Performance descriptors of nanostructured metal catalysts for acetylene hydrochlorination
Removal of mercuric chloride in catalyzed acetylene hydrochlorination is imperative for sustainable polyvinyl chloride production. To rationalize the performance hierarchies of alternative nanostructured metal catalysts we derived a unified performance descriptor, based on acetylene interaction.
Controlling the precise architecture of supported metals, down to the single-atom limit, is central to unlock their full potential in catalysis. In the last decade, we have witnessed many examples of such nanostructured or single-atom catalysts (SACs) disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our molecular understanding of surface reactions.1 In combination with in-depth characterization and theoretical investigations, the enhanced synthetic control over the active sites opens new perspectives to solve long-standing challenges and build on historically established activity correlations using descriptors as design parameters.
A timely example is acetylene hydrochlorination, a key industrial process for the production of polyvinyl chloride (PVC), which has recently regained significant attention due to the urgency to identify suitable alternatives to the industrially applied catalyst based on mercuric chloride.2 Following the regulatory incentive of the “Minamata convention” in 2013, which effectively banned the use of this highly toxic catalyst, as soon as an economically viable alternative is identified, our group, among many others, joined this quest in 2016.
At this point, the search for mercury-free hydrochlorination catalysts had been going on for decades. Already in the 1960s catalytic descriptors had been sought after for this reaction to effectively guide catalyst discovery. In these early studies, Smith3 and Shinoda4 correlated the activities of a broad range of supported metal chlorides to the electron affinity of the metal cations and to the electron affinity divided by the metal valence, respectively. A decade later, Hutchings et al. introduced the standard electrode potential as the state-of-the-art descriptor, directing research towards the precious metals, and in particular to gold-based catalysts.5 With the availability of advanced characterization methods with atomic resolution in recent years, molecular level insights could be gained over the latter materials, revealing a clear dependence of the activity on the applied synthesis conditions and thus pinpointing at a strong structure sensitivity of acetylene hydrochlorination.6 These results marked an important turning point for catalyst development efforts in this reaction from the traditional Au nanoparticle-based systems towards SACs. More importantly, it anticipated an oversimplification of the traditional activity correlations, which were not taking the structure sensitivity of individual metals into account.
With our group’s strong interests in SACs and halogen chemistry, we set out to explore this metal-dependent structure-sensitivity in acetylene hydrochlorination, which led us on an exciting journey over the course of the past six years. Thereby, we thoroughly examined the potential of nanostructured gold7 and ruthenium-based8 systems and identified platinum SACs9 as a new promising candidate for this reaction, due to their unprecedented stability with respect to any other metal. We also witnessed that each of these metals exhibits their own, unique structure-sensitivity and interaction with the carbon support,10 determining their distinct behaviour under reaction conditions and thus overall performance in acetylene hydrochlorination.
Figure 1. Speciation-performance analysis of M/C catalysts in acetylene hydrochlorination. a, Initial catalytic activity of M/C catalysts with varying metal nanostructure (SA: single atom, NP: nanoparticle) and host functionalization (AC: activated carbon, NC: N-doped carbon), expressed as the turnover frequency (TOF), as a function of the standard electrode potential of the respective metal chloride. b, Representation of the optimal active site structure for each metal.
Taking each metal’s individual “personality” into account, we were intrigued to explore whether a common denominator could be identified in their reactivity in acetylene hydrochlorination. To identify such speciation-sensitive global performance descriptor, we further extended the metal scope and generated a comprehensive platform of Au, Pt, Ru, Ir, Rh, and Pd single atoms and nanoparticles supported on functionalized carbons to assess their evolution during synthesis and under reaction conditions. Integrating the results of electron microscopy, X-ray absorption-, and X-ray photoelectron spectroscopy with kinetic studies and modelling, we revealed a diversity in active site structure and activity hierarchy that no previous correlation could account for (Figure 1).
The real challenge then lied in identifying a descriptor that could embrace this diversity and accurately describe the performance hierarchies. For this task, a team of different experts was needed, dedicated to perform steady-state and transient kinetic studies, chemisorption analysis, and density functional theory on our comprehensive catalyst platform. To our great delight, everyone’s hard work paid off the moment all results from the different techniques pointed towards the same conclusion (Figure 2).
Figure 2. Performance descriptor for M/C catalysts in acetylene hydrochlorination. Experimentally determined TOF as a function of a,b, the ratio of adsorption (kads,eff) and desorption (kdes) constants of HCl and C2H2, as determined by TAP, c, the acetylene adsorption capacity, as determined by volumetric chemisorption at reaction temperature, and d, the computed acetylene adsorption energy over modelled active sites, including single-atom sites with varying degree of chlorination (MCl1-3) and N-coordination environment (SA-N4), as well as metal nanoparticles (represented by the most stable (111) surface) and metal oxides (rutile most stable surfaces (110)). The SA sites represented in d are AuCl, PtCl2, PdCl2, IrCl3, RhCl3 anchored on bi-epoxidic (AC) or tri-pyrrolic (NC) defects. The grey dotted line is a guide to the eye. Colored regions indicate too low (grey) and too high (red) acetylene interaction.
Consistently, all the techniques identify the acetylene adsorption energy as a speciation-sensitive activity descriptor, further determining the catalyst’s selectivity with respect to coke formation. The stability of the different metal nanostructures is governed by the interplay between single atom-support interaction and the chlorine affinity, promoting redispersion or agglomeration, respectively. Despite the intrinsic features of different methodologies, their complementarity can ensure robustness in descriptor identification, providing relevant implications for catalyst design.
Going beyond acetylene hydrochlorination, the herein presented strategy integrating precision material synthesis, advanced characterization tools, and theory to control, systematically assess and rationalize nuclearity, coordination, and host effects is generally applicable to supported metal-catalyzed reactions and thus provides a unique framework to derive catalytic descriptors for a wide range of applications.
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