Selectivity and Reactivity of Pd-Rich PdGa Surfaces toward Selective Hydrogenation of Acetylene: Interplay of Surface Roughness and Ensemble Effect

Recent experiments have shown that PdGa nanocrystallites act as highly selective and reactive catalyst for selective hydrogenation of acetylene to ethylene. Motivated by these experimental results we have studied the mechanism and energetics of the above reaction on low indexed (100) and (110) PdGa surfaces using first-principles density functional theory based calculations. We find that the energetically favorable (100) surface created by cleaving the crystal in the less dense region shows reasonably good selectivity and high reactivity. The reactivity on this surface is comparable to that observed on Pd(111) surfaces. Since this surface termination is stable over a wide range of Ga chemical potential and hence is likely to occupy a substantial fraction of the surface area of PdGa nanocrystallites, we suggest this termination is responsible for the selectivity and reactivity exhibited by PdGa. In contrast to other surfaces where hydrogen adsorption and dissociation is followed by acetylene adsorption and hydrogenation, on this surface we identify a novel reaction mechanism in which hydrogen dissociation occurs in the presence of acetylene. A careful analysis of the factors determining the selectivity shows that selectivity results due to an interplay between surface roughness and chemical nature of the reactive ensemble

Cadmium Vacancy Minority Defects as Luminescence Centers in Size and Strain Dependent Photoluminescence Shifts in CdS Nanotubes

We have studied the absorbance and photoluminescence properties of cadmium sulfide nanotubes with overall size beyond the quantum confinement regime. While the absorption spectra are unaffected by the change in size there is an anomalous red-shift in the photoluminescence spectra with increase in size. Using density functional calculations, we have identified that the shift in the emission peak of the photoluminescence spectra is a result of the interplay between Cd vacancies on the surface of these nanotubes and the crystalline strain which was incorporated in these nanotubes during their growth process. Most importantly, our results show that rather than the defect concentration, it is the nature of the defect which plays a crucial role in determining the optical properties of these nanotubes. For this particular case of CdS nanotubes we find that though S interstitials are the most abundant ones, however, it is the Cd vacancies with second lowest formation energies which significantly affect the photoluminesence spectra

Fluxionality of Au Clusters at Ceria Surfaces during CO Oxidation: Relationships among Reactivity, Size, Cohesion, and Surface Defects from DFT Simulations

Density functional theory (DFT) calculations are used to identify correlations among reactivity, structural stability, cohesion, size, and morphology of small Au clusters supported on stoichiometric and defective CeO₂(111) surfaces. Molecular adsorption significantly affects the cluster morphology and in some cases induces cluster dissociation into smaller particles and deactivation. We present a thermodynamic rationalization of these effects and identify Au₃ as the smallest stable nanoparticle that can sustain catalytic cycles for CO oxidation without incurring structural/morphological changes that jeopardize its reactivity. The proposed Mars van Krevelen reaction pathway displays a low activation energy, which we explain in terms of the cluster fluxionality and of labile CO₂ intermediates at the Au/ceria interface. These findings shed light on the importance of cluster dynamics during reaction and provide key guidelines for engineering more efficient metal–oxide interfaces in catalysis

Hydrogenation of Ferrimagnetic Graphene on a Co Surface: Significant Enhancement of Spin Moments by C–H Functionality

Using ab initio density functional theory, we present a novel way of simultaneously enhancing the induced magnetic moment and opening up the band gap of a graphene sheet supported on ferromagnetic transition metal surface. Specifically, we have demonstrated that by simply hydrogenating graphene supported on ferromagnetic Co surface at saturation coverage, (i) there is a <b>six</b>-fold increase in the magnitude of the induced magnetic moment compared with the pristine graphene on the Co surface and (ii) for both the spin-up and the spin-down channels there is a band gap opening at the K-point of the Brillouin zone

Microscopic Insights into Hydrogen Permeation Through a Model PdCu Membrane from First-Principles Investigations

