CO-PROX Reaction Research Articles

Ce0.97Cu0.03O2 Nanocatalysts Synthesized via Microwave-assisted Hydrothermal Method: Characterization and CO-PROX Catalytic Efficiency

ABSTRACTIn the work presented here, Ce0.97Cu0.03O2 nanoparticles were synthesized by a microwave-assisted hydrothermal method under different synthesis temperatures. The obtained nanoparticles were tested as catalysts in preferential oxidation of CO to obtain CO-free H2 (PROX reaction). The samples were characterized by X-ray diffraction, transmission electron microscopy (TEM), electron paramagnetic resonance spectroscopy (EPR) and temperature-programmed reduction (TPR). X-ray diffraction measurements detected the presence of pure cubic CeO2 for all synthesized samples. TEM images of the Ce0.97Cu0.03O2 nanoparticles revealed that samples synthesized at 80°C are composed mainly of nanospheres with an average size of 20 nm. The formation of some nanorods with an average diameter of 8 nm and 40 nm in length, and the size reduction of the nanoparticles from 20 to approximately 15 nm is observed with increasing synthesis temperature. EPR spectra indicated that copper is found well dispersed in sample synthesized at 160°C, located predominant in surface sites of ceria. For samples synthesized at 80 and 120°C, the species are less dispersed than in the other one, resulting in the formation of Cu2+−Cu2+ dimmers at the surface of ceria. TPR profiles presented two reduction peaks, one below 400°C attributed to the reduction of different copper species and a second peak around 800°C attributed to the reduction of Ce4+→ Ce3+ species located in the volume of the nanoparticles. The peak related to the reduction of copper species shifts to lower temperatures with increasing synthesis temperature, i.e., the sample synthesized at 160°C is more easily reduced than the ones synthesized at 120 and 80°C. The nanoparticles showed active as catalysts for the CO-PROX reaction. The microwave-assisted method revealed efficient for the synthesis of Ce0.97Cu0.03O2 nanoparticles with copper species selective for the CO-PROX reaction, which reaches CO conversions up to 92% for the sample synthesized at 160°C.

Bismuth effect on the Co–Mn/Ce0.85Zr0.15O2 nanoparticulate for CO preferential oxidation in simulated syngas

The novel and efficient bismuth modified supported Co–Mn catalysts were prepared and employed to catalyze the preferential oxidation of CO (CO PROX) in simulated syngas. The effects of introducing-methods and loadings of bismuth on both catalytic performance and catalyst nature were investigated. The N2 adsorption/desorption measurement, X-ray diffraction (XRD) and H2-temperature programmed reduction (H2-TPR), and O2-TPD (O2-temperature programmed desorption) characterization techniques were performed to reveal the relationship between the catalytic properties and the nature of the catalysts. Results demonstrate that the as-prepared Bi modified supported Co–Mn catalyst exhibits excellent catalytic performance, depending on the introducing method and loadings of Bi. The enhancement of Bi addition into supported Co–Mn catalyst in the catalytic performance for CO PROX reaction is mainly ascribed to the dramatically improved reducibility of the Bi modified sample. Moreover, the decrease in hydrogen transformation over the Bi modified samples can be observed, suggesting the introduction of Bi can compress the catalytic activity for hydrogen oxidation. This study definitely demonstrates the existence of synergistic effect between the added bismuth and Co–Mn/Ce0.85Zr0.15O2 in the Bi modified supported Co–Mn catalyst for CO PROX reaction. The developed Co3O4–MnOx/Ce0.85Zr0.15O2–Bi2O3 catalyst with bismuth content of 4.2 wt.% presents the outstanding catalytic activity, selectivity, and durability for CO PROX reaction in the simulated syngas, and it can be considered as a promising candidate for highly efficient CO elimination from H2-rich stream.

