Multicomponent Catalysts: Limitations and Prospects - ACS Catalysis

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Multicomponent Catalysts: Limitations and Prospects Gaurav Kumar, Eranda Nikolla, Suljo Linic, J. Will Medlin, and Michael J. Janik ACS Catal., Just Accepted Manuscript • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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Multicomponent Catalysts: Limitations and Prospects Gaurav Kumar,a Eranda Nikolla,b* Suljo Linic,c* J. Will Medlin,d,* and Michael J. Janika,* a – Department of Chemical Engineering, Pennsylvania State University, University Park, PA, 16802, USA b - Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI, 48202, USA c – Department of Chemical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA d – Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO, 80309, USA * corresponding author emails, Janik: [email protected]; Medlin: [email protected]; Linic: [email protected]; Nikolla: [email protected] Abstract There has been a recent surge of interest in multicomponent catalysts that combine properties of chemically diverse materials. A major factor in this increased interest is the widespread recognition that the scaling relationships for adsorption and transition state energies of reactions place significant constraints on making step-change improvements in catalyst performance using monofunctional catalysts. In this perspective, we review the fundamental rationale for multicomponent materials, and describe several classes of materials that offer promise for improving activity and selectivity in catalysis. Our focus is on illustrating how

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recent advances in the ability to prepare precisely controlled multicomponent nanostructures have the potential to enhance the capability to design highly active and selective catalysts. Keywords: multicomponent, cascade, microkinetics, interface, electrocatalysis, plasmon 1. Introduction The two objectives of catalyst design are to maximize (i) the conversion of the reactants and (ii) the selectivity toward the desired product. The ideal catalyst would allow the reaction rate for the targeted reaction to reach its transport limitation, while the rate of the undesired reaction would be zero. Reaching this limit is often challenged by fundamental factors that limit the reaction rate on a single-component catalytic site.1 The Sabatier principle provides a qualitative description of an optimal catalyst, describing it as a material that moderately binds a reactive intermediate. Increasing the ability to activate reactants through strong surfaceintermediate binding is generally accompanied by a decrease in the availability of the free surface sites due to strong adsorption, leading to a balance in the optimal catalyst. The tradeoff in activating reactants rapidly versus binding them too strongly to complete product formation leads to the ubiquitous volcano relationships, where the optimal catalyst offers moderate binding of relevant intermediates.

Scaling relationships, which constrain the

relations among different adsorbate-catalyst binding energies, further limit the potential for tuning catalyst activity or selectivity in catalytic sequences involving multiple intermediates.2 The rate at the “peak” of the volcano is often well below the transport limitation. However, there are catalytic systems comprised of multiple components that exceed the singlecomponent rate and/or selectivity to the desired products, such as cascade enzymatic systems that incrementally progress along a reaction sequence to optimize product formation.3-4


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The purpose of this Perspective is to outline several ways in which multicomponent materials can offer a route to heterogeneous catalysts that exceed bounds imposed by the single component volcano relationship. Though multi-component catalysts are not rare—after all, even simply supporting a metal catalyst provides two potentially active materials— delineating the mechanistic function of each component, and how the components “communicate”, is often challenging. Some opportunities for multifunctional catalyst surfaces include (i) the design of cooperative sites at interfaces between chemically disparate materials,5 (ii) use of external stimuli that provide non-thermal pathways for energy transfer, and (iii) utilization of near-surface non-covalent interactions to stabilize particular adsorbates. As discussed below, diverse opportunities exist within each of these areas for realizing highperformance catalysts, and approaches from the different categories can in some cases be synergistic.

2. A microkinetic framework for analyzing multicomponent catalysts It is useful to begin our analysis by briefly outlining the limitations imposed by catalysts consisting of homogeneous active sites. Moreover, this analysis provides insights into what types of multifunctional materials are likely to lead to significant gains in activity or selectivity. To begin, we will consider the scenario consisting of two elementary steps occurring on different sites constituting a heterogeneous “cascade”. Intermediates produced at the first site diffuse to the second site, where they react to complete the reaction. In other words, the two catalyst sites operate independently of one another except that they “communicate” through the flow of reactive intermediates. Such a model two site system has been subjected to


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microkinetic analysis previously. Savchenko and Dadayan suggested conditions under which such “spillover” could cause “superadditive activity” in 1995.6

Reuter and co-workers7-8

recently showed through microkinetic modeling that there are inherent limitations in the use of dual-site catalysts to improve the overall reaction rate.

