Extreme Ionization Leading to Coulomb Explosion of Small Palladium

May 4, 2012 - Extreme Ionization Leading to Coulomb Explosion of Small. Palladium and Zirconium Oxide Clusters and Reactivity with Carbon. Monoxide...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCA

Extreme Ionization Leading to Coulomb Explosion of Small Palladium and Zirconium Oxide Clusters and Reactivity with Carbon Monoxide Matt W. Ross and A. W. Castleman, Jr.* Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Reported herein are strong-field ionization studies of small, neutral PdxOy and ZrxOy clusters made using ultrafast laser pulses (∼100 fs) centered at 624 nm. An enhancement in ionization of nearly 1.5 orders of magnitude lower in laser intensity than predicted from literature values is observed for both systems due to clustering. The change in enhancement upon addition of carbon monoxide at different pressures was also studied. Enhancement of high charge states of palladium was found to decrease upon CO addition, whereas in the case of the zirconium system, high charge states of zirconium were observed to increase. Pd and ZrO showed similar reactivity trends with CO and were found to have similar reactivity ratios in accord with their isovalent nature. found similar rates for Pd+ and ZrO+ reacting with ethane and propane and showed the primary mechanism to be the scission of a C−C bond in each hydrocarbon species. Density functional calculations of Pd+ and ZrO+ showed similar highest occupied molecular orbitals for each species, and they postulated that this was likely the reason that similar reactivity was observed. Similarly, ZrO2+ is expected to be the superatom counterpart of PdO+ and ZrO3+ the counterpart of PdO2+ due to each species having the same number of valence electrons. The highest occupied molecular orbital of each of these species, however, has a dissimilar shape. Studying the electronic properties of larger ZrxOy/PdxOy clusters is critical to understanding how these compounds may behave isovalently. Previous studies have shown that clusters can exhibit an enhancement in ionization in which lower laser intensity requirements for the production of high atomic charge states are observed from that predicted from tunneling rate equations upon exposure to ultrafast light pulses leading to Coulomb explosion.13,14 In this study, we investigate the Coulomb

1. INTRODUCTION Late-group transition metals are very useful catalysts for effecting many classes of reactions due to their high activity and mostly filled d orbitals.1−6 Specifically, group 10 metals are efficient catalysts used for many processes such as the oxidation of carbon monoxide to carbon dioxide. These metals, however, are expensive rare-earth elements, and searching for costeffective alternatives has become an area of extensive research.7−9 In particular, experiments have been undertaken to investigate if molecules with similar valence-shell configurations as group 10 metals will exhibit similar reactivity (termed “superatom” compounds).10 Reactivity trends between isovalent “superatomic” species have generated much interest due to the prospect of development of less expensive catalytic materials. ZrO and Pd are known to be isovalent species and display similar reactivity, but there is a paucity of information on the reactivity of larger oxides as well as the influence of bound reactants on their behavior. Pepernick et al. performed photoelectron studies on Pd−/ZrO− and WC−/Pt− (each having a total of 10 d electrons) to investigate their molecular orbital structure.11 It was found that Pd− and ZrO− displayed similar binding-energy photoelectron spectra, and ground-state excitations were assigned for each species. Tyo et al. extended this to investigate the reactivity rate of Pd+ versus ZrO+ with small hydrocarbons and to elucidate the mechanism.12 They © 2012 American Chemical Society

Special Issue: Peter B. Armentrout Festschrift Received: March 12, 2012 Revised: May 4, 2012 Published: May 4, 2012 1030

dx.doi.org/10.1021/jp302394a | J. Phys. Chem. A 2013, 117, 1030−1034

The Journal of Physical Chemistry A

Article

Figure 1. (a) High charge states of palladium subsequent to Coulomb explosion of palladium oxide clusters. Niobium oxide is coproduced in the system for comparison. (b) High charge states of zirconium subsequent to Coulomb explosion of zirconium oxide clusters.

detecting by multichannel plate detectors. To probe neutral clusters, the ion optics were held at static voltages that allow only noncharged species to enter the extraction region and interact with the femtosecond laser beam. Linearly polarized laser pulses (with a pulse width of 100 fs and energy of ∼1 mJ) from a previously described17 colliding pulse mode-locked laser centered at 624 nm were used to excite the neutral clusters and were detected using a multichannel plate detector. Carbon monoxide gas (99%, Matheson Tri-Gas) and acetylene gas (99.9%, VWR) were individually introduced into the cluster beam via a pick-up source in the vacuum chamber prior to ionization using a needle valve to control the flow rate.

explosion of small, neutral palladium oxide and zirconium oxide clusters to ascertain the maximum high charge states observed and how this distribution changes upon interaction with carbon monoxide gas. This provides insight into the stability of PdxOy versus ZrxOy clusters.

