Kinetic instabilities in the NO/H₂ reaction on platinum

Abstract

We have studied the catalytic reduction of nitric oxide (NO) with hydrogen (H 2 ) on platinum using both field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS). Under certain conditions of temperature and gas pressure this reaction exhibits non-linear behaviour, i.e. hysteresis effects and rate oscillations occur. Ensembles of kink sites of the <100> zones are found to act as pacemakers, which is in line with earlier studies demonstrating f012g planes to ignite in a quasi-concerted manner.1,2 Experiments with tips of varying radius of curvature always lead to the same sequence of image patterns. Large variations in the H 2 partial pressure has little influence on the frequency of the rate oscillations.Using PFDMS under the reaction conditions we show that NO + rather than H 2 O + /H 3 O + (the neutrals are formed during the reaction process) is the imaging species in FIM. Interestingly, PtO n+ x .x ≤ 3/ are seen in the mass spectra, indicating that the reaction is controlled by a vacant site mechanism with the inclusion of partial oxidation of the Pt surface.

Field emission techniques for studying surface reactions: Applying them to NO–H2 interaction with Pd tips

“…Indeed, the present atom-probe results have revealed small amounts of (NO) 2 + supporting the above assumption. Similar observations were previously made for Au tips as well [40]. Such dimer species may originate from a gem-dicarbonyl precursor state which for sterical reasons should be preferentially bound to steps/kinks.…”

“…This interpretation is also in good agreement with TPD measurements on various Pd single crystal surfaces [19,20]. In contrast to earlier studies with platinum tips [21], the dissociative adsorption of NO on Pd seems to occur on the whole visible surface area without significant preference towards particular facets. Increasing the H 2 :NO ratio to 3.6 is found to cause immediate changes in the total brightness of the imaged tip apex as can be seen in Fig.…”

Adsorption of small molecules on gold single crystal surfaces

“…Kruse and co-workers [53] studied the adsorption of NO on Au 3D hemispherical crystals (''field emitter tips'') by means of pulsed field desorption mass spectrometry (PFDMS) under dynamic gas flow conditions and at 300 K. Mullins's group also showed that the NO uptake on the Au(111) surface is enhanced by the presence of oxygen adatoms, likely due to electronic or structural Hussain et al [50] used DFT to study NO adsorption on Au(111), (100), (110) and (310) surfaces, as well as on adatoms on Au(100). The results clearly show that the adsorption energy of the molecules increases considerably with increasing the degree of coordinative unsaturation of the gold atoms to which the molecules bind.…”

Field Ion and Field Desorption Microscopy: Surface Chemistry Applications

“…The detection of small amounts of (NO) 2 + , in turn, indicates that the appearance of N 2 O is caused mainly by the formation of (NO) 2 dimers and their dissociation toward N 2 O and O ad , 61 similarly as it was observed earlier for Au tips. 62 An important process that accompanies often the surface reactions is the change of the oxidation state and of the morphology of the sample. Here, FIM allows again a direct in situ observation, whereas a direct comparison of the local behavior of crystallographically differently oriented facets is possible (Figure 9).…”