Preferential oxidation of CO on Ni/CeO2 catalysts in the presence of excess H2 and CO2

Preferential oxidation of CO (CO-PROX) was carried out over Ni supported on CeO2 prepared by the co-precipitation method. The influence of metal loadings (2.5, 5 and 10 wt.% Ni) and the reaction conditions such as reaction temperature and feed composition on CO oxidation and oxidation selectivity were evaluated by using dry reformate gas. No other reactions like CO or CO2 methanation, coking, reverse water gas shift (RWGS) reaction is observed in the temperature range of 100–200 °C on these catalysts. Hydrogen oxidation dominates over CO oxidation above the temperature of 200 °C. An increase in oxygen leads to an increase in CO conversion but a simultaneous decrease in the O2 selectivity. It has been noticed that 5 and 10 % Ni/CeO2 show better catalytic activity towards CO-PROX reaction. These catalysts were characterized by SBET, XRD, TEM, XPS and H2-TPR.

CO–PROX reactions on copper cerium oxide catalysts prepared by melt infiltration

A series of copper cerium oxide catalysts was prepared by melt infiltration routes using silica hollow spheres (SHS) as the support material. Their catalytic activity in CO oxidation and preferential oxidation of CO in H2 stream (PROX) strongly depend on the Cu/Ce ratio. Lower ratios resulted in higher activity, indicating that finely dispersed CuO clusters that strongly interact with ceria are the active sites. The highest activity is found for an atomic Cu/Ce ratio of 1/8. The catalysts prepared by melt infiltration are more active than their counterparts prepared by conventional solution impregnation methods. The difference is attributed to the higher proportion of finely dispersed CuO clusters strongly interacting with ceria in the melt infiltrated catalysts.

Transition metal promoters in CuO/CeO2 catalysts for CO removal from hydrogen streams

Either cobalt or molybdenum was added to CuO/CeO2 support by incipient wetness impregnation, with Cu/Me (Me=Co, Mo) atom ratio of 3. The as-prepared catalysts were characterized and tested in CO-PROX, WGS and OWGS reactions. Both compounds are mainly as Co3O4 and MoO3, although part of molybdenum was detected as Mo+5. Addition of molybdenum modified the lattice of ceria and as well as the textural properties, as a fraction of molybdenum was inserted in ceria lattice. In addition, molybdena assisted the sintering of ceria. Addition of cobalt did not modify substantially the structure and texture of ceria. The concentration of surface oxygen vacancies decreased after addition of Co or Mo, as found by Raman spectroscopy. It was found that CuCe sample has the highest concentration of small, well-dispersed copper species among all the samples.There was found a variety of copper species in different interaction degree with the surroundings. The less reducible sample was a molybdenum-containing sample, while below 125°C the highest CO uptake to H2 uptake ratio corresponds to CoCuCe sample, which correlated with its highest selectivity in CO-PROX. CO conversion of CuCe and CoCuCe catalysts in CO-PROX, WGS and OWGS reactions were similar, but cobalt-containing catalysts was more selective towards CO2 in CO-PROX, suggesting that the active copper species for both reactions should be different. Activity of MoCuCe sample is suppressed in CO-PROX, WGS and OWGS reactions. Cobalt was found to suppress H2 oxidation in CO-PROX reaction. CoCuCe catalyst is highly selective and active catalyst for CO-PROX, but its activity in WGS and OWGS does not overcome the activity of CuCe catalyst.

EFFECT OF PREPARATION METHODS OF CeO2-MnOx MIXED OXIDES ON PREFERENTIAL OXIDATION OF CO IN H2-RICH GASES OVER CuO-BASED CATALYSTS

