Emission Mossbauer

RBa2Cu307 (R = rare earth element or Y), La2 (5r,.Cu04 (0 < X < 0.3) Eu-155(Gd-155) emission Mossbauer spectroscopy, EFG tensor at R sites, in good agreement with point charge model when holes are supposed to be mainly in sublattices of the chain and at oxygen in Cu-O plane... 

About 20 HTe superconducting compounds and copper oxidic systems Correlations of the Cu NQR/NMR data with the Cu( Zn) emission Mossbauer data for HTSC lattices as a tool for the determination of atomic charges... 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

In situ emission Mossbauer spectroscopy provides valuable information on the chemical structure of dilute metal ions at the metal oxide/aqueous solution interface The principles of the method are described with some experimental results on divalent Co-57 and pentavalent Sb-119 adsorbed on hematite. 

In spite of the development of physicochemical techniques for surface analysis, spectroscopic methods applicable to the study of bonding between adsorbed metal ion species and substrate are limited, especially those applicable to in situ measurement at interfaces between solid and aqueous phases (1,2). In previous papers, we showed that emission Mossbauer measurement is useful in clarifying the chemical bonding environment of dilute metal ions adsorbed on magnetic metal oxide surfaces (3,1 ) ... 

We now extend the work to in situ measurements on metal ions adsorbed at the metal oxide/aqueous solution interface. In this report, our previous results are combined with new measurements to yield specific information on the chemical structure of adsorbed species at the solid/aqueous solution interface. Here, we describe the principles of emission Mossbauer spectroscopy, experimental techniques, and some results on divalent Co-57 and pentavalent Sb-119 ions adsorbed at the interface between hematite (a-Fe203) and aqueous solutions. 

Such highly ionized species have been detected for Cl-37 produced by the EC decay of Ar-37 in gaseous phase ((>). In solids, however, such anomalous states are not realized or their life time is much shorter than the half-life of the Mossbauer level (Fe-57 98 ns and Sn-119 17-8 ns) because of fast electron transfer, and usually species in ordinary valence states (2+, 3+ for Fe-57 and 2+, 4+ for Sn-119) are observed in emission Mossbauer spectra (7,8). The distribution of Fe-57 and Sn-119 between the two valence states depends on the physical and chemical environments of the decaying atom in a very complicated way, and detection of the counterparts of the redox reaction is generally very difficult. The recoil energy associated with the EC decays of Co-57 and Sb-119 is estimated to be insufficient to induce displacement of the atom in solids. 

In absorption Mossbauer spectroscopy, a source nuclide in a standard form (usually in a metallic matrix) is coupled with a sample to be investigated. This method requires at least 100 pg of Fe or Sn in the usual experimental setup even if a Mossbauer sensitive enriched stable isotope Fe-57 or Sn-119 is employed. In emission Mossbauer spectroscopy, however, 1 mCi of Co-57 or Sb-119, which corresponds nominally to 120 ng of Co-57 or 1.4 ng of Sb-119, is sufficient to permit measurement. This technique enables study of very dilute systems, especially those with ions directly bound to the substrate. 

The hematite with adsorbed Co-57 or Sb-119 along with the solution was subjected to emission Mossbauer measurement at 24 1°C with the experimental setup shown in Figure 2. The absorber, Fe-57-enriched potassium ferrocyanide (0.5 mg Fe-57/cm2) or barium stannate (0.9 mg Sn-119/cm2), was driven by a Hanger 700-series Mossbauer spectrometer connected to a Tracor-Northern TN-7200 multi-channel analyzer. The Mosssbauer gamma-rays of Co-57 and Sb-119 were detected respectively with a Kr(+3% carbon dioxide)-filled proportional counter and with a 2 mm-thick Nal(Tl) scintillation counter through 65 pm-thick Pd critical absorber for Sn K X-rays. The integral errors in the relative velocity were estimated to be of the order of 0.05 mm/s by repeated calibration measurements using standard absorbers. 

In Situ Mossbauer Measurement on Hematite/Divalent Co-57. The adsorption behavior of cobaltous ions on hematite surfaces was essentially the same as that on silica reported by James and Healy (12). Appreciable adsorption begins at about pH 4 followed by an abrupt increase in adsorption between pH 6 and 8. Beyond pH 9, adsorption is practically complete. Emission Mossbauer spectra of Fe-57 arising from the divalent Co-57 ions at the interface between hematite particles and the 0.1 mol/dm3 NaCl solutions of different pH at room temperature are shown in Figure 3 The emission spectra show a marked dependence on the pH of the aqueous phase. No emission lines ascribable to paramagnetic iron species are recognized in... 

Figure 2. Experimental setup for in situ emission Mossbauer measurement. Setup for Co-57 is shown.

Figure 3 In situ emission Mossbauer spectra of Fe-57 arising from divalent Co-57 at the hematite/0.1 mol dm"3 NaCl solution interface for various pH values of the aqueous phase (measurement at room temperature) (A) pH 5.7, (B) pH 7.4, (C) pH 9.6, (D) pH 11.0, (E) pH 12.7, (F) pH 3.0. (F) was measured...

In order to study the effect of the amount of pentavalent Sb ions on adsorption state, in situ emission Mossbauer measurement was made on Sb-119 adsorbed on hematite with non-radioactive pentavalent... 

Figure 6. Effects of preheating of sample suspensions at 98°C for 30 min on the in situ emission Mossbauer spectra of Sn-119 arising from pentavalent Sb-119 at the hematite/0.25 mol dm 3 LiCl solution interface (measurement at room temperature) (A1) Before heating, pH 6.6 and (A2) after heating, pH 7.9 (B1)...

