Hydrogen activation by magnesia catalysts

Journal Article · Wed Jul 01 00:00:00 EDT 1992 · Journal of Catalysis; (United States)

OSTI ID:7142140

Gu, Yulin; Brenner, A ^[1]

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1. Wayne State Univ., Detroit, MI (United States)

Hydrogen activation by magnesia catalysts was investigated by studying H{sub 2}-D{sub 2} exchange. The number of active sites, surface composition, rate, and mechanism of the reaction were investigated as a function of the activation temperature. Behavior of a commercial magnesia and a catalyst synthesized from Mg(OH){sub 2} were similar, although the latter catalyst had a much larger surface area. The upper limit for the number of active sites was determined in the classical manner by using selective poisoning by CO. A novel technique involving monitoring the stoichiometric reaction between magnesia and D{sub 2}(g) enabled the very unusual determination of the lower limit for the number of active sites. Thus, the true number of active sites is bracketed within the upper and lower limits. Maximal activity occurred after activation at 700 C. The number of active sites is about 10{sup 12}/cm{sup 2}, which is 10{sup 3}-fold higher than formerly reported on the basis of EPR data. The turnover frequency at 273 K and a partial pressure of 20 Torr of an equimolar mixture of H{sub 2}-D{sub 2} is 4 s{sup {minus}1}, roughly 10{sup 3}-fold less than previously reported. The site density and activity are now consistent with expected values, rather than the anomalous values previously reported. The hydroxyl coverage of the surface was determined in a novel manner using thermogravimetric analysis over the temperature range of 300 to 1,400 K. The catalysts are of low activity when the surface is either of very high or very low hydroxyl content. A mechanism in which the active site includes an ensemble consisting of a Mg{sup 2+} center and neighboring surface OH and O{sup 2{minus}} is proposed.

• OSTI ID: 7142140
• Journal Information: Journal of Catalysis; (United States), Vol. 136:1; ISSN 0021-9517
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
HYDROGEN
CHEMICAL ACTIVATION
MAGNESIUM OXIDES
CATALYTIC EFFECTS
CARBON MONOXIDE
CATALYSIS
CHEMICAL PREPARATION
CHEMICAL REACTION KINETICS
DEUTERIUM
ELECTRON SPIN RESONANCE
HYDROXYL RADICALS
ISOTOPIC EXCHANGE
MATHEMATICAL MODELS
POISONING
SURFACE AREA
SURFACE PROPERTIES
TEMPERATURE DEPENDENCE
THERMAL GRAVIMETRIC ANALYSIS
ALKALINE EARTH METAL COMPOUNDS
CARBON COMPOUNDS
CARBON OXIDES
CHALCOGENIDES
CHEMICAL ANALYSIS
ELEMENTS
GRAVIMETRIC ANALYSIS
HYDROGEN ISOTOPES
ISOTOPES
KINETICS
LIGHT NUCLEI
MAGNESIUM COMPOUNDS
MAGNETIC RESONANCE
NONMETALS
NUCLEI
ODD-ODD NUCLEI
OXIDES
OXYGEN COMPOUNDS
QUANTITATIVE CHEMICAL ANALYSIS
RADICALS
REACTION KINETICS
RESONANCE
STABLE ISOTOPES
SYNTHESIS
THERMAL ANALYSIS
400201* - Chemical & Physicochemical Properties