The Use of FT-IR to Analyze NOx Gases in Automobile Exhaust

Importance of the topic

The measurement and control of automotive exhaust emissions are central to meeting regulatory limits, improving air quality, and guiding engine and aftertreatment design. Modern emissions standards demand lower limits for CO, THC and NOx and require monitoring of additional species such as methanol and formaldehyde. Accurate, high time-resolution monitoring of a wide range of exhaust gases supports catalyst development, fuel formulation, and transient emissions profiling during drive cycles.

Objectives and overview of the study

This application note evaluates the use of Fourier transform infrared (FT-IR) spectroscopy, implemented in the Antaris IGS gas analyzer, for real-time analysis of automobile exhaust from an M85 (85% methanol) fuelled vehicle. The study aims to demonstrate (i) simultaneous speciation of NOx components at one-second resolution, (ii) the capability to measure raw exhaust without water removal, and (iii) how the system can be used to monitor catalyst behavior and transient reaction products during a US75 driving cycle.

Methodology

The experiment was performed in an automotive emissions laboratory using a chassis dynamometer to run a complete US75 driving cycle. Key methodological points:

• Raw tailpipe exhaust was sampled directly before and after the catalytic converter using two Antaris IGS FT-IR analyzers; no chemical pre-treatment or cold trapping was applied.
• Sampling interval: 1 second spectral acquisition and concentration calculation, enabling transient profiling of combustion and aftertreatment chemistry.
• Only a heated particulate filter was placed inline; water vapor remained in the gas stream and was included in the spectral analysis.
• Analyzer methods were optimized for M85 fuel and allowed calculation of up to ~40 gas species simultaneously; methods are extensible and raw spectral data are archived for later reanalysis.

Used instrumentation

• Antaris IGS gas analyzer (FT-IR based) — high-speed spectral acquisition and multicomponent fitting capability.
• Two analyzers deployed concurrently to monitor pre- and post-catalyst streams.
• Chassis dynamometer to reproduce the US75 driving cycle under controlled conditions.
• Heated particulate filter in sampling line to prevent particulate ingress; no cold trap or water removal.

Main results and discussion

The FT-IR system provided detailed, second-by-second insight into combustion and catalytic processes:

• NO and NO2 speciation: Pre-catalyst spectra show substantial NO and elevated NO2 when the engine is cold. NO2 concentrations decline as the engine warms. Post-catalyst spectra reveal that after the initial warm-up period (~30 s) NO2 is almost undetectable downstream of the catalyst while NO concentrations fall significantly once the catalyst reaches operating temperature.
• Catalyst warm-up dynamics: NO oxidation efficiency increases with catalyst temperature, exceeding 90% conversion of NO to N2 or benign products as the catalyst heats during the cycle.
• Transient reduction products: At catalyst cold-start, partial reduction of NO forms N2O. As the catalyst heats (around 55–60 s) N2O production declines and NH3 appears in the post-catalyst stream, indicating temperature- and surface-dependent changes in reduction pathways inside the converter. Neither N2O nor NH3 were detected upstream of the catalyst.
• Multi-species snapshot (example at 100 s, post-catalyst): The spectrum-derived snapshot included major components (H2O and CO2 at high concentrations), trace and regulated species such as CO (~166 ppm), NO (~127 ppm), NO2 (~1.6 ppm), N2O (~6.7 ppm), NH3 (~25 ppm), formaldehyde and methanol among others. Each concentration is reported with a fit-based error metric allowing evaluation of measurement quality.
• No water removal required: By using full-spectrum high-resolution FT-IR and multicomponent spectral fitting, interferences from water and overlapping absorptions are resolved sufficiently to avoid cold trapping or impingers that can alter gas composition or time resolution.

Benefits and practical applications of the method

• Real-time, high time-resolution speciation: One-second measurements capture transient emissions and catalyst light-off behavior, which bag sampling and laboratory chromatographic methods cannot provide.
• Comprehensive multicomponent analysis: FT-IR can quantify multiple regulated and non-regulated species simultaneously (including individual NOx species, carbonyls, alcohols, hydrocarbons), reducing the need for separate analyzers and sample collection systems.
• Reduced sampling preparation and artifacts: Direct analysis of raw exhaust avoids dilution bags, cold-traps, and impingers that can change gas composition and eliminate temporal resolution.
• Method flexibility and data reprocessing: Methods can be expanded to include new analytes and archived spectra can be reanalyzed post hoc to search for additional species without re-running tests.
• Diagnostic value for catalyst development: Time-resolved species profiles (e.g., NO → N2O → NH3 transitions) provide mechanistic insight into reduction/oxidation pathways and catalyst temperature-dependent behavior.

Future trends and potential uses

• Broader adoption of multicomponent FT-IR in development and regulatory testing as emissions standards expand to include oxygenated fuels and additional pollutants.
• Integration with engine control and on-board diagnostics for real-time emissions management and adaptive catalyst control strategies.
• Improved chemometric and spectral libraries enabling lower detection limits, faster fits, and automated interference correction for complex exhaust matrices.
• Hybrid measurement approaches combining FT-IR with fast GC or mass spectrometry to extend low-concentration speciation and isotopic analysis during transient events.
• Deployment in electric-hybrid and alternative-fuel vehicle testing where transient and cold-start emissions from auxiliary engines or fuel systems require high time resolution and multispecies capabilities.

Conclusions

The Antaris IGS FT-IR gas analyzer demonstrates clear advantages for automotive exhaust analysis: simultaneous, second-resolved speciation of NO and NO2 (and other nitrogen species), direct measurement of raw exhaust without water removal, and the ability to capture catalyst warm-up chemistry and transient reaction products. These capabilities make high-resolution FT-IR a powerful research tool for engine and aftertreatment development and for expanding emissions characterization beyond the limits of single-channel or bag-sampling techniques.

Reference

Thermo Fisher Scientific, Application Note 50649: The Use of FT-IR to Analyze NOx Gases in Automobile Exhaust, 2007. (Antaris IGS FT-IR gas analyzer application note describing M85 vehicle study and US75 driving cycle measurements.)