KINETIC ANALYSIS OF THE CATALYTIC DECOMPOSITION OF HYDRAZINE

KINETIC ANALYSIS OF THE CATALYTIC DECOMPOSITION OF HYDRAZINE

J.E. de MEDEIROS and G.P. VALENÇA^*

Department of Chemical Processes -School of Chemical Engineering

State University of Campinas - UNICAMP

C.P. 6066 - 13081-970 - Campinas, SP, Brazil. E-mail: [e-mail]

(Received: November 5, 1997; Accepted: February 12, 1998)

Abstract - The bond-order conservation method was used to study the catalytic decomposition of N₂H₄. Variation in the activation energy, E, of the most relevant steps was calculated as a function of the enthalpy of adsorption of N, Q_N, between 0 and 1250 kJmol^-1. Results suggest that below Q_N = 520 kJmol^-1 the catalytic decomposition of N₂H₄ produces mostly N₂ and H₂. Above Q_N = 520 kJmol^-1, NH₃ and N₂ are the main products. Near Q_N = 520 kJmol^-1 N₂, H₂ and NH₃ are obtained, in agreement with experimental results on different metals.

Keywords: Kinetic analysis, catalytic decomposition, activation energy.

INTRODUCTION

Catalytic decomposition of hydrazine occurs as either the N-H bond or the N-N bond breaks and the species formed recombine into products. The possible reaction intermediates are N₂H₃, N₂H₂, N₂H, NH₃, NH₂, NH, N, H, N₂ and H₂. When all possible species are combined, 74 possible elementary steps (Valença, 1993) are obtained which result in the two possible routes for hydrazine decomposition, as shown below:

N₂H₄® N₂ + H₂ (1)

3N₂H₄® N₂ + 4NH₃ (2)

(Contour and Pannetier, 1972; Dopheide et al., 1991; Alberas et al., 1992)

Analysis of the 74 steps on the (111) faces of Iridium, Nickel or Platinum resulted in 13 steps to describe both routes for N₂H₄ decomposition (Table 1). Two types of steps are involved, namely, surface dissociation and Langmuir-Hinshelwood steps. To choose between the different routes for hydrazine decomposition, a simple criteria was used. When two or more steps involving the same reacting species resulted in different products, that with the lowest activation energy was chosen as the preferred step. When two elementary steps had one common species and had similar values of activation energy (within 20 kJmol^-1), that with the largest value for the pre-exponential factor was chosen as the preferred step. Thus, dissociation steps (A~10¹⁰ s^-1) were preferred over Langmuir-Hinshelwood steps (A~10^-2 cm²s^-1) (Shustorovich, 1990; Boudart Djéga-Mariadassou, 1984). Then, the step with the largest activation energy was chosen as the rate-determining step in the sequence of 13 proposed elementary steps.

The present work studies the kinetics of the catalytic decomposition of hydrazine based on the bond-order conservation (BOC-MP) method developed by E. Shustorovich (1984, 1988, 1990). The method is based on experimental values of the enthalpy of adsorption of atoms on single crystal surfaces. The BOC-MP method predicts the enthalpy of adsorption of species containing more than one atom on the catalytic surface and the activation energy for elementary dissociation or recombination steps. The results are usually within 20% of the experimental value, in good agreement with experimental data. However, a limitation of the BOC-MP method is the need for adsorption data on different pairs of atoms and metal surfaces.

PROCEDURE

A general mathematical treatment, based on the equations of the BOC-MP method for each of the 13 selected steps studied previously (Valença, 1993) (Table 1), was used to study the energy profile for a broad range of surfaces. Thus, the objective of the present work was to study how the activation energy varied as a function of the enthalpy of adsorption of nitrogen (Q_N) on different metals. The values of Q_N were varied between 0 kJmol^-1 and 1250 kJmol^-1. This latter value was arbitrarily chosen as twice the highest value found in the literature (Table 2).

Table 1:

N2H4	®	NH2 + NH2
N2H4	®	N2H3 + H.
N2H3	®	NH + NH2
N2H3	®	HN=NH + H
HN=NH	®	NH + NH
HN=NH	®	N2H + H
N2H	®	NH + N
N2H	®	N2 + H
NH	®	N + H
NH + H	®	NH2
NH2 + H	®	NH3
H + H	®	H2
N + N	®	N2

Selected steps to study decomposition kinetics

Table 2:

Element	QN	Ref.	Element	QN	Ref.
Fe (110)	586	a	Ir (111)	531	a
Ni (111)	565	a	Ru(001)	565	b
Pd (111)	544	a	Rh (111)	531	c
W (110)	649	a	Pt (111)	485	c

Heat of chemisortion values of some catalytic support with N expressed in kJmol^-1

a – Rhodin and Ertl, 1979.

b – Shustorovich, 1991.

c – Shustorovich, 1990.

First, a mathematical expression for the enthalpy of adsorption of each possible reaction intermediate was written as a function of the Q_N and Q_H. Then, a mathematical expression for the activation energy of each of the 13 steps was written as a function of the enthalpy of adsorption of the possible chemically adsorbed species.