These authors concluded that

combining two catalysts limited by the same set of adsorbate scaling and Brønsted-EvansPolanyi (BEP) relationships9-10 cannot offer overall activities that exceed that of the optimal single material catalyst. Grabow and co-workers had earlier reached a similar conclusion in their analysis of two site systems for the CO oxidation reaction.11 To provide a conceptual framework for discussing bifunctional sites, consider the reaction energy diagram for an optimal single site catalyst—one that resides at the peak of a volcano plot. Assume that reactions 1 (A2 dissociation) and 2 (AB product formation) follow the same BEP relationship ‫ܧ‬௜௔௖௧ = ߙ௜ ‫ܧ‬௜௥௫௡ + ߚ௜

(1)

The optimal single-site catalyst, shown in green in Figure 1a, has the perfect “balance” between the two reaction steps (formation and consumption of the reactive intermediate) so that the reaction is not strictly limited by either the availability of active sites or by the activity for intermediate formation. In pursuit of a better catalyst, one could consider a dual-site mechanism in which there is a difference in binding strengths of the reactive intermediate on the two types of sites, such that the reactive intermediate is bound more strongly on the site used for production of an active intermediate, but weaker on the site from which it would be released to form a stable product. Such a scenario is shown in Figure 1a (red and purple), which illustrates the problem with such an approach: the individual barriers to both the first


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and second steps are decreased as desired, but the overall barrier to the second step is increased. Although the barrier for each individual reaction step has decreased, product formation requires that the intermediate overcome an additional barrier: the adsorption energy difference between sites. The apparent barrier, Eapp, therefore, would be greater than the optimal single site barrier as long as the BEP slope is less than 1. For an unphysical BEP slope, α, of greater than 1, the reaction barrier on the second site can be lowered by a much larger amount such that the Eapp becomes less than the single site optimal barrier. Detailed mathematical derivation of such a scenario has been provided in the SI.

a) Relative free energy

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c)

b) ૚࢙ ࡱ૛࢙ ࢇࢉ࢚૚ < ࡱࢇࢉ࢚

૚࢙ ࡱ૛࢙ ࢇࢉ࢚૚ < ࡱࢇࢉ࢚ ૚࢙ ࡱ૛࢙ ࢇ࢖࢖ > ࡱࢇࢉ࢚

૚࢙ ࡱ૛࢙ ࢇࢉ࢚૚ < ࡱࢇࢉ࢚

1

1

1 2’

2’

2’ 3

2

૚࢙ ࡱ૛࢙ ࢇ࢖࢖ < ࡱࢇࢉ࢚

૚࢙ ࡱ૛࢙ ࢇ࢖࢖ < ࡱࢇࢉ࢚

2

ࡱ૛࢙ ࢇ࢖࢖

3 ࡱ૛࢙ ࢇ࢖࢖

2

3 ࡱ૛࢙ ࢇ࢖࢖

Figure 1: Reaction energy diagram for a hypothetical reaction in which A2 dissociatively binds then reacts with B to form products, with reaction co-ordinates: 1 (A2 + 2B), 2 (2A* + 2B), 2’ (2A• + 2B), and 3 (2AB). Green diagrams in a and b reflect a single active site at the peak of a Sabatier “volcano.” Red and purple diagrams indicate a dual-site cascade where A binds to the first site of a cascade “*” stronger and to the second site “” weaker than on the optimal single site catalyst. The dual site cascade with a) two sites following the same BEP relations does not exceed the optimal single site rate. b) If the two sites follow drastically different BEP relations, all the reaction barriers (and apparent barriers) can be lower than the optimal single site barrier, giving a higher rate than the optimal single site. c) Two off-peak materials following different BEP relations can be combined to provide a higher rate than their individual rates by using one material (shown in red) to lower the barrier for dissociation and the other (shown in purple) to lower the reaction barrier.


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While combining two materials with similar chemical properties offers little hope of substantially improving catalytic rates beyond those limited by the BEP and scaling relations, materials with strong differences in BEP relationships can yield improved catalysts. If the two materials have dramatically different properties (Figure 1b), activation barriers may not scale equivalently with the individual reaction energies

