High-resolution scanning electron microscopy for the characterization of supported metal catalysts

A scanning electron microscope with a short-focal-length immersion lens and subnanometer resolution has been used to characterize several oxide-supported metal particle catalysts. Nanometer-sized metal particles in the Pt/TiO₂ and Pd/SiO₂ systems could be imaged with best clarity at the upper end of the operating voltage range (20–30 kV). However, visibility depended upon an adequate yield of secondary electrons relative to the support: small Pt particles on CeO₂ could not be located by secondary electron imaging. Best visibility of the surface topography of the support was obtained at lower accelerating voltages.     

High resolution Auger electron imaging of supported metal particles

High spatial resolution Auger electron spectra and images of supported metal particles have been obtained in a UHV scanning transmission electron microscope. An edge resolution < 3 nm has been achieved. The number of atoms in a small particle can be estimated from the integrated intensity of the Auger electrons. This method is very useful for detecting and measuring particles with sizes smaller than the incident probe size. Ag clusters containing less than 20 atoms have been detected when supported on a thin carbon film.     

Phosphorus promoted WS2/Al2O3 catalysts studied by transmission electron microscopy

A series of WS₂/Al₂O₃ catalysts containing a varied amount of phosphorus (0.0,2.5 and 6.0 wt% P₂O₅), sulfided at two different temperatures (873 and 1073 K), were studied by means of high-resolution transmission electron microscopy (HRTEM). The stacking of (002) layers in the WS₂-2H crystallites of the rag structure increases with the addition of phosphorus, whereas its length is kept almost constant. At high sulfidation temperatures phosphorus shows a stabilizing effect on the catalysts by retarding the stacking growth of WS₂ crystallites. Additionally, an aluminum phosphate crystalline phase is identified in the phosphorus promoted catalysts.     

Hydrocracking ofn-heptane on a sulfided NiMo-Y zeolite catalyst: effect of the sulfidation method

The activity and the selectivity of a bifunctional NiMo sulfides-Y zeolite catalyst forn-heptane hydrocracking depend very much on the sulfidation agent, namely hydrogen sulfide, or dimethyldisulfide in solution inn-heptane orn-decane. The presence of a hydrocarbon during the sulfidation lowers the acid activity of the catalyst because of coking of the zeolite acid sites, and also decreases the hydrogenating activity, due to a poorer sulfidation of the catalyst.     

Deformation behavior of nanocrystalline titania particles accessed by complementary in situ electron microscopy techniques

The mechanical behavior of nanostructured spherical submicrometer titania particles was studied by in situ uniaxial compression experiments in the scanning and transmission electron microscope (SEM and TEM). Mesoporous and amorphous titania particles were prepared by a wet chemical sol‐gel approach. To obtain nanocrystalline (nc) single‐phase anatase and rutile particles the amorphous particles were crystallized by high‐temperature annealing. For each sample the deformation behavior of at least 50 particles was investigated by in situ compression experiments in the SEM. In all cases an elastic – predominantly plastic deformation behavior accompanied by crack initiation at exceptionally high engineering strain values of several percent were observed. Crack propagation presumably along grain boundaries and a Weibull distributed fracture stress was shown for all nc particles. Complementary in situ TEM experiments and ex situ analysis of focused ion beam prepared particle cross‐sections were carried out to identify the underlying deformation mechanisms. Grain rotations and grain sliding are observed for nc anatase particles during in situ compression and are further identified to be linked to a densification of the mesoporous particle structure. Our dedicated preparation and quantitative in situ characterization methodology provides an excellent basis for a better understanding of the mechanical behavior of advanced ceramics.     

Transmission electron microscopy characterization of spark plasma sintered ZrB2 ceramic

Microstructures of ZrB₂ ceramics consolidated by hot-pressing and spark plasma sintering were investigated by transmission electron microscopy (TEM), combining energy dispersive X-ray spectroscopy (EDX). The microstructures of both ceramics were compared. Amount of impurities was lower for ZrB₂ consolidated by spark plasma sintering than for hot-pressed ZrB₂. In particular, oxygen impurity was not detected even at the grain-boundaries in ZrB₂ consolidated by spark plasma sintering. The cleaning effect generated on the powder surfaces during spark plasma sintering cycle was displayed. In addition, dislocations were present only in the spark plasma sintered ZrB₂ ceramic, as a result of localized high stresses.     

HRTEM observation of metal particles mobility on a Pt/γ-Al2O3 catalyst

High resolution transmission electron microscopy analysis of Pt/Al₂O₃ catalyst prepared by deposition of platinum from mesitylene solution shows that the smallest metal particles on edges and corners migrate under the influence of the electron beam. Sintering is also promoted.     

