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FIELD DESORPTION OF INTERMEDIATES IN THE RHODIUM CARBONYL FORMATION REACTION
G. Kellogg
To cite this version:
G. Kellogg. FIELD DESORPTION OF INTERMEDIATES IN THE RHODIUM CARBONYL FORMATION REACTION. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-233-C6-238.
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FIELD DESORPTION OF INTERMEDIATES IN THE RHODIUM CARBONYL FORMATION REACTION
G.L. Kellogg
Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.
Abstract
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The formation of Rh carbonyl reaction intermediates resulting from the interaction of gas phase CO with Rh field emitter tip surfaces has been investig$ted with the imaging atom-probe. A Rh tip was placed in a background of -10 Torr CO and heated to temperatures between 400 and 500 K. As the carbonyl formation reaction proceeded, surface species were field desorbed and mass analyzed. The primary species detected were singly-charged CO and doubly- charged Rh. Rh(C0) , Rh(C0) 2 , and Rh(C0) Measurement of the temperature d e p e n d e ~ ~ e of the various ion signad. indicated that all species except Rh(C0)2 desorbed with approximately the same activation energy (1.0-1.3 eV).This result, combined with a consistent CO/Rh ratio of 3 for all mass scans, suggests that these signas were produced by fragmentation of a Rh(C0)3 parent molecule. The Rh(C0) signal was found to disappear at higher temperatures suggesting that it was iue to an intermediate in a stepwise reaction sequence.
Over the narrow field range investigated (1.5
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2.0 v/A), the activation energies for desorption were independent of the applied field. Time-gated images of the various desorbed ions indicated enhanced desorption around the (100) pole, but no real structural sensitivity in the formation of the reaction intermediates could be identified.I
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IntroductionThe formation of compound species such as carbonyls on surfaces is of considerable concern in the area of heterogeneous catalysis because these compounds are often volatile at reaction temperatures. Vaporization of a metal by carbonyl formation can result in deactivation of an active catalyst due to several mechanisms /I/.
Unfortunately, for a metal such as Rh, which is a widely-used catalyst in automotive exhaust converters, little research has been done to characterize the formation of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987638
JOURNAL DE PHYSIQUE
carbonyl species at the atomic level. The most extensive work relating to carbonyl formation on Rh has been an attempt by numerous investigators to identify the electronic and geometric structure of Rh(C0)2 on alumina-supported Rh catalysts /2- 4/. However, this work has not addressed questions relating to the atomic steps involved in the formation of carbonyls on Rh surfaces nor to the further reaction of the dicarbonyl species to the volatile Rh4(CO)
.
Only very recently have investigations of effects such as the particle size13ependence of Rh volatilization by carbonyl formation been undertaken / 5 / .In this papc- -le results of a preliminary investigation to characterize the formation of caruonyl species on Rh field-emitter surfaces by imaging atom-probe mass spectroscopy are reported. Unlike studies carried out on supported particles, the Rh surface structure and morphology in this investigation were characterized at the atomic level by field ion microscopy. The primary purpose of the study was to determine the stability and the crystallographic specificity of intermediates in the reaction process. The procedures employed were similar to those used previously in field desorption mass spectroscopy investigations of carbonyl formation on Ni /6/
and Ru /7/. The unique feature of the imaging atom-probe in this type of investigation is the ability to determine directly the location on the surface where species identified in the field desorption mass spectra originate.
The apparatus used in this investigation was an all-metal, ultra-high vacuum imaging atom-probe with a background pressure of 1-2 x 10 O Torr. The design and operation of the imaging atom-probe has been described in detail in a review article by Panitz /8/. The sample tip was prepared from 0.127-mm diameter Rh wire and spotwelded to a 0.127-mm diameter Pt wire loop. The tip was heated by passing a dc current through the Pt loop, and the temperature was monitored by resistance measurements / 9 / . The temperature control electronics were isolated from ground potential which permitted high voltages to be applied at the same time the tip was being heated. The desorption of surface species was initiated by the application of a negative high- voltage pulse applied to a counterelectrode spaced 1-2 mm from the tip. The flight times of the desorbed ions from the tip to the channel-plate assembly were measured on the sweep of a fast, transient-waveform digitizer interfaced to a microcomputer.
To study carbonyl formation on
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e tip surface, CO was admitted to the imaging atom- probe at a pressure of 1-5 x 10 Torr. The tip was heated to temperatures in the range from 400-500 K. A dc bias voltage was applied to the tip and imaging atom- probe mass spectra were recorded upon application of 600 V pulses at a time interval of 1 sec. As shown below, the mass spectra contained mass peaks associated with CO, Rh, and various carbonyl species. The ion signal intensities were determined from the amplitudes of the peaks observed in the time-of-flight spectra. Time-gated images of various desorbed species were obtained by reducing the dc voltage on the second channel plate of the ion detector assembly from its normal value of 2.0 kV to 1.4 kV and applying gate-voltage pulses of 800 V amplitude during the arrival of specific ionic species. It should be noted that all surface reactions in this study took place in the presence of an applied electric field of at least 1.0 v/A.I11
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Results and DiscussionAn example of an imaging atom-probe mass spectrum obtained from a Rh tip in Torr CO at 400 K is shown in Fig. 1. The mass spectrum contains peaks identified as singly-charged CO, doubly-charged Rh, and doubly-charged Rh(C0)
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The carbonyl species did not appear at temperatures below 400 K and began ti-aisappear from the mass spectra at temperatures above 500 K. To gain information on the carbonyl formation process, the amplitudes of these peaks were measured as a function of temperature and field strength. Time-gating experiments were also carried out to identify possible crystallographic specificity in the formation of these intermediate species.0 5 1 0 1 5 2.0 1
FLIGHT TIME CMICROSECONDS)
Fig. 1 - - An imaging atom-probe mass spectrum taken from a Rh sample in l x 10 Torr CO at 400 K. CO, Rh, and several carbonyl species are detected
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The results of the temperature dependence measurements are shown in Fig. 2. The signal intensities for the various species detected are plotted in Arrhenius form.
