Investigations of Fe-Ru bimetallic catalysts by in situ Mössbauer and EXAFS studies

In situ Mössbauer and EXAFS investigations have shown that the reduction of iron in the monometallic Fe/SiO₂ catalyst is only partial, the reduction being mostly to a ferrous silicate phase. In the bimetallic Fe-Ru/SiO₂ catalysts, the proportion of the FeRu alloy formed on reduction increases markedly with the increase in Ru content; clearly, Ru significantly enhances the reduction of iron on SiO₂. In the Ru-rich compositions (Ru/Fe 1.0), most of the iron is present in the alloy phase and there is no segregation of -Fe. A comparative study of the different supports has shown that -Al₂O₃ and SiO₂ interact with iron strongly at low reduction temperatures while the TiO₂ support interacts at higher temperatures. The presence of traces of Fe³⁺ often found in reduced Fe-Ru catalysts is shown to arise from the oxidation of fine segregated iron particles on the support.Contribution No. 831 from the Solid State and Structural Chemistry Unit.     

Activity and selectivity control in iron catalyzed Fischer-Tropsch synthesis

We present results of a catalyst structure-function study that supports a CO hydrogenation model with -olefins formed as the principal primary products and n-paraffins formed during secondary hydrogenation reactions. The interplay of catalyst composition and reaction environment controls the extent of secondary reactions. Catalysts that contain mainly oxidic phases or carbides with large concentrations of excess matrix carbon favor secondary reactions. The relative concentrations of oxide and carbide phases depends on the ease of reduction of the catalyst, which can be changed by cation substitutions. For example, cobalt substitution into Fe₃O₄ lowers the reduction temperature by 20 ° C. Excess matrix carbon has been intentionally introduced (by treatment in high temperature H₂/CO) into model iron carbide catalysts produced by laser synthesis. Increased paraffin selectivity as matrix carbon is introduced suggests the influence of the diffusion constraints on product selectivity. Alkali promotion will affect secondary hydrogenation pathways. We illustrate how catalysts with low levels of alkali become increasingly more selective to paraffins at high conversions (and high effective H₂/CO ratios).Reaction environment also controls catalyst composition and selectivity. Mossbauer spectroscopy on spent catalysts suggests that oxide/carbide phase formation in iron catalysts are sensitive to reactor configuration (extent of backmixing). In integral fixed bed reactors, catalysts partition into carbide phases in the front of the bed but show increasing amounts of oxide near the exit, whereas the catalysts in the stirred tank reactor remain all carbides. Product selectivities reflect the phase differences.Other examples illustrating secondary hydrogenation phenomena will be presented.     

Surface coordinate geometry of iron catalysts: hydrogenation of CO2 over Fe/ZrO2 prepared by a novel method

Zirconia-supported iron oxide catalyst has been designed following the approach to the Fe/TiO₂ catalyst. The catalyst prepared by reducing the precursor obtained from incipient wetness impregnation in H₂ at the proper temperature exhibits good activity (CO₂ conversion >20%) and selectivity (68%) in the selective synthesis of hydrocarbons (C₂-C₅) from CO₂ and H₂. The catalytic activity has been found to vary with iron weight loadings in a two maxima fashion and is also affected by reduction temperature. Mössbauer and EXAFS analyses suggest that the active phases are coordinatively unsaturated ferric cations and -Fe. No ferrous cations are observed. A good geometric arrangement for the two phases on the catalyst is thought to give the highest catalytic activity.     

CO2 reforming of CH4 over Ni/Mg–Al oxide catalysts prepared by solid phase crystallization method from Mg–Al hydrotalcite-like precursors

Ni supported catalysts were prepared by the solid phase crystallization (spc) method starting from hydrotalcite (HT) anionic clay based on [Mg₆Al₂(OH)₁₆CO₃ ^2–]H₂O as the precursor. The precursors were prepared by the co-precipitation method from nitrates of the metal components, and then thermally decomposed, in situ reduced to form Ni supported catalysts (spc-Ni/Mg–Al) and used for the CO₂ reforming of CH₄ to synthesis gas. Ni²⁺ can well replace the Mg²⁺ site in the hydrotalcite, resulting in the formation of highly dispersed Ni metal particles on spc-Ni/Mg–Al. The spc-catalyst thus prepared showed higher activity than those prepared by the conventional impregnation (imp) method such as Ni/-Al₂O₃ and Ni/MgO. When Ni was supported by impregnation of Mg–Al mixed oxide prepared from Mg–Al HT, the activity of imp-Ni/Mg–Al thus prepared was not so low as those of Ni/-Al₂O₃ and Ni/MgO but close to that of spc-Ni/Mg–Al. The relatively high activity of imp-Ni/Mg–Al may be due to the regeneration of the Mg–Al HT phase from the mixed oxide during the preparation, resulting in an occurring of the incorporation of Ni²⁺ in the Mg²⁺ site in the HT as seen in the spc-method. Such an effect may give rise to the formation of highly dispersed Ni metal species and afford high activity on the imp-Ni/Mg–Al.     

