Internal Heterogeneity in Catalysts – Methods of Preparation and their Influence on Final Activity and Selectivity

Abstract

Solid catalysts often have a non‐uniform internal composition as a result of their method of preparation and working conditions. Such products sometimes command interest because of their better performance. Variables influencing internal distribution during their preparation are considered on the basis of experimental and theoretical studies. Activity and selectivity of these catalysts depend upon the internal distribution of the active species. Activation energies, reaction orders, and apparent diffusion coefficients are masked in relation to their real values, due to the mutual interdependence between mass transport phenomena and chemical reaction.

Characterization of Palladium (Pd) on Alumina Catalysts Prepared Using Liquid Carbon Dioxide

“…The chemical and physical properties of supported metal catalysts, and their performance in catalytic reactions, depends on the method of preparation. , Preparation of supported metal catalysts using supercritical carbon dioxide (scCO 2 ) has received considerable attention recently. – The unusual physical properties of scCO 2 , including zero surface tension, low viscosity, and tunable density, make it an interesting alternative medium for metal deposition, with the potential to overcome many of the disadvantages associated with traditional, aqueous catalyst preparation techniques. These disadvantages include slow diffusion of the metal salt into the pores of the support due to the high viscosity of water, changes in the structure of the support during drying due to the high surface tension of water, poor metal dispersion, and redistribution or inhomogeneous distribution of the active compound. ,– …”

“…These disadvantages include slow diffusion of the metal salt into the pores of the support due to the high viscosity of water, changes in the structure of the support during drying due to the high surface tension of water, poor metal dispersion, and redistribution or inhomogeneous distribution of the active compound. 1,[16][17][18] Catalysts with very small metal particle sizes and high activity have been prepared from scCO 2 . For example, Saquing et al reported that 1.6-3.1 nm diameter Pt particles were synthesized on a high-surface-area carbon aerogel (629-741 m 2 /g) by impregnating dimethyl(1,5-cyclooctadiene) platinum(II) (PtMe 2 (cod)) using scCO 2 at 27.6 MPa and 80 °C and subsequently reducing the organometallic compound to metallic Pt by heat treatment between 300-800 °C.…”

Characterization of Ru on Activated Carbon Catalysts Prepared by Supercritical Carbon Dioxide Impregnation

“…The chemical and physical properties of supported metal catalysts and their performance in catalytic reactions, depends on the method of preparation [1,2].Preparation of supported metal catalysts using supercritical carbon dioxide (SC CO 2 ) has received considerable attention recently [8][9][10][11][12].The unusual physical properties of SC CO 2 , including zero surface tension, low viscosity, and tunable density, make it an interesting alternative medium for metal deposition, with the potential to overcome the most disadvantages associated with traditional, aqueous catalyst preparation techniques. These disadvantages include the slow diffusion of metal salt into the pores of support due to the high viscosity of water, the structure changes of support during drying due to the high surface tension of water, the poor metal dispersion, and the redistribution or inhomogeneous distribution of the active compounds [1,[13][14].Although organometallic precursors possess appreciable solubility in SCFs such as SC CO 2 and have been used extensively for preparation of supported metal nanocomposites [15,16], there are few studies have been reported on the solubility of inorganometallic complexes in SC CO 2. So it would be valuable to utilize the inorganometallic precursor in SC CO 2 for the preparation of supported catalyst.…”

Supported Pd Catalyst Preparation Using Liquid Carbon Dioxide

“…Most commercial supported metal catalysts are prepared by impregnating a porous support with an aqueous solution containing a dissolved salt of the desired metal, followed by drying to remove the solvent and activation of the catalyst by high-temperature treatment. Traditional preparative methods have drawbacks, including slow diffusion of the metal salt into the pores of the support during impregnation because of the high viscosity of water, poor dispersion of metal, redistribution or inhomogeneous distribution of the active compound, and changes in the structure of the support during drying because of high capillary flow resulting from the high surface tension of water. − Deficiencies such as these have led to a search for alternative methods of catalyst synthesis. For example, chemical vapor deposition (CVD) involves the adsorption of gaseous organometallic compounds onto the support, with subsequent heat treatment to activate the catalyst.…”