J.W. Geus

The reduction behavior of supported iron catalysts in hydrogen or carbon monoxide atmospheres

Abstract The reduction behavior of iron catalysts supported on magnesia or alumina was investigated using in situ high-field magnetization measurements, thermomagnetic analysis, temperature-programmed reduction, and X-ray diffraction. The complete disappearance of ferro- or ferrimagnetic behavior preceding the reduction to α-Fe during the reduction by hydrogen of iron/magnesia catalysts provided evidence for the presence of a well-stabilized FeO phase at temperatures where bulk FeO is metastable. The concept that a temporary stabilization of an FeO phase during reduction is indicative of a considerable metal(oxide)-support interaction appeared to be useful in the characterization of iron/alumina catalysts. To determine the presence of an iron oxide species which has no marked interaction with the support, thermomagnetic analysis was performed during temperature-programmed reduction by hydrogen or carbon monoxide. During reduction with carbon monoxide the ferri- or ferromagnetism displayed by the iron/alumina catalysts investigated disappeared completely, while the formation of iron carbides preceding the reduction beyond FeO was not observed. Unsupported α-FeOOH or α-Fe 2 O 3 , on the other hand, reacted to θ-Fe 3 C prior to the disappearance of the ferro- or ferrimagnetism.

Preparation of silica-supported copper catalysts by means of deposition-precipitation

Abstract Silica-supported copper catalysts have been prepared by means of deposition-precipitation under both atmospheric and hydrothermal conditions. Characterization of the catalyst precursor indicates the formation of a highly dispersed copper hydrosilicate with structural properties similar to the mineral chrysocolla. Increasing the metal loading from 5 to 40 wt.-% causes the specific surface area of the catalyst precursors to rise from 210 to 520 m2/g. The precipitation conditions affect both the reduction behavior and the texture of the catalyst precursors. Hydrothermal synthesis gives rise to formation to chrysocolla-like catalyst precursors, which exhibit a decreased ease of reduction and a higher pore volume, Due to the formation of a copper hydrosilicate of a high specific surface area, this preparation method facilitates the formation of highly dispersed copper-on-silica particles in the reduced catalyst, even at elevated metal loadings. After reduction, the catalysts show a mean metal particle size gradually increasing from 3 to 8 nm as the metal loading increases.

The reduction behavior of silica-supported and alumina-supported iron catalysts: A Mössbauer and infrared spectroscopic study

The reduction behavior of Fe/Al2O3 and Fe/SiO2 catalysts is investigated using in situ Mossbauer spectroscopy and infrared spectra of adsorbed CO. Controlled injection at constant pH (= 6) of an acidified iron(III)nitrate solution into a suspension of the support leads to the formation of highly dispersed supported α-FeOOH particles (2–8 nm). Reduction of the Fe/Al2O3 catalyst is found to proceed via a stabilized Fe1−xO phase according to the sequence α-FeOOH/Fe3O4/Fe1−xO/α-Fe. Reduction for 15 h at 873 K leads to iron crystallites 20–30 nm in diameter. It is shown that these surfaces still contain traces of oxygen. This is in accordance with single crystal investigations which indicate that the reduction of iron surfaces is a surface-sensitive process: full reduction of small particles (consisting of more open surfaces) is virtually impossible. Moreover, it is established that upon high-temperature evacuation following hydrogen reduction the iron surfaces are severely oxidized by water molecules formed by hydrocondensation of hydroxyl groups from the support. Reduction of the Fe/SiO2 catalyst proceeds via an iron(II)silicate phase, which at higher temperatures is partly reduced to α-Fe. Small iron particles (10 nm) densely and homogeneously distributed over the support are obtained by prolonged reduction (up to 150 h) at 723 K, whereas severe sintering occurs upon reduction for relatively short periods (16 h) at temperatures above 823 K. With the silica-supported iron catalyst the iron crystallites appear to be (partly) encapsulated by an iron(II)silicate layer. The encapsulation of the silica-supported iron particles, the incomplete reduction of the alumina-supported iron particles, and the reoxidation occurring upon high-temperature evacuation presumably account for the small extent of room temperature hydrogen chemisorption usually found with supported iron catalysts.

Characterization of silica-supported copper catalysts by means of temperature-programmed reduction

Abstract The effect of the preparation procedure on the reduction behavior of silica-supported copper catalysts was studied. The reduction process was monitored by measurement of the hydrogen consumption during temperature-programmed reduction (TPR). The catalysts were prepared by a variety of methods, viz. impregnation, ion-exchange, homogeneous deposition-precipitation, and co-precipitation. The results show that the reduction behavior of the catalysts strongly depends on the preparation method as well as on the thermal pretreatment. Comparison of the TPR profiles of the catalysts with results obtained on mineral copper hydrosilicates and bulk copper oxide facilitated identification of the copper precursor species present in the catalysts.

Interaction of nickel ions with a γ-Al2O3 support during deposition from aqueous solution

Abstract The interaction of nickel ions with a γ-Al 2 O 3 support was studied using potentiometric titration during deposition from aqueous solution and temperature-programmed reduction of calcined samples. It was found that in calcined samples of low nickel content, nickel is present exclusively as a “surface aluminate.” With higher metal loadings a separate NiO phase was detected. From potentiometric titration experiments it was concluded that a precursor of the surface spinel, viz., a mixed hydroxide surface compound, is already formed in aqueous solution. The initial interaction of nickel ions with the alumina support is discussed in terms of specific adsorption of divalent metal ions.

