In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

Abstract

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Figure 1.

Room temperature diffuse reflectance spectrum of a calcined *BEA zeolite (Si/Al=12.5) loaded with 0.3 wt. % Fe (black curve) to contain nearly single site square planar coordinated Fe²⁺. The spectrum is deconvoluted with five gaussian band shapes (blue), the sum of the gaussian bands is given by the red curve. The insert gives the parameters of the gaussian bands; obtained from experiment. Copyright 2016 Springer Nature.

Figure 2.

(A) The HOMO and LUMO and HOMO-LUMO gap of a homogeneous aryl-Ni^IV complex and the role of its LUMO in C-H bond activation. (B) A CdS nanorod photocatalyst with a bulge along its z-axis, with band gaps in the UV-VIS-NIR range. The band gap is affected along the z-axis by the presence of the bulge. (C) The SyrB2 non-heme Fe^IV-oxo active site, similar to other non-heme Fe^IV-oxo sites in homogeneous, enzymatic and heterogeneous catalysis, with its NIR-accessible excited state playing a key role in H atom abstraction. Adapted from refs. ^–. Copyrights 2019 Springer nature, 2014 and 2022 American chemical society.

Figure 3.

(A) Visualization of the interaction between the incident light and the rough surface. (B) The suspension of α-Al₂O₃ in a stirred liquid medium to enable diffuse reflectance mode measurement. Copyright 2022 Royal society of chemistry.

Figure 4.

Scheme of an in situ/operando spectroscopy set up. Reproduced from ref. . Copyright 2010 Royal Society of Chemistry.

Figure 5.

Variable temperature UV-VIS-NIR absorption of $\left[{\mathrm{P}\mathrm{h}\mathrm{B}\mathrm{P}}^{{\mathrm{P}\mathrm{h}}_3}\right]\mathrm{F}\mathrm{e}\left({\mathrm{N}\mathrm{P}\mathrm{P}\mathrm{h}}_3\right)$ showing its temperature dependent transition from a low spin state to a high spin state. Adapted from ref. Copyright 2016 American chemical society.

Figure 6.

Magnetic circular dichroism C-term intensity with increasing field at low temperature. Absorption of left circularly polarized (lcp) light (green), absorption of right circularly polarized (rcp) light (purple), and the differential absorption (solid black) due to the differences in Boltzmann population of the m = +1 and m = −1 levels as the Zeeman splitting increases towards the right.

Figure 7.

UV-VIS-NIR absorption spectra of the Ru^II(DMSO)₄Cl₂ complex recorded at 293 K and 85 K. The low temperature spectrum resolves the vibronic coupling in the 390 nm absorption band. Adapted from ref. . Copyright 2011 Royal Society of Chemistry.

Figure 8.

(A) Stopped-flow UV-VIS absorption spectra of Fe^II-tryptophan hydroxylase with ${\mathrm{O}}_2$. From 0 – 175 ms (top) and 0.175 – 2 s (bottom). (B) Speciation curve for formation and decay of intermediates. Adapted from ref. . Copyright 2021 National Academy of Sciences.

Figure 9.

Timescales of some different processes related to in-situ UV-VIS-NIR absorption spectroscopy, and specialized techniques used to access these time scales. Fs-TA = femtosecond transient absorption, OPA = optical parametric amplifier, BBO = beta barium borate, CCD = charge-coupled device, ns-TA = nanosecond transient absorption, YAG = Yttrium Aluminum Garnet, PMT = photomultiplier module, MFC = mass flow controller, 4WV = four-way valve, MS = mass spectrometer, GC = gas chromatographer. Adapter from refs. ^, . Copyrights 2023 and 2019 Royal Society of Chemistry.

Figure 10.

(A) Tauc plots of pure TiO₂, MO and MO+TiO₂ samples with different MO addition (a, b, c), including the Tauc lines. (B) Tauc plots after subtraction of the MO contribution from the spectrum. C) Tauc plot of MO+TiO₂ with baseline fitting. (D) Table with the optical band gaps obtained from the methods in the plots of panels A-C. Adapted from ref. Copyright 2018 American Chemical Society.

