A sulfur-modified pore-blocking method to enhance the electrocatalytic stability of carbon-supported platinum nanoparticles.

Currently, the durability of electrode materials remains a big obstacle to the widespread adoption of proton exchange membrane fuel cells (PEMFCs). Herein thiourea and sodium dodecyl benzene sulfonate (SDS) were employed as sulfur source and carbon source to modify the pristine carbon black (Ketjen black EC300 J). A highly durable carbon supported Pt nanosized catalyst with higher platinum utilization for oxygen reduction reaction (ORR) in PEMFCs was produced by doping elemental sulfur into carbon supports and decreasing the carbon pore sizes and volume through a successive impregnation technique. The catalyst exhibits an initial activity of 0.167 A mgPt -1 at 0.90 V and demonstrates minimal activity loss after acceleration stress test (30,000 cycles of AST). The half-wave potential loss for representative sample (Pt/S-C-3) is only 14 mV with only 21.8 % ECSA decrease, 27.5 % MA loss and 5.9 % SA loss. A sintering test at various temperature shows a minor average size increase for sulfur-doped carbon (S-C) supported one (from 2.09 to 2.52 nm). In single-cell test, the MEA sample employing the platinum catalyst on modified carbon as cathode exhibited almost negligible performance loss after 30,000 cycles of AST.

Thrust Vectoring Control of a Novel Tilt-Rotor UAV Based on Backstepping Sliding Model Method.

In this paper, a control method of a novel tilt-rotor UAV with a blended wing body layout is studied. The novel UAV is capable of vertical take-off and landing and has strong stealth capabilities that can be applied to carrier-borne reconnaissance aircraft. However, the high aspect ratio of blended wing body UAVs leads to a wingtip or oar-tip touchdown problem when adopting the conventional position-attitude control (CPAC) scheme with a large crosswind disturbance. Moreover, when the UAV is subject to interference during reconnaissance, aerial photography, and landing missions, the conventional scheme cannot provide both attitude stability and track accuracy. First, a direct thrust vectoring control (DTVC) scheme is proposed. The control authority of the rotor tilt mechanism was added to enable the decoupling of the attitude and trajectory and to improve the response rate and response bandwidth of the flight trajectory. Second, considering the problems of strong couplings and parameter uncertainties and the nonlinear features and mismatched perturbations that are inevitable in the tilt-rotor, we designed a robust UAV controller based on the backstepping sliding mode control method and determined the stability of the control system through the Lyapunov function. Finally, in the case of crosswire interference during vertical takeoff and landing and the aerial photography missions of the UAV, the numerical simulation of the CPAC scheme and the DTVC scheme was carried out, respectively, and the Monte Carlo random test method was introduced to conduct the statistical test of the landing accuracy. The simulation results show that the DTVC scheme improves the landing accuracy and speed compared to the CAPC scheme and decouples the position control loop from the attitude control loop, finally enabling the UAV to complete the flight control in the VTOL phase.

Study of Modeling and Optimal Take-Off Scheme for a Novel Tilt-Rotor UAV.

The optimal trajectory planning for a novel tilt-rotor unmanned aerial vehicle (UAV) in different take-off schemes was studied. A novel tilt-rotor UAV that possesses characteristics of both tilt-rotors and a blended wing body is introduced. The aerodynamic modeling of the rotor based on blade element momentum theory (BEMT) is established. An analytical method for determining the taking-off envelope of tilt angle versus airspeed is presented. A novel takeoff-tilting scheme, namely tilting take-off (TTO), is developed, and its optimal trajectory is designed based on the direct collocation method. Parameters such as the rotor thrust, tilt angle of rotor and angle of attack are chosen as control variables, and the forward velocity, vertical velocity and altitude are selected as state variables. The time and the energy consumption are considered in the performance optimization indexes. The optimal trajectories of the TTO scheme and other conventional schemes including vertical take-off (VTO) and short take-off (STO) are compared and analyzed. Simulation results indicate that the TTO scheme consumes 47 percent less time and 75 percent less energy than the VTO scheme. Moreover, with minor differences in time and energy consumption compared to the STO scheme, but without the need for sliding distance, TTO is the optimal take-off scheme to satisfy the flight constraints of a novel tilt-rotor UAV.

