Molecular Heterogeneous Catalysis, Wiley (2006), 352729662X

406 Chapter 9

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410 Chapter 10

that have more than one surface atom, dissociation will only occur when the specific ensemble and arrangement of surface atoms is available for chemical bond activation. In such a case, the reaction is structure sensitive for both bond cleavage and bond association. The activation energy of the surface reaction is lowered in both directions since the reactant and the product states become more stabilized. For reactions that occur over a single metal atom, bond activation is enhanced when the metal surface atom is less coordinatively saturated. This lowers the barrier for bond activation but increases the reverse barrier for recombination. For a late transition state, the change in the activation energy for recombination will be less than the change in the activation energy of the bond-breaking reaction, which then will be most surface sensitive.

Brønsted–Evans–Polanyi Relation

Early and Late Transition States. Chapters 2 and 3

The Brønsted–Evans–Polanyi (BEP) relationship relates the changes in the activation energy for a particular reaction over di erent catalysts to the changes in their corresponding heat of reaction (or overall reaction energy). Similar relationships have also been developed for changes in the reactant molecule rather than changes in catalyst. The BEP equation is a simple empirical relationship, in which a non-equilibrium property, such as the activation energy, is related to an equilibrium property such as the reaction energy. The proportionality constant, α, in the BEP relationship is close to one when the transition-state conformation is close to product geometry (late transition state). The value of α is close to zero when the transition state is near the reactant state configuration (early transition state). For simplicity, the value of α in the literature is often approximated as 0.5.

Promotion

Chapters 2, 3 and 5

–Alloys: Mixed metal surfaces can promote the selectivity of a reaction, by o ering reduced ensemble sizes for adsorption, by poisoning reactions at surface edges and/or by o ering coadsorption sites. In addition, biand multi-metallic systems can also enhance the activity of the surface by changing the strength of the metal–adsorbate surface bond. These changes can occur through an “ensemble” or geometric e ect where the adsorbates on the surface bind preferentially to sites which maximize their surface adsorbate bond strength. The preference for one metal type over the second, in some cases, is even stronger than its coordination site preference. Therefore, alloying the surface can drive adsorbates to specific sites which can either promote or poison the site’s reactivity. The changes in activity can also occur through electronic or ligand e ects, which refer to the changes in the electronic structure at the active site due to changes in its nearest-neighbor ligands. Pseudomorphic metal overlayers and near-surface alloys demonstrate this point rather nicely in that the reactivity on these surfaces can be quite di erent even though the surface metal layer is the same. The changes in their reactivity can be explained by changes in the electronic properties of the surface that result from charge transfer or changes in the lattice spacing. More explicitly, the changes in binding energies and activation barriers are nicely correlated with shifts in the d-band center of the metal substrate.

–Mixed oxides: Mixed oxide surfaces can help to promote reactions by utilizing a combination of di erent types of sites with di erent functionality to aid the reaction.

Postscript 411

These include oxidation sites with di erent oxidation states, as well as Brønsted and Lewis acid and base sites. In addition, the sites responsible for O2 activation may be di erent from those for oxygen insertion or dehydrogenation. In order to stabilize particular reaction centers or catalyst phases, the formation of compounds with nonreactive cations is sometimes desirable.

–Mixed sulfides: Create enhanced activity by stabilizing sulfide vacancies

–Coadsorbed molecular moderators: Much of the chemistry on metal surfaces is twodimensional in nature with no participation of the weaker stabilizing van der Waal’s interactions from organic functional groups such as those which make up the enzyme cavity and no influence of the pore wall as one may have in zeolites. Certain heterogeneous catalyzed reactions carried out on metal surfaces, however, have shown significant improvement on the addition of molecular modifiers that can coadsorb along with the reactant. Upon adsorption, they can begin to create partial three-

dimensional pockets on the metal substrate. The well-known example here is the addition of Cinchona alkaloids to modify Pt and promote its ability to carry out hydrogenation of α-keto esters with high enantioselectivity.

