Enhancement of Methanol Oxidation Performance Over Pd/CeO2 Derived from MOF and Mechanism Investigation via In Situ Studies

A Pd/CeO₂-M with high methanol oxidation performance was prepared from metal–organic framework (MOF). The reaction results indicate that the Pd/CeO₂-M could achieve 100% methanol oxidation at 200 °C over the traditional Pd/CeO₂-C at 350 °C. The high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and Raman show that the Pd was uniformly distributed over the CeO₂ that was rich in oxygen vacancies. The methanol temperature program surface reaction (MeOH-TPSR) demonstrates that the Pd/CeO₂-M could effectively decompose methanol at a lower temperature with the assistance of oxygen vacancies compared with Pd/CeO₂-C. Methanol oxidation was conducted with in-situ Raman, CO diffuse reflectance infrared Fourier transformed spectroscopy (CO-DRIFTS) and in-situ DRIFTS to elucidate the reaction mechanism and establish the structure–activity relationship. The oxygen vacancies have participated in the assistance of methanol oxidation, which was revealed via in-situ Raman. The CO-DRIFTS indicates that the chemisorbed CO was more in linear form, which demonstrates that the Pd particles were over CeO₂-M with a smaller size. The in-situ DRIFTS experiments show the methanol could be further effectively oxidized to bidentate formate over Pd/CeO₂-M with the assistance of oxygen vacancies. The Pd/CeO₂-M catalysis and its enhancement mechanism studies provided a potential strategy in the VOC removal catalysis development.

Graphical Abstract

     

催化剂Au/CeO_2/MCM-41的制备及其活性研究

文章报道了一种新型催化剂Au/CeO2/MCM-41的制备,并对它的比表面积、晶体结构、表面元素价态、储释氧能力及催化活性进行表征。通过催化剂的活性测试,发现催化剂Au/CeO2/MCM-41对CO氧化具有很高的活性,但催化剂对煅烧温度比较敏感,随煅烧温度上升,催化活性下降很快,可能是因为一方面较高的温度煅烧使金纳米颗粒迅速长大,从而活性很快的下降;另一方面高温煅烧使催化剂发生烧结,从而比表面积下降引起活性的降低。     

二氧化铈载金催化剂催化CO氧化的研究进展

将金纳米颗粒负载在二氧化铈上,形成负载型催化剂,能应用于多种反应体系,表现出良好的催化活性,有着广阔的发展前景。综述了二氧化铈载金催化剂在一氧化碳催化氧化中的应用研究进展,介绍了二氧化铈载金催化剂中金的结构、价态和纳米尺寸,载体二氧化铈的形貌、结构、尺寸和氧空位以及金和载体二氧化铈两者之间的相互作用对一氧化碳催化氧化性能的影响。     

CuO/ZnO/CeO_2-ZrO_2催化甲醇水蒸气重整制氢性能

以液体燃料甲醇分布式现场重整制氢系统开发为研究目的,根据非对称耦合的思想,将Cu O/Zn O/Ce O2-Zr O2甲醇水蒸气重整催化剂和Pt/Al2O3催化燃烧催化剂应用于套筒式小型制氢反应器中,实现了甲醇水蒸气重整反应和燃烧反应的耦合。实验考察了Cu O/Zn O/Ce O2-Zr O2催化剂在套筒式小型制氢反应器中的性能。结果表明:在套筒式小型制氢反应器内,Cu O/Zn O/Ce O2-Zr O2催化剂的甲醇水蒸气重整活性比商业Cu O/Zn O/Al2O3催化剂高出30%左右;并且多次开停车和改变反应条件,均未对催化剂和反应器产生明显影响,150 h内催化剂和反应器稳定性良好。当反应温度为230~260℃,甲醇气体空速为300~1200 h-1,水醇物质的量比(S/M)为1.2时,最大氢产率达162.8 L/h,可为百瓦级质子交换膜燃料电池提供氢源。     

Methanol Partial Oxidation on MoO3/SiO2 Catalysts: Application of Vibrational Spectroscopic Imaging Techniques in a High Throughput Operando Reactor

A home-built, high-throughput operando (HTO) reactor was applied to study methanol partial oxidation reaction over MoO₃/SiO₂ catalysts. This HTO reactor combines Fourier transform infrared (FT-IR) imaging and Raman spectroscopy for high throughput catalyst evaluation and simultaneously for catalyst characterization under operando conditions. The catalyst activity and selectivity of all parallel reaction channels were followed at a time resolution of 2–20 s by the FT-IR imaging system that offers a spatial resolution of 16,384 pixels over a 2 × 2 inches illuminated cross-section area. Six specialized Raman probes were used to simultaneously collect Raman spectra of the catalyst surfaces and reaction intermediates under operando conditions. The structural variation of the MoO₃/SiO₂ catalysts with different molybdenum loadings and their catalytic performance at various temperatures were determined. The HTO reactor with the integrated imaging techniques allowed us to track the catalytic activities and the surface morphologies for multiple samples under various operando conditions.     

