NO Oxidation Activity Research Articles

Exploring the dynamic evolution of lattice oxygen on exsolved-Mn2O3@SmMn2O5 interfaces for NO Oxidation

Lattice oxygen in metal oxides plays an important role in the reaction of diesel oxidation catalysts, but the atomic-level understanding of structural evolution during the catalytic process remains elusive. Here, we develop a Mn2O3/SmMn2O5 catalyst using a non-stoichiometric exsolution method to explore the roles of lattice oxygen in NO oxidation. The enhanced covalency of Mn–O bond and increased electron density at Mn3+ sites, induced by the interface between exsolved Mn2O3 and mullite, lead to the formation of highly active lattice oxygen adjacent to Mn3+ sites. Near-ambient pressure X-ray photoelectron and absorption spectroscopies show that the activated lattice oxygen enables reversible changes in Mn valence states and Mn-O bond covalency during redox cycles, reducing energy barriers for NO oxidation and promoting NO2 desorption via the cooperative Mars-van Krevelen mechanism. Therefore, the Mn2O3/SmMn2O5 exhibits higher NO oxidation activity and better resistance to hydrothermal aging compared to a commercial Pt/Al2O3 catalyst.

Unveiling the enhanced reactivity of NO ozonation on NH4-SAPO-34 zeolite: Ab initio molecular dynamics combined with experimental characteristics

An environmental-friendly NH4-SAPO-34 catalyst, easily obtained via ion exchange, was prepared and tested for its catalytic ozonation activity in NO oxidation under O3-deficient conditions. Characterization results revealed the successful loading of NH4+ on the Brønsted acidity of SAPO-34 after NH4NO3 ion exchange. Surprisingly, the NH4-SAPO-34 zeolite exhibited superior performance in the catalytic ozonation of NO. Ab initio molecular dynamics simulations demonstrated that the facile binding of NO and O3, along with rapid dissociation of the intermediate (NO2 dimer), contributed to the enhanced catalytic performance of NH4-SAPO-34 compared to H-SAPO-34. Additionally, the impact of various ammonium salts on the catalytic activity was investigated, stating clearly the NH4+ coordination in the eight-membered ring windows as the active site. Thus, this study deepens our understanding of the NO ozonation dynamics catalyzed by NH4-SAPO-34 and provides a promising method for synthesizing catalysts without expensive transition metals.

Remarkable improvement in photocatalytic activity of g-C3N4 for NO oxidation through surface hydroxylation

The nitrogen oxide emissions originating from combustion pose significant risks to the environment. Photocatalysis is considered an efficient and environmentally friendly strategy to alleviate this problem. Graphitic carbon nitride (g-C3N4) is regarded as one of the most promising organic photocatalytic materials for environmental purification. However, its small specific surface area, weak adsorption and high recombination rate of charge carriers result in low intrinsic photocatalytic activity. To overcome these obstacles, a hydrothermal treatment of dicyandiamide-derived g-C3N4 (DCN) at different temperatures (140–200 °C) was employed to enhance the photocatalytic activity for NO oxidation. The experimental results demonstrated a significant improvement in the photocatalytic oxidation removal rate of NO after a hydrothermal treatment. The optimal photocatalyst, DCN-180, treated at 180 °C, demonstrated the highest NO removal efficiency (65.0 %), which is twice the value of pristine DCN (32.5 %). Additionally, the formation of the toxic intermediate NO2 was effectively suppressed during the reaction. Photoelectrochemical tests revealed that DCN-180 exhibited higher photocurrent density and smaller impedance radius compared to the untreated g-C3N4 sample. Moreover, density functional theory (DFT) calculations confirmed that the DCN-180 sample showed a stronger ability to adsorb O2 and NO. The enhanced photocatalytic NO oxidation performance of DCN-180 has been primarily attributed to its enlarged specific surface area (from 10.7 to 35.5 m2 g−1), local polarization effect, reduced interfacial charge transfer resistance, and improved adsorption abilities for NO and O2 molecules. This study provides valuable insights for designing and preparing highly efficient g-C3N4 based photocatalysts through surface modification for photocatalytic NO purification.

