Tetraalkylammonium Salts of Platinum Nitrato Complexes: Isolation, Structure, and Relevance to the Preparation of PtOx/CeO2 Catalysts for Low-Temperature CO Oxidation

Preparation of trans,trans-[Pt(py)2(N3)2(OH)2] via Photoinduced Reactivity of [Pt(NO3)6]2- Anion

The photoinduced reaction of [Pt(NO3)6]2- with pyridine or its derivatives (L) was found to result in the formation of [PtL4](NO3)2 salts in high yield. This transformation was successfully probed for methyl- and carboxyethyl-substituted pyridines, and the corresponding [PtL4](NO3)2 salts were isolated and fully characterized using single-crystal X-ray diffraction (SCXRD). Anation of the [Pt(py)4]2+ cationic complex with N3- was studied by 1H NMR spectroscopy in aqueous and water/dimethyl sulfoxide solutions of [Pt(py)4](NO3)2. A mixture of cis- and trans-[Pt(py)2(N3)2] complexes was determined as the final product of this interaction with the domination of the trans-isomer (cis to trans ratio is about 1:8) due to its preferable formation from the transient [Pt(py)3(N3)]+ cationic complex. The difference observed for the experimentally determined activation parameters of trans- and cis-paths of anation was supported by DFT calculations. Finally, the new three-stage Ag-free synthetic procedure for the preparation of the trans,trans-[Pt(py)2(N3)2(OH)2] prodrug (potential agent for the photodynamic anticancer therapy) was found using (i) light-induced formation of [Pt(py)4](NO3)2 from (Bu4N)2[Pt(NO3)6] followed by (ii) anation of [Pt(py)4]2+ with azide and (iii) accomplished by oxidation of the resulting mixture of cis- and trans-[Pt(py)2(N3)2] with H2O2. Efficient separation of cis,trans-[Pt(py)2(OH)2(N3)2] and trans,trans-[Pt(py)2(N3)2(OH)2] produced at the last stage was achieved by simple recrystallization from water.

(Me4N)2[Pt(CO3)2(OH)2]: The Isolated PtIVO6 Carbonato-Complex

The first fully inorganic Pt(IV) carbonato-complex trans-[Pt(CO3)2(OH)2]2- with a {PtO6} coordination sphere was isolated as the (Me4N)2[Pt(CO3)2(OH)2] (1) salt. The compound 1 was characterized using single-crystal and powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy (FTIR), electrospray ionization mass spectrometry (ESI-MS), nuclear magnetic resonance spectroscopy (NMR), and thermogravimetric analysis (TG). Density functional theory (DFT) calculations were also performed to analyze the spectral features of the complex. 1 crystallizes in the triclinic system (P-1) with a Z of 1. The trans-[Pt(CO3)2(OH)2]2- anion has axial hydroxo ligands and κ2-CO3 ligands, which form an equatorial plane. This anionic complex exhibits notable stability in aqueous solutions, while the axial hydroxo ligand can be readily modified, as exemplified by the acylation of the trans-[Pt(CO3)2(OH)2]2- into trans-[Pt(CO3)2(OAc)2]2- anion. Furthermore, it has been shown that rigid and glittering platinum coatings can be electrochemically deposited from an aqueous solution of 1 without the addition of surfactants.

Pd-Ce-Ox/MWCNTs and Pt-Ce-Ox/MWCNTs Composite Materials: Morphology, Microstructure, and Catalytic Properties

The composite nanomaterials based on noble metals, reducible oxides, and nanostructured carbon are considered to be perspective catalysts for many useful reactions. In the present work, multi-walled carbon nanotubes (MWCNTs) were used for the preparation of Pd-Ce-Ox/MWCNTs and Pt-Ce-Ox/MWCNTs catalysts comprising the active components (6 wt%Pd, 6 wt%Pt, 20 wt%CeO2) as highly dispersed nanoparticles, clusters, and single atoms. The application of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) provided analysis of the samples’ morphology and structure at the atomic level. For Pd-Ce-Ox/MWCNTs samples, the formation of PdO nanoparticles with an average crystallite size of ~8 nm was shown. Pt-Ce-Ox/MWCNTs catalysts comprised single Pt2+ ions and PtOx clusters less than 1 nm. A comparison of the catalytic properties of the samples showed higher activity of Pd-based catalysts in CO and CH4 oxidation reactions in a low-temperature range (T50 = 100 °C and T50 = 295 °C, respectively). However, oxidative pretreatment of the samples resulted in a remarkable enhancement of CO oxidation activity of Pt-Ce-Ox/MWCNTs catalyst at T < 20 °C (33% of CO conversion at T = 0 °C), while no changes were detected for the Pd-Ce-Ox/MWCNTs sample. The revealed catalytic effect was discussed in terms of the capability of the Pt-Ce-Ox/MWCNTs system to form unique PtOx clusters providing high catalytic activity in low-temperature CO oxidation.