Figure 11.

Absorption spectra and Boltzmann sigmoidal curve fitting with indication of the derived direct and indirect optical bandgap energies for crystalline Germanium. Adapted from ref. . Copyright 2019 Springer Nature.

Figure 12.

Au plasmon position changes as determined from in situ UV-VIS absorption spectroscopy after exposure to alternating ${\mathrm{H}}_2$ and ${\mathrm{O}}_2$ flows at 398 K. Reproduced from . Copyright 2021 Elsevier.

Figure 13.

(A) Integrated UV reflectance in the 750–950 nm range as a function of time during CO adsorption, ${\mathrm{O}}_2$ adsorption and CO oxidation at 20 and 100 °C. (B) UV-VIS DRS spectra of TiO₂ and Au/TiO₂. Reproduced from . Copyright 2022 Springer Nature.

Figure 14.

In situ UV-VIS absorption spectra as a function of the temperature of (A) TiO₂ P25 and (B) Au/TiO₂ catalyst during activation under ${\mathrm{H}}_2$ (orange) and RWGS reaction (blue). Reprinted with permission from . Copyright 2018 American Chemical Society.

Figure 15.

Au/TiO₂ UV-VIS-NIR difference absorption spectra of before and after reaction, for different reaction temperatures. Reprinted with permission from . Copyright 2018 American Chemical Society.

Figure 16.

In situ UV-VIS-NIR absorption spectra of Au/CeO₂ samples during ${\mathrm{H}}_2$ pretreatment, ${\mathrm{C}\mathrm{O}}_2$ pretreatment and RWGS reaction. Reproduced from . Copyright 2022 Elsevier.

Figure 17.

Catalytic activity (black), time-dependent Raman (blue) and UV-Vis bands (brown) evolution for the CeO₂ subsurface reduction state in a 0.5 wt% Au/CeO₂ catalyst, (A) after two reaction cycles, (B) during CO oxidation after reducing pretreatment. Reproduced from ref. .

Figure 18.

In situ UV-VIS-NIR absorption difference spectra of a 3% V2O₅/TiO₂ catalyst during the gas flow change from ${\mathrm{O}}_2$ (green) to NO+NH₃ (pink). Reproduced from . Copyright 2019 Elsevier.

Figure 19.

In situ UV-VIS-NIR absorption spectra of (A) VO_x/TiO₂ after reduction with ethanol at different temperatures for 1 h; (B) VO_x/N-TiO₂ catalysts with varying nitrogen content, after reduction with ethanol at 200 °C for 1 h. Inset: UV-VIS-NIR absorption intensity at reduced centers as a function of N/V mol ratio. Adapted from . Copyright 2018 Elsevier.

Figure 20.

(A) In situ and operando VIS 700 nm absorption band evolution and ${\mathrm{C}\mathrm{O}}_2$ conversion during ${\mathrm{O}}_2$ and ${\mathrm{H}}_2$ exposure at 130 °C followed by RWGS reaction from 130 to 280 °C. (B) Top: In situ and operando UV-VIS-NIR absorption results for In₂O₃ under different gas exposures at 250 °C. Bottom: Gas-phase IR analysis during UV-VIS-NIR absorption measurements. Reproduced from . Copyright 2022 Wiley.

Figure 21.

Framework structures and exchange sites of zeolite topologies mentioned in this review. Colors of the exchange sites match their naming in the legend. Adapted from ref. . Copyright 2021 Elsevier.

Figure 22.

Reaction schemes and intermediates of methane to methanol over Cu-zeolites with (left) [CuOCu]²⁺ and (right) [CuIIOH]⁺ a active sites, including the known UV-VIS-NIR absorption bands.

Figure 23.

(A) Binding modes of N₂O on the binuclear Cu⁺ center. (B) UV-VIS spectra of autoreduced Cu-MFI during ${\mathrm{O}}_2$ treatment at room temperature (top) and subsequent heating in He atmosphere to form [CuOCu]²⁺ (bottom). Adapted from refs. ^, . Copyright 2014 and 2010 American Chemical Society.

