Preferential CO oxidation over activated carbon supported catalysts in H2-rich gas streams containing CO2 and H2O

Selective CO oxidation (PROX) was studied at 423 K over 1% Pt–0.25% SnO_x and 1% Pt–1% CeO_x catalysts supported on un-oxidized and oxidized activated carbon (AC) using feed mixtures simulating the reformate coming from fuel processors. Effects of the addition of 15% CO₂ or (15% CO₂ + 10% H₂O) into feed mixtures containing 1% CO, 1% O₂, 60% H₂ and He were determined for nine different AC-supported catalysts, and the results were compared with those obtained with pure H₂-rich feed. Unlike other PROX catalysts having oxide supports, introduction of CO₂ into pure feed drastically increased CO conversion on all nine catalysts supported on oxidized or un-oxidized AC regardless of impregnation strategy.1% Pt–0.25% SnO_x supported on HNO₃-oxidized AC stands out as a potential candidate for commercial use in PROX since it yields 100% CO conversion under realistic feed conditions. 1% Pt–1% CeO_x catalysts prepared by sequential or co-impregnation and supported on air-oxidized AC also give 100% CO conversion in H₂-rich feed containing (CO₂ + H₂O) during extended run times and hence hold promise as PROX catalysts.     

Au/Ti-SBA-15 catalysts in CO and preferential (PROX) CO oxidation

The Au/Ti-SBA-15 catalysts were found promising for CO oxidation, including preferential CO oxidation in the presence of H₂ (PROX). The catalytic performance and Au particle size depending on the Ti content: 100% selectivity to CO₂ in PROX at 90% CO conversion was found for the catalysts of the Ti content below 1.32 wt.% Ti.     

Mesoporous CuO/TixZr1 − xO2 catalysts for low-temperature CO oxidation

Mesoporous CuO/Ti_xZr_1 − xO₂ catalysts were prepared by a surfactant-assisted method, and characterized by N₂ adsorption/desorption, TEM, XPS, in-situ FTIR and H₂-TPR. The catalysts exhibited high specific surface area (S_BET = 241 m²/g) and uniform pore size distribution. XPS and in-situ FTIR displayed that Cu⁺ and Cu²⁺ species coexisted in the catalysts. The CuO/Ti_xZr_1 − xO₂ catalysts presented obviously higher activity in CO oxidation reaction than the CuO/TiO₂ and CuO/ZrO₂ catalysts. Effect of molar ratios of Ti to Zr and calcination temperature on catalytic activity was investigated. The CuO/Ti_0.6Zr_0.4O₂ catalyst calcined at 400 °C exhibited excellent activity with 100% CO conversion at 140 °C.     

Hierarchically nanoporous Co–Mn–O/FeOx as a high performance catalyst for CO preferential oxidation in H2-rich stream

A novel and highly-efficient hierarchically nanoporous Co–Mn–O/FeOx catalyst fabricated by a hard/soft dual-templating and subsequent deposition–precipitation (HSDT/DP) approach demonstrates unexpectedly high catalytic activity with 100% CO conversion at 75 °C and wide temperature window of 75–200 °C with complete CO removal for CO preferential oxidation (CO PROX), ascribed to the unique microstructure and strong interaction between finely dispersed cobalt–manganese and FeO_x. The excellent catalytic performance allows it to be a practical candidate for CO elimination from H₂-rich stream.     

Activity and stability of Pd/MMnOx (M = Co,Ni, Fe and Cu) supported on cordierite as CO oxidation catalysts

Several catalysts of the general formula, MMnO_x (M = Co, Ni, Fe and Cu), were synthesised through the impregnation method; their activities were shown to be enhanced by the addition of a small amount of Pd (0.01–0.1 wt%). These catalysts exhibit different activities for the catalytic oxidation of CO, due to the different valence states of various transition metal oxides. The introduction of Pd prominently enhanced both the reduction and oxidation capabilities of the catalysts. These catalysts were optimised for oxidation activities by designing orthogonal experiments. Based upon the catalysts’ properties, the stability of these samples and their ability to resist steam over Pd/CoMnO_x/cordierite were investigated.     

