| At first, two kinds of fabrication method of Pt nanoparticles supported on carbon carriers Vulcan XC-72R were studied systematically. Then Polytetrafluoroethylene (PTFE) and Pt/C catalysts were supported on foamed nickel (FN) to obtain hydrophobic Pt/C/FN catalysts. The correlations between Pt microstructure, other fabrication processes and the catalytic activities of the hydrophobic catalysts were determined for hydrogen-water liquid phase exchange reaction (LPCE). Based on quantum mechanics computation theory, the reaction mechanisms of LPCE were studied on Pt, PtIr, PtRu, and PtFe based hydrophobic catalysts. According to the computation results, metal species added into Pt were selected and Pt based binary hydrophobic catalysts were prepared. The catalytic activities of the hydrophobic catalysts with different atom ratios of Pt to Ir, Ru, or Fe were examined for LPCE, which confirmed the theory computation results.Firstly, Pt/C catalysts were prepared in the dispersant ethylene glycol instead of isopropyl alcohol by improved impregnation-reduction method (Pt/C-I), using HCHO as reducing agent. Because ethylene glycol can arrest the growth of Pt particles, the average size of Pt particles is smaller than that of Pt/C catalysts fabricated by general impregnation-reduction method (Pt/C-G) markedly. Higher pH value, reaction temperature, and HCHO dosage are favorable for the fabrication of high-dispersed Pt/C catalysts, and Pt nanoparticles size can be controlled by changing these fabrication conditions. Then Pt/C catalysts were prepared by microwave heating (Pt/C-M) of ethylene glycol solution of platinum salt in high temperature, 0.5-2.5 MPa, and airproof containers rather than open containers in literatures. The Pt particles are dispersed on Vulcan XC-72R uniformly with a narrow size distribution. Because of higher reaction temperature, the average size of Pt particles is smaller than literature values. The Pt particles of Pt/C catalysts fabricated by dissolving [PtCl6]2- in ethylene glycol have a narrow size distribution, while the Pt particles sizes of Pt/C catalysts fabricated by dissolving [PtCl6]2- in water have difference distinctively. The average size of Pt particles increases with the increase of H2O content of reaction solvent.Then Pt/C catalysts and PTFE were loaded on foamed nickel carrier with high BET surface area by natural impregnation method. A kind of new hydrophobic Pt/C/FN catalyst was prepared. Pt/C catalysts and PTFE are dispersed on some sunken places of foamed nickel surface. They mix in uniformity mostly and have strong conjunct strength with foamed nickel surface. The catalytic activity of hydrophobic catalysts increases with the decrease of the average size of Pt particles for LPCE. Pt/C catalysts were reduced in 300℃for 2 h, or 500℃for 1 h. The catalytic activities of the hydrophobic catalysts increase because of the increase of Pt (0) contents, and Pt/C-500/FN has the highest catalytic activity. Pt/C-I/FN and Pt/C-M/FN hydrophobic catalysts have higher catalytic activity than Pt/C-G/FN, and Pt/C-I/FN have the highest catalytic activity because of higher Pt (0) content of Pt/C-I than that of Pt/C-M. 10%, 20%, 30% Pt/C catalysts were used to prepare hydrophobic catalysts, respectively. In the experimental conditions, Pt10/C/FN has the highest catalytic activity for the differences of average sizes of Pt particles, catalytic membrane resistance to hydrogen and water, and hydrophilicity of Pt/C catalysts. Because of low hydrophobicity, Pt/C/FN catalyst of too low PTFE content has low activity, and its catalytic activity would increase with the increase of PTFE content for higher hydrophobicity, but more Pt active sites would be covered by PTFE and interior diffusion effect of hydrogen and water would increase simultaneously. The catalytic activity of Pt/C/FN decreases with the increase of PTFE content when PTFE content increases to a certain extent.Based on electron and vibration approximate means and the density functional theory B3LYP, with LanL2DZ basis sets for