Bowker M., Davies P.R. (Eds.) Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis

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In Situ STM Studies of Model Catalysts

Fan Yang and D. Wayne Goodman

3.1

Introduction

The surface science approach to studying heterogeneous catalysis dates back to the

pioneering work of Langmuir [1] in the 1910s that addressed the adsorption of gases

on catalyst surfaces. Since then surface science studies of catalytic processes have

played a central role in our understanding of catalysis and have aided in the design

and improvement of catalysts for energy and environmental uses. The goal of surface

science investigations has been to provide structural and spectroscopic information

of catalyst surfaces at the spatial and temporal limit. Scanning tunneling microscopy

(STM), with the capacity to reach the spatial limit at the atomic level, has ignited

considerable interest since its discovery and has become a widely used tool in catalytic

science.

By following a selected area or a molecule at a model catalyst surface, in situ STM

can provide temporal measurements regarding the elementary steps of catalytic

transformations. The capabilities of in situ STM allow one to follow the dynamic

change of surface species and identify what Taylor described as the active site in

catalytic reactions. Such studies also provide kinetic measurements at the atomic

scale, enabling the most precise modeling of macroscopic reactions. With the

development of STM techniques, a large body of in situ STM work has emerged

in the past decade regarding catalytic processes such as adsorption and diffusion,

surface reaction, and catalyst deactivation. These experiments provide invaluable

insights into the fundamental issues of catalysis.

The timescale of catalytically important processes ranges from 10

12

to 10

s, with

the chemisorptions and reactions taking place within picoseconds whereas catalyst

deactivation occurs in seconds or minutes. Although the timescale of the funda-

mental process of adsorption and reaction is beyond the time resolution of STM,

information about the pathway and energetics of adsorption/reaction can be acquired

with STM by monitoring the change in the spatial distribution of surface adsorbates.

Indeed, in situ STM can be combined with femtolasers to probe surface processes at

both the spatial and the temporal limits. The feasibility of combining these two

techniques has been demonstrated in a recent study by Bartels et al. [2]. Such studies

are extremely promising with respect to our understanding of surface chemical

processes. In this chapter, we show how in situ STM has helped to visualize

elementary steps of chemical reaction and to elucidate mechanisms of catalyzed

processes.

Like most surface science techniques, conventional in situ STM studies have been

carried out in UHV on model catalysts consisting of extended planar surfaces. When

extrapolating the information obtained in UHV surface science studies to real-world

catalysis, two issues have generally concerned the catalysis community, namely, the

pressure and material gaps.

The pressure gap refers to the fact that surface science studies are conducted under

UHVconditions (10

10

–10

14

bar), whereasindustrial catalytic reactions typically are

carried out at high pressures (1–1000 bar). Over 10 orders of magnitude difference in

the pressure of reactant gases can drastically change the interaction of reactants on

the catalyst surface, a process essential to a catalytic reaction. The material gap refers

to the gap between the surface structure of metal single crystals often studied in

surface scienceand that of technical catalysts. Real-world catalysts usually consist of

small metal clusters ranging from 1 to 100 nm in size, ﬁnely dispersed onto a high-

surface-area oxide support. These metal clusters can have structures and properties

that are quite different from the bulk metal. In catalytic research, it is well

documented that the reactivity and selectivity of catalysts often depend on the size

and shape of supported metal clusters [3]. Furthermore, the presence of the oxide

support can modify the structure and properties of supported metal clusters.

The effect of cluster size and metal support interaction cannot be addressed in

surface science studies on well-deﬁned single crystal metal surfaces. As a local

structural probe, STM has the advantage of addressing these two issues. Although the

operational range of STM extends from UHV to high pressures, there are challenges

in maintaining the stability of STM at catalytically realistic operating temperatures

and pressures. Nevertheless, it is possible to apply STM to study catalytic reactions

under realistic conditions. To bridge the material gap, supported model catalysts,

consisting of small metal clusters supported on planar oxide surfaces, have been

introduced into surface science studies. STM is ideally suited to precisely charac-

terize the structure of these supported model catalysts. In this chapter, we show the

recent progress in bridging the pressure and material gaps by applying in situ STM

to the study of model catalysts under realistic reaction conditions.

