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3 Chemistry on the Nanoscale

3 Chemistry on the Nanoscale

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Chemistry on the Nanoscale




Fig. 10.25 (a) High-resolution transmission electron micrograph (HRTEM) of tris (cyclopentadienyl)cerium (CeCp3 ) in a single-walled carbon nanotube SWNT (CeCp3 @SWNT). (b), (c)

HRTEM of the sample transformed at 1000◦ C in vacuo into double-walled carbon nanotubes

(DWNT) containing cerium (Ce@DWNT). The undulating lines in between the outer tube wall

indicate an inner tube, while the dark dots are from cerium ions. Scale bar is 1 nm. (Reprinted with

permission from [10.48]. © 2009 American Physical Society)

For the application of nano test tube chemistry, the 1D quantized electronic

levels of SWNTs can be manipulated by doping [10.48], e.g., with tris(cyclopentadienyl)cerium (CeCp3 ; see Fig. 10.25a).

10.3.2 Dynamics in Water Nanodroplets

There are many examples in the fields of biology, geochemistry, tribology, and

nanofluidics, where water molecules are not present as a bulk liquid, but in confined geometries [10.49]. Near a surface, ordering of water molecules into layers

occurs [10.50] as shown by x-ray diffraction. This ordering was found to extend up

to several molecular diameters into the liquid. In the case of small water droplets, the

confinement is 3D, and the overall structure and dynamics of the water are affected.

A solution of nanometer-sized droplets (see Sect. 7.9) forms when preparing an emulsion of water in an apolar solvent by addition of a surfactant. The

anionic lipid surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT) is known

to form monodisperse micelles with radii ranging from 0.2 to 4.5 nm [10.49],

depending of the water-to-AOT ratio, conventionally denoted by the parameter

w0 = [H2 O] [AOT]. The dynamics of water molecules in nanodroplets was studied

by using ultrafast mid-infrared pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted water (HDO in D2 O) contained in reverse micelles. In




these experiments, the contributions of core and interfacial water molecules can be

observed separately, demonstrating that the molecular mobilities are surprisingly


When the size of the water droplets is increased, the amplitude of the slow component in the vibrational relaxation, which can be assigned to the water molecules

at the droplet interface, strongly decreases. From a quantitative analysis, a surface

coordination of about six hydrogen bonds per surfactant molecule is derived. This

number of hydrogen bonds is consistent with the fact that there are six lone electron

pairs located at the oxygens of the sulfonate anion of the AOT surfactant molecule,

which can accept one hydrogen bond each. Whereas the core water molecules reorient on a timescale close to that of bulk water (2–4 ps), the interfacial water is highly

immobile with reorientation times >15 ps [10.49].

The strongly different orientational mobilities of core and interfacial water

molecules have to be explained from their different intermolecular interactions and

very different geometric arrangements. Molecular reorientation involves the subsequent breaking and formation of hydrogen bonds. For core water molecules (as for

bulk water), the activation energy for reorientation is substantially lowered, because

these molecules can break a hydrogen bond while simultaneously forming a new

bond with another water molecule. For interfacial water molecules this process is

inhibited because these molecules are hydrogen bonded to an immobile surfactant

molecule. The slow orientational dynamics for the interfacial water molecules is

supported by molecular dynamics simulations on micellar systems [10.51]. Water

molecules with full bulk-like character only start to appear in the core of a cluster

of at least 2,000 water molecules (ω0 = 12) [10.49].

10.3.3 Targeted Delivery and Reaction of Single Molecules

Scanning probe microscopy-based techniques can be used to manipulate and deliver

single molecules in a precisely controlled manner to a specific target. Reactive

polymer molecules, attached at one end to an atomic force microscope (AFM) tip

(Fig. 10.26a), are brought into contact with a modified silicon substrate to which

they become linked by a chemical reaction. When the AFM tip is pulled away from

the surface, the mechanical force causes the weakest bond – the one between the

tip and the polymer – to break. This process transfers the polymer molecule to a

precisely defined site on the substrate where it can be modified by further chemical

reactions [10.52].

