Hydrogen production by ammonia decomposition using high surface area Mo2N and Co3Mo3N catalysts Seetharamulu Podilab, Sharif F. Zamana,b,*, Yahia A. Alhameda,b, Abdulrahim A. Al-Zahrania,b, Hafedh Drissb, Lachezar A. Petrovb a Chemical b SABIC
and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah 21589, Saudi Arabia. Chair of Catalysis, Chemical and Materials Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia. *Dr. Sharif F. Zaman, email:
[email protected], Tel: +966563063594 , Fax: +966126952257
Introduction: Introduction: Ammonia is a very important hydrogen storage medium. Compare with all organic compounds ammonia has the highest hydrogen content of 17.75 wt%. It is non-flammable and nonexplosive. Hydrogen produced by catalytic ammonia decomposition, (NH3 0.5N2 + 1.5H2), does not contain carbon monoxide or other catalytic poisons. For that reason, its use in fuel cells for energy production has very good prospects. Transition metal nitrides belong to a class of interstitial compounds, in which nitrogen atoms replace oxygen atoms in the metal oxide crystal structure. Owing to their similarity with group VIII noble metals, these materials have attracted much attention as potential catalysts for many reactions [1-2]. Here we are presenting a new method for preparation of high surface area bulk Mo2N catalyst and also 1,3,5 wt% Co promoted Mo2N catalysts and their activity results for ammonia decomposition reaction.
Catalyst preparation and characterization: characterization: High surface area Mo2N catalyst was prepared by dissolving calculated amount of ammonium heptamolybdate ((NH4)6Mo7O24.4H2O) in deionized water. Citric Acid (CA) was added to the Mo salt solution to obtain a 1:1 of CA: Mo molar ratio. Cobalt containing catalysts were prepared by the addition of required amount of cobalt nitrate salt to ammonium heptamolybdate solution before the addition of Citric Acid and remaining procedure is same as described above. Catalysts were characterized by BET, XRD, TPR, XPS, SEM, and TEM techniques. Catalytic activity tests were performed in a fixed bed quartz reactor under atmospheric pressure. Gas analysis was performed by an online gas chromatograph equipped with a thermal conductivity detector and a Poropak Q column. Characterization results are shown in Table 1 and Figure 1, 2 and 3 which confirm successfully formation of Mo2N and Co3Mo3N phases.
Fig.1: X-ray diffraction patterns of Mo2N, 1CoMo2N, 3CoMo2N and 5CoMo2N catalysts.
Table 1. BET surface area and average particle crystallites size of the studied catalysts BET surface area m2g–1
Average Average Average Relative proportional crystallite crystallite crystallite weightb, % Catalyst size of size of size of a b γ-Mo2N γ-Mo2N Co3Mo3N MoO2 Mo2N Co3Mo3N nm nm nm Mo2N 77.3 6.5 6.7 – 54.0 46.0 – 1CoMo2N 80.6 10.8 12.0 8.0 27.4 71.0 1.5 3CoMo2N 83.0 8.4 10.0 14.0 20.2 75.3 4.0 5CoMo2N 72.5 23.4 20.0 20.0 3.6 89.6 6.9 a b Crystal size calculated from TEM analysis; Relative proportional weight (%) calculated from XRD using Rietveld refinement analysis.
Fig.3: XPS spectra in Mo 3d region of (a) Mo2N catalyst (b)1CoMo2N catalyst (c) 3CoMo2N catalyst (d) 5CoMo2N catalyst.
Fig.2: (a) SEM of Mo2N sample (b) HRTEM of γ-Mo2N (c) Selected-Area Electron Diffraction pattern of a γ-Mo2N platelet (d) HRTEM of γ-Mo2N and corresponding SAED pattern.
Activity Results :
100
80
10
90
9
80
8
NH3 Conversion (%)
60
70
7
50 40
60
6
30
50
5
40
20
0 300
; ; ; ;
30
10
4
Conversion(%) ; H2 formation rate -
MoN 1CoMoN 3CoMoN 5CoMoN
400
450
500
550
600
Catalyst MoN 1CoMoN 3CoMoN 5CoMoN
-1
Ea (KJ/mol) 131.2 99.7 92.8 97.6
-2
-3
-4
MoN 1CoMoN 3CoMoN 5CoMoN
-5
3 -6
20
350
Bulk Mo2N catalyst showed the highest activation energy of 131.2 kJ mol−1 for NH3 decomposition. Cobalt addition to the catalysts caused a substantial decrease of the activation energy. The activation energy for the 3 wt% Co-Mo2N catalyst is 92.8 kJ mol−1, which suggests the promotional effect of cobalt. 0
H 2 formation (mmol/g cat min )
70
100
MoN 5CoMoN 3CoMoN 1CoMoN NH3 conversi on (%)
90
The stability performance and hydrogen production rate over all the catalysts under investigation were preformed for 30 hrs. No catalytic deactivation was observed and hydrogen production rate was also constant.
Ln(k)
The Mo2N catalyst showed >95% conversion at 600°C. The effect of cobalt promotion was evidenced at lower temperature in the range of 450–550°C. i.e. conversion over 3wt%CoMo2N is 10% higher than that on Mo2N catalyst at 550oC.
2 1
5
10
15
20
25
30
Temperature(°C)
Time (h)
Fig. Fig.4: Activity studies on Mo2N, 1CoMo2N, 3CoMo2N and 5CoMo2N catalysts at temperature range 300-600 °C and GHSV-6000h-1.
Fig. Fig.5: Stability performance studies over Mo2N, 1CoMo2N, 3CoMo2N and 5CoMo2N catalysts at 600°C and GHSV = 6000h-1.
1.2
1.3
-1
1.4
1.5
1/T*1000 (K )
Fig.6: Arrhenius plots for NH3 decomposition over Mo2N, 1CoMo2N, 3CoMo2N and 5CoMo2N catalysts
Conclusions:
> High surface area bulk molybdenum nitride showed high activity towards ammonia decomposition. > All catalysts showed more than 95% conversion of NH3 at 600 °C and quite stable at this high temperature. > NH3 conversion was enhanced by the addition of Co compare to the bulk Mo2N catalyst at lower temperatures (≤ 550 °C). The 3 wt% wt% CoCo-Mo2N catalyst showed highest activity among all catalysts investigated in this research due to uniform dispersion of Co3Mo3N phase on γ-Mo2N platelets with lower crystal size.
References:
[1]
P. M. Patterson, T. K. Das, B. H. Davis, Appl. Catal. A, 251 (2003) 449–455. [2] L. Leclercq, M. Provost, H. Pastor, G. Leclercq, J. Catal., 117 (1989) 384–395.
Acknowledgement : This work was supported by the SABIC Chair of Catalysis, King Abdulaziz University, Jeddah, Saudi Arabia.