Nitrogen‐containing microporous carbon with specific morphology for non‐metallic catalytic NO oxidation at room temperature: The effect of morphology and nitrogen doping

AbstractThe optimization of the gas diffusion path and surface coordination environment through morphology control can improve the intrinsic activity of the catalyst in NO oxidation reactions. Microporous nanosheets, nanowires, and spheres of carbon were constructed using resorcinol and formaldehyde as carbon sources, melamine as nitrogen source, and graphene oxide or carbon nanowires as structure‐directing agents to reveal the effects of morphology and nitrogen‐doping on NO oxidation activity at room temperature. With the increase of coating thickness, the ultramicroporous structure becomes pronounced and the nitrogen content increases, which contribute to the improvement of steady‐state NO conversion. The 2D microporous nanosheets (TDC‐200) with sheet structure shows prominent diffusion and adsorption capability than 1D nanowires and sphere, which shortens the gas diffusion path and enhances the efficient utilization of ultramicropores, thereby presenting the highest NO oxidation activity of 78.4% at room temperature. The results of DFT calculations further demonstrate that doping of nitrogen atoms could significantly reduce the (2NO + O2)ads energy barrier and accelerate the reaction. This study provides a deeper understanding of the NO oxidation on non‐metallic catalyst.

La-Co-Mn perovskite (LCMO) for NO and Hg0 oxidation: Experimental and DFT investigations of adsorption processes

To achieve superior catalytic performance, the development of plausible regulation of pure perovskite structure is necessary, which has become a hot research topic. In this study, manganese (Mn) ions were employed to dope cobalt (Co) ions at the B-site of LaCoO3, yielding a series of LaCo1-xMnxO3 (x = 0.1–0.5) perovskite (LCMO). The oxidation activity of NO and Hg0 was conducted, evaluating the effect of Mn-doping. The results indicated a minor change in NO oxidation activity and a significant enhancement in Hg0 oxidation, reaching above 92% at 100°C–300°C for LCMO. Structural characterization revealed a gradual transition in the phase structure of LCMO from the R-3C diamond phase to the Pm-3m cubic phase with increasing Mn doping. Furthermore, electronegativity properties of LCMO evolved with Co ions at the B-site exhibiting a preference for generating Co2+ over the more active Co3+. Finally, DFT calculations were conducted to analyze the adsorption energies of NO and Hg0 at different active sites on the surfaces of LaCoO3 (110) and Mn-LaCoO3 (110). The results indicated that Mn doping increased the active site amounts and strengthened the adsorption energy of Hg0 on the Mn-LaCoO3 (110) surface. However, the adsorption energy of NO remained largely unchanged, validating the oxidation results. This study offered insights for selecting B-site ions to adjust the perovskite structure of excellent performance, facilitating its industrial application.

Simultaneous catalytic oxidation of Hg0 and NO over CoxCuy metal oxides catalyst

In this work, Co3O4, CoxCuy and CuO metal oxides catalysts were synthesized by coprecipitation method and tested in catalytic oxidation of Hg0 and NO. The Co3Cu1 catalyst exhibited nearly 100% Hg0 oxidation efficiency at 100–400 ℃ and 84% NO oxidation efficiency at 250 ℃. Respectively, 1155.2 μg g−1 Hg0 adsorption capacity and 92.9 μmol g−1 NOx adsorption capacity over Co3Cu1 catalyst were measured by Hg0-TPD and NOx-TPD. The largest BET specific surface area of Co3Cu1 catalyst was obtained, reaching 26.2 m2 g−1. And H2-TPR demonstrated that the first reduction peak temperature (192 ℃) of Co3Cu1 catalyst was much lower than that of other catalysts. O2-TPD illustrated that the oxygen storage capacity and oxygen mobility ability of Co3Cu1 catalyst were higher than that of other catalysts. These results were consistent with the results of Hg0 and NO oxidation activity tests. In addition, in-situ DRIFTS spectra of NO adsorption indicated that the band at 1390 cm−1 over Co3Cu1 catalyst disappeared after the pretreatment of Hg0. And the transient experiments of Hg0 adsorption suggested that Hg0 could also be extruded from the surface of Co3Cu1 catalyst after the introduction of NO. It was speculated that there might be competitive adsorption between Hg0 and NO over Co3Cu1 catalyst. Finally, the XPS characterization demonstrated that the Oα and the redox cycle of Co3+ + Cu+ ↔ Co2+ + Cu2+ were involved in Hg0-NO simultaneous oxidation. The Co3Cu1 catalyst was an excellent candidate for Hg0-NO oxidation reaction.