3.2

Instrumentation

To visualize the fundamental steps of chemisorptions and reactions that occur at

surfaces, in situ STM investigations typically monitor the diffusion or transformation

of adsorbed molecules. A series of snapshots of preselected surface regions,

compiled into a STM movie, can reveal the evolution of surface phenomena.

On metal surfaces, the surface diffusion of adsorbates is usually so rapid that the

3 In Situ STM Studies of Model Catalysts

surface temperature must be lowered below room temperature (RT) for successful

STM viewing. Low-temperature (LT) STM, developed in the mid-1990s, not only

potentially allows the determination of reaction intermediates and pathways but also

enables the precise control and measurement of the bond activation processes using

the STM tip.

To measure reaction kinetics, STM should have the capability to resolve adsorbates

at temperatures relevant to catalytic reactions. For this purpose, a variable temper-

ature (VT) STM is required, as well as capabilities for rapid scanning. VT STM with a

typical scan rate of one frame per minute was developed in the mid-1990s.

Considering the scanning probe is a mechanical probe driven by electronics, the

acquisition time of STM images is typically restricted by the mechanical behavior of

the scanning components and the performance of the electronics. In the mid-1990s, a

few STM groups achieved a fast scan rate of approximately 20 frames/s on extended

model catalyst surfaces [4–6]. Working on the compact design of the scanner probe

and using high performance electronics, Frenken and coworkers [7] have recently

pushed the scan rate above the video rate (50 frame/s) on a graphite surface, with

atomic resolution and an image with 256 256 pixels.

Recently, a few groups have taken up the challenge to extend the in situ STM

investigations to high pressures. A major challenge in imaging surfaces with STM

over a wide pressure range is the sensitivity of the tunneling current to extremely

small changes at the tunneling junction resulting from induced instabilities by the

ambient gas. Efforts have emphasized the design of a STM that can work at high

temperatures and pressures with greater stabilities [8–11].

For in situ STM studies at high temperature and pressures, the inability of being

able to track a preselected surface area is often the limiting factor given the tunnel

junction instabilities and sample drifts. To overcome this challenge and to maintain

contact with a speciﬁc surface region, it is important to develop experimental

approaches that pattern the surface without inﬂuencing the kinetics and dynamics

of the particular areas under study. A shadowing technique (Figure 3.1a) has been

developed where metal atoms are dosed with the STM tip in the tunneling position

with the collimated metal ﬂux creating a shadow of the tip on the substrate [12, 13].

For metal clusters supported on an oxide surface, tip manipulation is another method

of choice (Figure 3.1c). This technique removes clusters from a speciﬁc area through

aggressive scanning. Using the STM tip to pattern the surface, it is now possible to

monitor a preselected surface area at elevated temperatures while changing the gas

pressure over 12 orders of magnitude.

In addition to the instrumental performance, the STM tip is of primary importance

for in situ STM measurements. Methods for the preparation of STM tips have been

extensively studied with a goal of preparing an atomically sharp tip [14–19]. The STM

tip is important for high-pressure studies with respect to two aspects, tip selection

and in situ tip regeneration. To ensure a continuous DOS near the Fermi level,

transition metals are usually selected to prepare STM tips. In the presence of reactant

gases, especially under high-pressure and high-temperature conditions, the chem-

ical and thermal stability of the STM tip becomes the ultimate limit for reaction

studies and thus the major concern in tip selection. Tungsten tips are very stable in