Palladium-based alloys are commonly used in industry as a membrane material for the purification of hydrogen. In this work, we report a systematic theoretical study of all of the processes associated with permeation of H through a model PdCu membrane. The surface of the membrane is modeled using the most stable (110) surface. Our calculations show that the nuclear quantum effects due to the light mass of the H atom can significantly affect the stability and reaction rates. On the basis of a microkinetic model of the permeation process, we find that the permeation can be limited by diffusion of H in the membrane bulk or the reassociation of atomic H to form H₂ on the permeate side of the membrane depending on the operation temperature and membrane thickness. At typical operating conditions, for membranes thinner than 0.5 μm, the permeation at high temperature (<i>T</i> > 500 K) is limited by surface processes, whereas at lower temperatures it can be either diffusion-limited or reassociation-limited

Microscopic Insights into Hydrogen Permeation Through a Model PdCu Membrane from First-Principles Investigations

Palladium-based alloys are commonly used in industry as a membrane material for the purification of hydrogen. In this work, we report a systematic theoretical study of all of the processes associated with permeation of H through a model PdCu membrane. The surface of the membrane is modeled using the most stable (110) surface. Our calculations show that the nuclear quantum effects due to the light mass of the H atom can significantly affect the stability and reaction rates. On the basis of a microkinetic model of the permeation process, we find that the permeation can be limited by diffusion of H in the membrane bulk or the reassociation of atomic H to form H₂ on the permeate side of the membrane depending on the operation temperature and membrane thickness. At typical operating conditions, for membranes thinner than 0.5 μm, the permeation at high temperature (<i>T</i> > 500 K) is limited by surface processes, whereas at lower temperatures it can be either diffusion-limited or reassociation-limited

Computational Insight into a Gold(I) N-Heterocyclic Carbene Mediated Alkyne Hydroamination Reaction

A gold­(I) N-heterocyclic carbene (NHC) complex mediated hydroamination of an alkyne has been modeled using density functional theory (DFT) study. In this regard, alkyne and amine coordination pathways have been investigated for the hydroamination reaction between two representative substrates, namely, MeCCH and PhNH₂, catalyzed by a gold­(I) NHC based (NHC)­AuCl-type precatalyst, namely, [1,3-dimethylimidazol-2-ylidene]­gold chloride. The amine coordination pathway displayed a lower activation barrier than the alkyne coordination pathway. The catalytic cycle is proposed to proceed via a crucial proton-transfer step occurring between the intermediates [(NHC)­AuCHCMeNH₂Ph]⁺ (<b>D</b>) and [(NHC)­Au­(PhNHMeCCH₂)]⁺ (<b>E</b>), the activation barrier of which was found to be significantly reduced by a proton relay mechanism process assisted by the presence of any adventitious H₂O molecule or even by any of the reacting PhNH₂ substrates. The final hydroaminated enamine product, PhNHMeCCH₂, was further seen to be stabilized in its tautomeric imine form PhNCMe₂

Computational Insight Into the Hydroamination of an Activated Olefin, As Catalyzed by a 1,2,4-Triazole-Derived Nickel(II) N‑Heterocyclic Carbene Complex

A density functional theory (DFT) investigation performed at the B3LYP/TZVP//B3LYP/6-31G­(d)-LANL2DZ level of theory on the hydroamination of dimethylamine (Me₂NH) on an activated olefin (namely, acrylonitrile (CH₂CHCN)), as catalyzed by a 1,2,4-triazol based nickel­(II) N-heterocyclic carbene complex (namely, [1,4-dimethyl-1,2,4-triazole-5-ylidene]₂ nickel dichloride) revealed that the olefin coordination pathway is favorable over the amine coordination pathway, although the initial olefin coordination step is higher in energy than the initial amine coordination step. Significantly enough, the reaction involved a crucial 1,3-proton transfer step between the resonance intermediates, i.e., the C-bound [(NHC)₂Ni­(CH­(CN)­CH₂NHMe₂)]⁺ (<b>D</b>) species or N-bound [(NHC)₂Ni­(NCCHCH₂NHMe₂)]⁺ (<b>E</b>) species and the intermediate [(NHC)₂Ni­(NCCH₂CH₂NMe₂)]⁺ (<b>F</b>), depicting the cleavage of a N–H bond and the formation of a C–H bond facilitated by a water-assisted/amine-assisted proton shuttle. Overall, among the various pathways explored, the lowest energy pathway involved alkene coordination, followed by an amine-assisted 1,3-proton transfer step

