J. Rodríguez-Mirasol

Heterogeneous catalytic decomposition of nitrous oxide

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N2O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.

NO and N2O decomposition over coal char at fluidized-bed combustion conditions

The decomposition of NO and N2O by chars has been studied in a fixed bed reactor under steady state conditions. The reactions were studied at temperatures from 673 to 1223 K and at partial NO and N2O pressures in the range of 0.62–3.08 and 0.19–1.92 mbar, respectively. The reactor pressure was 2.5 bar and the space time W/FNO,o(N2O,o)) was either 79280 or 100140 g s/mol for NO and N2O, respectively. Four coals ranging from lignite to anthracite were used as char precursors. N2O was more readily reduced on a coal char surface than NO. The kinetic parameters for the NO and N2O decomposition on a char surface were rather dependent on char structure. Apparent activation energies in the range of 111–181 and 66–104 kJ/mol were obtained for NO and N2O decomposition, respectively. Apparent reaction orders for NO decomposition on the chars ranged from 0.22 to 0.43. In the case of N2O, an order of ∼ 0.6 was found except for DE53 char that showed an almost zero order at lower temperatures. A large confidence interval was found for the reaction order, which indicates that it is changing with temperature. The rate of NO decomposition by char is strongly enhanced by the presence of CO and N2O; this is ascribed to surface oxygen removal. CO had no effect on N2O decomposition, since N2O itself acts as a surface oxygen liberating species. When O2 was added to the NO/char reaction, the NO decomposition increased at the beginning of the addition, but this effect decreased with time because of the rapid combustion of the char. O2 did not affect the N2O/char reaction.

Decomposition of nitrous oxide over ZSM-5 catalysts

Abstract Comparative kinetic and in-situ DRIFT studies of the N 2 O decomposition over Co-, Fe- and Cu-ZSM-5 have been performed. The implications of the presence of O 2 , CO, NO, H 2 O and SO 2 on the catalyst activity and stability and on the mechanism are evaluated.

Phosphorus functionalization for the rapid preparation of highly nanoporous submicron-diameter carbon fibers by electrospinning of lignin solutions

This work presents a fast and versatile method to prepare carbon fibers from lignin. It involves the production of submicron-sized phosphorus-functionalized lignin fibers in only one step by electrospinning of lignin/H3PO4 solutions. The phosphorus functionalities enable shortening of the conventional stabilization process from more than 90 h to only 2 h thus avoiding fiber fusion or even stabilizing the lignin fibers in an inert atmosphere. The incorporation of H3PO4 into the initial lignin solution produces more oxidized spun lignin fibers, due to the reaction of phosphoric acid with the dissolved lignin, generating phosphate (and/or polyphosphate) esters throughout the structure of lignin fibers. These phosphate groups seem to be responsible for the production of cross-linking reactions during the stabilization step that are, in this case, very active and effective in increasing the glass transition temperature of the lignin fibers, reducing the time needed for the stabilization step and improving this process. Moreover, they promote the chemical activation of lignin fibers and greatly increase their oxidation resistance, avoiding their complete combustion during carbonization under a low concentration of O2 at temperatures as high as 900 °C. The resulting carbon fibers gather different interesting properties, such as sub-micron diameters (≤1 μm), large surface area (≈2000 m2 g−1), relatively high performance in relation to their mechanical properties for functional applications and a rich variety of uniformly distributed O and P surface functionalities, which make them very attractive for heterogeneous catalysis, adsorption and electrochemical applications.

Carbon/H-ZSM-5 composites as supports for bi-functional Fischer–Tropsch synthesis catalysts

Mesoporous H-ZSM-5–carbon composites, prepared via tetrapropylammonium hydroxide (TPAOH) post treatment of H-ZSM-5 followed by deposition of pyrolytic carbon, have been used as the support for the preparation of Co-based Fischer–Tropsch catalysts. The resulting catalysts display an improved performance during Fischer–Tropsch synthesis (FTS), with higher activity, higher selectivity towards C5–C9 (gasoline range) hydrocarbons and lower selectivity towards C1 (and C2) than Co/mesoH-ZSM5 (without pyrolytic carbon). This is due to the weaker metal–support interaction caused by the deposited carbon (as revealed by XPS) leading to a higher reducibility of the Co species. Further, the partial deactivation of the Bronsted acid sites by pyrolytic carbon deposition, as was observed by NH3-TPD, allows the modification of the zeolite acidity. Both the olefin to paraffin (O/P) and the isoparaffin to normal paraffin (I/N) ratios decrease with the increase in the carbon content, opening the door to further tune the catalytic performance in multifunctional FTS operations.