Temperature CO Oxidation Research Articles

High Throughput Screening of Low Temperature CO Oxidation Catalysts Using IR Thermography

The catalytic oxidation of carbon monoxide to carbon dioxide is an important process used in several areas such as respiratory protection, industrial air purification, automotive emissions control, CO clean-up of flue gases and fuel cells. Research in this area has mainly focused on the improvement of catalytic activity at low temperatures. Numerous catalyst systems have been proposed, including those based on Pt, Pd, Rh, Ru, Au, Ag, and Cu, supported on refractory or reducible carriers or dispersed in perovskites. Well known commercial catalyst formulations for room temperature CO oxidation are based on CuMn2O4 (hopcalite) and CuCoAgMnOx mixed oxides. We have applied high-throughput and combinatorial methodologies to the discovery of more efficient catalysts for low temperature CO oxidation. The screening approach was based on a hierarchy of qualitative and semi-quantitative primary screens for the discovery of hits, and quantitative secondary screens for hit confirmation, lead optimization and scale-up. Parallel IR thermography was the primary screen, allowing one wafer-formatted library of 256 catalysts to be screened in approximately 1 hour. Multi-channel fixed bed reactors equipped with imaging reflection FTIR spectroscopy or GC were used for secondary screening. Novel RuCoCe compositions were discovered and optimized for CO oxidation and the effect of doping was investigated for supported and bulk mixed oxide catalysts. Another family of active hits that compare favorably with the Pt/Al2O3 benchmark is based on RuSn, where Sn can be used as a dopant (e.g. RuSn/SiO2) and/or as a high surface area carrier (e.g., SnO2 or Sn containing mixed metal oxides). Also, RuCu binary compositions were found to be active after a reduction pretreatment with hydrogen.

Mesoporous Silica and Titania by Glycol-Modified Precursors

ABSTRACTEthylene glycol modified precursors, such as tetrakis(2-hydroxyethyl)orthosilicate (EGMS) or bis(2-hydroxyethyl)titanate (EGMT), have distinct advantages in the synthesis of mesoporous materials by sol-gel processing compared to the commercially available tetraalkoxide precursors. The glycols released upon hydrolysis have proven to be compatible with lyotropic surfactant mesophases and in addition, these precursors allow for processing in purely aqueous conditions. Besides the standard characterization of the resulting titania and silica-based materials by XRD, electron microscopy, and nitrogen sorption, the potential of the titania-based materials for catalytic applications was tested using Au/TiO2 catalysts in low temperature CO oxidation reactions.

Low Temperature CO Oxidation over Unsupported Nanoporous Gold

Supported Au nanoclusters are well-known for their unusual properties in catalysis. We describe here that nanostructured porous Au made via dealloying represents a new class of unsupported catalysts with extraordinary activities in important reactions such as CO oxidation. Although nanoporous Au may contain some oxides on the surface, our results demonstrate that it is metallic Au that plays the main role in this catalytic reaction. Furthermore, this material has good low-temperature catalytic stability and is extremely CO tolerant.

Constructing surface active centres using Pd–Fe–O on zeolite for CO oxidation

Newly active centres for CO oxidation using Pd–M–O (M=Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, and Zr) on zeolite have been constructed. The derived co-impregnated Pd–Fe–Ox/NaZSM-5 catalyst not only showed high catalytic activity for low temperature CO oxidation, but was also a good kind of electrocatalyst for CO electrochemical stripping. It was probably because that the active centres including special oxygen species exhibit the high activity for CO oxidation. The typical 50% and 100% conversion temperatures of the co-impregnated Pd–Fe–Ox/NaZSM-5 catalyst for low temperature CO oxidation (T50 and T100) were 29.1 and 47°C, respectively. XRD and XPS characterizations indicated that the addition of Fe increased the dispersion of the active Pd species and Pd species were easy to enrich on the surface of the catalyst. A functional interface, bimetallic catalytic active centre, was formed on the surface of the catalyst, which contained a certain quantity of special oxygen species. The direct proofs of the existence of this kind of active oxygen species were given by our H2-TPR and O2-TPD experiments.

The preparation of Au/CeO2 catalysts and their activities for low-temperature CO oxidation

Au/CeO2 catalysts prepared by co-precipitation (CP) and deposition-precipitation (DP) methods were tested for low temperature CO oxidation reaction. The structural characters and redox features of the catalysts were investigated by XRD, XPS and H2-TPR. Their catalytic performances for low temperature CO oxidation were studied by means of a microreactor -GC system. It showed that the catalytic activities of Au/CeO2 catalysts greatly depended on the preparation method. The catalysts prepared by DP method exhibited a surprisingly higher activity towards CO oxidation than that prepared by CP method. This may arise from the differences in the particle sizes of Au and redox properties of the catalysts. The low Au loading and the resistance to high temperature of DP-prepared catalyst made it more applicable.