Impact of mild hydrothermal aging on kinetics of NH[formula omitted], NO, SO[formula omitted] and CO oxidation reactions on Cu/SSZ-13 catalyst

Cu/SSZ-13 is a catalyst commonly used for selective catalytic reduction (SCR) of nitrogen oxides (NOx) in diesel engine exhaust. Standard configurations are based on SCR preceded by an oxidation catalyst, however, a close-coupled SCR configuration appears relevant for upcoming regulations addressing cold-start emissions. Two often discussed ion-exchanged Cu sites in Cu/SSZ-13 are denoted ZCuOH and Z2Cu, where Z represents an Al atom in the zeolite framework. Mild hydrothermal aging (HTA) causes Cu redistribution within the catalyst; ZCuOH sites transform into more stable Z2Cu species. In this work, the effect of mild HTA on NH3, NO, SO2, and CO oxidation is studied on a commercial Cu/SSZ-13 sample. Temperature-programmed reaction experiments, performed in a lab reactor, and previously published catalyst characterization data using H2-TPR, NH3-TPD, and DRIFTS were used to build and validate a mathematical model. It couples the kinetics of the oxidation reactions with ZCuOH and Z2Cu concentration evolution during mild HTA. The model uses two distinct rate coefficients for ZCuOH and Z2Cu sites, allowing prediction of the catalyst activity during aging. The results suggest that the rate coefficients for the studied reactions on Z2Cu are effectively nil, so the observed activity in NH3, NO, SO2, and CO oxidation can be attributed solely to ZCuOH sites.

Distinct effects of Na on Cu-based Beta zeolites for the selective catalytic reduction of NOx with NH3

The distinct impact of Na on the NH3-SCR activity of Cu-based Beta zeolites was explored by incorporating Cu onto Na and H-type Beta zeolites via ion-exchange and impregnation methods. The characterization results revealed that an appropriate amount of Na facilitated the exchange of Cu ions into the Beta zeolites, leading to enhanced acidity and NOx adsorption properties. The CuNa-Beta catalysts obtained through ion-exchange, exhibited decreased NH3 oxidation activity at elevated temperatures while demonstrated enhanced NO oxidation activity at lower temperatures, resulting in the improved NOx conversion. However, when the Cu loading was conducted by the impregnation method, Na remained in the Cu/Beta zeolites caused the transformation of Cu to CuOx, resulting in the inferior NH3-SCR performance. These findings demonstrated the significant influence of co-cation Na and preparation methods on Cu locations and NH3-SCR performance of Cu-based Beta zeolites.

Promoting the photocatalytic NO oxidation activity of hierarchical porous g-C3N4 by introduction of nitrogen vacancies and charge channels

Sluggish charge kinetics and moderate adsorption–desorption ability of gas molecules are major limitations for photocatalytic NOx elimination of bulk g-C3N4. A hierarchical porous g-C3N4 photocatalyst modified with N vacancies and charge channels (KCNN) was prepared by thermal polymerisation in KCl medium followed by quenching to increase the photocatalytic efficiency. The optimized KCNN sample exhibits highly enhanced photocatalytic NO removal rate (70.5%), which is superior to those of bulk g-C3N4 (38.1%), porous g-C3N4 (54.5%) and K-doped g-C3N4 (58.6%), respectively. X-ray photoelectron spectroscopy and electron paramagnetic resonance data reveal the successful formation of N vacancy in g-C3N4 framework. The enhanced activity of KCNN is ascribed to the enlarged surface area, expanded light absorption, low charge recombination efficiency and strong oxidation capability, respectively. In situ DRIFTS and density functional theory results suggest that the introduction of N vacancies and K+ ions enable control over NO adsorption and activation, leading to the implementation of a preferred pathway (NO → NO+ → NO3‾) and reduction in the emission of toxic intermediates. This work presents a potential idea for improving the charge transfer of layered materials and optimising the diffusion/adsorption/activation of gas molecules for photocatalytic NO oxidation.

