Interface Physics

 

 

ReactorSXRD

 
It has been known for many years that there is a strong interplay between the surface structure of a catalyst and the actual catalytic reaction process. Figure 1 shows the dramatic effect of the catalytic reaction conditions on the surface structure of a catalyst particle. The Pt particle has served as a CO oxidation catalyst under realistic pressure and temperature conditions. One can clearly see that the catalyst surface has been altered strongly by the presence of the reactants and the product. The process that has caused this alteration is not revealed by such post-reaction observations. The only way to actually ‘see’ and understand surface rearrangements and correlate them with the catalytic mechanism at work, is to ‘look’ at the surface of the catalyst during the reaction, and while all reactant and product gasses are present. It is expected that similar structure modifications take place during many catalytic reactions on a wide variety of catalyst surfaces, i.e. that the alteration of the catalyst during and due to the reaction is a general effect in catalysis.
 
pt stephanopoulos
Figure 1: Pt catalyst particle before (left panel) and after exposure to elevated pressures of CO and O2 (middle and right panel). The dramatic effect of the catalytic conditions on the surface morphology is obvious. Reproduced from: Flytzani-Stephanopoulos et al., J. of Catalysis, 49 (1), (1977).

 

Traditionally, in surface science, the characterization of surface structure or morphology is carried out under ultrahigh vacuum (UHV, pressure < 10-9mbar) conditions, allowing researchers to work with clean and ordered surfaces and to control the type and number of molecules interacting with the surface accurately. This is advantageous when trying to understand the fundamental interaction of molecules with singlecrystalline surfaces. In UHV conditions electrons and ions also have a very long mean free path compared to ambient pressure conditions, allowing one to use techniques that require these long mean free paths (e.g. LEED, LEIS, SEM and TEM). However, in ‘real life’ catalysis the vast majority of processes takes place at elevated temperatures and at high pressures (> 0.1 bar). This constitutes a formidable pressure gap of easily nine or more orders of magnitude between realistic and laboratory conditions. In addition, there is also an important materials gap, since a catalyst is usually not a single-crystal surface but very often it consists of oxide supported, nanometer size particles. This means that particle size effects and particle-support interactions possibly contribute to and influence the catalytic mechanisms and performance.

 

The interface physics group uses multiple techniques to study model catalysts at elevated pressures: two scanning probe microscopy techniques, namely Reactor-STM and Reactor-AFM, and two x-ray based techniques, surface x-ray diffraction (SXRD) and grazing incidence small angle x-ray scattering (GISAXS). Since the x-ray based techniques require very intense x-ray radiation they are performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.  

 

Instrument development
In collaboration with the beamline staff of the ID3 surface diffraction beamline at the ESRF we have developped a high pressure/UHV flow reactor, shown in fig. 2. This setup allows us to prepare model catalyst surfaces under UHV conditions after which they can be studied under realistic conditions (pressure up to 1200 mbar and temperatures up to 950 K). The UHV system is also equipped with a Quadrupole Mass Spectrometer (QMS), with which we can analyze the composition of the gas in the reactor. In this way we can correlate the atomic-scale surface structure with the chemical activity of a model catalyst surface. A full technical description of the setup can be found here. If you are interested in buying the instrument, go to the Leiden Probe Microscopy website.

chamber photo
Figure 2: Flow reactor mounted on the diffractometer at the ID3 beamline.

 

Research
The central question of our research is: what is, under realistic conditions, the structure of the catalyst surface and how does this, in turn influence the reaction mechanism and, thus, the reactivity and selectivity?

Most of the experiments up to now have focussed on the study of CO oxidation and NO reduction on single crystals of Pt-group metals and role surface oxides play in these reactions. Several (co-)publications on the subjects can be found below or on the groups publication list.

 

 

