Florence Epron

Palladium and platinum-based catalysts in the catalytic reduction of nitrate in water: effect of copper, silver, or gold addition

Abstract Supported bimetallic palladium and platinum catalysts promoted by metals of group 11 (Cu, Ag, and Au) were prepared by control surface deposition and tested in the liquid-phase reduction of nitrates. Whereas bimetallic catalysts promoted by gold are totally inactive, copper or silver deposition leads to bimetallic catalysts active for nitrate reduction. The promoting effect of the second metal can be related to its redox properties, confirming that nitrate reduction occurs through a bifunctionnal mechanism following (i) a direct redox mechanism between promoter and nitrate and (ii) a catalytic reaction between hydrogen, chemisorbed on the noble metal, and intermediate nitrite. TEM experiments, TPR, and FTIR of chemisorbed CO studies of the different systems have been used to evidence the metal–metal interaction and the localization of the promoter. The characterization results have been correlated with the catalytic behavior of the materials.

Heterogeneous catalytic hydrogenation of biobased levulinic and succinic acids in aqueous solutions.

Supported noble-metal catalysts (Ru, Pd or Pt) and the corresponding Re-promoted catalysts exhibit a high activity for the hydrogenation of biobased carboxylic acids. Levulinic acid and succinic acid are converted into the lactones or the diols depending on the nature of the catalyst and the reaction conditions. The highest selectivity to 1,4-pentanediol of 82 % is achieved at 140 °C in the presence of the 1.9 % Ru-3.6 % Re/C catalyst.

Design of nanocatalysts for green hydrogen production from bioethanol.

Bioethanol is an interesting feedstock that may be used for hydrogen production by steam or autothermal reforming. However, the impurities (heavy alcohols, esters, acids, N compounds) contained in the raw feedstock require a costly purification, as they have a dramatic impact on catalyst activity and stability. Thus, a method that can utilize the raw feedstock without severe degradation of the catalyst would be desirable. In this Minireview, the composition of bioethanol from first and second generation biomass, the reactions involved in the catalytic ethanol steam reforming process and the design of catalysts adapted for hydrogen production from a real bioethanol feed are surveyed.

Nitrate reduction in water: Influence of the addition of a second metal on the performances of the Pd/CeO2 catalyst

An attempt is made to improve the catalytic nitrate reduction on Pd/CeO(2) catalysts by the addition of a second metal. The influence of the second metal such as Sn, In and Ag on the Pd/CeO(2) for nitrate reduction is explored. The second metal is introduced over monometallic Pd/CeO(2) by a redox reaction. Pd/CeO(2) is more active than the bimetallic catalysts under pure hydrogen flow. Whereas in presence of CO(2) the monometallic Pd/CeO(2) is inactive for nitrate reduction, bimetallic catalysts are found to be more active than under pure hydrogen flow and also than the monometallic catalyst with a low selectivity towards ammonium ions, undesired product of the reaction. The Pd-Sn/CeO(2) catalyst is comparatively the most suited for nitrate reduction.

Catalytic and electrocatalytic oxidation of oxalic acid in aqueous solutions of different compositions

Oxalic acid oxidation in aqueous solutions of different compositions (different anions and pH), with a platinum catalyst–electrode, was studied. Different experimental methods were compared: the presence or absence of gaseous oxygen, external potential control or spontaneous establishment of the potential in the presence of the reactants. The intrinsic catalytic activity of platinum was determined by analytical techniques. The presence of strongly adsorbed anions in the solution induces a decrease of the catalytic activity of platinum. The weak ‘anion-like’ adsorption of oxalic acid, occurring in competition with the adsorption of different anions, can account for this result. Undissociated oxalic acid is more reactive than oxalate and dioxalate anions. The activity of platinum measured at a given potential, is higher in catalytic than in electrocatalytic oxidation of oxalic acid, suggesting that different adsorbed active oxygen species are involved in these two oxidation processes.

