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Topics in Catalysis

, Volume 61, Issue 12–13, pp 1210–1217 | Cite as

Surface Science Approach to the Molecular Level Integration of the Principles in Heterogeneous, Homogeneous, and Enzymatic Catalysis

  • Tyler J. Hurlburt
  • Wen-Chi Liu
  • Rong Ye
  • Gabor A. Somorjai
Original Paper
  • 283 Downloads

Abstract

Heterogeneous, homogeneous, and enzymatic catalysis have generally been treated and studied as three separate fields. However all three fields have many aspects that unify them, therefore it is useful to study catalysts from each field in similar manners. Heterogeneous catalysts have been studied extensively under reaction conditions to monitor dynamic changes that occur during catalytic reactions, their atomic and molecular structure, and composition and oxidation state with high spatial and time resolution. The techniques used to monitor these catalysts include sum frequency generation vibrational spectroscopy, high pressure scanning tunneling microscopy, and ambient pressure X-ray photoelectron spectroscopy. In order to use these techniques to study homogeneous catalysts and enzymes under reaction conditions, we have heterogenized homogeneous catalysts by encapsulating small metal clusters in dendrimers and immobilized enzymes through the use of DNA tethers. By studying all three fields under reaction conditions with the same techniques we aim to show that heterogeneous, homogeneous, and enzymatic catalysts all behave similarly at the molecular level.

Keywords

Catalysis Heterogeneous Homogeneous Enzyme Nanoparticles Surface chemistry 

1 Introduction

Catalysis has long been separated into three distinct fields—heterogeneous, homogeneous, and enzymatic—all studied independently. Heterogeneous catalysis operates with the catalyst in a different phase than the reactants and products (e.g. solid–liquid, solid–gas); homogeneous catalysis has both catalyst and reactants/products in the same phase (almost exclusively liquid); enzymatic catalysis uses the active site of proteins to carry out the catalysis needed for biological systems.

Instead of treating each field as its own distinct discipline, we aim to unify all three types of catalysis and show that they all behave similarly on the molecular scale. All three fields have the capability to carry out the same broad class of reactions. Such as the oxidation of primary alcohols where bimetallic Au–Pd nanoparticles supported on TiO2 (heterogeneous), a Cu–azo complex (homogeneous), and horse liver alcohol dehydrogenase (enzyme) all have an affinity for the oxidation of benzyl alcohol [1, 2, 3]. Other examples of catalysts from each field catalyzing the same reaction can be seen in Table 1.

Table 1

Catalysts from each of the three fields of catalysis that catalyze the same reactions [1, 2, 3, 4, 5, 6, 7, 8, 9]

 

Alcohol oxidation

Isomerization

Esterification

Heterogeneous

Au–Pd bimetallic nanoparticles on TiO2

Zn based coordination network

Sulfated ZrO2

Homogeneous

Cu–azo complex

Bulky palladium hydride complex

Bulky diarlyammonium arenesulfonate

Enzyme

Alcohol dehydrogenase

Peptidyl prolyl cis–trans isomerase

Lipase

In most cases, the active catalyst, whether heterogeneous, homogeneous, or enzyme, is under 10 nm in scale (Fig. 1). As such, all of these catalysts could be considered nanoparticles. Besides the connection in their sizes, it has also been shown that the oxidation states of noble metal nanoparticles, including Pt and Rh, increase with decreasing sizes [10, 11]. When the nanoparticles are sufficiently small, their oxidation states eventually approach that of the metal complexes, which are extensively used as catalysts in the homogeneous catalysis.

Fig. 1

An example catalyst from each field of catalysis. Each catalyst is smaller than 10 nm in each dimension. These catalysts catalyze CO oxidation, olefin polymerization, and protein hydrolyzation respectively

As in the case of heterogeneous catalysis, the development of colloidal chemistry provides enormous possibilities towards producing various metal and metal oxide nanoparticles. By carefully controlling the precursors, surfactants, solvents, reducing agents, and the reaction temperature used during the synthesis, a wide range of nanoparticles with well-defined sizes, shapes, and compositions could be obtained [12, 13, 14].

