9 Search Results

Supported platinum nanoparticles are known to convert polyolefins to high-quality liquid hydrocarbons with hydrogen under relatively mild conditions. However, no systematic study has been undertaken using bimetallic catalysts for polyethylene upcycling. Specifically, a total of 98 monometallic and bimetallic combinations (Ag, Cr, Co, Cu, Fe, Ga, In, Mn, Ni, Pd, Pt, Rh, Ru, Zr) on alumina were synthesized utilizing surface organometallic chemistry (SOMC) technique via robotic platform. These were investigated at a small scale (10 mg of catalyst and 50 mg of polyethylene) for their activity for the hydrogenolysis of polyethylene in a high-throughput batch reactor. Combinations of Ni andmore » Co were selected as candidates with high activity toward conversion into paraffin oils. Reaction conditions were optimized with Ni/Co/Al₂O₃ catalyst at a larger scale (300 mg catalyst and 3 g polyethylene) to obtain a high yield (93.1%) of paraffin wax with desired properties (M_n = 380 Da) and low polydispersity (Đ = 1.2). Ni/Co/Al₂O₃ was compared against Co/Ni/Al₂O₃ to understand the role of the deposition sequence. When Co is deposited before Ni, a layer of cobalt aluminate is formed upon reduction, stabilizing the deposition of 5 nm metallic Ni particles. When nickel is deposited before Co, particles are larger (average >20 nm) and more oxidized (Ni^δ+ in NiAl₂O₄), decreasing the availability of the catalytically active metallic Ni. In conclusion, the difference in electronic environments was also described by DFT calculations, which revealed that smaller 3D clusters of Ni are preferred on CoAl2O4 over the 3D clusters on NiAl₂O₄ and that these smaller clusters are more reducible, as confirmed experimentally.« less

Selective ethylene oligomerization via oxidative cyclization, forming metalacyclic intermediates, is typically catalyzed by molecular titanium and chromium complexes to produce butenes, hexenes, or octenes, depending on the supporting ligand framework. However, this mechanism requires significant electron density at the metal active site and is not known to be generalizable to other first-row transition metals. Here, we computationally investigate the electronic modulation of five transition metals (Mn, Fe, Co, Ni, and Cu) supported on titania (TiO₂) through reductive lithium intercalation to promote selective oligomerization via oxidative cyclization, using density functional theory (DFT). Our findings predict that Mn/LiTiO₂ exhibits high catalytic activitymore » due to the exergonic nature of oxidative cyclization with two ethylene molecules. Additionally, lithium titanate (LiTiO₂) supports enhance catalytic performance compared to TiO₂. Experimental validation confirms that Mn/LiTiO₂ achieves higher conversion rates and improved selectivity towards hexene (C₄:C₆ = 1:2.6). The enhanced activity is attributed to lithiation, which alters the electronic environment around Mn active sites. Mechanistic studies reveal that the formation of a seven-membered ring, a key intermediate for hexene formation, is more favorable on LiTiO₂ than TiO₂. This work provides the first evidence of Mn catalyzing selective ethylene oligomerization via oxidative cyclization in either homogeneous or heterogeneous catalysis.« less

Synthesis of twenty-seven bimetallic catalysts consisting of nickel and one of nine different dopants (B, Co, Cu, Fe, Mg, Mn, Sn, V, and Zn) supported on three different metal oxides (Al₂O₃, CeO₂, and SiO₂) is carried out via organometallic grafting. The catalysts are evaluated for their activity and selectivity for the CO₂ methanation reaction at a feed ratio of H₂/CO₂ of 4 at 300 °C in a high-throughput flow reactor system. After in situ pre-activation (500 °C in H₂), Ni/Co/CeO₂ exhibited high conversion (84.3%) and selectivity for methane (99.6%). Ni/Co/CeO₂ was characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectronmore » spectroscopy (XPS), X-ray diffraction, H₂-temperature-programmed reduction (H₂-TPR), and CO₂-temperature-programmed desorption (CO₂-TPD). HRTEM showed the presence of single Ni and Co atoms on ceria after pre-reduction at 500 °C and after the methanation reaction at 300 °C for 15 h. XPS determined that the strong interaction between Ni, Co, and ceria increased after the reduction, leading to a charge transfer between Ni and Ce that created oxygen vacancies in ceria. Nickel was found to be Ni²⁺ in the as-prepared material and was partially reduced in the presence of cobalt and after the activation in H₂ at 500 °C. The DFT results show that both nickel and cerium exhibit lower Bader charges in the Ni/Co/CeO₂ system, confirming that the presence of cobalt enhances the reduction of both Ni and Ce through electronic interactions. This indicates that single cationic Ni atoms are highly effective for the methanation reaction. In conclusion, the organometallic grafting technique is found to be efficient for synthesizing catalysts with highly homogeneous dispersed species at low metal loadings (0.16 wt % Ni–0.15 wt % Co), which leads to high turnover frequency (up to 248.7 h^–1) and durability for methanation.« less