Figure 24.

(A) Decay of the 22 700 cm⁻¹ absorption band in time during reaction of [CuOCu]²⁺ in Cu-MFI with methane at 90 °C (left) and Eyring plot for the reaction of CH₄ with [CuOCu]²⁺ in Cu-MFI (right). (B) Decay of the 22 400 cm⁻¹ absorption band in time during reaction of [CuOCu]²⁺ in Cu-CHA with methane at 80 °C (left) and Eyring plot for the reaction of CH₄ with [CuOCu]²⁺ in Cu-CHA (right). (C) Decay of the 20 700 cm⁻¹ absorption band in time during reaction of [Cu^IIOH]⁺ in Cu-MOR with methane at 130 °C (left) and Eyring plot for the reaction of CH₄ with [Cu^IIOH]⁺ in Cu-MOR (right). (D) Schematic of bidentate oxygen ligation of [CuOCu]²⁺ cores in Cu-MFI (green box) and Cu-CHA (purple box) assigning out of plane (OOP) and in plane (IP) ligation with respect to the CuOCu plane. (E) DFT-calculated potential energy surface of [CuOCu]²⁺ (purple) and of localized structure (green). Adapted from refs. ^{, ,} . Copyright 2021 American Chemical Society, 2009 Proceedings of the National Academy of Sciences and 2022 American Chemical Society.

Figure 25.

UV-VIS absorption spectra of Cu-CHA after ${\mathrm{O}}_2$ treatment at 723 K (black) followed by 2 h CH₄ treatment at 473 K (red). The difference spectrum is shown in green. Adapted from ref. Copyright 2017 American Chemical Society.

Figure 26.

A proposed reaction scheme and intermediates of standard low temperature NH₃-SCR with Cu-zeolites, including suggested UV-VIS-NIR absorption bands, based on the spectroscopic studies in ref. .

Figure 27.

(A) (left) In situ UV-VIS absorption spectra of Cu-CHA during reduction half cycle. The catalyst was first oxidized under ${\mathrm{O}}_2$, followed by exposure to NH₃ and then to NO. Time dependence of d-d transition band intensity of Cu²⁺ species and MS intensity of N₂ during (middle) reduction half cycle and during (right) NH_3-SCR - reduction - NH₃-SCR transient reaction. (B) Cu-speciation in the Cu-CHA catalyst as a function of Si/Al and Cu/Al. (C) In situ UV-VIS-NIR absorption spectra of Cu-AEI under (left) different gas mixtures at 473 K and (right) NH₃-SCR at 473 K with different ${\mathrm{O}}_2$ concentration. Adapted from refs. ^{, , ,} . Copyright 2020 Wiley, 2022 American Chemical Society, and 2020 American Chemical Society.

Figure 28.

UV-VIS-NIR DR absorption spectra of Cu-CHA exposure to (left) NH₃/He and (right) NO/He at 200 °C after forming the [Cu₂O₂(NH₃)₄]²⁺ complexes. Top and bottom panels on the right are initial and subsequent evolution of DR UV-VIS-NIR absorption spectra respectively. Blue thick line: after NH₃/He exposure, red thick line: spectrum collected after forming the [Cu₂O₂(NH₃)₄]²⁺ complexes, grey thin lines: intermediates, light blue thick line: after NO/He exposure, orange thick line: the final spectrum in the panel. Adapted from ref. . Copyright 2020 American Chemical Society.

Figure 29.

(A) (left) UV-VIS absorption spectra of Fe-MOR during exposure to NO + NH₃ at 300 °C. (middle) Changes in the UV–VIS absorption intensity at 265 nm in NO + NH₃ and the MS signal of N₂. (right) Time course of the UV-VIS absorption intensity at 265 nm under ${\mathrm{O}}_2$ or NO+O₂. (B) UV-VIS absorption spectra of Fe-*BEA during (left) heating in ${\mathrm{O}}_2$ and NH₃–SCR reaction at different temperatures. (right) Deconvolution results of UV-VIS spectra of Fe-*BEA during NH₃–SCR reaction at different temperatures. Adapted from refs. ^, . Copyright 2022 and 2013 American Chemical Society.

