Kinetics of CO and H2 oxidation over CuO-CeO2 catalyst in H2 mixtures with CO2 and H2O

The kinetics of CO and H₂ oxidation over a CuO-CeO₂ catalyst were simultaneously investigated under reaction conditions of preferential CO oxidation (PROX) in hydrogen-rich mixtures with CO₂ and H₂O. An integral packed-bed tubular reactor was used to produce kinetic data for power-law kinetics for both CO and H₂ oxidations. The experimental results showed that the CO oxidation rate was essentially independent of H₂ and O₂ concentrations, while the H₂ oxidation rate was practically independent of CO and O₂ concentrations. In the CO oxidation, the reaction orders were 0.91, −0.37 and −0.62 with respect to the partial pressure of CO, CO₂ and H₂O, respectively. In the H₂ oxidation, the orders were 1.0, −0.48 and −0.69 with respect to the partial pressure of H₂, CO₂ and H₂O, respectively. The activation energies of the CO oxidation and the H₂ oxidation were 94.4 and 142 kJ/mol, respectively. The rate expressions of both oxidations were able to predict the performance of the PROX reactor with accuracy. The independence between the CO and the H₂ oxidation suggested different sites for CO and H₂ adsorption on the CuO-CeO₂ catalyst. Based on the results, we proposed a new reaction model for the preferential CO oxidation. The model assumes that CO adsorbs selectively on the Cu⁺ sites; H₂ dissociates and adsorbs on the Cu⁰ sites; the adsorbed species migrates to the interface between the copper components and the ceria support, and reacts there with the oxygen supplied by the ceria support; and the oxygen deficiency on the support is replenished by the oxygen in the reaction mixture.     

Enhancing preferential oxidation of CO in H2 on Au/α-Fe2O3 catalyst via combination with APTES/SBA-15 CO2-sorbent

Au/α-Fe₂O₃ was combined with a CO₂-sorbent (3-aminopropyltriethoxysilane (APTES) grafted on SBA-15 and hereafter denoted as APTES/SBA-15) to enhance preferential oxidation (PROX) of CO in H₂. The CO₂ molecules could be rapidly adsorbed on APTES/SBA-15 at low temperatures below 50 °C with a capacity of 0.68 mmol CO₂/g-sample, and desorbed at a temperature range of 50 °C–80 °C. Three different configurations of the Au/α-Fe₂O₃ catalyst and the CO₂-sorbent were tested in the PROX reaction, namely (i) the sorbent-free (catalyst//SBA-15//catalyst) configuration, (ii) the packed three-layer configuration (catalyst//CO₂-sorbent//catalyst), and (iii) the mechanically mixed catalyst and CO₂-sorbent configuration. Compared to configuration (i), configuration (ii) achieved an average 10% higher CO conversion at 50 °C and a GHSV of 65000 h⁻¹. However, the CO concentration could not be lowered to below 70 ppm from 2000 ppm using configuration (ii) at a GHSV of 10000 h⁻¹. Thus, a 5-layer configuration (catalyst//CO₂-sorbent//catalyst//CO₂-sorbent//catalyst) was used, and the CO concentration was lowered to ca. 25 ppm. The mechanism for enhancement of the PROX reaction by the continuous removal of CO₂ by the CO₂-sorbent is discussed and attributed to reduction of the surface carbonate on the Au/α-Fe₂O₃ catalyst formed during the PROX process.     

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeO x /Pt/TiO2 catalyst

Oxidation of CO on the FeO _x /Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O at 60 °C, but no such enhancement is observed on the Pt/TiO₂ catalyst, but shift reaction (CO + H₂O → H₂ + CO₂) does not occur on the FeO _x /Pt/TiO₂ catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO _x loaded Pt/TiO₂ catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O₂, and the reaction of the bridge-bonded CO with O₂ as well as of the linearly bonded CO is markedly enhanced by adding H₂ to a flow of CO + O₂. From these results, we deduced that the promoting effect of H₂ and/or H₂O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO _x /Pt/TiO₂ catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H₂O.      

