Effects of Ligands, Cluster Size, and Charge State in Gas-Phase Catalysis: A Happy Marriage of Experimental and Computational Studies
- First Online:
- Cite this article as:
- Schlangen, M. & Schwarz, H. Catal Lett (2012) 142: 1265. doi:10.1007/s10562-012-0892-3
- 1.6k Downloads
We present selected examples of gas-phase reactions which are of timely interest for the activation of small molecules. Due to the very nature of the experiments, detailed insight in the active site of catalysts is provided and—in combination with computational chemistry—mechanistic aspects of as well as the elementary steps involved in the making and breaking of chemical bonds are revealed.
KeywordsBond activation Catalysis Transition metals Reaction mechanisms Elementary steps
Since the seminal publication of Kappes and Staley in 1981 on “Gas-Phase Oxidation by Transition-Metal Cations” , various aspects of this topical problem have been addressed in numerous reviews [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. The enormous interest is due to the fact that gas-phase studies on ‘isolated’ reactants provide an ideal arena for probing experimentally the energetics and kinetics of a chemical reaction in an unperturbed environment at a strictly molecular level without being obscured by difficult-to-control or poorly understood solvation, aggregation, counterions and other effects, thus providing an opportunity to explore the concept of single-site catalysts directly [26, 27, 28, 29, 30, 31, 32, 33]. Further, in these experiments reactive intermediates can be characterized in detail, mechanisms uncovered, and questions addressed on how factors such as cluster size and dimensionality, stoichiometry, oxidation state, degree of coordinative saturation, aggregation, or charge state affect the outcome of a chemical process. Active or single-sites in heterogeneous catalysis are usually rather ill-defined and often characterized by dangling bonds, kinks, steps, defects, or nano-sized particles; probing them experimentally is all but trivial [31, 32, 34] and their identification constitutes one of the intellectual cornerstones in contemporary catalysis. As ‘naked’ gas-phase species are, in general, much more reactive than their condensed-phase counterparts, these studies will, in principle, of course never account for the precise kinetic and mechanistic details which prevail at a surface or in the condensed phase. Yet, complemented by appropriate computational studies, gas-phase experiments have proved meaningful, on the ground that they permit a systematic approach to address the above mentioned questions and provide a conceptual framework. The DEGUSSA process, that is the platinum-mediated coupling of CH4 and NH3 to generate HCN , may serve as a good example. Mass-spectrometry based experiments [36, 37] suggested (i) the key role of CH2NH as a crucial transient, and (ii) the advantage of using a bimetallic rather than a pure platinum-based catalysts for the C–N coupling step in competition with undesired soot formation; the existence of CH2NH was later confirmed by in situ photoionization studies  and currently used catalysts contain silver–platinum alloys. Obviously, each and every information and insight that help to optimize or improve the often trial-and-error based strategies on catalyst developments  are highly welcome.
In this invited perspective, we focus on selected aspects of four gas-phase catalytic reactions all of which are mediated by ionic species under thermal conditions; they encompass (i) the coupling of carbon–carbon bonds, (ii) the CO → CO2 conversion at ambient conditions, (iii) the activation of hydrocarbons, and (iv) the selective oxidation of methanol to formaldehyde.
While we will refrain from describing the various experimental techniques (which are available from the references given), we will rather focus on the elucidation of the often intriguing mechanisms.
2 Metal-mediated Formation of Carbon–Carbon Bonds
Cyclooligomerizations of unsaturated hydrocarbons, in particular assembling them to form benzene, are versatile reactions for the synthesis of aromatic compounds . Although these reactions are quite exothermic, they are usually hampered by large barriers if non-activated hydrocarbons are employed. Transition-metal complexes have been found to facilitate these processes in the condensed phase, and even single Ag, Rh, and Pt atoms supported on a MgO(001) surface were found to bring about acetylene trimerization at ambient conditions . Also in the gas phase, certain ‘bare’ transition-metal cations M+ affect these cyclization processes, and the catalytic reactions are often accompanied by dehydrogenation steps. The most classical example of the stepwise route  correspond to the dehydrogenative gas-phase trimerization of C2H4 by atomic W+ , U+ , Fe+ [45, 46, 47], or Fen+ cluster [48, 49, 50, 51]. The unique reactivity of the Fe4+ cluster, in comparison to other cluster sizes of iron or the complete absence of reactivity of Ni4+ towards C2H4 already illustrates the often-noted non-scalability of cluster properties—in fact, each atom counts !
