Combining XPS and ToF-SIMS for assessing the CO oxidation activity of Au/TiO2 catalysts

Crystal-Plane-Dependent Guaiacol Hydrodeoxygenation Performance of Au on Anatase TiO2

TiO2-supported catalysts have been widely used for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about the effect of their different crystalline surfaces on their activity during the hydrodeoxygenation process. In this work, Au supported on anatase TiO2, mainly exposing 101 or 001 facets, was investigated for the hydrodeoxygenation (HDO) of guaiacol. At 300 °C, the strong interaction between the Au and TiO2-101 surface resulted in the facile reduction of the TiO2-101 surface with concomitant formation of oxygen vacancies, as shown by the H2-TPR and H2-TPD profiles. Meanwhile, the formation of Auδ−, as determined by CO-DRIFT spectra and in situ XPS, was found to promote the demethylation of guaiacol producing methane. However, this strong interaction was absent on the Au/TiO2-001 catalyst since TiO2-001 was relatively difficult to be reduced compared with TiO2-101. The Au on TiO2-001 just served as the active site for the dissociation of hydrogen without the formation of Auδ−. The hydrogen atoms spilled over to the surface of TiO2-001 to form a small amount of oxygen vacancies, which resulted in lower activity than that over Au/TiO2-101. The catalytic activity of the Au/TiO2 catalyst for hydrodeoxygenation will be controlled by tuning the crystal plane of the TiO2 support.

Controlling the Size of Au Nanoparticles on Reducible Oxides with the Electrochemical Potential

Controlling the size of Au nanoparticles (NPs) and their interaction with the oxide support is important for their catalytic performance in chemical reactions, such as CO oxidation and water-gas shift. It is known that the oxygen vacancies at the surface of support oxides form strong chemical bonding with the Au NPs and inhibit their coarsening and deactivation. The resulting Au/oxygen vacancy interface also acts as an active site for oxidation reactions. Hence, small Au NPs are needed to increase the density of the Au/oxide interface. A dynamic way to control the size of the Au NPs on an oxide support is desirable but has been missing in the field. Here, we demonstrate an electrochemical method to control the size of the Au NPs by controlling the surface oxygen vacancy concentration of the support oxide. Oxides with different reducibilities, La_0.8Ca_0.2MnO_3±δ and Pr_0.1Ce_0.9O_2-δ, are used as model support oxides. By applying the electrochemical potential, we achieve a wide range of effective oxygen pressures, pO₂ (10^-37-10¹⁴ atm), in the support oxides. Applying the cathodic potential creates a high concentration of oxygen vacancies and forms finely distributed Au NPs with sizes of 7-13 nm at 700-770 °C in 10 min, while the anodic potential oxidizes the surface and increases the size of the Au NPs. The onset cathodic potential required to create small Au NPs depends strongly on the reducibility of the support oxide. The Au NPs did not undergo sintering even at 700-770 °C under the cathodic potential and also were stable in catalytically relevant conditions without potential.

Co, Cu, Fe, and Ni Deposited over TiO2 and Their Photocatalytic Activity in the Degradation of 2,4-Dichlorophenol and 2,4-Dichlorophenoxyacetic Acid

Pure TiO2 synthesized by the sol-gel method and subsequently deposited at 5% by weight with Co, Cu, Fe, and Ni ions by the deposition–precipitation method were studied as photocatalysts. The nanomaterials were analyzed by SEM, TEM, UV-Vis DRS, DRX, Physisorption N2, and XPS. The SEM and TEM images present a semi-spherical shape with small agglomerations of particles and average size between 63 and 65 nm. UV-Vis results show that a reduction below 3.2 eV exhibits a redshift displacement and increment in the optical absorption of the nanoparticles promoting the absorption in the UV-visible region. XRD spectra and analysis SAED suggest the characteristic anatase phase in TiO2 and deposited materials according to JCPDS 21-1272. The specific surface area was calculated and the nanomaterial Ni/TiO2 (21.3 m2 g−1) presents a slight increment when comparing to TiO2 (20.37 m2g−1). The information generated by the XPS spectra present the deposition of metallic ions on the support and the presence of different valence states for each photocatalyst. The photocatalytic activity was carried out in an aqueous solution with 80 mg L−1 of 2,4-D or 2,4-DCP under UV light (285 nm) with 100 mg L−1 of each photocatalysts for 360 min. The nanomaterial that presented the best efficiency was Ni/TiO2, obtaining a degradation of 85.6% and 90.3% for 2,4-D and 2,4-DCP, respectively. Similarly, this material was the one that presented the highest mineralization, 68.3% and 86.5% for 2,4-D and 2,4-DCP, respectively. Photocatalytic reactions correspond to the pseudo-first-order Langmuir–Hinshelwood model.