Ni-BaMnO3 Perovskite Catalysts for NOx-Assisted Soot Oxidation: Analyzing the Effect of the Nickel Addition Method

In this study, we analyzed the role of a series of BaMn1−xNixO3 (x = 0, 0.2, and 0.4) mixed oxide catalysts, synthesized using the sol–gel method, in NOx-assisted diesel soot oxidation. ICP-OES, XRD, XPS, and H2-TPR techniques were used for characterization and Temperature-Programmed Reaction experiments (NOx-TPR and Soot-NOx-TPR), and isothermal reactions at 450 °C (for the most active sample) were carried out to determine the catalytic activity. All samples catalyzed NO and soot oxidation at temperatures below 400 °C, presenting nickel-containing catalysts with the highest soot conversion and selectivity to CO2. However, the nickel content did not significantly modify the catalytic performance, and in order to improve it, two catalysts (5 wt % in Ni) were synthesized via the hydrothermal method (BMN2H) and the impregnation of nickel on a BaMnO3 perovskite as support (M5). The two samples presented higher activity for NO and soot oxidation than BMN2E (obtained via the sol–gel method) as they presented more nickel on the surface (as determined via XPS). BMN2H was more active than M5 as it presented (i) more surface oxygen vacancies, which are active sites for oxidation reactions; (ii) improved redox properties; and (iii) a lower average crystal size for nickel (as NiO). As a consequence of these properties, BMN2H featured a high soot oxidation rate at 450 °C, which hindered the accumulation of soot during the reaction and, thus, the deactivation of the catalyst.

Influence of catalyst composition on NOx storage and reduction characteristics of Ag catalyst supported on MgO-doped alumina

A novel material (Ag/MgO-Al2O3) for storage of NOx under cold-start conditions and its reduction in the high-temperature stabilized phase is synthesized, and the influence of MgO and Ag loading on its NOx removal performance is investigated. A high MgO loading increases the NOx storage capacity but also decreases the stability of adsorbed NOx. Moreover, it decreases the adsorption rate at early times since NOx spillover is inhibited. The catalysts with the lowest Ag and MgO loading exhibit the highest NO oxidation activity. The amount of NOx storage increases with a decrease in Ag loading and is attributed to enhanced spillover of NOx to the support. Increasing the Ag and MgO content leads to an enhanced NH3 and C3H6 oxidation activity, which is explained by the presence of Ag metallic clusters. Further, the NH3-SCR and C3H6-SCR activity decrease with Ag and MgO loading. Thus, the catalyst composition can be tuned to obtain the desired activity for various oxidation and reduction reactions.

Sintering behavior of platinum and modification effect of ceria on silica-alumina supports for diesel soot oxidation

In this work, silica-alumina mixed oxides with different SiO2 contents (5% and 30%) were adopted as acidic supports for platinum catalysts for soot oxidation. The obtained catalysts were thermally aged in air at 550, 650 and 750 °C for 2 h, respectively. The catalysts were characterized by transmission electron microscopy (TEM), CO chemisorption, inductively coupled plasma (ICP), N2 adsorption/desorption, NH3 temperature-programmed desorption (TPD), H2 temperature-programmed reduction (TPR), infrared (IR) spectroscopy of CO and NH3 adsorption, and NO temperature-programmed oxidation (TPO). The acidic support Pt catalysts had higher catalytic activities for NO oxidation and soot oxidation. After aging at 750 ℃, severer sintering of Pt occurred on the acidic supports, and some Pt particles larger than 50 nm formed, resulting in serious deactivation. It was ascribed to metallic state of Pt on the acidic supports with weakened metal-support interactions (SMI). Thus, CeO2 was added to the silica-alumina support by ball milling to strengthen the SMI via forming Pt-O-Ce bonds. In this way, Pt nanoparticles were anchored to the modified support to inhibit sintering at high temperatures, and the obtained Pt/CeO2/SiO2-Al2O3 catalyst exhibited both high soot oxidation activity and superior thermal stability.

