Studies On Preparation And Electro-Catalytic Performance Of Amorphous TiO₂ Modified Graphene Supported Pt Catalysts

Metal oxide modified graphene support materials have attracted increasing attention due to their potential ability for enhancing electrochemical activity and CO poisoning tolerance CO of platinum-based catalyst in direct methanol fuel cell (DMFC) system. In this thesis, the amorphous TiO2 modified GO composite was used as the support of ultrafine Pt and PtNi alloy catalysts. The catalysts were studied in the the anodic methanol oxidation reaction (MOR) and cathodic oxygen reduction reaction (ORR) in DMFC. Furthermore, the relationships between structure and electrocatalytic performance of the catalysts were discussed. The thesis focuses on:(1) Amorphous TiO2 modified GO composite amTiO2/GO (20 wt.% TiO2) and amTiO2/GO-re(11 wt.% TiO2) were prepared by TTIP (Titanium isopropooxide) in-situ hydrolysis in GO sol system followed long time drying at room temperature. Amorphous TiO2 modified pre-reduced GO (prGO) composite amTiO2/prGO (20 wt.% TiO2) was also obtained based on pre-reduced GO support. The amTiO2/GO shows an excellent dispersion and a lower GO defects concentration (Id/Ig=0.94). The amorphous TiO2 was uniformly decorated on the surface of GO in amTiO2/GO attributed to the large amounts of oxygen containing functional groups.(2) The Pt/amTiO2/rGO-t (t=6,12,18,24 h) catalysts with 20 wt.% Pt were prepared by using ethylene glycol impregnation reduction method with varied reduction time over amTiO2/GO composite support. The phase of TiO2 is amorphous in all the obtained catalysts. The reduction of GO in the catalysts became more complete with increasing reduction time, and the Pt/amTiO2/rGO-24 shows the best rGO restored C structure (Id/Ig=1.29). The Pt/amTiO2/rGO-24 possesses smaller mean Pt particle size of 2.66 nm than that of reference sample Pt/rGO (3.76 nm) due to the dispersion effect of amorphous TiO2 decoration.(3) A series of Pt/amTiO2/rGO-t catalysts exhibit variously higher electrocatalytic activity and stability than Pt/C (JM) in acidic MOR system. The Pt/amTiO2/rGO-24 exhibits the best catalytic performance, and its MOR forward anodic peak current (If) was 2.02 and 3.54 times higher than that of Pt/rGO and Pt/C(JM), respectively, due to the high dispersion and ultrafine size of Pt particles, the surface TiO2-OH bonds of amTiO2 played a pivotal role in the facilitative bi-functional mechanism, the inhibitory effect of amTiO2 to the corrosion of rGO and the possibly synergistic effect between Pt nanoparticles and amTiO2 modified rGO.(4) A series of alloy catalysts PtxN1/amTiO2/rGO (x(=Pt/Ni)=1,2,3) with fixed PtNi (20 wt.%) content were synthesized by EG impregnation reduction over amTiO2/GO composite. The dimension D111 vulue is decreased with increasing Pt:Ni ratio, and Pt3Ni1/amTiO2/rGO show the smallest D111(2.56 nm), compared to Pt3Ni1/rGO (3.26 nm) and Pt/amTiO2/rGO-24 (2.66 nm), ascribing to the alloying function of Ni and the promotioin effect of amorphous. The rGO defects concentration in Pt3Ni1/amTiO2/rGO (Id/Ig=1.29) was also the lowest.(5) The PtxNi1/amTiO2/rGO exhibit variously higher electrocatalytic activity and stability than Pt/C (JM) in acidic MOR system. The Pt3Ni,/amTiO2/rGO exhibits the best catalytic performance, and its If is 3.22 times higher than that of Pt/C(JM), though close to Pt/amTiO2/rGO-24. Particularly, the Pt3Ni1/amTiO2/rGO shows obvious enhancement of CO tolerance (If/Ib=1.20) compared with that of Pt/amTiO2/rGO-24 (It/Ib=0.77). The pivotal issues are that the change of Pt geometric configuration induced by the formation of PtNi alloy, the promoted bi-functional mechanism by the existence of Ni and amTiO2 decoration, the enhanced support-metal strong interaction between PtNi alloy particle and amTiO2, and the isolated effect of amTiO2 on rGO.(6) The ORR activity study indicates that the acidic oxygen reduction current density of Pt3Ni|/amTiO2/rGO was 1.96 times higher than that of Pt/amTiO2/rGO-24 and close to that of Pt/C (JM). The Pt3Ni,/amTiO2/rGO shows a more positive onset potential than Pt/amTiO2/rGO-24 and Pt/C (JM), indicating the enhanced ORR activity of Pt3Ni1/amTiO2/rGO. Based on Koutecky-Levich equation, a dominant 4-electron transfer pathway is revealed for Pt3Nii/amTiO2/rGO, attributing to the change of Pt geometric configuration induced by the formation of PtNi alloy and the possibly existed synergistic effect between PtNi alloy particles, amTiO2 and rGO.(7) The present strategy upon amorphous TiO2 modified GO support for preparing Pt or PtNi alloy loaded catalysts is an effective route of synthesizing high efficient Pt electrocatalysts. The obtained catalysts with highly dispersed ultrafine Pt size show significantly improved MOR and ORR performance, and can be extended to the electro-oxidation of other small organic morecule, such as ethanol and formic acid, being of a potential DMFC catalyst and sheding new light on highly efficient usage of noble metal resources.