Synthesis And Catalytic Performance Of Nanoporous Amorphous Alloys

Nanoporous metals of large specific surface area and excellent catalytic performanceshow great potential applications in the fileds of petrochemical and energy. Thenanoporous metals are always fabricated by dealloying methods. However, for crystallinematerials, the scope of alloys which can form the solid solution is limited, and most arebased on noble metals (e.g. Au, Pt), thus leads to an increased cost for the applications.Conversely, the amorphous alloys exhibit disordered structure, homogeneous chemicalcompositions in wide ranges and metastable state, thus shows unusual surface propertiesand are ideal precursor materials for dealloying.In this thesis, Mg65Cu25Gd10and Pd40Cu30Ni10P20amorphous alloy strips have beenfabricated by using induction melting and copper mold spray-casting. Nanoporousamorphous alloys with various pore sizes are then obtained by chemically orelectrochemically dealloying these precursors in various corrosive media for differentdurations. The structure and catalytic performance have been systematically characterizedby X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Scanning electronmicroscope (SEM) coupled with EDS energy spectrum analysis, Specific surface areameter(BET), electrochemical workstation and High performance liquid chromatography(HPLC).Nanoporous alloys with different pore sizes (20,30and50nm) were successfullyprepared via chemical dealloying in various concentrations of sulfuric acid for variousimmersion times. Elements such as Mg and Gd with higher activities are first etched in thesolution, the remaining copper atoms form the frame structure with double continuouschannels. Higher acid concentration is more prone to obtain larger pore sizes of thenanoporpus alloys than prolonging the immersion times. The nanoporous alloys showedhigh catalytic activity in the degradation of phenol, i.e. the degradation rate of phenol reached99%within20min, which is2-4times higher than untreated amorphous alloy.This is attributed to the larger surface areas of the nanoporous alloy. Moreover, the poresize of nanoporous alloys has a significant effect on the catalytic property, i.e, sampleswith relatively larger pore size (e.g.50nm) possess higher catalytic activity than those ofsmaller pore sizes (20nm and30nm), although the former owns a smaller specific surfacearea. Analysts believe that this is mainly related to the nano-aperture and wetting behaviorwhich have coordinated effects on phenol degradation. The contact angle of larger poresize sample is relatively small, so that the surface wettability of solid-liquid contact areaexpanded; on the other hand, larger pore size cause reaction-diffusion more smoothly. Theoptimal reaction conditions for catalytic oxidation of phenol with nano-porous amorphouswere0.4mol/L H2O2concentration and60℃for the reaction temperature. Finally, themechanism for degradation of phenol is discussed. First, Hydrogen peroxide interact withnanoporous copper strip to produce hydroxyl radicals, hydroxyl radicals then attack on thebenzene ring which has larger electron cloud density to form catechol and hydroquinone,then further oxidation becoming benzoquinone, under the effect of hydroxyl radicalsphenol was finally been oxidized into CO2and H2O.Another nanoporous amorphous system (Pd40Cu30Ni10P20) has also been synthesizedvia electrochemical dealloying in a0.5M H2SO4solution for different immersion times(45-120mins). Relatively homogeneous nanoporous-structure is obtained in the conditionof longer durations. Nanoporous Pd-based amorphous alloys have excellentelectro-catalytic properties of methanol. The sample with the longest etching time (120min) has the largest electrochemically active area (44.54m2/g) and thus the bestelectro-catalytic performance, as evidenced by higher positive peak current density (jf) andreverse peak current density (jb) is178.17and34.63mA/cm2, respectively, also itsresistance to CO poisoning (in term of jf/jb) reaches as high as5.1. Conversely, jfand jbforuntreated Pd-based amorphous alloy are only0.58and0.03mA/cm2, respectively. Inaddition, nanoporous alloys also show high stability and cycling performance, where the current density only decreased for6%after5cycles. Therefore, this nanoporousamorphous materials have potential applications in direct alcohol fuel cell electrodes.