Interface Effects in Hydrogen Elimination Reaction from Isopropanol by Ni₁₃ Cluster on θ‑Al₂O₃(010) Surface

We present results of theoretical investigation on catalytic hydrogen elimination from isopropanol (C₃H₈O) by free and θ-Al₂O₃(010)-supported Ni₁₃ cluster. The specific role played by the perimeter interface between the nickel cluster and alumina support is discussed. It is demonstrated that dehydrogenation of C₃H₈O on the free Ni₁₃ cluster is a two-step process with the first hydrogen elimination from the alcohol hydroxyl group, followed by C–H bond cleavage. Our calculations show that H elimination from OH group of C₃H₈O to Ni₁₃ cluster is the rate-determining step with the barrier of 0.95 eV, while the C–H bond cleavage requires overcoming the barrier of 0.41 eV. In the case of Ni₁₃ cluster supported on θ-Al₂O₃(010) the isopropanol molecule adsorbs on top of the surface Al atom in the close vicinity of the nickel cluster, which results in considerable decrease in barrier for H elimination due to formation of the complementary adsorption sites at the metal/support interface. It is demonstrated that intermediate formation of the Ni–C bond considerably promotes C–H bond cleavage. The described mechanism provides fundamental understanding of the process of the oxidant-free catalytic hydrogen elimination from alcohols on supported nickel clusters and can serve as a tool for rational design of novel type of nanocatalysts based on abundant noble-metal-free materials

Volcano-Curves for Dehydrogenation of 2‑Propanol and Hydrogenation of Nitrobenzene by SiO₂‑Supported Metal Nanoparticles Catalysts As Described in Terms of a d‑Band Model

To confirm whether the activity trends in multistep organic reactions can be understood in terms of the Hammer–Nørskov d-band model in combination with the linear energy relations, we studied correlations between the reaction rates for dehydrogenation and hydrogenation reactions and the position of the d-band center (ε_d) relative to the Fermi energy (<i>E</i>_F), the ε_d – <i>E</i>_F value, of various metal catalysts. SiO₂-supported metal (M = Ag, Cu, Pt, Ir, Pd, Rh, Ru, Ni, and Co) catalysts with the same metal loading (5 wt %) and similar metal particle size (8.9–11.7 nm) were prepared. The dehydrogenation of adsorbed 2-propanol in a flow of He and the hydrogenation of adsorbed nitrobenzene in a flow of H₂ were tested as model reactions of organic reactions on the metal surface. As a test reaction of H₂ dissociation on the surface, SiOH/SiOD exchange on the M/SiO₂ catalysts in a flow of D₂ is carried out. The liquid phase hydrogenation of nitrobenzene under 3.0 MPa of H₂ is adopted as an organic reaction under realistic conditions. Generally, the activities show volcano-type dependences on the ε_d – <i>E</i>_F value, indicating that the ε_d – <i>E</i>_F value is useful as a qualitative activity descriptor in heterogeneous catalysis of metal nanoparticles for multistep organic reactions

Surface Oxygen Atom as a Cooperative Ligand in Pd Nanoparticle Catalysis for Selective Hydration of Nitriles to Amides in Water: Experimental and Theoretical Studies

On the basis of an insight in surface science that Pd surfaces partially covered with oxygen adatoms (O_ad) show higher reactivity for water dissociation than clean Pd surfaces, we studied the effect of O_ad on the activity of carbon-supported palladium metal nanoparticle catalysts (Pd/C) for the selective hydration of nitriles to amides in water. A series of Pd/C with the same Pd loading (5 wt %) and a similar particle size (5.3–6.5 nm) but with different surface coverage of O_ad were prepared and characterized by various spectroscopic methods. The freshly H₂-reduced Pd/C shows no catalytic activity for hydration of acetonitrile, indicating that clean Pd metal surfaces are inactive. Air exposure of this catalyst under ambient conditions results in the formation of Pd metal NPs partially covered with O_ad, which act as effective and recyclable heterogeneous catalysts for selective hydration of various nitriles to the corresponding amides. Theoretical studies based on density functional theory calculations clarified a cooperative mechanism between metallic Pd and O_ad, in which O_ad as a Brønsted base site plays an important role in the dissociation of water via hydrogen bonding, and the mechanism is verified by kinetic results (activation energy, H₂O/D₂O kinetic isotope effect, Hammett slope). The mechanistic finding demonstrates a new design strategy of metal nanoparticle catalysts based on a molecular-level understanding of catalysis on oxygen-adsorbed metal surfaces

