Nanostructured platinum based catalysts for electrooxidation of small organic molecules prepared by microemulsion method

Abstract

Development of novel materials that would enable the commercialization of fuel cell technology and contribute to reduction of fossil fuel usage in energy production is of great importance for the progress of modern society. As a result, the number of scientific papers devoted to this problem is in a significant increase over the past years. When it comes to the oxidation of small organic molecules (SOM) as potential fuel for the fuel cells, in addition to the problem of the high price of platinum used as a catalyst, there is a problem of poisoning the platinum surface by carbon-monoxide formed as an intermediate in the anodic reaction. In this research carbon supported platinum nanoparticles were synthesized by a water-in-oil microemulsion synthesis procedure. The effect of HCl as a capping agent on nanoparticle shape was investigated by adding up to 35 % HCl in the water phase of the microemulsion. The optimal HCl amount was found to be 25%, in which case platinum was synthesized in the form of cubic-shaped nanoparticles. Four prepared catalysts were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD) and (high resolution) transmission electron microscopy (HR)TEM. These analyses confirmed the change in particle shape induced by the capping agent. Metal loading in catalyst powder was determined to be close to 20%, and the particle size calculated from TEM data went from 4 to 8 nm, with the increase of the HCl amount used. The electrochemical characterization and the investigation of these samples in the reactions of formic acid and methanol oxidation revealed improved catalytic performance of the sample that predominantly contained cubic-shaped nanoparticles compared to conventional cube-octahedron shaped particles. The reason of the improved activity and stability of this catalyst lies in the facilitated CO removal from the catalyst surface, and thus improved tolerance to surface poisoning.

Pronalazak novih materijala koji bi omogućili komercijalizaciju gorivnih spregova i doprineli smanjenju upotrebe fosilnih goriva u proizvodnji energije je od velikog značaja za razvoj savremenog društva. Zbog toga je broj naučnih radova posvećen ovom problemu u značajnom porastu tokom proteklih godina. Kada je u pitanju oksidacija malih organskih molekula kao potencijalnih goriva za gorivni spreg, pored problema visoke cene platine koja se koristi kao katalizator, prisutan je i problem trovanja površine platine ugljen-monoksidom koji se formira kao intermedijar anodne reakcije. U ovom radu nanočestice platine na ugljeničnom nosaču sintetizovane su mikroemulzionim postupkom. Dodatkom različitih količina aditiva HCl tokom sinteze (od 0 do 35 %) ispitan je uticaj dodatka aditiva na oblik nanočestica. Pri optimalnoj koncentraciji od 25% HCl sintetizovane su nanočestice kubnog oblika. Četiri sintetizovana katalizatora okarakterisana su termogravimetrijskom analizom (TGA), rendgenskom difrakcijom X-zraka (XRD), transmisionom elektronskom mikroskopijom (TEM) i transmisionom elektronskom mikroskopijom visoke rezolucije (HRTEM). Ovim metodama potvrđena je promena oblika nanočestica izazvana dodatkom aditiva, utvrđen je udeo metala u katalizatoru (20%) i određena je veličina čestica, koja raste sa porastom udela aditiva (od 4 do 8 nm prema TEM analizi). Elektrohemijskom karakterizacijom i ispitivanjem reakcija oksidacije mravlje kiseline i metanola na ovim katalizatorima, ustanovljeno je da katalizator sa nanočesticama kubnog oblika, i većim udelom preferencijalno orijentisanih ravni (100) pokazuje bolju aktivnost za ove reakcije od katalizatora Pt sa konvencionalnim kub-oktaedarskim česticama. Razlog tome leži u lakšem uklanjanju adsorbovanog CO sa površine nanočestica sa zastupljenim kubnim oblikom.