The CeO 2 -MnO x mixed oxides were prepared by deposition-precipitation (DP) and surfactant-templated (CB) methods, and then used as the support of CuO/ CeO 2 -MnO x catalysts. The samples were characterized by means of XRD, BET, H 2 -TPR, CO-TPD and XPS. Results show that preparation methods of CeO 2 -MnO x support have a direct effect on the physicochemical properties and catalytic activities of CuO/CeO 2 -MnO x catalysts for the CO preferential oxidation in H 2 -rich gases(CO PROX). CuO/CeO 2 -MnO x (CB) catalyst exhibits higher CO conversion and stability in H 2 -rich gases than CuO/CeO 2 -MnO x (DP), with 100% conversion of CO at 140 °C after a temperature cycle, which indicates that CeO 2 -MnO x (CB) as support of CuO based catalyst is more useful for CO PROX reaction than CeO 2 -MnO x (DP). Compared with CuO/CeO 2 -MnO x (DP), there are richer oxygen vacancies and more Mn4+ species on the surface of CuO/CeO 2 -MnO x (CB), and stronger interaction between CeO 2 and MnO x in it. More amounts of active copper species and complicated transfer of electrons density on CuO/CeO 2 -MnO x (CB) also favour the activity of the catalyst.

Improvement of nano-particulate CexZr1−xO2composite oxides supported cobalt oxide catalysts for CO preferential oxidation in H2-rich gases

The Co3O4/CexZr1−xO2 is a potential catalyst for CO preferential oxidation (CO PROX) in excess hydrogen. This study is devoted to the optimization of the nano-particulate CeO2–ZrO2 supported cobalt oxide catalysts. The effects of Ce/(Ce + Zr) atomic ratio, Co3O4 loading, calcination temperature, as well as reaction conditions like addition of CO2 and H2O, gas hourly space velocity (GHSV) and O2 concentration on the catalytic properties were investigated. Moreover, the temperature programmed reduction (TPR) and the powder X-ray diffraction (XRD) techniques were used to reveal the relationship between catalyst nature and catalytic properties. Results demonstrate that the catalytic performance of Co3O4/CexZr1−xO2 catalysts is strongly dependent on the H2 uptake, reduction temperature and crystallite size affected by Ce/(Ce + Zr) atomic ratio, cobalt oxide loading and calcination temperature. It is also found that the developed catalyst possesses high catalytic stability, and no obvious decrease in either CO conversion or CO2 selectivity can be observed even with the existence of CO2 and H2O in the feed. 16 wt.%Co3O4/Ce0.85Zr0.15O2 calcined at 450 °C could be a promising catalyst for the CO PROX reaction to eliminate trace CO from H2-rich gas.

Mesoporous CexMn1−xO2 composites as novel alternative carriers of supported Co3O4 catalysts for CO preferential oxidation in H2 stream

The mesoporous Co3O4 supported catalysts on Ce–M–O (M = Mn, Zr, Sn, Fe and Ti) composites were prepared by surfactant-assisted co-precipitation with subsequent incipient wetness impregnation (SACP–IWI) method. The catalysts were employed to eliminate trace CO from H2-rich gases through CO preferential oxidation (CO PROX) reaction. Effects of M type in Ce–M–O support, atomic ratio of Ce/(Ce + Mn), Co3O4 loading and the presence of H2O and CO2 in feed were investigated. Among the studied Ce–M–O composites, the Ce–Mn–O is a superior carrier to the others for supported Co3O4 catalysts in CO PROX reaction. Co3O4/Ce0.9Mn0.1O2 with 25 wt.% loading exhibits excellent catalytic properties and the 100% CO conversion can be achieved at 125–200 °C. Even with 10 vol.% H2O and 10 vol.% CO2 in feed, the complete CO transformation can still be maintained at a wide temperature range of 190–225 °C. Characterization techniques containing N2 adsorption/desorption, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR) and scanning electron microscopy (SEM) were employed to reveal the relationship between the nature and catalytic performance of the developed catalysts. Results show that the specific surface area doesn’t obviously affect the catalytic performance of the supported cobalt catalysts, but the right M type in carrier with appropriate amount effectively improves the Co3O4 dispersibility and the redox behavior of the catalysts. The large reducible Co3+ amount and the high tolerance to reduction atmosphere resulted from the interfacial interaction between Co3O4 and Ce–Mn support may significantly contribute to the high catalytic performance for CO PROX reaction, even in the simulated syngas.