metal Pt, Ir, Ru, or Fe and 6-311G** basis sets for H, D, T and O, the thermodynamic functions and the equilibrium pressures of adsorption and dissociation reactions of water vapor and hydrogen on metal surfaces have been calculated in different temperatures. And the adsorption and dissociation actions of water vapor and hydrogen were analyzed. According to the thermodynamic functions, Hydrogen isn't adsorbed on metal surfaces in intact molecule, and it combines with metal atoms in single atom. Water vapor molecule exists on Pt surface in intact molecule, but it has a tendency to dissociate to OH (ads) and H (ads) on Ir, Ru, or Fe surface. Then the coadsorption actions of hydrogen and water molecules were analyzed, and reaction mechanisms of LPCE were determined on Pt, PtIr, PtRu, and PtFe metal surfaces. The reaction mechanism of LPCE is as follows (e.g. H-D exchanges) on Pt surface. HD molecules dissociate to H (ads) and D (ads), and two or more H2O molecules react with D (ads) to form (H2O)nD+ (ads) (n≥2) and e". Hydrogen isotope exchanges occur between hydration layer and D atoms which are adsorbed on Pt surface via intermediates (H2O)nD+ (ads) (n≥2). The other reaction path exits on PtIr, PtRu and PtFe binary catalysts. HD and H2O molecules dissociate to H (ads), D (ads) and OH (ads), H (ads), respectively. Then D (ads) and OH (ads) react to HDO (ads), and H (ads) and H (ads) react to H2 (ads). At last, HDO and H2 molecules are desorbed from metal surface and hydrogen isotope exchange process is carried through. Ir, Ru, or Fe has aidant effect on LPCE reaction with metal Pt, and rational intermingles with these elements can increase catalytic activity of Pt based hydrophobic catalysts.Coadsorption action of H2 and CO molecules was investigated on Pt surface in the same way, and poisoning mechanism of CO on Pt based hydrophobic catalyst was determined for LPCE. When CO molecules are adsorbed on Pt active sites, the dissociation adsorption of H2 would not be happened sequentially and LPCE reaction is broken down.According to theory computation results, PtIr (Ru or Fe)/C catalysts with different ratios of Pt to Ir (Ru or Fe) were prepared by microwave heating method, using HCHO as reducing agent. The catalytic activities of corresponding hydrophobic catalysts were investigated for LPCE. And the influences of the reduction of PtIr (Ru or Fe)/C catalysts by H2/N2 mixed gas in 500℃on catalytic activity was determined. When different contents of Ir (Ru or Fe) are intermingled with Pt metal, the average size of active metal particles of PtIr (Ru or Fe)/C catalysts changes little. PtFe metal is still present in the face centered cubic (fcc) phase. But PtIr (or Ru) metal has a tendency to form amorphous alloy with the decrease of Pt content. When rational content of Ir (Ru or Fe) is intermingled with Pt, the catalytic activity of Pt based binary hydrophobic catalyst increase distinctively. Hydrophobic catalysts have optimal catalytic activities, when the ratios of Pt to Ir, Ru, or Fe are 4:1, 1:1 and 2:1, respectively. If PtM/C weren't reduced in high temperature, the sequence of catalytic activities of is: PtFe/C/FN > PtIr/C/FN≈PtRu/C/FN. After reduction treatment, the sequence is: PtIr/C-500/FN > PtFe/C-500/FN > PtRu/C-500/FN. The reduction treatment results in the increases of catalytic activities of PtIr and PtFe binary hydrophobic catalysts, and the decrease of catalytic activity of PtRu hydrophobic catalyst.As a result, some important basic issues of how to prepare highly active hydrophobic catalysts were discussed, and a kind of theory means to select the other element of Pt based binary hydrophobic catalyst was determined. Several ways of increasing catalytic activity of hydrophobic catalysts include: decreasing average size of active metal particles by improving preparation method of carbon supported Pt based catalysts, rational intermingling with the other element into Pt, and increasing reducing degree of Pt and some intermingled element. |
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