Preferential CO Research Articles

Preferential CO oxidation over Au/ZnO and Au/ZnO–Fe 2O 3 catalysts prepared by photodeposition

Preferential CO oxidation in a H 2-rich stream was studied over Au/ZnO and Au/ZnO–Fe 2O 3 catalysts prepared by photodeposition under UV–vis light. The high catalytic activities of both Au/ZnO and Au/ZnO–Fe 2O 3 catalysts are presented over a temperature range of 30–130 °C. TEM results revealed that the average particle size of Au over the Au/ZnO and Au/ZnO–Fe 2O 3 catalysts, is in the range of 3–5 nm. Moreover, DR/UV–vis spectra showed that the prepared catalysts contained Au δ+ and Au 0 (active sites for the PROX reaction) on the catalyst support. Based on the experiment observations, it can be concluded that the catalysts prepared by a photodeposition exhibited excellent catalytic activity, even when both CO 2 and H 2O were added to the simulated stream.

Direct Evidence for the Interfacial Oxidation of CO with Hydroxyls Catalyzed by Pt/Oxide Nanocatalysts

By rational design of the FeO(111)/Pt(111) inverse model catalyst and the control experiments, we report for the first time direct experimental evidence for the interfacial CO(ads) + OH(ads) reaction to produce CO(2) at the Pt-oxide interface at low temperatures, providing deep insights into the reaction mechanism and active site of the important low-temperature water-gas shift and preferential CO oxidation reactions catalyzed by Pt/oxide nanocatalysts at the molecular level.

On the mechanism of hydrogen-promoted gold-catalyzed CO oxidation

The kinetics of CO oxidation, H 2 oxidation and preferential CO oxidation (PrOx) over Au/Al 2O 3 catalysts have been investigated. The catalysts with the smallest particles (∼2 nm) are the most active for all three reactions. As previously observed, the presence of H 2 greatly promotes CO oxidation, which becomes faster than CO-free H 2 oxidation at low temperature. From these results and on the basis of previous works, we propose a complete PrOx mechanism. The reaction involves Au–OOH, Au–OH and Au–H intermediates, also involved in H 2 oxidation, and benefits from the presence of low-coordination sites.

Au/ZnO and Au/ZnO−Fe2O3Prepared by Deposition−Precipitation and Their Activity in the Preferential Oxidation of CO

Preferential CO oxidation in a H2-rich stream over ZnO- and ZnO−Fe2O3-supported nanosize Au catalysts prepared by the deposition−precipitation method has been investigated. The reactant gas mixture contained 1% CO, 1% O2, 40% H2, 0−10% CO2, and 0−10% H2O, with the balance being He, at a total flow rate of 50 mL/min. The result showed that the Au/ZnO catalyst calcined at 500 °C displayed superior activity, giving complete CO conversion with the highest selectivity (∼75%) at 50 °C, whereas the Au/ZnO−Fe2O3 showed a marked improvement in activity under CO2 and H2O surroundings. In a two-stage reactor, it provided a comparable CO conversion and a better selectivity, about 10%, as compared to a single-stage reactor. The prepared catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscopy (TEM), diffuse reflectance/ultraviolet−visible (DR/UV−vis) spectroscopy, and temperature-programmed reduction (TPR).

Evaluating the impact of enhanced anode CO tolerance on performance of proton-exchange-membrane fuel cell systems fueled by liquid hydrocarbons

Recent advances in anode electrocatalysts for low-temperature PEM fuel cells are increasing tolerance for CO in the H 2-rich anode stream. This study explores the impact of potential improvements in CO-tolerant electrocatalysts on the system efficiency of low-temperature Nafion-based PEM fuel cell systems operating in conjunction with a hydrocarbon autothermal reformer and a preferential CO oxidation (PROx) reactor for CO clean-up. The incomplete H 2 clean-up by PROx reactors with partial CO removal can present conditions where CO-tolerant anode electrocatalysts significantly improve overall system efficiency. Empirical fuel cell performance models were based upon voltage–current characteristics from single-cell MEA tests at varying CO concentrations with new Pt–Mo alloy reformate-tolerant electrocatalysts tested in conjunction with this study. A system-level model for a liquid-fueled PEM fuel cell system with a 5 kW full power output is used to study the trade-offs between the improved performance with decreased CO concentration and the increased penalties from the air supply to the PROx reactor and associated reduction in H 2 partial pressures to the anode. As CO tolerance is increased over current state-of-the-art Pt alloy catalysts, system efficiencies improve due primarily to higher fuel cell voltages and to a lesser extent to reductions in parasitic loads. Furthermore, increasing CO tolerance of anode electrocatalysts allows for the potential for reduced system costs with minimal efficiency penalty by reducing PROx reactor size through reduced CO conversion requirements.

