Mössbauer laboratory

The Mössbauer spectroscopy in principle is based on the detection of the recoilless resonance-absorption of gamma rays originated from appropriate nucelar transitions.

The main advantage of the method is that it provides means to detect relative energy changes in 10-11 part relative accuracy. Shifts in the energies of nuclear transitions originated from interactions with the local electric and magnetic fields at the nucleus lay in this range. Thus, information can be obtained on the local environment by comparing the energies of the nuclear transitions of an emitter and an absorber nucleus.

Unfortunately, the list of isotopes suitable for convenient studies is short: 57Fe, 57Co, 119Sn. However, with extended facilities investigations on other nuclei (67Zn, 99Ru, 121Sb, 125Te, 129I, 151Eu, 155Gd, 193Ir, 197Au, etc.) can also be accomplished.

Further in depth information on the features of the method is available e.g. on the homepage devoted to Topics Related to Mössbauer Spectroscopy.

The  local feature is the facility to perform in-situ measurements in our laboratory. I.e., samples can be treated and subsequently measured under different conditions, without exposing them to air. Thus, the actual oxidation and coordination states of the samples can be preserved and studied. This feature is particularly advantageous when studying (heterogeneous) catalysts - most of the studies has been performed on this topic in our Department.

Measurements and treatments can be performed in the 77 – 700 K temperature range in variuos ambient atmospheres , e.g. under vacuum (10–2 Pa) or in various reactant gases and in mixtures of them (most frequently hydrogen, nitrogen, carbon monixide, etc.). As a further extension, these gas streams can be saturated with vapours of volatile fluids, as well.

The photo of the apparatus.

Fields of activity:

The laboratory was installed in 1979, various types of heterogeneous catalysts have been studied since then. For demonstration, some fields are listed here (see also [G1] in the list of selected publications ).

1. Decomposition of carbonyl complexes, formation of iron carbide, metallic iron:

1.1. The guiding presumption was that metallic particles can probably be prepared by decomposing carbonyl complexes, since the zerovalent state of the metal is already provided in the precursor molecules. In the case of iron however, immediate formation and stabilisation of iron carbides was detected [1/1, 1/2].

1.2. Nanoparticles of metallic iron can be prepared and stabilized in high dispersion by different methods, e.g. in zeolites by strong reducing agents. In the Fe3+-Y + NaN3 reaction metallic particles were stabilized inside the cages ( in 1 nm size) with simultaneous formation of 3 -7 nm iron particles [1/3, 1/4].

2. Bimetallic supported catalysts:

2.1. Alloys of iron:

· In bimetallic systems, the preparation of metallic phase from the appropriate carbonyl complexes was sometimes successfull; for instance, formation of high dispersion FeRu alloy on silica was demonstrated [2/1].

· Iron may act as a promoter, e.g. addition of iron to 2 wt% Pd/SiO2 catalyst in minor amounts raises the selectivity of formation of methanol from CO and H2 almost to 100 % [2/2, 2/3]. In these catalysts, formation of alloys was clearly demonstrated [2/4]. In Pd:Fe = 1 ratio carbide formation was found, whereas iron-rich FePd/SiO2 with Fe Pd = 10 promoted aromatisation [2/5].

2.2. Among alloys of tin supported PtSn systems were primarily studied

· Depending on the method of preparation the extent of platinum-tin interaction may be different. The interaction is more intimate when the precursor is prepared by impregnating a molecular complex of tin and platinum onto alumina, whereas using the conventional coimpregnation results in a less expressed interaction of the two metals [2/6].

· Very convincing demonstration of the power of the method is seen in recent studies: variations of composition of catalysts in room temperature CO oxidation are clearly evidenced. Contributions of various types of PtSn alloys in the Mössbauer spectra and Sn(IV) « (tin rich) PtSn conversion is manifested in the surface layers of high dispersion bimetallic particles [2/7].

3. Zeolites, micro- and mesoporous molecular sieves

3.1. Extra framework ions

· were introduced to faujasite cages by regular aqueous ion exchange. Fe(II) ions were identified in both tetrahedral and octahedral locations, furthermore, formation of associated clusters of them was detected as well [3/1].

· Lewis-acidic properties of the octahedral extra-framework Fe2+component are demonstrated in Fe-MFI catalysts of various Si:Fe ratios (27, 35, 67); the proportion of Fe2+(Oct) component is correlated to the catalytic activities found in conversions of toluene [3/2].

· Completion of solid-state ion exchange in the FeCl2 + NH4-Y reaction was followed upon heating [3/3], possibility for exchange starting fom iron(II) acetate and –oxalate salts was studied as well [3/4].

