The pre-exponential factor, A, is calculated from the intercept of the fits of Equation 1 to the rate versus inverse temperature data shown in the ESI (Figure ). This is often expressed as A = Zρ, where Z is the frequency or collision factor, and ρ is the steric factor. The steric factor represents the fraction of successful collisions and is usually less than unity. For TCP molecules between the α-Fe(110) surfaces, ln(A) increases with pressure from 25.4 ± 3.2 at 1 GPa and to 28.1 ± 3.86 at 5 GPa. For the Fe 3 C(010) surfaces, ln(A) increases from 25.4 ± 3.6 at 1 GPa to 27.8 ± 4.7 at 5 GPa, while it increases from 22.6 ± 4.5 to 25.6 ± 2.1 for the Fe 3 O 4 (100) surfaces. The low-pressure values are in good agreement with previous estimates of the pre-exponential factor for the thermal decomposition of TCP on stainless steel at atmospheric pressure, ln(A) = 22.7. Figure shows the variation of ln(A) with E a from the 2D fits to the rate versus temperature data. The concurrent increase in E a with ln(A), which is known as the kinetic compensation effect, is often observed in heterogeneously catalysed processes. Previous studies using ReaxFF to study the mechanochemical behaviours of trialkyl phosphates have also observed such an effect. The presence of a compensation effect suggests that the changes in these parameters may have a mathematical rather than physical origin and does not necessarily indicate any changes to the reaction pathway. Transition state theory 102 predicts a pre-exponential factor of about 10 13 s -1 for first-order reactions, while experimental preexponential factors for surface reactions vary from 10 10 -10 16 s -1 . 103 Some previous mechanochemistry studies have used a constant pre-exponential factor of 10 13 s -1 to determine the activation energy. If a constant pre-exponential factor of 10 13 s -1 (ln(A) = 30) is used for all three surfaces (horizontal red dashed line in Figure , 8 the activation energies of TCP decomposition for the three surfaces from the 2D fits increase in the order α-Fe(110) ∼ Fe 3 C(010) < Fe 3 O 4 (001). Thus, the more reactive surfaces towards TCP give lower activation energies, which is consistent with previous experiments of α-pinene tribopolymerisation.