Preparation and characterization of nanostructured Ce0.9Cu0.1O2− solid solution with high surface area and its application for low temperature CO oxidation

Abstract Nanostructured Ce 0.9 Cu 0.1 O 2− δ solid solution with high surface area was prepared by improved citrate sol–gel method with incorporation of thermal treatment under N 2 . The sample was characterized by TG–DSC, BET nitrogen adsorption, XRD and H 2 -TPR. Its catalytic activity for CO oxidation was tested. It was found that the improved method offered catalysts with higher surface area and smaller crystallite size, which led to higher catalytic activity for low temperature CO oxidation. H 2 -TPR measurement indicated that there were three CuO species in the Ce 0.9 Cu 0.1 O 2− δ solid solutions: finely dispersed CuO species on the surface of CeO 2 , partial Cu 2+ penetrated into CeO 2 lattice and bulk CuO phase. The finely dispersed CuO species was regarded as the active site for the low temperature CO oxidation.

CO oxidation catalysts based on copper and manganese or cobalt oxides supported on MgF 2 and Al 2O 3

The catalytic activity of a mixed phase of copper–cobalt and copper–manganese oxides supported on magnesium fluorine or alumina has been studied in low temperature CO oxidation at 30 °C. During calcination, the oxides studied partially react to form different type spinels depending on the calcination temperature. These spinels have different effect on the catalytic activity. In low temperature CO oxidation the copper–manganese catalysts are more active than the copper–cobalt ones.

Room temperature visible light oxidation of CO by high surface area rutile TiO2-supported metal photocatalyst

Visible light-active rutile TiO2 with a high surface area of 200m2/g was obtained by a low-temperature sol–gel synthesis, based on a long aging duration of a titania sol to stabilize the rutile phase. Decorated by an adequate amount of metallic nanoparticles, this non-doped TiO2 displays high and stable performances for the on-stream room temperature oxidation of CO by visible light photocatalysis.

The Role of Impurities on the Low Temperature CO Oxidation on Au/TiO2

A number of Au/TiO2 catalysts have been prepared by a variety of methods in order to assess the affect of catalysts preparation methods on performance, catalyst contamination and the interplay between them. Their activity was studied in a pulse flow microreactor and it was found that preparation methods designed to eliminate impurities lead to more active samples. The effect of chlorine is often cited to be detrimental in the literature, but we have quantified it using XPS. It was found that the activity decreased in a nearly linear fashion with an increasing amount of this element at the surface. It is generally considered that catalysts prepared by the incipient wetness (IW) technique are ineffective for CO oxidation, but we show here that, by appropriate preparation methods, high activity IW catalysts can be made.

Dispersion, reduction and catalytic properties of copper oxide supported on Ce 0.5Zr 0.5O 2 solid solution

Ce 0.5Zr 0.5O 2 solid solution was synthesized by a microwave method and used as support for preparing a series of CuO/Ce 0.5Zr 0.5O 2 catalysts. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and temperature-programmed reduction (TPR), and the results indicated that: (1) the dispersion capacity of copper oxide on Ce 0.5Zr 0.5O 2 is about 0.70 mmolCuO/100 m 2Ce 0.5Zr 0.5O 2; (2) there are two kinds of copper species in the catalysts, i.e., dispersed and crystalline copper species. The catalytic activities of CuO/Ce 0.5Zr 0.5O 2 for the low temperature (100 and 125 °C) CO oxidation were also tested, which showed that the sample with CuO loading amount of 1.2 mmolCuO/100 m 2Ce 0.5Zr 0.5O 2 presented the highest activity. Combined with the XRD and TPR results, it seems to suggest that the main active species in this system should be the surface and/or small particle copper oxide species.

Vapor phase synthesis and characterization of bimetallic alloy and supported nanoparticle catalysts

The laser vaporization controlled condensation (LVCC) technique coupled with a differential mobility analyzer (DMA) is used to synthesize size-selected alloy nanoparticles and nanoparticle catalyst systems. The formation of Au–Ag alloy nanoparticles is concluded from the observation of only one plasmon band. The maximum of the plasmon absorption is found to vary linearly with the gold mole fraction. For the Au–Pd system, the XRD data confirms the formation of the alloy nanoparticles with no evidence of any of the pure components. The Au/CeO2 nanoparticle catalyst prepared by the LVCC method is a promising catalyst for low temperature CO oxidation due to its high activity and stability.

