Investigation of CO and formaldehyde oxidation over mesoporous Ag/Co3O4 catalysts

CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co₃O₄ catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co₃O₄ catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co₃O₄ and Ag/Co₃O₄ catalysts for the both reactions was different. And the addition of silver on mesoporous Co₃O₄ did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co₃O₄ should be indispensable for the high activity of HCHO oxidation at low temperature.     

CO oxidation catalyzed by Ag/SBA-15 catalysts: Influence of the hydrothermal treatment

Four types of SBA-15 were prepared with different times and temperatures of treatment in order to obtain a range of micropore sizes. CO oxidation was used as a probe reaction in order to evaluate the nature of the active species when SBA-15s were doped with ca 10% Ag deposited from an AgNO₃ solution and calcined or reduced at 350 °C. The texture (TEM, nitrogen physisorption), structure (XRD) and reducibility (TPR) of the various catalysts (Ag/SBA-15) were studied and compared to those of a catalyst prepared by deposition of silver on fumed silica as a reference. These catalysts differ initially by the nature of silica and by pore sizes. In CO oxidation, pre-reduced catalysts are more active than pre-oxidised ones. This has to do with two phenomena, i.e. sintering, which produces large inactive silver particles, and formation of active silver species in the form of small Ag₂O particles.     

Alumina-supported cobalt phosphide as a new catalyst for preferential CO oxidation at high temperatures

Abstract

Novel alumina-supported cobalt phosphide catalysts (designated as CoP-3, CoP-10, CoP-20 and CoP-40) prepared from the precursors with Co loadings of 3, 10, 20 and 40?wt% by H₂-temperature-programmed reaction were investigated as potential catalysts for preferential CO oxidation (PROX) in excess H₂ at high temperatures. It was found that the catalytic activities of these Co₂P/γ-Al₂O₃ catalysts were related to their Co loadings. The CoP-10 catalyst showed the best PROX performance in temperature range of 220–240?°C, which was attributed to its optimal microstructures (high surface area, small particle size and big amount of active site).     

Mechanism of CO oxidation in excess H2 over CuO/CeO2 catalysts: ESR and TPD studies

The oxidation of CO with oxygen over (0.25–6.4)% CuO/CeO₂ catalysts in excess H₂ is studied. CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO₂ at 150°C. At T > 150°C, the CO conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorption of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO-Cu⁺ carbonyls on Cu₂O clusters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu⁰ of the reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O₂ on Cu⁰ at T ～ 150°C. The activation energies of CO, CO₂, and H₂O desorption are estimated, and the activation energy of CO adsorption yielding CO-Cu⁺ carbonyls is calculated in the framework of the Langmuir-Hinshelwood model.     

Ce改性的Pt/γ-AlO3催化剂用于富氢气氛下CO选择氧化

研究了Ce改性的Pt/γ-AlO3对于富氢气氛下CO选择氧化反应的催化行为考察了制备条件（共沉积沉淀法、分步沉积沉淀法以及沉积沉淀温度）对催化活性的影响．结果表明，在80℃时用共沉积沉淀方法制备的催化剂Pt/γ-AlO3-CP-80对CO氧化反应表现出良好的活性和选择性，CO转化率在120℃时可以达到85％．利用氢气程序升温还原和原位漫反射红外光谱对不同条件下制备的催化剂进行了表征，分析了Cc的促进作用．     

Avoiding Self‐Poisoning: A Key Feature for the High Activity of Au/Mg(OH)2 Catalysts in Continuous Low‐Temperature CO Oxidation

Au/Mg(OH)₂ catalysts have been reported to be far more active in the catalytic low‐temperature CO oxidation (below 0 °C) than the thoroughly investigated Au/TiO₂ catalysts. Based on kinetic and in situ infrared spectroscopy (DRIFTS) measurements, we demonstrate that the comparatively weak interaction of Au/Mg(OH)₂ with CO₂ formed during the low‐temperature reaction is the main reason for the superior catalyst performance. This feature enables rapid product desorption and hence continuous CO oxidation at temperatures well below 0 °C. At these temperatures, Au/TiO₂ also catalyzes CO₂ formation, but does not allow for CO₂ desorption, which results in self‐poisoning. At higher temperatures (above 0 °C), however, CO₂ formation is rate‐limiting, which results in a much higher activity for Au/TiO₂ under these reaction conditions.     