As indicated in Fig. 2(a) the CO, Rh, Rh(CO), and Rh(C0) ion signals all exhibit Arrhenius behavior indicating that field desorption
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these species is thermally activated. Moreover, the slopes of the plots are all the same within the experimental uncertainty indicating that these ionic species ,have the same activation energy of field desorption (ranging from 1.00-
1.02 eV). The fact that the activation energies are the same suggests that all four ionic species arise from a single parent molecular species on the surface. Given that the volatile carbonyl species on Rh is Rh (CO) and that the largest carbonyl species detected is Rh(C0)). we speculate tkat tA2 parent molecule is Rh(C0).
The remaining molecular species are believed to arise from field- induced 3fragmentation of Rh(C0) 3 . Additional support for this speculation is given below.The temperature dependence of the Rh(C0)2 signal is shown in Fig. 2(b). Note that the temperature dependence of this signal does not follow Arrhenius behavior. In fact the signal actually falls to zero at higher temperatures. This behavior indicates that the R~I(CO)~ observed in the mass spectra does not arise from
Fig. 2(a)
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Arrhenius plots of various Fig. 2(b)-
The Rh(C0) ++ signal does ion signal intensities vs. temperature. not exhibit Arrhenius gehavior.
The activation energies are nearly the same for the four species plotted.
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PHYSIQUEdissociation of the parent molecule Rh(C0) but rather is a true reaction intermediate in a stepwise reaction from ~h(2;)) to Rh(C0)
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Similar stepwise reactions have been proposed in the carbonyl formation reactqon on Ni and Ru field emitters / 6 , 7 / . The absence of Rh(C0) at higher temperatures can be attributed to the increased reaction rate of&(COP
to Rh(C0)3 which leads to a reduced surface concentration. To confirm this possibi?ity, measurements of the Rh(C0)2 signal intensity as a function of desorption pulse interval are required.The dependence of the ion signal intensities on applied electric field was also measured. Arrhenius plots were made at seven different field strengths between 1.5 and 2.0
v/A.
Within this narrow range of applied electric the activation energy for field desorption varied by only a few percent. If the activation energies for field desorption are truly independent of applied electric field, then the rate- determining step in the desorption process is the formation of the surface carbonyl species and the measured activation energies may correspond to the surface reaction and not the desorption process. Measurements over a wider range of applied field will be made to check this possibility.It was mentioned above that the temperature dependence of the ion signal intensities suggest the presence of a parent molecular species, R~I(CO)~, on the surface.
Further evidence for such a species is shown in Fig. 3. The graph is a plot of the total CO signal intensity (i.e., the sum of the CO signal and the carbonyl signals multiplied by the number of CO molecules per carbonyl) vs. the total Rh signal intensity (i.e., all signals containing Rh) for each mass spectrum recorded. The mass spectra were recorded over a temperature range from 400 to 500 K and a field range from 1.5 to 2.0 v/A. Although the total signal intensities vary over several orders of magnitude, the ratio of CO to Rh is consistently three indicating a 3/1 ratio of CO/Rh on the surface.
RHODIUM I O N S
Fig. 3
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A plot of the total CO signal intensity vs. the total Rh signal intensity from mass spectra recorded under various conditions of temper- ature and applied field. The ratio of CO to Rh is consistently 3.To determine whether or not the detected species exhibited any crystallographic specificity, time gating experiments were performed. As mentioned above, these experiments involve the application of high voltage pulses to the detector channel plates during the arrival of a selected species. These time-gated images are then compared to a field ion image of the substrate to determine the crystallographic origin of the selected species. A field ion image of the Rh substrate used in the present investigation is shown in Fig. 4(a). The major low-index +plane i b this image ibthe (100) plane. Time-gated images corresponding to the CO , Rh(C0) , and Rh(C0)3 signals are shown in Fig. 4(b-d). Although an enhancement of the signal in the region of the (100) plane is observed, the possibility that this is due to
' k
Rh(C0)3 Images, and the tip failed before time-gated images of the Rh(C0)2 signal were obtained.
IV - Summary
The results of this investigation indicate that, like Ni and Ru /6,7/, the formation of carbonyl species on Rh proceeds by a stepwise surface reaction. However, the temperature dependence of the detected species lead to a different interpretation as to the origin of the species. Whereas in the case of Ni and Ru it has been suggested that all the detected subcarbonyl species arise from surfyce intermediates, our results for Rh indicate that (with the exception of Rh(CO)Z ) , the detected species arise from field-induced fragmentation of a parent molecular species. The only surface intermediate of the stepwise reaction detected by field desorption is Rh(CO),. The measured CO/Rh ratio indicates that the parent species
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is Rh(C0I3. From a more general standpoint, the results of this study have shown that information relating to the reaction kinetics and the crystallographic specificity of the reaction process is accessible with the imaging-atom-probe technique. However, more extensive measurements are required before more definitive conclusions regarding the details of the reaction can be reached.
Fig. 4
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(a) field i e image of the+$h substrate. (b)-(d) Time-gated images for CO , Rh(C0) , and Rh(C0)3 , respectively.JOURNAL
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AcknowledwentpThis work was supported by t h e U. S. Department of Energy under Contract #DE-AC04- 76DP00789.
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