Facet-dependent activity of shape-controlled TiO2 supported Au nanoparticles for the water–gas shift reaction

Temperature-dependent interfacial catalysis of Au/TiO2 catalysts for the water–gas shift (WGS) reaction.

Photodegradation of 2,4-D (dichlorophenoxyacetic acid) with Rh/TiO2; comparative study with other noble metals (Ru, Pt, and Au)

In this work the effect of noble metal on the photodegradation of 2,4-dichlorophenoxyacetic acid herbicide using TiO₂ as support was studied. The metals and concentration were: Rh, Ru, Pt and Au and 1, 0.98, 1.89, and 1.91 wt% respectively. Rhodium was taken as reference for this experiment. The samples were characterized by X-Ray Diffraction (XRD), UV-vis absorption spectra, N₂ physisorption (BET Specific Surface Area), High Annular Angle Analysis Darkfield (HAADF) and Transmission Electron Microscopy Scanning (STEM), H₂ chemisorption, optical emission spectroscopy with inductive coupling plasma analysis (ICP-OES), solid fluorescence, X-ray Photoelectron Spectroscopy (XPS) and OH quantification. The presence of the anatase crystalline phase was mostly confirmed in all samples. The band gap decreased with the presence of metal (from 3.24 to 2.92 eV). The specific area was a function of the metal particle size. The metal particle diameter showed the following sequence Pt > Ru > Au > Rh. By XPS, TiO₂ does not manifest changes in oxidation states, but when impregnated with metals, only Pt shows the highest abundance of any oxidized state (Pt²⁺). The presence of metal reveals less electron–hole recombination compared with titanium oxide. The results of photocatalytic activity showed that Pt and Rh are the two metals with the highest mineralization (99.0 and 98.3%, respectively).

The optimum catalyst for the photocatalytic degradation of 2,4-D was Rh (1%)/TiO₂ by UV radiation. The Rh presents a strong metal-support interaction and improves the photocatalytic properties of TiO₂, modifying its band gap energy.

Recent advances in catalytic systems in the prism of physicochemical properties to remediate toxic CO pollutants: A state-of-the-art review

Carbon monoxide (CO) is the most harmful pollutant in the air, causing environmental issues and adversely affecting humans and the vegetation and then raises global warming indirectly. CO oxidation is one of the most effective methods of reducing CO by converting it into carbon dioxide (CO₂) using a suitable catalytic system, due to its simplicity and great value for pollution control. The CO oxidation reaction has been widely studied in various applications, including proton-exchange membrane fuel cell technology and catalytic converters. CO oxidation has also been of great academic interest over the last few decades as a model reaction. Many review studies have been produced on catalysts development for CO oxidation, emphasizing noble metal catalysts, the configuration of catalysts, process parameter influence, and the deactivation of catalysts. Nevertheless, there is still some gap in a state of the art knowledge devoted exclusively to synergistic interactions between catalytic activity and physicochemical properties. In an effort to fill this gap, this analysis updates and clarifies innovations for various latest developed catalytic CO oxidation systems with contemporary evaluation and the synergistic relationship between oxygen vacancies, strong metal-support interaction, particle size, metal dispersion, chemical composition acidity/basicity, reducibility, porosity, and surface area. This review study is useful for environmentalists, scientists, and experts working on mitigating the harmful effects of CO on both academic and commercial levels in the research and development sectors.

