R. Fiedorow

The sintering of supported metal catalysts: II. Comparison of sintering rates of supported Pt, Ir, and Rh catalysts in hydrogen and oxygen

Abstract Changes in dispersion of alumina supported Pt, Ir, and Rh catalysts due to thermal treatment (250–800 °C) in oxygen and hydrogen atmospheres were measured. In oxygen atmospheres the sequence of thermal stability was found to be Rh > Pt > Ir, while in hydrogen atmospheres the sequence was Ir > Rh > Pt. Increases in dispersion due to treatment in oxygen were observed for Pt and Ir catalysts. The observed relative stabilities are compared to qualitative predictions based on sintering mechanisms.

The sintering of supported metal catalysts: I. Redispersion of supported platinum in oxygen

The effects of treatment in oxygen at elevated temperatures on the dispersion of five Pt/MAl2O3 catalysts (0.5 to 4.0 wt% Pt) were determined. Treatment temperatures of 300 to 700 °C and treatment periods of 1 to 128 hr were studied. Significant increases in dispersion (at times a two- to threefold increase) were observed at ≤600 °C. At 500 and 550 °C the increases in dispersion were relatively independent of the treatment time. At higher temperatures, rapid, time-dependent decreases in dispersion occurred. The results are in agreement with a molecular migration mechanism.

The nature and catalytic influence of coke formed on alumina: Oxidative dehydrogenation of ethylbenzene

Abstract Catalytic properties of alumina doped with 0.1 to 1.0 wt% NaOH, as well as that of pure alumina, were studied for the oxidative dehydrogenation of ethylbenzene and for some acid-catalyzed model reactions. The largest contribution to the formation of coke catalytically active for styrene formation was found to come from sites of moderate and weak acid strengths. Stronger acid centers produce cokes lower in hydrogen content, whereas weaker centers form cokes of a more saturated chemical nature. On very very weak centers, cokes with the ratio of C H are formed and such cokes show low catalytic activity for oxidative dehydrogenation. Based on ESR measurements, it has been suggested that paramagnetic centers are the active sites for styrene formation in the process of ethylbenzene oxidative dehydrogenation.

Surface properties of sulphate-containing aluminas

Abstract The chemical properties of the surface of an alumina containing an admixture of basic aluminium sulphates were compared to those of a comparable pure alumina. The sulphated alumina exhibited increased total acidity and oxidizing properties. Strong Bronsted-acid sites were formed, such as are absent on pure alumina. Conversely,the presence of sulphates decreased the basicity and reducing ability. The catalytic properties of this sulphated alumina are noteworthy.

A study of the kinetics and mechanism of COS hydrolysis over alumina

The role of surface sites on γ-alumina in the mechanism of hydrolysis of carbonyl sulfide was investigated using a combination infrared cell-recycle flow reactor. Zero-order and first-order behavior were observed for H2O and COS, respectively. The use of acidic (acetic acid) and basic (NaOH, NH3, and pyridine) site-blocking agents reveal basic sites to be essential for the hydrolysis surface reaction. Competitive adsorption of COS and H2O base centers was evident; however, an Eley-Rideal surface reaction mechanism involving adsorbed H2O seemed most compatible with the experimental observations. Catalytic deactivation with time and with exposure to oxygen were evaluated and explanations put forward.

On the role of surface hydroxyls during the Cr/SiO2-catalyzed polymerization of ethylene

Abstract The onset of polymerization of ethylene over a 0.5% chromium supported on silica catalyst was studied using a diffraction-type IR spectrophotometer. Detailed investigations emphasizing the hydroxyl region of the IR spectrum suggested the importance of silanol groups in the processes of reaction initiation, termination, and catalyst deactivation. The formation of chromium-bound hydroxyls and their role in catalyst deactivation were suggested.

Base properties of zeolite catalysts

Abstract Distributions of base strengths were determined using indicator techniques and i.r. spectroscopic studies of acetic acid adsorbed on samples of faujasite-type zeolites. The results show that base sites of variou zeolites differ significantly in both their strength and nature. Among sodium forms of the zeolites, NaX appeared to exhibit higher strength and concentrations of base sites than NaY. For hydrogen-sodium forms, the situation was reversed: HNaY prevailed over HNaX. The total basicities of the hydrogen—sodium zeolites were considerably higher than those of the sodium forms, due to contributions from both hydroxyl groups and oxygen ions of (AlO 4 ) − tetrahedra; whereas the latter exhibited Lewis basicity alone. Nonetheless, the higher basicities of the HNaY and HNaX zeolites do not correlate with their catalytic activity for decomposition of diacetone alcohol to acetone, a model base-catalysed reaction. The relatively poor conversions to acetone are examined within the context of base strengths and the high acidity of HNaX and HNaY since another reaction pathway, acid-catalysed formation of mesityl oxide, was also observed.

Effect of γ-alumina crystallinity on the state of rhodium during high-temperature treatments in oxygen and hydrogen

Abstract Rhodium catalysts, supported on six γ-Al2O3 supports with different crystallinities, were exposed to sequential treatments in hydrogen at 500°C, in oxygen at 760°C, in hydrogen at 500°C and at 760°C, respectively. Samples were characterized by X-ray diffraction and hydrogen chemisorption at various stages in the sequential treatment. Based on the characterization results, it is concluded that the formation of crystalline Rh2O3 is a function of γ-Al2O3 crystallinity; formation of crystalline Rh2O3 increased with increasing crystallinity of γ-Al2O3 during treatment in oxygen at 760°C. The crystalline Rh2O3 formed during treatment in oxygen at 760°C was reduced to Rh metal by hydrogen at 500°C, but most of the Rh did not adsorb hydrogen at room temperature. Subsequent treatment in hydrogen at 760°C increased the hydrogen adsorption capacity by as much as a factor of three. X-ray line broadening measurements showed that oxygen treatment of reduced Rh/γ-Al2O3 at 760°C followed by hydrogen reduction at 500°C resulted in significant increases in Rh crystal size; further treatment in hydrogen at 760°C resulted in additional sintering of Rh.

A Study of The Catalytic Oxydehydrogenation of Ethylbenzene Using Nitrobenzene

During the oxydehydrogenation of ethylbenzene with nitrobenzene on either alumina or magnesium sulphate surfaces, carbonaceous deposits which exhibit dehydroge-nating activity are formed. The activity of these coke catalysts decays with time because growth of this non-porous coke limits access of reactant molecules to the residual external surface of the parent (alumina) catalyst. The MgSO 4 -based cokes were also tested using model reactions. Paramagnetic centers in the coke (unpaired electrons localized on carbon atoms) and phenolic-type hydroxyl groups are the likely active sites.