Preferential Oxidation In H2-rich Gases Research Articles

CuO/CeO2–MnO2 Catalyst Prepared by Redox Method for Preferential Oxidation of CO in H2-Rich Gases

CuO/CeO2–MnO2 (CuCeMn-4) catalyst was prepared by redox method and was tested in the CO preferential oxidation in H2-rich gases (CO PROX). CuO/CeO2–Mn2O3 (CuCeMn-3) catalyst was prepared and tested for comparison. CuCeMn-4 exhibits higher catalytic activity than CuCeMn-3. The complete CO conversion with CO2 selectivity of 92% can be obtained at 120 °C over CuCeMn-4. The catalysts were characterized by means of N2 adsorption/desorption, XRD, H2-TPR and XPS techniques. The results show that the interaction between MnO2 and CeO2 in catalyst is stronger than Mn2O3 and CeO2, which can promote the mobility of oxygen species from CeO2–MnO2 to active copper species. XPS characterization further revealed that CuCeMn-4 contains richer lattice oxygen, higher amount of Cu+ species and less carbonate species on the surface as compared with CuCeMn-3. All these should be greatly responsible for the high activity of CuCeMn-4 catalyst.

The Solid-State-Grinding Synthesis of Maganese-Modified Cobalt Oxides and Application in the Low-Temperature CO Preferential Oxidation in H2-Rich Gases

A series of MnOx modified cobalt oxides with different atomic molar ratios of Mn/(Mn + Co) were prepared by a soft reactive grinding route and investigated for CO preferential oxidation in H2. It was found that as-prepared Mn-doped cobalt oxides exhibited superior activity compared to the single constituted oxides, other Mn–Co–O mixed oxides synthesized by solution-based route, and other grinding-derived mixed metal oxides M–Co–O (M = Zn, Ni, Cu, Fe). The grinding-derived MnCo10 catalyst with Mn/(Mn + Co) molar ration of 10% showed the best CO oxidation activity and higher selectivity at low temperature. The surface richness of Co3+ was not found as increasing the Mn molar ratio in the present work. However, the incoporation of MnOx with proper amount into Co3O4 could produce high surface area, high structure defects, and rich surface active oxygen species, while the ability to supply the active oxygen species was suggested to play the crucial role in promoting the catalytic performance of Mn–Co–O mixed oxides.

Boosting Cu-Ce interaction in CuxO/CeO2 nanocube catalysts for enhanced catalytic performance of preferential oxidation of CO in H2-rich gases

In this paper, a facile strategy by etching CeO2 spheres pretreated by different concentrations of H2SO4 or NaOH solution with an ionic liquid (IL) of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) under hydrothermal condition was developed to achieve CeO2 nanocubes. By depositing CuxO clusters on CeO2 nanocubes via a deposition−precipitation method, CuxO/CeO2 nanocube catalysts loading with different amounts of CuxO clusters were synthesized and the CuxO clusters were highly dispersed onto the surface of CeO2 nanocubes. The Raman spectroscopy, H2-TPR, XPS, and DRIFTS results reveal that there is a strong interaction between CuxO and CeO2. Among these CuxO/CeO2 catalysts, the 10CuCe-2OH-ILs, which was obtained via immersing CeO2 spheres using 2M NaOH, and subsequently being etched by ILs, and then loading ∼10% of copper, exhibits the optimal catalytic performances and stability. The catalytic activity of different catalysts increases in the order of 10CuCe-ILs <10CuCe-1H-ILs <10CuCe-0.1OH-ILs <10CuCe-5OH-ILs <10CuCe-2OH-ILs. The result is in agreement with the order of the surface oxygen vacancy concentration, reducibility and Cu+ content, suggesting that the interaction between CuxO species and surface oxygen vacancies of these CuxO/CeO2 catalysts plays an important role for CO preferential oxidation in H2-rich gases.

Variation of redox activity and synergistic effect for improving the preferential oxidation of CO in H2-rich gases in porous Pt/CeO2–Co3O4 catalysts

The porous Pt/CeO2–Co3O4 catalysts show superior catalytic performance for CO preferential oxidation in H2-rich gases.

