Gold-197 Mossbauer spectroscopy

D. T, Burns, K., Titus, D.D., Girard, G.R., Reiff, W.M. and Mascavage, L.M. (2003) Dichloro (pyridine-2-carboxamido-N 1, N2)gold(III), a bis-nitrogen aurocycle syntheses, gold-197 Mossbauer spectroscopy, and X-ray crystal structure. Inorganica Chimica Acta, 346, 1. 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Three series of Au nanoparticles on oxidic iron catalysts were prepared by coprecipitation, characterized by Au Mossbauer spectroscopy, and tested for their catalytic activity in the room-temperature oxidation of CO. Evidence was found that the most active catalyst comprises a combination of a noncrys-taUine and possibly hydrated gold oxyhydroxide, AUOOH XH2O, and poorly crystalhzed ferrihydrate, FeH0g-4H20 [421]. This work represents the first study to positively identify gold oxyhydroxide as an active phase for CO oxidation. Later, it was confirmed that the activity in CO2 production is related with the presence of-OH species on the support [422]. 

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

A number of publications deal with Au Mossbauer studies of various gold alloys. As the primary goal of this review is directed towards applications of Mossbauer spectroscopy to chemical compounds rather than metallic systems, we present here a brief enumeration of most of the work by giving reference to relevant publications and providing a note concerning the essential points of interest in each case ... 

Although in most of the above reports structural information is based on X-ray data, Au Mossbauer spectroscopy has also been successfully employed for the investigation of two-, three-, and four-coordination in gold(I) complexes.2551 An increase in coordination numbers leads to a decrease in the isomer shift (IS) by 1-2mm s 1 (three-coordination) or 2-4mm s 1 (four-coordination) relative to two-coordination. For the same ligands, the quadrupole splitting (QS) for three-coordinated complexes is expected to be very similar to that for the two-coordinate derivatives, while that for complexes with Td symmetry should be zero. 

Spectroscopic studies, including vibrational spectroscopy, UV-visible absorption and MCD spectroscopy and 197Au Mossbauer spectroscopy, suggest that isocyanides act largely as a donors to gold(I) with very little d -p back-bonding,69,404,408 It is this polarization R—... 

There has been to some degree the belief that Mossbauer spectroscopy, although in principle an ideal technique for catalyst studies, for practical purposes can only be applied to problems in catalysis if the catalyst contains either iron or tin. Therefore, one of the main purposes of this review is to show how Mossbauer spectroscopy can be directly extended to many additional Mossbauer atoms or isotopes (such as antimony, europium, nickel, ruthenium, gold, and tungsten) and, perhaps more importantly, how the technique can be extended to obtain information about systems that do not contain a Mossbauer atom. ... 

Similar considerations hold for the Mossbauer spectroscopy of gold, although with the recently increased interest in small particles of gold in catalysis, several investigations have been undertaken one such study is reviewed in the following section. 

Gold must be fully reduced to be active. This usually requires the catalyst to have been thermally activated, and there are cases where only Au° has been detected by XPS,43,53,118 XAFS27,32 or Mossbauer spectroscopy,106 and the catalyst found to be active. 

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