Surface restructuring of palladium particles induced by CO adsorption

In order to characterize chemisorption induced reconstruction the surface of supported palladium particles (model catalyst) has been investigated during CO adsorption by SSIMS (static secondary ion mass spectrometry). The SSIMS signal ratio _n Pd _n CO⁺/Pd _n ⁺ has been used to monitor the CO adsorption kinetics. The relative occupancy of linear and bridged CO sites has been determined by the value of PdCO⁺/( _n Pd _n CO⁺) and the variation of the Pd-Pd next neighbour distance by plotting Pd ₂ ⁺ /Pd⁺ during CO exposure. It has been shown that CO chemisorption induces a surface restructuring by increasing the Pd-Pd distance in the particle surface. This phenomenon has the features of a cooperative phenomenon. On the reconstructed particle one bonding state has been identified which appeared to be a precursor of the CO dissociation.     

Isothermal titration calorimetry, transmission electron microscopy, and field emission scanning electron microscopy of a main-chain viologen polymer containing bromide as counterions

The interaction of a main-chain viologen polymer containing bromide as counterions with water and aqueous potassium bromide over a broad range of concentrations was studied with isothermal titration calorimetry. The dilution process of this polymer was endothermic as opposed to flexible poly(sodium acrylate) and poly(sodium styrenesulfonate). This result may be related to the different mechanism of hydration of pyridinium and bromide groups in the main chain. It also exhibited aggregation phenomenon in both water and aqueous potassium bromide solutions as detected by transmission electron microscopy like other flexible and rigid-rod polyelectrolytes. As the polymer concentrations in aqueous solutions increase, the aggregated polymer exhibited more defined ordered structures than random structures observed at low polymer concentrations. Field emission scanning electron microscopy also revealed the effect of variation of concentration of aqueous potassium bromide on the morphology of the polymer matrix. At increasing concentrations of aqueous potassium bromide, the polymer structures became more ordered than those in low concentrations.     

Particle size studies of supported metal catalysts: a comparative study by X-ray diffraction,EXAFS and electron microscopy

Supported ruthenium and iridium metal catalysts are studied by X-ray diffraction (XRD), EXAFS analysis and transmission electron microscopy (TEM). Estimates of the mean particle size from these techniques range from 20 to 25 Å from XRD, 13 to 16 Å from EXAFS and 25 to 32 Å from TEM. The strengths and weaknesses of these instrumental methods are discussed, as is the intrinsic comparability of these techniques. From a combination of these methods, the average particle size is estimated to be of the order of 20–30 Å.     

生物质焦表面形貌SEM研究

采用扫描电镜对生物质焦的表面形貌进行研究。扫描电镜实验结果表明,生物质在制焦后,表面粗糙度提高,比表面积增大,孔隙更加发达。焦样表面形貌的特征主要是由生物质种类决定的,在扫描电镜下,稻壳焦样表面有锯齿状不规则的突起,但是开口在表面的孔很少;树叶焦样表面有不规则的孔;玉米杆焦样有如同蜂窝状的表面结构,表面孔十分丰富;棉花杆焦样有着絮状的表面结构,表面孔也较明显。热解温度对焦样表面形貌的影响不大。     

Transmission electron microscopy on Hf- and Ta-carbides sintered with TaSi2

The microstructure of hot pressed Hf- and Ta-carbides with 15 vol% of TaSi₂ was characterized by X-ray diffraction, scanning and transmission electron microscopy in order to investigate the densification mechanisms.The microstructure of the carbides was constituted by squared grains and subgrains were recognizable only by transmission electron microscopy: the inner part was constituted by the original MC grain and the outer area by a (M,Ta)C solid solution which grew epitaxially on it. The compositional misfit and the difference of the coefficients of thermal expansion between the two regions were accommodated by 45° grain boundaries and dislocations. At the triple junctions, Ta₅Si₃ and Ta_4.8Si₃C_0.3, with Hf impurities were detected. The grain boundaries were observed to be clean.The microstructure of the composites containing TaSi₂ was subsequently compared to composites sintered with addition of the same amount of MoSi₂.     

高分子材料微观形态结构的扫描电镜研究

利用扫描电镜对聚乙烯(PE)、丙烯腈-丁二烯-苯乙烯共聚物(ABS)等高分子材料填料的分散性及表面与断裂界面进行了研究。结果表明,扫描电镜能够详细观察研究高分子材料的表面结构、微观相分离等,便于表征高分子材料微观结构形态,是分析高分子材料微观结构形态的有效手段。     

Atomic force microscopy, scanning electron microscopy and electrochemical characterization of Al alloys, conversion coatings, and primers used for aircraft

Characterization of the metal–coating interface is crucial to the understanding and prediction of the performance of corrosion protective coatings. To date, such characterization has been incomplete and performed on a scale of measurement that gives little microscopic-scale information. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) have the ability to provide such information. We present in this work the use of such methods to examine the interface between steel and marine coatings and to image the surface of untreated aircraft aluminum alloys. We have also imaged these aircraft alloys after chromate/phosphate pretreatment. AFM and SEM have also been used to investigate the changes in surface morphology, which accompany changes in the samples due to exposure. Electrochemical noise methods, electrochemical impedance spectroscopy measurements, and Prohesion^TM testing were performed in parallel with the AFM/SEM measurements. The results, along with implications for aircraft coatings, are discussed.     

