Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at Θ_O=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f_7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low Θ_O, and the peak shifts to higher temperatures for increasing oxygen coverages up to Θ_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at Θ_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ desorption of 30 kcal mol⁻¹ near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol⁻¹.     

Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O

The reaction of CO₂ and H₂O to form bicarbonate (HCO⁻₃) was examined on the nearly perfect and vacuum annealed surfaces of TiO₂(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO₂(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO₂ only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti⁴⁺ sites (TPD peak at 137 K) typical of the nearly perfect TiO₂(110) surface. Vibrational HREELS indicates that CO₂ is linearly bound at the latter sites with a ν_a(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H₂O to bridging OH groups which recombine to liberate H₂O in TPD at 490 K. No evidence for a reaction between CO₂ and H₂O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO₂ adsorption is blocked by the presence of preadsorbed H₂O, adsorbed CO₂ is displaced by postdosed H₂O, and there is little or no evidence for bicarbonate formation in either case. However, when CO₂ and H₂O are simultaneously dosed, a new CO₂ TPD state is observed at 213 K, and the 166 K state associated with CO₂ at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO₂ TPD state to a bicarbonate species. The 213 K CO₂ TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO₂+H₂O exposure. Sticking coefficient measurements suggest that CO₂ adsorption at 110 K is precursor-mediated, as is known to be the case for H₂O adsorption on TiO₂(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H₂O–CO₂ van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO₂ and H₂O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO₂, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution.     

The reaction of H2S with surface oxygen on Ni(110)

The reactions of H₂S with predosed surface oxygen on Ni(110) surfaces were studied for a variety of coverage conditions. The primary reaction product is H₂O, but the details of the water formation and desorption depends on the coverage of both O and H₂S.

For high coverages of oxygen (p(2 × 1)−O; 0.5 ML), the reaction to form water is quantitative. The loss of oxygen from the surface (as measured by AES) is equal to the increase in sulfur coverage. XPS and HREELS measurements indicate the presence of chemisorbed H₂O immediately following large exposures of H₂S on the oxygen predosed surface at 110 K. Deuterium incorporation results suggest that the primary mechanism for these coverage conditions involves direct transfer of hydrogen from SH or H₂S moieties to the oxygen.

A second mechanism involving reaction of surface hydroxyl groups with surface hydrogen was also identified. This mechanism is particularly important for high coverages of oxygen (0.5 ML) and low coverages of H₂S (0.15 ML), where water desorption was observed at 235 K, but was not observed spectroscopically at 110 K. The sequential addition of two surface hydrogen atoms to surface oxygen is not an important mechanism in this system.

These reactions were modeled using a bond-order conservation method, and the model successfully reproduced the important mechanistic conclusions.     

The decomposition of H2S on Ni(110)

Adsorbed H₂S decomposes on Ni(110) to form primarily surface S and H for coverages of less than 0.5 ML. The hydrogen evolves in two separate TPD peaks, characteristic of hydrogen recombination and desorption from the clean surface and from regions perturbed by chemisorbed sulfur. XPS and HREELS indicate the presence of SH and possibly H₂S groups on the surface at 110 K. The XPS data indicates that for coverages less than about 0.5 ML, the concentration of molecular H₂S is small, but it is difficult to asess the coverage of SH groups. However, all of the molecular species decompose prior to hydrogen desorption (for high coverage, 180 K). Physisorbed H₂S is observed on the surface for coverages greater than about 0.5 ML.

The sulfur Auger lineshape was observed to be a function of both coverage and temperature. The changes in the lineshape were attributed to perturbations in local bonding interactions between the S and the Ni surface, perhaps involving some change in either bonding sites or distances but not involving SH bond scission.

The decomposition reaction was modeled using a bond order conservation method which successfully reproduced the experimental results.     

