Effect Of Catalyst Preparation Research Articles

Effect of catalyst preparation on NiAl 2O 3 poisoning by thiophene

The influence of experimental conditions such as the temperature of calcination and pelleting pressure on NiAl 2O 3 deactivation by thiophene in benzene hydrogenation is outlined and discussed. It was found that benzene hydrogenation over unpoisoned nickel catalyst is a structure-insensitive reaction. However, its inhibition by thiophene produced structure sensitivity: the reaction rate was less sensitive to thiophene for large nickel crystallites (formed during catalyst calcination at high temperatures followed by reduction), than for small crystallites. Thus, the structure sensitivity of poison adsorption imparted an apparent structure sensitivity to the main reaction in the presence of a poison in the feedstream (secondary structure sensitivity). The pore structure of the catalyst pellet directly influences the degree of diffusion resistance of both the main and the poisoning reactions. It was found that higher mass transfer resistances increase the life-time of the pellets while a lower mass transfer resistance increases the overall effectiveness factor.

INFRARED INVESTIGATION OF NiO-Al2O3 AEROGEL CATALYSTS FOR THE NITROXIDATION OF PROPENE TO NITRILES

Infrared and kinetic studies were conducted for NiO-Al2O3 aerogel catalysts to determine the types and relative numbers of Ni sites responsible for catalytic activity and selectivity in the nitroxidation of propene to 2-propenenitrile. The evaluated catalysts had Ni to Al ratios of 0.04/1.0, 0.4/1.0 and 1.0/1.0. The effect of catalyst preparation on active Ni sites was determined by studying pairs of catalysts with the same Ni/Al ratio. The infrared studies were carried out using CO and NO as molecular probes, and by varying the temperature of pretreatment ( calcination and reduction). Bands for adsorbed CO were seen for all catalysts corresponding to weakly held CO on Ni2+ and Ni1+ sites. Both linear and bridged adsorption of CO on reduced NiO sites were also evident on catalysts reduced in H2 at 550°C. All catalysts showed bands arising from NO strongly held on Ni sites. The CO and NO bands varied in intensity depending on the temperature of pretreatment. Catalytic activity depends on both the number of...

The effect of catalyst preparation on the performance of alumina-supported ruthenium catalysts: I. The impact of catalytic precursor on particle size and catalytic activity

The effect of preparation method and the choice of metallic precursor on the performance of a series of Ru Al 2O 3 catalysts were studied. Wet impregnation and incipient wetness were the methods employed; ruthenium nitrosylnitrate and ruthenium trichloride were the reagents. In the latter case, either Ru(III) Ru(IV) chlorospecies or mixtures of Ru(II) hydrazine complexes were the catalytic precursors. The series of Ru Al 2O 3 catalysts, with metal loadings from 0.7–5% by weight, were subjected to a battery of performance tests: CO temperature-programmed reaction, steadystate CO hydrogenation, and temperature-programmed surface reaction. The methanation activity and carbon deposited during steady-state reaction varied systematically with the dispersion of Ru on the alumina. High rates of methane production were found on catalysts containing a large reservoir of carbon-containing reaction intermediates. The performance of these catalysts depended on the precursor used in their preparation. The effects of weight loading, method of preparation, and variations in the impregnant pH were small within a group prepared from a Common precursor.

The effect of catalyst preparation on the performance of alumina-supported ruthenium catalysts: II. The impact of residual chloride

The impact of residual chloride on the properties and performance of a series of Ru Al 2O 3 catalysts has been studied. This work departs from previous studies in that it examines the poisoning and promoting effects of this electronegative element in CO hydrogenation as a function of the dispersion of the catalyst. The dispersion and structure of the particles are regulated by the choice of the precursor used during preparation and by the impregnation conditions. We find that, in general, chloride residues poison catalysts with a high dispersion but provide a promotional effect for catalysts with a poor dispersion.

Oxidative coupling of methane: I. Alkaline earth compound catalysts

The catalytic performance of oxides, sulfates, phosphates, carbonates, fluorides, and aluminates of Be, Mg, Ca, Sr, and Ba was studied for the oxidative coupling of methane to C x hydrocarbons ( n ≥ 2). To compare the various catalytic compounds, normalized reaction conditions were applied: T = 740 ° C, P o CH4 = 660 mbar, P o O2 = 80 mbar, P o N2 = 260 mbar at a total pressure of 1 bar. C 2+ selectivity increased from BeO (22%) via MgO (22 to 53%) and CaO (47 to 55%) to SrO and BaO (70 to 72%). Among different calcium compounds C 2+ selectivity varied in the order CaO (47 to 55%), CaSO 4 (53%), Ca 2SiO 4 (43%), CaAl 2O 3 (18%), CaF 2 (14%), and Ca 3(PO 4) 2 (11%). Furthermore, the effect of catalyst preparation and operating conditions on coupling selectivity was investigated. The results are discussed on the basis of surface basicity of the catalysts.

