Active control of catalysis and product selectivity by a fuel cell system

A new concept to control catalysis and catalytic reaction through partial oxidation of alkenes with O₂ is described. Oxidation of alkenes was studied by alkene/Pd-anode/H₃PO₄-electrolyte/cathode/O₂ fuel cell (FC). An idea based on electrocatalysis and electrochemical reactions to control reaction rates and product selectivity was proposed and proven through the oxidation of propylene, Wacker and π-allyl oxidation. The oxidation rate and the product selectivity to the Wacker and the π-allyl oxidations could be controlled by changing electrode potentials. We could active control oxidation states of Pd on the anode, Pd(II) or Pd(0), during the oxidation from outer circuit. The oxidation states of Pd on the anode decided the product selectivity.     

Supramolecular origins of product selectivity for methanol-to-olefin catalysis on HSAPO-34.

Ethylene selectivity in methanol-to-olefin (MTO) catalysis is related to the number of methyl groups on benzene rings trapped in the nanocages of the preferred catalyst HSAPO-34. By correlating the time evolutions of the catalysts' 13C NMR spectra and the volatile product distribution following abrupt cessation of methanol flow, we discovered that (in the absence of other adsorbates) propene is favored by methylbenzenes with four to six methyl groups but ethylene is predominant from those with two or three methyl groups. We substantially increased ethylene selectivity by operating at lower methanol partial pressures or higher temperatures, either of which reduces the steady-state average methyl substitution. As a step toward a kinetic analysis of the MTO reaction on HSAPO-34, we treated each nanocage with a methylbenzene molecule as a supramolecule capable of unimolecular dissociation into ethylene or propene and a less highly substituted methylbenzene. Addition of a water molecule to a nanocage containing a methylbenzene produces a distinct supramolecule with unique properties. Indeed, co-feeding water with methanol significantly increased the average number of methyl groups per ring at steady state relative to identical conditions without additional water, and also increased ethylene selectivity, apparently through transition state shape selectivity.     

通过表面钨掺杂大幅提升钯纳米立方体的燃料电池催化性能（英文）

发展兼具高活性和高稳定性的规整非铂电化学催化剂无论对于燃料电池的推广应用还是基础研究都具有重要意义.我们将钯纳米立方体(Pd nanocubes)作为晶种,使用表面掺杂的手段制备了一种表面结构规整的钨掺杂钯纳米立方体(W-doped Pd nanocubes).通过改变合成过程中所加入羰基钨前驱体的量以调控表面钨的原子比例,继而获得了钨原子比例分别为0%,0.8%,1.2%,1.5%的纳米立方体.所制W-doped Pd nanocubes/C催化剂在碱性条件下的氧还原反应中表现出优异性能,其中1.2%W-doped Pd nanocubes/C催化剂性能最佳,在0.9 VRHE时比活性达1.18 mA cm~(-2),质量活性达0.25 A mg~(-1)Pd,分别是商业Pt/C催化剂的4.7倍和2.5倍.研究表明,随着钨的掺杂量从0%增至1.5%,钨掺杂钯纳米立方体的d带中心从-2.49 eV逐渐降至-3.08 eV.同时,光电子能谱结果表明,随着钨掺杂量的增加,钯的3d峰位向低能逐渐偏移,说明了钨掺杂导致了电荷由钨转向钯.而d带中心的下移能够将更多的反键态拉下费米能级,继而导致反应中间体的吸附减弱.因此,由钨到钯的电荷转移导致的d带中心的下移,继而引起的反应中间体对催化剂的吸附作用变弱是氧还原催化活性增强的原因.而过高的W掺杂(1.5%)导致活性的降低也可以用Sabatier规则解释.在循环测试10000圈之后,1.2%W-doped Pd nanocubes/C催化剂的质量活性仅仅减少了14.8%,而商业Pt/C催化剂减少了40%,可见其具有极佳的稳定性.而且循环测试之后的透射电镜表征显示,相比于团聚严重的商业Pt/C催化剂,1.2%W-doped Pd nanocubes/C催化剂仍然分散良好,其形貌也几乎没有发生变化.此外,该催化剂对乙醇氧化反应也表现出优异的性能.在1.0 mol L~(-1)氢氧化钾和1.0 mol L~(-1)乙醇混合溶液中,测试峰电流达6.6 A mg~(-1)Pd,是Pd nanocubes/C催化剂的2.2倍,商业Pd/C催化剂的5.1倍.这同样得益于适量钨掺杂所导致的催化剂d带中心—下移引起的含碳中间体吸附的削弱.经过1000 s的稳定性测试,1.2%W-doped Pd nanocubes/C同样表现出高于商业Pd/C催化剂的稳定性.优异的氧还原和乙醇氧化性能表明所制1.2%W-doped Pd nanocubes/C是一种极具潜力的双功能燃料电池催化剂.     

