CO In H2-rich Stream Research Articles

Ru/TiO2 for the preferential oxidation of CO in H2-rich stream: Effects of catalyst pre-treatments and reconstruction of Ru sites

The preferential oxidation (PROX) of CO is a promising strategy for trace CO clean up in H2-rich stream to fuel cells. In the present study, a series of TiO2 supported clusters were prepared and studied for the PROX of CO. Amongst, Ru/TiO2 catalyst exhibited remarkably high PROX activity in the operation temperature range of fuel cells. The effects of catalyst preparation and pre-treatment on the catalytic performance of Ru/TiO2 were investigated in detail. Ru/TiO2 catalyst prepared by photo-deposition and pre-treated under H2–CO atmosphere was found to be the most promising one and complete CO oxidation could be achieved at >373K. Ru/TiO2 pre-treated under different reducing atmospheres were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) of CO adsorption. The surface reconstruction of Ru sites during catalyst pre-treatment was observed and isolated metallic Ru species was identified as preferred active sites for PROX reaction. Based on the catalytic and characterization results, the possible mechanism for PROX of CO over Ru/TiO2 was proposed.

Promoted effect of PANI on the preferential oxidation of CO in the presence of H2 over Au/TiO2 under visible light irradiation

A Polyaniline (PANI) assembled Au/TiO2 catalyst (Au/TiO2–PANI) was prepared by deposition–precipitation with the assistance of in-situ polymerization assistance, and was performed for oxidizing CO in H2-rich stream under visible light irradiation. It is found that the as-prepared Au/TiO2–PANI exhibited a higher activity of CO preferential oxidation (PROX) than the Au/TiO2 without PANI decoration at room temperature under visible light irradiation. Based on the results of structure characterization, photo-absorbance, photo-electrochemical, electron paramagnetic resonance, and adsorbing CO and H2 of Au/TiO2–PANI, it is proposed that the photo-excited electrons of PANI could transfer to TiO2 accompanied by suppressing the transfer of excited electrons induced by the localized surface plasmon resonance of Au nanoparticles to TiO2, resulting in the increases in surface electron densities of both Au and TiO2 sites. This will promote the activation of CO adsorbed at Au sites and the formation of O2− species at TiO2 sites, and then the oxidation of CO. Meanwhile, the protonation of N atoms in PANI could cause a spillover of the dissociative H at Au sites into PANI, resulting in the suppression of H2 oxidation.

Improving the CO-PROX Performance of Inverse CeO2/CuO Catalysts: Doping of the CuO Component with Zn

An inverse CeO2/CuO catalyst in which the CuO component has been doped with Zn is examined in comparison with the analogous Zn-free catalyst with the aim of enhancing the performance of this type of systems for preferential oxidation of CO in H2-rich stream (CO-PROX). The catalysts are characterized by XRD, Raman spectroscopy, XANES-EXAFS, XPS, and HRTEM. Their catalytic properties are explored in conjunction with redox properties studied by XANES, XRD, and DRIFTS under reaction conditions with MS detection. It is shown that the presence of zinc enhances the CO-PROX performance of the inverse system through decreasing its H2 oxidation activity, while the CO oxidation activity does not practically become affected. This is related to an increase in the CO2 selectivity of the catalyst that has been attributed to a hindering of the reduction of the CuO component, thus preventing the formation of H2 oxidation active sites under reaction conditions. A model of the structural and catalytic effects induced by the...

Photo-assisted catalytic methanation of CO in H2-rich stream over Ru/TiO2

Ultraviolet (UV) light was introduced into the process of hydrogenation of CO over Ru/TiO2 catalyst which were prepared by a facile impregnation–reduction method. It is found that UV irradiation can promote the conversion of CO into CH4 over Ru/TiO2 at 150–220°C. Based on the testing results of XPS, FT-IR and TPSR for Ru/TiO2 under UV irradiation, it is proposed that the photo-generated electrons induced by the band-gap excitation of TiO2 can transfer to Ru along with the increase of surface electron density of Ru sites. This will further promote the adsorption of CO and its activation at Ru sites, resulting in the photo-enhancement of the methanation of CO.

