Change Of Metal Oxidation State At High Temperature, Impact Of Temperature And Reactive Gases

Author

Yegor Nikitin, CUNY City CollegeFollow

Date of Award

2024

Document Type

Dissertation

Department

Chemical Engineering

First Advisor

Marco J. Castaldi

Keywords

High Temperature, Corrosion, Catalysis, Catalyst, Metal Oxidation

Abstract

The fundamental understanding of gas-solid reactions enables a wide range of applications to be developed and improved. Specifically, the use of metal-based materials is critical to ensuring robust, safe, and long-term operations. For example, in power systems where boilers are used to generate steam for heat or electricity generation the use of stainless metal alloys is critical to allow high quality steam to be produced in a very harsh environment. Alternatively, in catalysis, the use of metal doped heterogeneous catalysts are essential in achieving selectivity and conversion performance for chemical synthesis to emissions abatement. Typically, these metal alloys must withstand temperatures up to 1000 °C at pressures near 30 bar continuously for 8000 hours or more. In the power system arena, waste to energy (WtE) combustors represent one of the harshest environments due to the remaining 8% O2 as well as the presence of HCl gas up to 1000 ppm in combination with SO2 at 15-240 ppm levels in the flue gas. These components combine with moisture in the flue gas and particulate matter that contains a wide range of elements; from Pb at 200-19,000 mg/kg to Cd, Zn, Ca that are in the range of 25-41,000 mg/kg forming eutectics that accelerate corrosion on the metal alloys present. Similarly in catalysis, there are reactive intermediates that remain attached to the catalytic metal surfaces resulting in volatile metal complexes forming leading to losses via elevated vapor pressures at nominally 1000 °C operating temperatures. To probe how catalyst activity changes as the result of changes in oxidation state due to high temperatures in the presence of oxygen, decomposition of N2O over an Ir/Al2O3 catalyst was selected. During the reaction O2 and N2 chemical products are produced, whereby the oxygen product can further react with iridium to form volatile iridium oxides, Ir2O3 and IrO3, with vapor pressures up to 1 mbar, leading to the catalyst deactivation.

This research explored how the composition of the stainless alloys impacts their corrosion resistance performance at 550-1000 °C temperatures. Based on the transient experimental results, a protective corrosion scale forms up to 1000 °C in WtE and cement kiln environments. Upon further inspection, this was attributed to alloyed nickel and manganese metals, while iron and chrome had adverse effects on the corrosion resistance, which was unexpected. This was attributed to the formation of soluble alkaline chromates that dissolve in molten chloride salts. Also, corrosion performance of the alloys as the function of their composition correlations will be discussed. Since the laboratory test conditions approximate the industry conditions, these correlations can be used for estimates in cost-to-performance analysis when making decisions related to alloy installations.

In addition, the experimental techniques and literature knowledge developed from the corrosion work was applied to hypothesize an iridium catalyst deactivation mechanism during exothermic nitrous oxide decomposition. Through the understanding of the oxidation mechanisms of iridium catalysts, proper industrial techniques can be utilized to mitigate the catalyst deactivation and prolong catalyst life. Overall, the common investigative thread is the oxidation of metal surfaces in high temperature oxidizing environments.

Recommended Citation

Nikitin, Yegor, "Change Of Metal Oxidation State At High Temperature, Impact Of Temperature And Reactive Gases" (2024). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/1279