Reaction of Formic Acid on Zn-Modified Pd(111)

Dehydrogenation and dehydration of formic acid over orthorhombic molybdenum carbide

•

Formic acid (HCOOH) adsorption on β-Mo₂C is exothermic and favours a configuration parallel to the surface.

•

Once adsorbed, thermodynamics favour cleavage of the H—COOH bond to form CO.

•

CO bonds strongly to the surface, potentially poisoning the catalyst.

•

Therefore, kinetics favour dehydrogenation mechanism with CO₂ continuously formed.

The dehydrogenation and dehydration of formic acid is investigated on the β-Mo₂C (100) catalyst surface using time independent density functional theory. The energetics of the two mechanisms are calculated, and the thermochemistry and kinetics are discussed using the transition state theory. Subsequently, microkinetic modelling of the system is conducted, considering the batch reactor model. The potential energy landscape of the reaction shows a thermodynamically favourable cleavage of H—COOH to form CO; however, the kinetics show that the dehydrogenation mechanism is faster and CO₂ is continuously formed. The effect of HCOOH adsorption on the surface is also analysed, in a temperature-programmed desorption, with the conversion proceeding at under 350 K and desorption of CO₂ is observed with a selectivity of about 100 %, in line with the experimental reports.

Carbon-Doping as Efficient Strategy for Improving Photocatalytic Activity of Polysilicon Supported Pd in Hydrogen Evolution from Formic Acid

Interest in cost-effective materials pushes researchers to the inexpensive and abundant semiconductors to use photons' energy for generating electrons and holes required for photocatalytic transformations. At the same time, polysilicon is one of the economic semiconductors with a disadvantage of high bandgap which could be solved by carbon-doping. We employed this strategy to the synthesis of carbon-doped polysilicon by a new approach starting from citric acid and methyltrimethoxysilane. The nanocomposite obtained was utterly characterized, and compared with bare polysilicon; increased UV-Vis absorbance and shift to higher wavelengths were the most notable characteristics of the synthesized catalyst. The carbon-doped polysilicon was modified with Pd nanoparticles to obtain a new heterogeneous photocatalyst for the formic acid degradation. The decomposition of formic acid was photocatalyzed by the obtained nanocomposite with a hydrogen production turnover frequency of up to 690 h-1. Moreover, it was demonstrated that the catalyst is stable and recyclable.

Effect of thermal treatment of Pd decorated Fe/C nanocatalysts on their catalytic performance for formic acid oxidation

The thermal treatment of bimetallic nanocatalysts plays an important role in determining their catalytic performance. Here tuning the surface oxidized metal species of bimetallic Pd-Fe electrocatalysts for the formic acid (FA) oxidation reaction is reported and a correlation between the surface oxidized metal species of the Pd-Fe nanoparticles and their catalytic activities is proposed. The structural details of the Pd-Fe/C catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy and high-sensitivity low-energy ion scattering (HS-LEIS). Cyclic voltammetry measurements demonstrated that the mass activity of the Pd-Fe nanoparticles with a molar ratio of Pd/Fe = 1/15 is about 7.4 times higher than that of Pd/C. This enhancement could be attributed to the synergistic effect between Pd(0) and Pd oxidized species on the surface of the Pd-Fe/C treated sample and electronic effects. This finding demonstrates the importance of surface oxidized metal species at the nanoscale in harnessing the true electrocatalytic potential of bimetallic nanoparticles and opens up strategies for the development of highly active bimetallic nanoparticles for electrochemical energy conversion.

Catalytic Reforming of Oxygenates: State of the Art and Future Prospects

This Review describes recent advances in the design, synthesis, reactivity, selectivity, structural, and electronic properties of the catalysts for reforming of a variety of oxygenates (e.g., from simple monoalcohols to higher polyols, then to sugars, phenols, and finally complicated mixtures like bio-oil). A comprehensive exploration of the structure-activity relationship in catalytic reforming of oxygenates is carried out, assisted by state-of-the-art characterization techniques and computational tools. Critical emphasis has been given on the mechanisms of these heterogeneous-catalyzed reactions and especially on the nature of the active catalytic sites and reaction pathways. Similarities and differences (reaction mechanisms, design and synthesis of catalysts, as well as catalytic systems) in the reforming process of these oxygenates will also be discussed. A critical overview is then provided regarding the challenges and opportunities for research in this area with a focus on the roles that systems of heterogeneous catalysis, reaction engineering, and materials science can play in the near future. This Review aims to present insights into the intrinsic mechanism involved in catalytic reforming and provides guidance to the development of novel catalysts and processes for the efficient utilization of oxygenates for energy and environmental purposes.