Heterogeneous Catalytic Aziridination of Styrene Using Transition-Metal-Exchanged Zeolite Y

Hydrogen for fuel cells can be produced by reforming hydrocarbons. The H₂-rich reformate typically contains about 1 mol% CO which will poison the anode of polymer electrolyte fuel cells. The CO concentration can be reduced by preferential oxidation (PROX) using near-stoichiometric amounts of O₂. The conversion of CO should be over 99% while minimizing oxidation of H₂. Supported Pt catalysts with and without promotion by Ce were compared for the catalytic oxidation of CO by O₂ in a H₂ stream. With unsupported Pt catalysts, selectivity (to CO₂ as opposed to H₂O) was highest at low temperatures and low O₂/CO ratios, however conversion was low. Addition of Ce significantly improved CO conversion under these conditions.     

Recent progress in catalytical CO purification of H2-rich reformate for proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) fueled by hydrogen-rich gas are promising systems to substitute fossil fuel resources. This review evaluates state of the art and perspectives for the catalysts such as supported Ni, noble metals, and base metal oxide catalysts used for catalytical CO purification (SMET, PROX) in H₂-rich reformates for PEMFCs applications. The factors affecting activity like support effect, metal size effect, promoters, and metal-support interaction are assessed and thoroughly discussed. It is remarked that the challenges for their practical applications are to (i) achieve acceptable CO outlet concentration in ppm level with wide temperature window (ii) minimize undesired loss of H₂ and inhibit the occurrence of side reactions (iii) develop high performance catalysts with high resistance to CO₂ and steam under realistic conditions. Developing novel catalysts for catalytical CO purification based on the structure-activity relationship will resolve the challenges required for their practical applications.     

Modulating Cu species in copper–ceria catalyst for boosting CO preferential oxidation: Elucidating the role of ceria crystal plane

Copper–ceria has been regarded as an active catalyst for CO preferential oxidation (CO-PROX). However, modulation of Cu species by regulating the ceria crystal plane has remained largely underdeveloped. Herein, CuO/CeO₂-X catalysts (X stands for treating temperature) were synthesized by the freeze-drying impregnation method using CeO₂-X nanorods with variational termination planes as supports and employed to boost CO-PROX. The results from various characterization techniques reveal that the mass transfer between CuO_x clusters and CeO₂-X weakened, and the sintering of CuO_x enhanced as the stability of CeO₂-X exposed planes increased. It is manifested that the content and the effective diameter of CuO_x clusters increase, and the cerium doping in CuO_x clusters decreases. The CuO/CeO₂-400 catalyst prepared from CeO₂-400 with suitable crystal plane stability induces the most significant amount of Cu⁺–CO during the CO-PROX reaction, displaying optimal reactivity. This work expounds on the relationship between the initially exposed crystal plane of CeO₂ and the structure of Cu species after loading CuO, which is of great significance for designing efficient catalysts for CO-PROX.     

Metal-oxide catalysts for CO TOX and PROX processes in the Pt–Cr/Mo/W systems

The properties of Pt-based catalysts could be modified by the addition of second metal and formation of an alloy or metal-oxide interface. In the present work we studied the effect of Cr, Mo and W doping on the Pt performance in CO TOX and PROX reactions. [Pt(NH₃)₄]MO₄ (M = Mo, W) salts and solid solutions [Pt(NH₃)₄](M’O₄)_x(M″O₄)_1-x (M’ = Cr,Mo,W; M’‘ = Cr,Mo,W) were used as a single-source catalyst precursors. They were synthesized and characterized by a number of physicochemical methods of analysis. The thermal properties of obtained salts were studied in an oxidative atmosphere. The process of the decomposition is multistage and ends with a formation of platinum and the corresponding oxides of base metals (Cr/Mo/W).