The importance of passivation in the study of iron Fischer-Tropsch catalysts

In the study of iron catalysts, careful passivation is necessary for study of microstructure by ex situ analytical techniques. The passivation procedure used in our study consists of heating the sample in He at the reaction temperature, cooling to room temperature and introducing small amounts of O₂ (<1%) in a flowing He stream. A properly passivated sample shows no more than a few nm of surface Fe₃O₄ on-Fe, when examined in a high resolution TEM. Proper passivation is also characterized by an exotherm of no more than 2–3 K. We show that a Fischer-Tropsch catalyst carbided in CO will show substantial amounts of magnetite, if exposed to air without proper passivation. Such surface oxidation may cause errors in determining the relative amounts of the magnetite and carbide phases in Fischer-Tropsch catalysts, which are important for proper identification of the catalytically active phase.     

Iron-ruthenium catalyst for the water-gas shift reaction

Iron-ruthenium catalysts prepared by impregnation of calcination products of -, , -and -iron oxide-hydroxides with either ruthenium chloride or ruthenium red were tested for the activity for the water-gas shift reaction. The effect of support, ruthenium containing impregnation agent and thermal treatment on catalyst performance was discussed.     

The use of alkali carbonates in carbon concentration cells

The feasibility of using alkali carbonates as electrolytes in carbon concentration cells has been investigated. The following cell was set up: The test electrode (LHS) was a carbon-permeable-iron membrane in contact with a gaseous or liquid metal environment whose carbon activity could be varied. Experiments involving argon and liquid sodium environments at 970 K showed that the potential of the-Fe test electrode was a function of its carbon activity.The potential of the electrode,-Fe, C ¦ CO ₃ ^2– , was also measured as a function of carbonate ion activity and current.It was concluded that the predominant electrode reaction at the iron electrode was reversible and involved carbon and carbonate species or species with which they were in equilibrium.     

Metal effects in the Fischer-Tropsch synthesis: Bond-order-conservation-morse-potential approach

We have applied the BOC-MP method to theoretically analyze the metal effects in the Fischer-Tropsch (FT) synthesis by calculating the energetics of conceivable elementary steps (the relevant heats of chemisorption and the reaction activation barriers) during CO hydrogenation over the periodic series Fe(110), Ni(111), Pt(111), Cu(111). The basic steps such as dissociation of CO, hydrogenation of carbidic carbon, C-C chain growth by insertion of CH₂ versus CO into the metal-alkyl bonds, and chain termination leading to hydrocarbons (alkanes versus -olefins) or oxygenates are discussed in detail. It is shown that the periodic trends in the ability of metal surfaces to dissociate chemical bonds and those to recombine the bonds are always opposite. In particular, we argue that metallic Fe is necessary to produce the abundance of carbidic carbon from CO but the synthesis of hydrocarbons and oxygenates can effectively proceed only on carbided Fe surfaces which resemble the less active metals such as Pt. More specifically, we project that the C-C chain growth should occur predominantly via CH₂ insertion into the metal-alkyl bond and the primary FT products should be -olefins. These and other model projections are in agreement with experiment.     

A Novel KCaNi/α-Al2O3 Catalyst for CH4 Reforming with CO2

A potassium and calcium co-promoted nickel catalyst (KCaNi/-Al₂O₃) prepared by a direct impregnation method possessed a high activity, high stability and excellent coke resistance properties in CH₄ reforming with CO₂. XRD, XPS and H₂-TPR characterizations indicated that (i) Ca and K strengthened the interaction between Ni and -Al₂O₃ and promoted the formation of a unique NiAl₂O₄ phase on the surface of the catalyst and (ii) Ca and K increased the dispersion of Ni and retarded its sintering. Coking reactions (CH₄ temperature-programmed decomposition and O₂-TPO) disclosed that K reduced carbon formation via CH₄ decomposition.     

Carbon oxides hydrogenation on RhNa-Y: Addition of 1-butene

At 4.0 MPa, CO₂ hydrogenation over a RhNA-Y catalyst yields only saturated and essentially linear hydrocarbons (up to C₇). In contrast, CO hydrogenation gives a more complex mixture including olefins ( and ), paraffins (linear and branched) and oxygenates. Addition of 1-butene provides a plausible interpretation of these differences.     

