Activation and Catalytic Dehydrogenation of Methane on Small Pdx+

Article pubs.acs.org/JPCC

Activation and Catalytic Dehydrogenation of Methane on Small Pdx+ and PdxO+ Clusters Sandra M. Lang, Anja Frank, and Thorsten M. Bernhardt* Institute of Surface Chemistry and Catalysis, University of Ulm, Albert-Einstein-Allee 47, 89069 Ulm, Germany ABSTRACT: The temperature-dependent adsorption as well as the possible activation and dehydrogenation of methane mediated by small free palladium Pdx+ (x = 2−4) and palladium mono-oxide PdxO+ (x = 2−4) clusters have been studied in an ion trap experiment under multicollision conditions. The smallest clusters Pd2+ and Pd3+ are found to readily dehydrogenate methane, while Pd4+ is completely nonreactive within the whole investigated temperature range between 220 and 300 K. In the case of the oxides PdxO+, a critical number of at least three coadsorbed CD4 is required before methane dehydrogenation can be observed. Temperature-dependent kinetic studies of the reaction of Pd2+ and Pd2O+ with methane allow for the determination of catalytic reaction mechanisms that involve C−D bond cleavage and C−C bond formation processes. The catalytic activity of these two clusters is compared in terms of active species, cooperative effects, and activation barriers. Finally, the possible methane oxidation on PdxO+ is discussed. toward methane revealing for the first time a strongly cluster size dependent activity for methane dehydrogenation.16 In the study presented here we have extended these temperature-dependent mass spectrometric and reaction kinetic studies to bare palladium clusters Pdx+ (x = 2−4) to elucidate similarities and differences in the ability of palladium and palladium mono-oxide clusters for methane activation and dehydrogenation.

1. INTRODUCTION Metal and metal oxide particle catalysts represent an important class of solid catalytic materials in large-scale industrial processes.1 Such industrial heterogeneous catalysts are usually highly complex systems, which renders their educated optimization with respect to efficiency, product selectivity, energy demand, and prevention of problematic byproducts extremely difficult. To design tailor-made catalytic materials, a molecular level understanding of pivotal parameters and elementary reaction mechanisms is mandatory which can only be achieved through the investigation of simplified model systems. In this context, we utilized gas-phase palladium and palladium mono-oxide clusters as model systems to study the intrinsic propensity of small metal and metal-oxide particles in the activation and catalytic dehydrogenation of methane to form larger hydrocarbons. Generally, the homo- and heterolytic C−H bond cleavage as well as functionalization are regarded as a longstanding central problem for which no suitable catalysts are available so far.2,3 Investigations of methane activation by gas-phase palladium model systems are scarce and have mainly been limited to the Pd atom4−10 and the cation Pd+,11−13 which both were found to be nonreactive toward CH4. In contrast, neutral clusters of Pdx (x = 2−24), with the exception of Pd3 and Pd4, were experimentally observed to adsorb methane. Yet, the reactivity of all cluster sizes was generally rather low.4 In addition, no details concerning the reaction products and the possibility for methane activation and dehydrogenation have been presented in this study. The reactivity contrast between the dimer (reactive toward methane) and the trimer (nonreactive) has also been confirmed by theoretical simulations.14,15 Furthermore, a recent contribution from this laboratory reported on the reactivity of small palladium oxides PdxO+ (x = 2−4) © 2013 American Chemical Society

2. EXPERIMENTAL METHOD Small palladium Pdx+ and palladium mono-oxide clusters PdxO+ are generated by sputtering preoxidized palladium targets with high energetic Xe ion beams produced in a CORDIS (cold reflex discharge ion source).17 The formed clusters are thermalized in a helium-filled ion guide prior to mass selection in a first quadrupole mass spectrometer. The mass-selected cluster ion beam is then transferred into a home-built octopole ion trap, which is prefilled with about 1 Pa helium buffer gas and a small, well-defined fraction of deuterated methane CD4. The absolute pressure inside the trap is measured by a Baratron capacitance manometer (MKS, Typ 627B). Furthermore, the ion trap is attached to a combination of a closed cycle helium cryostat and a resistive heater that allows for temperature adjustment in the range between 20 and 300 K. Thermal equilibration of the clusters is achieved within a few milliseconds (about 103 collisions) under our experimental conditions,18 while the clusters are stored for a considerably longer time, typically between 0.1 s and several seconds. After the chosen reaction time tR, i.e., the storage time in the ion trap, Received: December 30, 2012 Revised: April 19, 2013 Published: May 7, 2013 9791

