Temperature CO Oxidation Research Articles

Synthesis of nanocrystalline gold–carbon nanotube composites and evaluation of their sorption and catalytic properties

Gold particles the size of nanometer scale supported on various substrates have gained enormous attention in the surface science and catalysis community. The more interesting application is the low temperature oxidation of CO. In the current work we proceed to the doping of commercial single carbon nanotubes with Au nanoparticles. The samples were characterized by X-ray diffraction and TEM. The characteristic XRD peaks denote the presence of the noble metal nanoparticles as well as a rough approximation of crystallite size and structure. TEM images gave us a clue about the size, the shape and dispersion of these particles on carbon nanotube surface. Sorption experiments of CO, CO 2 and O 2 at different temperatures ranging between 273 and 323 K were conducted to both carbon nanotubes and doped carbon nanotubes to elucidate the special interaction of gases with the noble metal nanoparticles and examine the role of the support in the conversion efficiency of CO. The activity of supported gold for CO oxidation in low temperatures was investigated by catalytic experiments in a specially developed continuous flow reactor, involving a packed bed of doped nanotubes. The reactor was interfaced with a gas chromatograph equipped with thermal conductivity detector and gas sampling valve.

Gold nanoparticles deposited on manganese(III) oxide as novel efficient catalyst for low temperature CO oxidation

Nanostructured α-Mn 2O 3 materials have been employed as supports for anchoring gold nanoparticles, which exhibited high activities for low-temperature CO oxidation in the absence or presence of excess H 2. The specific activity was found to increase with gold loading up to 5 wt%. Comparing the specific rates for CO oxidation (ca. 1.50 mmol CO g −1 Au s −1 at 25 °C) with the highly active Au catalysts in the current literature confirmed the exceptionally high activity of these new materials. The rate of CO oxidation showed about half-order dependence of CO partial pressure and was nearly independent of O 2 concentration. X-ray photoelectron spectroscopy combined with in situ CO adsorption studies revealed the presence of both metallic and positively charged gold species in the catalyst under reaction conditions. As compared with the coprecipitation-derived Au–MnO x catalyst, the superior activities of Au/ α-Mn 2O 3 have been attributed to its unique redox properties and the facile formation of activated oxygen species on the surface.

Synthesis of high performance hydroxyapatite-gold catalysts for CO oxidation

Catalysts for low temperature CO oxidation were prepared by decorating hydroxyapatite (HAp) ceramic foam scaffolds with highly dispersed gold nanocrystals using a deposition-precipitation (DP) process. Catalytic activity, microstructure and crystallinity were studied as a function of reagent pH (4–12) and aging time (10, 30, 60 min) for powders and porous supports. Superior products with small (≤ 5 nm) gold crystals distributed homogeneously over HAp foam were obtained at pH 8–9. Larger crystal sizes and colloidal gold agglomeration appeared at longer aging times. The optimized catalyst prepared by reaction at pH 9 for 30 min showed 100% CO conversion to CO2 at 150°C. The Au-HAp composite demonstrated excellent durability by retaining structural and crystallographic integrity with no loss of activity when tested at 65°C out to 166 h.

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO2, ZrO2, Al2O3, SiO2, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO2 nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn2O3 nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.

Understanding the effect of halide poisoning in CO oxidation over Au/TiO2

The effect of halide poisoning of Au/TiO2 catalysts in low temperature CO oxidation was investigated using bromide as the poison and a combination of X-ray absorption spectroscopy (XANES and EXAFS), quantitative CO adsorption, and catalytic measurements. It was found that halide prevented full reduction of cationic Au by displacing oxyhydroxy ligands and remaining bound to Au during low temperature reduction, causing a reduction in catalytic activity. On reduced Au samples, bromide (likely as NaBr molecule) was preferentially adsorbed on Au and not on TiO2, and suppressed both the adsorption of CO and the catalytic activity. At low Br contents, each adsorbed Br suppressed adsorption of three CO, suggesting that Br was adsorbed on three-fold sites but the effect decreased with increasing Br content possibly due to crowding of adsorbed Br. When 5–10% of the Au was bound to Br, the catalytic activity was completely blocked, although ∼35% of the original CO adsorption capacity remained. The data suggest that not all CO adsorption sites are catalytic active sites, and are consistent with the perimeter Au atoms at/near the particle-support interface (perimeter) being active sites.

