Synchrotron and simulations techniques applied to problems in materials science: catalysts and Azul Maya pigments

Abstract

Development of synchrotron techniques for the determination of the structure of disordered, amorphous and surface materials has exploded over the past 20 years owing to the increasing availability of high-flux synchrotron radiation and the continuing development of increasingly powerful synchrotron techniques. These techniques are available to materials scientists who are not necessarily synchrotron scientists through interaction with effective user communities that exist at synchrotrons such as the Stanford Synchrotron Radiation Laboratory. In this article the application of multiple synchrotron characterization techniques to two classes of materials defined as 'surface compounds' is reviewed. One class of surface compounds are materials like MoS(2-x)C(x) that are widely used petroleum catalysts, used to improve the environmental properties of transportation fuels. These compounds may be viewed as 'sulfide-supported carbides' in their catalytically active states. The second class of 'surface compounds' are the 'Maya blue' pigments that are based on technology created by the ancient Maya. These compounds are organic/inorganic 'surface complexes' consisting of the dye indigo and palygorskite, common clay. The identification of both surface compounds relies on the application of synchrotron techniques as described here.

A combined synchrotron powder diffraction and vibrational study of the thermal treatment of palygorskite–indigo to produce Maya blue

“…• The surface silanols are not modified in the presence of indigo and rehydrate fully after MB formation, a result that is incompatible with the description of MB as a "surface compOlll1 d," as suggested in some works [18,21].…”

“…A second possible structural approach proposed by Kleber et al [17] would allow indigo to penetrate (partially or deeper) into the clay tunnels. It can be found literature supporting Van Olphen [18][19][20][21][22] and Kleber [13,[23][24][25][26] models. More recently, Hubbard et al [6] suggested the covering of the opening of nano-tmmels by the indigo molecules.…”

Organic/inorganic complex pigments: Ancient colors Maya Blue

“…The physical and chemical properties are characterized using various experimental techniques (optical microscopy, scanning electron microscopy (SEM), (HRTEM), X-ray powder diffraction (XRD), infrared absorption and Raman scattering spectroscopy, thermogravimetric analysis (TGA) and surface analysis) and these results are complimented by density functional theory (DFT) calculations. Details of the analytical techniques not described in this report can be found in reference [14,15]. The solid-state chemical interaction phenomenon that gives rise to the pigments stability is shown to be a surface interaction between the organic and inorganic components.…”

Raman and infrared studies of synthetic Maya pigments as a function of heating time and dye concentration

“…The disappearance of the crystalline indigo structure upon heating as determined with synchrotron X-ray diffraction, and the fact that the palygorskite structure undergoes little change with no visible peak broadening and shifting, lead to the conclusion that the organic/inorganic nanocomposite is a surface compound. 2 The organic molecules for a 16% wt indigo concentration are primarily bound to the surface of the palygorskite with only limited penetration of the channels because of the fibrous nature of the clay, which offers limited surface area for penetration at the ends of the fibers.…”

“…4,6 The most prevalent hypothesis states that as a result of the heating process, water molecules are eliminated from the entrances of the channels of fibrous palygorskite, and the dye molecules enter into these open channels. 2,4,5 The formation of H-bonds between the indigo carbonyl group and the nearest structural water was suggested as the cause of anchoring the dye inside the clay structure, and the justification for the pigment stability. 7 Another hypothesis involving the presence of metallic nanoparticles such as iron oxide and an amorphous phase of iron hydroxide was also suggested to explain the pigment stability.…”