Ammonia decomposition over praseodymium-doped nickel catalysts

Details

Supervisors

Gaigneaux, Eric M. ; Gamez Rivera, Sebastian Antonio

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

This research project investigated catalysts to increase the conversion of ammonia into hydrogen at low temperatures. Ammonia can be produced from hydrogen in regions that possess the abundant and cheap renewable energy sources required to do so sustainably at scale. The low volumetric energy density of hydrogen makes it challenging to transport economically over long distances. Ammonia can be used as a hydrogen carrier thanks to its high energy density and a pre-existing infrastructure. However, the decomposition of ammonia into hydrogen is an endothermic process. A catalyst is required to decrease the temperature at which this reaction is performed. Ni/Al2O3 catalysts are known to be effective and cheap catalysts to perform the ammonia decomposition reaction. Rare earths, such as praseodymium, have the potential to improve the reaction mechanism at the rate determining step by acting as electron donors, and could improve the reducibility of Ni. At first, Al2O3 was synthesised by a sol-gel process according to varying parameters to determine the most optimal settings for the ammonia decomposition reaction. Al2O3 with hydrothermal synthesis at 120 °C and calcination at 800 °C was found to yield the highest ammonia conversion at 550 °C. Two Ni addition methods were evaluated. It was determined that direct reduction of Ni onto Al2O3, using NaBH4 as a reducing agent, was more optimal than the incorporation of Ni into the bulk of Al2O3 by simultaneous synthesis. The potential of Pr as dopant was initially assessed by adding it to the catalyst in different orders. Adding Pr onto Al2O3 by direct reduction followed by adding Ni onto Pr/Al2O3 yielded the highest ammonia conversion in the absence of a pre-treatment. However, adding Ni and Pr simultaneously in a single direct reduction step yielded the absolute highest ammonia conversion in the case of a H2 reduction pre-treatment. Further research determined that 5 % was the optimal Pr loading for catalysts made by directly reducing Ni onto Pr/Al2O3. Such catalysts performed slightly less well than Ni/Al2O3 in 22h reaction cycles at temperatures increasing from 300 °C to 550 °C with a H2 reduction pre-treatment. However, the opposite was true in the case of 62h catalytic tests at 550 °C with no pre-treatment. In conclusion, Pr is a dopant which results in a slight improvement in the performance of Ni/Al2O3 catalysts under specific conditions. However, further research is required to truly identify the catalyst synthesis method using Pr which yields the best performance.