An Au-coated AFM tip is modified by poly-N-succinimidyl acrylate (PNSA)

and a silicon substrate with amino propyltrimethoxysilane. In an N,Ndimethylformamide solution containing 4-dimethylaminopyridine (DMAP), the

functionalized AFM tip is brought into contact with the substrate to covalently link

the polymer chains to the substrate (Fig. 10.26a). Upon retraction of the tip, the Au

(tip)-c (polymer) bond is the weakest link and the most likely candidate for breaking. Upon cleavage, the polymer chain remains covalently attached to the substrate

and can be easily modified by a wide range of nucleophilic compounds [10.52].


Chemistry on the Nanoscale


Fig. 10.26 (a) Molecule by molecule delivery process. Left: During the tip–sample contact a

chemical reaction occurs between the activated esters of a poly-N-succinimidyl acrylate (PNSA)

chain grafted to the tip and the amino groups of the substrate to form an amide bond, which covalently links the chain to the substrate. Right: When the tip is pulled away from the surface, the

mechanical force causes the weakest bond – the bond between the tip and the polymer – to break.

(b) Atomic force microscopy (AFM) topography images obtained in air after delivery. Image

obtained in the area where four (PNSA) chains were deposited, one at a time. The chains are

decorated by branched polyethyleneimine (PEI).The decorated molecules appear in an extended

shape with a maximum vertical height of 4 nm. (Reprinted with permission from [10.52]. © 2006

Nature Publishing Group)




For visualization, the single organic PNSA chain on the substrate is reacted with

branched polyethyleneimine (PEI). The resulting structure is a PNSA backbone with

PEI side chains, which can be imaged by AFM (see Fig. 10.26b).

10.4 Catalysis

Catalysts are materials that accelerate the pace of chemical reactions, without changing their thermodynamics, and without being consumed during the reaction (see

[10.53]). They come in many forms as nanocrystals, metal complexes, small organic

molecules, enzymes. An industrial heterogeneous catalyst (see [10.54]) is typically

a material of nanoparticles (see [10.55]) dispersed on a porous (often metal oxide)

support [10.56]. Because of their inherent complexity and dispersed nature, most

catalysts have been developed by trial and error methods in the past, but new challenges demand new concepts for designing and synthesizing catalysts. The altered

geometric and electronic structure of nanoclusters may be employed – together with

the potentials of density functional theory (DFT) ab initio calculations (see, e.g.,

[10.57–10.59]) – to develop novel and unique catalytic properties.

In heterogeneous catalysis, the reacting molecules are absorbed on the catalytically active solid surface. Chemical bonds are broken and formed on the surface and

eventually the products are released back into the liquid or gas phase. For the elucidation of elementary processes in catalysis in his pioneering work [10.60, 10.61],

Gerhard Ertl has been awarded the Nobel Prize in chemistry in 2007.

Most industrial reactions for chemical conversion are catalyzed [10.62].

Specifically, catalysis enables the cost-effective and environmentally sound production of fuels (e.g., transportation fuels) with about ten different catalysts during

its transformation from curde oil [10.58]. Furthermore, catalysts are used in the

catalytic converter in the exhaust of every car, for the production of chemicals

(ammonia, methanol, flavors, fragrances), foods (hydrogenated fats), and pharmaceuticals (pain relievers). The value of the goods manufactured annually in the

United States in processes that involve catalysts is estimated to be several trillion

dollars [10.62].

In the following, studies of atomic-scale catalytic activities will be outlined on

some metallic and nonmetallic nanocrystals, which can be specifically investigated

by scanning tunneling microscopy [10.56, 10.63], high-resolution transmission electron microscopy (HRTEM; see Fig. 10.27), x-ray diffraction [10.64], extended x-ray

absorption fine structure (EXAFS [10.65]), x-ray photoelectron spectroscopy (XPS

[10.66]), etc.

10.4.1 Au Nanocrystals

Although extended gold surfaces are generally considered chemically inert, nanosized (<5 nm) gold particles can be effective catalysts for a number of oxidation




Fig. 10.27 Lattice-resolved environmental transmission electron microscopy of a Au cluster during exposure to a cycle of (a) H2 , 2 mbar; (b) O2 , 2 mbar; and (c) again H2 , 2 mbar at ambient

temperature. The images show a reversible change from a faceted shape in hydrogen, to a more

spherical shape during oxygen exposure to the final state again in hydrogen in which the faceted

shape is recovered. (Reprinted with permission from [10.67]. © 2006 Elsevier)

reactions [10.68, 10.70], allowing significantly lowered reaction temperatures for

the development of energy-efficient processes (see [10.58]). The reasons for the catalytic activity of small Au particles are seen, in addition to other effects, particularly

in the role of low-coordinated Au atoms in Au nanocrystals.

