Catalyst deposition methods

The approach of coating photocatalysts varies depending on the t)q es of substrates used. Due to the wide application of Ti02 in photocatalytic fuel cells, emphasis will be given to the deposition of Ti02 on electrically conductive substrates. 

It should be noted that a Ti02 slurry is usually made before spin coating or screen printing. Previous photocatal5ftic fuel cell studies showed that Ti02 slurry can be easily prepared by using 

Although there is a huge variety of choice for the anode materials in the photocatal5hic fuel cell, the candidates for cathodes are very limited. So far, the most effective and widely used cathode materials are Pt-based electro-catalysts. Much research has directly used Pt wire or Pt foil as the cathode for proton reduction to produce hydrogen under anaerobic conditions. The involved reaction can be expressed as 2H -f 2e H2. 

However, due to the fact that the main focus in photocatal)Tic fuel cell research is the development of the photoanode, most of the current work uses the commercially available cathode 

Compared with proton reduction, oxygen reduction reactions (ORR) are preferred in the photocatalytic fuel cell to enhance the fuel cell performance. This is due to the higher redox potential of the oxygen reduction reaction than that of the proton reduction. In the aerobic environment, the oxygen can be reduced to water by reacting with protons and electrons. The involved reaction is expressed as O2 -f 2 H -f e — H2O. 

---

Porous metal membranes are commercially available in stainless steel and some other alloys (e.g.. Inconel, Hastelloy) and they are characterized by a macroporous structure. On the other hand, porous ceramic membranes can be found commercially in various oxides and combination of oxides (e.g., Al203,li02,Zr02, Si02) and pore size families in the mesopore and macropore ranges (e.g., from 1 nm to several microns). Most of the literature studies on three-phase catalytic membrane reactors have been carried out by developing catalytic ceramic membranes. The deposition techniques for the preparation of catalytic ceramic membranes involve methods widely used for the preparation of traditional supported catalysts (Pinna, 1998), and methods specifically developed for the preparation of structured catalysts (Cybulski and Moulijn, 2006). Other methods to introduce a catalytic species on a porous support include the chemical vapour deposition and physical vapour deposition (Daub et al, 2001). The catalyst deposition method has a strong influence on the catalytic membrane reactor performance. 

By dropping au aromatic acid either alone or mixed with an aliphatic acid into a tube containing a thoria catalyst deposited on pumice and heated to 400-450°. This method is generally employed for the preparation of mixed aromatic - aliphatic ketones. Excess of the aUphatic acid is usually present since this leads to by-products which are easily separated and also tends to increase the yield of the desired ketone at the expense of the symmetrical ketone of the aromatic acid. Thus —... 

VOx supported on TiOi showed good catalytic activity in the selective oxidation of H2S to ammonium thiosulfate and elemental sulfur. V0x/Ti02 catalysts prepared by the precipitation-deposition method can achieve higher vanadium dispersions, and higher H2S conversions compared to those prepared by the impregnation method. 

Direct Chemical or Electrochemical Deposition of the Disperse Catalyst This method of direct deposition from a solution of its salt on a suitable conducting substrate is simpler and more practical than the preparation of electrodes from the hnished powders. It has the merit of being able to provide better contact between the catalyst and substrate, and multicomponent metal catalysts can be deposited from a solution containing a mixture of salts of several metals. 

Direct metal deposition from metallic sources has been extensively used for model catalyst deposition for high-throughput and combinatorial studies. However, these methods are also increasingly used to deposit practical electrocatalyst materials. The best known approach is the one developed by 3M researchers have used physical vapor deposition to deposit Pt and Ft alloys onto nanostructured (NS) films composed of perylene red whiskers. The approach has been recently been reviewed by Debe. ... 

Although the sputter deposition technique can provide a cheap and directly controlled deposition method, the performance of PEM fuel cells with sputtered CLs is still inferior to that of conventional ink-based fuel cells. In addition, other issues arise related to the physical properties of sputtered catalyst layers, such as low lateral electrical conductivity of the thin metallic films [96,108]. Furthermore, the smaller particle size of sputter-deposited Ft can hinder water transport because of the high resistance to water transport in a thick, dense, sputtered Ft layer [108]. Currently, the sputter deposition method is not considered an economically viable alternative for large-scale electrode fabrication [82] and further research is underway to improve methods. 