dx.doi.org/10.1021/jp312852r | J. Phys. Chem. C 2013, 117, 9791−9800

The Journal of Physical Chemistry C

Article

this mass range (blue bars). The calculated spectrum represents the sum of the cluster distributions of Pd4+, Pd4O+, and Pd4O2+ derived from the natural abundance of the palladium isotopes and shows that the broadening of the individual peaks can be attributed to this natural isotopic distribution, while a broadening due to contaminations with background gases like hydrogen or water can be excluded. For the reactivity studies in the ion trap, the cluster size of interest was mass-selected in the first quadrupole mass filter. Figure 1c displays a compilation of the separately recorded cluster ion signals of Pd4+ (solid line), Pd4O+ (dashed line), and Pd4O2+ (dotted line) obtained after mass selection in the first mass filter, storage in the ion trap without reactive gas, and subsequent mass analysis in the second quadrupole mass spectrometer. From this figure it is apparent that mass selection of the Pd4+ cluster or any of its oxides Pd4Oy+ under discrimination of the proximate clusters is unambiguously possible despite the slightly overlapping isotopic distributions. The bars in Figure 1c again illustrate the natural distribution of the palladium isotopes which are responsible for the broadening of the mass peaks. By recording all ion intensities (mass peak height) as a function of the reaction time tR in the ion trap, the kinetics at a well-defined reaction temperature can be obtained. The normalized kinetic traces are then evaluated by fitting integrated rate equations of potential reaction mechanisms to the experimental data by using the “Detmech” software.19 This leads to the determination of the simplest reaction mechanism that best fits the experimental data and of the corresponding rate constants k. Since the experiments are performed at a total pressure of about 1 Pa and thus in the kinetic low-pressure regime, the details of every association reaction step

all ionic reactants, intermediates, and products are extracted from the trap. The ion distribution is subsequently analyzed with a second quadrupole mass filter. Figure 1a displays a cluster ion distribution obtained after scanning the first quadrupole mass spectrometer and recording

Figure 1. (a) Palladium and palladium-oxide cluster distribution generated by the sputter source. The mass peaks denoted with (x,y) correspond to complexes of the stoichiometry PdxOy+. (b) Enlarged view of the Pd4+/Pd4O+/Pd4O2+ cluster signals (solid line) as well as the calculated cluster distribution in this mass range (blue bars) arising from the natural abundance of the palladium isotopes (sum of the cluster distributions of Pd4+, Pd4O+, and Pd4O2+). (c) Compilation of three separate mass spectra showing the cluster ion signals of Pd4Oy+ (y = 0−2) obtained after selection with the first mass filter, storage in the ion trap without reactive gas, and mass analysis with the second mass filter. The bars represent the calculated mass distributions of Pd4Oy+ derived from the natural abundance of the palladium isotopes. Please note that the isotopic distributions of the clusters were intentionally cut in the selection process with the first mass filter to unambiguously discriminate between the clusters.

PdxOy+ + CD4 → PdxOy CD4 +

k

(1)

can be described by the Lindemann energy transfer model for association reactions.20−22 PdxOy+ + CD4 ⇄ (PdxOy CD4 +)*

ka , k d

(2a)

(PdxOy CD4 +)* + He → PdxOy CD4 + + He*

ks

(2b)

+

According to this model the cluster PdxOy reacts with methane to form the energized complex (PdxOyCD4+)* (with the association rate constant ka). This complex can either decompose back to the reactants (decomposition rate constant kd) or be stabilized by collisions with the He buffer gas (stabilization rate constant ks). This means that the fitted rate constants of each association reaction step depend on the helium buffer gas. Because the concentrations of helium, [He], and methane, [CD4], are typically orders of magnitude larger than the cluster ion concentration and a steady flow of the reactants is ensured, the rate constants k are of pseudofirst-order, k = k(1) = k(3)[He][CD4], with the termolecular rate constant k(3). More details of the experimental setup and the data analysis procedure can be found elsewhere.18,22

the cluster ion current on a Faraday collector. This mass spectrum shows the preferred formation of mono-oxide palladium clusters, PdxO+, apart from bare palladium clusters, Pdx+, under these cluster source conditions. PdxO2+ signals are also apparent for clusters containing more than three palladium atoms. Due to an error in the mass calibration of the first quadrupole mass filter, in a previous study16 the prevailing PdxO+ clusters were erroneously taken for Pdx+ and reacted with methane. This error has been corrected, and here we present now a comprehensive comparative investigation of both Pdx+ and PdxO+ prepared under optimized cluster source conditions (which are mainly determined by the degree of oxidation of the sputter targets). Figure 1b shows an enlarged view of the mass spectrum displayed in Figure 1a in the mass range between 405 and 475 amu (solid line) as well as the calculated cluster distribution in

3. RESULTS 3.1. Room-Temperature Methane Adsorption. Figure 2 shows ion mass distributions obtained after the reaction of small palladium clusters Pdx+ (x = 2−4) and palladium monooxide clusters PdxO+ (x = 2−4) with deuterated methane CD4 at room temperature. The mass spectrum recorded in the case 9792



Article

Table 1. Termolecular Rate Constants k(3) for the Adsorption of a First Methane Molecule at Room Temperature k(3)/10−27 cm3 s−1 Pd2+ Pd3+ Pd4+

1.2 ± 0.2 2.3 ± 0.5 Pd2 > Pd4. In contrast to the bare palladium clusters, the mono-oxide clusters PdxO+ exhibit a much less pronounced cluster size dependence. At room temperature all investigated PdxO+ react with methane without indication for dehydrogenation, which is observed only at considerably lower temperatures due to cooperative effects occurring when more CD4 molecules are

