Segregation of boron and its reaction with oxygen on Rh

Abstract

The segregation of boron and its reactivity towards oxygen has been investigated by means of AES, XPS, UPS and ELS (in the electronic range) in the temperature range 11)0-1300 K. The segregation of boron in a Rh foil started from 700 K. The ~egregated boron produced a peak in XPS for the B(ls) level a~ 187.8 eV and emissions in UPS at 4,0 and 8,6-9.0 eV for B(2p) and B(2sp2), respectively, Analysis of the results suggested that the segregated boron on Rat foil mainly forms dimers or islands, instead of isolated monomers, without any significant charge transfer between rhodium and boron, Upon oxygen adsorption the B(ls) and O(ls) levels shifted to higher binding euergy (to 191,5 ~lnd 532.6 eV, respectively) and a new loss in the EELS was produced at 9,4 eV, demonstrating a strong chemical interaction between oxygen and boron. The interaction occurs at as low as 159 K, as indicated by tbe development of the 9.4 eV loss feature, It is assumed that boron suboxides are formed in which the B-B bond is retained, The cleavage of the B-B bond starts above 400 K attd is completed at 750 K, when the 2sp: hybrid state at 8.6-9.0 eV in the UPS, due to the B-B bond, is no longer detected. Formation of a polymer-like B:O3 species is proposed which reacts with elemental boron above 900 K to give B:O~. I. fail'eductionThe presence of adatoms (either as impurities or promot.rs) on metal surfaces can drastically influence the surface reactivity; this is in most cases attributed to an electronic interaction between the adatoms and the metals. However, there is increasing evidence that surface adatoms can interact directly with gaseous molecules, thereby strongly influencing the reactivity [1].We recently demonstrated that boron impurity segregating to a Rh surface dramatically alters the reactivity of the Rh surface towards N-and O-contain. ing moieties, such as CN [2], NO [3], CO 2 [41 and H~O 151, A possible reason for this phenomenon is that boron forms very strong bonds with N and O, which can promote the processes of surface dissociation of the adsorbed molecules. Following surface dissociation, we detected the formation of dissociation products via thermal desorption, and a new feature at 9,4 and 7.2 eV 0169-4332/89/$03.50

Preparation of a boron nitride single layer on a polycrystalline Rh surface

“…6B). The formation of a B O bond is also justified by the fine structure of B(KVV) and O(KVV) AES signal obtained in the interaction of oxygen with boron containing Rh at 300 K [32]. The AES transition of adsorbed oxygen on a B/Rh surface appeared at 513 eV at low exposure.…”

“…The segregation of boron was pointed out in earlier works [17][18][19][20][21][22][23][24]32]. It has been observed that boron is a major bulk impurity (17 ppm) in Rh.…”

“…The detection of this peak is doubtful, presumable due to its very low cross-section. The intensity of this photoemission at 8.6 eV increased and it shifted slightly to higher binding energy with increasing of the surface concentration of boron [32]. In a UPS study on iron borides, peak due to emission from the boron 2p, 2sp 2 and 2s states were identified [36].…”

Surface segregation of boron in dispersion-strengthened copper

“…The low BE peak is the larger of the two, and is assigned as above to elemental boron. The higher BE peak is believed to correspond to an oxide, but it is not clear whether it is the most common boron oxide, B203, or a defective suboxide or both; the BE of 191.2 eV has nearly the BE shift (4.1 eV, referenced to elemental boron) reported [5] in the literature for BzOs (4.6 eV).…”

“…The position of this peak is close to that assigned below to boron nitride, but no nitrogen was detected in this particular sample. Some authors have discussed boron suboxides like BxOz at surfaces [5]; perhaps there is a range of oxygen-deficient possibilities between B and B203.…”