Comparison of the activity of Au/CeO2 and Au/Fe2O3 catalysts for the CO oxidation and the water-gas shift reactions

We compare the activity and relevant gold species of nanostructured gold–cerium oxide and gold–iron oxide catalysts for the CO oxidation by dioxygen and water. Well dispersed gold nanoparticles in reduced form provide the active sites for the CO oxidation reaction on both oxide supports. On the other hand, oxidized gold species, strongly bound on the support catalyze the water-gas shift reaction. Gold species weakly bound to ceria (doped with lanthana) or iron oxide can be removed by sodium cyanide at pH ≥12. Both parent and leached catalysts were investigated. The activity of the leached gold–iron oxide catalyst in CO oxidation is approximately two orders of magnitude lower than that of the parent material. However, after exposure to H₂ up to 400 °C gold diffuses out and is in reduced form on the surface, a process accompanied by a dramatic enhancement of the CO oxidation activity. Similar results were found with the gold–ceria catalysts. On the other hand, pre-reduction of the calcined leached catalyst samples did not promote their water-gas shift activity. UV–Vis, XANES and XPS were used to probe the oxidation state of the catalysts after various treatments.     

氨催化氧化催化剂Pt/CeO_2的制备及性能表征

二氧化铈具有紫外光吸收能力,以其为载体,通过光催化还原沉积可实现贵金属负载。本研究通过光催化还原沉积制备了氨氧化用Pt/CeO_2催化剂,利用X射线衍射(XRD)、扫描电镜(SEM)、X射线能谱(EDS)、透射电镜(TEM)、BET比表面积、电耦合高频等离子发射光谱(ICP)、X射线光电子能谱(XPS)、NH_3-程序升温脱附(NH_3-TPD)对样品的物理性质进行了表征,并在φ4 mm×2 mm石英反应管中对其氨催化氧化性能进行了评价。结果表明,该法制备的Pt/CeO_2催化剂活性组分铂含量仅为0.079 7%,在载体表面分散性好;在反应温度800℃,原料中O_2和NH_3比为15的条件下,氨转化率为100%,氮氧化合物收率可达80%。     

Synergy of Components in CuZnO and CuZnO/Al2O3 on Methanol Synthesis: Analysis at the Site Level by SSITKA

In the present study, the effects of the individual components and an Al₂O₃ support on CuZnO for methanol (MeOH) synthesis were investigated at the site level for the first time using steady-state isotopic transient kinetic analysis and reaction at 250?°C and 1.8?atm. The presence of ZnO was found to decrease the hydrocarbon (CH₄ primarily) formation ability of Cu. By comparing the surface reaction parameters, it could be shown that Cu and ZnO catalysts (supported on Al₂O₃) exhibit lower MeOH formation rates compared to their combination in either CuZnO or CuZnO/Al₂O₃ due, especially, to lower intrinsic ??site?? activities. The synergy between Cu and ZnO was most obvious in the increase in MeOH TOF_ITK (a measure of site activity for MeOH formation), more than double that for Cu without ZnO. Al₂O₃ did not seem to impact MeOH synthesis in any way other than to increase dispersion of the CuZnO. However, it did furnish acid sites for the conversion of some MeOH to DME.     

Catalytic Partial Oxidation of Methanol and Methyl Mercaptan: Studies on the Selectivity of TiO2 and CeO2 Supported V2O5 Catalysts

Catalyst samples based on vanadium, cerium and titanium oxides were prepared using sol–gel method and wet impregnation. Several analytical techniques were used to characterize catalysts. The best activity and selectivity in the partial oxidation of methanol and methyl mercaptan were observed for the catalyst sample with the composition of 3 % V₂O₅/Ti_0.1–Ce_0.9O₂.     

Characterization and Catalytic Properties of Combustion Synthesized Au/CeO2 Catalyst

Ceria-supported Au catalyst has been synthesized by the solution combustion method for the first time and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Au is dispersed as Au⁰ as well as Au³⁺ states on CeO₂ surface of 20-30 nm crystallites. On heating the as-prepared 1% Au/CeO₂ in air, the concentration of Au³⁺ ions on CeO₂ increases at the expense of Au⁰. Catalytic activities for CO and hydrocarbon oxidation and NO reduction over the as-prepared and the heat-treated 1% Au/CeO₂ have been carried out using a temperature-programmed reaction technique in a packed bed tubular reactor. The results are compared with nano-sized Au metal particles dispersed on -Al₂O₂ substrate prepared by the same method. All the reactions over heat-treated Au/CeO₂ occur at lower temperature in comparison with the as-prepared Au/CeO₂ and Au/Al₂O₂. The rate of NO + CO reaction over as-prepared and heat-treated 1% Au/CeO₂ are 28.3 and 54.0 mol g^-1 s^-1 at 250 and 300 °C respectively. Activation energy (E _a) values are 106 and 90 kJ mol^-1 for CO + O₂ reaction respectively over as-prepared and heat-treated 1% Au/CeO₂ respectively.     