12-15

. When the BEP parameters for

elementary reaction steps vary between two sites “” and “*”—where, for example, “*” corresponds to the red profile in Figure 1(b) and “” to the purple—a combination of the two materials could provide a higher rate than the optimal single site catalyst, since a weaker adsorption energy for a second reactive site could still exhibit a lower reaction barrier. Mapping such a scenario onto specific materials for a simple small-molecule reaction is challenging; however, cascades combining materials with very different functionality, such as a combination of an acid site and a hydrogenation functionality, would offer different BEP relationships. The different BEP relationships would mean the activation barriers for the different functions do not scale equivalently with the binding energy of the same intermediate. To explore this, we consider a case where active site “” has a small BEP intercept, β, for an initial A2 dissociation and a large β for a subsequent AB formation step. Active site “*”, however, has a large β for A2 dissociation and a small β for AB formation. Such a system can yield an improved rate of reaction (above the single-site volcano curve), because now both the elementary step barrier and overall barrier to the second step decrease with the use of two materials (Figure 1b). Figure 2, which was produced from a microkinetic simulation (details in SI), shows the ratio of the rate of the multi-functional cascade to that of the optimal single-site materials (with either of two BEP parameter sets). Regions where the value is greater than 1


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show combinations of binding energies of A on the two sites that would give higher rates than the optimal single site material. For generating this plot, the difference in β value between the two sites is 0.3 eV, which is realistic when considering sites with different properties 12-15. When used as single site catalysts, the optimal material of class “” will bind A weakly, in order to lower the barrier of the subsequent reaction step that has a high β. The reciprocal argument can be made for the optimal material of class “*”, which will bind A strongly in order to lower the rate limiting A2 dissociation barrier with high β.

Figure 2: Contour plot of bifunctional rate gain (color legend) on a dual site cascade with unequal BEP parameters on each site. The rate gain is the ratio of the rate on the cascade to that on the optimal single site catalyst. Here the two sites “*” and “” have different BEP parameters, such that “” favors A2 dissociation and “*” favors the reaction step to form AB. The model was performed at 500 K, A2 and B pressures of 2 atm, AB pressure of 0 atm, ∆G=1 eV, Site 1: α1=0.7, β 1 =1.65, α2=0.5, β 2 =1.05; Site 2: α1=0.7, β 1=1.35, α2=0.5, β 2=1.35.

The discussion above focuses on catalyst activity, i.e. the rate of a single reaction. However, for many applications the primary emphasis is on improving selectivity, i.e. the relative rates of desired versus undesired reactions. One approach to enhancing the relative rate for a desired process is to again pair materials with strongly different BEP behavior. Here,


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it is useful if scaling/BEP relations differ substantially across the two sites or at their interface. For example, adding an acid catalyst site to a metal site enhances the rate of deoxygenation reactions without appreciably altering the rate of other reactions such as decarbonylation, leading to an improved selectivity16. This can be attributed to different scaling for metal versus acid catalysts, where reactive intermediates for the undesired process do not adsorb favorably on the second site, while the reactive intermediates for the desired process do. For reactions in which the desired and undesired processes involve chemically similar types of intermediates, the approach of site pairing may not be sufficient for enhancing selectivity, and additional levers for nanostructure control of interfaces must be brought to bear.

3. Promising multi-component catalytic systems 3.1. Tailored interfaces for interfacial reactions Below we present a few examples where the multifunctional characteristics of various nanostructured catalysts, often derived from their multicomponent design, were used to engineer an improved performance. To provide the best chance for identifying revolutionary effective bifunctional catalysts, one must focus on materials that couple chemically diverse functions. An example of such a combination of materials are bifunctional metal/oxide catalytic systems. These systems can overcome the limitations posed by the single component scaling/BEP relations via two different mechanisms.

The first is to use spillover of an

intermediate, i.e. one reaction step occurs on the metal or the oxide forming an intermediate, which is then spilled over onto the oxide or the metal, respectively, and reacts to complete the catalytic cycle.17 Since the activity on the metal and the oxide are governed by different


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scaling/BEP relationships, these systems can achieve activities beyond those predicted by the general scaling/BEP relations on metals, as described in the previous section. In the second mechanism (Scheme 1a), the interface between the two dissimilar materials offers catalytic sites with activity that exceeds a single-component volcano maximum. The water gas shift (WGS) reaction and CO oxidation provide example systems where the metal/metal oxide interface enhances catalytic performance (second mechanism). Extensive work on WGS using Au and Pt supported on ceria demonstrates that one can achieve high activity for this reaction due to ceria-metal interfacial sites, leading to activities beyond what can be achieved by metals alone.18-20 CO oxidation on Au/TiO2 catalysts also benefits from the bifunctional nature of the catalytic system, utilizing interfacial sites to exceed the reactivity

Scheme 1: (a) Schematic illustrating the activity on the metal and the metal/oxide interface governed by very different scaling/BEP relationships. (b) Schematic of controlled binding and introduction of interfacial active sites by nanoporous TiO2 films on Pd nanoparticles for hydrodeoxygenation of biomass-derived alcohols. Adapted with permission from Angew. Chemie, 56 (2017) 6594.26 Copyright 2017 Wiley-VCH Verlag GmbH.

