Is It Homogeneous or Heterogeneous Catalysis Derived from [RhCp*Cl₂]₂?In OperandoXAFS, Kinetic, and Crucial Kinetic Poisoning Evidence for Subnanometer Rh₄Cluster-Based Benzene Hydrogenation Catalysis

Abstract

Determining the true, kinetically dominant catalytically active species, in the classic benzene hydrogenation system pioneered by Maitlis and co-workers 34 years ago starting with [RhCp*Cl(2)](2) (Cp* = [η(5)-C(5)(CH(3))(5)]), has proven to be one of the most challenging case studies in the quest to distinguish single-metal-based "homogeneous" from polymetallic, "heterogeneous" catalysis. The reason, this study will show, is the previous failure to use the proper combination of: (i) in operando spectroscopy to determine the dominant form(s) of the precatalyst's mass under catalysis (i.e., operating) conditions, and then crucially also (ii) the previous lack of the necessary kinetic studies, catalysis being a "wholly kinetic phenomenon" as J. Halpern long ago noted. An important contribution from this study will be to reveal the power of quantitiative kinetic poisoning experiments for distinguishing single-metal, or in the present case subnanometer Rh(4) cluster-based catalysis, from larger, polymetallic Rh(0)(n) nanoparticle catalysis, at least under favorable conditions. The combined in operando X-ray absorption fine structure (XAFS) spectroscopy and kinetic evidence provide a compelling case for Rh(4)-based, with average stoichiometry "Rh(4)Cp*(2.4)Cl(4)H(c)", benzene hydrogenation catalysis in 2-propanol with added Et(3)N and at 100 °C and 50 atm initial H(2) pressure. The results also reveal, however, that if even ca. 1.4% of the total soluble Rh(0)(n) had formed nanoparticles, then those Rh(0)(n) nanoparticles would have been able to account for all the observed benzene hydrogenation catalytic rate (using commercial, ca. 2 nm, polyethyleneglycol-dodecylether hydrosol stabilized Rh(0)(n) nanoparticles as a model system). The results--especially the poisoning methodology developed and employed--are of significant, broader interest since determining the nature of the true catalyst continues to be a central, often vexing issue in any and all catalytic reactions. The results are also of fundamental interest in that they add to a growing body of evidence indicating that certain, appropriately ligated, coordinatively unsaturated, subnanometer M(4) transition-metal clusters can be relatively robust catalysts. Also demonstrated herein is that Rh(4) clusters are poisoned by Hg(0), demonstrating for the first time that the classic Hg(0) poisoning test of "homogeneous" vs "heterogeneous" catalysts cannot distinguish Rh(4)-based subnanometer catalysts from Rh(0)(n) nanoparticle catalysts, at least for the present examples of these two specific, Rh-based catalysts.

Pd‐Catalyzed Z‐Selective Semihydrogenation of Alkynes: Determining the Type of Active Species

“…When as trong-binding poison is applied to an efficientlya ctivatedm olecular catalyst, al inear negative relationship between the activity and the amount of poison is expected. [63,68] Indeed, plotting the activity versus the poison fraction yields astraight line with an egative slope (Figure 2), indicating that the applied system behaves as expected for am olecular catalyst. The fraction of the metal that is active can then be derived by the determinationo ft he slope of this line and calculating the intersection with the horizontal axis.…”

“…Only af raction of this may be active, since part of the surface is occupied by other coordinating species that solubilize the particles. [12,63,64] Hence, the observed NPs do not match well with the fractiono fa ctivem etal that was observed. Such am ismatch is an indication that other speciesm ay be active.…”

“…Generally, such av alue is indicative of catalytically active NPs. [63] In the quantitative poisoning experiment the activity does not decrease to zero in al inear fashion. This behavior is typically observedw hen the binding of the poisont ot he active catalysti sn ot infinitelyl arge with respect to the substrate.…”

“…[56,[63][64][65][66] The amount of active metal atoms is indicative for the type of the catalyst. Only the atoms on the outer sphere of NPs are catalytically active, leaving many non-participating metal atoms in the interioro ft he NP (Scheme1).…”

“…The percentageo ft he metal that is active in nanoclustersh as been scarcely studied. [63,83,104,105] If~40 %o fa ll Pd is in an active state, this is ah ighb ut not unrealistic value for nanoclusters, for which am uch larger amount of the Pd centers is exposed to the reaction medium at the surfaceo ft hese particlea sc ompared to NPs. Thev alue is especially high considering that part of the metal is present in aN Pf orm as well.…”