Oxygen-assisted reduction of Au species on Au/SiO2 catalyst in room temperature CO oxidation

An unexpected oxygen-assisted reduction of cationic Au species by CO was found on a Au/SiO(2) catalyst at room temperature and the produced metallic Au species plays an essential role in CO oxidation on Au/SiO(2).

Ordered mesoporous Co3O4 as highly active catalyst for low temperature CO-oxidation

Cubic ordered mesoporous Co3O4, prepared via the nanocasting pathway using KIT-6 as hard template, was found to be an excellent catalyst for low temperature CO oxidation, with the activity clearly depending on surface area and pore systems of the catalysts.

Neural network aided design of Pt-Co-Ce/Al2O3 catalyst for selective CO oxidation in hydrogen-rich streams

Abstract In this study, the design of Pt-Co-Ce/Al2O3 catalyst for the low temperature CO oxidation in hydrogen streams was modeled using artificial neural networks. The effects of five design parameters, namely Pt wt.%, Co wt.%, Ce wt.%, calcination temperature and calcination time, on CO conversion were investigated by modeling the experimental data obtained in our laboratory for 30 catalysts. Although 30 points data set can be considered as small for the neural network modeling, the results were quite satisfactory apparently due to the fact that the experimental data generated with response surface method were well balanced over the experimental region and it was very suitable for neural network modeling. The success of neural network modeling was more apparent when the number of data points was increased to 120 by using the time on stream as another input parameter. It was then concluded that the neural network modeling can be very helpful to improve the experimental works in catalyst design and it may be combined with the statistical experimental design techniques so that the successful models can be constructed using relatively small number of data points.

Synthesis, Characterization, Luminescence, and Electrochemistry of the Tetranuclear Gold(I) Amidinate Clusters, Precursors to CO Oxidation Catalysts: Au4[(ArNC(H)NAr)]4, Au4[(PhNC(Ph)NPh)]4 and Au4[PhNC(CH3)NPh)4

AbstractA summary of the chemistry of the tetranuclear Au(I) amidinate complexes is presented. Tetranuclear Au(I) amidinate clusters are produced by the reaction of the sodium salt of a amidine ligand with the gold precursor Au(THT)Cl in a (1:1) stoichiometry. The structures of the tetranuclear Au4[ArNC(H)NAr]4, Ar = C6H4‐4‐OMe, C6H3‐3,5‐Cl, C6H4‐4‐Me, C6H4‐3‐CF3, C6F5, C10H7 and the tetranuclear Au4[(PhNC(Ph)NPh]4 and Au4[PhNC(CH3)NPh]4 have been characterized by X‐ray crystallography. The average Au···Au distance between adjacent Au(I) atoms is ˜3.0 Å, typical of compounds having an aurophilic interaction. The four gold atoms are located at the corner of a rhomboid with the amidinate ligands bridged above and below the near plane of the four Au(I) atoms. The angles at Au···Au···Au in the cyclic units are between 70° and 116°. The tetranuclear gold(I) amidinate clusters each show different luminescence behavior. The tetranuclear clusters Au4[(ArNC(H)NAr]4, Ar = C6H4‐4‐OMe, Ar = C6H4‐3‐CF3, Ar = C6H4‐4‐Me and Ar = C6H4‐3,5‐Cl are the first tetranuclear gold(I) cluster species from group 11 elements that show fluorescence at room temperature. The tetranuclear naphthyl derivative Ar = C10H7 is luminescent only at 77 K. The pentafluorophenyl derivative Ar = C6F5 does not show any photoluminescence in the solid state nor in the solution. The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes. Electrochemical and chemical oxidation studies of the tetranuclear Au(I) amidinate clusters are presented. The tetranuclear complexes Au4[ArNC(H)NAr]4, Ar = C6H4‐4‐OMe, Ar = C6H4‐4‐Me, and Ar = C6H3‐3,5‐Cl, show three reversible waves at 0.75, 0.95, 1.09 V vs. Ag/AgCl at a scan rate of 500 mV/s in 0.1 M Bu4NPF6/CH2Cl2 at a Pt working electrode in CH2Cl2. Three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s are also observed for the tetranuclear complex Au4[PhNC(Ph)NPh]4 in CH2Cl2. The pentafluorophenyl amidinate derivative, Au4[ArNC(H)NAr]4, Ar = C6F5 shows no oxidation wave below 1.8 V. Recently it has been shown that Au4[ArNC(H)NAr]4 is a very effective catalyst precursor for room temperature CO oxidation.

