Palladium-alumina Cryogel with High Thermal Stability and CO Oxidation Activity

Abstract  

Palladium-alumina cryogel was prepared from palladium nitrate and aluminum sec-butoxide through a sol–gel processing and subsequent freeze drying. The cryogel showed higher thermal stability of palladium and catalytic CO oxidation activity than the corresponding xerogel and impregnation catalysts. The superior stability and activity were ascribed to the uniform distribution of palladium ions in the boehmite gel by the one-pot preparation of palladium-boehmite co-gel, and also to the suppression of transfer and aggregation of the metal during the subsequent freeze drying. As a result palladium ions were finely distributed and stabilized throughout the alumina cryogel support, which consequently suppressed the sintering of palladium at elevated temperatures.     

Low-temperature CO oxidation by Co3O4 nanocubes on the surface of Co(OH)2 nanosheets

CO oxidation has been performed on Co₃O₄ nanocubes and Co(OH)₂ nanosheets as model catalysts. The reaction rate of CO on Co₃O₄/Co(OH)₂ nanocomposites obtained by one-pot synthesis is about ~ two orders of magnitude higher than that on Co₃O₄ nanomaterials. The catalytic behaviors of different nanomaterials revealed that the assembly with nano building blocks cause the catalytic sites much more active for CO oxidation. The kinetic data showed that the activation energy for CO oxidation over Co₃O₄/Co(OH)₂ nanocomposites was lower than that of other nanomaterials. Since Co(OH)₂ nanosheets can prevent Co₃O₄ nanocubes aggregating, nanocomposites kept good catalytic stability.     

Facet-Dependent SERS Activity of Co3O4

Surface-enhanced Raman spectroscopy (SERS) is an ultra-sensitive and rapid technique that is able to significantly enhance the Raman signals of analytes absorbed on functional substrates by orders of magnitude. Recently, semiconductor-based SERS substrates have shown rapid progress due to their great cost-effectiveness, stability and biocompatibility. In this work, three types of faceted Co₃O₄ microcrystals with dominantly exposed {100} facets, {111} facets and co-exposed {100}-{111} facets (denoted as C-100, C-111 and C-both, respectively) are utilized as SERS substrates to detect the rhodamine 6G (R6G) molecule and nucleic acids (adenine and cytosine). C-100 exhibited the highest SERS sensitivity among these samples, and the lowest detection limits (LODs) to R6G and adenine can reach 10⁻⁷ M. First-principles density functional theory (DFT) simulations further unveiled a stronger photoinduced charge transfer (PICT) in C-100 than in C-111. This work provides new insights into the facet-dependent SERS for semiconductor materials.     

Preparation of Co3O4/ZrO2(Y2O3) Catalyst and its Enhanced Catalytic Oxidation of CO

Yttria-stabilized zirconia powders were prepared by the sol–gel method coupled with supercritical CO₂ fluid-drying technology, using ZrOCl₂·8H₂O as the precursor, urea as the precipitant, and yttria as the stabilizer. The particles were characterized by X-ray diffraction, TEM and BET. The Co₃O₄/ZrO₂(Y₂O₃) catalysts were prepared by the impregnation method. The content of cobalt was varied from 5 to 12 wt%. The prepared catalysts were calcined at 200–500 °C and the pretreating temperature was varied from 200–400 °C. The performance of CO catalytic oxidation was tested and the catalyst with 8% Co loading, calcined at 200 °C, and with a pretreating temperature of 300 °C, showed the highest catalytic activity. The temperature for 95% CO conversion was as low as 113 °C; and, the catalyst showed both good cycling stability and excellent long-term stability.     

Co3O4-Bi2O2CO3催化剂的制备及其光催化性能

以Bi(NO₃)₃·5H₂O、Co(CH₃COO)₂·4H₂O为原料,采用化学沉淀-水热法制备了Co₃O₄-Bi₂O₂CO₃异质结构复合半导体光催化剂,并通过X射线衍射仪（XRD）、扫描电镜（SEM）、X射线光电子能谱（XPS）、紫外可见漫反射光谱（DRS）、荧光光谱（PL）等手段对所合成的复合型催化剂进行了理化性能表征。研究结果表明：引入Co₃O₄没有改变Bi₂O₂CO₃物相结构,但促进了Bi₂O₂CO₃ 对可见光的吸收能力,提高了Bi₂O₂CO₃表面吸附氧物种的数量,抑制了光生载流子复合。复合光催化剂对罗丹明B（RhB）的光催化脱色实验显示引入Co₃O₄能够明显提高Bi₂O₂CO₃催化剂的光催化脱色能力。尤其是Co₃O₄引入量为0.6%的Co₃O₄-Bi₂O₂CO₃样品对罗丹明B染料的光催化脱色率可达到97%（模拟日光照射30min）。本文为复合型光催化剂制备提供了简单易行的技术路线,制备的新型半导体复合光催化剂Co₃O₄-Bi₂O₂CO₃在环境净化方面表现出了较好的应用前景。     