Ternary composite oxide catalysts CuO/Co3O4–CeO2 with wide temperature-window for the preferential oxidation of CO in H2-rich stream

A series of ternary composite oxide catalysts CuO/Co3O4–CeO2 with variable Ce/(Co+Ce) atomic ratios were prepared and employed for the preferential oxidation of CO (CO PROX). Many techniques such as N2-sorption, XRD, H2-TPR, O2-TPO, CO-TPD, O2-TPD, Cu K-edge XAFS (including EXAFS and XANES) and in situ DRIFTS were used for catalyst characterization. The catalyst CuO/Co3O4–CeO2 with Ce/(Ce+Co) ratio of 0.1 exhibits the best performance, showing not only the lowest temperature for the complete oxidation of CO (98°C), but also the broadest operating temperature window for full CO conversion (98–173°C). The results of N2-sorption and temperature-programmed characterizations including H2-TPR, O2-TPO, CO-TPD and O2-TPD show that the CuCoCe10 catalyst possesses the highest BET surface area, the best reducibility/oxidizability and the best performance for CO and O2 adsorption. Linear combination fitting of Cu K-edge XANES spectra reveals that multiple Cu species including Cu0, Cu+ and Cu2+ species co-exist in the spent catalyst CuCoCe10. Stable Cu+ carbonyl species are identified as the main active reaction intermediates as revealed by in situ DRIFTS. High temperature (>120°C) can lead to the reduction of Cu+ to Cu0, enhancing H2 oxidation; as a result, the selectivity of O2 towards CO2 is decreased. Based upon in situ DRIFTS results, a potential CO PROX mechanism over CuO/Co3O4–CeO2 catalysts is proposed.

Characterization of Active Sites/Entities and Redox/Catalytic Correlations in Copper-Ceria-Based Catalysts for Preferential Oxidation of CO in H2-Rich Streams

This article reviews work done at authors’ laboratories about catalysts based on combinations between copper and ceria for preferential oxidation of CO in H2-rich streams (CO-PROX). The main focus of this review is the characterization of active sites for the process on the basis of spectroscopic analysis of the systems under reaction conditions (operando techniques). On such a basis, it is exposed the state of the art in this field in connection with results obtained in other laboratories.

Selective oxidation of CO in H2-rich stream over Au/CeO2 and Cu/CeO2 catalysts: An insight on the effect of preparation method and catalyst pretreatment

Selective oxidation of CO in H2-rich stream was studied on Au/CeO2 and Cu/CeO2 catalysts, investigating the effect of both the preparation technique and the catalyst pretreatment towards the catalytic performance. It was found that in the case of Au catalysts the sample prepared by deposition–precipitation was more active than that prepared by co-precipitation. The opposite trend was instead observed on Cu samples. On all tested catalysts a higher pre-calcination temperature of catalysts resulted in a lower CO conversion. The extent of this effect was strong on Au samples and less evident on Cu ones. On both Au and Cu samples selectivity towards CO oxidation decreased on increasing reaction temperature, being almost independent from preparation and pretreatment. On the basis of characterisation data (H2-TPR, XRD, XPS, and BET surface area) the effect of preparation method and catalyst pretreatment was accounted for both the different particle size and surface amount of active species (Au or CuOx). According to a Mars-van Krevelen mechanism, implying lattice oxygens of the cerium oxide and CO (and H2) adsorbed on the active species, it was suggested that on Au/CeO2 the PROX reaction is strongly affected by the status of gold, which has a key role in the CO activation step, whereas on Cu/CeO2 the performance are significantly influenced by the ceria morphology/reactivity.