Steam Reforming of Methanol Over Cu/CeO2 Catalysts Studied in Comparison with Cu/ZnO and Cu/Zn(Al)O Catalysts

A series of Ce_1-xCu_xO_2- mixed oxides were synthesized using a co-precipitation method and tested as catalysts for the steam reforming of methanol. XRD patterns of the Ce_1-xCu_xO_2- mixed oxides indicated that Cu²⁺ ions were dissolved in CeO₂ lattices to form a solid solution by calcination at 773K when x < 0.2. A TPR (temperature-programmed reduction) investigation showed that the CeO₂ promotes the reduction of the Cu²⁺ species. Two reduction peaks were observed in the TPR profiles, which suggested that there were two different Cu²⁺ species in the Ce_1-xCu_xO_2- mixed oxides. The TPR peak at low temperature is attributed to the bulk Cu²⁺ species which dissolved into the CeO₂ lattices, and the peak at high temperature is due to the CuO species dispersed on the surface of CeO₂. The Ce_1-xCu_xO_2- mixed oxides were reduced to form Cu/CeO₂ catalysts for steam reforming of methanol, and were compared with Cu/ZnO, Cu/Zn(Al)O and Cu/AL₂O₃ catalysts. All the Cu-containing catalysts tested in this study showed high selectivities to CO₂ (over 97%) and H₂. A 3.8wt% Cu/CeO₂ catalyst showed a conversion of 53.9% for the steam reforming of methanol at 513K (W/F = 4.9 g h mol^-1), which was higher than that over Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%) and Cu/AL₂O₃ (11.2%) with the same Cu loading under the same reaction conditions. It is likely that the high activity of the Cu/CeO₂ catalysts may be due to the highly dispersed Cu metal particles and the strong metalsupport interaction between the Cu metal and CeO₂ support. Slow deactivations were observed over the 3.8wt% Cu/CeO₂ catalyst at 493 and 513K. The activity of the deactivated catalysts can be regenerated by calcination in air at 773K followed by reduction in H₂ at 673K, which indicated that a carbonaceous deposit on the catalyst surface caused the catalyst deactivation. Using the TPO (temperature-programmed oxidation) method, the amounts of coke on the 3.8wt% Cu/CeO₂ catalyst were 0.8wt% at 493K and 1.7wt% at 513K after 24h on stream.     

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.     

The Effect of SO2 and H2O on the Activity of Pd/CeO2 and Pd/Zr–CeO2 Diesel Oxidation Catalysts

Pd/CeO₂ and Pd/Zr-CeO₂ diesel oxidation catalysts were treated with sulphur, sulphur–water and water. According to the BET, TEM-EDS, XPS, ICP-OES analyses and catalytic activity tests, both studied catalysts were deactivated by sulphur due to formation of sulphates. Water treatment was found to have a promoting effect on the oxidation of CO and C₃H₆.     

Synergistic electronic and geometric effects of Au/CeO2 catalyst for oxidative esterification of methacrolein

Methyl methacrylate (MMA) is an important monomer for the production of acrylic plastics and polymer dispersions. In this study, we demonstrate a strategy to synergize the electronic and geometric effects of the Au/CeO₂ catalyst, achieving three orders of magnitude improvement in reaction rate for the oxidative esterification of methacrolein to MMA. The electronic properties of Au in terms of the valence electron population of 5d states are decoupled from the redox ability and acid–base properties of catalyst support as the main cause for the significant increase in the specific activity of the active site. Moreover, the Au edge site is identified as the main active site for this reaction, whose number reaches the maximum for the 1.6 nm-sized Au particles. Hence, the synergism between the electronic (lower valence electron population of 5d states) and geometric (more edge active sites) promotions of the Au-10/CeO₂ catalyst contributes to the highest catalytic activity.     

Influence of pretreatment atmospheres on the activity of Au/CeO2 catalyst for low-temperature CO oxidation

Au/CeO₂ catalyst was prepared via a deposition–precipitation method and further pretreated in different atmospheres prior to CO oxidation. A reductive atmosphere pretreatment slightly improved the low-temperature activity of Au/CeO₂. The greatest activity was achieved when Au/CeO₂ catalyst was subjected to an oxidative atmosphere followed by a reductive atmosphere pretreatment. Analysis using XRD, TEM and nitrogen sorption measurement showed that pretreatment retains the crystal phase, morphology and pore structure of CeO_2. However, there was a change in the interaction between gold species and CeO₂ support, as revealed by H₂-TPR measurement.