J. Phys. Chem. C 2008, 112, 17631–17634
17631
Enhanced Carbonate Formation on Gold Jinlong Gong and C. Buddie Mullins* Department of Chemical Engineering, Center for Nano and Molecular Science and Technology, and Texas Materials Institute, UniVersity of Texas at Austin, Austin, Texas 78712-0231 ReceiVed: July 3, 2008; ReVised Manuscript ReceiVed: September 9, 2008
We report the enhanced formation and decomposition of carbonate (CO3 T CO2 + Oa) from the reaction of C18O2 preadsorbed on Au(111) with an 16O atomic beam. The amount of formed carbonate increases significantly (by a factor of ∼4) compared to thermally accommodated C18O2 and 16Oa coadsorbed on the Au(111) surface. The results suggest that the reaction occurs prior to the accommodation of the incident atomic oxygen via a precursor-mediated mechanism. Introduction
Experimental Section
Mechanisms of gas-solid surface reactions are important and have been intensively investigated over the past several decades due to the numerous applications regarding catalysis in industrial processes.1 In particular, understanding of the formation of the carbonate anion is fundamentally and practically significant in heterogeneous catalysis.2-4 Fundamental investigations of carbonate on noble metals originate from identification of carbonate via infrared spectra5 and thermal desorption spectra from Ag(110).6-8
The experiments reported here were performed in a molecular beam surface scattering apparatus in which the ultrahigh vacuum scattering chamber has a base pressure of ∼ 1 × 10-10 Torr. Briefly, the scattering chamber is equipped with a reverse-view low energy electron diffractometer (LEED), an Auger electron spectrometer (AES), a quadrupole mass spectrometer (QMS), and an ion gun. More detailed descriptions of the apparatus are reported elsewhere.42-44 The beams for the experiments were produced by expanding the gas through a 200 µm diameter alumina nozzle, which is part of a supersonic, radio frequency (RF) plasma-jet atom source used for depositing atomic oxygen. Molecular beams result in a circular beam spot of diameter ∼3 mm on the surface at normal incidence; this is much smaller than the dimensions of the sample, thus minimizing gas interactions with other surfaces in the chamber. Beams are directed at a Au(111) single crystal (11 mm in diameter) mounted to a tantalum plate. Two tantalum wires are spot-welded to the tantalum plate to allow for computer-controlled, resistive heating of the sample. The tantalum heating wires are attached to two copper posts mounted on the sample probe that are in thermal contact with a liquid nitrogen reservoir. The sample temperature is measured by a chromel/ alumel thermocouple (type K) spot-welded to the top edge of the tantalum plate. The temperature can be accurately controlled by using a PID controller from 77 to 1200 K. The absolute temperature of the sample is calibrated with the known multilayer desorption temperature for water and/or recombinative desorption temperature of atomic oxygen. Temperature-programmed desorption experiments are conducted in an angle-integrated fashion. The crystal surface is within (0.5° of the (111) plane and contamination levels (primarily surface carbon) can be reduced to less than ∼0.02 monolayer (ML) by repeated Ar+-ion sputtering. This final small amount of carbon is routinely removed by exposing the sample to an atomic oxygen beam and then flashing to 700 K. An 8% (vol) O2 in argon mixture was used for generating atomic oxygen. Oxygen (16O and 18O) atoms were deposited on the surface employing our RF plasma-jet source with ∼ 40% dissociation fraction as determined via timeof-fight (TOF) measurements,45 which produced a supersonic beam of O atoms with an operational flux of about 0.02 ML/s at normal incidence (1 ML ≈ 1.387 × 1015 cm-2).43,46 Ionic
Gold catalysis has drawn tremendous attention since Haruta’s report in 1987 of the exceptional catalytic activity of supported nanoparticles (NPs) 2-5 nm in diameter.9 Since then several reactions have been shown to be effectively catalyzed on both Au NPs and single crystalline Au.10-31 Particular attention has been paid to low temperature CO oxidation17,20-22,25,31-36 on gold. However, regarding CO oxidation on gold, several details of the reaction mechanism remain unknown. One point needing clarification concerns the role of a carbonate species that has been considered in many mechanistic schemes to be an intermediate during CO oxidation.36-38 Unlike silver, until recently,39 carbonate has not been detected on gold surfaces and clusters.40,41 Therefore, investigation of the formation and decomposition of carbonate on Au(111) is of great interest to the catalysis community. Recently, our group has investigated carbonate formation and decomposition from the adsorption of oxygen-labeled carbon dioxide (C18O2) on an atomic oxygen (16O) precovered Au(111) surface.39 To further examine carbonate formation, herein we report on our investigations of the reaction of C18O2 preadsorbed on Au(111) with oxygen atoms provided from the gas phase via an atomic oxygen beam. The amount of formed carbonate with impinging gas-phase oxygen atoms is greatly increased (by a factor of ∼4) compared to the thermally driven case with C18O2 and 16Oa coadsorbed on the Au(111) surface. The results reported here suggest that carbonate formation occurs prior to the complete thermal accommodation of the incident atomic oxygen on the surface via a precursor-mediated mechanism. * To whom correspondence should be addressed. E-mail: mullins@ che.utexas.edu.