Recent Publications

"Generation of Pd model catalyst nanoparticles by spark discharge"
M.E. Messing, R. Westerström, B.O. Meuller, S. Blomberg, J. Gustafson, J.N. Andersen, E. Lundgren, R. van Rijn, O. Balmes, H. Bluhm, K. Deppert
Full Text [PDF 4 MB]
"Comment on "CO oxidation on Pt-group metals from ultrahigh vacuum to near atmospheric pressures. 2. Palladium and Platinum""
R. van Rijn, O. Balmes, R. Felici, J. Gustafson, D. Wermeille, R. Westerström, E. Lundgren, J.W.M. Frenken
J. Phys. Chem. C 114 (2010) 6875-6876 Full Text [PDF 707 kB]
"Ultrahigh vacuum/high-pressure flow reactor for surface x-ray diffraction and grazing incidence small angle x-ray scattering studies close to conditions for industrial catalysis"
R. van Rijn, M.D. Ackermann, O. Balmes, T. Dufrane, A. Geluk, H. Gonzalez, H. Isem, E. de Kuyper, L. Petit, V.A. Sole, D. Wermeille, R. Felici, J.W.M. Frenken
Rev. Sci. Instrum. 81 (2010) 014101 Full Text [PDF 821 kB]
"Catalytic activity of the Rh Surface Oxide: CO oxidation over rh(111) under realistic conditions"
J. Gustafson, R. Westerström, O. Balmes, A. Resta, R. van Rijn, X. Torrelles, C.T. Herbschleb, J.W.M. Frenken, E. Lundgren
J. Phys. Chem. C 114 (2010) 4580-4583 Full Text [PDF 609 kB]
"The ID03 surface diffraction beamline for in-situ and real-time X-ray investigations of catalytic reactions at surfaces"
O. Balmes, R. van Rijn, D. Wermeille, A. Resta, L.Petit, H. Isern, T. Dufrane, R. Felici
Catalysis Today 145 (2009) 220-226 Full Text [PDF 518 K]
 

 

ReactorSTM

 

With the Reactor-STM, we have taken part in the STRP project NanO2A novel approach to study the oxidation of nano-materials, within the Sixth Framework Program of the EC.

 

movie PHD THESIS and MOVIES 'Model Catalysts in Action' on the web

 

Much of the present-day fundamental understanding of catalysis has been obtained by surface science studies of model catalyst at well-defined, but strongly non-realistic conditions, such as ultrahigh vacuum. Yet, there lies an enormous gap, known as the "pressure gap", of typically ten orders of magnitude, between the pressures of traditional surface-science experiments and "real" catalysis. Under "real" conditions, i.e. high pressures and high temperatures, the structure, morphology of even the composition of the catalyst can be very much different. This, in turn, can have a dramatic effect on the catalyst' performance.

 

 

The Instrument

We have combined a scanning tunneling microscope with a flow-reactor, which allows us to "look" to a model catalyst surface at high pressures (1-5 bar) and elevated temperatures (300-425K), during a catalytic reaction. Since we operated the reactor in flow mode we can simultaneously monitor the gas-composition of the reaction products. 

 

 

The high-pressure flow-reactor STM part of the instrument looks like this: This device is integrated in an ultrahigh vacuum system, with standard surface-science preparation and analysis tools:
prototype HPSTMsetup

 

One of the tools on the UHV system is a Quadrupole Mass Spectrometer (QMS), with which we can analyze the composition of the gas that leaves the reactor. In this way we can relate (changes in) the atomic-scale surface structure with the chemical activity of a model catalyst surface.

 

Project: CO oxidation on Platinum (110)

The oxidation of carbon monoxide to carbon dioxide on platinum surfaces has been the subject of many studies. Beside the technical relevance for automotive catalysis, the relative simplicity of the reaction has made this system the "fruit fly" of catalysis. Both at low pressures and at atmospheric pressures spontaneous oscillations in the reaction rate have been observed. The change of the activity of the catalyst has been ascribed to changes in the surface structure or composition. We have used the Reactor-STM to record STM movies of a platinum(110) surface when we switched several times from a CO-rich flow to an oxygen-rich flow, and while measuring the pressures of COO2 and the reaction product CO2 simultaneously with the QMS. In the oxygen-rich flow the surface forms a thin surface oxide which resulted in a step-wise increase in the CO2 production. The results show that there is a strict one-to-one correspondence between the surface structure and the catalytic activity, and suggest a reaction mechanism which is not observed at low pressures.

 

The Movie:

The movie (click here to download, 1.6Mb) shows an example of simultaneously recorded STM images and partial pressures of reactants and a reaction product.
movie icon The upper part shows the STM images (210nm x 210 nm, 65 s/image). There was some thermal drift which made it impossible to keep track of the same surface area during the entire movie 

The lower part shows the partial pressures ofCO O2 and CO2, on a log-scale (vertical range: 10-4-1 bar).

The total recording time was125 min.

