Christopher M. Snively

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.

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.

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.

Multivariate and Univariate Analysis of Infrared Imaging Data for High-Throughput Studies of NH3 Decomposition and NOx Storage and Reduction Catalysts

The application of Fourier transform infrared (FTIR) spectroscopic imaging for the analysis of the reaction products from parallel reactors has been extended to the quantitative analysis of complex infrared (IR) spectra. Multivariate factor-based and univariate calibration models were developed to extract quantitative concentration information from highly overlapped IR spectra. The three multivariate factor-based models of principal component regression (PCR) and partial least squares 1 and 2 (PLS-1 and PLS-2) were employed. The effects of the number of coadded mirror scans used in the data collection and the number of factors used in the data analysis on the predictive ability of this multivariate approach were characterized. The effectiveness of these approaches is demonstrated through application to the high-throughput study of ammonia decomposition and NOx storage and reduction catalysts.

Performance and Application of a New Planar Array Infrared Spectrograph Operating in the Mid-Infrared (2000–975 cm -1 ) Fingerprint Region

A no-moving-part planar array infrared spectrograph (PA-IR) equipped with a 256 × 256 mercury cadmium telluride (MCT) focal plane array has been designed and constructed. The performance of the instrument, whose frequency range extends from 2000–975 cm−1, has been assessed in terms of resolution, bandwidth, and signal-to-noise ratio. The PA-IR spectrograph is able to record spectra with an 8.7 ms time resolution and has peak-to-peak noise levels as low as 2.4 × 10−4 A.U. As a demonstration of the potential of PA-IR, the dynamics of reorientation of a liquid crystalline sample exposed to a single electric field pulse has been studied. It was shown that PA-IR can be used for the simultaneous acquisition of two orthogonally polarized spectra. The advantages and limitations of PA-IR, step-scan Fourier transform infrared (FT-IR), and ultra-rapid-scanning FT-IR for real-time studies of reversible and irreversible phenomena are thoroughly discussed.

Experimental Aspects of Asynchronous Rapid-Scan Fourier Transform Infrared Imaging

In the asynchronous, rapid-scan approach to Fourier transform infrared (FT-IR) imaging, data sampling is not correlated with the zero crossings of the interference fringes of the HeNe reference laser. The success of this data collection scheme depends on the reproducibility of the clock signals driving the interferometer mirror and focal plane array data collection. In previous studies, it was shown that this implementation provides for markedly faster data acquisition without sacrificing data quality, as compared with step-scan imaging. This approach to data collection introduces some unique peculiarities to the acquisition and processing of imaging data. The purpose of this paper is to address a few of these concerns in terms of their effect on final data quality. Also, the practical aspects of implementing such an acquisition scheme are described in detail.

Development of a Planar Array Infrared Reflection Spectrograph for Reflection–Absorption Spectroscopy of Thin Films at Metal and Water Surfaces

Planar array infrared (PA-IR) spectroscopy offers several advantages over Fourier transform infrared (FT-IR) methods, including ultrafast speed (< 100 μs temporal resolution) and excellent sensitivity. However, obtaining spectra in the range of 1800 to 1000 cm−1 of films at the air–water interface remains difficult due to the poor IR reflectivity of water, the extremely low concentration of the thin film on the water subphase, and the interference of water bands. In this study, we report a new planar array infrared reflection spectrograph (PA-IRRS), which has several advantages over conventional approaches. This instrument can record sample and reference spectra simultaneously with an instrumental setup that is the same as that of a single-beam instrument by splitting the incident infrared beam into two sections on a plane mirror (H) or a water trough. With this design, the instrument can accommodate large infrared accessories, such as a water trough, without a loss of infrared beam intensity. Water bands can be subtracted to obtain a high-quality spectrum for poly(L-lactic acid) Langmuir film on the water subphase with a resolution of about 6 cm−1 in 10.8 s. Hence, this PA-IRRS system has great potential for investigating the time-resolved dynamics of a broad range of Langmuir films, such as cellular membranes or biopolymers, on the water subphase.

Real-time imaging of crystallization in polylactide enantiomeric monolayers at the air-water interface.

A newly developed planar array infrared reflection-absorption spectrograph (PA-IRRAS) offers significant advantages over conventional approaches including fast acquisition speed, excellent compensation for water vapor, and an excellent capacity for large infrared accessories, e.g., a water trough. In this study, the origin of stereocomplexation in a polylactide enantiomeric monolayer at the air-water interface was investigated using PA-IRRAS. PA-IRRAS was used as a probe to follow the real-time conformational changes associated with intermolecular interactions of polymer chains during the compression of the monolayers. It was found that a mixture of poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) (D/L) formed a stereocomplex when the two-dimensional monolayer developed at the air-water interface before film compression, indicating that there is no direct correlation between film compression and stereocomplexation. PA-IRRAS spectra of the stereocomplex exhibited distinct band shifts in crystalline sensitive components, e.g., the vas(C-O-C, h) mode, as well as amorphous-dependent components, e.g., the vs(C-O-C) mode, when compared with the spectra of PLLA alone. On the other hand, time-resolved PA-IRRAS spectra, which were obtained as the films were being compressed, revealed that both monolayers of PLLA and mixed PLLA/PDLA stereocomplex were crystallized into a 10(3)-helix and a 3(1)-helix, respectively, with a distinct band shift in crystalline sensitive components only. Fourier self-deconvolution of the spectra demonstrated that the band shift in crystalline sensitive components is correlated with the intermolecular interaction of polymer chains.

High-throughput reactor system with individual temperature control for the investigation of monolith catalysts

A high-throughput parallel reactor system has been designed and constructed to improve the reliability of results from large diameter catalysts such as monoliths. The system, which is expandable, consists of eight quartz reactors, 23.5 mm in diameter. The eight reactors were designed with separate K type thermocouples and radiant heaters, allowing for the independent measurement and control of each reactor temperature. This design gives steady state temperature distributions over the eight reactors within 0.5 degrees C of a common setpoint from 50 to 700 degrees C. Analysis of the effluent from these reactors is performed using rapid-scan Fourier transform infrared (FTIR) spectroscopic imaging. The integration of this technique to the reactor system allows a chemically specific, truly parallel analysis of the reactor effluents with a time resolution of approximately 8 s. The capabilities of this system were demonstrated via investigation of catalyst preparation conditions on the direct epoxidation of ethylene, i.e., on the ethylene conversion and the ethylene oxide selectivity. The ethylene, ethylene oxide, and carbon dioxide concentrations were calibrated based on spectra from FTIR imaging using univariate and multivariate chemometric techniques. The results from this analysis showed that the calcination conditions significantly affect the ethylene conversion, with a threefold increase in the conversion when the catalyst was calcined for 3 h versus 12 h at 400 degrees C.