Mn-N/C interfacial bridge accelerating electrons transfer of MnCO3/α-Ni(OH)2 to promote surface adsorbates activation for favorable photocatalytic NO oxidation

Photocatalytic technology has been proven to be an effective strategy for addressing NOx pollution. However, the lack of adsorbates activation ability of photocatalysts results in the formation of secondary toxic by-products. Herein, MnCO3/α-Ni(OH)2 composite photocatalyst was developed through one-pot hydrothermal method. The introduction of MnCO3 not only improves visible light absorption, but also generates new Mn-C and Mn-N interfacial bridge structures between the exposed Mn atoms of MnCO3 and the C/N functional groups of α-Ni(OH)2. This unique structure speeds up the migration of photogenerated electrons and holes, promoting surface-adsorbed O2 and NO co-activation. Photocatalytic activity test shows that MnCO3/α-Ni(OH)2 has enhanced NO oxidation activity without producing harmful NO2 by-products. This research offers a fresh perspective on the surface adsorbates activation to prevent secondary toxic by-product generation.

Metal-Doped Mesoporous MnO2-CeO2 Catalysts for Low-Temperature Pre-Oxidation of NO to NO2 in Fast SCR Process

Selective catalytic reduction (SCR) is an effective system for treating nitrogen oxides (NOx; mainly NO), and fast SCR requires the equimolar reactants of NO and NO2. This study focused on catalysts for oxidizing 50% of NO to NO2. A series of catalysts composed of a variety of components, such as mesoporous mMnO2-nCeO2 as carrier catalysts (m:n = 9:1 and 7:3) and transition metals (e.g., Fe, Co, Ni, Cu, and Cr), were synthesized and characterized using N2 adsorption, in situ XRD, TEM, and XPS. All samples had a mesoporous structure with pore size around 8 nm. XPS results demonstrated that addition of cerium ion increased the surface area and provided oxygen vacancy due to the formation of Ce3+ within the structure. NO oxidation activity was tested using a feed (205~300 ppm NO and 6% O2) that simulated typical flue gas conditions. Doped mesoporous mMnO2–nCeO2 has higher NO oxidation activity than pristine mMnO2–nCeO2. The doped mMnO2-nCeO2 catalyzed 50% of NO to NO2 at between 140 and 200 °C resulting in an equivalent amount of NO and NO2. Among the transition metals, Cu, Ni, Co, Fe, and Cr have the highest to lowest oxidation activity, respectively. The precatalytic oxidation of NO can potentially be combined with the current SCR system without changes to existing equipment and can be applied to the exhaust gas treatment for de-NOx.

Synthesis of CdS QDs/BaSnO3 nanocomposites via electrostatic self-assembly for visible light photocatalytic deep oxidation of NO

A BaSnO3 sample loaded with CdS quantum dots (CdS QDs/BaSnO3) was prepared by an electrostatic self-assembly method. The obtained CdS QDs/BaSnO3 nanocomposites exhibited an efficient visible light utilization and photocatalytic NO oxidation activity. The experimental results revealed that the loading of CdS QDs tremendously improved the light absorption range and the separation efficiency of photogenerated electron-hole pairs. In-situ DRIFTS tests demonstrated that the introduced Cd2+ could provide new active sites for NO adsorption and activation. Simultaneously, the EPR test showed that the introduction of CdS QDs induced the generation of ·O2–, which was conducive to promoting the deep oxidation of NO. This study provides a facile and effective synthesis method and strategy for improving the photocatalytic NO removal efficiency while significantly suppressing the NO2 concentration.

