NOVELTY - A method for separating hydrogen from a hydrogen-containing fluid, involves: separating hydrogen-containing fluid stream at 573 to about 1173 degrees K into a permeate stream comprising molecular hydrogen and retenate stream depleted of molecular hydrogen compared to hydrogen-containing fluid stream by permeating hydrogen through palladium-alloyed membrane having a nitrogen leakage growth rate at about 823 degrees K of not greater than 7x 10-12 (mol/m2/second/Pa)/hour, and where the palladium-alloyed membrane comprises palladium and at least one of ruthenium, rhodium, iridium, platinum, silver, gold and osmium.    USE - For separating hydrogen from a hydrogen-containing fluid (claimed) used as an alternative fuel.    ADVANTAGE - The method provides economic way to remove contaminants from hydrogen. The method efficiently uses palladium-alloyed membranes capable of selectively transporting hydrogen while withstanding temperature and pressure cycles encountered in typical applications in which hydrogen purification membranes are used. The method provides efficient and economical method for separating hydrogen. The method efficiently uses palladium-alloyed membranes having improved performance properties for separation of hydrogen. The palladium-alloyed membranes have improved performance properties at high operating temperatures. The improved high temperature performance properties allow for the palladium-alloyed membrane to be operated at, at least one of a high operating temperature, a stable permeate flux, a high permeate purity level, and a longer period of time. The molecular hydrogen producing reactors containing the palladium-alloyed membrane can be operated at, at least one of a higher conversation level, a higher space velocity, and under non-equilibrium conditions. The palladium-alloyed membranes having improved performance properties enhance molecular hydrogen separation processes. The separated hydrogen gas purity levels greater than or equal to 85 mole%, preferably greater than or equal to 90 mole%, more preferably greater than or equal to 92 mole%, especially greater than or equal to 95 mole%, particularly greater than or equal to 99.9 mole% at a temperature range of 573 degrees K to about 1173 degrees K.    DETAILED DESCRIPTION - A method for separating hydrogen from a hydrogen-containing fluid, involves: separating a hydrogen-containing fluid stream at a temperature of 573 to about 1173 degrees K into a permeate stream comprising molecular hydrogen and a retenate stream depleted of molecular hydrogen compared to the hydrogen-containing fluid stream by permeating hydrogen through a palladium-alloyed membrane having a nitrogen leakage growth rate at about 823 degrees K of not greater than 7x 10-12 (mol/m2/second/Pa)/hour, and where the palladium-alloyed membrane comprises palladium and at least one of ruthenium, rhodium, iridium, platinum, silver, gold and osmium. INDEPENDENT CLAIMS are included for the following:    (1) a metallic membrane (M1) comprising: a palladium-alloyed membrane having an average thickness of 1 to about 10 mu m and a nitrogen leakage growth rate at greater than or equal to 823 degrees K of not greater than 7x 10-12 (mol/m2/second/Pa)/hour, where the palladium-alloyed membrane comprises palladium and at least one of ruthenium, platinum, silver, gold and osmium;    (2) a method (P1) for purifying a fluid involving: providing a gaseous fluid stream comprising molecular hydrogen, water, and one or both of carbon dioxide and carbon monoxide; and contacting a palladium-alloyed membrane having a nitrogen leakage growth rate at about 823 degrees K of not greater than 7x 10-12 (mol/m2/second/Pa)/hour with the gaseous fluid stream, where the contacting of the gaseous fluid stream and the palladium-alloyed membrane: i) occurs at a temperature of 573 to about 1173 degrees K; and ii) separates the gaseous fluid stream into a permeate stream comprising molecular hydrogen and a retenate stream depleted of the molecular hydrogen; and    (3) a device comprising: a shell; a membrane positioned in the shell to form a permeate volume and a retenate volume; an inlet configured for introducing a first gaseous stream to the permeate volume; and a first outlet configured to exhaust pure molecular hydrogen from the permeate volume, where the membrane comprises a palladium-alloyed membrane having a nitrogen leakage growth rate at greater than or equal to 823 degrees K of not greater than 7x 10-12 (mol/m2/seconds/Pa)/hour.