Active control of methanol carbonylation selectivity over Au/carbon anode by electrochemical potential

Electrochemical oxidative carbonylation of methanol was studied over Au supported carbon anode in CO. The major carbonylation products were dimethyl oxalate (DMO) and dimethyl carbonate (DMC). The minor oxidation products were dimethoxy methane (DMM) and methyl formate (MF) from methanol and CO(2). Influences of various reaction conditions were studied on carbonylation activities and selectivities. The selectivities to DMO and DMC can be controlled by the electrochemical potential. Electrocatalysis of Au/carbon anode was studied by cyclic voltammetry (CV), stoichiometric reactions among Au(3+), methanol, and CO, and UV-vis spectra. The Au/carbon anode was characterized by XRD, SEM, and BE images before and after the carbonylation. These experimental facts strongly suggest that transition of oxidation states of Au affects changing of the carbonylation selectivities to DMO and DMC. Au(0) is the active species for the selective DMO formation by direct electrochemical carbonylation at low potentials (<+1.2 V (Ag/AgCl)). On the other hand, Au(3+) is the active spices for the selective DMC formation by indirect electrochemical carbonylation through Au(3+)/Au(+) redox at high potentials (>+1.3 V).     

Control of product selectivity by a styrene additive in ruthenium-catalyzed C-H arylation

A unique effect of styrene additive on product selectivity was observed for RuH(2)(CO)(PPh(3))(3)-catalyzed C-H arylation of acetophenone derivatives bearing two ortho C-H bonds. Without styrene, the C-H arylation with arylboronates gives diarylation products as the major products throughout the reaction, but the use of styrene as an additive switches the product selectivity and leads to selective formation of monoarylation products.     

An optimization and fast load-oriented control for current-based solid oxide fuel cell system

Journal of Solid State Electrochemistry - One of the key problems for a solid oxide fuel cell (SOFC), which is a high-temperature power-generation plant, is the cooperative control of safe...     

Active sites of the functionalized coals and carbons for oxygen reduction reaction in a fuel cell

The active sites of the coals and carbons functionalized with added nitrogen, oxygen and iron were studied for the oxygen reduction reaction (ORR) in a fuel cell. The catalysts were characterized based on the XPS, Raman, TEM, XRD and N₂ adsorption measurements. The ORR activity was promoted by the addition of iron and aluminum as the inorganic components of the ash to the ash-free brown coal. The ORR activity of the ash-components added to the ash-free brown coal was correlated to the I _D/I _G ratio (deficient carbon degree) and the pyridinic nitrogen based on the Raman and XPS analyses, respectively. The active sites of the brown coal were formed at the pyridinic nitrogen on parts of the defective carbons associated with iron on the alumina. On the other hand, for the nitrogen-doped carbons without iron, the ORR activity was related to the pyrrolic-NH, pyridinic nitrogen species and the defective carbon degree. Based on these results, the active sites of the iron-added and nitrogen-doped coals and carbons were the iron sites coordinated with the pyridinic nitrogen, while the active sites of the iron-free and nitrogen-doped carbons without iron were the pyrrolic-NH and pyridinic-NH⁺ sites of parts of the defective carbons. The difference between the active sites of the nitrogen-doped coals and carbons in the presence of iron and those in the absence of iron was discussed. These results suggested that the pyridinic N as a base site transformed into pyridinic-NH⁺ as an acid site by attack of the proton from the anode.     

Modeling of a control induced system for product formation in enzyme kinetics

Double substrate enzyme kinetics has a leading role for product quantification and optimization in different chemical and biochemical sectors. Mathematical approach for controlling these reactions in different stages by suitable parameters adds a new dimension in this interdisciplinary field of research. Applying control theoretic approach in the reversible backward stages of double substrate enzymatic model, time economization with regard to product formation is significant. In this article, we formulate a double substrate mathematical model of enzymatic dynamical reaction system with control measures with a view to observe the effect of changes of these measures with respect to the concentration of substrates, enzyme, complexes and finally product. Here, Pontryagin Minimum Principle is used for observing the effect of control measures in the system dynamics with the help of Hamiltonian. We compare the relevant numerical solutions for the substrates, enzyme, complexes and product concentration profile for a specified time interval with respect to control factors.     

Exceptional gas permeation selectivity of a glued Langmuir-Blodgett bilayer by pH control

The monolayer properties of 5,11,17,23,29,35-hexakis[(N,N,N-trimethylamonium)-N-methyl-37,38,39,40,41,42-hexakis-n-hexadecyloxy-calix[6]arene hexachloride (1) have been characterized over aqueous solutions of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) as a function of pH. At high pH values (e.g., pH 10), such monolayers show relatively low surface viscosities. At low pH (e.g., pH 4.4), these monolayers exhibit relatively high surface viscosities. The barrier properties of single Langmuir-Blodgett bilayers of 1, which have been ionically cross-linked (i.e., "glued together") with PAA were found to correlate with changes in surface viscosity. Thus, bilayers that were fabricated under low pH conditions exhibited high permeation barriers and high permeation selectivity with respect to He and N(2). Given the extreme thinness of these glued bilayers (ca. 6 nm), the optimized He/N(2) selectivity of ca. 1000 is extraordinary. These results, taken together, demonstrate the feasibility of fine tuning the surface viscosity of monolayers of 1, and also the barrier properties of corresponding glued bilayers, by adjusting the pH of an aqueous subphase that contains a weak polyacid.     

