The surface species whose decomposition gives rise to the high temperature peaks is a surface formate.

Rationale

The hydrogen evolved at low temperatures (approx. 50 % of the total desorbed hydrogen) presumably corresponds to the acidic hydrogen of the formic acid, indicating that the formic acid decomposes at low temperatures (possibly at the dosing temperature of 200 K and certainly below 250 K) to give the formate species and adsorbed atomic hydrogen.

HCOOH(g)   →   HCOOH(ads)   →  H(ads)   +   HCOO(ads)

Recombination and desorption of this hydrogen (at ca. 280 K)

H(ads)   +  H(ads)   →   H2 (g)

leaves only the formate species on the surface.

The formate species is stable to relatively high temperatures but eventually decomposes yielding the observed coincident desorption of hydrogen and carbon dioxide.

The hydrogen trace thus contains examples of both a desorption limited peak (at ca. 280 K) and a decomposition limited peak (at ca. 440-470 K)

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