*2.2. Synthesis of PdNPs*

PdNPs stabilized by 4MePy were synthesized in water, following the procedure [43,50]. Before NPs are used as catalysts they are dried because the presence of excess H2O in the reaction mixture might diminish the yield of diphenylurea formed during the carbonylation of aniline by CO/O2 (partial hydrolysis of urea may occur at high temperature) [1]. The TEM images presented in Figure 2 indicate agglomeration after drying (left panel versus middle panel), therefore, we also decided to synthesize NPs in ethanol. Right panel in Figure 2 demonstrates that palladium nanoparticles are not stable in ethanol and bigger aggregates are formed. A more appropriate name for these aggregates would be a palladium-based nanostructural material (PdNM) rather than nanoparticles.

**Figure 2.** (**A**) TEM image of the PdNPs stabilized by 4-methylpyridine (PdNPs/4MePy) from raw aqueous solution [51]. Pd: NaBH4 molar ratio=1:2, concentration of NaBH4 solution=1%. For synthesis conditions see Experimental Section. (**B**) TEM image of the PdNPs stabilized by 4-methylpyridine (PdNPs/4MePy) dried and re-suspended in distilled-deionized water. (**C**) TEM image of the PdNM (palladium-based nanostructural material) stabilized by 4-methylpyridine (PdNM/4MePy) obtained in ethanol.

### *2.3. Catalytic Activity of NPs Compared with Other Pd Species*

Conversion of aniline, selectivity towards DPU, and TOF for DPU formed in the presence of PdNPs/4MePy are presented in Table 3. Results obtained for PdNPs are compared with results obtained for commercially available Pdblack and two Pd(II) complexes: with 4MePy (the ligand that forms the most stable PdNPs) and with 2,4-Cl2Py (the ligand forming the most catalytically active complex of Pd(II)). TOF values of PdNPs, Pdblack, and Pd(II) complex, measured after 60 min demonstrate that all studied substances are effective pre-catalysts for carbonylation of aniline during the standard time of reaction. The highest yield observed for PdNPs indicates that PdNPs are either catalytically active species or the most efficient source of other catalytically active species e.g., [Pd(CO)3I]− [10]. Perhaps, when Pd(II) complex is applied as a pre-catalyst, it takes longer time to generate in situ catalytically active Pd(0) species from Pd(II), which might explain TOF for Pd(II) complexes being lower than for PdNPs. Our hypothesis, that active species responsible for catalytic activity are easily formed from PdNPs, is confirmed by results obtained for initial stages of the process (i.e., reaction carried out within first 15 min)-significant difference between TOF of PdNPs and TOF of Pd(II) complex is noticed. It is possible that, when carbonylation is carried out in the presence of PdNPs, catalytically active Pd(0) species are immediately present in the reaction from the beginning of the process. Differences in TOF values noticed for Pd(0) in the form of Pdblack (entry 4) and Pd(0) in the form of PdNM (entry 3) indicate that higher catalytic activity of PdNM might have an origin in the nanostructure of investigated material (image in the right panel of Figure 2 displays the PdNM formed from PdNPs with diameter ca. 10 nm). The lowest TOF value observed for Pdblack suggests that catalytically active species are not in the form of a heterogeneous bulk metal, but rather homogeneous complexes (with Pd0), as suggested in the literature [60]. Such a hypothesis that homogeneous complexes (with Pd0) are the real catalysts

can explain why TOF for Pd(II) is higher than for Pd(0) –formation of homogeneous Pd(0) species from bulky Pdblack is more difficult than from Pd(II).


**Table 3.** Parameters obtained for the carbonylation of aniline by CO/CO2/O2: conversion (CAN), selectivity (SDPU) and turnover frequency (TOFDPU), depending on the catalyst a.

Reaction conditions unless stated otherwise: Pd-based catalyst/Fe/I2 = 0.056/0.5/0.12 mmoL, AN = 54 mmoL, 1.5 MPa CO, 1.9 MPa CO2, 0.6 MPa O2, 20 mL EtOH, 60 min. Py = pyridine, AN = aniline, DPU = N,N-diphenylurea. b Selectivity toward DPU expressed as (mmoL DPU) × (mmoL converted AN)−<sup>1</sup> [%]. c TOFDPU (turnover frequency for AN) = [mmoL of AN reacted selectively to DPU] × [mmoL of Pd(II) complex used]−<sup>1</sup> × h−1. d 15 min. e PdNM = palladium-based nanostructural material was prepared in ethanol instead of water and raw solution of PdNM was introduced to the reactor (the volume of the solution was adjusted to obtain 0.056 mmoL of Pd). f 66 μL of 4MePy added.