Thus, the mechanism on the left-hand side of Figure 4g is proposed to be dominant at room temperature, while the righthand side mechanism becomes more and more prevailing as the temperature decreases. However, it should be noted that, although the overall reaction mechanism shown in Figure 4g satisfactorily fits the kinetics at all studied temperatures (as shown by the solid lines in Figure 4d and f), this represents a rather simple mechanism and due to the complicated product distribution and the different possibilities for formation of Pd2C3D8+ and Pd2C4D12+ alternative more complicated mechanisms (especially at intermediate temperatures) cannot be excluded. In marked contrast to Pd2+, the mono-oxide Pd2O+ does not dehydrogenate methane at room temperature but instead sequentially adsorbs two CD4 to form Pd2OCD4+ and Pd2O(CD4)2+ (cf. Figure 5a and b). However, at lower temperature the complex Pd2OC3D10+ appears, which can only be formed upon reaction with a third CD4 molecule yielding Pd2O(CD4)3+ and subsequent dehydrogenation. This latter intermediate is not visible in the mass spectrum obtained at 260 K (Figure 5c) but is clearly apparent in the mass spectrum obtained at 225 K (Figure 5e). The kinetics displayed in Figure 5d show that the intensity of the products Pd2O(CD4)2+ and Pd2OC3D10+ are constant for reaction times larger than about 1 s. As in the case of Pd2+ shown above, such kinetics can only be explained by an equilibrium reaction mechanism or a catalytic reaction cycle. Since there is no molecular deuterium present in the ion trap, an equilibrium reaction mechanism that includes the elimination of molecular hydrogen is not possible, and the products Pd2O(CD4)2+ and Pd2OC3D10+ must be connected via a catalytic reaction cycle as shown at the left-hand side of Figure 5g. According to this mechanism the catalytic reaction cycle is closed by elimination of an ethane molecule C2D6 from Pd2OC3D10+ under reformation of Pd2OCD4+. An alternative closing of the cycle assuming the elimination of CD2 and reformation of Pd2O(CD4)2+ cannot be completely excluded on the basis of the kinetic data; however, the liberation of the highly reactive carbene CD2 should be energetically unfavorable and is thus unlikely to occur. This assumption is also confirmed by the reactivity studies of Pd3+ shown in Figures 2b and 3b, which show the formation of carbene on the cluster, Pd3CD2+, which, however, cannot be eliminated. At even lower temperatures an additional product Pd2OC4D12+ containing dehydrogenated methane is detected. Figure 5e indicates a shoulder on the high mass side of this product peak which might be due to a second product, Pd2OC4D14+. This, however, cannot be resolved in the mass spectrum, and it is not possible to detect reliable kinetic data. Both complexes must be formed after reaction of a fourth methane molecule yielding Pd2O(CD4)4+ and subsequent dehydrogenation. The corresponding kinetics (Figure 5f) show that the intensity of Pd2OC3D10+, Pd2OC4D12+, Pd2O(CD4)3+, and Pd2O(CD4)4+ levels off after a reaction time of about 2 s. Again, due to the lack of D2 in the ion trap, an equilibrium reaction mechanism is not possible. Thus, at low temperatures a second catalytic reaction cycle, which is shown on the right-hand side of Figure 5g, is operative. This cycle can either be closed by elimination of ethylene, C2D4, and reformation of Pd2O(CD4)2+ or by elimination of propane, C3D8, and reformation of Pd2OCD4+. On the basis of the experimental data both reaction mechanisms are possible. 9796