high-resolution electron microscopy of aSr-containing sialon polytypoid phase

A new type of Sr-containing sialon polytypoid phase with the structural formula SrSi_10-xAl_18+xN_32-xO_x (x≈1) has been found in the Sr–Si–Al–O–N system. The phase was characterised by X-ray powder diffraction (XRPD), and its structure was investigated by electron diffraction (ED) and high resolution electron microscopy (HREM). It is considerably disordered, but the average structure has a rhombohedral unit cell with a=5·335(5)≈·a_AlN and c=79·1(1)Å≈30·c_AlN. The Sr atoms are located in layers M–Sr–M, M=(Si/Al), at the origin of the unit cell with 12 X=(O,N) atoms around it, at distances of 3 Å, forming a cubo-octahedron. The X atoms that form a hexagon around the Sr atom in the ab plane are corner shared by M=(Si/Al) tetrahedra with opposite polarity in adjacent layers in which 2/3 of the tetrahedra are occupied. The M–Sr–M layers alternate with normally eight-layer-thick AlN type blocks, although the thickness of these blocks frequently varies. The structural model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AlN blocks, similar to that proposed to occur in Si–Al–O–N polytypoid phases. The model with one Sr layer and 10 M=(Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and experimental HREM images.     

The interaction of oxygen with alumina-supported palladium particles

Utilizing a combination of molecular beam techniques and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions we have studied the interaction of oxygen with an alumina-supported Pd model catalyst as well as the influence of the oxygen pretreatment on the kinetics of the CO oxidation reaction. The Pd particles were deposited by metal evaporation in UHV onto a well-ordered alumina film prepared on a NiAl(110) single crystal. The particle density, morphology and structure are determined by STM both immediately after preparation and after oxygen adsorption and CO oxidation. The oxygen sticking coefficient and uptake in the temperature regime between 100 and 500 K and the kinetics of the CO oxidation reaction are quantitatively probed by molecular beam techniques. It is found that starting at temperatures below 300 K the Pd particles rapidly incorporate large amounts of oxygen, finally reaching stoichiometries of PdO_>0.5. STM shows, that neither the overall particle shape nor the dispersion is affected by the oxygen and CO treatment. Only after saturation of the bulk oxygen reservoir are stable CO oxidation conditions obtained. In the low-temperature regime (<500 K), only the surface oxygen, but not the bulk and subsurface oxygen is susceptible to the CO oxidation. The activation energies for the Langmuir–Hinshelwood step of the CO oxidation reaction were determined both in the regime of high CO coverage and high surface oxygen coverage. A comparison shows that the values are consistent with previous Pd(111) single crystal results. Thus, we conclude that, at least for the particle size under consideration in this study (5.5 nm), the LH activation energies are neither affected by the reduced size nor by the oxygen pretreatment.     

Scanning tunneling microscopy characterization of ammonia synthesis catalysts

The possibility of studying the texture of catalysts at the atomic level by Scanning Tunneling Microscopy (STM) is demonstrated. It is shown that high resolution pictures can be obtained even for a passivated ammonia synthesis catalyst using a scanning tunneling microscope operating in air. The STM results reveal that the catalyst has a spongy pore structure with both hole and slit shaped pores. The catalyst surfaces are observed to contain fine quite regular ridge structures with a direction that tentatively is related to the (110) direction of iron. The presence of the ridge structures is apparently the result of the topotactic nature of the reduction process and a preference to expose (111) Fe facets.     

贵金属SiO2核壳纳米材料的研究进展

从单金属、双金属或多金属的SiO2核壳体系出发,论述了几种典型的贵金属金、银、铂、钯、铑等为核或壳与SiO2包覆的核壳纳米材料的制备方法,改进的性质和在催化、光学、生物等方面的应用,并对未来的发展方向进行了展望.     

Hexairidium carbonyl clusters in the micropores of faujasite zeolites: evidence from transmission electron microscopy

[Ir₆(CO)₁₆] was formed in the pores of zeolite NaY by adsorption of [Ir(CO)₂(acac)] followed by treatment in CO + H₂. [Ir₆(CO)₁₅]²⁻ in zeolite NaX was prepared similarly. Each sample was characterized by high‐resolution transmission electron microscopy. The images indicate the presence of the iridium clusters in the zeolite micropores, with almost no scattering centers indicating iridium outside these pores. The supported [Ir₆(CO)₁₆] and [Ir₆(CO)₁₅]²⁻, which have previously been characterized by infrared and extended X‐ray absorption fine structure spectroscopies, are among the most uniform and structurally best defined supported metal clusters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Determination of lamellae in segmented polyurethanes by electron microscopy

The morphology of segmented polyurethane was studied with an electron microscope. Polymers were obtained by the prepolymer method from poly(ethylene adipate), 4,4-diphenylmethane diisocyanate and 1,4-butanediol. The unstained sample showed spherulites with fibrillar structure. Attempts were made to enhance the contrast in order to study the fibrillar structure more thoroughly. The staining method used to study the structure of lamellae is described.