Cyanosilylation of Aromatic Aldehydes by Cationic Ruthenium(II) Complexes of Benzimidazole-Derived O‑Functionalized N‑Heterocyclic Carbenes at Ambient Temperature under Solvent-Free Conditions

A series of ruthenium complexes, namely, [{1-(<i>N</i>-R₁-2-acetamido)-3-(R₂)-benzimidazol-2-ylidine}­Ru­(<i>p</i>-cymene)­Cl]­Cl, where {R₁ = 2,6-(<i>i</i>-Pr)₂C₆H₃, R₂ = <i>i</i>-Pr (<b>1c</b>); R₁ = 2,6-(<i>i</i>-Pr)₂C₆H₃, R₂ = Et (<b>2c</b>); R₁ = 2,4,6-(CH₃)₃C₆H₂, R₂ = Et (<b>3c</b>)}, of benzimidazole-derived N/O-functionalized N-heterocyclic carbene ligands successfully carried out the cyanosilylation reaction of aromatic aldehydes and heteroaryl aldehydes with trimethylsilyl cyanide, providing good to excellent yields (ca. 60–95%) at room temperature under solvent-free condition. The ruthenium (<b>1</b>–<b>3</b>)<b>c</b> complexes were synthesized from the silver (<b>1</b>–<b>3</b>)<b>b</b> analogues in ca. 67–80% yields. The silver (<b>1</b>–<b>3</b>)<b>b</b> complexes exhibited an argentophilic <i>d</i>¹⁰···<i>d</i>¹⁰ interaction in its dinuclear macrometallacyclic motif, as observed by a short Ag···Ag contact of 3.1894(3) Å in single-crystal X-ray diffraction studies for a representative silver complex <b>2b</b> and also in photoluminescence studies that showed characteristic emission band(s) at ca. 534–536 nm in the CHCl₃ solution and at ca. 482–487 and 530–533 nm in the solid state

Cyanosilylation of Aromatic Aldehydes by Cationic Ruthenium(II) Complexes of Benzimidazole-Derived O‑Functionalized N‑Heterocyclic Carbenes at Ambient Temperature under Solvent-Free Conditions

A series of ruthenium complexes, namely, [{1-(<i>N</i>-R₁-2-acetamido)-3-(R₂)-benzimidazol-2-ylidine}­Ru­(<i>p</i>-cymene)­Cl]­Cl, where {R₁ = 2,6-(<i>i</i>-Pr)₂C₆H₃, R₂ = <i>i</i>-Pr (<b>1c</b>); R₁ = 2,6-(<i>i</i>-Pr)₂C₆H₃, R₂ = Et (<b>2c</b>); R₁ = 2,4,6-(CH₃)₃C₆H₂, R₂ = Et (<b>3c</b>)}, of benzimidazole-derived N/O-functionalized N-heterocyclic carbene ligands successfully carried out the cyanosilylation reaction of aromatic aldehydes and heteroaryl aldehydes with trimethylsilyl cyanide, providing good to excellent yields (ca. 60–95%) at room temperature under solvent-free condition. The ruthenium (<b>1</b>–<b>3</b>)<b>c</b> complexes were synthesized from the silver (<b>1</b>–<b>3</b>)<b>b</b> analogues in ca. 67–80% yields. The silver (<b>1</b>–<b>3</b>)<b>b</b> complexes exhibited an argentophilic <i>d</i>¹⁰···<i>d</i>¹⁰ interaction in its dinuclear macrometallacyclic motif, as observed by a short Ag···Ag contact of 3.1894(3) Å in single-crystal X-ray diffraction studies for a representative silver complex <b>2b</b> and also in photoluminescence studies that showed characteristic emission band(s) at ca. 534–536 nm in the CHCl₃ solution and at ca. 482–487 and 530–533 nm in the solid state