expected if all elementary steps were to occur on an isolated Au particle.21-22 Green et al.21 found that reaction between CO and oxygen occurred in an interfacial zone despite the fact


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that the apparent mechanism involve adsorption of both oxygen and CO on TiO2. Water has also been proposed to participate in the reaction via a mechanism that involves interfacial sites;22 here, water adsorbed on TiO2 donates a proton to O2 to form OOH at the Au-TiO2 interface, which oxidizes CO adsorbed on Au. Though the two materials perform different mechanistic steps in what appears as a cascade, the essential component in increasing the rate is the interfacial site that offers a lower barrier to OOH formation than if the entire process had to occur on the Au particle. The essential feature of this system is that the interfacial site obeys very different BEP/scaling relations than Au itself. Interfacial sites between metals and oxides have also received attention in the context of biomass upgrading reactions. In many cases, an active hydrogenation catalyst, such as Pt or Pd, is combined with an “oxophilic” component to couple the two types of steps required for hydrodeoxygenation.23-25 Mechanistic work has again suggested that sites at the interfaces between such materials exhibit special properties that allow reactants bound across the interface to both accept H atoms and cleave C-O bonds. Recent developments in nanomaterials synthesis can provide opportunities for tuning active sites at interfaces in ways that can be more sophisticated than simply dispersing metal nanoparticles on supports. As one example from hydrodeoxygenation catalysis, Pd nanoparticles have been encapsulated within porous TiO2 shells of controlled porosity26 (Scheme 1,b). Pd@TiO2 catalysts with the smallest pores were found to exhibit essentially 100% selectivity to the desired deoxygenation pathway, while maintaining high turnover frequencies. This result was attributed both to an enhanced density of interfacial sites and to the elimination of sites associated with undesired pathways such as decarbonylation. Although such nanostructured materials show great promise in enhancing


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our ability to design bifunctional catalysts for enhanced performance, detailed characterization of such complex (and often “buried”) interfaces remains a challenge. Examples where interfacial sites are proposed to effectively lower the barrier for a key reaction step, relative to that given by a single material BEP, are not restricted to metal-support interfaces. Methanol/CO electrooxidation at the interface between Pt islands and Ru is accelerated due to interfacial sites that lower the apparent barrier for oxygen/hydroxide addition to CO27-29. The combination of spatially distributed acid and base sites for aldol condensation of acetone with 4-nitrobenzaldehyde takes advantage of simultaneous interaction of bound intermediates with the two functionalities to lower the activation barrier30-31. For each of these systems, multiple components act to create an interface with a lower activation barrier than that offered by a single-component catalyst with an equivalent intermediate binding energy. This has also been shown in the form of a microkinetic analysis on homogeneous metal alloys.2

3.2. Structures that break scaling relations via stabilizing transition states through non-covalent interactions Another multicomponent catalyst approach is to retain a single active site, which catalyzes bond-making and breaking steps, but to add extended components to alter the reactive environment such that it stabilizes a key transition state and lowers the barrier. It is well known that zeolite catalysts can lower transition state energies through interactions with the pore about the active site. For example, while reaction may occur at an acid site within the zeolite, the rate is greatly enhanced due to transition state stabilization by interaction with the


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zeolite framework32. According to this paradigm, although acidic zeolites are often considered a single site material, they actually contain two complementary functions: an “active” catalytic function and a more “passive” transition state stabilization function. Altering solvent properties to stabilize a transition state at a liquid/solid interface could be similarly classified.

a

b

R3 R4

H

O

R1

H

N+ R2

Scheme 2. (a) Non-covalent interactions between reactants and phenyl groups tethered to a Pt catalyst have been shown to dramatically improve selectivity for hydrogenation of cinnamaldehyde to cinnamyl alcohol. Pi-pi stacking interactions were proposed to stabilize a more “upright” form of adsorbates, favoring carbonyl hydrogenation over olefin hydrogenation. (b) Proposed mechanism for effect of amine on ketone hydrogenation over Pt catalysts. Amines such as L-proline were suggested to play a catalytic function such as serving as sites for the 34

transfer of H atoms to the ketone oxygen atom. Panel (a) reproduced from JACS 136 (2014) 520-526. Copyright 2014 American Chemical Society. Panel (b) adapted from JACS 137 (2015) 905-912.

35

Copyright 2015 American

Chemical Society.