2. EXPERIMENTAL SECTION The experimental details used to perform these experiments have been described previously.15 Briefly, palladium oxide clusters are created utilizing a laser vaporization (LaVa) source, in which the second harmonic of a Nd:YAG is used to ablate a rotating/translating palladium rod of 99.9% purity. A pulsed jet of ∼1% oxygen seeded in high-purity helium is passed over the rod as a carrier gas while maintaining a pressure of 1 × 10−6 Torr in the vacuum chamber. The resulting plasma of palladium, oxygen, and helium atoms undergoes supersonic expansion, leading to the formation of a cluster distribution of both neutral and ionic species. Zirconium oxide clusters are created in an analogous way using a 99.9% zirconium metal rod. The resulting cationic, neutral, and anionic clusters pass through a skimmer and thereafter travel toward a Wiley− McLaren16 time-of-flight (TOF) mass spectrometer. To gain insight into cluster size distributions, cationic clusters were observed by pulsing the TOF voltage grids and subsequently

3. RESULTS AND DISCUSSION 3A. Palladium Oxide Clusters. Small, neutral palladium oxide clusters were formed similar to those observed by Tyo et al.12 Clusters containing up to four palladiums are clearly observed, and the most stable peak of each series appears to be n palladiums (n = 2−4) bound to one oxygen in the initial cationic distribution, which is expected to be very similar to the neutral cluster distribution (see Supporting Information Figure S1). Direct ionization of palladium using a femtosecond laser centered at 624 nm showed only the first charge state of oxygen, indicating that the intensity of the laser can directly 1031

dx.doi.org/10.1021/jp302394a | J. Phys. Chem. A 2013, 117, 1030−1034

The Journal of Physical Chemistry A

Article

Figure 2. (a) Ratio of the intensity of high charge states of palladium to Pd+ as the pressure of CO is increased. (b) Ratio of the intensity of high charge states of zirconium to Zr+ as the pressure of CO is increased.

Table 1. Ionization Potentials of Relevant Species in eVa

ionize species up to ∼20 eV (Supporting Information Figure S2). Figure 1a shows the high charge atomic states of palladium and oxygen resulting from Coulomb explosion of PdxOy clusters. Ionization enhancement in clusters is a well-known phenomenon that refers to a lowering of the sequential ionization potential of an atom by space charge or collective electron effects. Ionization enhancement has previously been quantified 18 in small niobium oxide clusters that are coproduced alongside of palladium oxide clusters for direct comparison in this system. Pd5+, O3+, and Nb4+ are the maximum atomic charge states of each species observed subsequent to Coulomb explosion (it is likely that Nb5+ is produced; however, this peak is mass degenerate with water that is present in the system). Nb5+, Pd5+, and O3+ all have ion signal appearance potentials around 50 eV. The literature ionization potential (IP) values of each charge state of these atomic species are summarized in Table 1. While Nb+, Nb2+, Pd+, and Pd2+ showed different peak intensities dependent on the ionization potential of each species, the higher atomic charge states of niobium and palladium all had similar peak intensities. This indicates that these charge states are produced from Coulomb explosion of clusters. Previous studies on metal oxide clusters19 have shown enhanced ionization of niobium atomic charge states beginning with the Nb3+ ion signal (with slight enhancement of the Nb2+ ion signal compared to tunneling rate predictions). The maximum observed charge states of Nb5+ and Pd5+ in our system suggest an enhancement

charge state

Pd

Nb

Zr

O

I II III IV V

8.34 19.4 32.9 40.5 49.5

6.76 14.3 29.3 38.3 50.6

6.84 13.1 22.9 34.3

13.6 35.1 54.9

a

Charge states I−III for all species are taken from ref 20, and states IV and V for Pd and Nb were approximated using Slater’s rules from ref 21.

in ion signal nearly 1.5 orders of magnitude lower in laser intensity than that predicted by ADK tunneling theory19 based on direct, sequential ionization. In another series of experiments, different pressures of carbon monoxide gas were introduced into the system of neutral palladium oxide clusters prior to femtosecond ionization, and the ratios of the high atomic charge states of palladium to Pd+ were compared in order to investigate changes in ionization enhancement, which is directly linked to cluster stability. Figure 2a is a plot of the ratio of the high charge states of palladium to Pd+ versus CO pressure (Pd4+ and Pd5+ are excluded due to low ion signal intensities). Addition of carbon monoxide would be expected to decrease the number of larger palladium oxide clusters in the system (due to the weak binding energy between palladium and oxygen), resulting in a 1032

dx.doi.org/10.1021/jp302394a | J. Phys. Chem. A 2013, 117, 1030−1034

The Journal of Physical Chemistry A

Article

Figure 3. Ratio of the intensity of Zr2+/Zr+ and Pd2+/Pd+ with increasing CO pressure.