负载方式对Ag/Co_3O_4催化剂催化氧化甲醛活性的影响

采用硬模板法原位负载(insitu)和后浸渍(post)法制备了介孔Ag/Co3O4和不负载的介孔Co3O4催化剂,考察了它们对甲醛的催化氧化活性,其中后浸渍法负载的催化剂(Ag/Co3O4-post)展示了最优的活性。对各催化剂样品的表征发现,Ag/Co3O4-post在负载Ag的制备过程中介孔结构塌陷,但同时其还原能力增强,并且暴露出最多的表面Co3+和缺陷氧,进而使其具有最强的催化氧化甲醛的活性。     

Origin of ohmic losses at Co3O4/Ti electrodes

Co₃O₄/Ti and NiCo₂O₄/Ti electrodes were prepared in different ways to investigate the origin of the ohmic losses observed experimentally. In particular, titanium was pretreated in various ways including etching with a HF mixture, and reduction by cathodic hydrogen discharge prior to coating. Different types of commercial titanium and different concentrations of the precursors in solution were also tried. Some electrodes were prepared with a RuO₂ interlayer. Nickel and mild steel were also used as supports. Parameters to quantify the ohmic losses were the peak distance in voltammetric curves, and the initial slope and the deviation from linearity of current vs sweep rate plots. The experimental picture corroborates the view that the main component of the ohmic drop comes from the insulating barrier which forms at the support/oxide layer interface. The intrinsic conductivity of spinels does not appear to represent the main problem for thermal layers as usually prepared.Author to whom correspondence should be addressed.     

Effect of CeO2 Doping on Structure and Catalytic Performance of Co3O4 Catalyst for Low-Temperature CO Oxidation

The effect of CeO₂ doping on structure and catalytic performance of Co₃O₄ catalyst was studied for low-temperature CO oxidation. The Co₃O₄ catalyst was prepared by a precipitation method and the CeO₂/Co₃O₄ catalyst was prepared by an impregnation method. Their catalytic performance had been studied with a continuous flowing micro-reactor. The results reveal that the CeO₂/Co₃O₄ catalyst exhibits much better resistance to water vapor poisoning than the Co₃O₄ catalyst for CO oxidation. The CeO₂/Co₃O₄ catalyst can maintain CO complete conversion at least 8,400 min at 110 °C with 0.6% water vapor in the feed gas, while the Co₃O₄ catalyst can maintain at 100% for only 100 min. Characterizations with XRD, TEM and TPR suggest that the CeO₂/Co₃O₄ catalyst possesses higher dispersion degree, smaller particles and larger S_BET, due to the doping of Ceria, and exists the interaction between CeO₂ and Co₃O₄, which may contribute to the excellent water resistance for low-temperature CO oxidation. Furthermore, the H₂ detected in the reactor outlet gas seems to indicate that the water–gas shift reaction is the more direct reason.     

Synthesis of Co3O4 nanotubes and their catalytic applications in CO oxidation

Co₃O₄ nanotubes with the inner of about 8 nm and the length of 200–500 nm were synthesized by oxidizing Co nanowires. Analyses on the structural evolution of the intermediates at different intervals identified that the formation of Co₃O₄ nanotubes followed a nanoscale Kirkendall effect. The outward diffusion of cobalt species was faster than the inward diffusion of oxygen species, resulting in the formation of nanovoids at the initial stage and subsequently the tubular structure. The Co₃O₄ nanotubes showed a higher activity and stability than the spherical nanoparticles in CO oxidation, primarily due of the facile redox feature.     

