Dehydrogenation of benzene on Pt(111) surface

Synthesis and Characterization of Ligand-Linked Pt Nanoparticles: Tunable, Three-Dimensional, Porous Networks for Catalytic Hydrogen Sensing

Porous networks of Pt nanoparticles interlinked by bifunctional organic ligands have shown high potential as catalysts in micro-machined hydrogen gas sensors. By varying the ligand among p-phenylenediamine, benzidine, 4,4''-diamino-p-terphenyl, 1,5-diaminonaphthalene, and trans-1,4-diaminocyclohexane, new variants of such networks were synthesized. Inter-particle distances within the networks, determined via transmission electron microscopy tomography, varied from 0.8 to 1.4 nm in accordance with the nominal length of the respective ligand. While stable structures with intact and coordinatively bonded diamines were formed with all ligands, aromatic diamines showed superior thermal stability. The networks exhibited mesoporous structures depending on ligand and synthesis strategy and performed well as catalysts in hydrogen gas microsensors. They demonstrate the possibility of deliberately tuning micro- and mesoporosity and thereby transport properties and steric demands by choice of the right ligand also for other applications in heterogeneous catalysis.

Towards highly active Pd/CeO2 for alkene hydrogenation by tuning Pd dispersion and surface properties of the catalysts

Extensive applications of noble metals as heterogeneous catalysts are limited by their global reserve scarcity and exorbitant price. Identifying the intrinsic active nature of a catalyst benefits the designing of catalysts with trace amounts of noble metals; these catalysts display better or comparable overall catalytic efficiency than their heavily loaded counterparts. Herein, systematic studies on Pd dispersion and surface properties of a series of Pd/CeO₂ catalysts for styrene hydrogenation showed that high Pd dispersion and surface abundant defects of the catalysts are essential to realize superior activity. Highly dispersed subnanometric Pd clusters on porous nanorods of ceria with a large surface Ce³⁺ fraction of 27.4%, high Pd dispersion of 73.6%, and low Pd loading of 0.081 wt% delivered a very large turnover frequency of 103 233 h^-1 based on each exposed Pd atom for styrene hydrogenation at 1.0 MPa H₂ and 30 °C. Experimental data, kinetic analysis, and density functional theory calculations revealed that the highly dispersed Pd shows a low affinity for styrene and provides more exposed Pd sites for hydrogen activation. The surface abundant defects (oxygen vacancy) of Pd/CeO₂ catalysts can enrich the electron density of Pd, improve its capability for H₂ dissociation and lower the affinity of styrene for Pd.

Molecular Lifting, Twisting, and Curling during Metal-Assisted Polycyclic Hydrocarbon Dehydrogenation

The atomistic understanding of the dissociation mechanisms for large molecules adsorbed on surfaces is still a challenge in heterogeneous catalysis. This is especially true for polycyclic aromatic hydrocarbons, which represent an important class of organic compounds used to produce novel graphene-based architectures. Here, we show that coronene molecules adsorbed on Ir(111) undergo major conformational changes during dissociation. They first tilt upward with respect to the surface, still keeping their planar configuration, and subsequently experience a rotation, which changes the molecular axis orientation. Upon lifting, the internal C-C strain is initially relieved; as the dehydrogenation proceeds, the molecules experience a progressive increase in the average interatomic distance and gradually settle to form dome-shaped nanographene flakes. Our results provide important insight into the complex mechanism of molecular breakup, which could have implications in the synthesis of new carbon-based nanostructured materials.

Decomposition of multilayer benzene and n-hexane films on vanadium

Reactions of multilayer hydrocarbon films with a polycrystalline V substrate have been investigated using temperature-programmed desorption and time-of-flight secondary ion mass spectrometry. Most of the benzene molecules were dissociated on V, as evidenced by the strong depression in the thermal desorption yields of physisorbed species at 150 K. The reaction products dehydrogenated gradually after the multilayer film disappeared from the surface. Large amount of oxygen was needed to passivate the benzene decomposition on V. These behaviors indicate that the subsurface sites of V play a role in multilayer benzene decomposition. Decomposition of the n-hexane multilayer films is manifested by the desorption of methane at 105 K and gradual hydrogen desorption starting at this temperature, indicating that C-C bond scission precedes C-H bond cleavage. The n-hexane dissociation temperature is considerably lower than the thermal desorption temperature of the physisorbed species (140 K). The n-hexane multilayer morphology changes at the decomposition temperature, suggesting that a liquid-like phase formed after crystallization plays a role in the low-temperature decomposition of n-hexane.

Guaiacol hydrodeoxygenation mechanism on Pt(111): insights from density functional theory and linear free energy relations

Density functional theory is used to study the adsorption of guaiacol and its initial hydrodeoxygenation (HDO) reactions on Pt(111). Previous Brønsted-Evans-Polanyi (BEP) correlations for small open-chain molecules are inadequate in estimating the reaction barriers of phenolic compounds except for the side group (methoxy) carbon-dehydrogenation. New BEP relations are established using a select group of phenolic compounds. These relations are applied to construct a potential-energy surface of guaiacol-HDO to catechol. Analysis shows that catechol is mainly produced via dehydrogenation of the methoxy functional group followed by the CHx (x<3) removal of the functional group and hydrogenation of the ring carbon, in contrast to a hypothesis of a direct demethylation path. Dehydroxylation and demethoxylation are slow, implying that phenol is likely produced from catechol but not through its direct dehydroxylation followed by aromatic carbon-ring hydrogenation.

Adsorption of aromatics on the (111) surface of PtM and PtM3 (M = Fe, Ni) alloys

Tuning the Pt/M ratio tailors the adsorption characteristics of aromatics, similar to Pd/Fe systems, with applications for hydrodeoxygenation catalysis.

Integrated X-ray photoelectron spectroscopy and DFT characterization of benzene adsorption on Pt(111), Pt(355) and Pt(322) surfaces

We systematically investigate the adsorption of benzene on Pt(111), Pt(355) and Pt(322) surfaces by high-resolution X-ray photoelectron spectroscopy (XPS) and first-principle calculations based on density functional theory (DFT), including van der Waals corrections. By comparing the adsorption energies at 1/9, 1/16 and 1/25 ML on Pt(111), we find significant lateral interactions exist between the benzene molecules at 1/9 ML. The adsorption behavior on Pt(355) and Pt(322) is very different. While on Pt(355) a step species is clearly identified in the C 1s spectra at low coverages followed by occupation of a terrace species at high coverages, no evidence for a step species is found on Pt(322). These different adsorption sites are confirmed by extensive DFT calculations, where the most favorable adsorption configurations on Pt(355) and Pt(322) are also found to vary: a highly distorted across the step molecule is found on Pt(355) while a less distorted configuration adjacent to the step molecule is deduced for Pt(322). The theoretically proposed C 1s core level binding energy shifts between these most favorable configurations and the terrace species are found to correlate well with experiment: for Pt(355), two adsorbate states are found, separated by ~0.4 eV in XPS and 0.3 eV in the calculations, in contrast to only one state on Pt(322).