Figure 4. Comparison of the isoelectronic Pd+ and ZrO+ species. Both are shown to decrease upon addition of CO with a similar magnitude.

decrease in the ratio of high charge states produced due to cluster fragmentation prior to laser ionization. This trend is reflected in Figure 2a and shows that the ratio of high charge states to Pd+ decreases with increasing CO pressure. For comparison, acetylene was added to the palladium oxide cluster system, and a similar lowering of enhanced charge states was observed, indicating that collisions or reactions break apart larger PdxOy clusters (Supporting Information Figure S3). In contrast to the studies of Tyo et al.,12 an OPdCO+ peak was not observed subsequent to ionization, indicating that CO is not reacting with PdxOy clusters to a great extent and that the observed reduction in high charge states of palladium is collisional. 3B. Zirconium Oxide Clusters. Small, neutral zirconium oxide clusters were produced in a manner described above. Similar to palladium oxide clusters, up to four zirconium atoms bound to oxygen were clearly observed in the initial cationic cluster distribution, which is expected to be similar to the neutral cluster distribution (Supporting Information Figure S4). Figure 1b shows the atomic charge states of zirconium and oxygen observed upon ionization and subsequent Coulomb

explosion of zirconium oxide clusters. High atomic charge states of zirconium up to Zr4+ are observed, indicating a complete removal of valence electrons. Table 1 presents ionization potentials of zirconium taken from the literature; the Zr3+ charge state is expected to be enhanced in ionization rate based on previous studies.18 In contrast to palladium oxide clusters, the intensity ratio of ZrO+ to Zr+ in Figure 1b is higher than the intensity ratio of PdO+ to Pd+ in Figure 1a. These differing peak intensities reflect the weaker binding energy of PdO (2.43 eV)20 compared to that of ZrO (7.88 eV).20 Upon introducing carbon monoxide gas to zirconium oxide clusters prior to femtosecond ionization, the ratios of the high atomic charge states of zirconium to Zr+ show a uniform increase and are summarized in Figure 2b. The amount of atomic oxygen from the dissociation of clusters is found to increase in the system, as would be expected; however, the amount of ZrO+ species produced from fragmentation decreases as carbon monoxide is added to the system, which would not occur if larger clusters were fragmented by collisions of carbon monoxide with zirconium oxide clusters. This indicates that zirconium oxide clusters have stronger binding 1033

dx.doi.org/10.1021/jp302394a | J. Phys. Chem. A 2013, 117, 1030−1034

The Journal of Physical Chemistry A energies and are more resistant to fragmentation by CO. The mass spectrum in Figure 1b shows no evidence of carbon monoxide bound to zirconium oxide, indicating that the interaction between carbon monoxide and zirconium oxide clusters is weak. When no clustering is present, addition of CO (or acetylene) does not affect the direct ionization of Zr+ and Zr2+ (Supporting Information Figure S5). 3C. Comparison of Reactivity. Figure 3 shows a comparison of the ratio of the Pd2+ and Zr2+ charge states to Pd+ and Zr+, respectively, subsequent to adding CO gas to the system. The binding energies of PdO clusters are clearly lower than those of ZrO clusters. The bond dissociation energy between palladium and oxygen (2.43 eV)20 is lower than that between zirconium and oxygen (7.88 eV).20 Thus, this would imply a decrease in ionization enhancement due to molecular collisions of CO dissociating weakly bound PdxOy clusters prior to ionization and subsequent Coulomb explosion. PdO−CO is not observed in the system and is likely photodissociated in the system subsequent to Coulomb explosion and is not detected. The stronger binding energies of ZrxOy clusters cause them to be more resistant to dissociation upon collision with CO gas prior to ionization by the femtosecond laser. Thus, an increase in the high atomic charge states is observed due to interactions between carbon monoxide and zirconium oxide clusters. Therefore, even though larger oxide clusters of zirconium may be isovalent with palladium oxide clusters, they clearly have different binding energies and resistance to dissociation by carbon monoxide and acetylene at similar cluster size distributions. It is also interesting to note that the isovalent species of Pd and ZrO previously studied12 both show a decrease in signal intensity after adding CO gas to the system, as shown in Figure 4. With similar CO gas pressures, the ratio of each species decreases by about 10%, indicating the isovalent nature of these species.

