The effect of solution pH on the oscillatory electro-oxidation of methanol

Unraveling the Dynamic Processes of Methanol Electrooxidation at Isolated Rhodium Sites by In Situ Electrochemical Scanning Tunneling Microscopy

Materials with isolated single-atom Rh-N4 sites are emerging as promising and compelling catalysts for methanol electrooxidation. Herein, we carried out an in situ electrochemical scanning tunneling microscopy (ECSTM) investigation of the dynamic processes of methanol absorption and catalytic conversion in the rhodium octaethylporphyrin (RhOEP)-catalyzed methanol oxidation reaction at the molecular scale. The high-contrast RhOEP-CH3OH complex formed by methanol adsorption was visualized distinctly in the STM images. The Rh-C adsorption configuration of methanol on isolated rhodium sites was identified on the basis of a series of control experiments and theoretical simulation. The adsorption energy of methanol on RhOEP was obtained from quantitative analysis. In situ ECSTM experiments present an explicit description of the transformation of the intermediate species in the catalytic process. By qualitatively evaluating the rate constants of different stages in the reaction at the microscopic level, we considered the CO transformation/desorption as the critical step for determining the reaction dynamics. Methanol adsorption was found to be correlated with RhOEP oxidation in the initial stage of the reaction, and the dynamic information was revealed unambiguously by in situ potential step experiments. This work provides microscopic results for the catalytic mechanism of Rh-N4 sites for methanol electrooxidation, which is instructive for the rational design of the high-performance catalyst.

Oscillatory dynamics during the methanol electrooxidation reaction on Pt(111)

Despite several papers describing the oscillatory methanol electrooxidation reaction (OMOR) catalyzed by polycrystalline Pt, these dynamic instabilities are less explored on single crystalline surfaces. Herein, we observed and mapped for the first time the OMOR on Pt(111) in non-adsorbing anion solutions as well as in the presence of small amounts of sulfate anions. Period 1 oscillations with oscillation frequencies from 1.2 to 2.0 Hz were observed for methanol concentrations higher than 1.0 M, with no evolution to more complex patterns. These oscillations occur in the potential range in which PtOH is partially covering the surface without irreversible oxidation processes. Small changes in both the mean potential (E_m) and the poisoning rate along the time-series were observed, the so-called drift, and were explained in terms of the accumulation of intermediates at the interface. The presence of sulfate strongly inhibits the OMOR, and the results are discussed in terms of sulfate adlayer formation on {111} domains.

Engineering the morphology of palladium nanostructures to tune their electrocatalytic activity in formic acid oxidation reactions

Pd nanomaterials can be cheaper alternative catalysts for the electrocatalytic formic acid oxidation reaction (FAOR) in fuel cells. The size and shape of the nanoparticles and crystal engineering can play a crucial role in enhancing the catalytic activities of Pd nanostructures. A systematic study on the effect of varying the morphology of Pd nanostructures on their catalytic activities for FAOR is reported here. Palladium nanoparticles (Pd_0D), nanowires (Pd_1D) and nanosheets (Pd_2D) could be synthesized by using swollen liquid crystals as 'soft' templates. Swollen liquid crystals are lyotropic liquid crystals that are formed from a quaternary mixture of a surfactant, cosurfactant, brine and Pd salt dissolved in oil. Pd_1D nanostructures exhibited 2.7 and 19 fold higher current density than Pd_0D and Pd_2D nanostructures in the FAOR. The Pd_1D nanostructure possess higher electrochemically active surface area (ECSA), better catalytic activity, stability, and lower impedance to charge transfer when compared to the Pd_0D and Pd_2D nanostructures. The presence of relatively higher amounts of crystal defects and enriched (100) crystal facets in the Pd_1D nanostructure were found to be the reasons for their enhanced catalytic activities.

Multivariate statistical analysis of chemical and electrochemical oscillators for an accurate frequency selection

The effect of experimental parameters on the frequency of chemical oscillators has been systematically studied since the first observations of clock reactions. The approach is mainly based on univariate changes in one specific parameter while others are kept constant. The frequency is then monitored and the effect of each parameter is discussed separately. This type of analysis, however, does not take into account the multiple interactions among the controllable parameters and the synergic responses on the oscillation frequency. We have carried out a multivariate statistical analysis of chemical (BZ-ferroin catalyzed reaction) and electrochemical (Cu/Cu₂O cathodic deposition) oscillators and identified the contributions of the experimental parameters on frequency variations. The BZ reaction presented a strong dependence on the initial concentration of sodium bromate and temperature, resulting in a frequency increase. The concentration of malonic acid, the organic substrate, affects the system but with lower intensity compared with the combination of sodium bromate and temperature. On the other hand, the Cu/Cu₂O electrochemical oscillator was shown to be less sensitive to changes in the temperature. The applied current density and pH were the two parameters which most perturbed the system. Interestingly, the frequency behaved nonmonotonically with a quadratic dependence. The multivariate analysis of both oscillators exhibited significant differences - while the homogenous oscillator displayed a linear dependence with the factors, the heterogeneous one revealed a more complex dependence with quadratic terms. Our results may contribute, for instance, in the synthesis of self-organized materials in which an accurate frequency selection is required and, depending on its value, different physicochemical properties are obtained.