CO-Free Hydrogen Production for Fuel Cell Applications over Au/CeO2 Catalysts: FTIR Insight into the Role of Dopant

The impact of ceria doping by Zn (atomic ratio Zn/(Zn + Ce) = 0.05) on the structural and catalytic properties of Au/CeO(2) catalyst was studied. The ceria modification influenced the catalytic activity toward purification of hydrogen via water-gas shift (WGS) and preferential CO oxidation (PROX) reactions in a different way: it diminished the WGS activity and improved the PROX performance. A characterization by FTIR spectroscopy was conducted to explain differences in the catalytic performance. The nature of gold active species after different pretreatments, under different atmospheres (H(2), D(2)), and after admission of CO and its subsequent interaction with (18)O(2) was investigated. Evidence has been found of the dissociation of hydrogen at room temperature on gold, producing on the oxidized sample a broad absorption assigned to Au-OH vibrations, whereas on the reduced one, bands at 3200 and 1800 cm(-1) ascribed, respectively, to Au-OH and Au-H species have been detected. For the first time, the formation of Au-hydride on supported heterogeneous catalysts was proposed. These features were stronger on the Au/CeO(2) sample than on the Au/Zn-CeO(2) sample. The availability of highly dispersed gold clusters in contact with oxygen vacancies on the ceria surface could contribute to higher WGS activity, whereas the steps of small gold particles are the active sites for both CO and oxygen activation during the PROX reaction.

The development of a fully integrated micro-channel fuel processor using low temperature co-fired ceramic (LTCC)

A fully integrated micro-channel fuel processor system consisting of vaporizer, steam reformer, heat exchanger and preferential CO oxidation (PROX) was developed using low temperature co-fired ceramic (LTCC). To fabricate a compact all-in-one system, each substrate was stacked to build a multilayered type fuel processor. A CuO/ZnO/Al 2O 3 catalyst and Pt-based catalyst prepared by wet impregnation were deposited inside the micro-channel of steam reformer and PROX, respectively. The performance of the fully integrated micro-channel reformer was measured at various conditions such as the ratio of the feed flow rate, the ratio of H 2O/CH 3OH and the operating temperature of the reactor. In parallel with the experiments, 3-D fluid dynamics simulation (Fluent) was conducted to verify the micro-reformer performance. The fully integrated micro-channel reformer has the dimensions of W: 130 mm × D: 50 mm × H: 3 mm. The fuel processor produced the gas composition of 71% H 2 and 25% CO 2, and more than 93% of methanol conversion was achieved at 300 °C and 2 cm 3/h of the feed flow rate when CO concentration was maintained below 100 ppm by PROX.

Activity, stability, and deactivation behavior of supported Au/TiO 2 catalysts in the CO oxidation and preferential CO oxidation reaction at elevated temperatures

The influence of elevated reaction temperatures on the activity, long-time stability, and deactivation behavior of Au/TiO 2 catalysts for CO oxidation was studied by kinetic and in situ IR measurements in the temperature range 80–180 °C, both in an idealized, H 2-free reaction atmosphere and in a H 2-rich gas mixture. The results are discussed in a molecular picture. Most important for practical applications (PROX reaction), the selectivity for CO oxidation in H 2-rich gas mixtures decreases sharply with increasing temperature, from ∼60% at 80 °C to ∼15% at 180 °C. The increasing tendency for H 2 oxidation is reflected also by a distinctly lower apparent activation energy for CO oxidation in the presence of H 2 (18 ± 3 kJ mol −1) compared to reaction in a H 2-free reaction mixture (30 ± 3 kJ mol −1). In all cases, carbonate formation is observed and proposed to contribute to the deactivation, but with different extents depending on the reaction conditions.