Surface species and gas phase products in the preferential oxidation of CO on TiO2-supported Au-Rh bimetallic catalysts

The oxidation of CO in the presence of hydrogen (PROX process) was investigated on bimetallic Au-Rh catalysts at 300–373 K by Fourier transform infrared spectroscopy and mass spectroscopy. The effects of catalyst composition, reaction temperature and composition of the reacting gas mixtures have been studied. The IR studies revealed the formation of bi- and monodentate carbonates, bicarbonates and hydrocarbonates on the catalysts surfaces; these surface species proved to be not involved in the surface reactions. The formation of adsorbed formaldehyde was observed on all surfaces, except 1% (0.25Au+0.75Rh)/TiO₂. Adsorbed CO₂ (as the surface product of CO oxidation) was not detected on any surface. The presence of both O₂ and H₂ reduced the surface concentration of CO adsorbed on the metallic sites. Mass spectroscopic analysis of the gas phase showed that gaseous CO₂ was formed in the highest amount in the CO+O₂ mixture, the presence of H₂ suppressed the amount of CO₂ produced. This negative effect of H₂ was the lowest on the 1% Rh/TiO₂ and 1% (0.25Au+0.75Rh)/TiO₂ catalysts.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]?n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

Laws of selective CO oxidation over a Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in the surface ignition regime: II. Transition states

The kinetics of selective CO oxidation (or individual CO or H₂ oxidation) over ruthenium catalysts are considerably as affected by the heat released by the reaction and specifics of the interaction of ruthenium with feed oxygen. In a reactor with reduced heat removal (a quartz reactor) under loads of ∼701 g_Cat⁻¹ h⁻¹ and reagent percentages of ∼1 vol % CO, ∼1 vol % O₂, ∼60 vol % H₂, and N₂ to the balance, the reaction can be carried out in the catalyst surface ignition regime. When catalyst temperatures are below ∼200°C, feed oxygen deactivates metallic ruthenium, the degree of deactivation being a function of temperature and treatment time. Accordingly, depending on the parameters of the experiment and the properties of the ruthenium catalyst, various scenarios of the behavior of the catalyst in selective CO oxidation are realized, including both steady and transition states: in a non-isothermal regime, a slow deactivation of the catalyst accompanied by a travel of the reaction zone through the catalyst bed along the reagent flow; activation of the catalyst; or the oscillation regime. The results of this study demonstrate that, for a strongly exothermic reaction (selective CO oxidation, or CO, or H₂ oxidation) occurring inside the catalyst bed, the specifics of the entrance of the reaction into the surface ignition regime and the effects of feed components on the catalyst activity should be taken into account.     

Preparation,Characterization, and Catalytic Behavior of Nanostructured Mesoporous CuO/ZrO2 Catalysts for Low-Temperature CO Oxidation

High-surface area mesoporous 20 mol% CuO/ZrO₂ catalyst was prepared by a surfactant-assisted method of nanocrystalline particle assembly, and characterized by x-ray powder diffraction (XRD), N₂ adsorption, transmission electron microscopy (TEM), H₂-TPR, TG-DTA, and x-ray photoelectron spectra (XPS) techniques. The catalytic properties of the CuO/ZrO₂ nanocatalysts calcined at different temperature were evaluated by low-temperature carbon monoxide oxidation using a CATLAB system. The results showed that these mesoporous nanostructured CuO/ZrO₂ catalysts were very active for low-temperature CO oxidation and the CuO/ZrO₂ catalyst calcined at 400°C exhibited the highest catalytic activity.     

Effect of transition metal oxide additives on the activity of an Ag/SiO2 catalyst in carbon monoxide oxidation

The catalysts of silver supported on mesoporous silica modified with Co₃O₄, CeO₂, and ZrO₂ were prepared by an impregnation method; characterized by X-ray diffraction analysis, temperature-programmed reduction, and low-temperature nitrogen adsorption; and studied in a model reaction of CO oxidation. It was found that the Ag/SiO₂ system exhibited high activity in the reaction of CO oxidation, and the addition of transition metal oxides led to reduction of the temperature of 50% CO conversion by 40°C. The modification of Ag/SiO₂ with cerium dioxide was found most effective because of the interaction of silver particles and CeO₂ on the surface of silica gel.     