Au deposits on graphite: On the nature of high temperature desorption peaks in CO thermal desorption spectra

Due to the discovery of Au as a catalyst for low temperature CO oxidation, the adsorption of CO on Au surfaces has attracted a lot of attention recently. On stepped and rough single crystal surfaces as well as on deposited particles two characteristic desorption states above 100 K have been observed via TPD. We have studied Au deposits on graphite in order to elucidate the nature of these desorption peaks in more detail. For this purpose, Au was deposited at 100 K and 300 K on HOPG as a weakly interacting support. In analogy to other supports, we obtain two desorption states (∼140 K and ∼170 K) whose relative intensities depend strongly on the deposition temperature with the high temperature peak being much more pronounced for the 100 K deposits. After annealing to 600 K, both states drastically lose intensity. XP spectra, on the other hand, show virtually no decrease of the Au 4f intensity as would be expected for desorption or significant changes of the particle morphologies. We conclude that both desorption peaks are defect-related and connected with under-coordinated Au atoms that are lost for the most part upon annealing. These sites could be located at the perimeter of dendritic islands or on small, defect-rich particles in addition to larger particles not adsorbing CO at 100 K. Preliminary STM results are in favour of the second interpretation.

XPS study of supported gold catalysts: the role of Au0 and Au+δ species as active sites

AbstractGold nanoparticles supported on different oxides (SiO2, CeO2 and TiO2) were prepared by the SMAD (solvated metal atom dispersion) and deposition–precipitation (DP) techniques. The physical and chemical characterization of the catalysts was performed by X‐ray photoelectron spectroscopy (XPS) and X‐ray diffraction (XRD) and the catalytic activity was tested during the reaction of low temperature CO oxidation. The structural and surface analyses evidenced the presence of small gold crystallites (cluster size ∼2–5 nm) in all the SMAD‐prepared samples and oxidized gold species in the case of the DP catalysts. A different surface distribution of ionic gold species was found on the different supports. By comparing the catalytic activities of the samples, the presence of Au+1 species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. The higher activity of Au/CeO2(DP) catalysts at T ≈ 250 K can be ascribed to a better stabilization of the AuO− species by the cerium oxide. Nanosized metallic gold particles exhibit a worse catalytic performance, both on ‘reducible’ and ‘inert’ supports, being significantly active only in the temperature range: 400–600 K. Copyright © 2006 John Wiley & Sons, Ltd.

Metal-support and preparation influence on the structural and electronic properties of gold catalysts

Nanostructured gold catalysts supported on CeO 2 and SiO 2 were prepared by the deposition–precipitation (DP) and the solvated metal atom dispersion (SMAD) techniques. The structural and electronic properties of the catalysts were investigated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). Gold was found as small metal nanoparticles (cluster size ∼2 nm) in the SMAD-prepared samples and in ionic state in the DP catalysts. The catalytic activity of the samples was tested in the reaction of low temperature CO oxidation. Gold nanosized particles in a pure metallic state exhibited a worse catalytic performance, both on ceria and silica. The presence of non-metallic Au species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. The higher activity of the Au/CeO 2 (DP) sample with respect to the Au/SiO 2 (DP) catalyst can be ascribed to a better stabilization of the Au +1 ions, probably as AuO - species, by the cerium oxide.

Copper-zinc oxide and ceria promoted copper-zinc oxide as highly active catalysts for low temperature oxidation of carbon monoxide

Copper-zinc oxide catalyst is prepared by co-precipitation technique. The effects of aging the precipitate in the parent solution and addition of CeO2 are investigated. CuO-ZnO catalyst containing 60% CuO and 40% ZnO is found to be active for the ambient temperature oxidation of CO; however the activity can be improved considerably by aging the catalyst during the precipitation process. Alternatively, the catalyst activity can be improved by the addition of CeO2 during the precipitation without aging the precipitate. A much higher CO oxidation activity is achieved for the CuO-ZnO catalyst system than that reported earlier for the same system. X-ray photoelectron spectra (XPS) and XRD results show that aging the catalyst during the precipitation process brings out more ZnO to the surface leading to higher CuO dispersion. This leads to the formation of relatively less crystalline or amorphous CuO that promotes the formation of more linear or weakly bonded CO on the catalyst surface as well as the creation of CuOx species under redox conditions. The metastable copper oxide species is a good electrophilic reagent with superior oxygen transfer capacity and the presence of weakly bonded surface CO ensures a low temperature oxidation activity for the catalyst. It is further seen that addition of ceria to the catalyst also brings out similar advantages. However, increase in the CeO2 content increases the CO bonding strength causing an increase in the light-off temperature (loss of low temperature oxidation activity), in spite of high overall reducibility and CO adsorption capacity of the catalyst. The reducibility and susceptibility to variation in the oxidation states of copper oxide are critical to the activity of the catalyst. It is also found that addition of CeO2 brings out a significant improvement in the catalyst stability over an extended reaction period as well as at elevated reaction temperature.