3.2. Iron(II)phthalocyanine complex was encapsulated into Y zeolite cages: the changes in the coordination around the central Fe(II) ion were clearly evidenced in the respective Mössbauer spectra recorded in subsequent steps of a mild oxidation process (hidroquinone to quinone by H2O2) [3/5].

3.3 Vide variety of framework substituted zeolite analogues have been studied.

· Changes in the coordination and oxidation states induced by reduction-oxidation treatments are evidenced in ferrisilicates Fe-MFI [3/6], Fe-Beta [3/7] and Fe-FER [3/8]; bonding strengths of various iron species are distinguished by estimating their Debye-temperatures [3/9], existence and role of dinuclear Feframework-O-Feextra-framework centres are demonstrated in Fe-FER [3/8] and Fe-MFI [3/10].

· The respective Sn-MFI, Sn-MEL, Sn-MTW stannisilicates are also characterized in redox treatments, the variations in the oxidation state and bonding strength (via the determination of the respective d ln A(300K / 77 K) / dT values) of stannic and stannous species are revealed [3/11].

3.4 Molecular sieves have also been characterized.

· Properties of microporous FeAPO-5, FeAPO-11, Fe-ETS-10 were determined and compared [3/12, 3/13], and the redox behaviours of iron ions in them are correlated to the structural differences of the hosts [3/14].

· Tin and iron ions were incorporated into mesoporous MCM-41. Sn4+ exhibits a loose bonding and facile and reversible Sn4+ « Sn2+ conversion in Sn-MCM-41 under the redox conditions [3/15]. Single iron ions exist in various types of coordinations in Fe-MCM-41 samples of high Si:Fe ratios (~130) (e.g. Fe3+(Oct), Fe3+(Trig), Fe2+(Tetr), Fe2+(Trig)). At smaller Si:Fe ratios (~ 20) iron ions are stabilized in associated forms as well, however they retain the very high dispersion. Intermediate valency of iron (between 2 and 3) can also be detected in samples exposed to CO treatments [3/16].

List of selected publications:


G1 K. Lázár,
In situ transmission Mössbauer spectroscopy studies for structure determination of supported catalysts,
Struct. Chem., 2 (1991) 245-265

1. Carbonyls, carbides, metallic iron

1/1 K. Lázár, Z. Schay and L. Guczi,
Direct evidence for the correlation between surface carbon and CO + H2 selectivity on iron and iron-ruthenium catalysts prepared from metal carbonyl clusters,
J. Mol. Cat., 17 (1982) 205-218

1/2 K. Lázár, K. Matusek, J. Mink, S. Dobos, L. Guczi, A. Vizi-Orosz, L. Markó and W.M. Reiff,
Spectroscopic and catalytic study on metal carbonyl clusters supported on Cab-O-Sil. I. Impregnation and decomposition of Fe3(CO)12
J. Catal., 87 (1984) 163-178

1/3 H.K. Beyer, G. Onyestyák, B.J. Jönsson, K. Matusek, K. Lázár,
Reduction of iron ions to the metallic state in X and Y zeolites by sodium azide,
in: Proc. 12th International Zeolite Conference (Eds: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins) Materials Research Society, 1999. pp. 2875-2880.

1/4 K. Lázár, H.K. Beyer, G. Onyestyák, B.J. Jönsson, L.K. Varga, S. Pronier,
Iron nanoparticles in X and Y zeolites,
Nanostructured Materials, 12 (1999) 155-158.

2. Bimetallic supported catalysts

2/1. K. Lázár, W.M. Reiff, W. Mörke and L. Guczi,
Spectroscopic and catalytic study on metal carbonyl clusters supported on Cab-O-Sil. III. Application of low temperature and high-field Mössbauer spectroscopy to the characterization of iron-ruthenium bimetallic catalysts,
J. Catal., 100 (1986) 118-129

2/2. B.M. Choudary, K. Lázár, K. Matusek and L. Guczi,
Iron or lanthanum promoters on the selectivity of palladium zeolites in methanol synthesis,
J. Chem. Soc., Chem. Commun., 1988, 592

2/3. G. Lietz, M. Nimz, J. Völter, K. Lázár and L. Guczi,
Double promotion of palladium/silica catalysts by iron and magnesium oxide in synthesis of methanol from carbon monoxide and hydrogen,
Appl. Catal., 45 (1988) 71-83

2/4 . K. Lázár, M. Nimz, G. Lietz, J. Völter and L. Guczi,
Formation of PdFe alloys on silica supported catalysts,
Hyperfine Int., 41 (1988) 657-660

2/5 M. Nimz, G. Lietz, J. Völter, K. Lázár and L. Guczi,
Direct conversion of syngas to aromatics on FePd/SiO2 catalysts,
Catal. Lett., 1 (1988) 93-98

2/6 C. Kappenstein, M. Guerin, K. Lázár, K. Matusek, Z. Paál,
Characterisation and activity in n-hexane rearrangement reactions of metallic phases on Pt-Sn/Al2O3 catalysts of different preparation,
J. Chem. Soc., Faraday Trans., 94 (1998) 2463-2473.

