Chun-Ran Chang

Ultrathin rhodium nanosheets

Despite significant advances in the fabrication and applications of graphene-like materials, it remains a challenge to prepare single-layered metallic materials, which have great potential applications in physics, chemistry and material science. Here we report the fabrication of poly(vinylpyrrolidone)-supported single-layered rhodium nanosheets using a facile solvothermal method. Atomic force microscope shows that the thickness of a rhodium nanosheet is <4 Å. Electron diffraction and X-ray absorption spectroscopy measurements suggest that the rhodium nanosheets are composed of planar single-atom-layered sheets of rhodium. Density functional theory studies reveal that the single-layered Rh nanosheet involves a δ-bonding framework, which stabilizes the single-layered structure together with the poly(vinylpyrrolidone) ligands. The poly(vinylpyrrolidone)-supported single-layered rhodium nanosheet represents a class of metallic two-dimensional structures that might inspire further fundamental advances in physics, chemistry and material science.

High Catalytic Activity and Chemoselectivity of Sub-nanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes

Sub-nanometric Pd clusters on porous nanorods of CeO2 (PN-CeO2) with a high Pd dispersion of 73.6% exhibit the highest catalytic activity and best chemoselectivity for hydrogenation of nitroarenes to date. For hydrogenation of 4-nitrophenol, the catalysts yield a TOF of ∼44059 h(-1) and a chemoselectivity to 4-aminophenol of >99.9%. The superior catalytic performance can be attributed to a cooperative effect between the highly dispersed sub-nanometric Pd clusters for hydrogen activation and unique surface sites of PN-CeO2 with a high concentration of oxygen vacancy for an energetically and geometrically preferential adsorption of nitroarenes via nitro group. The high concentration of surface defects of PN-CeO2 and large Pd dispersion contribute to the enhanced catalytic activity for the hydrogenation reactions. The high chemoselectivity is mainly governed by the high Pd dispersion on the support. The catalysts also deliver high catalytic activity and selectivity for nitroaromatics with various reducible substituents into the corresponding aminoarenes.

Design of N-Coordinated Dual-Metal Sites: A Stable and Active Pt-Free Catalyst for Acidic Oxygen Reduction Reaction

We develop a host-guest strategy to construct an electrocatalyst with Fe-Co dual sites embedded on N-doped porous carbon and demonstrate its activity for oxygen reduction reaction in acidic electrolyte. Our catalyst exhibits superior oxygen reduction reaction performance, with comparable onset potential (Eonset, 1.06 vs 1.03 V) and half-wave potential (E1/2, 0.863 vs 0.858 V) than commercial Pt/C. The fuel cell test reveals (Fe,Co)/N-C outperforms most reported Pt-free catalysts in H2/O2 and H2/air. In addition, this cathode catalyst with dual metal sites is stable in a long-term operation with 50 000 cycles for electrode measurement and 100 h for H2/air single cell operation. Density functional theory calculations reveal the dual sites is favored for activation of O-O, crucial for four-electron oxygen reduction.

Theoretical studies on gas-phase reactions of sulfuric acid catalyzed hydrolysis of formaldehyde and formaldehyde with sulfuric acid and H2SO4···H2O complex.

The gas-phase reactions of sulfuric acid catalyzed hydrolysis of formaldehyde and formaldehyde with sulfuric acid and H2SO4···H2O complex are investigated employing the high-level quantum chemical calculations with M06-2X and CCSD(T) theoretical methods and the conventional transition state theory (CTST) with Eckart tunneling correction. The calculated results show that the energy barrier of hydrolysis of formaldehyde in gas phase is lowered to 6.09 kcal/mol from 38.04 kcal/mol, when the sulfuric acid is acted as a catalyst at the CCSD(T)/aug-cc-pv(T+d)z//M06-2X/6-311++G(3df,3pd) level of theory. Furthermore, the rate constant of the sulfuric acid catalyzed hydrolysis of formaldehyde combined with the concentrations of the species in the atmosphere demonstrates that the gas-phase hydrolysis of formaldehyde of sulfuric acid catalyst is feasible and could be of great importance for the sink of formaldehyde, which is in previously forbidden hydrolysis reaction. However, it is shown that the gas-phase reactions of formaldehyde with sulfuric acid and H2SO4···H2O complex lead to the formation of H2C(OH)OSO3H, which is of minor importance in the atmosphere.

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

Methanol steam reforming (MSR) is a promising method for large-scale production of hydrogen. While significant efforts have been devoted to the reaction mechanisms, the function of water except serving as a reactant is little known. Here we present a density functional study of the catalytic role of water in the whole MSR process on the PdZn(111) surface. It is shown that water not only influences the adsorption of reaction species but also the kinetics of the elementary steps of MSR. The calculated adsorption energies of reaction species including CH3OH*, CH3O*, H2COOH*, HCOOH* and HCOO* increase by some 0.10–0.30 eV in the presence of co-adsorbed water. Importantly, water is found to substantially favor six dehydrogenation steps of MSR through lowering the activation barriers by 0.25–0.46 eV without being consumed, providing compelling evidence for the catalytic role of water in MSR. Depending on the manner in which water participates in the reactions, the catalytic mechanisms of water are classified into two categories, the solvation effect and the H-transfer effect. For the dehydrogenation steps involving O–H bond cleavage, both of the two mechanisms are applicable and show a slight difference in reducing the reaction barriers. However, for the dehydrogenation steps involving C–H bond cleavage, the catalytic function of water can only be realized through the solvation effect. These results uncover the catalytic role of water in MSR and are helpful in understanding the water effect in other chemical transformations.

