High performance rare earth oxides LnOx (Ln = La, Ce, Nd, Sm and Dy)-modified Pt/SiO2 catalysts for CO oxidation in the presence of H2

The catalytic activities of low-temperature CO oxidation and preferential CO oxidation (PROX) in rich-H₂ stream over the Pt/SiO₂ and rare earth oxides modified Pt/SiO₂ catalysts have been investigated. The addition of rare earth oxide additives to the Pt/SiO₂ catalyst can promote the catalytic performance remarkably. The catalytic activities for PROX are dependent on the basicities of these oxides. The higher basic strength leads to higher activity. For Pt–CeO₂/SiO₂, the catalytic activity depends strongly on the content of CeO₂ and on the mole ratio of CO/O₂. XRD and H₂-temperature-programmed reduction (TPR) results show that the additives not only improve the dispersion of metallic Pt on the support surface, but also increase the reducibility of PtOx in Pt–CeO₂/SiO₂. The formation of the Pt–Ce alloy decreases the electron back-donation from the Pt 5d orbital to the 2σ* orbital of CO, and consequently suppresses CO–Pt bonding, resulting in the lowered CO coverage.     

Ge-modified Pt/SiO2 catalysts used in preferential CO oxidation (CO-PROX)

A bimetallic PtGe catalyst was prepared by a controlled surface reaction and studied for the PROX reaction. The good activity of the bimetallic catalyst can be assigned to the presence of a “noble metal-oxidized metal promoter” ensemble site in close contact, the noble metal (Pt) being the CO adsorption site and the oxidized metal promoter (Ge) the O₂ adsorption site. The stability of the PtGe clusters is confirmed by the EXAFS spectra and activity measurements in the preferential oxidation reaction that show similar values for samples subjected to different oxidation–reduction cycles.     

Preferential CO oxidation over CuO–CeO2 in excess hydrogen: Effectiveness factors of catalyst particles and temperature window for CO removal

In the preferential oxidation of CO in hydrogen mixtures (PROX), CO and H₂ oxidation occur in parallel on the surface in a porous catalyst. The diffusion of the reactants into the pore structure of the catalyst can affect the catalyst performance significantly, and its effect can be accounted for in terms of the effectiveness factor. Conventional methods for estimating the effectiveness factor are not directly applicable because they have been developed for a single reaction in a catalyst particle. A novel method for a simultaneous estimation of the effectiveness factors of the two reactions was developed in this study. This method is based on the PROX kinetics over a CuO–CeO₂ catalyst and is applicable to the cases where the CO oxidation can be approximated by a first-order reaction and both oxidations are zero-order reactions with respect to the O₂ partial pressure. With the method, the performance of an isothermal PROX reactor was simulated to determine the effects of the feed flow rate, feed composition, reactor temperature and catalyst size on the CO clean-up.     

Three-dimensional simulation and optimization of an isothermal PROX microreactor for fuel cell applications

Three-dimensional numerical simulations of the reacting flow in rectangular micro-channel PROX reactors are performed. To solve the set of governing equations, a finite volume method is applied using an improved SIMPLE algorithm. A three-step surface kinetics for the chemical reactions is utilized that includes hydrogen oxidation, carbon monoxide oxidation, and water–gas shift reaction. The kinetics chosen are for a Pt–Fe/γ-Al₂O₃ catalyst and operating temperatures of about 100 °C. The PROX reactor is expected to remove the carbon monoxide content in a hydrogen-rich stream from about 2% to less than 10 ppm. Effects of the inlet steam content, oxygen to carbon monoxide ratio, reactor wall temperature, aspect ratio of the channel cross section, and the channel hydraulic diameter are investigated. It is found that increasing the steam content, oxygen to carbon monoxide ratio, or wall temperature may improve the performance of the microreactor. It is also shown that the rate of water–gas shift reaction or its reverse is much lower than the oxidation reactions. Finally, it is revealed that based on a modified CO yield definition, the optimum channel geometry is a square shape.     

A comparative study of differently prepared rare earths-modified ceria-supported gold catalysts for preferential oxidation of CO

The preferential oxidation of CO in H₂-rich gas was studied over gold catalysts supported on ceria modified by rare earths (RE = La, Sm, Gd and Y). The ceria supports were prepared by mechanochemical activation or co-precipitation. The amount of RE₂O₃ was 10 wt%. Gold (2 wt%) was added by the deposition-precipitation method. The samples were characterized using XRD, HRTEM, HAADF, TPR, and Raman spectroscopy. It was established that catalysts prepared by co-precipitation were more active than samples made by mechanochemical activation. A gold catalyst on yttrium-modified ceria, prepared by co-precipitation, exhibited the highest catalytic activity and selectivity, and high stability. No substantial differences in the size distribution and average size of the nanogold particles in the studied catalysts were observed. The main reason for the differences in PROX activity of these gold catalysts was searched into the role of the ceria supports, depending on the preparation method, and the nature of the modifier.     