Article

Pd2OC3D10+, Pd2OC4D12+, Pd3OC4D10+, and Pd4OC4D14+ lead to the conclusion that Pd2O+ must adsorb at least three CD4 molecules to enable dehydrogenation, while four molecules are necessary for Pd3O+ and Pd4O+. Since association reactions in the ion trap can be described by the Lindemann energy transfer mechanism (cf. reaction 2) the formation of stable reaction products requires the stabilization by a third body collision. This stabilization is more effective the lower the temperature, thus leading to increased reactivity with decreasing temperature. As for dehydrogenation mediated by PdxO+ a larger critical number of adsorbed methane molecules is required compared to Pdx+, dehydrogenation is only possible at temperatures significantly below 300 K. Most interestingly, similar effects (i.e., the lower reactivity of oxidized palladium clusters toward methane compared to the bare metal clusters) have also been observed for dissociative CH4 adsorption on Pd(110) and oxygen-covered Pd(110) surfaces. Molecular beam surface scattering studies revealed a decreasing dissociation probability with increasing oxygen precoverage of the surface.42,43 This behavior has been attributed to steric blocking of active adsorption sites on the Pd surface by oxygen preadsorption. Furthermore, theoretical studies on the palladium surface Pd(111) in comparison with the palladium oxide surfaces PdO(001), PdO(110), and PdO(100) showed that C−H bond breaking on the perfect PdO(001) and PdO(110) is unfavorable due to the lack of O vacancy sites.44 However, also in the presence of O vacancies CH4 dissociation was found to be hampered on the oxides which is reflected in a considerably weaker surface−C and surface−H bond on PdO(001) compared to Pd(111) in conjunction with a stronger C−H bond in the adsorbed CH4. Additionally, it was shown that the transition state of C−H bond breaking is formed mainly by coupling between the surface O atom of PdO and the H atom of CH4. Since in the case of free PdxO+ clusters low coordinated palladium atoms are available, and in particular, methane dehydrogenation is generally possible but only requires a larger critical number of adsorbed CD4, steric effects can be excluded. Instead, a change in the electronic structure of the palladium cluster due to the preadsorption of the strongly electronegative oxygen atom causing increased activation barriers for dehydrogenation can be considered as more likely. To our knowledge this represents the first experimental comparison of the reactivity between bare palladium and the corresponding palladium mono-oxide clusters toward methane. Furthermore, theoretical computations have only been performed on neutral Pd2 and Pd2O.40 However, these calculations predicted very similar adsorption energies of CH4 to Pd2 and Pd2O, similar activation barriers for H migration yielding HPd2CH3 and HPd2OCH3, respectively, as well as similar stability of these insertion complexes suggesting that methane adsorption and activation should be similar on the palladium dimer and its corresponding mono-oxide. However, neither the interaction of multiple CH4 molecules nor the reaction mechanism leading to H2 formation and elimination has been considered in this study, which renders the comparison with the present experimental study difficult. Furthermore, C−H bond cleavage under hydrogen atom transfer has been investigated on various polynuclear metal oxide clusters revealing that terminal oxygen atoms with localized high-spin densities act as active oxygen radical centers for C−H bond cleavage.3,45 However, recent theoretical studies46 on Pd4O+ as well as on neutral PdxO2 (x = 1−4)47

adsorbed than at room temperature (see the following section). Furthermore, the termolecular rate constants for the adsorption of a first CD4 molecule indicate a comparable reactivity for Pd2O+ and Pd4O+, while the reactivity is enhanced for Pd3O+. The increased reactivity of the trimer oxide is in agreement with the high reactivity of the bare palladium trimer. 4.1.2. Cooperative Effects in the Dehydrogenation Reactions. At room temperature the cluster Pd3+ reacts with methane to form the product Pd3CD2+ (cf. Figure 2b). This indicates that Pd3+ directly activates and dehydrogenates one single adsorbed CD 4 molecule efficiently because the intermediate adsorption complex Pd3CD4+ is not detected on the time scale of the experiment. In contrast, in the case of Pd2+, the association product Pd2CD4+ containing nondehydrogenated methane is detected in low intensity as well as Pd2C2D4+, which must be formed from Pd2(CD4)2+ via dehydrogenation (Figure 2a). This suggests that Pd2+ is not able to dehydrogenate a single adsorbed methane molecule. However, the coadsorption of a second CD4 molecule leads to a cooperative interaction of these molecules which then facilitates dehydrogenation on Pd2+. Similar cooperative methane activation effects have been observed in the ethylene formation catalysis mediated by Au2+.23 The binding energies of a first adsorbed methane molecule to Pd2+ (0.83 ± 0.10 eV16) and Pd3+ (0.78 ± 0.10 eV16) are comparable. However, Pd3+ comprises a larger number of vibrational degrees of freedom than Pd2+ which means that (1) the energized complex (Pd3CD4+)* contains a larger amount of internal energy than (Pd2CD4+)* which can help to overcome activation barriers and (2) the internal energy can be redistributed more easily in (Pd3CD4+)* and the probability for localizing a critical amount of energy in a certain bond is decreased. Effect (1) should lead to enhanced dehydrogenation activity of Pd3+, while effect (2) should lead to enhanced dehydrogenation activity of Pd2+. Since the absolute value of the difference in the internal energy arising from the different numbers of vibrational degrees of freedom of the two clusters is rather small (about 0.08 eV for Pd2+ compared to about 0.14 eV for Pd3+ at room temperature), effect (2) is likely to predominate, and a higher activity for Pd2+ in the dehydrogenation reaction is expected. This is, however, in contrast to the experimental finding that Pd3+ is more efficiently dehydrogenating methane to form CD2. Therefore, it can be concluded that the major reason for the high dehydrogenation activity results from particular low activation energies for C−D bond splitting on the trimer compared to the dimer. For Pd2O+ and Pd3O+ the termolecular rate constants for the adsorption of a first methane molecule (cf. Table 1) reveal that the room-temperature reactivity of these clusters toward a first CD4 is very similar to the reactivity of Pd2+ and Pd3+, respectively. In contrast, the reactivity of Pd4O+ is substantial compared to Pd4+, which does not react with methane at all. However, although Figure 2d−f shows that all investigated palladium mono-oxide clusters readily adsorb methane at room temperature, the mass distributions do not provide any indications for dehydrogenation reactions. This suggests that the dehydrogenation of methane is significantly hampered on the palladium mono-oxide clusters compared to the bare palladium clusters. Instead, dehydrogenation appears to be only possible after cooperative coadsorption of a critical number of methane molecules. The detected dehydrogenation products 9797