For example, for the step N₂H₃® NH + NH₂, the equation for the activation energy as a function of the enthalpy of adsorption of adsorbed species is:

(3)

where

(4)

(5)

(6)

(7)

D_Re is the gas-phase dissociation energy for all species involved in the reaction step, and D_AB is the gas-phase dissociation energy of molecule AB.

Similar expressions were obtained for all 13 steps. In each case E is always a function of Q_N. In the steps in which H is adsorbed, E is also a function of Q_H. However, the change in Q_H is at least four times smaller than the change in Q_N. Preliminary calculation showed that a variation in Q_H of 25 kJmol^-1 results in a change in E of less than 10 kJmol^-1, i.e., within the experimental error of the method. Therefore, the results of E vs. Q_N in this work were calculated for Q_H between 243 and 263 kJmol^-1 and are shown as an average of E in this interval. A coordination number n = 3 was chosen. To calculate the term -E/RT we chose T = 300 K.

RESULTS AND DISCUSSION

The first step in the catalytic decomposition of hydrazine may be either of the following:

N₂H₄® N₂H₃ + H (8)

N₂H₄® NH₂ + NH₂ (9)

Activation energy is zero for Q_N > 415 kJmol^-1 in step 8, and for Q_N > 520 kJmol^-1 in step 9. For Q_N < 415 kJmol^-1, E for step 8 is always smaller than E for step 9. (Figure 1)

Thus, when Q_N < 520 kJmol^-1, step 8 is preferred. However, when Q_N > 520 kJmol^-1, both steps are equally possible.

Decomposition of N₂H₃ may occur as follows:

N₂H₃® N₂H₂ + H (10)

N₂H₃® NH + NH₂ (11)

Variation in the E for the N₂H₃ dissociation as a function of Q_N shows an intersection point where there is a change in the preferential step (Figure 2). The intersection point depends on Q_H, but is close to Q_N = 530 kJmol^-1. Thus, when Q_N > 530 kJmol^-1, the dissociation of N₂H₃ results in the formation of NH₂ and NH (N-N bond breaking), while when Q_N is less than 530 kJmol^-1, N₂H₃ breaks into N₂H₂ and H (N-H bond breaking).

Figure 1: Hydrazine dissociation steps with Q_H = 243 kJmol^-1.

Figure 2: N₂H₃ dissociation steps with Q_H = 243 kJmol^-1.

When these results are combined with those of the hydrazine dissociation step, the net result is that when Q_N < 520 kJmol^-1 successive dehydrogenation steps, forming N₂H₃ and N₂H₂, occur. On the other hand, when Q_N > 530 kJmol^-1, the decomposition of N₂H₃ into NH₂ and NH occurs irrespective of the hydrazine dissociation step.

Thus, near the range of Q_N = 520-530 kJmol^-1 there is a change in the rate determining step and we may speculate that both steps participate in the overall reaction.

Once NH₂ is formed it hydrogenates easily, forming NH₃ (E = 0 kJmol^-1) according to the elementary step:

NH₂ + H ® NH₃ (12)

Similarly, HN=NH dissociation may occur as two possible steps (Figure 3): with the break of the N-N bond or with the break of the N-H bond. For Q_N < 590 kJmol^-1, the preferential step is the one in which the N-H bond breaks. As Q_H increases, the curve representing the change in E vs. Q_N for the dissociation of N₂H₂ into N₂H and H comes close to zero, making the energy difference between the two steps higher. For Q_N > 590 kJmol^-1, both steps are possible. The same phenomenon had occurred in the N₂H₄ decomposition step.

In N₂H dissociation, an intersection between both steps occurs close to Q_N = 650 kJmol^-1 (Figure 4). Catalysts with Q_N < 650 kJmol^-1 have the following preferential reaction:

N₂H ® N₂ + H (13)

Above Q_N = 650 kJmol^-1 there is a change in the preferential step to:

N₂H ® NH + N (14)

Figure 3: HN=NH dissociation steps with Q_H = 243 kJmol^-1.

Figure 4: N₂H dissociation steps with Q_H = 243 kJmol^-1.

The present analysis provides a guide to how each reaction route of N₂H₄ decomposition occurs on a metal surface. Figure 5 shows that when Q_N < 520 kJmol^-1, the N₂H₄ decomposition reaction occurs by a sequence of steps in which all bonds that are broken are N-H bonds (Table 3). The sum of these steps corresponds to route 1. This agrees with the experiment, as N₂H₄ decomposition on Pt catalysts — Q_N = 485 kJmol^-1 — produces mostly H₂ and N₂. (Maurel et al., 1973)

On metal surfaces where Q_N > 530 kJmol^-1, NH₃ formation is favored, in agreement with experimental results. For example, the N₂H₄ decomposition on Ni catalysts — Q_N = 565 kJmol^-1 — produces mostly NH₃ and N₂ (Volter and Kuhn, 1965). Both hydrazine dissociation steps are possible (steps 8 or 9), but the N₂H₃ dissociation step produces NH₂ and NH. The sequence of steps on a metal surface where Q_N > 530 kJmol^-1 is shown in Table 4 (Route 2).