Organic ligands can also serve as a passive catalyst component to alter the activity of a metal active site33. Pt group metals can still serve as the active sites for reactions, such as hydrogenation, but design of tethered ligands (e.g., thiols and amines) can be used to stabilize transition states via non-covalent interactions. So far, such organic ligands have mainly been used to improve catalytic selectivity, hypothetically by stabilizing the transition state for a desired reaction relative to an undesired reaction. Scheme 2b shows an example in which pi-pi


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stacking interactions between the ligands and reacting species were found to favor an “upright” configuration during cinnamaldehyde hydrogenation. This upright configuration was associated with selective conversion of the carbonyl function, whereas olefin hydrogenation was strongly suppressed. Work to date with organic monolayers has largely focused on improving selectivity, in part because the ligands often lower activity by blocking large numbers of active sites and/or electronically perturbing the surface. Development of materials that avoid these issues is thus important for improving both activity and selectivity. In some cases, materials with enhanced activity for a desired reaction have been identified. For example, Kunz and coworkers35 found that the mass-based activity of Pt catalysts for selective ketone hydrogenation was improved by modifying the catalysts with L-proline. Here, the L-proline was proposed to improve selectivity through steric effects, and to also improve activity by introducing another active site associated with the amine. As depicted in Scheme 2, the amine was proposed to assist in one of the elementary steps, namely H transfer to the oxygen atom of the ketone reactant. Here, by virtue of its different BEP relationship for such reactions, the Pt-amine system provides interfacial active sites to carry out the reaction more efficiently.

3.3. Structures that allow non-thermal delivery of energy


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Electrocatalysts and photocatalysts are multi-functional materials that combine surface reactivity with energy (electron/hole or plasmon) transfer to exceed the rate offered by the thermally-stimulated process.

Electrocatalytic processes are inherently multi-component, as

one catalyst generates the ions/electrons used at the other site (Scheme 3a), and offer the potential to increase the reaction rate through the input of electrical energy, or to control the selectivity through voltage control. Electrified surfaces (electrodes) can serve as the catalytic sites for bond making and breaking while delivering electrical charge (high energy electrons) to change the energetics of kinetically relevant steps. One well-studied example of this is CO2 electrolysis to CO or hydrocarbons.36 The non-electrochemical CO2 hydrogenation reaction requires high temperature to occur on a single site catalyst.37 Separating the oxidation and reduction processes in an electrochemical cell, however, allows reaction at room temperature by providing a mechanism for non-thermal energy input through an applied voltage difference. Similarly, low temperature ammonia synthesis uses electron/proton transfer to N2 to weaken the N-N triple bond, making the process of ammonia synthesis more facile at significantly lower

a)

B

-

e energy

b)

+

A

Energy (light)

A

B

AB Metal

Plasmonic Metal

Conducting support Scheme 3. A reaction rate may be accelerated past the “volcano peak” through the non-thermal delivery of energy. In a), this is accomplished by separating the reaction into two redox half reactions, enabling electrical energy to accelerate the rate. In b) photons provide energy to excite plasmons that can accelerate rate-limiting steps in a catalytic process.


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temperatures than those associated with the conventional Haber Bosch processes.38-39 Photo(electro)catalytic processes can similarly separate oxidation and reduction on different sites to provide a cascade that allows reactivity at lower temperature through non-thermal energy input.40 Such processes also require one of the materials to participate as a photon absorber, further taking advantage of advances in multi-component system construction. Another family of materials with multifunctional characteristics are plasmonic metal catalysts, which operate by transiently injecting charge carriers into adsorbates when illuminated with resonant light (Scheme 3b).41 This process of charge injections leads to the “energizing” of the reactant which can be activated at lower temperature on the surface of the plasmonic metal nanoparticles.42-43 There are a number of examples of oxidation reactions on plasmonic nanostructures which activate oxygen using these principles. For example, Linic and co-workers have demonstrated an increased rate of ethylene epoxidation on silver catalysts by driving the reaction with visible light.41 The plasmonic functionality of Ag is used to overcome the thermally-limited rate by lowering the activation barrier for the rate limiting step, in this case, O2 dissociation.

4. Research Goals: An Outlook Increased synthetic and characterization control provide potential to control the structure and distribution of multiple components with new levels of precision to enable multicomponent catalyst design. Though there is some ability to combine similar materials, for example bimetallics, to move to the top of the volcano curves, combinations of materials that have different chemical functionalities provide more exciting avenues for substantial


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breakthroughs in activity and selectivity. Such multifunctional (often multicomponent) catalytic materials have the potential to improve activity or selectivity beyond the “peak” of the optimal single component catalyst.

