The Effect Of Pd-doping And Pre-adsorbed Oxygen Atom On Pt Catalyzed Methanol Dehydrogenation: A DFT Study

Direct methanol fuel cell(DMFC) is one of the most attractive technologies due to its low pollution, high energy conversion rate and convenient operation. Platinum(Pt) has been widely used as the monometallic catalyst for the conversation of fuel in DMFCs. However, the cost and the availability of Pt catalyst are the obstacle for scaling up DMFCs. Based on the concerns of lowering down the catalyst cost, developing a multi-component catalyst by adding low-cost metals is needed. In this paper, we systematically studied the dehydrogenation of CH3 OH on Pt(111), Pd(111), Pt Pd3(111), and pre-adsorbed oxygen atom on Pt(111) by using density functional theory(DFT). The reaction mechanism of methanol decomposition was discussed at the atom level, which has a certain guiding significance to design new type of catalysts. Specific research contents and results are as follows:The dehydrogenation of methanol on P t(111) and Pd(111) were studied. The calculation results showed that: the different adsorption species had the different favorable adsorption sites, and the adsorption site was dependent on different metal surfaces. The adsorption energies on the metal Pd were larger than that on metal Pt. In both Pt(111) surface and Pd(111) surface, the path of CH3OHâ†' CH2 OH â†'CHOHâ†' CHOâ†' CO is the most competitive.Pt monolayer(PtML) which reduces Pt loading is considered to be an effective method. Therefore, we have calculated the behavior of methanol dehydrogenation generated CO and H2 on Pt Pd3(111) surface. CH3 OH, CH2 OH, CH3 O and CHO preferred to locate at the top site. COH, CO and H preferred to adsorb on the fcc site, while CH2 O and C HOH would choose the bridge site. Compare with the energy changes in the potential energy surface of these four possible paths, the initial O-H scission of CH3 OH is the most favorable reaction pathway than C-H breakage and the activation barrier was only 0.955 e V. The dehydrogenation mechanism from CH3 OH to CO via breading O-H bond is derived as CH3 OH â†' CH3 O â†' CH2 O â†' CHO â†' CO. The first dissociation step, CH3 OH converted into CH3 O and H is identified as the rate- limiting step. The theoretical results indicate that the proposed pathway for methanol dehydrogenation on Pt Pd3(111) surface are energetically favorable, and verified that the Pt Pd3 catalyst with Pt monolayer is a good candidate for methanol dissociation.Pre-adsorbed oxygen atom on Pt(111) surface has a significant impact on the methanol dehydrogenation reaction. O xygen atom may provide a more reliable means to generate CH3 O fragment. During the several possible routes to eliminate H in CH3 O to formaldehyde, and O atom can minimize the activation energy. For the behavior of formaldehyde on O-Pt(111) surface, the results indicate that part of the formaldehyde is oxidized to formic acid, the other is self-dehydrogenation into CHO; C HO substance will further dehydrogenate to the final produce CO and H2. CHO + Oâ†'CO + OH is identified as the rate- limiting step. The initial O-H scission of HCOOH to HCOO is a favorable reaction pathway than C-H breakage to COOH. HCOO will react to generate CO2 and H. With the addition of surface oxygen, almost all surface reaction steps have low barriers, suggesting that pre-adsorbed oxygen atom on Pt(111) could promote methanol dehydrogenation behavior.