Pd Supported on Pr-Rich Cerium–Zirconium–Praseodymium Mixed Oxides for Propane and CO Oxidation

Round 1

Reviewer 1 Report

1. The manuscript entitled Pd Supported on Pr-Rich Cerium-Zirconium-Praseodymium Mixed Oxides for Propane and CO Oxidation investigated Pd (<0.2 wt. %) supported on the Pr-rich CZP mixed oxides as a catalyst for the propane and CO oxidation. Fahed et al. observed that after initial calcination at 800 °C, Pd is mainly in the form of fully dispersed isolated cations in strong interaction with CZP45 and its inactive for propane oxidation and quite active for low-temperature CO oxidation. In contrast, pre-reduced Pd/CZP45 catalysts promote propane oxidation due to the Pd nanoparticles formation coupled with the very high oxygen mobility of CZP45. However, the re-oxidation of Pd nanoparticles and their partial re-dispersion, promoted by the fast oxygen mobility of the mixed oxide, rapidly deactivated the catalysts in lean conditions. Overall, the manuscript is well written with a detailed characterization explanation. The manuscript is suitable for the catalysts journal. However, the following minor revisions need to be done before publication.
   1. Did the authors try to characterise the materials with XPS analysis? The XPS analysis of Pd before and after reduction of Pd-HS might have directly confirmed the Pd oxidation states, and current studies indirectly support the Pd oxidation states. 
   2. Please check the sentence on page 1, Line no. 30.
   3. The author should include more details of the In-situ FTIR instrument and how the measurements were taken in the catalyst characterization section.
   4. The authors may cite the following important recent articles related to the Pt group metal supported on Ceria. Energy Environ. Sci., 2020, 13, 1231-1239 and Chem. Commun., 2021, 57, 7382-7385.
   5. It's good to include the CPZ45 support H2-Reduction (Fig.3) and C3H8-TPR (Fig.4) for the comparison. 
   6. In figure 2, the table quality needs to be improved.

Author Response

We would like to thank the reviewer for the comments and suggestions for the manuscript

Point 1: Did the authors try to characterise the materials with XPS analysis? The XPS analysis of Pd before and after reduction of Pd-HS might have directly confirmed the Pd oxidation states, and current studies indirectly support the Pd oxidation states. 

We have performed preliminary XPS measurements but the ultra-high vacuum in the analysis chamber was found to strongly modified the surface state of CZP oxides by fully reducing Pr⁴⁺ and also Ce⁴⁺ cations. In addition, photoemission peaks of Pd 3d are overlapped with photoemission peaks of Zr 3p, leading impossible the analysis of the Pd oxidation state.

Point 2: Please check the sentence on page 1, Line no. 30.

The sentence has been modified as follow:” These catalytic materials, developed for the aftertreatment of gasoline and Diesel engines, are prone to deactivation at high temperatures due to the PGM sintering and collapse of the support [1].”

Point 3: The author should include more details of the In-situ FTIR instrument and how the measurements were taken in the catalyst characterization section.

Details on the FTIR procedure were added in the experimental part: “Pd sites on Pd-HS were identified by using CO as a probe molecule using in-situ IR measurement. Catalysts (24-28 mg) were pressed (~1 ton.cm^-2) into cylindrical thin pellets with a diameter of 16 mm. However, the quantity of species adsorbed on the low SSA Pd-LS catalyst was too low to obtain relevant FTIR spectra. We focused our efforts on Pd-HS. This latter was pre-treated at 600 °C under pure O₂ (66 mbar) for 20 min, then cooling-down to RT still in presence of oxygen and exposed to high vacuum for 15 min to evacuate CO₂ from the IR cell. CO₂ was supposed to come from the decomposition of surface carbonates at 600°C. The evacuation under vacuum was performed at RT to avoid any surface reduction that could occur at 600°C. Oxygen was again introduced (66 mbar) at RT and the pellet was heated up to 600°C for 20 min for a second oxidation step on a carbonate-free surface, cooling down again in O₂ at RT where the oxygen was removed from the cell under high vacuum. Finally, CO (1.3 mbar) was adsorbed at RT for 1 min and IR spectra were recorded (64 scans). The CO adsorption state was limited to 1 min to avoid   a possible reduction and/or rearrangement of the catalyst. After that, a re-oxidation of the same pellet was then performed during two successive treatments at 600°C in O₂ (66 mbar) of 15 min (carbonate decomposition) and 45 min (oxidation of a carbonate-free surface), respectively. In between these two steps, the catalyst was exposed to high vacuum to remove CO₂. Then, the pellet was cooling down in O₂ to RT, evacuated in high vacuum before the CO adsorption measurement for 1 min.

