Enhanced catalytic performance of Au/CuO–ZnO catalysts containing low CuO content for preferential oxidation of carbon monoxide in hydrogen-rich streams for PEMFC

In this study, effects of Au and CuO loadings in Au/CuO–ZnO nanocatalysts for preferential oxidation of carbon monoxide in H₂-rich streams (PROX) are investigated. CuO–ZnO supports were synthesized by a co-precipitation method. Au was also incorporated into the catalysts by a deposition-precipitation procedure. The catalysts were characterized by XRD, BET surface area, FESEM, HRTEM, H₂-TPR, FTIR, and CO-TPD. 2–10 nm Au nanoparticles are dispersed on CuO–ZnO support and significantly enhance the reducibility of CuO. The Au/CuO–ZnO catalysts containing low amount of CuO were found to be more active for PROX compared to the Au/ZnO catalyst. Moreover, as more CuO is added to Au/ZnO, the CO₂ selectivity increases in the whole PROX temperature range. The catalyst containing 2 wt% Au and 1 wt% CuO on ZnO exhibited the highest activity and selectivity in the operating temperature range of PEM fuel cells. The activity of this catalyst also remained almost intact during 900 min of PROX time on stream at 80 °C.     

Synthesis of highly-dispersed CuO–CeO2 catalyst through a chemisorption-hydrolysis route for CO preferential oxidation in H2-rich stream

The CuO–CeO₂ catalyst (CuO loading: 15 wt%) was prepared by a novel chemisorption-hydrolysis method, and employed for the preferential oxidation of CO (CO PROX) in H₂-rich stream. For comparison, several other conventional methods such as impregnation, co-precipitation and deposition-precipitation were also used to prepare the catalyst. It is found that the CuO–CeO₂ catalyst prepared by chemisorption-hydrolysis method exhibits the best catalytic performance, giving not only the widest temperature window (120–170 °C) for CO complete conversion, but also the highest oxygen to CO₂ selectivity of 99.9% at 120 °C. The results of XRD, N₂O chemisorption and in-situ FT-IR conformably indicate that this catalyst possesses the highest dispersion of Cu species, which facilitates the formation of Cu⁺ carbonyl species, and simultaneously prevents the adsorption and oxidation of H₂. With the increase of reaction temperature, Cu⁺ is gradually reduced to Cu⁰, enhancing the adsorption and oxidation of H₂, as a result, the selectivity of oxygen towards CO₂ is lowered obviously. The presence of CO₂ and H₂O exhibits negative effects on the catalytic performance, shortening the activity window to 150–170 °C region and decreasing the CO₂ selectivity to 87% at the temperature for the initial 100% conversion of CO. Based on the above study, a potential reaction pathway for CO PROX over the CuO–CeO₂ catalyst is proposed.     

Comparative study of CeO2/CuO and CuO/CeO2 catalysts on catalytic performance for preferential CO oxidation

The CeO₂/CuO and CuO/CeO₂ catalysts were synthesized by the hydrothermal method and characterized via XRD, SEM, H₂-TPR, HRTEM, XPS and N₂ adsorption–desorption techniques. The study shows that the rod-like structure is self-assembled CeO₂, and both hydrothermal time and Ce/Cu molar ratio are important factors when the particle-like CeO₂ is being self-assembled into the rod-like CeO₂. The CuO is key active component in the CO-PROX reaction, and its reduction has a negative influence on the selective oxidation of CO. The advantage of the inverse CeO₂/CuO catalyst is that it still can provide sufficient CuO for CO oxidation before 200 °C in the hydrogen-rich reductive gasses. The traditional CuO/CeO₂ catalyst shows better activity at lower temperature and the inverse CeO₂/CuO catalysts present higher CO₂ selectivity when the CO conversion reaches 100%. The performance of mixed sample verifies that they might be complementary in the CO-PROX system.     

