Promotional effects of oxygen vacancies of spinel catalysts in CO 2 hydrogenation are reported in early works, but the mechanistic origins remain elusive. Here, CoAl 2 O 4 spinels with varying numbers of oxygen vacancies are deliberately designed by a sol-gel method and different post-treatments. By combining catalytic testing, advanced electron microscopic and spectroscopic characterizations, and computational studies, the unusual oxygen vacancy-dependent catalytic behaviors are rationalized. Our work reveals that i) perfect spinel crystals possessing least oxygen vacancies can effectively constrain the Co 2+ species at working conditions that are less active but selective to CO; and ii) vacancy-rich spinels promote both H 2 and CO 2 activations and COOH* formation, explaining the higher hydrogenation activity, but overwhelming vacancies cause Co 2+ reduction and promote direct CO 2 * dissociation to CO* and deep hydrogenation to CH 4 . These molecular-level understandings reinforce the idea of proper design of oxygen vacancies to achieve activity-selectivity balance.
The utilization of solar energy in photocatalytic CO 2 reduction technology has demonstrated a significant potential in addressing the challenges of environmental pollution and energy shortage issues. Precise modulating of the photoreduction CO 2 pathway to achieve desired products is of particular interest, while the design of cost-effective and robust catalysts remains crucial. Herein, a series of BiOBr-based photocatalysts with different Co species were synthesized through a facile simple hydrothermal method in combination with the mechanical mixing. The Co species, which can be precisely modulated from Co 3 O 4 to CoOOH by adjusting the concentration of H 2 O 2 , were anchored into the BiOBr nanospheres to establish rich Z-scheme heterojunction interfaces. The Co species modulating of BiOBr-based Z-scheme heterojunction can achieve the transform photoreduction CO 2 products from CO to CH 4 . The CO selectivity in CO 2 photoreduction to reached 98.10 % (54.10 μmol·g −1 ·h −1 ), while the CH 4 selectivity over CoOOH/BiOBr reached 72.59 % (20.06 μmol·g −1 ·h −1 ). The outstanding photocatalytic performance and selective regulation of products are ascribed to the modulation of Co species in the BiOBr-based heterojunctions, which enables the customization of the photoreduction CO 2 pathway to achieve the desired products transformation from CO to CH 4 . In situ experiments and theoretical calculations reveal that the excellent CO selectivity observed in Co 3 O 4 /BiOBr heterojunction can be ascribed to a lower energy for *CO species compared with that of hydrogenation to *HCOOH and *CHO intermediates. Conversely, the presence of *HCOOH and *CHO intermediates with lower energy levels compared with *CO species achieved superior CH 4 selectivity in CoOOH/BiOBr heterojunction. This precise design strategy insights into the meticulous customization of Co species in semiconductor-based heterostructures at the molecular level, and enlightens the selective regulation of desired products for catalysis applications.
Yuebing Xu,Wenda Yu,Hao Zhang,Jian Xin,Xiaohui He,Bing Liu,Feng Jiang,Xiaohao Liu
Various Co-based SAPO-34 catalysts were prepared using different methods, including ion exchange (IE), incipient-wetness impregnation (IWI), and solid-phase grinding (SPG), to correlate the chemical states of Co species with the C–H and C–C bond scissions in ethane dehydrogenation. The IE-prepared Co/SAPO-34 led to stable, unreducible, and isolated exchanged Co sites anchored on the zeolite framework with a structure of −AlF–O–Co–O– and showed the highest selectivity to ethylene of close to 98% at 600 °C, which suggests that these Co sites favors suppressing the C–C bond scission in ethane. In comparison, the IWI- and SPG-prepared Co/SAPO-34 catalysts, especially for those with a high Co loading, inevitably give Co oxide clusters that are easily reduced into metallic Co. Together with catalytic results, characterizations, and DFT calculations, it is confirmed that the reduced Co clusters, especially for those outside SAPO-34 channels without the confinement effect, favor both C–H and C–C bond scission, boosting the conversion of ethane into CH4 or/and coke; however, the ionic-state −Co–O– species can smoothly terminate the ethane dehydrogenation for the ethylene product due to relatively high energy barriers for both C–H and C–C bond scission, avoiding a deep dehydrogenation and C–C cracking. As expected, the unreducible −Co–O– sites are very stable in the title reaction without deanchoring from the zeolite framework in a 100 h cyclic test. This study not only demonstrates the stable −Mδ+–Oδ− structure favorable for suppressing C–C bond scission but also highlights a catalyst-constructing strategy for Co-based and similar metal-based catalysts for dehydrogenation of other light alkanes.
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