Mechanism of Low-Temperature CO Oxidation on Pt/Fe-Containing Alumina Catalysts Pretreated with Water

In a previous article (Catal. Commun. 2012, 17, 194), we reported that Pt/Fe-containing alumina catalysts pretreated with water could catalyze CO oxidation even below room temperature. To clarify the effect of the water pretreatment and the reaction mechanism of the novel catalytic system, in situ Fourier transform infrared (FT-IR), and X-ray absorption fine structure (XAFS) measurements during CO oxidation were conducted. From FT-IR measurements, it was revealed that the Pt surface of the catalyst was covered with CO and that the adsorbed CO molecules did not desorb easily, as in the case of conventional Pt/Al₂O₃ catalyst. Pt L_III XAFS results also suggested the presence of CO on the Pt surface during CO oxidation. Thresholds of Fe K X-ray absorption near-edge structure shifted with the change between oxidative (0.5% O₂/He) and reductive (1% CO/He) atmospheres, indicating that the Fe redox change Fe²⁺ ↔ Fe³⁺ can participate in the reaction. From the degree of the shifts and average Pt diameters derived from high-angle annular dark-field scanning transmission electron microscopy and metal dispersion measurements, it was concluded that PtNP/FeO_<i>x</i> boundaries were efficiently formed upon the water pretreatment. The enhanced reactivity of the water-pretreated catalyst can be attributed to the increased number of boundaries and Pt diameter

Substrate-Specific Heterogeneous Catalysis of CeO₂ by Entropic Effects via Multiple Interactions

Achieving complete substrate specificity through multiple interactions like an enzyme is one of the ultimate goals in catalytic studies. Herein, we demonstrate that multiple interactions between the CeO₂ surface and substrates are the origin of substrate-specific hydration of nitriles in water by CeO₂, which is exclusively applicable to the nitriles with a heteroatom (N or O) adjacent to the α-carbon of the CN group but is not applicable to the other nitriles. Kinetic studies reveal that CeO₂ reduces the entropic barrier (<i>T</i>Δ<i>S</i>‡) for the reaction of the former reactive substrate, leading to 10⁷-fold rate enhancement compared with the latter substrate. Density functional theory (DFT) calculations confirmed multiple interaction of the reactive substrate with CeO₂, as well as preferable approximation and alignment of the nitrile group of the substrate to the active OH group on CeO₂ surface. This can lead to the reduction of the entropic barrier. This is the first example of an entropy-driven substrate-specific catalysis of a nonporous metal oxide surface, which will provide a new design strategy for enzyme-inspired synthetic catalysts

Heterogeneous Ni Catalyst for Direct Synthesis of Primary Amines from Alcohols and Ammonia

This paper reports the synthesis of primary amines from alcohols and NH₃ by an Al₂O₃-supported Ni nanoparticle catalyst as the first example of heterogeneous and noble-metal-free catalytic system for this reaction without additional hydrogen sources under relatively mild conditions. Various aliphatic alcohols are tolerated, and turnover numbers were higher than those of Ru-based homogeneous catalysts. The catalyst was recoverable and was reused. The effects of the Ni oxidation states and the acid–base nature of support oxides on the catalytic activity are studied. It is clarified that the surface metallic Ni sites are the catalytically active species, and the copresence of acidic and basic sites on the support surface is also indispensable for this catalytic system

Toward Effective Utilization of Methane: Machine Learning Prediction of Adsorption Energies on Metal Alloys

The process employed to discover new materials for specific applications typically utilizes screening of large compound libraries. In this approach, the performance of a compound is correlated to the properties of elements referred to as descriptors. In the effort described below, we developed a simple and efficient machine learning (ML) model for predicting adsorption energies of CH₄ related species, namely, CH₃, CH₂, CH, C, and H on the Cu-based alloys. The developed ML model predicted the DFT-calculated adsorption energies with 12 descriptors, which are readily available values for the selected elements. The predictive accuracy of four regression methods (ordinary linear regression by least-squares (OLR), random forest regression (RFR), gradient boosting regression (GBR), and extra tree regression (ETR)) with different numbers of descriptors and different test-set/training-set ratios was quantitatively evaluated using statistical cross validations. Among four types of regression methods, we have found that ETR gave the best performance in predicting the adsorption energies with the average root mean squared errors (RMSEs) below 0.3 eV. Strikingly, despite its simplicity and low computational cost, this model can predict the adsorption energies on a range of Cu-based alloy models (46 in total number) as calculated by using DFT. In addition, we show the ML prediction for the differences in the adsorption energies of CH₃ and CH₂ on the same surface. This would be of great importance especially when designing the selective catalytic reaction processes to suppress the undesired over-reactions. The accuracy and simplicity of the developed system suggest that adsorption energies can be readily predicted without time-consuming DFT calculations, and eventually, this would allow us to predict the catalytic performances of the solid catalysts

