Sibunit-Supported Mono- and Bimetallic Catalysts Used in Aqueous-Phase Reforming of Xylitol.
ABSTRACT: Carbon-supported mono- and bimetallic catalysts prepared via incipient wetness impregnation were systematically studied in aqueous-phase reforming (APR) of xylitol aiming at hydrogen production from biomass. The catalytic performance of several VIII group metals and their combinations, such as Pt, Ni, Pt-Ni, Re, Pt-Re, Ru, Pt-Ru, and Pt-Co, was compared for xylitol APR in a fixed-bed reactor at 225 °C and 29.7 bar (N2). Ni/C, Ru/C, and Re/C catalysts displayed significantly lower activity compared to others. Activity and selectivity to H2 of bimetallic Pt-Ni/C, Pt-Co/C, and Pt-Ru/C catalysts were close to that of Pt/C. Pt-Re/C catalyst showed an outstanding performance which was accompanied by a shift of the reaction pathways to the alkane formation and thereby lower hydrogen selectivity. Addition of the second metal to Pt was not found to be beneficial for hydrogen production, thus leaving Pt/C as the optimum carbon-supported catalyst.
Project description:A disadvantage of the commercial Raney Ni is that the Ni active sites are prone to leaching and deactivation in the hydrogenation of xylose to xylitol. To explore a more stable and robust catalyst, activated carbon (AC) supported Ni-Re bimetallic catalysts (Ni-Re/AC) were synthesized and used to hydrogenate xylose and hemicellulosic hydrolysate into xylitol under mild reaction conditions. In contrast to the monometallic Ni/AC catalyst, bimetallic Ni-Re/AC exhibited better catalytic performances in the hydrogenation of xylose to xylitol. A high xylitol yield up to 98% was achieved over Ni-Re/AC (nNi:nRe = 1:1) at 140 °C for 1 h. In addition, these bimetallic catalysts also had superior hydrogenation performance in the conversion of the hydrolysate derived from the hydrolysis reaction of the hemicellulose of Camellia oleifera shell. The characterization results showed that the addition of Re led to the formation of Ni-Re alloy and improved the dispersion of Ni active sites. The recycled experimental results revealed that the monometallic Ni and the bimetallic Ni-Re catalysts tended to deactivate, but the introduction of Re was able to remarkably improve the catalyst's stability and reduce the Ni leaching during the hydrogenation reaction.
Project description:Single atom catalysts exhibit particularly high catalytic activities in contrast to regular nanomaterial-based catalysts. Until recently, research has been mostly focused on single atom catalysts, and it remains a great challenge to synthesize bimetallic dimer structures. Herein, we successfully prepare high-quality one-to-one A-B bimetallic dimer structures (Pt-Ru dimers) through an atomic layer deposition (ALD) process. The Pt-Ru dimers show much higher hydrogen evolution activity (more than 50 times) and excellent stability compared to commercial Pt/C catalysts. X-ray absorption spectroscopy indicates that the Pt-Ru dimers structure model contains one Pt-Ru bonding configuration. First principle calculations reveal that the Pt-Ru dimer generates a synergy effect by modulating the electronic structure, which results in the enhanced hydrogen evolution activity. This work paves the way for the rational design of bimetallic dimers with good activity and stability, which have a great potential to be applied in various catalytic reactions.
Project description:Mono- and bimetallic alloy Pt and Ru catalysts supported on ?-Al2O3 have been investigated for the reduction of pollutants (NO x , NH3, and CO) generated during the continuous combustion of an aqueous urea ammonium nitrate fuel. A Pt/Ru alloy with a Pt25/Ru75 atomic ratio has been found to have higher activity and selectivity than those of a 50/50 alloy and monometallic catalysts. Among monometallic catalysts, Ru was more selective toward N2 formation, whereas Pt showed a higher selectivity toward NH3 formation. For Ru, it was observed that the oxidizing atmosphere of NO x pollutants caused the formation of RuO2, whereas Ru in the Pt/Ru alloy was stable under these conditions. Temperature (250-500 °C) and pressure (1-8 MPa) studies over Ru and 25/75 Pt/Ru have concluded that the alloy catalyst at 400 °C and 5 MPa reduced the pollutants to a minimum level with high yields of N2 (99.7%) and CO2 (99.9%). It was also observed that the 25/75 Pt/Ru catalyst remained stable up to 100 h of thermal treatment at 400 °C. Minimal pollutants were obtained at a weight hourly space velocity = 11?822 h-1. Characterization studies of the spent catalyst showed that metal particles were sintered over a period of time (8 h) and the ?-Al2O3 support was transformed into ?- and ?-phases under the hydrothermal reaction conditions.
