Modification of the Catalytic Properties of Iron as Determined By Moessbauer Spectroscopy.
Borcar, Shrihari Vassudeva
1980
Abstract
The techniques of Conversion Electron Moessbauer Spectroscopy (CEMS), and Transmission Moessbauer Spectroscopy (TMES), were used in heterogeneous catalysis research, to study the influence of metallic additives to Fe. Two separate studies were conducted with CEMS, one dealing with ('57)Fe/('56)Fe/Quartz film s and wiches, and the other dealing with Fe, Fe-Pd, Fe-Au thin films. The TMES study was used, to determine the extent of bimetallic cluster formation, in supported bimetallic systems. The reactions studied were NH(,3) decomposition, NO decomposition and NO reduction by H(,2). In the s and wich film study, a 12(DEGREES)A ('57)Fe film which was vacuum evaporated on top of a thick ('56)Fe film, showed a CEMS doublet of superparamagnetic Fe(,2)O(,3). On H(,2) reduction at 300(DEGREES)C, the ('57)Fe film was reduced to Fe('0) and coupled magnetically to the underlying ('56)Fe film. An iron nitride of composition (zeta)-Fe(,2)N could be identified on this film, after NH(,3) decomposition at 300(DEGREES)C. Nitrogen in solution, or chemisorbed nitrogen species, were not detected because of the weak nature of their interactions with Fe. With a 98(DEGREES)A ('57)Fe/('56)Fe film s and wich, several nitrides such as (gamma)'-Fe(,4)N, (epsilon)-Fe(,3.2)N and (zeta)-Fe(,2)N were identified by CEMS after NH(,3) decomposition at 500(DEGREES)C. In the Fe, Fe-Pd, Fe-Au film study, 55(DEGREES)A ('57)Fe films were used, and a 1:1 atomic ratio of Fe:additive was maintained. On heat treatment, alloys of composition (TURNEQ) FePd, (TURNEQ)FeAu(,3) were detected by CEMS. Heterogeneous decomposition rates of NO were established between 200-500(DEGREES)C, by the amount of iron oxides formed on these films. The amount of NO interacting with the pure Fe film increased steadily between 200-400(DEGREES)C, but decreased at 500(DEGREES)C. The amount of NO interaction was the highest with the FePd film and the lowest with the FeAu film. NO reduction studies over the Fe film, showed residual Fe oxides at treatment temperatures below 400(DEGREES)C. Similar studies over the FePd alloy, showed no such oxides even at temperatures as low as 200(DEGREES)C. With the Fe-Au film (with the FeAu(,3) alloy), residual Fe oxides were seen at temperatures below 300(DEGREES)C. Supported catalysts were prepared by the incipient wetness impregnation technique, with (eta)-alumina (245 m('2)/gm s. area) as the support. The loading of ('57)Fe was 1 wt%, and the additives were Pd, Au, Ru, Rh, Mn, K(,2)O (50 at% each), Pt (66 at%) and Mn (80 at%). Bimetallic clusters were observed with the Pt, Ru, Rh additives, after H(,2) reduction at 650(DEGREES)C for 1-1/2 - 4 hours (as determined by TMES). No such lig and effect interactions were observed with the other additives. Fe with the Mn (80%) additive could be reduced to Fe('0), but supportmetal interactions prevented this from happening with the other catalysts. K(,2)O reacted preferentially with the active sites on alumina. In this case, a large particle size was observed for Fe. The NO decomposition and reduction reactions conducted over Fe catalysts with Au, Pd, Pt additives showed that the additives had modified the redox characteristics of Fe.Types
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