A similar CO adsorption was performed after a first oxidation step of a fresh pellet at 600°C for 15 min in O₂ (66 mbar) to desorb carbonates following by a cooling-down at 500°C still in O₂. At 500°C, the oxygen was removed under high vacuum and 133 mbar of pure H₂ were introduced for 1 h to reduce the catalyst and then evacuated in high vacuum. After a cooling-down in vacuum at RT, CO was introduced and adsorbed for 1 min and IR spectra recorded. Then, the same pellet was re-oxidized at 600°C in O₂ (66 mbar) in two steps as initially (15 min/ high vacuum / 45 min), cooled-down in O₂ to RT, where CO was chemisorbed again for 1 min after the evacuation of O₂ in high vacuum. A similar sequence of measurements was performed on a fresh pellet except that the reduction step occurred in presence CO at 500°C. Two successive reduction steps in 10 mbar of CO for 30 min each one were performed with an evacuation in high vacuum in between to remove CO₂ produced by the first reduction phase. The CO adsorption at RT was then analyzed by FTIR. A similar re-oxidation step at 600°C than after the H₂ reduction was carried out before adsorbing again CO at RT. The signature of CO in the phase gas was systemically subtracted from all FTIR spectra”

Point 4: The authors may cite the following important recent articles related to the Pt group metal supported on Ceria. Energy Environ. Sci., 2020, 13, 1231-1239 and Chem. Commun., 2021, 57, 7382-7385.

These two articles have been cited in the revised manuscript.

Point 5: It's good to include the CPZ45 support H2-Reduction (Fig.3) and C3H8-TPR (Fig.4) for the comparison. 

We have modified Figures 3 and 4 to include also CZP45 for comparison.

Point 6: In figure 2, the table quality needs to be improved.

 We have improved the quality of Figure 2.

Reviewer 2 Report

Dear Authors,

This Manuscript “Pd Supported on Pr-Rich Cerium-Zirconium-Praseodymium Mixed Oxides for Propane and CO Oxidation”

This paper describes preparation (Pd supported on Pr-rich Ce-Zr-Pr). The catalytic activity was estimated under Propane and CO Oxidation.

The manuscript is well written, and the results are very convincing.

The manuscript is very well written; clear, precise, and easy to understand.

The following Main points should be considered, Please

1.     Authors need to provide experimental data for the actual metal in fresh catalysts and catalysts after the reaction.

2.     The reaction should be checked by leaching or reusability studies.

3.     It is suggested that Pd(II) species may be active in the reaction. The heterogeneity of the reaction should be checked by hot-filtration and leaching or reusability studies.

Good luck

Author Response

We would like to thank the reviewer for the comments and suggestions for the manuscript

Point 1: Authors need to provide experimental data for the actual metal in fresh catalysts and catalysts after the reaction.

The actual Pd loading has been measured by ICP-OES (see Table 1) in fresh catalysts which was calcined at 800°C for 8 h. We have also checked that the Pd concentration was not modified after catalytic measurements. Metal lost in emission control catalysts is generally due to the volatilisation of metal oxides at high temperature, typically above 800°C in an oxidizing atmosphere. As our catalyst was not exposed during catalytic tests to temperatures above 600°C, the loss of Pd during catalyst tests was unlikely.

Point 2: The reaction should be checked by leaching or reusability studies.

Vehicles emission control involves gas phase reactions on solid catalysts and therefore no leaching issue is expected. However, as mentioned in the first point, volatilisation of metal oxides can take place at very high temperature in oxidizing temperatures as well as sintering of the oxide support and metal nanoparticles. However, incursions in these harsh conditions are scarce and the lifetime of metal supported catalysts such as Pd/CZP materials are suitable for the application.

 Point 3: It is suggested that Pd(II) species may be active in the reaction. The heterogeneity of the reaction should be checked by hot-filtration and leaching or reusability studies.

CO and propane oxidations do not involve any liquid phases but only gas/solid interfaces. Pd(II) cations are not in a solution but stabilized on the surface of CZP as small clusters or isolated cations. These latter are stable in oxidizing conditions even at high temperature.

Author Response File: Author Response.docx