Mesoporous Au/TiO2 Catalysts for Low Temperature CO Oxidation

The activity and stability of structurally well defined mesoporous Au/TiO2 catalysts with different support morphologies and pore sizes for low temperature CO oxidation was investigated by kinetic measurements and in-situ IR spectroscopy. The resulting catalysts with Au particle sizes of ∼3 nm exhibit a high activity for CO oxidation, similar to or exceeding that of highly active standard Au/TiO2 catalysts with similar size Au nanoparticles and loading, and a significantly lower tendency for deactivation. Possible reasons for the improved performance of these catalysts are discussed.

CO oxidation at low temperature over Pd supported on CeO 2-TiO 2 composite oxide

Low temperature CO oxidation was carried out over CeO 2-TiO 2 composite oxide and thereon supported Pd catalysts. The effects of Ce/Ti ratio and pre-treatments of calcination and reduction on the catalytic behaviour were investigated. The CO oxidation starts at about 220 °C over CeO 2-TiO 2 and the pre-reduction treatment has little influence on the catalytic activity. Pd supported on CeO 2-TiO 2 (Pd/CeO 2-TiO 2) exhibits high activity for CO oxidation and a complete conversion of CO to CO 2 can be achieved even at ambient temperature, which suggests a synergistic effect between Pd and CeO 2-TiO 2. The activity and stability of Pd/CeO 2-TiO 2 can be further improved by the pre-reduction treatment. Ce/Ti ratio influences the catalytic behaviour significantly; the catalyst Pd/CeO 2-TiO 2 with a Ce/Ti mole ratio of 0.20 (Pd/Ce20Ti) owns the highest activity and stability, which suggests an optimization of the Pd-Ce-Ti interaction in Pd/Ce20Ti. The calcined Pd/CeO 2-TiO 2 with a Ce/Ti mole ratio higher than 0.10 shows a distorted light-off profile with the temperature, which implies an alternation of the reaction mechanism with increasing temperature.

Low temperature CO oxidation on Au(111) and the role of adsorbed water

This paper presents results of an investigation of low-temperature CO oxidation and the role of moisture on an atomic oxygen covered Au(111) surface by employing molecular beam scattering techniques under ultrahigh vacuum (UHV) conditions. The effect of atomic oxygen precoverage on CO oxidation was examined at sample temperatures as low as 77 K. Prompt CO2 production was observed when the CO beam impinges on the sample followed by a rapid decay of CO2 production in all cases. At oxygen precoverages above 0.5 ML, CO2 production decreases with increasing oxygen precoverage primarily due to the decrease in CO uptake. CO oxidation at 77 K goes through a precursor mediated reaction mechanism, where CO is in a precursor or trapped state and oxygen atoms are in a chemisorbed state. The role of adsorbed water was studied by using isotopically labeled water [H 2 18 O] to distinguish the oxygen species from that used in oxygen atom exposures [16O]. Evidence is presented that shows activated water or OH groups formed from water can directly participate in oxidizing CO on an atomic oxygen covered Au(111) surface.

Electrochemical activation of Pt catalyst by potassium for low temperature CO deep oxidation

This study exposes, for the first time, the effect of Electrochemical Promotion (EPOC) by potassium cations on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization drastically increased the catalytic rate for more than 11 times by decreasing the light-off temperature by almost 40 °C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds on Pt surface that enhanced the adsorption of O 2 at the expense of CO. The achieved results demonstrated that Electrochemical Promotion of Catalysis could be applied to improve the performance of catalytic converters at lower temperatures.