Preparation and Characterization of Cu/Pt/BEA Catalyst for Low Temperature CO Oxidation

The low temperature CO oxidation over Pt-based bimetallic catalysts was investigated. Various additives such as Ni, Cu, Pd, Ag and Au were added to the Pt/BEA catalyst by an ion-exchange method and the addition of Cu to the calcined Pt/BEA catalyst was found to remarkably improve the catalytic performance. A calcination step for the Pt/BEA before Cu ion-exchange was especially important in forming the active phases for CO oxidation. XRD results indicated that the metallic Pt was not considered to be related to the active site for CO oxidation. Moreover, interactions between Pt and Cu on the Cu/Pt*/BEA catalyst could be confirmed by CO-TPD. Pt was found to be an active component for CO oxidation, but is easily deactivated by the adsorption of CO molecules. It is thought that Cu, interacting with Pt, plays an important role in preventing the deactivation of Pt.

Low Temperature Oxidation of CO over Cluster-Derived Platinum−Gold Catalysts

The structural and catalytic properties of SiO2- and TiO2 -supported Pt-Au bimetallic catalysts prepared by coimpregnation were compared with those of samples of similar composition synthesized from a Pt2Au4(C{triple bond}CBut)8 cluster precursor. The smallest metal particles were formed when the bimetallic cluster was used as a precursor and TiO2 as the support. FTIR data indicate that highly dispersed Au crystallites in these samples, presumably located in close proximity to Pt, are capable of linearly coordinating CO molecules with a characteristic vibration observed at 2111 cm(-1). The cluster-derived Pt2Au4/TiO2 samples were the only ones exhibiting low-temperature CO oxidation activity, indicating that both the high dispersion of Au and the nature of the support are important factors affecting the catalytic activity for this system.

Highly active gold-ceria catalyst for the room temperature oxidation of carbon monoxide

Ceria supported gold catalysts containing 0.1–5 wt.% gold are prepared by deposition–precipitation technique in which the precipitate is subjected to ultrasound treatment and aging in the parent solution at 65 °C for 2 h prior to filtration and drying. The catalysts are found to be very active for the room temperature oxidation of CO. Au/CeO 2 catalyst containing as low as 1 wt.% Au is found to be a highly active catalyst for the room temperature oxidation of CO to CO 2 with complete conversion of CO and specific activity up to 490 × 10 −4 mol CO s −1 g Au −1. The catalyst shows good stability and sustains its high activity for a prolonged period. Aging of the catalyst in the parent solution is found to be important in obtaining a highly active catalyst. The catalysts are characterized by XRD, SEM, TEM, XPS and TPR/TPD. TPR, TPD and XPS studies suggest that the active site for CO oxidation may be a combination of Au + OH − and metallic gold in interface with reduced Ce 3+ sites. In other words, the presence of highly dispersed gold particles in close contact with the ceria surface defects is vital for the superior performance of the catalyst.

Highly reproducible syntheses of active Au/TiO 2 catalysts for CO oxidation by deposition–precipitation or impregnation

Gold catalysts supported on TiO 2 were prepared by a deposition–precipitation (DP) method to investigate how highly reproducible performance of the gold catalysts in CO oxidation can be achieved. A protocol was established for synthesizing identically performing catalysts by different operators. The results show that for this synthesis route, the calcination step is not needed to form highly active Au/TiO 2 catalysts, but leads to decreased activity. Improved catalytic activity was observed when a high solution pH was adjusted during the precipitation. Surprisingly, wet impregnation followed by ammonia steam treatment and a washing step with water also leads to Au/TiO 2 with 2- to 4-nm individual gold particles highly dispersed on the TiO 2 surface. In addition, this catalyst is active for room temperature CO oxidation. The temperature for 50% conversion of CO is below 25 °C, which is comparable to that of the gold catalyst prepared by the DP method. Therefore, contrary to reports in the literature, the impregnation method can be used in the preparation of high-activity gold catalysts.

Poisoning effect of SO 2 on the catalytic activity of Au/TiO 2 investigated with XPS and in situ FT-IR

The suppression of the CO oxidation activity by SO 2 treatment in Au/TiO 2 catalyst was investigated by means of reaction experiments, in situ IR and XPS. The CO oxidation activity over Au/TiO 2 catalyst decreased after SO 2 treatment. The oxidation state of Au particles did not change after SO 2 treatment. The XP spectra in the S 2p region showed that sulfate was formed on TiO 2 supports after SO 2 treatment. The IR experiment revealed that SO 2 treatment increased the adsorption strength between Au and CO. Carbonate peak was absent for the SO 2-pretreated sample in the presence of CO. The migration of CO was completely blocked by surface sulfate compound. Strong and irreversible formation of sulfate at the perimeter is the main reason of the loss of CO oxidation activity at low temperature CO oxidation.