Project description:Mono-, and bimetallic Ni-, Ru-, and Pt-modified nanosized Beta zeolite catalysts were prepared by the post synthesis method and characterized by powder X-ray diffraction (XRD), nitrogen physisorption, HRTEM microscopy, temperature-programmed reduction (TPR-TGA), ATR FT-IR spectroscopy, and by solid-state MAS-NMR spectroscopy. The presence of nanosized nickel-oxide, ruthenium-oxide, and platinum species was detected on the catalysts. The presence of Brønsted and Lewis acid sites, and incorporation of nickel ions into zeolite lattice was proven by FT-IR of adsorbed pyridine. The structural changes in the catalyst matrix were investigated by solid state NMR spectroscopy. The catalysts were used in a gas-phase hydrodemethoxylation and dealkylation of 2-methoxy-4-propylphenol as a lignin derivative molecule for phenol synthesis.
Project description:Cellulose grains were carbonized and applied as catalyst supports for nickel- and magnetite-promoted bimetallic palladium- and platinum-containing catalysts. The bimetallic spherical aggregates of Pd and Pt particles were created to enhance the synergistic effect among the precious metals during catalytic processes. As a first step, the cellulose bead-based supports were impregnated by nitrate salts of nickel and iron and carbonized at 973 K. After this step, the nickel was in an elemental state, while the iron was in a magnetite form in the corresponding supports. Then, Pd and Pt particles were deposited onto the supports and the catalyst surface; precious metal nanoparticles (10-20 nm) were clustered inside spherical aggregated particles 500-600 nm in size. The final bimetallic catalysts (i.e., Pd-Pt/CCB, Pd-Pt/Ni-CCB, and Pd-Pt/Fe<sub>3</sub>O<sub>4</sub>-CCB) were tested in hydrogenation of chlorate ions in the aqueous phase. For the nickel-promoted Pd-Pt catalyst, a >99% chlorate conversion was reached after 45 min at 80 °C. In contrast, the magnetite-promoted sample reached an 84.6% chlorate conversion after 3 h. Reuse tests were also carried out with the catalysts, and in the case of Pd-Pt/Ni-CCB after five cycles, the catalytic activity only decreased by ~7% which proves the stability of the system.
Project description:Bimetallic Pt-Ni with Pt on the outermost layer and an innermost layer enriched in Ni, referred to as Pt<sub>3</sub>Ni(Pt-skin), is a promising configuration of an electrocatalyst for the oxygen reduction reaction (ORR) in fuel cells. We prepare a core (Pd)/shell (Pt<sub>3</sub>Ni(Pt-skin)) catalyst (Pt<sub>3</sub>Ni(Pt-skin)/Pd/C) from Zn underpotential deposition (UPD) on a Ni UPD modified Pd/C catalyst, facilitating Pt atomic layer-by-layer growth on the Ni surface through the galvanic replacement process. Pt<sub>3</sub>Ni(Pt-skin)/Pd/C shows the best ORR performance, with a Pt specific activity of 16.7 mA cm<sup>-2</sup> and Pt mass activity of 14.2 A mg<sub>Pt</sub><sup>-1</sup>, which are 90- and 156- fold improvements over commercial Pt/C catalysts. The Pt<sub>3</sub>Ni(Pt-skin) structure effectively inhibits Ni leaching to improve the durability in two accelerated durability test modes mimicking the catalyst lifetime and start-up/shut-down cycles.
Project description:Although electrochemical water splitting is an effective and green approach to produce oxygen and hydrogen, the realization of efficient bifunctional catalysts that are stable in variable electrolytes is still a significant challenge. Herein, we report a three-dimensional hierarchical assembly structure composed of an ultrathin Ru shell and a Ru-Ni alloy core as a catalyst functioning under universal pH conditions. Compared with the typical Ir/C-Pt/C system, superior catalytic performances and excellent durability of the overall water splitting under universal pH have been demonstrated. The introduction of Ni downshifts the d-band center of the Ru-Ni electrocatalysts, modulating the surface electronic environment. Density functional theory results reveal that the mutually restrictive d-band interaction lowers the binding of (Ru, Ni) and (H, O) for easier O-O and H-H formation. The structure-induced eg-dz2 misalignment leads to minimization of surface Coulomb repulsion to achieve a barrier-free water-splitting process.