Article

The reaction cycle in Figure 4g shows that Pd2+ adsorbs two CD4, yielding Pd2(CD4)2+ followed by the dehydrogenation reaction and formation of Pd2C2D4+ at room temperature. From Figure 3a it is apparent that the intensity of this product decreases with decreasing reaction temperature, while the intensity of the product Pd2(CD4)2+, which is the precursor complex for the hydrogen elimination reaction, increases. These temperature-dependent product distributions thus indicate that the dehydrogenation of methane involves an activation barrier (denoted as ΔE1 in Figure 4g), which cannot be surmounted anymore at low temperatures, and the reaction terminates at Pd2(CD4) 2+. Similarly, Pd2O+ forms the dehydrogenated products Pd2OC3D10+ and Pd2OC4D12+, the intensities of which decrease with decreasing temperature, while Pd2O(CD4)3+ and Pd2O(CD4)4+ appear (cf. Figure 3d) indicating activation barriers (labeled ΔE1 and ΔE3 in Figure 5g) for the formation of the hydrocarbons and/or the elimination D2. Furthermore, the ion mass distributions and kinetics shown in Figure 5 show rather high intensities of the dehydrogenated products Pd2OC3D10+ and Pd2OC4D12+ which indicates that also the subsequent liberation reactions of the adsorbed hydrocarbon products (C2D4, C2D6, and C3D8) are connected with activation barriers (labeled ΔE2 and ΔE4 in Figure 5g). Similarly, in the case of the prospective ethylene formation on Pd2+, the liberation of this molecule from the cluster must be associated with a very high activation barrier since there is no experimental evidence for this process (see also discussion in Section 3.3). 4.2.3. Methane Oxidation. Reactivity studies of the palladium mono-oxides PdxO+ with methane demonstrate the ability of these clusters to activate and dehydrogenate methane under certain reaction conditions (cf. Figure 3); however, the mass spectra do not provide any indications for C−O bond formation leading to the oxidation of methane. This finding is in marked contrast to PdO+, which has been observed to form and eliminate methanol, CH3OH, upon reaction with methane.49 Furthermore, preoxidized palladium cations PdxO+ and PdxO2+ have recently been found to react with CO to form CO2 and the bare clusters.50 However, it was also demonstrated that CO oxidation on Pd6O+ is hampered50 by activation barriers, while the tetroxide Pd6O4+ is very reactive toward CO.48 Enhanced reactivity of highly oxidized metal cations compared to the corresponding mono-oxides has also been observed previously, e.g., for MoOy+51 and CrOy+52 in the reaction with methane. Thus, also for palladium the preformation of highly oxidized species might be mandatory for successful methane oxidation. Recently, the reaction of Pd2O+ in the presence of a CD4/O2 mixture has been studied but just revealed coadsorption products without indications of methane oxidation.16 However, Pd2O+ only slowly reacts with molecular oxygen, and the formation of higher oxides is difficult. In contrast, the larger cluster Pd6+ was found to activate and dissociate O2 very fast,48 rendering this cluster size a suitable candidate for successfully catalyzing the CD4 oxidation reaction. Experimental studies of these systems are currently in progress.

resulted in structures for the oxides comprising 2- and 3-fold coordinated oxygen atoms, while structures with monocoordinated (terminal) oxygen were found to be instable. Consequently, the oxygen atoms do not represent oxygen radicals in these clusters and are thus most likely not active centers for C−H bond cleavage and hydrogen atom transfer. In fact, the addition of an oxygen atom not only increases the coordination number of the Pd atoms (with the exception of Pd2O+) but might also considerably change the oxidation state of the Pd atoms and the electronic structure of the cluster which in this case obviously reduces the ability for methane dehydrogenation. 4.1.3. C−D Bond Activation. So far, theoretical investigations have only addressed the activation of a first CH4 on Pd atoms as well as small neutral Pdx clusters. These studies indicated a strong dependence of a possible C−H bond cleavage mechanism to form hydrido-methyl complexes H− Pdx −CH 3 on the level of theory applied.6−9,11,13,14,40 Furthermore, coadsorption of multiple methane molecules has not been studied so far. Consequently, experimental insight into methane activation is desirable. Although the mass spectra shown in Figures 2 and 3 cannot provide this information directly, the corresponding kinetics can offer indirect insight, if a CD4 is adsorbed molecularly or dissociatively. For Pd2+ the kinetics obtained at room temperature reveal formation of Pd2CD4+ in a simple forward reaction mechanism Pd 2+ + CD4 → Pd 2CD4 +

(5)