The promotional role of water in heterogeneous catalysis: mechanism insights from computational modeling

Water is the key to life in our planet. As one of the most abundant resources on the earth, water may play important roles in chemical reactions in addition to being a reaction medium. In this review, we highlight recent advances in experimental observations and mechanistic understanding of the promotional role of water in chemical reactions, with an emphasis on the essential effects of water in heterogeneous catalysis. As water may exist in molecular state or dissociate to hydroxyl (OH) and hydrogen (H) species on catalyst surfaces, we have outlined the roles of water from three aspects: (1) the promotional role of molecular water including the solvation‐like effect and water‐mediated H‐transfer, (2) the promotional role of OH/OH − and H/H+ species, and (3) some miscellaneous effects of water, such as water‐assisted carbon removal, surface reconstruction, and active sites blocking. The results discussed here provide a fundamental understanding of the promotional role of water in heterogeneous reactions and may inspire the theoretical studies of water effects on other fields of chemistry and atmospheric science as well. WIREs Comput Mol Sci 2016, 6:679–693. doi: 10.1002/wcms.1272

Hydrogenation of molecular oxygen to hydroperoxyl: An alternative pathway for O2 activation on nanogold catalysts

Activation of molecular O2 is the most critical step in gold-catalyzed oxidation reactions; however, the underlying mechanisms of this process remain under debate. In this study, we propose an alternative O2 activation pathway with the assistance of hydrogen-containing substrates using density functional theory. It is demonstrated that the co-adsorbed H-containing substrates (R–H) not only enhance the adsorption of O2, but also transfer a hydrogen atom to the adjacent O2, leading to O2 activation by its transformation to a hydroperoxyl (OOH) radical species. The activation barriers of the H-transfer from 16 selected R–H compounds (H2O, CH3OH, NH2CHCOOH, CH3CH=CH2, (CH3)2SiH2, etc.) to the co-adsorbed O2 are lower than 0.50 eV in most cases, indicating the feasibility of the activation of O2 via OOH under mild conditions. The formed OOH oxidant, with an increased O–O bond length of ~1.45 Å, either participates directly in oxidation reactions through the end-on oxygen atom, or dissociates into atomic oxygen and hydroxyl (OH) by crossing a fairly low energy barrier of 0.24 eV. Using CO oxidation as a probe, we have found that OOH has superior activity than activated O2 and atomic oxygen. This study reveals a new pathway for the activation of O2, and may provide insight into the oxidation catalysis of nanosized gold.

Solid frustrated-Lewis-pair catalysts constructed by regulations on surface defects of porous nanorods of CeO2

Identification on catalytic sites of heterogeneous catalysts at atomic level is important to understand catalytic mechanism. Surface engineering on defects of metal oxides can construct new active sites and regulate catalytic activity and selectivity. Here we outline the strategy by controlling surface defects of nanoceria to create the solid frustrated Lewis pair (FLP) metal oxide for efficient hydrogenation of alkenes and alkynes. Porous nanorods of ceria (PN-CeO2) with a high concentration of surface defects construct new Lewis acidic sites by two adjacent surface Ce3+. The neighbouring surface lattice oxygen as Lewis base and constructed Lewis acid create solid FLP site due to the rigid lattice of ceria, which can easily dissociate H–H bond with low activation energy of 0.17 eV.

Theoretical study on the leaching of palladium in a CO atmosphere

We studied computationally the leaching of palladium from the ideal surface Pd(111) and its various structural defects at different coverages of CO, using density functional calculations on slab models. Accordingly, the energy required for leaching of a single Pd atom from a bare surface is quite large, at least ∼270 kJ mol−1. In a CO atmosphere at low density, PdCO is predicted to be the leaching species; this process was calculated to require at least 225 kJ mol−1, somewhat less than the leaching of a bare metal adatom, 268 kJ mol−1. The energies required for either leaching process (at low CO density), yielding single Pd atoms or PdCO subcarbonyl, correlate in a linear fashion with the coordination number of the Pd center to be leached. At high CO coverage, leaching of Pd subcarbonyl species, Pd(CO)x (x = 2, 3), was calculated to be thermodynamically favorable in several cases, providing direct theoretical evidence for the feasibility of Pd leaching in a dense CO atmosphere. In a qualitative fashion, we also explored possible leaching mechanisms, comprising one or two steps.

Step-Edge Assisted Direct Linear Alkane Coupling

Direct coupling of alkanes via C-H activation of terminal methyl groups has acquired tremendous interests both scientifically and technically. Herein we present the results of linear alkane-coupling at the step edges of Cu surfaces at modulated temperatures. Combining the observations of scanning tunneling microscopy (STM) with density functional theory plus dispersion (DFT-D) calculations, we elucidate the mechanism of the reaction and demonstrate that the low activation barrier relies on heterogeneous catalysis at the upper step edges, where low-coordinated surface atoms are present. We further reveal the generality of the reaction, so that it can be applied on the step edges of different facets of surfaces.