3.2 Instrumentation

CO but perform poorly in the presence of O

or mixtures of CO and O

. Platinum or

platinum alloy tips are stable in O

but suffer from adsorption of CO, especially when

the sample surface temperature is above 550 K [20]. Gold is stable in both CO and O

but unstable at high temperatures, especially in the presence of water. In addition to

tip selection, in situ tip regeneration or cleaning is also critical for STM studies in the

presence of high-pressure reactant gases because the STM tip is susceptible to

picking up poorly conducting components during extended measurements at

elevated pressures. In situ tip regeneration refers to the method of applying a large

voltage pulse (from a few to hundreds of volts) between tip and sample while the tip is

in tunneling range. This method induces ﬁeld emission, which cleans and regen-

erates the STM tip. Wintterlin and coworkers [21] recently reported a high-pressure

STM study, in which tungsten tips were used to study ethylene oxidation on Ag(1 1 1);

the tip could be recovered by applying high voltages to the tip (e.g., þ300 V).

Figure 3.1 Methods of patterning the surface

for in situ STM studies. (a) Schematics of

shadow technique. (Reprinted with

Wiley, Inc.) (b) STM image of the surface created

by shadow technique. The shadow area

uncovered by metal clusters is distinguished

from the area covered with metal clusters by the

white dash line. (c) Schematics of the tip

manipulation. (d) STM image of the surface

created by tip manipulation. The dash rectangle

in (d) shows the area where most clusters are

picked up by the STM tip. This area with lower

cluster densities can be distinguished from the

rest of the surface and serves as a nanomarker

for in situ STM studies.

3 In Situ STM Studies of Model Catalysts

Gas puriﬁcation is another important issue for high-pressure STM studies. When

the surface is exposed to high pressures, even highly diluted impurities may

completely contaminate the surface. Puriﬁcation of all reactant gases using liquid

can greatly improve the operating pressure range for high-pressure reaction

studies and prevent the electrical breakdown often induced by humidity when

backﬁlling the STM chamber [13]. For a ﬂow-reactor system, where puriﬁcation of

large volume of gases is required, a heated zeolite ﬁlter is effective in removing

carbonyl contaminants from the gas ﬂow [22, 23].

3.3

Visualizing the Pathway of Catalytic Reactions

With the introduction of LT and VT STM, it is now possible to monitor the

fundamental steps of chemical reactions, that is, reactant chemisorption, diffusion,

and catalytic transformation. A detailed review covering this subject was published by

Wintterlin in 2000 [24]. Since then, in situ STM studies have ﬂourished and expanded

to the visualization of the reaction pathway and kinetics of surface processes. In the

following section, we highlight selected examples of recent progress in using in situ

STM for studying fundamental catalytic processes.

3.3.1

Imaging of Adsorbates and Reaction Intermediates

STM can induce adsorption–desorption and dissociation processes nonthermally

and with spatial control. At low temperatures, with limited surface diffusion, the

motion of adsorbates can be controlled and allows the determination of surface

reaction intermediates. The catalytic oxidation of CO on precious metal surfaces is

one of the most important model reactions in heterogeneous catalysis. Hahn and

Ho [25] have recently used in situ STM to visualize the reaction pathway of CO

oxidation on Ag(1 1 0) at 4 K. Figure 3.2 depicts the pathway believed to be operative:

the Langmuir–Hinshelwood mechanism. Figure 3.2a and b shows a CO molecule

and a pair of oxygen atoms adsorbed on the Ag(1 1 0) surface, respectively. The oxygen

atom pair was formed by placing the tip over a molecularly adsorbed O

and raising

the sample bias to 0.47 V. Subsequently, the bond between two oxygen atoms

is broken and the two oxygen atoms adsorbed at the nearest fourfold sites of the

Ag(1 1 0) surface show slight elongation along the [1



1 0] direction in Figure 3.2b. The

STM tip was then placed over the CO molecule with a þ0.24 V bias applied

repeatedly, causing the molecule to diffuse across the surface. Eventually, the CO

molecule moved close to the pair of O atoms (Figure 3.2c) and then joined them to

form the OCOO complex (Figure 3.2e). With an additional pulse of the sample

bias over the CO molecule, the OCOO complex is decomposed, leaving an oxygen

atom on the surface with the CO

desorbing from the Ag surface.