Sb carrier ions. The Sb-119 ions were adsorbed on 30 mg of hematite from 10 cm3 of a 0.25 mol/dm3 KC1 solution containing about 1 mg of pentavalent Sb ions. About 0.3 mg of Sb was adsorbed at pH 2.5 and 4.0. The amounts of Sb adsorbed are less than that required to cover all the hematite surfaces as a monolayer. The emission Mossbauer spectra obtained are shown in Figure 7. It is seen from Figure 7 that the width of the emission Mossbauer spectrum at pH 2.5 is much smaller than that of the carrier-free one, while essentially no effect of carrier Sb ions is observed at pH 4.0. 

Figure 7. In situ emission Mossbauer spectra of pentavalent Sb-119 adsorbed on hematite with Sb carrier from 0.25 mol/dm3 KC1 solution (A) pH 4.0 and (B) 2.5.

Figure 8. In situ emission Mossbauer spectra of divalent Co-57 adsorbed on hematite with pentavalent Sb ions from 0.1 mol/dm3 KCI solution (A) pH 5.5, (B) pH 9.2. (C) From 0.3 M KOH.

Effects of Pentavalent Sb on the Adsorption of Divalent Co-57. The emission Mossbauer spectra of divalent Co-57 adsorbed on hematite with pentavalent Sb ions (Figure 8) are complex and we have not yet succeeded in their analysis. It is certain, however, from the spectra that trivalent Fe-57 ions produced by the EC decay of Co-57 are interacting magnetically with the ferric ions of the substrate. This means that the divalent Co-57 are not adsorbed on the pentavalent Sb ions, but on hematite directly. The [Sb(OH)g]- anions are considered to facilitate direct adsorption of divalent Co-57 ions on the positively charged surfaces of hematite in the acidic region. 

In situ emission Mossbauer spectroscopic measurement of the hyper-fine magnetic fields on trivalent Fe-57 and tetravalent Sn-119 arising from divalent Co-57 and pentavalent Sb—119, respectively, yields valuable information on the chemical structure of adsorbed metal ions at the interface between hematite and an aqueous solution. 

Figure 27. Distribution of cobalt among the various species of an activated HDS catalyst, as determined by emission Mossbauer spectroscopy. The catalyst, prepared by impregnation of ammonium heptamolybdate and 57Co nitrate, was as similar as possible to the commercial one used for obtaining the results presented in the previous figures (3wt% as CoO, 13 wt% as M0O3 on -/-alumina). All precautions were taken to avoid possible errors due to noninstantaneous charge compensation in the sequence of nuclear events in 57Co decay (this has to be taken into account in the nonconducting M0S2 matrix) [155, 156].

Kamnev, A.A., Antonyuk, L.P., Smirnova, V.E., Serebrennikova, O.B., Kulikov, L.A., Perfiliev, Y.D. Trace cobalt speciation in bacteria and at enzymic active sites using emission Mossbauer spectroscopy. Anal. Bioanal. Chem. 372, 431 35 (2002)... 

In many materials problems, for example at surfaces or interfaces, the chemical composition and nuclear coordinates are not fully known. Indeed, any information which can be obtained by theory on these basic structural properties will be useful, in conjunction with experiment. Spatially Resolved Electron Energy Loss Spectroscopy (SREELS), X-ray near-edge absorption (XANES) and emission, Mossbauer spectra, etc. provide site-specific probes which can be combined with theory to help resolve structures. 

Kamnev A A, Antonyuk LP et al (2003) Application of emission Mossbauer spectroscopy to the study of cobalt coordination in the active centers of bacterial glutamine synthetase. Dokl Biochem Biophys 393 321-325... 

Nath A, Harpold M et al (1968) Emission Mossbauer spectroscopy for biologically important molecules. Vitamin B12, its analogs, and cobalt phthalocyanine. Chem Phys Lett 2 471-476... 

Scherson DA, Gupta SL, Fierro C, Yeager E, Kordesch M, Eldridge J, Hoffman R (1983) Cobalt tetramethoxyphenyl porphyrin - emission Mossbauer spectroscopy and O2 reduction electrochemical studies. Electrochim Acta 28(9) 1205-1209... 

A substantial advantage of emission Mossbauer spectroscopy in comparison with the transmission technique is that if the material to be investigated contains heavy elements, then the required dopant concentration (e.g., Co) may be 1-2 orders of magnitude lower in the emission experiment than the Fe concentration in an analogous transmission experiment. This is in connection with the intensity loss of the Mossbauer radiation due to electronic absorption, which is always self absorption in the source and regular absorption in the absorber (Vertes and Homonnay 1997). Low dopant concentration is very important in impurity Mossbauer spectroscopy, where the investigated material does not contain a Mbssbauer element thus, a conveniently measurable Mbssbauer nuclide is introduced artificially as an impurity with a potential risk of perturbing the physicochemical properties of the host phase. 

However, this model has been the subject of controversial discussions and some researchers argued against this remote control mechanism. Indeed, using emission Mossbauer spectroscopy (EMS), Wivel and co-workers (21) have shown that even when the CogSg phase is not detected, a promoter effect is observed by the addition of Co (Co/Mo < 0.4). Nevertheless, in any case, this model was very helpful in understanding metal sulfides activity and has clearly shown that the alumina support does not play any role in the promotion effect and that promotion resides uniquely at the interface of the two phases M0S2 and CogSg. 

Besides that, a systematic study of CoMo catalysts with emission Mossbauer spectroscopy (EMS) reported by Tops0e and co-workers, showed the existence of three different Co compounds in the catalyst cobalt contained in alumina support as aluminate (type 1), cobalt contained in CogSg (type 2), and a third cobalt species (type 3) associated in small amounts with the M0S2 phase. Topspe and co-workers (25) located the cobalt inside or on the edges of M0S2 crystallites and called the type 3 Co the CoMoS phase... 

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