Experimental measurements of hydrogen and CO surface diffusion on rhodium

Surface diffusion of hydrogen, deuterium and CO on Rh(111) has been investigated by laser-induced thermal desorption (LITD) and compared with previous results for these species on Pt(111) and on other metals. As the coverage θ of deuterium increases from 0.02 to 0.33, the pre-exponential factor D₀ remains constant at 8×10^-2 cm²/s, but the diffusion activation energy E_diff rises from 3.7 to 4.3 kcal/mol. E_diff for hydrogen is 0.6 kcal/mol lower than for deuterium, consistent with the difference in zero-point energy. For CO₂ E_diff=7 kcal/mol at all coverages, but D₀ rises from 10^-3 to 10^-2 cm²/s between θ=0.01 and 0.40. E_diff is found to be nearly identical to the reaction activation energies for the CO and hydrogen oxidation reactions on Rh and on several other transition metal surfaces. This suggests that surface diffusion of the reducing agent may be the rate-limiting step in their bimolecular surface reactions.