Aggregation and catalysis in a nonionic surfactant-polyethylenimine-chloroform system

The formation of polymer-colloid complexes (PCCs) of hydroxybenzylated polyethylenimines (PEIs) with poly(ethylene glycol-600) monolaurate (PM) in chloroform is established through dielcometric titration, light scattering, and viscometry. The effect of PCC on the PEI reactivity in phosphorylation with 4-nitrophenyl bis(chloromethyl)phosphinate is studied with spectrophotometry. It is found that the character of aggregates and the degree of their influence depend on the PM-to-PEI concentration ratio, polymer structure, and reaction temperature.     

Improved efficiency and product selectivity in the photo-Claisen-type rearrangement of an aryl naphthylmethyl ether using a microreactor/flow system

Ultraviolet (UV) irradiation of 2-[(2,4,6-trimethylphenoxy)methyl]-1-(methoxycarbonyl)naphthalene promotes a photochemical reaction that gives a cyclohexa-2,4-dienone product arising from a photo-Claisen-type ortho-rearrangement and a phenol derivative arising from a meta-rearrangement, along with 1-methoxycarbonyl-2-methylnaphthalene and 1,2-bis[1-(methoxycarbonyl)naphthalen-2-yl]ethane. When this process is carried out in a microreactor/flow system, its efficiency is dramatically enhanced and selectivity of products is improved. The effects on efficiency and product selectivity caused by the microreactor/flow system are attributed to more efficient light absorption and the suppression of secondary reactions.     

Catalytic layers in a reversible system comprising an electrolyzing cell and a fuel cell based on solid polymer electrolyte

Electrocatalytic layers of a fuel cell-electrolyzing cell reversible system with solid polymer electrolyte are studied. The system may be used as both a dc generator and a water electrolyzing cell. It is shown that the way the polytetrafluoroethylene (PTFE) additive is introduced into the cathode’s catalytic layer affects the cathode performance. The PTFE introduction in the form of suspension in an alcohol solution of MF-4SK polymer enhanced performance. Characteristics of platinum, iridium, and platinum-iridium anode catalysts are compared. The best characteristics are obtained using a composition based on platinum black and iridium black, applied layer-by-layer, with an iridium-black layer facing the surface of a solid-polymer membrane.     

Molecular recognition in homogeneous transition metal catalysis: a biomimetic strategy for high selectivity

Traditional methods for selectivity control in homogeneous transition metal catalysis either employ steric effects in a binding pocket or chelate control. In a supramolecular strategy, encapsulation of the substrate can provide useful shape and size selectivity. A fully developed molecular recognition strategy involving hydrogen bonding or solvophobic forces has given almost completely regioselective functionalization of remote, unactivated C-H bonds.     

Toward selectivity control of a heme peptide electrode by modification with a phase-transition polymer.

Reduction currents for H2O2 at a heme peptide (HP)-modified electrodes are suppressed by inhibitors, such as imidazole derivatives. Although this inhibition effect allows determinations of the total inhibition ability of imidazole derivatives, it has no selectivity. In this study the selectivity control of HP-modified electrodes for imidazole derivatives was performed utilizing the thermoresponsive phase transition of poly(N-isopropylacrylamide), which was chemically immobilized on HP-modified electrodes. The inhibition ratios for imidazole derivatives appeared to be small at temperatures below the lower critical solution temperature (LCST), and to be large above the LCST. This change was ascribed to a steric hindrance caused by a phase transition of the polymer. On the other hand, the inhibition ratio for histamine, which has a larger molecular size relative to imidazole, was not significantly changed by the phase transition. Thus, the selectivity of the HP-modified electrode was found to be controllable using an immobilized phase-transition polymer.     

Modification of photochemical reactivity by cyclodextrin complexalion: product selectivity in photo-fries rearrangement

Cyclodextrin encapsulation, both in the solid state and in aqueous solution brings about a remarkable regulation of the photo-Fries rearrangement of phenyl esters and anilides. In comparison to the nonselective mixture of ortho and para-rearranged isomers along with the deacylated product obtained in organic solvents, the solid β-cyclodextrin complexes of unsubstituted esters and anilides show a remarkable ‘ortho-selectivity’. An impressive regio-selectivity’ among the two ortho-rearranged isomers is observed for meta-substituted esters and anilides upon irradiation as β-cyclodextrin complexes. Specific orientations of the unsubstituted and meta-substituted esters and anilides in the β-cyclodextrin cavity are suggested to be responsible for the observed selectivity.