Where does methanol lose hydrogen to trigger steam reforming? A revisit of methanol dehydrogenation on the PdZn alloy model obtained from kinetic Monte Carlo simulations

Pd/ZnO is a promising catalyst studied for methanol steam reforming (MSR) and the 1 : 1 PdZn alloy is demonstrated to be the active component. It is believed that MSR starts from methanol dehydrogenation to methoxy. Previous studies of methanol dehydrogenation on the ideal PdZn(111) surface show that methanol adsorbs weakly on the PdZn(111) surface and it is hard for methanol to transform into methoxy because of the high dehydrogenation barrier, indicating that the catalyst model is not appropriate for investigating the first step of MSR. Using the model derived from our recent kinetic Monte Carlo simulations, we examined the process CH3OH → CH3O → CH2O → CHO → CO. Compared with the ideal model, methanol adsorbs much more strongly and the barrier from CH3OH → CH3O is much lower on the kMC model. On the other hand, the C-H bond breaking of CH3O, CH2O and CHO becomes harder. We show that co-adsorbed water is important for refreshing the active sites. The present study shows that the first MSR step most likely takes place on three-fold hollow sites formed by Zn atoms, and the inhomogeneity of the PdZn alloy may exert significant influences on reactions.

A theoretical study on the catalytic role of water in methanol steam reforming on PdZn(111)

The catalytic role of water in the methanol steam reforming process on the PdZn(111) surface is explored theoretically.

Nanoparticulate PdZn--pathways towards the synthetic control of nanosurface properties

This paper reports an in-depth structural investigation of PdZn nanoparticulates prepared over an entire compositional range. By using a combination of HRTEM, ICP-OES, EDX and XPS alongside PXRD, we are able to show how a liquid-type reduction process can be exploited to target different PdZn bimetallic structures while maintaining reproducibly narrow particle size distributions and average particle diameters of approximately 3 nm. Samples have been further analyzed by quantitative phase analysis of the Rietveld refined diffraction data, providing indications as to how variations in specific surface compositions are obtained when Zn is used as the alloying metal. The influence of nanolattice strain is investigated by geometric analysis of TEM data. Results suggest, in conjunction with previously published catalytic data, how different compositions of this specific bimetallic system may be exploited in catalytic processes to control substrate/product affinity. We thus demonstrate a new and simplified approach to PdZn bimetallics, which may offer novel perspectives for applications in industrial catalysis.

Density functional investigations of methanol dehydrogenation on Pd-Zn surface alloy

Methanol dehydrogenation on Pd(111) and various Pd-Zn surface alloy films supported on Pd(111) have been investigated using density functional method in combination with periodic slab models. Calculations show that compared to Pd(111) the interaction between CH(3)O and the films is enhanced, whereas that for CH(2)O and CHO is weakened. Zn in top layer facilitates the CH(3)O stability. At variance, the subsurface Zn reduces the interaction of CH(2)O and CHO with the substrate significantly. Addition of Zn promotes the O-H breaking of CH(3)OH and the dehydrogenation of CHO but hinders the dehydrogenation of CH(3)O and CH(2)O. Comparison shows that the third-layer Zn atoms have essentially no effect on the reactions. Our calculations demonstrate that the experimentally observed 360 K desorption peak cannot be originated from CH(2)O adsorbed at flat Pd-Zn alloy surfaces, and it is very likely that CH(2)O combines preferentially with some species before decomposing into CHO during methanol steam reforming if CH(2)O is an intermediate. Finally, we show that the newly proposed relationship between the energy of the initial states and transition states exhibits better correlation than the classical BEP relation.