Glycine on Pt(111): a TDS and XPS study

The adsorption and desorption of in situ deposited glycine on Pt(111) were investigated with thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). Glycine adsorbs intact on Pt(111) at all coverages at temperatures below 250 K. The collected results suggest that the glycine molecules adsorb predominantly in the zwitterionic state both in the first monolayer and in multilayers. Upon heating, intact molecules start to desorb from multilayers around 325 K. The second (and possibly third) layer(s) are somewhat more strongly bound than the subsequent layers. The multilayer desorption follows zero order kinetics with an activation energy of 0.87 eV molecule⁻¹. From the first saturated monolayer approximately half of the molecules desorbs intact with a desorption peak at 360 K, while the other half dissociates before desorption. Below 0.25 monolayer all molecules dissociate upon heating. The dissociation reactions lead to H₂, CO₂, and H₂O desorption around 375 K and CO desorption around 450 K. This is well below the reported gas phase decomposition temperature of glycine, but well above the thermal desorption temperatures of the individual H₂, CO₂, and H₂O species on Pt(111), i.e. the dissociation is catalyzed by the surface and H₂, CO₂, and H₂O immediately desorb upon dissociation. For temperatures above 500 K the remaining residues of the dissociated molecules undergo a series of reactions leading to desorption of, for example, H₂CN, N₂ and C₂N₂, leaving only carbon left on the surface at 900 K. Comparison with previously reported studies of this system show substantial agreement but also distinct differences.     

Study of ion desorption induced by a resonant core-level excitation of condensed H2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H₂O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a₁ ← O 1s resonant transition. The AES at the 4a₁ ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H₂O, and the normal-AES of the bulk H₂O, where the 4a₁ electron is delocalized before Auger transitions. H⁺ is found to be the only ion species in AEPICO spectra measured at the 4a₁ ← O 1s resonance and at the O 1s ionization (hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a₁ ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a₁ ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H₂O, which is extracted from the AES at the 4a₁ ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a₁ ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H⁺ desorption at the 4a₁ ← O 1s resonance. The enhancement of the H⁺ yield is ascribed to the O---H anti-bonding character of the 4a₁ orbital.     

Adsorption and decomposition of H2S on the Ge(100) surface

The adsorption and decomposition of H₂S on the Ge(100) surface is investigated. H₂S is a simple sulfur containing molecule that eventually decomposes to yield hydrogen gas and deposits sulfur on the germanium surface. The surface reactions of H₂S are investigated by ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, and temperature programmed desorption. Room temperature exposure of H₂S to Ge(100) results in dissociative adsorption which can be followed easily by ultraviolet photoelectron spectroscopy. Warming the H₂S exposed surface results in some molecular desorption and further decomposition of the adsorbed species. At saturation, 0.25 ML of H₂S decomposes generating 0.5 ML of atomic hydrogen. Above the hydrogen desorption temperature some etching of the germanium surface is observed by sulfur. The etch product, GeS, is subsequently observed in temperature programmed desorption experiments. Exposure of H₂S to the Ge surface at elevated temperatures leads to higher sulfur coverages. A sulfur coverage approaching 0.5 ML can be deposited at the higher exposure temperatures.     

Li修饰的C$lt;sub$gt;6$lt;/sub$gt;分子对H$lt;sub$gt;2$lt;/sub$gt;O的吸附研究

采用密度泛函理论中的广义梯度近似研究C₆Li吸附H₂O分子并将之进行分解的催化过程. 几何优化发现：Li原子最稳定的吸附位置是位于C 原子顶位上方. 研究表明,第一个H₂O 分子吸附在C₆Li上需要克服1.77 eV的能量势垒,然后分解为H和OH且与Li原子成键. 当吸附第二个H₂O分子时,第二个H₂O分子需要克服1.2 eV的能量势垒分解为H和OH,其中H与Li原子上的H原子结合成H₂,OH则替代Li 原子上的H结合在Li原子上. 因此C₆Li 可以作为催化剂将H₂O分子进行分解得到H₂. 分析可知：C₆Li主要是通过Li原子与H₂O之间形成的偶极矩作用来吸附H₂O 分子,与C₆₀Li₁₂ 的储氢机制类似. 研究结果可为储氢材料的制备提供一个新的思路. 关键词： 6')" href="#">C₆ Li 2O')" href="#">H₂O 密度泛函理论     

TPD, HREELS and UPS study of the adsorption and reaction of methyl nitrite (CH3ONO) on Pt(111)

The adsorption and reaction of methyl nitrite (CH₃ONO, CD₃ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH₃ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H₂ at low coverages and methanol for CH₃ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K.     