Project description:The catalytic hydrogenation of lactic acid to 1,2-propanediol with supported Ru catalysts in water was investigated. The influence of catalyst support (activated carbon, ?-Al<sub>2</sub>O<sub>3</sub>, SiO<sub>2</sub>, TiO<sub>2</sub>, and CeO<sub>2</sub>) and promoters (Pd, Au, Mo, Re, Sn) on the catalytic performance was evaluated. Catalytic tests revealed that TiO<sub>2</sub> yields the best Ru catalysts. With a monometallic Ru/TiO<sub>2</sub> catalyst, a 1,2-propanediol yield of 70?% at 79?% lactic acid conversion was achieved at 130?°C after 20?h reaction. Minor byproducts of the hydrogenation reaction were propionic acid, ethanol, 1-propanol, and 2-propanol. For the bimetallic catalysts, the addition of Pd and Au slightly enhanced the performance of Ru/TiO<sub>2</sub>, whereas the addition of common hydrogenation promoters such as Re, Mo, and Sn impaired the activity.
Project description:Single-atom catalysts provide an effective approach to reduce the amount of precious metals meanwhile maintain their catalytic activity. However, the sluggish activity of the catalysts for alkaline water dissociation has hampered advances in highly efficient hydrogen production. Herein, we develop a single-atom platinum immobilized NiO/Ni heterostructure (Pt<sub>SA</sub>-NiO/Ni) as an alkaline hydrogen evolution catalyst. It is found that Pt single atom coupled with NiO/Ni heterostructure enables the tunable binding abilities of hydroxyl ions (OH*) and hydrogen (H*), which efficiently tailors the water dissociation energy and promotes the H* conversion for accelerating alkaline hydrogen evolution reaction. A further enhancement is achieved by constructing Pt<sub>SA</sub>-NiO/Ni nanosheets on Ag nanowires to form a hierarchical three-dimensional morphology. Consequently, the fabricated Pt<sub>SA</sub>-NiO/Ni catalyst displays high alkaline hydrogen evolution performances with a quite high mass activity of 20.6 A mg<sup>-1</sup> for Pt at the overpotential of 100 mV, significantly outperforming the reported catalysts.
Project description:A single-atom catalyst (SAC) has an electronic structure that is very different from its bulk counterparts, and has shown an unexpectedly high specific activity with a significant reduction in noble metal usage for CO oxidation, fuel cell and hydrogen evolution applications, although physical origins of such performance enhancements are still poorly understood. Herein, by means of density functional theory (DFT) calculations, we for the first time investigate the great potential of single atom catalysts for CO<sub>2</sub> electroreduction applications. In particular, we study a single transition metal atom anchored on defective graphene with single or double vacancies, denoted M@sv-Gr or M@dv-Gr, where M = Ag, Au, Co, Cu, Fe, Ir, Ni, Os, Pd, Pt, Rh or Ru, as a CO<sub>2</sub> reduction catalyst. Many SACs are indeed shown to be highly selective for the CO<sub>2</sub> reduction reaction over a competitive H<sub>2</sub> evolution reaction due to favorable adsorption of carboxyl (*COOH) or formate (*OCHO) over hydrogen (*H) on the catalysts. On the basis of free energy profiles, we identified several promising candidate materials for different products; Ni@dv-Gr (limiting potential <i>U</i><sub>L</sub> = -0.41 V) and Pt@dv-Gr (-0.27 V) for CH<sub>3</sub>OH production, and Os@dv-Gr (-0.52 V) and Ru@dv-Gr (-0.52 V) for CH<sub>4</sub> production. In particular, the Pt@dv-Gr catalyst shows remarkable reduction in the limiting potential for CH<sub>3</sub>OH production compared to any existing catalysts, synthesized or predicted. To understand the origin of the activity enhancement of SACs, we find that the lack of an atomic ensemble for adsorbate binding and the unique electronic structure of the single atom catalysts as well as orbital interaction play an important role, contributing to binding energies of SACs that deviate considerably from the conventional scaling relation of bulk transition metals.