Effect of Catalyst Preparation on Catalytic Activity: V. Chemical Structures on Nickel/Alumina Catalysts

In situ X-ray photoelectron spectroscopy (XPS or ESCA) was used to identify the nickel species on Ni/Al 2O 3 catalyst surfaces as a function of nickel weight loading. The ESCA results show that the only nickel species on the reduced catalyst surface is NiAl 2O 4 for low weight loading catalysts (< 1 wt.%) whereas both nickel and NiAl 2O 4 are present on high weight loading catalysts. The carbon monoxide temperature-programmed reduction (CO-TPR) results, after reduction of the catalysts in hydrogen, confirm that each type of nickel species acts as a different reaction center for methane production. The high-temperature methane peak observed only from CO-TPR spectra of low weight loading catalysts is due to NiAl 2O 4; a low-temperature peak, which appears as the weight loading is increased, is due to nickel. Steady-state reaction kinetics for methane production yields activation energies that increase with increasing weight loading of the catalyst. The apparent activation energies for catalysts with a single methane peak in their CO-TPR spectra were normally distributed. The apparent activation energies for catalysts with two methane peaks in their CO-TPR spectra were also normally distributed when the method of the catalysts' preparation was considered in testing the statistical nature of the distribution. This study suggests that the spread in the distribution of the apparent activation energies can be related to the structure of the reaction centers on the catalyst surface. The catalysts with a single methane peak in their CO-TPR spectra have uniformly dispersed NiAl 2O 4 species and result in the lowest spread (±0.2 kcal/mol, 0.84 kJ/mol). Catalysts prepared by incipient wetness with two methane peaks in their CO-TPR spectra have more uniform and larger “particle-like” nickel present on the surface than catalysts prepared by wet impregnation; the result is a spread of ±1.0 kcal/mol (4.2 kJ/mol) in the normal distribution in apparent activation energies. Catalysts prepared by wet impregnation have smaller nickel particle sizes and greater heterogeneity in reactive centers; the spread in the distribution of activation energies of catalysts prepared by this method is ±2.6 kcal/mol (10.9 kJ/mol).

Effect of Catalyst Preparation on Catalytic Activity: VI. Chemical Structures on Nickel/Alumina Catalysts: their Impact on the Rate-Determining Step in the Hydrogenation of Carbon Monoxide

Titration of the carbon pool subsequent to steady-state reaction of hydrogen and carbon monoxide over Ni/Al 2O 3 catalysts in conjunction with temperature-programmed surface reaction studies has been performed to assess the impact of nickel speciation on the rate-determining step in the methanation reaction. Mass balances for the desorption products reveal significant differences that depend on the weight loading of the catalyst. Low-weight loading catalysts prepared by incipient wetness consist of NiAl 2O 4 reactive centers as revealed by electron spectroscopy for chemical analysis and carbon monoxide temperature-programmed reaction experiments. Data are presented which support the hypothesis the rate of methanation is controlled by the rate of surface-bound carbon monoxide dissociation over the catalysts. On the other hand, high-weight loading catalysts prepared by this method consist of “particle-like” nickel. Over these catalysts, the rate of methanation is found to be controlled by the rate of hydrogenation of CH x . It is proposed that variation in catalyst preparation procedures can account for the wide-spread controversy in the determination of rate-determining step for methanation over Ni/Al 2O 3 catalysts.

Effect of catalyst preparation on catalytic activity: VII. The Chemical Structures on Nickel/Alumina Catalysts: Their Impact on the Formation of Metal—Support Interactions

In situ electron spectroscopy for chemical analysis (ESCA) experiments and microreactor studies have been used to examine the apparent occurrence of metal-support interaction on Ni/Al 2O 3 catalysts; suppression of hydrogen chemisorption does occur subsequent to hydrogen temperature-programmed desorption. The strong metal—support interaction type behavior can be reversed by exposure of the catalyst to oxygen or carbon monoxide. It is demonstrated that the catalyst in the SMSI type state is denuded of surface NiAl 2O 4 but surface nickel is detectable by ESCA; this nickel does not adsorb hydrogen. Depth profiling reveals an increase in the nickel/aluminum intensity ratio and the partial reappearance of NiAl 2O 4. A model is proposed to account for the suppression of hydrogen adsorption on nickel. The partial covering of the active center by [Al x O y ] n aggregates formed during the thermal disproportionation of NiAl 2O 4 forms the basis of this model.