Adsorption and reaction of oxygen and deuterium on a Pd(110) surface, studied by Δφ and TDS

The co-adsorption of oxygen and deuterium at 100 K on a Pd(110) surface has been studied by measurements of the change in work function (Δφ) and by thermal desorption spectroscopy (TDS). When the surface with co-adsorbed species is heated, the adsorbates O and D react to form D₂O which desorbs from the surface at T > 200 K. The D₂O desorption peaks shift continuously to lower temperatures as the surface D coverage (θ_D) increases. The maximum production of D₂O is estimated to be 0.26 ML (1 ML = 9.5 × 10¹⁴ atoms cm⁻²), resulting from reaction in a layer containing 0.65 ML D and 0.3 ML O. The maximum work function increase caused by adsorption of D to saturation onto oxygen precovered Pd(110) decreases almost linearly with Δφ_O of the oxygen precovered surface. On a surface with pre-adsorbed D however, the maximum Δφ increase contributed by oxygen adsorption decreases abruptly at Δφ_D > 200 mV. This sharp change occurs at θ_D > 1 ML and is believed to be associated with the development of the reconstructed (1 × 2) phase of D/Pd(110).     

Effects of potassium on the adsorption and dissociation of CH3Cl on Pd(100)

The effects of potassium on the adsorption and dissociation of CH₃Cl on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (in the electronic range EELS), temperature-programmed desorption (TPD) and work function change. In contrast to the clean surface, the adsorption of CH₃Cl caused a significant work function increase, 0.9-1.4 eV, of potassium-dosed Pd. Preadsorbed K enhanced the binding energy of CH₃C1 to the surface and induced the dissociation of adsorbed molecules. The extent of the dissociation increased almost linearly with the potassium content. The appearance of a new emission in the UPS spectrum at 9.2 eV, attributed to adsorbed CH₃ species, and the low-temperature formation of ethane suggest that a fraction of adsorbed CH₃Cl dissociates even at 115–125 K on potassium-dosed Pd(100). At the same time, a significant part of adsorbed CH₃ radical is stabilized, the reaction of which occurs only at 250–300 K. By means of TPD measurements, H₂, CH₄, C₂H₆, C₂H₄, KCl and K were detected in the desorbing gases. The results are interpreted by assuming a through-metal electronic interaction at low potassium coverage and by a direct interaction of the Cl in the adsorbed CH₃Cl with potassium at high potassium coverage. The latter proposal is supported by the electron excited Auger fine structure of the Cl signal and by the formation of KCl in the desorbing gases.     

The interaction of D2O with Zr(0001) at 80 K

The adsorption of D₂O on Zr(0001) at 80 K and its subsequent reactions at higher temperatures have been studied by thermal desorption spectroscopy (TDS), work-function measurements (Δф), nuclear reaction analysis (NRA), LEED, infrared reflection spectroscopy (FTIR-RAS), Auger electron spectroscopy (AES), and static secondary ion mass spectroscopy (SSIMS). D₂O adsorption on Zr(0001) at 80 K is accompanied by a Δф of −1.33 eV. The adsorbed D₂O can be characterized into three layers by TDS: a chemisorbed layer (up to 0.23 ML), a second adsorbed layer, and an ice layer. The chemisorbed D₂O dissociates into OD_ad and D_ad at 80 K (possibly also into O_ad) and no desorption products could be detected, implying that the reaction products dissolved into the zirconium at temperatures appropriate for each component. The ice layer and most of the second adsorbed layer desorb as molecular water during heating. The water adsorbed at 80 K did not form any long-range ordered structure, but a (2 × 2) LEED pattern that was formed by heating the sample to temperatures above 430 K is believed due to be an ordered oxygen superstructure.     

