Ion-molecule reactions of gas-phase chromium oxyanions: CrxOyH z − + O2

A. K. Gianotto 0 B. D. M. Hodges 0 P. de B. Harrington 0 A. D. Appelhans 0 J. E. Olson 0 G. S. Groenewold 0 0 Idaho National Engineering and Environmental Laboratory , Idaho Falls, Idaho, USA Chromium oxyanions, CrxOyHz , were generated in the gas-phase using a quadrupole ion trap secondary ion mass spectrometer (IT-SIMS), where they were reacted with O2. Only CrO2 of the Cr1OyHz envelope was observed to react with oxygen, producing primarily CrO3 . The rate constant for the reaction of CrO2 with O2 was 38% of the Langevin collision constant at 310 K. CrO3 , CrO4 , and CrO4H were unreactive with O2 in the ion trap. In contrast, Cr2O4 was observed to react with O2 producing CrO3 CrO3 via oxidative degradation at a rate that was 15% efficient. The presence of background water facilitated the reaction of Cr 2O4 H2O to form Cr2O5H2 ; the hydrated product ion Cr2O5H2 reacted with O2 to form Cr2O6 (with concurrent elimination of H2O) at a rate that was 6% efficient. Cr2O5 also reacted with O2 to form Cr2O7 (4% efficient) and Cr2O6 O (2% efficient); these reactions proceeded in parallel. By comparison, Cr2O6 was unreactive with O2, and in fact, no further O2 addition could be observed for any of the Cr2O6Hz anions. Generalizing, CrxOyHz species that have low coordinate, low oxidation state metal centers are susceptible to O2 oxidation. However, when the metal coordination is 3, or when the formal oxidation state is 5, reactivity stops. (J Am Soc Mass Spectrom 2003, 14, 1067-1075) 2003 American Society for Mass Spectrometry - R are important because they modify the surface eactions of metal oxides with atmospheric gases functionality, which in turn can alter the outcome of subsequent gaseous interactions. In the case of chromium, these changes can affect the macroscopic properties of toxicity and mobility in the atmosphere and in the geologic subsurface [13]. These behavioral attributes have motivated research into surface speciation of Cr-bearing surfaces, with the intent of correlating the explicit chemical form of the metal with transport and toxicity. Substantial research on Cr speciation has utilized desorption ionization mass spectrometry, either laser desorption mass spectrometry [4 16] or secondary ion mass spectrometry [14, 1720]. These studies showed that indeed Cr surface speciation could be correlated with the mass spectral fingerprint generated by the desorption ionization studies. However, it was also noted that relatively subtle changes in the composition of the surface adsorbates (i.e., water) would significantly alter the population of the desorbed ions. These observations pointed out that desorbed ions underwent reactions with neutral gases in the vicinity of the desorption event, which influenced the appearance of the spectra [10, 12, 16], and complicated interpretation of surface speciation. On the other hand, the reactions afford the opportunity to examine the reactivity behavior of ionic metal oxide species, provided that species-explicit, and timedependent control can be exerted over the experiment. Since desorption ionization processes produce a variety of potentially reactive metal oxyanions, those gas-phase species that participate in aggregation or dissociation reactions can be studied. In particular, metal oxyanions containing low-coordinate metal centers are frequently produced in abundance: these are of particular interest because they are responsible for many significant reactions both in the gas phase and in the condensed phase [2123]. Recently, Cr surface speciation has been revisited using an ion trap secondary ion mass spectrometer (IT-SIMS), which relies upon generation of gas-phase species by bombardment of surface species using a polyatomic primary particle [24 27]. The motivation behind reexamination of this topic was the possibility that the higher pressures extant in the ion trap (He bath gas) would serve to collisionally stabilize otherwise fragile molecular secondary ions [28], and would in the process amplify species-dependent mass spectral differences. However, the secondary CrxOyHz ions were observed to react with ambient H2O, and O2 in the ion trap. A systematic study of the reactions with H2O showed that ions containing undercoordinated metal centers would undergo either addition or radical abstraction reactions [29]. These reactions would proceed in series until a Cr coordination number of 4 was reached, except for CrO3 , which was unreactive. The reactions of the chromium oxyanions with O2 were also significant, and the present study describes these in detail. The O2 oxidation of small Cr-bearing species was previously examined using laser ablation in an O2 atmosphere, with product isolation and analysis in a frozen argon matrix [30]. A suite of neutral products having the compositions Cr1O2-4 and Cr2O2-4 were identified using FTIR, and ab initio calculations suggested structures containing rhombic Cr2O2 structural units [30, 31]. O2 reactions with ionic Cr oxide species were conducted by Bricker and Russell [32], who concluded that O2 would bind with Cr carbonyl anions in a molecular fashion, but gas-phase studies of Cr oxyanions more typical of the condensed phase have not been conducted. Here, O2 oxidation of Cr1,2OyHz species is examined using an ion trap SIMS instrument; this approach has been effective for elucidating reaction pathways and kinetics for ionic Al [33, 34], Si [34], and U oxides [35]. The results show that under the ambient conditions of the ion trap (1 104 torr He, 310 K), Cr1,2OyHz species containing low-coordinate, low oxidation state metal centers underwent oxidation. Instrumentation Molecular secondary ions were sputtered into the gas phase within the IT-SIMS from powdered potassium dichromate (Baker Chemical, Phillipsburg, NJ) samples that were attached to the end of a 2.7 mm probe tip with double-sided tape (3M, St. Paul, MN). The IT-SIMS instrument used in this study is a modified Finnigan ITMS (Finnigan Corp., San Jose, CA) previously described [36]. Modifications include incorporation of a perrhenate (ReO4 ) primary ion beam, an insertion lock for introduction of solid samples, and an offset dynode with multichannel plate detector. The primary ion gun and sample probe tip are collinear and located outside opposite end caps of the ion trap. The primary ion gun was operated at 4.5 keV and produced a ReO4 beam with a 1.25 mm diameter at a primary ion current ranging from 300 to 450 pA. The ReO4 beam was used because this type of ion beam is more efficient for sputtering larger cluster ions into the gas-phase compared to atomic particle bombardment [20, 24, 27, 37]. The data acquisition and control system uses the Teledyne Apogee ITMS Beta Build 18 software that controls routine ITMS functions and the filtered noise field (FNF) (Teledyne Electronic Technologies, Mountain View, CA). Data analysis was performed using SATURN 2000 software (version 1.4, Varian, Walnut Creek, CA). A variable lea (...truncated)