Confined Generation of Homogeneously Dispersed Au and SnO2 Nanoparticles in Layered Silicate as Synergistic Catalysts

With the aid of the confined conversion of layered silicate RUB-15, homogeneously dispersed Au and SnO₂ nanoparticles (NPs) were generated in the confined layer space of RUB-15. The Au-SnO₂/SiO₂ composite was obtained with the structure that ultrafine Au and SnO₂ NPs were supported on SiO₂ lamellas. Benefited by the Sn(II)-assisted in situ reduction strategy, Au NPs were highly uniformed and evenly distributed in/on the RUB-15. This Au-SnO₂/SiO₂ composite was employed as a catalyst to the reduction of 4-nitrophenol showing excellent catalytic activity. The catalytic rate constant at room temperature was calculated to be 6.64 min^-1, which was dramatically higher than that of Au/SiO₂ composite produced by reduction with hydrazine hydrate on the same support of layered silicate RUB-15. The interaction between Au and SnO₂ NPs increased the electron density around Au NPs, which was demonstrated to be an essential factor to the excellent catalytic activity of the Au-SnO₂/SiO₂ composite. The simple and universal synthesis method afforded precise control over the size/spatial arrangement of Au and SnO₂ NPs on SiO₂ lamellas. The high activity of the Au-SnO₂/SiO₂ composite demonstrated that the strategy used in this study has good potential application prospect. Furthermore, this work provided new perspective on the catalysis mechanism to the metal/semiconductor synergistic catalyst system.

Excellent visible light responsive photocatalytic behavior of N-doped TiO2 toward decontamination of organic pollutants

In this work, N-doped TiO₂ (N-TiO₂) with ample and tunable OVs was successfully synthesized, deriving from facile hydrothermal method and baked in the NH₃ atmosphere. N-doping boosts the amount of surface hydroxyl and superoxide (O₂^-) of TiO₂, demonstrated by XPS and nitroblue tetrazolium (NBT)-O₂^- quantitative reaction. Rich and tunable OVs were confirmed by low temperature electron spin resonance (ESR) results, demonstrating that doping of N into TiO₂ can definitely construct higher OVs than the reference TiO₂. Surface photovoltage spectrum (SPS) test, fluorescence experiments and electrochemical measurements all display that N-TiO₂ photocatalysts with OVs have a higher severance efficiency of photogenerated e-/h⁺ pairs than the pristine TiO₂. Photocatalytic evaluation results exhibit that N-TiO₂ photocatalysts demonstrate better performance than the reference TiO₂ toward decontamination of rhodamine B and tetracycline. TiO₂ treated in ammonia atmosphere for 1 h shows the highest photocatalytic property. The visible light responsive catalytic behavior of TiO₂ treated in ammonia atmosphere for 1 h is much higher than that of commercial TiO₂ (P25) and the pristine TiO₂, separately. The ameliorated visible light behavior of N-TiO₂ photocatalysts is attributable to rich oxygen vacancies produced through introducing N into TiO₂ and the boosted severance of photoactivated e-/h⁺.

Enhancement of sonocatalytic degradation of organic dye by using titanium dioxide (TiO2)/activated carbon (AC) derived from oil palm empty fruit bunch

A novel titanium dioxide/activated carbon (TiO₂/AC) composite where AC derived from oil palm empty fruit bunch (EFB) was synthesised by using sol-gel method. All the samples were characterised by using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray (SEM-EDX), Fourier transformed infrared (FTIR), thermogravimetric analysis (TGA) and surface analyser. SEM analysis showed that TiO₂ particles were successfully embedded and well distributed on the AC surface. The elemental composition analysis found that the TiO₂/AC composite contained titanium (Ti), oxygen (O) and carbon (C) atoms. Meanwhile, the appearance of new band at about 960 cm^-1 which assigned to the Ti-O stretching was observed in the FTIR spectra when the AC was incorporated into TiO₂. TGA analysis showed that the weight loss of 32 wt.% from 150 to 550 °C was due to the decomposition of amorphous carbon layers and loss of hydroxyl groups on TiO₂. It was found that the TiO₂/AC composite had better performance in the sonocatalytic degradation of malachite green as compared to the individual AC and TiO₂ because the TiO₂/AC composite had dual functionality and huge number of active sites which could promote the mass transfer of dye molecules towards catalyst surface. By using 1.5 g/L of TiO₂/AC composite which calcined at 700 °C on 100 mL of 200 mg/L of malachite green at solution pH of 7, a degradation efficiency of 87.11% had been achieved after 30 min of ultrasonic irradiation. A lower chemical oxygen demand (COD) removal (81.75%) was observed because the structured dye molecules underwent mineralisation process during the sonocatalytic degradation to generate intermediate compounds. The TiO₂/AC composite was able to be recycled and still achieved a high degradation efficiency of 76.78% after second catalytic cycle as compared to the fresh TiO₂/AC composite with degradation efficiency of 87.11%. In conclusion, the TiO₂/AC composite had high reusability and promising for practical applications in textile industry.