Combustion synthesis of copper catalysts for selective CO oxidation

Copper catalysts supported on ceria, zirconia and niobia were prepared by combustion method with urea, containing a CuO loading of 6 wt.%, and tested on selective oxidation of CO. The characterization of the samples by X-ray diffraction (XRD) presented the formation of solid solution on CuO–CeO₂ catalyst and a change in crystalline structure of the support with copper insertion on ZrO₂ and Nb₂O₅ catalysts. The analysis of temperature-programmed reduction (TPR) revealed different interaction degrees of copper with the supports, with reduction peaks between 222 and 390 °C. The temperature-programmed desorption of CO (TPD-CO) profiles showed formation of CO₂ and H₂ only for the ceria and zirconia catalysts. In relation to the catalytic tests, the CuO–CeO₂ catalyst presented the best performance, with CO conversion of 95% at 150 °C up to 45 h on stream, and CO₂ selectivity of 55%.     

Effect of combining metallic and acid functions in CZA/HZSM-5 desilicated zeolite catalysts on the DME steam reforming in a fluidized bed

A study has been carried out on the effect of combining the metallic (CuO–ZnO–Al₂O₃) and acid (HZSM-5 zeolite treated with NaOH) functions, and of their mass ratio, on the kinetic performance in the steam reforming of dimethyl ether (SRD). The runs have been conducted in a fluidized bed reactor in the 225–325 °C range, and the reaction indices (dimethyl ether and methanol conversions, and H₂ and CO yields) have been explained based on the physico-chemical properties of the catalyst.     

CeO2 nanoparticles supported on CuO with petal-like and sphere-flower morphologies for preferential CO oxidation

The CuO supports with different morphology were prepared using the precipitation method. The inverse CeO₂/CuO catalysts were synthesized by the impregnation method, and characterized via XRD, H₂-TPR, SEM, TEM, XPS and N₂ adsorption-desorption techniques. The study showed that CO oxidation took place at the interface of CeO₂–CuO catalysts. The CeO₂/CuO catalysts maintained their morphologies, structure and the length of periphery at the CeO₂–CuO interface in the hydrogen-rich reaction gasses during the reaction. The two-dimensional and homogeneous petal morphology of support was most favorable for the formation of long periphery at the CeO₂–CuO interface, therefore the CeO₂ supported on the CuO with petal morphology presented good catalytic activity.     

Influence of catalyst and temperature on gasification performance of pig compost for hydrogen-rich gas production

The catalytic steam gasification of pig compost (PC) for hydrogen-rich gas production was conducted in a fixed-bed reactor. The influence of the catalyst and reactor temperature on yield and product composition was studied at the temperature range of 700–850 °C, for weight hourly space velocity (WHSV) in the range of 0.30–0.60 h⁻¹. The results indicate that the developed NiO on modified dolomite (NiO/MD) catalyst reveals better catalytic performance on the tar elimination and hydrogen yield than calcined MD or NiO/γ-Al₂O₃ catalyst. Meanwhile, the lower WHSV and higher reactor temperature can contribute to more hydrogen production and gas yield. Moreover, the char from catalytic steam gasification of PC has a highest ash content of 75.84% at 850 °C. In conclusion, pig compost is a potential candidate for hydrogen gas production through catalytic steam gasification technology.     

Modified precipitation processes and optimized copper content of CuO–CeO2 catalysts for water–gas shift reaction

