Relationship between structure and CO oxidation activity of ceria-supported gold catalysts

Gold catalysts supported on cerium oxide were prepared by solvated metal atom dispersion (SMAD), by deposition-precipitation (DP), and by coprecipitation (CP) methods and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic activity was tested in the CO oxidation reaction. The structural and surface analyses evidenced the presence of a modified ceria phase in the case of the DP sample and the presence of pure ceria and gold metal crystallites in the case of the SMAD and CP samples. The DP sample, after a mild treatment in air at 393 K, exhibited only ionic gold, and it was very active below 273 K. By comparing the activities of the different catalysts, it is suggested that the presence of small gold particles, as obtained by the SMAD technique, is not the main requisite for the achievement of the highest CO conversion. The strong interaction between ionic gold and ceria, by enhancing the ceria surface oxygen reducibility, may determine the particularly high activity.     

Temperature-programmed reduction study on carbon-supported platinum-gold alloy catalysts

Alloy catalysts of Pt-Au/C with different Pt/Au ratios were prepared by the precipitation-deposition of metal chlorides and reduced by H(2) at 470 K. The surface composition of alloy crystallites deposited on the prepared catalysts was characterized by a technique of temperature-programmed reduction (TPR). In the characterization, O(2) was chemisorbed on the reduced catalysts and the chemisorbed O(2) was reduced by TPR. A low-temperature routine (LT) in the temperature range between 120 and 430 K was used for the TPR characterization. Monometallic catalysts of Au/C and Pt/C showed a reduction peak in the LT-TPR at reduction temperature (T(r))=145 and 240 K, respectively. T(r) from alloyed catalysts fell in the range and increased monotonously with their Pt/Au ratios. Interior Pt atoms in deposited alloy particles tended to segregate toward their surface during oxidation treatment at elevated temperatures.     

Fe-modified HZSM-5 catalysts for ethanol conversion into light olefins

A series of Fe/HZSM-5 catalysts with different iron loadings were prepared by impregnation method.Characterization was performed by N2 adsorption-desorption,X-ray diffraction(XRD),NH3 temperature-programmed desorption(NH3-TPD),temperature-programmed reduction (TPR),temperature-programmed oxidation(TPO)and thermogravimetry(TG)analysis.Iron content in the synthesized samples varied from 1.1 wt%to 20 wt%.The obtained samples have been used for ethanol conversion into light olefins.It was found that the amount of strong acidity at 300 -5 50-C on Fe-modified samples was decreased,going with another new acid site appearance at 550- 600-C and that Fe/HZSM-5 catalysts were highly selective towards light olefins,especially the 9FZ sample.In addition,Fe-modified catalysts suppressed the conversion of ethanol to aromatics and paraffins and enhanced their anti-carbon deposit ability.     

CeO_2-Al_2O_3负载金催化剂用于水煤气变换反应的催化活性

采用浸渍法和沉积-沉淀法制备了CeO2-Al2O3复合氧化物,比较了复合氧化物负载纳米金催化剂对水煤气变换反应的催化活性。通过N2物理吸附、XRD、TEM、H2-TPR等表征手段对复合氧化物及其负载金催化剂的物相和结构进行分析,发现复合氧化物的制备方法及其焙烧温度对其比表面积、孔结构及水煤气变换反应活性有明显的影响。与沉积-沉淀法相比,浸渍法制备的CeO2-Al2O3复合氧化物具有较大的CeO2晶粒尺寸,经500℃焙烧后再负载金,所得催化剂具有更高的活性,250℃时CO转化率可达78.1%。     

助剂CeO2对Co/Al2O3催化剂上F-T合成反应性能的影响

 在用于F－T合成的Co／Al2O3催化剂中加入少量助剂,能够提高CO转化率和C5＋烃选择性．主要考察了助剂CeO2添加量和催化剂焙烧温度等因素对F－T合成反应的影响,并通过程序升温还原、程序升温氧化及X射线衍射等手段对催化剂进行了表征．结果表明,在Co／Al2O3催化剂中加入少量CeO2（n（Ce）／n（Co）＝0．1～0．14）,能够有效提高催化剂的催化活性和C5＋烃选择性;焙烧温度则以相反的趋势控制F－T反应活性和链增长几率;助剂的加入降低了催化剂的起始还原温度,改善了催化剂的还原性能．但是,催化剂的积碳量有所增加,经10h反应后,催化剂上存在两种类型的积碳．     