一个灵活的水结构：来自拉曼光谱的证据

有关水结构的认识仍有较大争议,焦点主要在于氢键结构类型的划分及各类型比重的确定。为进一步探讨水的结构,分析了不同温度、H↔D同位素取代和氯离子浓度条件下H₂O/D₂O的拉曼光谱特征。随温度由253 K逐渐升至753 K,纯水的OH伸缩振动带带宽显著下降,主峰明显蓝移,肩峰对主峰的相对强度不断变化,表明水中存在多种氢键结构。结合高斯去卷积方法,将这些光谱特征归因于水中并存的五种主要的氢键结构：两种四面体,单供体、单氢键和自由水。四面体氢键构型是水的弯曲振动、平动+弯曲和频振动及分子间振动耦合产生的结构基础。根据四面体度数据,水结构具有较强的灵活性,即水中相当比重的氢键结构以非四面体形态存在。升温导致水中的四面体度显著下降,促进四面体构型向单供体、单氢键和自由水转变。同位素取代降低OH/OD伸缩带低频肩峰对主峰的相对强度,却增加高频肩峰对主峰的相对强度,尤其在513 K以上的高温下,V_H2O/V_D2O=1/4或4/1水的OH或OD伸缩带上的高频肩峰将转变为主峰。这些特征有力的支持了水的多结构体模式：同位素取代导致水中的氢键结构形态发生转变,由于O—D…O↔O—H…O氢键的转换,原始O—D…O（O—H…O）四面体或非四面体氢键比率下降。同位素取代强化自由水模式甚至在高温下促其转变为主峰,也进一步证明了水中氢键结构随温度升高而转化的事实。多结构下的多峰模式可有效解释H₂O/D₂O—NaCl溶液在不同温度下的OD/OH伸缩带特征。NaCl加入,433 K以下显著减少水中的四面体氢键构型,将之转变为单供体和单氢键水;而513 K以上轻微地促进水结构。基于宽广条件范围内的拉曼光谱特征给出的水结构细节的划分方案为含水地质流体的拉曼光谱学和结构性质探讨提供了理论依据。     

The NO + CO reaction on Pt(100)

The interaction of NO with CO and with H₂ on Pt(100) was studied by temperature programmed desorption (TPD), isothermal desorption mass spectrometry, and low energy electron diffraction (LEED), TPD of NO and CO coadsorbed at 120 K yields almost complete reaction with both N₂ and CO₂ products desorbing as sharp, simultaneous peaks at ≈ 410 K. with full widths at half maximum as narrow as 3 K. Isothermal desorption mass spectrometry yields N₂ and CO₂ rates that exhibit a maximum with time. Both experiments indicate that the reaction mechanism is autocatalytic. Annealing NO-CO adlayers formed at 120 K to temperatures above 300 K causes the subsequent N₂ and CO₂ TPD peaks to broaden.'TPD of NO coadsorbed with H₂ yields sharp N₂ and H₂O product peaks that closely resemble the N₂ and CO₂ peaks observed in the NO + CO reaction. LEED experiments during TPD and isothermal desorption showed that the (1 × 1) → hex substrate phase transformation sometimes accompanies desorption of N₂ and CO₂. The TPD and isothermal desorption results can be fit by two simple models: chemical autocatalysis, in which an intermediate chemical species participates in a “chain propagation” reaction, and structural autocatalysis, which involves the formation of a reactive intermediate structure involving Pt atom displacements.     

Sodium adsorption on a Ge(100)-(2 × 1) surface

The adsorption of Na on a Ge(100)-(2 × 1) surface has been studied by means of AES, LEED, EELS, TPD and work-function measurements. In the submonolayer coverage region the coverage dependencies of the desorption activation energy E(Θ) and desorption frequency v(Θ) have been determined using the threshold TPD method. Our experimental data show that after the completion of the first Na layer, 3D crystallites develop on the Na/Ge(100) surface (Stranski-Krastanov growth mode). For Θ > 1 ML, formation, followed by decomposition of a certain Na---Ge surface compound occurs in the temperature range 410–550 K.     

Vibrational study of silicon oxidation: H2O on Si(100)