Manganese-promoted cobalt oxide as efficient and stable non-noble metal catalyst for preferential oxidation of CO in H2 stream

The modification by MnOx significantly enhanced the activity and stability of Co3O4 for the preferential oxidation of CO in H2-rich stream. The MnOx-promoted Co3O4 catalyst with a Mn/Co molar ratio of 1/8 exhibited the best performance; CO conversion was 56% at 40°C and increased to >90% as the temperature was raised to >75°C. This catalyst could provide 100% CO conversion at 125–175°C. CO conversions of 97% and 100% remained unchanged for 100h at 100 and 125°C, respectively, whereas the deactivation was observed over Co3O4 alone. Our characterizations using XRD, XPS and TEM indicated that the modification of Co3O4 with MnOx caused the formation of smaller MnxCo3−xO4 solid-solution particles. The O2-TPD, CO-TPD, and pulse reaction studies suggest that the incorporation of MnOx into Co3O4 increased the amounts of reactive oxygen species and adsorbed CO species over catalyst surfaces and enhanced the regeneration ability of the reduced catalyst by O2. All these enhancements are proposed to be responsible for the improved catalytic performance of the MnOx-promoted Co3O4.

Selective CO removal over Au/CeFe and CeCu catalysts in microreactors studied through kinetic analysis and CFD simulations

A kinetic study of the preferential oxidation of CO in H2 rich streams (CO-PrOx) over a cerium–copper oxide (CeCu) and a gold catalyst supported on cerium–iron oxide (Au/CeFe) is presented. The gold catalyst is very active but the CeCu oxide is more selective. A kinetic model describing the CO-PrOx system with CO2 and H2O in the feed has been formulated considering the oxidation of CO and H2 and the reverse water-gas shift reaction. The rate equations have been implemented in computational fluid dynamics codes to study the influence of the operating variables on the CO-PrOx in microchannels and microslits. The CeCu catalyst is the only one capable of achieving final CO contents below 10–100ppmv. Due to the opposite effect of temperature on activity and selectivity there is an optimal temperature at which the CO content is minimal over CeCu. This temperature varies between 170 and 200°C as the GHSV increases from 10,000 to 50,000h−1. Simulations have evidenced the very good heat transfer performance of the microdevices showing that the CO-PrOx temperature can be controlled using air as cooling fluid although the inlet temperature and flow rate should be carefully controlled to avoid reaction extinction. Both microchannels and microslits behaved similarly. The fact that the microslits are much easier to fabricate may be an interesting advantage in favour of that geometry in this case.

Preferential oxidation of CO in H2-rich stream over CuO/Ce1−xTixO2 catalysts

CuO/Ce1−xTixO2 prepared by sol–gel impregnation was used as catalysts for the preferential oxidation (PROX) of CO in H2-rich stream. The effects of support composition, catalyst calcination temperature as well as the presence of H2O and CO2 in the reaction stream on the catalytic performance of CuO/Ce1−xTixO2 were investigated. The results indicated that the catalyst CuO/Ce0.8Ti0.2O2 exhibits the highest activity and the optimal temperature for the catalyst calcination is 500°C. The presence of H2O and CO2 in the reaction stream has a negative effect on the catalytic activity and stability of CuO/Ce0.8Ti0.2O2. The negative effect of CO2 on the catalyst stability is stronger than that of H2O, suggesting that the accumulation of carbonate species may be the main reason for catalyst deactivation. The characterization by means of XRD, HRTEM, and H2-TPR indicated that the doping of TiO2 in CeO2 enhances the surface area of the composite oxide support, decreases its particle size, and promotes the dispersion of active copper species. The strong interaction between TiO2 and CeO2 in the support as well as the interfacial interaction between CuO and the support may also contribute to the high catalytic activity of CuO/Ce0.8Ti0.2O2.

Spectroscopic Study on the Nature of Active Entities in Copper–Ceria CO-PROX Catalysts

A series of oxidised copper-ceria catalysts with varying copper loadings and prepared by two different methods (impregnation of ceria and coprecipitation of the two components within reverse microemulsions) have been examined with the aim of determining the nature of active entities for the two main (CO and H2) competing oxidation reactions that take place during preferential oxidation of CO in H2-rich streams over this type of catalysts. The analysis is mainly based on operando spectroscopic exploration by DRIFTS and XANES as well as in situ analysis by Raman. The results allow establishing a model of the catalytic behaviour of this type of catalysts which can provide keys to control their CO-PROX catalytic properties.

