Creating New-Generation Catalysts for CO2 Hydrogenation to Methanol

ABSTRACT

In efforts towards establishing a circular carbon economy, CO₂ hydrogenation to methanol (CO₂-to-MeOH) has received extensive interest from academia and industry. The current industrial catalyst for CO₂-to-MeOH is Cu/ZnO/Al₂O₃; however, this system suffers from catalyst instability owing to moisture induced deactivation, hampering long-term process operation. Consequently, several Cu-free metal oxides have recently emerged as a new class of catalysts owing to the excellent selectivity and stability of these systems, among which a ZnO-ZrO₂ solid solution catalyst (ZnZrO_x) is undergoing pilot-scale testing. One distinct feature of ZnZrO_x is its superior long-term stability, nonetheless, one downside is its lower activity compared to the industrial Cu/ZnO/Al₂O₃ catalyst. As such, achieving both high activity and long-term stability is a challenging mission for developing next-generation CO₂-to-MeOH catalysts. Here, we introduce a strategy to promote a ZnZrO_x methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrO_x and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrO_x system delivers hydrogen activated on Pd to a broad area on the ZnZrO_x surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrO_x catalyst. In CO₂-to-MeOH, Pd/CNT + ZnZrO_x exhibits drastically boosted activity—the highest among reported ZnZrO_x-based catalysts—and excellent stability over 600 h on stream test, showing potential for practical implementation. What is more, we find that our physical mixing strategy for promoting CO₂-to-MeOH catalysts is generalizable, as we present a new-age Cu-based system with state-of-the-art activity and excellent stability in preliminary long-term testing.

Transitional Flow Regimes Driven by Shear and Buoyancy Forces

ABSTRACT

This work investigates the transitional flow regimes and their transport properties arising from the interaction between shear and buoyancy forces. While the impact of these forces has been extensively studied in isolation over the past decades - shear-driven turbulent Couette/Poiseuille/pipe flows and buoyancy-driven flows in Rayleigh-Bénard convection, their combined effects remain poorly understood. We aim to address two fundamental questions such as: 1) How does shear influence convection, and 2) can buoyancy promote transition to turbulence? To explore these questions, we consider a Rayleigh-Bénard-Poiseuille (RBP) setup, where the fluid is confined between two infinitely parallel plates, driven by a Poiseuille flow, heated from the bottom and cool from the top. The behaviour of the fluid is controlled by the Reynolds (Re) and Rayleigh (Ra) numbers, representing the relative strength of shear and buoyancy respectively. In the transitional Ra-Re parameter space considered here, the fluid exhibits five distinct flow regimes: 1) Chaotic convection, 2) stationary convection rolls, 3) wavy convection rolls, 4) intermittent convection rolls and 5) turbulence. This talk will present a detailed analysis of these flow regimes, offering insights into the interaction of shear and buoyancy forces.