Figure 30.

Reaction schemes and intermediates of N₂O decomposition and C-H activation with Fe zeolites, including the known UV-VIS-NIR absorption bands.

Figure 31.

UV-VIS-NIR absorption spectroscopy of α- N₂O and its thermodynamics. (A) (left) In situ UV-VIS-NIR absorption difference spectra of Fe-*BEA at 308 K after high temperature He treatment with ${\mathrm{p}}_{({\mathrm{N}}_2\mathrm{O})}$ ranging from 0 to 0.05 atmosphere. (C) Van ‘t Hoff plot of data derived from the adsorption data from in situ UV-VIS-NIR absorption spectra analogous to those shown in the left. (B) (left) In situ UV-VIS-NIR absorption difference spectra of Fe-*BEA at 373 K following the transformation of α- N₂O to α-O over a time interval of 15 minutes in an atmosphere of 35 vol.% N₂O in He. (D) Eyring plot of data derived from in situ UV-VIS-NIR absorption spectra analogous to those shown in the left. Adapted from ref. . Copyright 2021 Springer Nature.

Figure 32.

(left in each panel) Deconvolution of the UV–VIS-NIR absorption spectra into two groups of bands by NNMF analysis. (right in each panel) The evolution of each group as a function of methanol loading on the catalyst material of HSSZ-13 catalyzed methanol conversion at (A) 573 K and (B) 723 K and HSAPO-34 catalyzed methanol conversion at (C) 573 K and (D) 673 K. (red: active hydrocarbon species, black: deactivating hydrocarbon species). Reproduced from refs. ^, . Copyright 2014 Wiley and copyright 2015 American Chemical Society.

Figure 33.

(A) In situ UV-VIS spectra during methanol conversion at 673 K and hydrocarbon species corresponding to the UV-VIS absorbance bands over the (left)CHA, (middle) DDR and (right) LEV catalysts. (B) (left) MCR-ALS components and (right) their respective contributions to the overall in situ UV-VIS spectra versus time for the conversion of methanol over DDR zeolite at a reaction temperature of (top) 623 K and (bottom) 723 K. The colors of the contribution plots correspond to the colors of the components. Reproduced from refs. ^, . Copyright 2017 and 2018 American Chemical Society.

Figure 34.

(A) Photo of the reactor bed after 2h MTO reaction and in situ UV-VIS absorption spectra of conversion of methanol or a mixture of propylene and ethylene at three positions along the reactor bed over (top) HZSM-5 and (bottom) MgZSM-5. (B) in situ UV-VIS absorption spectra collected during methanol conversion over (left) AE3, (middle) Z2 and (right) Z1 at 773 K. Reproduced from refs. ^, . Copyright 2018 Royal Society of Chemistry and copyright 2018 Springer Nature.

Figure 35.

(A) Schematic of reactants mixing via stopped-flow (UV-VIS), G-III-Br: third-generation Grubbs catalyst, [L]: exogenous ligand such as 3-Br-pyridine, EVE: ethyl vinyl ether, Ru=CHOEt: initiation product; (B) UV-VIS spectra of the reaction of G-III-Br+[L] with EVE. Adapted from ref. . Copyright 2019 Royal Society of Chemistry.

Figure 36.

5 K, NIR MCD spectra for the reaction of FeCl₂(SciOPP) with one equiv. (A), two equiv. (B), 20 equiv. (C), and 100 equiv of MesMgBr (D). Adapted with permission from ref. . Copyright 2014 American Chemical Society.

Figure 37.

CD (L.mol⁻¹.cm⁻¹) and UV (L.mol⁻¹.cm⁻¹) spectra of the reduction/oxidation induced switching between Cu^II and Cu^I. Adapted from ref. with permission from the Royal Society of Chemistry.

