Binhang Yan

Catalytic reduction of CO2 by H2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities

Ocean acidification and climate change are expected to be two of the most difficult scientific challenges of the 21st century. Converting CO2 into valuable chemicals and fuels is one of the most practical routes for reducing CO2 emissions while fossil fuels continue to dominate the energy sector. Reducing CO2 by H2 using heterogeneous catalysis has been studied extensively, but there are still significant challenges in developing active, selective and stable catalysts suitable for large-scale commercialization. The catalytic reduction of CO2 by H2 can lead to the formation of three types of products: CO through the reverse water–gas shift (RWGS) reaction, methanol via selective hydrogenation, and hydrocarbons through combination of CO2 reduction with Fischer–Tropsch (FT) reactions. Investigations into these routes reveal that the stabilization of key reaction intermediates is critically important for controlling catalytic selectivity. Furthermore, viability of these processes is contingent on the development of a CO2-free H2 source on a large enough scale to significantly reduce CO2 emissions.

Optimizing Binding Energies of Key Intermediates for CO2 Hydrogenation to Methanol over Oxide-Supported Copper

Rational optimization of catalytic performance has been one of the major challenges in catalysis. Here we report a bottom-up study on the ability of TiO2 and ZrO2 to optimize the CO2 conversion to methanol on Cu, using combined density functional theory (DFT) calculations, kinetic Monte Carlo (KMC) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, and steady-state flow reactor tests. The theoretical results from DFT and KMC agree with in situ DRIFTS measurements, showing that both TiO2 and ZrO2 help to promote methanol synthesis on Cu via carboxyl intermediates and the reverse water-gas-shift (RWGS) pathway; the formate intermediates, on the other hand, likely act as a spectator eventually. The origin of the superior promoting effect of ZrO2 is associated with the fine-tuning capability of reduced Zr(3+) at the interface, being able to bind the key reaction intermediates, e.g. *CO2, *CO, *HCO, and *H2CO, moderately to facilitate methanol formation. This study demonstrates the importance of synergy between theory and experiments to elucidate the complex reaction mechanisms of CO2 hydrogenation for the realization of a better catalyst by design.

CO2 Hydrogenation over Oxide-Supported PtCo Catalysts: The Role of the Oxide Support in Determining the Product Selectivity

By simply changing the oxide support, the selectivity of a metal-oxide catalysts can be tuned. For the CO2 hydrogenation over PtCo bimetallic catalysts supported on different reducible oxides (CeO2 , ZrO2 , and TiO2 ), replacing a TiO2 support by CeO2 or ZrO2 selectively strengthens the binding of C,O-bound and O-bound species at the PtCo-oxide interface, leading to a different product selectivity. These results reveal mechanistic insights into how the catalytic performance of metal-oxide catalysts can be fine-tuned.

Electrochemical reduction of CO2 to synthesis gas with controlled CO/H2 ratios

The electrochemical carbon dioxide reduction reaction (CO2RR) to simultaneously produce carbon monoxide (CO) and hydrogen (H2) has been achieved on carbon supported palladium (Pd/C) nanoparticles in an aqueous electrolyte. The synthesis gas product has a CO to H2 ratio between 0.5 and 1, which is in the desirable range for thermochemical synthesis of methanol and Fischer–Tropsch reactions using existing industrial processes. In situ X-ray absorption spectroscopy in both near-edge (XANES) and extended regions (EXAFS) and in situ X-ray diffraction show that Pd has transformed into β-phase palladium hydride (β-PdH) during the CO2RR. Density functional theory (DFT) calculations demonstrate that the binding energies of both adsorbed CO and H are significantly weakened on PdH than on Pd surfaces, and that these energies are potential descriptors to facilitate the search for more efficient electrocatalysts for syngas production through the CO2RR.

Combining CO 2 reduction with propane oxidative dehydrogenation over bimetallic catalysts

The inherent variability and insufficiencies in the co-production of propylene from steam crackers has raised concerns regarding the global propylene production gap and has directed industry to develop more on-purpose propylene technologies. The oxidative dehydrogenation of propane by CO2 (CO2-ODHP) can potentially fill this gap while consuming a greenhouse gas. Non-precious FeNi and precious NiPt catalysts supported on CeO2 have been identified as promising catalysts for CO2-ODHP and dry reforming, respectively, in flow reactor studies conducted at 823 K. In-situ X-ray absorption spectroscopy measurements revealed the oxidation states of metals under reaction conditions and density functional theory calculations were utilized to identify the most favorable reaction pathways over the two types of catalysts.The oxidative dehydrogenation of propane by CO2 (CO2-ODHP) can potentially fill the gap of propylene production while consuming a greenhouse gas. Here, the authors identify non-precious FeNi and precious NiPt catalysts supported on CeO2 as promising catalysts for CO2-ODHP and dry reforming, respectively, in flow reactor studies.

Active sites for tandem reactions of CO2 reduction and ethane dehydrogenation

Significance Catalytic activity or selectivity of a supported metal catalyst is predominantly determined by its active site structure. Rational optimization of supported metal catalysts requires fundamental insights into active sites and structure–function relationships. Here, we convincingly identify two types of metal–oxide active sites and successfully correlate them with the corresponding catalytic performance for CO2-assisted dehydrogenation of ethane. Controlled synthesis of the two distinct active sites enables rational manipulation of the activity and selectivity, offering an opportunity to efficiently convert the underutilized ethane from shale gas to value-added products while mitigating anthropogenic CO2 emissions. Ethylene (C2H4) is one of the most important raw materials for chemical industry. The tandem reactions of CO2-assisted dehydrogenation of ethane (C2H6) to ethylene creates an opportunity to effectively use the underutilized ethane from shale gas while mitigating anthropogenic CO2 emissions. Here we identify the most likely active sites over CeO2-supported NiFe catalysts by using combined in situ characterization with density-functional theory (DFT) calculations. The experimental and theoretical results reveal that the Ni–FeOx interfacial sites can selectively break the C–H bonds and preserve the C–C bond of C2H6 to produce ethylene, while the Ni–CeOx interfacial sites efficiently cleave all of the C–H and C–C bonds to produce synthesis gas. Controlled synthesis of the two distinct active sites enables rational enhancement of the ethylene selectivity for the CO2-assisted dehydrogenation of ethane.