Supported Ru catalysts prepared by two sonication-assisted methods for preferential oxidation of CO in H2

The preferential oxidation (PROX) of CO in the presence of H(2) is an important step in the production of pure H(2) for industrial applications. In this report, two sonochemical methods (S1 and S2) were used to prepare highly dispersed Ru catalysts supported on mesoporous TiO(2) (TiO(2)(MSP)) for the PROX reaction, in which a reaction gas mixture containing 1% CO + 1% O(2) + 18% CO(2) + 78% H(2) was used. The supported Ru catalysts performed better than the supported Au and Pt catalysts, and the S1 and S2 methods are superior to the impregnation method. The Ru/TiO(2)(MSP) catalysts were active for the PROX reaction below 200 °C and good for the methanation reactions of CO and CO(2) above 200 °C. The presence of residual chlorine in the catalysts severely suppressed their PROX reaction activity, and a higher dispersion of Ru particles led to better catalytic performances. The addition of Au in the Ru/TiO(2)(MSP) catalyst also caused a poorer catalytic activity for both the PROX and the methanation reactions. TPR results showed that in the active catalysts prepared by the S1 and S2 methods, the well dispersed Ru particles, after calcination in air, had a stronger interaction with the support than those in the catalyst prepared by the impregnation method and in the Au-Ru/TiO(2)(MSP) catalyst. In situ CO absorption experiments performed with the diffusion reflectance Fourier transform infra red (DRIFT) method showed that the bridged adsorbed CO species on isolated Ru(0) sites correlated with the catalytic performances, indicating that these isolated Ru(0) sites are the most active sites of the Ru/TiO(2)(MSP) catalysts in the PROX reaction.     

Study on chemisorption of H<Subscript>2</Subscript>, O<Subscript>2</Subscript>, CO and C<Subscript>2</Subscript>H<Subscript>4</Subscript>on Pt-Ag/SiO<Subscript>2</Subscript>catalysts by microcalorimetry and FTIR

Adsorption microcalorimetry has been employed to study the interaction of ethylene with the reduced and oxidized Pt-Ag/SiO₂catalysts with different Ag contents to elucidate the modified effect of Ag towards the hydrocarbon processing on platinum catalysts. In addition, microcalorimetric adsorption of H₂, O₂, CO and FTIR of CO adsorption were conducted to investigate the influence of Ag on the surface structure of Pt catalyst. It is found from the microcalorimetric results of H₂and O₂adsorption that the addition of Ag to Pt/SiO₂leads to the enrichment of Ag on the catalyst surface which decreases the size of Pt surface ensembles of Pt-Ag/SiO₂catalysts. The microcalorimetry and FTIR of CO adsorption indicates that there still exist sites for linear and bridged CO adsorption on the surface of platinum catalysts simultaneously although Ag was incorporated into Pt/SiO₂. The ethylene microcalorimetric results show that the decrease of ensemble size of Pt surface sites suppresses the formation of dissociative species (ethylidyne) upon the chemisorption of C₂H₄on Pt-Ag/SiO₂. The differential heat vs. uptake plots for C₂H₄adsorption on the oxygen-preadsorbed Pt/SiO₂and Pt-Ag/SiO₂catalysts suggest that the incorporation of Ag to Pt/SiO₂could decrease the ability for the oxidation of C₂H₄.     

Catalytic Removal of NOx from Gas Mixtures Simulating Emissions from the Combustion of Biofuels

The activity of knitted silica-fibre supported Pd, Pt, Pt-Ni, Pd-Ni and Pd-Pt-Ni catalysts as well as Pd based H-ZSM-5 and H-ZSM-35 catalysts was studied in the conversion of gas mixtures containing 200 ppm CH₄, 2500 ppm CO, 500 ppm pyridine (or 500 ppm NO), 10 vol.% O₂ (or 0.155 vol.% O₂), 12 vol.% CO₂, 12 vol.% H₂O, balanced with He at GHSV of 60000 h^–1. Pyridine was found to inhibit both CO and CH₄ oxidation. IR studies indicated that NO adsorbed on Pd²⁺ is the principal adsorbed species on the Pd/HZSM-5 catalyst.     