Figure 38.

(A) Reaction of rac-(ebthi)Zr(η² -Me₃SiC₂- SiMe₃) with ethylene; (B) Reaction of rac-(ebthi)Zr(η² -Me₃SiC₂- SiMe₃) with ethylene via intermediate B; (C) UV-VIS absorption spectra for the reaction of rac-(ebthi) Zr(η² -Me₃SiC₂- SiMe₃) with ethylene in toluene at r.t., k’ = 0.0365 s⁻¹ (the pseudo-rate constants k’ for the formation of 3-Zr as a function of ethylene concentration, rate constant k was determined from the slope of the linear regression); (D) UV-Vis spectra for the reaction of rac-(ebthi) Hf(η² -Me₃SiC₂- SiMe₃) with ethylene in toluene, at room temperzture, k’_obs.1 = 78.6 s⁻¹, k’_obs.2 = 2.4 s⁻¹). Adapted from ref. . Copyright 2010 Elsevier.

Figure 39.

(A) In situ UV-VIS absorption spectra of Cr(acac)₃ before and after adding PNP ligand; (B) Spectra of Cr(acac)₃/PNP in presence of different activators. Adapted from ref. . Copyright 2019 Wiley.

Figure 40.

(a) UV-VIS spectra of [(MeCN)₄]CuOTf complex in MeCN (b)+bpy (c) after 20 min bubbling ${\mathrm{O}}_2$ at 20 °C (d) +NMI (e) +NMI after 15 min (f) +BzOH (g) +TEMPO. Adapted from ref. . Copyright 2015 Wiley.

Figure 41.

(A) The formation of the solvate complex [Rh(Me-DuPHOS)(MeOH)₂]BF₄ via coe intermediate; (B) Reaction spectra for the hydrogenation of [Rh(Me-Du-PHOS)(cod)]BF₄ in MeOH at r.t. Adapted from ref. . Copyright 2010 Elsevier.

Figure 42.

UV-VIS-NIR absorption spectra of the reaction of CrO₃*py in presence of CHHP in α,α,α-trifluorotoluene at 5 °C. Insert: (blue line) Spectrum of the formed cyclohexylperoxychromium(VI) complex after 2 min; (shifted black line) decomposition after 10 min and (red line) full decomposition after 48 h. Adapted from ref. . Copyright 2018 Wiley.

Figure 43.

In situ UV-VIS absorption spectra obtained from Fe/PP₃ in THF, after addition of formic acid and further addition of NaCl. Adapted from ref. . Copyright 2014 American Chemical Society.

Figure 44.

(A) In situ UV-VIS absorption spectra for the reaction of N-methylphthalimide and Ph₂SiH₂ using TBAF as catalyst; (B) UV-VIS absorption spectra of N,N-dimethylbenzamide, PhSiH₃, and the boric acid and their mixtures in toluene; (C) in situ UV-VIS absorption spectra for the reaction of N,N-dimethylbenzamide and PhSiH₃ with the boric acid in toluene. Adapted from refs. ^, . Copyright 2011 Wiley and 2013 Wiley.

Figure 45.

In situ UV-Vis absorption spectra recorded during the (A) AlCl₃- and (B) TiCl₄-mediated reaction using ketenacetale (6a & 6b) in CH₂Cl_2. Adapted from ref. , using data with permission from refs. ^, . Copyrights 2013 American Chemical Society, 2011 Wiley and 2013 Elsevier.

Figure 46.

Change in the rate of decay of intermediate Q with varying CH₄ and CD₄ concentrations. (Inset) Stopped-flow UV-VIS absorption spectra of decay of intermediate Q from 4 to 30 s. Adapted from refs. ^, . Copyright 1993 The American Society for Biochemistry and Molecular Biology and copyright 1996 American Chemical Society.

Figure 47.

Density functional theory model of sMMO’s intermediate Q performing H-atom abstraction on CH₄. (Color scheme: red = O, green = Fe, grey = C, blue = N, and white = H). Adapted from ref. . Copyright 2021 American Chemical Society.

