J.H. Block

Copper oxidation and surface copper oxide stability investigated by pulsed field desorption mass spectrometry

Abstract Pulsed field desorption mass spectrometry (PFDMS) has been used to examine several types of oxide layers formed by the in situ oxidation of clean copper surfaces. Films of Cu 2 O, CuO/Cu 2 O and CuO were produced by direct oxidation of Cu field emitters and examined for their behavior at different temperatures and for the effects of high fields. The high positive field pulses applied to the Cu 2 O overlayers result in field-enhanced ion injection at the copper/oxide interface, field-enhanced Cu + ion desorption or cation vacancy injection at the oxide/vacuum interface and produce only Cu + ions in the mass spectrum. CuO produces only Cu + , CuO + and CuO 2 + ions and CuO/Cu 2 O bilayers produce these ions until the CuO overlayer is removed from the Cu 2 O layer by field desorption. Removal of the CuO overlayer re-establishes the Cu 2 O field ionization process, namely Cu + ions which occur at approximately one half the field strength necessary for CuO field desorption at the same temperature. The low field strength required to produce the Cu + ions from a Cu 2 O overlayer reflects its ion conduction character and the ionic binding state of the Cu + . Heating the tip covered with Cu 2 O overlayers to higher than 470 K causes the overlayer to decompose and results in loss of its characteristic mass spectrum. It is concluded that PFDMS can be used to study the nature of the oxide overlayers formed during the early stages of oxidation and is projected to be a useful tool to examine electrostatic field effects during oxide growth.

The kinetics of adsorption and thermal desorption of NO on stepped Pt single crystal surfaces

The kinetics of adsorption and thermal desorption of nitric oxide on Pt field emitter surfaces has been studied by means of pulsed field desorption mass spectrometry (PFDMS). Adsorption on the high index planes with terraces of (111) orientation occurs molecularly with high initial sticking probabilities S=0.65,...,0.9. By varying the repetition frequency of the desorbing field pulses, i.e., by varying the field free reaction time tR (100 μs to 1 s), the mean lifetimes before thermal desorption of the NOad have been measured. The temperature dependence of the mean lifetimes has been evaluated. An activation energy of 139 kJ/mol and a preexponential of 3.8×10−15 s are found. These rate parameters are dominated by adsorption at step sites as verified on the basis of a model comprising multiple state adsorption and desorption. Dissociation of NOad has not been observed on stepped Pt(111), but has been on stepped Pt(100).

Observation of Rh‐subcarbonyls on stepped Rh surfaces during catalytic reactions

The interaction of CO, CO2, and CH3OH with stepped Rh field emitter surfaces has been studied by means of pulsed field desorption mass spectrometry (PFDMS) at 300 K and pressures at or below 1.3×10−4 Pa. A common feature in the PFD mass spectra of the different adsorbed layers is the occurrence of CO+ and Rh(CO)n+x ions (x=1–3, n=1,2). These species indicate that a surface reaction takes place between chemisorbed CO and Rh step sites to form adsorbed Rh‐subcarbonyls. COad is formed by CO adsorption from the gas phase as well as CO2 and CH3OH decomposition on the surface. The relative and absolute abundances of the various species depend on the desorption field strength FD of the pulses. High index Rh(CO)x are observed in large quantities at low FD. Under these conditions only small amounts of chemisorbed CO are removed and the surface is kept close to the saturation coverage. At high FD, the surface layer is completely desorbed, so that the COad concentration which builds up during the field free reaction...

Elemental Steps During the Catalytic Decomposition of no over Stepped Single Crystal Surfaces of Platinum and Ruthenium

The interaction of NO with stepped single crystal surfaces of Pt and Ru field emitters has been studied. Pulsed Field Desorption Mass Spectrometry (PFDMS) is employed to perform time resolved measurements. It is found that the NO adsorption (p = 1.3×10 −5 to 6.7×10 −4 Pa, T = 523 to 605 K) occurs molecularly on the high index planes of Pt with (111) orientation of the terraces, whereas molecular as well as dissociative adsorption is found to occur on the high index planes with (001) orientation of the terraces. By varying the repetition rate of the field pulses from 1 Hz up to 1 kHz (corresponding to a field free reaction time between 1 s and 1 ms) kinetic data of the adsorption, thermal desorption and decomposition of NO are obtained. The rate parameters for the first order thermal desorption on stepped Pt(111) are: E d =139kJ/mol, τ 0 = 3×10 −14 s. The initial stages of NO dissociation on stepped Pt(001) follow a complex kinetic mechanism which can be understood on the basis of structural autocatalysis. On stepped Ru(001) the dissociation proceeds according to first order kinetics. In addition, the oxygen deposition and accumulation leads to strong oxidation of Ru. This is evidenced by desorption of RuO x 2+ ions (x up to 3). A field induced decomposition of NO on Pt is observed and leads to high ionic rates of N 2 O and N + 2 . No marked face specifity has been found so far for this reaction.

Pulsed field desorption mass spectrometric study of the oxidation of hydrogen-doped Ni3Zr

Abstract The oxidation of a Ni 3 Zr field emitter, electrochemically doped with hydrogen, has been studied over a temperature range from 100°C to 250°C in an oxygen atmosphere with pressures ranging from 10 −4 Torr to 3 × 10 −1 Torr using PFDMS. Water was observed immediately after the first oxidation at 100°C and 10 −4 Torr O 2 and was observed throughout the series of oxidations. Pulse field desorption produced a wide range of ions: Ni + , Ni 2+ , H + , O + , O 2+ , O 2 + , H 2 O + , Ni(OH) 2 + , NiOH + , NiOH 2+ , NiOH 3+ , Ni 2 O 2+ , Ni(CO) + , Ni(CO) 2 + , Ni(CO) 3 + , Zr + , ZrO 2 3+ , ZrO 2 2+ , ZrO 2 + , Zr 2 O + , ZrO 3+ , ZrO 2+ and ZrO + ; which differ substantially from an atom probe investigation of the oxidation (dry, no H abs ) of the same alloy. The data show, for all oxidation temperatures and pressures, an initial strong enrichment of Ni at the surface. At lower temperatures and oxygen pressures this layer appears to be very thin. Previous low temperature/pressure thermal oxidation experiments performed on Ni 3 Zr (no H abs ) resulted in the preferential oxidation and segregation of Zr to the surface. Only at higher temperatures and/or oxygen pressures has preferential Ni oxidation and segregation been previously observed. This behavior is indeed observed during these experiments after the 250°C and 3 × 10 −1 Torr O 2 oxidation. The unusual H-doped Ni 3 Zr behavior at low temperatures/pressures appears to involve the formation of a thin layer of Ni(OH) x which is more stable than NiO. The results from the study of H-doped Ni 3 Zr yield some important new findings regarding the effect of hydrogen absorption on the alloy oxidation. In addition, the reaction of absorbed hydrogen with oxygen provides a unique way of introducing water on a sample in a UHV environment.