10.1021/jp805871j CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
17632 J. Phys. Chem. C, Vol. 112, No. 45, 2008
Figure 1. TPD spectra of 16O18O (m/e ) 34) after (a) dosing 1.3 ML of 16O on clean Au(111); (b) backfilling 30 L of C18O2 on Au(111) precovered with 1.3 ML of 16O; (c) dosing 1.3 ML of 16O on Au(111) precovered with 30 L of C18O2 via backfilling. All doses and/or backfillings are done at 77 K, and the heating ramps are 3 K/s.
species are deflected out of the beam line using a charged plate biased at 3 kV. The C18O2 (Cambridge Isotopes Inc., 95% C18O2) is backfilled into the scattering chamber using a leak valve. Results and Disscussion The surface carbonate forms via a reaction of adsorbed C18O2 and impinging 16O atoms and decomposes to form either C18O2 or C18O16O while leaving either 16Oa or 18Oa adatoms on the surface, respectively. Upon heating, the oxygen atoms recombinatively desorb to form 16O2 (mass 32) and 16O18O (mass 34) (18O2 is negligible because of low surface concentration). Therefore, carbonate formation and decomposition can be measured by monitoring the presence of mass 34 (18O16O) in temperature programmed desorption (TPD) spectra. Figure 1a shows a TPD spectrum of 16O18O produced from 1.3 ML of 16O deposited on Au(111) employing our RF plasma-jet source without preadsorbed C18O2. As a control experiment, notably, only ∼0.5% of the total amount of oxygen desorbs as mass 34 when the Au(111) surface is precovered with 16O but with no exposure to C18O2 (due to the natural isotopic abundance of 18O). Additionally, from a separate control experiment we know that no mass 34 is produced when the Au(111) surface is exposed to C18O2 without preadsorbed atomic oxygen.39 Figure 1b displays the desorption of 16O18O produced from exposure of the oxygen precovered surface [1.3 ML 16Oa/Au(111)] to 30 L (1 Langmuir ) 1 × 10-6 Torr · s) of C18O2 at 77 K and then heating the surface to 700 K. Figure 1c shows the mass 34 produced from reversing the order of exposures of 16Oa and C18O2 [i.e., we impinged 1.3 ML 16Oa at 77 K on the Au(111) surface after a 30 L C18O2 exposure]. For the latter case, much more 16O18O desorbs upon heating the surface. Assuming a statistical distribution in the decomposition of the surface-bound carbonate C16O18O2, we estimate the reaction probability of carbonate formation for the case shown in Figure 1c to be ∼0.03 (uncertainties of (20%). The amount of 16O18O desorbing for the experiment depicted by Figure 1c is ∼4 times larger than that shown in Figure 1b. We note that, with the same CO2 exposure, the CO2 coverages on clean Au(111) are much less than those on the atomic oxygen
Gong and Mullins covered surface employing King and Wells measurements. Therefore, it appears that the decreased reactivity in the backfilling experiment (when O atoms are deposited first) is not due to the fact that less CO2 is on the surface when the surface is precovered with O atoms. The relatively high yield of carbonate indicated by Figure 1c suggests that the nature of the adsorbed atomic oxygen (i.e., excited or ground state) plays a significant role in carbonate formation reactions. It might also be possible that molecular oxygen in excited states (generated from the plasma-jet) plays a role in the reaction. However, we expect this contribution to be negligible compared to that involving incipient adsorbed oxygen adatoms. When an oxygen atom strikes the surface it does not thermally accommodate immediately. Rather, energy is exchanged between the various degrees of freedom of the system, including internal and translational degrees of freedom of the oxygen atom. Transport across the surface for either short or large distances is also possible depending on the nature of the surface-adsorbate interaction and the efficiency of energy transfer.47 During the thermalization (accommodation) process on the surface, oxygen atoms can be considered to be “hot” precursors with their chemical state between the gas phase and chemisorbed state.47 Ertl and co-workers investigated the dissociative chemisorption of O2 on Al(111) by scanning tunneling microscopy and showed that, upon dissociation of O2, at least part of the chemisorption energy transforms into translational energy parallel to the surface.48 This leads the two oxygen atoms to separate from each other by at least 80 Å before the excess energy is dissipated.48 Their result suggests the lifetime of a “hot” oxygen adatom is many vibrational periods. When a “hot” oxygen adatom encounters an adsorbed CO2 molecule, direct formation of the carbonate is possible. We can not rule out gas-phase oxygen atoms reacting upon direct collision with adsorbed CO2 via an Eley-Rideal (ER) mechanism. However, it is more likely that the formation reaction occurs after the incident atom has impinged, but before it has become fully accommodated