J. W. Niemantsverdriet

On the time-dependent behavior of iron catalysts in Fischer-Tropsch synthesis

Iron, cobalt, and nickel behave differently during the Fischer-Tropsch synthesis. Iron catalytic activity is initially low and increases slowly to a maximum while the catalytic activity of Co and Ni is essentially constant throughout the process. Three explanations denoted as the carbide model, competition model, and the slow activation model, have been previously advanced to explain this behavior. A brief survey of these explanations is included herein, and experiments and described whose results should define the catalytic behavior. Failure of the slow activation model and the carbide model in explaining two of the four experiments is taken as sufficient reason to reject these two models. The different rates of C diffusion into the metals will satisfactorily explain the difference between Fe and the other Fischer-Tropsch catalysts. Results of competition between the carbidation rate and the Fischer-Tropsch rate for the different catalysts leads to the assumption that the competition model most closely describes catalytic behavior. (BLM)

Characterization of surface phases in bimetallic FeRhSiO2 catalysts by in situ Mössbauer spectroscopy at cryogenic temperatures

Abstract Mossbauer spectra of FeRh SiO 2 catalysts (5 wt% metal, sol Fe Rh atomic ratio = 1) have been recorded in situ at liquid-helium, liquid-nitrogen, and room temperatures. After reduction in H 2 at 725 K, 80% of the iron in the catalyst is still in an Fe 3+ surface phase, while 20% of the iron is Fe 0 alloyed with at least 1.5 times as much Rh. Fischer-Tropsch synthesis in CO + 3.3 H 2 at 525 K converts a part of the surface Fe 3+ into an Fe 2+ compound. Exposure of this catalyst to air at room temperature results in oxidation of all Fe 2+ and nearly all of the Fe 0 to an Fe 3+ state. The results illustrate the information contained in the temperature dependence of the resonant absorption areas of the Mossbauer spectra and the need for low-temperature spectra in quantitative analysis.

MOSSBAUER AND X-RAY PHOTOELECTRON SPECTROSCOPIC EVIDENCE FOR THE STRUCTURE OF SUPPORTED BIMETALLIC CATALYSTS - FERU, FERH, FEPD, FELR, AND FEPT ON SIO2

Abstract Silica-supported bimetallic catalysts, consisting of iron and a more noble Group VIII metal M (Ru, Rh, Pd, Ir, Pt) with metal loading 5 wt% and molar ratio Fe:M = 1:1, have been investigated with in situ Mossbauer spectroscopy and X-ray photoelectron spectroscopy. Reduced FeRu, FeRh, FeIr, and FePt on SiO2 contain the noble metal M in the zero-valent state, whereas iron is only partially reduced to Fe0, the latter being present in an FeM alloy. Between 50 and 80% of the iron is present as Fe3+ in iron(III) oxide, which is resistant to reduction by H2 up to at least 875 K. The Mossbauer parameters of the ferric iron change upon chemisorption of CO at 295 K, indicating that the iron(III) oxide is highly dispersed. In contrast to the other Fe M SiO 2 catalysts, reduced FePd SiO 2 contains all Pd and almost all Fe in the zero-valent state. The presence of both bcc FePd alloy and α-Fe metal indicates that phase segregation has occurred. Passivation of the Fe M SiO 2 catalysts in air at 295 K results in oxidation of Fe0 to Fe3+, while the metal M remains reduced. An exception is FeRu SiO 2 , in which about half of the Ru is oxidized by air at 295 K. All passivated Fe M SiO 2 catalysts show reduction of Fe3+ to Fe2+ or Fe0 by H2 and by CO at 295 K, which is promoted by the noble metal. Implications of the results on models for the structure of a supported bimetallic catalyst are discussed.

Preparation of zirconium oxide on silica and characterization by X-ray photoelectron spectroscopy, secondary ion mass spectrometry, temperature programmed oxidation and infra-red spectroscopy

Well dispersed ZrO2/SiO2 catalysts with a satisfactory thermal stability have been prepared by reaction between zirconium ethoxide dissolved in ethanol and hydroxyl groups of the silica support, followed by calcination in air at temperatures up to 700°C. Characterization of the catalysts in intermediate stages of the preparation by secondary-ion mass spectrometry (SIMS), infra-red spectroscopy (IR) and temperature-programmed oxidation (TPO) gives a detailed picture of the formation of the ZrO2 from the ethoxide precursor. Intensity ratios of the zirconium and silicon X-ray photoelectron spectroscopy (XPS) signals have been used to estimate the dispersion of the catalysts and to investigate their thermal stability. The results obtained on the ethoxide-derived catalysts are compared with those on ZrO2/SiO2 catalysts prepared by incipient wetness impregnation from an aqueous solution of zirconium nitrate. The work illustrates how useful the combination of XPS, SIMS, IR and TPO is for investigating the genesis of catalysts.

