p0CH4=0.08;p0CO=0.115;p0CO2=0.0522;p0H2=0.551; p0H2O=0.0;
p=p0CH4+p0CO+p0CO2+p0H2+p0H2O;
t0=800+273;
v=0.1;
nsum0=p*101325*v/(8.314*t0);
n0H2=p0H2/p*nsum0;
n0CO=p0CO/p*nsum0;
n0CO2=p0CO2/p*nsum0;
n0H2O=p0H2O/p*nsum0;
n0CH4=p0CH4/p*nsum0;
mcat=10;

nsum=(nH2[t]+nCO[t]+nCO2[t]+nH2O[t]+nCH4[t]);
pH2=nH2[t]/nsum*p;
pCO=nCO[t]/nsum*p;
pCO2=nCO2[t]/nsum*p;
pH2O=nH2O[t]/nsum*p;
pCH4=nCH4[t]/nsum*p;

k1=9.49*1000000000000000*Exp[-240100/(8.314*t0)]/3600;
k2=4.39*1000000*Exp[-67130/(8.314*t0)]/3600;
k3=2.29*1000000000000000*Exp[-243900/(8.314*t0)]/3600;
keq1=Exp[-26830/t0+30.114];
keq2=Exp[4400/t0-4.036];
kCO=8.23*0.00001*Exp[-70650/(8.314*t0)];
kCH4=6.65*0.0001*Exp[-38280/(8.314*t0)];
kH2O=1.77*10000*Exp[80680/(8.314*t0)];
kH2=6.12*0.000000001*Exp[-82900/(8.314*t0)];
den=1+kCO*pCO+kH2*pH2+kCH4*pCH4+kH2O*pH2O/pH2;
rate1=k1*(pCH4*pH2O/(pH2^2.5)-pH2^0.5*pCO/keq1)/(den^2);
rate2=k2*(pCO*pH2O/pH2-pCO2/keq2)/(den^2);
rate3=k3*(pCH4*(pH2O^2)/(pH2^3.5)-(pH2^0.5)*pCO2/(keq1*keq2)/(den^2));

NDS=NDSolve[
{nH2'[t]==mcat*(3*rate1+rate2+4*rate3),
nCO2'[t]==mcat*(rate2+rate3),
nCO'[t]==mcat*(rate1-rate2),
nH2O'[t]==mcat*(-rate1-rate2-2*rate3),
nCH4'[t]==mcat*(-rate1-rate3),
nH2[0]==n0H2,nCO2[0]==n0CO2,nCO[0]==n0CO,nH2O[0]==n0H2O,nCH4[0]==n0CH4},
{nH2[t],nCO2[t],nCO[t],nH2O[t],nCH4[t]},
{t,0,1},MaxStepSize->0.01];

Plot[nH2O[t]/. NDS,{t,0,0.0000001}]

Table[(rate1)/.NDS,{t,0.001,0.001}]