Understanding structure-performance relationships of CoOx/CeO2 catalysts for NO catalytic oxidation: Facet tailoring and bimetallic interface designing

Crystalline structure and bimetallic interaction of metal oxides are essential factors to determine the catalytic activity. Herein, three different CoOx/CeO2 catalysts, employing CeO2 nanorods (predominately exposed {110 facet), CeO2 nanopolyhedra ({111} facet) and CeO2 nanocubes ({100} facet) as the supports, are successfully prepared for investigating the effect of exposed crystal facets and bimetallic interface interaction on NO oxidation. In comparison with the {111} and {100} facets, the exposed crystal facet {110} exists the best superiority to anchor and stabilize Co species. Moreover, ultra-small CoOx clusters composed of strong Co-O coordination shells with minor Co-O-Ce interaction are formed and uniformly dispersed on the CeO2 nanorods. Structural characterizations reveal that the active exposed crystal facet {110} and the strong bimetallic interface interaction in CoOx/CeO2-nanorods (R-CC) result in more structural defect, endowing it with abundant oxygen vacancies, excellent reducibility and strong adsorption capacity. The DRIFTs spectra further indicate that the exposed crystal facet {110} has a significant promoting effect on the strength of nitrates compared with {111} and {100} facets. The bimetallic interface interaction not only significantly facilitates the formation of nitrate species at lower temperature, but also effectively suppresses the generation of sulfate and lower the sulphation rate. Therefore, R-CC catalyst exhibits the maximum NO oxidation activity with the conversion of 86.4 % at 300 °C and still sustains its high activity under cyclic condition or 50 ppm SO2. The provided crystalline structure and interaction-enhanced strategy sheds light on the design of high-activity NO oxidation catalysts.

Unraveling the contribution of valence state to the NO oxidation activity of manganese oxides

Mn-based catalysts are of immense interest in NO oxidation—a crucial subsystem of many deNOx techniques. To meet the industrial requirement and shed light on the effect of Mn valence state on catalytic activity, we synthesized stoichiometric ε-MnO2, which achieved outstanding NO oxidation activity at low temperatures (84% NO conversion at 150 °C), and a series of manganese oxides with different ratios of Mn4+/Mn3+ as comparisons. X-ray absorption spectroscopy and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated Mn4+ could accelerate the elementary surface reaction. We show that the surface redox efficiency, defined as the average reduction temperature obtained from H2 temperature-programmed reduction (H2-TPR), dictates the rate of surface reaction and further the activity of NO oxidation for various Mn-based catalysts, which can be a reference for advanced catalyst selection in NO catalytic oxidation.

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO.

Atom trapping leads to catalysts with atomically dispersed Ru1O5 sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru1/CeO2 is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru1/CeO2 shows very high NOx storage properties due to formation of stable Ru-NO complexes as well as a high spill-over rate of NOx onto CeO2. Only ∼0.05 wt % of Ru is required for excellent NOx storage. Ru1O5 sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO2 nanoparticles. We clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru1/CeO2 for NO reduction by CO at low temperatures: only 0.1-0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru1/CeO2 and its propensity to form oxygen vacancies/Ce+3 sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.

Single Pd atoms supported on ultra-thin bismuth tungstate nanosheets with oxygen vacancies as an efficient photocatalyst

The O vacancies of ultra-thin bismuth tungstate confine Pd atoms to produce a Pd-VO-UBWO SAC, which exhibits high photocatalytic activity for NO oxidation and RhB degradation.

Co-Precipitated Mn0.15Ce0.85O2−δ Catalysts for NO Oxidation: Manganese Precursors and Mn-Ce Interactions

Two Mn0.15Ce0.85O2−δ mixed oxides were synthesized by a co-precipitation method using Mn(NO3)2 and KMnO4 as the manganese precursors, respectively. Structural analyses by X-ray powder diffraction and Raman spectroscopy reveal the formation of MnOx-CeO2 solid solutions. The Mn0.15Ce0.85O2−δ catalyst prepared from the high-valent manganese precursor exhibits higher activity for the catalytic oxidation of NO. The advantage of KMnO4 is related to the improved redox property of the catalyst as supported by H2 temperature-programmed reduction (TPR) and O2 temperature-programmed desorption (TPD). The Mn-Ce interactions create more Mn4+, Ce3+ and oxygen vacancies on the KMnO4-synthesized mixed oxides based on the Raman and X-ray photoelectron spectra (XPS).