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BRIDGING THE PRESSURE GAP New methods let chemists characterize surfaces under pressures at which interesting chemical reactions occur
Mitch Jacoby C&EN Chicago
D evelopments in surface analysis over the past few decades have filled the surface chemist's tool box
with a large collection of analytical techniques. Electron spectroscopies, surface crystallography, scattering methods, and other surface-sensitive probes have been used with atomic resolution to reveal the structure and nature of catalysts, semiconductors, and other technologically important materials. By selectively examining the outermost region of these substances, surface chemists have learned how a skin-sometimes no more than a few atomic layers thick—can govern the physical and chemical properties of bulk matter.
Most of the surface probing has been done under conditions of extremely low pressure—often less than a trillionth of an atmosphere—very different from the conditions at real-world surfaces where interesting reactions occur. Experiments have typically been performed in this so-called ultra-high-vacuum (UHV) regime for two main reasons. First, the thin vacuum atmosphere enables specimens to be cleaned and remain uncontaminated by adsorption layers from background gases long enough to permit analysis. Second, beams of atoms, ions, or electrons can more easily travel in an unimpeded manner from their source to a specimen or from a specimen to a detector under low gas pressure conditions.
Despite the detailed information derived from ever more sophisticated surface probes, one question continues to be asked about all vacuum methods: Do the surface properties measured at UHV accurately represent the nature of the surface under the high pressure typically present during surface reactions? Recently, chemists have begun answering the "pressure gap" question by using infrared-visible sum frequency generation (SFG), scanning tunneling microscopy
(STM), and ultraviolet Raman spectroscopy to study chemical reactions on surfaces under real-world conditions.
Gabor A. Somorjai, professor of chemistry at the University of California, Berkeley, has used surface methods to study heterogeneous catalysis since the 1960s. The earlier approach to studying high-pressure effects, he explains, was to clean and characterize a sample catalyst under UHV conditions, then carry out a high-pressure reaction in an isolated reactor—often interfaced to gas analysis
equipment—then reexamine the sample under UHV for reaction-induced surface modifications.
"Characterization at UHV remains important because it provides a surface reference state," Somorjai says. "But, clearly, valuable information—available only at high pressure—is missing from the UHV-only studies. Now we can use SFG and STM over a 13-order-of-magnitude pressure range to probe the sample, not only before and after, but also during chemical reaction."
Somorjai, graduate student Paul S. Cremer, and UC Berkeley physics professor Yuen Ron Shen have used SFG, an interface-specific vibrational technique, to study olefin hydrogenation and other surface reactions. The signal-to-noise ratio in SFG, unlike that in many vacuum methods, remains high even at high pressure. The researchers exploited this property to distinguish surface reaction intermediates from spectator molecules. The gas phase was analyzed simultaneously by gas chromatography to study reaction kinetics.
In the case of propylene hydrogena-
"Nested" missing rows
Hydrogen
Oxygen
IX* . r€K>iHK>x''^HrX>
>.< >< x x >~s x x x x ->o\ x y*\ sx X } \ /"v H j \ / ^ A . / V A ^ ' \ / "/{Vy(x K X HK)
x>̂ (111) Microfacets
X K » ^ X X K
38888880^^^
Carbon Monoxide
STM images (left) and associated schematics show the effect of high-pressure gas adsorption on the same area of an initially pristine platinum (110) surface. Features nearest to the surface appear lightest. At top, 1.6 atm of hydrogen forms "nested" missing rows. Atomic rows appear bent in the micrograph because of an imaging artifact. One atm of oxygen (middle) causes microfaceting along the (111) crystallographic direction. At bottom, 1 atm of carbon monoxide causes step formation. The terraces—or horizontal portions of the steps—retain the (110) orientation but are separated in height from other terraces.
44 AUGUST 18, 1997 C&EN
as
o
o
Somorjai: probe sample during reaction
tion, Somorjai says, the species turning over—that is, converted to product—is the weakly adsorbed 7i-bonded propylene. Stepwise addition of hydrogen first forms a 2-propyl surface intermediate, then propane.
Hydrogenation of ethylene and iso-butene also proceeds from weakly adsorbed 7i-bonded species, Somorjai adds. The concentration of 7C-bonded ethylene was found to be only 4% of a monolayer— that is, one 7i-bonded ethylene for every 25 exposed platinum atoms. Given this coverage, Somorjai quotes an absolute ethylene hydrogenation turnover rate 25 times greater than that conservatively estimated based on the number of surface platinum atoms.