Morphology effects of nanocrystalline CeO2 on the preferential CO oxidation in H2-rich gas over Au/CeO2 catalyst

Gold supported on CeO2 nanocrystals of rod, cubic and polyhedral shapes were used for CO preferential oxidation (CO-PROX) in H2-rich gas. The catalytic activity ranked by CeO2 nanocrystals as follows: rods > polyhedra > cubes. The results of X-ray photoelectron spectroscopy, temperature-programmed reduction/desorption and in situ Fourier transform infrared spectroscopy indicated that the surface gold species and also the adsorption/desorption properties for CO and oxygen species were closely related to the nature of exposed crystal planes of CeO2 nanocrystals. The gold catalyst on CeO2-rods with {1 0 0}/{1 1 0}-dominant surfaces showed the best CO-PROX performance.

Selective CO removal in a H 2-rich stream over supported Ru catalysts for the polymer electrolyte membrane fuel cell (PEMFC)

We prepared various Ru catalysts supported on different supports such as yttria-stabilized zirconia (YSZ), ZrO 2, TiO 2, SiO 2 and γ-Al 2O 3 with a wet impregnation method. We applied them to the selective CO removal in a hydrogen-rich stream via the preferential CO oxidation (PROX) and the selective CO methanation simultaneously. Among them, Ru/YSZ showed the highest CO conversion especially at low temperatures. Several measurements: the N 2 physisorption, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), the CO chemisorptions, the temperature-programmed oxidation (TPO), the temperature-programmed reduction (TPR), the temperature-programmed desorption (TPD) of CO 2 with mass spectroscopy and the transmission electron microscopy (TEM), were conducted to characterize the catalysts. No linear correlation can be found between the amount of CO chemisorbed at 300 K and the PROX activity. On the other hand, the facile activation of O 2 appeared to be closely related to the high PROX activity, judging by the TPO experiment. In addition, the strong adsorption of CO 2 suppressed the low-temperature PROX activity. Ru/YSZ can be easily oxidized and also reduced at low temperatures. It is found that Ru/YSZ uptakes only small amounts of CO 2, which can be desorbed at low temperatures. Ru/YSZ can reduce the high inlet CO concentration to be less than 10 ppm even in the presence of H 2O and CO 2.

IrFeO x/SiO 2—A highly active catalyst for preferential CO oxidation in H 2

A novel bifunctional catalyst IrFeO x/SiO 2, which was very active and selective for preferential oxidation of CO under H 2-rich atmosphere, was developed in this study. XRD, H 2-TPR, and chemisorption were applied to characterize three kinds of IrFeO x/SiO 2 catalysts prepared by different impregnation sequences. The results indicated that both FeO x and Ir species were highly dispersed on the SiO 2 support; the reduction of Ir species in the Fe-promoted Ir/SiO 2 catalyst became easier than those in the Ir/SiO 2, together with the partial reduction of Fe 2O 3; the saturation uptake of O 2 adsorption was greatly enhanced. In-situ DRIFTS, XPS, and Mössbauer techniques were further applied to detect the structural information on the selected Ir-FeO x/SiO 2 catalyst prepared by co-impregnation method. A non-competitive adsorption catalytic mechanism was proposed where CO adsorbed on Ir sites and O 2 adsorbed on FeO x sites; the reaction probably took place at the interface of Ir and FeO x or via a spill-over process.