Toluene abatement on Ag-CeO2/SBA-15 catalysts: synergistic effect of silver and ceria

As a typical volatile organic compound, toluene is a hazardous material for human health and the environment, and currently, the development of catalysts for its oxidation into CO₂ and water is crucial. The series of Ag-CeO₂/SBA-15 catalysts is synthesized by wetness impregnation techniques and characterized by a number of physical-chemical methods (nitrogen [N₂] physisorption, small angle X-ray scattering [SAXS], transmission electron microscopy [TEM], and temperature-programmed reduction [TPR]). The toluene sorption and catalytic properties in toluene oxidation are studied. Small silver [Ag] and cerium oxide [ceria, CeO_2] particles with sizes below 3 nm are predominantly formed in the ordered structure of Santa Barbara Amorphous-15 [SBA-15]. The interactions between the Ag and CeO₂ nanoparticles are established. Temperature-programmed desorption of toluene [TPD-C₇H₈] analysis shows that physical adsorption of toluene occurs on pristine SBA-15 material, while the introduction of either silver or ceria to SBA-15 leads to the appearance of additional strongly bound chemisorbed toluene on such sites. When both Ag and CeO₂ are introduced, only chemisorbed toluene is formed over the Ag-CeO₂/SBA-15 catalyst, and the highest catalytic activity in toluene oxidation is observed over this catalyst (T_98% = 233 °C, 0.2% C₆H₅CH₃) that is attributed to the synergistic effect of ceria [CeO_2] and silver [Ag].     

Reduction of NO2 in Flue Gas by CO and Propylene over CuO‐CeO2/SiO2 in the Presence of O2

Catalytic reduction of NO₂ with CO and/or propylene in the presence of NO and excess oxygen, a model mixture for flue gas, was studied over a series of CuO‐CeO₂/SiO₂ catalysts between 120–260 °C. The effect of HCl, an impurity in flue gas, on the activity of the catalysts was evaluated. It was found that a binary oxide catalyst, 2% CuO‐8% CeO₂/SiO₂, was active for the reduction of NO₂ by CO and/or propylene. CO was effective for selective reduction of NO₂ in the presence of NO and O₂ in a temperature window between 160–200 °C while propylene was effective at temperature higher than 200 °C. In the presence of HCl, the activity of the catalyst for reduction of NO₂ with CO was irreversibly deactivated. However, the activity for reduction of NO₂ with propylene was not influenced by HCl.     

Preferential CO oxidation over Pt- and Rh-catalysts under conditions of strong CO adsorption

Reaction of preferential CO oxidation is studied over Rh-, Pt-catalysts in a flow reactor made of quartz. In the experiments, the following parameters were monitored: catalyst bed temperature at the inlet to the catalyst bed and the outlet, residual CO content (with IR-analyzer), residual O₂ content (GC analysis). In CO absence from the reaction gas, H2 is easily oxidized over Rh and Pt, and the reaction proceeds in the mode of catalytic surface ignition (CSI) with the “hot spot” at the inlet to the catalyst bed. Appearance of CO in gas at temperatures at least below 200°C leads to slow deactivation of the catalysts. The possibility of CSI mode realization under PROX conditions over a Rh-catalyst is shown. The key component of the reaction in this case is O₂—its residual content in CSI mode is less than 100 ppm. It is found that in CSI mode, a decrease in temperature increases selectivity of the reaction. Disestablishment of CSI (extinction) is accompanied over a Rh-catalyst by oscillations in catalytic activity. Possible mechanism of this oscillation phenomenon is pro-posed in connection with catalyst deactivation by CO at the inlet to the catalyst bed and with CSI at the outlet.     

