Design of Pd-based pseudo-binary alloy catalysts for highly active and selective NO reduction

Drastic tuning of NO reduction activity and N2 selectivity based on the catalyst design with pseudo-binary alloy structures.

Note that the slope α ranges from 0.9 to 1.5, which is consistent with the reaction order suggested from the kinetic analysis in Table 2. Figure S9. Dependence of reaction rate on NO and CO partial pressures (P NO and P CO , respectively) in NO reduction by CO over Pd/Al 2 O 3 and PdIn/Al 2 O 3 .

Kinetic Analysis
Considering a Langmuir-Hinshelwood type mechanism for the NO-CO reaction, the reaction steps are described as follows: ( where, σ indicates an adsorption site. The steps (1) ~ (5) are identical to those considered in the conventional kinetic models for NO-CO reaction over Pd and Rh catalysts. The modified points are that N 2 O is once formed as an adsorbate (6) and that N 2 O decomposition and sorption are considered (7, 8), which are the crucial factors to determine N 2 selectivity in the present system.
We here considered an approximation that for steps (4) and (5), reverse reaction can be ignored under an atmospheric pressure condition. This approximation was supported also by DFT calculation, in which the reaction barriers of the reverse reactions (E A + ΔE) are much higher than those of the forward reactions (E A , Table #). Other steps can be regarded to be in equilibrium except the ratedetermining step. Therefore, the equilibrium constants are generally defined as follows: where, , , and are the partial pressure of X, coverage of X, and percentage of vacant site: (1 -) , respectively.
Assuming that NO adsorption (1) is the rate determining step, the overall reaction rate can be expressed using a rate constant k as follows: Here, is expressed using the equilibrium constants and P X as follows: (1 -) Based on these, the overall reaction rate can be descried as follows: 2 ) + 8 -1 2 ( 7 2 -1 + 1 ) This equation indicates the first-order dependence of r on and that the reaction order of ranges from −1 to 0, respectively. This does not agree with the experimental results.
Assuming that CO adsorption (2) is the rate determining step, the overall reaction rate can be expressed using a rate constant k as follows: This equation is described using the equilibrium constants and P X as follows: Here, we temporally used as the equilibrium constant of the step (5) to solve 5 = 2 (1 -) 2 / the rate equation. Considering that the rate of forward reaction of step (5) is much faster than the reverse reaction, we can introduce an approximation, .

5
-1 ≪ 1 Therefore, the rate equation can be simplified as follows: This equation indicates the first-order dependence of r on and that the reaction order of ranges from −1 to 1, respectively. This does not agree with the experimental results.
Next, we assume that NO dissociation (3) is the rate determining step, the overall reaction rate can be expressed using a rate constant k as follows: This equation is described using the equilibrium constants and P X as follows: Assuming that N 2 formation (4) is the rate determining step, the overall reaction rate can be expressed using a rate constant k as follows: This equation indicates that the reaction orders of and range from −6 to 0 and from −2 to 0, respectively, being inconsistent with the experimental positive orders.
Then, we assume that CO oxidation (5) is the rate determining step, affording the overall reaction rate expressed as follows: The reaction orders of and can range from −4 to 2 and from −1 to 1, respectively, which is well consistent with the experimental positive orders.
When N 2 O formation (6) is assumed as the rate determining step, the overall reaction rate is expressed as follows: The reaction orders of and range from −2 to 0 and from −1 to 0, respectively, which is inconsistent with the experimental positive orders.
When N 2 O decomposition (7) is assumed as the rate determining step, the overall reaction rate is expressed as follows: The reaction orders of and range from −2 to 0 and from −1 to 0, respectively, which is inconsistent with the experimental positive orders.
When N 2 O desorption (8) is assumed as the rate determining step, the overall reaction rate is expressed as follows: The reaction orders of and range from −1 to 1 and from −1 to 0, respectively, which is inconsistent with the experimental positive order of .
Thus, assuming CO oxidation (5) as the rate-determining step exclusively gave reaction orders consistent with the experiment. On the basis of this result, we concluded that the rate-determining step of NO-CO reaction over Pd-based catalysts is CO oxidation.