Shen, a pioneer of the SFG technique, explains that because of spectroscopic selection rules, the SFG signal cannot originate from a centrosymmetric medium. This symmetry is present in the bulk of the platinum crystal and in the gas phase where atoms and molecules are evenly distributed. At the interface, however, with gas above and solid below, this symmetry is broken. As a result, the observed spectrum emanates almost entirely from the interface.
"The output from a pulsed laser is split into two beams," Cremer says. "One beam is frequency-doubled to produce 532-nm [green] light and the other is sent to an optical parametric generator/amplifier to produce infrared light." Then through a coherent process, Cremer adds, the infrared and green light beams are combined on an adsorbate-covered surface and produce blue light with a frequency equal to the sum of the frequencies of the IR and
green beams. A vibrational spectrum is recorded by scanning the IR source—while holding the visible light constant—and measuring the intensity of the blue light.
University of Illinois, Chicago, chemistry professor Michael Trenary notes that the reflection-absorption infrared technique, another adsorbate-specific vibrational probe, is generally less expensive to set up and easier to apply than SFG. Also, the SFG IR source used in these experiments has an output of only 2,600 to 4,000 cm"1—a much narrower region than that produced by commercially available IR sources used in reflection IR spectroscopy.
"Just the same, SFG offers important advantages relative to reflection IR," Trenary indicates. In the reflection IR case— at high pressure—the gas absorbs IR light and this contribution must be canceled from the reflection IR signal. "There are techniques for signal cancellation," Trenary adds, "but unlike SFG, in reflection IR the diminished probe beam intensity— caused by gas-phase absorption—reduces the signal-to-noise ratio of the spectrum.
"Also, at high enough temperature, the sample begins to act like an IR emitter," Trenary points out. This complicates the IR measurements but is not an issue in SFG. Furthermore, optics and detectors for visible light—used to collect the SFG signal—are intrinsically more sensitive and easier to work with than IR optics.
Working with graduate student Brian J. Mclntyre and senior staff scientist
Miquel Salmeron of Lawrence Berkeley National Laboratory in California, Somorjai used STM, at elevated temperature and pressure, to examine the manner in which surface atoms rearrange themselves to accommodate adsorption layers. The chemists observed that the particular form of rearrangement depends on the chemical environment.
A pristine platinum crystal f a ce -identified by its (110) Miller indices-forms "nested" missing rows upon exposure to 1.6 atm of hydrogen. Oxygen at 1 atm causes atoms on the same surface to group in the (111) geometry in small patches known as microfacets. The initially flat (110) surface responds to 1 atm of carbon monoxide by forming many terraces. These features are individually smooth, but differ from one another in step height.
"Adsorbate-induced surface restructuring is the first step in any surface reaction," Somorjai points out. Understanding the details of this process is key to improving catalyst performance.
"Once you determine whether surface restructuring stabilizes the catalyst or destroys it, the catalyst preparation can be modified to include supports or additives that facilitate the changes if they are beneficial or inhibit them if they are detrimental. Traditionally, catalyst formulation was done empirically. The goal of these studies—and in some cases it's been realized already—is to use molecular-level surface science to prepare a better catalyst by design."
Possible propylene hydrogenation pathways
Propylene and
hydrogen
H H
H _C=j=C^ H HH
jt-Bonded propylene
LI H H u H \ / H \ / H
H—C. I X — H
H H
Propane
H-C—C- •H HH
Di-c-bonded propylene
H H H X / * C - H
K / C ^H
H x / - H C - H H
1-Propyl
V HH
Propylidene
Propylene adsorption on platinum I may result In ft-bonded or di-G-bond-
surface species, and hydrogenation to propane may proceed by way of a 2-propyl or
a 1-propyl intermediate* In addition, a 1,2 hydrogen shift may convert di-c-bonded propylene to propylidene before hydrogen addition. Sum frequency generation, a surface vibrational technique, reveals the actual reaction proceeds from ̂ -bonded propylene to propane by way of a 2-propyl intermediate.
AUGUST 18, 1997 C&EN 45
Paul S. Weiss, associate professor of chemistry at Pennsylvania State University, University Park, finds the STM experiments significant not only because they monitor surface restructuring under different chemical conditions, but because the details of the restructuring—microfacet formation, for example—can guide planning of future experiments.
