Catalytic wet air oxidation of olive mill wastewater

Au-based catalysts, known for ambient temperature CO oxidation, have to provide stable performance of up to 5000 h in order to be commercially applicable in automotive fuel cells. In this report, the on-line deactivation characteristics of Au/TiO₂ in unconventional PROX conditions are discussed. As opposed to CO removal from air, results in this report suggests that carbonates have a minor effect on deactivation of Au/TiO₂ in dry H₂-rich conditions. Also, no conclusive correlation between surface hydration and deactivation was observed. Rather, deactivation appeared to have occurred as a result of an intrinsic transformation in the oxidation state of the active species in the reducing operating conditions; a process which was reversible in an oxidizing atmosphere.     

Selective CO oxidation in the presence of hydrogen over supported Pt catalysts promoted with transition metals

Selective CO oxidation in the presence of excess hydrogen was studied over supported Pt catalysts promoted with various transition metal compounds such as Cr, Mn, Fe, Co, Ni, Cu, Zn, and Zr. CO chemisorption, XRD, TPR, and TPO were conducted to characterize active catalysts. Among them, Pt-Ni/γ-Al₂O₃ showed high CO conversions over wide reaction temperatures. For supported Pt-Ni catalysts, Alumina was superior to TiO₂ and ZrO₂ as a support. The catalytic activity at low temperatures increased with increasing the molar ratio of Ni/Pt. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Ni and Pt was determined to be 5. This Pt-Ni/γ A1₂O₃ showed no decrease in CO conversion and CO₂ selectivity for the selective CO oxidation in the presence of 2 vol% H₂O and 20 vol% CO₂. The bimetallic phase of Pt-Ni seems to give rise to stable activity with high CO₂ selectivity in selective oxidation of CO in H₂-rich stream.     

Effect of copper loading on copper-ceria catalysts performance in CO selective oxidation for fuel cell applications

Copper–ceria catalysts with three different Cu loadings (1, 7 and 15 wt%) were prepared by incipient wet impregnation, dried at 120 °C and calcined in air at 500 °C. The as-prepared catalysts were characterized by XRD, BET, Diffuse Reflectance Spectroscopy (DRS–UV–visible), Raman spectroscopy, CO and H₂-TPR, CO-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements (with CO and O₂ concentration step-changes). The results indicated a good dispersion of copper for catalysts with 1 and 7 wt% Cu; however, bulk CuO was present for catalyst with 15 wt% Cu loading. Catalyst with 7 wt% Cu was observed to have very high capacity to release lattice oxygen to oxidize CO at low temperature. Activity results for CO oxidation in the absence and in the presence of 60% H₂, demonstrated a very similar performance for catalysts with 7 and 15 wt% Cu (both with T₁₀₀ = 112 °C), and much better than that of catalyst loaded with 1 wt% Cu. Catalyst with 7 wt% of copper shows very high activity (100% in a wide temperature window) and selectivity (higher than 85%), which makes an attractive for its use in purification of hydrogen for fuel cell applications. The presence of a mixture of CO₂ and H₂O inhibited catalyst activity, with CuO/CeO₂ catalyst with 7 wt% Cu exhibiting the best performance in the overall reaction temperature range. This could be attributed to the presence of highly disperse copper, only part of it in deep interaction with ceria. The effect of O₂/CO ratio (λ) and the potential reversibility of the inhibitory effect of CO₂ and H₂O were also investigated.     

Selective CO oxidation with real methanol reformate over monolithic Pt group catalysts: PEMFC applications

Alumina supported Pt group metal monolithic catalysts were investigated for selective oxidation of CO in hydrogen-rich methanol reforming gas for proton exchange membrane fuel cell (PEMFC) applications. The results are described and discussed in the present paper and show that Pt/γ_Al2O3$\mathrm{Pt}/\gamma {\mathrm{Al}}_2{\mathrm{O}}_3$ was the most promising candidate to selectively oxidize CO from an amount of about 1 vol% to less than 100 ppm. We have investigated the effect of the O₂ to CO feed ratio, the feed concentration of CO, the presence of H₂O and/or CO₂, and the space velocity on the activity, selectivity and stability of Pt/Al₂O₃ monolithic catalysts. Afterwards, the Pt/Al₂O₃ catalyst was scaled up and applied in 5 kW hydrogen producing systems based on methanol steam reforming and autothermal reforming. The hydrogen produced was then used as fuel for an integrated PEMFC.     