Influences of preparation methods of ZrO2 support and treatment conditions of Cu/ZrO2 catalysts on synthesis of methanol via CO hydrogenation

ZrO₂ supports were prepared by different methods (conventional precipitation method, shortened as “CP”, and alcogel/thermal treated with nitrogen method, shortened as “AN”), and Cu/ZrO₂ catalysts were prepared by impregnation method. The supports and catalysts were characterized by BET, XRD, TEM and TPR. The effects of the preparation methods of ZrO₂ supports and the treatment conditions (calcination and reduction temperatures) of the catalyst precursors on the texture structures of the supports and catalysts as well as on the catalytic performances of Cu/ZrO₂ in CO hydrogenation were investigated. The results showed that the support ZrO₂-AN had larger BET specific surface area, cumulative pore volume and average pore size than the support ZrO₂-CP. Cu/ZrO₂-AN catalysts showed higher CO hydrogenation activity and selectivity of oxygenates (C₁–C₄ alcohols and dimethyl ether) than Cu/ZrO₂-CP catalysts. Calcination and reduction temperatures of supports and catalyst precursors affected the catalytic performance of Cu/ZrO₂. The conversion of CO and the STY of oxygenates were 12.7% and 229 g/kg h, respectively, over Cu/ZrO₂-AN-550 at the conditions of 300 °C, 6 MPa.     

On the structure dependence of CO oxidation over CeO2 nanocrystals with well-defined surface planes

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({1 1 0} + {1 0 0}), cubes ({1 0 0}), and octahedra ({1 1 1}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water–gas shift reaction, and CO disproportionation reaction, reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods > cubes > octahedra. The mobility of lattice oxygen also shows similar dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO₂, while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.     

Hydroxyls-induced oxygen activation on “inert” Au nanoparticles for low-temperature CO oxidation

The NaOH additive substantially enhances the catalytic activity of Au/SiO₂ catalyst inert in catalyzing CO oxidation at temperatures below 150 °C, and Au/NaOH/SiO₂ catalyst with a NaOH:Au atomic ratio of 6 is active at room temperature. Both the particle size distribution and the electronic structure of Au nanoparticles were found to be similar in Au/SiO₂ and Au/NaOH/SiO₂ catalysts, unambiguously proving that hydroxyls on “inert” Au nanoparticles can induce the activation of O₂ for CO oxidation at room temperature. The accompanying density functional theory (DFT) calculation results reveal the determining role of COOH(a) in hydroxyls-induced activation of O₂ on the Au(1 1 1) surface. Our results successfully elucidate the influence of hydroxyls on the intrinsic activity of Au nanoparticles in CO oxidation, providing novel insights into the role of hydroxyls in the catalytic activity of Au catalysts and advancing the fundamental understanding of oxidation reactions catalyzed by Au catalysts.     

An excellent support of Pd–Fe–Ox catalyst for low temperature CO oxidation: CeO2 with rich (200) facets

Pd–Fe–O_x catalysts for low temperature CO oxidation were supported on SBA-15, CeO₂ nano-particles with rich (111) facets and CeO₂ nano-rod with rich (200) facets, and characterized by X-ray diffraction, low-temperature nitrogen adsorption, transmission electron microscopy and temperature-programmed reduction. The results showed that when CeO₂ nano-rod was used as a support, Pd–Fe–O_x catalyst exhibits higher activity (T₁₀₀ = 10 °C), resulting from the rich (200) facets of CeO₂ nano-rod, which leads to a formation of large numbers of the oxygen vacancies on the surface of Pd–Fe–O_x catalysts.     

Steam reforming of methanol over copper–manganese spinel oxide catalysts

The steam reforming of methanol was studied over a series of copper–manganese spinel oxide catalysts prepared with the urea–nitrate combustion method. All catalysts showed high activity towards H₂ production with high selectivity. Synthesis parameters affected catalyst properties and, among the catalysts tested, the one prepared with 75% excess of urea and an atomic ratio Cu/(Cu + Mn) = 0.30 showed the highest activity. The results show that formation of the spinel Cu_xMn_3 − xO₄ phase in the oxidized catalysts is responsible for the high activity. Cu–Mn catalysts were found to be superior to CuO–CeO₂ catalysts prepared with the same technique.     