IrFeOx/SiO2—A highly active catalyst for preferential CO oxidation in H2

A novel bifunctional catalyst IrFeO_x/SiO₂, which was very active and selective for preferential oxidation of CO under H₂-rich atmosphere, was developed in this study. XRD, H₂-TPR, and chemisorption were applied to characterize three kinds of IrFeO_x/SiO₂ catalysts prepared by different impregnation sequences. The results indicated that both FeO_x and Ir species were highly dispersed on the SiO₂ support; the reduction of Ir species in the Fe-promoted Ir/SiO₂ catalyst became easier than those in the Ir/SiO₂, together with the partial reduction of Fe₂O₃; the saturation uptake of O₂ adsorption was greatly enhanced. In-situ DRIFTS, XPS, and Mössbauer techniques were further applied to detect the structural information on the selected Ir-FeO_x/SiO₂ catalyst prepared by co-impregnation method. A non-competitive adsorption catalytic mechanism was proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction probably took place at the interface of Ir and FeO_x or via a spill-over process.     

CuO–CeO2 catalysts synthesized by various methods: Comparative study of redox properties

Three different preparation methods have been used to synthesize CuO–CeO₂ catalysts with 7 wt.% copper loading: coprecipitation (CP), sol–gel (SG) and urea–nitrate combustion (UC). All the samples have been characterized by a series of techniques such as XRD, Raman, TPR, TPD and OSC, in order to understand their different performance in CO oxidation, as a function of redox properties and catalyst structure. Among the catalysts, clearly CuCe-CP shows the lower activity because it presents the structure of a solid solution. CuCe-SG and CuCe-UC catalysts show much better performance in CO oxidation, in accordance to their higher redox capacity.     

CeO2-supported Pt–Cu alloy nanoparticles synthesized by radiolytic process for highly selective CO oxidation

CeO₂-supported Pt–Cu bimetallic catalysts were synthesized by radiolytic process and their PROX activities were evaluated in relation to structural properties of the catalysts. Irradiating the aqueous precursor solution yielded Pt–Cu alloy nanoparticles and amorphous-like CuO on CeO₂ which are thermodynamically stable products formed from reduced Pt and Cu. Addition of Cu to Pt significantly improved CO selectivity in PROX reaction. The Pt–Cu catalysts had wide temperature window for 100% CO conversion in contrast to very narrow window for monometallic Pt and Cu catalysts. Much lower light-off temperature for Pt–Cu catalysts than Cu catalyst revealed that Pt-Cu alloy surface is the active center. Regardless of the amount of CuO phase, the bimetallic catalyst exhibited high catalytic performance, which further revealed that Cu in close contact with Pt is responsible for the improved selectivity. The CuO phase was suggested to promote oxygen supply to CO chemisorbed on Pt–Cu alloy surface.     

Strong electrostatic adsorption of Pt over CoOx/TiO2 dual-support: Effect of synthesis pH on the catalytic activity for the CO oxidation in presence of hydrogen

Highly dispersed Pt-based catalysts were prepared by Strong Electrostatic Adsorption (SEA) for the PROX reaction. The effect of the catalyst synthesis pH was investigated to control the metal-support interactions for the system: Pt supported onto cobalt oxide, previously supported on TiO₂. The Pt/CoO_x/TiO₂ catalysts were prepared in a pH range of 3.0–9.0, obtaining Pt dispersion values above 90% with Pt crystallite sizes of 1–2 nm. The effect of the synthesis pH was correlated with the concentration of (Pt-CoO_x)_i interfacial sites, and this, in turn, with the catalytic activity. The concentration of (Pt-CoO_x)_i interfacial sites was higher for the most active catalyst, which was prepared at a condition that ensures a higher selective-SEA of Pt over the co-support (pH = 6.0). This study shows that the synthesis pH can determine the concentration and type of active sites and paves the way to use SEA to control the activity of dual-supported catalysts.     

Activity of Au/ZnO catalysts prepared by photo-deposition for the preferential CO oxidation in a H2-rich gas

In this work, Au supported over ZnO prepared by photodeposition was applied to prepare nano-size Au catalysts by utilizing UV light for the preferential oxidation (PROX) of CO. The results demonstrated that Au can be dispersed homogeneously over ZnO in the size range of 1–2 nm with a narrow size distribution. It was clearly seen that the preparation parameters (i.e. irradiation time, precipitant concentration, calcination, and storage condition) had a significant effect on the catalytic activity. Among the variables studied, low concentrations of precipitant and long irradiation time were by far the most influential on the catalytic activity.