2/7 . J.L. Margitfalvi, I. Borbáth, M. Hegedûs, E. Tfirst, S. Gõbölös, K. Lázár,
Low-temperature CO oxidation over new types of Sn-Pt/SiO2 catalysts,
Journal of Catalysis, 196 (2000) 200-204.

3. 1. Zeolites

3/1. K. Lázár, I. Manninger and B.M. Choudary,
Associated and single iron ions in ion-exchanged faujasite zeolites,
Hyperfine Interact., 69 (1991) 747-750

3/2 Lázár K., M.-Szeleczky A., Vorbeck, G., Fricke, R., Vondrova, A., Cejka, J.,
In situ Mössbauer study of iron containing MFI ferrisilicates: Relations to catalytic properties,
Journal of Radioanalytical and Nuclear Chemistry, Articles, 190 (1995) 407-411,

3/3 Lázár K., Pál-Borbély G., Beyer H.K., Karge, H.G.,
Solid-State Ion Exchange in Zeolites, Part 5. NH4-Y - Iron(II) Chloride,
Journal of the Chemical Society, Faraday Transactions, 90 (1994) 1329-1334

3/4 Lázár K., Pál-Borbély G., Beyer H.K., Karge, H.G.,
Catalysts by solid-state ion exchange: Iron in zeolite,
Studies in Surface Science and Catalysis, 91 (1995) 551-559

3/5 Lázár K., M.-Szeleczky A., Notheisz F., Zsigmond A.,
Encaged iron phthalocyanine for oxygen transfer; Catalytic and Mössbauer spectroscopic study,
Studies in Surface Science and Catalysis, 94 (1995) 720-727

3/6 K. Lázár, G. Borbély and H. Beyer,
In situ Mössbauer study of framework substituted (Fe)ZSM-5 zeolites,
Zeolites, 11 (1991) 214-222

3/7 Raj, A., Sivasanker, S., Lázár K.,
Studies on the Stability of Fe3+ Ions in the Ferrisilicate Analog of Zeolite Beta
Journal of Catalysis, 147 (1994) 207-213

3/8 K. Lázár, G. Lejeune, R.K. Ahedi, S.S. Shevade, A.N. Kotasthane,
Interpreting the oxidative catalytic activity in iron-substituted ferrierites using in situ Mössbauer spectroscopy,
J. Phys. Chem. B 102 (1998) 4865-4870.

3/9 K. Lázár, A.N. Kotasthane, P. Fejes,
Oxygen transfer centers in Fe-FER and Fe-MFI zeolites: Redox behaviour and Debye temperature derived from in situ Mössbauer spectra,
Catalysis Letters, 57 (1999) 171-177.

3/10 P. Fejes, J.B. Nagy, K. Lázár, J. Halász,
Heat-treatment of isomorphously substituted ZSM-5 (MFI) zeolites
Appl. Catal., A:General, 190 (2000) 117-135.

3/11 K. Lázár, A.M. Szeleczky, N.K. Mal, A.V. Ramaswamy,
In situ 119Sn-Mössbauer spectroscopic study on MFI, MEL and MTW tin silicalites,
Zeolites, 19 (1997) 123-127.

3.2. Molecular sieves

3/12 K. Lázár, J. Cejka,
Valency and coordination of iron in FeAlPO molecular sieves: an in situ Mössbauer study,
Stud. Surf. Sci. Catal., 125 (1999) 213-220.

3/13 K. Lázár, T.K. Das, K. Chaudhari, A.J. Chandwadkar,
Site preference and reducibility of substituted ferric iron in Fe-ETS-10,
Stud. Surf. Sci. Catal., 125 (1999) 301-306.

3/14 K. Lázár, A.J. Chandwadkar, P. Fejes, J. Cejka, A.V. Ramaswamy,
Valency changes of iron and tin in framework-substituted molecular sieves investigated by in situ Mössbauer spectroscopy,
J. Radioanal. Nucl. Chem., 246 (2000) 143-148.

3/15 K. Chaudhari, T.K. Das, P.R. Rajmohanan, K. Lazar, S. Sivasanker, A.J. Chandwadkar,
Synthesis, characterization and catalytic properties of mesoporous tin-containing analogs of MCM-41,
Journal of Catalysis, 183 (1999) 281-291.

3/16 K. Lázár, G. Pál-Borbély, Á. Szegedi, H.K. Beyer,
submitted to Hypefine Interact.