The reversible decomposition of methane on a NiSiO2 catalyst

Abstract The interaction of CH 4 with a silica-supported nickel catalyst is studied at temperatures from 30 to 350 °C, both in continuous-flow and pulse-flow experiments. Even at 30 °C chemisorption is observed; the apparent activation energy for the chemisorption is estimated at 6 kcal mole −1 . At temperatures above 175 °C the methane which is adsorbed on the Ni catalyst dissociates completely into adsorbed carbon atoms and hydrogen. The hydrogen released shifts the equilibrium CH 4 (g) → C(ads) + 4H(ads) to the left side. The reactivity of the carbonaceous deposit with hydrogen is also investigated. At all temperatures in the range 30 to 450 °C the only product of the exothermic reaction is methane. The reactivity passes through a maximum at 200 °C and strongly decreases at temperatures above 300 °C.

Thermal stability of basic aluminum sulfate

The tridecameric aluminum polymer [AlO4Al12(OH)24(H2O)12]7+ is prepared by forced hydrolysis of an Al(NO3)3 solution by NaOH up to an OH:Al mol ratio of 2.2. Upon addition of sulfate the tridecamer crystallizes into macroscopic crystallites of the basic aluminum sulfate Na0.1[Al13O4(OH)24(H2O)12](SO4)3.55, which is characterized structurally by means of X-ray diffraction, 27Al solid-state magic angle spinning NMR, IR and chemically by inductively coupled plasma atomic emission Spectroscopy. The basic aluminum sulfate has a monoclinic unit cell with a = 20.188±0.045 A, b = 11.489±0.026A, c = 24.980±0.056 A, and β= 102.957±0.022°. With TG analysis, DTA and heating stage X-ray diffraction the thermal decomposition is studied. The tridecamer persists as a stable unit in the sulfate structure to temperatures of 80°C. Approximately 9 mol H2O are adsorbed in excess per one mol basic aluminum sulfate; these are easily lost by heating to 80°C. From 80 to 360°C the tridecamer unit will gradually decompose losing its 12 water and 24 hydroxyl groups, to finally become X-ray amorphous. From 360 to 950°C, with a maximum between 880 and 950°C, SO3 is removed, leaving behind primary aluminum oxide.

Strong metal-support interaction in Ni/TiO2 catalysts: the origin of TiOx moieties on the surface of nickel particles

Abstract The interaction of nickel ions with a TiO2 support during catalyst preparation has been monitored using potentiometric titration. The adsorption of nickel ions in aqueous solution was found to be limited to 2.5 wt%. Using temperature-programmed reduction of calcined samples, it was established that NiO and TiO2 inter-diffuse rapidly at temperatures higher than 573 K. The interdiffusion process ultimately led to the formation of NiTiO3. Chemisorption of CO, H2 and N2 and IR spectroscopy of adsorbed CO showed that reduction of NiTiO3 (or a precursor thereof) resulted in a nickel surface displaying all the characteristics of the SMSI-effect. We conclude that titanium ions can be transported to the metal surface area via the intermediate formation of a titanate (precursor) followed by the segregatin of TiOx upon reduction.

Chemisorption of methane on silica-supported nickel catalysts: a magnetic and infrared study

The adsorption of methane on silica-supported nickel catalysts was studied at various constant temperatures (30 °C < T < 100 °C) and at increasing temperatures (30 °C < T < 300 °C) using a low-field magnetic method and infrared spectroscopy. In the entire temperature range the chemisorption of CH4 was found to be dissociative according to the reaction CH4 + 7Ni → Ni3C (“surface nickel-carbide”) + 4Ni-H. It was observed that, per unit surface area, small nickel crystallites were more reactive toward methane than were large crystallites.

Chemisorption of methane on NiSiO2 catalysts and reactivity of the chemisorption products toward hydrogen

The Chemisorption of hydrogen both on bare and carburized NiSiO2 catalysts was studied using a low-field magnetic method, infrared spectroscopy, and mass spectrometry. With a freshly reduced and evacuated sample of one of the catalysts, H2 chemisorption was investigated as a function of the temperature (30 < T < 100 °C). It was found that the slope of the magnetization-volume isotherm decreased with increasing temperature, which does not agree with the theory of superparamagnetism. The smaller slope at more elevated temperatures was ascribed to a more extensive coverage of the smaller nickel particles after admission of the initial H2 doses. Carburization of the catalysts was established by the decomposition of CH4 at temperatures from 30 to 300 °C. At low surface coverages the carbon was deposited as Ni3C strongly affecting the magnetization. At higher surface coverages CHx-complexes without any effect on the magnetization were chemisorbed. After the decomposition of CH4 the catalysts were evacuated at 250 °C, which was found to result in the conversion of a part of the carbonaceous deposit into methane. Also with the subsequent chemisorption of hydrogen on the carburized catalysts (T = 30 °C) the reaction between chemisorbed H-atoms and deposited carbon was apparent from the production of CH4. From a comparison of the magnetization-volume isotherms for H2 Chemisorption before and after the deposition of small amounts of carbon it was derived that the decomposition of methane preferentially proceeds on small nickel crystallites. Finally it was found that hydrogen was adsorbed not only on bare nickel (with magnetic effect) but also on nickel carbide (without magnetic effect).