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

Article

S Supporting Information *

Supplemental figures of cationic cluster distributions and reactivity ratios of clusters with acetylene gas as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel: (814) 865-7242. Fax: (814) 8655235. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the Air Force Office of Scientific Research (AFOSR) Utah MURI, Grant Number FA 9550-08-1-0400, for financial support.

■

REFERENCES

(1) Chen, M.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science 2005, 310, 291−293. (2) Xai, Y. Science 2009, 324, 1302−1305. (3) Engel, T.; Ertl, G. J. Chem. Phys. 1978, 69, 1267−1281. (4) Robles, R.; Khanna, S. N. Phys. Rev. B 2010, 82, 085428/1− 085428/6. (5) Somorjai, G.A.. Science 2008, 322, 932−934. (6) Shimizu, T.; Wang, H. Proc. Combust. Inst. 2011, 33, 1859−1866. (7) Kuznetzov, B. N.; Yermakov, Y. I.; Boudart, M.; Collman, J. P. J. Mol. Catal. 1978, 4, 49−57. (8) Lee, J. S.; Boudart, M. Catal. Lett. 1991, 8, 107−114. (9) Ross, P. N.; Stonehart, P. J. Catal. 1975, 39, 298−301. (10) Castleman, A. W., Jr.; Khanna, S. N. J. Phys. Chem. C 2009, 113, 2664−2675. (11) Peppernick, S. J.; Gunaratne, K. D.; Castleman, A. W., Jr. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 975−980. (12) Reber, A. C.; Khanna, S. N.; Tyo, E. C.; Harmon, C. L.; Castleman, A. W., Jr. J. Phys. Chem. C 2011, 115, 16797−16802. (13) Blumling, D. E.; Sayres, S. G.; Castleman, A. W., Jr. Int. J. Mass Spectrom. 2011, 300, 74−80. (14) Sayres, S. G.; Ross, M. W.; Castleman, A. W., Jr. Phys. Chem. Chem. Phys. 2011, 13, 12231−12239. (15) Sayres, S. G.; Ross, M. W.; Castleman, A. W., Jr. Phys. Rev. A: At., Mol., Opt. Phys. 2010, 82, 033424/1−033424/11. (16) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1956, 26, 1150− 1157. (17) Purnell, J.; Wei, S.; Buzza, S. A.; Castleman, A. W., Jr. J. Phys. Chem. 1993, 97, 12530−12534. (18) Sayres, S. G.; Ross, M. W.; Castleman, A. W., Jr. J. Chem. Phys. 2011, 135, 054312/1−054312/7. (19) Ammosov, M. V.; Delone, N. B.; Krainov, V. P. Sov. Phys. JETP 1986, 44, 1191−1194. (20) CRC Handbook of Chemistry and Physics, 90th ed.; Lide, D. R., Ed.; CRC Press/Taylor and Francis” Boca Raton, FL, 2010; Internet Version. (21) Reed, J. J. Chem. Educ. 1999, 76, 802−804.

4. CONCLUSION Small, neutral palladium oxides and zirconium oxides have been studied in detail to look for similar patterns in ionization enhancement and reactivity. Comparison of palladium oxide clusters to the high charge states of previously studied niobium oxide clusters showed an enhancement in palladium atomic high charge states (up to Pd5+) of ∼1.5 orders magnitude lower in laser intensity than expected from direct sequential ionization predicted from ADK tunneling theory.18 Upon addition of CO gas to the system, palladium oxide clusters showed a decrease in the ratio of high charge states of palladium to Pd+, indicating that carbon monoxide likely fragmented weakly bound palladium oxide clusters via molecular collisions prior to cluster ionization and resulted in a decrease in ionization enhancement upon field ionization. Addition of CO gas to zirconium oxide clusters showed a clear increase in ionization enhancement of high charge states, reflecting the higher binding energies of zirconium oxide clusters and suggesting weak interactions between zirconium oxide clusters and CO gas. This suggests that the metal−oxygen bond strength within the cluster will ultimately affect the overall reactivity of each system with carbon monoxide. The signal intensity ratios of the isovalent species Pd and ZrO (that have similar highest occupied molecular orbitals) are both observed to decrease upon addition of carbon monoxide, indicating a similar mechanism of dissociation from larger clusters. 1034

dx.doi.org/10.1021/jp302394a | J. Phys. Chem. A 2013, 117, 1030−1034