Herbert Hofmeister

Supported gold nanoparticles: in-depth catalyst characterization and application in hydrogenation and oxidation reactions

Silica, titania, zirconia and alumina supported gold particles of 1–6 nm size, prepared by various synthetic routes (sol–gel technique, deposition–precipitation, metal organic-chemical vapor deposition, impregnation, dip-coating) were employed in the selective hydrogenation of acrolein, crotonaldehyde and 1,3-butadiene and in the low-temperature oxidation of carbon monoxide. In-depth characterization of their structural and electronic properties by transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) was aimed at disclosing the nature of the active sites controlling the hydrogenation and oxidation reactions. The structural characteristics such as mean particle size, size distribution and dispersion depend both on the synthetic method employed and the nature of the support. For extremely small gold particles on titania and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonance of the metal and paramagnetic F-centers (trapped electrons in oxygen vacancies) of the support were observed. The marked structure-sensitivity observed for hydrogenation properties with decreasing particle size may be attributed to structural and electronic properties due to the quantum-size effect of sufficiently small gold particles. Furthermore, the adaptability of gold particles in coatings is demonstrated for a microchannel reactor.

The influence of real structure of gold catalysts in the partial hydrogenation of acrolein

The hydrogenation of acrolein on supported gold catalysts has been used as test reaction to study several aspects of structure sensitivity, i.e., the dependence of activity and selectivity on the size of gold particles. We focused our work on the influence of the geometrical configuration of the supported gold nanoparticles, namely the occurrence of multiply twinned particles (MTPs) and the degree of rounding. A higher number of MTPs resulted in a lowering of selectivity to the desired product, allyl alcohol, as well as a lowering of the turnover frequency. The higher amount frequency of a gold catalyst supported on TiO2 compared to ZrO2 is attributed to a higher degree of rounding of the former.

Synthesis of nanosized silver particles in ion-exchanged glass by electron beam irradiation

Ag particles of 4.2 nm mean diameter have been formed inside a glass matrix, doped with silver by ion exchange, by electron beam irradiation of the glass cut into thin slices by ultramicrotomy. By this treatment, a high concentration of particles which are homogeneously arranged throughout the glass and exhibit a narrow size distribution is achieved (volume fraction of particulate silver: 3.5×10−2). The interface stress reflecting the particle/matrix interaction is comparable to that of isolated Ag particles. This new route of synthesis will allow to generate materials with strong third order nonlinear susceptibility.

Gold Catalysts for the Partial Hydrogenation of Acrolein

The manufacture of unsaturated alcohols through selective hydrogenation of α,β-unsaturated aldehydes continues to be a topical issue. The objective of this study is to investigate the use of supported gold catalysts for the partial hydrogenation of acrolein to allyl alcohol. So far, in catalyst research or even chemical engineering gold has not attracted much attention as a catalyst mainly because of its chemical inertness. The intrinsic inertness of gold, however, can be influenced when the metal is applied with a high dispersity to a suitable support.

Small silver particles in glass surface layers produced by sodium-silver ion exchange — their concentration and size depth profile

Small spherical silver particles in a surface layer of commercial flat glass were produced by means of sodium-silver ion exchange. In each volume element of the layer there is a Gaussian distribution of the particle diameters. The mean diameter increases with penetration depth. Within one individual sample it can vary from 4.5 nm immediately at the glass surface up to more than 50 nm at the end of the layer. Due to a special preparation technique the results were gained by microspectrophotometric measurements as well as by investigations carried out with the transmission electron microscope and the electron-probe microanalyzer on one and the same sample always as function of the penetration depth.

Composition and lattice structure of fivefold twinned nanorods of silver

Multiply twinned silver particles of rod-like shape and nanometer dimensions prepared by inert-gas aggregation technique have been studied by high-resolution electron microscopy. These pentagonal nanorods exhibit aspect ratio between 1.8 and 6 with the length of their fivefold axis ranging from 22 to 132 nm. Digital image processing and evaluation was utilised to characterise composition and lattice structure of the nanorods. Measuring the spacings of lattice plane fringes of nanorod subunits in various orientations revealed no deviation from the face centred cubic lattice type. There was also no indication of extended lattice defects found. Instead, a certain extent of non-regular lattice distortions recognised in the surface regions of the nanorods apparently is an effective means to achieve sufficient space filling.

