Catalysis gas-phase

Another example of gas-phase catalysis is the destruction of ozone (03) in the stratosphere, catalyzed by Cl atoms. Ultraviolet light in the upper atmosphere causes the dissociation of molecular oxygen, which maintains a significant concentration of ozone ... 

The main focus of the following considerations is on catalysis using inorganic materials. Similar considerations come into play for catalysis with molecular compounds as catalytic components of course, issues related to diffusion in porous systems are not applicable there as molecular catalysts, unless bound or attached to a solid material or contained in a polymeric entity, lack a porous system which could restrict mass transport to the active center. It is evident that the basic considerations for mass transport-related phenomena are also valid for liquid and liquid-gas-phase catalysis with inorganic materials. 

In the previous sections we have discussed two different applications of gas-phase catalysis. Although different in nature, both case studies had the common feature that (1) all employed educts and the obtained products were gaseous and (2) the change in volume of the gas phase resulting from the educt conversion is negligible. For a number of reactions, especially for reactions in the petrochemical industry both of the above-mentioned features cannot be neglected, we will discuss classic examples and present technical approaches to overcome the obstacles related to the chemical transformations. 

The HTE characteristics that apply for gas-phase reactions (i.e., measurement under nondiffusion-limited conditions, equal distribution of gas flows and temperature, avoidance of crosscontamination, etc.) also apply for catalytic reactions in the liquid-phase. In addition, in liquid phase reactions mass-transport phenomena of the reactants are a vital point, especially if one of the reactants is a gas. It is worth spending some time to reflect on the topic of mass transfer related to liquid-gas-phase reactions. As we discussed before, for gas-phase catalysis, a crucial point is the measurement of catalysts under conditions where mass transport is not limiting the reaction and yields true microkinetic data. As an additional factor for mass transport in liquid-gas-phase reactions, the rate of reaction gas saturation of the liquid can also determine the kinetics of the reaction [81], In order to avoid mass-transport limitations with regard to gas/liquid mass transport, the transfer rate of the gas into the liquid (saturation of the liquid with gas) must be higher than the consumption of the reactant gas by the reaction. Otherwise, it is not possible to obtain true kinetic data of the catalytic reaction, which allow a comparison of the different catalyst candidates on a microkinetic basis, as only the gas uptake of the liquid will govern the result of the experiment (see Figure 11.32a). In three-phase reactions (gas-liquid-solid), the transport of the reactants to the surface of the solid (and the transport from the resulting products from this surface) will also... 

Stocker, M. (2005) Gas phase catalysis by zeolites. Micropor. Mesopor. Mater., 82, 257-292. 

The electrooxidation of CO to C02 is, similar to its electroless counterpart in gas-phase catalysis, one of the most widely studied electrochemical reaction processes [131,152]. It is generally assumed that the electrooxidation of adsorbed CO proceeds primarily via a Langmuir-Hinshelwood type mechanism involving either adsorbed water molecules or, at higher electrode potentials, adsorbed surface hydroxides (see Figure 6.25) according to... 

The Richard Process - A Claus Alternative The Claus reaction process is by far the most common for recovering elemental sulfur from hydrogen sulfide. This heterogeneous gas phase catalysis over alumina of the redox reaction between H2S and SO2,... 

Challenges Connected to Catalyst Screening in Gas-phase Catalysis... 

Case Studies of Selected Examples in Gas-phase Catalysis in Stage II Screening I 35... 

Mattson, B. Fujita, J. Catahan, R. Cheng, W. Greimann, J. Khandhar, P. Mattson, A. Ra-jani, A. Sullivan, P. Perkins, R. Demonstrating Heterogeneous Gas-Phase Catalysis with the Gas Reaction Catalyst Tube, J. Chem. Educ. 2003, 80, 768-773. 

Textural and Spectroscopic Characterisation of vanadium MCM-41 materials. Application to gas-phase catalysis. 

Gas-phase catalysis was performed at a temperature of 550°C under atmospheric pressure. The composition of the gas before oxidation was 4% propane, 8% Oj and 88% of helium used as gas vector. The laser Raman spectra were recorded on a Bio-Rad spectrometer, model FT-Raman II, using the 1.064 nm line of a Nd YAG laser for excitation. The Raman spectra were corrected for instrumental response using a white light reference spectrum. 

Bohme DK, Schwarz H (2005) Gas-phase catalysis by atomic and cluster metal ions The ultimate single-site catalysts. Angew Chem Int Ed 44 2336... 

The aim of this chapter is to provide the reader with an overview of the potential of modern computational chemistry in studying catalytic and electro-catalytic reactions. This will take us from state-of-the-art electronic structure calculations of metal-adsorbate interactions, through (ab initio) molecular dynamics simulations of solvent effects in electrode reactions, to lattice-gas-based Monte Carlo simulations of surface reactions taking place on catalyst surfaces. Rather than extensively discussing all the different types of studies that have been carried out, we focus on what we believe to be a few representative examples. We also point out the more general theory principles to be drawn from these studies, as well as refer to some of the relevant experimental literature that supports these conclusions. Examples are primarily taken from our own work other recent review papers, mainly focused on gas-phase catalysis, can be found in [1-3]. 

The role of particle size in catalysis and electrocatalysis is a subject of longstanding interest. It is not our intention here to discuss in detail the available experimental and theoretical literature. Extensive reviews on particle-size effects in gas-phase catalysis and electrocatalysis can be found in the papers of Henry [25] and Kinoshita [26], respectively. Also, several monographs, reviews, and conference proceedings discuss particle-size effects from experimental, theoretical, and computational points of view [9,27,28]. 

The high dispersity inside the nano-honeycomb matrix and the high surface area of the nanopartides leads to very good electrocatalytic activity. The electrocatalytic activities of nanosized platinum particles for methanol, formic add and formaldehyde electrooxidation have been recently reported [215]. The sensitivity of the catalyst particles has been interpreted in terms of a catalyst ensemble effect but the detailed microscopic behaviour is incomplete. Martin and co-workers [216] have demonstrated the incorporation of catalytic metal nanopartides such as Pt, Ru and Pt/Ru into carbon nanotubes and further used them in the electrocatalysis of oxygen reduction, methanol electrooxidation and gas phase catalysis of hydrocarbons. A related work on the incorporation of platinum nanopartides in carbon nanotubes has recently been reported to show promising electrocatalytic activity for oxygen reduction [217]. 

Since the strength of adsorption depends on surface electronic interactions between adsorbent and catalyst, a correlation of reaction rate and d-band character of simple metals is expected [Fig. 9 (772)] similar to gas phase catalysis (23, 24). The correlation seems to fail within the noble metal group, and this may result from the existence of multiple adsorption states and the structure, history, and activity of the catalysts used. 

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