperature or, most commonly, during a temperature ramp e.g., temperature-programmed desorption). The third pillar of studies at the Elucidation Level involves the use of DFT calculations to assess the structures, stabilities, and reactivity of species adsorbed onto the surface sites with the sites being composed of clusters of atoms or as periodic slabs of atoms) [77 8]. These studies are used to help interpret the results obtained from spectroscopic studies of catalyst surfaces e.g., to predict the vibrational spectra of species adsorbed in different orientations on different sites), to calculate heats of adsorption for various intermediates in a reaction mechanism e.g., to predict which species are expected to be abundant on the catalyst under reaction conditions), to estimate the energy changes for possible steps in a reaction mechanism thereby eliminating from further consideration steps with very positive energy changes), and to determine activation energy barriers for steps that are suspected as being kinetically significant in the reaction scheme. Indeed, a key feature of these theoretical studies is the ability to predict how the surface properties are expected to change as the nature of the surface is altered e.g., by changing the surface structure, or by adding possible promoters). This in turn will provide feedback to the Materials Level with regards to new materials that should be synthesized and which are likely to lead to an improved catalyst performance. In addition, these theoretically based studies provide information about highly reactive intermediates which might be difficult to obtain by direct experimental measurements. Most importantly, studies conducted at the Elucidation Level provide a scientific basis about the working catalysis that may, in future, be used to design different reaction pathways...8 Catalyst Development Catalyst development typically involves testing a large numberofcatalystswithafeedbackloop,asitiscurrently difficult to design catalysts apriori. In thisrespect, catalyst development studies involve examining a large number of catalysts, for which recent advances in high-throughput References see page 4

12 ch. 2. Principles of Heterogeneous Catalysis testing have attracted considerable attention [82 87]. Catalyst development through the testing of a wide range of materials was first practiced in 909 by Mittasch at BASF who, according to Timm [88], issued the following directive to his team who at the time were developing the synthesis of ammonia: The search for a suitable catalyst necessitates carrying out experiments with a number of elements, together with numerous additives. The catalytic substances must be tested at high pressures and temperatures, just as in the case of Haber s experiments. A very large number of tests will be required. Ten years later, the number of tests conducted had exceeded 0 000, and more than 4000 catalysts had been studied. This extraordinary effort was also extraordinarily successful. What has changed since then, however, is the way in which the systematic search is assisted. Today, armed with an arsenal of principles, concepts, instrumentation and computers, it is possible to identify and to improve new catalytic materials in a much shorter time and with a smaller number of trial samples, especially with the possibility of advancedcharacterization methods especially in-situ techniques) and insights from theoretical calculations e.g., DFT calculations). The practical merit of this assisted catalyst design is clear, while its scientific dividend is the possibility of learning as the design proceeds, with the building of a data bank of rate constants and the formulation of more precise models of active sites. With new theoretical insights or principles, quantitative bases of catalyst preparation and reproducibility of catalyst behavior, the future of heterogeneous catalysis still looks very bright. The path to the design of an optimal catalytic process would be clear if the activity, selectivity and stability of the catalyst were to move in the same direction upon an increase in a single process variable, such as temperature. However, this simple behavior is not typically observed, and choices must be made in every instance. For example, while the activity of a catalyst may increase with temperature, its stability usually decreases with temperature. In addition, the relationship between catalytic activity and selectivity is typically very complex, and is not understood in detail until the surface chemistry of the catalytic process has been elucidated. Accordingly, selectivity, stability and activity must be considered together, and trade-offs may have to be negotiated, perhaps by using multi-functional reactors with catalytic distillation or catalytic membranes. Success in heterogeneous catalysis begins with chemistry, but ends with catalytic reaction engineering...9 Bridging Gaps in Heterogeneous Catalysis The above description of research and development into heterogeneous catalysis as being interdisciplinary in nature, involving studies at the levels of materials, catalyst performance and elucidation, can also be cast in the form of building bridges between various types of studies and different types of material. As depicted in Fig. 7, we often talk about bringing together the field of surface science which traditionally is focused on studies of single crystal surfaces at low pressures) with the field of heterogeneous catalysis which traditionally is focused on studies of high-surface area catalytic materials surfaces under highpressure reaction conditions). More recently, we have talked about bridging the materials gap, as we have attempted to use experimental results from studies of welldefined model materials to interpret the performance of more complex, high-surface area catalytic materials. Traditionally, these model materials have been single crystals, cut at various angles to expose surfaces containing different types of sites, such as surfaces with different symmetries and atoms present at terraces, steps, and kinks [89]. More recently, however, these model materials have become highly sophisticated, such as the deposition of nanoparticles with specific sizes and geometries on welldefined support surfaces e.g., metal nanoparticles supported on thin films of oxides deposited on single crystal metal surfaces, or non-metallic nanoparticles supported directly on single crystal metal surfaces) [73, 75, 76]. We also talk about bridging the pressure gap, as we attempt to use experimental results from studies conducted at low pressures less than 0 6 Torr) to interpret the performance of catalysts under high-pressure reaction Low P Pressure gap Surface science High m 2 Materials gap Low m 2 Heterogeneous catalysis High P Fig. 7 Bridging the gap between surface science and heterogeneous catalysis.