The influence of different iron carbides on the activity and selectivity of iron-based Fischer−Tropsch catalysts has been studied. Different iron carbide phases are obtained by the pretreatment of a binary Fe/SiO 2 model catalyst (prepared by coprecipitation method) to different gas atmospheres (syngas, CO, or H 2 ). The phase structures, compositions, and particle sizes of the catalysts are characterized systematically by XRD, XAFS, MES, and TEM. It is found that in the syngas-treated catalyst only χ-Fe 5 C 2 carbide is formed. In the CO-treated catalyst, Fe 7 C 3 and χ-Fe 5 C 2 with a bimodal particle size distribution are formed, while the H 2 -treated catalyst exhibits the bimodal size distributed ε-Fe 2 C and χ-Fe 5 C 2 after a Fischer−Tropsch synthesis (FTS) reaction. The intrinsic FTS activity is calculated and assigned to each corresponding iron carbide based on the phase composition and the particle size. It is identified that Fe 7 C 3 has the highest intrinsic activity (TOF = 4.59 × 10 −2 s −1 ) among the three candidate carbides (ε-Fe 2 C, Fe 7 C 3 , and χ-Fe 5 C 2 ) in typical medium-temperature Fischer−Tropsch (MTFT) conditions (260−300 °C, 2−3 MPa, and H 2 /CO = 2). Moreover, FTS over ε-Fe 2 C leads to the lowest methane selectivity.
We use Monte Carlo simulations to demonstrate generic scaling aspects of classical phase transitions approached through a quench (or annealing) protocol where the temperature changes as a function of time with velocity v. Using a generalized Kibble-Zurek ansatz, we demonstrate dynamic scaling for different types of stochastic dynamics (Metropolis, Swendsen-Wang, and Wolff) on Ising models in two and higher dimensions. We show that there are dual scaling functions governing the dynamic scaling, which together describe the scaling behavior in the entire velocity range v ∈ [0, ∞). These functions have asymptotics corresponding to the adiabatic and diabatic limit, and close to these limits they are perturbative in v and 1/v, respectively. Away from their perturbative domains, both functions cross over into the same universal power-law scaling form governed by the static and dynamic critical exponents (as well as an exponent characterizing the quench protocol). As a by-product of the scaling studies, we obtain high-precision estimates of the dynamic exponent z for the two-dimensional Ising model subject to the three variants of Monte Carlo dynamics; for single-spin Metropolis updates zM = 2.1767(5), for Swendsen-Wang multi-cluster updates zSW = 0.297(3), and for Wolff singlecluster updates zW = 0.30(2). For Wolff dynamics, we find an interesting behavior with a non-analytic breakdown of the quasi-adiabatic and diabatic scaling, instead of the generic smooth cross-over described by a power law. We interpret this disconnect between the two scaling regimes as a dynamic phase transition of the Wolff algorithm, caused by an effective sudden loss of ergodicity at high velocity.
A series of Pd−WO x /Al 2 O 3 catalysts with different contents of WO x were prepared by stepwise incipient wetness impregnations. The influence of WO x on the physicochemical properties of Pd−WO x /Al 2 O 3 catalysts, as well as their catalytic performance for the hydrogenolysis of glucose to 1,2-propanediol (1,2-PDO), was investigated. At low surface W density (0.3−2.1 W nm −2 ), distorted isolated WO x and oligomeric WO x are present on the Pd−WO x /Al 2 O 3 catalysts. Furthermore, isolated WO 4 are the dominating species on the Pd−WO x (5%)/Al 2 O 3 catalyst. When the W density increased to 3.1 W nm −2 , polymeric WO x species are dominant on the Pd−WO x (30%)/Al 2 O 3 catalyst. The Pd surface area decreased while the acid amount increased with increasing W density. Furthermore, increased Lewis acid sites are provided by isolated WO 4 and oligomeric WO x species whereas increased Brønsted acid sites exist on polymeric WO x species. Lewis acid sites promote glucose isomerization to fructose, which is an intermediate in glucose hydrogenolysis to 1,2-PDO. Metal sites catalyze CO hydrogenation and C−C hydrogenolysis, which avoid the coke formation on catalysts. 1,2-PDO selectivity is dependent on the synergy of Lewis acid and metal sites; however, Brønsted acid sites have no contribution to the 1,2-PDO production. Typically, the Pd−WO x (5%)/Al 2 O 3 catalyst possessing the optimal balance of Lewis acid and the metal site shows a 1,2-PDO selectivity of 60.8% at a glucose conversion of 92.2% and has a lifetime of over 200 h.
