Elsevier

Catalysis Today

Volume 68, Issue 4, 30 July 2001, Pages 263-381
Catalysis Today

IR spectroscopy in catalysis

https://doi.org/10.1016/S0920-5861(01)00334-0Get rights and content

Abstract

Infrared (IR) spectroscopy undoubtedly represents one of the most important tools in catalysis research. In this review, recent catalytic applications of the most popular IR techniques will be presented. Each section starts from the very general basis of the spectroscopic method applied. The last section is devoted to the adsorption of chelating compounds on surfaces of mineral oxides. The aim of adding a large number of illustrations and an appendix is to make the presented material more familiar for young researchers and postgraduate students. Most of the papers discussed have appeared in the last 5–6 years, because the older literature has been reviewed in earlier papers.

Introduction

After nearly 50 years of intensive application, infrared spectroscopy (IR) remains the most widely used, and usually most effective, spectroscopic method for characterisation of the surface chemistry of heterogeneous catalysts. IR always played an important role in characterisation of heterogeneous catalysts, as it permits direct monitoring of the interaction between sorbed molecules and the catalysts.

The goals of catalytic research are varied. Complete understanding of catalytic reaction mechanisms, including the nature of adsorbed intermediates, is, of course, highly desirable. Catalysis is primarily an applied science, however, and as such should reasonably be expected to provide major assistance in reaching the goals of better catalysts and improved catalytic processes, from a better fundamental understanding of catalyst surface chemistry. This is an area in which IR will undoubtedly make further major contributions.

A variety of IR techniques has been and can be used in order to obtain information on the surface chemistry of different solids. Special meaning have investigations carried out under the reaction conditions. In principle for in situ measurements, all forms of IR spectroscopy are suitable. For most practical experimental reasons, however, the transmission–absorption technique is best suited. This is more related to the design of cells that are to be used as reactor than with the principal problems of the other techniques.

Recently, several books have been published which are in some way directed at catalysis research. The contents of the books cover theoretical aspects of spectroscopy [1], different techniques and modern molecular spectroscopy [2], [3], practical aspects of spectrometers and spectrometry [4], base information related to FT-IR with references and/or recommended bibliography for further reading [5], IR group frequencies [6], IR and Raman of inorganic and coordination compounds [7], interpretation of IR spectra [8], [9], a handbook which provides unique data of IR and comparative Raman spectra of inorganic compounds and organic salts including some non-ionic compounds [10], and books for those beginning to work with IR and Raman Spectroscopy in the investigation of surfaces [11], [12]. From the internet, the Internet Journal of Vibrational Spectroscopy (IJVS) can be found, which is published free of charge exclusively on the World Wide Web [13]. The six editions appearing each year are divided into two parts. The first contains three or four papers on a theme, of graded sophistication, aimed to assist, interest, and also improve the performance of the non-specialist through to the dedicated experienced spectroscopist; news, views, and an unusual feature “Hot Sources” on an ever expanding body of spectroscopic subjects, and Spectroscopists’ Bookshelf, a list of recommended books. Spectroscopy links is a regular, expanding feature of IJVS, which compiles a list of Web sites with a spectroscopic content [14]. Finally, a series of excellent reviews has been published in 1996, in a special issue of Catalysis Today (vibrational spectroscopy of adsorbed molecules and surface species on metal oxides 27 (3–4)).

In the past few years, one can observe a growing interest in the application of IR techniques in catalytic investigations. One of the reasons, among the others, is their wide distribution (nowadays, IR and/or FT-IR spectrometers belong to the standard equipment of each scientific laboratory) and the relatively low costs (compared to the other modern physico-chemical techniques for surface characterisation) of the base instrument.

The aim of this review is to present current investigations in the area of heterogeneous catalysis where different IR techniques are applied.

Section snippets

Brief historical background

Since 1905, when Coblentz obtained the first IR spectrum, vibrational spectroscopy has become an important analytical tool in research. By 1940, there was a large body of knowledge concerning IR spectroscopy [15], [16], [17]. Three programs of great importance during World War II provided the impetus to begin the manufacture of IR instruments: the synthetic rubber program, largely a US project; the production of aviation fuel, primarily a UK project; and the penicillin program, a joint US–UK

Recent applications of IR in catalytic research, and IR cell reactors for in situ studies

Because IR spectroscopy is a regularly used technique for catalyst characterisation, compilations and reviews on the various experimental techniques are numerous. Transmission–absorption, diffuse reflectance, ATR, specular reflectance, and photoacoustic spectroscopy are among the most frequently used techniques [19]. The principal information obtained with all these techniques is equivalent, and local availability and experimental necessities, such as the sample particle size and the molecular

Transmission spectroscopy

Transmission spectroscopy is the simplest sampling technique in IR spectroscopy and is used for routine spectral measurements (see Fig. 20). A small amount, usually 1–3 mg, of finely ground solid sample is mixed with approximately 400 mg powdered potassium bromide and then pressed in an evacuated die under high pressure (see Appendix A). The resulting discs are transparent and yield good spectra. The vast majority of experiments are currently performed in the transmission–absorption mode.