3 Low-temperature, Catalytic Oxidation of CO
Catalytic conversion of harmful gases, produced in fossil-fuel combustion, such as CO or the oxides of nitrogen, into nitrogen and carbon dioxide, is of utmost importance both environmentally and economically. While these redox reactions are exothermic, for example ΔrH = −87.3 kcal mol−1 for the process N2O + CO → N2 + CO2, they do not occur directly to any measurable extent at either room or elevated temperatures due to high barriers exceeding 47 kcal mol−1 for the N2O/CO couple . Catalysts are required to reduce these barriers, and the first example of a homogeneous catalysis in the gas phase in which atomic transition-metal cations bring about efficient N2O reduction by CO was reported by Kappes and Staley as early as 1981 . Later, numerous other atomic main-group and transition-metal cations have been tested as catalysts [64, 65, 66, 67, 68]. Out of 59 atomic cations investigated, 26 systems for the catalysis of O-atom transport were shown to lie within the ‘thermodynamic window of opportunity’  defined by the oxygen affinities (OA) of N2 and CO, with OA(N2) = 40 and OA(CO) = 127 kcal mol−1. Catalytic activity, however, was observed with only ten atomic cations, namely Ca+, Fe+, Ge+, Sr+, Ba+, Os+, Ir+, Pt+, Eu+, and Y+. The remaining 16 cations, which meet the thermodynamic criteria for oxygen-atom transport (Cr+, Mn+, Co+, Ni+, Cu+, Se+, Mo+, Rn+, Rh+, Sn+, Te+, Re+, Pb+, Bi+, Tm+, and Lu+), reacted too slowly during either the formation of MO+ or its reduction by CO. As shown earlier , this is due to a kinetic barrier resulting from an inefficient, spin–orbit coupling mediated curve crossing that is required for the change in multiplicities .
In the context of ‘catalyst poisoning’, studies with platinum clusters revealed remarkable effects of both the cluster size and the charge state for the CO/N2O couple [71, 72, 73]. For example, for the Pt7+ cluster, the active species in the redox process are Pt7+, Pt7O+, Pt7O2+, and Pt7CO+ with a turnover number >500 in their thermal reaction with CO. Adsorption of more than one CO molecule to the Pt7+ cluster, however, completely quenches the catalytic activity, so that an elevated CO partial pressure has to be avoided . Pronounced charge-state effects were reported for the Pt4+/− clusters, which are known as the least reactive for the cationic and the most reactive one for anionic platinum clusters [60, 72, 74]. Also for the latter, the catalytic activity terminates as soon as two or more CO molecules are adsorbed on the cluster. The enormous reactivity differences for the anionic versus cationic Pt4 cluster ions have been addressed in theoretical studies. Some of the differences are due to geometrical features showing a near planar anion and a structurally distorted tetrahedral cation. The former provides significantly stronger bonds than Pt4+ with both reactants N2O and CO . In addition, for the Pt4+/CO/N2O system there are kinetic barriers for both the doublet and quartet spin states that prevent the reaction to occur under thermal conditions .
In the context of catalytic, low-temperature CO oxidation, experimental and computational studies of free gold clusters occupy a central position in the literature [10, 11, 12, 25]. This is due to several factors: (1) Generally, the reactivity of a heterogeneous process is a complex convolution of the properties of metal cluster and those of the support. Therefore, the investigations of free, gas-phase clusters may help to reveal the intrinsic chemical features of an, e.g. nano-cluster catalyst. (2) Highly dispersed gold particles supported on metal oxides bring about low-temperature CO oxidation ; the catalytic activity correlates with the degree of dispersion, and Au8 clusters bound to oxygen-vacancy F center defects on Mg(001) were found to be the smallest clusters to mediate this reaction at low temperature . (3) The reactivity of free gold cluster towards molecular oxygen, which is rightly considered as the ideal terminal oxidant, depends crucially on the charge state and the cluster size. While cationic gold clusters are completely inert toward O2, Aun− clusters react at room temperature and exhibit a notable odd/even alternation. For example, only cluster anions containing an even number of gold atoms (resulting in an odd number of valence electrons) were found to adsorb one O2 molecule [10, 81, 82, 83]; this reactivity pattern corresponds with the odd/even variations of the vertical detachment energy showing minima for Aun− (n = 4, 6, 8, …) . Thus, the charge and size dependent electronic structures of the gold clusters fundamentally affect the chemical reactions with adsorbate molecules, and it was suggested that the interplay between gas-phase cluster physics and surface chemistry is a promising strategy to uncover “mechanisms of elementary steps in nanocatalysis” .
