Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Abstract

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state-of-the-art DFT methods and by a combination of supersonic molecular beams with high-resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH(3) and CH(2) are specially stabilized at sites located at particles edges and corners by an amount of 50-80 kJ mol(-1). This stabilization is caused by an enhanced activity of low-coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH(3) and CH(2) formation steps are endothermic on Pt(111) but markedly exothermic on Pt(79). An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol(-1) for first C-H bond scission and 40 kJ mol(-1) for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO(2) film on Cu(111). It is shown that CH(3) generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single-crystal surfaces in which CH(3) was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.

Methane dissociation on Pt(111): Searching for a specific reaction parameter density functional

“…Viñes et al [90] reported a TS energy of 1.06 eV for the RPBE functional and a computational setup similar to the one that we have used. Here, we have also obtained a larger E b value for the RPBE functional (about 1.17 eV, see Table 3).…”

“…[90] would include surface relaxation effects, which is not explicitly mentioned. In fact, the upward motion of the surface atom below the dissociating molecule has been shown to be able to lower the barrier by up to 0.15 eV for this system [49].…”

Alkyl chain length-dependent surface reaction of dodecahydro-N-alkylcarbazoles on Pt model catalysts

“…Consequently, we assign the first step at ∼350 K to dealkylation at highly reactive low-coordinated defect sites on the metal particles. Previous studies on the dehydrogenation of ethylene 28 and methane 29 show that these low-coordinated sites are responsible for a substantial decrease of the activation barrier of dehydrogenation, as compared to Pt(111). The literature states that hydrocarbon activation is promoted by the presence of defect sites due to a stronger binding of the adsorbates.…”

“…The literature states that hydrocarbon activation is promoted by the presence of defect sites due to a stronger binding of the adsorbates. 29 As a consequence, the kinetics and thermodynamics of dehydrogenation and degradation are strongly affected. We suggest that these findings can be transferred to the present system, which is reflected by a lowered temperature threshold of H x -NBC dealkylation on the supported Pt catalyst.…”

The Influence of Slight Protuberances in a Micro-Tube Reactor on Methane/Moist Air Catalytic Combustion

“…Moreover, natural gas is an attractive alternative fossil fuel which could reduce pollutant emissions and lower the energy dependence on fossil oil [14,15,22]. However, methane is considered the most stable hydrocarbon and is difficult oxidize, and its catalytic ignition requires relatively high temperatures even on expensive Pt-or Pd-based catalysts [23][24][25][26][27]. Therefore, preheating the inlet mixture gases [28], adding hydrogen to the methane fuel [29,30] and improved catalyst layout designs [31,32] have been proposed in succession to improve the combustion efficiency in micro-reactors.…”