A. Barański

FT-IR and FT-Raman study of hydrothermally degradated cellulose

Abstract Deterioration of books and archive materials is due to degradation of cellulose that is caused by many factors such as acidic hydrolysis, oxidative agents, light, air pollution or presence of microorganisms. Recently, FT-IR and FT-Raman spectroscopy have been used for the characterization of cellulose and its degradation products. In this work, we present vibrational spectra obtained with different sampling techniques of pure cellulose from softwood and cotton that has been hydrothermally treated for maximum 21 days in extreme humid atmosphere (100% humidity, 100°C). We show that the obtained results strictly depend upon spectroscopic techniques used in measurements. We also present and discuss changes in vibrational spectra pointing out a role that is played by water molecules in this accelerated aging process of paper.

Stability and excitation of potassium promoter in iron catalysts – the role of KFeO2 and KAlO2 phases

Well‐characterized catalyst model compounds of KAlO2 and KFeO2 are investigated by thermal desorption of potassium from the material. The desorbing fluxes of ions, atoms and highly excited states (field ionizable Rydberg states) were studied with surface and field ionization detectors in a vacuum apparatus. From the Arrhenius plots the activation energies for desorption of K and K+ were determined. The chemical state of potassium at the surfaces is concluded to be: ionic on KAlO2 (with the K desorption barrier of 1.76 eV) and covalent on KFeO2 (barrier of 2.73 eV). These results agree with the data obtained earlier for industrial catalysts for ammonia and styrene production. They are interpreted in terms of the Schottky cycle, which is completed for KAlO2 and fails for KFeO2. This failure indicates a non‐equilibrium desorption process. K Rydberg states are only found to desorb from KFeO2, in agreement with the suggestion that such states in some way are responsible for the catalytic activity.

Kinetics of reduction of iron catalysts for ammonia synthesis

Abstract The reduction of industrial iron synthetic ammonia catalysts by a stoichiometric mixture of hydrogen and nitrogen has been investigated. The reduction was studied in the temperature range 450–550 °C and for several grain sizes using a spring microbalance in a flow system. It was found that the results satisfy the Seth and Ross kinetic equation for a mixed-control mechanism in which neither the surface reaction nor the diffusion determines the reaction rate. The parameters of the Seth and Ross equation were calculated by a least squares method.

Structural and Compositional Characterization of Mixed CdS ‐ CdSe Films Grown by Cathodic Electrodeposition

Caracterisation par diffraction RX, microscopie electronique par balayage et spectrometrie par retrodiffusion de Rutherford. Etude de la reponse photoelectrochimique de CdS−CdSe. La composition et la bande interdite varient avec la composition de la solution de deposition. Les couches minces a haute concentration de CdSe sont amorphes et presentent une resistivite superieure a ceux ne contenant que CdS

Sulfur Poisoning of Iron Ammonia Catalyst Probed by Potassium Desorption

The surface of an unpoisoned and sulfur-poisoned industrial iron ammonia catalysts is investigated by K, K+ thermal desorption. The K+ desorption energy increases while the K energy decreases upon poisoning. Presence of sulfur also suppresses the potassium desorption in electronically excited states.

Temperature-programmed desorption studies of the hydrogen-zinc oxide system

Abstract Temperature-programmed desorption (TPD) investigations of ZnO (Kadox 25 and exoxalate) reveal the existence of six or seven types of surface complexes of hydrogen. They are specified by a temperature range of the maximum rate of desorption. The activation energy of desorption was determined for four types. Three types were assigned by a comparison of TPD results with the published ir and conductivity data. It was found that the so-called high temperature chemisorption occurs also at low temperatures, even at −35 °C.

Kinetics of activation of the industrial and model fused iron catalysts for ammonia synthesis

Abstract The paper summarizes our results published for many years and also adds new information. Kinetic data concerning the reduction of the oxidized forms of model as well as industrial catalysts used in ammonia synthesis were obtained in dry and in wet atmosphere containing 1% water vapour. The data for the industrial catalyst have been reassessed using three kinetic models. Modifications applied to the classical Seth-Ross model of the shrinking-core type resulted in the best fitting of this equation to the experimental data. The failure of the crackling core model to describe the kinetic data in a quantitative way is tentatively explained. The reduction of model catalysts containing enhanced amounts of wustite and/or potassium proceeds initially linearly with time. The effect of promoters, and especially of potassium, is discussed in more detail. The magnetite-alumina subsystem is responsible for the retardation effect of the water vapour on the reduction rate. A hypothesis is formulated that — in the presence of wustite or potassium — the inhibitive, Al-rich, hydrated surface layer is not effective in hindering the progress of the reduction process.

Characterization of unreduced fused iron catalyst for ammonia synthesis

Abstract A procedure for the characterization of iron catalysts is proposed. For this purpose, some model iron catalysts (obtained by fusion) for ammonia synthesis, in unreduced form, were studied. In order to characterize the catalysts, the following properties have to be established: (i) the content of major and minor cations, including Fe2+, and the total amount of iron; (ii) phase composition, insofar as the major phases are considered; (iii) homogeneity on the macro- and micro-scale, insofar, at least, as the major cations and major phases are considered; (iv) non-stoichiometry of the major iron oxides. In order to check the proposed procedure, five samples of the model catalysts (unpromoted, potassium oxide-promoted, alumina-promoted and potassium oxide/alumina-promoted) were prepared under industrial conditions and analysed according to the programme outlined above. It was found that the Fe2+ content varies significantly on either the macro- or micro-scale, this property being reflected in the wustite abundance. Wustite is present in two separate phases, one being rich in iron. This feature is due to slow cooling of the fused material. On the other hand, the magnetite phase is almost stoichiometric and well crystallized. The magnitude of the effective fields acting on iron nuclei at room temperature is a measure of the amount of alumina built into the magnetite phase. Mossbauer spectroscopy was found to be a very useful tool for the determination of the composition of the major phases and of the stoichiometry of iron-containing phases. The contents of impurities can be well approximated by the content of unreducible oxides at their highest degree of oxidation.

On the usefulness of Campbell's concept of the rate-determining step

Abstract Campbells recent (1994) definition of the degree of rate control enables one to quantify the relative importance of steps that a mixed-control kinetic model takes into account – as shown by three examples. The relation between overall reaction rate and the velocity of the rate-determining step has been clarified.

The simultaneous effect of promoters and water on the reduction of an iron catalyst for ammonia synthesis

Abstract The rate of reduction of magnetite by gaseous hydrogen is slightly affected by water vapour (1%). However, this effect of water vapour is significant in iron catalysts for ammonia synthesis of the KM I type. Promoted magnetite is the main component of the iron catalyst and it is concluded therefore that the influence of water is applicable only when promoters are present. The validity of the core-and-shell reduction model, assuming a Langmuir-Hinshelwood kinetic equation which describes the reaction at the oxide/iron interface, is discussed on the basis of the kinetic data for unpromoted and promoted iron catalysts. It is found that the model is generally valid, except for the case of advanced reduction of promoted catalyst in a moist atmosphere.