Gold catalysts containing interstitial carbon atoms boost hydrogenation activity

Supported gold nanoparticles are emerging catalysts for heterogeneous catalytic reactions, including selective hydrogenation. The traditionally used supports such as silica do not favor the heterolytic dissociation of hydrogen on the surface of gold, thus limiting its hydrogenation activity. Here we use gold catalyst particles partially embedded in the pore walls of mesoporous carbon with carbon atoms occupying interstitial sites in the gold lattice. This catalyst allows improved electron transfer from carbon to gold and, when used for the chemoselective hydrogenation of 3-nitrostyrene, gives a three times higher turn-over frequency (TOF) than that for the well-established Au/TiO2 system. The d electron gain of Au is linearly related to the activation entropy and TOF. The catalyst is stable, and can be recycled ten times with negligible loss of both reaction rate and overall conversion. This strategy paves the way for optimizing noble metal catalysts to give an enhanced hydrogenation catalytic performance.

Au-TiO2 Synthesized by a Microwave- and Sonochemistry-Assisted Sol-Gel Method: Characterization and Application as Photocatalyst

Titanium dioxide (TiO2) is a widely used and well studied photocatalyst synthesized using different methodologies, including sol-gel, which allows one to modify the material in a one-pot step. By using a microwave- and sonochemistry-assisted sol-gel method, x wt.% Au-TiO2 photocatalysts were successfully synthesized. Physicochemical characterization of the photocatalysts shows an average crystallite size of 10.5 nm and an even morphological distribution of spherical particles with the sonochemistry synthesis method. For the microwave method an average value of crystallite size of 8.3 nm was found and it presents an increase with the amount of Au load. The cyclic voltammetric response and Mott-Schottky analysis are consistent with a semiconductor material containing metallic particles and for a heterophase junction of anatase and brookite with oxygen vacancies, respectively. The photocatalytic activity was assessed by paracetamol degradation in an aqueous solution as model. The sonochemistry-synthesized photocatalysts display the most promising results as they have a better paracetamol removal and the amount of gold in the catalyst (0.7 wt.%) was found to be optimal for this process.

Role of the Support in Gold-Containing Nanoparticles as Heterogeneous Catalysts

In this review, we discuss selected examples from recent literature on the role of the support on directing the nanostructures of Au-based monometallic and bimetallic nanoparticles. The role of support is then discussed in relation to the catalytic properties of Au-based monometallic and bimetallic nanoparticles using different gas phase and liquid phase reactions. The reactions discussed include CO oxidation, aerobic oxidation of monohydric and polyhydric alcohols, selective hydrogenation of alkynes, hydrogenation of nitroaromatics, CO₂ hydrogenation, C–C coupling, and methane oxidation. Only studies where the role of support has been explicitly studied in detail have been selected for discussion. However, the role of support is also examined using examples of reactions involving unsupported metal nanoparticles (i.e., colloidal nanoparticles). It is clear that the support functionality can play a crucial role in tuning the catalytic activity that is observed and that advanced theory and characterization add greatly to our understanding of these fascinating catalysts.

Configurations of lead(II)-benzohydroxamic acid complexes in colloid and interface: A new perspective

Lead(II)-benzohydroxamic acid (Pb-BHA) complex collectors perform well with respect to scheelite flotation, and, due to their structure, they are widely used for industrial purposes. This paper examines the controversial issue of whether "O, O" five-membered ring or "N, O" four-membered ring complexes are formed when BHA coordinates with Pb ions, with their structure being comprehensively studied from the aspect of colloid and interface science. The configurations of Pb-BHA complexes are examined in a solution and on a mineral surface with experimental and computational methods. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) revealed that the five-membered ring is the dominant form of Pb-BHA complexes in a solution, whereas four-membered ring complexes are the stronger electron acceptor of the two. Moreover, XPS and time-of-flight secondary ion mass spectrometry (TOF-SIMS) confirmed that the four-membered ring complexes are stable with respect to being adsorbed on the scheelite surface. Therefore, although the four-membered ring is not as stable as the five-membered ring in a solution, it offers advantages with respect to adsorption on an electron-rich mineral surface during short-flotation processes.