Combinatorial screening of 150 supported metal oxide (manganese and additives) catalysts was carried out via a high-throughput synthesis platform and parallel reactors for the oxidative dehydrogenation (ODH) of propane to propylene. Specifically, an organomanganese (0.05-2.5 Mn atoms/nm²) complex was grafted on metal oxide supports (Al₂O₃, SiO₂, TiO₂, and ZrO₂) premodified with either Lewis acid (Al, Ti, Zn, and Zr) or redox-active (Cu, Cr, Ga Ni, V) additives at various surface coverages (25, 50, and 75%). Catalysts were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and UV-vis spectroscopy. Catalysts 0.05 Mn/V(50%)/Al₂O₃ andmore » 0.05 Mn/Ni(50%)/ZrO₂ showed the highest combined propane conversion and propylene selectivities (31/41% and 15/85%), with excellent stability at 500 degrees C for 25 h. The presence of Ni in Mn/Ni/ZrO₂ resulted in a 6-fold increase in turnover frequency (TOF) over the Mn/ZrO₂. HRTEM identified single Mn atoms after 500 degrees C heat treatment. For the Mn/Ni/ZrO₂ system, Mn was incorporated into the support lattice due to the similar ionic radius of Mn²⁺ and Zr⁴⁺, which was also enhanced by the presence of Ni. For the Mn/V/Al₂O₃ system, highly active MnO was prevalent as observed by Raman. Both V and Mn contributed to an increase in mutual dispersion, but both species remained on the surface. Finally, it is proposed that the highly dispersed atom and interactions between Mn with either Ni or V are responsible for the ODH performance and stability.« less

Platinum has been shown to be an active catalyst for the dry reforming of methane (DRM), which converts CO₂ and CH₄ into 2CO and 2H₂ (synthesis gases) that can further be processed to produce valuable chemical feedstocks. Catalytic activity is often improved by the addition of promoter atoms, which are typically transition metals or noble metals, such as PtNi and PtSn. Recently, boron has shown to be an effective and low-cost catalyst promoter. Pt/B/SiO₂ catalysts were prepared for DRM catalysis and compared with Pt/SiO₂ catalysts without boron promotion. Both catalysts had similar surface concentrations of platinum, but the catalytic activitymore » at 750 °C after 14 h for boron-containing catalyst was very high, resulting in nearly 100% CO₂ conversion and a H₂/CO ratio close to unity, compared to 12% CO₂ conversion and H₂/CO of 0.35 for boron-free Pt/SiO₂. The catalysts were investigated with X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and CO₂ temperature-programmed desorption (CO₂-TPD) to identify the deactivating factors. It was determined that neither platinum nanoparticle sintering nor coking was a significant factor in catalyst deactivation; instead, boron had an effect on the reactive surface groups on the SiO₂ support. Finally, these surface groups, such as hydroxyls and surface basic sites, enhance the adsorption of CO₂ and potentially stabilize intermediate carbonate species, resulting in a high CO₂ conversion for boron-promoted platinum catalysts.« less