Transient Reactive Intermediates

Chapter 2

Much of what we know about the reactive intermediates for a particular reaction has been established from either in situ or ex situ spectroscopic analyses of the reaction surface. What is usually measured, however, is the most stable species on the surface and not necessarily the most reactive species. There are a growing number of examples which have shown, through either experiment or theory, that the reaction may be controlled by species that are very reactive on the surface. They have very short lifetimes, thus making it di cult to catch them “in action”. Some of the notable examples include the π-bound ethylene species on Pt and Pd which are more weakly bound than their di-σ- bound intermediate but also tend to be the more predominant reaction channels. Similarly, transient O2 − surface intermediates on Cu and La2O3 and also O− on di erent metal surfaces have been identified.

Cluster Size E ects

Chapters 2 and 5

The reactivity of supported metal, metal oxide and metal sulfide nanoparticles can change owing to changes in the size of the particles and changes in the support. In this section, we refer only to changes that are the direct result of particle size. Indirect e ects that arise from the particle support interactions are described in the next section. In moving from bulk particles down to nanometer size dimensions, metal, metal oxide and metal sulfide particles all show significant changes in their electronic structure as we move from their band structures for the bulk materials to distinct molecular states with unique electronic properties for the nanoparticles. Therefore, systems that were once conducting, insulating or semiconducting may now have very di erent properties. The unique electronic structures of these molecular states govern the chemical properties and hence catalytic reactivity. It is well established that metal clusters of with fewer than 20 atoms in the gas phase show distinct electronic properties, chemisorption characteristics and surface reactivity. In addition, the band gap for metal oxide particles is increased significantly on moving down to very small nanoparticles, which are on the order of 40 atoms or less.

412 Chapter 10

A second predominant e ect that occurs on shrinking the cluster size is related to the changes that occur in the relative composition of di erent exposed facets and defect sites. The percentage of exposed corner and edge sites increases substantially as the cluster size is decreased down to molecular scales. This change in site composition density can dramatically influence the reactivity at the particle surface.

Support E ects

Chapters 2 and 5

The support that is used to anchor the active metal, metal oxide, or metal sulfide particle can significantly influence the reactivity of the particle depending upon its structure, morphology and chemical properties at its interface with the particle along with its local environment. The acid/base characteristics, the composition of surface species such as hydroxyl groups and cation or anion defect can all act to change the properties of the nanoparticle. First, the surface properties of the oxide can control the size, shape and morphology of the nanoparticle that forms. Second, the support can act as a ligand and control the electronic properties of catalytic particle via charge transfer. The electronic properties of the naked gas-phase particles can be significantly altered through electron transfer either from the support to the particle or from the particle to the support, thus leading to particles that are electron-rich or electron-poor. This will depend upon the bonding between the metal and the support and the presence of defects sites. Third, the interface formed between the nanoparticle and the support creates uniquely active sites such as in the formation of new mixed metal oxide sites (M1–O–M2) where the metal or metal oxide nanoparticles attach to their support, or new mixed metal sulfide (M1– S–M2) sites where the metal sulfide attaches to its support. In addition, new sites can be created by local charge transfer at the nanoparticle/support interface, the creation of bifunctional sites that cooperate across the interface and the formation of Brønsted acid sites to accommodate bridges between cations of di erent charges. The properties of the particles that form can be very di erent from those of the naked gas-phase particles. The factors that control the catalytic activity of nanometer metal and nanometer metal oxide particles on di erent supports are still a subject of great debate.

The Material and Pressure Gaps

Chapters 2, 3, 4, 5 and 6

The material and the pressure gaps refer to the two greatest di erences between experiments performed under ultrahigh vacuum conditions over single crystal surfaces and those carried out over actual catalysts run at near operating conditions. These same issues relate to the extrapolation of ideal theoretical results to catalysis over supported particles under industrial conditions. The ability to bridge the materials gap requires a more complete understanding of the active sites and models to represent them. For systems with well-defined structures such as organometallic clusters, heterpolyacids and many zeolites, there is no real materials gap since we know the location of the atoms and we can develop reasonable models to capture the primary structure. Simulating supported metals, on the other hand, requires modeling, or at least understanding, how changes in (1) metalsupport interactions, (2) surface structure, (3) defect sites and (4) particle shape, size and composition all influence catalytic reactivity.