Molecular mechanisms in partial oxidation of methane on Ir/α-Al2O3: reactivity dependence on catalyst properties and transport phenomena limitations

An in situ DRIFT and mass spectrometric study of catalytic partial oxidation of methane with Ir/-Al₂O₃ has enlightened relationships between the formation of surface metal carbonyl clusters and residence time and temperature conditions. Some cluster species produced during catalytic partial oxidation were also originated during CO₂ hydrogenation and CO₂ reforming experiments described in previous literature. An EXAFS analysis of the catalyst precursor, prepared through a solid-liquid reaction between Ir₄(CO)₁₂ clusters and the reactive surface sites of-Al₂O₃, is also included to discuss clusters structure produced at the surfaces.     

Preparation and Characterization of Copper Catalysts Supported on Mesoporous Al2O3 Nanofibers for N2O Reduction to N2

The gas-phase hydrogenation of benzene to cyclohexane over Ce_{1 - x} Pt _x O_{2 -} (x = 0.01, 0.02) catalyst was investigated in the temperature range 80-200 °C. A 42% conversion of benzene to cyclohexane with 100% specificity was observed at 100 °C over Ce_0.98Pt_0.02O_{2 -} with a catalyst residence time of 1.22 × 10⁴ g s/mol of benzene. The activity of the catalyst was compared with those of Pt metal, combustion-synthesized Pt/-Al₂O₃ and Pt/-Al₂O₃. The turnover frequency value of Ce_0.98Pt_0.02O_{2 -} is 0.292, which is an order of magnitude higher than those of the other Pt catalysts investigated. The kinetics of reaction and the deactivation behavior of the catalyst were studied and a regeneration methodology was suggested. The deactivation kinetics and structural evidence from XRD, XPS, TGA and H₂ uptake studies suggest that the oxidized Pt in Ce_0.98Pt_0.02O_{2 -} is responsible for the high catalytic activity towards benzene hydrogenation.     

Mechanisms of Iron Activation on Fe-Containing Zeolites and the Charge of α-Oxygen

According to previous Mössbauer data [1] -sites formation at the activation of Fe-containing zeolites is accompanied by irreversible self-reduction of the iron, proceeding without participation of an external reducing agent. Reduced Fe²⁺ ions are inert to O₂ but are reversibly oxidized to Fe³⁺ by N₂O, generating the -oxygen species, O, which provide selective oxidation of hydrocarbons.In this work, the mechanism of -sites formation was studied via quantitative measurement of the dioxygen amount desorbed into the gas phase at the step of self-reduction. A prominent role of the zeolite matrix chemical composition has been revealed. For example, with zeolites of Al–Si composition (FeZSM-5 and Fe-), heating to 900 °C in a closed vacuum space leads to irreversible evolution of O₂, which is accompanied by the immediate formation of -sites. Similar heating of B–Si and Ti–Si zeolites also leads to dioxygen evolution; however, this evolution is reversible and is not accompanied by formation of -sites. Activation of these zeolites occurs only in the presence of water vapor. Stoichiometric measurements showed that in terms of charge one regular O^2- ion, removed at the activation, is equivalent to two -oxygen atoms. So, -oxygen is identified as an ion-radical species O ^-., whose unique oxidation properties still distinguish it from the generally observed O^-. radicals.The mechanism of -sites formation is proposed, in which the process of strong chemical stabilization of reduced Fe²⁺ atoms in the zeolite structure is a key step, making impossible the reoxidation of the iron with O₂.     

The importance of modifier pH in the generation of enantioselective nickel catalysts

Enantioselective Ni/SiO₂ catalysts have been prepared by modification with aqueous solutions of (R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of a prochiral -keto ester (methylacetoacetate) to a -hydroxy ester ((R)-(-)-methyl-3-hydroxybutyrate). The simultaneous adsorption of TA and corrosive leaching of surface nickel metal are graphically illustrated and the progress of TA buildup on the catalyst surface with the duration of modification is presented. Variations of modifier pH were found to strongly affect the modification process and influence the ultimate hydrogenation rate and enantioselectivity. The surface coverage by TA is correlated to the asymmetric activity and an optimum fractional coverage of 0.2 is identified; at higher coverages modification with basic TA solutions yielded superior enantioselectivities. While TA treatment in basic media was less corrosive, a proportion of the surface enantioselective nickel sites was leached into solution during the hydrogenation step. The difference in the response of nickel precursors, prepared by impregnation and homogeneous precipitation/deposition, to TA treatment is compared and discussed in terms of metal/support interaction.     