Ultrasonic-assisted synthesis of highly active catalyst Au/MnO –CeO2 used for the preferential oxidation of CO in H2-rich stream

Abstract A series of nano-gold catalysts supported on binary oxides MOx–CeO2 (atomic ratio M/Ce = 1:1, M = Mn, Fe, Co, Ni) are prepared by deposition–precipitation (DP). An innovative and rather convenient ultrasonic pretreatment of the support is employed for Au/MnOx–CeO2 preparation. It is found that for preferential CO oxidation Au/MnOx–CeO2 is more active than Au/CeO2. Ultrasonic pretreatment of MnOx–CeO2 further promotes the performance of Au/MnOx–CeO2, with CO conversion increased by 24 % at 120 °C. Meanwhile, the selectivity of oxygen to CO2 is promoted in the whole temperature range, especially in 80–120 °C, the selectivity is increased by 15–21%. HR-TEM and XRD results indicate that ultrasonic pretreatment is favorable to the formation of much smaller gold nanoparticles (

Cobalt and Copper Composite Oxides as Efficient Catalysts for Preferential Oxidation of CO in H2-Rich Stream

A series of Co–Cu composite oxides with different Co/Cu atomic ratios were prepared by a co-precipitation method. XRD, N2 sorption, TEM, XPS, H2-TPR, CO-TPR, CO-TPD and O2-TPD were used to characterize the structure and redox properties of the composite oxides. Only spinel structure of Co3O4 phase was confirmed for the Co–Cu composite oxides with Co/Cu ratios of 4/1 and 2/1, but the particle sizes of these composite oxides decreased evidently compared with Co3O4. These composite oxides could be reduced at lower temperatures than Co3O4 by either H2 or CO. CO and O2 adsorption amounts over the composite oxides were significantly higher than those over Co3O4. These results indicated a strong interaction between cobalt and copper species in the composite samples, possibly suggesting the formation of Cu x Co3−x O4 solid solution. For the preferential oxidation of CO in a H2-rich stream, the Co–Cu composite oxides (Co/Cu = 4/1–1/1) showed distinctly higher catalytic activities than both Co3O4 and CuO, and the formation of Cu x Co3−x O4 solid solution was proposed to contribute to the high catalytic activity of the composite catalysts. The Co–Cu composite oxide was found to exhibit higher catalytic activity than several other Co3O4-based binary oxides including Co–Ce, Co–Ni, Co–Fe and Co–Zn oxides.

Preferential oxidation of CO in H2 rich stream over Au/CeO2–Co3O4 catalysts

Au/CeO2–Co3O4 catalysts with a Ce/Co atomic ratio from 0.1 to 0.6 were prepared by deposition–precipitation; their catalytic performance in preferential oxidation (PROX) of CO in H2 rich stream was investigated. Au/CeO2–Co3O4 exhibits much higher catalytic activity in CO PROX than Au/Co3O4 and Au/CeO2; over Au/CeO2–Co3O4, 100% CO conversion with 75% selectivity to CO2 can be achieved even at ambient temperature. Typically, for CO PROX at 80 °C over oxidation pretreated Au/CeO2–Co3O4 (Ce/Co = 0.2), 91% CO conversion and 51% selectivity to CO2 can last for 260 h.

Nanosized Pt-Co Catalysts for the Preferential CO Oxidation

The preferential CO oxidation in the presence of excess hydrogen was studied over Pt-Co/gamma-Al2O3. CO chemisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectrometer (EDX) and temperature programmed reduction (TPR) were conducted to characterize active catalysts. The catalytic activity for CO oxidation and methanation at low temperatures increased with the amounts of cobalt in Pt-Co/gamma-Al2O3. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Co and Pt was determined to be 10. The co-impregnated Pt-Co/gamma-Al2O3 appeared to be superior to Pt/Co/gamma-Al2O3 and Co/Pt/gamma-Al2O3. The reductive pretreatment at high temperature such as 773 K increased the CO2 selectivity over a wide reaction temperature. The bimetallic phase of Pt-Co seems to give rise to high catalytic activity in selective oxidation of CO in H2-rich stream.

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.