The lack of a backward reaction step indicates strong binding of CD4 with potential subsequent activation of a C−D bond. In contrast, the room-temperature kinetics of the palladium monooxides are best described by equilibrium reaction mechanisms PdxO+ + CD4 ⇄ PdxOCD4 +

for x = 2−4

PdxOCD4 + + CD4 ⇄ PdxO(CD4 )2+

for x = 2, 3

(6) (7)

indicating molecular adsorption of up to two methane molecules and rendering the formation of hydrido-methyl complexes unlikely. 4.2. Catalytic C−C Bond Formation on Pd2+ and Pd2O+. 4.2.1. Active Species and Cooperative Effects. The reaction mechanisms obtained for Pd2+ and Pd2O+ (cf. Figures 4g and 5g) indicate that the bare clusters are not part of the catalytic reaction cycles. The reason for this, as already discussed in Section 4.1, is that the dimers cannot dehydrogenate a first adsorbed CD4 molecule (liberate D2 from the complex), but the coadsorption of a second methane molecule leads to cooperative interaction which then enables hydrogen elimination. Alternative reaction mechanisms including cycle closings by reformation of Pd2+ and Pd2O+, respectively, have been tested and led to inferior fit quality in both cases. Similar results have recently also been obtained for ethylene formation mediated by Au2+ (where Au2CH4+ is part of the cycle instead of Au2+)23 as well as for the CO oxidation on Pd6+ which revealed the tetroxide Pd6O4+ as the catalytically active species.48 4.2.2. Activation Barriers. Since methane dehydrogenation via formation and elimination of larger hydrocarbons includes a variety of different bond breaking and bond formation processes, it is appropriate to assume that these reactions involve several activation barriers which can at least partly be accessed by the temperature-dependent reactivity studies.

5. CONCLUSION We have presented the first direct experimental comparison of the reactivity of small palladium cluster cations Pdx+ (x = 2−4) and their corresponding mono-oxide cluster PdxO+ (x = 2−4) in the adsorption and dehydrogenation of methane. Cluster size 9798



Article

Thermochemistry of Pd-Ligand Complexes. Int. J. Mass Spectrom. Ion Proc. 1997, 167/168, 195−212. (13) Blomberg, M. R. A.; Siegahn, P. E. M.; Svensson, M. Reaction of Second-Row Transition-Metal Cations with Methane. J. Phys. Chem. 1994, 98, 2062−2071. (14) Blomberg, M. R. A.; Siegahn, P. E. M.; Svensson, M. A Theoretical Study of the Reactivity of Pd Clusters with Methane. J. Phys. Chem. 1992, 96, 5783−5789. (15) Broclawik, E.; Haber, J.; Endou, A.; Stirling, A.; Yamauchi, R.; Kubo, M.; Miyamoto, A. Electronic Structure and Adsorption Properties of Precious Metals and Their Oxides: Density Functional Calculations. J. Mol. Catal. A 1997, 119, 35−44. (16) Lang, S. M.; Bernhardt, T. M. Methane Activation and Partial Oxidation on Free Gold and Palladium Clusters: Mechanistic Insights into Cooperative and Highly Selective Cluster Catalysis. Faraday Discuss. 2011, 152, 337−351. (17) Keller, R.; Nöhmeier, F.; Spädtke, P.; Schönenberg, M. H. CORDIS - An Improved High-Current Ion Source for Gases. Vacuum 1984, 34, 31−35. (18) Bernhardt, T. M. Gas Phase Reaction Kinetics of Small Gold and Silver Clusters. Int. J. Mass Spectrom. 2005, 243, 1−29. (19) Schumacher, E. DETMECH - Chemical Reaction Kinetics Software; University of Bern, Chemistry Department: Switzerland, 2003. (20) Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. Chemical Kinetics and Dynamics, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1999. (21) Laidler, K. J. Chemical Kinetics, 3rd ed.; HarperCollins: New York, 1987. (22) Bernhardt, T. M.; Hagen, J.; Lang, S. M.; Popolan, D. M.; Socaciu-Siebert, L.; Wöste, L. Binding Energies of O2 and CO to Small Gold, Silver, and Binary Silver-Gold Cluster Anions from Temperature Dependent Reaction Kinetics Measurements. J. Phys. Chem. A 2009, 113, 2724−2733. (23) Lang, S. M.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. Methane Activation and Catalytic Ethylene Formation on Free Au2+. Angew. Chem., Int. Ed. 2010, 49, 980−983. (24) Andersin, J.; Lopez, N.; Honkala, K. DFT Study on the Complex Reaction Networks in the Conversion of Ethylene to Ethylidyne on Flat and Stepped Pd. J. Phys. Chem. C 2009, 113, 8278− 8286. (25) Sock, M.; Eichler, A.; Surnev, S.; Andersen, J. N.; Klotzer, B.; Hayek, K.; Ramsey, M. G.; Netzer, F. P. High-Resolution Electron Spectroscopy of Different Adsorption States of Ethylene on Pd(111). Surf. Sci. 2003, 545, 122−136. (26) Calaza, F.; Gao, F.; Li, Z.; Tysoe, W. T. The Adsorption of Ethylene on Au/Pd(111) Alloy Surfaces. Surf. Sci. 2007, 601, 714− 722. (27) Ge, Q.; Neurock, M. Correlation of Adsorption Energy with Surface Structure: Ethylene Adsorption on Pd Surfaces. Chem. Phys. Lett. 2002, 358, 377−382. (28) Moskaleva, L. V.; Chen, Z.-X.; Aleksandrov, H. A.; Mohammed, A. B.; Sun, Q.; Rösch, N. Ethylene Conversion to Ethylidyne over Pd(111): Revisiting the Mechanism with First-Principles Calculations. J. Phys. Chem. C 2009, 113, 2512−2520. (29) Watson, G. W.; Fearon, J. Re-Evaluation of the Adsorption Mode of Ethene on the {111} Surface of Palladium Using Density Functional Theory. Surf. Sci. 2003, 547, L853−L858. (30) Bernardo, C. G. P. M.; Gomes, J. A. N. F. The Adsorption of Ethylene on the (100) Surfaces of Platinum, Palladium and Nickel: A DFT Study. J. Mol. Struct.: Theochem 2001, 542, 263−271. (31) Wetterer, M. S.; Lavrich, D. J.; Cummings, T.; Bernasek, S. L.; Scoles, G. Energetics and Kinetics of the Physisorption of Hydrocarbons on Au(111). J. Phys. Chem. B 1998, 102, 9266−9275. (32) Fahmi, A.; van Santen, R. A. Density Functional Study of Ethylene Adsorption on Palladium Clusters. J. Phys. Chem. 1996, 100, 5676−5680. (33) Lyalin, A.; Taketsugu, T. Adsorption of Ethylene on Neutral, Anionic, and Cationic Gold Clusters. J. Phys. Chem. C 2010, 114, 2484−2493.