Structures of Pb-BHA Complexes Adsorbed on Scheelite Surface

Previous studies have shown that Pb-BHA complexes (lead complexes of benzohydroxamic acid) have better collecting ability and can be used in flotation experiments with BHA acting as a collector and lead ions acting as activators. However, the structures of Pb-BHA complexes adsorbed on a mineral surface remain unclear. In this work, the adsorption behavior of Pb-BHA complexes on the scheelite surface was studied by flotation experiments and adsorption capacity measurements, and the structures of the adsorbed Pb-BHA complexes were determined using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The adsorption capacity results showed that more BHA was adsorbed on the scheelite surface in Pb-BHA flotation, and the XPS and TOF-SIMS analysis showed that the species of Pb-BHA complexes adsorbed on the scheelite surface were similar in activation flotation and Pb-BHA flotation. Therefore, the different contents of the complexes on the scheelite surface were responsible for the flotation behavior. XPS and TOF-SIMS showed that BHA combined with lead ions to form complexes with different structures, such as five- and four-membered ring structures. Structure fragment inference based on the measurements indicated that lead ions formed monomer complexes with two BHAs, and that lead hydroxide polymers with a certain degree of polymerization bonded with oxygen atoms in the complexes. The Pb-BHA complexes combine with oxygen atoms on the scheelite surface to form an adsorbate, rendering the surface hydrophobic.

Differences in the Catalytic Behavior of Au-Metalized TiO2 Systems During Phenol Photo-Degradation and CO Oxidation

For this present work, a series of Au-metallized TiO2 catalysts were synthesized and characterized in order to compare their performance in two different catalytic environments: the phenol degradation that occurs during the liquid phase and in the CO oxidation phase, which proceeds the gas phase. The obtained materials were analyzed by different techniques such as XRF, SBET, XRD, TEM, XPS, and UV-Vis DRS. Although the metallization was not totally efficient in all cases, the amount of noble metal loaded depended strongly on the deposition time. Furthermore, the differences in the amount of loaded gold were important factors influencing the physicochemical properties of the catalysts, and consequently, their performances in the studied reactors. The addition of gold represented a considerable increase in the phenol conversion when compared with that of the TiO2, despite the small amount of noble metal loaded. However, this was not the case in the CO oxidation reaction. Beyond the differences in the phase where the reaction occurred, the loss of catalytic activity during the CO oxidation reaction was directly related to the sintering of the gold nanoparticles.

A Solvothermal Synthesis of TiO₂ Nanoparticles in a Non-Polar Medium to Prepare Highly Stable Nanofluids with Improved Thermal Properties

Nanofluids are systems with several interesting heat transfer applications, but it can be a challenge to obtain highly stable suspensions. One way to overcome this challenge is to create the appropriate conditions to disperse the nanomaterial in the fluid. However, when the heat transfer fluid used is a non-polar organic oil, there are complications due to the low polarity of this solvent. Therefore, this study introduces a method to synthesize TiO₂ nanoparticles inside a non-polar fluid typically used in heat transfer applications. Nanoparticles produced were characterized for their structural and chemical properties using techniques such as X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The nanofluid showed a high stability, which was analyzed by means of UV-vis spectroscopy and by measuring its particle size and ζ potential. So, this nanofluid will have many possible applications. In this work, the use as heat transfer fluid was tested. In this sense, nanofluid also presented enhanced isobaric specific heat and thermal conductivity values with regard to the base fluid, which led to the heat transfer coefficient increasing by 14.4%. Thus, the nanofluid prepared could be a promising alternative to typical HTFs thanks to its improved thermal properties and high stability resulting from the synthesis procedure.

TiO2-nanosheet-assembled microspheres as Pd-catalyst support for highly-stable low-temperature CO oxidation

The Pd-embedded-in-TiO₂ structure could improve the activity and stability of the Pd/TiO₂ catalyst.