Supported noble metal catalysts (Pt, Pd, Ag, Rh, Ni, etc.) are an important class of catalysts. Depositing noble metals on high-area oxide supports (alumina, silica, zeolites) disperses the metal over the surface so that nearly every metal atom is on the surface. A critical property of supported catalysts is that they have high dispersion (fraction of atoms on the surface), and this is a strong function of support, method of preparation, and treatment conditions. Since noble metals are very expensive, this reduces the cost of catalyst. It is fairly common to have situations where the noble metals in a catalyst cost more than 100,000 in a typical reactor. Fortunately, these metals can usually be recovered and recycled when the catalyst has become deactivated and needs to be replaced. 

A GCE coated with a film of Prussian Blue (Fe4[Fe(CN)6]3) mimics HRP as catalyst for selective electrochemical reduction of H2O2 in the presence of O2. A careful deposition method has to be followed to prevent leakage of the oxidized form of the pigment from the coating into the solution. Amperometric determinations in phosphate buffer at pH 6.0, with an apphed potential of —50 mV vs. SCSE, shows Unearity in the 0.1 to 100 p,M range. ... 

Hi) Chemical Vapor Deposition Method All commercial production of MWCNTs is based on catalytic CVD processes. The CVD processes are basically of two types the catalyst is either deposited on a substrate or the catalyst precursor is continuously fed with the gas stream. 

The main characteristics of the various titanium oxide catalysts used in this chapter are summarized in Table 1. Titanium oxide thin film photocatalysts were prepared using an ionized cluster beam (ICB) deposition method [13-16]. In ICB deposition method, the titanium metal target was heated to 2200 K in a crucible and Ti vapor was introduced into the high vacuum chamber to produce Ti clusters. These clusters then reacted with O2 in die chamber and stoichiometric titanium oxide clusters were formed. Tlie ionized titanium oxide clusters formed by electron beam irradiation were accelerated by a high electric field and bombarded onto the glass substrate to form titanium oxide thin films. 

The direct electrochemical deposition methods for the preparation of electrocatalysts allow to localize the catalyst particles on the top surface of the carbon support, as close as possible to the solid polymer electrolyte and does not need heat (oxidative and/or reducing) treatment, as most of the chemical methods do, in order to clean the catalytic particles from surfactant contamination [27,28], This will prevent catalyst sintering due to the agglomeration of nanoparticles under thermal treatment. 

The hydrides of the heavier congeners of the Group 14 to 16 elements have weak E-H bonds and they can be decomposed under mild conditions to yield the pure element or a low-oxidation-state hydride (in many cases of ill-defined chemical composition and structure). This tendency, which also applies to the E-C bonds, underlies the usefulness of hydrides in many gas and vapor phase deposition methods.3 There is still, however, a need for catalysts, particularly to control the specificity of dehydrocoupling for example, the ability to make rings of a particular size or isomeric composition, or the ability to avoid cyclic products altogether. In addition, it is desirable to control homo- vs hetero-dehydrocoupling selectivity, something difficult to do by noncatalytic methods. 

Our experiment has demonstrated the possibility of the carbon nanotubes synthesis by low temperature (substrate temperature was 500°C) vapor deposition method from the ethanol vapor. Field emission samples were produced with different catalyst distribution on the substrate surface. 

Au/ZrC>2 catalysts are less active than Au/TiC>2 catalysts, whatever method of preparation is used deposition of colloidal gold,83,91 DP12 or laser vaporisation.70 Activity depends on the method used (Table 6.12), and appears to be due only to the presence of Au°. The reason for the difference between zirconia and titania is not understood Zr4+ is more difficult to reduce than Ti4+, so anion defects may be harder to form. The lattice structures also differ in monoclinic zirconia (baddleyite) the Zr4+ ion is unusually seven coordinate, and phase transitions into tetragonal and cubic structures occur at >1370 and >2570 K, respectively. However, the... 

There are two main steps in catalyst preparation The hrst consists of depositing the active component precursor, as a divided form, on the support and the seeond of transforming this precursor into the required active component which depending on the reaction to be catalyzed can be found in the oxide sullided or metallic state A large majority o( deposition methods involve aqueous solutions and the liquid solid intei face In some cases, deposition can be also performed trom the gas phase and involves the gas solid interlace... 

---