 
 
 
The Cartoon:
A selection of STM images from the movie is displayed below. Labels A-H in the panel which shows the partial pressures correspond to the labels of STM images.
e4s e30s e36s e43s
A : In a CO flow the surface consists of flat 1x1-terraces separated by monatomic steps. B: In a CO+O2 flow the surface initially had the same structure as in CO only. There was a modest CO2 production on this metallic surface (R-low)             C: But, when the CO pressure in the CO+O2mixture was below 15 mbar the surface oxidized. This coincided with a step up (~factor 3) in the CO2 production (R-high). D: The formation of the roughness implies that the CO reacted with the oxygen which was stored in the surface oxide. During the reaction the surface was continuously oxidized and reduced. 
e68s e86s f4s f16s
E: After switching back to the CO flow the oxide was removed and the surface smoothened again. F: Similar to image B, there was no change in structure in a flow of CO+O2, for CO pressures above 15 mbar.  G: The oxidic surface with the high reactivity roughened even faster, when we shortly increased the CO pressure (the peak at t=6500s).   H: By increasing the CO pressure above 24 mbar the oxide was removed. The decay of several residual adatom islands could be observed
Partial pressures of the reactants CO O2 and the reaction product CO2. The labels A-H correspond to the STM images (210nm x 210nm).The flow was 3.0 ml/min. at a total pressure of 0.5 bar and a temperature of 425 K. 

The details can be found in publication [3]


[1] "The 'Reactor STM': A scanning tunneling microscope for investigation of catalytic surfaces at semi-industrial reaction conditions", P. B. Rasmussen, B. L. M. Hendriksen, H. Zeijlemaker, H. G. Ficke, J. W. M. Frenken 
Rev. Sci. Instrum. 69 (1998) 3879
[2] "Hoge druk scanning tunneling microscopie voor katalytisch onderzoek: ontwikkeling en prestaties voor de Reactor-STM", B.L.M. Hendriksen, G.S. Verhoeven, E. de Kuyper, L. Crama, J.W.M. Frenken, P.B. Rasmussen, H. Zeijlemaker, W. Barsingerhorn, H.G. Ficke
Nevacblad 38 (2000) 5-9 (in Dutch)
[3] "CO oxidation on Pt(110): Scanning Tunneling Microscopy inside a flow-reactor", B.L.M. Hendriksen, J.W.M. Frenken
Phys. Rev. Lett. 89, (2002) 046101
[4] More publications about STM on metal surfaces in the group's publication list.
 
 

 

ReactorSTM, -AFM and -SXRD

 

ReactorSPM
The Interface Physics Group has developed two classes of instruments for the investigation of catalytic reactions on model catalysts under realistic or semi-realistic conditions. The ReactorSTM and ReactorAFM are two scanning probe microscopes that allow us to prepare and characterize single-crystal surfaces under ultrahigh vacuum conditions, after which they are transferred to a tiny reactor cell in which they are simultaneously exposed to a high-pressure gas flow at high temperatures and imaged with the tip of an STM or AFM. More information about this ReactorSPM approach can be found by clickling here.

 

ReactorSXRD
Similarly, we have developed a ReactorSXRD setup, which is a dedicated combination of sample preparation in UHV with Surface X-Ray Diffraction for surface structure determination under high-pressure, high-temperature gas-flow conditions. This work is carried out in collaboration with the group of Dr. Roberto Felici at beamline ID03 of the European Synctrotron Radiation Facility (ESRF) in Grenoble. More information about this ReactorSXRD activity can be found by clicking here.

 

Why do this?
There is a good reason for going to the conditions of high pressures and high temperatures with surface structure sensitive techniques, such as scanning probe microscopy and surface X-ray diffraction. Most techniques that have been developed for the investigation of the surfaces of materials with high, e.g. atomic, precision work best under ultrahigh vacuum conditions. This is why much of our present-day fundamental knowledge about catalytic processes at surfaces has been obtained from experiments at very low pressures, for example up between 10-9 and 10-5 mbar. However, it may well be that between these low pressures of traditional surface science and the much higher pressures of 10 mbar - 100 bar of industrial catalysis processes surfaces change their structure and composition. In addition to changes in the equilibrium structure of the catalyst structure and composition, the catalytic process may even keep the surface permanently out of equilibrium, leading to a dynamic rather than a thermodynamics state for the surface. The only way to find out whether or not this is so is to investigate catalytic systems under conditions that are as close as possible to those that are used in practice. Although we may recognize overall trends in the behavior, the investigation of (model) catalysts under realistic conditions needs to be carried out on a case-by-case basis.