Synthesis, Characterization, Luminescence, and Electrochemistry of New Tetranuclear Gold(I) Amidinate Clusters: Au4[PhNC(Ph)NPh]4, Au4[PhNC(CH3)NPh]4, and Au4[ArNC(H)NAr

The syntheses and the X-ray structures of the tetranuclear gold(I) benzamidinate, Au4[PhNC(Ph)NPh]4, and the tetranuclear gold(I) acetamidinate, Au4[PhNC(CH3)NPh]4, clusters are reported. The clusters are produced by the reaction of the sodium salt of an amidine ligand with the gold precursor Au(THT)Cl in a (1:1) stoichiometry. The average Au...Au distance between adjacent Au(I) atoms is ∼2.9 A, typical of compounds having an aurophilic interaction. The four gold atoms are arranged in a square (Au...Au...Au... = 88–91°) in the acetamidinate and in a distorted square (Au...Au...Au... = 82–97°) in the benzamidinate derivative. Electrochemical oxidation of the tetranuclear complex Au4[PhNC(Ph)NPh]4 show three reversible waves at 0.87, 1.19, 1.42 V vs. Ag/AgCl at a scan rate of 100 mV/s in CH2Cl2 similar to the three reversible waves seen before from the tetranuclear complexes Au4[ArNC(H)NAr]4, Ar = C6H4-4-OMe, Ar = C6H4-4-Me, and Ar = C6H3-3,5-Cl. A summary of the chemistry of the tetranuclear Au(I) amidinate complexes Au4[ArNC(H)NAr]4, Ar = C6H4-4-OMe, C6H3-3,5-Cl, C6H4-4-Me, C6H4-3-CF3, C6F5, C10H7 also is presented. The tetranuclear clusters Au4[ArNC(H)NAr]4, Ar = C6H4-4-OMe, Ar = C6H4-3-CF3, Ar = C6H4-4-Me and Ar = C6H4-3,5-Cl are the first tetranuclear gold(I) cluster species from group 11 elements to show fluorescence at room temperature. The lifetimes of the naphthyl and trifluoromethylphenyl complexes are in the millisecond range indicating phosphorescent processes. Recently it has been shown that Au4[ArNC(H)NAr]4 are very effective catalysts upon calcination for room temperature CO oxidation.

On-line deactivation of Au/TiO 2 for CO oxidation in H 2-rich gas streams

Au-based catalysts, known for ambient temperature CO oxidation, have to provide stable performance of up to 5000 h in order to be commercially applicable in automotive fuel cells. In this report, the on-line deactivation characteristics of Au/TiO 2 in unconventional PROX conditions are discussed. As opposed to CO removal from air, results in this report suggests that carbonates have a minor effect on deactivation of Au/TiO 2 in dry H 2-rich conditions. Also, no conclusive correlation between surface hydration and deactivation was observed. Rather, deactivation appeared to have occurred as a result of an intrinsic transformation in the oxidation state of the active species in the reducing operating conditions; a process which was reversible in an oxidizing atmosphere.

Structural and chemical promoter effects of alkali (earth) and cerium oxides in CO oxidation on supported gold

Nanostructured Au/Al 2O 3 catalysts prepared by deposition–precipitation with urea were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) and tested for low temperature CO oxidation. The paper is focused on the effect of two different kinds of additives on the structure and reactivity of Au/Al 2O 3 for CO oxidation: (1) alkali (earth) metal oxides and (2) ceria. The structural properties of nanosized Au particles are drastically affected by the presence of additives. The main role of the alkali (earth) metal oxides is to stabilize the small Au particles against sintering, such that when BaO is added to Au/Al 2O 3, full CO conversion is already achieved at room temperature. For this reaction ceria addition does not result in a higher activity. However, when both BaO and CeO x are added to Au/Al 2O 3, the catalytic performance of the multicomponent catalyst resembles that of Au/BaO/Al 2O 3.