Supported platinum nanoparticle catalysts are known to convert polyolefins to high-quality liquid hydrocarbons using hydrogen under relatively mild conditions. To date, few studies using platinum grafted onto various metal oxide (M_xO_y) supports have been undertaken to understand the role of the acidity of the oxide support in the carbon-carbon bond cleavage of polyethylene under consistent catalytic conditions. Specifically, two Pt/M_xO_y catalysts (M_xO_y = SrTiO₃ and SiO₂-Al₂O₃; Al = 3.0 wt %, target Pt loading 2 wt % Pt similar to 1.5 nm), under identical catalytic polyethylene hydrogenolysis conditions (T = 300 degree celsius, P(H₂) = 170 psi, t = 24more » h; M_w = similar to 3,800 g/mol, M_n = similar to 1,100 g/mol, D = 3.45, N_branch/100C = 1.0), yielded a narrow distribution of hydrocarbons with molecular weights in the range of lubricants (M_w = < 600 g/mol; M_n < 400 g/mol; D = 1.5). While Pt/SrTiO₃ formed saturated hydrocarbons with negligible branching, Pt/SiO₂-Al₂O₃ formed partially unsaturated hydrocarbons (<1 mol % alkenes and similar to 4 mol % alkyl aromatics) with increased branch density (N_branch/100C = 5.5). Further investigations suggest evidence for a competitive hydrocracking mechanism occurring alongside hydrogenolysis, stemming from the increased acidity of Pt/SiO₂-Al₂O₃ compared to Pt/SrTiO₃. Additionally, the products of these polymer deconstruction reactions were found to be independent of the polyethylene feedstock, allowing the potential to upcycle polyethylenes with various properties into a value-added product.« less

We report Isolated Pd atoms supported on high surface area MnO₂, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O₂, 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO₂ in facilitating redox turnovers.

High-throughput synthesis of a series of monometallic and bimetallic catalysts (45 bimetallic and 50 monometallic samples) consisting of nickel and one of nine different metal promoters (B, Co, Cu, Fe, Mg, Mn, Sn, V and Zn) supported on one of six different metal oxides alumina, ceria, magnesia, silica and titania) is carried out via organometallic grafting using a robotic platform. The catalysts are evaluated for their activity and selectivity for the dry reforming of methane at a feed ratio of CH₄:CO₂ of 1 at 650–800 °C in a parallel flow reactor system. The type of oxide support prevails over themore » type of additive for both catalyst activity and stability. On Al₂O₃ and MgO, Fe was found to be the best promoter; on SiO₂, Cu is the best promoter at 700 °C and higher, while on TiO₂, Mn is found to enhance the conversion at 800 °C. On CeO₂, all additives except Fe have beneficial effects. Twenty-five catalysts show > 90% methane conversion with ten catalysts showing > 95% conversion at 800 °C with the H₂:CO ratios ranging from 0.8 to 1.2. Amongst the ten highest performers, NiFe/Al₂O₃ and NiFe/MgO are more active than Ni/Al₂O₃ and Ni/MgO, respectively and were stable over a period of 25 h at 800 °C. Characterization on the as-prepared samples reveals highly dispersed phase, while after reduction in H₂, highly dispersed and reduced nickel particles up to 10 nm are formed. The particles do not increase in size under dry reforming reaction conditions at 800 °C. An increased hydrogen consumption observed during H₂-TPR of the nickel particles is positively correlated with methane conversion for Al₂O₃-based catalysts. The resistance to deactivation by coking and variation in coke structure are investigated by spectroscopic and microscopic methods to identify the relationship between metal promoters, alloy formation, and type of surface carbon deposits. Carbon whiskers were observed on the ten selected spent samples and are preferentially deposited on Ni rather than on the promoters. Carbon nanotube formation and metal particle removal from support were not observed to cause deactivation while amorphous carbon formation was clearly linked to catalyst deactivation, as amorphous carbon could encapsulate nickel, either on the support or at the end of the carbon nanotube. Furthermore, the organometallic grafting technique is an efficient and suitable technique for synthesizing highly dispersed and homogeneous phases which lead to high conversion and high durability for dry reforming of methane.« less

Platinum-based supported catalysts for hydrocarbon conversion are among the most effective for selective dehydrogenation and isomerization processes. However, high process temperatures and the possibility of coke formation require catalyst modifications to mitigate such effects. One of the emerging approaches to prevent platinum catalyst deactivation is the use of boron additives that have been proposed to prevent coking. Despite such a valuable property of boron, the mechanisms for extending the catalyst lifetime and the decrease in coke formation based on this method are still poorly understood. The type and transformations of boron species on silica surface were investigated as a functionmore » of boron introduction, platinum addition, catalyst activation, and catalytic reactivity by a combination of X-ray photoelectron spectroscopy, electron microscopy, solid-state nuclear magnetic resonance spectroscopy, and density functional theory calculations to uncover the possible role of boron modification in improving the catalytic performance. Catalytic nonoxidative dehydrogenation of n-butane revealed that incorporation of boron improved the catalytic activity (similar to 3x) and stability of Pt/SiO₂. The role of boron in enhancing catalytic performance was attributed to facilitating the migration of alkyl groups from platinum catalytic centers to tetrahedrally coordinated boron sites.« less