The precipitation processes of CuO–CeO₂ catalysts preparation were modified, and their optimized copper content were also studied in detail. As indicated by the experimental results, the WGS catalytic activities of the CuO–CeO₂ catalysts can be ranked as: stepwise precipitation (SP) > deposition–precipitation (DP) > co-precipitation (CP), suggesting that stepwise precipitation (i.e., a modified DP) is a convenient and effective method to prepare CuO–CeO₂ WGS catalysts. The optimized copper contents of CuO–CeO₂–SP and CuO–CeO₂–CP catalysts are 20 wt.% and 25 wt.%, respectively. Their catalytic activities can be strongly correlated with the results from XRD, XPS, Raman, N₂-physisorption, N₂O chemisorption and H₂-TPR. For DP and SP, a certain amount of copper has substitutively incorporated into ceria lattice with multiple Ce³⁺ and oxygen vacancies, which is considered as one relatively strong interaction between copper and ceria support. The substitutional incorporation of copper creates larger lattice distortion, lattice defect (embodying as larger lattice cell contraction of CeO₂, microstrain and Raman shift) and stronger reducibility (i.e., lower reduction temperature of H₂-TPR), as a consequence, higher surface energy and catalytic activity for WGS reaction. While for CP, copper nitrate and cerium nitrate were simultaneously precipitated, resulting in the burial of many copper species in ceria supports, i.e., occupancy incorporation of isolated copper into ceria lattice vacant site, which is considered as another weak interaction between copper and ceria support. Accordingly, combined with N₂O chemisorption results, it is demonstrated that surface copper species of CuO–CeO₂–CP are fewer and less active than those of CuO–CeO₂–DP and CuO–CeO₂–SP. Lastly, there is a direct relationship between the pore volume along with most probable pore size of the as-synthesized catalysts and their corresponding catalytic activities for WGS reaction.     

Autothermal reforming of iso-octane and gasoline over Rh-based catalysts: Influence of CeO2/γ-Al2O3-based mixed oxides on hydrogen production

Autothermal reforming (ATR) of iso-octane in the presence of Rh-based catalysts (0.5 wt% of Rh) supported onto γ-Al₂O₃, CeO₂, and ZrO₂ were initially carried out at 700 °C with a S/C ratio of 2.0, an O/C ratio of 0.84, and a gas hourly space velocity (GHSV) of 20,000 h⁻¹. The activity of Rh/γ-Al₂O₃ was found to be higher than Rh/CeO₂ and Rh/ZrO₂, with H₂ and (H₂ + CO) yields of 1.98 and 2.48 mol/mol C, respectively, after 10 h. This Rh/γ-Al₂O₃ material, however, was potentially susceptible to carbon coking and produced 3.5 wt% of carbon deposits following the reforming reaction, as evidenced by C, H, N, and S elemental analysis. In contrast, Rh/CeO₂ catalyst exhibited lower activity but higher stability than Rh/γ-Al₂O₃, with nearly no carbon being formed within 10 h. To combine the superior activity originated from Rh/γ-Al₂O₃ with high stability from Rh/CeO₂, Rh/CeO₂/γ-Al₂O₃ catalysts with different CeO₂ contents were synthesized and examined for the ATR reactions of iso-octane. Compared to Rh/γ-Al₂O₃, the newly prepared Rh/CeO₂/γ-Al₂O₃ catalysts (0.5 wt% of Rh and 20 wt% of CeO₂) showed even enhanced activity during 10 h, and H₂ and (H₂ + CO) yields were calculated to be 2.08 and 2.62 mol/mol C, respectively. In addition, as observed with Rh/CeO₂, the catalyst was further found to be stable with less than 0.3 wt% of carbon deposition after 10 h. The Rh/γ-Al₂O₃ and Rh/CeO₂/γ-Al₂O₃ catalysts were eventually tested for ATR reactions using commercial gasoline that contained sulfur, aromatics, and other impurities. The Rh/γ-Al₂O₃ catalyst was significantly deactivated, showing decreased activity after 4 h, while the Rh/CeO₂/γ-Al₂O₃ catalyst proved to be excellent in terms of stability against coke formation as well as activity towards the desired reforming reaction, maintaining its ability for H₂ production for 100 h.     