The complexity of these multi-component systems makes

mechanistic attribution of enhanced performance challenging. The single-component-Sabatier framework suggests a classification of how multi-component catalysts improve activity. 1 – combine two functionalities with different BEP/scaling relationships for the same intermediates in the same reaction path to improve the rate through spillover and separate mechanistic functions 2 – create interfacial bifunctional sites that alter the BEP relationship to lower barriers for key reaction steps 3 – control the reactive 3D environment using non-covalent interactions to stabilize transition states and alter/improve BEP relationships and lower barriers 4 – direct energy to reactive modes to exceed the reaction rate of the thermally equilibrated system We have provided examples of each of these above, though this list is not exhaustive and other modes of cooperating between multi-components could exist. For example, a multicomponent system may push a reaction through a different mechanism or control transport rather than active site properties. Advances towards synthesizing systems with a high density and mono-dispersity of multi-component sites can further guide multi-component catalyst design. Characterization and theory can aid in delineating the mechanistic contribution of the multi-material systems. A key goal of these efforts will be to identify quantitative descriptors that allow rational tuning of multi-component catalysts, and a few examples of descriptor-


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based design of the materials platforms described here already exist. As one example related to ligand-modified catalysts, the energy of non-covalent interactions between ligands and reactants or transition states has been used to predict the relationship between alcohol dehydration rates and ligand structure on phosphonate-coated TiO2 catalysts.44 However, development of rational approaches to catalyst design is frequently limited by our understanding of the structure of complex multi-component structures.

5. Supporting Information Details of the micro-kinetic model used for dual-site cascades, a graphical explanation of why the scaling/BEP constrained cascade cannot offer a rate that exceeds the best single site catalyst, proof that the BEP slope must exceed 1 for the cascade to improve the rate under scaling/BEP constraints 6. Acknowledgements The authors acknowledge support from the National Science Foundation for funding this research through DMREF Grant #1436206.

GK acknowledges training provided by the

Computational Materials Education and Training (CoMET) NSF Research Traineeship (Grant # DGE-1449785).

7. References 1. Holewinski, A.; Xin, H. L.; Nikolla, E.; Linic, S., Identifying Optimal Active Sites for Heterogeneous Catalysis by Metal Alloys Based on Molecular Descriptors and Electronic Structure Engineering. Curr. Opin. Chem. Eng. 2013, 2, 312-319. 2. Schweitzer, N.; Xin, H. L.; Nikolla, E.; Miller, J. T.; Linic, S., Establishing Relationships Between the Geometric Structure and Chemical Reactivity of Alloy Catalysts Based on Their Measured Electronic Structure. Top. Catal. 2010, 53, 348-356.