Influence of Synthetic Parameters and Reaction Conditions on Properties of CexZr1-xO2 for CO Preferential Oxidation Reaction in H2-rich Gases

CexZr1-xO2 (x=0-1) catalysts were prepared by urea grind combustion method (UGC) and tested for CO preferential oxidation in H2-rich gases. The catalytic performances of CexZr1-xO2-UGC-400 are comparable to CexZr1-xO2-SG-400 and higher than CexZr1-xO2-SA-400. The optimal Ce/Zr molar ratio, calcination temperature and urea addition (R value) for synthesizing CexZr1-xO2 catalysts are Ce/Zr = 4:1, 300 oC and R = 1.0, respectively. Ce0.8Zr0.2O2 catalyst prepared with this route can well adapt the change of gas hourly space velocity (GHSV), and no any obvious change is observed both in CO conversion and O2 selectivity with GHSV increasing from 12000 to 48000 h-1. Both CO conversion and O2 selectivity of Ce0.8Zr0.2O2 maintains relative stability during 72h time-on-stream test.

Modulating the operation temperature window of CO preferential oxidation in H2-rich gases on three dimensionally ordered macroporous CeO2–CuO catalysts by tuning their composition and incooperating Fe2O3 and Co3O4

Three dimensionally ordered (3DOM) copper-ceria based catalysts (CeO2–CuO, CeO2–CuO–Fe2O3, and CeO2–CuO–Co3O4) with diverse Ce/Cu atomic ratios and different doping amounts of Fe2O3 and Co3O4 are synthesized via a precursor thermal decomposition-assisted colloidal crystal templating method. The as-obtained CeO2–CuO, CeO2–CuO–Fe2O3 and CeO2–CuO–Co3O4 catalysts possess three dimensionally ordered macroporous structures. And their particle sizes, chemical compositions, and surface elemental states are investigated according to the variations of Ce/Cu atomic ratios and doping amounts of Fe2O3 and Co3O4. The catalytic performance of CO preferential oxidation in H2-rich gases over 3DOM CeO2–CuO, CeO2–CuO–Fe2O3, and CeO2–CuO–Co3O4 catalysts are mainly relevant to their particle sizes, chemical compositions, surface states, and synergetic interactions between different components. By modulating the Ce/Cu atomic ratios and the doping amounts of Fe2O3 and Co3O4, the superior catalytic performance of 3DOM CeO2–CuO catalysts with high CO conversion, CO2 selectivity and longer-term stability in a wide operation temperature window is achieved. This synthetic strategy provides a new methodology for the fabrication of 3DOM structured highly efficient catalysts, which would be very useful in the actual working conditions for proton exchange membrane fuel cells and water-gas shift reactions.

Ordered mesoporous Cu–Ce–O catalysts for CO preferential oxidation in H2-rich gases: Influence of copper content and pretreatment conditions

Highly ordered mesoporous Cu–Ce–O catalysts with different Cu contents have been synthesized by using ordered mesoporous silica KIT-6 as a hard template. The mesostructural order of the negative replica is influenced by the ratio of Cu to Ce. Using XRD, HR-SEM, TEM and EDX analysis, it was found that the ordered mesostructures of the nanocomposites degenerate with increasing Cu concentration, due to CuO leaching during the template removal process and a phase separation at high Cu concentration. Cu ions can replace Ce-ion in the structure of CeO2 at Cu concentrations below 40mol%. However, the Cu concentration in the final materials is lower than expected from the ratio used in the synthesis. The activity in preferential oxidation of CO in H2-rich gases (PROX) was tested at a space velocity of 60,000mLh−1gcat−1. The activity of the mesoporous catalysts increases with the concentration of Cu and becomes stable for Cu concentrations higher than 20mol%. A CO conversion around 100 % can be attained with Cu0.20Ce0.80O2 as catalyst at 160°C. The exit CO concentration can be as low as 70ppm under these conditions. The CO2 selectivity can reach 100 % at low temperature (60 – 80°C). Direct loading of CuO on the surface of mesoporous CeO2 leads to large CuO crystals and correspondingly low activity. The influence of the pretreatment atmosphere on activity was also studied. Oxidation–reduction–reoxidation cycling can improve the catalytic activity of the catalysts.

