Jochen A. Lauterbach

Effects of boundaries on pattern formation : catalytic oxidation of CO on platinum

The effect of boundaries on pattern formation was studied for the catalytic oxidation of carbon monoxide on platinum surfaces. Photolithography was used to create microscopic reacting domains on polycrystalline foils and single-crystal platinum (110) surfaces with inert titanium overlayers. Certain domain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems.

Fourier-Transform Infrared Imaging Using A Rapid-Scan Spectrometer

We present a major improvement to the Fourier-transform infrared (FTIR) imaging technique brought about by replacement of the commonly used step-scan spectrometer with a rapid-scanning spectrometer. This advancement dramatically decreases the time required for data collection without decreasing the data quality. With this new instrumental setup, an imaging data set consisting of 64x64 spectra with a 4-cm (-1) spectral resolution over a 1360-cm (-1) spectral range can be collected in 34 s. As a practical example, we demonstrate what we believe to be the first application of FTIR imaging to the screening of adsorbates on the elements of a combinatorial library containing different supported catalyst materials in the same reactant feed.

Chemically Sensitive Parallel Analysis of Combinatorial Catalyst Libraries

Abstract This study demonstrates how Fourier transform infrared imaging (FTIR) can be employed as a powerful spectroscopic tool for the parallel investigation of multiple member heterogeneous catalyst systems. FTIR imaging combines the chemical specificity and high sensitivity of infrared spectroscopy with the ability to rapidly analyze multiple samples simultaneously. A new implementation, using a rapid-scan FTIR spectrometer instead of a step-scan FTIR spectrometer, allows much improved data collection times without sacrificing data quality. Using CO adsorption and CO oxidation as model systems, it was established that FTIR imaging is very well suited to high-throughput parallel analysis of adsorbates and reaction products from supported catalyst libraries.

Subsurface Oxygen On Pt(100): Kinetics Of The Transition From Chemisorbed To Subsurface State And Its Reaction With CO, H2, and O2

Abstract The photoemission electron microscope (PEEM) makes it possible to image a surface via its work function. On a CO-covered Pt(100) surface, we prepared oxygen islands which appear dark in the PEEM image due to their higher work function. As the surface is heated to temperatures above 650 K we observe the conversion of these dark islands into very bright ones with work functions much lower than even that of the clean surface arising from an inverted dipole moment of oxygen atoms beneath the surface. We found an activation energy for this conversion of about 15 kcal/mol. Partially transformed oxygen islands were used to study the reactivity and the formation of this species further. CO and H 2 both react with subsurface oxygen rather slowly, while at the interface between the subsurface and the chemisorbed phase, both adsorbents accelerate the conversion of parts of the remaining oxygen atoms towards the subsurface state. In contrast, additional oxygen adsorption does not contribute to a further transformation. We propose a qualitative microscopic model for the formation of subsurface oxygen based on experimental evidence.

The Formation of Subsurface Oxygen on Pt(100)

PhotoEmission Electron Microscopy (PEEM) enables imaging a surface via its work function. If a CO covered Pt(100) surface is exposed to oxygen patches are formed which appear dark in the PEEM image due to their high work function. As the surface is heated to temperatures above 650 K we observe the conversion of these dark islands into very bright ones with work functions much lower than even that of the clean surface. These findings are attributed to a change in the dipole moment of the adsorbed oxygen induced by their migration beneath the surface. A total work-function decrease of up to 1.2 eV has been evaluated independently using a Scanning Photoemission Microscope (SPM). The properties of this new kind of oxygen were also further investigated with thermal desorption spectroscopy and with Auger-electron spectroscopy.

