(754f) Rate/Concentration Kinetic Petals for Microkinetic Examination of Catalyst Surface Processes

In a conventional steady-state
experiment the surface coverage is fixed by the gas phase concentration.  In a dynamic experiment the gas and surface
concentrations are decoupled and their dependences can
be used to derive microkinetic parameters as a basis for comparing catalysts at
a fundamental level.  Transient pulse
response experiments are used to construct rate/concentration dependencies, RC petals, that can be used to
distinguish the timing and interplay of adsorption, surface reaction and
product formation on different materials. 
A petal shape arises as the dynamic ‘reaction-diffusion’ experiment
forces the concentration and reaction rate to return to zero.  Ammonia decomposition was used as a probe
reaction to demonstrate intrinsic microkinetic differences for polycrystalline
iron, cobalt and a bimetallic CoFe catalyst
preparation.  Experimental testing on
these relatively simple materials aids in connecting microkinetic observations
to predictions from density functional theory. 
These methods, however, can be readily applied to complex industrial
catalysts.

On all three materials, the rate-concentration
dependences for NH₃ conversion exhibit petal shapes (RC Petals), and apparent rate constants
in the low coverage regime are compared.  The figure shows the 3D dependence of the NH₃
transformation rate as a function of the gas and surface N-species
concentration to demonstrate subtle microkinetic distinctions of CoFe and Co.  For all
materials, we found H₂ and N₂ product formation was
dependent on the concentration of surface intermediates with one exception: for
cobalt, two reaction pathways could be distinguished in the transient
experiment.  The fast reaction pathway
(8.0·10⁸
1/s) was dependent on the reactant gas concentration, the slow reaction pathway
(2.3·10⁸
1/s) was dependent on recombination of surface H-species.  N₂ formation was only observed
over CoFe and Co and a self-inhibition property was
observed on the CoFe sample.  These and other microkinetic features derived
from this new perspective will be discussed for understanding why similar
catalyst compositions perform different at a global level.