A second reactionpathway wasalso illustratedby moving aCO molecule toward the

absorbedoxygenatoms. In Figure3.3a,anSTMtipwith a COmolecule adsorbedatthe

3.3 Visualizing the Pathway of Catalytic Reactions

tip end was positioned over anoxygen atom, which lined up with another oxygen atom

fromthe O

dissociation.Thetwooxygenatomswereseparatedfromeachotherbytwo

lattice spacings. With a pulse of þ0.47 V sample bias, the CO molecule is detached

from the STM tip and reacted with the oxygen below the tip to form CO

. Figure 3.3b

shows the two pulses of tunneling current, corresponding to the CO molecule,

impinging on the surface and reacting with the adsorbed oxygen, and to CO

desorption, respectively. Figure 3.3c shows the oxygen atom left on the Ag(1 1 0)

surface. The combined imaging, manipulation, and spectroscopic capabilities of the

STM provide direct visualization of reaction pathways at the single molecule level.

At low temperatures, tip manipulation has been regularly used to promote surface

diffusion, to activate bonds, and to synthesize molecules. Recent progress along these

lines is described by Hla and Rieder [26, 27].

Figure 3.2 STM images obtained with a

CO-terminated tip, V

¼70 mV and I

¼1 nA.

(a) Isolated CO molecule, (b) two O atoms

(adsorbed on the nearest fourfold hollow sites

along the [1



1 0] direction), (c) CO and two

O atoms separated by 6.1 Å along the [0 0 1]

direction, and (e) OCOO complex. Grid

lines are drawn through the silver surface

atoms. Scan area of (a–c) and (e) is 25 Å 25 Å.

Schematic diagrams for adsorption geometries

of (c) and (e) are shown in (d) and (f ),

respectively; a linear atop and a tilted off-site CO

are implicated. The black (red) circles represent

carbon (oxygen) atoms and the large gray circles

are silver atoms. The sizes of the circles are

scaled to the atomic covalent radii. (Reprinted

The American Physical Society.)

3 In Situ STM Studies of Model Catalysts

3.3.2

Imaging Chemisorption on Metals

Under catalytic reaction conditions, adsorbates are usually mobile on the surface. The

diffusion of adsorbates has been studied both on metal surfaces and on oxide surfaces

by in situ STM. On metal surfaces, it has been shown that at low surface coverage,

adsorbates are mobile and are distributed randomly on the surface. As the surface

coverage is increased, the interaction between adsorbates also changes such that an

attractive interaction begins to appear, leading to the formation of adsorbate islands.

These adsorbate islands are in equilibrium with the diffusing adsorbates (2D gas) at

the surface. Eventually, with an increase in the adsorbate coverage, the adsorbate

islands grow into an adsorbate overlayer. In situ STM studies by Wintterlin et al. [4]

and by Wong et al. [28] both illustrate that the diffusion rate of adsorbates on metal

surfaces depend on their coverage and/or nearest neighbors. The surface diffusion

coefﬁcient measured for adsorbates on metal surfaces is meaningful only at very low

surface coverages where the adsorbate–adsorbate interaction has a negligible inﬂu-

ence on adsorbate diffusion.

The structure of the adsorbate layer formed at high surface coverage is more

relevant to catalytic reactions at high pressures. Besenbacher and coworkers [29–33]

found for CO on Pt(1 1 0) and Pt(1 1 1) and NO on Pd(1 1 1) that the structure of high

Figure 3.3 Reaction of a CO molecule released

from a CO-terminated tip with an O atom

adsorbed on the surface. (a) STM image, taken

with a CO-terminated tip, of two O atoms

separated by two lattice spacings (2 2.89 Å)

along the ½1



10 direction. Grid lines are drawn

through the silver surface atoms. (b) Tunneling

current during a 1470 mV sample bias pulse

with the CO-terminated tip over one of the two

O atoms (denoted by 



). Two current rises

(at 250 and 310 ms) indicate the moments of

desorption and reaction of CO from the tip and

the moment of desorption of CO

into vacuum.

the pulse, showing CO on the tip has reacted

away. Scan area of (a) and (c) is 25 Å 25Å.