Boosting the Activity of Single-Atom Pt1/CeO2 via Co Doping for Low-Temperature Catalytic Oxidation of CO

The properties of supports have a significant effect on the activity of noble metal single atoms. In this work, Co-doped CeO₂-supported single-atom Pt catalysts (Pt₁/Co-CeO₂) have been acquired by a synchronous pyrolysis/deposition route and demonstrated to promote low-temperature oxidation of CO. Revealed by a model reaction of 1% CO + 1% O₂ + 98% He at a space velocity of 12,000 mL/g_cat/h, CO conversion (100 °C) acquired on a (0.5% Pt)/(10% Co-CeO₂) catalyst (36.6%) was 3.6 and 4.9 times those of 0.5% Pt/CeO₂ (10.0%) and 10% Co-CeO₂ (7.4%) catalysts and 2.1 times that of their conversion sum (17.4%), confirming the positive role of the Co dopant in boosting the low-temperature oxidation of CO. The consistent results are also verified in the comparison of Pt₁/Co-ZnO with Pt₁/ZnO and Pt₁/Co-Al₂O₃ with Pt₁/Al₂O₃. In addition, the activity of single-atom Pt₁/Co-CeO₂ catalysts can be facilely modified by changing the loading of Pt and/or doping amount of Co. These reasonable data will provide a methodology to access more applicable catalysts for CO oxidation at low temperature.

Sulfuric Acid Solutions of [Pt(OH)4(H2O)2]: A Platinum Speciation Survey and Hydrated Pt(IV) Oxide Formation for Practical Use

The systematic study of the platinum speciation in sulfuric acid solutions of platinum (IV) hydroxide {[Pt(OH)₄(H₂O)₂], HHPA} was performed with the use of a combination of methods. Depending on the prevailing Pt form, the three regions of H₂SO₄ concentration were marked: (1) up to 3 M H₂SO₄ forms unstable solutions gradually generating the PtO₂·xH₂O particles; (2) 4-12 M H₂SO₄, where the series of mononuclear aqua-sulfato complexes ([Pt(SO₄)_n(H₂O)_6-n]^4-2n, where n = 0···4) dominate; and (3) 12 M and above, where, along with [Pt(SO₄)_n(H₂O)_6-n]^4-2n species, the polynuclear Pt(IV) species and complexes with a bidentate coordination mode of the sulfato ligand are formed. For the first time, the salts of the aqua-hydroxo Pt(IV) cation [Pt(OH)₂(H₂O)₄]SO₄ (triclinic and monoclinic phases) were isolated and studied with a combination of methods, including the single-crystal X-ray diffraction. The formation of PtO₂·xH₂O particles in sulfuric acid solutions (1-3 M) of HHPA and their spectral characteristics and morphology were studied. The deposition of PtO₂·xH₂O was highlighted as a convenient method to prepare various Pt-containing heterogeneous catalysts. This possibility was illustrated by the preparation of Pt/g-C₃N₄ catalysts, which show an excellent performance in catalytic H₂ generation under visible light irradiation with a quantum efficiency up to 5% and a rate of H₂ evolution up to 6.2 mol·h^-1 per gram of loaded platinum.

Effects of Subsurface Oxide on Cu1/CeO2 Single-Atom Catalysts for CO Oxidation: A Theoretical Investigation

Supported atomic dispersion metals are of great interest, and the interfacial effect between isolated metal atoms and supports is crucial in heterogeneous catalysis. Herein, the behavior of single-atom Cu catalysts dispersed on CeO₂ (100), (110), and (111) surfaces has been studied by DFT + U calculations. The interactions between ceria crystal planes and isolated Cu atoms together with their corresponding catalytic activities for CO oxidation are investigated. The CeO₂ (100) and (111) surfaces can stabilize active Cu⁺ species, while Cu exists as Cu²⁺ on the (110) surface. Cu⁺ is certified as the most active site for CO adsorption, which can promote the formation of the reaction intermediates and reduce reaction energy barriers. For the CeO₂ (100) surface, the interaction between CO and Cu is weak and the CO adsorbate is more likely to activate the subsurface oxygen. The catalytic performance is closely related to the binding strength of CO to the active Cu single atoms on the different subsurfaces. These results bring a significant insight into the rational design of single metal atoms on ceria and other reducible oxides.

Effect of TiO2 Calcination Pretreatment on the Performance of Pt/TiO2 Catalyst for CO Oxidation

In order to improve the CO catalytic oxidation performance of a Pt/TiO₂ catalyst, a series of Pt/TiO₂ catalysts were prepared via an impregnation method in this study, and various characterization methods were used to explore the effect of TiO₂ calcination pretreatment on the CO catalytic oxidation performance of the catalysts. The results revealed that Pt/TiO₂ (700 °C) prepared by TiO₂ after calcination pretreatment at 700 °C exhibits a superior CO oxidation activity at low temperatures. After calcination pretreatment, the catalyst exhibited a suitable specific surface area and pore structure, which is beneficial to the diffusion of reactants and reaction products. At the same time, the proportion of adsorbed oxygen on the catalyst surface was increased, which promoted the oxidation of CO. After calcination pretreatment, the adsorption capacity of the catalyst for CO and CO₂ decreased, which was beneficial for the simultaneous inhibition of the CO self-poisoning of Pt sites. In addition, the Pt species exhibited a higher degree of dispersion and a smaller particle size, thereby increasing the CO oxidation activity of the Pt/TiO₂ (700 °C) catalyst.