as discussed above. Such a mechanism is between the limiting LangmuirHinshelwood (L-H) and Eley-Rideal process and is described by Harris and Kasemo as a precursor-mediated mechanism.47 Similar phenomena have been reported regarding H + Cl/ Au(111)49 and for O + CO on Pt(111),43,50-52 Ir(111),43 and Ru(001).43 The reaction probability is calculated based on the number of oxygen atoms reacting with preadsorbed C18O2 to form carbonate compared to the total amount of atomic oxygen impinged on the Au(111) surface. It is possible that during impingement incident oxygen atoms can react with oxygen adatoms on the Au(111) surface, which would affect the accuracy of our calculation of reaction probability. Indeed, we made scattering measurements in which we monitored gas-phase mass 34 when impinging 18O on an 16O precovered Au(111) surface and our results show that only ∼4% of the impinging 18O atoms react with predeposited 16O. This result suggests that an oxygen depletion reaction via this channel would have negligible effect on our calculation of reaction probability and so we ignore it. Likewise, preadsorbed CO2 may desorb from impinging atomic oxygen due to the heat of adsorption,22,45,53 which could cause significant uncertainties in the calculated reaction probability. Surprisingly, we found this effect to be negligible in a control experiment in
Enhanced Carbonate Formation on Gold
J. Phys. Chem. C, Vol. 112, No. 45, 2008 17633 oxygen via a precursor-mediated mechanism. This result may have relevance to other surface chemistry; for example, the reaction between CO and adsorbed molecular oxygen O2,a to make CO2 may produce a “hot” oxygen adatom that could encounter other adsorbed moieties on real or model catalysts and react further. Acknowledgment. The authors thank T. Yan for experimental assistance. We also greatly appreciate the helpful comments of the referees. Finally, we acknowledge the Department of Energy (DE-FG02-04ER15587), Welch Foundation (F-1436), National Science Foundation (CTS-0553243) and the donors of the Petroleum Research Fund, administered by the American Chemical Society, for their financial support. References and Notes
Figure 2. Reaction probability as a function of C18O2 exposure (0.5-30 L). The reaction probability determined from experiments of impinging 1.3 ML oxygen atoms on Au(111) at 77 K after C18O2 exposure on Au(111) at 77 K. The reaction probability increases continuously with increasing C18O2 pre-exposure.
which no mass 44 (C16O2) was detected when impinging 16O on C16O2 preadsorbed Au(111). To further explore the carbonate formation reaction from impinging O atoms on the CO2 covered surface, we have investigated the effect of CO2 precoverage between 0.5 and 30 L. As shown in Figure 2, the reaction probability increases continuously with increasing C18O2 pre-exposure. This result supports a precursor-mediated mechanism. At low CO2 coverage, “hot” oxygen adatoms (precursors) must travel relatively long distances on average before encountering a CO2 molecule. The most likely scenario for this case is that on average an atomic oxygen (excited) precursor more likely quenches prior to reacting due to the distance required to encounter a CO2 ad-molecule. This thermally accommodated chemisorbed Oa species is much less reactive compared to those in a transient, excited adsorbed state. In contrast, it seems reasonable that, as the CO2 coverage increases, the “hot” Oa precursors would more frequently encounter an adsorbed CO2 on Au(111) and react before being thermalized. Previous investigations of gas-phase radical surface chemistry have shown very little surface temperature dependence in the reaction probability. Yates and co-workers studied the chemistry of impinging atomic hydrogen with halogenterminated Si(100) surfaces and showed increasing temperature resulted in a small increase in the reaction rate: for the Br-Si(100) surface the apparent activation energy was 0.07 ( 0.009 eV and for Cl-Si(100) the apparent activation energy was 0.09 ( 0.009 eV.54 Additionally, Wheeler et al. investigated CO oxidation from impinging oxygen atoms on CO covered Pt(111) and Ir(111).43 Apparent activation energies of -0.003 and -0.006 eV were obtained for Pt(111) and Ir(111), respectively.43 Unfortunately, we are unable to study the surface temperature dependence of the experiments reported here since CO2 desorption starts at ∼90 K on Au(111)35 leading to drastic changes in CO2 coverage above 90 K and the lowest surface temperature we can obtain is 77 K. In summary, we have presented experimental results regarding the enhanced formation of adsorbed carbonate from the reaction of C18O2 preadsorbed on Au(111) with an 16O atomic beam. Our results suggest that the reaction occurs prior to the thermal accommodation of the incident atomic
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