Promoting effect of the addition of Ce and Fe on manganese oxide catalyst for 1,2-dichlorobenzene catalytic combustion

Mn-based mixed-oxide (MnO_x) catalysts were modified with Fe, Ce, and Ce + Fe, and its catalytic oxidation activity was tested by using 1,2-dichlorobenzene (o-DCB) as models of chlorinated volatile organic compounds. Addition of Ce or Ce + Fe into MnO_x promoted their crystals to turn into amorphous powder, enhanced their specific surface area and changed their redox property. The catalytic activity of MnO_x improved remarkably by adding Ce or Ce + Fe indicating Ce plays an important role. Both Mn-Ce and Mn-Ce-Fe catalysts exhibited good stability for catalytic oxidation of o-DCB, indicating that the introduction of promoter is an important method to improve the catalytic performance.     

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/Al₂O₃ 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 H₂O/CH₃OH 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% H₂ and 25% CO₂, and more than 93% of methanol conversion was achieved at 300 °C and 2 cm³/h of the feed flow rate when CO concentration was maintained below 100 ppm by PROX.     

A novel catalyst for diesel soot oxidation

In this study, cobalt and lead based mixed oxide catalysts were tested for their soot oxidation ability. In addition to a mixed oxide formerly marketed as ceramic paint, a home made set was also prepared by incipient wetness impregnation of a cobalt oxide powder with a lead acetate solution and subsequent calcination. The materials investigated in this study were shown to decrease the peak combustion temperature of home made soot from 500 to 385 °C in air. Soot oxidation tests under inert (N₂) atmospheres revealed that the oxidation took place by using the lattice oxygen of the catalyst. Reaction temperature could be further decreased when these mixed oxide catalysts were impregnated with platinum. An optimum platinum loading was determined as 0.5 wt% based on the peak combustion temperature of the soot. The role of Pt was to assist the oxygen transfer from the gas phase to the lattice. It was observed that NO₂ is a better oxidizing agent as compared to air whereas NO had hardly any activity against soot oxidation reaction. When the mixed oxide catalyst was impregnated with platinum, the peak combustion temperature was measured as 310 °C in the presence of NO_x and air. The catalyst's unique performance was in terms of the rate of soot oxidation. Under the experimental conditions studied here, the soot oxidation was so facile that the oxygen in the gas phase was completely depleted. This stream of oxygen depleted and CO enriched gas phase can be used to reduce NO_x in the presence of a downstream or a co-catalyst.     

Oxy-methane reforming over high temperature stable NiCoMgCeOx and NiCoMgOx supported on zirconia–haffnia catalysts: Accelerated sulfur deactivation and regeneration

NiCoMgO_x and NiCoMgCeO_x on commercial low surface area zirconia–haffnia catalysts have unusually high thermal stability (⩾2000 °C) for syngas generation via the methane partial oxidation process (J. Catal., 233, 36, 2005). Herein we report the results on accelerated sulfur deactivation (0.74 mol% sulfur in feed) and corresponding regeneration (at 800 °C in 1:1 O₂ + N₂ flow) over these catalysts. The NiCoMgCeO_x catalyst, due to a larger mobility of lattice oxygen, showed a considerably higher resistance to sulfur poisoning; the higher mobility of the lattice oxygen in case of the NiCoMgCeO_x catalyst may be related to the presence of CeO₂. During the deactivation process, the selectivity for H₂ was decreased to a much greater extent than that for CO. Regeneration studies showed that even after complete deactivation of the catalysts, the original activity/selectivity of both the catalysts could be completely restored after a simple regeneration process. Based on their exceptionally high thermal stability, high activity/selectivity and easily regenerability, the NiCoMgO_x and NiCoMgCeO_x catalysts appear to be very promising candidates for the CPOM process.     

Gold-base catalysts supported on carbonate for low-temperature CO oxidation

Au/MeCO₃ (Me = Ca, Sr, Ba) catalysts prepared by co-precipitation method were studied for low-temperature CO oxidation in the presence/absence of water in the feed stream. The type of support and the calcination temperatures have considerable effect on the activity of the catalysts. Among them, Au/BaCO₃ calcined at 473 K shows the highest activity, full conversion of CO can be obtained at ambient reaction temperature in the presence of moisture. The addition of water vapor in feed stream has positive influence on the activity of Au/MeCO₃ catalysts for CO oxidation, which might be due to water can participate CO oxidation reaction directly.     