A possible explanation for this enhancement of co-adsorption activity occurring in an Eley–Rideal mechanism is that the first adsorbate affects the electronic structure of the cluster thus causing it to appear electronically different to the second approaching molecule. Accordingly, CO binds much more tightly to neutral Aun than to Aun− (n = 2, 4, 6, …). Consequently, an Au cluster anion with a preadsorbed, one-electron acceptor O2 molecule will appear to be neutral to the approaching CO molecule because of the charge transfer that takes place from the Aun− cluster to the antibonding 2π* orbital of the O2 adsorbate. The analogy to the surface-catalyzed oxidation  of CO becomes clear in that the excess electron in Aun− is crucial for the reaction to occur, and the neutral supported clusters acquire this electron by charge transfer from the support. In the gas phase, a turnover frequency of approximately 100 CO2 molecules per Au atom per second has been estimated  for the reaction catalyzed by Aun− (n = 10). This efficiency is two(!) orders of magnitude greater than that observed for the commercial gold catalyst. Similar, temperature-dependent cooperative effects were reported for the Au3−/CO/O2 system. While Au3− was found to be inert toward O2 in the temperature regime 100–200 K, pre-adsorption of CO resulted in a charge transfer from the metal cluster’s HOMO into the 2π* antibonding orbital of CO ; this is accompanied with an isomerization of the Au3− cluster from a linear to a triangular geometry. As the latter exhibits a significantly lower electron detachment energy, charge transfer to O2 is possible resulting in the experimentally observed co-adsorption products Au3(CO)(O2)2− .
Even cationic gold clusters which, in general, are inert toward molecular oxygen [81, 83, 91], can be activated by pre-adsorption of molecular hydrogen . Molecular binding of H2 in for example Au4(H2)4+ brings about charge transfer from the H2 ligands to the Au4+ core thus enabling the cluster to coadsorb O2 by donation of 0.14 e to the adsorbed O2 molecule. Similar effects were observed for Aun+ (n = 2, 16) , as well as for preoxidized Pdn+ clusters (n = 2–7)  or the oxides of both cationic and anionic gold cluster ions [93, 94, 95]. Once more, these (and other) examples clearly demonstrate that for the chemistry and physics of small cluster systems the motto holds true that “each atom counts!” .
4 Oxygen-centered Radicals as Active Sites in Catalytic Hydrocarbon Activation
5 Mechanistic Aspects of Catalytic CH3OH → CH2O Conversion
This metal-dependent selectivity of O–H versus C–H bond activation of CH3OH has its origin in the genesis by which the precursor species are formed. For iron, in the initial step a Fe(OCH3)(CH3OH)n+ (n ≤ 8) cluster is generated via solvolysis of FeX2 by the nucleophilic solvent CH3OH. For the co-generation of isomeric [Co,C,H3,O]+, two pathways have been identified. The one, resulting in the Co(OCH3)+ complex, is analogous to that for the iron system starting from Co(OCH3)(CH3OH)n+ (n = ≤8). However, this precursor, in competition with sequential CH3OH evaporation, undergoes loss of CH2O to generate Co(H)(CH3OH)+. This intermediate, in a spin-allowed elimination involving the Co–H bond and a hydrogen atom from the methyl group of the CH3OH ligand, then decomposes to H2 and Co(CH2OH)+. For the exclusive generation of Ni(CH2OH)+, two pathways are operative, both involving NiX(CH3OH)+ (X = H, Br) as precursors; in the subsequent evaporation of HX, based on labeling experiments, the hydrogen atom originates specifically from the methyl group of CH3OH.
