Temperature Programmed Oxidation: Mass Spectrometry Strategies for Catalyst Deactivation Studies

TPO characterises deactivated catalysts through following the controlled, temperature-dependent oxidation of deactivation species on the catalyst surface. A catalyst sample is exposed to an oxidising atmosphere, typically oxygen diluted in an inert carrier gas, while the temperature is increased at a constant linear rate. As the temperature rises, carbonaceous deposits and other poisons oxidise, producing characteristic gaseous species that show distinct oxidation temperatures and evolution profiles. The temperature at which oxidation occurs is highly informative. Light, amorphous carbon deposits oxidise at lower temperatures, whereas more ordered or graphitic coke requires higher temperatures because it is more strongly bound to the catalyst surface and has limited exposure to oxygen. Moreover, sulfur-containing species exhibit distinct oxidation behaviour that can often be separated thermally from carbon oxidation. Since oxidation temperature provides key insights on the nature of deactivating species, a linear heating ramp must be maintained to ensure reproducibility and allow meaningful comparison of oxidation temperatures, peak shapes, and gas evolution profiles across different catalyst deactivation studies.

Speciation through mass-resolved detection

MS extends TPO beyond bulk oxidation measurements by enabling the simultaneous detection of multiple gas species in catalyst deactivation studies. By monitoring specific mass-to-charge ratios, MS resolves carbon monoxide (CO), carbon dioxide (CO2), water (H2O), and sulfur oxides (SO?) independently during a single temperature ramp through mass-resolved detection of characteristic ion fragments associated with each compound. This capability allows researchers to associate individual oxidation events with specific deactivating species rather than relying on total oxygen consumption alone.

Isotopic tracing in TPO experiments

Isotopically labelled oxygen, such as ¹8O2, provides additional mechanistic insight into TPO experiments by enabling the origin of oxygen involved in oxidation processes to be traced. MS distinguishes labelled from unlabelled products, allowing researchers to determine whether oxygen is supplied directly from the gas phase or from the catalyst lattice itself. Such a distinction is particularly important for catalyst deactivation studies involving redox-active catalysts, where participation of lattice oxygen can influence both the extent of deactivation and the mechanisms governing catalyst regeneration.

Transient kinetic resolution

Oxidation processes during TPO can occur on short timescales, notably when loosely bound carbon species ignite or previously blocked pores become accessible. Capturing these rapid events is critical in catalyst deactivation studies, as they often reflect the accessibility, location, and reactivity of deactivating species. MS delivers the temporal resolution required to track this transient oxidation behaviour in real time, preserving kinetic information that slower detectors cannot resolve and ensuring that short-lived oxidation phenomena are accurately represented in deactivation profiles.

Resolving overlapping oxidation processes

Catalysts often contain multiple deactivating species that oxidise over similar temperature ranges during TPO. MS helps resolve such complexity by distinguishing compounds based on their characteristic ion signals and fragmentation behaviour. As a result, overlapping oxidation features can be separated, enabling independent assessment of sulfur oxidation, heavy coke combustion, and water evolution within a single experiment.

Linking gas ratios to physical deactivation

The ratio of CO2 to CO measured by MS during TPO provides insight into the physical nature of coke deposits. Higher CO2 formation typically indicates more complete oxidation associated with surface-bound carbon, while elevated CO signals may reflect diffusion limitations or pore blockage. When interpreted alongside oxidation temperature, these ratios help determine whether catalyst deactivation is dominated by surface coverage or internal pore obstruction.

Quantifying deposited carbon

MS enables quantitative analysis in TPO through calibrating evolved gas signals against known standards, allowing absolute amounts of CO and CO2 to be identified. Integrating such signals over the temperature programme provides a direct measure of the total carbon removed from the catalyst. The quantitative data from MS-TPO form the basis of comparative catalyst deactivation studies, supporting systematic evaluation of deactivation rates, feedstock composition effects, and the impact of operating conditions on catalyst lifetime.

Assessing regenerability

One important objective of catalyst deactivation studies is defining safe and effective regeneration conditions for restoring catalyst activity. MS-TPO profiles identify the minimum temperature required to remove deactivating species. This precision avoids unnecessary thermal exposure that can cause sintering or structural damage, preserving catalyst performance over multiple regeneration cycles.