In catalysts where Q_N is close to the 520-530 kJmol^-1 interval both routes are possible. For example, for Ir or Rh — Q_N = 531 kJmol^-1 — both reactions occur. (Maurel et al., 1973)

Figure 5:
Overall analysis.

Table 3:

N2H4	®	N2H3 + H
N2H3	®	HN=NH + H
HN=NH	®	N2H + H
N2H	®	N2 + H
H + H	®	H2

Hydrazine decomposition: formation of H₂ and N₂ (route 1)

Table 4:

N2H4	®	N2H3 + H
N2H4	®	NH2 + NH2
N2H3	®	NH2 + NH
NH	®	N + H
NH + H	®	NH2
NH2 + H	®	NH3
N + N	®	N2

Hydrazine decomposition: formation of NH₃ and N₂ (route 2)

CONCLUSIONS

The BOC-MP method was used to study the catalytic decomposition of N₂H₄. This reaction is simple enough to allow the analysis of the most important steps. Yet, it is complex enough to use for the study of catalytic selectivity. This example shows how the BOC-MP method can be used to choose among catalysts for a given selectivity for a desired reaction. In the present case there are three ranges of enthalpy of adsorption of N, corresponding to different catalysts, each of which favors one route. In the first range Q_N varies from 0 to 520 kJmol^-1, and the catalysts will promote N₂H₄ decomposition into N₂ and H₂. In the second range, Q_N > 530 kJmol^-1, ammonia formation is easier. Finally, near 520 to 530 kJmol^-1 both reactions are equally possible.

NOMENCLATURE

A

Pre-exponential factor, s^-1 or cm²s^-1

BOC-MP

Bond-order conservation - Morse potential

D _AB

Gas-phase dissociation energy of molecule AB

D _Re

Gas-phase dissociation energy for all species involved in the reaction step

E

Activation energy, kJmol^-1

n

Coordination number

Q _H

Enthalpy of adsorption of hydrogen, kJmol^-1

Q _N

Enthalpy of adsorption of nitrogen, kJmol^-1

Q _0N

Morse constant, kJmol^-1

Q _NH

Enthalpy of adsorption of NH, kJmol^-1

Q _NH2

Enthalpy of adsorption of NH₂, kJmol^-1

Q _N2H3

Enthalpy of adsorption of N₂H₃, kJmol^-1

R

Gas-law constant, J mol^-1 K^-1

T

Temperature, K

• Alberas, D.J.; Kiss, J.; Liu, Z-M. and Wite, J. M., Surface Chemistry of Hydrazine on Pt (111). Surface Science, 278, 51-61 (1992).
• Boudart, M. and Djéga-Mariadassou, G., Kinetics of Heterogeneous Catalytic Reactions, New Jersey, Princeton University Press (1984).
• Contour, J.P. and Pannetier, G., Hydrazine Decomposition over Supported Iridium Catalyst, Journal of Catalysis, 24, 434-445 (1972).
• Dopheide, R.; Schröter, L. and Zacharias, H., Adsorption and Decomposition of Hydrazine on Pd (100) - Surface Science, 257, 86-96 (1991).
• Maurel, R., Menezo, J-C. and Barrault, J., Décomposition de L´Hydrazine sur les Métaux du Groupe VIII et du Groupe I b. Journal de Chimie Physique, 9, 1221-1226 (1973).
• Rhodin, T.N. and Ertl, G., The Nature of the Surface Chemical Bond, Amsterdam, North-Holland (1979).
• Shustorovich, E., Activation Barrier for Adsorbate Surface Diffusion, Heat of Chemisorption, and Adsorbate Registry: Theoretical Interrelations. Journal of American Chemical Society, 106, 6479-6481 (1984).
• Shustorovich, E., Chemisorption Theory: In Search of the Elephant. Accounts of Chemical Research, 21 (5), 183-189 (1988).
• Shustorovich, E., The Bond-Order Conservation Approach to Chemisorption and Heterogeneous Catalysis: Applications and Implications. Advances in Catalysis, 37, 101-163 (1990).
• Shustorovich, E. Synthesis and Decomposition of Ammonia on Transition Metal Surfaces: Bond-Order Conservation Morse Potential Analysis. Surface Science, 259, L791-L796 (1991).
• Valença, G.P., Decomposiçăo Catalítica de Hidrazina: Teoria e Experięncia. Anais do 7^o Seminário Brasileiro de Catálise, IBP, 1, Gramado/RS, 375 (1993).
• Völter, J. and Kuhn, W., Katalyse des Hydrazinzerfalls und der Benzolhydrierung an Nickel auf dotiertem Magnesiumoxid Zeitschrift für Anorganische und Allgemeine Chemie, 340, 216 (1965).

* To whom correspondence should be addressed.

To whom correspondence should be addressed.

Publication Dates

• Publication in this collection
  09 Oct 1998
• Date of issue
  June 1998

History

• Received
  05 Nov 1997
• Accepted
  12 Feb 1998