Heterogeneous Metal Catalysts for Oxidation Reactions
Table 11
Major methods of catalysts synthesis.
Method
Brief description
Limitations
References
Deposition-precipitation
(a) Deposition-precipitation method is easier for the synthesis of various supported metal catalyst complexes in presence of excess alkali. (b) In alkaline media the [Au(en)2]3 + cations are deposited on anionic oxide (TiO2, Fe2O3, Al2O3, ZrO2, and CeO2) surfaces having high isoelectric point (PI > 7.00). (c) Functionalization of oxides may take part in the reaction as co-catalysts for the enhancement of the catalytic activity. (d) It is a very good method for the oxidation of alkanes to epoxides.
(a) It is a multistep processes for the deposition of metal onto the oxide surface. (b) It cannot integrate AuNPs on metal oxides of low isoelectric point (IEP ~2) such as SiO2. (c) It is limited to maximum 1 wt% Au-loading. (d) It requires multiple washing steps to eliminate excess chloride
(a) It simultaneously forms mesostructure to anchor gold. (b) It easily forms hexagonal array of mesopores and metal crystallites of 3–18 nm in diameter. (c) It is a simple method to insert gold nanoparticles onto the surface of oxides. (d) It permits the formation of particles in metallic state surrounded by chloride ions. These Cl− ions are the basic species for catalysts activation during acetonylacetone (AcAc) transformation (cyclization/dehydration) in gaseous state and also act as promoters for electron transfer to O2 during NO reduction with propene in presence of oxygen.
(a) The surface area of catalysts, prepared by this method, is low.
(a) Aqueous anions (sulfate, arsenates, and anionic functional groups of biomolecules) are adsorbed on the electrically charged metal oxide surfaces (b) Optimum gold loading takes place at 80°C. (c) It is a simple method, with no need for expensive instrumentations and expert personnel.
(a) Catalyst can be prepared at room temperature to avoid decomposition of the metal complex and reduction of gold. (b) Higher loading of gold (3 wt%) can be achieved and cation adsorption with metal leads to smaller particles (~2 nm) when the solution/support contact time is moderate (1 h)
(a) In general, the Au loading did not exceed 2 wt%.
(a) Interaction of gold precursors and the support surface takes place between the oxygen atoms of Me2Au (acetonylacetone) and the OH groups of the SiO2 surface at high temperature (~300°C). (b) Strong interaction between the metal catalyst and support oxides. Thus catalyst is not easily lost.
(a) The chlorides on support promote the aggregation of AuNPs and frequently poison the active sites of the catalyst. (b) Low pH (1) and high temperature are prerequisite (300°C). Contains higher amount of chloride impurities. (c) It cannot produce homogeneous and stable particles.
(a) it is an attractive method to control the aggregation of AuNPs. (b) Particle size is preserved during the immobilization step. (c) Particles size can easily be controlled. (d) It is highly selective and efficient.
(a) It requires extensive washing steps to remove excess chloride impurities.
(a) Supports are evacuated in vacuum at 200°C for 4 h to remove the adsorbed water (b) In general, OMCVD method involved in a system where the proportion between the substrate area and gas phase volume gets larger, so that the surface reactions hold a key parameter.
(a) It is expensive, requires special equipment, and the amount of metal incorporated by this method is somehow limited by pore volume of inert solid support.