Promoting effect of MnOx on the performance of CuO/CeO2 catalysts for preferential oxidation of CO in H2-rich gases

A series of CuO/CeO2–MnOx catalysts with the Mn/(Ce + Mn) atomic ratios of 0, 0.2, 0.4, 0.6 and 1.0 were prepared and characterized by means of N2 adsorption, XRD, H2-TPR and XPS. The promoting effect of MnOx on the performance of CuO/CeO2 catalyst for the CO preferential oxidation in H2-rich gases (CO PROX) was investigated. The results show that the addition of MnOx can improve the catalytic activity of CuO/CeO2 catalyst. CuO/CeO2–MnOx catalysts exhibit much higher activity in the preferential oxidation of CO than CuO/CeO2 and CuO/MnOx. The CuO/CeO2–MnOx catalyst with the Mn/Ce + Mn atomic ratio of 0.4 is the most active, over which 100 % conversion of CO with selectivity of 83 % is obtained at 140 °C. The presence of MnOx in CuO/CeO2–MnOx catalysts can promote the release of oxygen species from ceria to active copper species and increase the highly dispersed copper species. XPS characterization further revealed that the surface hydroxyls/carbonates of catalysts decrease with the amount of Mn, which should be greatly responsible for the higher activity of CuO/CeO2–MnOx catalysts.

Three dimensionally ordered macroporous Au/CeO2 catalysts synthesized via different methods for enhanced CO preferential oxidation in H2-rich gases

Three dimensionally ordered macroporous (3DOM) Au/CeO2 catalysts were synthesized via a thermal decomposition-assisted colloidal crystal templating method following different synthetic procedures using citric acid and oxalic acid as chelating ligands and CeCl3 and Ce(NO3)3 as cerium salt precursors. All 3DOM Au/CeO2 catalysts possess well-defined 3DOM skeletons composed of well-crystallized CeO2 nanoparticles, based on the different synthetic procedures, the 3DOM Au/CeO2 catalysts show variations in surface elemental compositions, particle sizes of CeO2, macroporous and mesoporous structures, and valence states of Au. The mesoporous walls with nanopores ∼3–4 nm on 3DOM Au/CeO2 skeletons can be created when using Ce(NO3)3 as precursor. The catalytic performance of 3DOM Au/CeO2 catalysts for CO preferential oxidation in H2-rich gases was systematically investigated. The catalytic performance is closely correlated to surface elemental compositions, particle sizes of CeO2, macroporous and mesoporous structures, and valence states of Au due to the different synthetic procedures. The 3DOM Au/CeO2 catalysts prepared using oxalic acid as chelating ligand and Ce(NO3)3 as cerium salt precursor show the highest catalytic activity for CO preferential oxidation in H2-rich gases with 90.1% CO conversion and 59.9% CO2 selectivity at 50 °C, and 88.3% CO conversion and 59.3% CO2 selectivity at 80 °C, respectively. The obtained 3DOM Au/CeO2 catalysts may be of importance for guiding the design of efficient catalysts with desired porous structures that are potentially applicable in polymer electrolyte membrane fuel cells.

Three-dimensionally ordered macroporous Au/CeO2–Co3O4 catalysts with mesoporous walls for enhanced CO preferential oxidation in H2-rich gases

Three-dimensionally ordered macroporous (3DOM) Au/CeO2–Co3O4 catalysts with well-defined macroporous skeletons and mesoporous walls were created via a precursor thermal decomposition-assisted colloidal crystal templating method, and their catalytic performance for CO preferential oxidation in H2-rich gases was systematically investigated. The results showed that the 3DOM Au/CeO2–Co3O4 catalysts possessed well-defined 3DOM skeletons composed of well-crystallized CeO2 and Co3O4 nanoparticles and mesoporous walls with nanopores ∼3–4nm. Their catalytic performance was closely correlated to their compositions, phase structures, porous architectures, surface elemental compositions, and valence states. In comparison with the Au supported CeO2 or Ce3O4 catalysts previously reported, the 3DOM Au/CeO2–Co3O4 catalysts showed superior catalytic conversion, selectivity, and stability—in particular, longer-term stability—for CO preferential oxidation in H2-rich gases, making them potentially applicable in polymer electrolyte membrane fuel cells.

