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

The adsorption of NO and its reaction with H2 over Pd tips were investigated by means of field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) in the 10−3 Pa pressure range and at sample temperatures between 400 and 600 K. By varying the H2 partial pressure while keeping the other control parameters constant, the NO+H2 reaction over Pd crystallites is shown to exhibit a strong hysteresis effect. The hysteresis region narrows with increase in temperature and the H2 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+H2/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 H2 pressures (the “hydrogen-side”), H atoms seem to diffuse subsurface. This process is blocked at lower H2 pressure (the “NO-side”) due to NOad and Oad accumulation on the surface. Probe-hole measurements with field pulses indicate that the Pd surface undergoes oxidation as revealed by the occurrence of PdO2+ species in the mass spectra.

Research Highlights
► Palladium crystallites provide bright FIM micrographs when imaged by nitric oxide (NO gas).
► The addition of hydrogen gas to NO gas causes abrupt changes of brightness at well-repeatable PNO/PH2 ratios.
► Brightness changes exhibit hysteresis behaviour and are correlated to modifications of the local chemical composition of the surface.
► A kinetic phase diagram has been established for the NO+H2/Pd system between 450 and 575 K.
► It is advanced that the Hadsorbed⇌Hbulk equilibrium is modified by the presence of Oad resulting from NO dissociation.