Controlled Synthesis of High Surface Area Pd and Pt/SiO2(core)@ZrO2(shell) Catalysts for Low Temperature Oxidation Applications

Chih-Han  Liu, University at Buffalo

Controlled Synthesis of High Surface Area Pd and Pt/SiO₂(core)@ZrO₂(shell) Catalysts for Low Temperature Oxidation Applications

Chih Han Liu¹, Junjie Chen¹, Todd J. Toops², Eleni A. Kyriakidou^1,*

¹Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, USA

²National Transportation Research Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

*elenikyr@buffalo.edu

 

Future diesel oxidation catalysts will need to perform effectively at increasingly low exhaust temperatures; this so-called “150ºC challenge” (i.e., achieve over 90% conversion below 150^oC) arises from continued improvements in engines efficiency.  Palladium supported on ZrO₂ catalysts have been tested for their low temperature diesel oxidation performance where 50 and 90% conversions were achieved at 160, 180ºC for CO and 180, 195ºC for C₃H₆, respectively [1].  Uncontrolled incorporation of ZrO₂ on high surface area SiO₂, resulted in incomplete coverage of SiO₂ by ZrO₂, leading to decreased performance compared to Pd/ZrO₂ catalysts.  Complete coverage of SiO₂ by ZrO₂ was thus introduced and SiO₂(core)@ZrO₂(shell) supports with an enhanced surface area of 210 m²/g were synthesized [2].  Herein, SiO₂@ZrO₂ supports with controlled sizes were synthesized by varying the NH₄OH concentration and feed rate, resulting in SiO₂@ZrO₂ spheres with average diameters of 450 nm and 202 nm (Fig. 1A, B).  The surface areas of the synthesized SiO₂@ZrO₂ supports increased with decreasing SiO₂@ZrO₂ diameter: 147 and 293 m²/g for SiO₂@ZrO₂ supports with 450 and 202 nm diameter, respectively.

The catalytic performance of 1 wt.% Pd/SiO₂@ZrO₂ monometallic catalysts was evaluated using the Crosscut Lean Exhaust Emissions Reduction Simulations (CLEERS) protocol (333 mL/min, 12% O₂, 6% H₂O, 6% CO₂, 400 ppm H₂, 2000 ppm CO, 100 ppm NO, 250 ppm C₂H₄, 100 ppm C₃H₆ ,33.33 ppm C₃H₈ and 210 ppm C₁₀H₂₂, Ar balance) under degreened and hydrothermally aged conditions [3].  Comparable T₅₀’s of CO were observed over 1 wt. % Pd/SiO₂@ZrO₂ with 450 and 202 nm support diameter, while Pd/SiO₂@ZrO₂ (202 nm) had a lower T₅₀ of THCs by 17^oC (Fig. 1C).  Improved performance was achieved after hydrothermal aging at 800^oC/10h, with the T₅₀’s of 1 wt. % Pd/SiO₂@ZrO₂ (450 nm) decreasing to 187^oC (CO) and 240ºC (THCs), while even lower T₅₀’s (176^oC (CO), 222ºC (THCs)) were achieved over the smaller support diameter (202 nm) 1 wt. % Pd/SiO₂@ZrO₂ catalyst.  Monometallic 1.8 wt.% Pt/SiO₂@ZrO₂, as well as bimetallic Pd-Pt/SiO₂@ZrO₂ catalysts with Pd/Pt ratios varying from 1/3 to 3 were synthesized for an immediate comparison with the performance of 1 wt.% Pd/SiO₂@ZrO₂.  This work illustrates the potential of developing Pd-based oxidation catalysts with enhanced durability and low-temperature activity using SiO₂@ZrO₂ core@shell shaped mixed oxide supports with controlled sizes.

Figure 1.  TEM image of SiO₂@ZrO₂ supports with (A) 450 nm (B) 202 nm diameters, and (C) T_50,90 of 1 wt.% Pd/SiO₂@ZrO₂ catalysts with 202, 450 nm support diameters evaluated with the simulated diesel exhaust CLEERS protocol.

 

[1] M.-Y. Kim, E.A. Kyriakidou, J.-S. Choi, T.J. Toops, A.J. Binder, C. Thomas, J.E. Parks II, V. Schwartz, J. Chen, D.K. Hensley, Appl. Catal., B, 187, 181-194 (2016).

[2] E.A. Kyriakidou, T.J. Toops, J.-S. Choi, M.J. Lance, J.E. Parks II, US Patent Publication US20180250659A1 (2018).

[3] Aftertreatment Protocols for Catalyst Characterization and Performance Evaluation: Low-Temperature Storage Catalyst Test Protocol: https://cleers.org/wp-content/uploads/2018/03/2018_ LTAT _ Low-Temperature-Storage-Protocol.pdf