Preferential CO Oxidation on a Highly Active Cu Single Atom Catalyst Supported by Ce−TiO_x

Abstract

A heterogeneous single-atom catalyst (SAC) based on a tiny Cu loading (0.05 wt.%) supported on CeO 2 À TiO 2 (Ce/Ti = 0.18) shows very high Cu mass-normalized CO oxidation activity, 100 % selectivity for CO 2 between 60 and 100 °C and essentially no deactivation during 17 h on stream in the preferential oxidation of CO (PROX) at 120 °C. Based on state of the art, this catalyst is among the most active and PROX-selective catalysts. Employing a combination of ex-situ and operando methods, we infer that the isolated Cu single sites are incorporated with redox-active Cu 2 + À OÀ Ce 4 + .Cu + À &À Ce 3 + moieties connected by labile oxygen and are exposed on the surface of highly dispersed ceria. This high dispersion is promoted by TiO 2 which itself does not participate in redox steps during PROX, as Ti ions remain essentially tetravalent while Cu and Ce undergo reversible redox shuttles.

Controlling Activity of Heterogeneous Cu Single‐Atom Catalysts by Fine‐Tuning the Redox Properties of CeO2‐TiO2 Supports

“…Finally, we would like to note that both insights from H 2 ‐TPR as well as the EELs on the electronic properties of CeOx species are in good agreement with NAP‐XPS results obtained on similar catalyst during CO oxidation (see Ref [14] . ) and during preferential CO oxidation (PROX) (see results in Ref [23] …”

Mechanism and selectivity of MOF-supported Cu single-atom catalysts for preferential CO oxidation

“…[8][9][10] With their well-dened molecular structure, MOF-based SACs can be considered as a model system for fundamental understanding of heterogeneous SACs, which is hardly accessible for oxide-supported catalysts. 11,12 Additionally, the use of MOFs as supports allows for loading higher density of isolated metal single sites compared to what can be achieved with typical catalyst supports, where limited loading in the latter case is essential to prevent sintering during reactions. [12][13][14][15] As an example, between 1.6 and 2 wt% of atomically dispersed Cu can be loaded into the UiO-66 framework, 16,17 compared to #0.05 wt% on CeO 2 -TiO 2 supports needed to ensure atomic dispersion of the loaded metal.…”

“…[12][13][14][15] As an example, between 1.6 and 2 wt% of atomically dispersed Cu can be loaded into the UiO-66 framework, 16,17 compared to #0.05 wt% on CeO 2 -TiO 2 supports needed to ensure atomic dispersion of the loaded metal. 11,12,16 This enhances the gravimetric and volumetric density of these catalysts which is necessary for their practical applications, where size and mass of the reactor are decisive. The tradeoff between the total catalyst-mass based activity and gravimetric efficiency in both cases should be optimized for possible technical applications.…”

“…Heterogeneous SACs have been widely investigated for CO oxidation, both in exhaust catalysis and in preferential CO oxidation (PROX). [11][12][13]15,18 The PROX process is important for the removal of CO from hydrogen-rich feed gases (produced from hydrocarbon reforming) that are used in low-temperature fuel cells or in ammonia power plants. 19,20 Considering the sustainable production of hydrogen through the reforming of renewable hydrocarbon sources (e.g., biomass or synthetic natural gas), the PROX process would also play a crucial role in the processing of solar fuels derived from renewable resources.…”