Surface E a (kJ mol -1 ) ln(A) (s -1 ) ∆V* (Å To investigate the combined influence of temperature and stress on reactivity, the decomposition rate data was also plotted as a 3D surface and fitted to Equation 1. This approach provided significantly reduced uncertainties in the activation volume, activation energy, and pre-exponential factor compared to 2D fits in our previous NEMD study of trialkyl phosphate mechanochemistry. To avoid biasing the 3D fits towards the higher decomposition rates, fits to the logarithm of the rate were employed. The 3D fits for α-Fe(110), Fe 3 C(010) and Fe 3 O 4 (100) are presented in Figures ), and c), respectively. Using the 3D surfaces, we calculated ∆V * , E a , and A values across the entire range of temperatures and pressures studied, as shown in Table . Unlike the 2D plots, the 3D fitting method did not capture the changes in the parameters with temperature and pressure. However, compared to the 2D fits, the 3D fits led to reduced uncertainties in all three parameters in the Bell model, as shown in the ESI (Figures and). The 3D fits suggest that the activation energy increases in the order: Fe 3 O 4 (100) < Fe 3 C(010) ∼ α-Fe(110), which is opposite to that expected from the relative reactivity of the surfaces. The smaller energy barrier for the less reactive oxide surfaces may be due to the availability of oxygen atoms to promote P-O dissociation through nucleophilic substitution reactions at the phosphorus atoms. On the other hand, the pre-exponential factor is also much smaller for the iron oxide surface, suggesting that the fraction of successful collisions is also much lower. This may be due to the decreased availability of iron atoms at the surface, which are essential to promoting the dissociation reactions. Moreover, the ∆V * values increase in the order: Fe 3 O 4 (100) < Fe 3 C(010) < α-Fe(110), suggesting that the more reactive surfaces lead to an increased shear stress dependence of the reactivity. The larger activation volume for α-Fe(110) than Fe 3 C(010) and particularly Fe 3 O 4 (100) could be because the more reactive surfaces anchor the TCP molecules, leading to more efficient transmission of stress from the surfaces to the molecules, as suggested from previous NEMD simulations of different systems. This observation contrasts with previous experiments α-pinene tribopolymerisation on a range of surfaces, which showed that surfaces that promoted α-pinene chemisorption (e.g. copper oxide) gave a smaller activation volume for the mechanochemical reaction than the ones only capable of physisorption (e.g. silicon oxide). Another factor contributing to the difference in activation volumes between the three surfaces could be the contact stiffness. In our NEMD simulations, at a given temperature, the stiffness of the three surfaces is quite similar (within 10 %), as shown in the ESI (Table ). At most temperatures, the contact stiffness increases in the order Fe 3 C(010) < α-Fe(110) < Fe 3 O 4 (100). Since the activation volume is expected to be inversely proportional to the contact stiffness, the increased stiffness of the Fe 3 O 4 (100) surface may also contribute to its smaller activation volume compared to the other two surfaces. However, the differences in contact stiffness probably play a secondary role compared to the adsorption strength in determining the variation in activation volume between the three surfaces.