The nobility of a metal is well illustrated by its ability to chemisorb oxygen

dissociatively as characterized by the oxygen chemisorption energy. As shown in

Fig. 10.28a, Au is the only metal with an endothermic chemisorption energy, illustrating its inert behavior in an oxygen atmosphere. This is due to the electron

d-bond states in Au which are so low in energy that the interaction with oxygen

2p states is net repulsive. Nevertheless, the oxidation of CO is one of the reactions

where Au nanoparticles with sizes <5 nm are a very good catalyst (Fig. 10.28b)

on different support materials. This can be understood by the results of DFT calculations to simulate the absorption of molecules on ten-atom free Au clusters

[10.58] (which also can be prepared experimentally [10.69]): CO oxidation by these

clusters is possible at room temperature and, most importantly, the binding energy

decreases to exothermic values with the coordination number (CN) of the Au atoms

(Fig. 10.28c) to which the reactants bind. Au atoms on a close-packed surface have

nine neighbors, at steps on a surface seven, and at the corners of small particles it

can be as low as three to four. The increase in the fraction of corner atoms with

decreasing Au particle size (Fig. 10.28d) coincides with the observed increase in

CO oxidation activity (Fig. 10.28b). This indicates the enhanced reactivity of lowcoordinated Au atoms which are abundant on the smallest nanometer-sized Au

nanoparticles [10.58]. It, furthermore, has been shown – based on a set of DFT

calculations of the full pathway of CO oxidation – that although platinum and

palladium are the most active catalysts for extended surfaces at high temperatures

(600 K), gold is the most active for very small particles at low temperatures (273 K)





Fig. 10.28 Enhanced catalytic activity of low-coordinated atoms of Au nanoclusters. (a) The

dissociative chemisorption energies for oxygen on transition metal surfaces with respect to a

molecule in vacuum calculated by density functional theory (DFT) . All results are for adsorption at either a body-centered cubic (210) surface (for Fe, Mo, W) or a face-centered cubic (211)

surface (other metals). (b) Reported catalytic activities for CO oxidation at 273 K as a function of

gold nanoparticle size d for different support materials. The solid curve shows the calculated fraction of atoms located at the corners of nanoparticles as a function of d for uniform particles shaped

as the top half of a regular cuboctahedron. (c) The correlation between the binding energies for O2 ,

O, and CO on Au and the coordination number of the Au atoms in different surfaces and clusters.

The binding energies are calculated using density functional theory (DFT), for the experimental

values see [10.58]. (d) Calculated fractions of Au atoms at corners (red), edges (blue), and crystal

faces (green) in uniform nanoparticles with the shape of the top half of a truncated octahedron (see

insert) as a function of Au particle diameter. (Reprinted with permission from [10.58]. © 2007


At present, the most serious problem associated with Au nanocatalysts is probably their short long-term stability which appears to be enhanced by atomic defects

in the support surface as shown by STM studies of Au clusters on rutile TiO2 (110)

surfaces [10.56]. Under reducing conditions (r-TiO2 ), vacancies of the bridging oxygen (Obr ) atoms can be introduced (see Fig. 10.29), by water doping (h-TiO2 ) H

adatoms can cap some of the Obr atoms, and by oxidation (o-TiO2 ) O adatoms (Oot )

are introduced. Obr vacancies and Oot atoms can stabilize small Au clusters on the

terraces and heating experiments demonstrate that Au clusters bind most strongly to

the o-TiO2 surface which is corroborated by DFT calculations [10.56].




Fig. 10.29 Structure of a TiO2 (110) surface with various types of atomic defects. (a) Ball model

of the surface. Large red balls represent O atoms, medium-sized black balls sixfold coordinated

Ti (6f-Ti), and medium-sized gray balls fivefold coordinated surface Ti atoms (5f-Ti). Small light

balls indicate H atoms. The bridge-bonded O species (Obr ), single oxygen vacancies (V0 ), and

on-top bonded O species (Oot ) are also indicated. Scanning tunneling microscopy (STM) images

of reduced r- (b), water-doped h- (c) and oxidized o- (d) TiO2 (110) surfaces before Au exposure.