Steam reforming and oxidative steam reforming of methanol over CuO–CeO2 catalysts

Steam reforming (SRM) and oxidative steam reforming of methanol (OSRM) were carried out over a series of coprecipitated CuO–CeO₂ catalysts with varying copper content in the range of 30–80 at.% Cu (= 100 × Cu/(Cu + Ce)). The effects of copper content, reaction temperature and O₂ concentration on catalytic activity were investigated. The activity of CuO–CeO₂ catalysts for SRM and OSRM increased with the copper content and 70 at.% CuO–CeO₂ catalyst showed the highest activity in the temperature range of 160–300 °C for both SRM and OSRM. After SRM or OSRM, the copper species in the catalysts observed by XRD were mainly metallic copper with small amount of CuO and Cu₂O, an indication that metallic copper is an active species in the catalysis of both SRM and OSRM. It was observed that the methanol conversion increased considerably with the addition of O₂ into the feed stream, indicating that the partial oxidation of methanol (POM) is much faster than SRM. The optimum 70 at.% CuO–CeO₂ catalyst showed stable activities for both SRM and OSRM reactions at 300 °C.     

Oxygen-enhanced water gas shift over ceria-supported Au–Cu bimetallic catalysts prepared by wet impregnation and deposition–precipitation

Au–Cu/ceria bimetallic catalysts were prepared incorporating Au by incipient wetness impregnation (IWI) and deposition-precipitation (DP) methods (with loadings of 1 wt.% and 7 wt.% of Au and Cu, respectively). The as-prepared catalysts were characterized by techniques such as BET, XRD, Raman, XPS, H₂-TPR, CO-TPD and Oxygen Storage Capacity (OSC) measurements. The results indicated a good dispersion of gold and copper for copper ceria catalyst and Au–Cu bimetallic catalysts. Addition of Au to CuO/CeO₂ increases highly the capacity to release lattice oxygen to oxidized CO at low temperatures compared to pure CuO/CeO₂. Au/CeO₂ and Au–CuO/CeO₂ catalyst prepared by DP show higher OSC value than counterparts prepared by IWI, either at 120 and 250 °C. Also, gold-containing catalysts prepared by DP show lower temperature of reduction that the samples prepared by IWI as a consequence of the higher dispersion of gold in the former samples. The presence of gold at different oxidation states was observed by XPS analysis. Preparation method strongly affects to the atom ratio of Au and Au + Cu with respect to surface ceria. The gold incorporation method was a key factor that enhances the redox properties and activity in both WGS and OWGS reactions. The present study shows the gas phase oxygen enhanced the activity of monometallic CuO/ceria and bimetallic Au–Cu/ceria prepared by IWI and DP methods in both WGS and OWGS reactions. AuCC catalyst prepared by DP shows higher hydrogen yield and also higher CO conversion than other prepared by IWI during OWGS reaction.     

Hydrogen production by steam reforming of dimethyl ether and CO-PrOx in a metal foam micro-reactor

A bi-function catalyst containing CuZnAlCr and HZSM-5 was used to generate hydrogen by stream reforming of dimethyl ether (SRD) in a metal foam micro-reactor and a fix-bed reactor. Dimethyl ether conversion of 99% and hydrogen yield of >95% was reached with HZSM-5/CuZnAlCr (mass ratio of 1:1) in the micro-reactor. A suitable balance between the dimethyl ether hydrolysis and methanol reforming steps requires the proper acidity and the metal sites. The CuZnAlCr/HZSM-5 properties, effect of CuZnAlCr to HZSM-5 mass ratio were investigated in the metal foam micro-reactor. Moreover, CO was removed from hydrogen-rich gas by preferential oxidation reaction (CO-PrOx) with PtFe/γ-Al₂O₃ catalyst in a similar metal foam micro-reactor follows the SRD stage. With the optimized O₂/CO ratio and reaction temperature, the CO concentration dropped to <10 ppm and hydrogen yield of ∼90% were achieved in the new-type SRD-COPrOx system. The SRD-COPrOx system provide a constant hydrogen production with CO concentration lower than 10 ppm during 20 h. The results indicate that metal foam micro-reactor has the great potential in the DME steam reforming to supply hydrogen for low-temperature fuel cells.     