Role of modifiers in multi-component MgO-supported Au catalysts designed for preferential CO oxidation

MgO-supported gold catalysts modified by V, Pb, Sm, La and Y have been investigated in order to elucidate the role of modifiers in preferential CO oxidation. All catalysts have been prepared by means of homogeneous precipitation of nitrate precursors using urea as a precipitating agent. Catalytic properties and characteristic features of catalysts were investigated in detail. Different spectroscopic techniques (XPS, FTIR and UV–Vis) and HRTEM were applied for catalyst characterization. The coverage of gold nanoparticles by small MgO clusters has been evidenced. Despite the high MgO coverage of gold, the parent Au/MgO catalysts showed high activity in CO oxidation indicating the beneficial effect of MgO in atomic closeness to Au. It has been revealed that in Pb-modified catalysts, gold has been alloyed with Pb. The results definitely indicate that Pb is responsible for the decrease in the number of gold sites involved in the activation of hydrogen. It has been shown that in multi-component catalysts MgO is decorated by nanoclusters of different metal oxides, and the Pb–Au nanoparticles are located in intimate contact to these oxides. In multi-component catalysts, the interfaces formed in this way provide new type of active site ensembles that can adsorb and activate CO and oxygen, while suppressing the activation of hydrogen.

Adsorption of CO 2 from dry gases on MCM-41 silica at ambient temperature and high pressure. 2: Adsorption of CO 2/N 2, CO 2/CH 4 and CO 2/H 2 binary mixtures

Adsorption equilibrium capacity of CO 2, CH 4, N 2, H 2 and O 2 on periodic mesoporous MCM-41 silica was measured gravimetrically at room temperature and pressure up to 25 bar. The ideal adsorption solution theory (IAST) was validated and used for the prediction of CO 2/N 2, CO 2/CH 4, CO 2/H 2 binary mixture adsorption equilibria on MCM-41 using single components adsorption data. In all cases, MCM-41 showed preferential CO 2 adsorption in comparison to the other gases, in agreement with CO 2/N 2, CO 2/CH 4, CO 2/H 2 selectivity determined using IAST. In comparison to well known benchmark CO 2 adsorbents like activated carbons, zeolites and metal-organic frameworks (MOFs), MCM-41 showed good CO 2 separation performances from CO 2/N 2, CO 2/CH 4 and CO 2/H 2 binary mixtures at high pressure, via pressure swing adsorption by utilizing a medium pressure desorption process (PSA-H/M). The working CO 2 capacity of MCM-41 in the aforementioned binary mixtures using PSA-H/M is generally higher than 13X zeolite and comparable to different activated carbons.

Role of Preparation Techniques in the Activity of Au/TiO2 Nanostructures Stabilised on SiO2: CO and Preferential CO Oxidation

Au colloid and titania in different sequence or Au-oxide nano ensembles preformed in hydrosol were deposited on inert amorphous silica or mesoporous SBA-15. It was compared with gold colloid adsorbed on TiO2 or silica support. The formation of the Au/TiO2 interface is discussed in terms of surface charges. Preferential CO oxidation in the presence of hydrogen (PROX) has been correlated with the CO oxidation activity and structural properties, perimeter and the influence of the TiO2 morphology on the catalytic activity has been demonstrated.

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 (

Preferential CO oxidation over supported noble metal catalysts

We conducted a comparative study for the preferential CO oxidation (PROX) over supported noble metal catalysts. The CO chemisorption in the absence and presence of H 2, the O 2 chemisorption, the temperature-programmed oxidation (TPO), the temperature-programmed reduction (TPR) after O 2 chemisorption and the transmission electron microscopy (TEM) were conducted to characterize the catalysts. Commercial catalysts such as 1 wt.% Pt/γ-Al 2O 3, 0.5 wt.% Ru/γ-Al 2O 3, 0.5 wt.% Rh/γ-Al 2O 3, 5 wt.% Pt/γ-Al 2O 3, 5 wt.% Ru/γ-Al 2O 3, and 5 wt.% Rh/γ-Al 2O 3 were utilized. Among them, 5 wt.% Ru/γ-Al 2O 3 showed the highest PROX activity. This catalyst can be considered to be promising because it can reduce the high inlet CO concentration to be less than 10 ppm at low temperatures where the reverse water–gas shift reaction can be minimized. No detectable amount of chemisorbed O 2 was measured at 373 K over 1 wt.% Pt/γ-Al 2O 3, 0.5 wt.% Rh/γ-Al 2O 3 and 5 wt.% Pt/γ-Al 2O 3. The moderate degree of CO and O 2 chemisorption at reaction temperatures appeared to be essential for the optimum PROX activity. Although 0.5 wt.% Ru/γ-Al 2O 3 and 5 wt.% Ru/γ-Al 2O 3 have the similar particle size distribution based on TEM analysis, the latter catalyst with smaller amount of chemisorbed CO and O 2 per Ru metal showed much better PROX activity. Based on TPO and TPR result, 0.5 wt.% Ru/γ-Al 2O 3 can be oxidized at lower temperatures but can be reduced at higher temperatures compared with 5 wt.% Ru/γ-Al 2O 3. Therefore, the easily reducible Ru species can give us the better PROX activity over a wide reaction temperature for Ru catalysts.