High Catalytic Activity in CO Oxidation over MnO x Nanocrystals

Manganese oxides of various stoichiometry were prepared via Mn-oxalate precipitation followed by thermal decomposition in the presence of oxygen. A non-stoichiometric manganese oxide, MnO _x (x = 1.61…1.67) was obtained by annealing at 633 K and demonstrated superior CO oxidation activity, i.e. full CO conversion at room temperature and below. The activity gradually decreased with time-on-stream of the reactants but could be easily recovered by heating at 633 K in the presence of oxygen. CO oxidation over MnO _x in the absence of oxygen proved to be possible with reduced rates and demonstrated a Mars—van Krevelen—type mechanism to be in operation. A TEM structural analysis showed the MnO _x phase to form microrods with large aspect ratio which broke up into nanocrystalline manganese oxide (MnO _x ) particles with diameters below 3 nm and a BET specific surface area of 525 m²/g. Annealing at 798 K rather than 633 K produced well crystalline Mn₂O₃ which showed lower CO oxidation activity, i.e. 100% CO conversion at 335 K. The catalytic performance in CO oxidation of various Mn-oxides either studied in this work or elsewhere was compared on the basis of specific reaction rates.     

焙烧温度对CuO/Co_3O_4-CeO_2催化剂CO优先氧化性能的影响

采用浸渍法制备了Cu O/Co3O4-Ce O2(Cu Co Ce10)催化剂,考察了在不同温度(250、300、350、400和450℃)焙烧后的样品在富氢气氛中对CO优先氧化反应的催化性能。应用BET、XRD、H2-TPR及XAFS技术详细表征了催化剂的结构与性能。结果表明,不同温度焙烧的催化剂中,铜物种均主要以Cu O相存在;350℃焙烧的Cu Co Ce10催化剂具有最大的比表面积和最好的氧化还原性能,在98~173℃的温度范围内能够将CO完全转化为CO2,呈现出最宽的CO优先氧化可操作温度窗口,以及良好的催化活性稳定性。     

Room temperature CO sensing by polyaniline/Co3O4 nanocomposite

Polyaniline/cobalt oxide (PANI/Co₃O₄) nanocomposites have been investigated for their sensitivity towards carbon monoxide (CO) gas at room temperature. The Co₃O₄ nanoparticles were prepared by ultrasound assisted coprecipitation method and then incorporated into the PANI matrix. Fourier transform infrared spectroscopy and ultraviolet–visible spectroscopy, powder X‐ray diffraction, and field emission scanning electron microscopy have been used to characterize the nanomaterials. The PANI/Co₃O₄ nanocomposite sensors were found to be highly selective to CO gas at room temperature. A significantly high response of 0.81 has been obtained for 75 ppm CO concentration with a response time of 40 s. Based on the observations of the sensing study, a mechanism for CO sensing by the nanocomposite has been proposed. Influence of humidity on the sensor response towards CO has also been studied and the results presented. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44115.     

Influence of dry operating conditions: observation of oscillations and low temperature CO oxidation over Co3O4 and Au/Co3O4 catalysts

A Sm(III) exchanged NaY zeolite prepared from aqueous SmCl₃ was modified with various amounts of fluorine using NH₄F. These fluorinated zeolites exhibit enhanced catalytic activity for the dealkylation of cumene. The evaluation of acid sites by infrared spectroscopy and pyridine adsorption was correlated with fluorine content.     

The CO methanation over NaY-zeolite supported Ni/Co3O4, Ni/ZrO2, Co3O4/ZrO2 and Ni/Co3O4/ZrO2 catalysts

The CO methanation was studied over zeolite NaY supported Ni, Co₃O₄, ZrO₂ catalysts. The XRD, N₂ physisorption and SEM analysis were used in order to characterize the catalysts. Catalytic activities were carried out under a feed composition of 1% CO, 50% H₂ and 49% He between the 125 °C to 375 °C. Except for the Ni/Co₃O₄/NaY catalyst, all catalysts gave high surface area because of the presence of zeolite NaY. Average pore diameter of the catalysts fell into the mesopore diameter range. The highest CO methanation activity was obtained with Ni/ZrO₂/NaY catalyst at which the CO methanation was started after 175 °C and 100% CO conversion was obtained at 275 °C using the same catalyst.     