Adsorption of CO at Ni(100) surfaces: a FTIRAS - TDS study

Abstract The adsorption of CO at Ni(100) surfaces was studied in the temperature range 125–350 K using TDS, FTIRAS and LEED. With TDS, three desorption states, β 2 , β 1 and α were observed at 450, 350 and 280 K corresponding to completion of coverages θ = 0.25, θ = 0.5, and θ = 0.67 (± 0.03), respectively. At coverages below θ = 0.5 CO adsorbs at terminal and bridge sites with coverage dependent C-O stretch frequencies of 2016 (low θ) to 2048 cm −1 (high θ) and 1885 to 1924 cm −1 , respectively. The population of the sites is temperature dependent, bridge sites are favored over top sites at low temperature and vice versa. At θ = 0.5 both, terminal and bridge sites are occupied, top site occupation exceeding bridge site occupation. Above θ = 0.5 FTIRA spectra exhibit in the terminal region a broad band from 2000 to 2100 cm −1 , which contains at least 3 components and suggests that a distinct terminal site does not exist at these coverages. On the contrary, in the bridge site region a single peak at 1968 cm −1 , with a width of 25 cm −1 , signals that bridge type or almost bridge type adsorption sites are still present at the CO covered surface. The results are in accordance with the assumption that CO molecules are arranged in incommensurate or weakly incommensurate adsorbate layers above half monolayer coverage.

Spatio-temporal pattern formation on polycrystalline platinum surfaces during catalytic CO oxidation

The individual grains of a polycrystalline platinum surface as well as the spatio-temporal concentration patterns associated with catalytic oxidation of carbon monoxide were imaged by means of photoemission electron microscopy (PEEM). For CO and O2 partial pressures in the 10−4 mbar range and at temperatures between 420 and 560 K different categories of pattern formation are observed which may be correlated with similar features observed previously with low-index single crystal planes, namely bistable behavior, propagating reaction fronts as well as spirals and target patterns. A novel effect concerns the appearance of regions with work functions considerably below those of the respective clean surfaces which are presumably due to the formation of subsurface oxygen species.

Catalyst design: knowledge extraction from high-throughput experimentation

We present a new framework for catalyst design that integrates computer-aided extraction of knowledge with high-throughput experimentation (HTE) and expert knowledge to realize the full benefit of HTE. We describe the current state of HTE and illustrate its speed and accuracy using an FTIR imaging system for oxidation of CO over metals. However, data is just information and not knowledge. In order to more effectively extract knowledge from HTE data, we propose a framework that, through advanced models and novel software architectures, strives to approximate the thought processes of the human expert. In the forward model the underlying chemistry is described as rules and the data or predictions as features. We discuss how our modeling framework—via a knowledge extraction (KE) engine— transparently maps rules-to-equations-to-parameters-to-features as part of the forward model. We show that our KE engine is capable of robust, automated model refinement, when modeled features do not match the experimental features. Further, when multiple models exist that can describe experimental data, new sets of HTE can be suggested. Thus, the KE engine improves (i) selection of chemistry rules and (ii) the completeness of the HTE data set as the model and data converge. We demonstrate the validity of the KE engine and model refinement capabilities using the production of aromatics from propane on H-ZSM-5. We also discuss how the framework applies to the inverse model, in order to meet the design challenge of predicting catalyst compositions for desired performance.  2003 Elsevier Science (USA). All rights reserved.

The adsorption of CO on Ir(111) investigated with FT-IRAS

Abstract The adsorption of CO on Ir(111) has been investigated with Fourier transform infrared reflection-absorption spectroscopy, temperature programmed desorption, and low-energy electron diffraction. At sample temperatures between 90 and 350 K, only a single absorption band, above 2000 cm−1, has been observed at all CO coverages. For fractional coverages above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm−1. The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages. The coverage-dependent frequency shift of the IR band can be described quantitatively using an improved dipolar coupling model. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. The chemical shift is positive with a constant value of approximately 12 cm−1 for all coverages, whereas the dipole shift increases with coverage up to a value of 36 cm−1 at a coverage of 0.5 ML.

A novel reactor system for high throughput catalyst testing under realistic conditions

This article describes a 16-channel reactor system specifically designed for the high throughput study of supported heterogeneous catalysts under well controlled conditions. Each of the individual channels of the reactor has been designed to operate as an autonomous plug flow reactor without thermal or product cross-talk often associated with other reactors specifically designed for high throughput experimentation (HTE) studies. The flowrate difference between reactors was kept to a minimum through the use of orifices, and the temperature of each channel is continuously monitored. This level of control allows for the measurement of kinetically significant parameters in a high throughput manner. The system is also capable of studying transients occurring on the order of seconds, to further assist in kinetic analysis and understanding. Results are shown for the determination of reaction order during carbon monoxide oxidation over various supported metal catalysts measured under differential conversion.