Chemoselective Access to γ-Ketoesters with Stereogenic Quaternary α-Center or γ-Keto Nitriles by Aerobic Reaction of α-Cyanoesters and Styrenes.

Chemoselective access to either γ-ketoesters with a quaternary all-carbon α-stereogenic center or γ-keto nitriles is described by copper-catalyzed aerobic reaction of styrenes with α-cyanoesters. Formal oxo-enolation or oxo-cyanomethylation of styrenes is achieved via a sequence of addition of enolate (or cyanomethyl) radical to olefin and oxidation of the resulting radical adduct. This method starts from abundant and cheap feedstock under aerobic conditions, without any prefunctionalization or the production of stoichiometric metal salts waste, making it very attractive for practical use.

Divergent synthesis of indoles, oxindoles, isocoumarins and isoquinolinones by general Pd-catalyzed retro-aldol/α-arylation.

Divergent synthesis of indoles, oxindoles, isocoumarins and isoquinolinones is described in this report by using a general Pd-catalyzed tandem reaction of ß-hydroxy carbonyl compounds with aryl halides bearing an ortho-nitro, -ester or -cyano substituent. A key retro-aldol/α-arylation reaction is involved that merges classic Pd cross-coupling chemistry with novel Pd-promoted retro-aldol C-C activation to produce α-arylated ketones or esters. Subsequent intramolecular condensation of the carbonyl with the ortho-synthon gives target heterocycles. The use of common, commercially available and cheap substrates and catalyst system adds additional synthetic advantages to the conceptual significance.

C−C Activation by Retro-Aldol Reaction of Two ß­Hydroxy Carbonyl Compounds: Synergy with Pd-Catalyzed Cross-Coupling To Access Mono-α-arylated Ketones and Esters.

A retro-aldol reaction of two ß-hydroxy compounds in synergy with Pd-catalyzed cross-coupling of aryl halides is reported herein, which produces selectively mono-α-arylated ketones and esters in good yields. This reaction is compatible with a broad range of aryl iodides, bromides, chlorides, and triflates and can tolerate an array of functional groups on the aromatic ring. Ready scale-up of this reaction to gram level is applicable without an appreciable decrease in the reaction yield. Furthermore, concise syntheses of biologically active isocoumarin and indole derivatives have been achieved to greatly demonstrate the synthetic value of this retro-aldol reaction. Finally, the reaction mechanism has been discussed on the basis of experimental observations and DFT computational results. A regulated six-membered-ring transition structure has been located for the key retro-aldol C−C cleavage, which constitutes the rate-determining step of a full catalytic cycle. The concept of C−C activation by retro-aldol reaction may also find applications in other fundamental reactions.

Trade-offs between the metabolic rate and population density of plants.

The energetic equivalence rule, which is based on a combination of metabolic theory and the self-thinning rule, is one of the fundamental laws of nature. However, there is a progressively increasing body of evidence that scaling relationships of metabolic rate vs. body mass and population density vs. body mass are variable and deviate from their respective theoretical values of 3/4 and -3/4 or -2/3. These findings questioned the previous hypotheses of energetic equivalence rule in plants. Here we examined the allometric relationships between photosynthetic mass (M(p)) or leaf mass (M(L)) vs. body mass (beta); population density vs. body mass (delta); and leaf mass vs. population density, for desert shrubs, trees, and herbaceous plants, respectively. As expected, the allometric relationships for both photosynthetic mass (i.e. metabolic rate) and population density varied with the environmental conditions. However, the ratio between the two exponents was -1 (i.e. beta/delta = -1) and followed the trade-off principle when local resources were limited. Our results demonstrate for the first time that the energetic equivalence rule of plants is based on trade-offs between the variable metabolic rate and population density rather than their constant allometric exponents.