The synthesized nanoparticles, either deposited on a two-dimensional surface or encapsulated into a three-dimensional porous material, are utilized in heterogeneous catalysis (Fig. 2). Following this approach, heterogenizing homogeneous and enzyme catalysts are also actively sought after, which will be discussed in detail in later sections.

Fig. 2

Nanoparticle catalysts can be deposited onto a two-dimensional surface through assembly by Langmuir–Blodgett film (left) or encapsulated in a three-dimensional support such as a mesoporous silica (right)

1.1 Characterization of Nanoparticle Catalysts Under Reaction Conditions

To truly understand these nanoparticle catalysts it is not enough to study the catalyst before and after the reaction, but to actually follow the catalyst while the reaction is ongoing. For example, it has been demonstrated repeatedly that the catalysts undergo structure reconstruction and changes in oxidation states upon contact with the reaction atmosphere, and that the active sites are often formed in-situ under reaction conditions [15, 16, 17, 18]. In addition, since the catalytic reactions often take place on the surface of the nanoparticle catalysts, characterization techniques equipped with superior surface sensitivities would be extremely powerful. Much of the work that has been done in studying catalysis under reaction conditions has focused on heterogeneous catalysis due to limitations of the spectroscopic and microscopic techniques used. These techniques include: sum frequency generation (SFG) vibrational spectroscopy, high pressure scanning tunneling microscopy (STM), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and nanodiode hot electron detection.

SFG is an inherently surface sensitive spectroscopic technique, making it a particularly good tool for studying adsorbed species under catalytic reaction conditions [19, 20, 21, 22]. SFG requires the spatial and temporal overlap of a fixed wavelength visible beam and a variable IR beam (Fig. 3a). When the IR frequency matches a vibrational mode of the surface there is a resulting outgoing beam that has a frequency equal to the sum of the frequencies of the two incoming beams (Fig. 3b). During ethylene hydrogenation reactions on Pt(111) we have observed several molecular species on the surface of the catalysts [21]. These include ethylidyne bound perpendicularly to the surface and ethylene bound parallel to the surface via either π-bonded or di-σ-bonded (Fig. 3c). Knowing the orientation and bonding of these molecules makes it possible to determine, under reaction conditions, the molecular details of the mechanisms of these reactions.

Fig. 3

a Scheme of an SFG system showing the overlap of the incoming fixed visible beam (green) and a variable IR beam (red) and the outgoing SF beam (blue). b Energy level diagram of SFG showing the frequency of the outgoing signal is equal to the sum of the frequencies of the two incoming beams. c SFG spectra of ethylene hydrogenation on Pt(111) showing the orientation of various molecular species on the surface of the catalyst [21]

Using high pressure STM it is seen that the species adsorbed on a solid catalyst are mobile, not just stuck in one active site [23, 24, 25]. Studies of cyclohexene hydrogenation on Pt(111) show streaky, diffuse STM images while the reaction is ongoing (Fig. 4a, b). However, upon the addition of carbon monoxide (which poisons any catalytic turnover) clear, ordered structures are seen (Fig. 4c, d). This suggests that when the catalyst is active and the reaction is ongoing the adsorbed species move faster than the tip of the STM (100 Å/ms) [25].

Fig. 4

a 200 Å × 200 Å high pressure STM image of Pt(111) in the presence of 20 mTorr cyclohexene and 20 mTorr hydrogen at 350 K. Blurry, streaky image suggests adsorbed species move faster than the STM tip. b Pressures of cyclohexene (black squares), cyclohexane (red triangles), and benzene (green diamonds) during cyclohexene hydrogenation. c 90 Å × 90 Å high pressure STM image of Pt(111) in the presence of 20 mTorr cyclohexene, 200 mTorr hydrogen, and 5 mTorr CO at 300 K (CO poisons the catalyst). Yellow rhombus represents the unit cell. d Pressures of cyclohexene, cyclohexane, and benzene during cyclohexene hydrogenation on poisoned catalyst [25]

