Catalysts surface area per unit mass

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

Catalyst Surface Area Per Unit Mass of Catalyst versus Mass Ratio of Platinum to Carbon Support... 

Platinum mass loading in terms of catalyst surface area per unit mass of catalyst particles is obtained from experimental evaluation of active layer structure. A typical empirical correlation for such a structure is given as (Marr and Li, 1999 Secanell et al., 2007)... 

Catalyst surface area per unit mass of catalyst particle Area of platinum in electrode... 

It will be demonstrated in this section that a narrow pore structure limits the reaction rate to an extent which casues the reaction rate to be either proportional to the square root of the specific surface area (per unit mass) or independent of it, depending on the mode of diffusion within the pore structure. Lest this departure of the reaction rate from direct proportionality with specific surface area might be thought to be accounted for in terms of a non-uniform distribution of surface energy over the catalyst surface, it should be pointed out that such in situ heterogeneity is usually only a small fraction of the total chemically active surface and cannot therefore explain the observed effects. 

As stated previously, another distinction usually made is between slurry and supported catalyst reactors. In slurry photocatalytic reactors the catalyst is present in the form of small particles suspended in the water being treated. These reactors generally tend to be more efficient than supported catalyst reactors, because the semiconductor particles provide a larger contact surface area per unit mass. In fact, the state of the photocatalyst is important both to increase contaminant adsorption and to improve the distribution of absorbed radiation. In a slurry unit the photocatalyst has a better contact with the dissolved molecules and is allowed to absorb radiation in a more homogeneous manner over the reaction volume. Using suspended catalyst has been the usual practice in PTC, CPC, and other types of tubular reactors. The drawback of this reactor design is the requirement for separation and recovery of the very small particles at the end of the water treatment process. This may eventually complicate and slow down the water throughput. 

Another catalyst type is the heterogeneous catalyst, which remains as a solid and promotes chemistry at the surface. To function well, they require high surface areas per unit mass. Metal oxides and hydroxides are common examples. A vanadium(V) oxide is employed in the formation of ammonia from nitrogen and hydrogen under elevated temperature and pressure, for example. Polyoxometallate metal clusters, which are oxo-ligand coordination complexes employing dominantly 02 and HO as ligands, have some catalytic roles. 

Most important, heterogeneous surface-catalyzed chemical reaction rates are written in pseudo-homogeneous (i.e., volumetric) form and they are included in the mass transfer equation instead of the boundary conditions. Details of the porosity and tortuosity of a catalytic pellet are included in the effective diffusion coefficient used to calculate the intrapellet Damkohler number. The parameters (i.e., internal surface area per unit mass of catalyst) and Papp (i.e., apparent pellet density, which includes the internal void volume), whose product has units of inverse length, allow one to express the kinetic rate laws in pseudo-volumetric form, as required by the mass transfer equation. Hence, the mass balance for homogeneous diffusion and multiple pseudo-volumetric chemical reactions in one catalytic pellet is... 

Sintering is the coalescence or growth of catalyst particles on a support or the fusion of grains of the support itself The catalyst activity is thereby decreased because larger particles have less surface area per unit mass. When the catalyst has very small metal crystallites (t/ < 10 nm) on a support, the crystallites diffuse randomly on the surface of the support and... 

The rate of mass transfer to the surface is expressed using either concentration or partial pressure as a driving force. The term a is the external surface area per unit mass of catalyst ... 

Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina (AI2O3) or silica (Si02). (a) Why is this an effective way of utilizing the catalyst material compared to having powdered metals (b) How does the surface area affect the rate of reaction ... 

Direct methanol fuel cells (DMFCs) are attracting much more attention for their potential as clean and mobile power sources for the near future [1-8], Generally, platinum (Pt)- or platinum-alloy-hased nanocluster-impregnated carbon supports are the best electrocatalysts for anodic and cathodic fuel cell reactions. These materials are veiy expensive, and thus there is a need to minimize catalyst loading without sacrificing electro-catalytic activity. Because the catalytic reaction is performed by fuel gas or fuel solution, one way to maximize catalyst utilization is to enhance the external Pt surface area per unit mass of Pt. The most efficient way to achieve this goal is to reduce the size of the Pt clusters. 

In these equations, a is the external surface area per unit mass of catalyst (m /g), k is the gas-phase mass transfer coefficient based on unit external surface area (m/s), k r is the apparent rate constant of the surface reaction per unit external surface area (m/s), and Qs and Cbs are the surface concentrations of A and B (mol/m ), respectively. 

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