Nano Metal or Metal Oxide Catalysts - Chemical Engineering PDF Download

Nano metal or metal oxide catalysts
Nano particles consisting of small metal or metal oxide crystallites in the range of few nanometers (1-100 nm) are important for catalysis and adsorption. Nanoparticles have higher effective surface area and improved physico-chemical properties giving better performance. Uniform shape and sized catalysts are particularly important for structure sensitive reactions where different type of surface metal atoms such as corners, edges or terrace atoms possesses quite different properties.

 Menezes et al. [1] studied the effect of particle size on catalytic activity of titania supported Au-Ag (1:1) nanoparticles for CO oxidation. They observed that reactivity of the catalysts increased as size of the nanometals decreased. They reported turn over frequency of 0.02, 0.04 and 0.08 s^-1 for nanoparticles having average size of 3,12 and 19 nm respectively.

Synthesis of nanoparticles
It is a challenge to prepare nanoparticles of uniform shape and size by conventional methods. Various new preparation methods are reported that give nanoparticles with narrow size distribution. Some of the reported methods are :

• Hydrothermal  technique
• Microwave assisted synthesis
• Dendrimer assisted method
• Reverse micelle method
• Chemical vapor deposition

Hydrothermal method

In a hydrothermal process precursor solution, often in presence of an alkali, is autoclaved at certain temperature for specific time. In a typical process, aqueous solution of precursor and KOH is mixed and placed in teflon-lined stainless-steel autoclave. Autoclave is maintained at 70- 200°C for 10-24 h depending on process and then air-cooled to room temperature. Resulting precipitate is collected by filtration, washed and finally dried. 

For example, nickel ferrite particles prepared by hydrothermal process resulted in particles of size distribution from 40-90 nm as reported by Cheng et al. [2]. 

Microwave-irradiated synthesis
Microwave irradiated synthesis method is a new promising technique for the preparation of size controlled metallic nanostructures. Xu et al. [3] reported preparation of Pt nanoparticles supported on CNTs. In this method solutions of Pt precursor (H₂PtCl₆. 6H₂O), ethylene glycol and KOH are mixed in vial and CNTs are uniformly dispersed in mixed solution. Closed vial is then placed in microwave oven (2450 MHz, 800 W) and heated for the required time. The resulting suspension filtered and dried at 120°C. This preparation method resulted in Pt nanoparticles on surface of CNTs having uniform spherical shape with diameter of 15 ± 3 nm.

Dendrimer assisted synthesis
Dendrimer is highly ordered branched poly-amidoamine macromolecule with tree-like topology and is used in preparation of nano-material. Dendrimer have multiple coordination sites that are utilized to coordinate metal ions and synthesize metal clusters. In this method metal ions in solution are complexed to dendrimer, mostly with amine groups in the outer shell of OH-terminated poly-amidoamine. Complexed metal ions are subsequently reduced to metal atoms which agglomerate into metal particle. Men et al. [4] reported synthesis of dendritic polyphenylazomethines (DPA) dendron encapsulated gold clusters having average cluster diameter of 2.2 nm.

Fig 1. Gold clusters with DPA dendron

Reverse micelle method
Reverse micelles are formed when surfactant molecules, possessing polar head group and hydrophobic tail, are placed in non-polar solvent. In reverse micelle structure surfactant molecules aggregate into nano sized spherical structures having core of polar head groups and shell of hydrophobic tails. If small amount of polar solvent is present in the mixture, it would be enclosed in core of reverse micelles. If this polar solvent contains an oxide precursor then controlled hydrolysis of precursor will be achieved by mixing small amount of water and small particles of metal oxide will form at core. Preparation of nano Al₂O₃ by reverse micelle method is discussed below.

Fig 2. Schematics showing formation of nano-sized particles of aluminium hydroxide by reverse micelle method.

A solution of inverse micelles is first formed by adding a long chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminium alkoxy amine adduct results in hydrolysis of the aluminium alkoxide adduct and formation of nano-sized particles of aluminium hydroxide after drying. 

Reverse micelle synthesis method is also reported for preparation of supported metal catalysts. Cheney et al. [5] reported synthesis of alumina supported Pt/Ni bimetallic catalysts by reverse micelle synthesis method. In this method two microemulsions were created by mixing 15% water, 10% surfactant and 75 % hydrocarbon( cyclohexane and propanol). Ni and Pt precursors were added to microemulsion -1 and hydrazine to the microemulsion -2. Each mixture was stirred separately for 1 h to allow micelles to equilibrate. In microemulsion-1 reverse micelles were formed having aqueous core and the Pt and Ni precursors were dissolved in aqueous polar core of the reverse micelles. Thereafter, the microemulsion –2 containing the reducing agent was added to the microemulsion -1 for insitu chemical reduction of the metals at reverse micelle core. The alumina support was added to the solution and titrated with acetone to disrupt the micelles and precipitate the nanoparticles onto the support. The supernatant was decanted and the catalyst powder was rinsed with acetone. Residual surfactant was removed by giving heat treatment in oxygen environment. By this method particles were obtained in the range of 1.4-2.8 nm.

Application of nanocatalysts

1. Gold nanocatalysts
Supported gold nanoparticles have been reported to be very effective catalysts for oxidation of alcohols. Gold supported on graphite can oxidize glycerol to glycerate under  relatively mild conditions. Under same conditions conventional supported Pt or Pd oxidation catalysts are much less selective.

2. Sulfated zirconia nanocatalysts   
These catalysts are highly effective in refining processes replacing liquid acid catalysts.These are used in acid promoted catalytic reactions such as alkylation,dehydration, isomerization etc.

3. Dendrimer encapsulated nanoparticles (DENs)
Pd-DENs catalysts are studied as carbon-carbon coupling catalysts. In Heck coupling reaction, involving C-C coupling between aryl halides or vinyl halides and activated alkenes in the presence of a base, the Pd-DEN catalysts show higher activity and selectivity relative to other colloidal Pd catalysts.

4. Tungsten oxide and titanium oxide nanocatalysts
Tungsten oxide and titanium oxide due to their visible-ultraviolet band gap have been extensively studied in electro-catalytic and photocatalytic applications.

5. Transition metal or metal oxide nanoparticlesTransition metal or metal oxide nanoparticles have found important application in catalyzing growth of carbon nanotubes, carbon fibres, nanowires etc. Cobalt or cobalt oxide nanoparticles mixed with Ni is used in synthesis of single walled carbon nanotubes.