CuO–CeO2 catalysts synthesized in one-step: Characterization and PROX performance

PEM fuel cells seem to be the most affordable and commercially viable hydrogen-based cells, the biggest challenge being to obtain CO-free H₂ (<100 ppm) as the fuel. In this study, the use of CuO–CeO₂ catalysts in preferential oxidation of CO to obtain CO-free H₂ (PROX reaction) was investigated. Ce_1−xCu_xO₂ catalysts, with x (mol%) = 0, 0.01, 0.03, 0.05 and 0.10, were synthesized in one-step by the polymeric precursor method, to obtain a very fine dispersion and strong metal-support interaction, to favor active copper species and a preference for the PROX reaction. The results obtained from catalyzed reactions and characterization of the catalysts by XRD, Rietveld refinement, BET surface area, UV–Vis and TPR, suggest that this one-step synthesis method gives rise to catalysts with copper species selective for the PROX reaction, which reaches a maximum rate on Ce_0.97Cu_0.03O₂ catalyst.     

Promoting Role of Fe in the Preferential Oxidation of CO Over Ir/Al2O3

A novel catalyst Ir–FeO _x /Al₂O₃ designed for the preferential oxidation (PROX) of CO under excess hydrogen was developed in this work. To clarify the promoting role of Fe species, three different impregnation sequences were employed, and the resultant catalysts were characterized by various techniques. The results showed that the Ir–FeO _x /Al₂O₃ catalyst, which was prepared by the co-impregnation procedure, exhibited the best performance for the PROX. The partially exposed and highly dispersed Ir sites and the FeO _x sites allowed good adsorption for both CO and O₂ over the Ir–FeO _x /Al₂O₃, which was believed to be responsible for the enhanced activity.     

Alumina supported Au/Y-doped ceria catalysts for pure hydrogen production via PROX

Gold catalysts on Y-doped ceria dispersed on high surface area γ-Al₂O₃ were synthesized and tested in preferential CO oxidation in hydrogen rich stream (PROX). The effect of ceria loading (10, 20 or 30 wt%) was studied. The gold catalyst with the lowest ceria amount exhibited the highest PROX activity. The addition of Y₂O₃ (1 wt%) led to improved performance. The most favorable effect was observed in the sample with 20 wt% ceria amount. This gold catalyst showed good PROX activity and stability in the presence of CO₂ and water. Catalysts characterization by XRD, HRTEM/HAADF, XPS and H₂-TPR was used to elucidate the relationship between the chemical composition, state of gold, support features and catalytic properties.     

Oxidation Catalysis by Supported Gold Nano-Clusters

The historical notion regarding the inability of gold to catalyze reactions has been discarded in view of recent studies, which have clearly demonstrated the high catalytic efficiency of supported nano-gold catalysts. Although nano-Au catalysts are known to catalyze a variety of reactions, the major focus has been on CO oxidation catalysis. In this work we focus on the important aspects related to the CO oxidation reaction. Special emphasis is placed on the studies undertaken on model nano-Au systems as these studies have considerably enhanced the understanding of the oxidation process. Gold in a highly dispersed state can selectively oxidize CO in the presence of excess hydrogen (of tremendous interest to state-of-the-art low-temperature fuel cells); related studies are addressed in this review. The nano-gold catalysts have also been investigated for the direct vapor-phase oxidation of propylene to propylene oxide in the presence of molecular oxygen; these investigations are highlighted in this work.     

Methanol reformate treatment in a PEM fuel cell-reactor

The treatment of methanol reformate, containing up to 2500 ppm CO, by the anode of a PEM fuel cell, operating as a preferential oxidation (PROX) reactor, was investigated in order to examine the possibility of electrochemically promoting the water–gas-shift (WGS) reaction and thus making the gas mixture suitable for anodic oxidation. It was found that the electrochemical promotion effect plays a significant role in a normal fuel cell operation (air at the cathode) but not in a hydrogen pumping operation (H₂ at the cathode). This implies that the role of oxygen crossover in the electropromotion (EP) of the WGS reaction and in the CO oxidation is vital. During fuel cell operation, the increase in the rate of CO consumption over a Pt/C anode is 2.5 times larger than the electrochemical rate, I/2F of CO consumption, while for oxygen bleeding conditions (fuel mixture + 1% O₂ at the anode) the increase is up to five times larger than I/2F, i.e. the Faradaic efficiency is up to 5. This shows that the catalytic properties of the Pt anode are significantly modified by varying catalyst potential and by the extent of O₂ crossover.

The effect of temperature, gas composition, membrane thickness and Pt anode alloying with Cu was studied. It was found that the rate of CO consumption is significantly enhanced by increasing T, p_H2 and increasing O₂ crossover rate. Also the Faradaic efficiency reaches even higher values (up to 9) when using PtCu/C anodes. However, the Faradaic efficiency drops in general below 100% at high current densities and CO conversion levels.     

CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

A series of Au/TiO₂ catalysts has been prepared from precursors of various metal-organic gold complexes (Au _n , n = 2–4) and their catalytic activity for CO oxidation studied. The Au/TiO₂ catalyst synthesized from a tetranuclear gold complex shows the best performance for CO oxidation with transmission electron microscopy of this catalyst indicating an average gold particle size of 3.1 nm.