Particle support interaction

Eq. (1) one might expect stronger bonding of metal particles to the basal than to the edge plane. However, the above-cited experimental data point to the opposite fact, suggesting that the gain in the particle-support interaction is provided by the last term of Eq. (1) namely by a considerable decrease in the surface tension at the metal-carbon interface and by an increase in the contact area due to the lateral interaction of the particle with the steps at the support surface. 

This leads us to the concept called nanocatalysis, and specifically to nanofabricated model catalysts, as an approach to bridge the structure gap. In Fig. 4.4, some examples of planar model structures of increasing complexity are depicted, which fulfill these criteria. At the top, there is a simple array of catalyst particles on an inactive support. The inactive support can be replaced by an active support (second picture from the top), meaning a support that significantly affects the properties of the nanoparticles via particle-support interactions (a clear distinction between inactive and active is not easy or not even possible—there is always some influence of the support on the supported particle). In some cases, the size of the support particle has an influence on the overall catalytic activity. This is, for example, the case when there is a spillover or capture zone for reactants or intermediates, which move by diffusion from the catalyst nanoparticle to the support or vice versa. In order to study such effects, one may want to systematically vary the radius of the... 

The indirect effects related to the particle-support interactions include the wetting of the particle on the support, which influence both the shape and size of the particles that form, and the stress or strain that the support imparts on the electronic structure of the particle. 

It was concluded that gold particle-support interaction is required together with careful selection of the titania-silica support and control of the gold-particle size. The use of pH 7.0 in the deposition-precipitation method was also recommended, together with calcination at 573 K. Propene oxide has recently been obtained via a two-stage process involving dehydrogenation of propane to propene (selectivity of 57%), followed by a propene to propene oxide oxidation with a selectivity of 8%, and... 

DFT is a powerful method for determining reaction mechanisms over metal-oxide systems. We have chosen to review studies that focus on developing catalysts for the water-gas-shift reaction because this is a particularly active research area with numerous examples of DFT application to supported metal-oxide catalysis. The studies first considered herein assess the activity of unsupported gold and copper metal clusters, which can then be compared directly to studies over the analogous oxide-supported systems. The importance of considering particle-support interactions is emphasized, because the oxide support can often play an active role in catalytic mechanisms. 

Recent DFT studies have characterized the nature of active sites on metal-oxide catalysts for WGS, which emphasize the unique aspects of particle-support interactions in Cu/ceria," Au/ceria systems," and Pt/ceria " (among numerous others). The primary intent of this section is to exhibit the utility and limitations of DFT for investigating the many aspects of metal-oxide catalysis, rather than to serve as an exhaustive review of computational work on WGS catalysis in the literature. For a more detailed discussion of the subject, we refer the reader to the recent review of computational work on Au/ ceria that can be found in a perspective article by Zhang, Michaelides, and Jenkins." ... 

Liu, Rodriguez, and coworkers" " " applied DFT methods together with experimental studies to investigate how particle-support interactions affect the WGS activity of Au and Cu nanoparticles supported on reducible oxides, such... 

Background. The formalism presented in the previous section for predicting the stability of oxide surfaces in equilibrium with a multi-component gas phase is readily extended to systems that contain catalytic metal particles supported on oxide surfaces. Identifying stable particle-support constructions is indispensable for predicting the catalytic activity of the particle-support interface. This section will outline studies on reducible oxides (Ti02 and Ce02) that display unique particle-support interactions where the oxide support plays an active role in the catalytic mechanism. These examples demonstrate the ability of ab initio thermodynamics to determine the stability of metal clusters on oxide supports under realistic catalytic conditions. Such calculations can be used in concert with DFT reactivity studies... 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

The interactions between metals and supports in conventional supported metal catalysts have been the focus of extensive research [12,30]. The subject is complex, and much attention has been focused on so-called strong metal-support interactions, which may involve reactions of the support with the metal particles, for example, leading to the formation of fragments of an oxide (e.g., Ti02) that creep onto the metal and partially cover it [31]. Such species on a metal may inhibit catalysis by covering sites, but they may also improve catalytic performance, perhaps playing a promoter-like role. 

The literature of metal-support interactions includes httle about the possible chemical bonding of metal clusters or particles to supports. Supported molecular metal clusters with carbonyl ligands removed have afforded opportunities to understand the metal-support interface in some detail, and the results provide insights into the bonding of clusters to supports that appear to be generalizable beyond the small clusters to the larger particles of conventional supported metal catalysts [6]. 

Size reduction of metal particles results in several changes of the physico-chemical properties. The primary change is observed in the electronic properties of the metal particles which can be characterized by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively) as well as Auger-electron spectroscopy (AES) measurements. Furthermore, morphology of the metal nanoparticles is highly sensitive to the environment, such as ion-metal interaction (e.g. metal-support interaction)... 

The valence band structure of very small metal crystallites is expected to differ from that of an infinite crystal for a number of reasons (a) with a ratio of surface to bulk atoms approaching unity (ca. 2 nm diameter), the potential seen by the nearly free valence electrons will be very different from the periodic potential of an infinite crystal (b) surface states, if they exist, would be expected to dominate the electronic density of states (DOS) (c) the electronic DOS of very small metal crystallites on a support surface will be affected by the metal-support interactions. It is essential to determine at what crystallite size (or number of atoms per crystallite) the electronic density of sates begins to depart from that of the infinite crystal, as the material state of the catalyst particle can affect changes in the surface thermodynamics which may control the catalysis and electro-catalysis of heterogeneous reactions as well as the physical properties of the catalyst particle [26]. 

The presence of shielding compounds interferes with subsequent processes, as the formation of metal-support interactions is able to stabilize supported particles. Moreover, the shielding effect of the colloid protectors prevents the contact of metal particles with the reacting molecules, thus avoiding the use of unsupported colloidal particles as a catalytic system [11]. 

Concerning the Fischer-Tropsch synthesis, carbon nanomaterials have already been successfully employed as catalyst support media on a laboratory scale. The main attention in literature has been paid so far to subjects such as the comparison of functionalization techniques,9-11 the influence of promoters on the catalytic performance,1 12 and the investigations of metal particle size effects7,8 as well as of metal-support interactions.14,15 However, research was focused on one nanomaterial type only in each of these studies. Yu et al.16 compared the performance of two different kinds of nanofibers (herringbones and platelets) in the Fischer-Tropsch synthesis. A direct comparison between nanotubes and nanofibers as catalyst support media has not yet been an issue of discussion in Fischer-Tropsch investigations. In addition, a comparison with commercially used FT catalysts has up to now not been published. 

There is mounting evidence to suggest that water effects during FTS may also be linked to Co particle size, as determined by the support interaction [18,19], even though the turnover frequencies of CO over the surface Co metal atoms on... 

---