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18 3. Oroz-Guinea, I.; Garcia-Junceda, E., Enzyme Catalysed Tandem Reactions. Curr. Opin. Chem. Biol. 2013, 17, 236-249. 4. Wheeldon, I.; Minteer, S. D.; Banta, S.; Barton, S. C.; Atanassov, P.; Sigman, M., Substrate Channelling as an Approach to Cascade Reactions. Nat. Chem. 2016, 8, 299-309. 5. Margelefsky, E. L.; Zeidan, R. K.; Davis, M. E., Cooperative Catalysis by Silica-Supported Organic Functional Groups. Chem. Soc. Rev. 2008, 37, 1118-1126. 6. Savchenko, V. I.; Dadayan, K. A., On the Possible Role of Adparticles Spillover on a Heterophase Surface in Causing Superadditive Activity. React. Kinet. Catal. Lett. 1995, 55, 33-40. 7. Andersen, M.; Medford, A. J.; Nørskov, J. K.; Reuter, K., Scaling-Relation-Based Analysis of Bifunctional Catalysis: The Case for Homogeneous Bimetallic Alloys. ACS Catal. 2017, 3960-3967. 8. Andersen, M.; Medford, A. J.; Nørskov, J. K.; Reuter, K., Analyzing the Case for Bifunctional Catalysis. Angew. Chem. Int. Ed. 2016, 55, 5210-5214. 9. Bronsted, J. N., Acid and Basic Catalysis. Chem. Rev. 1928, 5, 231-338. 10. Evans, M. G.; Polanyi, M., Further Considerations on the Thermodynamics of Chemical Equilibria and Reaction Rates. Trans. Faraday Soc. 1936, 32, 1333-1359. 11. Doan, H. Exploring Design Opportunities of Bifunctional Catalysts Using Density Functional Theory and Microkinetic Modeling. MS, University of Housing, University of Houston, 2012. 12. Wang, S.; Petzold, V.; Tripkovic, V.; Kleis, J.; Howalt, J. G.; Skulason, E.; Fernandez, E. M.; Hvolbaek, B.; Jones, G.; Toftelund, A.; Falsig, H.; Bjorketun, M.; Studt, F.; Abild-Pedersen, F.; Rossmeisl, J.; Norskov, J. K.; Bligaard, T., Universal Transition State Scaling Relations for (De)hydrogenation over Transition Metals. Phys. Chem. Chem. Phys. 2011, 13, 20760-20765. 13. Wang, S.; Temel, B.; Shen, J.; Jones, G.; Grabow, L. C.; Studt, F.; Bligaard, T.; Abild-Pedersen, F.; Christensen, C. H.; Norskov, J. K., Universal Bronsted-Evans-Polanyi Relations for C-C, C-O, C-N, N-O, N-N, and O-O Dissociation Reactions. Catal. Lett. 2011, 141, 370-373. 14. Vojvodic, A.; Calle-Vallejo, F.; Guo, W.; Wang, S.; Toftelund, A.; Studt, F.; Martinez, J. I.; Shen, J.; Man, I. C.; Rossmeisl, J.; Bligaard, T.; Norskov, J. K.; Abild-Pedersen, F., On the Behavior of BronstedEvans-Polanyi Relations for Transition Metal Oxides. J.Chem. Phys. 2011, 134. 15. Vines, F.; Vojvodic, A.; Abild-Pedersen, F.; Illas, F., Bronsted-Evans-Polanyi Relationship for Transition Metal Carbide and Transition Metal Oxide Surfaces. J. Phys. Chem. C 2013, 117, 4168-4171. 16. Robinson, A. M.; Hensley, J. E.; Medlin, J. W., Bifunctional Catalysts for Upgrading of BiomassDerived Oxygenates: A Review. ACS Catal. 2016, 6, 5026-5043. 17. Conner, W. C.; Falconer, J. L., Spillover in Heterogeneous Catalysis. Chem. Rev. 1995, 95, 759788. 18. Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M., Active Nonmetallic Au and Pt Species on Ceria-Based Water-Gas Shift Catalysts. Science 2003, 301, 935-938. 19. Fu, Q.; Kudriavtseva, S.; Saltsburg, H.; Flytzani-Stephanopoulos, M., Gold-Ceria Catalysts for LowTemperature Water-Gas Shift Reaction. Chem. Eng. J. 2003, 93, 41-53. 20. Deng, W. L.; De Jesus, J.; Saltsburg, H.; Flytzani-Stephanopoulos, M., Low-Content Gold-Ceria Catalysts for the Water-Gas Shift and Preferential CO Oxidation Reactions. Appl. Catal. A-Gen. 2005, 291, 126-135. 21. Green, I. X.; Tang, W. J.; Neurock, M.; Yates, J. T., Spectroscopic Observation of Dual Catalytic Sites During Oxidation of CO on a Au/TiO2 Catalyst. Science 2011, 333, 736-739. 22. Saavedra, J.; Doan, H. A.; Pursell, C. J.; Grabow, L. C.; Chandler, B. D., The Critical Role of Water at the Gold-Titania Interface in Catalytic CO Oxidation. Science 2014, 345, 1599-1602. 23. Griffin, M. B.; Ferguson, G. A.; Ruddy, D. A.; Biddy, M. J.; Beckham, G. T.; Schaidle, J. A., Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of m-Cresol over Pt/C and Pt/TiO2 Catalysts. ACS Catal. 2016, 6, 2715-2727.


