Catalyst Activity Down-selection

The greatest advantage of the optical method is simplicity when screening large catalyst libraries, because this technique requires only aqueous indicator solutions and a hand-held UV lamp. However, this method has two major drawbacks it is insensitive to minor differences in electrode activity and it does not provide direct electrochemical measurement, which is required for complete characterization. 

Besides SECM, other speetroseopie mefliods, such as scanning differential electrochemical mass spectrometry (SDEMS) and IR thermography, were also used as combinatorial screening methods for fuel cell electrocatalysis. The principle of SDEMS is to use mass spectrometry to locally measure dissolved gases and volatile liquid species near flie surfaces of catalyst arrays. IR thermography is based on reaction heat mapping. The heat results from the fuel cell electrochemical reactions on the catalyst arrays. Both methods can obtain reaction 

The activity and performance of fuel cell electrocatalysts need to be evaluated in terms of electrochemieal parameters, including current density and electrode potential. Electrochemieal sereening methods have been identified as ideal direct approaches for combinatorial studies of fuel cell catalysts. Two types of electrochemical measurement systems have been developed for combinatorial screening of fuel cell catalysts the array half-cell system and the array single-cell system. 

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Early reports on the use of Pt alloys as ORR catalysts for PEM fuel cells were published by a group of scientists at Texas A M University in the early 1990s [4-6]. They created a series of Pt alloys, including Pt-Ni, Pt-Co, Pt-Cr, Pt-Mn, and Pt-Fe, at high temperature (900 °C) under an inert atmosphere. These alloys can be expressed as Pt-M (where M is the non-noble metal alloying component). In the process of activity down-selection, a composition of 75(Pt) 25(M) was found to be... 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

In 1976, Lalancette et al. studied228 the catalytic activity of graphite intercalated AICI3 and compared it with neat AICI3 in solution-phase alkylations. Whereas the rate of alkylation slowed down using the intercalated catalyst, a higher selectivity toward monoalkylation was found (Table 5.13). 

Thus, thermodynamic analysis of ideal models reveals that dispersing the active catalyst phase down to particles of no larger than 10 nm may affect considerably both the adsorption equilibrium as well as the rate (para meters SCA and TOF) and selectivity of the catalytic reaction. The neces sary condition here is the participation of either the matter of the dispersed active phase (active catalyst component) or an intermediate to be dissolved in the dispersed active phase (see [5]) in the catalytic transformations. 

The PPR and LFR are also applied in a more recently developed dedicated process for NOx removal from off-gases. The Shell low-temperature NO reduction process is based on the reaction of nitrogen oxides with ammonia (reactions iv and v), catalyzed by a highly active and selective catalyst, consisting of vanadium and titania on a silica carrier [18]. The high activity of this catalyst allows the reaction of NO with ammonia (known as selective catalytic reduction) to be carried out not only at the usual temperatures around 300°C, but at substantially lower temperatures down to 130°C. The catalyst is commercially manufactured and applied in the form of spheres (S-995) or as granules (S-095) [19]. 

Both activity and selectivity respond in a very sensitive manner to the extent of catalyst alkalization (normally doping by means of K2O). It appears that the chemisorption of the reactants and the speed of all CO-consuming reactions (CO reduction, water-gas shift reaction, surface-carbide formation, etc.) are increased. While in former times the liquefaction result (amount of liquid gasolines) was the quality measure of a Fischer-Tropsch catalyst, nowadays it is narrow product distributions into which research puts its efforts. To this end, the mechanistic question has maintained focal importance. The oil crisis in the 1970s initiated intensive work in order to narrow down the Fischer-Tropsch product spectrum. 

In order to determine the catalyst stability two experiments were carried out. Firstly, the catalyst was tested in 10 °C intervals, from 450-600 °C. The catalyst temperature was than lowered in 10 °C intervals back down to 450 °C. No significant change in activity or product distribution was noted on the downward cycle. Secondly, the catalyst was tested for 50 hours in methane rich conditions. Again with no significant change in activity or selectivity. Hence, the catalyst was deemed to be stable under the experimental conditions employed here. 

Operational flexibility in terms of selected catalyst activity and partial load operation (down to 50 %) was successfully demonstrated. 

In the exploring catalysts for fuel cell cathode ORR, besides new catalyst synthesis, catalyst characterization using electrochemical measurements such as RDE and RRDE techniques seems necessary in order to validate the catalytic ORR activity of the synthesized catalysts, and the down-selection of new catalyst designs. 

Transport Criteria in PBRs In laboratory catalytic reactors, basic problems are related to scaling down in order to eliminate all diffusional gradients so that the reactor performance reflects chemical phenomena only [24, 25]. Evaluation of catalyst performance, kinetic modeling, and hence reactor scale-up depend on data that show the steady-state chemical activity and selectivity correctly. The criteria to be satisfied for achieving this goal are defined both at the reactor scale (macroscale) and at the catalyst particle scale (microscale). External and internal transport effects existing around and within catalyst particles distort intrinsic chemical data, and catalyst evaluation based on such data can mislead the decision to be made on an industrial catalyst or generate irrelevant data and felse rate equations in a kinetic study. The elimination of microscale transport effects from experiments on intrinsic kinetics is discussed in detail in Sections 2.3 and 2.4 of this chapter. 

Monolith perovskites prepared from ultradispersed powders of mixed oxides of rare earths (La-Ce or Dy-Y) and transition metals (Ni, Fe, and Mn) have recently been used in methane combustion [65], These preshaped structures were seen to be active and selective in the target reaction over a wide range of temperatures. The scale of the specific activity and apparent activation energy for monoliths paralleled that found for powdered samples. The catalyst decreased the ignition temperature down to 200 K to achieve a 10% methane conversion, and enhanced selectivity to CO2 with respect to the uncatalyzed process. 

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