MOx (M = Mn, Fe, Ni or Cr) improved supported Co3O4 catalysts on ceria–zirconia nanoparticulate for CO preferential oxidation in H2-rich gases

This approach dealt with the modification of our developed Co3O4/Ce0.85Zr0.15O2 catalysts with transition metal oxides MOx (M=Mn, Fe, Ni or Cr) for CO preferential oxidation (CO PROX) in H2-rich gases. Results showed that just MnOx modification remarkably broadened the temperature window of 100% CO conversion, also better than those in the report about Co-Mn composite catalysts from another research group. And even in the presence of H2O and CO2, the 175–225°C temperature range for almost 100% CO conversion can be achieved over the optimized catalyst. Moreover, the addition of MnOx significantly improved the catalytic activity and selectivity, which was dramatically better than those of the supported MnOx or Co3O4 catalysts on ceria–zirconia nanoparticulate, indicating the existence of the remarkable Co-Mn synergistic effect. The MnOx modified supported Co3O4 catalysts exhibited outstanding catalytic properties for CO PROX reaction, which was strongly dependent on the reducibility and dispersion of the active species affected by the Co/Mn atomic ratio and loading. The XRD and H2-TPR techniques were performed to reveal the effect of structure–activity relationship on the catalytic properties. The analytic results presented that Co3+ was the main active species for CO PROX reaction, and the existence of Mn4+ and Mn3+ was also favorable to the reaction. The XRD and H2-TPR characterization results affirmed the existence of strong interaction between Co and Mn, increasing the ratio of Co3+/Co2+ resulted from the electron transfer between Co2+ and Mn4+. Besides the improvement of Co3O4 dispersion and the enlargement of the Co3+/Co2+ ratio through Co-Mn interaction, the addition of MnOx could improve the stability of the ceria–zirconia support through the metal–support interaction. As a result, the CO selective oxidation reaction was efficiently improved. The 16wt.%Co3O4-MnOx/Ce0.85Zr0.15O2 (Co/Mn=8:1) could be a quite potential catalyst for eliminating trace CO from H2-rich gases.

A high efficient two phase CuO/Cu2(OH)3NO3(Co2+/Fe3+) composite catalyst for CO-PROX reaction

CuO, Co3O4, Fe2O3 oxides, Cu2(OH)3NO3 and Cu2(OH)3NO3(Co2 +/Fe3 +) composite were prepared by solution technique, characterized by PXRD, N2 physisorption, SEM-EDX and were tested in the CO-PROX reaction. The pretreatment of Cu2(OH)3NO3(Co2 +/Fe3 +) composite in a gas mixture leads to formation of an active and highly selective two-phase CuO/Cu2(OH)3NO3(Co2 +/Fe3 +) composite catalyst. The H2 conversion over the composite catalyst reaches the value of 3% at 438 K when CO is converted to CO2 completely. The addition of Co2 + and Fe3 + to Cu2(OH)3NO3 stabilizes this phase in the composite and improves the catalytic activity in CO oxidation and the reaction selectivity.

Influence of the calcination temperature on the structure and reducibility of nanoceria obtained from crystalline Ce(OH)CO 3 precursor

A high yield green method was developed for the preparation of reactive nanotextured ceria (CeO 2). The preparation method is based on the oxidation of a crystalline Ce(OH)CO 3 precursor that decompose at relative low temperature (ca. 250 °C) yielding CeO 2 nanocrystals initially rich in Ce 3+. After increasing calcination temperatures (in the range 350–650 °C), PXRD analysis show a slight crystal growth after calcination temperatures up to 550 °C, however cell contraction in such case denotes the definitive oxidation of remnant Ce 3+ centers. XPS results confirm Ce 3+ fraction diminution as calcination temperature increases. TPR profiles of ceria samples show two reduction events being the low temperature one (at ca. 500 °C) related to a surface process in which approximately only one cerium monolayer is involved. Catalytic activity tests for COPROX reaction were performed under differential reactor conditions to evaluate their activity in the temperature range 100–300 °C. The optimum activity recorded for the sample calcined at 450 °C accounts for the compromise between oxide’s activation and surface preservation.