Ambient pressure XPS can be used to determine the surface composition and oxidation states of bimetallic nanoparticles under reaction relevant conditions rather than the ultra-high vacuum conditions typically needed for XPS (Fig. 5a). In the presence of an oxidizing gas (nitric oxide) 15 nm rhodium-palladium nanoparticles preferentially segregate rhodium to the surface, with ~ 94% of the Rh being in the oxide form. Upon the addition of a reducing gas (carbon monoxide) to mimic the reaction conditions, the surface composition becomes much more equal and about 76% of the remaining surface Rh are reduced to its metallic state. This trend continues through cycles of oxidizing and reducing conditions (Fig. 5b) [26].

Fig. 5

a Scheme of ambient pressure XPS setup. b Surface atomic fraction of Rh and Pd in bimetallic nanoparticles under oxidizing (NO) and reducing/catalytic (NO + CO) conditions [26]

Besides the characterization techniques that probe the surface structure and composition as well as the adsorbed species, the detection of hot electrons generated during exothermic catalytic reactions with nanodiodes grants access to the charge transfer process on the catalytic interfaces during the reaction [27, 28]. For example, the turnover rates of H2 and CO oxidation were shown to be directly correlated to the strength of the chemicurrent detected at the metal-support interface, which provides important insights on the essence of the strong metal-support interaction (SMSI) effect [29].

1.2 Heterogenization of Homogeneous and Enzymatic Catalysts

In order to be able to use these surface based techniques to study homogeneous and enzymatic catalysts we have immobilized catalysts from both fields onto solid supports. This includes small metal clusters in dendrimers supported onto mesoporous supports to study homogenous catalysis and enzymes tethered to glass slides to study enzymatic catalysis.

Metal clusters contain two to several hundred atoms. Theoretical work by Nørskov’s group suggests that above 560 atoms (~ 2.7 nm), the surface chemical activity of Au clusters transforms from a molecular to bulk behavior [30]. For instance, below that size, finite-size effects are observable, and facets become small enough for the charge redistribution upon oxygen adsorption to start reaching the edge of the particle (Fig. 6a). Additionally, we have shown that the oxidation state of platinum nanoparticles changes with the size of the particle. As the particle goes from 1.5 to 0.8 nm, the platinum changes from being almost entirely metallic to being primarily oxidized [31] (Fig. 6b). These two studies show that the electronic structure of metal nanoparticles is dependent on the size of the particle.

Fig. 6

a Charge redistribution of gold nanoparticles upon oxygen adsorption from top and side views. Blue and red coloring represents charge accumulation and depletion respectively. Number of atoms in particles are listed above the particles—far right is bulk gold(111). b XPS spectra of 0.8 and 1.5 nm Pt nanoparticles showing that the 0.8 nm particle is primarily oxidized and the 1.5 nm particle is mostly metallic [30]

We can synthesize the small nanoclusters in dendrimers, a class of uniform polymers with the connectivity of fractal trees and generally radial symmetry. The dense multivalency, shape persistence, and structural uniformity of dendrimers render them versatile scaffolds for the synthesis and stabilization of small nanoclusters. These dendrimer-encapsulated metal clusters (DEMCs) are then loaded into mesoporous silica to generate solid catalysts (Fig. 7). These catalysts can catalyze reactions that had been challenging for conventional heterogeneous catalysts (e.g. aldol reactions and π-bond activation). In addition, supported DEMCs also show high activities for typical heterogeneous reactions, including hydrogenation and alkane hydroisomerization, which are listed in Table 2 [17, 32, 33, 34, 35, 36, 37, 38]. Critically, studies confirmed that these supported DEMCs are truly heterogeneous and stable against leaching. Thus, we have heterogenized homogeneous catalysts, which can be defined as the modification of homogeneous catalysts to render them in a separable (solid) phase from the starting materials and products.

