**4. Conclusions**

In this work, the effect of CO2 on the rate of carbonylation of aniline to N,N'-diphenylurea was investigated. Taking into account results obtained for various types of Pd-based pre-catalysts—namely, PdNPs, PdNM, Pd(0) and Pd(II) complexes—the highest TOF is observed for PdNPs. Thus, we sugges<sup>t</sup> that PdNPs act as active species or as the most efficient source of other catalytically active Pd(0) species. High yields observed for Pd(II) complexes support proposed mechanism where Pd(II) is easily reduced to catalytically active Pd(0) during catalytic cycle. We conclude that introduction of PdNPs to the reaction mixture instead of Pdblack or Pd(II) results in higher conversion and turnover frequency. We believe that obtained results may be helpful in elucidating the role of metallic nanoparticles in other organic processes.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/10/8/877/s1, Figure S1: TG curve for centrifuged and dried PdNPs/4MePy (obtained in water), Figure S2: Effect of derivatives of pyridine on the conversion (Conv.), yield and selectivity (Select.) of the catalyst. Reaction conditions: 54 mmol AN, PdCl2(XnPy)2/Fe/I2 = 0.056/2.68/0.118 mmol, 15 atm CO, 19 atm CO2, 6 atm O2, 20 ml EtOH, 100◦C, 60 min.

**Author Contributions:** D.M.: Investigation, Visualization, Writing—original draft, Writing—review & editing. A.K.: Visualization, Formal analysis, Writing—review & editing. P.P.: Methodology, Data curation, Writing—original draft, Writing—review & editing. A.K.-S.: Conceptualization, Investigation, Data curation, Formal analysis, Funding acquisition, Methodology, Project administration, Resources, Supervision, Visualization, Writing—original draft, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Centre for Research and Development-Poland, gran<sup>t</sup> LIDER no. LIDER/34/0102/L-7/15/NCBR/2016.

**Conflicts of Interest:** This manuscript has not been submitted elsewhere for consideration, and the authors declare no competing financial interests.
