(412a) Pulsed Temperature Activation in Heterogeneous Catalysis

The deliberate excitation of unsteady state phenomena in chemical engineering has been discussed since the late 1960's. The idea is to exploit the nonlinearities in the system such that time average values are achieved that cannot be reached through steady state operation. This field was dubbed periodic control [1].

Typically two different forms of periodic excitation are distinguished: concentration forcing and temperature forcing. Though some authors believe the strong nonlinearities in temperature show potential [3], the vast majority of literature is on concentration forcing. One reason for this is that it is easier to significantly vary the inlet concentration in time than it is to vary the temperature by a significant amount in short time. The periods encountered in fast temperature forcing are at least in the multi-second range [2].

This paper focuses on temperature forcing, but at a completely different timescale, creating a novel concept of operation we call pulsed activation. This concept applies to heterogeneous catalytic reactions.

  The pulsed activation concept

For these catalytic reactions it is known where reactions are taking place: at the catalytic surface. That is why in stead of heating the whole reactor, it is chosen to heat just the catalytically active part. Since the amount of mass that is heated is low, fast transients can be created. In our case jumps of up to a thousand Kelvin are generated in as little time as 20 microseconds. Through the design of the reactor the catalytic surface cools down to the original temperature in around 100 microseconds. In this way pulses of temperature are created which are used to activate a chemical reaction at the surface.

The conditions are chosen such that at the low base temperature the necessary reactants can settle at the surface yet there is no reaction. When pulsing to the high temperature the reaction is activated. If the temperature pulses are faster than the time needed for the surface reactants to desorb and establish the equilibrium corresponding to the high temperature conditions are temporarily created which could never exist under steady state operation.

Figure 1 conceptually shows how these pulses increase the available space of operation of a chemical process. A base condition is chosen, to which temperature pulses of a chosen amplitude and frequency can be added. By selecting an amplitude of zero the steady state operation is obtained, so the steady state operation is included in the pulsed activation as well. The aim is to find a more profitable way of operation in the region that is added by pulsed activation.

Figure 1: Increasing the available modes of operation through pulsed activation

In stead of having a continuous reaction rate, this concept effectively converts small batches of up to one monolayer of reactants each pulse. At high temperatures the steady state will typically have an almost empty surface, resulting in a low reaction rate even if the reactivity of the adsorbates is high. This is reflected by the well known volcano curve from catalytic reactions. By pulsing the temperature faster than the surface molecules can desorb a unique situation is created with a high coverage of highly reactive adsorbates.

In this way the extra flexibility added by the pulsed activation concept is used to precisely control the rate of a reaction. A specific benefit over steady state operation is that the reaction rate can be controlled nearly instantaneously. By stopping the pulses the reaction stops nearly immediately. Another interesting path is to investigate selectivity issues in more complex reaction schemes. Undesired side- or consecutive reactions could be prevented by the short duration of the pulses, especially if these take place in the gas phase.

One of the key reaction complexes that will be investigated is the effect of pulsed activation on the Fisher-Tropsch synthesis reaction complex which may take a very important role in our society's future energy cycle. By variation of the amplitude and frequency of the pulses extra handles are introduced to influence the product distribution.   Design of a micro reactor for pulsed activation

To test the pulsed activation concept in practice a micro reactor has been built. Figure 2 schematically shows the reactor design. At the core of the design is a silicon wafer with a 200 nm platinum strip acting as a catalytic surface.

Figure 2: Microreactor design for pulsed activation

Contact pads are deposited on the wafer, and a high voltage is applied forcing a high current through the catalytic layer. The resistive losses heat up the catalytic layer. Because the current density in the layer is very high (10¹¹ A/m²) the temperature gradient is also very high and hence the heating is very fast. The layer cools down through conduction of heat to the much cooler wafer. Figure 3 shows the catalytic surface temperature from a simulation of the heat distribution in the reactor.

Figure 3: Simulation of catalytic surface temperature in time

As designed, the catalyst used in the reactor is polycrystalline platinum which is active for many reaction complexes. Through electrochemical deposition or by spin coating of nano particles different catalytically active surfaces can be created on top of the platinum layer. For instance, in case of the Fischer-Tropsch reaction iron nano particles will be used.

In the reactor three scenarios are investigated:

·        Normal steady state operation

·        Adding energy as pulses to the catalytic layer

·        Adding the same amount of energy to the catalyst in DC

Since adding energy locally to the catalyst may give a temperature gradient in the reactor and thus a slightly higher catalytic surface temperature, the case of adding energy through pulses needs to be compared to adding the same amount of energy with a DC source.

At the moment of submission the testing of the reactor is complete and the first measurements are being taken. For CO oxidation significant differences can be measured between the three scenarios. Results will be presented at the conference.   Simulation results

This work relied on simulations of heat distribution and surface reactions to design the micro reactor and develop the pulsed activation concept. Prior to building the reactor the concept has been tested in theory. This part shows how the pulsed activation concept affects a very simple theoretical surface reaction complex. Say that we have the following reactions:

1)         A + * à A*

2)         A* à A + *

3)         2A* à B + 2*

A reactant can adsorb, and when it does eventually it will either desorb or react and create B, which instantaneously desorbs. For the adsorption a collision based model is used, while the reaction and desorption terms are activated and follow an Arrhenius curve. For this system the steady state solution is easily computed. If it is assumed that the partial pressure of A doesn't vary significantly during a pulse the solution for an ideal periodic two step temperature pulse can be solved analytically.

Figure 4 shows that result if the activation energy for the reaction is higher than the one for desorption. The top two plots show the coverage and the rate in steady state, which results in a volcano curve. The bottom two plots show the pulsed activation for pulses from 300 to 800 K. During each pulse the surface is almost completely emptied and around 12% of the A is converted to B while the rest just desorbs. Since this process is repeated 1000 times per second, the time average rate (turn over number) of the pulsed reaction is 61 [1/s] while the maximum rate that could possibly be achieved using steady state operation is around 7 [1/s].

Figure 4: Comparison between pulsed and steady state reaction rate demonstrating a higher time average reaction rate can be accomplished using pulsed activation

For more complex reaction schemes it is more difficult to find analytical solutions but using a numerical approach also there examples can easily be found where pulsed activation shows higher conversion or different selectivity.

  Conclusion

A new way of operating heterogeneous catalytic reactions has been introduced which we named pulsed activation. This method can be categorized as a form of process intensification through periodic operation, though the periods are far shorter than usually encountered. A reactor has been built in which pulses of hundreds of Kelvin can be applied in around 20 microseconds. Testing and measurements are underway and results will be presented at the conference. Simulation suggests there are reaction schemes for which this form of operation fundamentally enhances the results of steady state operation.   References

[1]        J.E. Bailey. Chemical Reactor Theory, A Review, chapter 12, Periodic Phenomena, pages 758?813. Prentice-Hall, 1977.

[2]        J.J. Brandner, G. Emig, M.A. Liauw, and K. Shubert. Fast temperature cycling in microstructure devices. Chemical Engineering Journal, 101:217?224, 2004.

[3]        P.L. Silveston and R.R. Hudgins. Periodic temperature forcing of catalytic reactions. Chemical Engineering Science, 59:4043?4053, 2004.