Heterogeneous single-metal-site catalysts (HSMSCs) have attracted considerable interest, but most studies have focused on the metal atoms in the active site while ignoring the key role of ligands. The unique coordination environment of a single-site catalyst is crucial for realizing its potential. Constructing this kind of catalyst via a feasible and practical fabrication method is challenging. Herein, a single-site Pd catalyst with iodide ligands supported on activated carbon (Pd1/AC) was successfully fabricated by atomic dispersion of large Pd nanoparticles (NPs). Intermediate I• radicals were detected during the atomic dispersion process of Pd NPs by in situ imaging photoelectron photoion coincidence spectroscopy (in situ iPEPICO) with vacuum ultraviolet synchrotron radiation. The molecular structure of single-site Pd was established as [Pd­(CO)­I4(OAC)]2– through combined characterization. Alkyne dialkoxycarbonylation with high selectivity toward 1,4-dicarboxylic acid esters (>94%) and high acetylene conversion (>99%) was achieved. A sulfonic promoter on the Pd1/AC catalyst for alkyne dialkoxycarbonylation was avoided because of the iodide ligand. Good durability and a broad substrate scope were successfully achieved.

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Sulfur poisoning is a severe problem in industrial applications, attracting broad interest in fundamental research studies. Although a number of studies about sulfur resistance have been implemented in many reactions on nanoparticle catalysts, few investigations focus on carbonylation reactions using heterogeneous single-metal-site catalysts (HSMSCs). Herein, we present an unanticipated sulfur-promoted performance in olefin hydrocarboxylation reactions on a single-Rh-site catalyst supported on porous ionic polymers (Rh1/PIPs) with 1000 ppm H2S in CO feed. Ex situ EXAFS and in situ DRIFTS revealed a ternary cycle mechanism of olefin hydrocarboxylation reactions with Rh–H complexes as predominant active species in both pure and H2S-containing feedstock. Moreover, the transformation of the Rh mononuclear complex with the addition of H2S was also demonstrated. Density functional theory studies were performed to verify the feasibility of the proposed pathway and confirm that the energy barriers of transition states with the sulfur ligand were much lower than those in normal feed, for example, a decline of 3.4 kcal/mol for the rate-determining step of migration and insertion of CO. This work provides a distinctive example for the insight of sulfur effect on carbonylation, which could be potentially beneficial for further applications of HSMSCs.

Heterogeneous single‐metal‐site catalysts usually suffer from poor stability, thereby limiting industrial applications. Dual Pd1−Ru1 single‐atom‐sites supported on porous ionic polymers (Pd1−Ru1/PIPs) were constructed using a wetness impregnation method. The two isolated metal species in the form of a binuclear complex were immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single Pd‐ or Ru‐site catalyst, the dual single‐atom system exhibits higher activity with 98 % acetylene conversion and near 100 % selectivity to dialkoxycarbonylation products, as well as better cycling stability for ten cycles without obvious decay. Based on DFT calculations, it was found that the single‐Ru site exhibited a strong CO adsorption energy of −1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1−Ru1/PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1/PIPs for the rate‐determining step. The synergetic effect between neighboring single sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized PdII active sites. The discovery of synergetic effects between single sites can deepen our understanding of single‐site catalysts at the molecular level.

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Heterogeneous single‐metal‐site catalysts usually suffer from poor stability, thereby limiting industrial applications. Dual Pd1−Ru1 single‐atom‐sites supported on porous ionic polymers (Pd1−Ru1/PIPs) were constructed using a wetness impregnation method. The two isolated metal species in the form of a binuclear complex were immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single Pd‐ or Ru‐site catalyst, the dual single‐atom system exhibits higher activity with 98 % acetylene conversion and near 100 % selectivity to dialkoxycarbonylation products, as well as better cycling stability for ten cycles without obvious decay. Based on DFT calculations, it was found that the single‐Ru site exhibited a strong CO adsorption energy of −1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1−Ru1/PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1/PIPs for the rate‐determining step. The synergetic effect between neighboring single sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized PdII active sites. The discovery of synergetic effects between single sites can deepen our understanding of single‐site catalysts at the molecular level.

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Sulfur poisoning is a challenge for most nanoparticle metal catalysts. A trace amount of sulfur contaminants could result in dramatic catalytic activity reduction or even irreversible deactivation1-5. Therefore, new approaches to enhance the catalyst sulfur-resistance have continuously attracted attention from academia and industry. Herein, a role reversal of sulfur from poison to promotor is presented for an Rh-based heterogeneous catalyst from supported Rh nanoparticles (NPs) to its single-site catalysts (Rh1/AC, AC: activated carbon) in methanol carbonylation, ethylene and acetylene hydrocarboxylic reaction with a feed containing 1000 ppm H2S (S-feed). In situ free-electron laser/time of flight mass spectrometry (In situ FEL/TOF MS) indicated that H2S could be quickly transformed into catalyst-friendly CH3SCH3 and/or CH3SH on the Rh1/AC, which coordinated with the Rh ions and promoted its methanol carbonylation reaction, possessing a lower energy barrier based on DFT calculations. On the contrary, strong adsorption of H2S on the surface of Rh NPs inhibited the reaction of reactants.