Papers by Author: Carmen M. Rangel

Abstract: Direct electrochemical reduction of CO₂ is a process that could contribute to the reduction of the emission of greenhouse gases by using CO₂ as a raw material for fuel production. This paper focuses on voltammetric studies of functionalized electrodes for the electrochemical conversion of CO₂ and reports on its use as a tool for electrode screening and optimization. Nickel substrates modified with copper and ruthenium/copper electrodeposits were studied. Voltammetric experiments indicate that CO₂ electroreduction follows a nickel type mechanism in which this electrochemical reaction occurs simultaneously and in competition with hydrogen evolution. A significant inhibition of hydrogen evolution reaction is observed in nickel modified electrodes. Inhibition characteristics and the onset of carbon dioxide conversion are dependent of the type of electrode functionalization. Voltammetry is thus a powerful tool to evaluate electrode modifications and for tuning electrodes for an optimized electrocatalytic performance.

Abstract: Depending on the energy level used during mechanical alloying, the constitution of the resulting products can vary extensively. With high energy input, full transformation to the equilibrium phase, FeTi, is achieved. In contrast, for low levels of energy input, the process is akin to mixing without any phase transformation even for extended milling periods. In the present work, nanostructured FeTi powders were produced by mechanical alloying, avoiding the unfavourable agglomeration problem, by using a relatively low level of energy (e.g. 300 rpm) to mill the pure metallic constituents, Fe and Ti, followed by subsequent heat treatment at 800°C. A major achievement of this research was to show that, by modulating the milling intensity and total milling time, the high temperature synthesis reaction of FeTi (1100°C) can be partially or totally suppressed, reverting instead to a metastable reaction path at low temperature (650°C). The mechanical “activation” modifies the reactivity of the system, producing a very thin Ti /Fe layers. That in conjunction with a high level of defects induced mechanically may be responsible for the metastability. Partial substitution of Fe with Ni (10%) resulted essentially in the same phase constitution, indicating solid solution of Ni in FeTi replacing partially Fe lattice positions.

Abstract: FeTi intermetallic powders are very promising media for reversible hydrogen storage. However, difficult activation treatments including annealing at elevated temperatures in high pressure H2 gas atmosphere are mandatory. In the present work nanostructured FeTi powders were produced and activated in situ at room temperature using mechanical alloying/milling (MA/MM) of pure metallic constituents, Fe and Ti, added with sodium borohydride. The resultant powders, FeTiHx, already H2 pre-charged, absorbed a significant amount of H2 but require optimization for reversible absorption/desorption. This system has one of the highest volumetric storage capacities and can be produced at low cost. Several parameters of the as-milled powders were controlled. The phase constitution of the reaction products was characterized by X-ray diffraction and scanning electron microscopy and the absorption isotherms of the activated powders were determined.

Abstract: The reactors used for Selective Catalytic Reduction (SCR) of NOx require low pressure drop structured catalyst packing. Structured packings, such as ceramic foams, are gaining increasing interest for application in low pressure drop reactors, membrane reactors and catalytic distillation units. In this work, cobalt ion exchanged mordenite (Co-HMOR)-coated cordierite-based foams produced by the replication method were evaluated for catalytic reduction of NOx with methane. The addition of 0.3 wt.% Pd to 2 wt.% Co-HMOR leads to a material that can convert 50 % NOx to N2 at 450 °C in a reaction mixture containing 2000 ppm CH4, 1000 ppm NOx, 5 % O2 and balance helium, at GHSV=17000 h-1. Although in an early stage of development, an efficient coating procedure was explored and different ways of exchange of Co and Pd cations into mordenite (Si/Al=10) were studied. Additions of 2 wt.% fumed silica enhanced adhesion of the zeolite onto the ceramic foam. Pd-exchanged Co-HMOR showed to be very sensitive to steam. A 50 % decrease in NOx conversion to N2 was observed after Pd/Co-HMOR samples were exposed at 450 °C to a reaction mixture containing 2 vol% H2O. Although further research is needed to ascertain the mechanism of this deactivation behaviour, agglomeration of Pd forming PdO particles is envisaged.

Abstract: Degussa P-25 TiO2 powder was used as a catalyst in the photocatalytic oxidation of organics. With the objective of promoting a more effective electron-hole separation upon irradiation, the catalyst was modified with small amounts of platinum. Characterisation of the powders was done by X-Ray diffraction and SEM. Chloroform was used as a model molecule and its degradation followed using selective electrodes, with excellent results for the modified titania. Langmuir-Hinselwood kinetic was used to describe the heterogeneous oxidation of chloroform on TiO2, for low solute concentrations.