(d–f) are the schematic diagrams for (a–c),

respectively. (Reprinted with permission from

Society.)

3.3 Visualizing the Pathway of Catalytic Reactions

adsorbate coverages, formed at low-temperature and low-pressure conditions, is

identical to the structure formed at room temperature and high pressures. This

ﬁnding suggests that on metal surfaces, reaction studies at high surface coverage

conducted at low temperature likely connect with real catalytic processes at high

pressures.

By studying the diffusion of surface adsorbates (or adsorbate vacancies), in situ

STM can be used to determine the active site for chemisorptions. Mitsui et al. [34–36]

studied the process of hydrogen dissociation on Pd(1 1 1) using in situ STM. Pd is a

catalyst widely used in hydrogenation and dehydrogenation reactions. Exposure of Pd

to H

leads to dissociative adsorption. At approximately 65 K and in the presence of

2 10

7

Torr of H

, Pd(1 1 1) is nearly saturated with H atoms leaving only a few

vacancies as sites for the dissociation of adsorbed H

molecules (Figure 3.4). Due to

the inversion of the image contrast caused by the adsorption of H atoms on the STM

tip, the empty surface sites are imaged as protrusions. This surface can be used to

model the Pd(1 1 1) surface under high-pressure H

at room temperature.

Figure 3.5 shows a sequence of STM images acquired for the same surface region.

Figure 3.5a depicts a number of isolated vacancies (bright spots) separated by more

than one Pd lattice, as well as two aggregates of vacancies (marked by dashed circles).

Each vacancy aggregate consists of a pair of vacancies (dimer) occupying neighboring

fcc sites. The dimers in Figure 3.5 are always imaged as a three-lobed object because

of the fast diffusion of a neighbor H atom, which can hop over bridging sites to

occupy the vacancy pairs without getting close to other H atoms. The vacancy pairs or

dimers are most frequently encountered in the STM study. Isolated vacancies can hop

Figure 3.4 The 6.5 nm 6.5 nm STM image of Pd(1 1 1) with a H

coverage near one monolayer. Numerous H vacancies, visible as

bright protrusions, are present. V

¼45 mV and I

¼2.7 nA.

Streaks and fractional protrusions are due to vacancies moving

while the tip is scanning over them. (Reprinted with permission

3 In Situ STM Studies of Model Catalysts

randomly on the Pd surface and occasionally coalesce to form aggregates, as shown in

Figure 3.5b. Vacancies A and B aggregate to form a dimer while vacancies C, D, and E

coalesce to form a three-vacancy aggregate. The vacancy dimer remains together for

several minutes and eventually disintegrates back to isolated vacancies. However, the

three-vacancy aggregate disappears and leaves only one vacancy on the surface

(Figure 3.5c). The authors concluded then that two vacancies in the three-vacancy

aggregate are occupied by H atoms from the dissociation of adsorbed H

. The authors

Figure 3.5 STM images from a movie showing

the formation, separation, and annihilation of

H-vacancy clusters. The images on the left

(3 nm 2.5 nm) are repeated on the right with

annotations. (a) Five vacancies near the center

are labeled (A–E) and two triangular vacancy

pairs (2V) are marked with dashed triangles for

reference. (b) Vacancies A and B have formed a

2V cluster indicated by the triangle containing

the number 2. Vacancies C–E have formed a

three-vacancy (3V) cluster, indicated by the

larger triangle containing the number 3.

vacancies A and B, while the 3V cluster has been

annihilated by dissociative adsorption of a H

molecule, leaving a single remaining vacancy C.

The other 2V clusters separated a few frames

later. (Reprinted with permission from Ref. [34].

3.3 Visualizing the Pathway of Catalytic Reactions