Jeremy J. Venter

Carbon-supported FeMn and KFeMn clusters for the synthesis of C2C4 olefins from CO and H2: I. Chemisorption and catalytic behavior

Abstract Highly dispersed FeMn bimetallic particles were obtained on a high-surface-area amorphous carbon black support using FeMn and KFeMn carbonyl clusters. These catalysts were characterized by hydrogen adsorption and by CO chemisorption at 195 and 300 K, and their kinetic behavior for CO hydrogenation was studied at 1 atm in a differential, plug-flow microreactor. The K-promoted clusters with stoichiometries of KMnFe and KMnFe 2 gave 85–90 wt% ethylene, propylene, and butene with methane as the only other detectable hydrocarbon product. The KFe 3 cluster also had a high selectivity to olefins but showed somewhat more chain growth. The nonpromoted MnFe catalysts with Fe Mn = 2 , prepared from either stoichiometric mixed-metal carbonyl clusters or coimpregnation of the separate Fe and Mn carbonyl clusters, also had a high selectivity to light olefins; however, this selectivity was strongly dependent upon the pretreatment. The properties of the FeMn clusters without K is consistent with a proposed model in which a surface spinel, (Fe l - y Mn y ) 3 O 4 , plays a principal role in providing high selectivity to light olefins.

Carbon-supported FeMn and KFeMn clusters for the synthesis of C2C4 olefins from CO and H2: II. Activity and selectivity maintenance and regenerability

Abstract Highly dispersed carbon-supported MnFe and KMnFe catalysts were prepared which showed activity stabilization after a loss of approximately 50% of the initial activity during 24–110 h on-stream. The deactivation was attributed to carbon deposition, rather than sintering, and could be reversed by a treatment in hydrogen at reaction temperatures. Precursors with Fe Mn = 2 optimized the olefin selectivity, and mixed-metal clusters of this type gave higher selectivities than their coimpregnated counterparts. A low reduction temperature (473 K) for the unpromoted NEt 4 [Fe 2 Mn(CO) 13 ]catalyst gave a high selectivity to olefins which remained stable during the 24-h period the catalyst was maintained under reaction conditions. The particularly high C 2 C 4 olefin yields obtained with the K[Fe 2 Mn(CO) 13 ] catalyst were sustained throughout the 26-h activity maintenance run.

The genesis of carbon-supported Fe-Mn and K-Fe-Mn catalysts from stoichiometric metal carbonyl clusters. III. Characterization by chemisorption, calorimetry, and kinetic analysis

Abstract Carbon-supported Fe, FeMn, and KFeMn catalysts derived from stoichiometric mixedmetal carbonyl clusters were pretreated at either 473 or 673 K in H 2 after which their chemisorption behavior and catalytic properties for CO hydrogenation were determined. The iron remained welldispersed at all times except after high temperature reduction when potassium was present. The single promotion by either Mn or K increased the olefin/paraffin ratio, and the doubly promoted catalyst gave very high selectivity to light olefins. Integral CO heats of adsorption at 300 K were measured, and they increased from 15 kcal/mole on the Fe C catalysts to nearly 17 kcal/mole on each singly promoted sample to 21 kcal/mole on the doubly promoted catalyst. A model for the decomposition of these carbonyl clusters is proposed based on these results combined with previous studies utilizing Mossbauer effect spectroscopy, transmission election microscopy/energy dispersive spectroscopy, and diffuse reflectance Fourier transform infrared spectroscopy. The state of the MnO x , and K phases on the iron surface, as well as Fe crystallite size, appears to play a dominant role in determining catalytic behavior.

Carbon-supported Fe3(CO)12, Ru3(CO)12 and Os3(CO)12: Decomposition rates, CO adsorption properties and CO hydrogenation behavior

Abstract The thermal decomposition of the three dodecacarbonyl clusters, Fe 3 (CO) 12 , Ru 3 (CO) 12 and Os 3 (CO) 12 , on a clean carbon surface has been monitored by DRIFTS (Diffuse Reflectance Fourier Transform Infrared Spectroscopy) at different temperatures under a He or H 2 atmosphere. First-order rate constants and activation energies were determined. Different intermediate species were observed, and the absence of oxygen species on the surface allowed the formation of zero-valent metal particles during decomposition. The subsequent highly dispersed, carbon-supported metal catalysts have been characterized by the calorimetric measurement of the heats of adsorption of CO and by their kinetic behavior in the CO hydrogenation reaction. The CO heats of adsorption and the activation energies for both decarbonylation and methanation showed the same trend among the clusters as the initial metal-CO bond strengths.

The genesis of carbon-supported FeMn and KFeMn catalysts from stoichiometric mixed-metal carbonyl clusters: II. Characterization by Mössbauer spectroscopy and TEMEDS

Abstract Mossbauer effect spectroscopy (MES) and transmission electron microscopy/energy-dispersive X-ray spectroscopy ( TEM EDS ) were used to study the chemistry of the iron particles produced by the thermal decomposition of Stoichiometric KFe, FeMn, and KFeMn mixed-metal carbonyl clusters on a high surface area carbon. Intermediate chemical states during the cluster decomposition process below 473 K were identified using MES, and they indicated that decarbonylation occurred via the formation of Fe(CO) 5 and [Fe 4 (CO) 13 ] − during heating in H 2 . Following decomposition at 473 K, the principal final phase was the D-structure, which has been associated with superparamagnetic Fe combined with an Fe 2+ state. Additionally, a doublet characteristic of Fe 3+ oxide and/or superparamagnetic carbide appeared in the spectrum. No evidence for a mixed spinel such as Fe 2 MnO 4 was obtained, but the Fe and Mn appeared to remain in contact, presumably as MnO x on top of small Fe crystallites. A subsequent treatment in H 2 at 673 K caused the K-promoted catalysts to sinter and form separate phases of Mn oxide and large particles of α-Fe, as detected by MES and TEM EDS . For the unpromoted FeMn sample, little sintering occurred under H 2 at 673 K, and the particles existed in a phase which has been previously found in Fe-only Carbon-supported catalysts.

Preparation of Carbon-Supported K-Fe-Mn and Fe-Mn Catalysts Using Carbonyl Clusters

Highly dispersed Fe-Mn bimetallic particles have been formed on a high surface area amorphous carbon black support using Fe-Mn and K-Fe-Mn carbonyl clusters containing different Fe/Mn ratios. These catalysts have been characterized by hydrogen adsorption and CO chemisorption at 195K and 300K, and by their kinetic behavior for CO hydrogenation at 1 atm. The K-promoted KMnFe and KMnFe2 catalysts gave 85-90 wt? ethylene, propylene, and butene with CHH as the only other detectable hydrocarbon product. Comparison with other Mn-Fe catalysts reported in the literature shows that these catalysts not only have a much higher selectivity to light olefins but also have higher activity per gram iron.