Dehydrogenation or oxidative dehydrogenation (ODH) of alkanes to produce alkenes directly from natural gas/shale gas is gaining in importance.Ti 3 AlC 2 ,aMAX phase, which hitherto had not been used in catalysis,e fficiently catalyzest he ODH of n-butane to butenes and butadiene, which are important intermediates for the synthesis of polymers and other compounds.The catalyst, which combines both metallic and ceramic properties,i ss table for at least 30 ho n stream, even at lowO 2 :butane ratios,w ithout suffering from coking.T his material has neither lattice oxygens nor noble metals,y et au nique combination of numerous defects and at hin surface Ti 1Ày Al y O 2Ày/2 layer that is rich in oxygen vacancies makes it an active catalyst. Given the large number of compositions available,MAX phases may find applications in several heterogeneously catalyzedreactions.
MAXphases,atermcoinedinthelate1990s,areafamilyofternary carbides and nitrides with layered hexagonal crystal structures.T heir name reflects their chemical formula: M n+1 AX n ,w here Mi sa ne arly transition metal, Ai sa n element (mostly from Groups 13 and 14), Xiscarbon and/or nitrogen, and n = 1, 2, or 3( see Figure 1). Most of these phases were discovered in powder form already in the 1960s, though the synthesis of phase-pure bulk samples was only achieved in 1996.[1] TheMAX phases are interesting because they exhibit au nique combination of ceramic and metallic properties.[2-4] Their bonding is acombination of covalent and metallic.[5] Thed ensity of states at the Fermi level is substantial and dominated by the d-d orbitals of the Melement.[5] Similarly,their electrical resistivity is metal-like in that it drops linearly with decreasing temperatures.T hey conduct heat and electricity like metals,y et they are elastically stiff,s trong,b rittle,a nd some are heat-tolerant like ceramics. [6] Numerous studies have been published on the electrical, thermal and mechanical properties of the MAX phases. [7][8][9][10][11][12] However,t othe best of our knowledge,n oc atalytic applications have been reported so far. Considering the combination of metallic and ceramic properties and high stability,w e anticipated that these materials could be good catalysts. Moreover,t he fact that they differ from most conventional