Preparation and catalytic performance of Co3O4 catalysts for low-temperature CO oxidation

Without use of any surfactant or oxidant, a series of Co₃O₄ catalysts have been prepared from cobalt nitrate aqueous solution via a very simple liquid-precipitation method with ammonium acid carbonate followed by calcination at various temperatures. The catalytic performance of the Co₃O₄ for CO oxidation has been studied with a continuous flowing laboratory microreactor system. The results show that the CO conversion of all the samples can reach 100% at ambient temperature. The catalyst calcined at 300 °C is able to maintain its activity for CO complete oxidation more than 500 min at 25 °C and about 240 min even at −78 °C. High reaction temperature results in a high catalytic stability, while the catalytic stability decreases with further increasing the reaction temperature. Characterizations with X-ray powder diffraction and transmission electron microscopy suggest that all the samples exist as a pure Co₃O₄ phase with the spinel structure, the samples are apt to aggregate and the specific surface area gradually decreases with increasing the calcination temperature, which directly leads to the decrease of catalytic stability. Furthermore, the amount of active oxygen species measured by CO titration experiments appears to be critical for catalytic performance.     

Origin of Capacity Fading in Nano-Sized Co3O4 Electrodes: Electrochemical Impedance Spectroscopy Study

Transition metal oxides have been suggested as innovative, high-energy electrode materials for lithium-ion batteries because their electrochemical conversion reactions can transfer two to six electrons. However, nano-sized transition metal oxides, especially Co₃O₄, exhibit drastic capacity decay during discharge/charge cycling, which hinders their practical use in lithium-ion batteries. Herein, we prepared nano-sized Co₃O₄ with high crystallinity using a simple citrate-gel method and used electrochemical impedance spectroscopy method to examine the origin for the drastic capacity fading observed in the nano-sized Co₃O₄ anode system. During cycling, AC impedance responses were collected at the first discharged state and at every subsequent tenth discharged state until the 100th cycle. By examining the separable relaxation time of each electrochemical reaction and the goodness-of-fit results, a direct relation between the charge transfer process and cycling performance was clearly observed.     

Low-temperature CO oxidation over CuO/Fe2O3 catalysts

Copper iron composite oxide catalysts have been prepared by co-precipitation method. The catalytic activity and stability of the catalysts on CO oxidation were evaluated by using a microreactor-GC system. The results indicated that the copper iron composite oxide catalysts exhibited obviously high stability and catalytic activity on CO oxidation at low temperature. The effect of the calcination temperature, the molar ratios of copper to iron, the specific surface areas and the particle sizes on the catalytic activity of the catalysts was investigated in this paper.     

Low-temperature Oxidation of Carbon Monoxide on Co/ZrO2

A 10%Co/ZrO₂ catalyst prepared by impregnation was tested for its activity for the oxidation of CO to CO₂ in excess oxygen. Activity tests showed that conversion could be obtained at temperatures as low as 20 °C. Time-on-stream studies showed no loss of activity in these experiments, indicating that this catalyst is stable in the experimental oxidizing conditions. The activation energy for the CO to CO₂ oxidation reaction was calculated as E_a = 54 kJ/mol over this catalyst. Characterization of the material by thermogravimetric analysis, temperature-programmed techniques, X-ray photoelectron spectroscopy, and laser Raman spectroscopy indicate that Co₃O₄ is present on monoclinic ZrO₂ after the calcination of the catalyst.     

Low-temperature synthesis one-dimensional Ag2CO3/SnFe2O4 Z-scheme with excellent visible-light photoactivity

In this study, Ag₂CO₃/SnFe₂O₄ (Ag₂CO₃/SFO) photocatalyst was prepared by a simple hydrothermal-ultrasonic method for the efficient degradation of ciprofloxacin and phenol. The SFO nanoparticles were attached on the surface of Ag₂CO₃ rods synthesized by a low-temperature precipitation method, resulting a unique 1D/0D morphology, which increased the number of active sites. Due to introduction of magnetic SFO, Ag₂CO₃/SFO exhibited excellent magnetic recovery performance. When the mass fraction of SFO was 5%, the degradation efficiency of composite photocatalyst was the highest, the degradation rate for ciprofloxacin was 6.5 and 1.5 times higher than pure SFO and Ag₂CO₃, respectively. The improved photoactivity of Ag₂CO₃/SFO should be attributed to the construction of heterojunction with tight interface, which boosts the separation and transfer of photoinduced electron and hole pairs. On the basis of experimental results, a possible Z-scheme photocatalytic mechanism was discussed. Additionally, the excellent photostability of Ag₂CO₃/SFO was proved by a cycle experiment.