Ba deposition and oxidation on theta-Al2O3/NiAl(100) ultrathin films. Part I: Anaerobic deposition conditions

Room-temperature Ba deposition on an oxygen-terminated theta-Al(2)O(3)/NiAl(100) ultrathin film substrate under ultrahigh vacuum (UHV) conditions is studied using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) techniques. In addition, Ba oxidation by the ions of the alumina substrate at 300 K < T < 1200 K in the absence of a gas-phase oxidizing agent is investigated. Our results indicate that at room temperature Ba grows in a layer-by-layer fashion for the first two layers, and Ba is partially oxidized. Annealing at T < 700 K results in further oxidation of the Ba species, whereas annealing at higher temperatures leads to loss of Ba from the surface via desorption and subsurface diffusion.     

Characterization of CuO supported on tetragonal ZrO2 catalysts for N2O decomposition to N2

A series of tetragonal zirconia-supported CuO oxide catalysts with various CuO loadings were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), ultraviolet and visible diffuse reflectance spectroscopy (UV/vis-DRS), and temperature-programmed reduction (TPR) measurements. The results indicate that the dispersion capacity of copper oxide on this support is approximately 8.6 Cu(2+) ions/nm(2) ZrO(2). The state of the resulting supported copper species depends on the CuO loading. At CuO loadings below the dispersion capacity, only highly dispersed copper ion species are present on the surface of t-ZrO(2). In particular, isolated Cu ions are the predominant species at low loadings. In contrast, pair Cu ions become the most abundant species at loadings near the dispersion capacity. It has been proposed that these dispersed CuO (isolated and paired Cu ions) have a symmetric 5-fold-oxygen-coordination symmetry (C(3)(v) symmetry) and can be described as distorted octahedra with a missing corner or a trigonal bipyramids. Finally, at CuO loadings above the dispersion capacity the formation of crystalline CuO is observed. TPR results reveal that the dispersed Cu ion species have a different reducibility from CuO crystallites, presumably due to strong interactions between these species and the t-ZrO(2) support. The catalytic activity of these CuO/t-ZrO(2) catalysts for the decomposition of N(2)O can also be directly correlated to CuO dispersion, with paired Cu ions being the most active species for this reaction.     

Characterization and catalytic functionalities of copper oxide catalysts supported on zirconia

A series of zirconia supported copper oxide catalysts with varying copper loadings (1.2-19.1 wt %) were prepared by impregnation method. The catalysts were characterized by X-ray diffraction, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and temperature-programmed desorption of CO2. Copper dispersion and metal area were determined by N2O decomposition method. X-ray diffraction patterns indicate the presence of crystalline CuO phase beyond 2.7 wt % of Cu on zirconia. UV-visible diffuse reflectance spectra suggest the presence of two types of copper species on the ZrO2 support. XPS peaks intensity ratio of Cu 2p3/2 and Zr 3d5/2 was compared with Cu dispersion calculated from N2O decomposition. TPR patterns reveal the presence of highly dispersed copper oxide at lower temperatures and bulk CuO at higher temperatures. The basicity of the catalysts was found to increase with Cu loading, and the activity of the catalysts was also found to increase with the increase in Cu loading up to 2.7 wt % Cu loading. The catalytic properties were evaluated for the dehydrogenation of cyclohexanol to cyclohexanone and were related to surface properties of the copper species supported on zirconia.     

焙烧温度对含Mg助剂的铁基催化剂F-T合成反应性能的影响

采用连续共沉淀和喷雾干燥技术相结合的方法制备了Mg助剂的Fe/Cu/K/SiO2催化剂,采用N2物理吸附、XRD、MES 和H2-TPR等表征手段,考察了焙烧温度对催化剂比表面积、体相结构和还原性能的影响。结果表明,随着焙烧温度的升高,催化剂的比表面积降低,平均孔径增大,体相中α-Fe2O3晶粒逐渐增大,催化剂变的越来越难还原,其结构更加稳定。在H2/CO （摩尔比）= 2.2、250 ℃、2.0 MPa和2 000 h－1于固定床反应器考察了焙烧温度对该催化剂F-T合成反应性能的影响,结果表明,随着焙烧温度的升高,催化剂的F-T合成反应活性降低,在运行过程中反应活性逐渐增加直至达到平稳,但达到平稳所需的诱导期越来越长;提高焙烧温度使烃产物分布向重质烃方向转移,有利于降低CH4的选择性,促进重质烃的生成。     

Characterization of surface composition of platinum and ruthenium nanoalloys dispersed on active carbon