The effect of catalyst preparation on catalytic activity: I. The catalytic activity of Ni/Al 2O 3 catalysts prepared by wet impregnation

A systematic study to examine the relationship between catalyst preparation procedures and the structure, dispersion, activity, and selectivity of the finished catalyst is reported. Nickel catalysts supported on gamma-alumina prepared by wet impregnation [Appl. Catal., 24 (1986) 241] were characterized by H 2 TPD, CO TPR, steady-state CO hydrogenation, and TPSR. The impregnant pH, nickel concentration, and ionic strength were the solution variables employed during wet impregnation. It was found that the dispersion based on H2 uptake decreases with increasing weight loading. The dispersion based on CO uptake decreases with increasing weight loading up to 2.1 wt%, but is relatively constant as the weight loading is further increased. A single peak in H 2-TPD and CO-TPR for the 0.8 wt% Ni/Al 2O 3 catalyst suggests that the active Ni sites on this catalyst predominantly result from NiAl 2O 4-like species. The methanation activity of the catalysts increases with increasing CO uptake. The carbon-containing residues after steady-state CO hydrogenation were analyzed by TPSR experiments. For low weight loading catalysts, two types of carbon-containing species were observed. Higher weight loading catalysts have three types of carbon-containing species. Analysis showed, however, the average number of carbon-containing residues was about 4 C/Ni, independent of weight loading. The catalytic performance was found to correlate with the metal loading which, in turn, is predictable based solely on the properties of the electrolyte. A companion paper will provide design criteria for producing and reproducing active Ni/Al 2O 3 catalysts.

The effect of catalyst preparation on catalytic activity: II. The design of Ni/Al 2O 3 catalysts prepared by wet impregnation

The performance of catalysts has been shown to be strongly dependent on their methods of preparation. We have previously reported the effect of preparation procedures including metal concentration, pH, and ionic strength of the impregnation solution on the catalytic performance of the finished catalyst. Here design equations are presented for Ni/Al 2O 3 catalysts prepared by wet impregnation from nickel nitrate solution in contact with a γ-Al 2O 3 support. The metal dispersion, activity for C 1, C 2 and C 3 formation under synthesis conditions, and the carbon deposited during reaction are shown to be predictable based solely on the properties of the electrolytes from which these catalysts were formed.

Effect of catalyst preparation on selectivity of high temperaturepropane oxidation to CO and H 2

High temperature oxidation of propane by air, catalysed by nickeldeposited within porous mullite supports was studied. Catalysts were prepared by impregnation, using techniques of single and multiple immersion of supports in solutions of different concentrations of Ni-nitrate (0.5-2.5 mol Ni dm −3) at 298 K, followed by drying, calcination and reduction. Pore structure (i.e., pore volume, pore volume distribution and surface area) of the prepared support and samples was determined. The samples with the most developed surface area were obtained from the most concentrated immersion solutions. The highest selectivity for the reaction 2C 3H 8 + 3O 2 = 6CO + 8H 2 at temperatures 1123 and 1173 K, was obtained when the catalyst was prepared by 6-fold immersion in the solution of 0.75 mol Ni dm −3.

The effect of catalyst preparation on catalytic activity: III. Thecatalytic activity of Ni/Al 2O 3 catalysts prepared byincipient wetness

A series of nickel catalysts supported on gamma-alumina prepared by incipient wetness procedures was characterized by H 2 TPD, CO TPR, steady-state CO hydrogenation, and TPSR. The impregnant pH and nickel concentration were the solution variables used to generate five groups of catalysts ranging in weight loading from 0.9 to 6.23 wt% at initial electrolyte pH's of 1, 3 and 5. The impregnant pH and weight loading independently affected the catalytic performance. The structure, dispersion, activity, selectivity and total carbon deposited during steady-state reaction were found to correlate with the metal weight loading and the initial pH of the impregnant. The results of this study provide the preparation procedures necessary to produce and reproduce Ni/Al 2O 3 catalysts with desired catalytic performance. In a companion paper the design of catalysts prepared by incipient wetness techniques will be presented.

The effect of catalyst preparation on catalytic activity: IV. The design of Ni/Al 2O 3 catalysts prepared by incipient wetness

The catalytic performance of Ni/Al 2O 3 catalysts prepared by incipient wetness has been shown to be strongly dependent on their preparation conditions. It has been demonstrated that the nickel weight loading and impregnant pH independently affect the catalytic performance of the catalysts. Herein design contour plots for Ni/Al 2O 3 catalysts with desired metal dispersion, activity for C 1, C 2 and C 3 formation under steady-state synthesis conditions, and the amount of total carbon deposited after reaction are presented.