Figure 48.

(A) Arthropod hemocyanin with active site highlighted. (B) Resonance Raman spectrum of oxy-hemocyanin. (C) Resonance Raman excitation profile of the 286 cm⁻¹ (Cu-Cu) and 744 cm⁻¹ (O-O) vibrations of hemocyanin. Adapted with permission from refs. ^, . Copyright 1994 and 2014 American Chemical Society.

Figure 49.

(A) Conversion of L-tyrosine to L-DOPAquinone by the enzyme tyrosinase in melanin biosynthesis. (B) The main vibrational modes of the Cu₂O₂ core in tyrosinase. Resonance Raman spectra of oxy-Ty (blue) and the ternary intermediate of tyrosinase (red). (C) Dependence of $k_{obs}$ on substrate concentration in ${\mathrm{H}}_2\mathrm{O}$ (solid blue) and D₂O (solid red) with a linear fit of the low substrate regime (dashed). (D) QM/MM reaction coordinate for the monooxygenation of phenol by tyrosinase. Adapted from ref. . Copyright 2022 National Academy of Sciences.

Figure 50.

(Top) low-temperature absorption and (bottom) low-temperature MCD of plastocyanin. Adapted with permission from ref. . Copyright 2004 American Chemical Society.

Figure 51.

(Top) Saturation magnetization VTVH MCD for an S = 2 system. Adapted with permission from ref. . Copyright 2013 American Chemical Society.

Figure 52.

Absorption (top) and 7 T VT MCD (bottom) of the Br-Fe^IV=O intermediate in SyrB2. The lowest energy bands are labeled with roman numerals. *Minor heme contaminant. Adapted from ref. . Copyright 2016 American Chemical Society.

Figure 53.

(A) VT MCD spectrum of band I, the ${\mathrm{d}}_{\mathrm{x}\mathrm{z}/\mathrm{y}\mathrm{z}}{\mathrm{\pi }}^{\mathrm{*}}\to {\mathrm{d}}_{{\mathrm{z}}^{\mathrm{R}\mathrm{E}\mathrm{F}\_\mathrm{R}\mathrm{e}\mathrm{f}189754136\backslash \#0\backslash \mathrm{h}\backslash \mathrm{*}\mathrm{M}\mathrm{E}\mathrm{R}\mathrm{G}\mathrm{E}\mathrm{F}\mathrm{O}\mathrm{R}\mathrm{M}\mathrm{A}\mathrm{T}2}}\mathrm{\sigma }$ transitions in the Fe^IV=O intermediate in SyrB2. (B) Potential energy surfaces of the ground state and two lowest energy excited states. (C) FMO contours of the Fe^IV=O intermediate at the transition state with potential approach of the C-H bond in the $\mathrm{y}\mathrm{z}$ plane (top) and $\mathrm{x}\mathrm{z}$ plane (bottom). X refers to the halide. Adapted from ref. . Copyright 2016 American Chemical Society.

Figure 54.

Change in the promotion energy from the dπ* to dσ* orbital with elongation of the Fe^IV=O bond. Ground state dσ* LUMO with O character (bottom left) and excited state dπ* LUMO with O character (top right). Adapted from ref. . Copyright 2020 American Chemical Society.

Figure 55.

DFT model of the Cl-Fe^III-OH SyrB2 intermediate chlorination (green) vs hydroxylation (red) after HAA reaction coordinate through the π channel. Adapted from ref. . Copyright 2017 American Chemical Society.

Figure 56.

UV-VIS-NIR absorption data on (A) UU-100(Co) thin film switching between −1.5 and −0.05 V, and (C) COF-366-Co at steady-state at different applied potentials. (B) Optical Absorbance kinetics curve of the UU-100(Co) thin films measured at 520 and 670 nm by switching the potential from −0.05 to −1.5 V. Adapted from ref. ^, . Copyrights 2019 American Chemical Society and 2015 American Association for the Advancement of Science.

Figure 57.

