Diffuse reflectance infrared Fourier transform spectroscopic investigation of the decomposition of carbon-supported iron carbonyl clusters

Abstract

The thermal decomposition of Fe3(CO)12 has been studied for the first time by dispersing this cluster on an oxygen-free carbon surface and monitoring its behavior by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The Fe3(CO)12 clusters on carbon decomposed to Fe(CO)5 in either He or H2; consequently, the decarbonylation of both clusters could be followed. First-order rate constants of decomposition in He or H2 were determined for each cluster and compared to literature kx values for nucleophilic substitution reactions in solution. The kx values for each reaction were very similar for Fe3(CO)12, but the kx value for decomposition was much higher for Fe(CO)s. The rate-determining step in either the substitution or the decomposition reaction appears to be the removal of the first CO ligand. No stable hydrido-iron clusters were formed. The activation energy of decomposition of Fe3(CO)12 was near 18 kcal/mol in He and 21 kcal/mol in H2 while that for Fe(CO)5 was near 15.5 kcal/mol in either gas. These activation energies are lower than those observed for substitution reactions in solution, but this can be explained by Fe-Fe bond formation during the decomposition process to create very small Fe crystallites. Large amounts of chemisorbed CO confirmed the formation of well-dispersed Fe on carbon following decomposition at 673 K under either He or H2; however, DRIFTS spectra of CO at 195 K detected no IR-active CO species and at 300 K only Fe(CO)s was present. The heat of adsorption of CO on these small Fe particles was measured calorimetrically at 300 K and found to be 15.0 ± 1.6 kcal/mol. This study represents a portion of the first successful application of an IR spectroscopic technique to characterize carbon-supported metal catalysts.

Carbon-supported iron catalysts: influence of support porosity and preparation techniques on crystallite size and catalytic behavior

“…Although there is some uncertainty regarding the adsorption stoichiometry of c o , a ratio of CO,d/Fe, = 1/2 at 195 K has been found to give crystallite sizes in good agreement with other techniques (Jung et al, 1982;Boudart et al, 1975). At 300 K CO adsorption is complicated by the possibility of carbonyl formation on very small particles Chen et al, 1989) and dissociation on rougher Fe surfaces Cameron and Dwyer, 1988;Moon et al, 1987); regardless, an assumed stoichiometry of CO,d/Fe, = 1 seems to provide reasonable estimates of particle size (Jung et al, 1982;Venter et al, 1989). Hydrogen chemisorption is activated on iron, particularly on very small crystallites (Topsoe et al, 1981); thus the technique employed by Amelse et al (1981) was used to minimize this difficulty.…”

“…Although a number of Fe/C catalyst systems have been studied (Jung et al, 1982a,b; Kaminsky et al, 1985;Chen et al, 1986;Venter et al, 1987Venter et al, , 1989Jones et al, 1986; * Author to whom correspondence should-be addressed. + Current address: Departamento de Quimica Inorginica e Ingenieria Quimica, Universidad de Alicante, Alicante, Spain.…”

Stabilization of iron by manganese promoters in uniform bimetallic FeMn Fischer–Tropsch model catalysts prepared from colloidal nanoparticles

“…[10][11][12] It has also been reported that manganese acts not only as a chemical promoter to alter the chemisorption of the reactants on the catalyst but also as a structural promoter to enhance dispersion of active iron and to stabilise the catalyst during the FTS process. [13][14][15] Stabilising the catalysts in their active state for a longer time is often an important challenge and understanding how structural promoters enhance catalyst stability is of great importance. Detailed characterisation of conventional porous heterogeneous catalysts is usually difficult, because the relevant structural, morphological and chemical changes take place on the active metal nanoparticles, situated inside the pores of the support.…”

Mechanistic Investigation of Heterogeneous Catalysis by Transient Infrared Methods

“…Infrared (IR) spectroscopy has been one of the most important means of studying adsorbates on supported catalysts [2,[24][25][26][27]. The development of in situ IR cells has permitted investigation of IR-observable adsorbates during reaction [28][29][30][31][32][33][34][35][36][37]. Transient in situ IR experiments have been employed to study the dynamic behavior of adsorbates [14,29,30,38,39].…”