Advanced Nanomaterials in Biomedical, Sensor and Energy Applications by Jayeeta Chattopadhyay & Rohit Srivastava

Author:Jayeeta Chattopadhyay & Rohit Srivastava
Language: eng
Format: epub
Publisher: Springer Singapore, Singapore

3 Nanocatalysts

Transition metal-based nano-sized catalysts are able to display high catalytic activity and selectivity when compared to the bulk. This is attributed to their large surface-to-volume ratio, shape- and size-dependent properties, as well as large number of under-coordinated active sites on the surface. Nanostructure of Co-B catalyst displays outstanding performance for catalytic H2 production of chemical hydride.

3.1 Nanoparticle-Assembled Thin Film

Pulsed laser deposition (PLD) is a single-step deposition technique for developing a thin film with nanoparticles (NPs) embedded on the surface. In addition, the deposition parameters during PLD can be varied to bring about morphological and structural changes in the film and can be optimized for any given application [14]. Catalysts in the form of thin film coatings are easily recoverable and can be reused. They are thus suitable for use as on/off switch for H2 generation. Co-B nanoparticle-assembled thin films displays catalytic activity much higher than Co-B powder with five to six times higher hydrogen generation rate (HGR). When Co-B is deposited by PLD, the spherical shaped Co NPs are embedded in a boron matrix (Co-NP-B-MA) having a narrow size distribution (Fig. 4a and 4b). The size and density can be varied by tuning the laser fluence. When the laser fluence increases from 3 to 9 J/cm2, the average particle diameter (D) of Co NPs increases from about 11 to 15 nm (Fig. 4b) [15]. This feature of the Co-B film is due to a phenomenon called phase explosion induced by laser irradiation, which occurs under extreme conditions of pressure and high temperature over the target in vacuum [16]. The irradiating laser beam with high energy density superheats the region just under the target surface. The temperature crosses the thermodynamic stability limit of the material (~90% of Tc, the thermodynamic critical temperature) that induces a homogeneous nucleation of a vapor bubble. There is a rapid transition of the super-heated liquid to a matrix of nano-sized liquid droplets and vapors that leave the irradiated surface and is deposited on the substrate. The emitted liquid has a homogeneous composition of Co-B as the process occurs in a super-heated liquid state near Tc. During the flight from target on to the substrate, phase separation occurs due to cooling of Co NPs that are embedded in a matrix of B. As seen in the HRTEM, each Co NP is made of nano-sized domain with a set of atomic planes separated by grain boundaries of width 0.5-1.0 nm (Fig. 5a). Fast Fourier transform (FFT) (Fig. 5b) and the SAED pattern (Fig. 5c) confirm that the Co NPs are polycrystalline. When Co NPs are compared with a noble metal catalyst like Pt/C and Pd/C that are commercially available, the catalytic activity for H2 production is equivalent to that obtained for Pt metal and is greater than that of Pd metal catalyst (Fig. 6). The Rmax (maximum H2 generation rate) obtained for SBH (or AB) is 18.1 (or 10.2) L of H2/min/(g of Co) and 18.2 (or 10.5) L H2/min/(g of Pt) with Co-NP-B-MA catalyst and Pt/C powder, respectively (Fig.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.