The pressure gap refers to the di erence in pressure between reactions run under ultrahigh vacuum conditions and those run under actual industrial conditions. This di erence

Postscript 413

in pressure can be of the order 1010-fold, which will significantly influence the surface coverages and can a ect the surface reactivity and selectivity. We have shown that the most stable surface structures for oxides, sulfides and metals are strongly dependent on the reaction conditions and can readily change throughout the course of reaction. This requires the application of phase diagrams to map out the lowest energy surface structures as a function of chemical potential. The dynamic changes that occur as the result of reaction can be simulated using kinetic Monte Carlo simulations, provided that the elementary processes have been measured experimentally or calculated quantum mechanically. The simulations of catalytic surface reactions on both metals and oxides show that the system is strongly dependent upon the total pressure, partial pressures and the temperature as they significantly influence the total surface coverage, the relative surface compositions and the potential reactivity.

Lock and Key Related Molecular Recognition Concepts

Chapters 2, 4 and 7

The concept of pretransition-state orientation is general to molecular catalysis. A catalyst has to provide the optimum opportunity to stabilize reactants in a configuration that, with minimum movement, can lead to the respective transition state. Maximum stabilization of the transition-state free energy has to occur for the transition state. Optimum stabilization of the pretransition-state structure requires a steric match between the shape of the pretransition-state configuration and the catalyst cavity or the catalyst surface. This involves a molecular recognition process.

The flexibility of the catalyst framework helps to accommodate the di erent steric requirements of reactant, transition and product states. The steric match of the pretransition state and the transition state should not be so tight that it prevents the entropic movement of substrate. The product that forms must be unfavorably bound so that it will desorb from the active site once it forms.

The induced lock and key principle refers to a flexible catalyst lattice that adapts its shape to that of the substrate. The anti-lock and key principle refers to enantioselective catalytic systems where the state of most unfavorable binding yields the preferred product. The entropy di erence here determines the selectivity.

Selectivity

Chapters 2, 3, 4, 5, 6

There are many factors that determine the rate of a reaction sequence that lead to a particular product. Within the same catalytic system, reaction sequences leading to di erent products may compete. The two key parameters, which are important to the selectivity of a catalytic reaction, are the di erence of the rate constants of elementary reaction steps controlled by electronic, geometric or steric parameters and the overlayer composition of the reactive catalytic surface or occupancy of complex or cavity. This a ects the relative probability for product molecule formation from the recombination or dissociation of reaction intermediates generated during the catalytic cycle. The relative stability of the fragment molecules determines their concentration and, hence the probability that they are present at high enough concentration to result in a finite quantity for recombination. Site occupancy controls also the probability of surface vacancies necessary for dissociation. The last, for instance, is an important parameter that discriminates between associative

414 Chapter 10

and dissociative reaction steps. In addition, the di erences in elementary rate constants determine also the selectivity.

Structure-Sensitive and -Insensitive Reactions

Ensemble E ect. Chapter 3

The surface structure can a ect surface reactivity electronically by changes in the degree of coordinative unsaturation of the surface atoms, and geometrically by creating ensembles of surface sites with di erent topologies.

Elementary steps which proceed through transition states in which the product atoms remain bonded to the same metal atom are typically insensitive to the binding site topology. This occurs for the activation of C–H and N–H bonds in molecules. Elementary steps, however, that generate atoms or fragments that demand higher fold coordination sites as products are typically much more sensitive to surface binding site configurations, and hence structure sensitive. For instance, this tends to occur for reactions involving the activation of diatomics that are strongly bound to the surface such as CO and NO. Alloying a reactive metal with an inert metal decreases the size of the reactive metal surface ensembles that form. The activation of the adsorbed molecule is therefore suppressed.