The

ATR spectroscopy

Internal reflection spectroscopy (IRS) became a popular spectroscopic technique in the early 1960s. It has become more widely known by the name ATR spectroscopy (see Fig. 59). ATR spectroscopy permits any surface to be brought in to contact with a high index of refraction internal reflection element (IRE). However, since the radiation is trapped by total internal reflection inside the IRE and only interacts with the sample surface, it is not propagated through it. The ATR technique has found

Diffuse reflectance spectroscopy

The optical phenomenon known as diffuse reflectance is commonly used in the UV–Vis, NIR, and MIR regions to obtain molecular spectroscopic information (see Fig. 62). When it is applied in MIR area with an Fourier transform it is known as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is usually used to obtain spectra of powders with minimum sample preparation. The collection and analysis of surface-reflected electromagnetic radiation as a function of frequency or

Emission spectroscopy

There are numerous examples of manufacturing processes where the ability to perform a rapid, non-destructive determination of product quality is of great value. IR spectral data of a sample contain compositional as well as physical property information hence can often provide the information required determining product quality. Infrared emission spectroscopy (IRES) is a technique that can often be applied to conveniently collect IR data from a sample in a process (see Fig. 76). All that is

Photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) measures a sample’s absorbance spectrum directly with a controllable sampling depth and with little or no sample preparation (see Fig. 80). This rapid direct analysis capability is applicable to nearly all samples encompassing a wide range of absorbance strengths and physical forms. Among the other key features of PAS are that it is non-destructive, noncontact, applicable to macrosamples and microsamples, insensitive to surface morphology. It has a spectral

Hyphenated and miscellaneous IR techniques

Determination of molecular identities and quantities in complex mixtures may be the most common problem in chemistry. In most instances, the result of synthetic chemical operations is not a single product, but a distribution of multiple products. Because of the difficulty and importance of the task, multiple analytical techniques are combined to form what are often called hyphenated techniques. Combined analytical techniques utilising FT-IR have been employed for solid, liquid, and gas-phase

Second-harmonic generation

Second-harmonic generation (SHG) and sum-frequency generation (SFG) are two second-order nonlinear optical spectroscopy techniques that, because of their high surface specificity, have recently become the important tools for the study of surfaces and interfaces [765], [766], [767], [768], [769], [770], [771].

Unlike traditional FT-IR, SFG is sensitive only to interfaces and requires no bulk subtraction. SFG has many advantages over FT-IR with respect to surface/interface studies. This is

Adsorption of chelating compounds on mineral oxides surface

Ethylenediaminetetraacetic acid (EDTA) was first synthesised in Germany during the 1930 s. Since that time, EDTA and its salts have become important industrial chelating agents [837]. For a long time they were applied in analytical chemistry or with complex metals as chelated micro-nutrients. In a practical sense, metal micro-nutrients may be chemically changed and protected by forming a cage-like structure around the metal ions. This cage-like protective structure will prevent unwanted and

Conclusions

This review tries to present as complete a picture as possible of the actual and the potential use of IR spectroscopic techniques in catalytic research. Recently, Kalinkova [890] has published a paper where she gave a schematic presentation of the application of IR spectroscopy in pharmacy. The application of IR in catalysis and surface science can be presented in a similar way (Fig. 98). It is a useful way to summarise the material presented here.

There is no doubt that we can observe an

Acknowledgements

The author gratefully acknowledges the suggestions given by Prof. Julian Ross, Limerick University, Ireland, Dr. Ben Nieuwenhuys, Leiden University, and also special thanks to Dr. Michael Gagan, Open Manchester University, for his kind help in linguistic improvements. This paper is dedicated to all my co-workers at the Department.

Glossary

This glossary contains definitions of many important FT-IR terms [5,899].

100% line
is calculated by rationing two background spectra taken under identical conditions. Ideally, the result is a flat line at 100% transmittance. The slope and noise of 100% lines are measured to ascertain the quality of spectra and the health of an instrument.
Absorbance
units used to measure the amount of IR radiation absorbed by a sample. Absorbance is commonly used as the y-axis unit in IR spectra. Absorbance is defined by Beer’s law, and is linearly proportional to concentration. This

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