Supported Co3O4-CeO2 catalysts on modified activated carbon for CO preferential oxidation in H2-rich gases

CO preferential oxidation (PROX) reactions were performed over the supported Co3O4-CeO2 catalysts on modified activated carbon (AC) for eliminating the trace CO from H2-rich gases. The effects of support modification by H2O2 oxidation treatment, catalyst calcination temperature, Ce/Co atomic ratio (nCe/Co), Co3O4-CeO2 loading and reaction parameters on catalytic properties of the Co3O4-CeO2/AC catalysts were investigated. Various characterization techniques like scanning electron microscopy (SEM), X-ray diffraction (XRD) and H2 temperature-programmed reduction (H2-TPR) were employed to reveal the relationship between catalysts nature and catalytic performance. Results illustrate that the supported Co3O4-CeO2 catalyst on modified AC exhibits excellent catalytic properties, which highly depends on dispersity and reducibility of Co3O4 affected by the time of support treatment (tp), calcination temperature, nCe/Co and loading. The supported 35wt% Co3O4-CeO2 catalyst (1:8 of nCe/Co) on the modified AC with H2O2 oxidation treatment for 6h demonstrates the best catalytic properties and the almost complete CO transformation takes place in a wide temperature range of 125–190°C. Moreover, it is also found that the developed catalyst exhibits an outstanding catalytic stability, and 100% CO conversion can be maintained as the time on stream evolutes up to 1800min even in the presence of CO2 and H2O in the feed. The optimized Co3O4-CeO2/AC may be a robust and promising catalyst for eliminating trace CO from H2-rich gases.

Activity of rhodium-based catalysts for CO preferential oxidation in H2-rich gases

The CO preferential oxidation (CO-PROX) process can lead to a reduction of the CO concentration in the hydrogen-rich gas from WGS process of hydrocarbons reformate down to at least 10 ppmv or below, so as to enable its direct feeding to standard PEM fuel cells. Rh-based catalysts supported on 3A zeolite, alumina, titania or ceria were prepared and tested for potential application in CO-PROX operating in a temperature range compatible with PEM FCs (80–100 °C). Among the prepared catalysts, 1% Rh-3A zeolite catalyst was found to be the most suitable one for the CO-PROX at low temperature: it reduced the inlet CO concentration below 10 ppmv within a temperature range of at least 80–120 °C without the appearance of undesirable side reactions. In order to improve the oxygen selectivity toward the CO complete oxidation with a lowest as possible hydrogen parasitic oxidation, the oxygen amount in the feed gas composition was decreased. With 1% Rh-3A zeolite catalyst the lowest O2 feed concentration was found to be 3 times the corresponding value of the CO stoichiometric oxidation (λ = 2O2/CO = 3). Finally, with the goal to reduce the noble metal costs, tests at lower Rh load (from 1 to 0.5%) were carried out. No significant variations in both activity and width of complete CO conversion temperature range resulted for 0.5% Rh-3A catalyst. Therefore, this catalyst operating at λ = 3 could potentially be used for the CO-PROX reaction at low temperature.

Catalytic performance of Au–TiO2 catalysts prepared by deposition–precipitation for CO preferential oxidation in H2-rich gases

CO preferential oxidation (CO-PROX) can lead to a reduction of the CO content in the hydrogen-rich gas derived from hydrocarbon reforming down to at least 10 ppmv or below, so as to enable its direct feeding to standard PEM fuel cells. Au-based catalysts supported on titania were prepared and tested as potential applications in CO-PROX operating over a temperature range compatible with PEM FCs (80–100 °C). Au–TiO2 catalysts were prepared by means of two deposition–precipitation methods, varying pH (with NaOH) and some other parameters during the preparation procedure. Also the calcinations temperature was a varied parameter to evaluate the most promising catalyst. Catalyst prepared by deposition–precipitation method, using lower pH and calcined at 300 °C was found to be the most suitable one for the CO-PROX at low temperature: it reduced the inlet CO concentration below 10 ppmv within a temperature range of at least 60–80 °C. Tests at progressively lower O-to-CO feed ratio (λ) and the same WSV value were carried out for the sake of reducing H2 consumption and improve CO-PROX selectivity. For 2%Au–TiO2 the minimum λ value, ensuring a sufficiently wide temperature range of a nearly complete CO conversion at temperatures compatible with PEM FCs operation was found to be equal to 4. Tests doubling WSV were also carried out, but a complete CO conversion was not reached. Finally, CO inlet concentration was reduced to 0.5% (possible CO outlet concentration from WGS stage); parasite RWGS reaction seemed to take place leading to worse performance.