[NiEn3](MoO4)0.5(WO4)0.5 Co-Crystals as Single-Source Precursors for Ternary Refractory Ni-Mo-W Alloys

The co-crystallisation of [NiEn₃](NO₃)₂ (En = ethylenediamine) with Na₂MoO₄ and Na₂WO₄ from a water solution results in the formation of [NiEn₃](MoO₄)_0.5(WO₄)_0.5 co-crystals. According to the X-ray diffraction analysis of eight single crystals, the parameters of the hexagonal unit cell (space group P–31c, Z = 2) vary in the following intervals: a = 9.2332(3)–9.2566(6); c = 9.9512(12)–9.9753(7) Å with the Mo/W ratio changing from 0.513(3)/0.487(3) to 0.078(4)/0.895(9). The thermal decomposition of [NiEn₃](MoO₄)_0.5(WO₄)_0.5 individual crystals obtained by co-crystallisation was performed in He and H₂ atmospheres. The ex situ X-ray study of thermal decomposition products shows the formation of nanocrystalline refractory alloys and carbide composites containing ternary Ni–Mo–W phases. The formation of carbon–nitride phases at certain stages of heating up to 1000 °C were shown.

Speciation and separation of platinum(iv) polynuclear complexes in concentrated nitric acid solutions

Understanding the solution chemistry of Pt(iv) is crucial for the hydrometallurgy of precious metals. To gain such an understanding, the speciation and separation of Pt(iv) complexes in concentrated HNO₃ solutions were investigated via Pt L_III edge X-ray absorption fine structure (XAFS) spectroscopy. The XAFS results for concentrated HNO₃ solutions of Na₂Pt(OH)₆ revealed the dominant presence of Pt polynuclear complexes, wherein the formation of Pt(iv) polynuclear complexes depended on the metal concentration and the Na₂Pt(OH)₆ dissolution temperature. The dominant species present in a heated nitrate solution of 0.90 g-Pt L^-1 and a non-heated nitrate solution of 3.2 g-Pt L^-1 were dinuclear Pt(iv) complexes, whereas those in a heated solution of 3.0 g-Pt L^-1 were predominantly larger polynuclear complexes, such as, tetra- and hexa-nuclear complexes. The presence of larger Pt(iv) complexes was confirmed via XAFS spectroscopy, wherein the adsorption of Pt(iv) ions from a 10 M HNO₃ solution by a chelating resin functionalised with iminodiacetic acid and a strongly basic anion-exchange resin bearing trimethyl ammonium nitrate was examined. The adsorption of 50 mg L^-1 of Pt(iv) by the two resins was tested using aqueous solutions diluted from heated HNO₃ solutions with varying metal concentrations, and also from a non-heated solution. We found that Pt(iv) complexes from heating solutions containing high Pt(iv) concentrations displayed high adsorption percentages. In addition, the selective adsorption of Pt(iv) over Pd(ii), Ag(i), Cu(ii), Ni(ii), and Fe(iii) from a 10 M HNO₃ solution was achieved using a strongly basic anion-exchange resin.

Effect of CeO2 Presence on the Electronic Structure and the Activity for Ethanol Oxidation of Carbon Supported Pt

Pt/CeO2/C electrocatalysts in different compositions were prepared and their structural characteristics and activities for ethanol oxidation in alkaline media were evaluated. In the presence of CeO2, an increase in the platinum particle size was observed. XANES measurements indicated that the Pt d-band vacancies increased with increasing CeO2 amounts. For the first time, the decrease in electro activity was described to an electronic effect for high CeO2 contents. The dependence of the activity for ethanol oxidation on CeO2 content went to a maximum, due to the counteracting bifunctional and electronic effects of the metal oxide.

Photoinduced Deposition of Platinum from (Bu4N)2[Pt(NO3)6] for a Low Pt-Loading Pt/TiO2 Hydrogen Photogeneration Catalyst

An efficient method for the deposition of ionic platinum species PtO_x onto a TiO₂ surface was developed on the basis of light-induced activation of the [Pt(NO₃)₆]^2- anion. The deposited PtO_x species with an effective Pt oxidation state between +4 and +2 have an oxygen-made environment and include single ion centers {PtO_n} and polyatomic ensembles {Pt_nO_m} connected to a TiO₂ surface with Pt-O-Ti bonds. The resulting PtO_x/TiO₂ materials were tested as photocatalysts for the hydrogen evolution reaction (HER) from a water ethanol mixture and have shown uniquely high activity with the rate of H₂ evolution achieving 11 mol h^-1 per gram of Pt, which is the highest result for such materials reported to date. A combination of spectral methods shows that, under HER conditions, reduction of the supported PtO_x species leads to the formation of well-dispersed nanoparticles of metallic platinum attached on the surface of TiO₂ by Ti-O-Pt bonds. The high activity of the PtO_x/TiO₂ materials is believed to result from a combination of uniform distribution of small platinum nanoparticles over the titania surface and their close interaction with TiO₂.