Supported samples of 8 wt % monometallic Pt/C and Ru/C, as well as 12 wt % bimetallic Pt50Ru50/C, were prepared by the method of incipient wetness impregnation. Impregnated samples were subsequently reduced by hydrogen and then oxidized in air at different To temperatures. TEM and XRD examinations indicated that metal crystallites were finely dispersed with a diameter of dM < or = 3 nm on the reduced samples. Reductive behavior of the oxidized samples by hydrogen was pursued with the technique of temperature programmed reduction (TPR). The temperature of the reduction peaks (Tr) noticed in the TPR profiles varied with the metal composition of catalysts and To temperature of oxidation. At To = 300 K, oxidation was confined to the surface layer of metallic crystallites. As a result, Pts O (with a peak at Tr = 230 K) or PtsO2 (Tr = 250 K) was formed on monometallic Pt/C while RusO2 (Tr approximately 380 K) was formed on Ru/C. A reductive peak with Tr = 250 K was found from the bimetallic sample from Pt50Ru50/C oxidized at To = 300 K. The reductive peak suggests bimetallic crystallites were dispersed with cherry type structure, with Pt exposed at the surface and Ru in the core. On increasing the To temperature of oxidation treatment to 370 K and higher, Tr peaks between 270 and 350 K were gradually noticed on the oxidized bimetallic sample. Peaks in this Tr region are assigned to reduction of the oxidized alloy surface (AsO). Evidently, a segregation of Ru to the surface of the bimetallic crystallites is indicated upon oxidation at To > 380 K.     

Effects of the Different Supports on the Activity and Selectivity of Iron-Cobalt Bimetallic Catalyst for Fischer-Tropsch Synthesis

Silica, alumina, and activated carbon supported iron-cobalt catalysts were prepared by incipient wetness impregnation. These catalysts have been characterized by BET, X-ray diffraction (XRD), and temperature-programmed reduction (TPR). Activity and selectivity of iron-cobalt supported on different carriers for CO hydrogenation were studied under the conditions of 1.5 MPa, 493 K, 630 h-1, and H2/CO ratio of 1.6. The results indicate that the activity, C4 olefin/(C4 olefin C4 paraffin) ratio, and C5 olefin/(C5 olefin C5 paraffin) decrease in the order of Fe-Co/SiO2, Fe-Co/AC1, Fe-Co/Al2O3 and Fe-Co/AC2. The activity of Fe-Co/SiO2 reached a maximum. The results of TPR show that the Fe-Co/SiO2 catalyst is to some extent different. XRD patterns show that the Fe-Co/SiO2 catalyst differs significantly from the others; it has two diffraction peaks. The active spinel phase is correlated with the supports.     

Nanostructured Cu(x)Ce1-xO2-y mixed oxide catalysts: characterization and WGS activity tests

Cu(x)Ce(1-x)O(2-y) mixed oxide catalysts were prepared by different preparation procedures: co-precipitation, the sol-gel peroxide route, and the sol-gel citric acid-assisted route. The resulting solids were investigated by means of XRD, BET, H(2) and CO temperature-programmed reduction (TPR), oxidation (TPO) and desorption (TPD) analyses, and N(2)O pulse selective reaction. It was confirmed that H(2) (CO) consumed for complete reduction of well-dispersed and bulk-like CuO phases to Cu(0), reduction of surface ceria and H(2) (CO) adsorption on the catalyst surface contribute to the total H(2) (CO) consumption. Among catalysts examined, the Cu(0.15)Ce(0.85)O(2-y) mixed oxide sample prepared by means of co-precipitation method exhibits the highest activity and stability for water-gas shift (WGS) pulse reaction in the range of employed operating conditions. WGS activity of copper-ceria mixed oxide catalysts is determined by the extent of surface ceria reduction and dispersion of copper species.     

