Explore the vast range of titles available.

Supported nanoparticles are broadly employed in industrial catalytic processes, where the active sites can be tuned by metal-support interactions (MSIs). Although it is well accepted that supports can modify the chemistry of metal nanoparticles, systematic utilization of MSIs for achieving desired catalytic performance is still challenging. The developments of supports with appropriate chemical properties and identification of the resulting active sites are the main barriers. Here, we develop two-dimensional transition metal carbides (MXenes) supported platinum as efficient catalysts for light alkane dehydrogenations. Ordered Pt 3 Ti and surface Pt 3 Nb intermetallic compound nanoparticles are formed via reactive metal-support interactions on Pt/Ti 3 C 2 T x and Pt/Nb 2 CT x catalysts, respectively. MXene supports modulate the nature of the active sites, making them highly selective toward C–H activation. Such exploitation of the MSIs makes MXenes promising platforms with versatile chemical reactivity and tunability for facile design of supported intermetallic nanoparticles over a wide range of compositions and structures. The performance of supported metal nanoparticle catalysts can be tailored by metal-support interactions, but their use in catalyst design is still challenging. Here, the authors develop two-dimensional transition metal carbides as platforms for designing intermetallic compound catalysts that are efficient for light alkane dehydrogenations.

Single-site Ga/SiO 2 catalysts exhibit up to 99% C 3 H 6 selectivity at 4% propane conversion with an initial rate of 5.4 × 10 −4 (mole C 3 H 6 ) (mole Ga) −1  s −1 during propane dehydrogenation (PDH) at 550 °C. Following pre-treatment in H 2 at 550 °C, only four-coordinate, Ga 3+ –O Lewis acid sites are observed under reaction conditions. At 650 °C in H 2 , an additional isolated Ga site with lower Ga–O coordination ( N Ga−O  < 4) is formed and leads to a 30% decrease in the initial PDH rate per total moles of Ga. The PDH rates are equivalent when normalized by the amount of surface, four-coordinate Ga 3+ –O, regardless of catalyst pre-treatment conditions, which indicates that these isolated Ga 3+ centers are the catalytically relevant sites. Graphical Abstract Isolated, Lewis acidic Ga 3+ cations present as four-coordinate Ga 3+ –O centers exhibit up to 99% C 3 H 6 selectivity during propane dehydrogenation (PDH) at 550 °C. An additional isolated Ga site with lower Ga–O coordination is formed during H 2 treatment at elevated temperatures, but is inactive for PDH and reversibly decomposes under reaction conditions.

A hybrid hydrogen-carbon (H₂CAR) process for the production of liquid hydrocarbon fuels is proposed wherein biomass is the carbon source and hydrogen is supplied from carbon-free energy. To implement this concept, a process has been designed to co-feed a biomass gasifier with H₂ and CO₂ recycled from the H₂-CO to liquid conversion reactor. Modeling of this biomass to liquids process has identified several major advantages of the H₂CAR process. (i) The land area needed to grow the biomass is <40% of that needed by other routes that solely use biomass to support the entire transportation sector. (ii) Whereas the literature estimates known processes to be able to produce [almost equal to]30% of the United States transportation fuel from the annual biomass of 1.366 billion tons, the H₂CAR process shows the potential to supply the entire United States transportation sector from that quantity of biomass. (iii) The synthesized liquid provides H₂ storage in an open loop system. (iv) Reduction to practice of the H₂CAR route has the potential to provide the transportation sector for the foreseeable future, using the existing infrastructure. The rationale of using H₂ in the H₂CAR process is explained by the significantly higher annualized average solar energy conversion efficiency for hydrogen generation versus that for biomass growth. For coal to liquids, the advantage of H₂CAR is that there is no additional CO₂ release to the atmosphere due to the replacement of petroleum with coal, thus eliminating the need to sequester CO₂.

Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NOₓ) with ammonia (NH₃), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O₂)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

Copper ions in zeolites help remove noxious nitrogen oxides from diesel exhaust by catalyzing their reaction with ammonia and oxygen. Paolucci et al. found that these copper ions may move about during the reaction (see the Perspective by Janssens and Vennestrom). Zeolite catalysts generally fix metals in place while the reacting partners flow in and out of their cagelike structures. In this case, though, x-ray absorption spectroscopy suggested that the ammonia was mobilizing the copper ions to pair up as they activated oxygen during the catalytic cycle. Science , this issue p. 898 ; see also p. 866 Copper ions can move about and pair up in a zeolite framework as they catalyze nitric oxide removal from diesel exhaust. Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NO x ) with ammonia (NH 3 ), but the low-temperature rate dependence on copper (Cu) volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O 2 )–mediated Cu I →Cu II redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.