Size is 13 × 13 nm2 . In the STM images, the symbols indicate Obr vacancies (square), H adatoms

on Obr sites (hexagon), and Oot atoms in the Ti troughs (circle). Insets (3 × 3 nm2 ) show the

point defects of interest enlarged [10.56, 10.71]. (Reprinted with permission from [10.56]. © 2007


10.4.2 Pt Nanocatalysts

Considerable research has been stimulated to use nanometer and sub-nanometer

(Pt8–12 ) Pt clusters as a catalyst [10.72–10.75] with high performance and utilization efficiency in the production of hydrogen and particularly in the direct methanol

fuel cell (see [10.76]). It has been shown that high-index planes generally exhibit

much higher catalytic activity than the stable planes, such as {111} and {100},

because the high-index planes have a high density of atomic steps, ledges, and kinks,

which usually serve as active sites for breaking chemical bonds (see [10.77]). Pt

nanocrystals with a tetrahexahedral (THH) shape [10.77] are enclosed by 24 highindex facets as {730}, {210}, and/or {520} surfaces (see Fig. 10.30a, c) that have

a large density of atomic steps. These high-energy surfaces are stable to 800◦ C

and exhibit much enhanced (up to 400%) catalytic activity for equivalent Pt surface




Fig. 10.30 (a) Transmission electron micrograph (TEM) of a tetrahexahedral (THH) Pt nanocrystal with {730} surfaces. The inset is a projected model of the THH. (b) Corresponding selected area

electron diffraction (SAED) pattern, showing the single-crystal structure of the THH Pt nanocrystal. (c) High-resolution transmission electron micrograph (HRTEM) of a THH Pt nanocrystal

showing the surface atomic steps made of (210) and (310) subfacets [10.77]. (d) Hollow Pt shell

with nanochannels [10.76]. (Reprinted with permission from [10.77] (a–c) and [10.76] (d). © 2007

AAAS (a–c) and © 2008 American Chemical Society)

areas for electro-oxidation of small organic fuels as formic acid and ethanol [10.77].

The catalytic activity is also enhanced in the case of hollow Pt nanospheres with

nanochannels (Fig. 10.30d). High thermal stability of Pt nanoparticles as catalysts

for ethylene hydrogenation and CO oxidation can be achieved by coating with a

mesoporous silica shell [10.78]. Enhanced catalytic activity for hydrogenation reactions in the aqueous phase was demonstrated for very small Pt nanocrystals with a

diameter of 2–3 nm [10.79]. The (111) facets of tetrahedral Pt nanoparticles favor

selectively the formation of cis-2-butene, despite the fact that, in the gas phase, it is

energetically less stable than trans-2-butene, but more stable when adsorbed on the

Pt (111) surface. DFT calculations indicate that the Pt (111) surface reconstruction

is central to the adsorption of the isomers [10.80].

10.4.3 Pd Nanocatalysts

A TEM image of Pd nanocrystals on an Al2 O3 support is shown in Fig. 10.31. The

catalyst composition and oxidation state can be determined by x-ray photoelectron

spectroscopy (XPS) . The properties of a catalyst may significantly change during

a catalytic reaction due to surface restructuring by the adsorption of reactants or

products, due to the formation of Pd hydrides during hydrogenation reactions, due




Fig. 10.31 (a) Transmission electron micrograph of a technological Pd–Al2 O3 catalyst (5 wt%

Pd). (b) Atomic structure of a cuboctahedral Pd nanoparticle by high-resolution imaging. (c) PdO

nanoparticle of an initial Pd–Al2 O3 catalyst used for methane combustion at ambient pressure at

about 825 K. (Reprinted with permission from [10.81]. © 2007 Elsevier)

to carbon laydown, due to oxide formation (Fig. 10.31c), or due to changes of the

surface composition in the case of bimetallic catalyst nanoparticles [10.81].

By alloying Pd nanoparticles with Ag, a higher fraction of CO can bind to single

Pd atoms. Isolated Pd atoms in bimetallic Pd–Ag nanoparticles are also crucial for

the selective hydrogenation of acetylene C2 H2 to ethylene C2 H4 [10.81]. When the

surface of Pd–Au nanoparticles exhibits a maximum number of pairs of isolated Pd

atoms, the rate of vinyl acetate synthesis shows a maximum, suggesting that the Pd

monomer pair is the active site [10.81].