Low-temperature continuous wet oxidation of trichloroethylene over CoOx/TiO2 catalysts

TiO₂-supported metal oxides such as CoO_x, CuO_x, NiO_x and FeO_x have been used for catalytic wet oxidation of trichloroethylene (TCE) in a continuous flow type fixed-bed reactor system, and the most promising catalyst for this wet catalysis has been characterized using XPS and XRD techniques. All the supported catalysts gave relatively low conversions for the wet oxidation at 36 °C, except for 5 wt% CoO_x/TiO₂ which exhibited a steady-state conversion of 45% via a transient activity behavior up to 1 h on stream. XPS measurements yielded that a Co 2p_3/2 main peak at 779.8 eV appeared with the 5 wt% CoO_x/TiO₂ catalyst after the continuous wet TCE oxidation at 36 °C for ca. 6 h (spent catalyst) and this binding energy value was equal to that of Co₃O₄ among reference Co compounds used here, while the catalyst calcined at 570 °C (fresh catalyst) possessed a main peak at 781.3 eV, very similar to that for CoTiO_x species such as CoTiO₃ and Co₂TiO₄. Only characteristic reflections for Co₃O₄ were indicated upon XRD measurements even with the fresh catalyst sample. The simplest model, based on these XPS and XRD results, for nanosized Co₃O₄ particles existing with the fresh catalyst could reasonably explain the transient activity behavior observed upon the wet TCE oxidation.     

Role of ceria in the improvement of SO2 resistance of LaxCe1 − xFeO3 catalysts for catalytic reduction of NO with CO

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_1 − xFeO₃ compositions were prepared by sol–gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_1 − xFeO3 composite oxides.     

Simultaneous catalytic removal of NOx and soot particulate over Co–Al mixed oxide catalysts derived from hydrotalcites

Co–Al mixed oxides (CAO) was prepared by co-precipitation method from hydrotalcites (HT) as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NO_x and diesel soot particulates by the temperature-programmed reaction (TPR) technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500 °C and 800 °C, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800 °C were higher than that at 500 °C. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.     

Effects of WO3 doping on stability and N2O escape of MnOx–CeO2 mixed oxides as a low-temperature SCR catalyst

MnO_x–WO_x–CeO₂ catalysts synthesized using a sol–gel method were investigated for the low-temperature NH₃-SCR reaction. Among them, W_0.1Mn_0.4Ce_0.5 mixed oxides exhibited above 80% NO_x conversion from 140 to 300 °C. In addition, this catalyst exhibited high stability and CO₂ tolerance in a 50 h activity test at 150 °C. Substantially reduced N₂O production and enhanced N₂ selectivity were achieved by WO₃ doping, which was due to the weakened reducibility and increased number of acid sites. The decreased SO₂ oxidation activity as well as the reduced formation of ammonium and manganese sulfates resulted in a high SO₂ resistance of this catalyst.     

Impact of metal doping on the activity of Au/CeO2 catalysts for catalytic abatement of VOCs and CO in waste gases

The catalytic performance in the total oxidation of CO and methanol over gold catalysts supported on ceria doped by different metal oxides (Me = Fe, Mn and Co) was studied and a strong influence of the nature of dopant was observed. The activity towards the oxidation of CO and CH₃OH was in the order: AuCeCo > AuCe > AuCeFe > AuCeMn. The characterization by XRD and HRTEM evidenced differences in the average size and the distribution of gold particles. AuCeCo catalyst exhibited superior low-temperature CO oxidation activity (100% conversion degree was obtained at 25 °C) and almost 100% total oxidation of CH₃OH at about 40 °C. Higher hydrogen consumption was estimated by means of TPR over this catalyst. The effect of modification with Co₃O₄ of Au/CeO₂ catalysts on their CO oxidation activity was further studied by varying of the dopant content (5, 10 and 15 wt.% Co₃O₄).