The effect of support on sulfur tolerance and regenerability under reducing environments was investigated by rate measurements for ethylene hydrogenation, hydrogen chemisorption, and extended X-ray absorption fine structure (EXAFS). Catalysts, 1 % Pt/Al 2 O 3 and 1 % Pt/P25 (TiO 2 ), were tested after sulfidation in H 2 S/H 2 at 250 °C followed by regeneration treatments in H 2 at 250, 350 and 450 °C. Our combined results showed a 20–27 times decrease in the rate of ethylene hydrogenation on both sulfided catalysts, accompanied by a 4–6 times drop in the Pt surface area. Regenerations up to 450 °C were unable to remove all the sulfur, as evidenced by the presence of Pt–S bonds by EXAFS at about 2.25–2.33 Å, characteristic lengths for chemisorbed sulfur and bulk-type PtS. However, a partial recovery of the hydrogenation rate per mole of Pt was observed on sulfided Pt/Al 2 O 3 after reduction at 450 °C, while the induction of strong metal support interactions (SMSI) at reduction temperature above 350 °C was observed on Pt/P25, regardless of the presence of sulfur. For Pt/P25, the reversal of the SMSI state together with sulfur removal by mild oxidation suggests that sequential reduction/oxidation treatments may be more effective in restoring the S-free state of TiO 2 -supported catalysts. Graphical Abstract Pt/Al 2 O 3 and Pt/TiO 2 (P25) sulfur tolerance and regenerability were evaluated after reduction treatments in H 2 . Both catalysts were equally poisoned by sulfur based on C 2 H 4 hydrogenation. Reduction treatments up to 450 °C were not able to remove sulfur on either catalyst. Sulfur on Pt may inhibit the formation of the SMSI state on Pt/TiO 2 , especially below 350 °C.

Single-site Ga/SiO.sub.2 catalysts exhibit up to 99% C.sub.3H.sub.6 selectivity at 4% propane conversion with an initial rate of 5.4 x 10.sup.-4 (mole C.sub.3H.sub.6) (mole Ga).sup.-1 s.sup.-1 during propane dehydrogenation (PDH) at 550 °C. Following pre-treatment in H.sub.2 at 550 °C, only four-coordinate, Ga.sup.3+-O Lewis acid sites are observed under reaction conditions. At 650 °C in H.sub.2, an additional isolated Ga site with lower Ga-O coordination (N.sub.Ga-O < 4) is formed and leads to a 30% decrease in the initial PDH rate per total moles of Ga. The PDH rates are equivalent when normalized by the amount of surface, four-coordinate Ga.sup.3+-O, regardless of catalyst pre-treatment conditions, which indicates that these isolated Ga.sup.3+ centers are the catalytically relevant sites.

Isothermal oscillations in the rate of decomposition of N2O were studied on an over-exchanged Cu-ZSM-5 catalyst by mass spectroscopy and in situ transient FTIR. Oscillations in the production of O2 and N2 were observed to occur in a temperature range of 410–490°C at a total pressure of 1.0 Torr pure N2O. FTIR has provided the first spectroscopic evidence that surface nitrate species are present under oscillatory conditions. This study confirmed a previously proposed model that predicts a slow build-up of surface nitrates, followed by a rapid nitrate decomposition coupled with an increase in the rate of N2O decomposition. The IR signature of the surface nitrates suggests they are monodentate nitrate species bound to Cu2+ ions. Temperature-programmed desorption studies reveal a strong correlation between the stability of the surface nitrate species and the temperature range in which oscillations occur.

Atomic force microscopy (AFM) has been used to study the morphology of an ultrafine gold‐on‐titania catalyst. By using TappingModeTM AFM (TMAFM) and SuperSharp silicon probes to minimize tip radius artifacts, we determined values for the average Au particle diameter and the gold loading in good agreement with high‐resolution transmission electron microscopy (HRTEM) results. These results demonstrate the ability of AFM to characterize real supported metal catalysts with small metal particles (<5 nm) and low metal loadings, achieving resolution comparable to HRTEM, but in the ambient environment.