In another example of helping surface science move from UHV and model catalysts to real-world conditions, chemistry professor Peter C. Stair of Northwestern University, Evanston, HI., has demonstrated that by using UV excitation in place of the visible light traditionally used, Raman scattering provides strong, diagnostic spectra— under reaction conditions—from samples that may be impossible to probe with any other method. Such samples include coke-covered catalysts, chemical vapor deposition (CVD) diamond films, and deeply colored materials such as coal, graphite electrodes, and black pigments.
"Switching from the visible to the UV provides the ability to access electronic transitions in many materials via the resonance Raman process," says University of
Pittsburgh chemistry professor Sanford A. Asher. This may result in an enhancement of the Raman scattering probability— or cross section—by as much as a factor of 100 million. In addition, fluorescence interference, which often buries the small Raman signal under an enormous background, is always avoided using UV light.
Catalyst deactivation often results from carbonaceous films that are deposited on catalyst surfaces during reaction. Using the UV Raman technique, Stair suggests, catalyst manufacturers can uncover details of the coking process, enabling them to design longer life catalysts.
Stair and postdoctoral researcher Can Li used UV Raman spectroscopy to study coke formation on ZSM-5, a commercial zeolite catalyst used for hydrocarbon conversion. The catalyst was coked by heating in
Comparison of UV and visible Raman excitation
Intensity
257-nm excitation
Diamond band (1,325 cm 1)
Amorphous carbon band
/
488-nm excitation
1,000 1,200 1,400 1,600 Raman shift, cm"1
1,800 2,000
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Impurities in diamond films grown by chemical vapor deposition cause a fluorescence signal that obscures the Raman spectrum if the sample is excited with visible light (488 nm), Excitation with UV light (257 nm) reveals distinct diamond and nondiamond Raman bands*
propylene. Initially the researchers observed olefinic Raman bands. With continued heating, the olefin was observed to convert to a surface polyolefin. Ultimately, the coke was transformed to an aromatic species. Probing similar systems with light in the visible region produces broad featureless fluorescence that hides the Raman spectrum, Stair says.
"I am very optimistic that [Stair's] technique will provide new and useful information," says Jeffrey T. Miller, a catalyst researcher at Amoco Oil Co., Naper-ville, 111. "We already see distinct Raman spectra from various refining catalysts." Now the task is to conduct control experiments that will aid in spectral interpretation, Miller adds.
Hyun-Soo Hong, gear oils technology manager at Lubrizol Corp., Wickliffe, Ohio, also expresses enthusiasm about the Raman technique. Hong's group evaluates lubricant additive performance by analyzing films formed on metal surfaces during friction and wear tests. Using the UV Raman method in addition to photoelectron spectroscopy and other UHV techniques, Hong says, provides a very complete picture of the reaction process.
"The UV Raman probe can be used to study the plasma CVD diamond growth process during diamond deposition," remarks Calum H. Munro, a University of Pittsburgh postdoctoral researcher who works with Asher. Diamond's hardness and low friction coefficient make it ideal
46 AUGUST 18, 1997 C&EN
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Stair: Raman excitation in the uitraviolet
for coating tool tips. Its thermal properties may be used to conduct heat away from electronic components. And the films have optical applications as well. "It's amazing the diamond signal can be detected, if you consider the number of light sources in the CVD plasma," says Munro. "The Raman diamond band is easily detected and may be used to make real-time changes to the plasma gas composition to improve film quality."
Despite the advantages of exciting Raman spectra in the UV, several problems have limited the technique's popularity, Stair says. To begin with, a continuous wave (CW) UV laser is required. Pulsed UV lasers have been available commercially for many years, but they cause thermal breakdown in solid samples. In addition, high-quality filters—to block the Rayleigh scattered light—and high-quality UV detectors have been hard to find, Stair notes. Just recently though, Asher's collaboration with laser manufacturer Coherent Laser Group, Santa Clara, Calif., has resulted in a commercially available CW UV laser. In addition, Renishaw pic, Wotton-under-Edge, England, has just unveiled a UV Raman instrument. Stair expects that this will increase the technique's popularity.
Says Penn State's Weiss: "The development of surface techniques that permit investigation throughout this enormous pressure range—from UHV to atmospheres—allows chemists to address the basic surface questions whose answers were, more or less, promised to us when UHV surface science was being developed. By bridging the pressure gap, surface science is finally beginning to deliver on that promise."^
48 AUGUST 18, 1997 C&EN
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