Light Alkane Activation on Rh(111), an Imperfectly Perfect Surface

Steam reforming is one of the most important industrial chemical processes. However, it is usually performed in large-scale reactors and has high energy demand. Catalytic partial oxidation of methane, on the other hand, can be carried out in compact and low capital-cost reactors to produce syngas. Rh is generally considered the best metal catalyst for partial oxidation.
Effusive molecular beam experiments were used to measure CH4 and C2H6 dissociative sticking coefficients, S(Tg, Ts ; θ) on a Rh(111) crystal, for which the impinging gas temperature, Tg, and surface temperature, Ts, could be independently varied, along with the angle of incidence, θ, of the impinging gas. The 500 – 900 K temperature range explored is relevant to heterogeneous catalytic processes such as methane partial oxidation. A dynamically biased precursor mediated microcanonical trapping (PMMT) model of dissociative chemisorption was used to analyze the experimental results. Modelling indicates that unlike on the Pt (111) surface where (111) terrace site reactivity dominates, methane reactivity on Rh (111) indicates that Rh step sites are not easily poisoned by C accumulation and can contribute substantially to the overall methane reactivity, especially at lower temperatures. Threshold energies for dissociative chemisorption on the terraces and steps sites were optimally modeled as 74.3 kJ/mol and 36.7 kJ/mol. Translations parallel to the surface and rotations were treated as spectator degrees of freedoms. The efficacy of vibrational energy to promote reactivity relative to normal translational energy was ηv=0.55 and one surface oscillator participated in energy exchange within the collisionally formed precursor complexes. A two-channel Arrhenius model restricted to only the thermal dissociative sticking coefficient measured along the direction of surface normal, Sn(T=Tg=Ts), yielded apparent activation energies of 70.6 and 25.5 kJ/mol which could be attributed to terrace and step sites, respectively. Such multidimensional reactivity studies allow for relatively facile designation of the terrace and step activity which allows us to reconcile single crystal and Rh foil reactivity studies. PMMT modeling of the step site reactivity on Rh(111) could be extrapolated to replicate the thermal dissociative sticking coefficient of the “defect dominated” Rh film surfaces measured by Ehrlich at temperatures in the 250 -350 K range where much of the elevated kinetic isotope effect (9 to 15) could be attributed to quantum mechanical tunneling through the reactive barrier.
In ethane dosing experiments, it is discovered that carbon migration happens at a significant rate at 900 K and carbon segregation in each direct would interfere with sticking coefficient measurements. C2 species might favor the formation of polymeric C or graphene on the surface which was not observed for methane, which may further complicate carbon behavior on Rh(111) surface.