Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.

Hydrogen Sensors - A review

Lewis Acids: From Conventional Homogeneous to Green Homogeneous and Heterogeneous Catalysis

Micro-Raman and FTIR studies of synthetic and natural apatites

The present overview is intended to point the readers' attention to the important subject of calcium orthophosphates. This type of materials is of special significance for human beings, because they represent the inorganic part of major normal (bones, teeth and antlers) and pathological (i.e., those appearing due to various diseases) calcified tissues of mammals. For example, atherosclerosis results in blood vessel blockage caused by a solid composite of cholesterol with calcium orthophosphates, while dental caries and osteoporosis mean a partial decalcification of teeth and bones, respectively, that results in replacement of a less soluble and harder biological apatite by more soluble and softer calcium hydrogenphosphates. Therefore, the processes of both normal and pathological calcifications are just an in vivo crystallization of calcium orthophosphates. Similarly, dental caries and osteoporosis might be considered an in vivo dissolution of calcium orthophosphates. Thus, calcium orthophosphates hold a great significance for humankind, and in this paper, an overview on the current knowledge on this subject is provided.

https://www.tandfonline.com/doi/pdf/10.4161/biom.18790

Calcium orthophosphates: Occurrence, properties, biomineralization, pathological calcification and biomimetic applications

Bauxite residue (Red mud) is produced in alumina plants by the Bayer process in which Al-containing minerals are dissolved in hot NaOH The global residue inventory reached an estimated 35 billion tons in 2014, increasing by approximately 120 million tons per annum The appropriate management of bauxite residue is becoming a global environmental concern following increased awareness of the need for environmental protection Establishment of a vegetation cover is the most promising way forward for the management of bauxite residue, although its physical and chemical properties can limit plant growth due to high alkalinity and salinity, low hydraulic conductivity, trace element toxicity (Al and Fe), and deficiencies in organic matter and nutrition concentrations This paper discusses the various revegetation and rehabilitation strategies Studies of the rehabilitation of bauxite residues have mainly focused on two approaches, amelioration of the surface layer and screening of tolerant plants and soil microorganisms Amendment with gypsum can reduce the high alkalinity and salinity, promote soil aggregation, and increase the hydraulic conductivity of bauxite residues Organic matter can provide a source of plant nutrients, form stable complexes with metal cations, promote hydraulic conductivity, stabilize soil structure, and provide an energy source for soil organisms Tolerant plants and microorganisms such as halophytes and alkaliphilic microbes show the greatest potential to ameliorate bauxite residues However, during restoration or as a result of natural vegetation establishment, soil formation becomes a critical issue and an improved understanding of the various pedogenic processes are required, and future direction should focus on this area

A review of the characterization and revegetation of bauxite residues (Red mud)

A study on the structure and energetics of hydroxyapatite has been carried out using generalized gradient approximation density functional theory. The previously proposed $P{6}_{3}∕m$ and $P{6}_{3}$ hexagonal structural models have been found to be energetically unfavorable as compared with the previously proposed $P{2}_{1}∕b$ symmetry and a newly proposed monoclinic $P{2}_{1}$ structural model. A detailed analysis of the symmetry restrictions and inconsistencies between the reported physical properties, such as diffraction and birefringence, with hexagonal symmetry question the validity of these models. Rietveld analysis carried out on a synthetic sample, reported to be predominantly hexagonal at room temperature, shows that the diffraction pattern of this material can be interpreted as a mixed monoclinic phase, 23% monoclinic $P{2}_{1}∕b$ and 77% monoclinic $P{2}_{1}$. This interpretation reconciles many anomalies in reported experimental observations.

Structure and stability of hydroxyapatite: Density functional calculation and Rietveld analysis

Chemistry of bauxite residue neutralisation and aspects to implementation

Boron oxide catalysts supported on alumina have been prepared with boria loadings in the range 0–20 wt.-%. These materials were characterized by B.E.T. surface area measurements, X-ray diffraction and temperature-programmed desorption of ammonia and were used as catalysts for the Beckmann rearrangement of cyclohexanone oxime to caprolactam by passing the oxime vapour through a reactor containing the catalysts at 300°C. The surface areas of the samples decreased as the boria loading was increased, and all catalysts tested for the rearrangement reaction diminished in activity with time-onstream. There was a direct relationship between the amount of coke which formed on the surface and the loss in catalytic activity. This loss was least for the highest boron oxide loading tested, and this catalyst also featured the least amount of coke formation. Selectivity to caprolactam was associated with the presence of surface acidic sites of intermediate strength. A correlation was observed between the concentration of these surface sites and the selectivity to caprolactam.

Influence of boria loading on the acidity of B2O3/Al2O3 catalysts for the conversion of cyclohexanone oxime to caprolactam

Disorder of the OH− ion orientation is inherent in the currently accepted structure of hydroxyapatite (HA), hexagonal P63/m. In the case of local ordering in the OH− ion orientation, the resulting structure will be a monoclinic P21/b, with an alternate orientation of OH− columns in the b-direction. Both of these proposed structures are electrically non polar and fail to explain the polarisation phenomena observed in HA. Here we report some theoretical considerations of structural order in HA and show that in addition to the non polar P21/b phase HA can also exist as a polar phase. Various physical properties including static dielectric constant and polarisation are simulated from empirical interatomic potential calculations and used to explain the observed dielectric anomaly in sintered hydroxyapatite.

Structural order and dielectric behaviour of hydroxyapatite

The Beckmann rearrangement of cyclohexanone oxime to caprolactam has been studied over a range of aluminas modified by the addition of chloride, phosphate, sulphate, sodium or boron oxide. The catalysts were characterized by temperature-programmed desorption of ammonia and carbon dioxide and by pyridine adsorption monitored by infrared spectroscopy. Conversion of the oxime declined with time-on-stream for all catalysts studied, but selectivity to caprolactam remained constant during the first five hours of operation. In this period a relationship was observed between the selectivity to cap- rolactam and the surface concentration of intermediate strength acid sites, wherein the selectivity increased as the surface concentration of these sites increased. The boron oxide modified alumina presented the largest concentration of intermediate strength acid sites, and the low level of coke formation on this material was associated with the total absence of surface basicity. A mechanism for the rear- rangement reaction on the modified aluminas is presented.

J.B. McMonagle

Papers

Structure and stability of hydroxyapatite: Density functional calculation and Rietveld analysis

Chemistry of bauxite residue neutralisation and aspects to implementation

Influence of boria loading on the acidity of B2O3/Al2O3 catalysts for the conversion of cyclohexanone oxime to caprolactam

Structural order and dielectric behaviour of hydroxyapatite

Factors affecting selectivity in the rearrangement of cyclohexanone oxime to caprolactam over modified aluminas