Paper, reflectance spectroscopy

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

The most important UV/VIS applications have been in the fields of color measurement and color matching, areas of great importance to the dye, paint, paper, textile, and printing industries. The pharmaceutical industry has similar interests in that the use of coloring agents in formulations requires specification. Reflectance spectroscopy has been used, however, by a number of workers to study the kinetics and mechanisms associated with a variety of reactions that were found to take place in the solid state. 

Previous reviews on solid-state infra-red spectroscopy are, e.g. by Vedder and Hornig I7) which contains a relatively complete bibliography of all the most important work up to 1960 and by Mitra 18), while Wilks and Hirsh-feld 19) have written a review of internal reflection spectroscopy. Orville-Thomas 20) wrote a review on I. R. spectroscopy as a diagnostic tool, and Annual Reviews of Physical Chemistry 21) often contain review papers on infra-red spectroscopy which also refer to the solid state. 

The process of loading zeolites with organometallie complexes always brings to the forefront the question of internal versus external confinement of the metal guest. In this paper we present some experiments based on size exclusion, metal loading and intrazeolite chemistry which in conjunction with FT-FAR-IR, EPR and UV-visible reflectance spectroscopy, critically probe the question of internal versus external location for the case of five representative organometallics,... 

In an effort to gain a better understanding of the initial fouling step leading to bacterial attachment and biofilm formation, we have investigated the adsorption of protein and polysaccharides onto thin metallic films and uncoated internal reflection elements from flowing solutions using attenuated total reflectance spectroscopy. The preliminary results will be described in this paper. 

Recently, 2-hydroxypropyl cellulose (HPC) was demonstrated to be a very suitable matrix for UV/Vis absorption study of photoyellowing of milled-wood lignin in the solid state [10] where clean kinetics of discolouration were established after different chemical treatments of the lignin in solution. This indicates the importance in the discolouration process of phenolic structures without carbonyl groups and conjugated double bonds. The present communication describes a similar photochemical study on quinones and hydroquinones incorporated in HPC films under comparative concentrations ( 6.5xl0"5 mol/g HPC). For comparison, we also use UV/Vis reflectance spectroscopy to examine the behaviour of these compounds adsorbed on filter paper after light-exposure. 

And it was in 1967 that a detailed review and description of the state of the art of internal reflection spectroscopy was given in a short form by Hirschfeld and Wilks (18) and more comprehensively in the well known book of Harrick (19). In the following years up till now there were many papers on this topic, but with very few exceptions (which will be mentioned later) all of them deal with applications in the infrared region. 

In the present paper, the main objectives are (i) to prepare reactive peroxocomplexes in situ at the material s surface starting from a precursor material, and (ii) to control the catalytic properties (activity, selectivity, oxidant efficiency) via modification of the micro-environment of the catalytic center through variation of the anion population. The catalyst precursors and the in situ formed peroxocomplexes are characterized by means of XRD, IR, TGA/DTA and UV-Vis reflectance spectroscopy. 

Porsskahl and J Janson. Sequential Treatment of Mechanical and Chemimechanical Pulps with Light and Heat. Part 1. UV-VIS Reflectance Spectroscopy. Nordic Pulp Paper J. 6 118-126, 1991. 

Phenylhydrazine is able to react with carbonyl groups to give coloured compounds. Paper sheets can be reacted with this reagent or modifications of it (e.g. 4-nitrophenylhydrazine) and the colour produced can be measured by reflectance spectroscopy. The colour may also be measured by dissolving the paper and measuring the absorbance of the solution. Lignin interferes badly with this test. 

For a discussion of reflectance spectroscopy, two types of reflectance must be defined, specular and diffuse. Specular reflectance is simply mirrorlike reflectance from a surface and is sometimes called regular reflectance it has a well-defined reflectance angle. Diffuse reflectance is defined as reflected radiant energy that has been partially absorbed and partially scattered by a surface with no defined angle of reflectance. The diffuse reflectance technique is widely used today for industrial applications involving textiles, plastics, paints, dyestuffs, inks, paper, food, and building materials. In the area of basic research, diffuse reflectance spectroscopy has been used in studies of solid-solid reactions, of species absorbed on metal surfaces, of radiation transfer, and of slightly soluble species. 

It can be argued that other methods permit access to the low wavenumber spectral region, such as Raman spectroscopy and diffuse reflectance spectroscopy (DRIFTS). It is beyond the scope of this paper to comment on the deficiencies of these methods there are many when compared to the weU understood transmission method, both from the experimental and theoretical points of view. 

In this paper, the basic principles of reflectance spectroscopy will be first discussed, emphasizing what is particular to each method electronic spectra for UV-visible Reflectance Spectroscopy (UVERS), vibrational fingerprints for Electrochemically Modulated Infrared Reflectance Spectroscopy (EMIRS). After a short presentation of the experimental set-up for each technique, various examples, taken mainly from our laboratory, will be given. 

Diffuse reflection spectroscopy, also known as diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, enables the analysis of many samples for which traditional techniques fail, to be made with little or no sample preparation. Many substances in their natural. state. e.spe-cially powders but also any solid with a rough surface, such as dyed textiles and printed papers, exhibit diffuse reflection, i.e., incident light is... 

The second method (B) used is based on an equation for external reflection spectroscopy on metal substrates given in the paper of Qiabal [12], covering both IRRAS spectra and ATR of thin films on highly reflective metal substrates ... 

NIR and MIR reflectance spectroscopy were used to characterize Whatman No. 1 filter paper coated with solutions of poly(hydrogenmethylsiloxane) and dichlorodimethylsilane. The paper was coated to improve the extraction of polycyclic aromatic hydrocarbons (PAHs) from water [104]. 

NIRS in internal reflection mode has also been used to study in situ low-level surfactant adsorption reactions (sub-monolayer coverage) using reactive internal reflection elements [175]. Standard errors of 0.04% were reported for the determination of 0.25-1.25% additives in nylon (cubes and films) [294]. Turley et al [309] used NIRS to determine ethylene oxide content and glycerin additive concentrations in ethylene oxide/propylene oxide copolymers. NIR diffuse reflectance spectroscopy was used to analyse up to 10% paper (cellulose) in agglomerate plastic waste (PE 60%, PS 20%, PP 15%, PVC 4%) [161]. For pigment applications, cfr, ref. [310]. 

IR diffuse reflectance spectroscopy seems to be the more appropriate method for observing OH stretch frequencies as well as their overtones and combination bands in the near infrared, especially with high-silica zeolites, i.e. at low hydroxyl concentrations, thus acquiring additional information on the shape of the potential curve [17,26,27]. In these papers an attempt is made to draw a unifying picture of the coinciding OH frequencies of silica-rich zeolites such as mordenites and ZSM type zeolites and the different and more complicated spectra of... 

LSPR Reflectance Spectroscopy, quantitative multivariate analysis and time constant analysis was performed as described in our papers (10, 11). 

---