Methanol synthesis over Pd/SiO2 with narrow Pd size distribution prepared by using MCM-41 as a support precursor

All silicious MCM-41 was investigated as a support or a support precursor for Pd/SiO₂ and prepared catalysts were tested for methanol synthesis from CO and H₂. The methods of Pd loading on the MCM-41 were impregnation, seed impregnation and chemical vapor deposition (CVD). For both impregnations, most Pd existed outside of the pore as large particles, and only a small part of Pd was inserted into the pore of MCM-41 retaining the initial structure. On the contrary, in the catalyst prepared by CVD method, the MCM-41 structure was completely destroyed to become amorphous SiO₂. Yet the average Pd particle size in this catalyst was smaller and its distribution was narrower than those of the catalysts prepared by impregnation methods. In the methanol synthesis from CO hydrogenation the catalyst prepared by CVD showed higher methanol selectivity than other MCM-41-derived catalysts. This result was considered to be due to the more uniform distribution of the Pd particle size.     

Isotopic Studies on the Mechanism of Partial Oxidation of CH4 to Syngas over a Ni/Al2O3 Catalyst

Silica-supported PdZn catalysts have been studied in CO and CO₂ hydrogenation and in ethylene hydroformylation. The dilution of surface Pd by Zn lowers the hydrogenating capability of the catalysts and favours the production of higher hydrocarbons in CO hydrogenation. The catalyst with a molar ratio Pd:Zn = 3 showed an enhanced ability to insert CO into an M–alkyl bond; this catalyst produced higher oxygenates in the CO hydrogenation and was the most active in all reactions studied.     

Structures and catalytic properties of magnesium molybdate in the oxidative dehydrogenation of alkanes

Magnesium molybdates have been prepared from an aqueous solution of magnesium nitrate and ammonium paramolybdate under various pH conditions and used for the selective oxidation of propane and isobutane with gaseous oxygen under atmospheric pressure in the temperature range of 360–520°C. The structure analyses by XRD and FT-IR showed the formation of three phases, -MgMoO₄, -MgMoO₄ and Mg₂Mo₃O₁₁, in the catalysts calcined below 550°C. The catalyst prepared at pH=5.7 showed the highest activity for the oxidative dehydrogenation of the alkanes as well as the strongest acidity. By XPS measurements, an excess amount of Mo compared to Mg was observed over the active catalysts. It is likely that the excess molybdenum species is present as molybdate and creates acidic sites over the catalyst surface.     

Monte Carlo simulations combined with UHV-atmospheric pressure reaction studies on CO hydrogenation on cobalt

We show that useful information on catalytic reactions can be obtained using Monte Carlo simulations combined with experimental data from model catalysts. The experimental rate dependencies of CO hydrogenation on the partial pressures were used to guide the selection of different parameter values used in the simulations. The results give the following picture of the reaction conditions on the surface: hydrogen and carbon monoxide occupy different adsorption sites, the diffusion of hydrogen and the growth of hydrocarbon chains are fast processes, and the rate-limiting elementary reaction step is the termination of the hydrocarbon chains (-hydrogenation). The formation of longer chain hydrocarbons falls onto the line defined by the Anderson-Flory-Schulz distribution but the value of the chain growth parameter , obtained in the simulations, is higher than the experimental value.     

Effect of impregnation sequence on performance of SiO2 supported Cu-Fe catalysts for higher alcohols synthesis from syngas

The effects of different impregnation sequences of copper and iron on the performance of Cu-Fe/SiO₂ catalysts for higher alcohols synthesis from syngas were investigated by N₂ adsorption, XRD, H₂-TPR, CO-IR, XPS, and CO hydrogenation reaction. The results indicate that the catalyst prepared by impregnation of support first with Fe and then with Cu exhibits the highest selectivity (36.1%) and space time yield (153.3 g·kg_cat^− 1·h^− 1) of alcohols. The CO conversion and alcohol selectivity of the catalysts was closely related to the content of surface Cu, and the ratio of surface contents of Cu to Fe, respectively.     

Carbon monoxide hydrogenation on Fe2O3/ZrO2 catalysts

Fe₂O₃/ZrO₂ catalysts prepared by impregnation and coprecipitation methods were used for catalytic hydrogenation of CO. It was shown that the structure, reduction behavior of iron species, and catalytic properties of the catalysts were obviously affected by the preparation methods. For the Fe₂O₃/ZrO₂ catalyst prepared by the impregnation method, the Fischer-Tropsch catalytic activity and the selectivity to light olefins were much higher than those of the corresponding catalyst prepared by the coprecipitation method, the formation of methane was suppressed and the selectivity to light olefins was enhanced. Various intermediates formed during the successive steps of reduction of the catalysts were studied by using temperature-programmed reduction combined with in situ Mössbauer spectroscopy. The role of zirconia in the catalysts was discussed.