Formation of silver particles and periodic precipitate layers in silicate glass induced by thermally assisted hydrogen permeation

Nanoscale silver particles embedded in sodium silicate glass were produced by Na/Ag ion exchange and subsequent thermal treatment in a hydrogen atmosphere. Their structure and spatial distribution were studied by conventional and high-resolution electron microscopy (HREM). Two different mechanisms of particle formation could be identified: (i) reduction of ionic silver by hydrogen and formation of mostly defective particles (twinned) within a near-surface region; and (ii) formation of single-crystalline particles in the interior of the glass resulting from reduction by means of polyvalent iron ions. Electron microscopy investigation revealed the completion of periodic layers of silver particles in near-surface regions with high silver concentration induced by thermally assisted hydrogen permeation. The self-organized periodic layer formation may be explained in terms of Ostwalds supersaturation theory, assuming interdiffusion of two mobile species. Analysis of lattice plane spacings from HREM images of silver particles revealed the typical size-dependent lattice contraction. The extent of this, however, was found to be different for particles formed by hydrogen permeation and those formed by interaction with polyvalent iron ions. These differences reflect different influences of the surrounding glass matrix, probably originating from the conditions of particle formation (thermal history).

Identification of active sites and influence of real structure of gold catalysts in the selective hydrogenation of acrolein to allyl alcohol

We have successfully employed oxide supported gold catalysts in the gas-phase hydrogenation of acrolein, the α,β-unsaturated aldehyde being most difficult to hydrogenate at the carbonyl group. The relation of structural characteristics and surface state of these catalysts with respect to their activity and selectivity have been studied by surface analytical techniques and high-resolution transmission electron microscopy which was carried out to elucidate the real crystal structures of gold particles. The performance of gold catalysts formed by various preparation and pretreatment procedures was strongly dependent on the reduction time: Increasing the latter resulted in a high number of multiple twinned gold particles which decrease both the selectivity to the desired product, allyl alcohol, and the turnover frequency. Furthermore, it has been demonstrated for the first time that the addition of a second metal (indium) results in a selective decoration of the faces of nanosized gold particles leaving edges free. The active sites of supported gold catalysts, favoring the adsorption of C=O group of acrolein and subsequent reaction to allyl alcohol, have been identified as edges of gold nanoparticles. The zinc oxide supported bimetallic Au-In catalyst produced allyl alcohol with a selectivity of 63% which is comparable to the best hydrogenation catalysts for this reaction.

Strong visible photoluminescence from hollow silica nanoparticles

Starting with crystalline silicon nanoparticles, which were produced by CO2 laser pyrolysis of silane in a gas flow reactor, we have synthesized amorphous silica nanoparticles via oxidation. Upon excitation with UV light, the novel nanostructured material gives rise to an intense red photoluminescence (PL) which resembles that of some silicon nanostructures. Transmission electron microscopy studies and electron energy loss spectroscopy confirm that the nanoparticles are composed of amorphous silica and that the majority of them are hollow. The strong red PL is attributed to defects or molecular species located at the inner and outer surfaces of the hollow nanoparticles. Its similarity to the PL of nanostructured silicon seems fortuitous.

Shape and internal structure of silver nanoparticles embedded in glass

The structural characteristics of silver nanoparticles embedded in glass by various routes of fabrication were studied in detail using high-resolution electron microscopy to find out if they are influenced by interaction with the surrounding glass matrix. Besides the formation conditions, the strength of the interaction between metal and glass governs the size-dependent changes of lattice spacings in such nanoparticles. However, determination of these changes is not straightforward because of complicated particle configurations and the interference nature of the lattice imaging technique. Imaging of lattice plane fringes and careful diffractogram analysis allowed the exclusion of any kind of tetragonal lattice distortion or transformation to hexagonal lattice type that may be deduced at first sight. Instead, the formation of twin faults in these nanoparticles turned out to be the essential structural feature and the main source of confusion about the lattice structure observed. The variety of particle forms is comparable to particles supported on oxide carriers. It is composed of single-crystalline particles of nearly cuboctahedron shape, particles containing single twin faults, multiple twinned particles containing parallel twin lamellae, and multiple twinned particles composed of cyclic twinned segments arranged around axes of 5-fold symmetry. The more twin planes involved in the particle composition, the more complicated is the interpretation of lattice spacings and lattice fringe patterns due to superposition of several twin segments.