(A) Absorption changes of (right) IrO_x and (left) Ir_molecular on mesoporous indium tin oxide upon applying an oxidative potential in 0.1 MHClO₄ aqueous solution at pH 1.2. (B) Normalized deconvolution of the spectroelectrochemical data of Ir_molecular and IrO_x. (C) Absorption decay after turning the potential off. Adapted from refs. ^–. Copyrights 2021 and 2022 American Chemical Society.

Figure 58.

Cyclic voltammograms (CVs) (Left) and UV-VIS-NIR absorption spectra (Right) of the (A) & (B) pure Ni and (C) & (D) 25% Fe films. Data points in the CVs correspond to the absorbance at 2.5 eV with the same color coding as in the spectra. Adapted from ref. . Copyright 2017 National Academy of Sciences.

Figure 59.

Change in UV-VIS-NIR absorption spectra of each sample; (A) after activation with respect to the absorbance before activation, and (B) after catalysis onset with respect to the absorbance before catalytic process. Adapted from ref. . Copyright 2020 Royal Society of Chemistry.

Figure 60.

Schematic of a pump-probe transient absorption experiment. At $t_0$ the ground state absorption spectrum is measured, before applying a pump pulse. This is used as the reference spectrum ${A(\lambda )}_{groundstate}$ in eq 17. At a later time $t_1$, the probe pulse is applied selectively exciting electrons from the ground state to an excited state (in this case from VB to CB). Then with delay $\tau$, the absorption spectrum is measured again to get $A(\lambda ,\tau )$ of eq 17. These pump-probe sequences are repeated for different $\tau$ to produce a time series of TA spectra.

Figure 61.

(A) Schematic illustration of CdS-C3, CdS-C6, and CdS-C9. (B) The steady-state absorption spectrum of CdS-C6 (red line, upper) and transient absorption spectra of CdS-C6 after 400 nm excitation (bottom). TA dynamics probed at 460 nm of CdS-Cx QDs in the (C) absence and (D) presence of lignin model compound PPol. Reproduced from ref. . Copyright 2019 American Chemical Society.

Figure 62.

The normalized transient decays from TA for (A) anatase and (B) rutile probed at 900 nm. Summary of different time scales from TA dynamics of the photocatalytic processes with (C) anatase and (D) rutile. Reproduced from ref. . Copyright 2015 American Chemical Society.

Figure 63.

(A) Schematic illustration of three distinct exciton states (X1, X2, and X3) in CdSe/CdS-Pt nanocrystals (upper panel) and their corresponding energy level diagram (lower panel). X1, X2, and X3 refer to excitons localized in the CdS rod, the CdS bulb region surrounding the CdSe seed, and the CdSe seed, respectively. Excitation energy decreases in the order X1 > X2 >X3. (B) TA spectra after 400 nm excitation for 0.2 to 1 000 ps. Reproduced from refs. ^, . Copyright 2013 American Chemical Society.

Figure 64.

UV-VIS absorption spectra (Kubelka-Munk) of Cu/TiO₂ after different irradiation times in (A) 100% ${\mathrm{C}\mathrm{O}}_2$, (B) 0.1 vol % O₂/CO₂, and (C) 0.5 vol % O₂/CO₂. (D) Proposed mechanism for photocatalytic conversion of ${\mathrm{C}\mathrm{O}}_2$ over Cu/TiO₂ and the nature of active copper species. Reproduced from ref. . Copyright 2019 Cell Press.

Figure 65.

(A) Schematic illustration of Re-COF structure and the proposed catalytic mechanism for ${\mathrm{C}\mathrm{O}}_2$ reduction over Re-COF. (B) Production of CO as a function of time from the photocatalytic conversion of ${\mathrm{C}\mathrm{O}}_2$ over Re-COF. (C) The in situ DRS UV–VIS absorption spectra of Re-COF under photocatalytic conditions within 15 min (upper panel) and 3 h (lower panel). The inset in lower panel shows the in situ spectra collected from 5 to 9 h. Reproduced from ref. . Copyright 2018 American Chemical Society.