Low temperature CO oxidation on differently prepared TiO2(110) supported Au catalysts

The preparation of Au/TiO2(110) planar catalysts and the low temperature (LT) oxidation of CO (in H2) were investigated by Scanning Tunnelling Microscopy (STM), Auger-electron Spectroscopy (AES) and Thermal Programmed Reaction (TPR). The distribution and the mean size of the Au particles were modified by different pretreatments of the support: (A) Ar+ bombardment; deposition to (B) 0.05 ML of K or (C) 0.5 ML of Mo. It was shown that the mean size of the Au particles formed at RT is 3-4 nm on the clean TiO2(110) surface and this value deacreased to 1.5-2 nm for all pretreated supports. These 2D catalysts were tested in the LT oxidation (PROX) of CO. The same reaction conditions were kept for all experiments: the reaction chamber was filled up to 0.5 hPa O2, 0.5 hPa CO and 9 hPa H2 and the temperature of the catalyst was increased from 310 K to 400 K, 450 K or 500K for 60 minutes. The clean, Ar+pretreated, K or Mo preexposed oxide surfaces exhibited no activity in the PROX reaction. By the deposition of Au of 1-3 monolayer on to these surfaces, however, an enhancement of CO2 in the gas phase was clearly detected. The clean TiO2(110) surface prooved to be the best support for gold, any modification of the support resulted in a loss of the activity. For all cases dependently on the pretreatments, the mean size of the Au particles increased significantly up to 4-8 nm in the PROX at 500 K.

Effect of interparticle interaction on the low temperature oxidation of CO over size-selected Au nanocatalysts supported on ultrathin TiC films

This work aims to get insight into the influence of interparticle interactions on catalysis. The low temperature CO oxidation is used as a model reaction. A strong dependence of the catalytic activity and stability of gold nanoparticles uniformly dispersed on polycrystalline TiC films was observed as a function of the interparticle distance. Two samples with similar height distributions (∼2 nm), but with different average interparticle distances (∼30 and ∼80 nm), were synthesized using diblock copolymer encapsulation. Their chemical reactivity was investigated by temperature programmed desorption (TPD), and reactive coarsening and subsequent deactivation was observed for the sample with the smallest interparticle distance. The system with the largest average interparticle distance showed higher stability towards agglomeration and longer lifetime.

Discovery of Novel Catalytic Materials for Emissions Control Using High Throughput Scanning Mass Spectrometry

High-throughput approaches were applied to the discovery of more efficient catalysts for various applications in emissions control. The screening approach was based on a hierarchy of qualitative or semi-quantitative primary screens for discovery of hits and quantitative secondary screens for confirmation and scale-up of leads. In this work, primary screening was carried out by fast scanning mass spectrometry (SMS) for NO(x) abatement, low temperature CO oxidation, VOC removal, CO(x) methanation and the water gas shift (WGS) reaction.

High activity supported gold catalysts by incipient wetness impregnation

It is generally thought that catalysts produced by incipient wetness impregnation (IW) are very poor for low temperature CO oxidation, and that it is necessary to use methods such as deposition–precipitation (DP) to make high activity materials. The former is true, indeed such IW catalysts are poor, and we present reactor data, XPS and TEM analysis which show that this is due to the very negative effect of the chloride anion involved in the preparation, which results in poisoning and excessive sintering of the Au particles. With the DP method, the chloride is largely removed during the preparation and so poisoning and sintering are avoided. However, we show here that, contrary to previous considerations, high activity catalysts can indeed be prepared by the incipient wetness method, if care is taken to remove the chloride ion during the process. This is achieved by using the double impregnation method (DIM). In this a double impregnation of chloroauric acid and a base are made to precipitate out gold hydroxide within the pores of the catalyst, followed by limited washing. This results in a much more active catalyst, which is active for CO oxidation at ambient temperature. The results for DIM and DP are compared, and it is proposed that the DIM method may represent an environmentally and economically more favorable route to high activity gold catalyst production.