Steam reforming of dimethyl ether over Cu–Ni/γ-Al2O3 bi-functional catalyst prepared by deposition–precipitation method

Cu–Ni/γ-Al₂O₃ catalysts with different metal contents for dimethyl ether steam reforming (DME SR) were prepared by the method of deposition–precipitation. Characterization of specific surface area measurement (BET), X-ray diffraction (XRD) and hydrogen temperature-programmed reduction (H₂-TPR) revealed that nickel improved the dispersion of copper, increased the interaction between copper and γ-Al₂O₃, and therefore, inhibited the sintering of copper. Ammonia temperature-programmed desorption (NH₃-TPD) showed that metal particles could occupy the acid sites, leading to the decrease in acid amount and acid strength of Cu–Ni/γ-Al₂O₃ catalyst. Kinetic measurements indicated that γ-Al₂O₃ is vital for DME SR and a higher content of γ-Al₂O₃ in catalyst was needed. The addition of nickel suppressed the water gas shift (WGS) reaction. Initial durability testing showed that the conversion of DME over Cu–Ni/γ-Al₂O₃ catalyst was always almost complete during the 30 h experimental reaction time. Therefore, Cu–Ni/γ-Al₂O₃ could be a potential DME SR catalyst for the production of hydrogen.     

Study of partial oxidation reforming of methane to syngas over self-sustained electrochemical promotion catalyst

A self-sustained electrochemical promotion (SSEP) catalyst is synthesized for partial oxidation reforming (POXR) of CH₄ to produce syngas (H₂ and CO) at a relatively low temperature ranging from 350 to 650 °C. The SSEP catalyst is comprised of 4 components: microscopic Ni/Cu/CeO₂ anode, La_0.9Sr_0.1MnO₃ cathode, copper as electron conductor, and yttria-stabilized-zirconia as oxygen ion conductor, which form microscopic electrochemical cells to enable the self-sustained electrochemical promotion for the POXR process. The SSEP catalyst exhibited much better catalytic performance in POXR of CH₄ than a Ni–Cu–CeO₂ catalyst and a commercial Pt–CeO₂ catalyst. The CH₄ conversion over the SSEP catalyst is 29.4% at 350 °C and reaches 100% at 550 °C and the maximum selectivity to H₂ is on the level of 90% at 450–650 °C under a GHSV of 42,000 h⁻¹. The mechanism of the SSEP is discussed.     

Influence of crystallite size and interface on the catalytic performance over the CeO2/CuO catalysts

The solvothermal method was used to prepare the CuO precursor with cotton-ball-like morphology in order to obtain the CeO₂/CuO catalysts with high BET surface area. The catalysts were characterized via SEM, XRD, H₂-TPR, ICP, HRTEM and N₂ adsorption–desorption techniques. The study shows that CeO₂ and CuO interact on the contact interface. The interaction of oxides switches on CO oxidation at 55 °C and the synergistic effect of interaction also improves H₂ oxidation at 95 °C. CO oxidation takes place at the contact interface of CeO₂ and CuO. The high BET surface area and good dispersion of catalysts can be more helpful for the presence of accumulated long periphery at interface of CeO₂ and CuO than the larger CeO₂ particles when most of CeO₂ particles pile into the small clusters and distribute on the bulk CuO. The CeO₂/CuO catalyst with 1:2 Ce/Cu molar ratio has the highest BET surface area and better dispersion of CeO₂ among the catalysts, therefore it display good catalytic activity, selectivity and stability.     

A kinetic study on the structural and functional roles of lanthana in iron-based high temperature water–gas shift catalysts

The structural and functional roles of varying amounts of lanthana in co-precipitated high temperature Fe₂O₃/Cr₂O₃/CuO water–gas shift catalysts were studied at 1 atm and 350–425 °C temperature range.     

