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

Abstract

a b s t r a c tThe adsorption of NO and its reaction with H 2 over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10 À 3 Pa pressure range and at sample temperatures between 400 and 600 K. By varying the H 2 partial pressure while keeping the other control parameters constant, the NO +H 2 reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H 2 pressures delimiting this hysteresis decrease as well. Abrupt transformations of the micrographs are observed by FIM from bright to dark patterns and vice versa. These transformations define the hysteresis region. The collected data allow establishing a novel kinetic phase diagram of the NO + H 2 /Pd system within the range of temperatures and pressures indicated. The observed features are correlated with a local chemical analysis by means of field pulses. NO + seems to be the dominating imaging species under all conditions. At high relative H 2 pressures (the ''hydrogen-side''), H atoms seem to diffuse subsurface. This process is blocked at lower H 2 pressure (the ''NO-side'') due to NO ad and O ad accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO 2 + species in the mass spectra.

Field Ion and Field Desorption Microscopy: Surface Chemistry Applications

“…58-60 FIM brightness analysis provided data about the hysteresis behavior, and a corresponding kinetic phase diagram consisting of the NO-covered steady state, hysteresis region, and H 2 -covered steady state in the temperature region 450-575 K could be determined. 60 Local chemical probing suggests that processes in the subsurface region play a significant role, for example, the exchange of the hydrogen atoms between the adsorbed layer and palladium bulk and Pd surface oxidation (PdO 2 + ions were detected). 60 On the NO side of the reaction, N 2 O + ions were observed, suggesting the presence of N 2 O ad species on the surface.…”

“…60 Local chemical probing suggests that processes in the subsurface region play a significant role, for example, the exchange of the hydrogen atoms between the adsorbed layer and palladium bulk and Pd surface oxidation (PdO 2 + ions were detected). 60 On the NO side of the reaction, N 2 O + ions were observed, suggesting the presence of N 2 O ad species on the surface. 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.…”

Hydrogenation of NO and NO2over palladium and platinum nanocrystallites: case studies using field emission techniques

“…The details of the experimental procedure aiming at determining the hysteresis effect over field emitter tips have been described earlier. 3 Briefly, the tip temperature is raised to values of interest (E400-600 K) and pure NO is introduced at pressures from 10 À4 to 10 À2 Pa. The tip voltage is raised so as to develop an electric field of approximately +10 V nm À1 at the tip apex to produce a visible field ion pattern.…”

Improving the Understanding of Catalytic Processes using Field Emission Techniques: The Case of NO+H2 on Pd-Au.

“…Due to the dissociation of the NO gas species, the presence of adsorbate and subsurface oxygen atoms is possible and might be responsible for the observed reconstruction. It must be mentioned that experiments performed on pure Pd [2] and pure Au [3] showed the presence of NO2 + and (NO)2 + , and indicates that a disproportionation process must be considered in the mechanism of NO dissociation. The presence of both Pd and Au in the system allows the dissociation of NO to occur and could be due by the synergy between Au and Pd atoms.…”