Oxidation of Silanes to Silanols on Pd Nanoparticles: H₂ Desorption Accelerated by Surface Oxygen Atom

The oxidation of silane to silanol on the clean and oxygen-covered Pd(111) surface is investigated with periodic density functional theory calculations to gain a better understanding of the effect of surface oxygen atom on Pd nanoparticle catalysts. The calculations confirmed that this catalytic reaction is initiated by the dissociative adsorption of silane on the Pd surface. The resultant silyl group is attacked by a water molecule to form silanol and an H atom on the Pd surface with inversion of configuration at the Si center. An activation energy of 11.3 kcal/mol is required for the water addition, and the transition state for this step is energetically highest in the entire reaction profile. These computational results are in good agreement with our stereochemical and kinetic studies. The H atoms on the Pd surface inhibit further reaction, and therefore, they should be removed to achieve the catalytic activity experimentally. We found that the role of the surface oxygen atom is to facilitate the desorption of H₂ from the Pd surface without the formation of OH and H₂O. The introduction of surface oxygen atoms can enhance the catalytic ability of metal nanoparticles for green organic reactions

Heterogeneous Ni Catalysts for N‑Alkylation of Amines with Alcohols

Nickel nanoparticles loaded onto various supports (Ni/MO_<i>x</i>) have been prepared and studied for the N-alkylation of amines with alcohols. Among the catalysts, Ni/θ-Al₂O₃ prepared by in situ H₂-reduction of NiO/θ-Al₂O₃ shows the highest activity, and it acts as reusable heterogeneous catalyst for the alkylation of anilines and aliphatic amines with various alcohols (benzyl and aliphatic alcohols) under additive free conditions. Primary amines are converted into secondary amines and secondary amines into tertiary amines. For the reaction of aniline with an aliphatic alcohol the catalyst shows higher turnover number (TON) than precious metal-based state-of-the-art catalysts. Mechanistic studies suggest that the reaction proceeds through a hydrogen-borrowing mechanism. The activity of Ni catalysts depends on the nature of support materials; acid–base bifunctional supports give higher activity than basic or acidic supports, indicating that acid–base sites on supports are necessary. The presence of basic (pyridine) or acidic (acetic acid) additive in the solution decreased the activity of Ni/θ-Al₂O₃, which suggests the cooperation of the acid–base site of θ-Al₂O₃. For a series of Ni/θ-Al₂O₃ catalysts with different particle size, the turnover frequency (TOF) per surface Ni increases with decreasing Ni mean particle size, indicating that low-coordinated Ni species and/or metal–support interface are active sites. From these results, we propose that the active site for this reaction is metal–support interface, where low-coordinated Ni⁰ atoms are adjacent to the acid–base sites of alumina

Cooperative H₂ Activation at Ag Cluster/θ-Al₂O₃(110) Dual Perimeter Sites: A Density Functional Theory Study

H₂ dissociation by Ag clusters supported on the θ-Al₂O₃(110) surface has been investigated using density functional theory calculations. The crucial role of the dual perimeter site of Ag cluster and the surface oxygen (O) site of the alumina support is demonstrated with three theoretical models: anchored cluster, isolated cluster, and anchored cluster on hydroxylated alumina. The heterolytic cleavage of H₂ at the silver–alumina interface, yielding Ag–H^δ− and O–H^δ+, is thermodynamically and kinetically preferred compared with H₂ cleavage at two Ag atomic sites on top of the Al₂O₃-supported Ag cluster and the homolytic cleavage of H₂ on the isolated Ag cluster. The hydroxylation at the O site of the alumina reduces the H₂ dissociation activity, which indicates that the interfacial bare O site is indispensible. It is concluded that the interfacial cooperative mechanism between the Ag cluster and Lewis acid–base pair site (bare Al–O site) is essentially relevant for the H₂ activation over Ag-loaded Al₂O₃ catalysts