The vibrational spectrum of water dissociatively adsorbed on Si(100) surfaces is obtained with surface infrared absorption spectroscopy. Low frequency spectra (< 1450 cm⁻¹ are acquired using a buried CoSi₂ layer as an internal mirror to perform external reflection spectroscopy. On clean Si(100), water dissociates into H and OH surface species as evidenced by EELS results [1] in the literature which show a Si---H stretching vibration (2082 cm⁻¹), and SiO---H vibrations (O---H stretch at 3660 cm⁻¹ and the Si---O---H bend and Si---O stretch of the hydroxyl group centered around 820 cm⁻¹). In this paper, infrared (IR) measurements are presented which confirm and resolve the issue of a puzzling isotopic shift for the Si---O mode of the surface hydroxyl group, namely, that the Si---O stretch of the O---H surface species formed upon H₂O exposure occurs at 825 cm⁻¹, while the Si---O stretch of the ---OD surface species formed upon D₂O exposure shifts to 840 cm⁻¹, contrary to what is expected for simple reduced mass arguments. The higher resolution of IR measurements versus typical EELS measurements makes it possible to identify a new mode at 898 cm⁻¹, which is an important piece of evidence in understanding the anomalous frequency shift. By comparing the results of measurements for adsorption of H¹⁶₂O, H¹⁸₂O and D₂O with the results from recently performed first-principles calculations, it can be shown that a strong vibrational interaction between the Si---O stretching and Si---O---H bending functional group vibrations of the hydroxyl group accounts for the observed isotopic shifts.     

基于光谱分析技术的宣纸用铝盐施胶沉淀剂作用机理研究

为了明确铝盐沉淀剂在书画宣纸表面施胶过程中的作用机理,利用Ferron逐时络合分光光谱、高场27Al 核磁共振波谱(27Al-NMR)以及衰减全反射红外光谱技术(ATR-FTIR)研究了明矾(Alum)、聚合氯化铝(PAC)及聚合硫酸铝(PAS)三种常用铝盐施胶沉淀剂的水解聚合铝形态、与胶料混合后在宣纸表面施胶时的铝形态分布变化。(1) Ferron逐时络合分光光谱和27Al-NMR分析表明,明矾及聚合硫酸铝的水解产物主要为单核铝Al(H₂O)3+₆(Al₁),AlSO+₄和多核铝[Al₃₀O₈(OH)₅₆(H₂O)₂₄]18+(Al₃₀);聚合氯化铝除Al₁,Al₃₀外,还存在笼式结构的多核铝[AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]7+(Al₁₃);(2)27Al-NMR分析表明,铝盐与明胶混合后单核铝、多核铝形态的共振峰强度均有所降低,结合ATR-FTIR分析结果可知,降低的各正价态水解聚合产物很可能与明胶微粒中羟基(-OH)或羧基(-COOH)产生了键合,形成了网状络合物,将原本带负电的明胶粒子转化为带正电的明胶粒子,促使明胶微粒沉淀在带负电的纤维表面,起到施胶沉淀剂的作用。施胶后,明胶胶原蛋白的羟基、一部分氨基和羧基与植物纤维表面的非离子区域的羧基能形成众多的分子间的氢键,提高宣纸抗水性。因此,Ferron逐时络合分光光度法、高场27Al-NMR及ATR-FTIR技术相结合可迅速判断各类铝盐沉淀剂在宣纸表面施胶过程中的化学形态变化, 是研究施胶机理的有效手段。     

Chemisorption and reactivity of furan on Pd{111}

XPS, HREELS, ARUPS and Δø data show that furan chemisorbs non-dissociatively on Pd{111} at 175 K, the molecular plane being significantly tilted with respect to the surface normal. Bonding involves both the oxygen lone pair and significant π interaction with the substrate. The degree of decomposition that accompanies molecular desorption is a strong function of coverage: 40% of the adsorbate desorbs molecularly from the saturated monolayer. Decomposition occurs via decarbonylation to yield CO_a and H_a followed by desorption rate limited loss of H₂ and CO. It seems probable that an adsorbed C₃H₃ species formed during this process undergoes subsequent stepwise dehydrogenation ultimately yielding H₂ and C_a.     

Structural modifications of dodecatungstophosphoric acid hexahydrate induced by temperature in the 10–358 K range. In situ high-resolution neutron powder diffraction investigation

Dodecatungstophosphoric acid hexahydrate H₃PW₁₂O₄₀·6H₂O crystal structure has been investigated by neutron powder diffraction (NPD) at different temperatures in the 10–358 K range. A nonconvergent reversible phase transition has been noticed at about 320 K. This transition is associated with a change in dynamic equilibrium of hydrate species and partial reduction/oxidation (redox) W⁶⁺↔W⁵⁺. Expressive structure changes lie in the P---O bonding inside Keggin's anion and the H₅O₂⁺ conformational angle.