Recently, it was observed that not only the nature of the metal, but also the ligand L for a given metal M matters with regard to the course of competitive C–H versus O–H bond activation (Schlangen M, unpublished results). For example, the system Ni(OH)(CD3OH)+ gives rise to the formation of H2O/HDO in a ratio 33:1, for the electronically related complex Ni(Cl)(CD3OH)+ the ratio HCl/DCl drops to 2:1, for Ni(Br)(CD3OH)+ HBr/DBr loss amounts to only <0.05, and, finally, the celebrated Ni(H)(OH)+ species [121, 122] in its reaction with CD3OH undergoes exclusive elimination of HD, thus pointing to clean activation of the methyl C–D bond (Schlangen M, unpublished results). Clearly, these puzzling experimental findings constitute a challenge for computational chemistry to account for a highly metal- and ligand-dependent behavior.
For both cycles the anionic complex Mo2(O6)(OCHR2)− (R = H, alkyl) serves as central intermediate , and three elementary steps matter: (1) condensation of the complex with the alcohols R2CHOH and elimination of H2O to produce an alkoxo-bound cluster; (2) oxidation of the alkoxo ligand and its liberation as an aldehyde or a ketone in a step which is rate-limiting and requires the supply of external energy through collision-induced dissociation; (3) regeneration of the catalyst by oxidation with nitromethane. The second cycle is similar, but differs in the order of the reaction with the alcohol and the use of nitromethane as the terminal oxidant.
The crucial role of the binuclear metal center in these redox processes was assessed by examination of the relative reactivities of the mononuclear MO3(OH)− and binuclear M2O6(OH)− complexes (M = Cr, Mo, W). The molybdenum and tungsten binuclear centers (M = Mo, W) were reactive towards alcohols, but the chromium complex was not; this finding is consistent with the order of basicity of the hydroxo ligand in these anionic complexes. However, the tungsten complex W2O6(OCHR2)− prefers a redox-neutral elimination of an alkene rather than oxidation of the alkoxo ligand to form an aldehyde or a ketone. This observation is in keeping with the oxidizing power of the anions. Interestingly, each of the mononuclear anions MO3(OH)− (M = Cr, Mo, W) was inert to reaction with methanol, which highlights the importance of the second MO3 unit in the catalytic cycles. Clearly, only the bimolybdate center has the appropriate balance of electronic properties that allows it to participate in each of the three steps; these gas-phase studies with well-defined cluster anions correspond to the unique role of molybdenum(VI) trioxide (MoO3) in the industrial oxidation of methanol to formaldehyde at 300–400 °C .
In addition to the topics addressed in this perspective there are numerous other examples for using gas-phase experiments with ‘isolated’ reagents as models for mimicking catalytic reactions in the condensed phase, and they include inter alia: (1) the mechanistic understanding of the Cytochrome P-450 mediated C–H bond oxygenation [130, 131, 132, 133] based on a detailed analysis of the most simple system, that is FeO+/H2 [134, 135], (2) the relationship between the rich gas-phase chemistry of bare PtO2+  and the extraordinary features exhibited by high-valent platinum oxides , (3) the gas-phase CH4 → CH3OH or C6H6 → C6H5OH conversions in fully thermal catalytic cycles [137, 138], (4) the efficient catalytic gas-phase dehydration of acetic acid to ketene , or (5) the elegant experimental/computational gas-phase investigation on the reactions of bare Ag2O+ with olefins which, in many ways, revealed crucial details of the large-scale heterogeneous olefin epoxidation .
There is indeed good reason to argue that an integrated approach employing the whole arsenal of seemingly esoteric gas-phase work in conjunction with appropriate computational studies will help to bridge the gap between chemistry and physics conducted at a strictly atomic level in the gas phase [8, 11, 16, 20, 22, 25] and the most complex behavior that prevails at surfaces [31, 32, 34, 89] or in solution [141, 142] and, at long last, may thus provide insight in the nature of active sites in catalysis.
The work conducted at the TU Berlin has been generously funded by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft within the “Cluster of Excellence: Unifying Concepts in Catalysis”. We are grateful to Andrea Beck for technical assistance.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.