A Mössbauer study of surface effects on iron Fischer-Tropsch catalysts

Abstract Unsupported iron catalysts show contributions of iron oxide in their Mossbauer spectra after Fischer-Tropsch synthesis when the spectra are recorded at 4.2 K. This iro n oxide is characterized by a low recoilles fraction, corresponding to a Debye temperature of about 50 K and by a broad distribution in magnetic hyperfine fields, ranging from 370 to 544 kOe, the magnetic field of bulk α-Fe 2 O 3 . It is argued that this oxide is located at the surface of the catalysts. In situ Mossbauer experiments confirm that iron surface oxide can indeed by formed during Fischer-Tropsch synthesis. Implications for the carburization of iron are discussed.

Mössbauer Spectroscopy of Iron and Iron Alloy Fischer-Tropsch Catalysts

Mossbauer spectroscopy is an excellent technique for in situ investigations of iron-containing catalysts, because of the high penetrating power of the y-radiation and the high sensitivity of the spectral parameters to the chemical State and the local environment of the Mossbauer atom. Applications of Mossbauer spectroscopy in the field of catalysis up to 1980 have extensively been reviewed by Dumesic and Topsoe [1], and by Topsoe, Dumesic, and Morfrup [2]. Here we will review Mossbauer investigations of iron and iron alloy Fischer-Tropsch catalysts, carried out at the Interuniversitair Reactor Instituut at Delft.

Influence of particle motion on the Mössbauer effect in microcrystals α-FeOOH and α-Fe2O3

Abstract Particle motion due to thermal agitation causes the recoilless fraction, ƒ, of microcrystals α-FeOOH to decrease more strongly with temperature than expected on the base of lattice vibrations only. Hence, Debye temperatures, θ D , derived from ƒ(T) data are too low. Analysis of the second-order Doppler shift yields the correct values of θ D . For bulk α-FeOOH both methods yield identical values of θ D .

Characterization of FeRu/TiO2 and Fe/TiO2 catalysts after reduction and Fischer-Tropsch synthesis by Mossbauer spectroscopy

Abstract TiO2-supported bimetallic catalysts, consisting of iron and ruthenium with metal loading 5 wt% and molar ratios Fe:Ru = ∞, 10:1, 3:1, 1:1 and 1:3 have been investigated with in situ Mo˝ssbauer spectroscopy. Ruthenium has a strong influence on the behaviour of the iron in the catalysts. Partial reduction of Fe3+ to Fe2+ takes place at a lower temperature for FeRu/TiO2 than for Fe/TiO2. For reduced Fe/TiO2 and 10:1 FeRu/TiO2 catalysts a magnetic sextuplet of α-Fe and bcc-FeRu alloy has been observed and carburization of the iron in these catalysts to χFe5C2 during Fischer-Tropsch synthesis. The 3:1, 1:1 and 1:3 FeRu/TiO2 catalysts are only partially reduced to FeO, the latter being present in a hcp-FeRu alloy. With increasing ruthenium concentration and increasing reduction temperature, the initially formed Fe2+-phase is transformed back to an Fe3+ phase. This Fe3+-phase changes partially to an Fe2+-phase upon chemisoption of CO at 295 K, indicating that the Fe3+-phase is highly dispersed. Passivation of the catalysts in air at 295 K results in oxidation of Fe0 and Fe2+ to Fe3+. All passivated FeRu/TiO2 catalysts show reduction of Fe3+ to Fe2+ by H2 and by CO at 295 K, which is promoted by the noble metal ruthenium.

Moessbauer spectroscopy of supported bimetallic catalysts: 1:5 FeM/SiO2 (M = Ru, Rh, Pd, Ir, Pt)

Mössbauer spectra of SiO2-supported bimetallic FeM (M=Ru, Rh, Pd, Ir, and Pt) with FeM=1∶5 arter treatments such as reduction, exposure to CO and passivation in air are described and compared with previous results obtained on 1∶1 FeM/SiO2 catalysts.

Methanol from synthesis gas over bimetallic FePd catalysts

Bimetallic FePd/SiO2 catalysts exhibit higher activities in the formation of methanol from synthesis gas than Pd/SiO2. The catalysts are a complex mixture of bcc and fcc FePd alloy, α-Fe and some unreduced iron.