The adsorption properties of nanoparticles and nanomaterials.

Important in the study of physical and chemical properties of nanoparticles and nanomaterials is the study of the adsorption properties. The results of these studies will help to develop new effective adsorbents for a variety of practical applications: for chromatography purification filters, gas analyzers, etc.

One of the most convenient methods for studying the adsorption properties is inverse gas chromatography. It provides an opportunity to determine the thermodynamic parameters of the sorption of test substance and from these data to identify the ability of the sorbent to different intermolecular interactions.

Inverse gas chromatography is a physical characterization technique that is used in the analysis of the surfaces of solids. While in gas chromatography a sample containing multiple species is separated into its components on a stationary phase, inverse gas chromatography uses injection of a single species to investigate the characteristics of a stationary phase sample.

There are several works dedicated to gas chromatographic investigation of various materials, including, nanostructured materials. As we already know, the properties of such objects differ from those of bulk materials. Due to their highly surface properties of the nanoparticles are highly adsorbent, i.e. capable of adsorbing its weight per unit many times more adsorbable substances than macroscopic dispersion. Therefore, the study of the adsorption properties is practically and theoretically important task.

By nanostructured materials include zeolites. Tümsek F and Inel O. by inverse gas chromatography studied the thermodynamics of adsorption of normal alkanes (C6-C8) in 3 Å, 4 Å and 5 Å zeolites. They have determined that with increasing number of carbon atoms increases linearly adsorption parameters observed compensating thermodynamic effects; In addition, the heat of adsorption on zeolite 3 Å higher than 5 Å zeolite. (Å=0,1 nm)

Bilgic C. and Askın A. studied the adsorption of n-hexane, cyclohexane and benzene 3 Å and 5 Å zeolites as well as alumina. It was shown that the best observed on zeolites benzene sorption than alumina with the alumina hexane sorption at higher than molecular sieves.

Change entropy, enthalpy potentials, Gibbs adsorption energies show that the determining factor for the sorption of normal alkanes on zeolites is a zeolite topology, that is, the parameters of its "cellular structure". The most striking manifestation of the "cellular effect" (the best adsorption of small molecules than the large) is observed for zeolites with the size of "windows" <0.47 nm.

To predict the adsorption and catalytic properties of zeolites used computer modeling techniques. The literature describes the values of the prediction method of sorption enthalpies of linear and branched alkanes on the different zeolites called Graph Machines method. Models developed using this method, allow satisfactorily predict the enthalpies of adsorption of zeolite, even when using a very limited training sample (10 molecules). Efficacy in such models is that they allow adsorption model complex systems, such as zeolite ZSM-22 using only a few experimental data. This illustrates the potential of this approach in screening for chromatographic zeolites and catalytic purposes.

With gas chromatography, the adsorption properties of carbon nanotubes have been studied. The nanotubes obtained by high-temperature annealing and thermal oxidation have a similar surface structure, but vary considerably concentration of polar groups on the surface. The initial values of dispersion interactions and modified carbon nanotubes are very similar, but modified nanotubes have great contribution of specific interactions, approximately 10% and 30% of the total surface energy.

Important is the study of gas adsorption on metal nanoparticles. Fedorov A.S, Serzhantov M.V and Kuzubov A.A observed the features of hydrogen sorption at the surface in the magnesium nanoparticles.

There is a problem of a massive destruction of the crystal lattice of magnesium in the hydrogen chemisorption. Due to the minimum of defects and small size of nanoparticles of magnesium lattice they are stable, it opens opportunities for the use of magnesium for the creation of hydrogen accumulators. The presence of titanium clusters allows easier adsorption of hydrogen on magnesium.

Of particular interest are the metal nanoparticles deposited on oxide supports. Such materials can be used as catalysts in the gas-sensitive sensors. For example, a sensor based on gold and platinum nanoparticles supported on alumina is quite sensitive to certain organic substances.

Experimental and Quantum-chemical study of adsorption of alkanes (C6-C9) on zirconium oxide modified gold nanoparticles and nickel showed that the values of the heats of adsorption on the modified samples higher than pure media by an average of 7 kJ / mol.

Surfacing partially dehydroxylated silica silver nanoparticles reduces surface hydrophilicity adsorption surface appearance with a very high potential and dispersive forces nonspecific sorption surface increase. Greatly increases the selectivity of the adsorbent with respect to homologues.

For adsorbents based nanoparticles, they are applied to carriers - "substrate". The polymers and carbon materials can be used as a "substrate" for the application of the nanoparticles. Introduction of silver nanoparticles in a polymeric or carbon matrix increases the value of the differential heat of adsorption of almost all classes of compounds, and contributes to the partial blockage of active adsorption centers, resulting in the contribution of the dispersion and electron interactions that silver acts as an acceptor of electron density of adsorbate molecules.

Adsorptive properties of the nanoparticles are different on different substrates. For example, the sorption capacity of the silver nanoparticles obtained by radiation-chemical reduction of metal ions from the reverse micellar solutions, on the surface of silica gel, and porous polyethylene varies considerably. In the adsorption properties and selectivity of adsorbents containing metal nanoparticles, the particle size also influences. Thus, Qian L., Lv X., Ren Y., Wang H., Chen G., Wang Y., et al. held a chromatographic study of mesoporous silica nanoparticles modified nickel. It has been found that reducing the size of the nanoparticles increases adsorption of aromatic compounds; this effect, according to the authors,called coordination-unsaturated state of metal atoms on the surface of the nanoparticles.

Control questions

1. Explain two properties of metal based nanoparticles.

2. Analyze some factors, that explain the change in the thermodynamic variables of Nano dispersed particles.

3. Why are important the adsorption properties of nanoparticles and nanomaterials?