Cooperative effect between copper and gold on ceria for CO-PROX reaction

1.1 wt% Cu and 1 wt% Au/CeO 2 catalysts were prepared by impregnation of a high-surface area ceria (>200 m 2 g −1) with copper nitrate and dimethyl(acetylacetonate) gold(III), respectively. They were dried and calcined in air at 300 °C. They were denoted Au/CeO 2 and CuO x/CeO 2. The same procedure was used to prepare a composite catalyst by impregnation of gold on the CuO x/CeO 2 catalyst (Au-CuO x/CeO 2). XRD reveals that ceria is poorly crystallized (crystallite particle size of 11.4 nm) while Au and CuO x cannot be detected. Redox properties were evaluated by temperature-programmed reduction (TPR) and oxygen storage capacity (OSC) measurements. While gold is easily reduced below 100 °C, copper oxide reduces to Cu 0 at 266 °C. Copper and gold promote the surface reduction of ceria. OSC measurements confirm that about one monolayer of ceria can be reduced at 400 °C. The performances of the catalysts in PROX reaction (2%CO + 2%O 2 + 70%H 2 in He) were measured between room temperature and 300 °C. At low temperature (below 120 °C), there is an increase in CO conversion up to 100%. Au/CeO 2 is about 9 times more active than CuO/CeO 2 but it is much less selective (50–60% of selectivity for Au instead of 100% for CuO x). Au-CuO x/CeO 2 exhibits an intermediary behavior with both good activity and good selectivity. A decrease in CO conversion is observed above 120 °C for all the catalysts. It is due to the reverse water gas shift reaction (RWGS) tending to reform CO from CO 2 and H 2. This behavior is more marked for Au than for CuO x and Au-CuO x catalysts, which shows that gold is very active in RWGS. The three catalysts are deactivated by CO 2 while only copper is very sensitive to steam. An interesting cooperative effect is shown for the Au-CuO x/CeO 2 catalyst: (i) moderate and totally reversible inhibition by CO 2 and (ii) absence of inhibition by steam.

High catalytic activity in CO PROX reaction of low cobalt-oxide loading catalysts supported on nano-particulate CeO2–ZrO2 oxides

Abstract Co3O4/NP-ZrO2, Co3O4/NP-CeO2 and Co3O4/NP-Ce0.8Zr0.2O2 catalysts were prepared via a reverse microemulsion/incipient wetness impregnation (RM–IWI) method. The catalytic properties for CO preferential oxidation (CO PROX) reaction in H2-rich stream were investigated. The Co3O4/NP-Ce0.8Zr0.2O2 catalyst with 1.8 wt.% Co3O4 loading has exhibited higher catalytic activity than that of the other two catalysts. The higher catalytic activity might be attributed to the combination effect of the highly dispersed cobalt oxide, the improvement in CeO2 reducibility due to ZrO2 incorporation in CeO2 structures, and the strong cobalt oxide-support interaction.

CuO/CeO 2 washcoated ceramic monoliths for CO-PROX reaction

CuO/CeO 2 catalysts, with 7 and 9 wt.% copper loading was prepared by wet impregnation, and later was washcoated over cordierite monoliths in order to study their performance in CO-PROX reaction. The powder sample with low copper loading shows high concentration of easily reducible copper species. SEM images of the monoliths show an average thickness of 9 and 12 μm of the catalyst layer for 7% and 9% copper containing samples. EDX analyses suggested that copper has been deposited non-uniformly on the ceria surface. In the absence of CO 2 and H 2O, monolith washcoated with 7% Cu loaded catalyst shows better performance than its 9% counterpart in terms of activity and selectivity towards CO 2 in CO-PROX reaction. However, CO 2 substantially inhibits the activity and selectivity of both monoliths, in particular 7% Cu loaded catalyst. Also, H 2O inhibits the activity and selectivity of both monoliths, although in lower extent than that of CO 2.

