Pulse flow system

CO chemisorption measurements were performed at 25°C using a home-made pulse flow system. Prior to the measurements, the samples were subjected to a pretreatment involving exposure to hydrogen flow for 1 h at 25°C, followed by He purge for 2 h at the same temperature of reduction. 

Smob is determined in the pulse-flow system from the radioactivity of fraction containing the reversibly taken up and the desorbed, by flowing gas (e g., by H2 or N2) of I3 radioactivity collected between injections of two hydrogen sulfide pulses and calculated by the expression ... 

H2 chemisorption measurements were performed at 298K using a pulse flow system. Before measurements, catalysts were heated (lOK/min) in a H2 stream (40 ml/min) at 673K for 2 h. After reduction the siirface was cleaned for 2 h with an Ar stream, then cooled at 298K. The chemisorption measurements were performed with 5% diluted H2 in Ar. 

The centrifugal pump is a versatile unit in the process plant, since its ease of control, non-pulsing flow, pressure limiting operation fits many small and large flow systems. 

The use of IR pulse technique was reported for the first time around the year 2000 in order to study a catalytic reaction by transient mode [126-131], A little amount of reactant can be quickly added on the continuous flow using an injection loop and then introduce a transient perturbation to the system. Figure 4.10 illustrates the experimental system used for transient pulse reaction. It generally consists in (1) the gas flow system with mass flow controllers, (2) the six-ports valve with the injection loop, (3) the in situ IR reactor cell with self-supporting catalyst wafer, (4) the analysis section with a FTIR spectrometer for recording spectra of adsorbed species and (5) a quadruple MS for the gas analysis of reactants and products. 

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

Another system is the plug pulse system seen in Fig. 11. The discharge valve is alternatively activated with a gas pulse. This system works well for free flowing materials. The minipot is a variation of the plug pulse system The operation of the minipot is depicted in Fig. 12. 

It is used in IC systems when the amperometric process confers selectivity to the determination of the analytes. The operative modes employed in the amperometric techniques for detection in flow systems include those at (1) constant potential, where the current is measured in continuous mode, (2) at pulsed potential with sampling of the current at dehned periods of time (pulsed amperometry, PAD), or (3) at pulsed potential with integration of the current at defined periods of time (integrated pulsed amperometry, IPAD). Amperometric techniques are successfully employed for the determination of carbohydrates, catecholamines, phenols, cyanide, iodide, amines, etc., even if, for optimal detection, it is often required to change the mobile-phase conditions. This is the case of the detection of biogenic amines separated by cation-exchange in acidic eluent and detected by IPAD at the Au electrode after the post-column addition of a pH modiher (NaOH) [262]. 

The concept of a leaky dead zone is illustrated in Figure 6.9. A complete mix reactor is connected to a leaky dead zone through the inflow and outflow discharges to and from the dead zone, Qd. The dead zone is, in itself, a complete mix reactor, but it is not part of the main flow system with discharge Q. The independent parameters that can be ht to a tracer pulse or front are the volume of the primary complete mix reactor, Vi, the volume of the dead zone, Vd, and the discharge into and out of the dead zone, Qd. 

In this section we discuss the major experimental methods used to determine absolute rate constants for gas-phase reactions relevant to atmospheric chemistry. These include fast-flow systems (FFS), flash photolysis (FP), static reaction systems, and pulse radiolysis. The determination of relative rate constants is discussed in Section C. 

Flow systems are frequently required when samples are exposed to many laser pulses, when the substrate concentrations are very low (certainly <10 AT), or when the products of photolysis are efficient quenchers or strongly absorbing. 

This correlation corresponds to an exponential decay model, k = koe aY. This expression differs from the conventional exponential model often used in continuous-flow systems 22, 23), k = koe at, in that the analog to time in a pulsed reactor is pulse number or its equivalent, cumulative feed introduced. In our case the correlating quantity is cumulative feed converted, Y. If one assumes that deactivation is caused by coke, the amount of which is proportional to hexane actually converted, this... 

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