Energetics and kinetics of CO and NO adsorption on Pt{100}: Restructuring and lateral interactions

Abstract

Calorimetric heats of adsorption and sticking probabilities are reported for NO and CO on both the reconstructed hex and the unreconstructed ͑1ϫ1͒ surfaces of Pt͕100͖ by single crystal adsorption calorimetry ͑SCAC͒, at room temperature. The hex surface reverts to the ͑1ϫ1͒ structure during adsorption of both gases, as previously reported. The initial heat of adsorption on the ͑1ϫ1͒ surface is 215 kJ/mol for CO and 200 kJ/mol for NO. Adsorbate-adsorbate interactions determine not only the dependence of the heat of adsorption on coverage but also the formation of different ordered structures. A model is suggested to explain the observed dependence of the differential heat on coverage and the LEED patterns, and a Monte Carlo simulation is performed to derive the corresponding differential heat, thus allowing estimates to be made of the magnitude of adsorbateadsorbate interactions. For CO adsorption, the critical contribution is the pairwise interaction energy ⑀ d between molecules in nnn sites while for NO triplet formation is suggested with significant repulsive interaction between molecules in the same triplet ͑⑀ t ͒ and an even stronger repulsion between triplet pairs ͑⑀ tt ͒. NO-NO repulsive interactions ͑⑀ t ϭ20 kJ/mol, ⑀ tt ϭ80 kJ/mol͒ are considerably stronger than CO-CO interactions ͑⑀ d ϭ5 kJ/mol͒; thus, at half monolayer coverage CO gives rise to a c͑2ϫ2͒ pattern while NO gives a c͑2ϫ4͒ pattern. Moreover, with CO the coverage can be increased to 0.75 ML, with the formation of compressed structures, while for NO the saturation coverage is just 0.5 ML. The differential heat on the hex surface is also discussed showing the possible role of adsorption at defect sites in the energetics of the system. The surface energy difference between the clean ͑1ϫ1͒ and hex surfaces is obtained as 20 kJ͑mol Pt s ͒ Ϫ1 by comparing the integral heats of adsorption of CO on both surfaces at ϭ0.5, when the final states of the two systems are identical.

First-Principles-Based Kinetic Monte Carlo Simulation of Nitric Oxide Reduction over Platinum Nanoparticles under Lean-Burn Conditions

“…Experimental studies of NO temperature-programmed desorption and calorimetric measurements on different Pt surfaces have shown that lateral interactions are significant and cannot be neglected. 23,24 The strong repulsive interactions between the O-O and the O-NO pairs on Pt surfaces have been studied using first-principles calculations. [14][15][16]25,26 The through-space interactions are modeled herein by using a van der Waals (vdW) interaction model taken from the Merck molecular force field.…”

“…The pre-exponential factor for bimolecular A* + B* surface reaction, where A* and B* are immobile, was calculated to be 10 -2 or 10 -3 cm 2 /s at 500 K and scaled by (T/500) 1/2 . 24 The deterministic rate equation for a bimolecular reaction system, however, must also be multiplied by the site density (10 15 sites/cm 2 ) in order to provide the pre-exponential factor in terms of s -1…”

“…The sticking coefficients of NO on the (100) and the (111) facets were taken to be 0.7 and 0.6, respectively. 24,45 The sticking coefficients of O 2 on the (100) and (111) facets were taken to be 0.1 and 0.02, respectively, based on experimental measurements. 46,47 The sticking coefficients of NO and O 2 on the edge (100)/(111) sites are assumed to be the same as those on the (111) facet.…”

Bridge-bonded adsorbates on fcc(100) and fcc(111) surfaces: A kinetic Monte Carlo study

“…49 There have been extensive discussions on the exact nature of the c͑4 ϫ 2͒ ordered structure, and other studies have suggested a different coverage of NO of 0.50 ML. 42,45,47,50 This coverage was proposed based on missing peaks in the LEED pattern, and the fact that on most other metals the saturation coverage of NO and CO is much higher than 0.25 ML. Notwithstanding these arguments we are of the opinion that the STM images presented in Ref.…”

Atomistic potential for adsorbate/surface systems: CO on Pt

“…For instance, Yeo et al 20 find, using calorimetric measurements, an adsorption energy of Ϫ1.94 eV for CO on Pt͕111͖, whereas Ertl et al 21 from isosteric measurements find Ϫ1.40 eV. The calorimetric measurements were performed for a series of Pt surfaces, including the Pt͕100͖-hex surface, 22 which makes, at least, comparison of the relative differences of the adsorption energies physically reliable and hence suitable to use in the fitting procedure. Unfortunately it is experimentally not possible to determine the difference in adsorption energy between the atop and bridge sites in a straightforward manner.…”