In conjunction with an industrial partner (Sasol Technology UK), the applicants have previously used INS measurements performed on the MAPS spectrometer to characterize a series of iron based Fischer-Tropsch synthesis catalysts after various regimes of reaction testing. Most recently (RB1610413 + RB1810122) the programme of work has been extended to examine the Fischer Tropsch-to-olefins (FTTO) process, which requires the addition of specific chemical promoters to shift the product distribution from high molecular weight linear alkanes to low molecular weight olefins. This proposal seeks to use INS to examine the nature and form of the hydrocarbonaceous overlayer associated with time-on-stream for a test reaction (CO hydrogenation at 623 K) that includes the use of CO2 in the feed stream. This arrangement closely mimics the FTTO industrial scenario.