specific reaction products with CD4 and activities for dehydrogenation are observed for both species. Furthermore, Pd2+ and Pd3+ are found to efficiently dehydrogenate methane at room temperature, while the activity of all mono-oxides PdxO+ is considerably lower. Using the example of the dimers Pd2+ and Pd2O+, details of the temperature-dependent catalytic reaction mechanism have been investigated, and the catalytic activity has been discussed in terms of active species, cooperative effects, and activation barriers. These studies demonstrate that small palladium particles can be effective for low-temperature methane activation and dehydrogenation, while, however, the corresponding oxides are less suitable. This finding on simplified model systems may represent an important step for the tailormade design of low-temperature Pd-based catalysts for C−H bond cleavage and C−H bond functionalization.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +49-731-50-25455. Fax: +49-731-50-25452. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. In particular, SML is grateful to the European Social Fund Baden-Württemberg for a Margarete von Wrangell fellowship.

■

REFERENCES

(1) Gates, B. C. Catalytic Chemistry; John Wiley & Sons, Inc.: New York, Singapore, 1992. (2) Crabtree, R. H. Organometallic Alkane CH Activation. J. Organomet. Chem. 2004, 689, 4083−4091. (3) Schwarz, H. Chemistry with Methane: Concepts Rather than Recipes. Angew. Chem., Int. Ed. 2011, 50, 10096−10115. (4) Fayet, P.; Kaldor, A.; Cox, D. M. Palladium Clusters: H2, D2, N2, CH4, CD4, C2H3, and C2H6 Reactivity and D2 Saturation Studies. J. Chem. Phys. 1990, 92, 254−261. (5) Carroll, J. J.; Weisshaar, J. C. Reactions of Neutral Palladium Atoms in the Gas Phase: Formation of Stable Pd(alkane) Complexes at 300 K. J. Am. Chem. Soc. 1993, 115, 800−801. (6) Carroll, J. J.; Haug, K. L.; Weisshaar, J. C.; Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. Gas Phase Reactions of Second-Row Transition Metal Atoms with Small Hydrocarbons: Experiment and Theory. J. Phys. Chem. 1995, 99, 13955−13969. (7) Blomberg, M. R. A.; Siegbahn, P. E. M.; Svensson, M. Mechanisms for the Reactions between Methane and the Neutral Transition Metal Atoms from Yttrium to Palladium. J. Am. Chem. Soc. 1992, 114, 6095−6102. (8) Diefenbach, A.; Bickelhaupt, F. M. Oxidative Addition of Pd to C−H, C−C and C−Cl Bonds: Importance of Relativistic Effects in DFT Calculations. J. Chem. Phys. 2001, 115, 4030−4040. (9) Cui, Q.; Musaev, D. G.; Morokuma, K. Molecular Orbital Study of H2 and CH4 Activation on Small Metal Clusters. I. Pt, Pd, Pt2, and Pd2. J. Chem. Phys. 1998, 108, 8418−8428. (10) Zhang, W.; Wang, L. The Effect of Cluster Thickness on the Adsorption of CH4 on Pdn. Comp. Theor. Chem. 2011, 963, 236−244. (11) Schlangen, M.; Schwarz, H. Thermal Activation of Methane by Group 10 Metal Hydrides MH+: The Same and Not the Same. Angew. Chem., Int. Ed. 2007, 46, 5614−5617. (12) Chen, Y.-M.; Sievers, M. R.; Armentrout, P. B. Activation of CH4, C2H6, C3H8, and c-C3H6 by Gas-Phase Pd+ and the 9799



