Coupling of Methane and Ammonia by Dinuclear Bimetallic Platinum–Coinage‐Metal Cations PtM⁺

Abstract

The activation and functionalization of methane constitute continuing challenges for catalysis. One example of their industrial realization is given by the Degussa process for the large-scale production of hydrogen cyanide from methane and ammonia using platinum as heterogeneous catalyst [Eq. (1)].Our aim is the development of a gas-phase model for this process that may enhance the current mechanistic knowledge [1,2] and thus ultimately contribute to rational catalyst improvement. So far, atomic Pt + , homonuclear clusters Pt m + as well as small heteronuclear clusters Pt m Au n + ions have been studied. [3][4][5][6][7] Whereas all the homonuclear Pt m + clusters investigated achieve CH 4 activation in the first reaction step, [8,9] the C À N coupling of the resulting carbene species Pt m CH 2 + with NH 3 only works for m = 1. [3][4][5] In contrast, the dinuclear cluster mediates CÀN bond formation if one platinum atom is replaced by a gold atom.[6] This finding was ascribed to a particularly well-balanced binding situation in the PtAuCH 2 + species. The bonding is strong enough to afford spontaneous activation of CH 4 on the one hand but does not prevent the CH 2 fragment from coupling with NH 3 on the other.[6] Recent results have shown, however, that the larger bimetallic Pt m Au n + clusters do not mediate the coupling step between the CH 2 moiety and NH 3 , thus pointing to special features of PtAu + . [10] To further elucidate the remarkable role of the second metal in the dinuclear cluster, we have extended our studies to PtCu + and PtAg + . The change from gold to its lighter congeners copper and silver is straightforward in view of their electronic and chemical similarities (note, however, that gold is distinguished from both copper and silver by the operation of much stronger relativistic effects [11] ). As with PtAu + , the reactions with CH 4 , O 2 , and NH 3 are investigated; for comparison, the reactivities of Cu 2 + and Ag 2 + are included

Chemical properties of mass-selected coinage metal cluster anions: towards obtaining molecular-level understanding of nanocatalysis

“…Previous quantum chemical calculations have found very strong d-sp mixing for Au, and the UPS spectra of the pure Au cluster anions also agree with these calculations [60,[79][80][81][82]. The stronger d characters of Au nanoclusters are most likely related to relativistic effects [83][84][85]. As the atomic nuclear charge increases, electrons penetrating to the nucleus increase their velocity, and consequently their mass due to relativity.…”

“…The relativistic effect causes the s electrons to be in smaller orbitals than one might classically expect. The s electrons are therefore more strongly bound, shielding d electrons more effectively [83][84][85]. Ample evidence can be found in the literature that the relativistic effect plays a pivotal role in the chemistry of heavy metal atoms like Au [83,85].…”

Theory meets experiment: Gas-phase chemistry of coinage metals

“…Theoretical methods are meanwhile even able to address systems as complex as mixed transition-metal clusters. Thus, experimental investigations of methane activation by heteronuclear Pt m M n + cluster cations (M = Cu, Ag, Au) and the subsequent coupling reactions with gaseous ammonia [237][238][239] were rapidly followed by detailed theoretical studies of the bare metal clusters themselves, the products formed with methane, and the subsequent reaction with NH 3 , which provided a reasonable basis for understanding all of effects observed experimentally [240][241][242][243][244] -a remarkable performance of theory in almost incredibly short time!…”

Bimetallic Au-Pt nanoparticles synthesized by radiolysis: Application in electro-catalysis

“…Cationic Au-Pt clusters have shown unique reactivity for C-N coupling of methane and ammonia. 11,12 Recently it has been reported that carbon-supported Au-Pt bimetallic NPs display performances better or similar to those of pure platinum supported on carbon for oxygen reduction, 13,14 provided alloyed and not phasesegregated Au-Pt NPs are used. It has been also shown that platinum could be partially substituted by gold for application in methanol oxidation: higher resistance to poisoning was observed in acidic media with Au-Pt bimetallic NPs, while in alkaline environment a significant overall improvement in performance was reported.…”