Ethanol Decomposition and Dehydrogenation for Hydrogen Production: A Review of Heterogeneous Catalysts

Abstract

Biowaste derived ethanol is an important feedstock for carbon neutral hydrogen production. It is liquid at atmospheric conditions, has a high hydrogen to carbon ratio, and it can be generated from biomass, lignocellulosic wastes, and sewage sludge, offering opportunities for converting waste to hydrogen as an alternative to fossil fuels-based sources of hydrogen. A large-scale application of bioethanol for hydrogen production has the potential to considerably reduce CO2 emissions. Recent reviews on ethanol reforming for hydrogen production show a growth in research interest in this topic, although primarily focused on ethanol steam reforming (ESR) in which steam, in addition to ethanol, also acts as an additional source of hydrogen. Ethanol partial oxidation (POx) is another relatively well-studied route, which is exothermic in nature; however, hydrogen selectivity can be compromised due to the presence of oxygen leading to water formation. Ethanol decomposition and dehydrogenation (ED) are relatively less studied compared to the other two routes, possibly due to coke formation that often leads to catalyst deactivation. The past decade has seen an increasing number of papers on ED to exploit it for the production of aldehydes, acetates, and ethers, as well as various forms of carbon along with hydrogen. The goal of this article is to provide a review on the recent development in nonoxidative ethanol dehydrogenation and ethanol decomposition catalysis for hydrogen production. Catalytic studies over the past few decades have benefitted from the unprecedented growth in experimental techniques, particularly from in situ transmission electron microscopy (in situ TEM) and near ambient X-ray photoelectron spectroscopy (NA-XPS), allowing researchers to obtain dynamic structural information caused by reactant-catalyst interactions leading to greater insights based on more realistic information. A review incorporating the current status will allow us to better understand the structure-performance relationship, leading to the implementation of practical and realistic considerations for catalysts synthesis and reactor design for stable performance. The present review is an attempt to put together the recent progress, mainly in the past decade, on the catalysts for ethanol dehydrogenation, experimental conditions for effective extraction of hydrogen and targeted products, and metal-support interactions and their contribution in directing the reaction pathways. For the ease of reading and comprehension, the article is divided into subsections based on metals (transition metals and noble metals) and supports. The review concludes with a table listing the catalysts, synthesis method, reaction environments, and key findings for a quick and easy access to the critical systems assessed during the review process.