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.
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:|
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 CO, O2 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 (click here to download, 1.6Mb) shows an example of simultaneously recorded STM images and partial pressures of reactants and a reaction product.|
|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.
|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.|
|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.|
|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 
|||"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
|||"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)
|||"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
|||More publications about STM on metal surfaces in the group's publication list.|