There is an important di erence in the activation barriers that result when the adatoms that initially form upon dissociation share bonding to one or more surface metal atoms and when they do not share bonding with the same metal surface atoms. The transitionstate energy is substantially lowered for the forward reaction when product fragments do not bind to the same metal atoms. The barrier, however, is also reduced for the reverse recombination reaction. The reactivity of the (100) surface as compared with the (111) surface and the reaction at step or kink sites as compared with those on terrace sites two good examples of this. Reactions that proceed by activation over a single metal atom tend to be less structure sensitive. The activation barrier will change when the surface metal atom changes coordinative unsaturation. When the reactivity of the metal atom varies, the activation barriers for the forward and the reverse reaction move in di erent directions.

Lateral Interaction E ects and Surface Reconstruction

Chapter 3

When two (or more) adsorbed atoms bond to the same surface atom(s), they experience a repulsive interaction. When two adsorbed atoms bond to two di erent neighboring metal atoms that share a metal–metal bond, they tend to experience attractive interactions. These two rules can readily be deduced from the Bond Order Conservation principle which indicates that the atom-surface bond strength decreases with an increase in the number of adatoms bonded to the same surface metal atom. This change does not occur linearly with the number of neighboring atoms or molecules, but instead tends to vary exponentially.

The formation of an overlayer of adatoms or molecules can lead to reconstruction of the surface metal layers. This will reduce strain in the surface layer due to the altered metal–metal atom interactions. Often ordered surface phases are formed, in which the adatoms have reduced reactivity, because of the increased interaction with the reconstructed surface atom overlayer. The reactivity of the adsorbate overlayer is then limited to the boundary atoms of the overlayer surface islands. Once ordered overlayers are formed and the surface concentration of adatoms or molecules is further increased, bonding in

Postscript 415

the surface overlayer becomes weakened because more unfavorable bonding sites are occupied. The catalytically active surface species sometimes are the more weakly bonded species in the surface overlayer.

Orbital Symmetry Control

Chapter 3

Substrate bond cleavage reactions occur with low activation energies when the unoccupied adsorbate antibonding molecular orbitals become populated with a finite electron density in the transition state. This is the result of electron transfer from highest occupied states within the catalyst to the lowest unoccupied states of the reactant. Substrate bond cleavage and associative reaction steps often occur along a reaction coordinate that is nearly parallel to the surface so as to stabilize the interactions between the two fragment and the catalyst surface in the transition state. The corresponding antibonding orbital is therefore usually antisymmetric with respect to the surface normal. Electron transfer between this substrate orbital and the surface requires an interaction between this state and surface orbitals of the same symmetry. At a local level, the symmetry of substrate orbitals and local orbital fragments have to match. The corresponding local surface orbital fragments are called group orbitals. For a substrate chemical bond crossing a surface metal atom, the dxz or dyz atomic orbitals which are antisymmetric with respect to surface normal can interact with the antibonding unoccupied substrate orbital. The surface atomic dz2 , s and pz orbitals are symmetric. Hence, atop s-atomic orbitals do not interact with the antibonding substrate orbital fragment. This is only possible in a valley or bridging position, where the interaction occurs now with group orbitals, which are linear combinations of atomic orbitals. On metals with small d-orbital extensions, crossing over valleys tends to be preferred. The interaction with the antisymmetric s-atomic orbital combinations then dominates.

Universal Relationships

Chapter 3

Diatomic molecules adsorbed to transition-metal surfaces dissociate through tight transition states. A general Brønsted–Evans–Polanyi relationship can then be defined which is valid for nearly all diatomic molecules that dissociate along a similar reaction path:

Eact = E0 + αEreact

The parameters E0 and α are adjustable.

Carbenium and Carbonium Ions in Zeolites and Solid Acids

Chapters 4 and 5

Transition states in proton-catalyzed reactions that occur in zeolites and other solid acids proceed through activated intermediates close to the carbenium or carbonium ions found in superacid solutions. A carbenium ion is a positively charged ion, that can be formed by the protonation of an alkene. The positive charge is localized on an sp2-hybridized C atom. A carbonium ion is a protonated saturated alkane, that forms non-classical valencies such as a protonated σ C-C bond or a five-coordinated C atom. In zeolites, the positively charged intermediates are compensated for by the negatively charged zeolite framework