 

A striking example of the strong interplay between the surface structure of a catalyst and the actual catalytic reaction process is shown by the picture below, which shows three scanning electron microscopy (SEM) images of a Pt particle that serves as the catalyst for the oxidation of carbon monoxide, CO, at elevated temperatures and pressures. The image on the left shows the structure before reaction, while the two images on the right, taken after the reaction, show that the catalyst surface has been altered strongly by having been in intimate conact with the reactants, O2 and CO, and the product, CO2.

pt stephanopoulos

Pt catalyst particle before (left panel) and after exposure to elevated pressures of CO and O2 (middle and right panel). The effect of the catalytic conditions on the surface morphology is dramatic. Reproduced from: Flytzani-Stephanopoulos et al., Journal of Catalysis, 49 (1), 1977.

 

A recent result
We have used the combination of ReactorSTM and ReactorSXRD measurements to reveal the presence of spontaneous reaction oscillations in the catalytic oxidation of CO at atmospheric pressures. The measurements show that the driving force for the oscillations is the periodic build-up and decay of roughness of the catalyst surface that makes the surface switch back and forth between a metallic state and an oxidic structure. These results have appeared in Nature Chemistry. More about the high-pressure reaction oscillations can be found by clicking here.

 

 

High-Speed Variable-Temperature STM

 

The Scanning Probe experiments in our group rely heavily on the use of home-built, special-purpose Scanning Probe Microscopes. The first special SPM instrument developed by the Interface Physics Group is a High-Speed, Variable-Temperature Scanning Tunneling Microscope. The technical aspects of this microscope are described in [1,2,3,4].

 

The high speed of the STM is obtained by a combination of fast analog electronics and a fast digital control system, which is interfaced to an ensemble of three T800-transputers and a Silicon Graphics workstation. This system acquires up to 100.000 pixels per second, which can be configured into e.g. 10 images per second of 100 x 100 pixels per image. A clever scheduling of tasks between the transputers ensures that the images are recorded without any time delay between the last pixel of an image and the first pixel of the next image. Currently, we are working on the development of a fully digitally controlled analog feedback and control system that will go up to video rates, i.e. 25 images/s, at a more mature image size of 256 x 256 pixels. This system should be operational in the course of 2002.

 

The temperature range available to our present STM is 30 K - 800 K. Anywhere in this temperature range, the temperature can be swept over 300 K, while a single area on the surface can be kept "in view" of the tip, without the need for mechanical position adjustments of the tip, either parallel or perpendicular to the surface. This superb thermal stability has been reached as the result of a computer optimization of the design. This was performed in the form of a collection extensive finite-element computer calculations of the thermal behavior of the microscope+sample combination, during temperature ramps of the sample.

STMdesign

The figure shows the result of the computer optimization. A central tube scanner is used to scan the sample surface (pink). The coarse approach is performed via a pivoting mechanism on the sample holder. A wobble stick can be used to exchange the entire scanner unit, and the sample holder can be exchanged by use of the same wobble stick. the photograph below shows the microscope in reality.

stm

Below is a photograph of the symmetrically expanding and contracting sample holder, with the pivoting mechanism for the coarse approach. The holder contains a heating element and a thermocouple or Pt-resistance thermometer.

sampleholder 

The microscope is built into an ultrahigh vacuum system, with LEED, Auger, and other tools for surface preparation and diagnostics.

vacuumsystem

Here's a picture of Raoul van Gastel, pretending to do useful things with it, and to have fun with it at the same time...

raoulwithSTM 

We're still working on the variable-temperature STM, and hope to improve its thermal stability even further in the near future...

 

 

 


 

[1] "Surfaces in motion: a variable-temperature scanning tunneling microscopy study"  
M.S. Hoogeman  
Ph.D. thesis: Leiden University, June, 1998
[2] "Design and performance of a programmable-temperature scanning tunneling microscope"  
M.S. Hoogeman, D. Glastra van Loon, R.W.M. Loos, H.G. Ficke, E. de Haas, J.J. van der Linden, H. Zeijlemaker, L. Kuipers, M.F. Chang, M.A.J. Klik, and J.W.M. Frenken  
Rev. Sci. Instrum. 69 (1998) 2072
[3] "Surface Dynamics Studied with a High-Temperature High-Speed Scanning Tunneling Microscope"  
L. Kuipers  
Ph.D. thesis: Amsterdam University, June, 1994
[4] "Design and Performance of a High-Temperature High-Speed STM"  
L. Kuipers, R.W.M. Loos, H. Neerings, J. ter Horst, G.J. Ruwiel, A.P. de Jongh and J.W.M. Frenken  
Rev. Sci. Instr. 66 (1995) 4557
[5] More publications in the group's publication list.