Reduction of copper in porous matrixes. Stepwise and autocatalytic reduction routes

The reduction of Cu(II) oxide species in siliceous matrixes of different porosity (MFI, FAU, MCM-48) and in alumosilicate MFI was studied by temperature-programmed reduction in hydrogen (TPR), by X-ray absorption fine structure (after stationary hydrogen treatments), and by transmission electron microscopy. It was found that the reduction may proceed in one or in two reduction steps. The two-step scheme known for zeolites was observed also for Cu(II) in siliceous microporous matrixes, with similar temperature of Cu(II) reduction onset as for the alumosilicate MFI. Therefore, the two-step scheme cannot be explained by the stabilization of Cu ions by intra-zeolite electrical fields. CuOx clusters in MCM-48 were reduced in a one-step scheme (similar to bulk CuO) at high Cu content (6 wt %) but in a two-step scheme at low Cu content (1 wt %). The two reduction steps observed with most samples cannot be identified with the transitions of all Cu(II) to Cu(I) and of Cu(I) to Cu(0). Instead, Cu(0) nuclei were observed already at low reduction temperatures and were found to coexist with Cu ions over temperature ranges of different extension. This coexistence range was narrow in materials that favor aggregation of the Cu nuclei into particles: Cu-MCM-48 of low Cu content and Cu-ZSM-5. In the latter, metal segregation from the pore system was found to be accompanied by an autocatalytic initiation of the second reduction step. In the siliceous microporous matrixes, the Cu(0) nuclei were observed to coexist with Cu ions over wide temperature ranges (100 K for MFI) at temperatures far above that of Cu reduction in the bulk oxide. These observations suggest that oligomeric Cu metal nuclei which may have been formed, e.g., at the intersections of the MFI channel system, may be unable to activate hydrogen, which would be required for rapid reduction of the coexisting Cu ions.     

Agglomeration, sputtering, and carbon monoxide adsorption behavior for Au/Al(2)O(3) prepared by Au(n)(+) deposition on Al(2)O(3)/NiAl(110)

Size-selected gold clusters, Au(n)(+) (n = 1, 3, 4), were deposited on an ordered Al(2)O(3) film grown on NiAl(110), and changes in morphology and electronic properties with deposition/annealing temperature and cluster size were investigated by X-ray photoelectron spectroscopy (XPS) and ion-scattering spectroscopy (ISS). Extensive agglomeration was observed by ISS for annealing temperatures above 300 K, accompanied by large shifts in the Au XPS binding energy. Agglomeration is more extensive in room-temperature deposition, compared to samples prepared by low-temperature deposition, then annealed to room temperature. Agglomeration is also observed to be dependent on deposited cluster size. CO adsorption was studied by ISS and temperature-programmed desorption, and we looked for CO oxidation under conditions where substantial activity is seen for Au(n)/TiO(2). No activity was observed for Au(n)/Al(2)O(3). The differences between the two systems are interpreted in terms of the nature of the metal-support interactions.     

Evidence for a compensation effect during the temperature-programmed reduction of copper oxide in hydrogen

The reduction of copper oxide derived from basic Cu-carbonate in hydrogen has been studied under temperature-programmed conditions (TPR) and the TPR patterns were analyzed by means of Arrhenius plots at constant conversion (Friedman plots). These plots indicate that the reduction process cannot be described on the basis of constant kinetic parameters and reveal the presence of isokinetic temperatures. These suggest the presence of a compensation effect requiring a modification of the rate equation.     

Dispersion and reduction of copper oxide supported on WO3-modified Ce(0.5)Zr(0.5)O2 solid solution

XRD (X-ray diffraction), BET (Brunauer-Emmett-Teller), LRS (laser Raman spectra), XPS (X-ray photoelectron spectroscopy), and TPR (temperature-programmed reduction) are used to investigate the surface properties of CuO/WO3/Ce(0.5)Zr(0.5)O2 samples. The results indicate that (1) tungsten oxide can be highly dispersed on Ce(0.5)Zr(0.5)O2 (denoted as CZ hereafter) solid solution, with a dispersion capacity of about 0.8 mmol WO(3)/(100 m2 CZ), and comparatively, the supported tungsten oxide species are preferentially interacted with ceria component on the surface of CZ; (2) for CuO/WO3/CZ samples with a half-monolayer WO3 loading, i.e., xCu-0.4W-CZ series, the surface of CZ is only partially covered by the preloaded WO3) and the supported copper oxide species are dispersed on the remaining surface vacant sites on CZ as well as on top of the preloaded tungsten oxide, while for the samples preloaded with a full-monolayer WO3, i.e., xCu-0.8W-CZ series, only dispersed on the top of the preloaded tungsten oxide monolayer; (3) the effect of the loading amount of WO3 on the reduction property of Cu2+ ions in a series of CuO/WO3/CZ samples has been observed and tentatively attributed to the formation of WO3 monolayer on CZ and the different coordination environments of the dispersed Cu2+ ions are discussed on the basis of the consideration of the incorporation model proposed previously (Chen, Y.; Zhang, L. Catal. Lett. 1992, 12, 51).     