Article

(34) Kang, G.-J.; Chen, Z.-X.; Li, Z. Theoretical Studies of the Interactions of Ethylene and Formaldehyde with Gold Clusters. J. Chem. Phys. 2009, 131, 034710. (35) Achatz, U.; Berg, C.; Joos, S.; Fox, B. S.; Beyer, M. K.; NiednerSchatteburg, G.; Bondybey, V. E. Methane Activation by Platinum Cluster Ions in the Gas Phase: Effects of Cluster Charge on the Pt4 Tetramer. Chem. Phys. Lett. 2000, 320, 53−58. (36) Koszinowski, K.; Schröder, D.; Schwarz, H. Reactivity of Small Cationic Platinum Clusters. J. Chem. Phys. A 2003, 107, 4999−5006. (37) Harding, D. J.; Kerpal, C.; Meijer, G.; Fielicke, A. Activated Methane on Small Cationic Platinum Clusters. Angew. Chem., Int. Ed. 2012, 51, 817−819. (38) Lv, L. L.; Wang, Y. C.; Wang, Q.; Liu, H. W. Why is Pt4+ the Least Efficient Cationic Cluster in Activating the C−H Bond in Methane? Two-State Reaction Computational Investigation. J. Phys. Chem. C 2010, 114, 17610−17620. (39) Kalita, B.; Deka, R. C. DFT Study of CO Adsorption on Neutral and Charged Pdn (n = 1−7) Clusters. Eur. Phys. J. D 2009, 53, 51−58. (40) Broclawik, E.; Yamauchi, R.; Endou, A.; Kubo, M.; Miyamoto, A. Density Functional Study on the Activation of Methane Over Pd2, PdO, and Pd2O Clusters. Int. J. Quantum Chem. 1997, 61, 673−682. (41) Bertani, V.; Cavallotti, C.; Masi, M.; Carrà, S. Density Functional Study of the Interaction of Palladium Clusters with Hydrogen and CHx Species. J. Phys. Chem. A 2000, 104, 11390− 11397. (42) Valden, M.; Pere, J.; Xiang, N.; Pessa, M. Influence of Preadsorbed Oxygen on Activated Chemisorption of Methane on Pd(110). Chem. Phys. Lett. 1996, 257, 289−296. (43) Valden, M.; Pere, J.; Hirsimäki, M.; Suhonen, S.; Pessa, M. Activated Adsorption of Methane on Clean and Oxygen-Modified Pt(111) and Pd(110). Surf. Sci. 1997, 377−379, 605−609. (44) Li, H.-Y.; Guo, Y.-L.; Guo, Y.; Lu, G.-Z.; Hu, P. C−H Bond Activation over Metal Oxides: A New Insight into the Dissociation Kinetics from Density Functional Theory. J. Chem. Phys. 2008, 128, 051101. (45) Dietl, N.; Schlangen, M.; Schwarz, H. Thermal Hydrogen-Atom Transfer from Methane: The Role of Radicals and Spin States in OxoCluster Chemistry. Angew. Chem., Int. Ed. 2012, 51, 5544−5555. (46) Kalita, B.; Deka, R. C. Reaction Intermediates of CO Oxidation on Gas Phase Pd4 Clusters: A Density Functional Study. J. Am. Chem. Soc. 2009, 131, 13252−13254. (47) Huber, B.; Häkkinen, H.; Landman, U.; Moseler, M. Oxidation of Small Gas Phase Pd Clusters: A Density Functional Study. Comput. Mater. Sci. 2006, 35, 371−374. (48) Lang, S.; Fleischer, I.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. Pd6O4+: An Oxidation Resistant yet Highly Catalytically Active Nano-Oxide Cluster. J. Am. Chem. Soc. 2012, 134, 20654− 20659. (49) Bozovic, A.; Feil, S.; Koyanagi, G. K.; Viggiano, A. A.; Zhang, X.; Schlangen, M.; Schwarz, H.; Bohme, D. K. Conversion of Methane to Methanol: Nickel, Palladium, and Platinum (d9) Cations as Catalysts for the Oxidation of Methane by Ozone at Room Temperature. Chem.Eur. J. 2010, 16, 11605−11610. (50) Lang, S. M.; Schnabel, T.; Bernhardt, T. M. Reactions of Carbon Monoxide with Free Palladium Oxide Clusters: Strongly Size Dependent Competition Between Adsorption and Combustion. Phys. Chem. Chem. Phys. 2012, 14, 9364−9370. (51) Kretzschmar, I.; Fiedler, A.; Harvey, J. N.; Schröder, D.; Schwarz, H. Effects of Sequential Ligation of Molybdenum Cation by Chalcogenides on Electronic Structures and Gas-Phase Reactivity. J. Phys. Chem. A 1997, 101, 6252−6264. (52) Fiedler, A.; Kretzschmar, I.; Schröder, D.; Schwarz, H. Chromium Dioxide Cation OCrO+ in the Gas Phase: Structure, Electronic States, and the Reactivity with Hydrogen and Hydrocarbons. J. Am. Chem. Soc. 1996, 118, 9941−9952.

9800


Activation and Catalytic Dehydrogenation of Methane on Small Pdx+

Recommend Documents