无氯 Cu/AC 催化剂的制备及其催化气相甲醇氧化羰基化反应性能

 以 Cu2(NO3)(OH)3/AC (活性碳) 为催化剂前驱体, 在惰性气氛中于不同温度热处理分别制得无氯的 CuO/AC, Cu2O/AC 和 Cu0/AC 催化剂, 并用于甲醇直接气相氧化羰基化合成碳酸二甲酯 (DMC) 反应. 结果表明, 200 °C 处理制得的催化剂中, Cu 物种以 CuO 为主. 随着处理温度的升高, 催化剂中 CuO 含量逐渐降低, 而 Cu2O 含量增加; 400 °C 制备的催化剂中, Cu 物种仅以 Cu2O 形式存在; 而 450 °C 以上处理时则以 Cu0 形式存在. 随着热处理温度的提高, 相应催化剂活性逐渐增加, 表明 CuO, Cu2O 和 Cu0 均具有催化活性, 其活性大小的顺序为 CuO < Cu2O < Cu0. 在 140 °C, CO:MeOH:O2 = 4:10:1, SV = 5 600 h1 条件下, 450 °C 处理制备的 Cu0/AC 催化剂表现出较高的催化甲醇氧化羰基化活性, 其中甲醇转化率达 11.5%, DMC 的时空收率和选择性分别为 261.9 mg/(g•h) 和 76.0%.     

Relationship between Pt particle size and catalyst activity for catalytic oxidation of ultrahigh‐concentration formaldehyde

The effects of particle size and kinetics of Pt/activated carbon (AC) catalysts on catalytic oxidation of formaldehyde (HCHO) were investigated. AC, f‐SiO₂ and MCM‐41 were used as supports to prepare low‐Pt‐content catalysts using H₂ reduction. Pt/AC catalyst shows the highest activity with the largest Pt particle size. By contrast, 0.1 wt% Pt/AC reduced using KBH₄ has much higher activity than that reduced using H₂, which can oxidize HCHO completely over 6000 ppm at 60°C in a fixed bed reactor. Transmission electron microscopy and X‐ray photoelectron spectroscopy results indicate that Pt/AC‐KBH₄ has larger Pt particles and lower valence state than Pt/AC‐H₂, which may be attributed to the ligand effect between Pt⁴⁺ and the AC support. The result of O₂ temperature‐programmed oxidation suggests that highly dispersed Pt⁴⁺ ions have stronger interaction with AC support and thus are harder to be reduced by H₂. Furthermore, Pt/AC is structure‐sensitive and larger‐sized Pt particles result in a high conversion of HCHO. Investigation of kinetics indicated that it is a zero‐order reaction for such a high HCHO concentration condition for Pt/AC‐KBH₄.     

Fe-Ag/Al_2O_3催化丙烯还原NO的实验研究

以蜂窝状陶瓷为载体,采用溶胶凝胶法和浸渍法制备了不同Fe/Ag负载量的Fe-Ag/Al_2O_3催化剂。以C_3H_6为还原剂,在模拟烟气条件下和200-700℃范围内,程序控温的陶瓷管流动反应器上进行了催化还原NO的性能评估。结果表明,7.2Fe/1.9Ag/20Al_2O_3/CM在500和550℃时催化C_3H_6还原NO的脱硝效率分别超过90%和达到100%。铁离子能有效地提高Ag/20Al_2O_3/CM催化剂抵抗烟气中的SO_2和H_2O的能力。结果表明,当烟气中含有体积分数为0.02%的SO2和8%的H_2O时,在500℃时7.2Fe/1.9Ag/20Al_2O_3/CM催化C_3H_6还原NO的脱硝效率不受影响,在6 h的连续实验中保持90%的脱硝效率而没有下降。而未经铁离子修饰的2Ag/20Al_2O_3/CM的催化活性则受烟气中的SO2和H_2O影响很大,0.02%的SO2和8%的H_2O分别使2Ag/20Al_2O_3/CM在500℃时催化C_3H_6还原NO的脱硝效率迅速从70%分别下降至46%和25%。XRD和SEM表征结果表明,经铁离子修饰后的7.2Fe/1.9Ag/20Al_2O_3/CM催化剂中,形成了AgFeO_2以及Fe~(3+),催化剂表面变得疏松多孔,形成以Fe_3O_4为主的针状和片状晶体。H_2-TPR结果表明,7.2Fe/1.9Ag/20Al_2O_3/CM比Ag/20Al_2O_3/CM在更宽的温度范围内具有更好的还原特性。吡啶吸附红外光谱(Py-FTIR)实验结果显示,Fe增加了催化剂表面的Lewis酸性位。