(144d) Needle in a Haystack Catalysis - An Experimental Study Using Temporal Analysis of Products (TAP)

Abstract:

New results from a series of heterogeneous CO oxidation experiments using a single 350 µm diameter platinum particle packed in a bed of 100,000 inert particles (?platinum needle in a quartz haystack?) are presented.  Detailed kinetic characterization under high vacuum and atmospheric pressures was performed using the Temporal Analysis of Products (TAP-2) reactor.  The results provide a direct correspondence between data obtained in non-steady-state vacuum experiments and steady flow experiments carried out at atmospheric pressures.  This correspondence provides the basis for bridging the pressure gap.

Keywords: CO Oxidation, Single Particle Catalysis, TAP.

1. Introduction

A key goal of catalysis research is to understand the relationship between catalyst structure and performance.  For example, experiments using single crystals and surface science techniques have helped establish that some crystal planes are more active and/or selective than others.  Ultimately, fundamental structure-performance studies are intended to illuminate the operation of industrial catalysts and the nature of active sites on complex catalytic surfaces.  However, a key problem in relating fundamental studies with real catalyst performance is the so called ?pressure-materials gap? between surface science experiments and research at industrial conditions.

The TAP reactor provides the opportunity to perform experiments on real catalysts at industrial conditions and at high vacuum conditions close to the surface science regime.  This paper reports results of new experiments in which TAP experiments are performed using a single micron-sized particle surrounded by a bed of inert particles.  Using a single particle has a number of unique advantages.  For example, a single particle eliminates non-uniformity in catalyst composition and temperature profiles within the reactor.  A single particle can be moved to different axial and radial positions and the influence of catalyst position on kinetic characteristics can be explored.  Single particle experiments also allow very creative experiments probing the relationship between catalyst structure and performance.  For example, by changing the particle position, the length of the inert particle bed or the size of the inert particles the number of gas-surface collisions can be precisely controlled.  Also, by using a single catalyst particle, the number of actives sites on the catalyst can be more precisely quantified.  Single particles can be single crystals, deposited films, or industrial catalysts.  Experiments can be performed at both vacuum conditions and atmospheric pressures in sequence, and results between the two regimes can be directly compared.

2. Experimental 

Single particle experiments were performed using the Temporal Analysis of Products (TAP-2) reactor system [1,2].  Single catalyst particles were held in a stainless steel, cylindrical micro-reactor 4.19 cm in length and 0.64 cm in diameter.  The reactor contains an internal thermocouple and a resistance heating system capable of 0.1° accuracy.  Particles were obtained from a variety of sources including platinum foil, platinum powder, and platinum supported on alumina and silica.  In this paper we report results using a 350 μm diameter high-purity (99.9%) platinum powder particle packed in a bed of 100,000 inert quartz particles with diameters between 210-250 μm.  Atmospheric pressure and TAP vacuum pulse response experiments were performed at a variety of reactor temperatures.  Atmospheric pressure experiments were conducted using stoichiometric mixtures of O₂ and CO (20 cc/min each) while ramping the reactor temperature up and down.   Prior to performing TAP vacuum pulse response experiments, the catalyst was pre-treated by heating the particle in an oxygen flow.  Subsequently, the catalyst was titrated using a series of alternating CO/Ar and O₂/Ar pulses.  This is known as a type of TAP pulse response experiment called the pump-probe method. 

Typical pulse intensities used were on the order of 1x10¹⁴ molecules per pulse.  Conversion in both TAP and normal flow conditions was determined as the ratio between the CO reacted to form CO₂ and the amount originally pulsed or flowed into the reactor.  The temperature dependence of CO conversion in the two regimes allowed the direct comparison of flow and TAP experiments over a wide range of operating conditions.  Our estimates show that in both experiments, the temperature gradients within one particle are not significant.   

3. Results and discussion

CO conversion is much higher under TAP vacuum conditions than atmospheric flow pressures.    This result is mostly due to the transport properties inside the reactor at these two pressure regimes.  Under TAP conditions, a maximum in CO conversion of nearly 95% is observed at 170 °C.  At this same temperature, a conversion of only 15% is obtained under atmospheric flow conditions.  At 170 ˚C, this is the turning point in both pressure regimes where there is a transition in coverage of surface species from either a CO dominated state or an oxygen dominated state depending on the direction of temperature increase or decrease.  In the region of the CO₂ maximum, the areas under the CO₂ response curves corresponding to the O₂ and CO pulses are approximately the same, indicating nearly equal coverages for O₂ and CO.

 
Figure 1.  (A) CO₂ exit response curves obtained from single Pt particle TAP vacuum experiment.  There is a shift shift in the amount of CO₂ produced during both CO and oxygen pulses as temperature increases. (B) CO₂ yield in atmospheric flow experiment with temperature hysteresis. (C) CO₂ production from TAP vacuum experiment. Black line indicates total CO₂ production.  Red and blue lines indicate CO₂ yield on the O₂ and CO pulses, respectively. 

4. Conclusions

The correspondence in ?turning points? indicates that the coverage in vacuum and atmospheric pressure experiments is the same and intrinsic kinetic data obtained in vacuum experiments can be used to describe kinetic behavior in the atmospheric pressure domain.  The ability to relate data in the atmospheric domain to data obtained in vacuum pulse response experiments is a bridging step across the pressure gap [5].

References

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