Using the example of Zn-doped La2CuO4, we demonstrate that a spinless impurity doped into a non-frustrated antiferromagnet can induce substantial frustrating interactions among the spins surrounding it. This counterintuitive result is the key to resolving discrepancies between experimental data and earlier theories. Analytic and quantum Monte Carlo studies of the impurity-induced frustration are in a close accord with each other and experiments. The mechanism proposed here should be common to other correlated oxides as well. PACS numbers: 75.10.Jm, 75.30.Ds, 78.70.Nx Impurities are known to be an effective tool to locally perturb quantum systems, thereby revealing important information about their microscopic interactions and correlations . A well studied example of a strongly correlated quantum system in which effects of such impurity doping can be investigated is La 2 CuO 4 -one of the most important cuprate superconductor parent compounds. In its pristine form, this material is a two-dimensional (2D) spin-1 2 Heisenberg antiferromagnet (AF) . It is believed that the substitution of Cu 2+ (S = 1 2 ) ions by spinless Zn 2+ represents a good realization of the site-diluted Heisenberg hamiltonian [3,4,5,6]. In this Letter, we demonstrate that there exists a significant qualitative correction to the dilution picture. Impurities can induce substantial frustrating interactions between nearby spins. Not only does this effect explain discrepancies between experimental data and the dilution-only theories for La 2 Cu 1−x Zn x O 4 , but it may also be important for a variety of other phenomena in diluted magnets and doped Mott insulators. Our mechanism for such an effect should be common to many charge-transfer insulators, including oxides of transition metals.We propose that the presence of extra degrees of freedom due to oxygen orbitals necessarily results in frustrating terms in the corresponding low-energy spin hamiltonian of the Zndoped system, which are absent in the dilution-only models. Utilizing quantum Monte Carlo (QMC) and analytic T -matrix approaches, we calculate the doping dependence of the staggered magnetization for such a low-energy model. We show that this model, with the parameters appropriate for the CuO 2 planes given by a three-band Hubbard model calculation, naturally explains experimental data.Experiments and theories.-Comprehensive studies of the problem of La 2 CuO 4 diluted by spinless Zn impurities have been performed using neutron scattering, magnetometry, and NMR (NQR) on the experimental side [3,4], and QMC and T -matrix approaches of the diluted Heisenberg model on the theoretical side [5,6,7]. These studies allow for extensive cross-checks. The unbiased QMC data agree with the Tmatrix results closely up to x ≃ 15%, supporting the validity of the latter in the low-doping regime [5,6]. However, there are serious discrepancies between theoretical and experimental results. Fig. 1 shows the average magnetic moment M per Cu site versus the Zn doping fraction x. The experimental data a...
The monogenic genes Ol-1, ol-2, and Ol-4 confer resistance to tomato powdery mildew Oidium neolycopersici via different mechanisms. The biochemical mechanisms involved in these monogenic resistances were studied by monitoring through time the association of H2O2 and callose accumulation with hypersensitive response (HR) and papilla formation. Our results showed that H2O2 and callose accumulation are coupled with both Ol-1- and Ol-4-mediated HR-associated resistance as well as with the ol-2-mediated papillae-associated resistance. Further, the transcriptomal changes related to these monogenic resistances were studied by using cDNA-amplification fragment length polymorphism. The expression profiling clarified that 81% of DE-TDF (differentially expressed transcript-derived fragments) were up-regulated upon inoculation with O. neolycopersici in both the compatible and Ol-1-mediated incompatible interactions, though with a difference in expression timing. Of these DE-TDF, more than 70% were not detected in the Ol-4-mediated resistance, while 58% were expressed in the ol-2-mediated resistance, generally at later timepoints. Sequence information suggested that most of these DE-TDF are related to genes involved in either basal defense or establishment of compatibility. In addition, DE-TDF (19%) specifically expressed in different incompatible interactions were identified. Expression patterns of some DE-TDF and marker gene GluB suggested that papillae-associated resistance exploits a different defense pathway from that of HR-associated resistance.
Chutes and Ladders is an exciting up-and-down-again game in which players race to be the first to the top of the board. Along the way, they will find ladders to help them advance, and chutes that will cause them to move backwards. The development of nucleoside analogs for clinical treatment of hepatitis C presents a similar scenario in which taking shortcuts may help quickly advance a program, but there is always a tremendous risk of being sent backwards as one competes for the finish line. In recent years the treatment options for chronic hepatitis C virus (HCV) infection have expand due to the development of a replicon based in vitro evaluation system, allowing for the identification of multiple drugable viral targets along with a concerted and substantial drug discovery effort. Three major drug targets have reached clinical study for chronic HCV infection: the NS3/4A serine protease, the large phosphoprotein NS5A, and the NS5B RNA-dependent RNA polymerase. Recently, two oral HCV protease inhibitors were approved by the FDA and were the first direct acting anti-HCV agents to result from the substantial research in this area. There are currently many new chemical entities from several different target classes that are being evaluated worldwide in clinical trials for their effectiveness at achieving a sustained virologic response (SVR) (Pham et al., 2004; Radkowski et al., 2005). Clearly the goal is to develop therapies leading to a cure that are safe, widely accessible and available, and effective against all HCV genotypes (GT), and all stages of the disease. Nucleoside analogs that target the HCV NS5B polymerase that have reached human clinical trials is the focus of this review as they have demonstrated significant advantages in the clinic with broader activity against the various HCV GT and a higher barrier to the development of resistant viruses when compared to all other classes of HCV inhibitors.
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