10.4.4 MoS2 Nanocatalysts as Model Catalysts

for Hydrodesulfurization (HDS)

Driven primarily by strict environmental regulations, a substantial decrease in the

sulfur level in transportation fuels has been achieved. However, there is an urgent

need for even cleaner fuels and better HDS catalysts (see [10.56]). The HDS catalyst, which separates S embedded in organic compounds in crude oil, consists of

2–3 nm diameter MoS2 nanoparticles promoted for activity enhancement with Co

or Ni and supported on a porous alumina carrier. STM studies have made it possible

to investigate the cluster morphology, the atomic scale structure of the catalytically

important MoS2 cluster edges, their active sites, and the location of promoter atoms

[10.56], as discussed below.




Fig. 10.32 (a) Atom-resolved STM image of a single-layer MoS2 nanocluster (4.1×4.1 nm2 ). (b)

Ball model (top view above and side view below) of the edge structure determined as the Mo edge

covered with S2 dimers. Mo atoms are shown in blue and S atoms are yellow. (c) STM images

of a MoS2 nanocluster after exposure to hydrogen and the test molecule thiophene (C4 H4 S);

5×5 nm2 . (d) STM image and ball model of a single-layered Co–Mo–S nanoparticle; image size:

5.1×5.2 nm2 . Color code for the ball model: S – yellow, Mo – blue, Co – red. (Reprinted with

permission from [10.56]. © 2007 Elsevier)

As an HDS model catalyst, MoS2 nanoclusters are synthesized on a Au substrate [10.56]. In the STM image of Fig. 10.32a, a characteristic bright brim of

high-electron state density is observed to extend all the way around the cluster

edge. The electronic edge state is metallic in character, whereas bulk MoS2 is a

semiconductor with a bandgap of 1.2 eV. Experiments with hydrogenation of thiophene (C4 H4 S) to C4 H7 S on the MoS2 nanocatalysts (see Fig. 10.32c) suggest

together with DFT studies that the activation occurs on the metallic brim states on

S vacancies.

Co and Ni are known to act as promoters for the MoS2 -based HDS catalysis.

The Co-promoted nanoclusters adopt truncated shapes (Fig. 10.32d, e) relative to

the triangular morphology of the unpromoted particles (Fig. 10.32a). The truncated

shape implies that new types of edge terminations are present with the metallic




brim as a new type of active site for hydrogenation that in combination with sulfur

vacancies can facilitate the full desulfurization of S-containing molecules [10.56].

10.4.5 In Situ Phase Analysis of a Catalyst

To gain insight into the mechanisms of heterogeneous catalysts, which would guide

the design of novel catalysts, it is necessary to have a detailed characterization of

the physicochemical composition of heterogeneous catalysts in their working state

at the nanometer scale [10.82]. For this goal, a nanoreactor together with scanning

transmission x-ray microscopy (STXM) is used at atmospheric pressure and up to

350◦ C to monitor in situ phase changes in an iron-based Fischer–Tropsch catalyst.

In the Fischer–Tropsch synthesis (FTS), synthesis gas (CO + H2 ) is converted

into hydrocarbon chains through a surface polymerization reaction for the production of chemicals and fuels from sources other than crude oil, most notably natural

gas, coal, and biomass. The catalyst consists of an iron oxide phase dispersed on

SiO2 . During FTS, iron oxide and metallic iron usually coexist with the iron phases

largely converted into iron carbides. For in situ characterization of the catalyst,

the x-ray absorption (see Sects. 2.7 and 8.1) at the carbon K edge (284.2 eV), at

the oxygen K edge (543.1 eV), and at the iron L2 and L3 edges (706.8 eV and

719.9 eV, respectively) are used with a spatial resolution of ∼15 nm [10.82]. Before

the in situ experiments the iron phase of the catalyst is mainly present as α-Fe2 O3

(Fig. 10.33a). After a 2 h exposure to H2 at 350◦ C, a conversion to a mixture of iron

Fig. 10.33 Chemical contour maps (400 nm × 750 nm) derived by scanning transmission x-ray

absorption microscopy (STXM). (a) Before treatment at 25◦ C 105 Pa He, (b) after 2 h in H2

(∼ 105 Pa), and (c) after 4 h in synthesis gas (∼105 Pa) at 250◦ C. The squares indicate the sites

where STXM spectra at the iron L2 and L3 edges and at the oxygen K edge are given in. (Reprinted

with permission from [10.82]. © 2008 Nature Publishing Group)

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