Hydrogen production from low temperature WGS reaction on co-precipitated Cu–CeO2 catalysts: An optimization of Cu loading

Low temperature water–gas shift (WGS) reaction has been carried out at the gas hourly space velocity of 72,152 h⁻¹ over Cu–CeO₂ catalyst prepared by a co-precipitation method. Cu loading was optimized to obtain highly active co-precipitated Cu–CeO₂ catalysts for low temperature WGS. 80 wt% Cu–CeO₂ exhibited the highest CO conversion as well as the most stable activity (X_CO > 46% at 240 °C for 100 h). The excellent catalytic performance is mainly due to a strong metal to support interaction, resulting in the prevention of Cu sintering.     

Study of Ce-Pt/γ-Al2O3 for the selective oxidation of CO in H2 for application to PEFCs: Effect of gases

In order to supply pure hydrogen to proton exchange membrane (PEM) fuel cells and avoid CO poisoning, selective CO oxidation in H₂ was studied over Ce-Pt/γ-Al₂O₃. Adding the Ce promoted the CO conversion and selectivity of Pt/γ-Al₂O₃ with changing loading weights of Pt and Ce, oxygen concentration, residence time, and the composition of gases (H₂O, CO₂, and N₂). At 250 °C, adding H₂O to the feed gas enhanced the CO conversion due to the water–gas shift reaction. While, adding CO₂ to the feed gas suppressed the CO conversion due to the reversible water–gas shift reaction. In situ BET and XRD tests showed that well-dispersed metallic Pt particles (−2 nm) existed on the Ce oxide over the alumina support, which helps to supply oxygen to the Pt for a high activity of CO oxidation and selectivity.     

Meso–macroporous monolithic CuO–CeO2/γ/α-Al2O3 catalysts for CO preferential oxidation in hydrogen-rich gas: Effect of loading methods

Meso–macroporous alumina supported CuO–CeO₂ catalysts were prepared by citrate, urea combustion and impregnation methods. The effect of loading methods on the microstructure of the catalysts, the interaction between copper and ceria and the catalytic performance for preferential oxidation of CO in hydrogen-rich gases was investigated. The prepared monolithic catalysts were characterized by using techniques of N₂ adsorption and desorption, SEM, XRD, HRTEM and TPR. The results showed that the loading methods markedly influenced the catalyst structure and the catalytic performance. The citrate and urea combustion methods favored the formation of the interaction between copper and ceria. Compared with the urea combustion method, the citrate method led to smaller ceria particles on the alumina support. The meso–macroporous monolithic catalysts prepared by the citrate method maintained the structural characteristics of the highly active CuO–CeO₂ catalysts, and showed good catalytic performance in CO preferential oxidation in the simulated reformate gases containing water and CO₂.     

Development of highly effective supported nickel catalysts for pre-reforming of liquefied petroleum gas under low steam to carbon molar ratios

The pre-reforming of commercial liquefied petroleum gas (LPG) was investigated over Ni–CeO₂ catalysts at low steam to carbon (S/C) molar ratios less than 1.0. It was found that the catalytic activity and selectivity depended strongly on the nature of the support and the interaction between Ni and CeO₂. The Ni–CeO₂/Al₂O₃ catalysts, which were prepared by impregnating boehmite (AlOOH) with an aqueous solution of cerium and nickel nitrates, exhibited the optimal catalytic activity and remarkable stability for the steam reforming of LPG in the temperature range of 275–375 °C. The effects of CeO₂ loading, reaction temperature and S/C ratio on the catalytic behavior of the Ni–CeO₂/Al₂O₃ catalysts were discussed in detail. The results showed that the catalysts with 10 wt.% CeO₂ had the highest catalytic activity, and higher S/C ratios contributed to LPG reforming and the methanation of carbon oxides and hydrogen. The XRD and H₂-TPR analyses revealed that the strong interaction between Ni and CeO₂ resulted in the formation of CeAlO₃ in the Ni–CeO₂/Al₂O₃ catalysts reduced. The stability tests of 15Ni–10CeO₂/Al₂O₃ catalyst at 350 °C indicated that the catalyst was stable, and the stability could be enhanced by increasing S/C ratio.