Cu/SiO2催化剂结构对乙酸甲酯加氢性能的影响

采用尿素水解法制备了Cu/SiO₂催化剂, 探究其用于乙酸甲酯(MA)加氢制取乙醇的催化性能, 并通过N₂物理吸附、X射线衍射(XRD)、程序升温还原(TPR)、透射电子显微镜(TEM)和X射线光电子能谱(XPS)等表征方法分析了催化剂的物理化学特性, 探究了铜负载量和还原温度等对催化剂结构的影响, 以及与催化活性之间的关系. 发现在铜负载量分别为10%、20%和30% (质量分数, w)的催化剂中, 铜负载量为20%的催化剂因具有较多且分散均匀的活性组分而表现出最佳的加氢效果. 接着在铜负载量为20%的催化剂上研究了还原温度(270, 350, 450 ℃)对催化性能的影响, 发现在350 ℃下还原的催化剂活性最高, 在最佳的反应条件下, 乙酸甲酯转化率达到97.8%, 乙醇选择性达到64.9% (理论最大值为66.6%), 主要归属于它具有较高的铜物种分散度, 最合适的Cu⁰/(Cu⁰+Cu⁺)摩尔比例, 同时实现了解离氢气和活化乙酸甲酯的功能.     

Au/CeO2-TiO2表面物种在还原和CO氧化过程中的变化

采用溶胶-凝胶法制备了CeO2-TiO2载体, 再用沉积沉淀法制备了Au/CeO2-TiO2催化剂. 利用原位漫反射红外(FT-IR), 程序升温还原(TPR)、X射线衍射(XRD)、N2吸附-脱附方法考察了催化剂的结构和表面性能. 结果表明, CeO2的存在有效抑制TiO2晶粒的长大, 增加TiO2的比表面积和晶格应变, 从而增强了Au和载体中TiO2的相互作用, 使得催化剂表面的氧化能力显著增强. 结合原位CO吸附的FT-IR结果表明, 不同温度的还原预处理能有效改变催化剂表面氧物种的组成, 并对不同氧物种在CO低温氧化过程中的作用进行了分析.     

Rh-promoted methanol decomposition on cerium oxide thin films

Methanol adsorption and reaction have been studied on Rh-deposited cerium oxide thin films under UHV conditions using temperature-programmed desorption and synchrotron soft X-ray photoelectron spectroscopy. The methanol behavior was examined as a function of the Ce oxidation state, methanol exposure, and Rh particle size and coverage. When Rh nanoparticles were deposited on the ceria films, methanol decomposed on Rh to CO and H below 200 K. H atoms recombined and desorbed between 200 and 300 K. CO evolved from Rh deposited on fully oxidized ceria between 400 and 500 K. However, on reduced ceria films, the CO on Rh further decomposed to atomic C. Methanol adsorbed on the ceria films deprotonated to form methoxy as the only intermediate on the surface. This methoxy decomposed and desorbed as CO and H2 at higher temperatures regardless of the ceria oxidation state. Compared with the methanol reaction on Rh-free ceria thin films, formaldehyde formation from methoxy was completely suppressed after Rh deposition. Our results indicate that Rh can promote the decomposition of methoxy adsorbed on the ceria and that decomposition of methoxy intermediates occurred at the metal/oxide interfaces. On the other hand, the reduced ceria can promote total methanol decomposition on Rh.     

Dispersion state of CuO on CeO2

XRD and XPS are used to study the dispersion state of CuO on ceria surface. The dispersion capacity values of CuO measured by the two methods are consistent, which are of 1.20 mmol CuO/100 m² CeO₂. In addition, the results reveal that highly dispersed Cu²⁺ ions are formed at low CuO loadings and that increasing the CuO content to a value higher than its dispersion capacity produces crystalline CuO after the surface vacant sites on CeO₂ are filled. The atomic composition of the outermost layer of the CuO/CeO₂ samples has been probed by using static secondary ion mass spectroscopy (SSIMS), and the ratim of Cu/Ce are found to be 0.93 and 0.46 for the 1.22 and 0.61 mmol CuO/CeO₂ samples respectively. Temperature-programmed reduction (TPR) profile with two reduction peaks at 156 and 165°C suggests that the reduction of highly dispersed Cu²⁺ ions consists of two steps and is easier than that of CuO crystallites, in which the TPR profile has only one reduction peak at about 249°C. The above experimental results are in good agreement with the prediction of the incorporation model. Project supported by the National Natural Science Foundation of China.