Improvement of catalyst for oxidation of carbon monoxide in the presence of hydrogen

Abstract

The polymer electrolyte membrane fuel cell (PEMFC) is an energy conversion device that uses hydrogen as fuel to produce electrical energy. The fuel cell is, however, very sensitive toward CO, thus, amounts over 10 ppm will poison the Pt-based anode part of the fuel cell. Preferential oxidation of carbon monoxide (CO-PROX) is considered to be the most efficient way of eliminating CO to such low concentrations.
This thesis project aims to find a suitable catalyst that shows high selectivity and activity towards the CO-PROX reaction and achieves the requirement of <10 ppm CO. The new catalyst formulations will be compared to a reference catalyst consisting of 3% Pt, and 12% Co mounted on ɣ-Al2O3.
The work was performed by firstly investigating different catalysts through literature studies. Moreover, through the findings in the literature, new formulations were chosen and manufactured. In this work, fifteen different catalysts were prepared and investigated. The first thirteen catalysts were produced by the incipient wetness method by varying the concentrations of the active phase and the choice of promoters. The last two catalysts were produced by a method where the catalyst was pre-soaked in distilled water before the impregnation of the active phase.
The tests were performed in a reactor containing 4.8 mL of catalyst and 4.8 mL of inert. The measurements were performed at ambient pressure, the reactor feed composition was 0.5% CO, 72.3% H2, 17.7% CO2 and 9.5% H2O. The O:CO ratio and gas hourly space velocity (GHSV) varied slightly during the different runs. The dry gas from the reactor was analyzed at temperatures between 100-200 ̊C by a CO analysis measurement tool.
The reference catalyst was analyzed through scanning electron microscopy (SEM) and it was concluded that all the active sites were situated on the surface of the catalyst. Additionally, by tableting the reference material on site it was concluded that with the reactor conditions the reaction occurred on the surface of the catalyst, thus it was not limited by mass transfer inside the pores. Moreover, through the new preparation method, a catalyst containing 0.2% Pt and 6% Co mounted on ɣ-Al2O3 was prepared. Results show that the new catalyst formulation exceeds the activity of the reference catalyst giving approximately 15 ppm CO at 150 ̊C. By investigating the undesired reverse water gas shift (RWGS) reaction, it was concluded that the new catalyst formulation is highly selective towards CO-PROX thus it fully converts the CO. However, as the concentration of CO is reduced, the equilibrium reaction (RWGS) is more favored leading to its small production of CO. Nevertheless, the RWGS reaction is unfavored at lower temperatures and lower residence times thus more work can possibly be done to decrease the effects of the RGWS reaction.
The aim was partially fulfilled. By changing the preparation method, the Pt and Co concentrations were decreased by 93% and 50% respectively and reached higher CO conversion compared to the reference catalyst. However, because of the RWGS reaction, the catalyst consisting of 0.2% Pt and 6% Co does not achieve the requirement of < 10 ppm CO although it is very close.

Popular Abstract

With the concern about air pollution and global warming, the prospect of using fuel cells as energy conversion system is on the topic today. To use hydrogen as feed and convert the chemical energy within the bonds of the hydrogen molecule into electrical power causes no pollutant generation. Usually, the feed stream needs to be purified in advance since it often contains carbon monoxide, which poisons the anode part of the fuel cell. One promising path is to oxidize the carbon monoxide to form carbon dioxide. However, this comes with significant challenges as the hydrogen content in the feed stream is high. Therefore, a very selective and active catalyst is needed.
This thesis project chose a reference catalyst of 3% platinum and 12% cobalt mounted on gamma-alumina, and 15 different catalyst formulations were prepared.
The oxidation reaction is considered to be very fast. Therefore, an egg-shell impregnated catalyst, i.e., a catalyst with its active phase positioned on the surface, is beneficial for this application. Using a different preparation method resulted in a thinner layer of platinum mounted on cobalt gamma-alumina spheres. The new catalyst showed great catalytic performance; thus, the platinum and cobalt contents were decreased by 93% and 50%, respectively, and still reached as low carbon monoxide contents as the reference catalyst. Further investigations showed that a side reaction took part and produced very small amounts of carbon monoxide. The thesis gives numerous suggestions for future work to suppress the side reaction further and reduce the carbon monoxide content.

• author: Ridell, Maria
• supervisor: Christian Hulteberg
• organization: Chemical Engineering (M.Sc.Eng.)
• course: KETM05 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Chemical Engineering, Kemiteknik, catalyst, PEMFC, polymer electrolyte membrane fuel cell, fuel cell, CO-PROX, PROX, Preferential oxidation, purification, CO elimination
• language: English
• id: 9092927
• date added to LUP: 2022-06-28 09:13:45
• date last changed: 2022-06-28 09:13:45