Flame‐Synthesized Ceria‐Supported Copper Dimers for Preferential Oxidation of CO

AbstractRapid synthesis of CuCeO2 catalysts by flame spray pyrolysis produces highly active Cu dimer morphologies without the need for additional catalyst pretreatment. The active Cu component is enriched onto the CeO2 surface at concentrations higher than the nominal loading with no evidence of amorphous or crystalline CuO phase. Increasing the Cu content results in a morphological transition from isolated Cu monomers to oxygen‐bridged dimers and an associated increase in oxygen vacancy concentration. Dimer‐containing CuCeO2 catalysts display high levels of activity and selectivity in the low‐temperature preferential oxidation of CO. Experimental measurements and simulations suggest that the geometry of the dimer presents a comparatively ionic CuO bond at the catalyst surface. Further studies indicate that these ionic dimer species promote preferential CO oxidation at lower temperatures than observed for monomeric Cu species. This is the first report to explicitly propose and demonstrate that the structural distortion associated with the formation of Cu dimers directly induces increased bond ionicity at the catalyst surface and that these changes are responsible for improved catalytic activity.

Preferential CO oxidation in a H2-rich stream promoted by ReOx species attached to Pt metal particles

Clusters of ReO(x) are formed and attached to Pt metal particles during H(2) reduction treatment on Pt-ReO(x)/SiO(2), and the synergy between Pt metal surface and neighboring ReO(x) clusters gives high catalytic performance in the preferential CO oxidation in the presence of H(2)O and CO(2) as more realistic reaction conditions at low reaction temperature.

Investigation of Double-Stage Preferential CO Oxidation Reactor over Bimetallic Au−Pt Supported on A-Zeolite Catalyst

This article presents experiment data on the preferential CO oxidation (PROX) in a hydrogen-rich stream, currently the most practical method for CO removal induced by a double-stage process. The reaction was carried out in a microreactor testing unit using Au and Pt supported on an A zeolite catalyst in the temperature range of 50−300 °C under atmospheric pressure. The reactant gas mixture contained CO (1%), O2 (1%), H2 (40%), CO2 (0−10%), and H2O (0−10%), with the balance being He, at a total flow rate of 50 mL/min. The results clearly show that the addition of the second stage significantly reduces the H2 loss and the O2 requirements while greatly increasing the CO selectivity from ∼48 to ∼58%. The addition of CO2 and H2O in the feed slightly reduces the CO oxidation activity both in single- and double-stage processes. Moreover, the double-stage process shows a CO removal efficiency of over 99% during 60 h of testing at a reaction temperature of 170 °C in the simulated reformed gas (CO2- and H2O-free).

Silicon-based micro-reactor for preferential CO oxidation

Abstract A silicon-based micro-reactor was fabricated using a micro-electromechanical system (MEMS) process and silicon micromachining technology (SMT) to allow preferential CO oxidation (PrOx) to be applied to compact portable devices. To design the micro-PrOx reactor, the effects of channel length, channel width, channel array, and O2 content in the reactant gas on the CO conversion and selectivity for CO were investigated. The single micro-PrOx reactor required an O2 concentration four times higher than the theoretical quantity for a complete conversion of 1% CO into CO2. The optimum operating temperature to obtain high CO conversion, by suppressing the reverse water–gas shift (r-WGS) reaction, depended considerably on the channel lengths of the micro-reactor rather than on the channel widths or the channel arrays. The micro-PrOx reactor showed 99.4% conversion of CO and 44.14% selectivity for CO at 260 °C when operating with 1% CO, 50% H2, 35% CO2, N2 (balance), and 2% O2.