The chemisorption of hydrogen on the (111) and (110)-(1×2) surfaces of iridium and platinum

Abstract

The chemisorption of hydrogen on both the Ir(111) and Pt(110)-(1×2) surfaces has been examined under ultrahigh vacuum conditions with thermal desorption mass spectrometry, LEED, and contact potential difference measurements. No ordered adsorbate superstructures were observed on either surface at any fractional coverage and at surface temperatures from 100 to 700 K, and the (1×2) reconstruction of the Pt(110) surface was stable in all cases. Hydrogen adsorbs dissociatively on the Ir(111) surface, the adsorption reaction described by second-order Langmuir kinetics with an initial probability of adsorption of 7×10−3. The rate parameters describing the second-order desorption reaction of hydrogen from the Ir(111) surface are weakly dependent on coverage between fractional coverages of 0.1 and 0.3, and are given by Ed ≂12.6 kcal mol−1 and k(2)d ≂2×10−6 cm2 s−1. Beyond a fractional coverage of 0.3, however, both rate parameters decrease with increasing coverage. Hydrogen adsorbs dissociatively on the Pt(110)-(1×2) surface into two distinct β2 and β1 adstates, and the ratio of the saturation densities of these two states, β2:β1, is 1:2. Adsorption into the higher binding energy β2 adstate is described by first-order Langmuir kinetics with an initial probability of adsorption of 0.46, whereas adsorption into the β1 adstate is described by second-order Langmuir kinetics with an ‘‘initial’’ probability of adsorption of 0.022. The rate parameters describing the desorption reaction of hydrogen from the Pt(110)-(1×2) surface are strongly dependent on the coverage. In the coverage regime characteristic of the β2 adstate (θ≤0.32) the rate parameters are approximately symmetric about one-half of saturation of this state. Specifically, from the values for the zero-coverage limit of Ed ≂18 kcal mol−1 and k(2)d ≂10−4 cm2 s−1, the parameters first increase to maximum values of Ed ≂26.5 kcal mol−1 and k(2)d ≂0.3 cm2 s−1 at θ=0.15, and subsequently decrease approximately to the values for the zero-coverage limit at θ=0.32 In the coverage regime characteristic of the β1 adstate (θ>0.32), the activation energy decreases continuously with increasing coverage from a value of Ed ≂17 kcal mol−1 at θ=0.35, whereas the preexponential factor remains essentially constant with a value of 3×10−4 cm2 s−1. The contact potential difference for hydrogen on Pt(110)-(1×2) increases continuously with coverage to a value of 0.17 eV at θ=0.30. As the coverage increases further, however, it decreases continuously approaching a value of −0.50 eV at saturation. Probable binding states for the β2 and β1 adstates on the Pt(110)-(1×2) surface are inferred from both the adsorption and desorption kinetics and the contact potential difference measurements. Comparisons of the results obtained on the (111) and (110)-(1×2) surfaces of both iridium and platinum suggest strongly that local surface structure (e.g., ‘‘step’’ sites vs terrace sites) has a profound influence on the kinetics of adsorption of hydrogen on these surfaces. Surface structure apparently also has a profound influence on the desorption kinetics of hydrogen via the mediation of adatom–adatom interactions. Whereas both attractive and repulsive interactions are clearly manifest within the β2 adstates on the (110)-(1×2) surfaces, only repulsive interactions are apparent on the (111) surfaces and for the β1 adstates on the (110)-(1×2) surfaces.