Effect of Au promoter in CuO/CeO 2 catalysts for the oxygen-assisted WGS reaction

The present work focuses on the development of bimetallic Au–Cu catalyst supported on CeO 2 for the oxygen-assisted water-gas-shift (OWGS) reaction. Au–CuO/CeO 2 catalyst with Au loadings of 0.5, 1 and 1.5 wt.% and Cu loadings of 7, 11 and 20 wt.% were prepared by incipient wetness impregnation. The as-prepared catalysts were characterized by means of XRD, BET, and a set of temperature programmed methods. The catalysts activity was evaluated in CO-PROX, WGS and OWGS reactions. For WGS reaction the gold addition did not improve the activity of the better 7% CuO/CeO 2 catalyst. Addition of 1% of gold to CuO/CeO 2 improved the catalyst activity in CO-PROX and widened the operation window. Either with λ = 1 and 2, complete CO conversion is achieved at low temperature, usually below 120 °C, with high selectivities with all gold-containing catalysts. When small oxygen amount is added to the WGS mixture, the activities of all the catalysts tested were improved in terms of CO conversion, particularly catalysts containing both Au and CuO, although parallel hydrogen oxidation also is slightly improved. Catalyst with 1% Au and 7% CuO supported on ceria achieves almost equilibrium CO conversion at 220 °C when O 2/CO = 0.5 is added to the feedstream.

Promotion effect of Sn in alumina-supported Pt catalysts for CO-PROX

Abstract In this work we have studied the effect of the addition of Sn to alumina-supported Pt catalysts towards the catalytic performance in CO-PROX reaction. Monometallic Pt and Sn catalysts supported on alumina, and bimetallic Pt–Sn supported on alumina (with Pt/Sn atomic ratios of 1.92, 0.53 and 0.28) was prepared by successive impregnation, with high dispersion of the metal. The addition of Sn to Pt does not substantially increase the activity in CO-PROX at low temperatures; however, the temperature interval where the CO conversion is maximum was significantly increased. The optimum Pt/Sn atomic ratio was found to be 0.53. In a wide operation window with respect to temperature, the catalyst with optimum Pt to Sn ratio shows a maximum CO conversion of 78% for λ = 2 with constant selectivity (about 40%) and with 31%CO yield. In the presence of either CO2 or H2O the performance of Sn promoted catalyst was seen to show improved activity.

Hydrocarbons valorisation to cleaner fuels: H 2-rich gas production via fuel processors

This paper presents a panoramic overview of the experimental work carried out at Politecnico di Torino on the development of structured catalytic reactors for the production of H 2-rich streams from hydrocarbons, to be fed to PEM-FCs: short contact time catalytic partial oxidation (SCT-CPO) reactor for syngas production, micro-channelled reactors for syngas CO clean-up, water gas shift (WGS) and preferential oxidation (PROX). Concerning the reactor for methane SCT-CPO, 10% Ni deposited on irregular γ-Al 2O 3 particles presented the best performance (CH 4 conversion > 90%; H 2 selectivity > 95%) compared to 0.5% Rh deposited over γ-Al 2O 3 spheres. As regards the HT-WGS reaction, the best performance was obtained with the catalyst 1% Pt/(CeO 2 + TiO 2): it reached the thermodynamic equilibrium (inlet CO: 10% b.v.; WHSV = 40 Nl h −1 g cat −1) in the temperature range 450–525 °C, allowing the abatement of almost 60% of the CO present in the fed gas stream. Finally, the best performance towards CO-PROX reaction was obtained with the catalyst 1% Rh/(Al 2O 3 + 3A zeolite): it reached the complete CO conversion (inlet CO: 0.5% b.v.; WHSV = 40 Nl h −1 g cat −1; λ = 3) in the temperature range 130–160 °C, thus assuring an easy controllability of the system.

