Nickel has been considered a promising nonprecious metal for propane dehydrogenation (PDH) because of its appealing ability to activate alkane molecules; however, synthesis of selective and stable Ni sites remains a challenge. Herein, we describe an alumina-supported isolated Ni(II) site by settling Ni2+ cations into Al3+ vacancy on γ-Al2O3 as a selective and stable Ni-based catalyst for PDH. Based on the results from combined characterizations, including in situ X-ray adsorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), atomically dispersed Ni(II) sites with bonding to oxygen ions from the alumina support are demonstrated. PDH catalyst tests shows that the Ni(II) single-site catalyst delivers superior performance compared to a supported metallic Ni NP catalyst, possessing >93% propylene selectivity at considerable propane conversions (15–45%), which surpasses Ni nanoparticle (NP) catalysts. The correlation of selectivity to propylene on different Ni structures with the structural characterizations suggest that the coordinatively unsaturated tetrahedral Ni(II) sites facilitate the desorption of propylene, which inhibits the side reactions for coking. Moreover, the atomically dispersed Ni(II) sites remain in its local structure in the reaction-regeneration cycles, as evidenced by in situ XAS. This study on alumina-supported nickel catalysts affords insights into the nature of selective and stable nickel sites involved in the PDH reaction. 

Insights into the Nature of Selective Nickel Sites on Ni/Al2O3 Catalysts for Propane Dehydrogenation

Kinetic measurements for silica-supported Pt and Pt–Mo catalysts were collected in vapor-phase acetic acid hydrodeoxygenation by varying the hydrogen partial pressure between 18 and 72 kPa and the acetic acid partial pressure between 7 and 18 kPa at 423–473 K. Under all testing conditions, the Pt–Mo catalyst was more active and selective. In addition, the apparent activation energy for Pt–Mo of 76 ± 3 kJ/mol was lower than that of 84 ± 4 kJ/mol for Pt. The apparent reaction orders were also different. The order in hydrogen of 0.8 ± 0.1 for Pt–Mo changed to zero at higher hydrogen partial pressures while that for Pt remained constant at 0.6 ± 0.1. A near-zero order in acetic acid for Pt–Mo changed to −2.1 at higher acetic acid pressures while that for Pt remained constant at −2.9 ± 0.3. These differences in reaction kinetics as well as in selectivity trends with changes in temperature and feed composition indicated a change in the reaction mechanism for Pt–Mo. The catalysts were characterized with hydrogen temperature programmed desorption, oxygen temperature programmed oxidation and transmission electron microscopy with energy-dispersed X-ray spectroscopy elemental mapping. Mo was present in the form of subnanometer-size clusters on the surface of Pt nanoparticles. Both Pt and Pt–Mo catalysts were stable under the reaction conditions for 10 h, and the size and structure of Pt and Pt–Mo particles remained mostly unchanged, without coke accumulation. Density functional theory calculations show that neighboring Pt–Mo surface atoms act as a single active site where Mo serves as a preferential binding anchor for O atoms. The presence of Mo changes the structure and reactivity of hydrocarbon surface species, leading to a change in the reaction mechanism. In contrast with Pt, C–O bond breaking reactions become more favorable on Pt–Mo and, conversely, C–C bond breaking reactions become less favorable on Pt–Mo, explaining the experimentally observed higher activities and selectivities of Pt–Mo. 

Kinetics and Reaction Mechanisms of Acetic Acid Hydrodeoxygenation over Pt and Pt–Mo Catalysts

Adsorption, Surface Reactions and Hydrodeoxygenation of Acetic Acid on Platinum and Nickel Catalysts

A biosensor utilizing long-period fiber gratings (LPFG) functionalized with nanoporous coated structures was developed for the rapid detection of Staphylococcus aureus (S. aureus) bacteria. The nanoporous structure coatings on the LPFG surface facilitated specific adhesion and interaction with S. aureus, resulting in an instantaneous shift in the resonance wavelength (RW) in the transmission spectrum of the LPFG. The LPFG with nanoporous polyelectrolyte coatings exhibited an approximately seven-fold RW shift compared to the bare LPFG under the optimal experiment conditions. By tracking the RW shifts, we were able to monitor the real-time S. aureus adhesion to study the interaction process in detail. The bacterial differentiation and S. aureus specificity of the method was confirmed through a series of studies using Escherichia coli (E. coli). This nanoporous structure-enabled LPFG-based biosensor scheme holds significant promise for rapid, reliable, and low-cost detection of S. aureus for biomedical applications.

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Bacterial Detection and Differentiation of Staphylococcus aureus and Escherichia coli Utilizing Long-Period Fiber Gratings Functionalized with Nanoporous Coated Structures

Coordination environment tuning of Ni-Sn catalysts by glu-carbon to optimize cellulose hydrogenolysis to oxygenated chemicals

Temperature programmed reaction (TPR) measurements with propane over silica-supported Ni, Ni−Sn and Sn catalysts show that the reaction products change significantly from mostly methane, hydrogen and surface carbon over Ni to propylene and hydrogen over Ni−Sn. Propylene formation over Ni−Sn starts at a moderate temperature of 630 K. Since the activity of Sn by itself is low, Sn serves as a promoter for Ni. The promoter effects are attributed to a lower adsorption energy of molecularly adsorbed propylene and suppression of propylidyne formation on Ni−Sn based on temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) measurements as well as density functional theory (DFT) calculations for propylene adsorption on Ni(110) and c(2×2)-Sn/Ni(110) single-crystal surfaces. On Ni, propylene forms a π-bonded structure with ν(C=C) at 1500 cm−1, which desorbs at 170 K, and a di-σ-bonded structure with ν(C=C) at 1416 cm−1, which desorbs at 245 K. The di-σ-bonded structure is asymmetric, with the methylene C atom being in the middle of the Ni−Ni bridge site, and the methylidyne C atom being above one of these Ni atoms. Therefore, this structure can also be characterized as a hybrid between di-σ- and π-bonded structures. Only a fraction of propylene desorbs from Ni because propylene can convert into propylidyne, which decomposes further. In contrast, propylene forms only a π-bonded structure on Ni−Sn with ν(C=C) at 1506 cm−1, which desorbs at 125 K. The low stability of this structure enables propylene to desorb fully, resulting in high reaction selectivity in propane dehydrogenation to propylene over the Ni−Sn catalyst. 

Stephen C. Newman

Papers

Propane Dehydrogenation to Propylene and Propylene Adsorption on Ni and Ni‐Sn Catalysts