Silver–Gold Bimetal-Loaded TiO₂ Photocatalysts for CO₂ Reduction

Abstract

Bimetallic Au−Ag/TiO 2 materials have been investigated as promising catalysts for the photocatalytic conversion of CO 2 with water as the reducing agent. The deposition of both metals on the TiO 2 surface was performed using a deposition method in independent steps. Characterization studies reveal a parallel distribution of both metals, with a preferential surface exposure of silver nanoparticles (NPs). The close contact between metal NPs and TiO 2 allows an efficient charge transfer between both phases upon excitation, which results in a distinct reaction mechanism under ultraviolet (UV) and visible illumination. Under UV light, bimetallic catalysts are able to shift the reaction selectivity toward methane, compared to the solely syngas (CO + H 2 ) production over bare TiO 2 . This effect is ascribed to the capacity of the metal NPs to extract photoexcited electrons. Under visible light, this selectivity modification is not observed, whereas bimetallic catalysts are active toward syngas production, significantly improving the residual activity of TiO 2 . Under these conditions, an additional strong interfacial interaction between metal NPs and TiO 2 allows the photoexcitation of electrons from surface states in the band gap, leading to the occurrence of near-surface electron−hole pairs.

Experimental methods in chemical engineering:X‐ray absorption spectroscopy—XAS,XANES,EXAFS

“…With this approach (operando spaceresolution EXAFS analysis), Dann et al [88] studied how F I G U R E 1 9 Ag K-edge Xray absorption near edge structure (XANES) spectra during (A) exposure to the Xray beam and during UV light switching under reaction conditions (B) during a cycle of successive on/off external blue illumination. Reproduced from Reñones et al [60] Copyright American Chemical Society 2020 the ignition of CO oxidation reactions coincide with the oxidation of Pd nanoparticles on catalyst surfaces, confirming the Langmuir-Hinshelwood reaction mechanism. Beale and coworkers [89] produced 3D images of a carbon-supported Mo-Pt industrial catalyst with scanning XANES tomography to visualize the distribution of both Pt (active) and Mo (promoter) phases.…”

“…A similar experimental procedure was applied to study photocatalysis with Ag nanoparticles supported on TiO 2 to reduce CO 2 . [60] The operando XANES study (Figure 19) identified how charge transferred in UV and visible light: UV light promotes electrons from the titania valence band to the conduction band (3d states of Ti) and then to Ag. The charge accumulated in the Ag-centres reduce the CO 2 to CH 4 .…”

Recent Progress in the Synthesis and Applications of Composite Photocatalysts: A Critical Review

“…In another report, bimetallic Au-Ag/TiO 2 composite photocatalyst revealed enhanced activity for CO 2 photoreduction, which was accredited to the promoted charge separation via the strong interfacial contact between the bimetal nanoparticles and TiO 2 . [25] Plasmonic Ag@TiO 2 core-shell composite also revealed excellent performance for selective CO 2 conversion to CH 4 . [379] According to Mahyoub and coworkers, [380] 3D flowerlike plasmonic Ag-CeO 2 -ZnO Z-scheme ternary composite showed superior activity for CO 2 conversion which was accredited to the remarkably improved charge carrier's separation and transfer.…”

“…Beside the UV counterpart, about 46% of visible and 48% infrared light accounts for the total solar spectrum. [20,21] A variety of wide and narrow band gap photocatalysts and their composites, such as SrTiO 3 , [22,23] TiO 2 , [24,25] ZnO, [26,27] BiVO 4 , [28,29] Bi 2 WO 6 , [30,31] Cu 2 O, [32] CdS, [33,34] r-GO, [35,36] g-C 3 N 4 , [4,[37][38][39] MoS 2 , [40,41] MoSe 2 , [42][43][44] WS 2 , [45][46][47] Fe 2 O 3 , [48,49] LaFeO 3 , [10,50] BiFeO 3 , [50,51] CulnS 2 , [52] copper chalcogenide, [53,54] double-layered hydroxides (LDH), [55,56] and semiconductor quantum dots, [57] etc., have been assessed so far in photocatalysis for water reduction, CO 2 conversion and pollutant degradation. Thus, a single semiconductor photocatalyst always exhibit low photocatalytic efficiency owing to the low solar energy harvesting and poor charge carrier's separation.…”

Improving Photoelectrochemical Activity of ZnO/TiO2 Core–Shell Nanostructure through Ag Nanoparticle Integration

“…In the application of PEC water splitting, nanostructured semiconductor photoelectrode possesses a large surface-to-volume ratio (SVR), thus reducing the migration distance of photo-excited carriers and facilitating their transfer to surface. The heterojunctions integrating ZnO or TiO 2 with another semiconductor [26][27][28][29][30][31][32] or metal co-catalyst [33][34][35][36][37][38][39][40][41][42][43] have been explored by many researchers. Moreover, heterogeneous structures which combine more than two semiconductors or semiconductor-metal junctions have also been employed as an effective strategy to promote carrier separation, avoid the carrier recombination and improve solar energy conversion efficiency [44][45][46][47][48][49][50].…”