Design and testing of a microchannel reactor for the PROX reaction

The different steps for manufacturing a microchannel reactor for the PROX reaction are discussed. Transient Liquid Phase bonding (TLP) using a Ni–B–Si amorphous melt spun is used for joining micromilled Al-alloyed ferritic stainless steel plates followed by recrystallization at 1200°C for 5h. A CuOx–CeO2 catalyst synthesized by the coprecipitation method was washcoated on the microchannel block resulting in a homogenous 20–30μm thick layer. The catalytic activity for CO-PROX reaction is similar in both the powder catalyst and the microchannel coated reactor but the selectivity is higher in the microchannel reactor.

Preferential CO oxidation in excess of hydrogen over Au/HMS catalysts modified by Ce, Fe and Ti oxides

The effect of the modification of HMS (hexagonal mesoporous silica) support with Fe3+, Ce4+ or Ti4+ ions on the physico-chemical and catalytic properties of mesoporous Au/HMS catalyst was evaluated in the preferential oxidation (PROX) of CO, in hydrogen-rich stream (50vol.% H2) and in the total CO oxidation (TOX) reaction. The bare supports were prepared via neutral S0I0 templating route, employing one pot synthesis, whereas the supported gold catalysts were prepared by the deposition–precipitation method. The reduced and spent Au catalysts have been studied by different techniques such as N2 physisorption, oxygen storage capacity (OSC), XRD, HRTEM, hydrogen chemisorption (by TPD-H2), DRS UV–vis, Mössbauer and XPS spectroscopic techniques. Among the catalysts studied, Au/HMS–Fe recorded the highest activity and stability for total CO oxidation. The XPS and DRS UV–vis data suggest that the active-sites configuration of this catalyst during this reaction could be small gold particles and ionic gold species stabilized by the support. The Au/HMS–Fe catalyst was also able to oxidize CO selectively in a hydrogen-rich gas mixture (PROX reaction) at a relevant temperature to hydrogen fuel cell applications (T50=83°C). Additionally, the Au/HMS–Fe sample recorded the lowest hydrogen oxidation (undesired reaction) during the CO-PROX reaction in excess hydrogen. From the catalyst activity–structure correlation, the main factors influencing the superior catalytic behaviour of this sample in the CO-PROX reaction are (i) its largest Au species surface exposure, as determined by XPS for the spent catalysts; (ii) the largest population of small Au particles, as determined by HRTEM for the freshly reduced samples; (iii) the large oxygen and hydrogen storage capacities, and (iv) the lowest deactivation by the formation of surface coke species, as determined by coke burning followed by TGA technique.

Au–Cu alloy nanoparticles supported on silica gel as catalyst for CO oxidation: Effects of Au/Cu ratios

Au–Cu bimetallic catalysts with Au/Cu ratios ranging from 3/1 to 20/1 were prepared on silica gel support by a two-step method. The catalysts were characterized by ICP, XRD and TEM. The results showed that, irrespective of Au/Cu ratios, all the bimetallic nanoparticles had significantly reduced particle sizes (3.0–3.6 nm) in comparison with monometallic gold catalysts (5.7 nm). Both CO oxidation and PROX reactions were employed to evaluate the catalytic activities of Au–Cu bimetallic catalysts. For CO oxidation, the alloy catalysts show non-monotonic temperature dependence showing a valley in the intermediate temperature range. The catalyst with Au/Cu ratio of 20/1 gave the highest activity at room temperature, but its activity showed the deepest valley with increasing the reaction temperature. On the other hand, the catalyst with Au/Cu ratio of 3/1 exhibited the best performance for PROX reaction. For the Au/Cu ratios investigated, the bimetallic catalysts showed superior performance to monometallic gold catalysts, demonstrating the synergy between gold and copper.