Fig. 7

Synthesis of dendrimer-encapsulated metal clusters and loading into mesoporous silica (SBA-15) [32]

Table 2

A list of reactions studied with supported DEMCs in our group

Dendrimer-encapsulated Au clusters are able to catalyze the Hayashi–Ito aldol reaction of methyl isocyanoacetate (MI) and benzaldehydes, a classic homogeneous Au(I)-catalyzed reaction [34]. The Au clusters showed higher turnover rates than the homogeneous equivalents, yet they allowed for tunable selectivity to the products in the trans or cis configurations. When these Au clusters are loaded in mesoporous silica with relatively large pores, the selectivity to the trans product could be as high as 100%, while the cis product selectivity is higher when the pore size was limited. Another example is the ring-opening of cyclopropane derivatives under hydrogen, a model reaction of C–C bond activation [33]. Dendrimer-encapsulated rhodium clusters are able to catalyze the ring-opening of phenylcyclopropane at room temperature with 100% selectivity to the linear product, while the Wilkinson’s catalyst, a classic homogeneous catalyst for this reaction, gives much lower turnover rates with a mixture of linear and branched products. The differences of activities and selectivity of the clusters and typical homogeneous catalysts are attributed to different reaction mechanisms.

To heterogenize enzymes, we have begun studying DNA directed immobilization (DDI) [39, 40, 41]. In this method a short (20–25 base) DNA strand is conjugated to an enzyme. The complementary strand is attached to a solid surface. The two strands can then hybridize with each other giving an immobilized enzyme system (Fig. 8a). Specifically, in the method we use, the enzyme is conjugated to the DNA strand at the N-terminus and no other locations. This allows for control in the orientation of the immobilized enzymes.

Fig. 8

a General scheme of DNA directed immobilization. A 20–25 base DNA strand is attached to a glass surface and the complimentary strand is conjugated to a protein. Upon hybridization of the DNA strands, an ordered array of enzymes are tethered to the surface. b The reaction catalyzed by aldolase. c Concentration of products over time in the presence of DNA-aldolase in solution (green), DNA-aldolase immobilized on the surface (pink), DNA-aldolase non-specifically adsorbed on the surface (blue). d Relative rates of activity of DNA-aldolase in solution (green) and immobilized DNA-aldolase (pink) after reuse cycles [42]

We have used DDI to immobilize aldolase onto glass slides [42]. Aldolase is an enzyme central to the glycolysis pathway that catalyzes the C–C bond breaking reverse aldol reaction. Specifically aldolase catalyzes the conversion of fructose 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate (Fig. 8b). Upon immobilization aldolase retains its activity and there is a minimal activity coming from any non-specifically adsorbed enzyme (Fig. 8c). Additionally, the immobilized aldolase is easily reusable and is more stable than the free enzyme in solution through multiple cycles (Fig. 8d). The ordering of the enzymes achieved by this immobilization technique is required for studying the enzyme using SFG vibrational spectroscopy.

Now that we have shown the ability to heterogenize both homogeneous and enzymatic catalysts, we are working on using the surface techniques typically reserved for studying heterogeneous catalysis to understand these other fields. By using these same techniques to study catalysts on the molecular level from each field while they are catalyzing the same reactions, we hope to unify all three fields of catalysis.

Notes

Acknowledgements

The work shown in this article was supported by the Director, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences and Biosciences Division of the US Department of Energy under Contract DE-AC02-05CH11231.

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© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  • Tyler J. Hurlburt
    • 1
    • 2
    • 3
  • Wen-Chi Liu
    • 1
    • 2
    • 3
  • Rong Ye
    • 1
    • 2
    • 3
  • Gabor A. Somorjai
    • 1
    • 2
    • 3
  1. 1.Department of ChemistryUniversity of CaliforniaBerkeleyUSA
  2. 2.Kavli Energy NanoScience InstituteUniversity of CaliforniaBerkeleyUSA
  3. 3.Chemical Sciences DivisionLawrence Berkeley National LaboratoriesBerkeleyUSA

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