Insights into the Oxidation State and Location of Rhenium in Re‐Pd/TiO2 Catalysts for Aqueous‐Phase Selective Hydrogenation of Succinic Acid to 1,4‐Butanediol as a Function of Palladium and Rhenium Deposition Methods

ReOx‐Pd/TiO2 catalysts prepared from different 2 wt %Pd/TiO2 catalysts using two protocols for the deposition of the Re promoter (successive impregnation and catalytic reduction) were characterized by different techniques to better understand the nature of the active and selective sites implied in the aqueous‐phase hydrogenation of succinic acid to 1,4‐butanediol. Regardless of the support and Re introduction method, it was established that varying amounts of Pd and Re were in very close proximity without electronic interaction in the reduced catalysts. A high fraction of Re always remained partially oxidized to generate a bimetallic catalyst that can provide the necessary bifunctional sites to enable the selective hydrogenolysis of the intermediate γ‐butyrolactone to 1,4‐butanediol. Depending on the method of promotion, the ReOx species that interact with Pd were deposited as clusters with different spatial Re–Re interactions.

Synthesis, characterization and catalytic properties of polypyrrole-supported catalysts

Polypyrrole, a conductive polymer, was used as support of noble metals, namely, copper and palladium, for application in heterogeneous catalysis. Catalysts were characterized by TEM coupled with EDS and XRD. Their activity and selectivity were determined in the reduction of nitrate and intermediate nitrite in water under 1 bar of hydrogen and at room temperature. In nitrite reduction, these novel catalysts demonstrated a better activity and a higher selectivity towards nitrogen formation than their Pd/Al2O3 counterpart. On the other hand, the bimetallic catalyst tested for nitrate reduction showed an activity similar to the one of a classical Pd–Cu/Al2O3 catalyst, but no intermediate nitrite was observed.

Bioethanol reforming for H2 production. A comparison with hydrocarbon reforming

Hydrogen is essentially produced by steam reforming (SR) of hydrocarbon fractions (natural gas, naphtha, …) on an industrial scale. Replacing fossil fuels by biofuels for H2 production has attracted much attention with an increased interest for bioethanol steam reforming. Kinetics and mechanisms of hydrocarbon-SR and alcohol-SR present some similarities but also some very important differences due to alcohol reactivity much more complex than that of hydrocarbons. The scope of this report is to compare the two processes in terms of reaction mechanisms. Attention will also be paid to the case of crude bioethanol.

Study of the main reactions involved in reforming of exhaust gas recirculation (REGR) in gasoline engines

In a previous paper, it was demonstrated that a 1 wt% Rh catalyst supported on modified ZrO2 exhibits a high activity and stability in REGR conditions. The purpose of the present study was to investigate the main reactions involved in REGR conditions in the presence of this catalyst, namely steam reforming (SR), dry reforming (DR), and the reactions producing methane, i.e. methanation, decomposition and hydrogenolysis. Isooctane (C8H18) was used as a molecule representative of gasoline, and a mixture of N2, H2O, CO2 and O2 as model exhaust gas. A reaction scheme was proposed for REGR conditions. Based on the study of individual reactions (SR, DR and methane formation) at 450 °C, 520 °C and 580 °C, the complex system can be described following three consecutive steps: reforming reactions, yielding H2 and CO2 as primary products; reverse water gas shift (RWGS), yielding CO as secondary product; and finally methanation reactions, specially with CO, yielding CH4 as tertiary product.

Comparison in Dimethyl Ether Steam Reforming of Conventional Cu-ZnO-Al2O3 and Supported Pt Metal Catalysts

The catalytic performance of platinum based catalysts supported on CeO2/Al2O3 (with different proportion of both oxides) was compared to the conventional Cu-ZnO-Al2O3 (CZA) catalyst for methanol steam reforming (MeOH-SR), which is the second stage in the dimethyl ether steam reforming (SRD) process for obtaining H-2. The catalytic performances of Pt based catalysts were notably influenced by the addition of CeO2 to the support: a higher content of CeO2 improved the catalyst activity and selectivity to H-2, and limited to some extent CO and CH4 formation. At low temperatures (below 350 degrees C), the copper based CZA catalyst provided the best results in MeOH-SR, but at high temperature (350-500 degrees C) Pt/CeO2 and CZA showed similar catalytic performance. Consequently, Pt/CeO2 can be envisioned as a proper metallic function to be used in the bifunctional catalyst for SRD together with gamma-Al2O3 acid function, which requires temperatures above 350 degrees C for attaining high conversion in dimethyl ether hydrolysis.