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19 24. Hong, Y.; Zhang, H.; Sun, J.; Ayman, K. M.; Hensley, A. J. R.; Gu, M.; Engelhard, M. H.; McEwen, J.-S.; Wang, Y., Synergistic Catalysis between Pd and Fe in Gas Phase Hydrodeoxygenation of m-Cresol. ACS Catal. 2014, 4, 3335-3345. 25. Chia, M.; Pagán-Torres, Y. J.; Hibbitts, D.; Tan, Q.; Pham, H. N.; Datye, A. K.; Neurock, M.; Davis, R. J.; Dumesic, J. A., Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium–Rhenium Catalysts. J. Am. Chem. Soc. 2011, 133, 12675-12689. 26. Zhang, J.; Wang, B.; Nikolla, E.; Medlin, J. W., Directing Reaction Pathways through Controlled Reactant Binding at Pd–TiO2 Interfaces. Angew. Chem. Int. Ed. 2017, 56, 6594-6598. 27. Koper, M. T. M.; Lebedeva, N. P.; Hermse, C. G. M., Dynamics of CO at the Solid/Liquid Interface Studied by Modeling and Simulation of CO Electro-Oxidation on Pt and PtRu Electrodes. Faraday Discuss. 2002, 121, 301-311. 28. Lu, C.; Rice, C.; Masel, R. I.; Babu, P. K.; Waszczuk, P.; Kim, H. S.; Oldfield, E.; Wieckowski, A., UHV, Electrochemical NMR, and Electrochemical Studies of Platinum/Ruthenium Fuel Cell Catalysts. J. Phys. Chem. B 2002, 106, 9581-9589. 29. Stolbov, S.; Ortigoza, M. A.; Adzic, R.; Rahman, T. S., High CO Tolerance of Pt/Ru Nanocatalyst: Insight from First Principles Calculations. J. Chem. Phys. 2009, 130. 30. Zeidan, R. K.; Davis, M. E., The Effect of Acid–Base Pairing on Catalysis: An Efficient Acid–Base Functionalized Catalyst for Aldol Condensation. J. Catal. 2007, 247, 379-382. 31. Zeidan, R. K.; Hwang, S.-J.; Davis, M. E., Multifunctional Heterogeneous Catalysts: SBA-15Containing Primary Amines and Sulfonic Acids. Angew. Chem. Int. Ed. 2006, 45, 6332-6335. 32. Gounder, R.; Iglesia, E., The Roles of Entropy and Enthalpy in Stabilizing Ion-Pairs at Transition States in Zeolite Acid Catalysis. Acc. Chem. Res. 2012, 45, 229-238. 33. Schoenbaum, C. A.; Schwartz, D. K.; Medlin, J. W., Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers. Acc. Chem. Res. 2014, 47, 1438-1445. 34. Kahsar, K. R.; Schwartz, D. K.; Medlin, J. W., Control of Metal Catalyst Selectivity through Specific Noncovalent Molecular Interactions. J. Am. Chem. Soc. 2014, 136, 520-526. 35. Schrader, I.; Warneke, J.; Backenköhler, J.; Kunz, S., Functionalization of Platinum Nanoparticles with l-Proline: Simultaneous Enhancements of Catalytic Activity and Selectivity. J. Am. Chem. Soc. 2015, 137, 905-912. 36. Qiao, J. L.; Liu, Y. Y.; Hong, F.; Zhang, J. J., A Review of Catalysts for the Electroreduction of Carbon Dioxide to Produce Low-Carbon Fuels. Chem. Soc. Rev. 2014, 43, 631-675. 37. Porosoff, M. D.; Yan, B. H.; Chen, J. G. G., Catalytic reduction of CO2 by H2 for Synthesis of CO, Methanol and Hydrocarbons: Challenges and Opportunities. Energy Environ. Sci. 2016, 9, 62-73. 38. Rod, T. H.; Logadottir, A.; Norskov, J. K., Ammonia Synthesis at Low Temperatures. J. Chem. Phys. 2000, 112, 5343-5347. 39. Licht, S.; Cui, B. C.; Wang, B. H.; Li, F. F.; Lau, J.; Liu, S. Z., Ammonia Synthesis by N2 and Steam Electrolysis in Molten Hydroxide Suspensions of Nanoscale Fe2O3. Science 2014, 345, 637-640. 40. Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.; Kubiak, C. P., Photochemical and Photoelectrochemical Reduction of CO2. In Annual Review of Physical Chemistry, Vol 63, Johnson, M. A.; Martinez, T. J., Eds. Annual Reviews: Palo Alto, 2012; Vol. 63, pp 541-+. 41. Christopher, P.; Xin, H. L.; Linic, S., Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures. Nat. Chem. 2011, 3, 467-472. 42. Aslam, U.; Chavez, S.; Linic, S., Controlling Energy Flow in Multimetallic Nanostructures for Plasmonic Catalysis. Nat. Nanotechnol. 2017, 12, 1000-1005. 43. Boerigter, C.; Aslam, U.; Linic, S., Mechanism of Charge Transfer from Plasmonic Nanostructures to Chemically Attached Materials. ACS Nano 2016, 10, 6108-6115. 44. Ellis, L. D.; Trottier, R. M.; Musgrave, C. B.; Schwartz, D. K.; Medlin, J. W., Controlling the Surface Reactivity of Titania via Electronic Tuning of Self-Assembled Monolayers. ACS Catal. 2017, 7, 8351-8357.


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Multicomponent Catalysts: Limitations and Prospects - ACS Catalysis

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